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
}}ÄÄ��UU��
Microstructural Analysis of Lead-free Solders
with Under Bump Metallization
¤¤))ìì±±VV����nnFFss��pptt��ÿÿ��¨̈11ÞÞ
èèÌÌ��ëë
Submitted to
Department of Electronic Engineering
=W`ÓÓ
in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy
C�ÑS
by
Md. Nazrul Islam
January 2006
2??�£+Ü
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
3.3.1 Results and discussions 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
5.1 Introduction 132
5.2 Effects of In-containing solder with thin Ni metallization on Cu pads 134
5.2.1 Results and discussions 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 Results and discussions 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
6.1 Introduction 161
6.2 Results and discussions 163
6.3 Summary 177
References 178
Chapter 7: Investigation of Sn0.7Cu solder with Au/Ni/Cu UBM
7.1 Introduction 181
7.2 Results and discussions 182
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
8.1 Introduction 212
8.2 Results and discussions 214
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
Figure 4.6 : 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
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
Figure 4.9 : 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
after 120 minutes reflow
120
Figure 4.10 : Interfacial structure of- a) Ni/Sn-Ag, b) NiP/Sn-Ag, c) Ni/Sn-
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
Figure 5.6 : Electrolytic Ni/solder joints after 30 minutes reflow- a)
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
Figure 5.13 : Interfacial reactions of NiP/solder after 120 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.
149
Figure 5.14 : Interfacial reactions of NiP/solder after 180 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.
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
Figure 6.5 : SEM images showing the solder-Cu interfaces after 6 days of
aging for a) Sn-2.8Ag-0.5Cu and b) Sn-2.8Ag-0.5Cu-1.0Bi
solders.
168
Figure 6.6 : SEM images showing the solder-Cu interfaces after 14 days of
aging for a) Sn-2.8Ag-0.5Cu and b) Sn-2.8Ag-0.5Cu-1.0Bi
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
Figure 7.7 : Electrolytic Ni/solder joints after 120 minutes reflow- a)
Hexagonal Cu-rich TIMCs and b) Interface of the Sn-Cu
solder joints.
190
Figure 7.8 : Electrolytic Ni/solder joints after 180 minutes reflow- a)
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