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INTERFACIAL REACTIONS DURING SOLDERING OF Sn-Ag-Cu LEAD FREE
SOLDERS ON IMMERSION SILVER AND ELECTROLESS NICKEL/
IMMERSION GOLD SURFACE FINISHES
SITI RABIATULL AISHA BINTI IDRIS
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Engineering (Mechanical)
Faculty of Mechanical Engineering
Universiti Teknologi Malaysia
NOVEMBER 2008
iii
To my beloved parents, sisters and brothers,
for their endless love, support and tolerance.
iv
ACKNOWLEDGEMENTS
First and foremost, I would like to express my sincere appreciation to my
thesis supervisors, Associate Professor Dr. Ali Ourdjini and Dr. Astuty Amrin for the
encouragement, guidance and motivation. Throughout the years, they had given me
faith to up bring this project and to deliver it according to the expectations. Their
patience and advice has walked me through all the difficulties I have met.
My heartfelt thanks go to Dr Azmah Hanim who has provided the
unconditional support, guidance and advice throughout the project. I shall never
forget the endless encouragement and assistance she has provided. I also would like
to thank Intel Technology (Malaysia) for providing the resources and funding my
research.
My special thanks also go to all Materials Science Laboratory technicians
especially Mr. Jefri and Mr. Ayub for providing the hardware and technical support
needed to complete this work. It would not have been possible for me to complete
this project without their help. Also, I would like to express my deepest gratitude to
my family members especially mummy and papa, and friends especially Zulkhairry,
Ieyja and Lia for their care and encouragement which have kept me confident and
motivated. Last but not least, the successful completion of this project would have
been impossible without the contribution from the above individuals who have lent
their helping hands. Thus, I would like to express my gratitude to all the individuals
mentioned above for their support and continuous encouragements.
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ABSTRACT
In the on-going trend towards miniaturization in the electronics packaging industry, the increasing popularity of ultra fine line technologies has brought into question the physical aspects of pad topography and metallization. As the solder joints shrink in size, the thickness of the pad metallization available can be very small, thus rendering close control of the soldering process and development of intermetallic compounds at the solder joint is essential. Interfacial reactions and the structure of intermetallics at the solder/substrate interface play an important role in solder joint reliability and the present study was undertaken to investigate these interfacial reactions in order to have a better understanding on the formation of reactions and their growth. In this study, interfacial reactions between Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu solders and immersion silver (ImAg) and electroless nickel/immersion gold (ENIG) surface finishes were investigated. Emphasis is made on the effect of solder size, subsequent ageing of solder joints on the interfacial microstructures. Several techniques of materials characterization including optical, image analysis, scanning electron microscopy and energy dispersive X-ray analysis were used to examine and quantify the intermetallics in terms of composition, thickness and morphology. It was found that after soldering on ImAg only scallop-type Cu6Sn5 layer was formed and that its thickness increases with decreasing solder size. Subsequent ageing produced a second layer of Cu3Sn that forms between the Cu substrate and Cu6Sn5 layer. Growth kinetics showed that the Cu3Sn layer grew at a faster rate than the Cu6Sn5 and that Kirkendall voids were also observed within this Cu3Sn indicating that Cu diffuses much faster in the Cu3Sn than Sn in the Cu6Sn5. When soldering on ENIG finish, the reaction layer was found to consist of only one layer of (Cu, Ni)6Sn5 in the larger solders, while in the smallest solder (200 µm) both (Ni, Cu)3Sn and (Cu, Ni)6Sn5 were formed. These results reconciled well with the current theory of a critical Cu concentration determining the type of intermetallic layer that would form. The Ag content in the solder also affected the nucleation and growth of Ag3Sn plates as well as Cu-Sn intermetallic. Higher Ag containing Sn-Ag-Cu solder promoted growth of Cu6Sn5 rods and large Ag3Sn plates. Subjecting the solder joint to isothermal ageing induced thickening and coarsening of the intermetallics as well as changed in their morphologies. The results showed that the thickness of intermetallics increases with increasing the duration of ageing for both solders investigated and for all solder sphere sizes.
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ABSTRAK
Dalam menuju ke arah pengecilan dalam industri pembungkusan elektronik,
peningkatan kepopularan teknololgi ultra halus telah menimbulkan tanda tanya terhadap aspek fizikal topografi pad dan perlogaman. Sebaik sahaja saiz sambungan pateri mengecut, ketebalan perlogaman pad yang ada boleh menjadi kecil, maka ini menjadikan pengawalan tertutup proses pematerian dan perkembangan sebatian antara logam pada sambungan pateri adalah penting. Tindakbalas antara muka dan struktur sebatian antara logam pada pateri/ antara muka substrat memainkan peranan penting dalam keboleharapan sambungan pateri dan dengan itu, kajian ini dijalankan untuk menyelidik tindakbalas antara muka ini untuk mendapatkan pemahaman yang lebih baik terhadap pembentukan tindakbalas dan penumbuhannya. Dalam kajian ini, tindakbalas antara muka antara pateri Sn-4Ag-0.5Cu dan Sn-3Ag-0.5Cu dan kemasan permukaan immersion silver (ImAg) dan electroless nickel/ immersion gold (ENIG) telah diselidik. Penekanan diberikan kepada kesan saiz pateri, diikuti dengan penuaan sambungan pateri ke atas mikrostruktur antara muka. Beberapa teknik pencirian bahan telah digunakan untuk memeriksa dan menjumlahkan sebatian antara logam yang berkaitan dengan komposisi, ketebalan dan morfologi iaitu kaedah optik, analisis imej, scanning electron microscopy dan energy dispersive X-ray analysis. Didapati bahawa selepas pematerian ke atas ImAg, hanya lapisan Cu6Sn5 jenis scallop dijumpai dan ketebalannya meningkat dengan peningkatan saiz pateri. Proses penuaan menghasilkan lapisan kedua iaitu Cu3Sn yang terbentuk di antara Cu di dalam substrat dan lapisan Cu6Sn5. Kinetik pertumbuhan menunjukkan lapisan Cu3Sn tumbuh dengan kadar yang lebih cepat berbanding Cu6Sn5 dan Kirkendall voids juga didapati terbentuk di dalam Cu3Sn menunjukkan Cu meresap lebih cepat di dalam Cu3Sn berbanding Sn di dalam Cu6Sn5. Apabila pematerian ke atas ENIG dilakukan lapisan tindakbalas didapati terdiri daripada hanya satu lapisan (Cu, Ni)6Sn5 di dalam pateri yang besar manakala di dalam pateri yang kecil (Ø200 µm) kedua-dua (Ni, Cu)3Sn4 dan (Cu, Ni)6Sn5 terbentuk. Keputusan ini bersesuaian dengan teori yang digunakan iaitu kepekatan Cu kritikal menentukan jenis lapisan sebatian antara logam yang akan terbentuk. Kandungan Ag di dalam pateri juga memberikan kesan ke atas penukleusan dan pertumbuhan kepingan Ag3Sn dan juga sebatian antara logam Cu-Sn. Pateri Sn-Ag-Cu yang mengandungi Ag yang tinggi menggalakkan pertumbuhan rod Cu6Sn5 dan kepingan Ag3Sn yang besar. Penuaan ke atas sambungan pateri menggalakkan peningkatan ketebalan dan pengasaran sebatian antara logam dan juga perubahan ke atas morfologinya. Keputusan menunjukkan ketebalan sebatian antara logam meningkat dengan peningkatan masa penuaan untuk kedua-dua jenis pateri dan kesemua saiz pateri logam yang dikaji.
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENTS iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xiii
LIST OF ABBREVIATIONS xviii
LIST OF APPENDICES xx
1 INTRODUCTION 1
1.1 Introduction 1
1.2 Field of Research 4
1.3 Objectives of the Research 6
1.4 Scopes of the Research 6
1.5 Importance of the Research 7
1.6 Structure of Thesis 7
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2 LITERATURE REVIEW - ELECTRONIC PACKAGING 8
2.1 Introduction 8
2.1.1 Electronic Package Hierarchy 9
2.1.2 Purpose of Electronic Packaging 10
2.1.3 Requirement of the Electronic Packaging 11
2.1.4 Interconnection Implementation 12
2.1.4.1 Wire Bonding Interconnection 12
2.1.4.2 Tape-Automated Bonding 14
2.1.4.3 Flip Chip Bonding 16
2.2 Flip Chip Interconnect 21
2.2.1 Solder Bump Structure for Flip Chip
Interconnection 22
2.2.1.1 Under Bump Metallurgy (UBM) 23
2.2.1.2 Top Surface Metallurgy (TSM) 24
3 LITERATURE REVIEW - SURFACE FINISH SYSTEMS 25
3.1 Introduction 25
3.2. Thickness of Surface Finish Layer 26
3.3. Surface Finish Systems 29
3.3.1 Hot-Air Solder Leveling (HASL) 30
3.3.2 Organic Solderability Preservative (OSP) Finish 31
3.3.3 Electroless Nickel/ Immersion Gold 32
3.3.4 Nickel/ Palladium/ Gold Finish 34
3.3.5 Immersion Silver 36
3.3.6 Immersion Tin 38
3.3.7 Summary 39
4 LITERATURE REVIEW – SOLDERING 41
4.1 Introduction 41
4.2 Material 43
4.3 Soldering Techniques 44
4.3.1 Reflow Soldering 44
4.3.2 Wave Soldering 49
4.3.3 Hand Soldering 50
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4.4 Solder Materials 51
4.4.1 Lead-based Solders 54
4.4.2 Lead-free Solders 56
4.4.2.1 Characteristic of Lead Free Solders 59
4.4.2.2 Melting Temperature 61
4.4.2.3 Microstructure 62
4.5 Flux 63
4.5.1 Flux Functions 64
4.5.2 Flux components 65
4.5.3 Types of Flux 66
4.5.3.1 Resin fluxes 66
4.5.3.2 Water soluble flux 67
4.5.3.3 No clean fluxes 68
4.6 Solderability 70
4.7 Intermetallic Compounds 72
4.7.1 Factors Affecting the Growth of IMC 75
4.7.2 Effects of IMC 77
4.8 Isothermal Aging Treatment 78
5 RESEARCH METHODOLOGY 80
5.1 Introduction 80
5.2 Substrate Material 81
5.3 Plating Process 83
5.3.1 Pretreatment of copper substrate 83
5.3.2 Plating Equipment Setup 84
5.3.3 Electroless Nickel Plating 86
5.3.4 Immersion Gold Plating 87
5.3.5 immersion Silver Plating 88
5.4 Reflow Soldering 90
5.4.1 Solder Masking 90
5.4.2 Flux 90
5.4.3 Solder Bump Formation 91
5.5 Isothermal Aging 92
5.6 Materials Characterisation 92
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5.6.1 Characterization of Specimens Cross Section 93
5.6.2 Characterization of Specimens Top Surface 94
6 RESULTS AND DISCUSSION 95
6.1 Introduction 95
6.2 Top Surface Metallurgy (TSM) Deposition 96
6.3 Identification of Intermetallics in Solder Joints 97
6.4 Composition and Surface Morphologies of IMC 100
6.4.1 Intermetallics between SAC and ImAg 100
6.4.1.1 Reflow Soldering 100
6.4.1.2 Isothermal Ageing 106
6.4.1.3 Formation of Kirkendall Voids 116
6.4.2 Intermetallics between SAC and ENIG 118
6.4.2.1 Reflow Soldering 118
6.4.2.2 Isothermal Ageing 133
6.5 Thickness of Intermetallic Compound 145
6.5.1 Effect of Solder Volume on IMC Thickness 145
6.5.2 Effect of Surface Finishes on IMC Thickness 149
6.5.3 Growth Kinetics of IMC on ImAg Finish 150
6.5.4 Effect of Ag Concentration on IMC Thickness 154
6.5.5 Effect of Ageing Duration on IMC Thickness 159
7 CONCLUSIONS AND FUTURE WORKS 161
7.1 Conclusion 161
7.2 Future Works 162
REFERENCES 164
APPENDIX 175
PUBLISHED PAPERS 191
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LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Levels of interconnection for general electronic system 10 2.2 The advantages and disadvantages of wire bonding
interconnection 13 2.3 The advantages and disadvantages of TAB over the wire
bonding technology 15 2.4 Comparison of Interconnection Implementation 19 3.1 Comparison between different surface finish 39 4.1 Summary of reflow profiling 46 4.2 Benefits and limitations for vary reflow method 48 4.3 Melting properties of some common solder alloys 51 4.4 Lead-Free Solders for CSP Applications 56 4.5 Properties of Hard and Soft Solder Alloys 60 4.6 Lead-free solders with liquidus (T1), solidus (T2) and
eutectic (Te) 61 4.7 Solderability of different base metal 72 4.8 Potential IMC formation and un-compatibility between
solder and common substrates 73 5.1 The Swan and Gostin bath 87 5.2 Immersion silver bath formulation 89 5.3 Chemical composition of Klemm Solution II 93
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5.4 Etching time for cross sectional deep etching 94 6.1 Atomic number of elements 99 6.2 Atomic percentage of predicted IMCs 99 6.3 Compositions of the interfacial reaction products after reflow
soldering and ageing for 2000 hours at 150 oC 134 6.4 Intermetallic Thickness (µm) on ImAg surface finish 146 6.5 Intermetallic Thickness (µm) on ENIG surface finish 146 6.6 Calculation of the growth rate coefficient (D) for
SAC405/ ImAg 152 6.7 Calculation of the growth rate coefficient (D) for
SAC305/ ImAg 152
xiii
LIST OF FIGURES
FIGURE NO. TITLE PAGE
1.1 Schematic showing the main parts of an electronic package 2 2.1 Electronic packaging hierarchy 9 2.2 The wire bonding assembly shows how a bare chip is
interconnected to a substrate or another chip using a wire conductor 13
2.3 Schematic of TAB 14 2.4 Example of TAB devices 15 2.5 (a) Standard flip chip array with solder bumps.
(b) Cross-section of flip chip bonding 17 2.6 Solder Bump Structure 22 3.1 Dissolution rates of a few typical base metals in tin 27 3.2 Schematic diagram of the HASL technique 31 4.1 Typical solder reflow profile for eutectic Pb-Sn solder 45 4.2 The main heating options in reflow soldering 47 4.3 Typical wave soldering machine 49 4.4 The principle of hand soldering 50 4.5 Phase Diagram of Pb-Sn Alloy 54 4.6 The wetting angle 71
xiv
4.7 Cross section through a soldered joint, made with eutectic solder 74
4.8 Types of intermetallics formed between Cu and Sn 74 4.9 Needle-like Cu6Sn5 intermetallics 74 4.10 Schematic diagrams of the layers before and after isothermal
ageing 79
5.1 (a) Plan view and (b) Side view of copper substrate 81
5.2 Process flowchart of reflow soldering and specimen analysis 82 5.3 (a) Experimental set-up in the plating bath, (b) The plating
bath and (c) Schematic set-up of electroless nickel and immersion gold plating process 85
5.4 Schematic set-up of immersion silver plating 85 5.5 Commercial medium phosphorous concentration
electroless nickel plating solution: NIMUDEN 5X 86 5.6 Schematic process of electroless nickel plating 86 5.7 Schematic process of immersion gold plating on nickel 87 5.8 Immersion silver plating steps 88 5.9 Schematic process of immersion silver plating 89 5.10 Process flowchart for Immersion Silver 89 5.11 Solder joint formations for ENIG surface finish 91 5.12 Reflow profile fro Sn-Ag-Cu 92 5.13 IMCs formed from the top surface view 94 6.1 Copper substrate before plating (after pretreatment process) 96 6.2 Copper substrate plated with silver coating 97 6.3 FESEM-EDX results of IAg on Cu 97 6.4 Example of weight percentage calculation 98 6.5 Cross-sectional optical images after reflow: a) SAC405/ ImAg,
b) SAC305/ImAg 101
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6.6 Cross-sectional SEM images after reflow: a) SAC405/ImAg, b) SAC305/ImAg 101
6.7 Top view micrographs formed during reflow between SAC405
solder and ImAg. (a) 200µm, (b) 300µm, (c) 500µm and (d) 700µm 102
6.8 Top view micrographs formed during reflow between SAC305 solder and ImAg. (a) 200µm, (b) 300µm, (c) 500µm and (d) 700µm 103
6.9 Top view SEM images showing formation of large Ag3Sn
plates and Cu6Sn5 rods in SAC405/ ImAg (a, b) and Cu6Sn5 rods on SAC305/ ImAg (c) 104
6.10 Formation of Ag3Sn during reflow between SAC405
solder and ImAg:(a, b, c) Top surface morphology of the solder joint and (d) Cross section (x500) 105
6.11 Optical micrographs of cross-sectional views of SAC405/ ImAg (a-c) and SAC305/ImAg (d-f). (a, d): after reflow and (b, e): after ageing at 150oC for 250 hours and (c,f) after ageing at 150oC for 2000 hours 107 6.12 SEM images of cross-sectional views showing the effect of
ageing on the interfacial morphology. (a) SAC405/ImAg and (b) SAC305/ ImAg 109
6.13 Morphology of Cu6Sn5 on ImAg for 200µm solder bump of
SAC405, (a) Ageing 250 hours, (b) Ageing 500 hours, (c) Ageing 1000 hours and (d) Ageing 2000 hours 110
6.14 Morphology of Cu6Sn5 on ImAg for 200µm solder bump of
SAC305, (a) Ageing 250 hours, (b) Ageing 500 hours, (c) Ageing 1000 hours and (d) Ageing 2000 hours 111
6.15 Morphology of Cu6Sn5 on ImAg for 700µm solder bump of
SAC405, (a) Ageing 250 hours, (b) Ageing 500 hours, (c) Ageing 1000 hours and (d) Ageing 2000 hours 112
6.16 Morphology of Cu6Sn5 on ImAg for 700µm solder bump of
SAC305, (a) Ageing 250 hours, (b) Ageing 500 hours, (c) Ageing 1000 hours and (d) Ageing 2000 hours 113
6.17 Schematic of Ag3Sn particles embedded during intermetallic
growth 113 6.18 Ag3Sn on ImAg using 700µm solder (a) After reflow and
(b) After ageing for 500 hours 115
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6.19 Schematic diagram of IMCs growth in Cu/Au specimen: (a) dissolution of Ag layer into molten solder, (b) formation of Cu6Sn5 during reflow soldering and (c) Conversion of Cu3Sn and Ag3Sn after isothermal ageing 116
6.20 SEM image of cross-sectional view of 500µm SAC405
solder/ ImAg after ageing for 500 hours 117 6.21 SEM image of cross-sectional view of 700µm SAC405
solder/ ImAg after ageing for 1000 hours 117 6.22 The mechanism of Kirkendall Voids formation 118 6.23 Cross-section views of the intermetallics formed between
ENIG and SAC405 (a-c) and SAC305 (d-f) solders (X500) 119 6.24 Cross section and top views of (Cu, Ni)6Sn5 IMC formed
during reflow between ENIG and SAC405 solder. (a, d) 300 µm, (b, e) 500 µm and (c, f) 700 µm 122
6.25 Cross section and top views of (Cu, Ni)6Sn5 IMC formed
during reflow between ENIG and SAC305 solder. (a, d) 300 µm, (b, e) 500 µm and (c, f) 700 µm 123
6.26 Top view of intermetallics formed between ENIG and
200 µm SAC405 (top) and SAC305 (bottom) solders 126 6.27 SEM images of cross sections of intermetallic formed between
ENIG and SAC405 for (a) 500 µm and (b) 700 µm solders 127 6.28 SEM images of cross sections of intermetallic formed between
ENIG and SAC305 for (a) 300 µm and (b) 500 µm solders 128 6.29a EDX results of interface intermetallic formed between ENIG
and 500 µm SAC405 solder during reflow 129 6.29b EDX results of interface intermetallic formed between ENIG
and 700 µm SAC405 solder during reflow 130 6.30a EDX results of interface intermetallic formed between ENIG
and 300 µm SAC305 solder during reflow 131 6.30b EDX results of interface intermetallic formed between ENIG
and 500 µm SAC305 solder during reflow 132 6.31 Cross sections of IMCs formed between ENIG and SAC405
solder. (a) reflow (500 µm) and (b) after 2000 hrs ageing (500 µm),(c) reflow (700 µm) and (d) after 2000 hrs ageing (700 µm) 134
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6.32 SAC305 (a) after reflow and (b) ageing (2000 hrs) 135 6.33 Effect of ageing time on intermetallics formed between ENIG
and SAC405 solder with different solder sizes. 300 µm solder: a: reflow, b: ageing for 250 hours, g: ageing for 500 hours and h: ageing for 2000 hours. 500 µm solder: c: reflow, d: ageing for 250 hours, i: ageing for 500 hours and j: ageing for 2000 hours. 700 µm solder: e: reflow, f: ageing for 250 hours, 136
6.34 Top view of intermetallics formed between ENIG and 200 µm
SAC405: a: reflow, b: ageing for 250 hours, and c: ageing for 1000 hours 138
6.35 Formation of Ag3Sn in the 700 microns solder bump after reflow
soldering (SAC405). (a, b) Top surface morphology of the solder joint and (c, d) Cross section 141
6.36 SEM image showing different morphology of intermetallics
between center and periphery of solder joint 142 6.37 Different morphologies of IMC which form circular boundary
regions in Sn-Ag-Cu solder joint, (a) Sn-3Ag-0.5Cu solder and (b) Sn-4Ag-0.5Cu solder 143
6.38 Effect of Ageing on the morphology of Ag3Sn intermetallic.
(a) After reflow soldering and (b) After 500 hours ageing 144 6.39 Intermetallic thickness versus solder bump size for ImAg
surface finish as function of ageing time. (a) Sn-4Ag-0.5Cu and (b) Sn-3Ag-0.5Cu 147
6.40 Intermetallic thickness versus solder bump size for ENIG surface
finish as function of ageing time. (a) SAC405 and (b) SAC305 148 6.41 Intermetallic thickness versus ageing time between SAC405
and ImAg surface finish: (top) Cu3Sn and (bottom) Cu6Sn5 layer 153
6.42 Intermetallic thickness versus ageing time between SAC305
and ImAg surface finish: (top) Cu3Sn and (bottom) Cu6Sn5 layer 154
6.43 SEM top views of Ag3Sn intermetallic for SAC405/ ImAg
(a, b) and SAC305/ ImAg (c) 157 6.44 Ag3Sn IMC formation in ImAg surface finish 159
xviii
LIST OF ABBREVIATIONS
ASTM - American Society for Testing and Materials
BGA - Ball grid array
BLM - Ball limiting metallurgy
C4 - Controlled collapse chip connection
CSP - Chip scale package
COB - Chip on board
DIG - Direct immersion gold
EDX - Energy dispersive spectrum
ENEPIG - Electroless nickel/ electroless palladium/ immersion gold
ENIG - Electroless nickel /immersion gold
EU - European Union
FC - Flip chip
FESEM - Field emission scanning electron microscope
HASL - Hot air soldered levelled
IC - Integrated circuit
IMC - Intermetallic compound
I/ O - Input/ output
xix
OSP - Organic solderable presevatives
PCB - Printed circuit board
PWB - Printed wire bonding
R - Pure rosin flux
RA - Activated rosin flux
RMA - Midly activated rosin flux
SEM - Scanning electron microscope
SMD - Surface mount devices
SMT - Surface mount technology
TAB - Tape automated bonding
TSM - Top surface metallurgy
WB - Wire bonding
WEEE Waste from electrical and electronic equipments
WW - Water white rosin flux
XRD - X-ray diffraction
xx
LIST OF APPENDICES
APPENDIX TITLE PAGE
A FESEM/ EDX results (selected samples only) 175 B Tables and Graphs 185
CHAPTER 1
INTRODUCTION
1.1 Introduction
Integrated circuit (IC) chips are the heart of electronic system controls, and
since they are typically sensitive to electrical, mechanical, physical and chemical
influences, they require special considerations by the packaging engineer. Today’s
circuit and system-level requirements of high performance, high reliability and low
cost have placed greater demands on the packaging engineer to have a better
understanding of the existing and emerging IC packaging technologies.
The number of input/output (I/O) pin count for high-end use is increasing all
the time with decreasing package size. On these advanced high pin count chips, the
electrode pads are arranged into an area array with narrow pad pitch and bumps are
formed on each pad for flip chip interconnection. Assembly in flip chip
interconnection is a direct electrical and mechanical connection face down of a bare
die onto the printed circuit board (PWB) by means of solder bumps. As shown in
Figure 1.1 the entire interconnection system consists of four parts: under bump
metallurgy (UBM) on the die side, solder balls, and substrate metallization pad with
top surface metallurgy (TSM). Both UBM and TSM provide adhesion, diffusion
2
barriers and protection layers. During assembly, and hence soldering, the solder
melts and an intermetallic layer forms between the solder and metallurgy pads on
both sides of the package.
Flipped Solder balls Chip
Upper Bump Metallurgy (UBM)
Underfill epoxy
Die
Solder Ball • Conductive path • Thermal conductivity • Mechanical support
Solder Top Surface Metallurgy (TSM) /
Surface Finish • Adhesion layer • Solder wettable layer • Diffusion barrier layer • Oxidation barrier layer
Substrate
Figure 1.1: Schematic showing the main parts of an electronic package
The demand has recently increased for new bump formation technologies
which enable the simultaneous formation of large numbers of bumps with a narrow
bump pitch at low cost and short tact processing. However, some reliability issues
may be arising from the utilization of smaller solder bump size.
Due to its excellent conductivity and surface for soldering, copper has been
widely used as the substrate materials. However, several types of metal coating must
also be deposited on copper surfaces as board finishes for the purpose of providing
3
wetting surfaces and protection against the environment. The selection of a
metallurgical system (solder – top surface metallurgy) is very important because of
its influence on the reliability of electronic assemblies. Typical surface finish
metallurgy consists of two main layers: 1) a solderable layer in contact with the
underlying copper and 2) a protective layer on top of the solderable layer. The
purpose of the solderable layer is to provide the surface to which the liquid solder
wets and then adheres upon solidification. This same solderable layer also acts as the
diffusion barrier by preventing diffusion of the solder to the copper substrate. The
protective layer serves to protect the solderability of the solderable layer from
degradation due to exposure to ambient environment until reflow soldering occurs.
During reflow the solder melts and the protective layer dissolves into the molten
solder exposing in the process the solderable layer to the molten solder. The
solderable layer now is also subjected to dissolution by the molten solder until
solidification is complete. This results in the formation of an intermetallic layer
between the solderable layer and solder. This intermetallic layer will grow in
thickness during subsequent thermal ageing after assembly due to solid – solid
reaction between the solderable layer and the solder by solid-state diffusion.
An important aspect of solder joint processing is a good understanding of the
solder – substrate metallization reaction. The intermetallic layer, which develops
from this reaction, is essential in order to achieve strong and reliable solder joints.
However, excessive growth of this intermetallic layer may lead to degradation of
solder joint reliability. Also the morphology of the intermetallic layer after soldering
and subsequent solid-state thermal processes may also affect the reliability of the
solder joint.
4
1.2 Field of Research
While the majority of surface finishes can be considered as multifunctional in
nature, their primary function remains that of providing either solderability
protection to the underlying copper basis material or to act as a solderable surface to
which the solder joint will be formed. There are many factors that can influence the
choice or rejection of a surface finish – cost, availability and even misinformation –
but as a minimum a good surface finish should have or meet the following criteria: 1)
It must have reasonable shelf-life; 2) It must be capable of withstanding multiple
process steps; 3) It should be compatible with existing assembly equipment.
Among these board finishes, electroless nickel/ immersion gold (ENIG) has
gained a significant level of interest in the last few years. This metal coating is
deposited directly on copper to provide a diffusion barrier (to prevent copper
dissolution in liquid solder), is very solderable, provides flat board finishes and
protects against oxidation. Also, compared to other metals, growth of intermetallics
between Ni and Sn is much slower. However, as the usage of ENIG increased, a
problem of void formation which leads to brittle failure was found. These voids are
also known as the Kirkendall voids. It is believed that these voids are formed
because of the fast diffusion of Ni from the phosphorus rich nickel layer, which is a
by-product layer of Ni-Sn interfacial reaction. This concern of voiding is further
magnified with lead-free soldering. Thus, the search for a surface finish that
produces high reliability of solder joints continues.
An ideal surface finish is still eluding most researchers and manufacturers
especially with the imminent global usage of lead free solders. Thus, the need to find
the most suitable and low cost surface finish has become necessary. Recently, silver
coatings, including the new immersion silver coatings are used in numerous
electronic components (Arra, M., et al., 2003). This is due to its lower material cost,
wire-bondable and silver coating itself does not melt, but instead, it dissolves into the
molten solder, which may decrease the speed of wetting. Apart from that, immersion
5
silver can develop single element coating (which can reduce cost); result in relatively
thin layers (typically less than 1µm) because the deposition process halts when the
substrate surface is completely covered with the coated material.
Based on the challenges discussed above, this research aims at understanding
the interfacial reactions occurring during soldering and subsequent thermal ageing
between different lead-free solders and different surface finishes metallurgies. Many
research studies have been performed on the interfacial reactions between lead-free
solders and various surface finish metallurgies, such as Cu, and ENIG but very little
research has been done on immersion silver finish and thus knowledge on interfacial
reactions during reflow and ageing is lacking. The current research addresses in
particular the effect of solder bump sizes, solder composition, and thermal ageing on
the type, size and morphologies of intermetallics formed on immersion silver (ImAg)
and electroless nickel/immersion gold (ENIG) finishes. The solder alloys
investigated are Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu with spheres having diameters of
700, 500, 300 and 200 µm. In order to quantify the effect of temperature on the
growth of intermetallics the solder joins formed after reflow are subjected to thermal
ageing at 150 oC for up to 2000 hours. To address the above issues and achieve the
research aims reflow soldering experiments have been conducted and several
characterization techniques were used in including, optical microcopy, image
analysis, scanning electron microscopy (SEM) and energy dispersive X-ray analysis
(EDX). Special focus will also be on the formation and growth of interfacial voids
and their influence on the reliability of solder joints.
6
1.3 Objectives of the Research
The followings are the objectives of the project:
To identify the types of intermetallic compounds formed during the
interfacial reaction between lead-free solders and several surface finish
metallurgies, mainly electroless nickel/immersion gold (ENIG) and
immersion silver (ImAg).
To quantify the effect of solid state ageing duration on the formation and
growth of intermetallics both at the solder/ substrate interface and bulk
solder.
To establish the effect of solder bump size, Ag concentration of the solder
alloy and reflow soldering time on the interfacial reactions.
1.4 Scope of the Research
The project consists of two main tasks:
1. Deposition of the surface finish, ENIG and ImAg. This task will involve the
use of the electroless and immersion plating processes to deposit the desired
thickness of the finish layers on a copper substrate.
2. The second task aims at conducting experimental work of soldering between
liquid lead-free solder on the two surface finishes described above and
evaluate the effect of factors such as, solid state aging, reflow soldering time
and cooling rates on the formation and growth of intermetallics both at the
solder/ substrate interface and bulk solder. Characterization will involve the
type, morphology and thickness of intermetallics as well as the volume
fraction of these intermetallics both at the interface and bulk solder.
7
1.5 Importance of the Research
In this research, we will try to obtain more knowledge on a new kind of
surface finish, the immersion silver (ImAg) with lead free solders, i.e., Sn-4Ag-
0.5Cu and Sn-3Ag-0.5Cu. Other parameters will also be taken into consideration
like the sizes of the solder ball, i.e., 200µm, 300µm, 500µm and 700µm. ENIG also
will be conducted in this research as a comparison to IAg. We would also study the
effect of different ageing time on the solder joints such as 0 hour, 250 hours, 500
hours, 1000 hours and 2000 hours. We hope that the results from this research would
provide a clearer view of what is happening in the solder joint and to what extent the
intermetallics may become detrimental to the solder joint so that more consideration
factors for future design and material selection could be provided.
1.6 Structure of the Thesis
This thesis comprises five chapters. Chapter one is an introduction in which
problem statement, objective of the research and scope of work are presented. The
literature review is divided into three parts. Part one is presented in chapter two
which covers the basics on electronic packaging and methods of bonding such as flip
chip bonding, wire bonding and tape automated bonding. The second part of
literature review is in chapter three and covers the surface finish systems for TSM,
coating technology and plating techniques. Whereas the third part of literature
review (chapter four) discusses the soldering methods as well as the intermetallic
compounds formation in the solder joint during soldering. In chapter five, the
detailed experimental procedure and techniques employed in the current research are
presented and discussed. In chapter six, results and discussion, the author presents all
experimental results obtained and evidence to support them. Finally, in chapter
seven, a set of conclusions and future work is presented.
CHAPTER 2
LITERATURE REVIEW
ELECTRONIC PACKAGING
2.1 Introduction
Microelectronics devices contain many electronic components within an
active silicon chip, such as transistors, capacitors, resistors, etc. To form a usable
device, a silicon chip requires protection from the environment as well as both
electrical and mechanical connections to the surrounding components. The
technology dealing with these requirements is called electronic packaging. The
physical design of an electronic package starts from the functions of the integrated
circuits on the semiconductor chips and components. The design must provide
access to all the terminals on the chips for input power and signal transmission.
Secondly the design must provide the electrical wiring for interconnection. In
addition, thermal energy transformed from electrical energy must be dissipated, and
all the circuits must be protected from damage during next level assembly and its
service life (Lau, 1994).
9
Most electronic applications require increased reliability and performance as
well as lower cost, weight and size. All of these factors depend on the capabilities
related to making more integrated components, which in turn depend on advanced
assembly equipment that can put a large number of small components into smaller
and smaller areas.
2.1.1 Electronic Package Hierarchy
A general electronic system could be classified into four packaging or
interconnect levels, as shown in Figure 2.1 (Gilleo, 2002). These packaging and
interconnection levels are:
Zero level packaging Chip connection
First level packaging IC packaging
Second level packaging Printed circuit board (PCB)
Third level packaging Circuit card to mother board
Figure 2.1: Electronic packaging hierarchy
10
Table 2.1: Levels of interconnection for general electronic system
Level Description
Level - 0 This level involves interconnecting different electronic elements such as
transistors, resistors, capacitors, etc, on the same chip with no packaging.
Physically, this microelectronic circuit is called a `bare die' or `bare chip.'
Level - 1 Pertains to all processes (i.e. mounting, bonding and encapsulating)
involved in packaging a bare die to produce an integrated circuit
(IC).Wiring the die to a package usually involves one of the
interconnection methods discussed in the next section.
Level - 2 Pertains to all the technologies employed in interconnecting a number of
such `integrated circuits' on a printed circuit board (PCB).
Level - 3 Pertains to the interconnection of the boards into a cabinet system
Level - 4 Pertains to the cabling interconnections and housing of the final system.
A typical microelectronic package is designed to provide the following
functions:
• Connections for signal lines leading onto and off the silicon chip.
• Connections for power lines that powers the circuits on the chip.
• Connections of signal lines between the system components and
interconnections for input/output terminals.
• Removal of the heat generated by the circuits.
• Support and protection of the bare chip.
2.1.2 Purpose of Electronic Packaging
The followings are the functions of electronic packaging:
a) Signal passage – Provide a path for the electrical current that empowers the
circuits on the chip.
11
b) Power distribution – Distribute the signals onto and off of the silicon chip.
c) Heat dissipation – Remove the heat generated by the circuit.
d) Protection – Support and protect the chip from hostile environment.
2.1.3 Requirement of the Electronic Packaging
There are a large number of requirements that an electronic package has to fulfill,
such as:
a) Mechanical requirements
These may involve constraints on the structure and thermal characteristics of the
supporting substrate. The substrate is a base material that provides a supporting
surface for deposited or etched wiring patterns - for attachment of component
parts or for fabrication of a semiconductor device.
b) Input/Output (I/O) requirements
These vary significantly depending on the system of interest. For example, the
I/O ports for a hand-held calculator are the keyboard and the display. By
contrast, in a large computer system the I/O ports may include tape drives, disk
drives, printers, etc.
c) Environmental requirements
Tolerance of the packaging to operating conditions such as air humidity and
exposure to chemicals.
12
d) Reliability requirements
The system's ability to operate for many years with very few problems
e) Interconnection requirements
The number of I/O ports that are needed to provide inter-chip communication,
i.e. between chips and the supporting substrate.
2.1.4 Interconnection Implementation
An interconnection is the conductive path required to achieve connection
from one circuit element to another or to the rest of the circuit system. Such
interconnections may be pins, terminals, formed conductors, or any other mating
system. At the chip level, interconnects are needed to connect the different
electronic circuit elements implemented on or in the chip such as transistors,
capacitors, etc. There are three types of interconnection in electronic packaging:
2.1.4.1 Wire Bonding Interconnection
Wire bonding is a method used to connect a fine wire between an on-chip pad
and a substrate pad. This substrate may simply be the ceramic base of a package or
another chip. The structure of a wire bond assembly is shown in Figure 2.2.
Common wire materials include gold and aluminium. Besides, wire bonding also
provides some advantages and disadvantages, as shown in Table 2.2.
13
Figure 2.2: The wire bonding assembly shows how a bare chip is interconnected to a
substrate or another chip using a wire conductor. Hence, the `substrate pad' may
either be a package pad or a pad on another chip (Abtew, M., 2000).
Table 2.2: The advantages and disadvantages of wire bonding interconnection
Advantages Disadvantages
• Low cost
• It is commodity unlike the
advanced die attach platforms for
flip chip bonding
• It is extremely flexible-changes in
die size can be accommodated
without noticeable additional costs
• Low I/O counts due to
technology limitations
• Large bonding pads in order of
100 x 100 µm2
• Large bonding pitch in order of
200 µm
• The requirement for relatively
large quantities of gold
• Production rat
• Relatively poor electrical
performance
• Variations in bond geometry
• Robustness and reliability
problems brought about by
environmental conditions
14
2.1.4.2 Tape-Automated Bonding
Tape-automated bonding (TAB) is an approach to fine the pitch
interconnection of a chip to a lead-frame. The interconnections are patterned on a
multilayer polymer tape. The tape is positioned above the `bare die' so that the metal
tracks (on the polymer tape) correspond to the bonding sites on the die, as shown in
Figure 2.3 and Figure 2.4. The connections from the tape to the chip are called inner
lead bonds. These bonds are made by dissolving some of the polymer away so the
electrical connections are left as small cantilever beams that are bonded to the pads
on the chip. Connections from the tape to the substrate are called the outer lead
bonds, and are made by soldering or by thermal compression bonding (Donald et al.,
1989). A new version of TAB, referred to as `area TAB', borrows a good idea from a
bonding technique called `bump bonding'. Besides that, tape-automated bonding
technology provides several advantages over the wire bonding technology, as shown
in Table 2.3.
Figure 2.3: Schematic of TAB (Duffek, 1974)
15
Figure 2.4: Example of TAB devices (Duffek, 1974)
Table 2.3: The advantages and disadvantages of TAB over the wire bonding
technology
Advantages Disadvantages
• Smaller bonding pad
• Have smaller on-chip bonding
pitch in the rang of 100 µm
• A decrease in the quantity of gold
used for bonding
• The reduction of variations in
bond geometry
• An increase in production rate
because of ‘gang’ bonding
• A stronger and more uniform
inner lead bonding strength
• Better electrical performance and
higher I/O counts (up to 850 pins)
• Lower labour costs
• Lighter weight and greater
densities
• Chip can be attached in face up
and face down configuration
• Cost and time consuming in
designing and fabricating the tape
• High capital expense of the TAB
bonding equipment
• Limited to high volume
production applications because
of each die must have its own
tape patterned for its bonding
configuration
In addition to better electrical performance (noise and frequency), lower
labour costs, higher I/O counts (up to 850 pins) and lighter weight, greater densities
16
are achievable and the chip can be attached in a face-up or face-down configuration
(Sergent, J. E., and Herper, C.A., 1995). On the other hand, the disadvantages of
TAB technology include the time and cost of designing and fabricating the tape and
the capital expense of the TAB bonding equipment. In addition, each die must have
its own tape patterned for its bonding configuration. For these reasons, TAB has
typically been limited to high-volume production applications.
2.1.4.3 Flip Chip Bonding
In the development of packaging of electronics the aim is to lower cost,
increase the packaging density and improve the performance while still maintaining
or even improving the reliability of the circuits. The concept of flip chip process
where the semiconductor chip is assembled face down onto circuit board is ideal for
size considerations, because there is no extra area needed for contacting on the sides
of the component (Baldwin, 2002). The performance in high frequency applications
is superior to other interconnection methods, because the length of the connection
path is minimized. Also reliability is better than with packaged components due to
decreased number of connections. In flip chip joining, there is only one level of
connections between the chip and the circuit board.
The length of the electrical connections between the chip and the substrate
can be minimized by:
(a) Placing solder bumps on the die
(b) Flipping the die over
(c) Aligning the solder bumps with the contact pads on the substrate
(d) Reflowing the solder balls in a furnace to establish the bonding between the
die and the substrate.
17
(a)
Die Epoxy Underfill
Solder ball
Substrate Mold cap
(b)
Figure 2.5: (a) Standard flip chip array with solder bumps (Gilleo, 2002). (b) Cross-
section of flip chip bonding
The flip chip bonding technology provides several advantages. These advantages
include:
a) Smaller size:
Smaller IC footprint (only about 5% of that of packaged IC e.g. quad flat
pack), reduced height and weight.
b) Increased functionality:
The use of flip chip allows an increase in the number of I/O. I/O is not limited
to the perimeter of the chip as in wire bonding. An area array pad layout
18
enables more signal, power and ground connections in less space. Flip chip
can easily handle more than 400 pads.
c) Improved performance:
Short interconnect delivers low inductance, resistance and capacitance, small
electrical delays, good high frequency characteristics, thermal path from the
back side of the die.
d) Improved reliability:
Epoxy underfill in large chips ensures high reliability. Flip chip can reduce
the number connections per pin from three to one. Improved thermal
capabilities: Because flip chips are not encapsulated, the back side of the chip
can be used for efficient cooling.
e) Low cost:
Batch bumping process, reduced cost of bumping, and cost reductions in the
underfill-process.
Besides that, flip chip also has some disadvantages. The followings are the
disadvantages of a flip chip:
1. Difficult testing of bare die.
2. Limited availability of bumped chips.
3. Challenge for PCB technology as pitches become very fine and bump counts
are high.
4. For inspection of hidden joints x-ray equipment is needed.
5. Handling of bare chips is difficult.
6. High assembly accuracy needed.
7. With present day materials under filling process with a considerable curing
time needed.
8. Low reliability for some substrates.
9. Repairing is difficult or impossible.
19
Table 2.4: Comparison of Interconnection Implementation
Wire Bonding Tape-Automated
Bonding
Flip Chip Bonding
Properties
Substrate
metallization
Not critical: thin or
thick film gold
Not critical: Au or
solder
Low CTE required
for large chips
(>170 mils) solder
bond pads (size
critical)
Extra IC
processing
Not required Au or solder bumped
I/O (wafer level)
Solder bumped I/O
pads (wafer level)
Interconnect Gold or Al wire Au weld on solder
Cu tape leads (Au
plated/welded or
soldered)
Solder
Bonding process Thermocompressio
n thermosonic
ultrasonic
Chip-specific tape
TAB ILB/OLB gang
or single point
Solder reflow
(batch) Heat, inert
atmosphere
Capital
equipment
Manual or
automatic wire
bonder
Pick and place, TAB
form and exercise,
ILB and OLB
Face down
alignment bonder
Die attachment Conductive/noncon
ductive adhesion or
metallurgical
Conductive/noncond
uctive adhesive
Nonconductive
adhesive or no
adhesive
I/O pitch (min.)
I/O location
6 mils proved
Staggered
4-8 mils
Perimeter
8 mils
Perimeter on area
Reworkability Yes OLB-yes Difficult (solder
volume control
oxidation without
flux)
20
Table 2.4 (continued): Comparison between different surface finishes
Wire Bonding Tape-Automated
Bonding
Flip Chip
Bonding
Chip-to-chip
spacing
50-70 mils 100 mils standard,
70 mils feasible
10-30 mils feasible
(rework limited)
Chip
pretestability
N/A Possible N/A
Reliability issues Reliability proven
nonconductive pull
testing
Visual inspection
possible bond
integrity being
addressed
Inspection difficult
CTE matched
substrate required
to reduce solder
fatigue Difficult to
clean beneath die
Electrical Speed a power
limited
Low inductance
resistance
Best performance
Heat dissipation Best for
metallurgical
attachment
Good-through
adhesive and Cu
leads
Poor-through
solder I/Os
(depends on total
I/O area)
Technology
maturity
Very mature Demonstrated by
many, most
application single-
chip, high volume
Demonstrated in
production: IBM,
Delco, Motorola (8
mil pitch min.)
Key issue Device pretesting
at speed difficult
IC bumping
required Tape,
hand tooling
required for each
IC type Tape
quality/delivery
delays
Device pretesting,
heat dissipation,
inspectability,
rework IC bumping
required
21
2.2 Flip Chip Interconnections
Flip chip joining is not a new technology. IBM has driven the technology for
mainframe computer applications and has processed many millions of chips on
ceramic substrates since the end of 1960’s. At the beginning of the 1970’s, the
automotive industry also began to use flip chips on ceramics. Today, flip chips are
widely used for watches, mobile phones, portable communicators, disk drives,
hearing aids, liquid crystal display (LCD) display, automotive engine controllers as
well as the main frame computers. The number of flip chips assembled was over 500
million in year 1995 and close to 600 million flip chips were consumed in 1997
(Rymaszewski and Tummala, 1989).
Flip chip describes the method of electrically connecting the die to the
package carrier. The package carrier, either substrate or lead-frame, then provides
the connection from the die to the exterior of the package. In “standard” packaging,
the interconnection between the die and the carrier is made using wire. The die is
attached to the carrier face up, and then a wire is bonded first to the die, then looped
and bonded to the carrier. Wires are typically 1-5 mm in length, and 25-35 µm in
diameter. In contrast, the connection between the die and carrier in flip chip
packaging is made through a conductive “bump” that is placed directly on the die
surface. The bumped die is then “flipped over” and placed face down, with the
bumps connecting to the carrier directly. A bump is typically 70-100 µm high, and
100-125 µm in diameter.
The flip chip connection is generally formed either using solder or using
conductive adhesive. The solder bump is attached to a substrate by a solder reflow
process, very similar to the process used to attach BGA balls to the package exterior.
After the die is soldered, underfill is added between the die and the substrate.
Underfill is a specially engineered epoxy that fills the area between the die and the
carrier, surrounding the solder bumps. It is deigned to control the stress in the solder
joints caused by the difference in thermal expansion between the silicon die and the
22
carrier. Once cured, the underfill absorbs the stress, reducing the strain on the solder
bumps, greatly increasing the life of the finished package. The chip attach and
underfill steps are the basics of flip chip interconnect. Beyond this, the reminders of
package construction surrounding the die can take many forms and can generally
utilize existing manufacturing processes and package formats.
2.2.1 Solder Bump Structure for Flip Chip Interconnection
Solder bump structure consists of the following:
1. Under Bump Metallurgy (UBM) (on the die side)
2. Top Surface Metallurgy (TSM) (on the substrate side)
Chip
Upper BumpMetallurgy
(UBM)
Solder Bump
Top SurfaceMetallurgy(TSM)
Substrate
Figure 2.6: Solder Bump Structure
23
2.2.1.1 Under Bump Metallurgy (UBM)
In all flip-chip processes, to ensure a low and stable contact resistance at the
solder bump-bond pad interface, the copper bond pads must be re-metallized to
eliminate non-conductive copper oxides. This remetallization is the reason for
depositing an under bump metallization (UBM), and consists of three metal layers:
an adhesive, a diffusion barrier and a bonding layer. The solder bump is typically
made of high melting point alloy, enabling subsequent manufacturing processes to
use lower temperature soldering.
Critical properties of the UBM adhesive layer include good electrical contact
with the metallization pad on the die, strong adhesion to the pad and the passivation
layer surrounding it and selective etchability of the metal later to enable the use of
photolithography techniques during fabrication. The UBM barrier layer must also
have good solderability and the capability to prevent inter-diffusion between the
solder and the pad metallization. Finally, the UBM bonding layer must provide an
inert surface during bonding and protect the barrier layer from oxidation during
storage.
In the UBM structure, chromium and titanium are common adhesion layer
metals; copper, palladium, platinum and nickel are barrier layer metals; and gold is a
common bonding layer and gold as an upper bonding layer. Multilayer ceramic
substrates usually use a flash of gold on nickel film.
24
2.2.1.2 Top Surface Metallurgy (TSM)
The lower barrier layer of the TSM, which is also known as under bump
metallization on copper is an essential part of low-cost solder flip chip technology.
Its main functions are to provide excellent solderable surface and to act as a diffusion
barrier to protect the underlying copper from reacting with the solder. In the absence
of TSM, the solder reacts with copper and forms intermetallic compounds rapidly
(Kumar et al., 2004a). Gold/ platinum, silver/ palladium, silver/ palladium / gold and
silver/ platinum are used as thick-film TSM pads. Various surface finishes of TSM
will be studied thoroughly in this project.
The three major steps involved in manufacturing flip chip bonds are die
bumping and TSM structure manufacture on the substrate, alignment of the die and
substrate and assembly. The TSM structure and solder bump manufacturing, called
the bumping process, can be implemented using a variety of methods, including
metal masking, photolithography, electroless and electroplating and ultrasonic
soldering, maskless bumping and copper bumping.
CHAPTER 3
SURFACE FINISH SYSTEMS
3.1 Introduction
In flip chip bonding technology, the silicon chip is flipped and mounted onto
a substrate directly. As the substrate has a direct contact with the solder bumps,
diffusion and chemical reactions will occur between the substrate materials and the
solder bumps. Thus, one of the final steps in the substrate manufacturing process is
to provide a surface finish on exposed parts of the board. This surface finish, which
may consist of several layers of metal, has three main functions:
1. To protect the underlying copper from oxidation. Copper has excellent
solderability but it oxidizes easily. The formation of the oxide layer on
the copper surface will reduce the solderability significantly.
2. To provide a solderable surface on which to apply the components in
subsequent assembly steps, which also means to improve the wettability
and flowability of molten solder on the base material.
3. To act as a diffusion barrier to prevent excessive intermetallic compound
(IMC) formation between the solder and the base material.
26
However, the thickness of barrier layer must be closely controlled. If the
layer is too thin, it will be fully consumed during the soldering process when it is in
contact with the molten solder (Beauvilier, 2002). However, excessive thickness
involves high cost, and creates higher residual stress. Basic requirements for printed
wiring board (PWB) surface finish are:
1) Good metallurgical bonding properties to copper and solder
2) Inert property at ambient and/or reflow condition over time
3) Excellent wetting properties with solder
4) Flatness and controlled thickness
3.2. Thickness of Surface Finish Layer
The thickness of the solderable and protective layers must be closely
controlled. The more-important limit to be followed for the solderable layer is that
of maintaining a specified minimum layer thickness. As noted above, the molten
solder will begin to dissolve the solderable finish on contact. Should the layer be
too thin, for the prescribed time interval that the molten solder contacts it, it will be
fully consumed during the soldering process, thereby exposing the molten solder to
the underlying base material surface. Because the purpose of the solderable layer is
to accommodate an unsolderable base material surface, there is no reason to believe
that the base material surface has been made solderable, either intentionally or by
means of the plating process. In fact, it should always be presumed that the
underlying base material surface is not solderable. Once the molten solder reaches
the unsolderable base material surface, the molten solder de-wets from the surfaces
(Vianco, 1998).
It should be noted that dissolution of the solderable finish could potentially
degrade the properties of the molten and solidified solder. However, such effects are
27
rarely observed, due mainly to the fact that traditional solderable finishes (e.g. Ni,
Pd, and Fe) have relatively low dissolution rates in molten solder. Figure 3.1 shows
the dissolution rates of several metals in tin (Cullen, 1998).
Figure 3.1: Dissolution rates of a few typical base metals in tin (Cullen, 1998)
Excessively thick solderable coatings have two major drawbacks. The first
detriment is technically based. All coatings are deposited with some level of
residual stress and contamination, the latter arising from bath components. The
magnitude of each of these factors increases as the layer thickens. Overly thick
layers have higher residual stresses that can be a source of delamination by the layer
from the base material surface. Delamination may occur immediately after
deposition, or take places as a result of the thermal stresses caused by subsequent
soldering processes. Excessive amounts of organic contaminants, initially hidden in
thick layers, will volatize under the temperature rise of the soldering process. A
minimal effect will simply be an increase in void formation in the solder. However,
the violent volatilization of the organic components can actually break up the
coating and/or aggravate delamination defects. The second drawback of excessively
thick solderable coatings is economically based. Simply, thick coatings result in an
28
unnecessary cost penalty caused by both prolonged manufacturing time and
excessive material waste.
Optimization of the thickness of the protective layer is also based on a
number of premises, several of which are unique to this structure. Too thin a
protective coating can allow small pin-holes, fissures, and other breaches to
compromise the layer, causing oxidation and/or contamination of the underlying
solderable layer surface that leads to poor solderability performance. Also,
electroplated films are subjected to thickness fluctuations caused by variations in the
throwing power of the bath over the part surface. Such fluctuations, when combined
with very thin films, can result in too thin a layer, or no layer at all, to provide
adequate protection over some portions of the underlying surface. Such variations
are less prevalent with electroless (autocatalytic) and immersion (conversion)
coatings.
Some drawbacks of excessively thick protective finishes are the same as
those described for the case of solderable layers. Unnecessarily long manufacturing
time and costly material waste is also applicable to protective finishes. In fact, cost
penalties can become particularly burdensome because many protective layers
comprise the precious metals Au, Pt, Pd, or Ag. Also, residual stresses in the
coatings will increase as the layers thicken. Although precious metal coatings are
sufficiently ductile ("soft") to relieve such stresses, thicker layers increase the
chance that they will delaminate from the underlying solderable finish.
Delamination of the protective finish exposes the surface of the solderable layer to
oxidation and/or environmental contamination that will degrade the latter's
solderability.
A more important consequence of overly thick protective finishes is their
effect on the properties of the solder and thus, the performance of the solder joint.
The protective finish is fully dissolved into the molten solder during soldering. If
the coating composition differs from that of the molten solder, then the coating
29
material becomes, in fact, a contaminant to the solder in the joint. As the coating
becomes thicker relative to the quantity of solder comprising the solder joint
volume, it will have a greater effect on the composition and therefore, the properties
of the solder once it has dissolved. The compositional change can cause an increase
in the solder's liquidus temperature, resulting in premature solidification of the
solder before the joint is completely formed. Contaminants may also decrease the
solidus temperature of the solder, thus reducing the maximum service temperature to
which the joint can be exposed. High contamination levels can lead to reductions in
the mechanical strength and ductility properties of the solder after solidification (e.g.
Au embrittlement). These problems can be particularly acute for solder that must
fill confined geometries, such as gaps and holes or solder joints for fine-pitched,
surface mount circuit boards. The small quantities of solder will experience higher
contamination levels from the dissolved layers, resulting in a greater effect on their
properties and that of the joint, before and after solidification,
3.3 Surface Finish Systems
A number of finishes have been available for some time as alternatives to the
Hot Air Soldered Leveled (HASL) finish (Beauvilier, 2002). These finishes have
been developed to meet different needs than compliance with lead-free soldering,
and have been driven by surface mount component yield issues. These finishes
include organic solderable preservatives (OSP), electroless nickel/ immersion gold
(ENIG), electroless nickel/ electroless palladium/ immersion gold (ENEPIG),
Immersion Silver (ImmAg), Immersion Tin (ImmSn), etc. Several commonly used
surface finishes and structures are described in the following sections. However, a
good metal coating must be able to bond good metallurgical properties to substrate
and solder; inert property at ambient and/or reflow condition over time; excellent
wetting properties with solder; and flatness and controllable thickness (Yee and
Ladhar, 1998). Ultimately, the selection of surface finish belongs to the end-user or
customer and not the PCB manufacturers. Selection of the appropriate surface finish
30
plays an important role in producing reliable electronic packages. This is especially
so with the shift to lead-free soldering.
3.3.1 Hot-Air Solder Leveling (HASL)
Hot air solder leveling (HASL) consists of the same solder alloy used for
soldering. It basically involves applying a solder the appropriate printed circuit
surface by the immersion of that surface into a bath of the molten alloy. Hot-solder
dipped finishes require that the underlying base metal surfaces be solderable, not
only for the finish to coat the surface, but also because the hot-dipped finish is
dissolved into the molten (process) solder so that the final joint is made to the base
metal underneath it. According to Yee and Ladhar (1998), accurate thickness
control is difficult in this process. With the use of lead-free soldering, the
temperature of the molten solder in a HASL system may reach up to 260 oC.
Figure 3.3 shows schematically the process of HASL. The circuit board is
first coated with flux and then immersed into the bath of molten solder. The board is
then withdrawn and immediately passed between two hot air jets which blow away
excess molten solder from the pads. A thin, coating of solder is left on the exposed,
conductive features. Even after exposure to heat and humidity, HASL provides a
highly solderable surface, with a long shelf life (Bastecki, 1996).
31
Figure 3.2: Schematic diagram of the HASL technique (Bastecki, 1996)
3.3.2 Organic Solderability Preservative (OSP) Finish
Organic solderability preservatives are an important group of protective
finishes for bare Cu. OSP provide flat, planar pads and deliver sufficient protection
to pad surfaces when highly active, water soluble fluxes are used (Bastecki, 1996).
In OSP, three classes of compounds are currently used by the electronics industry: a)
benzotirazole, b) imidazole and c) benzimidazole compounds. These compounds
bond to the exposed Cu surface and protect its surface against oxidation until
soldering can occur, by forming a chemical bond with the Cu. The solderability of
the underlying Cu surface must be excellent, since that surface will ultimately
support the wetting and spreading of molten solder during assembly. Fortunately,
the Cu solderability often goes hand-in-hand with the adhesion of the OSP layer at
the time that the coating is applied. Therefore, poor OSP adhesion during the
coating process provides an initial indication of a significant contamination of the
Cu surface that would also deteriorate its solderability later on (Vianco, 1998).
32
The organic solder preservatives can be classified as either monolayer
coating or multi-layer coatings. Currently, OSP has been widely used in the
electronics industry due to its very low cost and achievable flatness. The major
issue with OSP is its relatively poor solderability, dependence on fluxes and
tendency to decompose during assembly (Yee and Ladhar, 1998).
3.3.3 Electroless Nickel/Immersion Gold (ENIG)
The Ni/ Au surface finish has been used extensively by the electronics
industry as a solderability coating for difficult to-solder lead materials. The Au
layer is the protective finish, and is dissolved by the molten solder while the Ni layer
is added between the Cu and Au layers to prevent potential inter-diffusion between
Sn in the solder and Cu. Both Ni and Au can be deposited by electroless
(autocatalytic) plating processes. Layer thicknesses as recommended by MIL-STD-
1276D are: Ni, 1.3-3.8 µm 2.5-5.0 µm (if necessary) and Au, 0.05-0.2 µm (Vianco,
1998).
The use of electroless nickel/ immersion gold finish for PCB’s has grown
significantly in the last two decades. ENIG is compatible with a wide range of
component assembly methods, including reflow and wave soldering. Circuit board
applications use “electroless” processes almost exclusively for both Ni and Au
finish. That is because the layers are deposited after the circuit board had been
fabricated, when the Cu features are not all electrically connected as would be
required for an electroplating process. Baudrand and Bengston (1995) commented
that electroless nickel would provide both solderable surface and good barrier
protection. Figure 3.1 shows that nickel has the slowest dissolution rate in liquid
solder and the slowest intermetallic compound formation rate compared with gold,
silver, cobalt, or palladium (Cullen, 1998). Also, unlike copper, nickel will not
allow tin diffusion. The Ni/ Au surface finish generally exhibits excellent
33
solderability, both on laboratory test coupons as well as on prototype circuit boards.
However, the finish is susceptible to a defect called interfacial fracture, black line
nickel or black pad (Houghton, 2002). Basically, ENIG is used to:
a) Prevent copper migration to gold.
b) Prevent gold migration into nickel.
c) A source of nickel for Ni3Sn4 intermetallic formation.
In the case of the Au layer, it may be deposited by electroless plating or
immersion plating. An immersion Au layer is very dense and offers similar
solderability protection, as do the thicker electroless plating. The thickness of gold
is crucial, since too much of gold will embrittle the intermetallic compounds (Yee
and Ladhar, 1998). According to Vianco (1998) the Au dissolved into the solder
during soldering is prone to re-deposition at the solder/ Ni interface as Au-Sn
intermetallic compound. A drop in the strength of the solder joints was
subsequently observed after accelerated aging at an elevated temperature of 150°C
and for a period of 14 days. Being based on solid-state diffusion processes the
redeposition mechanism will be very sensitive to time and temperature environments
as well as to the relative quantity of Au in the joint.
The interface between the Ni layer and the solder is susceptible to
intermetallic compound layer growth by solid-state diffusion processes, which are
accelerated by elevated temperatures. The extent of growth is determined by the
duration of such exposure. When compared to Cu, the growth rate of intermetallic
compounds between Sn-based solders and Ni is relatively slow.
Electroless nickel plating is conducted by immersing the objects with a
catalytic coating in a solution containing nickel ions (source of Ni) and a suitable
reducing agent, which may include hypophosphite, borohydride or hydrazine at
temperature in excess of 90 oC. Some other organic complexing agents for nickel
34
ions, buffers, and stabilizers are also added in the solution. The electrochemical
mechanism, where catalytic oxidation of the hypophosphite yield electrons at the
catalytic surface which in turn reduces nickel and hydrogen ions is illustrated as
follows (Gutzeit 1959, 1960):
H2PO-2 + H2O → H2PO-
3 + 2H+ + 2e- (3.1)
Ni2+ + 2e- → Ni (3.2)
2H+ + 2e- → H2 (3.3)
H2PO-2 + 2H+ + e- → P + 2H2O (3.4)
The atomic hydrogen mechanism, where atomic hydrogen is released as the
result of the catalytic dehydrogenation of hypophosphite molecule adsorbed at the
surface is illustrated as follows:
H2PO-2 + H2O → HPO3
2- + H+2Hads (3.5)
2Hads + Ni2+ → Ni + 2H+ (3.6)
H2PO2 + Hads → H2O + OH- + P (3.7)
The adsorbed active hydrogen, (3.6) then reduces nickel at the surface of the
catalyst.
(H2PO2)2- + H2O → H+ + (HPO3)2- + H2 (3.8)
3.3.4 Electroless Nickel/ Electroless Palladium/ Immersion Gold (ENEPIG)
The Ni/Pd finish has also received considerable interest as a surface finish
for the Au features on printed circuit board. This surface finish system is being
35
targeted to replace the traditional HASL coating because the former systems leave a
very flat layer over Cu that will not interfere with the placement and integrity of
fine-pitch and ultra-fine-pitch, peripheral leaded packages and chip components.
Ni/P/Au surface finish process is similar to the ENIG process, except that a
Pd layer is deposited after the Ni layer and before the Au layer. The Pd layer can be
deposited by an electroless (autocatalytic) process so that unlimited thicknesses are
theoretically possible. Typically, the deposit also contains a 6-7 percent cent of
phosphorous (P), the source of which is the reducing agent required during the
plating process. The thickness can range from as little as 0.15 µm to as high as 1.5
µm, but the typical value is between 0.25-0.51 µm. Within this thickness range,
circuit board features show excellent solderability. Since Pd is harder than Au, the
strength of the surface finish is increased when the Pd layer is added oxidation
resistance of the Ni layer is improved.
Palladium coatings alone do not appear to offer the level of solderability
protection required for circuit boards that are exposed to multiple reflow cycles
(two-sided and/or mixed technology). Potential degradation mechanisms include Cu
diffusion from the underlying conductor and arriving at the surface of the Pd layer to
be subsequently oxidized, resulting in the degradation to solderability. As a second
mechanism, Pd can develop an oxide layer that potentially inhibits wetting and
spreading by the molten solder. In answer to this second scenario, a thin layer of
immersion-deposited Au (0.05-0.2 µm) is deposited over the Pd to maintain
adequate solderability when storage and processing conditions are too severe for the
exposed Pd surface. This is the basis of the Ni/Pd/Au finish (Vianco, 1998).
36
3.3.5 Immersion Silver (ImAg)
Immersion silver is deposited directly on the copper surface by a chemical
displacement reaction in which silver ions are exchanged for copper ions and
deposited on the exposed surface. The deposits provide a thin silver layer of 0.1 to
0.4 µm, which may be deposited with an organic material to reduce tarnish of the
immersion silver. This helps seal the surface and allow for extended shelf life.
Silver offers a flat and extremely solderable surface compared to HASL finish, Pb-
free inspection at assembly, lack of solder mask attack and surface contact
functionality (Cullen and Milad, 2004). The surface is also bondable for both
aluminum and gold wire (Gilleo, 2002; Clyde, 2001 and Milad, 2007). Unlike
ENIG, immersion silver is also a low temperature process. During soldering, the
molten solder wets and spreads on the surface of silver coating. The silver then
dissolves into the molten solder allowing the formation of a copper-tin intermetallic
solder joint, similar to HASL and OSPs. The silver from the coating forms silver-tin
(Ag3Sn) intermetallics in the solder.
One of the disadvantages with immersion silver has always been silver
migration in electronic environments (Vianco, 1998). This is due to the property of
silver to form water-soluble salts when exposed to moisture and electrical bias. The
incorporation of organics into the immersion silver minimizes this phenomenon.
Aside from that, occasional voiding in the solder joint was also reported. Studies are
being done to determine whether this problem comes from the excessive silver
thickness. If that is so, then an upper thickness limitation has to be set (Clyde, 2001
and Milad, 2007). The immersion silver also is an active surface and readily
combines with sulfur from the environment. Silver sulfide tarnishes the surface and
creates doubt about the integrity of the finish at inspection. Proper packaging of
immersion silver finished boards are critical to control sulfurization. The key in
packaging is to minimize contact of the surface with the environment and to ensure
all materials used in packaging and during storage are sulfur free (Milad, 2007).
37
Immersion plating or galvanic displacement, are terms that are usually
restricted to processes that are based on chemical replacement of the base metal by
the more noble metal coating. In this case the reaction will, in principle, either
completely cease or else proceed at an immeasurably slow rate once the base metal
surface is effectively masked by a thin coating. Solutions in this particular category
are only capable of yielding deposits of limited thickness as compared to electroless
plating. Although the theory of immersion plating is quite simple and the general
effects of all operating parameters are well known, at least quantitatively, the narrow
range of conditions under which useful coatings may be produced requires that any
new possibility must be tested by trial and error to determine how critical each of the
above factors are, since the end effect is mainly decorative, there has been little
stimulus to develop new processes for mass production.
The primary advantage of employing the immersion method of metal
deposition is that the bath can avoid the use of chemical reducers because the
chemical reduction of the metal from solution is driven by the oxidation of the
underlying metal. Once the metal from solution deposits to such a thickness that the
surface metal cannot be efficiently dissolved, the reaction slows. Therefore, the
immersion metal coatings are very thin. Silver is generally deposited to a maximum
thickness of about 0.5µm (Cullen, 2003). Immersion silver is deposited from a
dilute solution containing silver salts, metal complexors and organic surface
modifiers. The electromotive potential between silver and copper is 0.456 volts, so
the reaction is straightforward.
2Ag+ + Cu0 → 2Ag0 + Cu++ (3.9)
Organic compounds are added to the formula to inhibit tarnish and to prevent
electromigration. In some cases, an organic may be added to prevent the
precipitation of silver from the chemical bath. Precipitation would ordinarily occur
due to the interaction of silver ions and light.
38
3.3.6 Immersion Tin (ImSn)
The immersion tin process also uses the displacement reaction that
exchanges tin ions for copper ions to directly deposit a dense layer on the exposed
copper surface. It is one of a logical replacement for HASL for two reasons; first, it
is flat and co-planar, and second, it is lead-free. However, tin readily forms a
copper-tin intermetallic, namely Cu3Sn and Cu6Sn5, whose growth may affect the
soldering performance. A thicker Cu-Sn intermetallic is known to occur when
soldering with lead-free alloys on immersion tin finish. Thus, the tin thickness is
directly related to this intermetallic formation. A thick deposit of 1.0 µm (Clyde,
2001) or in a range between 0.76 to 1.27µm (30 to 50µinch) (Milad, 2007) is
feasible, thus ensuring a copper-free tin surface.
Immersion tin provides highly solderable surface and a dense uniform
coating with superior hole-wall lubricity. This characteristic makes it the choice for
backplane panels that are assembled by pin insertion. Immersion tin is a viable lead-
free finish option for some applications, but, how this finish will survive high
temperature assembly associated with lead free Sn-Ag-Cu solder alloy remains to be
seen. The solder joint intermetallic should not be a problem; however, the higher
temperature profile could accelerate the intermetallic formation compromising the
solderability of the surface (Clyde, 2001 and Milad, 2007).
Immersion tin also offers other disadvantages. The bath makeup entails the
use of thiourea, which is banned in certain geographical locations for environmental
reasons. During processing, the primary byproduct in the bath is copper thiourea.
Waste treatment allowance must be made for the containment of the thiourea and its
copper salt by-product. The shelf life of the surface is, to some extent, limited (less
than a year). This is due to the progression of the copper-tin intermetallic until it
reaches the surface and renders the product non-solderable. This could be
accelerated under excessive temperature and humidity conditions (Clyde, 2001 and
Milad, 2007).
39
Another issue with immersion tin is its propensity to form whiskers at room
temperature. Immersion tin whiskers do not grow as a result of exposure to heat,
vacuum, pressure, humidity or bias voltage. They grow naturally over time, which
would seem to indicate, that the primary source is Cu6Sn5 migration stress (Milad,
2007).
3.3.7 Summary
A good surface finish, however, is not just one that provides a flat and
solderable surface. It must also provide: (i) compatibility with other metals such
nickel, gold, and tin, (ii) strong consistent solder joint strength, (iii) consistent
solderability and (iv) long term electrical reliability (Stafstrom, 2000). Some of the
characteristics of the surface finishes are shown in Table 3.1 (Barbeta, 2004).
Table 3.1: Comparison between different surface finish
Surface finish Advantages Disadvantages
HASL Nothing solders like solder. Easily applied. Lots of industry experience. Easily rework. Good bong strength. Withstands multiple thermal cycles.
Huge co-planarity differences. Contain lead. Not suitable for high aspect ratios. Not suited for <0.5mm pitch. PWB dimensional stability issues. Bridging problems on fine pitch assemblies. Inconsistent coating thickness.
OSP Flat, coplanar pads. Reworkable. Doesn’t affect final whole size. Short, easy process. Cu-Sn intermetallic formed has been reported to be stronger and more robust than Ni-Sn intermetallic from Ni-Au.
Require changes in the assembly line. Question remains over the reliability of exposed Copper after assembly. Limited thermal cycles. Cannot be reworked by the assembler. Limited shelf life. Test pins cut coating, leaving exposed copper. Limited in circuit testability. Not inspectable at assembly.
ENIG Planar surface. Consistent thickness. Withstands multiple thermal cycles. Long shelf life. Good for line pitch product.
Not Au wire bondable. Expensive. Should not be used on < 1mm pitch; black pad issues. Waste treatment of nickel. Cannot be reworked at PWB fabricator. Nickel is a suspected carcinogen. Not optimal for higher speed signals.
40
Table 3.1 (continue): Comparison between difference surface finish
Surface finish Advantages Disadvantages
ENEPIG Pd keeps Ni from passivating in presence of “porous” gold coating. Al and Au wire bondable. Planar surface. Good for fine pitch product.
Additional processing step. Adds cost. Dip tank process. Evidence that Pd poisons the solder paste after reflow. Waste treatment. Very complex processing steps.
Immersion
Silver
Good for fine pitch product. Planar surface. No black pad concerns. Short, easy process cycle. Eliminates nickel. Doesn’t affect final whole size. Long shelf life. Can be reworked/ reapplied by the fabricator. Inexpensive. Drop-in process for the assembler. Good for ultra-high speed signals.
Friction coefficient; may not be suited for compliant pin insertion. Some systems cannot throw into blind vias with aspect ratios > 1:1. Tarnishing must be controlled.
Immersion Tin Good for fine pitch product. Planar surface. Eliminates nickel. Can substitute for reflowed solder in selective strip. Inexpensive.
Form whiskers at room temperature. Short shelf life. Problem with soldermask compatibility.
CHAPTER 4
SOLDERING
4.1 Introduction
Soldering is a well known and widely used process where two or more metal
items are joined together using a fusible alloy with a melting temperature that is
lower than their own. The most commonly used solder is a fusible alloy consisting
essentially of a tin and lead mixture. It is the solvent action (the solder actually
dissolves a small amount of the metals surface, at a temperature that’s well below its
melting point and joins with it) of the solder alloy that causes it to fuse with and
attach to the surface of the metal items being joined. The solvent action that takes
place, between the solder and the metal, makes the joint chemical (not just physical)
in nature and causes the properties of the joint to differ from the original solders
properties and from those of the surface of the metal items being joined. When metal
parts are joined by solder, a metallic continuity is established as a result of the
interfaces where the solder is bonded to the metallic surfaces.
42
The metal joining process that is generally referred to as soldering (or soft
soldering) requires temperatures between 183oC and 450oC. The joining of metals at
temperatures above 450oC (and below the melting point of the metals being joined) is
more commonly referred to as brazing (or hard soldering). The actual melting and
fusing of the metal items that are being joined together is considered welding. There
are, of coarse overlapping situations that may occur when classifying a process. The
actual joining characteristics that take place are physically different in each of these
processes. Soft solders attach to metals by what is referred to as a solvent action that
takes place at relatively low temperatures. Hard solders or brazing alloys contain
metals that require higher temperatures to cause the solvent action to take place and
fuse the alloy with the metal being joined. Because welding involves melting and
fusing the surface of the metals that are being joined together, a filler or fusible
material is not always used.
Soldering is used primarily when the expected operating temperature of a
joint will not exceed around 149oC and thermal or electrical continuity can not be
adequately achieved, or maintained, by the use of a mechanical joint. It is one of the
most ideal methods available for the creation of a physical, electrical, or hermetically
sealed bond between various metal items that are being joined together. Soldering is
quite often used, in addition to other mechanical methods (twisting, crimping, etc.) to
improve electrical continuity, to help protect the joint from the effects of vibration, or
to encapsulate the joined metals preventing oxidation. Although soldering may be
used to provide some minor support to an assembly, the solder should not (excluding
sheet metal applications) be used as the primary mechanical support of a finished
joint.
43
4.2 Materials
The soldering process may be accomplished in a wide variety of ways, but
the four primary ingredients required will remain the same. They are;
1. The base metal (or metal items being joined)
The base metal is the metal that is in contact with the solder and forms an
intermediate alloy. There are many metals that will react willingly with
solders to form a strong chemical and physical bond, while others can be very
difficult, or even impossible to solder.
2. A type of flux (or a method of cleaning and maintaining the surface to be
soldered)
Flux is used to eliminate minor surface oxidation and to prevent further
oxidation of the base metals surface during the heating process. Although
there are many types of flux, each will include two basic parts, chemicals and
solvents. The chemical includes the active portion, while the solvent is
actually the carrying agent. It is the solvent that determines the cleaning
method required to remove the remaining residue after soldering.
3. The solder
Solder is the alloy used to create the solvent action, which generates the bond
between the base metals. The type and form of the solder is very important
and must be determined by the individual application being performed, as
well as the base metals and soldering method being employed.
4. Heat
When an alloy is heated it typically goes thorough multiple phases. It goes
from a solid state to what is known as a pasty stage, sort of halfway between
a liquid and a solid, and then to a liquid state. In soldering it is difficult to
work with a substance that goes through a pasty stage. Eutectic solder is
often used for this reason. A eutectic alloy is one that goes directly from a
solid state to a liquid state without a pasty stage. The eutectic tin-lead alloy is
44
made up of 63% tin and 37% lead. Eutectic tin-lead solder can be applied as
a liquid just above the melting point, and then as it cools it will transform
directly into a solid. This makes it possible to form solid solder joints very
quickly. Sometimes a 60% tin and 40% lead alloy is used. This alloy
exhibits a nearly eutectic change from solid state to a liquid state and can be
produced at a lower cost (Barker, 1993).
It is important to match the soldering method and the equipment that will be used, to
the soldering application that is being considered.
4.3 Soldering Techniques
There are three main methods used in soldering process: The difference is the
sequence in which solder, flux and heat are brought to the joint, and in the way in
which the soldering heat is brought to the joint or joints:
1. Reflow Soldering
2. Wave Soldering
3. Hand Soldering
4.3.1 Reflow Soldering
Reflow soldering is a metallurgical joining method and is a much older
process than wave soldering, going far back into antiquity; under the name of ‘sweat
soldering’ it is used in plumbing to this day. With the advent of hybrid technology
45
more than thirty years ago, sweat soldering was recognized as the logical way of
joining surface mount devices (SMD’s), which were specifically developed for
hybrids, to the metallic conductor pattern of the ceramic substrate.
The soldering process involves four basic elements: the base metal (in our
case the substrate surface finish metals), soldering flux, solder alloy and heat
(temperature). To begin with, solder and flux are placed on one or both joint
surfaces, either together in the form of a solder paste or separately, first the solder in
the form of a metallic coating and then the flux at a later stage. Subsequently the
joints are put together. The important point is that all this happens at room
temperature though with some procedures, the solder may have been pre-deposited
on one or both joint surfaces by a hot-tinning method. With all reflow strategies, the
assembled joints are finally heated to a temperature high enough to melt the solder,
and for long enough to let it tin the joint surfaces and fill all the joint gaps. Then,
heating is discontinued and the solder is allowed to solidify, the faster the better
(Strauss, 1994). Figure 4.1 and Table 4.1 shows the sequence steps and summary of
the reflow soldering profile.
Figure 4.1: Typical solder reflow profile for eutectic Pb-Sn solder (Diodes
Incorporated, 2005)
PREHEAT To relieve the volatile solvent
SOAK Activation of the flux
REFLOW Peak temperature is reached to ensure the entire assembly acquires sufficient heat for reflow.
COOLING
• Slow cooling
• Excessive brittle IMCs layer
• Larger grain size
• Rapid cooling
• Induce stresses to the solder joints.
46
Table 4.1: Summary of reflow profiling (Wood and Rupprecht, 2004)
STAGE DESCRIPTION
Preheat To get heat into the assembly and bring it up to flux activation
temperature. A typical target would be to reach 120°C within 30 to
90s, depending on the type of board.
Soak To allow solvents evaporate from the solder paste and fluxes to
activate at an elevated temperature, removing oxides and
contaminants prior to soldering. Time is important because an
overly extended soak time may “dry” out the solder paste, causing
reoxidation. A typical soak would range from 120 to 170°C and
would last between 30 and 90s.
Reflow To bring the component exceeds the liquidus points, to a
temperature between 200 and 210°C to allow soldering (63Sn/
37Pb melts at 183°C). This will wet both the component leads and
the board pads. The surface tension effects occur, minimizing
wetted volume. This time generally would be between 30 and 60s.
Cool Down Having a cooling stage is important because overly extended reflow
times will cause changes in the solder joint structure that could
affect reliability. There is also a practical side to ensuring that the
board is back below reflow temperature before operator handling.
The rework system should direct cold air through a separate air
chamber because continuation through the heaters, even after they
have been switched off, will promote extended reflow.
The available reflow methods for solder paste utilize the heat sources such as
conduction, infrared, vapor phase condensation, hot gas, convection, induction,
resistance and laser (Strauss, 1994). Each of these methods posses features which
are particularly suitable for specific assemblies, although there may be a preference,
depending on the volume of production, cost, type of components involved, type of
materials involved and other processing parameters. Figure 4.2 shows the diagram
of different types of reflow method. Table 4.2 summarizes the strengths and
limitations of each method.
47
Figure 4.2: The main heating options in reflow soldering
48
Table 4.2: Benefits and limitations for vary reflow method (Strauss, 1994)
Reflow Benefits Limitations
Conduction • Low equipment capital
• Rapid temperature changeover
• Visibility during reflow
• Planar surface and
single-side attachment
• Limited surface area
Infrared • High throughput
• Versatile temperature profiling
and processing parameter
• Mass, geometry
dependence
Vapor Phase
Condensation
• Uniform temperature
• Geometry independence
• High throughput
• Difficult to change
temperature
• Temperature limitation
• Relatively high
operation cost
Hot Gas • Low cost
• Fast heating rate
• Localized heating
• Temperature control
• Low throughput
Convention • High throughput • Slow heating
Induction • Fast heating rate
• High temperature capability
• Application to non-
magnetic metal parts
only
Laser • Localized heating with high
intensity
• Short reflow time
• Superior solder joint
• Package crack prevention
• High equipment capital
• Specialized paste
requirement
• Limit in mass soldering
49
4.3.2 Wave Soldering
Wave soldering is ideally suited for high throughput production of SMT
assembly. Wave soldering is a conductive heat-transfer system in which molten tin-
lead solder is used as the medium to transfer heat to the work piece assembly as well
as being the source and means of applying solder to the assembly. The assembly
travel along on a conveyor belt at the predetermine rate of speed through a carefully
controlled wave of liquid solder, which forms the electrical and, for surface mounted
components, the mechanical interconnections between the component and PWB
(Beh, 2000).
A typical wave soldering system includes station that first apply flux to the
bottom surface of the assembly and up to the plated through-holes, a second station
that preheats the assembly by radiation, and a third station that applies the solder in
the form of a wave, which also supplies the bulk of heat to the joints by conduction
as shown in Figure 4.3 (Hwang, J. S., 2002). The preheat process in the system is to
allow faster conveyor speeds resulting in less heat stress on the components. The
soldering temperature is maintained at the lower end of its temperature range to
avoid leaking. Mostly, the wave is split into two sections. One is a narrow, turbulent
wave to wet all the components and two is a smooth wave which removes excess
solder. For best results, the solid contents of the solder flux should be minimized
(Skipp, 1988).
Figure 4.3: Typical wave soldering machine (Hwang, J. S., 2002)
50
4.3.3 Hand Soldering
With hand soldering, the heat source is the top of a soldering iron, which is
heated to 300-350oC. A small amount of flux may have been applied to the joint
members before they are placed together. The assembled joint is heated by placing
the tip of the soldering iron on it or close to it. Solder and flux are then applied
together, in the form of a hollow solder wire, which carries a core of flux, commonly
based on rosin.
The end of the core wire is placed against the entry into the joint gap. As
soon as its temperature has reached about 100oC, the rosin melts and flows out of the
solder wire into the joint. Soon afterwards, the joint temperature will have risen
above 183oC; the solder begins to melt too, and follows the flux into the joint gap
(Figure 4.4). After the joint is satisfactory filled, the soldering iron is lifted clear,
and the joint is allowed to solidify. Thus, with hand soldering, the sequence of
requirements is as follows:
a) Sometimes, a small amount of flux
b) Heat, transmitted by conduction
c) Solder, together with the bulk of the flux
Figure 4.4: The principle of hand soldering (Hwang, J. S., 2002)
51
4.4 Solder Materials
Solder is a metal or metallic alloy used, when melted, to join metallic surface
together. All solders function as solvents for the metal surfaces to be joined. The
solder permits the creation of a liquid-phase intermetallic at a temperature far below
the normal melting temperature of the metals to be joined (Minogue, 2002). The
selection of solder alloy is driven primarily by the assembly process and the
materials to be joined.
The most common material is the 63Sn/ 37Pb tin-lead eutectic alloy. As a
close alternative, some manufacturers prefer to use Sn-Pb-Ag due to fatigue
resistance and the dispersion and solution hardening assumed to be imparted by the
addition of silver. The materials properties of the most common electronic solder
alloys are summarized in Table 4.3. Tin and indium are the two low melting
elements that have the basic property to wet and bond to metallic substrate, in view
of the very high cost and limited availability of indium, all proposed lead-free solders
have been based on tin. According to Harrison et al. (2001), due to the compatibility
with current processes and avoidance of PCB and component damage require a
melting point not higher that about 230°C, so a eutectic or near-eutectic composition
is desirable.
Table 4.3: Melting properties of some common solder alloys
Solidus oC Liquidus oC Alloy
118
118
118
118
141
143
157
179
125
118 (eutectic)
131
145
237
143 (eutectic)
157
179 (eutectic)
50In/ 50Sn
52In/ 48Sn
52In/ 48Sn
58In/ 42Sn
90In/ 10Ag
97In/ 3Ag
100In
62Sn/ 36Pb/ 2Ag
52
Table 4.3 (continue): Melting properties of some common solder alloys
Solidus oC Liquidus oC Alloy
183
188
219
217
217
221
221
221
221
221
227
227
227
227
232
232
232
232
234
280
275
183 (eutectic)
188 (eutectic)
219 (eutectic)
222
222
295
240
221(eutectic)
221
226
349
300
227
232
232
240
238
232
245
280
303
63Sn/ 37Pb
77.2Sn/ 2.8Ag/ 20In
96.2Sn/ 0.5Sb/ 0.8Cu/ 2.5Ag
95.75Sn/ 3.5Ag/ 0.75Cu
95.5Sn/ 4Ag/ 0.5Cu
90Sn/ 10Ag
95Sn/ 5Ag
96.5Sn/ 3.5Ag
96Sn/ 4Ag
97.5Sn/ 2.5Ag
95.5Sn/ 0.5Ag/ 4Cu
97Sn/ 3Cu
99.3Sn/ 0.7Cu
65Sn/ 1Cu
65Sn/ 25Ag/ 10Sb
95Sn/ 5Sb
97Sn/ 3Sb
100Sn
91.5Sn/ 8.5Sb
20Sn/ 80Au
10Sn/ 90Pb
Solder material has remained an almost constant system parameter despite
electronic innovations. The wide variety of electronic products is mostly
manufactured with near-eutectic tin/lead based solders. Widely used soft solders
must fulfill different requirements:
i. Melting point
The thermal stability of components and printed circuit material, which are
currently inexpensive, limits maximum temperature of the soldering process
to 225oC over a period of 8 seconds.
53
ii. Process compatibility
Alternative alloys must be compatible with no-clean flux and suitable for the
production of bar solder, solder paste and solder wire. The temperature
resistance of printed circuit boards must be tested.
iii. Toxicity
The alloys should contain neither cadmium, antimony nor any other elements
classified as hazardous materials.
iv. Physical, mechanical and electrochemical properties
The main physical properties are determined by electrical and thermal
conducting capacity, density, surface tension and wetting reaction. The
mechanical quality refers to flow properties and fatigue reaction. Corrosion,
oxidation and migration tendency determine electrochemical quality.
v. Costs and availability
Metals make about 60% of the costs for bar solder in case of a 63Sn/37Pb
alloy, but in the case of solder pastes, only 5-8%. Production steps such as
mixing and filling create the main costs for solder pastes.
Solders used with copper alloys usually consist of tin and lead, with a
60%Sn-40%Pb mixture being the most common. The eutectic composition, 63%Sn-
37%Pb provides the lowest solder melting temperature at 183oC, well below that of
either Sn or Pb. Other elements contain in the solder are either impurities that can be
detrimental to the process, or elements added to improve the bond strength or alter
some other characteristic in a beneficial manner. Recently, driven by environmental
factors, interest in lead-free alternative soldering alloys has revived and evaluation of
such solders is being actively pursued. So, solders are commonly divided into two
main categories:
a) Lead-based solders
b) Lead-free solders
54
4.4.1 Lead-based Solders
The eutectic combination of two or three constituent metals has a lowest
liquidus temperature in the phase diagram. The liquid solution will freeze solid from
the liquid state to solid at a distinct liquidus-solidus transition temperature, like the
pure metals. Eutectic compositions have lower viscosity and higher degree of
fluidity than the other similar, but noneutectic compositions comprising of the same
metals. This is due to the noneutectic alloys’ gap between the solidus and the
liquidus points that is referred to as the pasty range. Phase diagram of Sn-Pb is
shown in Figure 4.5.
Figure 4.5: Phase Diagram of Pb-Sn Alloy
The high liquidus temperature (303oC) limits its direct application in the
direct reflow primarily to ceramic systems. This high lead solders must be handled
carefully not to mechanically dent or damage the spheres, because it is soft. The
high lead-containing spheres also have a greater propensity to darken during
handling. However, this high lead-containing solder is metallurgically compatible
55
with the eutectic 63Sn/37Pb and is commonly employed in the joining application.
This can be accomplished by jumping the die with high lead solder then joining it to
an organic board with eutectic Sn/Pb. Meanwhile 10Sn/90Pb is also compatible with
63Sn/36Pb/2Ag, which the additional silver provides a better fatigue resistance to the
solder ball.
With a melting temperature of 183oC, the Sn-Pb binary system allows
soldering conditions that are compatible with most substrate materials and SMT
devices. As one of the primary components of eutectic solders, lead impacts the
following effects:
i. It reduces the surface tension of pure Sn (550 dyne/cm at 232oC), the lower
surface tension of solder (470 dyne/cm 63Sn-37Pb at 176oC) facilities
wetting.
ii. As an impurity in Sn at levels as low as 0.1%, lead prevents the
transformation of white or β-tin to gray or α-tin upon cooling past 176oC.
The reaction results in a 26% increases in volume and the transformation
causes lost of structural integrity to the tin.
iii. Lead serves as a solvent metal, enabling the other joint constituents, such as
Sn and Cu, to form intermetallic bond rapidly.
When these factors combined with lead, it will be an available and low cost
metal and thus make it an ideal alloying element with tin. The SMT soldering
system that is mainly based on eutectic and near-eutectic tin-lead solders has been
well developed and refined after many years of experience.
56
4.4.2 Lead-free Solders
Tin-lead solder is the most commonly used solder for electronic assembly.
However, there are concerns about the use of lead due to its adverse effects on
human health. Lead is linked to health hazards such as disorders of the nervous and
reproductive systems and delayed neurological and physical development. Lead
poisoning is particularly damaging to the neurological development of young
children. Lead-free soldering for electronics is a global trend toward a lead-free
environment, and the favored Pb-free solder alternatives vary from region to region.
In general, however, high-tin alloys are preferred. These include Sn/Ag, Sn/Cu,
Sn/Ag/Cu, Sn/Ag/Bi and various versions of those alloys with small amounts of
other elements, such as Sb.
Sn/Ag/Bi systems are currently employed in some Japanese products.
However, Sn/Ag/Cu systems are more tolerant toward Pb contamination than
bismuth-containing systems and are therefore more compatible with the existing
infrastructure during this transition stage. The application will dictate the specific
Pb-free alloy chosen for different applications. Pb-free solder systems suitable for
CSP (Chip Scale Packages) soldering applications are primarily alloys of Sn with
Ag, Bi, Cu, Sb, In or Zn, as shown in Table 4.4. These alloys may serve as
substitutes for eutectic Sn/Pb solders in CSP interconnects. However, substitutes for
high-melting-temperature solders have not yet been developed.
Table 4.4: Lead-Free Solders for CSP Applications (Yang et al., 2001)
Melting Temperature Range o(C) Solder Alloy
227 99.3Sn0.7Cu
221 96.5Sn3.5Ag
221-226 98Sn2Ag
205-213 93.5Sn3.5Ag3Bi
207-212 90.5Sn7.5Bi2Ag
57
Table 4.4 (continue): Lead-Free Solders for CSP Applications
200-216 91.8Sn3.4Ag4.8Bi
226-228 97Sn2Cu0.8Sb0.2Ag
213-218 96.2Sn2.5Ag0.8Cu0.5Sb
232-240 95Sn5Sb
189-199 89Sn8Zn3Bi
138 58Bi42Sn
217-219 95.5Sn4Ag0.5Cu
216-218 93.6Sn4.7Ag1.7Cu
217-219 95.5Sn3.8Ag0.7Cu
217-218 96.3Sn3.2Ag0.5Cu
217-219 95Sn4Ag1Cu
Many of the systems are based on adding a small quantity of a third or fourth
element to binary alloy systems to lower the alloy's melting point and increase
wetting and reliability. Researchers have reported that, with increasing amounts of
additive elements, the melting point of the system first decreases. The bond strength
then rapidly decreases, almost levels off, and then decreases again (Abtew, 2000).
Finally, the wettability increases rapidly at first, reaching its maximum at a
composition corresponding to the midpoint of the plateau of bond strength, before it
decreases (Beh, 2000). The followings are examples of different solder alloys that
are mainly used in the industry nowadays:
i. 99.3Sn/0.7Cu
99.3Sn/0.7Cu (227oC) is reported to have soldering qualities equal to eutectic
Sn/Pb in telephone manufacturing (Beh, 2000). However, in air reflow, the
wettability is reduced and the fillet exhibits a rough and textured appearance.
Thus composition is probably the “poorest” in mechanical properties
available from all Pb-free solders. This is best suited for use in wave
soldering because the materials cost and the inverting of waves is not costly.
58
ii. 96.5Sn/3.5Ag
96.5Sn/3.5Ag (221oC) is considered one of the most promising solder alloys.
However, Abtew and Selvaduray (2000) reported that it offers the poorest
wetting for reflow soldering among high-Sn alloys.
iii. Sn/Ag/Cu
This is a ternary eutectic at 217oC, although the exact composition is to be
clarified. Cu is added to Sn/Ag to slow the Cu dissolution and lower the
melting temperature. This improves wettability, creep and thermal fatigue
characteristics. Some researchers reported that this alloy has better reliability
and solderability than Sn/Ag and Sn/Cu (Abtew and Selvaduray, 2000). It
also has a better yield than eutectic Sn/Pb. They also recommended this alloy
for general purpose use.
iv. Sn/Ag/Cu/X
96.2Sn/2.5Ag/0.8Cu/0.5Sb (213-218oC) is reported by the International Tin
Research Institute, to have greater fatigue performance than a eutectic Sn/Pb
alloy. Some other researchers reported that a 0.5% Sb addition may
strengthen the alloy further (Hwang, 2002).
v. Sn/Ag/Bi/X
The addition of less than 5% Bi lowers the melting point and improves the
wettability of Sn/Ag systems. With this alloy, solderability is the best among
a range of Pb-free materials (Frear, D., 2002). The addition of a large amount
(~5-20%) of Bi lowers the melting point of eutectic Sn/Pb solders but looses
the good properties of eutectic Sn/Ag systems.
vi. Sn/Sb
95Sn/5Sb (232-240oC) offers poor wetting, although better than
96.5Sn/3.5Ag and its liquidus temperature is too high.
vii. Sn/Zn/X
91Sn/9Zn (eutectic 199oC) is fairly reactive since Zn causes oxidation and
corrosion and reacts with flux to form a hardened paste. In 89Sn/8Zn/3Bi, Bi
59
replaces Zn to reduce the Zn corrosion in humid conditions. Sn/Zn/Bi alloys
can have a melting point close to eutectic Sn/Pb. This alloy was developed
primarily by home electronics manufacturers targeting low-cost products.
4.4.2.1 Characteristic of Lead-free Solders
Solder is a readily fusible alloy used to join the surfaces of metals. There are
two types of solders which are:
• Soft Solders
A soft solder is usually a lead-tin mixture. The tensile strength of solder is
greatest with 72.5% of lead, but this alloy is not sufficiently fusible to be used
for general soldering. Any of the alloys that contain above 70% of lead have
a melting point too high to be used with a steel iron. The main uses of these
alloys are for coating iron or steel sheets for roofing and filling of hollow
casting. The alloy that contains about 67% of lead is used for plumber’s
work. The alloys containing 55 to 60% of lead melt from about 215 to 230oC,
and are sufficiently fusible and freely flowing for ordinary soldering. Alloys
which contain 58% of lead are used to considerable extent for soldering joint
in electrical wiring because it is considered as easily flowing solder. But by
far, the favorite alloy for soldering is that which contains 50% of lead and
50% of tin where it is known as “half and half”. This alloy melts rapidly,
flows freely and presents a bright surface when the joint is finished. It can be
used for every purpose to which soft solder is applicable, except in plumbing
(Lau, 1994).
60
• Hard Solders
Hard solder is used for joining such metals as copper, silver and gold, and
alloys such as brass and gunmetal. When applied to copper, iron, brass
and similar metals in the operation, it is known as brazing, while when
applied to precious metals such as silver, it is known as silver soldering
(Lau, 1994).
The distinction between these two classes of solders is important. Most
alloys have a tendency to recrystallize at homologous temperatures of 50 to 60% of
their melting points in oK. At or above room temperature, soft solders tend to
undergo recrystallization (annealing) spontaneously. In the proper metallurgical
sense, soft solders cannot work hardened, rather they remain ductile under all
conditions. Table 4.5 shows the properties of some hard and soft solder alloys.
Table 4.5: Properties of Hard and Soft Solder Alloys (Guo et al. 1992)
Yield Strength (Mpa) at
Alloy
Melting
Point (oC)
Thermal
Conductivity (W/moC)
Coefficient of Thermal
Expansion (10-6/oC) 23oC 100oC 150oC
Hard Solders
Au-20Sn 250 57.3 15.9 275 217 165
Au-12Ge 356 44.4 13.3 185 177 170
Au-3Si 363 27.2 12.3 220 207 195
Soft Solders
Pb-63Sn 183-188 50.6 24.7 35 15 4
Pb-5Sn 308-312 23 29.8 14 10 5
1 pascal (Pa) = 1 newton/m2 = 0.145 x 10-3 psi
A lead-free alternative solder that could be a drop-in replacement for eutectic
tin-lead solder has yet to be found. Low melting point metals are the obvious
starting points in the search for lead-free solders. Of these metals, cadmium, mercury,
lead and thallium have intrinsic toxicity. Despite their low melting point, the alkali
61
metals tend to form water-soluble oxides and extremely reactive. Therefore, this
group of elements must be excluded from consideration.
Gallium is a relatively rare element with an insufficient world supply.
Magnesium is a very reactive element and Sn-Mg alloys are known to tarnish readily
in air. Nevertheless, there are alternative lead-free solders that have the performance
potential to be replacements for tin-lead solder. The performance characteristics that
are of importance include the melting temperature, microstructure, surface tension,
coefficient of thermal expansion (CTE), electrical resistivity, oxidation behaviour,
corrosion behaviour, elastic modulus, yield strength, shear strength, fatigue
behaviour and creep behaviour.
4.4.2.2 Melting Temperature
The melting temperature of the solder determines the maximum allowable
temperature a product can be exposed to in service and the maximum processing
temperature that devices and substrate can withstand during soldering. The solidus
and liquidus temperatures of various lead-free solders are provided in Table 4.6.
Some of alloys are ternaries and quaternaries. Many of the lead-free alloys shown in
the table have been studied extensively.
Table 4.6: Lead-free solders with liquidus (T1), solidus (T2) and eutectic (Te)
62
Table 4.6 (continue): Lead-free solders with liquidus (T1), solidus (T2) and eutectic
(Te)
4.4.2.3 Microstructure
Generally, for a material of a given chemical composition, the microstructure
is not constant and varies greatly, depending on processing and service conditions.
In surface mount assembly, the time and temperature dependent soldering profile
affects the microstructure of the number intermetallic phases in the solder joint.
Cooling rate will affect the initial microstructure in the surface mount assembly.
One of the deniable properties of potential lead-free solders for SMT applications is
having eutectic behaviour, where both phases solidify concurrently at a single
temperature, rather than use a range of temperatures.
63
Slow cooling rate, where the solidification reaction proceeds more slowly,
results in wider inter-lamellar spacing. At a sufficiently fast cooling rate, the eutectic
structure looses its lamellar character and a characteristic with fine, uniform
dispersion of phases within small eutectic colonies or grain sizes is formed. Faster
cooling rate enhances the number of colony nuclei formed and therefore colony size
increase with a slower cooling rate. This microstructural variation, due to cooling
rate, can drastically affect the fatigue of the solder joint. Because room temperature
is a high homologous temperature for solder, the initial eutectic microstructure
evolves over time.
The lamellar structure of the eutectic becomes coarser to attain an
energetically more favorable morphology. There is energy tied opal any grain
boundary, a solder joint with a coarse grain structure has lower energy content than
one consisting of fine grain size because of surface area and free energy relations.
This energy difference is the driving force for the grain coarsening, which proceeds
throughout the mechanism of solid-state diffusion.
4.5 Flux
Today’s electronic assembly has tighter lead spacing. The use of BGA
components where it is difficult to visualize the condition of the solder joint after the
reflow process makes it even more essential than ever to develop an understanding of
the paste chemistry and what occurs during the reflow process. The understanding
can enable an engineer to optimize the reflow conditions to suit the chemistry of the
colder paste. The net result of this optimization is increased yields, less soldering
defects, reduced costs and increased reliability. The electronic assembly market has,
in fact, become in the last decade mature and surface mount assembly in one part of
the world is no different than surface mount assembly in another part of global
64
markets. Maintaining an efficient soldering operation is essential for overall
competitiveness (Biocca, 1994).
The flux chemistry has also undergone major changes, improvements in
formulation to accommodate the more stringent needs of the industry. The ideal
material for flux must be matched to the application, without undesirable side effects.
Chemical corrosion, electric leakage and attack on plastics and components are but
few of the problems to be avoided (Manco, 1986).
4.5.1 Flux Functions
Fluxes are used in soldering industry to improve quality of the soldering.
During soldering, flux is used to remove metal oxides from the surface being
soldered so that the solder will adhere to the metallization pad. The chemical
behavior of the flux is very important in the formation of defect-free solder joints and
to the long-term interconnection reliability. A good solder flux must serve several
functions:
1) It must be able to remove the oxide film, tarnish fingerprints and dirt.
2) Must coat the newly cleaned area to prevent re-oxidation at the soldering
temperatures.
3) Must be thermally activated. The flux act as an efficient medium of heat
distribution and heat transfer during the soldering process. This ensures a
uniform solder joint free of porosity or other defects.
4) The flux must reduce the surface tension between the solder and the base
metal, which promotes wetting of the solder onto the base metal and permits
the formation of a defect-free solder joint.
65
4.5.2 Flux components
In a general way, the flux is made with four classes of ingredients: activators,
vehicles, solvents and additives. The activators are used to strengthen the fluxing
properties of rosin (Manko, 1986). The activator will remove the oxidation or tarnish
on the base metal. These may be highly corrosive acids, basic amines, or mildly
reactive organic acids. The activators may be aggressive at soldering temperatures
(~250°C), but minimal corrosive at room temperatures.
The second constituent is the vehicle, which is the solid or non-volatile liquid
that coats the base metal. It carries the removed grime and oxides away from the
solder joint. Additionally, the vehicle acts as the heat transfer medium to allow a fast
and homogeneous heat distribution towards the surface being soldered. The vehicle
also acts as a blanket to keep air from causing reoxidation when it comes in contact
with the metal cleaned by the activators. The vehicle may be a rosin, glycerol, resin,
glycol, polyglycol or polyglycol surfactant.
Solvents constitute an additional flux constituent. The solvent carries the
other flux ingredients in ambient temperature and to bring ones uniformly to the
precise place of the soldering. During pre-heat, the solvent volatilizes and
evaporates. The solvent is typically an alcohol, alphatic hydrocarbon, terpene
hydrocarbon, glycol, glycol ester or glycol ether.
The final category of flux constituents contains the remaining components of
the flux. This may be plasticizers, foaming promoters (used in wave soldering
applications), dyes, thermal stabilizers, corrosion inhibitors, surfactants, rheological
control agents, tackifiers, rheological agents or perfume. These are to improve the
existing function of flux. They are always merged with the vehicles.
66
4.5.3 Types of Flux
There are three different flux families available in market today: resin flux,
water soluble flux and no clean flux. Each type is suited for specific types of
assemblies. Electronic assemblies for extreme reliability such as avionics, space and
medical applications may still require the circuitry to be cleaned of flux residues after
soldering. Cleaning may also be required if circuits are conformal coated.
4.5.3.1 Resin fluxes
In this category, there are resinous flux and synthetic fluxes. The most used
resin in resinous flux is the rosin, which is made with resin of pine which consists of
90% resin acids and 10% ester. The melting point of rosin is located between 125˚C
and 130˚C. To improve the activity of these flows, activators are added. These are
traditional fluxes used by military assemblers and are still used today. The flux
system is based on natural or modified gum rosin with the addition of organic acids
and incremental additions of organic halides. These additives are used to increase
the wetting efficiency of the solder, since rosin alone is too weak an activator to
promote rapid wetting of solder. Three activator categories are distinguished:
1. R (rosin only): This type of flux is the least active and is generally
recommended for use on surfaces that are all ready very clean. It is intended
for this type of flux to leave virtually no residue behind.
2. RMA (rosin mildly activated): This type of flux contains activators that have
been added in order to enhance its cleaning and deoxidizing abilities. It will
leave a minimal amount of inert residue behind. That residue should be non-
corrosive, tack free and be substantially free from ionic contamination after
cleaning.
67
3. RA (rosin activated): This type of flux also contains activators that have been
added and is the most aggressive of the rosin-based fluxes. Although it
leaves the most residues behind, these residues can be easily removed by
using the appropriate type of flux cleaners.
It is necessary to know the temperature to which the activators act and their
aggressiveness degree which will influence the cleaning process after soldering.
RMA is the most used flux in the industry. These fluxes usually give amber flux
residues and are removable in solvents or water/ saponifier blends of cleaners. The
rosin flux systems used traditionally have higher solids contents in the range of 70-
80%. During the reflow process, the higher solid content, contributed heavily by the
rosin content act to shield the activators add to the flux and enable the flux to sustain
slight extremes in heating. These rosin based fluxes can tolerate slightly higher soak
temperatures, higher peak temperatures and time above the alloy’s liquidus (Biocca,
1994). Parenthetically, there is not any resin in synthetic fluxes. This flux is made
with some organic or carboxylic. They are obtained by organic synthesis.
4.5.3.2 Water soluble flux
Water soluble fluxes (sometimes termed organic acid fluxes or organic
intermediate fluxes) usually are based on the oxidation removing qualities of an
organic acid such as citric or glutamic acid and inorganic halide additives as their
main activators. The amount of acid reactant is more generous thus these pastes are
most suitable for difficult soldering. Metals which are oxidized or more difficult to
wet by molten solder are joined more easily. These residues are corrosive and
conductive in nature and must be removed after the soldering process. Due to the
corrosiveness of these flux residues, a hot water wash following solder reflow is
necessary and ionic contamination testing of the board after washing is highly
recommended. Boards stored without washing for more than an hour after exiting
68
the reflow oven may have serious oxidation buildup. These water washable fluxes
are used extensively by contract assemblers and electronic assemblies, which cannot
tolerate any remaining flux residues.
Water washable solder pastes due to the more active or acidic flux systems
used in their preparation are less affected by excesses in temperature in the soak zone
and less affected by excessive time above the alloy liquidus temperature, or the
melting temperature of the alloy. A typical flux formulation in weight percentages
for a water-soluble flux is defined below:
a. Water soluble high molecular weight resin – 60%
b. High boiling point solvents – 26%
c. Wetting agents – 5%
d. Gelling agents – 5%
e. Activators – 4% (halide and organic acid)
4.5.3.3 No clean fluxes
Millions of printed circuit boards are manufactured that are never cleaned
following solder reflow. This is especially true in consumer product electronic
assemblies, primarily to save money. Inert, bob-corrosive flux residues are left on
the boards and do not cause problems. By virtue of the fact that the boards are not
cleaned, it can be said the fluxes used are no clean fluxes. However, in recent years
the term no clean has come to refer to a type of flux specifically designed so flux
residues are minimized. No clean fluxes generally have the same aggressiveness as
RMA fluxes.
69
Typically no-clean fluxes are designed intentionally to be of lower activity.
Activity can be defined as the chemical efficiency of the acidic components of the
flux to remove the oxides present on the surfaces to be joined. More active fluxes
enable the removal of more tenacious oxide layers and improve the overall soldering
process. More active fluxes contain a higher concentration of organic acids or may
contain the addition of organic acids or organic halides but the amount used is
always incremental as to insure the non-corrosive nature of the flux residue is not
jeopardized (Biocca, 1994).
Lower residue is achieved in no clean fluxes by using lower solids content
than in other fluxes. Solids content refers to the ratio of solvent thinner to solid
component in the flux. A typical no clean flux has less than 15% solids, compared
with 50% in other types of flux. Since the solids in a flux are the basis for its
effectiveness, reducing the solids content tightens the process window for no clean
fluxes. Flux density control is therefore more important than with other fluxes. The
solids in solder paste flux chemistry are comprised of the resin, gelling agents,
wetting agents and activators or acid components. These ingredients will remain as
part of the flux residue after soldering, the activator may remain in its reacted form
and some of it will have volatized (Biocca, 1994). Normally, lower solids require a
nitrogen atmosphere in the reflow oven to achieve optimum soldering results
especially when it is used with fine pitch surface mount devices. Nitrogen gas
eliminates oxygen from the heat chamber of the reflow oven so that oxidation is
minimized.
No-clean fluxes, due to the choice of acids that have to be chosen to insure a
non-corrosive, non-conductive residue, weak organic acids and often weakly reactive
organic hydro-halides are added to promote better soldering. These components are
more affected by the temperatures encountered in the reflow process and special
attention must be executed when profiling the oven to not expose the flux to high
temperatures in the soak zone and also in avoiding excessive time above the melting
temperature of the alloy (Biocca, 1994).
70
No clean fluxes can be either rosin or resin based. The most noticeable
difference is that rosin based no clean flux produces a thin layer of sticky residue that
can be removed if desired. Resin based no clean flux produces a thin residue that is
hard and not sticky, but cannot be removed by water or other solvents. This can
occasionally be a consideration if rework is necessary. The amount of resin
incorporated in no-clean flux systems can also vary and impact on the amount of
residue left on around solder joints after soldering. Lower amounts of resin will
normally give cleaner looking boards due to the lesser resin left after reflow. This
resin is a high boiling, high molecular weight mix of organic compounds, which does
not volatilize at the temperatures encountered in surface mount technology (SMT)
processes and remains on the board.
4.6 Solderability
During reflow soldering the liquid solder alloy wets the surface of both
metals to be joined and upon solidification forms the solder joint. This solder joint is
significantly dependent on the solderability (Vianco, 1998). The solderability of
components and board lands/ traces can be defined as: (Blackwell, 2000):
Wettability. The nature of component terminations and board
metallisations must be such that the surface is wetted with molten solder
within the specified time available for soldering, without subsequent
dewetting.
Metallization dissolution. The component and board metallisations must
be able to withstand soldering times and temperatures without dissolving
or leaching.
Thermal demand. The mass of the traces, leads and thermal aspects of
packages must allow the joint areas to heat to the necessary soldering
temperature without adversely affecting the component or board
materials.
71
The term wetting in this case refers to the liquid behavior towards a solid
surface when they come into contact. Wetting is the word used to describe the
extents that solder flows over the surfaces to be bonded. Poor wetting is the result of
solder particles in the paste bonding to themselves to form a sphere, so the edges of a
poorly wetted solder joint are rounded rather than flat. Wetting is measured by the
angle made where the edge of a layer of solder paste meets the pad on a circuit
board. Poor wetting leaves a thick edge and a large angle (Figure 4.6). The wetting
angle can be interpreted in terms of the three surface energies involved, the molten
solder, the solid and the interface between the two.
Figure 4.6: The wetting angle
Moreover, the base metal is highly correlated with solderability. In the case
of soldering printed circuit board, the component’s leads or pins and board’s metallic
circuitry are the base metals that will contact the solder. Metals, such as aluminum,
high alloy steels, cast iron and titanium have very low solderability. These materials
are important because they provide choices of material in the construction of
soldering machine, and also as temporary covers for components that are not to be
soldered (Tai, 2003). Table 4.7 describes the general solderability of different base
materials.
Ideal: Ө <45°
Poor Ө >90°
Acceptable Ө <90°
ӨӨ Ө
72
Table 4.7: Solderability of different base metal
SOLDERABILITY BASE METAL REMARKS
Excellent Tin, Cadmium, Gold, Silver,
Palladium, Rhodium
Noble metals dissolve easily in
solders, resulting in brittle joints
Good Copper, Bronze, Brass, Lead,
Nickel-Silver, Beryllium-
Copper
High thermal conductivity of
these metals requires high heat
input during soldering. Oxidizes
quickly so proper flux must be
used
Fair Carbon Steels, Low-Alloy
Steels, Zinc, Nickel
Solder joints become brittle in
sulfur-rich environments. Avoid
higher temperatures in the
presence of lubricants (which
contain sulfur).
High-Alloy Steels, Stainless
Steels
Too much chromium oxide the
surface needs to be cleaned with
an aggressive flux.
Poor
High temperature
applications
Good high temperature
properties, good fatigue strength.
Medium or low flow properties
Very Difficult Cast Iron, Chromium,
Titanium, Tantalum,
Magnesium
Require pre-plating with a
solderable metal
4.7 Intermetallic Compounds
When a solid solution solidifies, alloys of metals which have a limited mutual
solubility may form new phases at certain ratios. These new phases possess crystal
structures different from either component and are called intermetallic compounds
73
(IMCs). The intermetallic layer properties in the created joint differ chemically and
physically from both the solder and the substrate metal (Minogue, 2002), exhibiting
less metallic characteristics such as reduced density, ductility and conductivity
(Hwang, 1992). Table 4.8 lists potential intermetallic compounds which have been
established in the equilibrium phase diagrams and identified in the assemblies.
Table 4.8: Potential IMC formation and un-compatibility between solder and
common substrates (Hwang, 1992)
Solder Substrate Intermetallic Compounds or Incompatibility
Sn-containing Cu Cu6Sn5, Cu3Sn
Sn-containing Au-based AuSn, AuSn2, AuSn4
Sn-containing Ag-based (Intermetallic phases)
Sn-containing Pd-based PdSn4
Sn-containing Ni Ni2Sn4
In-containing Cu Cu6In5 (Intermetallic phase)
In-containing Au-based AuIn, AuIn2
In-containing Ag-based Ag3In, Ag2In, AgIn2
In-containing Ni Ni3In2, Ni2In, NiIn, Ni2In3
Sb-containing Brass or Zn-containing ZnSb, ZnSb3
Sn/Bi Pb-containing (Sn/Pb
Solder)
(Low-melting ternary
phase)
High In-containing Sn-containing (Sn/Pb
Solder)
(Low-melting Sn/In alloy)
The key intermetallic system for both tin-lead alloys and the tin containing
lead-free solders is the tin-copper system (Figure 4.7). A number of intermetallic
stoichiometries between tin and copper can exist depending on the soldering
conditions employed. For example, copper in contact with liquid solder forms two
distinct intermetallics between copper and the tin contained in the solder, forming a
layer of Cu3Sn phase next to the copper, and a thicker Cu6Sn5 above it, as shown in
74
Figure 4.8. Cu3Sn phase is normally only found on the copper surface at the solder
joint interface, but Cu6Sn5 IMC tends to float away from the surface and are found in
the melt as rods and needle-shaped as shown in Figure 4.9.
Cu
Solder
Copper
Cu3Sn (ε)
Diffusion zone Cu5Sn6 (η)
Solder
Diffusion zone
Copper
Figure 4.7: Cross section through a soldered joint, made with eutectic solder
Figure 4.8: Types of intermetallics formed between Cu and Sn (Moon et al., 2000)
Figure 4.9: Needle-like Cu6Sn5 intermetallics (Tan C. L. 2006)
75
4.7.1 Factors Affecting the Growth of Intermetallic Compounds
In addition to the material compatibility, external factors also affect the rate
of intermetallic compound formation, namely, the temperature of exposure and the
time at the elevated temperature. Thus the solder reflow conditions, such as the peak
temperature, the time at the peak temperature, and the total residence time at elevate
temperatures are expected to influence the rate and extent of intermetallic growth. In
storage and service, the temperature exposure of the assembly is obviously another
external factor for the intermetallic growth in the “vulnerable” combination systems.
With the continuous shift towards miniaturization of electronic packaging the
thickness of the metallization pads is sometimes limited to few microns and
achieving reliable solder joints becomes a challenge to the industry since close
control of the soldering process and joint interfacial microstructure are required.
Although the formation of a thin layer of intermetallic during soldering is desirable
to achieve good metallurgical bond, excessive growth of these intermetallics affects
the mechanical reliability of the joint (Kumar, 2004a). As reported by Hwang (1992)
and Kumar (2004a), the formation of intermetallic compounds which are very brittle
in nature has been identified as one of the main sources of solder joint failure. When
an appreciable thickness of intermetallic compound is developed along the solder-
substrate interface, cracks are often initiated around the interfacial area under
stressful conditions. On the other hand, having insufficient intermetallics can also
cause joint failure. It has been reported by He et al. (2004) that micro-cracks and
oxidation of the base metal may occur after plating. If this occurs, either a thinner
layer of intermetallic on the oxidized surface is formed, or the plating simply
dissolves in the solder without forming any metallurgical bond at all. The resulting
solder joint is much weaker, resulting in failure. Such cracks generally originate at
the high stress regions and propagate along the solder-lead interface instead of
through the bulk of the joint.
76
Growth of the intermetallic layer in the solid state due during high
temperature storage or other thermal exposures depends to a large extent on the
initial layer that formed during soldering. Measures to prevent excessive growth of
the intermetallic layer include the deposition of a barrier layer on the Cu base in
contact with the liquid solder. In ENIG surface finish, for example, Ni is deposited
on the Cu substrate to act as a diffusion barrier. During soldering the Ni layer does
not dissolve in liquid solder to the same extent as copper, so that the contact is
between the tin and the nickel, rather than to the base copper. The metallurgical
bond is formed through the intermetallic Ni3Sn4, which has a monoclinic structure
and lower strength than copper-tin intermetallics (Islam et al., 2005).
Several factors may affect the thickness of the intermetallic layer present in
the solder joint. Among these factors: composition of the solder alloy, temperature
and time of soldering, composition of base material (substrate), service condition and
the volume of solder. The solder volume is very important since once the solder
reaches saturation the dissolution rate decreases and further dissolution of the base
material contributes only to the growth of the intermetallic layer. The effect of
solder volume on the amount of Cu dissolution and intermetallic thickness was
studied by Sharif et al. (2003) using Sn-Pb eutectic solder. They reported that
increasing the solder volume results in thinner intermetallic but higher dissolution
rate of the Cu substrate, while in smaller solder volume the intermetallic layer
thickness was higher. The reason for the observed results was attributed to the early
saturation of the solder with Cu in the small solder volume. Very little research on
the effect of solder volume or solder bump size using lead-free solders and
alternative surface finishes has been reported in the literature.
The formation of appreciable intermetallic compounds has been shown to
adversely affect solder joint strength and integrity. This metallurgical reaction is
dependent on time and temperature. With an understanding of the metallurgical
reactions, in conjunction with the utilization of phase diagrams, control of the time
and temperature is the way to minimize the intermetallic compound growth. A rough
77
estimation for calculating the total intermetallic layer thickness in the solid state
system is as follows:
X02 = Dt = D0t exp (-Q/RT) (4.1)
Where the diffusion coefficient, D0 is approximately 10-6 m2S-1 and the activation
energy for the intermetallic layer growth, Q, is about 80 kJ/mol.
4.7.2 Effects of Intermetallic Compounds
Reliability of solder joints is sensitive to the type and thickness of
intermetallic phase. The composition of intermetallic is dependent primarily on the
solder composition. The formation of IMC layer provides a chemical bonding
between the solder and the substrate. It is necessary for the joint strength. But an
overgrown IMC layer is known to be detrimental to the joint strength due to the
brittle nature of IMCs.
Sn-4Ag-0.5Cu solder bumps on Ni have interfacial intermetallics with Ni-Cu-
Sn ternary composition and the Sn-4Ag-0.5Cu solder bumps on Cu have Cu6Sn5. It
is widely reported that the Sn-3.5Ag solders without Cu forms a binary compound
Ni3Sn4. IMC grown in Sn-Ag-Cu appeared to be of two layers, which consist of the
top layer with a higher Cu content than Ni and the bottom layer attached to the Ni
layer with a higher Ni content than Cu. The Sn-4Ag-0.5Cu solder bumps on Ni
characterized intermetallics of Ni-Cu-Sn ternary compositions as (Ni, Cu)6Sn5
intermetallics from EDS analysis. The intermetallic phases were analyzed using
XRD as well as SEM and EDS in the present investigation.
78
The effects of alloy composition on microstructural, especially the formation
of large intermetallic compounds, and mechanical properties of various Sn-Ag-Cu
solder joints were investigated by Jang and Frear, (2000) and Jeon et al., (2003). The
range of Ag-Cu content of Sn-Ag-Cu alloys was from 3wt%Ag-0.5wt%Cu to
3.9wt%Ag-0.7wt%Cu. The high Ag content alloys exhibit the formation of large
Ag3Sn platelets especially at the solder-reaction layer interfaces, regardless of the
kind of substrate. Long Cu6Sn5 whiskers are formed in all the solder, Cu joints and
the high Cu content solder joints. Those whiskers have two shapes of needle and
hollow types. For the high Cu content solder joints with a 42 alloy substrate, long
Ni-Cu-Sn whiskers are also formed at the interface and in the solder. The presence
of large Ag3Sn platelets does not degrade strength directly, but affects fracture mode.
Large Ag3Sn platelets induce brittle fracture at an interface and provide crack
initiation sites. In order to avoid the formation of the large intermetallic compounds,
especially Ag3Sn platelets, the optimum composition of Sn-Ag-Cu alloy lies in the
lower Ag-Cu content.
The intermetallic layers can have detrimental effects not only on solderability
but also on the mechanical properties of the solder joint because the intermetallic
compound layers are brittle as compared to solder or tin. However, intermetallics are
necessary for strong and reliable solder bonding. But then, it must not grow thicker,
otherwise will cause brittle joint which may lead to failure of the bonding.
4.8 Isothermal Aging Treatment
Isothermal aging plays an important role in testing the reliability of
assembled electronic devices and in particular, for flip chip technology which is used
on computer chip packaging (Tai, 2003). This treatment actually simulates the
accelerated service condition of the components. The IMC layer formed during
soldering between the under plate and liquid solder will eventually develop during
79
the solid state thermal ageing, its thickness increases with time under certain
temperature and its morphology changes. Figure 4.10 schematically illustrates how
the IMC formed during soldering increases its thickness after thermal ageing
consuming more of the solderable layer in the process.
Figure 4.10: Schematic diagrams of the layers before and after isothermal ageing.
Although IMC is essential in a solder joint, formation of thick or new
intermetallic compounds might cause reliability problems to the products. As
mentioned before, the IMC growth is strongly influenced by the operating
temperature of the device which can be generally divided into the following three
categories (Suganuma, 2001):
40°C to 100°C : most consumer electronics (TV, PC, freezer, etc.)
40°C to 125°C : reliable consumer electronics (mobile phone, notebook PC ,
etc.)
40°C to 150°C : vehicles (especially in engine room) and factory equipment.
The author had chosen 150°C as the isothermal ageing temperature in this project.
Solder
Solder
Base Material
Solderable layer
IMC
Base Material
Solderable layer Protective layer (remaining)
Solder/ protective layer
(IMC)
Thicker solder/ protective layer (IMC)
IMC/ solderable layer interface
CHAPTER 5
RESEARCH METHODOLOGY
5.1 Introduction
The main objective of this research study is to investigate the effect of
different surface finishes and solder bump size (solder volume) on intermetallic
formation and growth during soldering using lead-free solders (Sn-4Ag-0.5Cu and
Sn-3Ag-0.5Cu). From this point SAC405 and SAC305 will be used throughout the
thesis. The surface finishes used are: Immersion Sil2y (TSM) which consists of
layers of Ag and also Cu/Ni/Au. After preparing the substrate surface finish, lead-
free SAC405 and SAC305 solder balls in various sizes (Ø200µm, Ø300µm, Ø500µm
and Ø700µm) were placed onto the substrate and the whole structure was subjected
to reflow soldering. Then, the substrates will undergo isothermal solid state ageing
at a temperature of 150oC for different ageing time. Analysis of the results is mainly
focused on the characterization of the IMCs formed between TSM and solders. The
detail steps for the research approach taken in this thesis are described in the
following sections.
81
5.2 Substrate Material
The substrate material used in the experiments consists of FR4 (epoxy glass)
material sandwiched between two electroplated copper layers. The dimensions of the
substrate are shown in Figure 5.1. Two types of surface finishes will be deposited:
immersion silver and electroless nickel with medium phosphorus/ immersion gold.
35 mm
2 mm
50 mm
Copper
Polymer
Copper
2
( a ) ( b )
Figure 5.1: (a) Plan view and (b) Side view of copper substrate.
The copper substrate will first undergo a pretreatment process before plating
in order to remove the oxide and activate the copper surface. After plating, the
specimens are ready for reflow soldering and also isothermal ageing process. Figure
5.2 shows the flowchart of this research.
82
PRETREATMENT
ELECTROLESS NICKEL IMMERSION SILVER
IMMERSION GOLD
REFLOW SOLDERING
AGEING
MICROSTRUCTURAL ETCHING TOP SURFACE
IMAGE ANALYZER FESEM FESEM - EDXOPTICAL MICROSCOPE
IMCs distribution IMCs thickness IMCs composition IMCs interface, 3D morphology
Figure 5.2: Process flowchart of reflow soldering and specimen analysis
83
5.3 Plating process
5.3.1 Pretreatment of copper substrate
Before electroless plating, the substrate surface was cleaned in order to
remove oxides, dirt, grease or oil and also to activate the copper surface to make sure
that the copper is clean and ready for plating. This is because, poor surface
preparation can cause lack of adhesion, deposit porosity, roughness, non-uniform
coatings and/or dark deposits (Aleksinas, 1990). The pretreatment process for ENIG
surface finish followed in the present study is as follows:
i. Grind the copper substrate with silicon carbide paper of 4000 grit size to
physically remove the oxide layer.
ii. Rinse in distilled water.
iii. Medium alkalinity soaks clean at 70-90°C to remove dirt.
iv. Rinse in distilled water.
v. Alkalinity cleans with 10% NaOH at 60°C for 3 minutes to degrease.
vi. Rinse in distilled water.
vii. Soak in acid solution containing 10% H2SO4 for 1 minute to deoxidize the
copper surface.
viii. Rinse in distilled water.
ix. Dip into Palladium chloride + HCL solution for 3 minutes to activate the
copper surface.
x. Rinse in distilled water.
While for immersion silver plating, the pretreatment steps are as follows:
i. Medium alkalinity soaks clean at 70ºC-90 ºC for 4 minutes to remove
dust
ii. Rinse in distilled water
84
iii. Microetching in pre-sulphate solution at50 - 60 ºC for 2 minutes to
remove any surface copper oxide.
iv. Rinse in distilled water
v. Acidic clean with 10%H2SO4 for 1 minute to remove traces of cleaning
and/or microecthing composition and thus activate the copper substrate.
vi. Rinsed in distilled water
5.3.2 Plating equipment set-up
The experimental set up for the electroless nickel/immersion gold and
immersion silver plating are shown in Figure 5.3 and Figure 5.4 respectively. The
set-up of the experiment includes the use of a plating bath equipped with a stainless
steel heating coil, and which provides constant temperature control. Plating is
conducted in a 1L Pyrex beaker placed inside the bath to maximize the amount of
solution used for plating. A hanger was placed in the beaker to hold the samples,
which in turn were hooked by an aluminium wire as shown in Figure 5.3 (c) and
Figure 5.4. However, before starting the immersion silver plating, a pre-plating step
is needed in order to remove any dirt or contamination from the pretreatment process
which may lead to poor adhesion between the Ag layer and Cu (Figure 5.4).
The sealing of the plating bath is highly recommended. The plating bath was
covered to avoid evaporation of the plating solution (Tai, 2003). The aqueous
solution may evaporate during the plating process especially if the plating is
conducted for longer times at high temperatures such as electroless nickel bath and
immersion gold bath. When evaporation does occur during plating, water is lost
which will affect the original concentration of plating solution, causing instability of
the plating solution. This has been overcome by filling the plating bath with distilled
water.
85
(b)(a)
Aluminum hook Hanger
Beaker
Heater
Copper substrate Plating
solution
Hot water
(c)
Figure 5.3: (a) Experimental set-up in the plating bath, (b) The plating bath and, (c)
Schematic set-up of electroless nickel and immersion gold plating process.
Plating 7min
Pre-plating 1min
Hot water silver bath
Copper substrate
Heater
Beaker
Hanger Aluminum clip
Figure 5.4: Schematic set-up of immersion silver plating
86
5.3.3 Electroless nickel plating
In this study, a medium phosphorous nickel solution was used to plate a layer
of nickel onto the copper surface. The medium phosphorous solution is a
commercial one and is called NIMUDEN 5X, a high temperature acidic solution (90-
92°C, pH = 4.3-4.5). It is usually used in the industry to produce coating with
phosphorus content between 5wt%-10wt%. This medium phosphorous solution was
supplied by a vendor. To make-up the solution, four parts of distilled water are
mixed into one part of NIMUDEN 5X. The solution is then placed into the plating
bath. Sodium hydroxide and sulfuric acid is used to control the pH value to the
required range. If the pH value is below 4.0, plating speed decreases and if the value
is above 4.6, the plating solution sometimes becomes muddy. The plating time was
predetermined to be about 10 minutes in order to produce 4-5µm nickel layer.
Figure 5.5: Commercial medium phosphorous concentration electroless nickel
plating solution: NIMUDEN 5X
Ni
Cu
Ni++
Figure 5.6: Schematic process of electroless nickel plating
87
5.3.4 Immersion gold plating
The second step in the deposition of TSM was to coat a layer of gold on top
of the nickel layer for oxidation protection. The immersion gold plating is conducted
immediately after electroless nickel with no pretreatment, except rinsing in running
tap water. The plating time used is 3 minutes to produce 0.3µm layer of gold. The
combination for the immersion gold solution is that of Swan and Gostin bath
(hyphophosphite bath) as shown in Table 5.1. According to Okinaka (1974), the
immersion gold deposition occurs by a galvanic displacement reaction. That is,
noble metals such as gold spontaneously exchange places with less noble metals. In
the case of ENIG, for example, the nickel ionizes and dissolves into solution as the
gold cations in solution obtain electrons from the nickel and form a thin gold layer
over the existing nickel (Coyle et al., 2001). A schematic process is shown in Figure
5.7.
Table 5.1: The Swan and Gostin bath (Okinaka, 1974)
Chemical Reagent/Parameter Quantity
Potassium cyanoaurate, KAuCN2 2g/L
Ammonium chloride 75g/L
Sodium citrate, dehydrate 50g/L
Sodium hyphophosphite, dehydrate 10g/L
pH 7.0-7.5
Temperature 93±1°C
Au+
Ni
Figure 5.7: Schematic process of immersion gold plating on nickel
88
5.3.5 Immersion silver plating
Immersion plating is also called a galvanic displacement reaction, in which
the process will stop when the entire Cu surface is displaced with Ag (Figure 5.9).
Immersion silver is only capable of providing a maximum thickness of Ag layer.
Beyond that, the extended Ag plating time will not increase the plating thickness. In
this study, the desired thickness was obtained by varying the plating time. Similar
with previous plating procedures, the substrate was rinsed with running tap water
after being removed from previous solution. Few combinations of immersion
solution have been tried before a most stable combination was identified. Figure
5.10 shows the steps followed in the present study to achieve the desired silver layer.
The combination selected in the present study is a neutral solution with the plating
time 8 minutes to produced a Ag coating of ~0.27µm and 12 minutes plating time to
produce a Ag coating of ~ 0.78µm as shown in Table 5.2.
1 2
Figure 5.8: Immersion silver plating steps
89
Table 5.2: Immersion silver bath formulation
Chemical Reagent/Parameter Quantity
Ethylene diamine tetra-acetic acid (EDTA) 50 gm/L
Sodium Hydroxide 20.4 gm/L
perature
Silver Nitrate 1 gm/L
pH 6.46
Tem 40oC
Figure 5.9:
Schematic process of immersion silver plating
Ag+
A
C
Figure 5.10: Process flowchart for Immersion Silver
90
5.4 Reflow S
.4.1 Solder Masking
In practice, the use of solder mask is intended to limit the solder distribution
on a PC
.4.2 Flux
Cleanliness is essential for efficient and effective soldering. Solder will not
adhere
oldering
5
B during the reflow soldering or wave soldering process to just the land area
where the component being attached to the board for mechanical and electrical
connection (Strauss, 1998). Prior to reflow soldering, the substrates were first
laminated with a layer of dry solder mask to restrict the molten solder from flat
spreading during reflow, especially for the smaller solders. The surface should be
clean and dry before applying the solder mask. Next, the solder mask together with
the printed negative film were exposed to ultraviolet (UV) light for 40 seconds in the
UV unit. After removing the film, the substrates were soaked in the developer which
contains potassium carbonate mixed with water at a ratio of 25:1 to form a layer of
solder mask with small pad openings of desired diameter for all small solder bump
sizes investigated. The solder mask was then exposed to UV light again for 120
seconds for further curing and hardening.
5
to dirty, greasy, or oxidized surfaces. Heated metals tend to oxidize rapidly.
The primary function of a soldering flux is to remove oxidation and tarnish from the
surface being joined (surface finish and solder ball), and prevent further oxidation as
well as improve the wetting of molten solder during reflow soldering. A no clean
flux was used in this research. The exact chemistry is unknown to the researcher as
the sponsor of the present work supplied the flux under confidential circumstances.
91
After applying solder mask, a thin layer of no clean flux was applied onto the
substrates to ensure the solderability of the surface finish.
5.4.3 Solder Bump Formation
The solder compositions used in the present study include lead-free Sn-
4.0Ag-0.5Cu and Sn-3.0Ag-0.5Cu. They have a melting temperature of 220˚C and
reflow temperature around 250˚C. Solder bump formation was performed on all
surface finishes: ImAg and ENIG. After applying the flux, the substrates were
manually populated with solder balls with different sizes, namely 200, 300, 500 and
700µm in diameter. The solders with the same size were arranged in an array of 9 ×
5 on each of the substrate. Bonding to form the solder joints was made by subjecting
the substrates to a reflow soldering in a resistance furnace with the peak reflow
temperature set at ~250˚C. Figure 5.11 shows the solder joint formation for ENIG
surface finish. While Figure 5.12 shows the reflow profile used in this research.
Figure 5.11: Solder joint formation for ENIG surface finish
92
3025 15 2010
50
250200
150 100
T, oC
Time (minutes)
Figure 5.12: Reflow profile for Sn-Ag-Cu
5.5 Isothermal Aging
Isothermal aging is one of the processes to test the reliability of the
component and also accelerate the growth of intermetallic compounds. After reflow
soldering, each specimen was subjected to isothermal aging at a fixed temperature of
150 oC for different aging durations, which were 0 hour (as reflow), 250 hours, 500
hours, 1000 hours and 2000 hours respectively in order to examine the effect of long
term exposure to high temperature on the growth of intermetallics.
5.6 Specimens Characterization
Characterization was conducted to determine the type or composition and
morphologies of the IMCs formed in the solder joints. Several techniques of
characterization tools were employed for analysis and measurement. Optical
93
microscope, SEM (scanning electron microscopy) and FESEM (Field emission
scanning electron microscope) equipped with energy dispersive X-ray (EDX) were
used to characterize the type of IMC formed in terms of type and composition.
While the measurement of the IMC thickness was performed using the image
analyzer (IA). In order to examine the 3-D morphology of intermetallics, solder
joints were subjected to a selective deep etching in acidic solution. This technique
allows the solder to be etched away leaving only the intermetallics to be examined.
Selective etching was performed both on solder joints cross-sections and top view
surface. Conventional etching was also conducted on cross-sections in order to
examine the microstructures and also measure the intermetallics thickness.
5.6.1 Characterization of Specimens Cross Section
After soldering and solid state ageing, the substrates were cooled to room
temperature. Samples containing four to five solder joints were cross-sectioned, cold
mounted, polished and etched. The cold-mounted samples were first ground on
successive silicon carbide papers starting with the coarse 320 grit up to the finer
4000 grit, followed by polishing and finally etching to reveal the interface and
internal microstructure of the solder joints. The intermetallics formed during
soldering and after aging treatment are investigated in terms of their type (elements
combining to form such intermetallics), morphology (shape and distribution) and
thickness. Table 5.3 lists the different chemical etchants used to reveal the cross
sectional microstructures.
Table 5.3: Chemical composition of Klemm Solution II
Chemical Composition
Stock Solution* 100 ml K2S2O5 5 g
* Stock Solution = 1 kg Na2S2O3. 5H2O (Sodium Thiosulphate) in 300ml water
94
The cross sections were observed by using Nikon optical microscope at
various magnifications at different locations. The thickness of the IMCs layer at the
solder joint was measured using the image analyzer. After that, the selected cross
sectional specimens were subjected to deep etching using 10% hydrochloric acid and
90% ethanol. Different duration of etching time was used for specimens with
different solder size (Table 5.4). This process was followed by examined the
samples using Scanning Electron Microscope (SEM) and the composition of the
IMCs was determined by Energy Dispersive X-ray Analysis (EDX).
Table 5.4: Etching time for cross sectional deep etching
Solder size (µm) Etching time (minutes)
200 15 – 20 300 30 – 35 500 40 – 45 700 60
5.6.2 Characterization of Specimens Top Surface
The specimens were deep etched to determine the morphologies and types of
IMCs formed using 10% hydrochloric acid and 90% ethanol where it etched away
the solder in order to reveal the IMCs at the solder joint (Figure 5.13). Then, the
specimens were ultrasonically cleaned followed by sputtered with a thin coating of
gold before the specimens were characterized by SEM and EDX.
Solder SEM/ EDX
CuSF/ TSM
IMC IMC
SF/ TSMCu
Figure5.13: IMCs formed from the top surface view
CHAPTER 6
RESULTS AND DISCUSSION
6.1 Introduction
This chapter discusses the results of surface finish deposition, intermetallic
compound formation and growth during soldering between of Sn-Ag-Cu lead free
solders and two different surface finishes, i.e.; Immersion Silver (IAg) and
Electroless Nickel/ Immersion Gold (ENIG). This chapter also discusses the effect
of solder bump size (volume) (200µm, 300µm, 500µm and 700µm diameter solder
spheres), silver concentration in the solder alloy (Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu),
and also isothermal aging process on the composition and morphology of
intermetallics. Special focus will also be on the formation and growth of interfacial
voids.
96
6.2 Top Surface Metallurgy (TSM) Deposition
The under bump metallurgy consists of various types of surface finishes
which are ENIG, ENEPIG, ImAg, ISn, DIG and many more. However, in this
research, there will be special focus on the new type of surface finish that is used in
the industry nowadays, which is Immersion Silver (ImAg). While ENIG will be used
as a reference to the results obtained. To achieve one of the objectives of the current
study, an attempt was made to deposit the silver layer through immersion plating
process. Several plating solutions were attempted in order to plate a silver layer with
the desired thickness, which is <1µm. The solutions range from acidic to alkaline
with different process parameters. After several attempts, one neutral solution
capable of depositing a layer of silver with the thickness of 0.27µm after 8 minutes
plating time and 0.78µm after 12 minutes plating time was successfully identified.
Figure 6.1 shows the results of copper substrate before plating (after pretreatment
process). While Figure 6.2 shows the results of immersion silver plating.
Copper microstructure at 5 K X magnification
10 K X
Cu = ~100%
Figure 6.1: Copper substrate before plating (after pretreatment process)
97
Figure 6.2: Copper substrate plated with silver coating
From the EDX analysis of the sample shown in Figure 6.2, the results clearly
show that the silver layer deposited using the neutral solution, which operates at a pH
value of 6.46, fully covered the copper substrate (Figure 6.3).
Ag = 75.13%
Cu = 24.87%
Figure 6.3: FESEM-EDX results of IAg on Cu
6.3 Identification of Intermetallic in solder joint
It is important to identify the type or composition of the IMCs in the solder
joint before continuing with the study of the effect of the various parameters on the
98
IMCs formation in the solder joint. This information would be used as a reference in
later analysis. Generally, the IMCs formed in the solder joint can be identified
through ratio calculation and comparison with EDX results obtained from SEM or
FESEM.
Firstly the intermetallics present in a solder joint must be identified by
determining their compositions. Atomic percentage of each element that takes part
in the formation of a particular intermetallic is determined by doing a simple
calculation. For example, the most common intermetallic formed in a solder joint
between Ni and Sn solder is Ni3Sn4, which is a combination of a three nickel atoms
and four tins atoms, with the atomic number of 28 and 50 respectively (Table 6.1).
Figure 6.4 illustrates a simple calculation by using Ni3Sn4. As a result, tin dominates
with 72.9 at% in the total atomic number of this intermetallic compared to 27.06 at%
for nickel. Table 6.2 illustrates the predicted atomic percentage of elements formed
in the solder joints.
Figure 6.4: Example of atomic percentage calculation
99
The same calculation was repeated to all possible types of IMCs and every
atomic percentage of elements for each particular IMC has been summarized in
Table 6.2. By comparing the atomic percentage obtained from EDX analysis with
the calculated atomic percentages, the types of IMC can be determined.
Table 6.1: Atomic number of elements
Elements Atomic Number Ni
Sn
Ag
Cu
Au
28
50
47
29
79
Table 6.2: Atomic percentage of predicted IMCs
100
6.4 Composition and Surface Morphologies of Intermetallics
Sn-Ag-Cu solder alloys are considered to have the highest potential lead-free
standard alloy systems to replace Sn-Pb solder alloys. The main advantages of this
alloy system are lower melting temperatures compared to Sn-Ag binary eutectic
alloys and superior mechanical properties as well as relatively good solderability.
For these reasons, Sn-4.0wt%Ag-0.5wt%Cu and Sn-3.0wt%Ag-0.5wt%Cu solder
alloys were selected for this research.
During reflow soldering, the formation of the solder joint exists when there
are interactions between liquid solder with the base metal, copper or nickel. This
interfacial reaction results in the formation of intermetallic compounds, which are
identified using techniques such as EDX. The solid-state reactions and growth of
intermetallics during long term exposure to high temperatures are also important
because the morphology, distribution and thickness of these intermetallics will affect
the solder joint reliability. The morphologies of the various intermetallics formed in
the solder joint during soldering and after solid state ageing were examined on all
specimens using selective or deep etching of the solder. The results for Pb-free
solders are presented in the following sections with different surface finishes and
solder bump sizes.
6.4.1 Intermetallics between Sn-Ag-Cu solders and ImAg Surface Finish
6.4.1.1 Interfacial Reactions after Reflow Soldering
As the SAC solders/ immersion silver is reflowed, the Ag layer will dissolve
into the molten solder exposing the Cu substrate to the solder. Once the solder is
101
melted, the Sn in molten solder will quickly react with the solid Cu and develops
small nuclei which quickly grow and form a scallop like islands of the IMC at the
interface. When this type of intermetallic becomes large enough and connect
together to form a continuous layer of Cu-Sn intermetallics, the formation of the
IMC involves two steps; 1) The diffusion of Cu through the newly formed IMC layer
and, 2) Chemical reaction between Sn and Cu to form intermetallics. The cross-
sectional optical and SEM micrographs of Figure 6.5 and Figure 6.6, for both
SAC405 and SAC305 solders, reveal that the Ag layer disappeared completely,
confirming that it was entirely dissolved into the molten solder during reflow. The
interface intermetallic was confirmed by EDX analysis that it has the Cu6Sn5
structure. Observation of these two Figures also clearly shows that the Cu6Sn5 were
scallop-like after reflow.
Ag6Sn Cu6Sn5Cu6Sn5
X500 X500
Figure 6.5: Cross-sectional optical images after reflow: a) SAC405/ ImAg, b)
SAC305/ImAg
Figure 6.6: Cross-sectional SEM images after reflow: a) SAC405/ImAg, b)
SAC305/ImAg
102
SEM images of the 3-D morphology of the intermetallic formed after reflow
are shown in Figure 6.7 and Figure 6.8 for SAC405 and SAC305 solders
respectively. It is very obvious and evident that the Cu6Sn5 intermetallic have the
scallop-type morphologies and that their size decrease as the solder bump size
increases indicating the formation of thicker intermetallic in the smaller solder bump
(This will be discussed later in Section 6.5).
Cu6Sn5(a)
Cu6Sn5(b)
Cu6Sn5(c)
Cu6Sn5(d)
Figure 6.7: Top view micrographs formed during reflow between SAC405 solder
and ImAg. (a) 200µm, (b) 300µm, (c) 500µm and (d) 700µm
103
Cu6Sn5Cu6Sn5(a) (b)
Cu6Sn5(c)
Cu6Sn5(d)
Figure 6.8: Top view micrographs formed during reflow between SAC305 solder
and ImAg. (a) 200µm, (b) 300µm, (c) 500µm and (d) 700µm
According to Laurila (2005), the dissolution rates of Cu along the Cu surface
are not necessarily uniform. There are areas where all the Cu atoms have not been
used in the formation of Cu6Sn5 IMC and this Cu surplus may lead to the formation
of long tubes or fibres of Cu6Sn5. This is indeed confirmed in Figure 6.9 which
shows the presence of large rods of Cu6Sn5 when soldering with SAC405 and
SAC305 solders on ImAg. Figure 6.9 also shows the formation of large Ag3Sn
plates in the SAC405/ ImAg but no such large Ag3Sn were formed when soldering
with the SAC305, which is consistent with previous research (Kim et al. 2003) who
reported that the formation of large Ag3Sn can be prevented by reducing the amount
of Ag in the solder. In their work it was reported that these large Ag3Sn formed in
both Sn3.5Ag0.7Cu and Sn3.9Ag0.6Cu, results which are in good agreement with
the present study when soldering with SAC405.
104
(c)
(a)
Ag3Sn
Cu6Sn5
(b) Cu6Sn5
Ag3Sn
Figure 6.9: Top view SEM images showing formation of large Ag3Sn plates and
Cu6Sn5 rods in SAC405/ ImAg (a, b) and Cu6Sn5 rods on SAC305/ ImAg (c)
105
Ag3Sn intermetallic formed both at the interface and in the bulk solder of
SAC405 with different morphologies such as large dendrite-like, plates and blocky
shape as shown in Figure 6.10. These large Ag3Sn intermetallics are quite brittle,
which may lead to serious problems under stressed conditions in the actual service
for PCB. Zeng and Tu (2002) mentioned that these Ag3Sn crystals will easily lead to
failure if formed in a high stress area, such as corner region between solder bump
and top surface metallurgy (TSM), where cracks can be initiated and propagated
along the interface between Ag3Sn and solder.
(b) (a)
Cu6Sn5
Ag3Sn
X500
(d)
Ag3Sn
Ag3Sn
(c)
Figure 6.10: Formation of Ag3Sn during reflow between SAC405 solder and ImAg:
(a, b, c) Top surface morphology of the solder joint and (d) Cross section (x500)
106
6.4.1.2 Effect of Isothermal Ageing on Intermetallics
Isothermal aging was conducted for 250 hours, 500 hours, 1000 hours and
2000 hours at 150oC. During isothermal aging, the IMCs continue to grow by solid-
state inter-diffusion between the Sn in the solder and Cu in the substrate. During
reflow soldering the molten solder dissolves the Ag layer and gets in direct contact
with the Cu substrate. The solder reaction occurs at the interface to form Cu6Sn5.
During further heating in the solid state, this Cu6Sn5 becomes a continuous layer and
a new IMC phase forms between the Cu and Cu6Sn5 as shown by the thin grey layer
in Figure 6.11 which is known as Cu3Sn. However, this layer cannot be seen on the
top surface of the IMCs because it has been covered with Cu6Sn5 IMC from the top.
Thus, it can only be observed through the cross section. It can also be clearly
revealed from Figure 6.11 that the Cu3Sn layer is more even than the Cu6Sn5 layer.
With increasing ageing time it is also shown that the Cu3Sn grows at a faster rate
compared to Cu6Sn5 layer. For the solder joint aged for 2000 hours the thickness of
Cu3Sn is almost equal to that of Cu6Sn5 layer (Figure 6.11).
It has been established (Laurila et al. 2005) that during soldering at the first
phase to form is Cu6Sn5 at the Cu/Sn interface. This reaction is believed to be very
fast and is controlled mainly by the dissolution of Cu in the molten solder followed
by chemical reaction. The Cu3Sn layer instead will only form if a long contact time
is allowed with the liquid solder. Research work by Yu et al. (2004) claimed that
grain boundary diffusion is the predominant mechanism for the IMCs growth during
heating or ageing. Between Cu6Sn5 grains, molten solder channels extend all the
way to the Cu3Sn/Cu interface. Since Cu3Sn compound layer is so thin, these
channels serve as fast diffusion and dissolution paths of copper in solder to form
interfacial reactions.
As shown in Figure 6.11 it is clear that the Cu3Sn grows as the ageing time is
increased. This can be explained by the fact that during solid-state aging as Sn and
Cu diffuse to the interface and react. It was claimed by (Peng et al. 2007) that after
107
the Cu atoms diffuse to the interface of Cu3Sn/Cu6Sn5 through the grain boundaries
of Cu3Sn, Cu6Sn5 is converted to Cu3Sn.
(f)
(e)
(d)
(c)
(b)
(a)
Cu6Sn5
Cu6Sn5
500X500X
500X
500X 500X
500X
Cu3Sn
Cu3Sn
Figure 6.11: Optical micrographs of cross-sectional views of SAC405/ ImAg (a-c)
and SAC305/ImAg (d-f). (a, d): after reflow and (b, e): after ageing at 150oC for 250
hours and (c,f) after ageing at 150oC for 2000 hours
108
Thus the growth of Cu6Sn5 is now dependent on the Cu available in the bulk
solder. Since most of this Cu has already been consumed in the formation of Cu6Sn5,
there is little Cu that can diffuse to the Cu6Sn5/ solder interface, significantly limiting
the growth of Cu6Sn5. Cu3Sn can grow quite rapidly at the Cu6Sn5/Cu3Sn interface
by consuming Cu form Cu6Sn5. The results of intermetallics thicknesses as well as
the growth rates of Cu6Sn5 and Cu3Sn will be discussed in Section 6.5.
Figure 6.12 shows SEM micrographs of solder/ ImAg interface of the
SAC405 and SAC305 solder joints after ageing at 150 oC for 1000 hours (SAC405)
and to 2000 hours (SAC305). From these micrographs it can be revealed that the
morphology of Cu6Sn5 layer has transformed from the scallop-type to layer-type
during ageing.
The effect of solid state aging on the size and morphology of intermetallics
can also be seen in Figure 6.13 to Figure 6.16. Observation reveals that the
intermetallic has coarsened and changed to layer-type morphology instead of the
scallop-type observed after reflow soldering for both SAC405 and SAC305 solders.
The same trend is observed in both smaller and larger solder bump sizes. Thus, the
aged SAC405 and SAC305 solder joints comprise, in addition to the Cu substrate,
Cu6Sn5 and Cu3Sn, particles of Ag3Sn embedded within the Cu6Sn5 intermetallic
layer. The Ag3Sn embedded in the Cu6Sn5 intermetallic after ageing for both
SAC405 and SAC305 solders investigated is particularly evident when soldering
with the smaller solder balls (Figure 6.13 and Figure 6.14). Such features have been
observed in previous studies (Huang et al., 2006) and their formation is believed to
be the result of a higher content of Ag in the solder and not in the substrate as Ag3Sn
particles embedded in Cu6Sn5 were also observed when soldering on Cu (Siti
Rabiatull Aisha (2006) and Yoon et al. (2008)) indicating that there is no effect from
the silver in the substrate. The presence of Ag3Sn particles embedded within the
Cu6Sn5 intermetallic is believed to be related to the depletion of Sn in the solder
close to the Cu6Sn5 layer by solid state reaction during ageing (Yoon et al. 2008).
The reason why more embedded Ag3Sn were observed in the smaller solder balls
compared to the larger solders has been given by Huang et al. (2006). As the
109
intermetallic grows into the solder, the faster the growth rate the more Ag3Sn
particles will be embedded into the interface intermetallic (Cu6Sn5 in this case) as
shown schematically in Figure 6.17. Since the intermetallic layer growth is faster in
the smaller solder more Ag3Sn were found to be embedded within the Cu6Sn5 layer.
The embedded Ag3Sn particles also exhibited coarsening due to ageing treatment.
These results are also consistent with previous findings (Choi et al., 1999).
Cu3Sn
Cu3Sn
Cu6Sn5
Cu6Sn5
Figure 6.12: SEM images of cross-sectional views showing the effect of ageing on
the interfacial morphology. (a) SAC405/ImAg and (b) SAC305/ ImAg
110
Cu6Sn5
Ag3Sn
(a)
Ag3Sn
(b)
Cu6Sn5
(d)
Cu6Sn5
(c) Ag3Sn
Figure 6.13: Morphology of Cu6Sn5 on ImAg for 200µm solder bump of SAC405,
(a) Ageing 250 hours, (b) Ageing 500 hours, (c) Ageing 1000 hours and (d) Ageing
2000 hours
111
Cu6Sn5
Ag3Sn
(d) (c)
(b) (a)
Figure 6.14: Morphology of Cu6Sn5 on ImAg for 200µm solder bump of SAC305,
(a) Ageing 250 hours, (b) Ageing 500 hours, (c) Ageing 1000 hours and (d) Ageing
2000 hours
112
Cu6Sn5
Ag3Sn
(d) (c)
(b) (a)
Figure 6.15: Morphology of Cu6Sn5 on ImAg for 700µm solder bump of SAC405,
(a) Ageing 250 hours, (b) Ageing 500 hours, (c) Ageing 1000 hours and (d) Ageing
2000 hours
113
Cu6Sn5
(d) (c) Ag3Sn
(b) (a)
Figure 6.16: Morphology of Cu6Sn5 on ImAg for 700µm solder bump of SAC305,
(a) Ageing 250 hours, (b) Ageing 500 hours, (c) Ageing 1000 hours and (d) Ageing
2000 hours
Substrate
Solder
Ag IMC
Ag3Sn
(a) Before ageing (b) After ageing
Figure 6.17: Schematic of Ag3Sn particles embedded during intermetallic growth
(Huang et al. 2006)
114
From Figure 6.13 to Figure 6.16 it is also very clear that as the ageing time is
increased the intermetallic layer has coarsened for a given solder bump size. This
indicates that the Cu6Sn5 intermetallic layer has indeed increased in thickness. Also
it can be seen that the grain size for smaller solder bump size is bigger compared to
the larger solder bump size for both SAC405 and SAC305 solders particularly after
reflow and ageing for shorter time (250 hours) indicating that thicker intermetallics
grow in smaller solders. For example the measured grain size for SAC405 solder
size of 200 µm and 700 µm after reflow were 5.075 µm and 2.034 µm respectively.
These values have increased after ageing for 2000 hours to 10.80 µm and 6.433 µm
respectively.
The present study also showed that more Ag3Sn of the block-type was formed
ahead of the interface when soldering with larger solder balls as shown in Figure
6.18a. The Ag3Sn also transforms to a more round and spherical shape during aging
instead of the faceted and blocky-shaped phase observed after reflow soldering
(Figure 6.18b). According to Wenger and Furrow (2000), Ag layer will dissolve
rapidly into the molten solder during reflow soldering because of the high solubility
of Ag in Sn. However, in the solid-solder condition, Ag atoms must come out of the
solder because the solubility of Ag in Sn is nearly zero (Jeon et al. (2003)). Then,
Ag atoms slowly accumulate on top of the Cu6Sn5 intermetallic. Figure 6.19
schematically illustrates the IMCs growth on Cu/Ag surface finish in as-reflow
(Figure 6. 19a and Figure 6.19b) and after aging process (Figure 6.19c)
115
(a) (a)
Ag3Sn
(b)
Figure 6.18: Ag3Sn on ImAg using 700µm solder (a) After reflow and (b) After
ageing for 500 hours
116
Substrate
Solder
Ag
(a)
Cu6Sn5
Cu
Sn
(b)
Cu3Sn Cu
(c)
Ag3SnAg
Cu6Sn5
Figure 6.19: Schematic diagram of IMCs growth in Cu/Au specimen: (a) dissolution
of Ag layer into molten solder, (b) formation of Cu6Sn5 during reflow soldering and
(c) Conversion of Cu3Sn and Ag3Sn after isothermal ageing
6.4.1.3 Formation of Kirkendall Voids
Figure 6.20 and Figure 6.21 show that Kirkendall voids formed inside the
Cu3Sn layer at the SAC solder/ ImAg interface. The Kirkendall voids start to form in
the Cu3Sn layer after ageing for 500 hours, probably attributed to the fast diffusion
rate of Cu and the fast reaction to form IMC at the interface. Research done by Xiao
et al. (2001) also showed that these Kirkendall voids formed in the same layer during
long-time ageing. It is important first to review some pointers on the Kirkendall
voids formation in solder joints. As mentioned before, the main diffusion element in
Cu6Sn5 is Sn and the main diffusion element in Cu3Sn is Cu. It seems that the
diffusion of Sn in Cu6Sn5 is slower, leading to a shortage of Sn to react with Cu in
the Cu3Sn layer. Thus, the lacking Sn in the lattice spaces in Cu3Sn can therefore
results in the formation of Kirkendall voids. This is also in agreement with the work
done by Laurila et al. (2005) where they mentioned that during the formation of
Cu3Sn IMC, both components, (Cu and Sn) diffuse into the Cu3Sn-phase but the
diffusion of Cu has been measured to be ~3 times faster, so the voids in the Sn-Ag-
Cu/Cu reaction couple are at the right location and thus could constitute a Kirkendall
plane. The growth rate of these voids is exponential with temperature, therefore
117
increasing significantly at higher temperature (particularly 125oC and above) during
ageing (Raiyo, 2005) as shown in Figure 6.22.
Figure 6.20: SEM image of cross-sectional view of 500µm SAC405 solder/ ImAg
after ageing for 500 hours.
Kirkendall voids
Kirkendall voids
Cu6Sn5
Cu3Sn
Cu6Sn5
Cu3Sn
Ag3Sn
Ag3Sn
Figure 6.21: SEM image of cross-sectional view of 700µm SAC405 solder/ ImAg
after ageing for 1000 hours.
118
Figure 6.22: The mechanism of Kirkendall Voids formation (Raiyo, 2005).
6.4.2 Intermetallics between Sn-Ag-Cu solders and ENIG Surface Finish
6.4.2.1 Interfacial Reactions during Reflow Soldering
Over the past several years there has been consistent growth in the use of
electroless nickel/immersion gold (ENIG) as a final finish. This finish offers
diffusion barrier in printed circuit board for the electronic packages. Since the
reaction rate of Ni with liquid Sn solder is slower than that of Cu, it acts to prevent
the rapid interfacial reaction between solder and Cu conductor in electronic devices.
This in turn, results in thinner intermetallic compounds. During reflow soldering, the
molten lead-free Sn-Ag-Cu solder alloy dissolves the entire Au layer into the liquid
solder, allowing Sn from the molten solder to react with the Ni layer to form Ni-Sn
intermetallics. In this research, the electroless nickel layer in the ENIG finish has
been deposited with a medium content of phosphorous of around 7 wt%.
In the present study the morphology and evolution of interfacial reactions
between ENIG and SAC405 and SAC305 lead-free solders were investigated using
cross-sectional and top surface views by the aid of optical and electron microscopy.
Figure 6.23 shows the optical micrographs of cross-sections of solder joint made
from SAC405 and SAC305 solders with three different solder bump sizes, 300, 500
119
and 700 µm after reflow soldering. What can be observed from these micrographs is
that the intermetallics formed are thinner compared to those formed on ImAg surface
finish as described in section 6.4.1.2. As will be discussed later the thickness of
these intermetallics is also smaller in the bigger solder and vice versa.
0
0
0 0
F
S
c
a
b
0
igure 6.23: Cross-section views of the inte
AC405 (a-c) and SAC305 (d-f) solders (X50
f
d
e
0
rmetallics formed between EN
0).
X50
X50X50
X50
X50
X50
IG and
120
It has also been established (Laurila et al. 2005) that there are three main
IMCs in the Ni-Sn system, which are stable at temperature of interest, i.e. below
260oC: Ni3Sn, Ni3Sn2 and Ni3Sn4. However, since ternary reactions among Ni, Cu
and Sn at the interface are involved and complicated, several different results in
terms of IMC phases and compositions on ENIG surface finish were reported. In
previous works by Li et al. (2005), Sharif et al. (2005) and Jeon et al. (2003) it was
reported that only the (Cu, Ni)6Sn5 phase was detected at the interface after reflow
soldering at temperatures below 300°C. Huang et al. (2006) studied the interfacial
reactions between Sn-0.7Cu and Sn-3.5Ag-0.7Cu solders on ENIG and reported that
after reflow soldering at 250oC for 1minute only needle-shaped (Cu, Ni)6Sn5
intermetallic formed at the interface.
There has been several publications detailing the formation of intermetallics
between Cu-containing lead-free solders and Ni-based surface metallization (Chen et
al. (2002), Ho et al. (2002), Tsai et al. (2003), Ho et al. (2006) based on the Sn-Cu-
Ni phase diagram proposed by Lin et al. (Lin et al. (2002), while other researchers
(Hsu et al. (2003) and Wang et al. (2003) used an earlier version of the phase
diagram. According to Vuorinen et al. (2008) none of these authors took into
account the effect of super-saturation of the molten solder with either Ni or Cu, or
indeed the change in temperature. When soldering Cu-bearing Pb-free solders such
as Sn-Ag-Cu solders and Ni-based metallization, the interfacial reactions formed
consist of ternary intermetallics, (Cu, Ni)6Sn5, (Ni, Cu)3Sn4 or both (Cu, Ni)6Sn5 and
(Ni, Cu)3Sn4 depending on the content of Cu in the solder (Ho et al. (2002), Jeon et
al. (2003))
Ho et al. (2002) studied the reactions between Ni and Sn-Ag-Cu solders at
250 oC. In the solder composition they kept the Ag concentration constant at 3.9
wt% but the Cu content was varied between 0 wt% and 3.0 wt%. They observed that
when the Cu content in Sn-Ag solders is below 0.5 wt%, a continuous layer of (Ni,
Cu)3Sn4 is formed above which a small amount of discontinuous (Cu, Ni)6Sn5
particles is formed between the Ni layer and solder. When the Cu content increased
to 0.5 wt%, the (Cu, Ni)6Sn5 becomes the continuous layer over the (Ni, Cu)3Sn4
121
layer. At higher concentrations of Cu (higher than 0.5 wt%) only a continuous layer
of (Cu, Ni)6Sn5 is formed. Several other studies also reported similar findings albeit
with a different threshold of Cu concentration that separates between whether Ni or
Cu based intermetallics are formed. Quite recently, however, Vuorinen et al. (2008)
reported almost the same results as Ho et al. (2002). Based on both thermodynamics
and kinetics they reported that if the Cu content of the solder is less than 0.4 wt%,
only (Ni, Cu)3Sn4 nucleates at the Ni/solder interface at 250 oC soldering temperature
and is more stable than (Cu, Ni)6Sn5. They also reported that the critical Cu
concentration is temperature dependent.
In order to reveal the morphology and type of intermetallics formed during
reflow selective etching of the solders was performed on the samples. Figure 6.24
and Figure 6.25 show the cross sections and top views of the interface intermetallics
in the SAC405/ ENIG and SAC305/ ENIG for the same solder joints shown in
Figure 6.23 after reflow soldering. Chemical analysis by energy dispersive X-ray
analysis (EDX) revealed that the intermetallics formed during reflow soldering the
SAC405 joints using 300, 500 and 700 µm solders were of the type (Cu, Ni)6Sn5
with needle-shape as shown in Figure 6.24 (d-f). The initial chemical composition of
the (Cu, Ni)6Sn5 intermetallics for both SAC405 and SAC305 solders was with the
range: Cu (24-31 at%), Ni (12-17 at%) and Sn (30-47 at%). Selected EDX analysis
results are given in Appendix A. Thus, the results of the present study are in good
agreement with the results reported by Ho et al. (2002), Young et al. (2003), Zeng et
al. (2004), Huang et al. (2006) and Vuorinen et al. (2008). This indeed is clear from
the cross-sectional as well as top view morphologies that only a continuous layer of
(Cu, Ni)6Sn5 has formed during reflow soldering with either SAC405 or SAC305
solders with the Cu concentration being at 0.5 wt%. Soldering of the solder joint in
the present study was performed at a temperature below 250 oC and based on the
experimental results of Vuorinen et al. (2008) (Cu, Ni)6Sn5 is expected to be more
stable in Sn-Ag-Cu solders containing around 0.5 wt% (like in the present study).
This is exactly what was observed here in all solder bump sizes investigated except
for the smallest solder bump size of 200 µm.
122
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5
e
d
f
b
a
c
X500
X500
X500
Figure 6.24: Cross section and top views of (Cu, Ni)6Sn5 intermetallic formed during
reflow between ENIG and SAC405 solder. (a, d) 300 µm, (b, e) 500 µm and (c, f)
700 µm.
123
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5fc
d
e
a
b
X500
X500
X500
Figure 6.25: Cross section and top views of (Cu, Ni)6Sn5 intermetallic formed during
reflow between ENIG and SAC305 solder. (a, d) 300 µm, (b, e) 500 µm and (c, f)
700 µm.
124
When soldering with the smaller solder balls of 200 µm, however, the results
obtained showed that (Ni, Cu)3Sn4 intermetallic formed at the interface instead of
(Cu, Ni)6Sn5 for both SAC405 and SAC305 solders (Figure 6.26). The composition
of this (Ni, Cu)3Sn4 intermetallic was: Ni (30-37 at%), Cu (5-9 at%) and Sn (43-47
at%). This observation is supported by the explanation proposed by Vuorinen et al.
(2008) based on super-saturation of the molten solder at the interface near the solid
base metal with both Ni and Cu during soldering.
As the solder reaches saturation the dissolution rate decreases and from this
stage on the dissolution of the metallization contributes only for the growth of the
intermetallic layer at the solder joint interface. This super-saturation, they claimed,
would also lead to the formation of a thick two phase solidification structure namely
(Ni, Cu)3Sn4 and (Cu, Ni)6Sn5. Since a smaller solder volume (smaller solder bump)
will be supersaturated with both Ni and Cu very quickly during soldering, then both
ternary Cu-based and Ni-based intermetallics would form as observed in the present
study. (Cu, Ni)6Sn5 intermetallic is a stable phase and the presence of Ni will
stabilize it further very strongly (Laurila et al. (2005)), and it was reported by Huang
et al. (2006) that as much as 25 at% of Ni may replace Cu atoms in the (Cu, Ni)6Sn5.
To provide further evidence for the results observed, cross-sections of solder
joints were selectively deep etched to reveal the morphology and analyse the
chemical composition of the interfacial reactions formed during soldering using all
solder ball sizes for both solders investigated. EDX analysis confirmed that when
soldering with 300, 500 and 700 µm solder ball only one intermetallic was formed,
which is (Cu, Ni)6Sn5 while in the 200 µm solder ball both (Ni, Cu)3Sn4 and (Cu,
Ni)6Sn5 formed at the interface as shown in Figure 6.27 and Figure 6.28 for SAC405
and SAC305 solders respectively. The EDX analysis results for the respective
micrographs are given in Figure 6.29 and Figure 6.30.
The presence of only one intermetallic phase, (Cu, Ni)6Sn5, after soldering
with larger SAC405 and SAC305 solders (300, 500 and 700 µm) can be attributed to
125
what is called by Huang et a. (2006) as the “Cu concentration effect”. As discussed
above the a value of 0.5 wt% of Cu was found to be the critical value that determine
the transition between (Cu, Ni)6Sn5 to (Ni, Cu)3Sn4. Since the Cu content in both
solders used in the present study is fixed at 0.5 wt%, it was expected that both (Cu,
Ni)6Sn5 to (Ni, Cu)3Sn4 would form in all solder bump sizes used. The Cu content is
solder joints is limited and during soldering, this Cu is involved in the interfacial
reaction. As this interfacial reaction or intermetallic grows in thickness Cu in the
solder is consumed and thus its concentration might change compared to the initial
solder composition. The fact that for small solder joints made with 200 µm solder
bump (Ni, Cu)3Sn4 formed at the interface is an indication that the Cu content at the
interface has decreased since saturation with both Ni and Cu occurs rapidly after
melting making dissolution of more Cu from the substrate almost impossible.
However, in larger solder joints more Cu can be dissolved in the solder before
saturation is achieved. In this situation the Cu concentration at the interface may not
decrease below the critical value resulting in the formation of only (Cu, Ni)6Sn5
intermetallic.
126
(Ni, Cu)3Sn4
(Ni, Cu)3Sn4
Ni3Sn2
Figure 6.26: Top view of intermetallics formed between ENIG and 200 µm SAC405
(top) and SAC305 (bottom) solders.
127
(Cu, Ni)6Sn5
Ag3Sn
(Cu, Ni)6Sn5
b
a
Figure 6.27: SEM images of cross sections of intermetallic formed between ENIG
and SAC405 for (a) 500 µm and (b) 700 µm solders.
128
(Cu, Ni)6Sn5
(Cu, Ni)6Sn5
b
a
Figure 6.28: SEM images of cross sections of intermetallic formed between ENIG
and SAC305 for (a) 300 µm and (b) 500 µm solders.
129
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 1.45 0.7713 12.84 1.40 19.43 Cu L 2.39 0.5637 28.88 1.26 40.38 Sn L 6.22 0.9055 46.79 1.44 35.02 Au M 1.32 0.7856 11.48 1.20 5.18 Totals 100.00
Figure 6.29a: EDX results of interface intermetallic formed between ENIG and 500
µm SAC405 solder during reflow (EDX for Figure 6.27a).
130
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 1.01 0.7742 9.28 1.36 14.71 Cu L 2.35 0.5827 28.47 1.24 41.71 Sn L 5.84 0.9083 45.43 1.40 35.63 Au M 1.90 0.7987 16.83 1.23 7.96 Totals 100.00
Figure 6.29b: EDX results of interface intermetallic formed between ENIG and 700
µm SAC405 solder during reflow (EDX for Figure 6.27b).
131
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 1.64 0.7733 15.19 1.67 22.89 Cu L 2.08 0.5535 27.01 1.52 37.62 Sn L 5.74 0.9031 45.65 1.69 34.03 Au M 1.33 0.7850 12.15 1.38 5.46 Totals 100.00
Figure 6.30a: EDX results of interface intermetallic formed between ENIG and 300
µm SAC305 solder during reflow (EDX for Figure 6.28a).
132
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 1.98 0.7769 19.32 4.24 28.75 Cu L 1.72 0.5359 24.28 4.02 33.38 Sn L 5.21 0.8986 43.91 4.42 32.32 Au M 1.29 0.7820 12.49 3.73 5.54 Totals 100.00
Figure 6.30b: EDX results of interface intermetallic formed between ENIG and 500
µm SAC305 solder during reflow (EDX for Figure 6.28b).
133
6.4.2.2 Effect of Isothermal Aging on Intermetallics
Intermetallics will continue to grow when subjected to heating or ageing.
Figure 6.31 shows typical cross sectional of the solder joints after reflow and after
ageing up to 2000 hours at 150oC for both SAC405 and SAC305 solders with the
solder bump sizes being 500 and 700 µm. In contrast to the intermetallics observed
after solder reflowing, ageing of the solder joints led to the intermetallics gradually
transforming to a layered structure as shown clearly by the SEM images of the cross-
sectional solder joints in Figure 6.32b and also the top views of the intermetallics
morphologies after reflow and ageing for up to 2000 hours for the SAC405 solder
(Figure 6.33). There is also an indication that the intermetallic layer grew in
thickness with increasing ageing time (detailed discussion on intermetallics thickness
is presented later in this chapter), which is consistent with previous research work.
Figure 6.33 and Figure 6.34 clearly show that due to ageing coarsening of the
intermetallics has occurred.
The present results also clearly showed that there was no change in the type
of intermetallics formed after the joints were subjected to ageing. That is the same
type of intermetallics observed after reflow were detected after ageing in the
respective solder bump sizes. Only (Cu, Ni)6Sn5 intermetallic layer was observed in
the 300, 500 and 700 µm solders whereas two layers: (Ni, Cu)3Sn4 and (Cu, Ni)6Sn5
intermetallics were formed in the smaller solder bump size of 200 µm as confirmed
by EDX analysis. The quantitative results of EDX analyses for the intermetallics
after reflow and ageing for both solders and for all solder bump sizes are shown in
Table 6.3.
For the 300, 500 and 700 µm SAC405 and SAC305 solders, the thickening
and coarsening of intermetallics appears to be slow than that in the 200 µm solders.
For example, after ageing for only 250 hours at 150oC the (Ni, Cu)3Sn4 showed quite
a significant coarsening compared to the 700 µm solder as shown in Figure 6.33. As
134
the ageing time increased, both (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 intermetallics
coarsened but it is quite evident that the (Ni, Cu)3Sn4 grains coarsened at a faster rate
as shown by the larger grains compared to the (Cu, Ni)6Sn5 (compare Figure 6.33
with Figure 6.34). These observations are consistent with previous studies (Huang
et al. (2006).
Table 6.3: Compositions of the interfacial reaction products after reflow soldering and ageing for 2000 hours at 150oC. Solder
Reaction Product after reflow Reaction product after ageing
for 2000 hours at 150oC 200 µm (Ni80, Cu20)3Sn4/ (Cu, Ni)6Sn5 (Ni85, Cu15)3Sn4/ (Cu60, Ni40)6Sn5
300 µm (Cu58, Ni42)3Sn4 (Cu61, Ni39)6Sn5
500 µm (Cu68, Ni32)6Sn5 (Cu78, Ni22)6Sn5
SAC405
700 µm (Cu74, Ni26)6Sn5 (Cu80, Ni20)6Sn5
200 µm (Ni69, Cu11)3Sn4/ (Cu, Ni)6Sn5 (Cu60, Ni40)6Sn5
300 µm (Cu53, Ni47)6Sn5 (Cu53, Ni47)6Sn5
500 µm (Cu66, Ni34)6Sn5 (Cu61, Ni39)6Sn5
SAC305
700 µm (Cu75, Ni25)6Sn5 (Cu70, Ni30)6Sn5
X500X500
X500X500
dc
ba
Figure 6.31: Cross sections of intermetallics formed between ENIG and SAC405
solder. (a) reflow (500 µm) and (b) after 2000 hours ageing (500 µm), (c) reflow
(700 µm) and (d) after 2000 hours ageing (700 µm).
135
(Ni,Cu)3Sn4
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5
b
a
Figure 6.32: SAC305 (a) after reflow and (b) ageing (2000 hrs)
136
(Ni,Cu)3Sn4
(Cu,Ni)6Sn5
(Ni,Cu)3Sn4
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5
(Ni,Cu)3Sn4
(Cu,Ni)6Sn5(Cu,Ni)6Sn5
fe
d
b
c
a
Figure 6.33: Effect of ageing time on intermetallics formed between ENIG and
SAC405 solder with different solder sizes. 300 µm solder: a: reflow, b: ageing for
250 hours, g: ageing for 500 hours and h: ageing for 2000 hours. 500 µm solder: c:
reflow, d: ageing for 250 hours, i: ageing for 500 hours and j: ageing for 2000 hours.
700 µm solder: e: reflow, f: ageing for 250 hours, k: ageing for 500 hours and l:
ageing for 2000 hours.
137
Ag3Sn
(Cu,Ni)6Sn5 (Cu23,Au44,Ni8)6Sn5lk
h
j
(Cu, Ni)6Sn5
(Cu,Ni)6Sn5
Ag3Sn
(Cu20,Au32,Ni17)6Sn5
(Cu28,Au27,Ni17)6Sn5
i
g
Figure 6.33 (Continued): Effect of ageing time on intermetallics formed between
ENIG and SAC405 solder with different solder sizes. 300 µm solder: a: reflow, b:
ageing for 250 hours, g: ageing for 500 hours and h: ageing for 2000 hours. 500 µm
solder: c: reflow, d: ageing for 250 hours, i: ageing for 500 hours and j: ageing for
2000 hours. 700 µm solder: e: reflow, f: ageing for 250 hours, k: ageing for 500
hours and l: ageing for 2000 hours.
138
(Ni,Cu)3Sn4
(Ni,Cu)3Sn4
(Ni,Cu)3Sn4
(c)
(b)
(a)
Figure 6.34: Top view of intermetallics formed between ENIG and 200 µm
SAC405: a: reflow, b: ageing for 250 hours, and c: ageing for 1000 hours.
139
In addition, when the solder joints were aged for 1000 hours and 2000 hours,
a small amount of Au was found in the (Cu, Ni)6Sn5 layer forming Cu-Au Ni-Sn at
the interface of the IMC (Figure 6.33h and Figure 6.33l). Dae et al., (2005) reported
that the resettlement of Au at the interface is due to the following: (1) the pre-
deposited Au layer dissolves or reacts with the solder leaving no observable Au at
the interface, (2) during solidification, the dissolved Au forms a binary AuSn4
compound within the solder region and appears as needle-like or platelet in structure,
(3) the binary AuSn4 compound in the solder bump gradually disappears with aging
time and re-settles at the interface as a ternary compound (Au, Ni)Sn4.
However, according to Chen et al. (2007), the re-deposition of AuSn4 does
not occur when sufficient Cu is present in the solder, because Au is incorporated in
the Cu6Sn5-based IMC structure. The theory is that, during aging, some of the AuSn4
IMC in the bulk solder decomposes to release Au atoms so that they can diffuse
towards the interface to supply Au for the growth of (Cu, Au, Ni)6Sn5. Once all of
the Cu is consumed at the interfacial reactions, Au is re-deposited as (Au, Ni)Sn4 at
the interface. The fact that Sn-Ag-Cu solder joints with larger solder volume did not
exhibit the re-deposition behavior at the interface because the Cu content was still
high enough to suppress the re-deposition even after long aging time as compared to
the smaller solder joints provide evidence and support the above theory. From
Figure 6.33h and Figure 6.33l, it can be clearly shown that after aging there is some
Au incorporated into the Cu6Sn5-based intermetallic for all solder sizes but, with
different rates. Smaller solder size start to have (Cu28, Au27, Ni17)6Sn5 after 500
hours ageing while in the larger solder size, the (Cu23, Au44, Ni8)6Sn5 only formed
after prolonged ageing time of 2000 hours. Moreover, according to Duan et al.
(2003), the formation of these (Cu, Au, Ni,)6Sn5 particles could be attributed to the
presence of Cu in the solder alloy so that Au can readily diffuse into the Cu6Sn5
precipitates in the bulk. Besides, they also reported that the Au concentration is
critical in the formation of intermetallic compounds and their mechanical strength.
They concluded that a proper amount of (Cu, Au, Ni,)6Sn5 precipitate can reinforce
the solder matrix while a large coalescence of AuSn4 in the course of aging
deteriorates the solder joint reliability even if it appears in the solder bulk. Recently,
140
Islam and Chan (2005) reported that the dispersed Cu-Sn and Ag-Sn intermetallics in
the bulk solder would induce higher mechanical strength.
It has been suggested by Ho et al. (2002) that the presence of Ag in SAC
solders does not have any effect because it does not dissolve in neither (Cu, Ni)6Sn5
nor (Ni, Cu)3Sn4 intermetallics. The present results also confirm these findings as no
Ag was detected in the interfacial layer as it is not directly involved in the interface
intermetallic formation. Ag is frequently observed, however, to form Ag3Sn in the
bulk solder in plate-like morphology and also at the interface in the shape of particles
after reflow soldering (Figure 6.35). Ag3Sn intermetallic is formed during the initial
reflow process and does not evolve in the subsequent ageing processes due to its high
chemical stability, except transforming into more rounded shape in order to reduce
its surface energy. Plate-like Ag3Sn IMC which precipitates in the Sn-matrix is
commonly observed in Sn-Ag-Cu soldered package as reported by Duan et al. (2003)
and Li et al. (2005). According to Kim et al. (2003), cooling rate during reflow also
plays an important role in the changes of Ag3Sn structure and alters the solder joint
strength. The Ag3Sn phase will become larger by decreasing the cooling rate.
Previous research also proved that the formation of large primary Ag3Sn precipitates
will exhibit degradation in ductility and induce reliability problems. Zeng and Tu
(2002) presented that the large Ag3Sn platelet which forms in high stress areas such
as the corner region between solder bump and under-bump-metallization will easily
initiate cracks and enable propagation along the interface between the Ag3Sn and
solder which may lead to failure. However, the small Ag3Sn particles inside the
solder can be beneficial to strengthen the solder interconnects (Dezhi et al., 2005a).
This is because, Ag3Sn is more brittle compared to the matrix of the solder alloy,
therefore the small and evenly distributed Ag3Sn IMC particles in the solder may
improve the mechanical properties and interconnect integrity.
Of all the different sizes of solder bumps investigated, more Ag3Sn just ahead
of the interface were observed in the larger solder ball size (700 µm) for SAC405
solder. The Ag3Sn is also formed in different morphologies, such as plates and
block-type as shown in Figure 6.35. For the other solder bump sizes, namely 200
141
µm, 300 µm and 500 µm, only few of Ag3Sn were observed to form at the interface
ahead of the intermetallic layer.
(d)
500X 500X
(b)
(c)
Ag3Sn
(a) Ag3Sn
(Ni,Cu)3Sn4
Ag3Sn
Figure 6.35: Formation of Ag3Sn in the 700 microns solder bump after reflow
soldering (SAC405). (a, b) Top surface morphology of the solder joint and (c, d)
Cross section.
An interesting observation made in the present study is the presence of a ring-
shaped of the intermetallic layer found during soldering between the SAC solders/
ENIG couple. Close analysis of the top view morphology of the intermetallic layer
revealed the formation of multiple intermetallic regions (Figure 6.36). Previous
research reported by Cheu Li (2006) and Azmah (2007) also observed the same
pattern when soldering with SAC solder on ENIG but no such ring shape pattern was
observed when soldering with Sn-Pb solder alloy. The formation of such pattern is
not clear and requires further study. However, it is believed that the reasons for this
formation are due to the wetting properties of the lead free solder composition
142
ranging from the edge to the center of the solder bump and probably also to the non-
homogeneity of the solder alloy itself since unlike the Sn-Pb alloy, the SAC solders
contain three main elements, Sn, Cu and Ag. Yoon et al. (2004) claimed that this
situation occurred probably due to discrepancy of Cu content in the solder matrix
between the centre and edge during solidification. In other words, the site at the
centre containing relatively more Cu atoms would be a nucleation site for the (Cu,
Ni)6Sn5 intermetallic. However, the author believes that the formation of these
different grains with varying sizes and even composition may also be correlated with
other factors such as the temperature gradient along the radial direction on the
solder/pad contact area and solder ball contact to the substrate metallization pad
during reflow and/or cooling of the solder joint.
Towards centre
Figure 6.36: SEM image showing different morphology of intermetallics between
center and periphery of solder joint.
An example of such variation in the grain size of intermetallic is shown in
Figure 6.37 for the SAC solder. The intermetallic formed is similar in type which is
(Cu, Ni)6Sn5 but the grains vary in size.
143
(Cu,Ni)6Sn5
Figure 6.37: Different morphologies of IMC which form circular boundary regions
in Sn-Ag-Cu solder joint, (a) Sn-3Ag-0.5Cu solder and (b) Sn-4Ag-0.5Cu solder.
Apart from that, the aging process also induced changes in the morphology of
the Ag3Sn block-type intermetallic into more spherical and non-faceted phase.
While the underneath intermetallics formed were observed to be more compact,
uniform and coarse. Recently, Li et al. (2005) reported that the Ag3Sn was
predominately formed from the reflow process and as such it is not evolved in the
subsequent aging process due to its high chemical stability. Figure 6.38 compares
the intermetallics before and after aging.
144
Ag3Sn
Ag3Sn
(b)
(a)
Figure 6.38: Effect of Ageing on the morphology of Ag3Sn intermetallic, (a) After
reflow soldering and (b) After 500 hours ageing.
500X 500X
145
6.5 Intermetallic Compounds Thickness
Generally, the consumption of the copper substrate will decrease when there
are other layers applied, which prevent further diffusion between copper and solder
material. Different types of surface finishes will basically produce various types of
intermetallics. Another objective of this research was to investigate the effect of
solder bump size on the growth of intermetallics formed during soldering between
two lead-free solders: Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu on immersion silver and
ENIG surface finishes. Because of the continuous miniaturization in electronic
products, the size of electronic components and hence the size of solder joints is
being scaled down continuously. The issue of intermetallics formed at the interface
of these smaller solder joints and their effect on the solder joint reliability has
become a challenge. In the present research, an attempt has been made to investigate
the effect of reducing solder bump size on the type and size of intermetallics formed
during soldering. These intermetallics may affect the reliability of the solder joint
especially after being exposed to the long term ageing. Thus, the author had
conducted ageing to quantify the average thickness of the intermetallics formed in
order to examine the effect of these intermetallics on the reliability of the solder
joint.
6.5.1 Effect of Solder Volume on Intermetallic Compound Thickness
On both ImAg and ENIG surface finish, a thin layer of intermetallics can
form in a very short time and meet the soldering requirements. Once the solder melts
during the reflow process, Sn in the molten solder immediately reacts with Ni layer
or Cu substrate to form scallop-like island of intermetallics at the interface and the
formation of intermetallics in a molten solder can be defined in two steps. The first
step is the diffusion of Ni or Cu substrate through the newly formed intermetallic
146
layer, and the second reaction is the chemical reaction between Sn/Ni or Cu/Sn and
any other substances to form the intermetallics.
The measured average thicknesses of intermetallics thickness as function of
ageing time are listed in Table 6.4 and Table 6.5 for SAC405 and SAC305 solders on
immersion silver and ENIG surface finishes respectively. It must be noted that for
ImAg surface finish, the thicknesses are listed in Table 6.4 is the total thickness of
Cu6Sn5 and Cu3Sn.
Table 6.4: Intermetallic Thickness (µm) on ImAg surface finish
Sn-4Ag-0.5Cu Sn-3Ag-0.5Cu Solder
Size (صm)
Ageing time (hours) Ageing time (hours)
0 250 500 1000 2000 0 250 500 1000 2000
200 3.575 5.984 6.486 7.311 8.775 3.795 5.704 6.226 8.956 9.478
300 2.692 5.627 6.437 8.002 8.514 2.691 5.482 5.743 7.029 9.920
500 2.330 5.110 7.413 7.59 8.072 1.958 4.442 4.856 6.854 9.247
700 1.022 4.900 7.832 7.149 7.255 1.449 2.857 3.655 5.211 8.493
Table 6.5: Intermetallic Thickness (µm) on ENIG surface finish
Sn-4Ag-0.5Cu Sn-3Ag-0.5Cu Solder
Size (صm)
Ageing time (hours) Ageing time (hours)
0 250 500 1000 2000 0 250 500 1000 2000
200 2.651 2.852 2.895 2.906 3.105 2.325 2.531 2.676 2.789 2.812
300 1.458 1.528 1.981 1.968 2.411 1.647 2.010 2.501 2.664 2.788
500 1.365 1.502 1.889 1.929 1.990 1.084 1.285 1.743 1.885 1.989
700 1.049 1.408 1.749 1.779 1.846 0.766 1.237 1.568 1.647 1.781
Figure 6.39 and Figure 6.40 show the IMC thickness for SAC405 and
SAC305 solders on ImAg and ENIG finishes respectively. The results are inclusive
of all solder ball sizes investigated: 200µm, 300µm, 500µm and 700µm solder
bumps. A clear trend can be observed here, where the intermetallic layer gets thinner
147
as the solder bump size is increased from 200µm to 700µm for both ImAg and ENIG
surface finishes. These results reveal that the mean thickness of intermetallics for
solders with smaller solder size (200µm) was thicker than that of the bigger solder
size (700µm) using the surface finishes investigated in the present study. This is in
agreement with previous results obtained by J. Koh Swee Fong (2006) and Sung et
al. (2003).
IMC Thickness VS Bump Size
0
2
4
6
8
10
12
200µm 300µm 500µm 700µm
Bump Size (µm)
IMC
Thi
ckne
ss (µ
m)
0 hours 250 hours 500 hours 1000 hours 2000 hours
(a)
IMC Thickness VS Bump Size
0
2
4
6
8
10
12
200µm 300µm 500µm 700µm
Bump Size (µm)
IMC
Thi
ckne
ss (µ
m)
0 hours 250 hours 500 hours 1000 hours 2000 hours
(b)
Figure 6.39: Intermetallic thickness (Cu6Sn5+Cu3Sn) versus solder bump size for
ImAg surface finish as function of ageing time; (a) Sn-4Ag-0.5Cu and (b) Sn-3Ag-
0.5Cu.
148
IMC Thickness VS Bump Size
00.5
11.5
22.5
33.5
200µm 300µm 500µm 700µm
Bump Size (µm)
IMC
Thi
ckne
ss (µ
m)
0 hours 250 hours 500 hours 1000 hours 2000 hours
(a)
IMC Thickness VS Bump Size
00.5
11.5
22.5
33.5
200µm 300µm 500µm 700µm
Bump Size (µm)
IMC
Thi
ckne
ss (µ
m)
0 hours 250 hours 500 hours 1000 hours 2000 hours
(b)
Figure 6.40: Intermetallic thickness versus solder bump size for ENIG surface finish
as function of ageing time; (a) SAC405 and (b) SAC305.
The results of intermetallic layer thickness in Figure 6.39 and Figure 6.40 are
plotted in Figure I in Appendix B as function of the solder size. The intermetallic
layer on the ImAg system is the total thickness of Cu6Sn5 and Cu3Sn layers. The
average thickness of the intermetallic layer is found to decrease as the solder size is
increased for both solder and surface finishes investigated. With smaller solder
sizes (related to solder volume) the metallization pads (Cu or Ni) will decrease as the
solder size is reduced because saturation is achieved rapidly whereas in larger solder
149
sizes rate of dissolution of Cu or Ni is higher so that more solute is dissolved before
saturation is achieved. As explained above the first stage of intermetallic layer
formation starts with Cu/ Ni dissolution followed by growth of this layer by
diffusion. The higher intermetallic thickness observed in the smaller solder sizes is
attributed to the early saturation of the solder.
Previous research by M. Schaefer et al. (1997) also reported that the
dissolution phenomena of metals involved in the interfacial reactions are controlled
by the ratio of solder volume or size to contact pad area, (V/A). With an increase in
this V/A ratio the diffusion distance for Cu to saturate the liquid solder increases and
thus, resulting in slower interfacial reactions. This explanation could be applied in
the present study. Since the metal pad used is constant for all specimens and was
fixed at 380µm and the solder ball diameter was in the range of 200µm to 700µm,
the V/A ratio increases with the increase of solder ball diameter. This confirms our
finding that with decreasing the solder volume or joint size, i.e; decreasing the V/A
ratio will lead to slow intermetallics growth. The current findings on ImAg surface
finish are also in agreement with previous research (Sharif et al., 2005, W. K. Choi et
al., 2002). In one of the discussion in Sung et al. (2003), they explained that, during
solidification of intermetallic layer thickness for the smaller solder size, the
probability of intermetallic formation on the existing IMC at the interface is much
higher than that of the solder with the bigger size. This is due to thermo-dynamical
reaction where the new intermetallics form on the existing intermetallics posses less
free energy than those associated with the new nuclei. Thus, the chances of finding
existing intermetallic sites are higher for the solder with smaller bulk volume.
6.5.2 Effect of Surface Finish on Intermetallic Compound Thickness
Based on the measured intermetallic thicknesses in Figure I (Appendix B) the
growth of the Cu-Sn intermetallics is faster than that of the Ni-Sn intermetallics as
150
shown by the higher intermetallic thickness observed when soldering on ImAg
compared with the ENIG solder joints. After reflow ImAg surface finish produced
thicker intermetallics which decrease with an increase in the solder bump size. The
fact that the intermetallics grow faster when soldering on ImAg compared to ENIG is
expected since the interfacial reaction is faster between liquid Sn solder and Cu on
ImAg finish (the Ag layer will dissolve upon melting the solder) in contrast to that
between Ni and Sn on ENIG finish. The Ni acts to prevent the rapid interfacial
reaction between solder and Cu conductor in electronic devices, which in turn,
results in thinner intermetallic compounds. Nevertheless, the same trend of IMC
thickness versus solder size has been observed when soldering on ENIG.
6.5.3 Growth Kinetics of Intermetallics on Immersion Silver Finish
As discussed in section 6.4, the intermetallics formed on ImAg finish,
particularly after ageing consist of two layers: Cu6Sn5 and Cu3Sn and it appear from
the microstructural observations that the Cu3Sn layer grows faster than the Cu6Sn5
layer as the ageing time is increased. An attempt is made therefore to quantify the
growth rates of both layers as a function of ageing time. The average thicknesses of
Cu6Sn5 and Cu3Sn are listed in Table 6.6 and Table 6.7 and plotted in Figure 6.41
and Figure 6.42 for SAC405 and SAC305 solders respectively. It is shown that the
Cu3Sn thickness increased significantly with increasing ageing time, whereas the
Cu6Sn5 increased only slightly. For example for the 200 µm solder the Cu3Sn has
grown from 2.25 µm to 4.88 µm after 2000 hours ageing at 150oC. It is a growth of
2.63 µm compared to the Cu6Sn5 which had only grown by 0.2 µm after 2000 hours
ageing. That is the Cu3Sn has grown 13 times faster than the Cu6Sn5 intermetallic
layer. Similar trend is observed for the other solder sizes investigated and for both
SAC405 and SAC305 solders. However, after 500 hours ageing, there is a
significant drop for the 200 µm solder size. It is probably due to the fact that the
existing intermetallic slows down the diffusion rate of Cu in Sn. Thus, the
151
intermetallic thickness will be decreased. Besides, it is also likely that there is some
inconsistency during the measurement of the intermetallic thickness which may
affect the values obtained as can be seen in Figure 6.41 and Figure 6.42.
The growth of Cu6Sn5 intermetallic depends on the amount of Cu available in
the solder alloy, and since most of this Cu is used to form Cu6Sn5 particles in the
bulk solder the amount of Cu that can diffuse to the solder/ Cu6Sn5 is small, resulting
in a limited growth o the Cu6Sn5. During ageing, the Cu3Sn intermetallic expanded
on both sides, resulting in the shifting of the Cu/ Cu3Sn interface towards the Cu
substrate and the Cu6Sn5 /Cu3Sn interface towards the Cu6Sn5 layer. This is the
reason why Cu3Sn grew much faster than the Cu6Sn5 as shown in Figure 6.41 and
Figure 6.42. The intermetallic thickness as function of time is expressed as:
DtYY += 0
Where Y is the intermetallic thickness at time t, Y0 is the initial intermetallic layer
thickness (after reflow) and D is the diffusion coefficient (related to the growth rate
constant).
152
Table 6.6: Calculation of the growth rate coefficient (D) for SAC405/ ImAg.
Solder size Initial Thickness
(Y0) Ageing time,
hours (t) Thickness after ageing
(Y) D (µm2/s) 200 0 250 2.249 5.62E-06
0 500 3.6743 7.5E-06 0 1000 3.775 3.96E-06 0 2000 4.8797 3.31E-06
300 0 250 2.213 5.44E-06 0 500 3.464 6.67E-06 0 1000 3.253 2.94E-06 0 2000 3.855 2.06E-06
500 0 250 2.331 6.04E-06 0 500 3.495 6.79E-06 0 1000 3.614 3.63E-06 0 2000 4.056 2.28E-06
700 0 250 1.767 3.47E-06 0 500 3.093 5.31E-06 0 1000 3.213 2.87E-06 0 2000 3.351 1.56E-06
Table 6.7: Calculation of the growth rate coefficient (D) for SAC305/ ImAg.
Solder size Initial Thickness
(Y0) Ageing time,
hours (t) Thickness after ageing
(Y) D (µm2/s) 200 3.575 250 3.735 2.84E-08
3.575 500 2.8117 3.24E-07 3.575 1000 3.5357 4.29E-10 3.575 2000 3.8953 1.42E-08
300 2.692 250 3.414 5.79E-07 2.692 500 3.574 4.32E-07 2.692 1000 4.202 6.33E-07 2.692 2000 4.659 5.37E-07
500 2.33 250 2.779 2.24E-07 2.33 500 3.918 1.4E-06 2.33 1000 3.976 7.53E-07 2.33 2000 4.016 3.95E-07
700 1.022 250 3.173 5.14E-06 1.022 500 4.699 7.51E-06 1.022 1000 4.458 3.28E-06 1.022 2000 4.6685 1.85E-06
153
Thickness of Cu3Sn versus Ageing Time
6
IMC
Thi
ckne
ss (µ
m)
5 200µm 4 300µm
3 500µm 2 700µm 1
0
2000 0 250 500 1000
Ageing time (hours)
Thickness of Cu6Sn5 versus Ageing Time
6
IMC
Thi
ckne
ss (µ
m)
5
200µm 4 300µm
3 500µm 2 700µm
1 0
2000 0 250 500 1000 Ageing time (hours)
Figure 6.41: Intermetallic thickness versus ageing time between SAC405 and ImAg
surface finish: (top) Cu3Sn and (bottom) Cu6Sn5 layer.
154
Thickness of Cu3Sn versus Ageing Time
6 5
IMC
Thi
ckne
ss (µ
m)
200µm 4
300µm 3
500µm 2 700µm 1 0
0 250 500 1000 2000 Ageing time (hours)
Thickness of Cu6Sn5 versus Ageing Time
6
0 1 2 3 4 5
IMC
Thi
ckne
ss (µ
m)
200µm 300µm
500µm 700µm
2000 0 250 500 1000
Ageing time (hours)
Figure 6.42: Intermetallic thickness versus ageing time between SAC305 and ImAg
surface finish: (top) Cu3Sn and (bottom) Cu6Sn5 layer.
6.5.4 Effect of Ag Concentration on Intermetallic Compound Thickness
Due to reliability concerns in electronic packaging, one of the focused areas
is the impact of Ag component content of the Sn-Ag-Cu alloys on the interfacial
microstructures of the solder interconnects. The main concern is the presence of the
large Ag3Sn plates, which have inherently in-compatible geometry to the surrounding
microstructures, i.e., the bulk solder and the substrate (Frear et al., 2001 and Kang et
155
al., 2004). Using differential thermal analysis (DTA), Moon et al. (2000) reported
that the β-Sn phase required a large undercooling to begin nucleation and ensure
solidification while Ag3Sn plates nucleated and grew with a minimal amount of
undercooling. This difference in undercooling allows the Ag3Sn plates to nucleate
and grow relatively large before the nucleation and growth of solid Sn commence.
Henderson et al. (2002) and Lehman et al. (2003) suggested that, due to the
limitation of long-range diffusion to provide the needed constituents for forming the
Ag3Sn plates, the plates could grow to a large size only in the liquid phase before the
final solder joint solidification. According to Kim et al. (2002) who studied the
influence of Ag3Sn plates on the lead free joint mechanical performance, the
formation of large Ag3Sn plates must be avoided as they provide crack initiation sites
under tensile and shear stresses. In an attempt to explore the growth repression of
the Ag3Sn plates, Henderson et al. (2002) used a thermodynamic calculation in an
isopleth phase diagram of Sn-xAg−0.7Cu, and proposed that at a nominal Cu
concentration of 0.7 wt% in the liquid, the Ag composition of the liquidus at 20oC
undercooling is approximately 2.7 wt%. Hence, for Ag compositions equal to or less
than 2.7 wt%, the Ag3Sn plates cannot grow within the liquid phase in a solder joint
without undercoolings in excess of 20oC (Kim et al., 2002). Besides, they also
mentioned that cooling rate of the solidification also influence the growth of Ag3Sn
plates, where a fast cooling rate could effectively retard the growth of the plates
(Kang et al., 2003 and Jeong et al., 2004). However, in this research, the author
tends to choose slow cooling rate because fast cooling rate may induce more stresses
to the solder joint as mentioned by Kim et al. (2002).
In the present study the Ag3Sn was observed to form into two types of
morphologies: large plates and particle type when soldering with SAC405 on ImAg
surface finish as shown in Figure 6.43(a, b). The large Ag3Sn plates forms at the
solder/ metallization interface (Figure 6.43a) whereas the Ag3Sn particles grow in the
solder just ahead of the interface. In the SAC405 solder both large Ag3Sn plates and
Cu6Sn5 rods were found to form during reflows (Figure 6.9). During reflow both Cu
and Ag from the substrate and surface finish are consumed into the liquid solder thus
increasing the Cu and Ag concentration in the solder. There are still some
ambiguities to whether there is a linked growth between the Cu6Sn5 rods and Ag3Sn
156
intermetallics. However, it has been reported quite recently that the Cu content has a
strong effect on the Ag3Sn growth (Lu et al. (2006)). At a lower Cu content it is
difficult for the large plates of Ag3Sn to grow, and only Ag3Sn particulates can grow.
As the Cu content is increased, the growth of Ag3Sn plates is possible. Lu et al.
(2006) during soldering as the Cu6Sn5 intermetallic is growing the Sn content
decreases in the surrounding area, while the Ag content increases. This is because
five Sn atoms are needed to form Cu6Sn5 while only one Sn atom is required to from
Ag3Sn. During further heating or ageing a local saturation of Ag builds up in the
vicinity of the grown Cu6Sn5 intermetallic, and the nucleation of Ag3Sn becomes
possible. This is indeed what happened when SAC405 is soldered on ImAg finish as
shown in Figure 6.43a.
In the present study, two types of solder were used: SAC405 and SAC305.
The Cu content is fixed at 0.5 wt% and the Ag content is 4 wt% and 3 wt%. From
the SEM observation, large plates of Ag3Sn were only observed in the SAC405
together with Ag3Sn particulates whereas only Ag3Sn particulates formed in the
SAC305 solder (Figure 6.43c). In view of the present results it is more likely that
both Cu and Ag content affect the growth of large plates of Ag3Sn and thus further
investigation is needed.
157
Nodule-like Ag3Sn
(a)
(b)
(c)
Blocky Ag3Sn
Plate-like Ag3Sn
Cu6Sn5
Cu6Sn5
Cu6Sn5
Figure 6.43: SEM top views of Ag3Sn intermetallic for SAC405/ ImAg (a, b) and
SAC305/ ImAg (c).
158
Furthermore, it was found that for ImAg surface finish, more Ag3Sn
particulates were observed in SAC405 solder for all solder sizes especially for the
bigger solder compared to SAC305 solders (Figure 6.13 – Figure 6.16). Similar
observation is made when soldering on ENIG surface finish but most of the smaller
solder bump size do not have Ag3Sn. It is believed that this situation occurs
probably due to the surface finish used. More Ag3Sn found at the interface of ImAg
surface finish because Ag layer dissolved into the solder during reflow soldering.
Then, instead of Ag in the solder itself, this Ag will react with Sn in the solder and
resettle at the interface producing more Ag3Sn for all solder sizes. Compared to
ENIG surface finish, there was no Ag layer involved, thus Ag3Sn produced at the
interface was directly came from the solder itself. However, the reason why less or
no Ag3Sn particulates were found in the smaller solders is related to the saturation
phenomenon as discussed above.
Besides that, in the present study, when the Ag content of the solder change
from 3 wt% to 4 w%, the thickness of intermetallics increased. According to Henry
et al. (2006), the microstructures and growth of Cu-Sn intermetallic are also
influenced by the Ag content but to a lesser degree. The Ag-promoted growth of
Cu6Sn5 in the bulk solder is thought to be due to a mechanism whereby a high Ag
content provided more nucleation sites for the Cu6Sn5 intermetallic. However,
according to the thermodynamic simulation work of Kim et al. (2002), when the Ag
content is beyond 3.5 wt.%, Ag3Sn will be the primary phase to nucleate and grow.
Chada et al. (2000), and Moon and Boettinger (2004) suggested similar results based
on their simulation work. Hence, for the alloy groups under study, the Ag3Sn
probably is the first phase to grow for the 3.9Ag group. The nucleation and growth of
Ag3Sn are extremely fast, forming oversized plates in the molten solder (Henderson
et al., 2002 and Kang et al., 2003). To form an Ag3Sn crystallite, three silver atoms
are required while only one tin atom is needed. As a result, excess Sn atoms are
accumulated in the vicinity of the Ag3Sn plates due to the rapid growth of the Ag3Sn
plates. Thus, the relative concentration balance between the Sn and Cu is broken. It
is this “hypoeutectic” concentration of the Sn-Cu system (or say, a “boosted” Cu
solubility), that drives more substrate copper atoms to diffuse through the interfacial
intermetallic and react with tin atoms forming Cu6Sn5 in the bulk solder. In terms of
159
end result, the growth of the Cu6Sn5 in the bulk solder is promoted either by attaining
the critical radii of the Cu6Sn5 crystallites earlier or by providing more nucleation
sites. Figure II and Figure III in Appendix B shows comparison of the intermetallics
thicknesses between SAC405 and SAC305 solders on ImAg and ENIG. It is noticed
that the preferred nucleation and growth sites for the Ag3Sn plates are along the
periphery of the interconnect (Figure 6.44). This can be explained as; (i) Compared
to the pad center, the peripheral line was in a higher energy state because of
geometrical change and material difference and, (ii) There was a more pronounced
growth of the Cu-Sn IMC (in the form of rods) along the pad boundary compared to
the pad center, and Cu-Sn rods provide a desirable environment for the Ag3Sn plates
to nucleate and grow (Lu et al., 2005).
Plate-like Ag3Sn
Figure 6.44: Ag3Sn IMC formation in ImAg surface finish.
6.5.5 Effect of Ageing Duration on Intermetallic Compound Thickness
During solid-state ageing, intermetallic growth is primarily a solid-state
diffusion and thus depends highly on temperature and time. The ageing temperature
used in this research is fixed at 150ºC, whilst four ageing durations were used, 0 hour
160
(as reflow), 250 hours, 500 hours, 1000 hours and 2000 hours. According to Laurila
et al. (2005), in the intermetallic formation model, Sn is the main diffusing species in
the Cu6Sn5–phase. Both Cu and Sn will diffuse into the Cu3Sn-phase, but the
diffusion of Cu is ~ 3 times faster than Sn. This is consistent with the well known
Cu3Sn rule. It is also an important fact that as the Cu3Sn-phase grows, it consumes
some of the Cu6Sn5–phase. Thus, the thickness of the Cu6Sn5–phase depends both
on Sn diffusion and the rate of consumption by the growing Cu3Sn-phase. While for
ENIG surface finish, the thickness of the intermetallics will be slightly different.
This is due to the existence of Ni layer between the solder and Cu which act as a
diffusion barrier layer.
The total intermetallics thickness comprising both Cu3Sn and Cu6Sn5 incrased
as the ageing time increase as shown in Figure IV and Figure V (Appendix B). The
intermetallics growth is significantly faster during the isothermal ageing for the
smaller solders compared to the larger ones. As discussed previously in section 6.5.1
in smaller solders the saturation in Cu is achieved quite rapidly as the temperature is
increased (whether during reflow or ageing) whereas in the larger solders more time
is needed for the solder volume to achieve saturation. Once saturation is achieved
the intermetallic will continue to grow in size. In the larger solder volume, however,
more Cu will continue to dissolve into the solder before any saturation is achieved,
resulting in thinner intermetallics. It can also be seen that the intermetallic growth
rate of all solder bump sizes grew mostly uniform throughout the ageing duration for
all types of surface finishes. When comparing between the ImAg and ENIG surface
finishes, the mean thickness of intermetallics for ImAg surface finish was found to
be thicker compared to ENIG surface finish. This is expected since the Cu-Sn
reaction is faster than that between Ni-Sn.
CHAPTER 7
CONCLUSIONS AND FUTURE WORKS
7.1 Conclusion
From the results of interfacial reactions between Sn-Ag-Cu lead-free solders and
immersion silver and electroless nickel / immersion gold finishes, the following
conclusions can be drawn:
1. Solder bump size (solder volume) has a significant effect on thickness of
intermetallic layer where smaller solder size produces thicker intermetallics
compared to larger solders. Saturation of smaller solders with Cu and/ or Ni
is the reason for thicker intermetallics.
2. When soldering on immersion silver, only Cu6Sn5 intermetallic layer formed
after reflow but with ageing at 150oC, Cu3Sn formed between the Cu6Sn5 and
Cu substrate. The results also showed that the Cu3Sn layer grows at a faster
rate than the Cu6Sn5 with increased ageing time.
3. Kirkendall voids were observed to form in the Cu3Sn indicating that Cu
diffuses faster in the Cu3Sn than the diffusion of Sn in Cu6Sn5.
162
4. The solder with higher Ag content (SAC405) produced both large Ag3Sn
plates and Cu-Sn rods in the solder compared to the lower Ag containing
solder (SAC305). Only Cu-Sn rods with no Ag3Sn plates observed in the
SAC305 solder when soldered on immersion silver
5. After ageing, more Ag3Sn embedded particles were observed in the smaller
solders since as the intermetallic layer grows faster in the smaller solders
more Ag3Sn particles that are formed in the solder just ahead of the joint
interface will be embedded in the Cu6Sn5 layer
6. Isothermal ageing treatment resulted in coarsening and thickening of
intermetallics in both ImAg and ENIG finishes. The ageing process also
transformed the Ag3Sn into more round and spherical morphology
7. Soldering on ENIG showed that in smaller solders (200 µm) both (Ni,
Cu)3Sn4 (Cu, Ni)6Sn5 forms during reflow but only (Cu, Ni)6Sn5 intermetallic
formed in the larger solders investigated (300, 500 and 700 µm). The
formation of either of these types of intermetallic reconciles well with the
current theory of critical Cu content in the solder
8. Thicker intermetallics were formed on ImAg compared to ENIG because Ni
in the ENIG acts as a diffusion barrier to the interfacial reaction between Cu
and Sn which is the main reaction in ImAg finish
7.2 Future Works
Future work that can be carried out would include the following:
1. Conduct mechanical shock testing.
163
2. Conduct more study on the effect of different Cu and Ag concentrations of
the solder alloy to the intermetallic compound formation.
3. Investigate the effect of doping lead-free solders, particularly the SAC
solders, with elements such as Bi and Sb on the interfacial reactions on
immersion silver finish
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APPENDIX A
FESEM EDX RESULTS (SELECTED SAMPLES ONLY)
176
EDX result of 700 µm solder SAC405/ ENIG - Reflow (Spot 1)
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 1.14 0.7822 10.90 1.44 17.24 Cu L 2.14 0.5793 27.62 1.29 40.36 Sn L 5.22 0.9049 43.19 1.43 33.78 Au M 1.95 0.7986 18.28 1.26 8.62 Totals 100.00
177
EDX result of 200 µm solder SAC405/ ENIG_1000 hours ageing (Spot 2)
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni K 4.48 1.1337 6.22 0.68 10.04 Cu L 7.03 0.4106 26.92 1.10 40.20 Sn L 31.77 0.9047 55.22 1.13 44.15 Au M 5.68 0.7681 11.64 0.90 5.61 Totals 100.00
178
EDX result of 500 µm solder SAC405/ ENIG_500 hours ageing (Spot 1)
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 0.90 0.7530 7.98 1.33 12.96 Cu L 2.26 0.5761 26.23 1.20 39.37 Sn L 6.77 0.9155 49.51 1.44 39.78 Au M 1.96 0.8048 16.29 1.22 7.89 Totals 100.00
179
EDX result of 500 µm solder SAC405/ ENIG_500 hours ageing (Spot 2)
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 0.71 0.7440 6.71 1.41 11.10 Cu L 2.07 0.5772 25.30 1.24 38.68 Sn L 6.68 0.9196 51.24 1.52 41.95 Au M 1.93 0.8098 16.75 1.27 8.26 Totals 100.00
180
EDX result of specimen (200 µm solder) SAC405/ ENIG- Reflow
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 1.64 0.6788 17.28 1.29 30.52 Cu L 0.30 0.4892 4.34 1.05 7.08 Sn L 7.93 0.9324 60.92 1.61 53.21 Au M 2.02 0.8274 17.46 1.31 9.19 Totals 100.00
181
EDX result of 300 µm SAC305/ ENIG- Reflow
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 0.64 0.7680 6.71 1.21 14.36 Cu L 0.64 0.6031 8.47 1.02 16.76 Sn L 4.03 0.9211 35.11 1.40 37.17 Au M 5.51 0.8884 49.71 1.56 31.71 Totals 100.00
182
EDX result of 200 µm SAC305/ ENIG – 2000 hours ageing
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Ni L 1.81 0.7725 16.66 1.85 24.99 Cu L 1.99 0.5459 25.91 1.61 35.92 Sn L 5.78 0.9020 45.48 1.80 33.75 Au M 1.32 0.7838 11.95 1.35 5.34 Totals 100.00
183
EDX result of 200 µm SAC305/ ImAg – 1000 hours aging
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Cu L 9.61 0.4394 38.52 0.90 53.92 Sn L 32.52 0.9312 61.48 0.90 46.08 Totals 100.00
184
EDX result of 200 µm SAC305/ ImAg - Reflow
Element App Intensity Weight% Weight% Atomic% Conc. Corrn. Sigma Cu L 2.19 0.6911 27.60 1.05 48.49 Sn L 2.92 0.9070 28.03 1.22 26.37 Au M 4.38 0.8574 44.36 1.38 25.14 Totals 100.00
APPENDIX B
TABLES AND GRAPHS
186
IMC Thickness VS Bump Size
10
90 hours
250 hours 8
IMC
Thi
ckne
ss (µ
m)
500 hours 7 ImAg1000 hours6
2000 hours5
0 hours
250 hours 4
500 hours ENIG3 1000 hours
22000 hours 1
0200µm 300µm 500µm
700µm
Bump Size (µm)
IMC Thickness VS Bump Size
12
0 hours10
0
2
4
6
8
200µm 300µm 500µm 700µm
Bump Size (µm)
250 hours500 hours1000 hours2000 hours0 hours250 hours500 hours1000 hours2000 hours
IMC
Thi
ckne
ss (µ
m)
ImAg
ENIG
Figure I: Intermetallic thickness versus solder bump size for ImAg and ENIG
surface finishes as function of ageing time: (top) SAC405 and (bottom) SAC305.
187
IMC thickness VS solder type (IAg)
0
1
2
3
4
SAC 405 SAC 305
solder type
IMC
thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC thickness VS solder type (IAg)
01234567
SAC 405 SAC 305
solder type
IMC
thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC thickness VS solder type (IAg)
0
2
4
6
8
10
SAC 405 SAC 305
solder typeIM
C th
ickn
ess
(µm
)200µm 300µm 500µm 700µm
(c)(b)
(a)
IMC thickness VS solder type (IAg)
0
2
4
6
8
10
SAC 405 SAC 305
solder type
IMC
thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC thickness VS solder type (IAg)
02468
1012
SAC 405 SAC 305
solder type
IMC
thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
(e)(d)
Figure II: Intermetallic thickness versus solder types for ImAg surface finish; (a) As
reflow, (b) 250 hours ageing, (c) 500 hours ageing, (d) 1000 hours ageing and (e)
2000 hours ageing.
188
IMC thickness VS solder type (ENIG)
00.5
11.5
22.5
3
SAC 405 SAC 305
solder type
IMC
thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC thickness VS solder type (ENIG)
00.5
11.5
22.5
3
SAC 405 SAC 305
solder type
IMC
thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC thickness VS solder type (ENIG)
00.5
11.5
22.5
33.5
SAC 405 SAC 305
solder typeIM
C th
ickn
ess
(µm
)200µm 300µm 500µm 700µm
IMC thickness VS solder type (ENIG)
00.5
11.5
22.5
33.5
SAC 405 SAC 305
solder type
IMC
thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC thickness VS solder type (ENIG)
00.5
11.5
22.5
33.5
SAC 405 SAC 305
solder type
IMC
thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
(e)(d)
(c)(b)
(a)
Figure III: Intermetallic thickness versus solder types for ENIG surface finish; (a)
As reflow, (b) 250 hours ageing, (c) 500 hours ageing, (d) 1000 hours ageing and (e)
2000 hours ageing.
189
IMC Thickness VS Surface Finish
0
1
2
3
4
IAg ENIG
Surface Finish
IMC
Thi
ckne
ss (µ
m)
200µm 300µm 500µm 700µm
IMC Thickness VS Surface Finish
01234567
IAg ENIG
Surface Finish
IMC
Thi
ckne
ss (µ
m)
200µm 300µm 500µm 700µm
IMC Thickness VS Surface Finish
0
2
4
6
8
10
IAg ENIG
Surface Finish
IMC
Thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC Thickness VS Surface Finish
0
2
4
6
8
10
IAg ENIG
Surface Finish
IMC
Thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC Thickness VS Surface Finish
02
46
810
12
IAg ENIG
Surface Finish
IMC
Thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
(e)(d)
(c)(b)
(a)
Figure IV: Intermetallic thickness versus surface finish using Sn-4Ag-0.5Cu; (a) As
reflow, (b) 250 hours ageing, (c) 500 hours ageing, (d) 1000 hours ageing and (e)
2000 hours ageing.
190
IMC Thickness VS Surface Finish
0
1
2
3
4
IAg ENIG
Surface Finish
IMC
Thi
ckne
ss (µ
m)
200µm 300
µm 500µm 700µm
IMC Thickness VS Surface Finish
0123456
IAg ENIG
Surface Finish
IMC
Thi
ckne
ss (µ
m)
200µm 300µm 500µm 700µm
(a)
(b)IMC Thickness VS Surface Finish
01234567
IAg ENIG
Surface FinishIM
C T
hick
ness
(µm
)200µm 300µm 500µm 700µm
IMC Thickness VS Surface Finish
0
2
4
6
8
10
IAg ENIG
Surface Finish
IMC
Thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
IMC Thickness VS Surface Finish
02
46
810
12
IAg ENIG
Surface Finish
IMC
Thic
knes
s (µ
m)
200µm 300µm 500µm 700µm
(e)(d)
(c)
Figure V: Intermetallic thickness versus surface finish using Sn-3Ag-0.5Cu; (a) As
reflow, (b) 250 hours ageing, (c) 500 hours ageing, (d) 1000 hours ageing and (e)
2000 hours ageing.
PUBLISHED PAPERS
NMC 2007, Johor Bahru, Malaysia National Metallurgical Conference
STUDY ON INTERFACIAL REACTION BETWEEN LEAD-FREE SOLDERS AND ALTERNATIVE SURFACE FINISHES
I. Siti Rabiatull Aisha, A. Ourdjini, Saliza A. Osman and A. Astuty
Faculty of Mechanical Engineering, University of Technology Malaysia Skudai, 81310, Johor Bahru, Malaysia
Abstract— This study investigates the interfacial reactions occurring during reflow soldering between Sn-Ag-Cu lead-free solder and two surface finishes: electroless nickel/ immersion gold (ENIG) and immersion silver (IAg). The study focuses on interfacial reactions evolution and growth kinetics of intermetallic compounds (IMC) formed during soldering and isothermal ageing at 150 oC for up to 2000 hours. Optical and scanning electron microscopy were used to measure IMC thickness and examine the morphology of IMC respectively, whereas the IMC phases were identified by energy dispersive X-ray analysis (EDX). The results showed that the IMC formed on ENIG finish is thinner compared to that formed on IAg finish. For IAg surface finish, Cu
6Sn
5 IMCs with scallop morphology
are formed at the solder/ surface finish interface after reflow while a second IMC, Cu
3Sn was formed
between the copper and Cu6Sn
5 IMC after the
isothermal ageing treatment. For ENIG surface finish both (Cu, Ni)
6Sn
5 and (Ni, Cu)
3Sn
4 are formed after
soldering. Isothermal aging of the solder joints formed on ENIG finish was found to have a significant effect on the morphology of the intermetallics by transforming to more spherical and denser morphology in addition to an increase in their thickness with increased ageing time. Keywords: lead-free; immersion silver; electroless nickel; ageing; intermetallic 1. Introduction
The harmful effects of lead (Pb) on the environment and human health have stimulated substantial research and development efforts to discover lead-free solder alloys for electronic application. The most promising solder alloy recommended by NEMI 2000 (National Electronic Manufacturing Initiative) is Sn-Ag-Cu ternary alloy which has advantages of good wetting property, superior interfacial properties, high creep resistance and low coarsening rate [1,2]. Apart from that, the other important issue for the development of Pb-free is to find a suitable surface finish on a printed circuit board (PCB), where it act as a diffusion barrier layer, wettable layer and a corrosion resistant layer. The surface finishes used in this study include immersion
silver (IAg), electroless nickel/immersion gold (ENIG) and copper as a reference.
The main reason of using IAg is because of the
higher co-planarity requirement for the fine pitch surface mount assemblies. Immersion finishes produce single element coating which result in relatively thin layers (typically less than 1 µm) because the deposition process halts when the substrate surface is completely covered with the coated material [3]. During soldering, the Ag coating does not melt. Instead, it dissolves into the molten solder, which may decrease the speed of the wetting. Some recent electromigration test results indicate that the migration of Ag is not a concern [4].The advantages of this surface finish are: lower material cost, wire-bondable, simpler operation, planar surface, long shelf life and also good for ultra-high speed signals.
Another type of surface finish studied in this
research is ENIG. Electroless nickel (Ni) layer is deposited on the Cu substrate as a diffusion barrier between Cu and solder materials [8]. While immersion gold (Au) layer is deposited onto the nickel layers in order to prevent oxidation and act as a solderable finishes in reflow soldering.
In the development of package materials system,
joint reliability should be considered as one of the most critical criteria [5,6]. This joint is produced during soldering where copper dissolve into the molten solder and react with Sn forming intermetallic compound (IMC) at the solder/copper interface. Although the formation of the IMC layer is desirable for good wetting and bonding, excessively thick layer is harmful because of its brittle nature that makes it prone to mechanical failures even at low loads [7].
In this study, we investigate the intermetallic growth and thickness between Sn-4Ag-0.5Cu solder alloy with different surface finishes such as immersion silver (IAg), and electroless nickel/ immersion gold (ENIG) during reflow soldering and after ageing treatment. The solder joint is evaluated in terms of IMC thickness.
2. Experimental details
Copper samples of size 45 mm × 50 mm × 1 mm were cut from sandwich copper sheet. The sample
192
NMC 2007, Johor Bahru, Malaysia National Metallurgical Conference
substrates were lightly grinded and then polished to remove any oxide of copper and to increase the wettability. The samples then were subjected to pretreatment process in order to remove the dirt, grease, oxide layer and also to activate the Cu surfaces before plating process. There are two types of surface finish used in this study, namely, IAg and ENIG. Table 1 shows the thickness for each surface finish. For ENIG samples, the electroless Ni layer contains ~13wt% phosphorous. After ENIG plating process, the samples were then being deposited with gold layer through immersion plating without any pretreatment except rinsing in running tap water. While for IAg surface finish, a few combinations of immersion solution have been tried before we had finally arrived at the most stable combination which is: 50g/L of ethylene diamine tetra-acetic acid (EDTA), 20.4g/L of sodium hydroxide and 1g/L of silver nitrate. Figure 1 shows the steps followed in the present study to achieve the suitable solution with the desired silver thickness. The operating temperature of the plating bath was set up to 45 ºC. After that, all the samples were laminated with a layer of solder mask to restrict the molten solder from flat spreading during reflow. Then, the solder mask together with the patterned film was cured by ultraviolet (UV) light in order to produce small openings of 0.38 mm in diameter. After curing the samples, a thin layer of no clean flux is applied onto the substrate to remove the oxide layer and also to improve the wetting of molten solder during reflow soldering. Then, the substrate was manually populated by the solder balls with 700 µm of diameter in an array of 9 x 5 on each substrate. The metallurgical bonding between the solder balls and substrate surface was formed by reflow soldering at temperature 250 ºC.
After reflow soldering, the samples were subjected to ageing treatment at 150 ºC for 500 hours, 1000 hours and 2000 hours. Then, all the samples were characterized both at the top surface and cross section. From top surface, 3-D morphology of these intermetallics will be examined by etching away the solder (deep etching). Both scanning electron microscope (SEM) and Energy dispersive X-ray (EDX) are used to study the intermetallics at the interface. Cross sections of the specimens are prepared using standard metallographic steps and examined using Nikon optical microscope, image analyzer, SEM-EDX and FESEM (Field emission scanning electron microscope).
Table 1: Thickness of layer of each surface finish
SURFACE FINISH THICKNESS DEPOSITED
(µm) IAg (Ag) ~0.3 ENIG (Ni/Au) ~4 – 5 / ~0.3
Figure 1: Process flowchart for Immersion Silver
3. Results and discussion To achieve one of the objectives of the current
study, an attempt was made to deposit the silver layer with very thin thickness. Several plating solutions were tried to plate a silver layer with the desired thickness. The solutions range from acidic to alkaline with different process parameters. After many attempts, we successfully identified one neutral solution capable of depositing a layer of silver with the thickness of 0.2744µm for thin coating and 0.7854µm for thick coating. Figure 2 shows the results of copper substrate before plating (after pretreatment process) while Figure 3 shows the results of immersion silver plating. From the EDX analysis, the results clearly shows that the silver layer deposited using the neutral solution, which operates at a pH value of 6.46, fully covered the copper substrate (Figure 3).
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Figure 2: Copper substrate before plating (after
pretreatment process)
Figure 3: FESEM-EDX results of IAg on Cu
3.1. Bare Cu
After reflow soldering, a uniform spherical shape of copper-tin intermetallic formed in the solder joint. As identified by EDX analysis, the reaction layer on Cu substrate is the Cu6Sn5 phase. This result similar to the previous worked done by Blaine Partee [8] and Amagai et al [9]. In addition to the Cu6Sn5 intermetallics, Ag3Sn IMCs also formed at the interface and in the bulk solder. Its morphology is that of large dendrite-like and plates as shown in Figure 6. These large Ag3Sn intermetallics are quite brittle, which may lead to serious problems under stressed conditions in the actual service for PCB. Zeng and Tu [1] mentioned that these Ag3Sn crystals
will easily lead to failure if formed in a high stress area, such as corner region between solder bump and top surface metallurgy (TSM), cracks can be initiated and propagated along the interface between Ag3Sn and solder.
During isothermal aging, the IMCs continue to grow by inter-diffusion between the Sn in the solder and Cu in the substrate. This solid state aging resulted in a significant increase in the thickness of the Cu6Sn5 IMCs (Figure 4). Figure 4b, shows that the IMC has coarsened and became more uniform instead of the scallop-type observed after reflow soldering. Moreover, after heating the solder joint, a new IMC phase has formed between the Cu and Cu6Sn5 IMC as shown by the thin grey layer in Figure 5 which is known as Cu3Sn. The possible mechanism for Cu3Sn can be proposed as follows: in solid state, Sn diffuses more slowly than Cu inside Cu6Sn5 IMC, so Cu accumulated at the interface between Cu and Cu6Sn5 resulting in the formation of Cu3Sn, which can consume some of Cu6Sn5 IMC at the beginning of the solid reactions. During ageing, the Cu diffuses towards the solder, so does Sn towards the Cu layer, resulting in the growth of both Cu3Sn and Cu6Sn5 IMC layers [10]
(a)
(a)Cu6Sn5
(b)
Ag = 75.13%
Cu = 24.87%
(b)Cu6Sn5
Ag3Sn
Figure 4: Morphology of Cu6Sn5 (a) After reflow, (b)
After ageing 1000 hours
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Figure 5: Formation of Cu3Sn at the solder interface after isothermal aging
Figure 6: Formation of Ag3Sn after reflow soldering 3.2. Immersion Silver
In the solder joint between Sn/4Ag/0.5Cu and IAg
surface finish, the same type of IMC can be observed in Figure 7 which is Cu6Sn5. The morphology is uniformly round and smooth spherical shape. However, the size of the IMC is smaller compared to bare copper. Apart from that, the small particles and large plate-like Ag3Sn IMC was also seen in these solder joints.
Moreover, similar to bare copper, after heating the solder joint, a new IMC phase has formed between the Cu and Cu6Sn5 IMC as shown by the thin grey layer in Figure 5 which is known as Cu3Sn when identified by EDX analyses. This layer exactly followed the morphology of the Cu substrate. However, there were numerous Kirkendall voids formed in the Cu3Sn layer (Figure 8). Xiao et al [11] showed that Kirkendall voids formed during the long-time ageing. The formation mechanism of Kirkendall voids in Cu3Sn layer appeared to be different compared to the voids in Ni3P. In such cases, the main diffusion element is Sn in Cu6Sn5 but Cu in Cu3Sn [12]. Diffusion of Sn in Cu6Sn5 is very slow, which determines the entire growth of the IMCs,
leading to shortage of Sn to react with Cu in Cu3Sn layer. The lacking Sn in the lattice spaces in Cu3Sn can therefore result in the formation of Kirkendall voids. However, in our study, no Kirkendall voids were found in the Cu3Sn layer for bare Cu, indeed, Kirkendall voids were observed if IAg was used as surface finish. This indicates an interconnection between Kirkendall voids and immersion silver, but the details of the reason is still unclear.
Cu6Sn5
Cu3Sn
500 X
Cu6Sn5
Figure 7: Morphology of Cu6Sn5 after reflow.
Ag3Sn
1
Figure 8: Kirkendall voids found in Cu3Sn IMC
during long time ageing treatment. 3.3. Electroless Nickel/ Immersion Gold
Different types of IMCs found when using this surface finish. It is believed that the reasons of this formation are due to wetting properties of the lead-free solders composition and inconsistently distributed composition on the solder itself [13]. Figure 9 shows the circular boundary regions which form at the solder interface when using this type of surface finish. During reflow, the Au layer dissolves into the solder, exposing the Ni layer to react with the solder to form the solder joint. Diffusion of the Ni layer into the solder will results in the formation of IMCs, consuming the Ni layer.
2 3 4
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Figure 9: Circular boundary regions which form in
the IMC layer of Sn/4Ag/0.5Cu.
Figure 10a shows the interface morphology at the solder joint which clearly shows that mainly needle-shaped (Ni,Cu)3Sn4 IMCs were observed after reflow. While (Cu, Ni)6Sn5 was observed to form between (Ni,Cu)3Sn4 and solder (Figure 11a). This results are in agreement with previous study [12] that the first phase to form and grow in the Ni-Sn systems is (Ni,Cu)3Sn4 or Ni3Sn4. Ni3Sn2 was believed to grow very slowly and probably requires a larger undercooling. Cheu Li [14] agree that this is the reason why normally Ni3Sn2 will be observed only near the edges of the solder joints. However, in this study, there was no Ni3Sn2 found.
Figure 10: Top views of ENIG finish (a) After reflow (b) After ageing 1000 hours
During ageing, IMC’s will continue to grow. Figure 10b and Figure 11b shows the top view and cross sectional images of solder bumps after aging process. In contrast to IMCs after reflow, all IMC’s were formed with a layered structure. There is also an indication that (Ni, Cu)3Sn4 grew thicker with isothermal ageing
(a)
(Ni,Cu)3Sn4
(Cu,Ni)6Sn5
(b)
(Ni,Cu)3Sn4
(a) (Cu,Ni)6Sn5(Ni,Cu)3Sn4
Figure 11: Cross section of Sn-Ag-Cu /ENIG finish (a) After reflow, (b) After ageing 2000
hours
3.4. Determination of IMC thickness
Thickening of intermetallics is a major concern in reliability issue of electronics packaging. This is because it will lead to the failure of the joint with fracture in the IMC itself or along the interface between the solder and IMC layer when expose to any mechanical forces, such as vibration, expansion and contraction caused by variation in temperature.
(b)(Ni,Cu)3Sn4
Figure 12 shows the effect of isothermal ageing on
the intermetallic thickness for all type of surface finishes studied. Generally, bare Cu substrate gave the greatest IMC thickness in both reflow and aging condition. This is probably due to the fast interaction between Cu and Sn. While IAg surface finish shows slower growth rate than bare copper, but higher than ENIG surface finish. However, up to certain extends,
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the thickness of IMCs decrease for IAg and bare copper finish. This is due to the existence of the IMC layer slows down the diffusion of the Cu at the interface. For ENIG surface finish, the growth of the IMCs keep increasing until ageing 2000 hours. ENIG produced the thinnest IMCs because there is Ni layer that act as a diffusion barrier layer between the solder and Cu. Thus, the diffusion will be slower.
IMC Thickness VS Surface Finish
0.000
2.0004.000
6.0008.000
10.000
IAg ENIG COPPER
Surface Finish
Thic
knes
s (µ
)
As-reflow 500 hours 1000 hours 2000 hours
Figure 12: IMC thickness after reflow and ageing for
all surface finishes studied 4. Conclusion
In this study, the microstructure of the IMCs has been studied when lead-free solders react with different surface finishes. On bare Cu and IAg, the Cu6Sn5 and Cu3Sn IMCs formed at the interface. The Cu3Sn grew due to the slow diffusion rate of Sn in Cu6Sn5 IMC after ageing treatment. Apart from that, the intermetallics are also thickened and coarsen after ageing. While on ENIG surface finish, the IMCs formed were (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 phases at the interface after reflow. These IMCs also become thicker due to the persistent consumption of Ni during aging. However, ENIG produced the thinnest IMCs as compared to IAg and bare Cu.
Acknowledgement The authors would like to acknowledge the financial support provided by INTEL (Malaysia) Research Student Fellowship (vot 73724) and UTM for providing the research facilities. References 1. K. Zeng, K.N. Tu, Six cases of reliability study
of Pb-free solder joints in electronic packaging technology, Mater. Sci. & Eng. R 38 (2002) 55-105.
2. http://www.nemi.org 3. P. T. Vianco, An Overview of Surface Finishes
and Their Role in Printed Circuit Board
S. Chada, Investigation of Immersion Silver PCB Finishes for Portable Product Applica
Solderability and Solder Joint Performance, Circuit World, Vol. 25, No. 1 (1999) 6-24.
4. tions,
5.
b. 46 (2006) 896-904.
Manufacturing Center of
9. l
10. namics of solid-state aging of eutectic
11. plating process on
12. ns between lead-free solders
13. ead-Free
14. ead-Free Solder and Different Surface
15. g, Proceedings of the
Proceedings of SMTA International, Chicago, IL, October 2001, pp. 604-611.
J. W. Yoon, S. W. Kim, S. B. Jung, Mater. Trans. 45 (2004) 727-733.
6. W. M. Chen, M. McCloskey, S. C. O’Mathuna, Microelectron. Relia
7. K. H. Prakash and T. Sritharan, Acta Mater, 49 (2001), 2481 – 2489.
8. B. Partee, Intermetallics In Electronic Soldering, National Electronics Excellence, June 2004. M. Amagai, M. Watanabe, M. Omiya, K. Kiahimoto, T. Shibuya, Mechanicacharacterization of Sn-Ag-based lead-free solders, Microelectron. Reliab. 42 (2002) 951-966. T.Y.Lee et al., Morphology, kinetics and thermodySnPb and Pb-free solders (Sn-3.5Ag, Sn-3.8Ag-0.7Cu and Sn-0.7Cu) on Cu Journal of Materials Research (2002), 291-301. G. W. Xiao et al., Effect of Cu stud microstruture and electrointermetallics compound growth and reliability of flip chip solder bump, IEEE Transactions on Components and Packaging Technologies, (2001), 682-690. T. Laurila, V. Vourine and J.K. Kivilathi, Interfacial reactioand common base materials, Materials Science & Engineering R-Reports (2005), 1-60. Seeling K. and Suraski D., (2001), Materials and Process Considerations for LElectronics Assembly, Circuits Assembly, 12, No. 12. T. Cheu Li, Interfacial Reaction Between Sn-Ag-Cu LFinishes (2006), 140-141. Huang, Y. W., Collier, P., Teo, K. et al. Wafer Bumping by Stencil PrintinPan Pacific Microelectronics Symposium. February 10-13, 1998. Kauai, HI, 455-460.
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Effect of solder bump size on interfacial reactions during soldering between Pb-free solder and Cu and Ni/Pd/Au surface finishes
F. Nor Akmal1, A. Ourdjini1, M.A. Azmah Hanim1, I. Siti Rabiatull Aisha1 and Y.T. Chin2.
1Faculty of Mechanical Engineering, University of Technology Malaysia (UTM)
81310, Johor Bahru, MALAYSIA [email protected]
2Intel Technology (M) Sdn. Bhd. Bayan Lepas, Penang
Abstract— Flip chip technology provides the ultimate in high I/O-density and count with superior electrical performance for interconnecting electronic components. Therefore, the study of the intermetallic compounds was conducted to investigate the effect of solder bumps sizes on several surface finishes which are copper and Electroless Nickel / Electroless Palladium / Immersion Gold (ENEPIG) which is widely used in electronics packaging as surface finish for flip-chip application nowadays. In this research, field emission scanning electron microscopy (FESEM) analysis was conducted to analyze the morphology and composition of intermetallic compounds (IMCs) formed at the interface between the solder and UBM. The IMCs between the SAC lead-free solder with Cu surface finish after reflow were mainly (Cu, Ni)6Sn5 and Cu6Sn5. While the main IMC’s formed between lead-free solder on ENEPIG surface finish are (Ni, Cu)3Sn4 dan Ni3Sn4. The results from FESEM with energy dispersive x-ray (EDX) have revealed that isothermal aging at 150ºC has caused the thickening and coarsening of IMCs as well as changing them into more spherical shape. The thickness of the intermetallic compounds in both finishes investigated was found to be higher in solders with smaller bump size. From the experimental results, it also appears that the growth rate of IMC’s is higher when soldering on copper compared to ENEPIG finish. Besides that, the results also showed that the thickness of intermetallic compounds was found to be proportional to isothermal aging duration. 1. Introduction The requirements of electronic packaging is toward higher I/O, greater performance, higher density, and lighter weight, the use of area array packaging technology is expected to increase. The type of packaging, Flip Chip, provides the ultimate in high I/O-density and count with superior electrical performance, and very small size [1]. It is well known that soldering involves a reaction between molten solder and substrate, which dissolves some of
the substrate and which forms some sort of intermetallic layer [2]. The interfacial chemical reactions enhance the wettability between the solder and the substrate. The intermetallic compound (IMC), grows at the solder and at the under bump metallurgy (UBM) interface at the practical operating temperature during the reflow process and eventually forms the solder bump. Due to its excellent conductivity and surface for soldering, copper has been widely used as the substrate materials. Though, surface finishes are still needed to be deposited onto the substrate surface as copper may oxidize easily. For electroless nickel/ electroless palladium/ immersion gold (ENEPIG) surface finish, nickel functions as the diffusion barrier with its low dissolution rate into tin. Meanwhile the palladium and gold layers can protect the underlying metals from oxidation. ENEPIG is formed by the deposition of electroless nickel (120 – 240 micro inches), followed by 5 to 15 micro inches of electroless palladium with an immersion gold flash (1 – 2 micro inches). The electroless palladium layer prevents any probability of nickel corrosion that may caused by the immersion gold deposition reaction. This layer is much harder than gold, providing added strength to the surface finish for wire bonding and connector attachment, while protecting the underlying nickel from oxidation [3].
Figure 1: The deposition layers of ENEPIG surface finish
The nickel/palladium/gold plated boards have a shelf life of up to two years or more. The process is almost similar to the nickel/gold process, except that it uses a palladium metal layer that is deposited after the
Au
Ni
Cu
Pd
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nickel layer, but prior to the final gold layer. ENEPIG is the finish that has the widest latitude for a variety of applications. It provides a flat co-planar surface. Sometimes referred to as the universal finish, it is a good soldering surface, a gold wire bondable surface, aluminium wire bondable surface, as well as a contacting surface. 2. Experimental Details In order to investigate the effect of solder bump size on intermetallic compound (IMC) formation between solder bump (SnAgCu) and substrate, three different sizes of solder balls, namely 300, 500, and 700 µm in diameter were soldered on two different finishes; Bare Copper surface finish and Electroless Nickel / Electroless Palladium / Immersion Gold (ENEPIG) finishes. The Copper substrate is the copper-polymer sandwiched substrates (FR4). The experiments must start with depositing the under bump metallurgy first. The ENEPIG is an under bump metallurgy which consists of layers of Nickel, Palladium and Gold towards copper plate. The first step in the ENEPIG process is to catalyze the copper surface for Ni deposition. The copper substrates were first subjected to grind mechanically to remove the oxide layer and rust followed by rinse with running tap water. Then, the copper substrates were subjected to a pretreatment cleaning process to remove dust, grease and oxide layers as well as to activate the copper surface for ENEPIG plating preparation. Since Ni is less noble than Cu, activation of the copper surface for Electroless Ni plating can be achieve by seeding a noble metal such as palladium chloride (or palladium sulfate [4]. The substrate was activated twice during the pretreatment before electroless nickel plating and before the electroless palladium plating was initiated with palladium solution. The immersion gold plating was performed right after electroless palladium without pretreatment except rinsing with water. The Nickel deposition is aided by a hyphophosphite (H2PO2) reducing agent that decomposes during this reaction and results in the decomposition of phosphorus in the electroless Ni layer [5]. In this research, medium phosphorus / medium phosphorus (7-10%) was used. Several combinations of electroless Nickel plating solutions have been tried out before arriving at one most stable plating succession. The components of optimum nickel plating solution are one part of NIMUDEN 5X and four parts of distilled water with 50 minutes plating times until reach the thickness of 5-6 µm at temperature of 950C and the range of pH is 4.3-4.5. For palladium plating the solution used consisted of 2g/L PdCl2, 200ml/L NH4OH (28% NH3), 3.5g/L Na2EDTA.2H2O, 20mg/L (8 drops) Thiodiglycollic
acid, and 8.5g/L NaH2PO2H2O where the operating temperature of the plating bath was set around 450C and a pH within the range of 8.0-9.0. The final step for ENEPIG plating is immersion gold where the gold layer acts as an oxidation barrier. The immersion gold plating is conducted immediately after electroless palladium with no pretreatment, except rinsing in running tap water. The combination for the immersion gold solution are 2g/L Potassium cyanoaurate (KAuCN2), 75g/L Ammonium chloride, 50g/L dehydrate Sodium citrate, and10g/L dehydrate Sodium hyphophosphite. The operating temperature of the plating bath was set around 93ºC, and a pH within the range of 7.0-7.5 After preparing the substrate and under bump metallurgy, the differences sizes of solder balls were placed onto the surface and then the whole structure was subjected to reflow soldering. This process was followed by an isothermal solid state aging at a temperature of 1500C for different aging times. Analysis of the results obtained focused mainly on characterization the intermetallic compounds formed between the Cu/Au under bump metallurgy and solders. This was made as function of process parameters such as thickness and morphology of IMC.
3. Results and Discussion 3.1. Effect of Solder Bump Size on Intermetallic Compound Thickness In order to study the effect to the intermetallics thickness, they were measured from the cross sections of different solder joints and it was shown in histograms in Figure 2. These histograms illustrate the effect of solder joint size on interfacial reactions occurring during soldering. It is clearly shown that the intermetallics grow faster when soldering with a smaller solder volume (small solder bump) compared with those formed in larger solder volume. The results are similar with all solder joints which is a clear trend that can be observed here, where the IMC layer thickness decreases with increasing solder size from 300µm to 700µm for ENEPIG and copper surface finishes. The dissolution phenomena of metals involved in the interfacial reactions are controlled by the ratio of solder volume to contact pad area [6]. With an increase in this V/A ratio the diffusion distance for Cu to saturate the liquid solder increases, thus resulting in slower interfacial reactions. This explanation could be applied in the present study. Even the metal pad used is different for all specimens and depends on the solder sizes, the ratio V/A still increases with increasing the solder ball diameter. This confirms the finding that with decreasing the
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solder volume i.e; increasing the V/A ratio will lead to high intermetallics growth [7].
IMC Thickness VS Solder Bump Size
012345678
300(µm) 500(µm) 700(µm)Solder Bump Size (µm)
IMC
thic
knes
s (µm
)
0 (hr)500(hrs)1000(hrs)
Figure 2.a: IMC thickness for Pb-free solder on Cu surface finish
IMC Thickness VS Solder Bump Size
00.5
11.5
2
2.53
300(µm) 500(µm) 700(µm)
Solder Bump Size (µm)
IMC
thic
knes
s (µm
)
0 (hr)500(hrs)1000(hrs)
Figure 2.b:IMC thickness for Pb-free solder on ENEPIG surface finish
On the other hand, from the figures 2, it is found that the Cu substrates formed thicker intermetallics than ENEPIG. Figure 2.b shows that the intermetallics thickness has not shown significant increase after 1000hrs. This is attributed to the fact that after soldering the IMC on copper finish has scallop-type morphology with a rough interface. However, due to the effect of the ageing process the IMC has somewhat leveled off and became more continuous and exhibit a smoother interface [7]. IMCs produced in ENEPIG also decrease when the solder bump size increases. This is due to the existence of the Ni diffusion barrier layer between the solder and substrate. Since the reaction rate of Ni with liquid Sn solder is smaller than that of Cu, it acts to prevent the rapid interfacial reaction between solder and Cu conductor in electronic devices. This in turn, results in thinner intermetallic compounds. During reflow soldering, the molten lead-free Sn-4.0Ag-0.5Cu solder alloy dissolves the entire Au and Pd layer into the liquid solder, allowing Sn from the molten solder to react with the Ni layer to form Ni-Sn intermetallics. 3.2. Composition and Morphology of Intermetallic Compounds Analysis. During reflow soldering, the formation of the solder joint exists when there are interactions between liquid solder with the base metal, copper or nickel.
This interfacial reaction results in the formation of intermetallic compounds. Besides, separate samples of all categories were aged at 1500C for 500 hours and 1000 hours. In all these samples several phases were identified by optical and electron microscopy together with EDX. The solid-state reactions and growth of intermetallics during long term exposure to high temperatures are also important because the morphology, distribution and thickness of these intermetallics will affect the solder joint reliability. The morphologies of the various intermetallics formed in the solder joint during soldering and after solid state ageing were examined on all specimens using the deep etching of the solder. It is well known that soldering involves a reaction between molten solder and substrate, which dissolves some of the substrate and which forms some sort of intermetallic layer [2]. The interfacial chemical reactions enhance the wettability between the solder and the substrate. When tin-containing solder comes in contact with the copper pad surface, a layer of Cu–Sn IMC, consisting of the Cu6Sn5 phase adjacent to the solder and the Cu3Sn phase next to the copper land pad surface is formed in between and serve as the bonding material for solder joint. There is an extra IMC layer observed between the Cu6Sn5 and the Cu pad in all solder bump sizes studied after ageing for up to 500 hrs (Figure 3.b) and 1000hrs. This layer is confirmed by EDX analysis as Cu3Sn. No such Cu3Sn is observed at the interface after reflow in the cross sectional optical micrographs used in this study (Figure 3.a). In the case of smaller solder, the bulk solder is saturated with Cu quickly, Sn supply also is limited than Cu from the substrate, the Cu6Sn5 layer that formed first will transform into Cu3Sn. The same effect also happened for the other solder bump sizes which are the 500 µm and 700 µm.
(a)
(b)
Cu6Sn5
500 X Cu6Sn5
Figure 3; Cu3Sn and Cu6Sn5 IMCs formed between lead-free
solder and bare copper for 300 µm solder bump after (a) reflow soldering and (b) 500 hrs aging.
Cu3Sn 500 X
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For the solder joint at Cu suface, there are Cu-Sn intermetallics compound at the interface as result of the diffusion of copper into the solder after reflow soldering. However, the dissolution rates of Cu along the Cu surface are not uniform. There are areas where all the Cu atoms have not been used in the formation of Cu6Sn5 IMC. This Cu surplus may lead to the formation of long tubes or fibres of Cu6Sn5. Figure 4 shows the IMCs at the top surface on Cu surface finish after reflow with different solder bump sizes. It is clearly shown that the same IMC morphology is observed for all solder bump sizes investigated. Thus, it can be concluded that difference in solder bump size does not have an effect on the type of intermetallic compound formed between copper surface finish and lead free solder.
(a)
(b)
(c)
Figure 4.5; Morphology of Cu6Sn5 formed between Sn-Ag-Cu solder and bare copper after reflow for difference solder sizes; (a)
300 µm, (b) 500 µm, and (c) 700 µm.
During isothermal aging, the IMCs continue to grow by inter-diffusion between the Sn in the solder and Cu in the substrate. This solid state aging resulted in a significant increase in the size of the Cu6Sn5 IMCs (Figure 5). Figure 5.b shows that after 500 hours aging, the IMC has coarsened and became more uniform and denser as well as the IMC morphology after 1000 hours instead of the scallop-type observed after reflow soldering. Moreover, after heating the solder joint, a new IMC phase has formed between the Cu substrate and Cu6Sn5 IMC as shown by the thin grey layer in Figure 3.b which is known as Cu3Sn. In the temperature range of interest (e.g. below 350 0C), the interfacial reaction with molten Sn-based solder results in the formation of Cu3Sn(ε) and Cu6Sn5(η) layers[8] (Figure 6). The same effect also happened with the solder bump sizes of 500 µm and 700 µm. In addition, there is the Ag3Sn IMCs which was observed in solder-Cu joint on the Cu6Sn5. The shapes formed vary but generally they were plate-like. After aging, Ag3Sn IMCs will become more spherical and smaller (Figure 5.b).
Cu6Sn5
(a)
Cu6Sn5
(b)
Ag3Sn
Cu6Sn5
Ag3Sn
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Cu6Sn
4000 X
Cu6Sn
4000 X
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(c)
Figure 5; Morphology on the top surface of IMCbetween Sn-Ag-Cu solder and bare copper for 30
bump with difference aging durations; (a) as reflowand (c) 1000 hrs
Figure 6: Cu–Sn binary phase diagram [
However, different phenomenon haENEPIG surface finish. In general, at tebelow 260 0C (reflow temperature is 217is the first phase to form at the liqconductor interface. The first stage of thethe dissolution of Ni in liquid solder, unsupersaturated with Ni. This is because thof gold and palladium dissolve into the bexposing the nickel layer to react with thform solder joint. In fact, there are mointermetallics were present at the inENEPIG Surface finish compared to thmaking it much more complicated to itypes of intermetallics that exist. This other than Sn, Ag and Cu from the soldpart in the formation of intermetallic after reflow. Lead free Sn-Ag-Cu solders are capabledifferent types of intermetallics, which arcircular boundary phases as shown in Ficontributing factor to the formation of thboundary regions is due to the differeproperties ranging from the edge to the c
solder bump. The different deep etching reaction at these regions also could be considered as one of the
Cu6Sn
s formed 0 µm solder , (b) 500 hrs,
9]
ppened to mperatures 0C) Ni3Sn4 uid Sn/Ni reaction is til solder is e thin layer ulk solder, e solder to re types of terface of
e Bare Cu, dentify the is because
er also take compounds
of forming e present in gure 7. The ese circular nt wetting
entre of the
contribution factor.
Inner Region
4000 X
n
Figure 7: Different morphologies of intermeta
circular boundary regions in lead free Sn-A
Several intermetallics were found in Ag-Cu solder joint which are (Ni, CCu6Sn5, and Ag3Sn. Neither goldcontaining intermetallics were founsolder joints. Although gold and padetected in EDX analysis, they arelow and are usually ignored in the dthe composition of intermetallic comp Generally, (Ni, Cu)3Sn4 and Ni3Sn4 of needle-like or rod shape afterblocky Ag3Sn intermetallics were above them at the centre region. The these intermetallics for the different sreflow are shown in Figure 8. It wAg3Sn were rarely found in the sol300 µm solder bumps. This is due smaller solder volume has relativelyin the solder and thus forming lessolder joint compared to that whisolder bump with higher solder volum
Generally, isothermal aging wmorphologies of intermetallic compoSn-Ag-Cu solder joint. Figure 9 (a)needle-like Ni3Sn4 were formedsoldering for sample using 300µmHowever, they changed to a morchunky shape after 500 hours aging t9 (b)). The intermetallics continuecoarsen when subjected to further ahours aging treatment. The same trend also applied to the so500µm and 700µm solder bumps whthe transformation of intermetallics fneedle-like shape to more compaspherical shape with aging duration.
Besides that, there is another intermformed at the outer region which is kThe same trend also applied at th
202
Outer Regio
llics which form the g-Cu solder joint.
the lead free Sn-u)3Sn4, Ni3Sn4 ,
nor palladium-d in lead free
lladium are also relatively very etermination of ounds.
, take the forms reflow. While usually formed morphologies of older sizes after
as observed that der joints using to the fact that
low Ag content s Ag3Sn in the ch uses bigger e.
ill affect the und in lead-free shows that the after reflow solder bumps. e spherical and reatment (Figure d to grow and ging from 1000
lder joints using ich experienced rom a loose and ct, coarse and
etallic that has nown as Ni3Sn.
is region where
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from the needle shape of Ni3Sn after reflow to a more spherical and chunky shape after 500 hours aging treatment. The intermetallics continued to grow and coarsen when subjected to further aging from 1000 hours aging treatment (Figure 10). At circular boundary phases, it shows different intermetallic which is Cu6Sn5 (Figure 10.a).
(a)
(b)
(c)
Figure 8; Top surface morphology of intermetallics formed in centre region after reflow using (a) 300µm (b) 500µm (c) 700µm
solder bumps
(a)
(b)
(c)
Figure 9; Top surface micrographs showing morphologies of in Sn-Ag-Cu solder joint
using 300µm solder bumps at inner region
(a) after reflow (b) 500 hrs (c) 1000 hrs
(a)
Ag3Sn
Ag3Sn
203
Cu Sn
Ni3Sn4
6 5
Ni Sn
(Ni,Cu)3Sn4
(Ni,Cu)3Sn4
3 4
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[2] Wassink, R.J.K. (1997). Soldering in Electronics, Second ed., Scotland: Electrochemical Publications Ltd., p. 141.
(b)
4
(c)
Figure 10; Top surface micrographs showing morpSn-Ag-Cu solder joint
using 500µm solder bumps
(a) after reflow (b)500 hrs (c) 1000 hr 4. Conclusions In this study, the mean thickness of the compounds was found to be thicker in smvolume compared to larger solder volumother hand, ENEPIG surface finish prodintermetallic compounds compared to surface finish and the main type of compounds formed are Ni3Sn4 and (Ni,Cintermetallic compounds type and mormore or less the same for the diffevolumes investigated in this research.surface finishes, aging resulted in growthin terms of overall thickness, coaintermetallic compounds and also morphology of intermetallic compounspherical shape. Acknowledgments The authors would like to thank UniversMalaysia for providing research facilities References [1] Lau,S.T. (1994). Chip On Board Technologie
Modules.New York: Van Nostrand Reinhold.
Ni3Sn
[3] Dusek,M., Nottay,J., and Hunt,C. (2001).Compatibility of Lead-Free Solder with PCB Materials. UK: Centre National Physical Laboratory
[4] Johal,K. (2001) Are You in Control of your Electroless Ni/Immersion Gold Process?. Chicago: SMTA International
[5] Fassel, W. M. and friends (1966) Electroless Plating of
Metals (U.S. Patent 3264199)
[6] Schaefer,M., Laub,W., Fournelle, R.A., and Liang,J. (1997)
Proc. Design Reliability Solder Interconnects. p.247
[7] Ourdjini,A., Azmah Hanim,M.A., Koh,, S.F.J., Siti Aisha
Idris, Tan, K.S., and Chin, Y.T. (2006) Effect of Solder
Volume on Interfacial Reactions between Eutectic Sn-Pb and
Sn-Ag-Cu Solders and Ni(P)-Au Surface Finish. Malaysia: 4
Ni3SnFaculty of Mechanical Engineering, University of
Technology Malaysia:
[8] Laurila,T., Vuorinen, V., and Kivilahti, J.K. (2005).
Materials Science and Engineering :Interfacial Reactions
Between Lead-Free Solders And Common Base Materials.
Laboratory of Electronics Production Technology. Helsinki
University of Technology.
[9] Massalski,T. (1996). Binary Alloy Phase Diagrams, ASM.
hologies of in at outer region s
intermetallic aller solder e. On the
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IEMT 2008, Penang, Malaysia International Electronic Manufacturing Technology
EFFECT OF SURFACE FINISH METALLURGY ON INTERMETALLIC
COMPOUNDS DURING SOLDERING WITH TIN-SILVER-COPPER SOLDERS
A. Ourdjini1, M.A. Azmah Hanim1,2, I. Siti Rabiatull Aisha1 and Y.T. Chin2
1Faculty of Mechanical Engineering, University of Technology Malaysia (UTM)
81310, Johor Bahru, MALAYSIA [email protected]
2Intel Technology (M) Sdn. Bhd. Bayan Lepas Free Industrial Zone Phase 3, Halaman Kampung Jawa
1900, Penang, MALAYSIA
Abstract— Solder joint reliability is dependent on both thickness and morphology of the intermetallics that form and grow at the solder joint interface during soldering and subsequent thermal ageing and examining the morphology of these intermetallics is of great importance. The focus of this paper is to present experimental results of a comprehensive study of the interfacial reactions during soldering of Sn-Ag-Cu lead-free solders on copper (Cu), immersion silver (ImAg), electroless nickel/ immersion gold (ENIG) and electroless nickel/ electroless palladium/ immersion gold (ENEPIG) surface finishes. Using scanning electron microscopy detailed a study of the 3-D morphology and grain size of the intermetallics was conducted. The results showed that when soldering on ENIG and ENEPIG finishes several morphologies of intermetallics with different grain sizes form at the solder joint interface compared to a single intermetallic morphology that forms when soldering on copper and immersion silver. An attempt was made to discuss the effect of several factors that may have an influence on the type of morphology the intermetallics may grow into. The results obtained in the present investigation also revealed that the technique of removing the solder by deep etching to examine the morphology of intermetallics is a convenient and efficient method to investigate the intermetallics formed at the solder joints. Keywords: lead-free solder, surface finish, intermetallics, solder joint
1. Introduction
The demand has recently increased for new bump formation technologies which enable the simultaneous formation of large numbers of bumps with a narrow bump pitch at low cost and short tact
processing. However, some reliability issues may be arising from the utilization of smaller solder bump size. Due to its excellent conductivity and surface for soldering, copper has been widely used as the substrate materials. However, several types of metal coating must also be deposited on copper surfaces as board finishes for the purpose of providing wetting surfaces and protection against the environment. The selection of a metallurgical system (solder – top surface metallurgy) is very important because of its influence on the reliability of electronic assemblies. Typical surface finish metallurgy consists of two main layers: 1) a solderable layer in contact with the underlying copper and 2) a protective layer on top of the solderable layer. The purpose of the solderable layer is to provide the surface to which the liquid solder wets and then adheres upon solidification. This same solderable layer also acts as the diffusion barrier by preventing diffusion of the solder to the copper substrate. The protective layer serves to protect the solderability of the solderable layer from degradation due to exposure to ambient environment until reflow soldering occurs. During reflow the solder melts and the protective layer dissolves into the molten solder exposing in the process the solderable layer to the molten solder. The solderable layer now is also subjected to dissolution by the molten solder until solidification is complete. This results in the formation of an intermetallic layer between the solderable layer and solder. This intermetallic layer will grow in thickness during subsequent thermal ageing after assembly due to solid – solid reaction between the solderable layer and the solder by solid-state diffusion.
In this paper we present results of an experimental investigation to illustrate the importance of surface finish metallurgy on the type and morphology of
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intermetallics formed during soldering with SAC solder.
2. Experimental Details Four different surface finish metallurgies were selected for this study: bare copper (Cu), immersion silver (ImAg), electroless nickel/ immersion gold (ENIG) and electroless nickel/ immersion gold (ENEPIG). The ImAg, ENIG and (ENEPIG) surface finishes were deposited on a free-oxygen pure copper substrate with the dimensions (width x length x thickness) of 45 x 50 x 1 mm. The copper substrate was first subjected to a pretreatment process to remove the oxide and activate the copper surface before the desired finish layers are deposited. Prior to soldering, a dry solder mask was laminated onto the plated substrates using a laminated machine. Then, the solder mask was exposed to the ultraviolet (UV) light through a patterned film. The exposure is to ensure an array of pads is made onto the solder mask upon the subsequent development stage in the developing solution. The substrates were then populated with Sn-4Ag-0.5Cu (SAC405) solder spheres. The solder spheres were arranged in several rows and bonding to form the solder joints was made by reflow in a furnace with the peak reflow temperature set at 250 oC. Before soldering, all substrates were treated with a no clean flux to remove surface oxide. In order to reveal the morphology of intermetallics formed during the soldering process a useful method of selective chemical etching of the top surface was employed and examination of the intermetallics was made by means of scanning electron microscopy. Energy dispersive x-ray (EDX) was used to identify the type and composition of intermetallics formed.
3. Results and Discussion
Thermodynamics is usually used to describe the intermetallics which form at the interface between metal pads in the surface finish and liquid Sn-based solders and thus phase diagrams are important to explain why such an intermetallic can form or not. The type of intermetallic formed depends on the surface metallurgy used. It is well established that when soldering on bare Cu and ImAg, Cu6Sn5 IMC is formed during reflow, while (Cu, Ni)6Sn5 is formed when soldering on ENIG and ENEPIG.
As the SAC solders/ immersion silver is reflowed the Ag layer will dissolve into the molten solder exposing the Cu substrate to the solder. Once the
solder is melted, the Sn in molten solder will quickly react with the solid Cu and develops small nuclei which quickly grow and form a scallop like islands of the IMC at the interface. Figure 1 and Figure 2 show the top surface morphologies of the Cu6Sn5 IMC formed on Cu and ImAg finishes respectively. It is quite clear when observing the grain size of the scallops that thinner IMC is formed on ImAg compared to when soldering on Cu. It is important to note that the morphology and indeed size of the scallop-like IMC observed in Figure 1 and Figure 2 was uniform over the entire surface of the pad area as shown in Figure 3.
Figure 1. Scallop-type Cu6Sn5 IMC formed on Cu after reflow
Figure 2. Scallop-type Cu6Sn5 IMC formed on ImAg
after reflow
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Figure 3: Scallop-type Cu6Sn5 IMC formed on Cu
As observed above in both Cu and ImAg finishes, the interfacial reaction occurs between the liquid Sn solder and Cu in the substrate pad. However, an interesting observation made in the present study is the presence of a ring-shape of the intermetallic layer found during soldering between the SAC solder/ENIG and to a lesser degree between SAC solder/ENIG couples. Figure 4 shows the top view morphology of the intermetallic layer formed between SAC405 and ENIG finish. It is interesting to notice the presence of a ring-shape with varying sizes of intermetallics. EDX analysis confirmed that the intermetallic is still (Cu, Ni)6Sn5. Figure 4: Ring-shape of intermetallics near the edge of the ENIG pad. Figure 5 and Figure 6 show more examples of the various sizes of the (Cu, Ni)6Sn5 intermetallic on ENIG and ENEPIG surface finishes respectively.
Figure 5: Different grain sizes of (Cu, Ni)6Sn5 formed on ENIG finish Figure 6: Different grain sizes of (Cu, Ni)6Sn5 formed on ENEPIG finish Previous researchers [2, 3] also observed the same pattern when soldering with SAC solder on ENIG but no such ring pattern was observed when soldering with Sn-Pb solder alloy. The reasons for the formation of such a pattern of varying grain sizes of the intermetallic phase and its possible effect on the
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solder joint reliability is not clear. Several factors may have an influence on such a scale of intermetallic morphology change. The elements involved in the interfacial reactions are the same: Sn, Ag, and Cu except that when soldering on ENIG and ENEPIG there is Ni involved. It is believed it is probably due to the wetting properties of the SAC lead-free solder composition ranging being different over the pad area. Another factor is the non-homogeneity of the solder alloy itself since unlike the Sn-Pb alloy, the SAC solders contain three main elements, Sn, Cu and Ag. Yoon et al. [4] claimed that this situation occurred probably due to discrepancy of Cu content in the solder matrix between the centre and edge during solidification. However, the authors believe that the formation of these different intermetallic grains with varying sizes may also be correlated with the solder sphere contact to the substrate metallization pad. In the Sn-Pb / ENIG system, for example, the interfacial reaction occurs mainly between liquid Sn and Ni whereas with SAC solder/ ENIG system, both Cu (from the solder) and Ni (in the surface finish) are involved in the reaction. Since the Ni-Sn reaction is slower than that between Cu-Sn, and coupled with varying wetting over the pad area, this may lead to faster growth of Cu-Sn based intermetallic in areas where there is more Cu supply. It has been reported by Laurila et al. [5] that the dissolution rates of Cu along the Cu pad surface are not necessarily uniform. The fact that even when using SAC solder on Cu and ImAg finishes (no Ni is involved in these systems) there was only one uniform intermetallic layer over the entire pad area, is evidence that competitive reactions between Cu and Ni with Sn may have an influence on the morphology and size of grains formed.
4. Conclusions
The results showed that the type of surface finish metallurgy used has a strong influence on the morphology and size of intermetallic grains formed during soldering with SAC-based solders. Having both Ni and Cu in the surface finish/ solder system may lead to competitive growth and hence results in varying grain sizes of the intermetallic layer. The results also revealed that the technique of removing the solder by selective etching to examine the morphology of intermetallics is a convenient and efficient method to investigate the intermetallics formed at the solder joints.
References
[1] M. Schaefer, W. Laub, R.A. Fournelle, and J. Liang, Proc. Design Reliability Solder Interconnects, eds, R.K. Mahidhara,
D.R. Frear, S.M.L. Sastry, K.L. Murthy, P.K. Liaw and W.L. Winterbottom (Warrendale, PA: TMS, 1997), p.247
[2] Tan, C.L., M. Eng. Thesis, Universiti Teknologi Malaysia, 2006
[3] Azmah. H. A., PhD Thesis, Universiti Teknologi Malaysia, 2007
[4] Yoon, J. W., Kim, S. W. and Jung, S. B. Intermetallic Morphology, Interfacial Reaction and Joint Reliability of Pb-free Sn-Ag-Cu Solder on Electrolytic Ni BGA Substrate”. Journal of Alloys and Compounds, 2004, 392, 247-252
[5] Laurila, T., Vuorinen, V. and Kivilahti, J. K. Interfacial Reactions between Lead-free Solders and Common Base Materials. Materials Science and Engineering,, 2005, R49: 1-60.
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EFFECT OF SOLDER VOLUME AND PAD AREA ON INTERMETALLIC
COMPOUNDS FORMATION DURING SOLDERING BETWEN Sn-4Ag-0.5Cu AND IMMERSION SILVER FINISH
I. Siti Rabiatull Aisha, A. Ourdjini and A. Astuty, O. Saliza Azlina
Faculty of Mechanical Engineering, University of Technology Malaysia
Skudai, 81310, Johor Bahru, Malaysia
Abstract Recently, silver as an electrochemical deposit on copper substrate has attracted much attention in the microelectronics field. To deposit silver particles on copper, immersion plating is used and it was characterized by field emission microscope (FESEM), energy dispersive X-ray analysis (EDX) and also image analyzer to determine the thickness of the silver plating. Apart from that, this paper also examines various sizes of Sn-4Ag-0.5Cu lead free solders which are Ø200µm, Ø300µm, Ø500µm and Ø700µm. With different solder joint sizes, the dissolution rate of top surface metallurgy (TSM) and intermetallic compound (IMC) growth kinetics will be different. The effect of solder volume/ pad metallization area (V/A) ratio on IMC growth was investigated during reflow soldering and solid state ageing. Higher V/A ratio produced thinner and more fragmented IMC morphology while lower V/A ratio produced better defined IMC layer at the interface after reflow soldering. After ageing at 150oC for up to 2000 hours, the initial scallop morphology of the Cu6Sn5 IMC layer changed to that of a more planar type and also became thicker. Besides, another IMC layer formed after ageing which is Cu3Sn. Several techniques of materials characterization including optical microscope, image analysis, scanning electron microscopy and energy dispersive X-ray analysis were used to examine and quantify the intermetallics in terms of composition, thickness and morphology. Keywords: intermetallics, lead-free solder, surface finish, immersion silver, soldering
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