On the spontaneous growth of soft metallic whiskers

The 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 solder technology. Recently it was shown that the driving force is most probably a reaction between oxygen and the sprouting metal (Barsoum, 2004). The resulting volume expansion creates a compressive stress that pushes the whiskers up. The model proposed explains observations on In and Sn whiskers as well as many past observations. Herein further evidence is presented for, and discussion of, the proposed model. Stresses, calculated using finite element modeling, are reasonable and in line with measured values. Based on this work, a potential solution to the whisker problem is in principle simple: either slow or prevent the diffusion of oxygen into the soft metal or, more practically and effectively, work with larger grained solder, which should reduce the magnitude of the compressive stresses.