Driving force and mechanism for spontaneous metal whisker formation

Physical Review Letters, 93(20): pp. 206104-1—206104-4. Retrieved September 19, 2006 from http://www.mse.drexel.edu/max/pdf%20references/drexel_pdfs/papers/WhiskesPRL_2004.pdf. DOI: http://dx.doi.org/10.1103/PhysRevLett.93.206104The room temperature spontaneous growth of low melting point metal whiskers, such as Sn, poses a serious reliability problem in the semiconducting industry; a problem that has become acute with the introduction of Pb-free technology. To date, this 50+ year old problem has resisted interpretation. Herein we show that the driving force is essentially a reaction between oxygen and the sprouting metal. The resulting volume expansion creates a compressive stress that pushes the whiskers up. The model proposed explains our observations on In and Sn whiskers and many past observations. The solution is in principle simple: diffusion of oxygen into the metal must be prevented or slowed down. This was demonstrated by coating the active surfaces with a polymer coating