Reliability Testing of Ni-Modified SnCu and SAC305 - Vibration. Keith Sweatman, Nihon Superior Joelle Arnold, DfR Solutions March 2008

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1 Reliability Testing of Ni-Modified SnCu and SAC305 - Vibration Keith Sweatman, Nihon Superior Joelle Arnold, DfR Solutions March

2 Previous Work Accelerated Life Testing of SN100C for Surface Mount Devices: A Novel Approach to Lead Free Solder Alloy Qualification IPC/JEDEC Lead Free Reliability & Reliability Testing for RoHs Lead Free Electronics Conference, April 10-12, 2007, Boston, MA Accelerated Reliability Testing of Ni-Modified SnCu and SAC305 IPC/JEDEC International Conference of Reliability, Rework and Repair of Lead-Free Electronics, March 11, 2008, Raleigh, NC Reference these reports for specific details on experimental procedure, use of board level power-cycling device, experimental results and experimental discussion

3 Objectives To provide some preliminary evidence that the Ni-modified SnCu alloy can be used with confidence as an alternative to SAC305 as a lead-free solder in a wide range of service environments To provide some benchmarking of the Nimodified SnCu alloy against Sn-37Pb and SAC

4 Background The Ni-modified Sn-0.7Cu alloy has been well established in commercial mass production since 1999 with more than 3000 wave and selective soldering lines and more than 300 Hot Air Solder Leveling lines in operation in more than 40 countries. More recently the alloy has found application in reflow soldering where the peak reflow temperature is 240 C and higher. Although there are no reported cases of failure in service companies considering the use of the Ni-modified Sn-0.7Cu alloy are seeking reliability test data the to support that decision The work reported in this paper is part of the program to obtain the test data required to support the increasing use of the Nimodified Sn-0.7Cu alloy

5 Agenda The Ni-modified SnCu Alloy Components and Test Vehicle Assembly Test Methods and Results Life Prediction Model Conclusions

6 The Ni-modified SnCu Alloy: Sn-0.7Cu Recommended by inemi as a possible leadfree solder Advantages of Sn-0.7Cu Low cost raw materials (no silver) Simple binary system that is easy to manage Low environmental impact (no silver) Does not complicate recycling Mechanical properties similar to those of Sn- 37Pb low flow stress/high ductility A eutectic like Sn-37Pb?

7 The Ni-modified SnCu Alloy: Sn-0.7Cu Melting point: 227 C Compare with 217 C melting point of SnAgCu eutectic Sn-0.7Cu alloy considered suitable only for wave soldering But difficult to use: Temperatures as high as 300 C required Joints dull and cracked High incidence of bridging

8 The Ni-modified SnCu Alloy: Sn-0.7Cu Problems with performance as solder result from non-eutectic behavior Primary β-sn dendrites Cooling rate ~ 5 C/sec

9 The Ni-modified SnCu Alloy: Sn-0.7Cu Tetsuro Nishimura discovered in 1998 that an addition of Ni at a very specific level dramatically changes the behavior of the Sn-0.7Cu alloy Sn-0.7Cu Ni

10 The Ni-modified SnCu Alloy: Sn-0.7Cu The Ni addition works by forcing the alloy to behave as a eutectic Sn-0.7Cu Ni

11 The Ni-modified SnCu Alloy: Sn-0.7Cu Sn-0.7Cu-0.05Ni +Ge SN100C Matches the Sn-37Pb in appearance and behavior Sn-37Pb SN100C

12 Ni Stabilisation of η Hexagonal Close Packed Cu 6 Sn 5 Tin-Copper Phase Diagram Phase transformation from hexagonal Cu 6 Sn 5 to monoclinic Cu 6 Sn 5 with volume change Nickel stabilizes the hexagonal close packed form of the Cu 6 Sn 5 ensuring the integrity of the intermetallic layer K. Nogita & T. Nishimura, Scripta Materialia 59, 2 (2008)

13 Ni Stabilisation of η Hexagonal Close Packed Cu 6 Sn 5 ~9% Ni in IMC Transmission Electron Micrograph Electron Diffraction Pattern Transmission Electron Micrograph Electron Diffraction Pattern K. Nogita & T. Nishimura, Scripta Materialia 59, 2 (2008)

14 Ni Stabilisation of η Hexagonal Close Packed Cu 6 Sn 5 η - η Cu 6 Sn 5, Heated/cooled at 1 C/min * Transformation from hexagonal to monoclinic 438±18 J/mol 186 C Ni-stabilized η (Cu,Ni) 6 Sn 5, Heated/cooled at 1 C/min** No transformation from hexagonal to monoclinic η - Cu 6 Sn 5 η Cu 6 Sn 5 No phase transformation K. Nogita & T. Nishimura, Scripta Materialia 59, 2 (2008) *Fig4 (b) from G. Ghosh and M. Asta: Journal of Materials Research, 20(2005) ** (Cu,Ni)6Sn5 taken from Sn0.7Cu0.05Ni alloy, DSC by Nihon Superior Co. Ltd

15 Ni Stabilisation of η Hexagonal Close Packed Cu 6 Sn 5 Synchrotron micro-xrf element mapping of Cu 6 Sn 5 phase formed in Sn-0.7Cu-0.05Ni T. Ventura, C.M. Gourlay, K. Nogita, T. Nishimura, M. Rappaz, A.K. Dahle, Journal of Electronic Materials, 37, 1 (2008) Ni segregates to the eutectic Cu 6 Sn 5 phase 2. Ni is relatively homogeneously distributed in the eutectic Cu 6 Sn

16 Ni Stabilisation of η Hexagonal Close Packed Cu 6 Sn 5 Sn-0.7Cu-0.05Ni OSP Substrate After 2 reflow cycles and C SAC305 Crack-free IMC stabilized by Ni Cracking of IMC due to phase change on cooling

17 Experimental Procedure

18 Test Vehicle Three SMT package styles simulate most interconnect geometries 2512 leadless resistors (0Ω, lead free) TSOP (44 gull wing leads, daisy chained, alloy 42) Fine pitch CSP (7.2mmx6.7mm, 0.5mm pitch, 96 balls) Custom SN100C samples Sample coupons OSP, High Tg FR-4 (180 C) samples per coupon Board level heating element

19 Test Vehicle, cont Resistors Heater circuit Monitoring channels

20 Test Vehicle, cont. Chip scale packages Heater circuit Monitoring channels

21 Test Vehicle, cont. TSOP components Heater circuit Monitoring channels

22 Experimental Procedure: Vibration Vibration (171.2Hz) Tested at resonant frequency of boards 30G ~10K cycles 20G ~100K cycles 12G ~1M cycles 6G ~10M cycles Hz 6G, 12G, 20G, 30G AnaTech event detector SnNiCu Resistor TSSOP CSP Resistor SAC TSSOP 2 CSP 1 SnPb Resistor TSSOP 1 2 CSP 1 SUM

23 Results: Vibration

24 Results: Vibration - Resistor 30G SN100C resistor samples for 30G (green) and 12G (blue). Failures did not occur in time exposed to vibration for 6G and 20G. 12G

25 Results: Vibration - TSOP 30G 20G 12G 6G SN100C TSOP samples for 6G (blue), 12G (green), 20G (red) and 30G (violet). Only four failures occurred in exposure time for 6G samples; it is likely that these were infant mortalities (defects) as the bulk of the samples survived past 1E+7 cycles

26 Results: Vibration - CSP 30G 20G 12G SN100C CSP samples for 12G (blue), 20G (green) and 30G (red). Failures did not occur in time exposed to vibration for 6G samples

27 Results: Vibration Resistor, all solders SAC SN100C SnPb Resistor samples exposed to 30G vibration for SAC (blue), SN100C (green) and SnPb (red). SN100C demonstrates an intermediate lifetime between SnPb and SAC

28 Results: Vibration TSOP, all solders SN100C SAC SnPb TSOP samples exposed to 30G vibration for SAC (blue), SN100C (green) and SnPb (red). SAC and SnPb show a relatively similar characteristic life (eta, η), however, SN100C demonstrates the most predictable wearout behavior with the highest shape factor (beta, β)

29 Results: Vibration CSP, all solders SAC SN100C SnPb CSP samples exposed to 30G vibration for SAC (blue), SN100C (green) and SnPb (red). SN100C demonstrates a comparable lifetime to that of SnPb. β 1 for SAC CSPs random wearout behavior, which may indicate brittle fracture

30 Vibration Results Discussion The difference in relative vibration behaviors (SnPb vs. SAC) for the TSOP and CSP devices may be explained by solder joint geometry and package design. CSP is very stiff, dominated by the silicon substrate, and will tend to induce displacement-driven degradation behavior SAC305 is much stiffer than SnPb, so under this displacement-driven regime the failure site is transferring from the bulk solder to the brittle intermetallic region. If failure were occurring in this area, it would potentially explain very low beta. TSOP leads are more compliant than the CSP interconnect geometry, allowing for more stress-driven degradation behavior. SAC305 should be better able to resist stress-driven inelastic deformation

31 Results: Vibration Cross Sectional Analysis SnPb SAC Sn100C TSOP lead, 30G, 100x

32 Results: Vibration Cross Sectional Analysis SnPb SAC Sn100C CSP solder ball, 30G, 500x

33 Discussion Vibration Results: Relative performance of SN100C to SAC/SnPb dependent upon package style

34 Life Prediction Model Times to Failure Data: *blue data indicates test was stopped without failures

35 Steinberg Model for High Cycle Fatigue N O = N C Z Z C O b N O is number of cycles to failure under operational conditions N C = 10 7 b = solder dependent factor (6.4 for SnPb) Model designed for high cycle fatigue (greater than 10 5 cycles)

36 Steinberg Model for High Cycle Fatigue, cont. Z O = deflection under operational conditions Z O = 9.8 G f 2 n Z C = deflection for critical number of cycles to failure Z C = ( ) C h r L B

37 Steinberg Fit to Data Vibration Load (G's) Steinberg Predictions for SnPb and Vibe Data Test terminated at 2.4 million cycles with no failures 0 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10 Cycles to Failure SN100C - CSP SN100C - TSOP SN100C - Resistor SAC - CSP SAC - TSOP SAC - Resistor SnPb - CSP SnPb - TSOP SnPb - Resistor CSP - SnPb Steinberg Prediction TSOP - SnPb Steinberg Prediction Resistor - SnPb Steinberg Prediction Test terminated at 77 million cycles with no failures Above 10 5 cycles to failure (blue line), Steinberg model is accurate predictor of SN100C failure behavior (though less conservative than for SnPb) Above 10 7 cycles to failure (gold line), Steinberg assumes failures not due to solder joint fatigue (existence of fatigue limit)

38 Steinberg Model with b = 9 Vibration Load (G's) Steinberg Predictions for b = 9 and Vibe Data Test terminated at 2.4 million cycles with no failures SN100C - CSP SN100C - TSOP SN100C - Resistor SAC - CSP SAC - TSOP SAC - Resistor SnPb - CSP SnPb - TSOP SnPb - Resistor CSP - SnPb Steinberg Prediction TSOP - SnPb Steinberg Prediction Resistor - SnPb Steinberg Prediction Test terminated at 77 million cycles with no failures Modification of b term (solder dependent exponent) makes Steinberg model more conservative predictor of high cycle fatigue time to failure 0 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10 Cycles to Failure

39 Conclusions Steinberg model for high cycle fatigue fits failure data Less conservative predictor than for SnPb Modification of solder dependent exponent (b) can make model more conservative For example, b = 9 Further data to generate a more accurate term forthcoming Finite Element Analysis required for unique package geometries