Wire Bonding in Microelectronics

Transcription

1 Wire Bonding in Microelectronics Materials, Processes, Reliability, and Yield George G. Harman Second Edition McGraw-Hill New York San Francisco Washington, D.C. Auckland Bogota Caracas Lisbon London Madrid Mexico City Milan Montreal New Delhi San Juan Singapore Sydney Tokyo Toronto

2 Contents Preface xi Acknowledgments xiv Chapter 1. Technical Introduction to the Second Edition Wedge- and Ball-Bonding Machine Operations How to Approach Bonding Problems Which materials can be ultrasonically bonded? Assessing the bondability and reliability of proposed new bond Systems Some unusual uses of wire bonds 10 Chapter 2. Ultrasonic Bonding Systems and Technologies (Including Ultrasonic Wire Bonding Mechanism) Introduction Ultrasonic Transducer and Tool Vibration Modes How Ultrasonic Bonds Are Made Bonding with High (er) Frequency Ultrasonic Energy In-Process (Real-Time) Bond Monitoring Wire-Bonding Technologies Thermocompression bonding Ultrasonic wedge bonding (small- and large-diameter wires) Thermosonic Ball and Wedge Bonding Choosing a wire-bonding technology Variations of Fine Wire-Bonding Technology Ribbon wire bonding Parallel gap and tweezer welding Major Chip Interconnection Alternatives to Wire Bonding (Flip Chip and TAB); Limits of Wire Bonding Flip Chip TAB Wire-bonding technology: a comparison 37

3 vi Contents Chapter 3. Some Aspects of Bonding Wire Characteristics that Can Affect Bonding, Reliability, and Testing Introduction Stress-Strain Characteristics of Bonding Wires Shelf-Life Aging of Bonding Wires General Discussion of Gold Bonding Wire Aluminum Wire for Ultrasonic Wedge Bonding Wire and Metallization Hardness Effect of EFO Polarity on Gold Wire and Its Metallurgy Metallurgical Fatigue of Bonding Wires Conductor Burnout Bonding wires Maximum allowable current for PCB and MCM conductors 63 Appendix WM-1: A Listing of Useful ASTM Standards and Specifications on Bonding Wire and Bond Testing 63 Chapter 4. Wire Bond Testing Introduction The Destructive Bond Pull Test Variables of the bond pull fest Failure predictions that are based on pull test data Effect of metallurgy and bonding processes on the bond pull force Effect of wire elongation on bond-pull force (largediameter AI, and Au wire used in ball bonding) Nondestructive Pull Test Introduction Metallurgical and Statistical Interpretation of NDP test Assessment of any NDP test-induced metallurgical defects Limitations of the NDP test The current Status of the NDPT (1996) for military and space applications Ball-Bond Shear Test Introduction Apparatus A manual shear probe as an aid in setting up a ball bonder Interferences to making accurate ball-shear test measurements Ball-shear force versus bonded area Effect of gold-aluminum intermetallics on the shear force Pluck test Comparison of the ball-shear and bond-pull tests Applications of the ball-shear test Shear test for wedge bonds Ball-shear test standardization Evaluating Both the Ball and the Wedge Bond on a Single Wire Thermal Stress Test for Al-Au Wire Bond Reliability Future Issues in Wire Bond Testing Appendix TA-1 110

4 Contents vii Chapter 5. Gold-Aluminum Intermetallic Compounds and Other Metallic Interface Reactions Encountered in Wire Bonding Gold-Aluminum Intermetallic Compound Formation and Classical Wire Bond Failures Introduction Intermetallic Compound formation The classical Au-Al Compound failure modes Reverslng metaliurgical interfaces The effect of dlffuslon Inhibitors and barriers Impurity-Accelerated Au-Al Bond Failures The effect of halogens on the Au-Al bond System Recommendations for removing or avoidlng halogen contamination Nonhalogen epoxy outgassing Induced bond failures Non-gold-aluminum Bond Interfaces Alumlnum-copper wire bond system Alumlnum metalllzation contalning copper Copper-gold wire bond system Palladium-Au and -AI bonding system (used for lead frames) The silver-aluminum wire bond system Alumlnum-nickel wire bond system Au-Au, AI-AI, Au-Ag, and less-used monometallic bonding Systems 145 Appendix IA-1 :Rapid Bond Failure in Poorly Welded Au-Al Bonds 149 Appendix IA-2:Various Bond-Related Corrosion Reactions 152 Chapter 6. Bond Failures Resulting from Gold-Plating Impurities and Conditions Introduction Specific Piating Impurities Hydrogen Gas in Piated Films Hydrogen-induced package problems Failure Symptoms that appear similar to gas entrapments: resistance drift Failures from Metallic impurities in or on Gold Films That Are Not an Intentional Part of Piating Baths Introduction Nickel Copper Chromium Titanium Tin Gold-Plating Standards Recommendations for reliable gold-plated films Electroless Autocatalytic Gold Nongold Platings Used in Electronics Packaging 178 Chapter 7. Cleaning to Improve Bondability and Reliability Introduction Molecular cleaning methods to enhance bondability and reliability 184

5 viii Contents Ultraviolet-ozone cleaning Plasma cleaning Plasma cleaning mechanism Discussion and evaluation of molecular and solvent cleaning methods Problems encountered in using molecular cleaning methods Burnishing The Sensitivity of Different Bonding Technologies to Surface Contamination 196 Appendix CL-1: Circuit Damage Caused by Plasma Cleaning during Packaging 198 Chapter 8. Mechanical Problems in Wire Bonding Cratering Introduction Bonding machine characteristics and setup Parameters Bonding force Tool wire-pad impact force Causes of cratering materials Intermetallics Silicon nodule-induced cratering Cratering over polysilicon Gallium arsenide cratering Cracks in the Heels of Ultrasonic Wedge Bonds The Effect of Acceleration, Vibrations, and Shock Centrifuge effects on wire bonds The effect of Ultrasonic cleaning on wire bonds The effect of shock and Vibration tests on wire bonds Effects of Power and Temperature Cycling of Wire Bonds 231 Appendix M-1: Fracture Toughness Defined 236 Chapter 9. High-Yield and Fine-Pitch Wire Bonding Introduction The Background Necessary to Achieve High-Yield Bonding The Requirements for High-Yield Bonding Clean, bondable metallization The bonding machine and its control Reliability for Small Numbers of Bonds (Small Sample Statistics) Package-Related Bond-Yield Issues Possible 6cr Yieid Enhancements and Problems that Need Further Study Other Conclusions that May Affect Device Yield Wire sweep Wire looping Fine-Pitch Ball and Wedge Bonding Introduction Fine-pitch ball and wedge bonding The Problems of Fine-Pitch Bonding Conclusions Acknowledgments 260

6 Contents ix Chapter 10. Wire Bonding to Multichip Modules and Other Soft Substrates Introduction Bonding to MCM-D Substrates Bonding to MCM-L Substrates The Effect of the Substrate's Material Properties on Wire Bonding Bonding Machine Considerations Additional Considerations When Using Wire Bonds in MCMs Running at High Clock Rates Inductance of wire bonds Skin-effect in typical MCM conductor metal structures Conclusions Acknowledgments 278 Glossary 281 Index 285