Results from Whisker investigations and the corresponding
Transcription
Results from Whisker investigations and the corresponding
Lead free: Whisker Results from Whisker investigations and the corresponding conclusions Dr. Werner A. Hügel, Dr. Verena Kirchner, Moheb Nayeri, Dr. Lothar Henneken, and Rolf Keller Robert Bosch GmbH, Stuttgart, Germany Automotive Electronics 1 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Motivation • Whiskers are known since 1946, when they were observed by H. L. Cobb • Until today no generally valid theory is available to explain the growth of whiskers Models: • Eshelby et al. Phys. Rev. 91, 775 (1953): Dislocation driven growth • Fisher et al. Acta Metall. 2, 368 (1954): Macroscopic stress driven growth • Furuta et al. Jap. J. of Appl. Phys. 8, 1404 (1969) Stored strain energy • Lindborg et al. Acta Metall. 24, 181 (1976): Growth induced by diffusion and stress • Tu et al. Phys. Rev. B 49, 2030 (1994): Stress driven growth induced by IMC Automotive Electronics 2 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Motivation Whisker test standard will be implemented in the near future • No further investigations necessary, most problems are solved? • Whisker growth mechanism still unknown • No accelerated test available • What do we test with this test (comparability of different results from different companies)? • What about the comparability with qualification results and conditions in serial production? • What are minor and major changes? Automotive Electronics 3 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Motivation • Impact of different bath chemistry • Delayed whisker growth can be observed • Control of the organic additives is important • What is the root cause of the whisker growth? ESA STR-223, 09.1987 B.D. Dunn Automotive Electronics 4 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Observations of whiskers: Fe/Ni alloy with Cu underlayer Base material: alloy 42 Underlayer: 4µm Cu Plating: 5µm Sn Post bake: - Storage: ambient conditions for 5 months Whiskers > 170µm no further growth of whiskers after 500T/C (-60°C/+60°C) Automotive Electronics 5 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Observations of whiskers: Fe/Ni alloy Base material Underlayer Sn plating Post bake Alloy 42 Storage Whisker length - 4µm - T/C 80µm Cu, 4µm 7µm - T/C 20µm Cu, 4µm 7µm - 12 months ambient 25µm Cu, 4µm 5µm - 5 months ambient 170µm Cu, 4µm 10µm - 5 months ambient 70µm • Underlayer can reduce whisker length after T/C • Careful usage of galvanic Cu underlayer, amplified whisker growth possible! Automotive Electronics 6 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Observations of whiskers: Cu alloy Base material: CuSn6 Underlayer: - Plating: 1-4µm Sn Post bake: - Storage: ambient conditions for 6 months Whisker can carry currents up to 60mA Automotive Electronics 7 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Observations of whiskers: Ag underlayer Base material: Cu Underlayer: 2µm Ag Plating: 6µm Sn Post bake 150°C for 1h Storage: 6 months ambient cond. 6 months +50°C uncontr. hum. Whiskers > 35µm Ag underlayer is not an effective countermeasure against whisker growth! Automotive Electronics 8 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Observations of whiskers: 3 x reflow Tpeak260°C No Ni in the area where whiskers are growing Base material: Cu Underlayer: 2µm Ni ??? Plating: 3-5µm Sn Post bake - Storage: 4 months RT 1 month 50°C/85% r.h. Whiskers > 110µm The soldering process is not prohibiting the growth of whisker! Automotive Electronics 9 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Observations of whiskers: after solder dipping (SnPb) SnPb Solder Whiskers are appearing close to the soldered area Base material: Cu Underlayer: - Plating: ~5µm Sn Post bake: 1h @ 150°C Storage: ambient 5 months 60°C/93% r.h. 2000h Sn plating Whisker > 50µm Heat impact of the solder dipping cannot prohibit the whisker growth in the unwetted area Automotive Electronics 10 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Observations of whiskers: Cu alloy Base material: C7025 Underlayer: - Plating: 5µm Sn Post bake: - Storage: ambient conditions for 4 months Whisker is not just a “longer” grain! Automotive Electronics 11 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Overview of observed whisker Base material Underlayer Sn plating Post bake CuSn6 C7025 Tammac C194 Cu alloy CuSn0.15 Storage Whisker - 4µm - 6 months ambient 130µm - 4µm - 12 months ambient 200µm - 5µm - 4 months ambient 70µm - 10µm - 6 months ambient 15µm 10µm Ni, 1µm 5µm - 6 months ambient & 500 TC Ni, 1µm 5µm - 4 months ambient no - 5µm - 500 TC 6µm Ni, 1µm 5µm - 500 TC 11µm - 8µm - 5 months ambient no - 8µm 150°C, 1h no Ag, 2µm 6µm 150°C, 1h 5 months ambient 6 months ambient & 6 months HT 35µm Ag, 2µm 4µm - 2 months ambient 15µm Automotive Electronics 12 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Whisker length / µm Effectiveness of Ni underlayer, storage at 20°C and 50%r.h. much longer whisker no saturation saturation Time / ww Automotive Electronics 13 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. CR/APT1 Results from Whisker investigations and the corresponding conclusions Growth of whisker: • growth of whisker has no uniform behavior • Linear growth (worst case) as well as square root like growth (diffusion) has to be assumed • a complete saturation of the whisker growth could not be observed within 2 years (100ww) • Samples without countermeasures are showing different distribution of length and growth rate for whiskers • risk assessment is for each plating system unique Automotive Electronics 14 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Estimation for ONE device: Fine Pitch Device (defect rate over 15 years) 10µm Sn on Cu lead frame without further countermeasures like postbake or Ni-underlayer Device 1 square root law 80-Pin Plastic QFP Defect rate 144-Pin Plastic LQFP (Fine Pitch) < 480 ppm Device 1 square root law Defect rate < 3600 ppm Device 1 time proportional < 98,9% Device 1 time proportional 100% Device 2 square root law 100% 100% Device 2 square root law Device 2 time proportional 100% Automotive Electronics 15 Device 2 time proportional AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. 100% Results from Whisker investigations and the corresponding conclusions Whisker length / µm Plating thickness and supplier of the galvanic chemistry stored at 20°C and 50%r.h. for 21 weeks CR/APT1 Automotive Electronics 16 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Whisker evaluation for lead free plating: Base material: Plating thickness: Plating system: alloy 42 5µm, 10µm, 15µm lead free Whisker definition: > 100µm Tests performed: Result: 1000TC (-85°C / +85°C) 10.000h (RT) 4000h (60°C / 93% r.h.) NO whisker found, evaluation and qualification passed ready for release? Automotive Electronics 17 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Facts and further consideration: • Any mechanism which causes whisker growth is following the laws of physics e.g. diffusion and relaxation • It is not possible to apply the test procedures for standard device qualification as a whisker acceleration test • Process control of the galvanic chemistry becomes more important for lead free production • Until a real test for measuring the propensity of whisker growth and a real knowledge of the growth mechanism behind the growth is available we need test durations of 4000 hours and a well implemented process control during the plating Automotive Electronics 18 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Parameters influencing whisker growth: • Alloy of lead frame or base material (SnZn, SnCu, …) • Plating conditions (current density, bath temperature, belt speed, …) • Plating chemistry (bath supplier, composition, …) • Control of the bath parameters (metals, additives, …) • Bath parameters defined by the bath supplier have to be continuously controlled at the supplier (SPC, OCAP, …) • Build-up of retained samples and implementation of a whisker monitoring at the supplier Automotive Electronics 19 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Tests for the future? Measurement according to Warren(1) • Measurement of the inhomogeneous broadened line widths due to the micro stress in the grains • The slope shows the stress distribution over grains 4µm Sn without post bake (1) B. E. Warren, X-ray diffraction, Dover Publications, Inc. New York, 1990 4µm Sn on 2µm Ni 4µm Sn with post bake Automotive Electronics 20 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Conclusion: • Qualification of the bath and the lead free samples is very important • Bath has to be qualified for the whole required parameter range • Whisker test has to be performed with the corner samples under conditions equal to the serial production • Base material with highest propensity of whisker growth has to be used as a reference for a qualification test • Pass criteria for a whisker test are that no whisker growth is observed on the samples for storage at constant temperature and 30µm after 500 T/C from -55°C / + 85°C • Long term study (over application life time) of whisker growth for countermeasures like postbake and Ni underlayer is necessary Automotive Electronics 21 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties. Results from Whisker investigations and the corresponding conclusions Thank you for your attention Contact: [email protected] Whiskers are they a problem? Automotive Electronics 22 AE/QMM-S5 | 5/30/2005 | © Robert Bosch GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as copying and passing on to third parties.