Lead-free Soldering Process Development and Reliability
Description
Lead-free Soldering Process Development and Reliability provides a comprehensive discussion of all modern topics in lead-free soldering. Perfect for process, quality, failure analysis and reliability engineers in production industries, this reference will help practitioners address issues in research, development and production.
Among other topics, the book addresses:
· Developments in process engineering (SMT, Wave, Rework, Paste Technology)
· Low temperature, high temperature and high reliability alloys
· Intermetallic compounds
· PCB surface finishes and laminates
· Underfills, encapsulants and conformal coatings
· Reliability assessments
In a regulatory environment that includes the adoption of mandatory lead-free requirements in a variety of countries, the book's explanations of high-temperature, low-temperature, and high-reliability lead-free alloys in terms of process and reliability implications are invaluable to working engineers.
Lead-free Soldering takes a forward-looking approach, with an eye towards developments likely to impact the industry in the coming years. These will include the introduction of lead-free requirements in high-reliability electronics products in the medical, automotive, and defense industries. The book provides practitioners in these and other segments of the industry with guidelines and information to help comply with these requirements.
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JASBIR BATH, is the owner of Bath Consultancy LLC which provides consulting and training services in the electronics manufacturing industry. He was the Corporate Lead Engineer with Solectron Corporation/ Flex for ten years with a role involving tin-lead and lead-free solder process development. Previously he was a Technical Officer at ITRI (International Tin Research Institute/ Tin Technology) Ltd in the U.K.
Content
List of Contributors xix
Introduction xxi
1 Lead-Free Surface Mount Technology 1
Jennifer Nguyen and Jasbir Bath
1.1 Introduction 1
1.2 Lead-Free Solder Paste Alloys 1
1.3 Solder Paste Printing 2
1.3.1 Introduction 2
1.3.2 Key Paste Printing Elements 2
1.4 Component Placement 5
1.4.1 Introduction 5
1.4.2 Key Placement Parameters 5
1.4.2.1 Nozzle 6
1.4.2.2 Vision System 6
1.4.2.3 PCB Support 6
1.4.2.4 Component Size, Packaging, and Feeder Capacity 6
1.4.2.5 Feeder Capacity 6
1.5 Reflow Process 7
1.5.1 Introduction 7
1.5.2 Key Parameters 7
1.5.2.1 Preheat 7
1.5.2.2 Soak 8
1.5.2.3 Reflow 8
1.5.2.4 Cooling 9
1.5.2.5 Reflow Atmosphere 9
1.6 Vacuum Soldering 9
1.7 Paste in Hole 10
1.8 Robotic Soldering 11
1.9 Advanced Technologies 12
1.9.1 Flip Chip 12
1.9.2 Package on Package 12
1.10 Inspection 13
1.10.1 Solder Paste Inspection (SPI) 13
1.10.2 Solder Joint Inspection 14
1.10.2.1 Automated Optical Inspection (AOI) 14
1.10.2.2 X-ray Inspection 15
1.11 Conclusions 16
References 17
2 Wave/Selective Soldering 19
Gerjan Diepstraten
2.1 Introduction 19
2.2 Flux 19
2.2.1 The Function of a Flux 19
2.2.2 Flux Contents 20
2.3 Amount of Flux Application on a Board 20
2.4 Flux Handling 21
2.5 Flux Application 21
2.5.1 Methods to Apply Flux (Wave Soldering) 21
2.5.2 Methods to Apply Flux (Selective Soldering) 23
2.6 Preheat 24
2.6.1 Preheat Process-Heating Methods 24
2.6.2 Preheat Temperatures 27
2.6.3 Preheat Time 28
2.6.4 Controlling Preheat Temperatures 28
2.6.5 BoardWarpage Compensation (Selective Soldering) 29
2.7 Selective Soldering 29
2.7.1 Different Selective Soldering Point to Point Nozzles (Selective Soldering) 29
2.7.2 Solder Temperatures (Selective Soldering) 30
2.7.3 Dip/Contact Times (Selective Soldering) 31
2.7.4 Drag Conditions (Selective Soldering) 31
2.7.5 Nitrogen Environment (Selective Soldering) 31
2.7.6 Wave Height Controls (Selective Soldering) 32
2.7.7 De-Bridging Tools (Selective Soldering) 32
2.7.8 Solder Pot (Selective Soldering) 33
2.7.9 Topside Heating during Soldering (Selective Soldering) 34
2.7.10 Selective Soldering Dip Process with Nozzle Plates (Selective Soldering) 34
2.7.11 Solder Temperatures for Multi-Wave Dip Soldering (Selective Soldering) 35
2.7.12 Nitrogen Environment (Selective Soldering) 35
2.7.13 Wave Height Control (Selective Soldering) 36
2.7.14 Dip Time - Contact Time with Solder (Selective Soldering) 36
2.7.15 Solder Flow Acceleration and Deceleration (Selective Soldering) 37
2.7.16 De-Bridging Tools (Selective Soldering) 37
2.7.17 Pallets (Selective Soldering) 38
2.7.18 Conveyor (Selective Soldering) 38
2.8 Wave Soldering 39
2.8.1 Wave Formers (Wave Soldering) 39
2.8.2 Pallets (Wave Soldering) 40
2.8.3 Nitrogen Environment (Wave Soldering) 40
2.8.4 Process Control (Wave Soldering) 41
2.8.5 Conveyor (Wave Soldering) 41
2.9 Conclusions 42
References 42
3 Lead-Free Rework 43
Jasbir Bath
3.1 Introduction 43
3.2 Hand Soldering Rework for SMT and PTH Components 43
3.2.1 Alloy and Flux Choices 43
3.2.1.1 Alloys 43
3.2.1.2 Flux 44
3.2.2 Soldering Iron Tip Life 44
3.2.3 Hand Soldering Temperatures and Times 47
3.3 BGA/CSP Rework 50
3.3.1 Alloy and Flux Choices 50
3.3.1.1 Alloys 50
3.3.1.2 Flux 50
3.3.2 BGA/CSP Rework Soldering Temperatures and Times 50
3.3.3 Component Temperatures in Relation to IPC/JEDEC J-STD-020 and Component/BoardWarpage Standards 52
3.3.3.1 IPC/JEDEC J-STD-020 Standard 52
3.3.3.2 ComponentWarpage Standards 52
3.3.3.3 BoardWarpage Standards 52
3.3.4 Equipment Updates for Lead-Free BGA/CSP Rework 53
3.3.5 Adjacent Component Temperatures 53
3.4 Non-standard Component Rework (Including BTC/QFN) 54
3.4.1 Alloy and Flux Choices 54
3.4.1.1 Alloys 54
3.4.1.2 Flux 54
3.4.2 Soldering Temperatures and Times 54
3.4.3 Non-standard Component Temperatures in Relation to IPC JEDEC J-STD-020 Standard and ComponentWarpage Standards 55
3.4.4 Equipment and Tooling Updates for Lead-Free Non-standard Component Rework 55
3.4.5 Adjacent Component Temperatures 56
3.4.6 Non-standard Component Rework Solder Joint Reliability 56
3.5 PTH (Pin-Through-Hole)Wave Rework 56
3.5.1 Alloy and Flux Choices 56
3.5.1.1 Alloys 56
3.5.1.2 Flux 57
3.5.2 Soldering Temperatures and Times 57
3.5.3 Component Temperatures in Relation to Industry and Board Standards During PTH Rework 67
3.5.3.1 Component Temperature Rating Standards 67
3.5.3.2 Bare Board Testing Standards and Methods for PTH Rework 67
3.5.4 Equipment Updates for PTH Component Rework 68
3.5.5 Adjacent Component Temperatures During PTH Rework 68
3.5.6 PTH Component Rework Solder Joint Reliability 68
3.5.6.1 Copper Dissolution 68
3.5.6.2 Holefill 69
3.6 Conclusions 69
References 70
4 Solder Paste and Flux Technology 73
Shantanu Joshi and Peter Borgesen
4.1 Introduction 73
4.2 Solder Paste 75
4.2.1 Water-Soluble Solder Paste 75
4.2.2 No-Clean Solder Paste 76
4.3 Flux Technology 77
4.3.1 Halide-Free and Halide-Containing 77
4.4 Composition of Solder Paste 79
4.4.1 Alloy 79
4.4.2 Flux 82
4.4.3 Solder Powder Type 83
4.4.3.1 Oxide Layer 84
4.5 Characteristics of a Solder Paste 84
4.5.1 Printing 84
4.5.1.1 Printing Parameters 85
4.5.2 Reflow 86
4.5.2.1 Wetting/Spreadability of Lead-Free Solder Paste 86
4.5.2.2 Bridging 86
4.5.2.3 Micro Solder Balls 86
4.5.2.4 Voiding 86
4.5.2.5 Head-on-Pillow Component Soldering Defect 88
4.5.2.6 Non-Wet Open 90
4.5.2.7 Tombstoning 90
4.5.3 In-Circuit Test (ICT) Probe Testability 90
4.5.4 Flux Reliability Issues 91
4.6 Conclusions 92
References 92
5 Low Temperature Lead-Free Alloys and Solder Pastes 95
Raiyo Aspandiar, Nilesh Badwe, and Kevin Byrd
5.1 Introduction 95
5.1.1 Definition of Low Temperature Solders 95
5.1.2 Benefits of Low Temperature Soldering 97
5.1.2.1 Reduced Manufacturing Cost 98
5.1.2.2 Power Use Savings 98
5.1.2.3 Environmental Benefits 99
5.1.2.4 Manufacturing Yield Improvements 100
5.1.3 Drawbacks 103
5.1.3.1 Brittleness 103
5.1.4 Other Low Temperature Metallurgical Systems 103
5.2 Development of Robust Bismuth-Based Low Temperature Solder Alloys 105
5.2.1 Bismuth-Tin (Bi-Sn) Phase Diagram 105
5.2.2 Mechanical Properties 107
5.2.3 Physical Properties 108
5.2.4 Alloy Development Progress 108
5.2.5 Fluxes for Low Temperature Solders 109
5.3 SMT Process Characterization of Sn-Bi Based Solder Pastes 111
5.3.1 Printability 111
5.3.2 Reflow Profiles 112
5.3.3 Rework 113
5.4 Polymeric Reinforcement of Sn-Bi Based Low Temperature Alloys 114
5.4.1 Current Polymeric Reinforcement Strategies 114
5.4.2 Joint Reinforced Pastes (JRP) 118
5.4.3 Polymeric Reinforcement Summary 128
5.5 Mixed SnAgCu-BiSn BGA Solder Joints 128
5.5.1 Formation Mechanism 128
5.5.2 Microstructural Features and Key Characteristics 133
5.5.3 Soldering Process Optimization 134
5.5.4 Possible Defects 135
5.6 Solder Joint Reliability 140
5.7 Conclusions 145
5.8 Future Development and Trends 146
References 149
6 High Temperature Lead-Free Bonding Materials - The Need, the Potential Candidates and the Challenges 155
Hongwen Zhang and Ning-Cheng Lee
6.1 Introduction 155
6.2 Solder Materials 159
6.2.1 Gold-Based Solders 159
6.2.2 Bismuth-Rich Solders 160
6.2.2.1 Design of Bismuth-Rich Solders 160
6.2.2.2 Mechanical Behavior of BiAgX 163
6.2.2.3 Microstructure and Microstructural Evolution of BiAgX Joint 167
6.2.3 Tin-Antimony (Sn-Sb) High Temperature Solders 174
6.2.4 Zinc-Aluminum Solders 176
6.3 Silver (Ag)-Sintering Materials 178
6.4 Transient Liquid Phase Bonding Materials/Technique 181
6.5 Summary 182
Acknowledgment 185
References 185
7 Lead (Pb)-Free Solders for High Reliability and High-Performance Applications 191
Richard J. Coyle
7.1 Evolution of Commercial Lead (Pb)-Free Solder Alloys 191
7.1.1 First Generation Commercial Pb-Free Solders 191
7.1.2 Second Generation Commercial Pb-Free Solders 192
7.1.3 Third Generation Commercial Pb-Free Solders 196
7.2 Third Generation Alloy Research and Development 196
7.2.1 Limitations of Sn-Ag-Cu Solder Alloys 196
7.2.2 Emergence of Commercial Third Generation Alloys 202
7.2.2.1 The Genesis of 3rd Generation Alloy Development 202
7.2.2.2 An Expanding Class of 3rd Generation Alloys 202
7.2.3 Metallurgical Considerations 203
7.2.3.1 Antimony (Sb) Additions to Tin (Sn) 206
7.2.3.2 Indium (In) Additions to Tin (Sn) 207
7.2.3.3 Bismuth (Bi) Additions to Tin (Sn) 209
7.3 Reliability Testing Third Generation Commercial Pb-Free Solders 210
7.3.1 Thermal Fatigue Evaluations 210
7.3.2 iNEMI/HDPUG Third Generation Alloy Pb-Free Thermal Fatigue Project 213
7.3.3 Microstructure and Reliability of Third Generation Alloys 219
7.4 Reliability Gaps and Suggestions for AdditionalWork 223
7.4.1 Root Cause of Interfacial Fractures 223
7.4.2 Effect of Component Attributes on Thermal Fatigue 224
7.4.3 Effect of Surface Finish on Thermal Fatigue 224
7.4.4 Thermomechanical Test Parameters and Test Outcomes 225
7.4.4.1 Thermal Cycling Dwell Time 225
7.4.4.2 Preconditioning (Isothermal Aging) 225
7.4.4.3 Thermal Cycling of Mixed Metallurgy BGA Assemblies 226
7.4.4.4 Thermal Shock or Aggressive Thermal Cycling 226
7.4.5 Reliability Under Mechanical Loading: Drop/Shock, and Vibration 227
7.4.6 Solder Alloy Microstructure and Reliability 230
7.4.7 Summary of Suggestions for Additional Investigation 231
7.5 Conclusions 232
Acknowledgments 234
References 234
8 Lead-Free Printed Wiring Board Surface Finishes 249
Rick Nichols
8.1 Introduction: Why a Surface Finish is Needed 249
8.2 Surface Finishes in the Market 250
8.3 Application Perspective 255
8.4 A Description of Final Finishes 261
8.4.1 Hot Air Solder Leveling (HASL) 263
8.4.1.1 Process Complexity 263
8.4.1.2 Process Description 265
8.4.1.3 Issues and Remedies 267
8.4.1.4 Summary 267
8.4.2 High Temperature OSP 267
8.4.2.1 Process Complexity 267
8.4.2.2 Process Description 269
8.4.2.3 Issues and Remedies 270
8.4.2.4 Summary 270
8.4.3 Immersion Tin 271
8.4.3.1 Process Complexity 271
8.4.3.2 Process Description 273
8.4.3.3 Issues and Remedies 275
8.4.3.4 Summary 276
8.4.4 Immersion Silver 276
8.4.4.1 Process Complexity 277
8.4.4.2 Process Description 279
8.4.4.3 Issues and Remedies 280
8.4.4.4 Summary 281
8.4.5 Electroless Nickel Immersion Gold (ENIG) 281
8.4.5.1 Process Complexity 281
8.4.5.2 Process Description 283
8.4.5.3 Issues and Remedies 285
8.4.5.4 Summary 286
8.4.6 Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) 287
8.4.6.1 Process Complexity 287
8.4.6.2 Process Description 289
8.4.6.3 Issues and Remedies 290
8.4.6.4 Summary 291
8.4.7 Electroless Nickel Autocatalytic Gold (ENAG) 291
8.4.7.1 Process Complexity 292
8.4.7.2 Process Description 293
8.4.7.3 Issues and Remedies 295
8.4.7.4 Summary 295
8.4.8 Electroless Palladium Autocatalytic Gold (EPAG) 295
8.4.8.1 Process Complexity 295
8.4.8.2 Process Description 297
8.4.8.3 Issues and Remedies 298
8.4.8.4 Summary 299
8.4.9 Electrolytic Nickel Electrolytic Gold 299
8.4.9.1 Process Complexity 299
8.4.9.2 Process Description 301
8.4.9.3 Issues and Remedies 301
8.4.9.4 Summary 302
8.5 Conclusions 303
References 304
9 PCB Laminates (Including High Speed Requirements) 307
Karl Sauter and Silvio Bertling
9.1 Introduction 307
9.2 Manufacturing Background 307
9.3 PCB Fabrication Design and Laminate Manufacturing Factors Affecting Yield and Reliability 308
9.3.1 High Frequency Loss 308
9.3.2 Mixed Dielectric 308
9.3.3 Back-Drilling 309
9.3.4 Aspect Ratio 309
9.3.5 PCB Fabrication 309
9.3.6 Press Lamination 310
9.3.7 Moisture Content 310
9.3.8 Laminate Material 311
9.4 Assembly Factors Affecting Yields and Long-Term Reliability for Laminate Materials 311
9.4.1 Reflow Temperature 311
9.4.2 Assembly Components 312
9.4.3 Thermal Stress 312
9.5 Copper Foil Trends (by Silvio Bertling) 312
9.6 High Frequency/High Speed and Other Trends Affecting Laminate Materials 316
9.6.1 High Speed Standards 316
9.6.2 Adhesion Treatment (Prior to Press Lamination) 317
9.6.3 Laminate Material Filler Content 317
9.6.4 GlassWeave Effect 317
9.6.5 Halogen-Free 318
9.7 Conclusions 318
References 319
10 Underfills and Encapsulants Used in Lead-Free Electronic Assembly 321
Brian J. Toleno
10.1 Introduction 321
10.2 Rheology 322
10.2.1 Rheological Response and Behavior 323
10.2.1.1 Thixotropy 325
10.2.2 Measuring Rheology 327
10.2.2.1 Spindle Type Viscometry 327
10.2.2.2 Cone and Plate Rheometry 328
10.3 Curing of Adhesive Systems 330
10.3.1 Thermal Cure 330
10.3.2 Ultraviolet (UV) Light Curing 335
10.3.3 Moisture Cure 338
10.4 Glass Transition Temperature 339
10.5 Coefficient of Thermal Expansion (CTE) 341
10.6 Young's Modulus (E) 343
10.7 Applications 344
10.7.1 Underfills 344
10.7.1.1 Capillary Underfill 345
10.7.1.2 Fluxing (No-Flow) Underfill 348
10.7.1.3 Removable/Reworkable Underfill 349
10.7.1.4 Staking or Corner Bond Underfill 349
10.7.2 Encapsulant Materials 350
10.7.2.1 Glob Top 351
10.7.2.2 Component Encapsulation 351
10.7.2.3 Application 353
10.7.2.4 Low-Pressure Molding 355
10.8 Conclusions 355
References 355
11 Thermal Cycling and General Reliability Considerations 359
Maxim Serebreni
11.1 Introduction to Thermal Cycling of Electronics 359
11.1.1 Influence of Solder Alloy Composition and Microstructure on Thermal Cycling Reliability 362
11.2 Influence of Package Type and Thermal Cycling Profile 363
11.2.1 Influence of Board and Pad Design 366
11.3 Fatigue Life Prediction Models 371
11.3.1 Empirical Models and Acceleration Factors 371
11.3.2 Semi-empirical Models 372
11.3.3 Finite Element Analysis (FEA) Based Fatigue Life Predictions 373
11.4 Conclusions 376
References 377
12 Intermetallic Compounds 381
Alyssa Yaeger, Travis Dale, Elizabeth McClamrock, Ganesh Subbarayan, and Carol Handwerker
12.1 Introduction 381
12.1.1 Solders 382
12.1.2 Interaction with Substrates 382
12.2 Setting the Stage 384
12.2.1 Mechanical and Thermomechanical Response of Solder Joints 386
12.3 Common Lead-Free Solder Alloy Systems 392
12.3.1 Solder Joints Formed Between Sn-Cu, Sn-Ag, and Sn-Ag-Cu Solder Alloys and Copper Surface Finishes 396
12.3.1.1 Sn-Cu Solder on Copper 396
12.3.1.2 Sn-Ag and Sn-Ag-Cu Solder Alloys on Copper 399
12.3.2 Solder Joints Formed Between Sn-Cu, Sn-Ag, and Sn-Ag-Cu Alloys and Nickel Surface Finishes 408
12.3.2.1 Ni-Sn 408
12.3.2.2 Sn-Ag Solder Alloys on Nickel 411
12.3.2.3 Spalling 415
12.3.2.4 Effects of Phosphorus Concentration in ENIG on Solder Joint Reliability 416
12.3.3 Au-Sn 417
12.4 High Lead - Exemption 422
12.5 Conclusions 423
References 423
13 Conformal Coatings 429
Jason Keeping
13.1 Introduction 429
13.2 Environmental, Health, and Safety (EHS) Requirements 430
13.3 Overview of Types of Conformal Coatings 430
13.3.1 Types of Conformal Coatings 431
13.3.1.1 Acrylic Resins (Type AR) 432
13.3.1.2 Urethane Resins (Type UR) 433
13.3.1.3 Epoxy Resins (Type ER) 433
13.3.1.4 Silicone Resins (Type SR) 435
13.3.1.5 Para-xylylene (Type XY) 436
13.3.1.6 Synthetic Rubber (Type SC) 437
13.3.1.7 Ultra-Thin (Type UT) 438
13.4 Preparatory Steps Necessary to Ensure a Successful Coating Process 440
13.4.1 Assembly Cleaning 440
13.4.2 Assembly Masking 440
13.4.3 Priming and Other Surface Treatments 441
13.4.3.1 Measuring Surface Energy 441
13.4.3.2 Water Drop Contact Angle 447
13.4.4 Bake-Out 448
13.5 Various Methods of Applying Conformal Coating 449
13.5.1 Manual Coating 449
13.5.2 Dip 449
13.5.3 Hand Spray 450
13.5.4 Automatic Spray 451
13.5.5 Selective Coating 451
13.5.6 Vapor Deposition 451
13.6 Aspects for Cure, Inspection, and Demasking 453
13.6.1 Cure 453
13.6.1.1 Solvent Evaporation 453
13.6.1.2 Room Temperature Vulcanization (RTV) 454
13.6.1.3 Heat Cure 454
13.6.1.4 UV Cure 454
13.6.1.5 Catalyzed 454
13.6.2 UV Inspection 455
13.6.3 Demasking 455
13.7 Repair and Rework Processes 456
13.7.1 Chemical 456
13.7.2 Thermal 456
13.7.3 Mechanical 457
13.7.4 Abrasion (Micro-Abrasion) 457
13.7.5 Plasma Etch 457
13.8 Design Guidance on When and Where Conformal Coating is Required, and Which Physical Characteristics and Properties are Important to Consider 457
13.8.1 Is Conformal Coating Required? 458
13.8.1.1 Why Use It? 458
13.8.1.2 Why Not Use Conformal Coating? 459
13.8.2 Desirable Material Properties 459
13.8.3 Areas to Mask 461
13.9 Long-Term Reliability and Testing 462
13.10 Conclusions 462
13.11 Future Work 463
References 463
Index 467
1
Lead-Free Surface Mount Technology
Jennifer Nguyen1 and Jasbir Bath2
1Flex, Milpitas, California, USA
2Bath Consultancy LLC, San Ramon, CA, USA
1.1 Introduction
Surface mount technology (SMT) involves the assembly or attachment of surface mount devices (SMDs) onto the printed circuit board (PCB). Today, the majority of the products are built using surface mount technology and lead-free process. This chapter will review the surface mount process for lead-free soldering, including printing, component placement, reflow, inspection, and test. The chapter also discusses some advanced miniaturization technologies used in the SMT process.
1.2 Lead-Free Solder Paste Alloys
Today, there are a variety of lead-free solder paste alloys available in the market. SnAgCu (SAC) materials with 3.0-4.0% Ag and 0.5-0.9% Cu and remainder Sn are widely accepted within the industry. Among them, Sn3.0Ag0.5Cu (SAC305) is still the most common alloy used in the SMT process. These SnAgCu alloys have the liquidus temperature of around 217 °C. As the cost of Ag has increased over the past years, the use of low Ag alloy materials such as Sn0.3-1.0AgCu or SnCu/SnCuNi has increased. These alloys have approximately 10 °C higher melting temperature than SAC305 and may need to be processed at slightly higher temperature during the reflow process.
Low temperature lead-free alloys which contain SnBi/SnBiAg are also used. These alloys have melting temperature around 140 °C and can be processed at 170-190 °C. These low temperature alloys usually have high bismuth content and they create some reliability concerns, especially on mechanical reliability. These low temperature alloys are used on certain applications such as light-emitting diode (LED)/TV products. In recent years, there is a desire for low temperature lead-free alloy alternatives with better reliability. The drivers for these low temperature alloys include component warpage, low energy consumption, and component or board sensitivity to the higher temperature lead-free process. These alloys typically have higher liquidus temperature than traditional SnBi/SnBiAg alloys, but they still have lower liquidus temperature than SAC305. These alloys have gained a lot of interest in the industry in the recent years, and some are available in the market and used in production.
1.3 Solder Paste Printing
1.3.1 Introduction
One of the most important processes of the surface mount assembly is the application of solder paste to the PCB. This process must accurately deposit the correct amount of solder paste onto each of the pads to be soldered. Screen-printing the solder paste through a foil or stencil is the most commonly used technique, although other technique such as jet printing is also used.
There is no major change to solder paste printing for lead-free processes. The same printer can be used for tin-lead and lead-free printing. In general, the same stencil design guidelines can be used for lead-free process.
1.3.2 Key Paste Printing Elements
Solder paste printing process is one of the most important processes in surface mount technology. This process can account for the majority of the assembly defects if it is not controlled properly. For effective solder paste printing, the following key factors need to be optimized and controlled:
- PCB support
- Squeegee (type, speed, pressure, angle)
- Stencil (thickness, aperture, cleanliness, snap off, separation speed)
- Solder paste (including type, viscosity)
PCB support is important to the printing process. Good PCB support holds the PCB flat against the stencil during the screen-printing process. PCB support is generally provided with the screen-printing machines. If the board is not properly supported, solder defects such as bridging, insufficient solder, and solder smearing can be seen. For fine pitch printing such 0.3/0.4 mm pitch chip scale package (CSP), 0201/01005 (Imperial) chip component, a dedicated custom-made fixture for printing or vacuum support should be used.
Squeegees, squeegee pressure, and speed are other critical parameters in the screen-printing process. Metal squeegees are commonly used for printing solder paste, and rubber or polyurethane squeegees are used for epoxy printing. A squeegee angle of 60 °C to the stencil is typically used [1]. Squeegee speed and squeegee pressure are critical for good printing. The speed of the squeegee determines how much time the solder paste can roll and settle into the apertures of the stencil and onto the pads of the PCB. In the beginning of lead-free conversion, a slower printing speed was used because the lead-free solder paste was stickier than tin-lead solder paste. Today, many lead-free solder pastes can print well at high speed.
The speed setting is widely varied from a typical range of 20-100 mm/s-1 depending on the size of the aperture, the size of PCB, and the quantity of boards being assembled, etc. Printing speed used depends on the solder paste supplier or is optimized by a Design of Experiment (DOE). It is typically between 40 and 80 mm s-1. During the solder paste printing, it is important to apply sufficient squeegee pressure and this pressure should be evenly distributed across the entire squeegees. Too little pressure can cause incomplete solder paste transfer to the PCB or paste smearing. Too much pressure can cause the paste to squeeze between the stencil and the pad.
Stencil is another key factor in the solder paste printing. Metal stencils are used in solder paste printing. Stainless steel material is commonly used; however, metal stencils can be made of copper, bronze, or nickel [2]. There are several types of screen-printing stencil, including chemical etch, laser cut, and electroformed [2]. The thickness of the stencil is typically 125 µm (5 mil) or 150 µm (6 mil). Stencils with the thickness of 100 µm (4 mil) or thinner have become more popular with the high density and fine pitch components such as 0201/01005 (Imperial) chip components or 0.4/0.3 mm pitch CSP or quad flat no-leads/bottom termination component (QFN/BTC) components. Thicker stencils than 150 µm are typically used when more paste is needed. Stencil thickness and aperture size determine the amount of paste deposited on the pad. In general, stencil aperture must be three times and preferably five times the diameter of the solder particles. To ensure the proper paste release and efficient printing, the aspect ratio should be greater than 1.5, and the area ratio should be greater 0.66.
The aspect ratio is defined by Eq. (1.1), and the area ratio is shown in Eq. (1.2).
(1.1) (1.2)Snap off and stencil separation speed are also important for good printing quality. Snap off is the distance between the stencil and the PCB. For metal stencil printing, the snap off should be zero. This is also called contact printing. A high snap off will result in a thicker layer of solder paste. Stencil separation speed is the speed of separation between the stencil and PCB after printing. Traditionally, high separation speed will result in clogging of the stencil apertures or tailing at edges around the solder paste deposited (Figure 1.1). However, lead-free pastes tend to have a higher adherence than tin-lead pastes and may prefer high separation speed than tin-lead solder paste. Separation speed varies depending on the solder pastes and its supplier, and the supplier's recommendation should generally be followed.
Figure 1.1 Example of tailing at the edge of the paste due to high separation speed.
Last but not least, the correct solder paste type and material should be used. The correct type of solder paste should be selected based upon the size of the apertures within the stencil. Type 3 was commonly used in the tin-lead process; however, Type 4 has become a more common lead-free solder paste type in the recent years due to the increase in miniaturized components on the printed circuit board. The release from the apertures of the stencil is affected by the particle size within the selected solder paste. Table 1.1 lists the particle size of different solder paste type.
Table 1.1 General solder paste type and particle sizes.
Paste type Particle size (µm) 3 45-25 4 38-20 5 25-15 6 15-5Both tin-lead and lead-free solder paste should be refrigerated while being stored to maintain its shelf life but must be brought to room temperature before use to maintain quality. Some new lead-free solder pastes require no refrigeration and can be stored at room temperature. The solder paste should be mixed...
