Results from Whisker investigations and the corresponding conclusions

Transcription

1 Lead free: Whisker Results from Whisker investigations and the Dr. Werner A. Hügel, Dr. Verena Kirchner, Moheb Nayeri, Dr. Lothar Henneken, and Rolf Keller Robert Bosch GmbH, Stuttgart, Germany 1

2 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 2

3 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? 3

4 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? 4 ESA STR-223, B.D. Dunn

5 Observations of whiskers: Fe/Ni alloy with Cu underlayer Base material: alloy 42 Underlayer: Plating: 4µm Cu 5µm Sn Post bake: - Storage: Whiskers > 170µm ambient conditions for 5 months no further growth of whiskers after 500T/C (-60 C/+60 C) 5

6 Observations of whiskers: Fe/Ni alloy Base material Underlayer Sn plating Post bake Storage Whisker length - 4µm - T/C 80µm Alloy 42 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! 6

7 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 7

8 Observations of whiskers: Ag underlayer Base material: Cu Underlayer: Plating: Post bake Storage: 2µm Ag 6µm Sn 150 C for 1h 6 months ambient cond. 6 months +50 C uncontr. hum. Whiskers > 35µm Ag underlayer is not an effective countermeasure against whisker growth! 8

9 Observations of whiskers: 3 x reflow T peak 260 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. The soldering process is not prohibiting the growth of whisker! Whiskers > 110µm 9

10 Observations of whiskers: after solder dipping (SnPb) SnPb Solder Base material: Cu Underlayer: - Whiskers are appearing close to the soldered area Sn plating Plating: ~5µm Sn Post bake: 150 C Storage: ambient 5 months 60 C/93% r.h. 2000h Whisker > 50µm Heat impact of the solder dipping cannot prohibit the whisker growth in the unwetted area 10

11 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! 11

12 Overview of observed whisker Base material Underlayer Sn plating Post bake Storage Whisker CuSn6-4µm - 6 months ambient 130µm - 4µm - 12 months ambient 200µm - 5µm - 4 months ambient 70µm C µm - 6 months ambient 15µm Ni, 1µm 5µm - 6 months ambient & 500 TC 10µm Ni, 1µm 5µm - 4 months ambient no Tammac - 5µm TC 6µm Ni, 1µm 5µm TC 11µm C194 Cu alloy - 8µm - 5 months ambient no - 8µm 150 C, 1h 5 months ambient no 6 months ambient & Ag, 2µm 6µm 150 C, 1h 6 months HT 35µm CuSn0.15 Ag, 2µm 4µm - 2 months ambient 15µm 12

13 Effectiveness of Ni underlayer, storage at 20 C and 50%r.h. Whisker length / µm much longer whisker no saturation saturation Time / ww CR/APT1 13

14 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 14

15 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 80-Pin Plastic QFP Device 1 square root law Device 1 time proportional Device 2 square root law Device 2 time proportional Defect rate < 480 ppm < 98,9% 100% 100% 144-Pin Plastic LQFP (Fine Pitch) Device 1 square root law Device 1 time proportional Device 2 square root law Device 2 time proportional Defect rate < 3600 ppm 100% 100% 100% 15

16 Plating thickness and supplier of the galvanic chemistry stored at 20 C and 50%r.h. for 21 weeks Whisker length / µm CR/APT1 16

17 Whisker evaluation for lead free plating: Base material: alloy 42 Plating thickness: 5µm, 10µm, 15µm Plating system: lead free Whisker definition: > 100µm Tests performed: 1000TC (-85 C / +85 C) h (RT) 4000h (60 C / 93% r.h.) Result: NO whisker found, evaluation and qualification passed ready for release? 17

18 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 18

19 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 19

20 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 20 (1) B. E. Warren, X-ray diffraction, Dover Publications, Inc. New York, µm Sn on 2µm Ni 4µm Sn with post bake

21 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 21

22 Thank you for your attention Contact: Whiskers are they a problem? 22