vii
TABLE OF CONTENTS
CHAPTER
TITLE
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
xi
LIST OF FIGURES
xiii
LIST OF ABBREVIATIONS
1
PAGE
xviii
LIST OF APPENDICES
xx
INTRODUCTION
1
1.1
Introduction
1
1.2
Field of Research
4
1.3
Objectives of the Research
6
1.4
Scopes of the Research
6
1.5
Importance of the Research
7
1.6
Structure of Thesis
7
viii
2
LITERATURE REVIEW - ELECTRONIC PACKAGING
8
2.1
Introduction
8
2.1.1
Electronic Package Hierarchy
9
2.1.2
Purpose of Electronic Packaging
10
2.1.3
Requirement of the Electronic Packaging
11
2.1.4
Interconnection Implementation
12
2.1.4.1 Wire Bonding Interconnection
12
2.1.4.2 Tape-Automated Bonding
14
2.1.4.3 Flip Chip Bonding
16
2.2
Flip Chip Interconnect
2.2.1
3
4
21
Solder Bump Structure for Flip Chip
Interconnection
22
2.2.1.1 Under Bump Metallurgy (UBM)
23
2.2.1.2 Top Surface Metallurgy (TSM)
24
LITERATURE REVIEW - SURFACE FINISH SYSTEMS
25
3.1
Introduction
25
3.2.
Thickness of Surface Finish Layer
26
3.3.
Surface Finish Systems
29
3.3.1
Hot-Air Solder Leveling (HASL)
30
3.3.2
Organic Solderability Preservative (OSP) Finish
31
3.3.3 Electroless Nickel/ Immersion Gold
32
3.3.4
Nickel/ Palladium/ Gold Finish
34
3.3.5
Immersion Silver
36
3.3.6
Immersion Tin
38
3.3.7
Summary
39
LITERATURE REVIEW – SOLDERING
41
4.1
Introduction
41
4.2
Material
43
4.3
Soldering Techniques
44
4.3.1
44
Reflow Soldering
4.3.2 Wave Soldering
49
4.3.3
50
Hand Soldering
ix
4.4
4.5
51
4.4.1 Lead-based Solders
54
4.4.2
Lead-free Solders
56
4.4.2.1 Characteristic of Lead Free Solders
59
4.4.2.2 Melting Temperature
61
4.4.2.3 Microstructure
62
Flux
63
4.5.1
Flux Functions
64
4.5.2
Flux components
65
4.5.3
Types of Flux
66
4.5.3.1 Resin fluxes
66
4.5.3.2 Water soluble flux
67
4.5.3.3 No clean fluxes
68
4.6
Solderability
70
4.7
Intermetallic Compounds
72
4.7.1 Factors Affecting the Growth of IMC
75
4.7.2 Effects of IMC
77
Isothermal Aging Treatment
78
RESEARCH METHODOLOGY
80
5.1
Introduction
80
5.2
Substrate Material
81
5.3
Plating Process
83
5.3.1
Pretreatment of copper substrate
83
5.3.2
Plating Equipment Setup
84
5.3.3
Electroless Nickel Plating
86
5.3.4
Immersion Gold Plating
87
5.3.5
immersion Silver Plating
88
4.8
5
Solder Materials
5.4
Reflow Soldering
90
5.4.1
Solder Masking
90
5.4.2
Flux
90
5.4.3
Solder Bump Formation
91
5.5
Isothermal Aging
92
5.6
Materials Characterisation
92
x
5.6.1
Characterization of Specimens Cross Section
5.6.2 Characterization of Specimens Top Surface
6
94
RESULTS AND DISCUSSION
95
6.1
Introduction
95
6.2
Top Surface Metallurgy (TSM) Deposition
96
6.3
Identification of Intermetallics in Solder Joints
97
6.4
Composition and Surface Morphologies of IMC
100
6.4.1
Intermetallics between SAC and ImAg
100
6.4.1.1 Reflow Soldering
100
6.4.1.2 Isothermal Ageing
106
6.4.1.3 Formation of Kirkendall Voids
116
Intermetallics between SAC and ENIG
118
6.4.2.1 Reflow Soldering
118
6.4.2.2 Isothermal Ageing
133
6.4.2
6.5
7
93
Thickness of Intermetallic Compound
145
6.5.1
145
Effect of Solder Volume on IMC Thickness
6.5.2 Effect of Surface Finishes on IMC Thickness
149
6.5.3 Growth Kinetics of IMC on ImAg Finish
150
6.5.4 Effect of Ag Concentration on IMC Thickness
154
6.5.5 Effect of Ageing Duration on IMC Thickness
159
CONCLUSIONS AND FUTURE WORKS
161
7.1
Conclusion
161
7.2
Future Works
162
REFERENCES
164
APPENDIX
175
PUBLISHED PAPERS
191
xi
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Levels of interconnection for general electronic system
10
2.2
The advantages and disadvantages of wire bonding
interconnection
13
The advantages and disadvantages of TAB over the wire
bonding technology
15
2.4
Comparison of Interconnection Implementation
19
3.1
Comparison between different surface finish
39
4.1
Summary of reflow profiling
46
4.2
Benefits and limitations for vary reflow method
48
4.3
Melting properties of some common solder alloys
51
4.4
Lead-Free Solders for CSP Applications
56
4.5
Properties of Hard and Soft Solder Alloys
60
4.6
Lead-free solders with liquidus (T1), solidus (T2) and
eutectic (Te)
61
4.7
Solderability of different base metal
72
4.8
Potential IMC formation and un-compatibility between
solder and common substrates
73
5.1
The Swan and Gostin bath
87
5.2
Immersion silver bath formulation
89
5.3
Chemical composition of Klemm Solution II
93
2.3
xii
5.4
Etching time for cross sectional deep etching
94
6.1
Atomic number of elements
99
6.2
Atomic percentage of predicted IMCs
99
6.3
Compositions of the interfacial reaction products after reflow
soldering and ageing for 2000 hours at 150 oC
134
6.4
Intermetallic Thickness (µm) on ImAg surface finish
146
6.5
Intermetallic Thickness (µm) on ENIG surface finish
146
6.6
Calculation of the growth rate coefficient (D) for
SAC405/ ImAg
152
Calculation of the growth rate coefficient (D) for
SAC305/ ImAg
152
6.7
xiii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
1.1
Schematic showing the main parts of an electronic package
2
2.1
Electronic packaging hierarchy
9
2.2
The wire bonding assembly shows how a bare chip is
interconnected to a substrate or another chip using a
wire conductor
13
2.3
Schematic of TAB
14
2.4
Example of TAB devices
15
2.5
(a) Standard flip chip array with solder bumps.
(b) Cross-section of flip chip bonding
17
2.6
Solder Bump Structure
22
3.1
Dissolution rates of a few typical base metals in tin
27
3.2
Schematic diagram of the HASL technique
31
4.1
Typical solder reflow profile for eutectic Pb-Sn solder
45
4.2
The main heating options in reflow soldering
47
4.3
Typical wave soldering machine
49
4.4
The principle of hand soldering
50
4.5
Phase Diagram of Pb-Sn Alloy
54
4.6
The wetting angle
71
xiv
4.7
Cross section through a soldered joint, made with eutectic
solder
74
4.8
Types of intermetallics formed between Cu and Sn
74
4.9
Needle-like Cu6Sn5 intermetallics
74
4.10
Schematic diagrams of the layers before and after isothermal
ageing
79
5.1
(a) Plan view and (b) Side view of copper substrate
81
5.2
Process flowchart of reflow soldering and specimen analysis
82
5.3
(a) Experimental set-up in the plating bath, (b) The plating
bath and (c) Schematic set-up of electroless nickel and
immersion gold plating process
85
5.4
Schematic set-up of immersion silver plating
85
5.5
Commercial medium phosphorous concentration
electroless nickel plating solution: NIMUDEN 5X
86
5.6
Schematic process of electroless nickel plating
86
5.7
Schematic process of immersion gold plating on nickel
87
5.8
Immersion silver plating steps
88
5.9
Schematic process of immersion silver plating
89
5.10
Process flowchart for Immersion Silver
89
5.11
Solder joint formations for ENIG surface finish
91
5.12
Reflow profile fro Sn-Ag-Cu
92
5.13
IMCs formed from the top surface view
94
6.1
Copper substrate before plating (after pretreatment process)
96
6.2
Copper substrate plated with silver coating
97
6.3
FESEM-EDX results of IAg on Cu
97
6.4
Example of weight percentage calculation
98
6.5
Cross-sectional optical images after reflow: a) SAC405/ ImAg,
b) SAC305/ImAg
101
xv
6.6
6.7
6.8
6.9
6.10
Cross-sectional SEM images after reflow: a) SAC405/ImAg,
b) SAC305/ImAg
101
Top view micrographs formed during reflow between SAC405
solder and ImAg. (a) 200µm, (b) 300µm, (c) 500µm and
(d) 700µm
102
Top view micrographs formed during reflow between SAC305
solder and ImAg. (a) 200µm, (b) 300µm, (c) 500µm and
(d) 700µm
103
Top view SEM images showing formation of large Ag3Sn
plates and Cu6Sn5 rods in SAC405/ ImAg (a, b) and Cu6Sn5
rods on SAC305/ ImAg (c)
104
Formation of Ag3Sn during reflow between SAC405
solder and ImAg:(a, b, c) Top surface morphology of the
solder joint and (d) Cross section (x500)
105
6.11
Optical micrographs of cross-sectional views of SAC405/ ImAg
(a-c) and SAC305/ImAg (d-f). (a, d): after reflow and (b, e):
after ageing at 150oC for 250 hours and (c,f) after ageing at
150oC for 2000 hours
107
6.12
SEM images of cross-sectional views showing the effect of
ageing on the interfacial morphology. (a) SAC405/ImAg and
(b) SAC305/ ImAg
109
Morphology of Cu6Sn5 on ImAg for 200µm solder bump of
SAC405, (a) Ageing 250 hours, (b) Ageing 500 hours,
(c) Ageing 1000 hours and (d) Ageing 2000 hours
110
Morphology of Cu6Sn5 on ImAg for 200µm solder bump of
SAC305, (a) Ageing 250 hours, (b) Ageing 500 hours,
(c) Ageing 1000 hours and (d) Ageing 2000 hours
111
Morphology of Cu6Sn5 on ImAg for 700µm solder bump of
SAC405, (a) Ageing 250 hours, (b) Ageing 500 hours,
(c) Ageing 1000 hours and (d) Ageing 2000 hours
112
Morphology of Cu6Sn5 on ImAg for 700µm solder bump of
SAC305, (a) Ageing 250 hours, (b) Ageing 500 hours,
(c) Ageing 1000 hours and (d) Ageing 2000 hours
113
Schematic of Ag3Sn particles embedded during intermetallic
growth
113
Ag3Sn on ImAg using 700µm solder (a) After reflow and
(b) After ageing for 500 hours
115
6.13
6.14
6.15
6.16
6.17
6.18
xvi
6.19
Schematic diagram of IMCs growth in Cu/Au specimen:
(a) dissolution of Ag layer into molten solder, (b) formation
of Cu6Sn5 during reflow soldering and (c) Conversion of
Cu3Sn and Ag3Sn after isothermal ageing
116
SEM image of cross-sectional view of 500µm SAC405
solder/ ImAg after ageing for 500 hours
117
SEM image of cross-sectional view of 700µm SAC405
solder/ ImAg after ageing for 1000 hours
117
6.22
The mechanism of Kirkendall Voids formation
118
6.23
Cross-section views of the intermetallics formed between
ENIG and SAC405 (a-c) and SAC305 (d-f) solders (X500)
119
Cross section and top views of (Cu, Ni)6Sn5 IMC formed
during reflow between ENIG and SAC405 solder.
(a, d) 300 µm, (b, e) 500 µm and (c, f) 700 µm
122
Cross section and top views of (Cu, Ni)6Sn5 IMC formed
during reflow between ENIG and SAC305 solder.
(a, d) 300 µm, (b, e) 500 µm and (c, f) 700 µm
123
Top view of intermetallics formed between ENIG and
200 µm SAC405 (top) and SAC305 (bottom) solders
126
SEM images of cross sections of intermetallic formed between
ENIG and SAC405 for (a) 500 µm and (b) 700 µm solders
127
SEM images of cross sections of intermetallic formed between
ENIG and SAC305 for (a) 300 µm and (b) 500 µm solders
128
EDX results of interface intermetallic formed between ENIG
and 500 µm SAC405 solder during reflow
129
EDX results of interface intermetallic formed between ENIG
and 700 µm SAC405 solder during reflow
130
EDX results of interface intermetallic formed between ENIG
and 300 µm SAC305 solder during reflow
131
EDX results of interface intermetallic formed between ENIG
and 500 µm SAC305 solder during reflow
132
Cross sections of IMCs formed between ENIG and SAC405
solder. (a) reflow (500 µm) and (b) after 2000 hrs ageing
(500 µm),(c) reflow (700 µm) and (d) after 2000 hrs ageing
(700 µm)
134
6.20
6.21
6.24
6.25
6.26
6.27
6.28
6.29a
6.29b
6.30a
6.30b
6.31
xvii
6.32
SAC305 (a) after reflow and (b) ageing (2000 hrs)
135
6.33
Effect of ageing time on intermetallics formed between ENIG
and SAC405 solder with different solder sizes. 300 µm solder:
a: reflow, b: ageing for 250 hours, g: ageing for 500 hours and
h: ageing for 2000 hours. 500 µm solder: c: reflow, d: ageing
for 250 hours, i: ageing for 500 hours and j: ageing for 2000
hours. 700 µm solder: e: reflow, f: ageing for 250 hours,
136
Top view of intermetallics formed between ENIG and 200 µm
SAC405: a: reflow, b: ageing for 250 hours, and c: ageing for
1000 hours
138
6.34
6.35
Formation of Ag3Sn in the 700 microns solder bump after reflow
soldering (SAC405). (a, b) Top surface morphology of the solder
joint and (c, d) Cross section
141
6.36
SEM image showing different morphology of intermetallics
between center and periphery of solder joint
142
Different morphologies of IMC which form circular boundary
regions in Sn-Ag-Cu solder joint, (a) Sn-3Ag-0.5Cu solder and
(b) Sn-4Ag-0.5Cu solder
143
Effect of Ageing on the morphology of Ag3Sn intermetallic.
(a) After reflow soldering and (b) After 500 hours ageing
144
Intermetallic thickness versus solder bump size for ImAg
surface finish as function of ageing time. (a) Sn-4Ag-0.5Cu
and (b) Sn-3Ag-0.5Cu
147
6.37
6.38
6.39
6.40
Intermetallic thickness versus solder bump size for ENIG surface
finish as function of ageing time. (a) SAC405 and (b) SAC305 148
6.41
Intermetallic thickness versus ageing time between SAC405
and ImAg surface finish: (top) Cu3Sn and (bottom)
Cu6Sn5 layer
153
Intermetallic thickness versus ageing time between SAC305
and ImAg surface finish: (top) Cu3Sn and (bottom)
Cu6Sn5 layer
154
SEM top views of Ag3Sn intermetallic for SAC405/ ImAg
(a, b) and SAC305/ ImAg (c)
157
Ag3Sn IMC formation in ImAg surface finish
159
6.42
6.43
6.44
xviii
LIST OF ABBREVIATIONS
ASTM
-
American Society for Testing and Materials
BGA
-
Ball grid array
BLM
-
Ball limiting metallurgy
C4
-
Controlled collapse chip connection
CSP
-
Chip scale package
COB
-
Chip on board
DIG
-
Direct immersion gold
EDX
-
Energy dispersive spectrum
ENEPIG
-
Electroless nickel/ electroless palladium/ immersion gold
ENIG
-
Electroless nickel /immersion gold
EU
-
European Union
FC
-
Flip chip
FESEM
-
Field emission scanning electron microscope
HASL
-
Hot air soldered levelled
IC
-
Integrated circuit
IMC
-
Intermetallic compound
I/ O
-
Input/ output
xix
OSP
-
Organic solderable presevatives
PCB
-
Printed circuit board
PWB
-
Printed wire bonding
R
-
Pure rosin flux
RA
-
Activated rosin flux
RMA
-
Midly activated rosin flux
SEM
-
Scanning electron microscope
SMD
-
Surface mount devices
SMT
-
Surface mount technology
TAB
-
Tape automated bonding
TSM
-
Top surface metallurgy
WB
-
Wire bonding
WEEE
Waste from electrical and electronic equipments
WW
-
Water white rosin flux
XRD
-
X-ray diffraction
xx
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
FESEM/ EDX results (selected samples only)
175
B
Tables and Graphs
185