Microstructural Analysis of Lead-free Solders with Under...

Microstructural Analysis of Lead-free

Solders with Under Bump Metallization

MD. NAZRUL ISLAM

DOCTOR OF PHILOSOPHY

CITY UNIVERSITY OF HONG KONG

January 2006

CITY UNIVERSITY OF HONG KONG

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Microstructural Analysis of Lead-free Solders

with Under Bump Metallization

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Submitted to

Department of Electronic Engineering

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in Partial Fulfillment of the Requirements

for the Degree of Doctor of Philosophy

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by

Md. Nazrul Islam

January 2006

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ii

ABSTRACT

The concurrent developments in emerging technology to realize increasingly

miniaturized electronic products with multifunctional capabilities have driven electronic

packaging to a high density, with the solder joints constantly shrinking towards much

smaller scales. Therefore, selection of appropriate combinations of under bump

metallization (UBM) of ball grid array (BGA) substrate and lead-free solder play an

important role in developing a reliable electronic product. Sn-Ag, Sn-Ag-Cu, Sn-Ag-

Cu-In, Sn-Ag-Cu-Bi, Sn-Cu, Sn-Zn and Sn-Zn-Bi solders have been selected for

soldering and aging with Cu, Ni/Au and NiP/Cu UBM. The interface between the solder

and the UBM affect the reliability of the solder joints. Microscopic analysis of these

solder joints have been conducted by scanning electron microscopy (SEM) equipped

with an energy dispersed X-ray (EDX) facility to investigate the composition and

properties of intermetallic compounds formed as a function of time and temperature.

An investigation has been carried out to compare the interfacial reactions of the

UBM of a BGA substrate with molten eutectic Sn-3.5%Ag-0.5%Cu solder having

different volumes. The Cu consumption was much higher for the Cu/Sn-Ag-Cu solder

system with a higher solder volume. The interfacial reaction rate for the Cu/solder

system is higher than that for the Ni/solder system. A change in solder volumes had no

significant effect on the dissolution rate of the Ni layer. However, for both cases, the

interfacial IMC thickness was higher with smaller solder balls. Higher spalling of

medium-Cu containing TIMC and resettlement of Au are observed in the smaller

volume solder ball/Ni system. Solder joints made with larger balls are more reliable than

those made with smaller solder balls.

The changes in the thickness of the Ni layer have a significant effect on the

growth rate of IMC and the dissolution rate of the Ni layer. Copper atoms can diffuse

through the thin Ni layer and TIMC layer, which change the microstructure and

morphology of the interface. However, a higher consumption rate of the Au/Ni/Cu UBM

iii

in the thin Ni/solder system affects the reliability of the solder joint. For long-term

reliability of the lead-free solder joint, a thin Ni layer should be avoided.

During reflow, the formation of Sn-Cu-Ni TIMCs on the Ni layer is much

quicker than the formation of Ni-Sn BIMCs. However, in the case of electroless NiP, the

growth rates of IMCs are almost the same. The dissolution of NiP layer and P-rich Ni

layer thickness in Sn-Ag-Cu solder joint are lower during 30 minutes reflow than those

of the Sn-Ag solder. Sn-Ag-Cu solders have higher shear loads with Ni layer but show a

relatively lower shear load with a NiP layer.

Cu-containing solder alloys (Sn-Cu, Sn-Ag-Cu, and Sn-Ag-Cu-In) have been

used to identify their interfacial reactions with NiP UBM. Some In-Sn-Au IMCs have

entrapped into the interfacial IMCs due to lower diffusion of Au in the In-containing

solder than that of the Sn-Cu and Sn-Ag-Cu solder. During extended reflow, high-Cu

containing TIMCs have a lower growth rate and consume less of the NiP layer. The

spalling of medium-Cu containing TIMCs in the Sn-Ag-Cu solder increases both the

growth rate of TIMCs and the consumption rate of the NiP layer. Low-Cu containing

QIMCs in the Sn-Ag-Cu-In solder are stable on P-rich Ni and reduce the dissolution rate

of the NiP layer. Consumption of the NiP layer can be reduced by adding Cu or In,

because of the changes of the interfacial IMCs phases which are stable and adhered well

to the P-rich Ni layer during reflow.

The addition of 1 wt% Bi into the Sn-2.8Ag-0.5Cu solder inhibits the excessive

formation of IMCs during the soldering reaction and thereafter during aging. A

significant increase of IMC layer thickness was observed for both solders where the

increasing tendency was lower for the Bi-containing solder. The formation of IMCs

during aging for both solders follows the diffusion control mechanism and the diffusion

of Cu is more pronounced for the Sn-2.8Ag-0.5Cu solder. The IMC growth rate

constants for Sn-2.8Ag-0.5Cu and Sn-2.8Ag-0.5Cu-1.0Bi solders were calculated as

2.21 x 10-17

m2/s and 1.91 x 10

-17 m

2/s respectively, which had a significant effect on the

growth behavior of IMCs during aging.

In the case of Ni/Sn0.7Cu solder system, Cu prevents the resettlement of Au to

the interface. The shear load of solder joints is relatively stable from 1.98 to 1.86 kgf

during long time reflow with high Ni and negligible amount of Au in the TIMCs at the

iv

interface. It is seen that shear load does not depend on the thickness of IMCs and reflow

time. The percentage of Au into the high Ni-containing ternary and quaternary

compounds is found to be low due to preferential diffusion of Au. The shear load during

aging is not stable due to the formation of more Au-containing compounds at the

interface. Cu and Ni play a significant role for interfacial reactions and formation of

different type of IMCs which affect the reliability of solder joint

The interfacial reactions of Sn-Zn based solders and a Sn-Ag-Cu solder have

been compared with an eutectic Sn-Pb solder. The morphologies of the IMCs are quite

different for different solder compositions. As-reflowed, the growth rates of IMCs in the

Sn-Zn based solder are higher than in the Sn-Ag-Cu and Sn-Pb solders. Bi offers

significant effects on the wetting, the growth rate of IMCs as well as on the size and

distribution of Zn-ULFK�SKDVHV�LQ�WKH��-Sn matrix. No Cu-Sn IMCs are found in the Sn-

Zn based solder during 20 minutes reflow. The consumption of Cu by the solders are

ranked as Sn-Zn-Bi>Sn-Ag-Cu>Sn-Zn>Sn-Pb. Despite the higher Cu-consumption rate,

Bi-containing solder may be a promising candidate for a lead-free solder in modern

electronic packaging, taking into account its lower soldering temperature and material

costs.

vi

CERTIFICATE OF COMPOSITION

I hereby certify that this thesis is unique and composed by myself embodying the

results for my study and research at the City University of Hong Kong. It has not been

presented for any other award to any other institute.

vii

Contents

Abstract ii

Acknowledgement v

Certification of Composition vi

Contents vii

Lists of Tables xii

List of Figures xiii

Abbreviations xxi

List of Symbols xxii

Chapter 1: Overview of Microelectronic Packaging Technology Page

1.1 Introduction 1

1.1.1 Microelectronic Packaging 1

1.1.2 Materials and Categories of packages 5

1.1.2.1 Materials/components 5

1.1.2.2 Categories of Packages 6

1.1.3 Application of Solder in Electronic Packaging 8

1.2 Alternative of Pb-containing solders 12

1.2.1 Interconnections 12

1.2.1.1 Interconnection Methods 13

1.2.1.1.1 Wire bonding 14

1.2.1.1.2 Tape Automated Bonding (TAB) 15

1.2.1.1.3 Ball Grid Array (BGA) 16

1.2.1.1.4 Flip chip interconnection methods 16

1.2.2 Conductive adhesive interconnection systems 18

1.2.3 Soldering 19

1.2.3.1 Intermetallic compound (IMC) growth kinetics 21

1.3 Selection of Pb-free solders 24

viii

1.4 Overview of the Research Plan and Objective 36

References 41

Chapter 2: Experimental Procedures 51

2.1 Materials and Manufacturing Process of 2-ML Flexible Substrate 52

2.2 Preparation of substrates for solder bonding 59

2.3 Soldering 60

2.4 Testing Methods 63

2.4.1 Ball Shear test 63

2.4.2 Microscopic Examinations 63

References 65

Chapter 3: Microstructural Analysis of Sn-Ag-Cu Solder with Cu and

Ni Metallization

3.1 Introduction 67

3.1.1 Effect of solder volume 67

3.1.2 Effect of Ni layer thickness 68

3.2 Effect of Sn-Ag-Cu solder volume on Cu metallization 70

3.2.1 Results and discussions 71

3.2.2 Summary 84

3.3 Effect of Sn-Ag-Cu solder volume on Ni metallization 85


3.3.2 Comparison of solder volume with Ni and Cu UBM 89

3.3.3 Summary 90

3.4 Effect of Ni layer thickness 92

3.4.1 Microstructure just after reflow 92

3.4.2 The Interfacial region after a long time molten reaction 93

3.4.3 Summary 99

References 101

ix

Chapter 4: Comparative Study of SnAg and SnAgCu Solders-UBM

4.1 Introduction 106

4.2 Results and Discussions 108

4.2.1 Ball shear load and fracture mode 108

4.2.2 Reaction kinetics and Cross-sectional studies of the interface 111

4.2.2.1 Microstructure after as reflowed 111

4.2.2.2 Interface after long-time reflow 115

4.2.3 Effect of elements during extended reflow 124

4.2.4 Correlations among the interfacial structure, the IMCs thickness

and the Ball shear Load

125

4.3 Summary 126

References 127

Chapter 5: Effects of Indium Addition into the Sn-Ag-Cu solder


5.2 Effects of In-containing solder with thin Ni metallization on Cu pads 134


5.2.1.1 Microstructure just after reflow 134

5.2.1.2 Interface after long time molten reaction 136

5.2.2 Summary 141

5.3 Interfacial Reactions of Cu-containing Lead-free solders with Au/NiP

Metallization

142


5.3.1.1 Microstructure just after as-reflowed 142

5.3.1.2 Interface after long-time molten reactions 145

5.3.1.3 Effect of elements and formation of new phases 153

5.3.2 Summary 155

References 157

x

Chapter 6: Effect of Addition of Bi on Microstructure of the Sn-Ag-Cu

solder


6.2 Results and discussions 163

6.3 Summary 177

References 178

Chapter 7: Investigation of Sn0.7Cu solder with Au/Ni/Cu UBM



7.2.1 Investigation during reflow 182

7.2.1.1 Shear load of solder joint 182

7.2.1.2 Reaction kinetics and cross-sectional studies of the interface 184

7.2.1.2.1 Interface after as-reflowed 184

7.2.1.2.2 Interface after long time reaction 186

7.2.1.2.3 Effect of elements during extended reflow 194

7.2.1.2.4 Correlations among the interfacial structure, the IMCs

thickness and the mechanical strength

195

7.2.1.3 Summary 197

7.2.2 Effects Aging of Ni/Sn-Cu solder joints 198

7.2.2.1 Microstructure of solder joint during aging 198

7.3 Comparison of reflow and aging of solder joint 204

7.4 Comparison of shear load during reflow and aging 206

7.5 Conclusions 207

References 208

xi

Chapter 8: Comparative study of Sn-Zn based and Sn-Ag-Cu solder

with Cu UBM



8.2.1 Microstructure of alloys during first reflow 215

8.2.2 Microstructures during long-time reflow 217

8.2.2.1 Effect of elements during extended reflow 226

8.3 Summary 227

References 229

Chapter 9: Conclusions and Recommendations for Future Work

9.1 Conclusions 232

9.1.1 Microstructure of the solder joints 232

9.1.2 Effect of solder volume with Ni and Cu metallization 232

9.1.3 Effect of Ni layer thickness 233

9.1.4 Cu-containing solder and Pb-containing solder 233

9.1.5 Elimination of Au-embrittlement 234

9.1.6 Effect of addition of forth elements in the Sn-Ag-Cu solder 235

9.1.7 Addition of Bi in the Sn-Zn based solder 235

9.2 Recommendations for Future Work 236

References 240

Publications 241

xii

List of Tables:

Table-1.1: Example of some products/components in three-level packaging. 3

Table-1.2: Difference between PTH and SMT. 7

Table-1.3: Global lead consumption. 9

Table-1.4: Comparison of joining techniques 13

Table-1.5: Example of UBM/solder system and interfacial IMC 21

Table-1.6: Alternative metals to Pb and their alloys. 25

Table 1.7: Classification of solder alloys. 26

Table-1.8: Maximum working temperature vs Homologous for three alloys. 31

Table-1.9: Choice of lead-free alloys by Industry. 37

Table-2.1: Summary of the experimental procedures 62

Table-4.1: The changes of shear load during extended times of reflow. 111

Table-5.1: Formation of IMCs during reflow. 154

Table-7.1: Atomic percentages of Sn, Cu, and Ni in the Sn-Cu-Ni IMCs (by

EDX analysis-spot 3) of the solder joints at different reflow time.

189

Table-7.2: Atomic percentages of Sn, Cu and Ni in the Sn-Cu-Ni TIMCs (by

EDX analysis-spot 3) of the solder joints at different aging time.

201

xiii

List of Figures:

Figure 1.1 : Microelectronic packaging systems 1

Figure 1.2 : Three-level Packaging Hierarchy 2

Figure 1.3 : Trends of electronic Packaging technology 4

Figure 1.4 : World IT and Microsystems market largest 4

Figure 1.5 : Discrete components 6

Figure 1.6 : Categories of packages 7

Figure 1.7 : a) SMT and b) PTH assembly 8

Figure 1.8 : The path of lead from soldered board to the human body

through drinking water

10

Figure 1.9 : Chip to substrate interconnection techniques 14

Figure 1.10 : Wire bonding between substrate pads to its associated chip

pads

15

Figure 1.11 : Example of TAB package 15

Figure 1.12 : a) Schematic of Flip chip solder interconnection and b) UBM 17

Figure 1.13 : Schematic diagram of Adhesive interconnection systems a)

ICA and b) ACA

18

Figure 1.14 : Conduction path of ICA joints 19

Figure 1.15 : Self-alignment of soldering process 20

Figure 1.16 : Equilibrium phase diagram of Au-Sn system 28

Figure 1.17 : Equilibrium phase diagram of Cu-Sn 29

Figure 1.18 : Equilibrium phase diagram of Ni-Sn 29

Figure 1.19 : Equilibrium phase diagram of Sn-Zn solder 34

Figure 1.20 : Equilibrium phase diagram of Sn-Zn-Bi solder 35

Figure 1.21 : Dissolution rate of metals in Sn-Pb solder as a function of

temperature.

36

Figure 1.22 : Basic combination of lead-free solder alloys. 37

xiv

Figure 2:1 : Schematic diagram of the cross-section of a TBGA package 51

Figure 2.2 : 90 degree peel strength of 2 Metal layers 52

Figure 2.3 : Manufacturing process of 2-metal layer flexible substrate used

for this study (courtesy of Compass Technology Co. Ltd)

54

Figure 2.4 : Cu build-up via formation (courtesy of Compass Technology

Co. Ltd)

56

Figure 2.5 : a) SEM image after electrolytic Ni/Au plating or NiP/Au

plating on Cu pad and b) Au layer on the Ni or NiP layer.

57

Figure 2.6 : Morphology of plated Cu pad, a) Ni/Au and b) NiP/Au 58

Figure 2.7 : Micrograph of SAM 58

Figure 2.8 : Schematic diagram of placing of solder on the pre-fluxed pad 60

Figure 2.9 : a) Joining of solder balls after reflow and b) Cross-section of a

solder joint

61

Figure 2.10 : Schematic diagram of Ball shear test 62

Figure 3.1 : Schematic diagram of solder ball attachment on Cu substrate

pad for a) solder S and b) solder L

70

Figure 3.2 : Schematic diagram of solder ball spreading on Cu plate after

reflow for a) solder S and b) solder L

70

Figure 3.3 : SEM backscattered electron micrograph illustrating the

dissolution of Cu substrate and Cu-Sn intermetallics reflowed

at 2300C for a) 5 minute, b) 10 minute, c) 20 minute reflow for

both solder S and solder L

72

Figure 3.4 : The consumed thickness of Cu vs. reflow time at three

different temperatures for a) solder S and b) solder L

74

Figure 3.5 : Average IMCs layer thickness as a function of reflow time for

a) solder S and b) solder L

75

Figure 3.6 : Schematic diagram of a) solder S and b) solder L 77

Figure 3.7 : SEM micrograph illustrating the interface of solder S reflowed

at 2500C for 20 minute on a) Cu plate and b) Cu pad with

78

xv

solder musk

Figure 3.8 : Cu–Sn IMCs with in the bulk solder after 10 minute reflow at

2300C for a) solder S and b) solder L

79

Figure 3.9 : Cu–Sn IMCs within the bulk solder in as-reflowed condition

at 2300C a) solder S and b) solder L

81

Figure 3.10 : Comparison of volume of IMCs versus reflow time at 2500C

with solder L and solder S a) both in the bulk and at the

interface, and b) at the interface

83

Figure 3.11 : SEM image after reflow- a) Solder S and b) Solder L 85

Figure 3.12 : Interfacial structures of solder joints after 1 minutes reflow at

240oC – a) solder S and c) solder L. Intermetallics within the

bulk solders after 1 minutes reflow at 240oC b) for solder S

and d) for solder L.

86

Figure 3.13 : Interfacial structure of a solder joint after 10 minutes reflow at

240oC - a) solder S and b) solder L

87

Figure 3.14 : Interfacial structure of a solder joint after 30 minutes reflow at

240oC- a) solder S, and b) solder L

88

Figure 3.15 : Electrolytic Ni/solders joint after 1 minute reflow at 250oC-

Interfacial IMC a) for thin Ni/ solder system and c) for thick

Ni/ solder system. IMC in the bulk solder - b) for thin Ni and

d) for thick Ni, (2000x).

92

Figure 3.16 : Electrolytic Ni/solder joints after 10 minutes reflow-

Interfacial IMC a) for thin Ni/ solder system and c) for thick

Ni/ solder system. IMCs in the bulk solder- b) for thin Ni and

d) for thick Ni, (2000x).

93

Figure 3.17 : IMCs thickness as a function of reflow time 94

Figure 3.18 : Electrolytic Ni/solder joints after 20 minutes reflow- a) thin Ni

layer / solder and b) thick Ni layer / solder

95

Figure 3.19 : Consumed Ni layer thickness with reflow time

96

xvi

Figure 3.20 : Electrolytic Ni/solder joints after 30 minutes reflow- a)

Interface of a thin Ni/Sn-Ag-Cu solder joint, and b)IMCs

within the bulk solder of thin Ni; c) Interface of a thick Ni/Sn-

Ag-Cu solder joints

98

Figure 4.1 : Shear load of a) Sn-Ag, Sn-Ag-Cu and Sn-Pb-Ag solder joints

with Ni and b) Sn-Ag and Sn-Ag-Cu solder joints with

electroless NiP

108

Figure 4.2 : Fracture surfaces of- (a) Ni/Sn-Ag solder joint, (b) NiP/Sn-Ag

solder joint, (c) Ni/Sn-Ag-Cu solder joint, and (d) NiP/Sn-Ag-

Cu solder joint after 180 minutes in the molten condition

109

Figure 4.3 : Interfacial structure of- a) Ni/Sn-Ag, b) NiP/Sn-Ag, c) Ni/Sn-

Ag-Cu, d) NiP/Sn-Ag-Cu, and e) Ni/Sn-Pb-Ag, solder joints

as reflowed

112

Figure 4.4 : Dissolution rate of electrolytic Ni and electroless NiP layer

with Sn-Ag-Cu solder

113

Figure 4.5 : Dissolution rate of electrolytic Ni and electroless NiP layer

with Sn-Ag solder

114


Ag-Cu, d) NiP/Sn-Ag-Cu, and e) Ni/Sn-Pb-Ag, solder joints

after 30 minutes reflow

116

Figure 4.7 : Thickness of IMCs of solder joints as a function of reflow

time- a) Sn-Ag, Sn-Ag-Cu and Sn-Pb-Ag solder joints with Ni

and b) Sn-Ag and Sn-Ag-Cu solders joints with NiP

118

Figure 4.8 : Thickness of P-rich Ni layer as a function of reflow time 119


Ag-Cu, d) NiP/Sn-Ag-Cu, and e) Ni/Sn-Pb-Ag solder joints

after 120 minutes reflow

120


Ag-Cu, and d) NiP/Sn-Ag-Cu solder joints after 180 minutes

reflow

122

xvii

Figure 5.1 : Electrolytic Ni/solder joints after 1 minute reflow- a) Interface

of a Sn-Ag-Cu-In solder joint and b) IMCs in the bulk of Sn-

Ag-Cu-In solder.

135

Figure 5.2 : Thin Ni/solder joints after 10 minutes reflow- a) Interface of a

In-containing solder joint and b) IMCs in the bulk In-

containing solder.

136

Figure 5.3 : IMCs thickness as a function of reflow time 136

Figure 5.4 : Interfacial structure of thin Ni/In-containing solder after 20

minutes reflow.

138

Figure 5.5 : Consumed Ni layer thickness with reflow time. 139


Interface of a Ni/Sn-Ag-Cu-In solder joint and b) IMCs within

the bulk In-containing solder.

140

Figure 5.7 : Interfacial reactions of NiP/solder as-reflowed - a) Interface of

a NiP/Sn-Cu solder joint, b) Interface of a NiP/Sn-Ag-Cu

solder joint, and c) Interface of a NiP/Sn-Ag-Cu-In solder

joint.

143

Figure 5.8 : Different types of IMCs in the Cu-containing solder as-

reflowed- a) Sn-Cu, b) Sn-Ag-Cu, and c) Sn-Ag-Cu-In

solders.

144

Figure 5.9 : Interfacial reactions of NiP/solder after 30 minutes reflow- a)

Interface of a NiP/Sn-Cu solder joint, b) Interface of a NiP/Sn-

Ag-Cu solder joint, and c) Interface of a NiP/Sn-Ag-Cu-In

solder joint.

145

Figure 5.10 : a) Cu6Sn5 IMCs with a specific shape in a Sn0.7Cu solder, and

b, c, d) Ni-containing IMCs in a Cu-containing solder after

30minutes reflow.

146

Figure 5.11 : a) Thickness of IMCs with reflow time and b) P-rich Ni layer

thickness with reflow time.

147

xviii

Figure 5.12 : Amount of NiP layer thickness consumed with reflow time. 148



Ag-Cu solder joint, and c) Interface of a NiP/Sn-Ag-Cu-In

solder joint.

149



Ag-Cu solder joint and c) Interface of a NiP/Sn-Ag-Cu-In

solder joint.

151

Figure 6.1 : Schematic diagram showing the soldering procedure 163

Figure 6.2 : SEM images of solder-Cu interfaces after soldering at 255 0C

for a) Sn-2.8Ag-0.5Cu and b) Sn-2.8Ag-0.5Cu-1.0Bi solders.

164

Figure 6.3 : Formation of intermetallic compound in the bulk of the solder

after soldering at 255 0C for a) Sn-2.8Ag-0.5Cu and b) Sn-

2.8Ag-0.5Cu-1.0Bi solders.

165

Figure 6.4 : SEM images showing the solder-Cu interfaces after 2 days of

aging for a) Sn-2.8Ag-0.5Cu and b) Sn-2.8Ag-0.5Cu-1.0Bi

solders.

166



solders.

168



solders.

169

Figure 6.7 : Thickness of intermetallic compound (IMC) layer as a

function of square root of aging time.

171

Figure 6.8 : Plot of ln(Yt-Y0) versus ln t to obtain the time exponent n. 173

Figure 6.9 : Cu-consumption by solder as a function of aging time. 174

Figure 6.10 : Formation of intermetallic compounds in the bulk of a) Sn-

2.8Ag-0.5Cu and b) Sn-2.8Ag-0.5Cu-1.0Bi solders after aging

174

xix

at 150 0C for i) 2 days, ii) 6 days iii) 10 days and iv) 14 days.

Figure 7.1 : Shear load of Sn-Pb-Ag and Sn0.7Cu solder joints with Ni. 183

Figure 7.2 : Fracture surface of Sn-Cu solder after ball shear test 183

Figure 7.3 : Electrolytic Ni/solder joints after 1 minute reflow- a) Bulk Sn-

Cu solder, b) Bulk Sn-Pb-Ag solder, and c) Interface of a Sn-

Cu solder joints.

185

Figure 7.4 : Interfacial structure of a Ni/Sn-Cu solders joints after 30

minutes reflow.

187

Figure 7.5 : IMCs thickness as a function of reflow time. 187

Figure 7.6 : Consumed Ni layer thickness with reflow time. 188


Hexagonal Cu-rich TIMCs and b) Interface of the Sn-Cu

solder joints.

190


Different TIMCs at the interface of the Sn-Cu solder joints,

and b) Interface of the Sn-Pb-Ag solder joints.

193

Figure 7.9 : Sn-Cu-Ni TIMCs vs. shear load of Sn-Cu solder during

extended time of reflow.

196

Figure 7.10 : TIMCs thickness of the solder joints during aging. 198

Figure 7.11 : After 4 days aging at 1750C- a) Bulk solder, and b) Interfacial

structure of solder joint.

199

Figure 7.12 : Fracture surfaces of the solder joints after- a) 4 days aging and

b) 12 days aging at 1750C.

200

Figure 7.13 : After 12 days aging at 1750C- a) Bulk solder, and b)

Interfacial structure of solder joint.

202

Figure 7.14 : After 16 days aging at 1750C- a) Bulk solder, and b)

Interfacial structure of solder joint.

204

Figure 7.15 : Shear load of solder joint during aging. 206

xx

Figure 8.1 : Solder joints with Cu substrates- a) Interface of a Sn-Zn, b)

Interface of a Sn-Zn-Bi, c) Interface of a Sn-Ag-Cu, and d)

Interface of a Sn-Pb solder joint.

216

Figure 8.2 : Interfacial reactions of Cu/solder joints after 1 minute reflow-

a) Interface of a Sn-Zn, b) Interface of a Sn-Zn-Bi, c)

Interface of a Sn-Ag-Cu, and d) Interface of a Sn-Pb solder

joint.

217

Figure 8.3 : Different types of IMCs in the bulk solders as reflowed- a) Sn-

Zn, b) Sn-Zn-Bi and c) Sn-Pb solders.

218

Figure 8.4 : Interfacial reactions of Cu/solder joints after 10 minutes

reflow- a) Interface of a Sn-Zn, b) Interface of a Sn-Zn-Bi and

c) Interface of a Sn-Pb solder joint.

220

Figure 8.5 : Different IMCs phases in solders after 10 minutes reflow- a)

In the bulk Sn-Zn and b) Sn-Zn-Bi solders.

221

Figure 8.6 : IMCs layer thicknesses as a function of reflow time. 222

Figure 8.7 : Amount of Cu layer thickness consumed as a function of

reflow time.

223

Figure 8.8 : Interfacial reactions of Cu/solder joints after 20 minutes

reflow- a) Interface of a Sn-Zn, b) Interface of a Sn-Zn-Bi, c)

Interface of a Sn-Pb solder joint.

224

Figure 8.9 : IMC phases in solders after 20minutes reflow- a) Sn-Zn, b)

Sn-Zn-Bi, and c) Sn-Pb solder.

225

xxi

Abbreviations

ACA Anisotropic conductive adhesive

BIMC Binary Intermetallic Compound

BSE Back Scattered Electron

BGA Ball Grid Array

CSP Chip Scale Package

CTE Coefficient of thermal expansion

CPU Central Processing Unit

DIP Dual-in-line packages

EDX Energy Dispersive X-ray

FC Flip Chip

IC Integrated Circuit

ICA Isotropic conductive adhesive

I/O Input/Output

IMC Intermetallic Compound

KGD Known-Good die

LCD Liquid Crystal Display

ML Metal Layer

PC Personal Computer

PCB Printed Circuit Board

PTH Pin-through hole

PWB Printed Wiring Board

PQFB Plastic Quad Flat Package

QFP Quad Flat Package

QIMC Quaternary Intermetallic Compound

RAM Random Asses Memory

SEM Scanning Electron Microscope

SE Secondary Electron

SIP System-in-Packages

xxii

SMT Surface Mount Technology

SOC System-on-Chip

SOP Small Outline Package

TBGA Tape Ball Grid Array

2 ML Two Metal Layer

TIMC Ternary Intermetallic Compound

UBM Under Bump Metallization

USB Universal serial Bus

WLP Wafer Level Package

List of Symbols

h∆ Consumed thickness of Cu,

A Total interfacial area between the solder and the Cu,

n Weight fraction of Cu in the liquid solder,

V Total volume of liquid solder,

Cuf Weight fraction of Cu in the Cu-Sn compound,

VC Total volume of the Cu-Sn intermetallics

Cuρ Density of Cu

Lρ Density of the liquid solder

Cρ Density of the Cu-Sn intermetallic compound

V1 Volume of IMCs in the interface

VB Volume of IMCs in the bulk

Y Thickness of the intermetallic layer

t Time

k Intermetallic growth rate constant

n Time exponent

µm Micrometre

mm Millimetre

m Metre

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