Mechanism and prevention of spontaneous tin whisker growth

Spontaneous Sn whisker growth on Cu leadframe finished with Pb-free solder is a serious reliability problem in electrical and electronic devices. Recently, Fortune magazine had an article to describe the urgency of the problem. The spontaneous growth is an irreversible process, in which there are two atomic fluxes driven by two kinds of driving force. There are a flux of Cu atoms and a flux of Sn atoms. The Cu atoms diffuse from the leadframe into the solder finish driven by chemical potential gradient to form intermetallic compound of Cu₆Sn₅ in the grain boundaries of the solder, and the growth of the compound at room temperature generates a compressive stress in the solder. To relieve the stress, a flux of Sn atoms driven by the stress gradient diffuses away to grow a spontaneous Sn whisker which is stress-free. The typical industry solution is to insert a diffusion barrier of Ni between the Cu and solder to prevent the diffusion of Cu into the solder. It is insufficient, because we have to uncouple the irreversible processes and stop both the fluxes of Cu and Sn. A solution is presented here.