Latest solder paste developments: Low Silver Alloys

Transcription

1 Latest solder paste developments: Low Silver Alloys Scanditron Technology Day, 11/09/2014 Petr Bettinec, AIM Solder

2 AIM Introduction AIM is a subsidiary of American Iron & Metal Founded in 1936 in Montreal, Canada Group employs over 2000 people worldwide AIM operates worldwide

3 Global presence Canada (HQ) Mfg: USA, Mexico, Brazil, China Other support offices in China, India, Singapore, Philippines EUROPE?

4 AIM Solder Europe Sp. z o.o. New facility in Lodz, PL Scheduled to ship 2014 Full line manufacturing AIM added 2 sales managers and technical application engineer in 2014

5 AIM Full line production Solder paste Bar solder Cored and solid wire solders Flux Cleaners Adhesives Underfills Reclamation services, laboratory

6 Production Testing of Ni- Modified SnCu Solder Paste Performed by: Karl Seelig, Timothy O Neill, Kevin Pigeon & Mehran Maaleckian AIM Solder Andy Monson & Walter Machado Hayward Industries, HRI Chrys Shea Shea Engineering Services

7 Outline/Agenda Introduction Case Study Descriptions Assembly & Test Solder Joint Analysis Summary Discussion and Conclusions Questions

8 Introduction Ni-Modified SnCu Solder Silver-free, lead-free alloy Less expensive than SAC305 Long, successful history in lead-free wave soldering and HAL PCB final finish Limited acceptance in SMT reflow due to higher melting temperature than SAC305 Concerns of cold, poorly wetted, or otherwise unreliable solder joints when reflowed on the low end of the SAC305 process window

9 Alloy Comparison Property SnCuNi SAC305 Composition Liquidus Point Solidus Point Appearance Sn-0.7Cu- 0.05Ni+Ge 227 C 227 C Shiny Sn-3.5Ag-0.5Cu 221 C 217 C Dull with primary Sn dendrites on surface Source: Nihon Superior Recommended peak reflow temperatures are typically higher than the solder alloy s liquidus temperature

10 Two Case Studies PCB assemblies with similar configurations Mix of SMT and PTH Low to moderate complexity Different reflow profiles One in the sweet spot One on the low end of the process window Comparison of results Assembly Inspection & Test Reliability Analysis

11 Test Vehicle Case 1 Case 1a Controller Board Approx. 6x6 Case 1b Display Board Approx. 3x3.5

12 Test Vehicle Case 2 Case 2 Controller Board Approx. 8x8

13 Reflow Profiles Case 1a Case 2 Peak Temp ~234C Peak Temp ~245C TA 217C: ~60sec TA 217C: ~75sec Low end of reflow window Sweet spot in reflow window PCB 1a reflowed near lower edge of process window

14 Reflow Profiles Case 1b Original Case 1b Modified Peak Temp ~234C Peak Temp ~237C TA 217C: ~60sec TA 217C: ~80C; TA 227C:~60sec Incomplete reflow in area of higher thermal mass Modified to achieve reflow in cold area PCB 1b modified slightly from production recipe

15 Other Assembly Process Info Case 1: 30 of each PCB for each alloy Printer: EKRA X4 Paste: Type 4 NC Stencil: 5mil Eform Placer: ASM SIPLACE D4 Reflow: BTU Pyramax 7-zone AOI: Mirtec MV-7 Wave solder: 5 sets of each alloy, all with SnCuNi wave alloy Case 2: 50 PCBs with each alloy Printer: MPM UP2000 Paste: Type 3 NC Stencil: 5mil lasercut, E-polished Placer: Mydata MY12 Reflow: Electrovert Omniflo 10-zone Wave solder: All 50 PCBs, with wave alloy matched to SMT alloy

16 Solder Joint Appearance, Case 1 Both alloys produced acceptable solder wetting and fillet formation

17 SnCuNi Appearance The cooler reflow process produced joints with less luster than usual:

18 Inspection & Test Case 1: All (120 total) PCBAs inspected visually and by AOI No defects found by inspectors or AOI machine SnCuNi SMT joints had slightly duller appearance than usual 5 of each alloy built up with PTH components, wave soldered (with SnCuNi) and assembled into chassis for functional test All 10 complete assemblies passed functional test Case 2: All (100 total) PCBAs inspected visually; no AOI Similar (undisclosed) defect rates, between alloys except for tombstones PCB design has a tendency for tombstones SnCuNi: no tombstones SAC305: avg 3 tombstones/bd All passed in-circuit test All passed functional test

19 Performed 100% on production line No defects found on SnCuNi or SAC305 SnCuNi & SAC305 on same program Samples sent to Mirtec lab for analysis Both alloys ran under same program with standard lighting No program or parameter tweaks required AOI, Case 1

20 X-Ray Analysis SMT, Case 1 The SnCuNi produced slightly more SMT voids than the SAC305, but within the acceptable range Voiding can be mitigated by adding soak to reflow profile

21 X-Ray Analysis PTH, Case 2 The SnCuNi produced less voids than the SAC305 Causes of PTH voiding not investigated

22 Wetting, Case 1- Least Thermal Exposure Both alloys showed good wetting

23 Intermetallic Compound (IMC), Case1 Both alloys show continuous IMCs approximately 3um thick - SnCuNi IMC is smoother and more - SAC305 s IMC is more dendritic nodular due to nickel s inhibition of grain structure of bulk solder is more intermetallic precipitation columnar - Grain structure of bulk solder is more equiaxed Both alloys showed good IMC formation

24 Thermal Aging, Case 1 Assemblies aged at 125 C for 96 hours (4 days) Cross sectioned No changes in IMC Shear tested 0805 components 15 angle Dage 400 series shear tester No decrease in shear strength Further aged at 150 C for 240 hours (10 days)

25 Thermal Aging & IMC Growth As Reflowed Age #1: 125 C 96 hrs Age #2: Age # C 240 hrs Initial thermal aging had no significant effect on IMC growth; continued aging at higher temperature showed similar effects

26 Thermal Aging & Shear Strength Thermal aging had no significant effect on joint strength

27 Thermal Cycling, Case 2 - Highest Complexity Assemblies cycled C 1 hour cycles Tested at 1000, 2000 and 3000 cycles Electrical test no defects found Cross-sections: no structural changes noticed in solder joints Shear tests: no significant changes in joint strength SnCuNi had unexplained lower average shear strengths at 1000 cycle interval, but typical strengths at 2000 & 3000 cycle measurements

28 Thermal Cycling & Shear Strength Thermal cycling had no significant impact on joint strength

29 Summary: Assembly and Test Process Case 1 Case 2 Build Quantity 30 each of controller and display PCBs with SnCuNi and SAC total 50 each with SnCuNi and SAC total Reflow Cool Moderate Wave solder 5 of each with SnCuNi only 20 total 50 each with SnCuNi and SAC total Visual Inspection No defects found Similar defect rates but more tombstones with SAC305 Automatic Optical Inspection No defects found on production line No program tweaks needed in laboratory analysis In-Circuit Test Pass Functional Test 5 of each pair pass Pass

30 Summary: Assembly and Test Process Case 1 Case 2 Visual Appearance Voiding Microstructure Thermal Aging 125 C for 96 hrs Additional 150 C for 240 hrs Thermal Cycling C, 1 hr Shear Strength Slightly duller than typical SnCuNi surface finish Acceptable wetting SnCuNi showed slightly more voiding in SMT. Both within acceptable limits Uniform, continuous IMC approx. 3um thick in both alloys SMT joints. Equiaxed grain structure in bulk solder of SnCuNi; dendritic grain structure in bulk solder of SAC305. Initial aging had no effect on IMC or grain structure or either alloy. Additional aging showed similar results. Typical surface finish and wetting SAC305 showed slightly more voiding in PTH Both within acceptable limits No electrical test failures found after 1k, 2k & 3k cycles No structural changes observed in cross sections at 1k, 2k & 3k cycles 0805 Resistors average 4-6kg shear strength, no significant difference between alloys before or after thermal aging Resistors average 4-6kg shear strength, only deviation noted was SnCuNi after 1k thermal cycles.

31 Discussion & Conclusions (1) Two case studies compared performance of SnCuNi and SAC305 in moderate and cool reflow profiles Moderate profile provided performance benchmark, cool profile provided critical process characterization information SnCuNi produced acceptable solder joints, even when processed at temperatures previously considered too cool for the alloy Visual appearance of SnCuNi joints were duller than when processed in hotter reflow profile, but good wetting and IMC formation were observed in cross sections AOI recognition without program adjustment eases implementation concerns on automatic inspection

32 Discussion & Conclusions (2) Voiding SMT voiding was slightly higher with SnCuNi alloy PTH voiding was higher with SAC305 alloy Both within acceptable ranges Shear strengths were in normal range, before and after thermal aging and thermal cycling Thermal cycling (3000 cycles) had no noted effects on solder joint electrical or mechanical integrity on Case 2 Forewent thermal cycling on Case 1 boards based on failure-free performance of Case 2 and result of other Case 1 solder joint analyses

33 Discussion & Conclusions (3) Thermal aging cycle had no significant effect on IMC growth rate Silver-free solder alloy can reduce solder paste costs by as much as 20% Concerns about low temperature processing have slowed its implementation & delayed cost savings Case 1 demonstrates the SnCuNi s full compatibility with the SAC305 reflow process window, even on the low end Can be considered a viable drop-in replacement for nearly any SMT process that uses a SAC or SACbased alloy

34 Questions? Thank You