Tin Whisker Growth

and

Mitigation with a Nanocrysytalline Nickel Coating

by

Szymon Janiuk

A thesis submitted in conformity with the requirements

For the degree of Master of Applied Science

Graduate Department of Material Science and Engineering

University of Toronto

© Copyright by Szymon Janiuk (2015)

ii

Tin Whisker Growth and Mitigation with a Nanocrysytalline Nickel Coating

Master of Applied Science, 2015

Szymon Janiuk

Materials Science and Engineering, University of Toronto

Abstract

Tin whiskers are a problem in the electronics industry since the EU banned the use of

lead in Pb-Sn solders as part of the Restriction of Hazardous Substances (RoHS). The biggest

concern with Sn whiskers is their ability to short-circuit electronics. High reliability applications

such as the aerospace, defense, healthcare, and automotive industries are at most risk. This

project explores Sn whisker mitigation and prevention with the use of nanocrystalline nickel

coating over Sn surfaces. Sn was plated onto a pure Cu substrate using electroplating. A high

temperature and high humidity condition, at 85°C and 85% RH, was effective at growing

whiskers. A nNi coating was plated over Sn/Cu coupons. After subjecting the nNi/ Sn/Cu

samples through 85°C/85% RH testing conditions, no whiskers were observed penetrating the

surface. These results make nNi a viable material to use as a coating to prevent the growth of Sn

whiskers in electronic assemblies.

iii

Acknowledgements

The author would like to extend his personal thanks to Dr. Doug Perovic (UofT) and Dr.

Polina Snugovsky (Celestica) for their supervision, advice, and assistance in this master’s thesis.

Special thanks are also extended to the Materials Science and Engineering department of the

University of Toronto for the ongoing support and training. And thanks are also extended to the

many people who contributed to this project through related work or equipment training:

(At University of Toronto)

Sal Boccia – SEM operation and training

Leonid Snugovsky – Experimental and analysis equipment

Dan Grozea – Experimental and analysis equipment

(At Celestica)

Eva Kosiba – Support work

Ivan Matijevic – Support work

(At Integran)

Dr. Gino Palumbo – Consulting and nickel coating

Neil Mahalanobis – Consulting and nickel coating

iv

Table of Contents

Abstract ..................................................................................................................................................... ii

Acknowledgements .................................................................................................................................. iii

List of Tables ........................................................................................................................................... vi

List of Figures ......................................................................................................................................... vii

List of Appendices ................................................................................................................................... ix

Nomenclature and Abbreviations.............................................................................................................. x

Chapter 1 ....................................................................................................................................................... 1

Introduction and Background ....................................................................................................................... 1

1.1 Tin Whiskers ....................................................................................................................................... 1

1.2 Factors Affecting Whisker Growth ..................................................................................................... 9

Chapter 2 ..................................................................................................................................................... 11

Literature Review ........................................................................................................................................ 11

2.1 Surface Coatings ............................................................................................................................... 11

2.2 Substrate and IMC Effects ................................................................................................................ 12

2.3 Grain Structure Effects ..................................................................................................................... 13

2.4 Oxidation Effects .............................................................................................................................. 15

2.5 Stress Effects ..................................................................................................................................... 16

2.6 Environmental Conditions ................................................................................................................ 17

2.7 Compositional Effects ....................................................................................................................... 17

2.8 Growth Mechanism and Driving Force Models and Theories .......................................................... 19

Objectives ................................................................................................................................................... 22

Chapter 3 ..................................................................................................................................................... 23

Materials Preparation Techniques ............................................................................................................... 23

3.1 Electroplating .................................................................................................................................... 23

3.2 Storage Conditions ............................................................................................................................ 25

3.2.1 High Temperature and Humidity Test ....................................................................................... 26

3.2.2 Thermal Cycling ........................................................................................................................ 26

Chapter 4 ..................................................................................................................................................... 28

Analysis Techniques ................................................................................................................................... 28

4.1 Optical Microscopy (OM) ................................................................................................................. 28

4.2 Scanning Electron Microscope (SEM) ............................................................................................. 28

v

4.3 Energy Dispersive Spectroscopy ...................................................................................................... 30

Chapter 5 ..................................................................................................................................................... 31

Results and Discussion ............................................................................................................................... 31

5.1 Electroplating .................................................................................................................................... 31

5.1.1 Electroplating Current Density .................................................................................................. 31

5.1.2 Temperature Effects ................................................................................................................... 33

5.1.3 Surface Cleaning ........................................................................................................................ 33

5.1.4 Plating Thickness ....................................................................................................................... 37

5.1.5 Sn coating Surface Defects ............................................................................................................ 39

5.2 Storage Conditions ............................................................................................................................ 40

5.2.1 Ambient Storage ............................................................................................................................ 41

5.2.2 Thermal Cycling ............................................................................................................................ 42

5.2.3 High Temperature and High Humidity Test .................................................................................. 44

5.2.4 EDS Analysis of Sn coatings and Sn whiskers .............................................................................. 48

5.2.5 Whisker Growth Mechanism ......................................................................................................... 50

5.3 Whisker Root Cause Analysis........................................................................................................... 51

5.4 Nickel Plated Tin Samples ................................................................................................................ 54

5.3.1 EDS Elemental analysis of nickel coated samples ..................................................................... 58

Chapter 6 ..................................................................................................................................................... 62

Conclusions and Recommendations ........................................................................................................... 62

Chapter 7 ..................................................................................................................................................... 63

Future Work ................................................................................................................................................ 63

References ................................................................................................................................................... 64

Appendix I Whisker Measurements ............................................................................................................ 72

Appendix II Plating time calculations ......................................................................................................... 74

Appendix III Phase Diagrams ..................................................................................................................... 75

vi

List of Tables

Caption Page

Table 1 Satellite failures that are attributed to Sn whiskers……………………………………...2

Table 2 CTE values of Sn, Cu, Ni, and Al………………………………………………………41

Table 3 Whisker density, length, and hillock density measurement results………………….....44

vii

List of Figures

Caption Page

Figure 1 Two SEM micrographs of Tin whiskers a) a straight whisker and b) a kinked whisker…1

Figure 2 Size comparison between a whisker and a strand of human hair…………………………4

Figure 3 A tin whisker growing from a finish and making a connection with a pin from a

connector……………………………………………………………………………….....4

Figure 4 Vianco’s process schematic of DRX from his paper……………………………………..9

Figure 5 Electroplating setup for plating Sn onto a Cu substrate ………………………………....25

Figure 6 a) Initial Sn surface after electroplating b) post heat treatment Sn surface……………...26

Figure 7 Picture of the thermal cycle set-up inside one of the oven chamber showing the Ni/Sn

coated Cu sample at the top right hanging from a metal cage…………………………...27

Figure 8 Sn Dendritic formation when plating with too high of a current density………………..32

Figure 9 Surface of a Sn deposition a day after plating a)100x b) 300x magnification…………..32

Figure 10 Figure 10: Sn surface after plating in three temperatures at 25 mA/cm2 A) 75 °C, B) 85

°C, C) 95 °C……………………………………………………………………………...33

Figure 11 Dry with Air as is right after plating a) 100x, b) 500x magnification…………………...34

Figure 12 Rinsing with tap water a) 25x, b) 1000x magnification……………………………….....35

Figure 13 Rinsing with alcohol a) 1000x, b) 2500x magnification…………………………………35

Figure 14 Rinsing with salt water a) 300x, b) 1000x magnification………………………………..36

Figure 15 Rinse with DI water a) 500x, b) 1000x magnification…………………………………..36

Figure 16 Sn deposit cross-section thickness measurement with an optical microscope…………..37

Figure 17 a) 500x plated at 45 mA/cm2 for 5 min b) 1000x plated at 45 mA/cm2 for 5 min……...38

Figure 18 a) 500x plated at 35 mA/cm2 for 5 min b) 1000x plated at 35 mA/cm2 for 5 min……...38

Figure 19 a) 500x plated at 25 mA/cm2 for 5 min b) 1000x plated at 25 mA/cm2 for 5 min……...38

Figure 20 a) Initial Sn plated surface, 0 days after plating b) Sn plated surface after a year in

ambient condition………………………………………………………………………..42

Figure 21 before and after images of thermal cycling Sn coated samples A) before 250x, and B)

after 300x ………………………………………………………………………………..43

viii

Caption Page

Figure 22 A few different whiskers as well as hillocks on a Sn coating surface from an 85°C 85RH

test………………………………………………………………………………………..45

Figure 23 Close-up of whiskers on a Sn coating surface from an 85°C 85RH test………………...45

Figure 24 Close-up of a kinked whisker on a Sn coating surface from an 85°C 85RH test………..46

Figure 25 Close-up of a straight whisker on a Sn coating surface from an 85°C 85RH test……….46

Figure 26 EDS analysis of Sn samples after 85°C and 85% RH treatment a) Sn surface, b) Sn

Hillock, c) Sn Hillock cluster, d) Base of Sn Whisker, e) at the body kink of Sn Whisker,

F) at the tip of a Sn whisker……………………………………………………………...49

Figure 27 Fishbone diagram of the factors that contribute to the growth of Sn whiskers………… 51

Figure 28 SEM Image of surface of Nickel coated sample 10µm and 50nm grain at 500x

magnification…………………………………………………………………………….55

Figure 29 SEM Image of surface of Nickel coated sample 50µm and 50nm grain at 500x

magnification…………………………………………………………………………….55

Figure 30 SEM Image of surface of Nickel coated sample 10µm and 10nm grain at 500x

magnification…………………………………………………………………………….56

Figure 31 a) SEM image b) EDS analysis of the selected surface area of the 10 µm thick coating

with 50 nm grains………………………………………………………………………..59

Figure 32 a) SEM image b) EDS analysis of the selected surface area of the 50 µm thick coating

with 50 nm grains………………………………………………………………………..60

Figure 33 a) SEM image b) EDS analysis of the selected surface area of the 50 µm thick coating

with 10 nm grains………………………………………………………………………..61

ix

List of Appendices

Description Page

Appendix I: Whisker Measurement………………………………………………………….……72

Appendix II: Plating Time Calculations…………………………………………………………...74

Appendix III: Phase Diagrams……………………………………………………………………...75

x

Nomenclature and Abbreviations

Al Aluminum

BSE Back Scattered Electrons

Cd Cadmium

Cu Copper

CXR Characteristic X-rays

DRX Dynamic Recrystallization

EDS Energy Dispersive Spectroscopy

EU European Union

i Current Density

Ni Nickel

nNi Nanocrystalline Nickel

nNi/Sn/Cu Nanocrystalline Nickel plated on Tin plated on a copper substrate

OM Optical Microscopy

pNi Polycrystalline Nickel

Pb Lead

RH Relative Humidity

RoHS Restriction of Hazardous Materials

RT Room Temperature

SE Secondary Electron

SEM Scanning Electron Microscope

Sn Tin

Sn/Cu Tin plated on a copper substrate

TH Homologous Temperature

1

Chapter 1

Introduction and Background

1.1 Tin Whiskers

Tin (Sn) whiskers are microscopic metal fibres that spontaneously grow from the surface

of Sn and Sn-based solders, coatings, and finishes. Two examples of Sn whiskers are shown in

figure 1, as observed in a scanning electron microscope (SEM). The dimensions of the whiskers

are microscopic and have slight variance, with diameters typically in the range of 0.5-5 µm and

can grow to over 20,000 µm in length. These dimensions, especially the length of the Sn

whiskers, can potentially create short circuits in electronics because Sn whiskers are electrically

conductive. Short circuits are potentially catastrophic in high reliability applications such as

satellites, military equipment, aeroplanes, cars, healthcare equipment, etc.

Figure 1: Two SEM micrographs of Tin whiskers A) a straight whisker and B) a kinked whisker

2

A number of case studies highlight some of the high reliability applications that have

suffered failures due to whiskers to date. The following case studies cover the automotive

industry, the aerospace industry, and the healthcare industry.

Beginning in 2003 some of Toyota Camries experienced sticky acceleration pedals that

led to accidents. The NASA Goddard Space Flight Center team investigated a failed Accelerator

Pedal Position (APP) Sensor. The NASA team found a whisker shorting between two terminals

in the acceleration pedal using electrical testing. This whisker shorting caused the pedal to be

sticky rendering the sedan undrivable and not safe for the road [1].

The following are commercial satellite cases where whiskers have caused satellites to fail

due to Sn whisker induced short circuits to their satellite control processors (SCP) where the Sn

whiskers grew on pure Sn plated electromagnetic relays. Each satellite had a primary SCP and

one redundant SCP. The failure of both SCPs results in a complete loss of the satellite [2]. The

following table lists the satellites that failed due to Sn whiskers.

Table 1: Satellite failures that are attributed to Sn whiskers

Satellite Name Launch Date First Satellite Control

Processor Failure

Redundant Satellite Control

Processor Failure

GALAXY VII 27 October

1992

13 June 1998 22 November 2000

GALAXY IV 24 June 1993 (not caused by 'tin

whiskers')

19 May 1998

SOLIDARIDAD

1

19 November

1993

28 April 1999 27 August 2000

GALAXY IIIR 15 December

1995

21 April 2001 15 January 2006

The following are Sn whisker induced failures reported in military applications. In 2002

Davy [2] studied the failure of relays on military aircraft models used by the air forces of several

3

countries. The cause of the failure was attributed to Sn whiskers growing from the relays of the

armature, plated with bright Sn, towards a terminal stud. This short circuit created a spectacular

failure by allowing high voltage to flow from the armature to the case causing melting of

components [3]. Secondly, Raytheon, a major missile manufacturer, responsible for the Patriot

missile of Desert Storm operations ran into intermittent misfire problems. These problems were

found to be due to Sn whiskers after an exhaustive failure analysis of their electronics [4].

Failure cases attributed to whiskers are also prevalent in the medical field. In 1986, Sn

whiskers were found to be the culprit that caused the recall of several models of a pacemaker. To

prevent this problem the FDA proposed the education of field personnel and manufacturers of

the Sn whisker issue [5]. In 1994 rotary switches, important to a medical-device manufacturer of

apnea monitors, were failing due to Zn whiskers. The zinc-plated parts were deemed unsuitable

in this low voltage application due to the formation of Zn whiskers [6].

To give the reader a better idea of the scale of Sn whiskers and their appearance, figures 2

and 3 give two examples of Sn whiskers. In figure 2 the scale of a Sn whisker is shown with

reference to a human hair. In figure 3 a Sn whisker is seen growing out of a Sn finished

connector contacting a copper pin which can cause a short circuit failure.

4

Figure 2: Size comparison between a whisker and a strand of human hair [17]

Figure 3: A tin whisker growing from a finish and making a connection with a pin from a connector [18]

Interestingly, the Sn whisker problem has been known since the 1940’s and was remedied

in the 1950’s by adding Pb to Sn. Around WWII cadmium (Cd) was used as the primary plating

material for electronic finishing; however, it was found that the cause of many failures in the

electrical components were due to Cd whisker formation, as summarized in by Cobb [7].

Sn Whisker

5

Therefore, in a search for a metal to replace Cd, Sn was considered to be a great

candidate. The change to pure Sn coatings was initiated by Bell Labs [8]. Unfortunately, after a

few years of having Sn in service, Sn was also found to grow whiskers. Then, in the 1950’s it

was discovered that Pb suppressed the growth of whiskers in Sn. Accordingly, the whisker

problem was deemed to be solved, until the early 2000’s. In 2003 the EU banned the use of Pb in

solders effective in the beginning of 2006 with the introduction of the Restriction of Hazardous

Substances (RoHS) initiative [9]. Since then many R&D initiatives have been pursued in

academia and industry to study tin whiskers, how and why they grow, and ways to prevent or

mitigate them in electronic assemblies.

Currently, the most accepted growth mechanism for tin whiskers is the recrystallization

model [10] (more specifically the dynamic recrystallization method as outlined by Vianco et al.

[11, 12]). The most accepted driving force for whisker growth is internal compressive stresses.

The recrystallization process involves the storage of internal stresses through internal defects in

metals such as dislocations, their recovery, then the recrystallization stage, and finally the grain

growth stage, which corresponds to the whisker growth stage. The driving force, or the internal

compressive stress, can be generated by many factors such as the plating process or the

solidification process, atomic mismatch between the Sn and the substrate (usually Cu),

intermetallic compound (IMC) growth, impurities, grain size, grain orientation, surface oxide,

external deformation, etc.

Early whisker growth theories drew an analogy with “squeezing toothpaste out of a tube”.

However, whisker growth is not simply an extrusion phenomenon; this theory was disproved by

Fisher et al. [13] with a more detailed model. There is an incubation stage followed by a rapid

growth step, and then a slower growth step. The total whisker length is independent of the

6

applied stress yet the applied stress does control the growth rate of the whisker. Also, bulk

diffusion is insufficient for the amount of mass transport that is required in whisker growth; there

is an enhanced diffusion mechanism required in order to move the required Sn atoms to grow the

Sn whiskers.

Recrystallization has been the most popular theory for whisker growth as it is

energetically favourable and also has a driving force about four orders of magnitude higher over

other grain growth processes. Vianco et al. proposed dynamic recrystallization (DRX) as a model

that describes whisker growth in metals and alloys. Dynamic Recrystallization (DRX) is a

special case of the recrystallization process where the initiation and grain growth happen during

deformation.

Early theories also suggested that an oxide layer played a pivotal role and was thought to

be a requirement for the incubation and growth of whiskers. However, recent studies have shown

that an oxide layer on the surface of a metal is not universally required for whisker growth since

Au whiskers have also been observed to grow as pointed out by Hannay et al. [14]. The studies

done on Sn-Cu coatings by Moon et al. [15] also disproved the necessity of an oxide layer for the

growth of Sn whiskers. The DRX model does not require an oxide layer in order to grow Sn

whiskers. DRX is controlled by strain rate, temperature, and grain size. The DRX model requires

two processes to function: first, the deformation mechanism that initiates DRX, and second, the

mass transport mechanism that sustains whisker growth.

The nucleation and growth of new grains occurs during deformation in DRX rather than

during heat treatment as in the case of static recrystallization. Sn is a perfect candidate for DRX

as it is hot worked at room temperature due to its melting temperature being only 232 °C.

7

Whisker growth is a special form of DRX that occurs at the surface of the metal. Tsuji et al. [16]

concluded that whisker growth is energetically favourable and that whiskers prefer to grow on

the surface rather that in the bulk of the material since there is no thermodynamic barrier to

whisker formation. Whisker growth also occurs at stress levels that are below the yield strength

of the alloy; the yield strength of Sn is around 9-14 MPa.

Compressive stress generates plastic deformation and increases the strain energy that

initiates the DRX process. DRX can be promoted by either time-dependent (creep) or time

independent (stress-strain). Smaller grain sizes also gives rise to more whisker initiation sites.

Once initiation occurs the whisker growth is determined by the grain growth step of the DRX

process. Therefore, thin coatings are more likely to grow whiskers because they usually have

smaller grains than thicker coatings. Also, thinner coatings cannot accommodate stresses as well

as thicker coatings. Sn has a homologous temperature, TH, of 0.59 at room temperature, which

easily activates the DRX process. Since most residual stresses in Sn coatings are below its yield

strength, there is a need for time-dependent (creep) deformation at low strain rates to support

DRX for whisker growth [11].

A high TH and low strain rates promote the DRX process, cyclic DRX with a grain

growth step to be precise. The incubation period builds up the deformation needed to start DRX;

therefore, grains that are larger than 10 µm hinder whisker formation. Whiskers also grow at

preferred initiation sites where the available slip systems in the BCT structure of Sn are available

[11].

The material that is needed for whisker growth comes from the self-diffusion of Sn atoms

which can occur with fast, very fast, and abnormal diffusion rates within the bulk of Sn [11].

8

Mass transport mechanisms for grain growth (whisker growth) include: dislocation slip,

dislocations piled up at grain boundary, dislocation climb, pipe diffusion, and fast/short circuit

diffusion facilitated by high-angle boundaries (as in the case of Coble creep). Very fast diffusion

mechanisms usually have low activation energies with ΔH less than 40kJ/mol. However, fast

diffusion doesn't always require low activation energy. The pre-exponential jump frequency

factor can change significantly for interstitial and substitutional mechanisms. Abnormally fast

diffusion can also happen along high-density dislocation paths like dislocation pile-ups, high-

angle grain boundary and twin boundaries. Fast mass transport in Sn can also occur in the bulk of

the grains, and does not require grain boundaries of other surfaces for quick mass transport [11].

Fig 4 summarizes the steps in the DRX model for whisker growth from Vianco et al. [11].

Compressive stresses create dislocations that pile up at already existing grain boundaries. The

strain energy increases where new grains are initiated as the DRX grain refinement step.

Whiskers do not grow from a pre-existing grain. The new grain (whisker grain) orientation does

not need to correlate exactly to the texture of the nearby grains. The orientation of the new grain

may have a second-order dependence. The new grain grows into the deformed material by

boundary migration with the new grain limited in size to that of the previously deformed grain.

Deformation continues under the compressive stress, which increases the driving force to reduce

the resulting strain energy. This is where whisker growth takes place. The grain boundary

between the new grain and the surrounding old grains remains the same, and because of this

grain growth constraint, the grain grows out of the surface as a whisker [11].

9

Figure 4: Description of Vianco’s process schematic of DRX [11]

It is important to recognize that Sn whiskers are inevitable and that only their incubation

time and growth speed can be mitigated but not prevented. There are many different Sn whisker

mitigation strategies that are being studied at the moment such as selective metal coatings,

conformal coatings, reflow, annealing, underplating, and alloying. This master’s project focused

on studying the use of a nanocrystalline nickel (nNi) coating on Sn surfaces to prevent the

growth of Sn whiskers. The intention was to eventually develop a coating process for the

electronics assembly industry that could effectively mitigate Sn whisker growth.

1.2 Factors Affecting Whisker Growth

The following is a list of the various factors that affect whisker growth. Many of these

factors affect the internal stress of the Sn coating, but there are also factors that increase the

10

diffusion rate of atoms within Sn and factors that increase the likelihood of DRX to occur. The

fact that there are over twenty factors is telling to how complex the Sn whisker phenomenon is.

1. Alloying Elements

2. Coating Thickness

3. IMC Formation

4. Substrate Composition

5. Substrate Roughness

6. External pressure

7. Impurities

8. Service Temperature

9. Humidity

10. Grain Size

11. Grain Morphology

12. Oxidation

13. Annealing

14. Cooling Rate

15. Solution Type

16. Plating Temperature

17. Current Density

18. Impurities and additives in bath

19. Surface Damage

20. Handling

21. Forming, Bending leads

22. Contmaination (cleanliness on

coating surface)

23. Corrosion

24. CTE

25. Underlayer

26. Solder Dip

27. Conformal Coating

28. Re-flow

29. Current and Voltage in Operation

11

Chapter 2

Literature Review

2.1 Surface Coatings

Reinbold and Kumar [19] plated a copper cap on one half of a tin plated sample, with

silicon as the substrate, to observe whisker growth penetration ability. Whiskers grew on the

exposed tin side and the whisker density grew outward from the border between the two metals

at a rate much faster than the spread of the intermetallic, confirming that whiskers do not need to

be near the intermetallic compound (IMC) particles in order for the whiskers to form.

Additionally, the copper cap was penetrated by tin whiskers leaving the copper on top of the

whisker, showing that copper is not a suitable protective coating for tin-based solders [19].

Crandall et al. [20] investigated the use of Pt, Au, Cr, and Ni as metal caps by sputter

deposition onto a sample coated with Sn. Only certain metal caps were effective at blocking Sn

whiskers. Ni and Pt metal cap films successfully blocked all Sn whiskers for at least one year.

All Au films were penetrated after one month and all Cr films were penetrated after two months.

Whiskers that penetrated the metal caps carry up a piece of the metal cap, which were used to

explain why only certain metal caps block whiskers. It is believed that the shear modulus of the

coating is the main influence in the penetrability of a metal cap [20].

Landman et al. [21], deposited Ni capping over Pb-free Sn and high percentage Sn-alloys,

but not on any insulating material of the electronic assembly. It is expected that this cap layer

will prevent the growth of tin whiskers permanently. Tin Whiskers haven’t been seen in over five

12

years on an assembly coated with 200 nm of Ni [21]. This process is contrasted with other

whisker mitigation methods such as polymer and ceramic conformal coatings.

2.2 Substrate and IMC Effects

Sony et al. [22] evaluated six different copper alloy substrates coated with pure tin to

study tin whisker growth. The influence of the IMC thickness growth between the Cu and Sn

whisker growth was evaluated. Bright tin was plated on Cu, CuBe, cartridge brass, phosphor

bronze, Cu-Ni-Si “7025”, and Cu-Ni-Sn “spinodal”. The samples were mechanically stressed

and then subjected to high temperature and humidity storage conditions for 1000 hrs. Five out of

six substrates grew whiskers; the Cu-Ni-Sn sample did not grow whiskers. It was hypothesized

that the Ni layer prevents the growth of Sn whiskers by preventing the Cu6Sn5 IMC from

forming. Nevertheless, the thickness of the IMC was found to have little effect on tin whisker

growth [22].

IMC growth rate at the Sn-Cu interface with or without a Ni underplate and its related

stresses were evaluated by real-time measurements using the flexure beam method. The sample

without a Ni underplate exhibited pyramid shaped Cu6Sn5 IMC growing along the Sn grain

boundaries, especially at the triple junctions, creating a compressive stress that saturated at -11

MPa. The sample with the Ni underplate formed platelet IMC grains of Ni3Sn4 and had a slight

tensile stress, which didn’t grow any Sn whiskers [23].

Tin was plated on copper, and brass. Whiskers only grew on the pure copper substrate

and not on the brass. The presence or lack of tin whiskers was postulated to be due to the

thermodynamic stability of the Cu6Sn5 IMC where Zn inhibits its formation between the brass

substrate and tin coating. Copper and tin form the Cu6Sn5 IMC at their interface and grows

13

preferentially at the tin grain boundaries forming a pyramid structure that induces a compressive

stress within the tin coating making it more prone to whiskering [24].

Han et al. [25] evaluated the propensity and growth of tin whiskers on Alloy 42, and Cu

lead-frames that were stored for four years in ambient conditions after a post-bake treatment and

stored at 55C/85% RH for 3000 hrs. Whiskers showed a preferred growth direction of [321]. No

whiskers were observed on Alloy 42 after four years, however many whiskers were observed on

the Cu substrate [25].

Rodekohr. et al. [26] found that smooth substrate surfaces encourage whisker growth,

which is contrary to conventional wisdom. Smoother substrate surfaces produced a higher

density and longer Sn whiskers. The connection of surface roughness to the growth of whiskers

is not well understood and needs further study [26].

A copper-tin IMC, Cu6Sn5, creates internal compressive stress inside the Sn layer. To

evaluate if interfacial IMC is required to grow whiskers, Sn and Sn-Cu films were coated on

tungsten (W) where no IMC is formed interfacially. At room temperature, conical Sn hillocks

grew on the Sn coating and Sn whiskers grew from the Sn-Cu alloy electrodeposit. These results

demonstrated that interfacial IMC is not required for initial whisker growth [27].

2.3 Grain Structure Effects

Yu and Hsieh [28] developed a pure tin deposition process that produced various tin grain

structures such as columnar, semi-columnar, and random structures to study tin whisker

formation. The FIB was used to study whisker formation, grain structures, and intermetallic

formation. Full columnar structures formed normal to the deposited surface, and semi-columnar

14

and random structure stress relaxation occurred parallel to the surface. Yu and Hsieh concluded

that stress is more likely to be rapidly relieved within deposits with fewer grain boundaries [28].

Sobiech et al. [29] investigated the interrelations of microstructural evolution, phase

transformation, residual stress development, and whiskering behavior in Sn and SnPb coatings

on Cu aged at room temperature. Pb changes the stress relaxation mechanism in the coating. In

pure Sn coatings the grain morphology is columnar and the stress relaxation is localized by

unidirectional grain growth from the surface of the coating (i.e. whiskers form). In SnPb

coatings, the grains are equiaxed, and the relaxation mechanism is through uniform grain

coarsening without whiskering [29].

Kakeshita et al. [30] examined tin and tin-lead electroplates from various solutions and

examined them using high voltage electron microscopy on the effect of grain size on tin whisker

growth. Coatings that were well polygonized and had grain sizes in the range of few microns

didn’t grow whiskers readily. The coatings with irregular grain structure and few microns in size

grew many whiskers readily. Dislocation loops were observed in the irregular-shaped grain,

which might be formed by clustering of vacancies or interstitial atoms upon electroplating [30].

Pei et al. [31] found that grains that form into whiskers and hillocks are present in the as-

deposited film (ie. not renucleated) and many have horizontal grain boundaries beneath them.

Grain rotation during whisker and hillock formation suggests that measurements performed after

the features grow do not indicate their initial grain orientations. It was found that hillocks grow

from (001) type grains [31].

Choi et al. [32] performed synchrotron radiation X-ray micro-diffraction analysis to

measure the local stress level, the orientation of the grains in the finish around a whisker, and the

15

growth direction of whiskers. The residual stress in the coating was found to be around 10 MPa

at the root of the whisker. The growth direction of whiskers is [001] with a preferred orientation

of [321] on the Sn finish. The grain just below the whisker was found to be [210] orientation

[32].

Treuting and Arnold [33] found the growth direction of tin whiskers to be <100>, <101>,

[001].

2.4 Oxidation Effects

Localized cracking of the surface oxide layer has been proposed as a necessary step in the

nucleation of Sn whiskers in Sn electroplated films. A bright Sn-Cu film was electrodeposited

and inserted into an ultrahigh vacuum Auger system and cleaned using an Ar+ ion beam to

remove the oxide film. Then, the samples were aged in the Auger system chamber. Whiskers and

other features present at the time of the Ar+ ion cleaning left visible “shadows” on the surface.

During aging new whiskers appeared, identified by the lack of “shadows”, nucleated and grew.

Therefore, the presence or absence of an oxide film has a minimal effect on Sn whisker

nucleation and growth [34].

Crandall et al. [35, 36] determined that oxides were not necessary for tin whisker growth

[35]. The role of oxygen on whisker growth was studied by Crandall et al. by exposing thin films

of Sn on brass to 1 atm of pure oxygen. A 9-fold increase in whisker growth was observed in the

pure O2 conditions compared to ambient conditions. The O2 exposed samples contained a higher

fraction of SnO/SnO2 compared to atmospheric conditions [36]. While oxygen might increase

the growth of tin whiskers oxygen is not required for the growth of whiskers [35, 36].

16

2.5 Stress Effects

Tin whiskers were observed to grow profusely on electrodeposited tin-manganese. They had

a short incubation time of a few hours, followed by quick growth of many whiskers. The tin-

manganese coating was in a tensile residual stress state. This shows that tin whiskers can grow in

any residual stress condition, not only with a residual compressive stress [37].

Tu et al. [38] claim that tin whisker growth is a surface relief phenomenon of creep, driven

by a compressive stress gradient. The stress is intrinsically generated by the chemical reaction

between Sn and Cu to form Cu6Sn5. In order to produce a stress gradient, a break in the oxide

layer is required because the free surface of the break is stress-free. Electromigration can be used

for accelerated whisker growth for faster and easier studies [38].

Y. Mizuguchi et al. studied Sn-2% Cu plated on Cu with a Ni underplate between the Cu and

Sn. Twinning and recrystallization was observed in their samples due to mechanically induced

stress. The major plane family observed was {301}, where it was 20 times more prevalent

compared to as-deposited coatings. It was concluded that twin formation plays a critical role in

mechanically induced formation of whiskers [39].

Sn whisker growth was characterized in compressive, tensile, and zero intrinsic stress states.

After three months of incubation 12,000 whiskers/cm2 grew in tensile conditions, 16,000

whiskers/cm2 in compression, and 4,000 whiskers/cm2 grew in zero stress. This study shows that

whiskers grow under any stress state and it’s a question of when, not if, whiskers will grow [40].

Southworth et al. [41] evaluated the role of imposed strain on whisker growth in matte tin

and employed deformation states and environments that facilitated the development of

accelerated test methodology to provide results representative of whisker growth. Imposed strain

17

and elevated temperatures lowered the incubation time and accelerated Sn whisker growth on a

Cu substrate. A strain of 7.2 % and a temperature around 50 ºC proved to grow the most

whiskers [41].

2.6 Environmental Conditions

The Sn on brass case at 85% RH produced 6-fold greater whisker densities than Sn on brass

exposed to pure O2, which in turn produced 9-fold greater whisker densities than Sn on brass

exposed to ambient room temperature/humidity [42].

Fortier et al. [43] studied whisker growth with bright tin electroplated on a brass substrate

stored at ambient and 95% humid environment. The whiskers that grew in the 95% humid

environment had a polycrystalline structure [43]. The distinct grains occur when the whisker

changes the direction in which it is growing and it will appear to have a kink in its structure.

Horvath et al. [44] studied the effects of humidity on tin whisker growth. Nickel and

silver underlayers between the copper and tin layers were used as whisker mitigation methods.

Samples were stored at 40°C/95% RH, 105°C/100% RH, and 50°C/25% RH for 4200 h. The

40°C/95% RH had the longest whiskers, however the 105°C/100% RH condition had the highest

density of whiskers [44].

2.7 Compositional Effects

Thin coatings of high purity bright Sn, Sn-Cu, and Sn-Pb layers were electrodeposited on

phosphor bronze cantilever beam 3 µm, 7 µm, and 16 µm thick. Beam deflection measurements

proved that all electrodeposits had in-plane compressive stress with Sn-Cu having the highest

stress, and Sn-Pb having the lowest stress. In a few days the Sn-Cu coating developed whiskers

18

200 µm in length and hillocks 50 µm in length. The Sn-Pb coating remained unchanged. The

precipitation of Cu6Sn5 increased the compressive stress in the Sn-Cu coating, and Pb decreased

the compressive stress in the Sn-Pb coating as compared to the pure Sn coating. Over time, creep

decreases the compressive stress inside the coatings. Uniform creep occurs in the Sn-Pb coating

because of its equiaxed crystal structure. Local hillocks and whiskers occur in Sn and Sn-Cu

coatings because of their columnar crystal structure. Compact hillocks formed on Sn because it’s

columnar grain boundaries were mobile. The formation of contorted whiskers and hillocks occur

in Sn-Pb because its columnar grain boundary motion is impeded [45].

This problem of Sn whiskers was mitigated in the past by adding Pb to Sn. Interestingly

enough Pb forms whiskers on its own as well but when alloyed with Sn the whiskers are

inhibited. Pb doesn’t totally make the Sn whiskers stop growing but it stops them from growing

to any harmful length about 30 µm maximum. Tin whiskers are mitigated when alloyed with Pb

because Pb reduces the internal stress of the tin. The reason Pb reduced the internal stress of Sn

is because Pb changes the grain structure of the Sn from columnar grains to equiaxed grains.

Equiaxed grains can accommodate internal stress better, they have more grain boundaries thus

their diffusion path is longer, and they have more potential directions at which whisker can grow

from [43, 45].

Dimitrovska et al. [46] studied the effect of micro-alloying of Sn plating for the purpose of

mitigating the growth of Sn whiskers. The alloys that were studied were Sn-Bi, Sn-Zn, and Sn-

Cu electrodeposited onto a brass substrate using pulse plating. Alloying proved to increase the

incubation period of whisker growth due to the change of grain structure of pure Sn, which is

columnar, into equiaxed grains for alloyed Sn [46].

19

Snugovsky et al. [47] studied the effect of ionic contamination from component and

assembly on whisker formation. Samples were contaminated with chloride, sulfate, bromide, and

nitrate. It was found that the contamination increased the growth of whiskers [47].

Stafford et al. [48] studied whisker growth in Sn, Sn-Cu, and Sn-Pb. In a few days the

Sn-Cu deposits developed 50 µm hillocks and 200 µm, the pure Sn grew 20 µm compact conical

hillocks, and the Sn-Pb remained unchanged [48].

2.8 Growth Mechanism and Driving Force Models and Theories

Tin whiskers are known to be single crystals, which grow spontaneously. Whisker growth

can be regarded as a grain growth phenomenon, which is part of the recrystallization process as

described in Sec. 1. Boguslavsky et al. [49] believe that recrystallization can be applied to tin

deposits because they contain sufficiently high densities of lattice defects such as dislocations

and vacancies, which resemble bulk materials after plastic deformation. Tin’s recrystallization

temperature is around room temperature and therefore it occurs spontaneously. Grain sizes of

around 1-15 μm are required to grow whiskers as well as a mix of high and low energy grain

boundaries. Sn grain boundaries are pinned by surface grooves allowing abnormal grain growth

(whiskers). Frank-Read multiplication of dislocations was proposed as the mechanism of whisker

growth. Short and long-range diffusion moves Sn atoms from the high-energy grains to the low-

energy grains. IMCs increase the overall stress of the coating over time although they are not

necessary for whisker growth. Other factors affecting whisker growth include impurities and

their concentrations, surface oxies, particle inclusions, etc. [49].

Tu et al. [50] studied whisker growth on beta-tin (β-Sn) as a surface relief phenomenon of

creep. It is driven by a compressive stress gradient and occurs at room temperature. Spontaneous

20

Sn whiskers are known to grow on matte Sn finish on Cu. The compressive stress needed for the

growth is self-generated; no externally applied stress is required. The three indispensable

conditions of Sn whisker growth are: (1) the fast room-temperature diffusion in Sn, (2) the room-

temperature reaction between Sn and Cu or another element to form IMC which generates the

compressive stress in Sn, and (3) the breaking of the protective surface oxide on Sn. The last

condition is needed in order to produce a compressive stress gradient for creep. When the oxide

is broken at a weak spot, the exposed free surface is stress-free, so a compressive stress gradient

is developed, and creep or the growth of a whisker can occur to relax the stress. The growth of

Sn whiskers is from the bottom, not from the top, since the morphology of the tip does not

change with whisker growth. The crystal structure of Sn is body-centered tetragonal with the

lattice constant “a”=0.58311 nm and “c”=0.31817 nm. The whisker growth direction, or the axis

along the length of the whisker, has been found mostly to be the “c” axis, but growth along other

axes such as [100] and [311] has also been found. Comparing the whiskers formed on eutectic

SnCu and pure Sn, it seems that the Cu in eutectic SnCu enhances Sn whisker growth. Although

the composition of eutectic SnCu consists of 98.7 at % Sn and 1.3 at % Cu, the small amount of

Cu seems to have a very large effect on whisker growth on the eutectic SnCu finish. The origin

of the compressive stress can be mechanical, thermal, and chemical. Compressive stress is

generated by interstitial diffusion of Cu into Sn and the formation of Cu6Sn5 in the grain

boundary of Sn. When the Cu atoms from the lead frame diffuse into the finish to grow the grain

boundary Cu6Sn5, the volume increase due to the IMC growth will exert a compressive stress to

the grains on both sides of the grain boundary [50].

21

Sobiech et al. [51] found that local in-plane residual strain gradients exist around the root

of spontaneously growing Sn whiskers on the surface of Sn coating deposited on Cu. Observed

in-plane residual strain-depth gradients provide the driving forces for whisker growth [51].

Lee et al. [52] studied spontaneous growth of Sn whiskers by electroplating Sn into

phosphor bronze substrate. A biaxial compressive stress of 8 MPa was found to be the driving

force created by the formation of the Cu6Sn5 IMC. This biaxial stress gives rise to a strain normal

to the coating [52].

Thomas et al. [53] studied the diffusion of Sn within itself. Long-range diffusion of tin is

thought to occur in order to supply the amount of tin required to form a whisker. The objective of

this study was to use tin isotopes and secondary ion mass spectroscopy (SIMS) to evaluate the

room temperature self-diffusion of tin within whisker-prone tin platings – both bright and matte

tin finishes. The lattice diffusion coefficient calculated for the bright tin appeared to be larger

than the literature values measured for a near perfect tin lattice. Whiskers growing from the matte

tin appeared to originate from the plating surface and not from the substrate/plating interface.

The whiskers had a diameter equivalent to the grain size and no nodule growth prior to the

formation of whiskers was observed. In contrast, whiskers on the bright tin plating grew from

large nodules, which must have formed by a recrystallization process. Long-range diffusion of

Sn118 isotope parallel to the substrate was observed during TOF-SIMS analysis of whiskers

growing on the matte Sn120 plating adjacent to a Sn118 source. Large amounts of Sn118 were

detected in the whiskers but not in the plating itself. This suggests that the diffusion of Sn118

occurred through the grain boundaries (or over the plating surface) and not through the tin lattice

[53].

22

Objectives

The main objective of this thesis is to study an innovative Sn whisker mitigation strategy

that would successfully deter their growth by coating nNi onto Sn surfaces in electronic circuit

board assemblies. However, since Sn whiskers can take months to grow the first objective was to

determine the most favourable conditions under which Sn whiskers would grow. The second

objective was to analyze and characterize the whiskers that developed. The third objective was to

see if nickel was a suitable material for Sn whisker mitigation by plating nNi on top of the

solder. The final objective was to coat nNi onto the soldered parts in an electronic assembly. Due

to time restrictions only the first three objectives were completed and the fourth will be

completed as future work.

23

Chapter 3

Materials Preparation Techniques

3.1 Electroplating

Electroplating was used to deposit very thin Sn coatings of about 2 to 3 µm in thickness.

Electroplating is a deposition technique that uses current to reduce dissolved metal cations to

form a coating on the cathode, and the anode is the metal that is used to replenish the positive

metal ions in the plating bath that are being plated onto the cathode.

The type of Sn coating that was electroplated onto the Cu substrate is matte Sn since

matte Sn has been reported to be favourable in Sn whisker growth due to a coarse grain size of

around 1-3 µm in contrast to bright Sn where the grain size is less than one micron. An alkaline

Sn bath with stannic ions was used.

The materials used in the deposition stage included the Cu substrate (used as the

cathode), the Sn anode, and the stannate bath. The substrate that was used was 99.9% Cu foil,

0.254 mm thick, from Alfa Aesar. The material used for the anode was 99.8% Sn Alfa Aesar

CAS #7440-31-5. The bath composition was made of sodium stannate with sodium hydroxide,

specifically 142.1 g/L (0.67M) of NaSnO3xH2O, and 15 g/L (0.375M) NaOH [62].

The following electroplating procedure was employed. The electroplating procedure

includes the preparation of the cathode, anode, bath, electroplating, post-plating cleaning, and

finally storage. First, the cathode was prepared by cutting the Cu sheet into 3x3 cm squares.

Second, 2x3 cm area of the copper is used for plating while the other 1x3 cm and the back of the

coupon is insulated by Microstop. Third, the surface of the Cu was then lightly polished with a

24

fine grit sand paper. Fourth, sodium hydroxide, NaOH, was used, heated to 50°C, to clean the

surface of the Cu substrate of any organic material. Fifth, concentrated sulfuric acid was used to

remove any oxides present on the Cu surface. At this point the Cu cathode is ready to be

connected to the power supply. Next, the anode was prepared by cutting the Sn ingot into ½ inch

pieces and these pieces were placed into a titanium mesh basket, which was connected to the

power supply. The bath was prepared by adding 142.1 g/L (0.67M) of NaSnO3xH2O, and 15 g/L

(0.375 M) NaOH into one litre of deionized (DI) water. This bath was then brought up to a

temperature of 85 °C and stirred with a magnetic stirring rod. Right before plating, the power

supply current is set so that the current density was 25 mA/cm2, for a 6 cm2 area; therefore, 150

mA was needed. The initial current density was taken from Miller [62] and modified

experimentally to get the best results with electroplating time and coating quality. The anode and

cathode are put into the bath and are connected to the power supply while the bath is kept at 85

°C. The power supply was turned on for 4.5 min to get a thickness of approximately 3 µm. Upon

completion of plating, the cathode was taken out of the bath and was rinsed with DI a few times

and air dried. The samples were stored in sealed plastic containers at ambient conditions to

prevent getting disturbed or contaminated prior to environmental conditioning in an oven at

various environmental conditions for different samples. Figure 5 shows the plating set-up in the

lab.

25

Figure 5: Electroplating setup for plating Sn onto a Cu substrate.

3.2 Storage Conditions

After electroplating Sn onto a Cu substrate the coupons were put through various

environmental conditions and stored at ambient conditions in sealed plastic containers. The first

condition involved control samples stored at ambient temperature and humidity. The second

environmental condition involved high temperature and humidity, an industrial standard of 85 °C

and 85% relative humidity. The third environmental condition was thermal cycling from -55 °C

to 85 °C. The high temperature and humidity, and the thermal cycling environmental tests

required a specialized oven and were conducted at Celestica’s facilities. These environmental

condition tests were used to determine the fastest way of growing whiskers to see if the nNi

coating could prevent whiskers from penetrating the metal cap. The nNi/Sn/Cu samples were

only put through the high temperature and high humidity conditions where the propensity for Sn

whisker growth on bare Sn/Cu samples was greatest.

26

3.2.1 High Temperature and Humidity Test

The test conditions required the temperature and humidity to be at 85 °C and 85% RH,

and the amount of time that the samples were in the chamber was about 1000 hours. This

environmental condition proved to be the most favourable condition for whisker growth and was

mainly used for the subsequent experiments where Sn whisker growth was required. Figure 6 a)

and b) demonstrate the evolution of the Sn surface before and after the 85 °C and 85% RH

environmental condition. In figure 6 a) the surface had a smooth surface with few surface

imperfections. Figure 6 b) shows there are visible hillocks growing from the surface as well as a

few small, but still visible Sn whiskers.

Figure 6: A) Initial Sn surface after electroplating B) post heat treatment Sn surface.

3.2.2 Thermal Cycling

The temperature at which the samples were cycled ranged between 85 °C and -55 °C.

The samples were in the chamber for 500 hours, which turned out to be about 1500 cycles. Each

27

cycle consisted of 20 minutes in total, 9.5 minutes of dwell time in the -55 °C section followed

by 0.5 minute to transport the stage to the 85 °C chamber for another 9.5 minutes of dwell time

after which it took another 0.5 minute to go back down to the low temperature section of the

oven. Figure 7 shows the samples on the top right hanging from the metal cage inside the oven

chamber.

Figure 7: The thermal cycle set-up inside one of the ovens showing the Ni/Sn/Cu samples.

28

Chapter 4

Analysis Techniques

4.1 Optical Microscopy (OM)

Optical microscopy uses visible light and a series of lenses to magnify images of desired

samples. Cameras are used to capture the images from the microscope. Optical microscopy is a

very inexpensive way to analyze samples, and is usually the first analysis method used after

preparing samples. This saves time and resources because Sn whiskers can usually be detected

by optical microscopy under special lighting conditions. The optical microscope was used to

screen samples for whisker growth before using the SEM. The models that were used were

Olympus PME 3 and Reichert MeF3 at magnifications ranging from 3x to 200x.

4.2 Scanning Electron Microscope (SEM)

The SEM is capable of inspecting samples at magnifications of up to 500,000x and

capturing their images by scanning a beam of electrons on the desired area of the sample. A

resolution of about 1 nm can be obtained with an SEM. Usually the samples are observed in high

vacuum, but there are SEMs that can operate in low vacuum and even in atmospheric conditions.

Different signals are produced by the interaction of the electron beam with the atoms in the

sample. Secondary electrons (SE) are the primary method of detection and image creation. Other

signals include back-scattered electrons (BSE), characteristic X-rays (EDS), light or

cathodoluminescence (CL), sample current, and transmitted electrons.

Most signals originate from the surface atoms or from the atoms very close to the surface

in the sample usually from a depth not exceeding 5 µm, this is called the interaction volume of

29

the beam in the sample which has the shape of a tear drop. SEs are emitted from a sample by the

non-elastic scattering caused by the electron beam interacting with the electrons orbiting the

atoms in the sample. BSEs are the elastic scattered electrons from the incident electron beam,

which are reflected by the atoms in the sample. BSEs are highly sensitive to the atomic number

of an element therefore they are used to distinguish between different elements in the sample. A

heavier atom will reflect more electrons back to the surface and will show up brighter in a BSE

image.

The main advantages of the SEM are its high magnification, many different signal

outputs which enables a wide variety of analysis techniques to be performed in one machine, and

the large depth-of-field (DOF). The magnification can range from 10x to 500,000x making the

SEM over 250x more magnification power than an OM. The large depth of field makes images

appear three dimensional.

Sample preparation and the size of the sample are the two main disadvantages of the

SEM. The samples that can be observed in an SEM have to be relatively small depending on the

specific SEM being used. Typically samples don’t exceed 1 inch by 1 inch in size. Therefore, the

first step in sample preparation is the cutting of the bulk material, which can alter the material

properties of the sample being observed. Electrical conductivity is a requirement for imaging,

otherwise electrons build up in the sample making imaging very difficult and cause damage to

the specimen. If the sample is nonconductive then it needs to be coated with a thin layer of

conducting material. Materials most often used for making the surface of the sample conductive

are graphite, gold, platinum and palladium. The surface also needs to be grounded so that the

electrons do not build up just on the surface. Grounding is achieved by attaching the sample to a

30

metal stub with carbon tape and having the tape touch the surface of the sample giving the

electrons a path from the surface to the ground.

The SEM was used to observe samples for tin whiskers at high magnification and to

classify their characteristics such as width, length, and density. The SEm models that were used

were the Hitachi S-2500, S-570 and mainly the S-4500 at magnifications ranging from 35x to

5000x.

4.3 Energy Dispersive Spectroscopy

Energy dispersive spectroscopy (EDS) was used to analyze the composition of the tin

coating, surface features such as hillocks, and tin whiskers. EDS is an analytical technique used

for chemical characterization and elemental analysis of materials. It works by detecting

characteristic X-rays emitted from a specimen. Every element has a unique atomic structure and

a unique set of peaks on the X-ray spectrum allowing for chemical identification. In order to

extract the characteristic X-rays from a specimen a high energy beam of electrons, in the case of

an SEM, is focused on the desired specimen. The beam excites the inner electrons and promotes

them to a higher energy level. An electron from a higher shell drops down to the lower energy

level hole, left behind by the excited electron, and the difference in energy is released as X-rays.

The difference in the energy between two shells in each element is unique allowing the element

to be identified. The detector measures the magnitude of the X-ray’s energy and the number of

X-rays that are emitted. A sensor detects the X-rays and converts their energy into voltages and

this information is sent to a pulse processor, which measures the X-rays and sends them to the

analyzer for data visualization and analysis.

31

Chapter 5

Results and Discussion

5.1 Electroplating

Plating Sn onto copper was not a trivial matter. If the current density, temperature of the

bath, cleaning after plating, or plating thickness is not optimal then the quality of the Sn coating

will be poor and Sn whiskers might take years to grow if at all. The optimum settings for whisker

growth were found to be at 25 mA/cm2, the temperature of the plating bath at 85 °C, cleaning

with DI water and air after plating, and a thin coating of 3 µm or less. These parameters were

found through literature and experimentation trials [54, 55, 62, 69].

5.1.1 Electroplating Current Density

The first electroplating property that was experimented with was the plating current

density (i). The industry standard current density for tin electroplating in an alkaline bath is

between 0.5 mA/cm2 and 3 mA/cm2 [54, 55]. Higher current densities are desired for faster

plating rates; however, above a certain current density the quality of the coating suffers. The

coating will not be continuous, and dendritic growth takes place, which is undesired as seen in

Figure 8. The current density with the best quality and quickest plating time was found to be

about 25 mA/cm2 with the resulting surface coating shown in Figure 9. The plating rate at 25

mA/cm2 of current density is approximately 1 µm of tin for 90 s of plating.

32

Figure 8: Sn Dendritic formation when plating with too high of a current density, at 50 mA/cm2

Figure 9: Surface of a Sn deposition a day after plating A) 100x B) 300x magnification

33

5.1.2 Temperature Effects

Temperature was the second parameter to be tested to see what role it had in the plating

of tin on copper. The industry standard temperature for plating Sn from an alkaline solution is

between 60-85 °C [54, 55]. Three temperatures were tested, 75 °C, 85 °C, and 95 °C. A

temperature of 75 ˚C yielded the best plating quality. It was found that at 95 °C the bath would

evaporate too quickly. This parameter was mostly derived from literature as it had little

discernible differences visually between the three temperatures when tested experimentally.

Figure 10 A, B, and C show the effects of bath temperature on the plating structure of the

surface. Some minor surface deformations can be observed and more so at the higher

temperatures.

Figure 10: Sn surface after plating in three temperatures at 25 mA/cm2 A) 75 °C, B) 85 °C, C) 95 °C.

5.1.3 Surface Cleaning

Cleaning the sample after plating also had a potential role in the formation of Sn

whiskers. Snugovsky et al. [56] studied the effect of ionic contamination on the formation of Sn

whiskers. Their study used chloride, sulfate bromide, nitrate, and three fluxes having different

halide contents to contaminate the surface of soldered electronic components. These electronic

34

components were then subjected to a 85°C/85% RH test. Snugovsky’s et al. study concluded that

whiskers grew from the contaminated Sn surfaces. Five different cleaning methods were tested,

air drying without rinsing, rinsing with tap water, rinsing with DI water, rinsing with alcohol,

and finally rinsing with salt water. Figures 11, 12, 13, 14, and 15 show the SEM images of the Sn

samples cleaned after the plating step using air drying without rinsing, rinsing with tap water,

rinsing with alcohol, rinsing with salt water, and finally rinsing with DI water respectively.

Whiskers were found to grow from the air dry method right after plating and alcohol rinsed

samples as seen in Figure 11 and 13; although, these results could not be reproduced. Due to the

consistent reproducibility of the DI rinse this method was chosen as the method for cleaning

samples after plating.

Figure 11: Dry with Air as is right after plating at 25 mA/cm2 A) 100x, B) 500x magnification

35

Figure 12: Rinsing with tap water at 25 mA/cm2 A) 25x, B) 1000x magnification

Figure 13: Rinsing with alcohol at 25 mA/cm2 A) 1000x, B) 2500x magnification

36

Figure 14: Rinsing with salt water at 25 mA/cm2 A) 300x, B) 1000x magnification

Figure 15: Rinse with DI water at 25 mA/cm2 A) 500x, B) 1000x magnification

37

5.1.4 Plating Thickness

The thickness of the Sn coating affects the speed at which whiskers grow therefore a

coating closest to 1 µm is desired [57]. By analyzing the cross sections of Sn coatings varying

from 1-10 µm, the 3 µm coating of Sn on Cu was selected due to the speed of whisker growth

and ease of analysis. The key reason that the coating was not made thinner than 3 µm was

because it was very difficult to prepare metallurgical Sn samples at below 2 μm plating thickness

with the techniques that were available at the time. Figure 15 shows an image of a cross-

sectioned sample that was observed with an optical microscope. The thickness of the coating is

observed to be around 2 to 3 µm. Figures 17, 18, and 19 show the different thicknesses that were

achieved by plating using different current densities for five minutes.

Figure 16: Sn deposit plated at 75 °C and 25 mA/cm2 cross-section thickness measurements with an optical microscope

38

Figure 17: a) 500x plated at 75 °C and 45 mA/cm2 for 5 min b) 1000x plated at 45 mA/cm2 for 5 min

Figure 18: a) 500x plated at 75 °C and 35 mA/cm2 for 5 min b) 1000x plated at 35 mA/cm2 for 5 min

Figure 19: a) 500x plated at 75 °C and 25 mA/cm2 for 5 min b) 1000x plated at 25 mA/cm2 for 5 min

39

Cheng et al. [57] performed studies on Sn film of different thicknesses to determine the

optimum thickness for enhanced whisker growth. They found that a coating of around 2 µm

resulted in the growth of the most whiskers. It was postulated that Sn atoms flowed through the

vertical grain boundaries to the interface of the whisker boundary. Therefore, the coating that

provides the least amount of grains, a single grained layer, will grow whiskers the fastest. Even

though the present study’s plating thickness was chosen to be around 3 µm, the actual plating

thickness was measured to be about 2 µm, which is coincidentally the more effective thickness to

grow whiskers according to Cheng et al. [57].

5.1.5 Sn coating Surface Defects

There are various types of surface defects on Sn and Sn alloy electrodeposits: whiskers, odd-

shape eruptions, and hillocks. Typically, whiskers and odd-shape eruptions are observed when

the grain boundaries of the Sn electrodeposit appear to be immobile (e.g., Sn–Cu deposits) while

hillocks are observed when the grain boundaries are mobile (e.g., pure Sn deposits). The

whiskers/hillock growth mechanism is diffusional grain boundary creep that feeds material to the

base of the whisker/hillock grain. The columnar structure typical of many electrodeposits

exacerbates the whisker problem. Pb additions breakup columnar structures into equiaxed

structures that enable a more uniform creep response in the deposit interior to relieve stress [70].

40

5.2 Storage Conditions

The next step in the experimentation phase was to put these optimized samples for

whisker growth through different environmental conditions at different temperatures and

humidity and for a set amount of time and store them at ambient conditions.

All of the steps in a product’s lifecycle can induce stresses to its components and the

environmental conditions where the product is stored are very important for stress development.

In a real world scenario, most electronics are stored at room temperature and ambient relative

humidity; this is known as an ambient condition. Another source of stress that induces whiskers

is the oxide layer that is exacerbated by humidity. Therefore, conditions that have higher

humidity can grow whiskers faster because the oxide layer helps to create a compressive stress

gradient in the Sn coating. Horvath et al. [58] studied the effects of humidity and temperature on

the growth of whiskers at 40 °C/95% RH, 105 °C/100% RH, and 50 °C/25% RH. They found

that most whiskers grew in the 105 °C/100% RH condition and at 50 °C/25% RH whiskers grew

the least. Osenbach et al. [59] studied whisker development and found the optimum conditions

for whisker growth to be at about 60C/87% RH.

Vianco et al. [10] believes that the growth mechanism of tin whiskers is dynamic-

recrystallization; therefore, elevated temperatures, up to a point, should increase the speed at

which the recrystallization process occurs. The optimum temperature for whisker growth has

been reported to be in the range 50 to 85°C [58, 59]. Once the product is in service the

temperature of the component can fluctuate anywhere between -65 °C to 175 °C depending on

the environmental and service conditions. The temperature changes induce thermal stress in the

components because the parts are made of different materials, which have different coefficient of

thermal expansion. The CTE values of Sn, Cu, Ni, and Al are shown in table 2 [59, 60, 61].

41

Table 2: CTE values of Sn, Cu, Ni, and Al.

Material Sn Cu Ni Al

CTE ppm/°C 23 17 13 23.1

Three environmental conditions were chosen in which to store the Sn/Cu samples. The

ambient condition was the first environmental condition, which took place at room temperature

and ambient relative humidity for control Sn/Cu samples. The second condition was thermal

cycling, which was conducted at Celestica. And the third condition was an elevated temperature

and humidity environment, which was also conducted at Celestica.

5.2.1 Ambient Storage

The results for the Sn/Cu samples that were stored at ambient conditions are presented.

After the samples were prepared with electroplating and cleaned they were stored in a

Tupperware container under ambient conditions. These samples were inspected every week

visually, and with an optical microscope to look for whiskers. Figure 20 shows the initial surface

right after the Sn was electroplated onto the Cu substrate. No whiskers were found after a year of

ambient storage; however, studies in the literature achieved whiskers within two weeks of

ambient storage.

42

Figure 20: A) Initial Sn plated surface, immediately after plating B) Sn plated surface after a year in ambient condition

Even though the samples in the ambient condition did not grow any whiskers after one year of

storage, hillocks were observed on the surface of these samples. This result was surprising since

Miller [62] observed Sn whiskers growing after only 10 days for Sn/Cu samples in room ambient

condition storage. The whiskers had lengths varying from 10-250 µm. Also, Han et al. [63],

achieved whisker growth at ambient conditions after four years on Cu lead frames, but not on

Alloy42 lead frames. This was attributed to the absence of the Sn/Cu IMC for the Alloy42 lead

frame.

5.2.2 Thermal Cycling

This section reports on the results after the thermal cycling tests were conducted. These

samples were put through 500 cycles between -55 °C and 90 °C. The samples were held at each

temperature for 9.5 min and took 30 sec to travel from each chamber resulting in the total time of

approximately 20 min for each cycle. There was no Sn whisker growth after the thermal cycling

43

of the Sn/Cu samples; however, there were hillocks evident from the large bulges on the Sn

coated surface. Dittes et al. [60] performed thermal cycling at various conditions on Sn//Cu, and

Sn//FeNi42 samples. They found diminishing Sn whisker growth rates when increasing the

number of cycles. Also, a linear relationship between whisker length and ΔT was found [60].

Figure 21: before and after images of thermal cycling Sn coated samples A) before 250x, and B) after 300x

44

5.2.3 High Temperature and High Humidity Test

The high temperature and humidity samples were put into an oven at Celestica set at

85 °C and 85% RH for three months. This was found to be the most reliable condition to get Sn

whiskers to form on Sn/Cu samples and is also the standard environmental testing condition in

the electronics industry. Figure 22 shows many hillocks and Sn whiskers formed during and after

the 85°C/85%RH condition. Figures 22 to 25 show the different whiskers that were seen after

going through the 85°C/85%RH condition. Sn whiskers grew in all shapes and sizes from

straight whiskers in Figures 22 and 24, to thicker and kinked whiskers as seen in Figures 23 and

24. Complex shaped, or kinked, whiskers can grow from controlled environmental conditions

such as a high temperature and high humidity condition. It is still not well understood why

whiskers change direction when growing and need to be studied further [44].

The whisker density, mean length, maximum length, minimum length and hillock density

were obtained from a few images that were then averaged. These results are summarized in

Table 3. A sample table of the data and a sample image from the measurements is provided in

appendix I.

Table 3: Whisker density, length, and hillock density measurement results

Whisker Density 338 whiskers/mm2

Whisker Mean Length 22.3 µm

Whisker Max Length 46.8 µm

Whisker Min Length 5.5 µm

Hillock Density 845 hillocks/mm2

45

Figure 22: A few different whiskers as well as hillocks on a Sn coating surface from an 85°C 85RH test

Figure 23: Close-up of whiskers on a Sn coating surface from an 85°C 85RH test

46

Figure 24: Close-up of a kinked whisker on a Sn coating surface from an 85°C 85RH test

Figure 25: Close-up of a straight whisker on a Sn coating surface from an 85°C 85RH test

47

The values for whisker density and length in table 3 are comparable to those found in the

literature. Snugovsky et al. [56] put their samples through 85°C and 85% RH conditions for 500,

1000, and 1500 h in order to grow whiskers. After 1500 h whiskers grew to a maximum length of

65 µm, and the density of whiskers after 1000 h of exposure was about 900 wh/mm2.

Fang et al. [64] reported a mean whisker length of 24, 25.7, and 26 µm after 8, 13, and 18

months of storage respectively. These whisker length were obtained after plating 5 µm of bright

tin over brass (type 260), then annealed one week after plating at 150 °C for 2 h followed by a 60

°C/95% RH for 2 weeks. The coupons were stored in ambient conditions after the heat

treatments for 8, 13, and 18 months. These whisker lengths are comparable to the ones that were

grown in the present study, however the time frame of the whisker growth in Fang’s et al. study

took considerably longer. This longer growth time of the Sn whiskers can be attributed to the

thickness of the Sn coating of 5 µm in Fang et al. study as opposed to the 2-3 µm in the present

study.

Horvath et al. [58] achieved average whisker lengths of 30 µm, 50 µm, and 35 µm after

4250 h in conditions of 105 °C/100%RH, 40 °C/95%RH Ag, and 40 °C/95%RH Ni respectively.

The whisker densities achieved at 105 °C/100%RH Ag, 40 °C/95%RH Ag, and 40 °C/95%RH

Ni conditions were 150,000 wh/mm2, 3000 wh/mm2, and 750 wh/mm2 respectively. The

maximum whisker length at 105 °C/100%RH Ag, 40 °C/95%RH Ag, and 40 °C/95%RH Ni

conditions were 50 µm, 82 µm, and 45 µm respectively. These whisker lengths are comparable

to the whiskers that were grown in the present study.

48

5.2.4 EDS Analysis of Sn coatings and Sn whiskers

The EDS analysis of the Sn whiskers of various Sn surfaces, hillocks, and at different

areas of a Sn whisker are shown in Figures 26 (a-f). The EDS area analysis shows that the Sn

coating is mostly made of Sn with trace amounts of Cu. The reason that Cu gets picked up is

because the Sn coating is very thin, and most likely not uniform, ranging from 2-3 µm in

thickness, plus there might be some Cu that diffuses into the Sn as they form the Cu6Sn5 IMC

together. The intermetallic compound, Cu6Sn5, forms at the Cu-Sn interface and grows

preferentially in the grain boundaries of the Sn coating; therefore, making Cu appear close to the

surface of the Sn coating. Furthermore, the interaction volume of the electron beam from the

SEM penetrates the sample to about 1 µm making the materials that are within the coating

detectable. With all of these factors combined the substrate in such a system will be partially

detected. No other element was detected with the EDS in these Sn/Cu samples.

The hillocks and surface features showed more Cu presence than the surface itself

measured with an area analysis. The Sn whisker also showed more Cu than the surface. The

whisker was analyzed in three different areas using spot analysis, at the base, in the middle, and

at the tip. The middle had the least copper and the tip had the most. The reason there is trace

amounts of Cu in the Sn whiskers is due to the Cu substrate diffusing into the Sn coating layer.

Copper diffuses relatively quickly through the bulk of tin and especially fast in the grain

boundaries of Sn. Therefore, when Sn is plated onto a Cu substrate some of the Cu will always

be found on the surface of the Sn coating and thus in the Sn whiskers when they end up growing

from the Sn surface. The amount of material that is required for growing whiskers is much more

that the volume in the immediate area; therefore, material diffuses from long range to the

location where the Sn whisker is growing.

49

Figure 26: EDS analysis of Sn samples after 85°C and 85% RH treatment A) Sn surface, B) Sn Hillock, C) Sn Hillock cluster, D) Base of Sn Whisker, E) at the body kink of Sn Whisker, F) at the tip of a Sn whisker.

50

5.2.5 Whisker Growth Mechanism

The findings from the present study seem to support the DRX model of whisker growth.

According to Vianco et al. [11] DRX is controlled mainly by three properties strain rate,

temperature, and initial grain size. A low strain rate, high temperature, and a small initial grain

size promotes cyclic DRX that grows whiskers. A small initial grain size, high temperature and

low strain rate are prime conditions in which whiskers grow as explained by the DRX model.

The low strain rate allows the deformation to build up in the bulk of the material mainly due to

dislocation build up at the grain boundaries this is also enhanced by having small grains [28]. In

the current study the stain rate was small since no external stresses were applied, the temperature

was high (especially for a low melting point metal like Sn) and the grain size was also small at

around 1-3 um which is similar to the thickness of the Sn coating using nominal conditions that

provide this correlation between the coating thickness and grain size and morphology. Having a

low strain rate also allows for the DRX process to happen multiple times as grains get refined

and build up additional deformation from the surrounding grains. At the high temperature and

humidity condition Sn has a homologous temperature of TH = 0.71, this promotes anomalously

fast diffusion that Sn is capable of due to its low melting temperature.

The small grain sizes also provide more potential grain nucleation sites for Sn whiskers to

grow [31-33]. The internal stress that drives the DRX process is caused by many factors that

could be seen in Figure 27. These stresses induce deformation that is the driving force behind

DRX, mostly by dislocation pile-up at the grain boundaries. In the present study the major

sources that contributed to the driving force of DRX are: the Cu substrate that in turn forms an

IMC with Sn [22-25], the oxide layer due to high humidity [34-36], and small grain size [28-32]

because the dislocations saturate faster in smaller grains

51

5.3 Whisker Root Cause Analysis Figure 32 shows a summary of the potential contributing factors that lead to Sn whisker growth. These factors are then ranked

by their severity, using literature and this study, in which they cause the Sn whiskers to grow and should be dealt with first.

Surface Damage

Handling

Forming, bending leads

Solder dip

Conformal coating

Re-flow

Roughness

Composition, CTE

missmatch

Underlayer

Sn Whisker

Formation

Temperature

Tin Coating Characteristics

Humidity

Processing Method External Stresses

Electronic Assembly Process Substrate Material Environmental Conditions

Contamination

Annealing

Impurities

IMC Formation

Cooling Rate

Solution Type

Grain Size

Temperature

Grain Morphology

Alloying elements

Oxidation

Current Density

Impurities or Additives

External Pressure

Current and Voltage

Corrosion

Figure 27: Fishbone diagram of the factors that contribute to the growth of Sn whiskers

52

Many factors are known to affect Sn whisker growth. Sn whisker growth is a stress

relaxation mechanism where grains grow out of the surface of the material in order to relieve

internal residual stresses. Therefore, any factor that contributes to the internal stress of Sn will

also encourage whisker growth.

The fishbone diagram gives many of the factors that contribute to the growth of Sn

whiskers. Sn whisker growth is not due to a single factor but many factors that each contributes

towards whisker growth. The main reason why whiskers grow is due to a material that has a

predisposition to grow metal whiskers; therefore the only way to get rid of whiskers is to avoid

the following pure metals that are known to be susceptible to whisker formation: Sn, Cd, Pb, Zn,

Ag, Au, Al, In.

The second factor that has a major effect on whisker growth is alloying. Pure Sn is more

prone to grow whiskers than alloyed Sn; therefore, this is a simple parameter to manipulate in

order to mitigate whisker growth.

Thirdly, the coating thickness of the Sn coating plays a significant role in the formation

of whiskers. Thin coatings have more internal stress than thick coatings because internal stress

can be accommodated more effectively by thicker coatings.

The next factor that induces a significant amount of stress into the coatings is IMC

growth due to the coating and substrate reacting; therefore, it is best not to use copper as a

substrate with Sn, instead Alloy 42 has been shown to mitigate whisker growth. If copper is

unavoidable, an underlayer for the solder can be used, such as a coating of Ni on a Cu substrate,

to mitigate whiskers.

53

The roughness of the substrate is also known to play a role in whisker formation. Smooth

substrates have been shown to grow more whiskers than rough substrates.

Next, any external pressure that can be alleviated from the coating surface could reduce

the probability of whiskers from forming. Any surface damage should be avoided such as

scratches or dents of the Sn coatings.

Some impurities can encourage whisker growth such as Cu as it can form an IMC in Sn

and increase the Sn coating’s internal stress. The cleanliness of the Sn surface has been shown to

affect whisker growth. A cleaner surface seems to mitigate whiskers. A contaminated surface

with Cl−, SO42−, NO3−, and/or Br− has been shown to promote whisker growth.

The temperature where the components are being stored or operated at should be close to

room temperature to avoid promoting whisker growth. The humidity should be as low as

possible to mitigate whisker growth.

The grain size of the Sn coating plays a role in the formation of whiskers. However,

controlling the grain size of the coating is a difficult task therefore this factor ranks fairly low on

mitigating Sn whiskers.

The grain morphology plays a role in the formation of whiskers. Coatings with equiaxed

grains grow whiskers slower because the grain boundary diffusion path is longer than a coating

with columnar grains where the diffusion path is short and leads straight to the surface.

The current and voltage that runs through the soldered parts has very little effect on Sn

whiskers and most of the time cannot be controlled for the mitigation of whiskers.

54

The oxide layer of the coating is a source of internal stress therefore oxidation can

exacerbate the growth of whiskers; however, getting rid of the oxide layer is almost impossible

making this factor the least controllable.

5.4 Nickel Plated Tin Samples

This section describes the results of the Sn/Cu samples that were plated with nNi. The

nNi plating process was outsourced to Integran Technologies. Their process for this project

involved electroplating the nNi onto the Sn/Cu samples that were provided to them by the author.

To get the nNi grains in the nanometer range an electroplating technique called pulse plating was

used [65]. Three different nickel coatings with different coating thicknesses and grain sizes were

tested for tin whisker penetrability. The three types of nickel coatings were plated at Integran

using their processes that would yield the following: 50 µm thick coating and 50 nm grains, 50

µm thick coating with 10 nm grains, and finally 10 µm thick coating with 50 nm grains. These

nNi/Sn/Cu samples were then placed in the conditions that produced the most whiskers in the

shortest amount of time. This condition was the 85 °C and 85 % relative humidity as whiskers

only grew from the Sn/Cu samples in this condition. SEM images of the nNi/Sn/Cu samples are

shown in Figures 28, 29, and 30. The nNi surface is observed to have a cauliflower-like structure

from the plating process, which does not correspond to their grain size.

SEM was used to check the quality of the nNi coating after the environmental testing plus

a year of storage in ambient conditions. From the images in figures 26 to 28 there are no Sn

whiskers growing that can be observed on the nNi surface. Therefore, with these observations it

can be concluded that these nNi coatings mitigated the growth of whiskers from the Sn/Cu

samples. This leads to the future work for this project where an electronic assembly would be

selectively coated with nNi into the soldered parts using an electroless plating method.

55

Figure 28: SEM Image of surface of Nickel coated sample 10µm and 50nm grain at 500x magnification

Figure 29: SEM Image of surface of Nickel coated sample 50µm and 50nm grain at 500x magnification

56

Figure 30: SEM Image of surface of Nickel coated sample 10µm and 10nm grain at 500x magnification

All three nNi coated samples showed no sign of whisker penetration after an

environmental condition at 85 °C and 85% RH and a year in storage. This nNi coating is

therefore successful at mitigating Sn whisker growth. The coating thickness, of 10 to 50 µm is

however in excess to what is theoretically needed to mitigate Sn whiskers. A polycrystalline

nickel (pNi) coating of around 200nm in thickness should be sufficient in mitigating whisker

growth as shown in the literature [66-67]. A nNi coating could reduce the thickness of the

coating even further; therefore use less material and have a faster plating time.

Crandall et al. [66] used hard metal cap layers to prevent the growth of Sn whiskers.

First, pure Sn films of 150 nm were deposited on Si wafer substrates. Second, metal cap layers of

Pt, Au, Cr, and Ni were sputter deposited on half of each sample with a thickness of 50, 100, and

57

200 nm. Then, the coupons were incubated in ambient conditions. After 100 days of storage the

Au and Cr coatings were penetrated by the Sn whiskers; however, the Pt and Ni coatings did not

show any penetration after 100 days except for the thinnest 50 nm Ni coating, which was

penetrated by the Sn whiskers. The rest of the Ni coatings, 100 nm and 200 nm, were again

observed after 510 days of storage showing no whiskers penetrating [66]. This study showed the

viability of a very thin pNi coating of at least 100 nm in thickness can be used for the prevention

of Sn whiskers.

Kim et al., [67] treated the surface of Sn platings with Ni, Au, and Pb for the prevention

of Sn whiskers. Ni was flash plated with thicknesses of 50 nm and 200 nm onto Sn/Cu and

Sn/Alloy42. The 50 nm Ni coating was still susceptible to whisker growth whereas the 200 nm

Ni coating showed no whisker growth after over two years [67]. This paper also shows that there

is a certain thickness of a Ni coating required in order to block Sn whiskers from penetration.

The minimum thickness for a pNi coating seems to be around 100-200 nm to prevent Sn

whiskers from penetrating. However, since nNi is significantly stronger that pNi, therefore the

thickness of a nNi coating could be even thinner than the pNi to prevent Sn whiskers from

growing [66-67].

The nNi coatings that were used in the current study had a grain size of 50 nm and 10 nm.

A material with a 10 nm grain size has theoretically reached [68] its highest strength due to grain

refinement. Nickel with 10 nm grains has been shown to have about ten times the strength of

nickel with a grain size of 4 µm [68]. Therefore, a coating with a thickness of only 10-20 nm nNi

should be sufficient to mitigate Sn whiskers, granted that it is a continuous coating. Also, a nNi

58

coating thickness of 10 to 20 nm the plating process roughly takes about 2 to 4 seconds

respectively to coat using industrial standard electroless deposition.

5.3.1 EDS Elemental analysis of nickel coated samples

The following section presents the elemental analysis of the nNi coated samples using

EDS. Figures 31, 32, and 33 show the EDS results for the 10 µm thick coating with 50 nm

grains, 50 µm coating and 50 nm grains, and finally 50 µm coating with 10 nm grains

respectively. The EDS analysis was done over a relatively large area, about 50x50 µm, on each

sample to get a good average for each sample’s surface chemical composition. All three of the

samples showed almost 100% nickel. However, there are some trace amounts of sulfur and

chloride impurities from the plating process and from the cleaning steps. Also, seen in the EDS

analysis for all three of the samples is the oxygen that shows up on the nNi layer most likely due

to some surface impurities from the plating process.

59

Figure 31: A) SEM image, B) EDS analysis of the selected surface area of the 10 µm thick coating with 50 nm grains.

60

Figure 32: A) SEM image B) EDS analysis of the selected surface area of the 50 µm thick coating with 50 nm grains.

61

Figure 33: A) SEM image B) EDS analysis of the selected surface area of the 50 µm thick coating with 10 nm grains.

62

Chapter 6

Conclusions and Recommendations

The growth of Sn whiskers is a spontaneous phenomenon that occurs in thin tin coatings

irrespective of the composition or environmental factors. Sn whiskers grow inevitability in Pb-

free solders and surface finishes; it is not a question of if but when. Tin whiskers are a fact of

nature and can only be mitigated and not prevented outright from the current knowledge of the

situation. In the past Pb was used to mitigate Sn whiskers and only around 3% Pb was needed as

an alloy addition in Sn in order to stunt their growth to a maximum length of about 20 µm.

However, since Pb was banned by the EU in the RoHS initiative in the beginning of 2006,

alternative methods to mitigate Sn whisker growth need to be developed.

This study found that whiskers grew the most when the Sn was plated less than 3 µm

coating on a Cu substrate from an electroplating bath that yields a matte finish. Also, the

electroplating bath that yielded the most whiskers was an alkaline bath with stannate ions, stirred

at 75 °C, and plated with a current density of i = 25 mA/cm2. The environmental condition that is

most prone to grow whiskers was found to be 85 °C and 85% relative humidity. The grain size of

the coating was achieved with plating a thickness that would correlate with the grain size and in

turn to the diameter of the whiskers. The size of the whiskers, 2-3 µm, in this study showed that

the coating that was plated should have a similar grain size at 2-3 µm and this in turn agrees well

with the thickness of the coating which was measured to be between 2-3 µm.

An effective way of mitigating whisker growth is with a metal coating usually using Ni.

This study found that an electroplated nNi coating prevents Sn whiskers from growing through

for over at least a year. One of the most promising new mitigation methods is selective nNi

63

coating of Sn soldered joints, and Sn finishes. This project showed that thin nNi can prevent the

penetration of Sn whiskers and is therefore a viable option for electronics manufacturers as a

whisker mitigation technique.

Chapter 7

Future Work

There is still much to be done in order to optimise nNi as a coating for the prevention of

Sn whiskers for the electronics industry. First, the thickness of the nNi coating needs to be

optimized to get the most efficient use of the material for the most amount of protection. The

minimum pNi coating thickness for preventing Sn whiskers penetrating is known to be about 100

nm. Since nNi is stronger than pNi the minimum thickness of the coating should be less than 100

nm and would therefore make electroless Ni-P plating a very quick process provided that the

coating is continuous at such small thicknesses. Selectively coating nNi onto Sn solder joints and

Sn finishes in an electronic assembly and still be functional afterwards is a requirement.

Electroless plating is the most promising process to achieve this requirement and needs to be

developed for the selective plating of nNi coatings onto Sn soldered parts and Sn finished

surfaces in electronic assemblies. Finally, an investigation into whether the mass transport leaves

voids in the Sn coating should be done.

64

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72

Appendix I

Whisker Measurements

Table 1 Appendix I – Whisker Length Measurement

Whisker Nr. Length (µm) Whisker Nr. Length (µm)

1 18.4 28 12.4

2 5.8 29 20.1

3 11.0 30 13.5

4 18.5 31 8.3

5 19.6 32 13.9

6 22.8 33 10.2

7 5.4 34 9.6

8 5.4 35 5.0

9 9.9 36 12.0

10 8.8 37 2.8

11 14.7 38 8.7

12 23.4 39 5.9

13 11.8 40 9.4

14 15.5 41 15.6

15 5.5 42 9.8

16 17.6 43 4.9

17 6.3 44 5.7

18 7.6 45 19.6

19 21.0 46 10.9

20 5.7 47 7.0

21 3.8 48 5.4

22 13.2 49 5.3

23 8.8 50 10.7

24 7.0 51 11.0 Mean

25 10.0 52 5.5 SD

26 19.6 53 2.8 Min

27 7.2 54 23.4 Max

These measurements were taken from an SEM image where the sample is on a small

angle so the numbers in the table above are the raw measurement numbers from the image. To

get an approximation of the true length of the whiskers the length of the whiskers from the above

table were multiplied by a factor of two.

73

Figure AI: Visible whisker growth on the Sn coating surface after 85°C/85% RH treatment. Example of image used for

whisker length and density measurements.

74

Appendix II

Plating time calculations

To get the time required to plate Sn from a stannate bath the following equation was used:

𝑡 =ℎ𝑎𝜌

𝑍𝐼

Where h is the height, or thickness, of the plating in cm, a is the area that is to be plated in cm, ρ is the

density of the material (7.3 g/cm3 for βSn), Z is the electrical equivalent of the material being plated

(3.076x10-4 g/C for Sn in a stannic solution), and finally I is the current that is being used to electroplate.

To plate about 1 μm of Sn it takes about 95 s, so to plate 3 μm the amount of plating time is 4 min and 45

s. When taking current efficiency of 90% into consideration the plating time increases to 106 s for 1um

and 318 s or 5 min and 18 s.

[Modern electroplating] M. Schlesinger, and M. Paunovic, Modern Electroplating. Hoboken NJ: John

Wiley and Sons, Inc., 2010, pg. 1-31(for plating time)

75

Appendix III

Phase Diagrams

This section of the appendix gives the relevant phase diagrams for this study. Cu-Sn, Ni-Sn, and Cu-Ni

phase diagrams are presented.

Figure AII: Phase diagram of Cu-Sn system

76

Figure AIII: Phase diagram of Ni-Sn system

77

Figure AIV: Phase diagram of Cu-Ni system