Quantifying the Rates of Sn Whisker Growth and Plastic Strain Relaxation Using Thermally-Induced Stress
Whiskers and hillocks that grow out of Sn-based coatings are a critical reliability issue in Pb-free electronics. Although their growth is widely regarded as a stress-relaxation mechanism, quantitative understanding of the relationship between the stress, growth kinetics, and strain relaxation is still lacking. In this work, the well-controlled strain induced by thermal-expansion mismatch was used to study the whiskering behavior of electroplated Sn films. Stress was quantified by monitoring wafer-curvature and the density of whiskers and hillocks was measured simultaneously by use of optical microscopy. Evolution of the volume of individual features was also measured by scanning electron microscopy after different periods of heating. The measurements were used to develop a model for temperature-dependent and stress-dependent growth kinetics of whiskers and hillocks and to determine the amount of strain relaxation which occurs as a result of their formation.
KeywordsTin whiskering thermal cycling stress relaxation grain boundary diffusion
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- 1.NHTSA–NASA Study of Unintended Acceleration in Toyota Vehicle, in: National Highway Traffic Safety Administration (NHTSA), 2011, p. 121.Google Scholar
- 2.NASA. Multiple examples of whisker-induced failures are documented on the NASA website. Available: http://nepp. nasa.gov/whisker/.
- 3.Directive 2002/95/Ec of the European Parliament and of the council of 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment., E. Union, 2003.Google Scholar
- 6.R.M. Fisher, L S. Darken, and K.G. Carroll, Acta Metall. 2, 368 (1954).Google Scholar
- 12.I. Boguslavsky and P. Bush, Proceedings of APEX Conference (Anaheim, CA, 2003), pp. S12.14.1–S12.14.10.Google Scholar
- 13.S. Sakuyama and M. Kutami, Fujitsu Sci. Tech. J 41, 217 (2005).Google Scholar
- 32.M. Sobiech, U. Welzel, R. Schuster, E.J. Mittemeijer, W. Hugel, A. Seekamp, and V. Muller, Proceedings of 57th Electronic Components and Technology Conference (Reno, Nevada, USA, 2007), pp. 192–197.Google Scholar
- 50.L.B. Freund and S. Suresh, Thin Film Materials: Stress, Defect Formation and Surface Evolution, 1st ed. (Cambridge: Cambridge University Press, 2009), pp. 86–93.Google Scholar
- 53.F. Pei, Ph.D. Thesis, School of Engineering, Brown University (2015).Google Scholar
- 58.E. Buchovecky, Ph.D. Thesis, School of Engineering, Brown University, 2010.Google Scholar