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Pb-FREE SnAgCu BALL GRID ARRAY (BGA) COMPONENTS IN
SnPb ASSEMBLY PROCESS Project
Leaders: Robert Kinyanjui and
Quyen Chu
October 11 , 2007iNEMI Council Meeting
1
OUTLINEOUTLINE•
Project Motivation
•
Project Objectives
•
Background Information
•
Phase 1A–
Results & Discussion
•
Phase 1B
–
Experimental Design
–
Results
•
Summary
2
PROJECT MOTIVATIONPROJECT MOTIVATION
•
The coming into effect of the European Union (EU) directive on Reduction of Hazardous Substances (RoHS)
–
Created supply chain constraints on SnPb components availability especially for the RoHS exempted sector of the industry
•
Has created the need for soldering Pb-free SnAgCu BGA solder ball terminations in a Sn-Pb soldering process without yield loss or reliability impact.
3
1)
To assess the process parameters for assembling Pb-free SnAgCu BGAs under the temperature constraints of a conventional tin-lead (SnPb) assembly process.
A.
Identify an appropriate process window for assembling Pb-free SAC BGAs in SnPb assembly process
B.
Determine the quality of the mixed alloy solder joints
2)
To understand the reliability of mixed-alloy (SnAgCu in SnPb) solder joints.
3)
To develop a “generic”
process guideline and risk assessment for assembling Pb-free BGAs in a SnPb assembly process.
Project ObjectivesProject Objectives
4
BACKGROUND INFORMATIONBACKGROUND INFORMATION
•
Reflow process challenges when assembling SAC BGA using a SnPb assembly process:–
Peak Temperature has to be within the SnPb assembly process (range: 205 through 215C – solder joint temperature)
–
Time above liquidus has to be in the range 45-90 seconds
–
Mix of different component sizes (pitch) in a typical assembly •
This may result in Ball Collapse issues
–
Non-mixing of the SAC solder balls and SnPb solder alloys at the typical SnPb reflow processing conditions (Non-uniform microstructure)
5
Phase1A of the ProjectPhase1A of the Project
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Phase 1A: Test Vehicle and Components Phase 1A: Test Vehicle and Components
PBGA313 PBGA256 PBGA324 CSP160
PBGA480
PBGA324
PBGA256CSP46 CSP156 CBGA256
CSP46 CSP208
μCSP50
µCSP50
CSP160
Objective: Characterization of peak temperature
and time above Liquidus
(TAL) for mixed (SnAgCu in SnPb) solder Joints
PBGA313 and PBGA324 were selected for examination
• Lead-free ball Alloy: SAC405•Considered 205, 225, 240oC peak temperatures and 60 seconds TAL
Test Vehicle Thickness: 62 milsLaminate Material: FR4Finish: ImmAgPaste: Sn63Pb, SAC305Stencil Thickness: 5mils Laser Cut Stencil aperture: Round (10% reduction)
7
Phase 1A: Impact of Peak temperature Phase 1A: Impact of Peak temperature
Observations•Solder balls started to melt from the board side.•The η
and ε
primary solids were often observed even above their melting temperature
η
ε
Large BGA Component (PBGA313)
Reflow @ 240oC; 60secsPrimary solid: none
Reflow @ 225oC; 60secsPrimary solid: η
Reflow @ 205oC; 60secsPrimary solids:(Sn) + η
+ ε
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Impact of Peak Reflow Temperature on the Microstructure of Impact of Peak Reflow Temperature on the Microstructure of the PBGA313the PBGA313
(Sn) + (Pb) + ε: starts to melt @ 177 ºC
(Sn) + η
+ ε: starts to melt @ 217 ºC
non-melted SAC
non melted SAC & partiallymelted (Sn) + (Pb) + ε
Large BGA Component (PBGA313) @ 205 ºCComponent side
Board sideObserved segregated regions in alloy phases
9
Elemental Compositional Maps of Large BGA (PBGA313); 205ºC
Note: Ag concentration shown in lower left mapis increasing due to dissolution of immersion Agsurface finish as well as dissolution of SAC.
(Sn)
(Ag)
(Cu)
(Pb)
Microstructure of the PBGA313 @ Microstructure of the PBGA313 @ 205ºC
10
Ag3
Sn
(Pb) + (Sn)+Ag3
SnCu6
Sn5
Large BGA (PBGA313); 225ºC
Microstructure of the PBGA313 @ Microstructure of the PBGA313 @ 225ºC
Observations•
(Sn)-Ag3
Sn binary eutectic structure.•
Abnormal IMC size of Ag3
Sn
11
Elemental Compositional Map of Large BGA(PBGA313); 225ºC
(Sn)
(Ag)
(Cu)
(Pb)
Microstructure of the PBGA313 @ Microstructure of the PBGA313 @ 225ºC
12
Large BGA (PBGA313) ; 240oC
The Ag3
Sn and Pb phases are found embedded in the interdendritic
spaces
Microstructure of the PBGA313 @ Microstructure of the PBGA313 @ 240ºC
13
(Sn)
(Ag)
(Cu)
(Pb)
Elemental Compositional Map of Large BGA (PBGA313); 240ºCMicrostructure of the PBGA313 @ Microstructure of the PBGA313 @ 240ºC
A high peak temperature provides a homogenized microstructure
14
PHASE 1A SUMMARYPHASE 1A SUMMARY•
Observed microstructures are in agreement with predictions based
on the calculated solidification path: especially, the Scheil
calculations.•
SAC-Pb formed the final frozen phases: (Sn) + (Pb) + ε
with melting point: ~177 ºC (calculated), and micro-pores were observed in the same area.
•
Further investigation of porosity effects may be required, which
results from different phase fractions compared to equilibrium microstructures.
•
At 205 ºC reflow temperature, there are significant differences in microstructure between the melted SAC-Pb and non-melted SAC.
In addition, the primary ε
phase will grow to an abnormal IMC size if the reflow temperature is not sufficient to melt it. These features may be unfavorable to the reliability of the solder joint.
•
The growth of the Cu6
Sn5
IMC has less influence on the microstructures than that of the Ag3
Sn IMC.
•
In order to obtain a homogenized SAC + Pb microstructure, further optimization of the reflow peak temperature
and TAL
is required.
15
Phase1B of the ProjectPhase1B of the Project
16
Phase 1B Study ObjectivesPhase 1B Study Objectives
•
To characterize the mixed alloy solder joint microstrutural homogeneity with the following considerations:
–
Package size/ball volume (Pitch)
–
Peak Reflow Temperature
–
Time Above Liquidus
–
Solder paste volume
17
EXPERIMENTAL
DESIGN
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Note:•Pb-free BGA ball alloy: SAC405• SnPb components of each type were used for baseline run.
SBGA600 PBGA324 CABGA288 CTBGA132
1)1) Characteristics of Components ConsiderationCharacteristics of Components Consideration•Four different package sizes/ball volumes (Pitch) were selected:
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2)2) Test VehicleTest Vehicle
•
PCB Dimensions:–6.800” x 4.075” x 0.093”
•
Finish–Electroless Nickel Immersion Gold (ENiG) –Copper OSP
•
Number of Layers –8 Internal board Layers
–Tg = 170oC–Td = 340oC
Bare Test Board
20
3)3) Assembly ConsiderationAssembly Consideration
•
Assembly Setup–
Two thermocouples per array package were placed along the diagonal line of the package PCB footprint on a bare PCB for profiling.
–
FIVE Reflow profiles were generated•
Peak Temp: 210, 215, and 235oC•
Time Above Liquidus (TAL): 60, 90, 120 seconds
•
Stencil–
6 mil foil thick–
Two aperture openings (1:1 and 10% Reduction)•
For the 0.5mm pitch component, only one aperture opening (used a 12mil square opening) was used
21
Assembly Process
Flow
Reflow Profile
Number
BGA Ball Alloy
Solder Paste Alloy
Peak Temp.
(oC)
Time Above
Liquidus (sec)
Stencil Aperture Opening
(%)
1 1 SnPb SnPb 210 60 90
2 1 SnAgCu SnPb 210 60 90
3 2 SnAgCu SnPb 210 90 90
4 3 SnAgCu SnPb 210 120 90
5 1 SnAgCu SnPb 210 60 100
6 2 SnAgCu SnPb 210 90 100
7 4 SnAgCu SnAgCu 235 60 90
8 5 SnAgCu SnPb 215 60 90
4)4) Assembly MatrixAssembly Matrix•Eight different assembly process flow were generated with FIVE different processing conditions of TALs and Peak Temperatures
•A total of fifteen boards were assembled for initial inspection and another set of Ninety Eight boards were assembled for Mechanical and ATC testing.
22
RESULTS
23
Representative Reflow ProfileRepresentative Reflow ProfileReflow Profile Conditions: Target Peak Temp. = 210oC; TAL = 60sec
NOTE: The ΔT for the largest package, SBGA600 is
2.5oC, while the ΔT for the CTBGA132 is
2.6oC.
24
1.27mm SBGA600 1.00mm PBGA324 0.80mm CABGA288 0.50mm CTBGA132
Component Ball Pitch (mm)
Ball Diameter as Rec’d (mm)
Ball Volume (mm3) x 10-2
90 % Aperture Opening 100 % Aperture Opening
Paste Volume (Metals) (mm3) x 10-2
Paste Volume (Metals) (mm3) x 10-2
SBGA600 1.27 0.760 22.98 1.90 2.33
PBGA324 1.00 0.630 13.09 1.14 1.42
CABGA288 0.80 0.460 5.10 0.68 0.87
CTBGA132 0.50 0.300 1.41 0.54 0.52
Representative Solder Paste Print DepositRepresentative Solder Paste Print Deposit
•
No serious issues were observed in the paste deposit process.
•
The average solder paste volume for each of the package types
25
Reflow Profile Conditions = Target Peak Temp. = 210oC; TAL = 60sec
Assembled Test BoardAssembled Test Board
26
Solder Joint Inspection after AssemblySolder Joint Inspection after Assembly
•
A representative solder joint formation for the SBGA600 component reflowed at peak temperature of 210oC with 120 seconds above liquidus (Profile # 3)
•
NOTE: At 120 TAL and Tpeak =210C, the SBGA600 package still displays a non-homogeneous microstructure between the SAC solder ball alloy and the SnPb solder paste.
Non-uniformSolder jointmicrostructureMixed
NonMixed
27
Initial PCBA AnalysisInitial PCBA Analysis
•
Analysis of as-reflowed BGA solder joints integrity through dye & pry and metallography
–One board from each of the reflow conditions was subjected to dye & pry followed by metallographic analysis:
•
Determine the solder joint structure
•
Examine the degree of mixing between SnPb and Sn-Ag-Cu solder alloys
•
Characterize the solder joint height/diameter
Dye & Pry
Metallography
Direction in reflow Oven
28
Representative Dye & Pry ResultsRepresentative Dye & Pry Results
1.0mm PBGA324
0.8mm CABGA288
PCB Side
Package Side
•
No dye intrusion was observed no signs of open
joints
29
What is the Extent or Degree of MixingWhat is the Extent or Degree of Mixing
Between SnPb and SnAgCu solder alloysBetween SnPb and SnAgCu solder alloys
after assemblyafter assembly
30
Determination of the Extent/Degree of Alloy Mixing Determination of the Extent/Degree of Alloy Mixing The extent of alloy mixing was determined from the ratio
Y2/Y1
solder joints obtained at 100x
(SAC+ Pb)
(SAC)
NOTED:Due to possible package and PCB warpage issues
that may interfere with the solder joint formation, the extent of solder alloy mixing was measured on center balls
on the outermost rows
for each of the package.
31
Effect of Package Size (Pitch)Effect of Package Size (Pitch)
1.0mm PBGA324
1.27mm SBGA600
0.5mm CTBGA132
0.8mm CABGA288
Peak Temp: 210C and TAL: 60sec
The extent of alloy mixing is observed to increase with increasing ball-to-
paste ratio for a peak temperature of 210oC and for all the TALs
tested.
It was noted that approximately 40% solder ball-to-paste volume ratio is needed in order to achieve 100% alloy mixing.
32
The degree of mixing increases linearly with the time above liquidus
of 60, 90, and 120secs for the 1.27,1.00 and 0.8mm pitch packages.
The Effect of Time Above The Effect of Time Above LiquidousLiquidous1.0mm PBGA324 1.27mm SBGA600
TAL:120secTAL: 60sec TAL:120secTAL: 60sec
0%
20%
40%
60%
80%
100%
40 60 80 100 120 140
Time Above Liquidus (sec)
Perc
ent M
ixed
(%)
1.2 mm
1.0 mm
0.8 mm
Linear (0.8 mm)
Linear (1.0 mm)
Linear (1.2 mm)
33
Little or no discernable difference was observed in the degree of mixing for the largest package (SBGA600) with pitch of 1.27mm.
However for the 1.0mm and 0.8mm pitch BGAs, it was generally observed that increasing the paste volume from a 90% to 100% aperture opening increased the degree of mixing.
90% Paste Volume 100% Paste Volume
1.0mm PBGA324 1.27mm SBGA600
90% Paste Volume 100% Paste Volume
0%
20%
40%
60%
80%
100%
120%
60 90 60 90 60 90
1.2 1.0 0.8
Package Pitch (mm), TAL (sec)
Perc
ent M
ixed
(%)
Aperture Opening 90%Aperture Opening 100%
Effect of Paste VolumeEffect of Paste Volume
34
Tpeak
= 215C,
TAL = 60sec
Increasing the peak temperature by 5oCwith TAL = 60 sec,
•
leads to an increased degree of mixing for both PBGA324 and SBGA600.
•
The extent of mixing increases from 50% to 100% for the PBGA324 package
1.0mm PBGA324 1.27mm SBGA600
Tpeak
= 210C,
TAL = 60sec
0%
20%
40%
60%
80%
100%
120%
1.2 1.0
Package Pitch (mm)
Perc
ent M
ixed
(%)
210C, 90%apt
210C, 100%Apt
215C, 90%Apt
Tpeak
= 215C,
TAL = 60sec
Tpeak
= 210C,
TAL = 60sec
Effect of Peak TemperatureEffect of Peak Temperature
35
Summary of ObservationsSummary of Observations
•
The larger the package/ball volume, the lower the degree of mixing observed.
–
CTBGA132 (0.5mm Pitch) showed a 100% mixing at 210C and 60sec time above liquidus.
•
Doubling the time above liquidus (from 60sec to 120sec), increases the degree of mixing.
•
For the small packages (reduced SAC solder ball volume), increased paste volume corresponds to increased degree of mixing.
–
However, for the largest (SBGA600) package no significant change in mixing was observed.
•
Increasing the peak temperature from 210C to 215C led to a significant increase in the degree of mixing (almost doubled) for the 1.0mm pitch package.
36
Project StatusProject Status
•
Assembly and initial analysis completed. Phase 1 data was presented at the 2007 SMTAI conference in Orlando, Florida
•
Accelerated Thermal Cycling (ATC) with a temperature profile: - 40C to 125C is completed (~3500 cycles).
•
ATC failure analysis still in progress.
•
Vibration test will now be performed in place of Drop test.
•
Project anticipated to be completed by end of Dec. ’07 and final industry paper/presentation will be presented at the IPC/APEX 2008 conference.
37
Tactics Milestones & Issues•
Phase 1: Characterize the peak temperature effects on the Pb-free BGAs in SnPb paste
•
Phase 2: Study the reliability of the Pb-free BGAs processed within the temperature constraints of SnPb assembly conditions
•
Phase 3: Develop a “generic” process guideline and risk assessment for assembling mixed –alloy solder joints
•
Phase 1……………………………………..COMPLETED•
Phase 2 …………………...…………………………Q407•
Phase 3: ………………………………... ……...…...Q108
•
ISSUES:•
This project covers the current issues of BGA having SAC 305/405 solder balls processed in SnPb. More recently, vendors are providing BGAs with very low Ag or no silver increasing melting range up to 227C. This will further complicate any solution/s found in this project.
Goal: Identify the process parameters for assembling Pb-free SnAgCu BGAs under the temperature constraints of a conventional tin-lead (SnPb) assembly process.
Goal: Identify the process parameters for assembling Pb-free SnAgCu BGAs under the temperature constraints of a conventional tin-lead (SnPb) assembly process.
Strategy:•
Conduct experiments to characterize mixing level of LF BGA in a typical SnPb process and understand the reliability impact.
•
Project Leads: Robert Kinyanjui (Sanmina-SCI), Quyen Chu (Jabil)
Thrust Area:Thrust Area: MiniaturizationMiniaturization PbPb--Free BGAs in SnPb Assemblies Free BGAs in SnPb Assemblies
ProjectProjectTIG:TIG: Board AssemblyBoard Assembly
PagePage 1 of 21 of 2
Start:Start: 0808--2525--20052005
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The project membership is also grateful for the iNEMI SecretariaThe project membership is also grateful for the iNEMI Secretariat support t support
AcknowledgementAcknowledgement
39
Thank You
Questions
