Thermal Fatigue Result for Low and No-Ag Alloys - Pb-Free Alloy Characterization Speaker: William Chao, Cisco Chair: Elizabeth Benedetto, HP October 26, 2012 inemi Session, IMPACT
Project Team Members 19 companies; 66 individuals Solder alloy suppliers, component suppliers, EMS providers, OEMs 2
Outline Background and Objectives Overall Program Structure Test Vehicle Establishing Procedures for Producing Comparable Data Assembly/ATC Weibull Plots Conclusions Acknowledgements 3
Background Team designed an experimental program intended to provide the industry with a clearer indication of the effects of varying silver content and microalloying additions on the reliability performance of alloys in thermal cycling, including acceleration behavior In this study, the team is using accelerated thermal cycling (ATC) to evaluate thermal fatigue performance of 12 commercial or developmental Pb-free solder alloys and the eutectic Sn-37Pb solder baseline 4
Background and Objectives Goals team to learn: Dependence of thermal fatigue resistance on Ag concentration Impact of common dopants Thermal fatigue performance of some new alloys Performance of low Ag alloys relative to Sn- 37Pb How alloy composition affects acceleration behavior Provide limited set of data on the impact of package design Impact of thermal aging on thermal fatigue performance of Pb-free alloys. Thermal fatigue data for multiple Pb-free alloys for use in modeling Sn-37Pb Soldered with Sn-37Pb Paste SAC405 Soldered with SAC305 Paste 5
Background and Objectives Increase in Pb-free solder alloy choice Variations in Ag content Microalloying Knowledge gap associated with new materials: thermal fatigue Lack of test data No data comparison to Sn-37Pb No acceleration models SAC205 + Ni Soldered with SAC305 Paste SAC0307 Soldered with SAC305 Paste 6
Background These initial findings from a subset of the inemi thermal cycling test matrix derived from the lower Ag alloys Data is presented for two different BGA components and comparisons of results are made among multiple temperature cycles Thermal cycling variables include different cyclic temperature ranges ( T) and temperature extremes The failure data are reported as characteristic life (number of cycles to 63.2% failure) and slope from a two-parameter Weibull analysis. The ATC data are discussed in terms of the alloy composition and the initial microstructures The relative performance of the various alloys under multiple ATC conditions, and how this may impact acceleration behavior, is discussed. 7
Overall Program Structure Alloy Selection Sn-37Pb chosen as a control SN100C chosen as a 0% Ag Pbfree alloy Various levels of silver (nominal 0-4%) Various microalloys/dopants Some new alloys In most cases the paste alloy is SAC 305. Decision based on: Simplification of the experimental matrix Recognition that in most cases today the paste alloy will be SAC305 with potentially multiple BGA ball alloys being used on the same board No. BGA Ball Alloy Trade Name or Designation Solder Paste Comments 1 Sn-37Pb Eutectic Sn-Pb Sn-37Pb Control 2 Sn-0.7Cu+0.05Ni+Ge SN100C SN100C 0% Ag joint 3 Sn-0.7Cu+0.05Ni+Ge SN100C SAC305 Impact of [Ag] 4 Sn-0.3Ag-0.7Cu SAC0307 SAC305 Impact of [Ag] 5 Sn-1.0Ag-0.5Cu SAC105 SAC305 Impact of [Ag] 6 Sn-2.0Ag-0.5Cu SAC205 SAC305 Impact of [Ag] 7 Sn-3.0Ag-0.5Cu SAC305 SAC305 Impact of [Ag] 8 Sn-4.0Ag-0.5cu SAC405 SAC305 Impact of [Ag] 9 Sn-1.0Ag-0.5Cu+0.05Ni SAC105+Ni SAC305 Impact of dopant 10 Sn-2.0Ag-0.5Cu+0.05Ni SAC205+Ni SAC305 Impact of dopant Impact of 11 Sn-1.0Ag-0.5Cu+0.03Mn SAC105+Mn+Ce SAC305 dopant Doped commercial 12 Sn-0.3Ag-0.7Cu + Bi SACX0307 SAC305 alloy 13 Sn-1.0Ag-0.5Cu SAC105 aged SAC305 Effect of aging 14 Sn-3.0Ag-0.5Cu SAC305 aged SAC305 Effect of aging 15 Sn-1.0Ag-0.7Cu SAC107 SAC305 Impact of [Cu] Doped commercial 16 Sn-1.7Ag-0.7Cu-0.4Sb SACi SAC305 alloy 8
Overall Program Structure Profile Selection Goal 1: Impact and interactions of: temperature range, maximum temperature, and dwell time (core DOE profiles 1-8) Goal 2: Performance for the most commonly used profiles: 0/100 C and - 40/125 C (profiles 1 and 10) Goal 3: Impact of long dwells (profile 9) Profile No. Company Cycle (Min/Max/Dwell) 1 ALU 0/100/10 2 IST 25/125/10 3 Henkel -40/100/10 4 Nihon -15/125/10 5 ALU 0/100/60 6 HP 25/125/60 7 HP -40/100/60 8 CALCE -15/125/60 Comment Core DOE 9 CALCE -40/100/120 Long Dwell 10 Delphi -40/125/10 Common; Auto 9
Test Vehicle PCB designed by Alcatel-Lucent Six layer daisy-chained board (LG451HR) 16 sites for the 192-I/O and 16 sites for the 84-I/O Copper OSP surface finish Components 0.8-mm pitch, 14 mm x 14 mm 192-I/O ChipArray ball-grid array (CABGA) 0.5-mm pitch, 7mm x 7mm 84- I/O ChipArray thin core ball-grid array (CTBGA) Land pads - electrolytic Ni/Au 10
Establishing Procedures for Producing Comparable Data Great care was taken to ensure traceability and consistency Three checklists Unification of materials, tools & instruments Unification on methods of ATC setup Checklist before start of ATC (profile, ramp rate, failure definition, etc.) Tracking of the alloys throughout each process Individual LGA substrate lots Ink dot pattern on every package per lot allowed tracking throughout each process 11
Board Assembly Assembled by Flextronics, San Jose 16 test cells 13 PCAs per cell 5-mil thick stencil 14-mil-diameter round apertures 192-I/O CABGA 12-mil x 12-mil square apertures 84-I/O CTBGA PCA Group Number of PCAs per Cell Allocations 1 8 Core DOE 2 2 Two additional profiles 3 1 Aging studies 4 1 Spares 5 1 Witness set 12
ATC Highlights Checklist #3 compared profiles site to site Criteria to ensure profiles were within tolerances Profiles presented to team prior to start of test Failure definition (IPC9701A) Event Detector vs. Data Logger Unified failure definition using IPC definition for event detectors (usable by data loggers) Failure: 10 events (measurements of resistance above the threshold value of 1000Ω) take place within 10% of cycles from the first event. The first event meeting this criterion is defined as the point of failure. 13
Alloys Tested Low-Ag and no-ag content (0-1% Ag) alloys included to study effect of Ag level A selection of microalloying additions Some variation on Cu content 14
Assembly Reflow Profile All lead-free alloys assembled with same profile 15
Thermal Cycles Only 10 minute dwells results available Profile No. Company Cycle (Min/Max/Dwell) Comment 1 ALU 0/100/10 2 IST 25/125/10 3 Henkel -40/100/10 4 Nihon/DfR -15/125/10 5 ALU 0/100/60 Core DOE 6 HP 25/125/60 7 HP -40/100/60 8 CALCE -15/125/60 9 CALCE -40/100/120 Long Dwell 10 Delphi -40/125/10 Common; Auto 16
Trends in Characteristic Life 17
Trends in Characteristic Life 18
Effect of Ag Levels on Sn-0.7Cu 0-100 C, 10 minute dwells CABGA192 Hardening effect of Ag is apparent 19
Low-Ag Alloys with Reference Alloys 0-100 C, 10 minute dwells CABGA192 20
Effect of Microalloying Additions 0-100 C, 10 minute dwells CABGA192 No significant effect of Ni on SAC105 in this test condition Significant reduction of characteristic life and spread of failures with Mn + Ce addition 21
Effect of Microalloying Bi & Rare Earths 0-100 C, 10 minute dwells CABGA192 No significant effect in this test condition 22
Effect of Cu Level 0-100 C, 10 minute dwells CABGA192 No significant effect in this test condition 23
Summary of Acceleration Factors Acceleration Factor Thermal Cycle -40-125 C/10 25-125 C/10-40-100 C/10-15-125 C/10 0/100 C/10 Average 4.23 1.41 Std Dev 0.7 0.29-40-125 C/10 Average 2.62 Std Dev 0.36-40-100 C/10 Average 1.03 Std Dev 0.13 24
Summary of Effect of Component Type Category Alloy/Paste Component Effect 0/100/10 0/100/10 CABGA192 CTBGA84 η Ratio η β η Β Reference Sn-37Pb/Sn-37Pb 1477 12.3 2310 11.0 1.6 Alloys SAC305/SAC305 5718 7.0 9819 7.0 1.7 No-Ag SN100C/SN100C 3101 8.7 5306 7.7 1.7 Very Low-Ag Low-Ag SN100C/SAC305 SAC105/SAC305 3067 4910 10.0 5.4 6625 6826 8.0 7.9 2.2 1.4 SAC0307/SAC305 SAC107/SAC305 4071 5000 9.2 5.2 5577 7255 11.6 7.4 1.4 1.5 SACX0307/SAC305 4194 7.0 7183 10.0 1.7 Microalloyed SAC105+Ni/SAC305 4707 6.6 7683 7.0 1.6 SAC105+Mn/SAC305 3396 4.1 6514 8.3 1.9 25
Category Summary of Effect of Peak Alloy/Paste Temperature 0/100/10 CABGA192 25/125/10 CABGA192 η β η β Reference Sn-37Pb/Sn-37Pb 1477 12.3 1527 9.7 Alloys SAC305/SAC305 5718 7.0 4632 7.4 No-Ag SN100C/SN100C 3101 8.7 2087 10.6 Very Low-Ag Low-Ag SN100C/SAC305 SAC105/SAC305 3067 4910 10.0 5.4 3197 3144 3.4 3.5 SAC0307/SAC305 SAC107/SAC305 4071 5000 9.2 5.2 2607 2660 7.2 7.1 SACX0307/SAC305 4194 7.0 3205 5.5 Microalloyed SAC105+Ni/SAC305 4707 6.6 3413 4.1 SAC105+Mn/SAC305 3396 4.1 2873 2.7 Acceleration Factor 0/100/10 to 25/125/10 1 1.2 1.5 1 1.6 1.6 1.9 1.3 1.4 1.2 26
Acceleration factors between the -40-125 C profile and the 0-100 C 27
Thermal Cycle Acceleration Factors 28
Thermal Cycle Acceleration Factors 29
Conclusions In short dwell (10 minute) thermal cycles there is a correlation between characteristic life and Ag content All of the Pb-free alloys perform better than Sn- 37Pb under the tested conditions Pb-free alloys have high acceleration factors (0-100 C vs -40-125 C) suggests low-ag alloys will perform better the Sn-37Pb in office or similarly controlled environments 30
Conclusions (continued) Stress generated by strain and exposure to elevated temperature increases, the differences between Pb-free alloys collapses and performance appears to converge towards that of Sn-37Pb. Results of thermal cycling with longer dwells will confirm whether the trend to convergence holds 31
Future Work Completion of all thermal cycles Detailed failure analysis to understand the evolution of the microstructure to failure 32
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