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