A framework for studying dynamics and stability of diffusive-reactive interfaces with application to Cu6Sn5 intermetallic compound growth.

Often during phase growth, the rate of accretion, on the one hand, is determined by a competition between bulk diffusion and surface reaction rate. The morphology of the phase interface, on the other hand, is determined by an interplay between surface diffusivity and surface reaction rate. In this study, a framework to predict the growth and the morphology of an interface by modelling the interplay between bulk diffusion, surface reaction rate and surface diffusion is developed. The framework is demonstrated using the example of Cu-Sn intermetallic compound growth that is of significance to modern microelectronic assemblies. In particular, the dynamics and stability of the interface created when Cu and Sn react to form the compound Cu6Sn5 is explored. Prior experimental observations of the Cu6Sn5-Sn interface have shown it to possess either a scalloped, flat or needle-shaped morphology. Diffuse interface simulations are carried out to elucidate the mechanism behind the interface formation. The computational model accounts for the bulk diffusion of Cu through the intermetallic compound, reaction at the interface to form Cu6Sn5, surface diffusion of Cu6Sn5 along the interface and the influence of the electric current density in accelerating the bulk diffusion of Cu. A stability analysis is performed to identify the conditions under which the interface evolves into a flat, scalloped or needle-shaped structure.