Spontaneous whisker growth on lead-free solder finishes

Spontaneous whisker growth on beta-Sn is a creep phenomenon near a surface, driven by compressive force. The compressive force is self-generated and the sample responses by growing stress-free whiskers. While a whisker grows out of a surface, it requires an oxidized surface and the oxide has to be protective. An oxide-free metal under compression in ultra-high vacuum will not grow whiskers. Due to a very limited temperature range, from room temperature to 60 °C, of spontaneous growth of Sn whiskers, a systematic study of the growth is difficult because if the temperature is lower, there is insufficient atomic diffusion and if the temperature is higher, there is no driving force because of stress relief. In this paper we review briefly the creep behavior of whisker growth on beta-Sn surfaces. We present a model of grain boundary fluid flow mechanism of growth of Sn whiskers. The agreement with experimental observation is discussed.