A multi-phase field study of the role of grain boundary diffusion in growth of Cu6Sn5 intermetallic compound during early stage of soldering reaction

During soldering process employing Sn-based lead-free solders, an intermetallic compound (IMC) layer forms between the molten solder and pad/under bump metallization (UBM), whose morphology and thickness play an important role in controlling the service performance of the solder joints, in particular for solder interconnects with the decreasing size where the interfacial IMC layer takes up a high volume fraction in the solder joint. Thus, characterizing the morphology change and growth kinetics of interfacial IMC layer is very important to optimize the soldering process and evaluate the reliability of solder interconnects. In this paper, a multi-phase field model is employed to intensively account for the effect of grain boundary diffusion on the morphology and growth kinetics as well as size distribution behavior of interfacial Cu₆Sn₅ IMC layer at the Sn/Cu interface during early stage of the soldering reaction. The simulation results show that there are three stages of IMC growth, including the initial stage associated with Cu₆Sn₅ grain broadening followed by the transition stage characterized by formation of scallop-shaped grains and the last normal growth stage dominated by IMC layer thickening and concurrently Cu₆Sn₅ grain coarsening. Increase in the grain boundary diffusion coefficient would increase the thickness of IMC layer while decreasing the average width of Cu₆Sn₅ grains. The relationship between the IMC layer thickness/grain width and soldering reaction time can be well fitted by exponential growth law, in which the high grain boundary diffusion coefficient can produce precise growth exponent close to that in the ideal solid-liquid interface reaction. The simulation results also suggest that size distribution of Cu₆Sn₅ grains can not be characterized by the classical LSW theory while showing good agreement with that predicted by the flux-drive- ripening (FDR) theory.