Irreversible processes of spontaneous whisker growth in bimetallic Cu-Sn thin-film reactions

Using the thermodynamics of irreversible processes, we examine spontaneous interfacial reaction at room temperature in bimetallic Cu-Sn thin films. The reaction forms an intermetallic compound and is interfacial reaction limited. The volume change in forming the compound produces a biaxial compressive stress in the Sn film. The stress results in the growth of Sn whiskers to accompany the reaction. This is a case of an irreversible process of involving the interaction of chemical affinity and mechanical stress. However, the atomic mobility can significantly affect the reaction product and morphology. If the reaction temperature is raised above 100 \ifmmode^\circ\else\textdegree\fi{}C, the compound formation becomes so fast that there is no time to grow whiskers, instead hillocks form. To model the whisker growth (or hillock growth), we assume that whiskers grow at weaker spots on the Sn surface where the surface oxide has been broken. These spots become localized stress relief centers, and they are surrounded by a long-range stress gradient needed for whisker growth.