Dynamic material characterisation of solder interconnects in microelectronic packaging

DYNAMIC MATERIAL CHARACTERIZATION OF
SOLDER INTERCONNECTS
IN MICROELECTRONIC PACKAGING

ONG KAI CHUAN
(B.Eng.(Hons.), NUS)

A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2005


Acknowledgement

ACKNOWLEDGEMENT
I would like take this opportunity to express my utmost gratitude to my supervisor for the
pass 5 years in NUS, Dr Vincent Tan, my co-supervisor Dr Lim Chwee Teck from NUS
and Dr Zhang Xiao Wu and Mr Wong Ee Wah from IME, Professor John Field from
Cavendish Lab, Cambridge.

I would like to show my gratitude for their patience,

valuable guidance and treasured advice throughout this few years of my quest for
knowledge and acquiring a understanding of the field of research.

Also I would like to thank staff from Dr Lu Li for giving me advice regarding the
material aspect of this research, NUS Materials lab for their warm hospitality and
allowing me to use their equipment, and also, bio-engineering lab and advance
manufacturing lab for letting me use their equipment during the period of my Masters of

Engineering degree.

Finally, the last but not least, the great people from Impact Mechanics Lab. Lab officers
Alvin and Joe, my fellow post-graduate friends and colleague, who have provided me
with more then just valuable aid at my hour of need, and brainstorming sessions when I
develop mental blocks, but you have provided me friendship and a wonderful time here
in NUS Impact Mechanics Lab. Thank you all.

i


Table of Contents

TABLE OF CONTENTS

Page No.

ACKNOWLEDGEMENT

i

TABLE OF CONTENTS

ii

SUMMARY

vi

LIST OF FIGURES

vii

LIST OF TABLES

xiii

LIST OF ACRONYMS

xv

CHAPTER1 INTRODUCTION

1

1.1

Dynamic Property of Solder

1

1.2

Lead-Free Solder

2

1.3

Objective

4

1.4

Scope

4

CHAPTER 2 LITERATURE REVIEW

6

2.1

Solder Material

6

2.2

Dynamic Material Properties of Solder

8

2.3

Split Hopkinson Pressure Bar Experiment (SHPB)

10

2.4

Solder Microstructure

13

CHAPTER 3 MICROSTRUCTURE OF SOLDER SPECIMEN

16

3.1 Specimen Preparation

16

3.1.1 Casting
3.1.2 Machining
3.1.3 Etching
3.1.4 Image Acquisition

16
19
19
20

ii


Table of Contents

3.2 Microstructure of Sn-37Pb Solder Specimens
3.2.1 Slow Cooling
3.2.2 Moderate Cooling
3.2.3 Quench Cooling
3.2.4 Solder Balls
3.3 Microstructure of Sn-3.5Ag Solder Specimens
3.3.1 Slow Cooling
3.3.2 Moderate Cooling
3.3.3 Quench Cooling
3.3.4 Solder Balls
3.4 Microstructure of Sn-3.8Ag-0.7Cu Solder Specimens
3.4.1 Slow Cooling
3.4.2 Moderate Cooling
3.4.3 Quench Cooling
3.4.4 Solder Balls

21
22
23
24
25
27
27
29
31
31
33
35
36
39

40

3.5 Chapter Summary

42

CHAPTER 4 QUASI-STATIC MATERIAL
PROPERTIES OF SOLDER SPECIMENS

44

4.1

44

Graphs of Quasi-Statically Compressed Solder Specimens

4.2 Young’s Modulus of Solder Specimens
4.2.1 Comparing Materials
4.2.2 Comparing Microstructure
4.3 Yield of Solder Specimens

46
47
47
48

4.3.1 Comparing Materials
4.3.2 Comparing Microstructure

49
50

4.4 Tangential Modulus of Solder Specimens

51

4.4.1 Comparing Materials
4.4.2 Comparing Microstructure

52
52

iii


Table of Contents
4.5 Chapter Summary

56

CHAPTER 5 DYNAMIC MATERIAL PROPERTIES
OF SOLDER SPECIMENS

57

5.1 Material Response of Sn-37Pb Solder Specimens

57

5.1.1 Slow Cooled
5.1.2 Moderately Cooled
5.1.3 Quench Cooled
5.1.4 Sn-37Pb Solder Summary
5.2 Material Response of Sn-3.5Ag Solder Specimens
5.1.1 Slow Cooled
5.1.2 Moderately Cooled
5.1.3 Quench Cooled
5.1.4 Sn-3.5Ag Solder Summary
5.1 Material Response of Sn-3.8Ag-0.7Cu Solder Specimens
5.1.1 Slow Cooled
5.1.2 Moderately Cooled
5.1.3 Quench Cooled
5.1.4 Sn-3.8Ag-0.7Cu Solder Summary

58
59
61
62
65
65
66
68
70
72
72
74
76
78

5.4 Chapter Summary

82

CHAPTER 6 COMPARISON OF BULK SOLDER PROPERTIES
WITH SOLDER BALL PROPERTIES

84

6.1 Solder Ball Experiments

84

6.1.1 Experimental Setup
6.1.2 Experimental Results
6.2 Solder Ball Simulation
6.2.1 Software
6.2.2 Simulation Setup
6.2.2.1 Material Definition
6.2.2.2 Interaction
6.2.2.3 Load / Boundary Condition
6.2.2.4 Explicit verses Implicit

84
85
86
87
87
87
88

89
90

iv


Table of Contents
6.2.2.5 Meshing Resolution
6.2.2.6 Analysis Precision
6.2.3 Local strain within solder ball during SHPB experiment
6.3 Comparison of Simulation and Experimental Results
6.3.1 Sn-37Pb
6.3.2 Sn-3.5Ag
6.3.3 Sn-3.8Ag-0.7Cu

90
92
93
94
96
98
100

6.4 Comparison and Prediction of Solder Ball Properties

103

CHAPTER 7 CONCLUSION AND RECOMMENDATIONS

105

7.1 Conclusion

105

7.2 Recommendations

107

LIST OF REFERENCES

108

APPENDIX A - SOLDER PHASE DIAGRAM

114

APPENDIX B - SPECIMEN PREPARATION FLOW CHART

116

APPENDIX C - SOLDER MICROSTRUCTURE

117

APPENDIX D - EXPERIMENTAL EQUIPMENT

123

v

Summary

SUMMARY
An Investigation of quasi-static and dynamic properties of Sn-37Pb solder and two leadfree solder materials, Sn-3.5Ag and Sn-3.8Ag-0.7Cu was carried out using the split
Hopkinson pressure bar (SHPB). Each solder was cast at three different cooling rates
(slow cooling, moderate cooling and quench cooling) to understand how microstructure
and material response change with the variation of rate of solidification of these solders.
A Finite Element analysis software simulation of the SHPB experiments on single balls
was performed using the bulk dynamic material properties to assess how well the bulk
material response obtained in experiments represents actual solder deformation.

All dynamically deformed materials show a distinct increase in yield strength and flow
stress as compared to their quasi-static properties. Sn-37Pb solder shows consistent
increase in flow stress as strain rate increases for all cooling rates. Whereas Sn-3.5Ag
solder generally displays negative strain rate sensitivity with the exception of moderately
cooled specimens. Sn-3.8Ag-0.7Cu solder formed via slow cooling shows positive strain
rate sensitivity whereas those formed by faster cooling rates have no strain rate
dependence.

Finite element simulation results obtained using purely quasi-static properties show
significant under-estimation of the strength of solder ball under high deformation rate.
Simulations using both dynamic and quasi-static material of solder demonstrate better
reflection of solder ball response in SHPB experiments.

vi


List of Figures

LIST OF FIGURES
Figure 2.1:

Schematic diagram of a compressive Split Hopkinson
Pressure Bar (SHPB) setup

10

Figure 3.1:

Polished and etched co-cast solder samples for optical /
SEM microscopy

20

Figure 3.2

Optical Micrographs of as-cast solder samples formed
via different cooling rates (a) By slow Cooling, (b) By
Moderate Cooling and (c) By Quench Cooling

21

Figure 3.3:

Optical micrographs of grain boundaries in Sn-37Pb,
SC sample at increasing magnifications
(a) 30X magnification
(b) 150X magnification

(c) 300X magnification
(d) 750X magnification

21

Figure 3.4:

Scanning electron micrographs of Sn-37Pb formed by
slow cooling at (a) 500X and (b) 2000X magnifications

22

Figure 3.5:

Scanning Electron Micrographs of Sn-37Pb formed by
Moderate Cooling at (a) 500X and (b) 2000X
magnifications

23

Figure 3.6:

Scanning electron micrographs of Sn-37Pb formed by
quench cooling at (a) 500X and (b) 2000X
magnification

24

Figure 3.7:

SEM micrographs of Sn-37Pb virgin solder balls at (a)
200X, (b) 500X, and (c) 2000X magnification

25

Figure 3.8:

SEM micrographs of Sn-37Pb solder balls after re-flow
at (a) 200X, (b) 500X, and (c) 2000X magnification

26

Figure 3.9:

Scanning electron micrographs of Sn-3.5Ag formed by
slow cooling at (a) 500X and (b) 2000X magnification

28

Figure 3.10:

Optical Micrographs of Sn-3.5Ag formed by slow
cooling at (a) 30X and (b) 300X magnification

28

Figure 3.11:

Optical Micrographs of Sn-3.5Ag formed by moderate
cooling at three different magnifications (a) 40X, (b)

and (c) at 350X and (d) 600X

29

vii


List of Figures
Figure 3.12:

SEM micrographs of bulk Sn-3.5Ag solder formed by
Moderate Cooling at (a) 500X and (b) 2000X
magnification

30

Figure 3.13:

SEM micrographs of Sn-3.5Ag bulk solder cast by
quench cooling at (a) 500X and (b) 2000X
magnification

32

Figure 3.14:

SEM micrographs of virgin Sn-3.5Ag solder balls at (a)
200X, (b) 500X and (c) 2000X magnification

32

Figure 3.15:

SEM micrographs of Sn-3.5Ag Solder balls after reflow at (a) 200X, (b) 500X and (c) 2000X
magnification

33

Figure 3.16:

Optical Micrographs of Sn-3.8Ag-0.7Cu bulk solder
cast by slow cooling at (a) 150X, (b) 250X, (c) 500X
and (d) 700X magnification

35

Figure 3.17:

SEM micrographs of Sn-3.8Ag-0.7Cu bulk solder cast
by slow cooling at (a) 500X and (b) 2000X
magnification

36

Figure 3.18:

Optical Micrographs of Sn-3.8Ag-0.7Cu bulk solder
cast by moderate cooling at (a) 50X, (b) 140X, (c)
250X and (d) 700X magnification

38

Figure 3.19:

SEM Micrographs of Sn-3.8Ag-0.7Cu bulk solder cast
by moderate cooling at (a) 500X and (b) 2000X
magnification

39

Figure 3.20:

SEM Micrographs of Sn-3.8Ag-0.7Cu bulk solder cast
by quench cooling at (a) 500X and (b) 2000X
magnification

40

Figure 3.21:

SEM micrographs of virgin Sn-3.8Ag-0.7Cu solder
balls at (a) 200X, (b) 500X and (c) 2000X
magnification

41

Figure 3.22:

SEM micrographs of Sn-3.8Ag-0.7Cu Solder Balls
after re-flow at (a) 200X, (b) 500X and (c) 2000X

magnification

41

Figure 4.1

Stress-strain curves of bulk Sn-37Pb solder under
quasi-static loading

45

viii


List of Figures
Figure 4.2:

Stress-strain curves of bulk Sn-3.5Ag solder under
quasi-static loading

45

Figure 4.3:

Stress-strain curves of Bulk Sn-3.8Ag-0.7Cu solder
under quasi-static loading

46

Figure 4.4:

Young’s modulus of bulk solder of three different
compositions

46

Figure 4.5:

Yield stresses of bulk solder (0.2% strain offset)

48

Figure 4.6:

Tangent modulus of bulk solder in plastic deformation
between 1% - 3% strain

51

Figure 4.7:

Charts showing quasi-static results of Sn-37Pb solder
flow stresses at (a) 1% strain and (b) 3% strain

53

Figure 4.8:

Charts showing quasi-static results of Sn-3.5Ag solder
flow stresses at (a) 1% strain and (b) 3% strain

54

Figure 4.9:

Charts showing quasi-static results of Sn-3.8Ag-0.7Cu
solder flow stresses at (a) 1% strain and (b) 3% strain

55

Figure 5.1:

Response of bulk Sn-37Pb SC solder in the SHPB
experiment up to 30% strain

58

Figure 5.2:

Response of bulk Sn-37Pb SC solder in the SHPB
experiment up to 80% strain

59

Figure 5.3:

Response of bulk Sn-37Pb MC solder in the SHPB
experiment up to 30% strain

60

Figure 5.4:

Response of bulk Sn-37Pb MC solder in the SHPB
experiment up to 80% strain

61

Figure 5.5:

Response of bulk Sn-37Pb QC solder in the SHPB
experiment up to 30% strain

62

Figure 5.6:

Response of bulk Sn-37Pb QC solder in the SHPB
experiment up to 80% strain

62

Figure 5.7:

Summary of true stress at 5%, 25% and 60% strain
from SHPB experiment for Sn-37Pb bulk solder cast
via SC, MC and QC

64

ix


List of Figures
Figure 5.8:

Response of bulk Sn-3.5Ag SC solder in the SHPB
experiment up to 30% strain

65

Figure 5.9:

Response of bulk Sn-3.5Ag SC solder in the SHPB
experiment up to 80% strain

66

Figure 5.10:

Response of bulk Sn-3.5Ag MC solder in the SHPB
experiment up to 30% strain

67

Figure 5.11:

Response of bulk Sn-3.5Ag MC solder in the SHPB
experiment up to 80% strain

67

Figure 5.12:

Response of bulk Sn-3.5Ag QC solder in the SHPB
experiment up to 30% strain

68

Figure 5.13:

Response of bulk Sn-3.5Ag QC solder in the SHPB
experiment up to 80% strain

69

Figure 5.14:

Response of bulk Sn-3.8Ag-0.7Cu SC solder in the
SHPB experiment up to 30% strain

73

Figure 5.15:

Response of bulk Sn-3.8Ag-0.7Cu SC solder in the
SHPB experiment up to 80% strain

73

Figure 5.16:

Response of bulk Sn-3.8Ag-0.7Cu MC solder in the
SHPB experiment up to 30% strain

74

Figure 5.17:

Response of bulk Sn-3.8Ag-0.7Cu MC solder in the
SHPB experiment up to 80% strain

75

Figure 5.18:

Flow Stress of high strain rate compression at 5%, 25%
and 60% strain of MC bulk Sn-3.8Ag-0.7Cu solder

76

Figure 5.19:

Response of bulk Sn-3.8Ag-0.7Cu QC solder in the
SHPB experiment up to 30% strain

77

Figure 5.20:

Response of bulk Sn-3.8Ag-0.7Cu QC solder in the
SHPB experiment up to 80% strain

77

Figure 6.1:

Force vs Displacement graph of virgin solder balls
undergoing slow (3.67x10-5 ms-1) and high strain rates
(12.5 ms-1)

85

x


List of Figures
Figure 6.2:

Plot of force required for 0.38mm deformation of
solder ball at different compression rates (Low strain
rate values obtained by using Instron Micro-Force
Tester, High strain rate values obtained from miniature
Hopkinson Bar experiment)

86

Figure 6.3:

Input Velocity profiles at 2.5 ms-1, 5.5 ms-1 and 7.5 ms-1

deformation rate.

90

Figure 6.4:

Enlarged view of the simulation mesh of solder ball
resting between the input and output rods of the split
Hopkinson pressure bar experiment

91

Figure 6.5:

Output Strain readings using Single and Double
precision data calculation

92

Figure 6.6:

Finite Element simulation visualization module of
strain distribution within the solder ball during
compression at (a) 0 μs, (b) 1.25 μs,(c) 2.5 μs, (d)
5.0μs, (e) 8.25 μs and (f) 11.75 μs

93

Figure 6.7:

Transmitted strain from SHPB experiment with Sn37Pb solder ball specimen with a deformation rate of
2.5 m/s

96

Figure 6.8:

Transmitted strain from SHPB experiment with Sn37Pb solder ball specimen with a deformation rate of
5.5 m/s

97

Figure 6.9:

Transmitted strain from SHPB experiment with Sn37Pb solder ball specimen with a deformation rate of
7.5 m/s

97

Figure 6.10:

Transmitted strain from SHPB experiment with Sn3.5Ag solder ball specimen with a deformation rate of
2.5 m/s

98

Figure 6.11:

Transmitted strain from SHPB experiment with Sn3.5Ag solder ball specimen with a deformation rate of
5.5 m/s

99

Figure 6.12:

Transmitted strain from SHPB experiment with Sn3.5Ag solder ball specimen with a deformation rate of
7.5 m/s

99

xi


List of Figures
Figure 6.13:

Transmitted strain from SHPB experiment with Sn3.8Ag-0.7Cu solder ball specimen with a deformation
rate of 2.5 m/s

100

Figure 6.14:

Transmitted strain from SHPB experiment with Sn3.8Ag-0.7Cu solder ball specimen with a deformation
rate of 5.5 m/s

101

Figure 6.15:

Transmitted strain from SHPB experiment with Sn3.8Ag-0.7Cu solder ball specimen with a deformation
rate of 7.5 m/s

101

xii


List of Tables

LIST OF TABLES
Table 1.1:

Project Scope

5

Table 2.1:

Properties of each selected solder composition

7

Table 3.1:

The three different cooling rates of solder specimen

18

Table 3.2:

Highlights of microstructure of each cooling rate

42

Table 3.3

Microstructure of bulk solder most similar to solder balls
before/after reflow

43

Table 4.1

Young’s modulus of solder specimens

47

Table 4.2

Yield stresses of solder specimens

48

Table 4.3

Tangential modulus of solder specimens between 1% and 3%
strain

51

Table 4.4

Observed correlations of quasi-static solder repose to different
cooling rates

56

Table 5.1

Features of high strain-rate response of Sn-37Pb solder

62

Table 5.2

Features of high strain-rate response of Sn-3.5Ag solder

70

Table 5.3

Features of high strain-rate response of Sn-3.8Ag-0.7Cu solder

78

Table 5.4

Summary of observations of the correlation of material properties
with cooling rate for all three solder compressed at high strain

rates

82

Table 6.1

Dimensions of parts in Finite Element simulations

91

Table 6.2

Material properties adopted for use in simulation

95

Table 6.3

Simulation results closest to experimental response of SHPB
experiment

102

Table 6.4

Microstructure of bulk solder most similar to solder balls
before/after reflow

103

Table 6.5

Microstructure and simulation comparison with actual virgin
solder balls

104

xiii


List of Tables
Table 7.1

Microstructure of bulk solder most similar to solder ball
before/after reflow

105

xiv


List of Acronyms

LIST OF ACRONYMS
A

:

Cross sectional area of Hopkinson Bars

As

:

Cross sectional area of specimen

Al

:

Aluminum

Al2O3 :

Aluminum Oxide

Ag

:

Silver

Bi

:

Bismuth

C

:

Elastic wave velocity

C0

:

Elastic wave velocity in Hopkinson Bar

Cp

:

Heat capacity

Cu

:

Element Copper

E

:

Young’s Modulus of Hopkinson Bar

HCL

:

Hydrochloric Acid

HNO3 :

Nitric Acid

L

:

Length of specimen in a Split Hopkinson Pressure Bar

Pb

:

Lead

Sn

:

Tin

t

:

Time

ΔT

:

Temperature rise

o

:

Rate of change in temperature (Cooling Rate)

C/s

β–Sn :

Beta phase of tin

δσ/δε :

Work hardening rate

dε

Strain interval.

:

xv


List of Acronyms

ε

:

Strain

εs

:

Strain of the specimen

εi

:

Magnitude of the incident strain passing through the input bar

εr

:

Magnitude of the reflected strain passing through the input bar

εt

:

Magnitude of the transmitted strain passing through the input bar

:

Strain Rate

εs

:

Strain rate experienced by the specimen in a Split Hopkinson Pressure Bar

ρ

:

Density

σ

:

Stress

σs

:

Stress experienced by the specimen

νi

:

Particle velocity of specimen in a Split Hopkinson Pressure Bar

.

ε
.

xvi


1. Introduction

CHAPTER 1

INTRODUCTION

1.1 Dynamic Property of Solder
The advancement of the portable electronics industry in the past ten to fifteen years has
been nothing short of astounding. In the past, it would be unimaginable to have portable
telephones, computers of the present size, functions and capabilities. Processing power
that once required a whole room to house can now fit onto the palm of your hand.
Greater portability also means that electronic devices are more prone to experiencing
severe physical shock than before. Consumer electronic devices for example experience

such physical shocks when they are being dropped or struck. The US Air Force estimates
that vibration and shock causes 20 percent of the mechanical failures in airborne
electronics [1].

The increasing global demand for both miniaturization and multi-functionality of
electronic devices has encouraged the development of Surface Mount Technology (SMT)
to replace of the less space efficient Through-Hole-Technology (THT) (both being
methods of using solder as interconnects to attach integrated circuit packages onto printed
circuit-boards).

With Chip Scale Packaging (CSP) and Ball Grid Array (BGA, a form of SMT) both
developing rapidly, the size of and pitch between interconnects has also shrunk.
As a result solder interconnects play a more significant role in providing physical
support. Zhu [2] found that an impact induced BGA (solder interconnects) crack is the
most dominant cause of failure in a portable phone drop and tumble verification test.

1


1. Introduction
As equipment in warfare and our everyday life become more dependent on electronics,
research in the dynamic (high strain rate) response of solder interconnects to make these
electronic devices more robust becomes more salient.

1.2 Lead-Free Solder
For more than 50 years, tin-lead (Sn-Pb) solder has been used almost exclusively
throughout the world in the electronics industry to attach electronic components onto the
printed circuit boards (PCBs). However, there have been concerns of the hazardous
effects of lead on the environment. Once the electronic devices are discarded, the fear is
that the lead will find its way into the garbage and landfill. From there it can leach into

the water supply and contaminate it. Although industrial scrap is normally recycled,
consumer waste cannot be controlled [3].

Thus, in June 2000, after five years of

consultations and documented drafts, the European Union (EU) penned the following
three legislations to minimize lead usage, and thus, promote the use of lead-free solder
[4]:

1. WEEE (Waste from Electrical and Electronic Equipment) – primarily concerned
with aspects of the end-of-life of electronic equipments to minimize waste and
maximize recycling.
2. RoHS (Restriction of Hazardous Substance) – restrictions on the use of certain
hazardous substances in electrical and electronic equipment. i.e. to ban certain
hazardous materials such as lead.

2


1. Introduction
3. EEE (Environment of Electrical and Electronic Equipment Directive) – concerned
with minimizing overall environmental impact by paying attention to aspects of
design and manufacture, without banning materials.

The directives were adopted by the member states in December 2002 and RoHS will be
enforced in July 1, 2006.

The EU is not alone in this campaign. In Japan, although no impending legislation on
material ban exists, public preference for “green” products is the incentive for going leadfree. Big brands such as NEC, Hitachi, and Sony were already marketing some lead -free
products since 2000 [4]. Hitachi, Sony, Fujitsu and Matsushita have turned lead-free

since 2002. In the United States of America, the National Electronics Manufacturing
Initiative (NEMI) have held “Lead-Free Initiative Meetings” since 1999.

In summary, consolidated efforts have been promising, as Dr Brian Richards from the
National Physical Laboratory has put it, “The inevitable conclusion is that the transition
to lead-free soldering is underway and will accelerate over the next few years” [4]. Thus,
research on the behaviour of lead-free solder will make important contributions towards a
smoother transition.

3


1. Introduction
1.3 Objectives
The objectives of this research are:
ƒ

To investigate the quasi-static and dynamic properties of three types of solder
material (e.g. Sn-37Pb, Sn-3.5Ag, Sn-3.8Ag-0.7Cu), each cast at three different
cooling rates, to give three different types of microstructure, and

ƒ

To find the type of bulk solder which best represents virgin solder balls (solder
balls before reflow) by comparing their microstructure and material response and
predict the type of solder that will best represent solder ball material after reflow.

1.4 Scope
Bulk solder specimens are produced from three different cooling rates per composition.
The microstructure of each of the specimens will be examined to find the best match with

microstructure of virgin and reflowed solder balls.

Quasi-static and dynamic (high-strain rate) compression tests are performed on both bulk
solder and virgin solder balls. The obtained bulk material behaviour (quasi-static and
dynamic) will be fed to finite element simulations of the Split Hopkinson Pressure Bar
experiments on a single solder ball.

Subsequently, the simulation outcome will be

compared with experimental results to find the type of bulk solders which best represents
virgin and reflowed solder balls during impact.

4


1. Introduction
A summary of the scope of this project is shown in table 1.1 below.
Table 1.1 Project Scope
Compression Tests

Microstructure

Quasi-Static
Bulk
Specimen

Solder
Ball

FEM

Dynamic

Slow Cooled,

Sn-37Pb

Sn-37Pb

Moderately Cooled,

Sn-3.5Ag,

Sn-3.5Ag,

Quenched Cooled.

Sn-3.8Ag-0.7Cu

Sn-3.8Ag-0.7Cu

Sn-37Pb

Sn-37Pb

Sn-37Pb

Sn-3.5Ag,

Sn-3.5Ag,

Sn-3.5Ag,

Sn-3.8Ag-0.7Cu

Sn-3.8Ag-0.7Cu

Sn-3.8Ag-0.7Cu

Virgin Solder Balls

5


2. Literature Review

CHAPTER 2

LITERATURE REVIEW

2.1 Solder Materials
After 50 years of using SnPb solder by the electronics industry, the first step towards
removing lead-containing solder is to the find a suitable replacement.

Many

organizations from Europe (IDEALS, ITRI), USA (NEMI), and Japan (JEITA) have been
doing research and have set up consulting agencies such as the National Institute of
Standards and Technology (NIST, Gaithersburg, MD), International Tin Research
Institute (ITRI, Uxbridge, England) and National Physical Laboratory (NPL, UK) to look

for the best lead-free replacement for eutectic Sn-37Pb solder.

Several solder

compositions were short-listed by these institutions and organizations. With reference to
their findings, two lead-free solders (one binary, Sn-3.5Ag and one ternary, Sn-3.8Ag0.7Cu) and one lead-containing solder (eutectic Sn-37Pb) were selected for the purpose
of this research. Eutectic Sn-37Pb solder was chosen as a benchmark to compare with
the two other lead-free solders. SnAgCu solder is chosen since it seems to be the most
anticipated lead-free solder to take over SnPb. The other lead-free solder chosen is the
SnAg. It is chosen due to its history of usage in the industry and could be a possible
alternative to SnAgCu solder.

Sn-Ag-Cu (Tin-Silver-Copper) close eutectic ternary solder is the most promising and
popular choice among many institutions [4, 5, 6]. The large volume telecommunication
industry has targeted this alloy [4]. Sn-3.8Ag-0.7Cu solder was identified by the
European IDEALS consortium as the best lead-free alloy for reflow due to its baseline
advantages of reduced melting temperature (as compared to Sn-3.5Ag) and additional

6


2. Literature Review
strengthening phase. It is also reported to have reliability equivalent to, if not better than
that of SnPb and SnPbAg solders [5].

The Tin-Silver (Sn3.5Ag) solder is another lead-free solder that is believed to have high
potential [5] along with others such as SnCu and SnAgBi [6]. Sn-3.5Ag solder is said to
have good fatigue resistance and overall good joint strength [7]. With one of the longest
history of use as a lead-free alloy, it also has good mechanical properties and better
solderability than SnCu.

Ford (Visteon Automotive Systems) has reported using

Sn3.5Ag solder successfully in production (module assembly) for wave soldering since
1989 [5]. This is due to its higher melting temperature (221oC) as compared to the Tinlead solder (183 oC).

SnAg has been used for many years in certain electronic

applications [6] and thermal fatigue testing of the alloy has often shown it to be more
reliable than SnPb solder.

Table 2.1 shows some of the properties of each of the three solders. Phase diagrams of
each composition are attached in appendix A.

Table 2.1 Properties of Each Selected Solder Composition
Solder Composition

Density (kg/m3)

Melting Point (oC)

Sn-37Pb

8400

183

Sn-3.5Ag

7360

221

Sn-3.8Ag-0.7Cu

7400

217

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2. Literature Review
2.2 Dynamic Material Properties of Solder
There has been many research on solder interconnects that focus on different aspects of
solder properties in the past decade. The emphasis is on the more dominant areas such as:
•

Product level tests [8, 9]

•

Board level tests and simulation involving
ƒ

Drop-tests [10, 11, 12], and

ƒ

Bending tests [13, 14]

•

Thermo-mechanical effects [12, 15, 16]

•

Tensile, low strain rate properties [16, 17, 18]

•

Creep and stress relaxation [19, 20, 21, 22]

•

Vibration [1, 23]

•

Microstructure [20, 21, 24]

In recent years, there has been rising interest and emphasis on board level and product
level drop tests due to increased awareness and major concern of possible failure caused
by drop impact of portable electronic devices. The ultimate aim is to be able to predict
the behaviour and response of electronic devices when subjected to such loads so as to
improve their reliability.

8