Preferential growth of intermetallics under temperature gradient at Cu-Sn interface during transient liquid phase bonding: insights from phase field simulation

Transient liquid phase bonding under temperature gradient in electronics interconnections yields intermetallic grains at the bonding interface with different morphological features compared with conventional soldering process. However, the interfacial reactions due to the thermal gradient that result in the preferential growth of intermetallics are yet to be fully understood. In this study, incorporating with the thermotransport effect a multiphase field model is developed to elaborate the fundamental growth mechanism of Cu6Sn5 intermetallic in the Sn/Cu solder interconnect under temperature gradient. We particularly account for the effect of orientation and anisotropic thermal conductivity of Cu6Sn5 intermetallic grains in relation to the temperature gradient during their growth, as observed thermal conductivity with the c-axis of Cu6Sn5 intermetallic parallel to the gradient can be 1.6 times of those perpendicular to the gradient. Simulation results show that the temperature gradient can accelerate the growth of the Cu6Sn5 phase, as reported in the experiments. The heat flux is mainly conducted through the intermetallic grain with c-axis parallel to the temperature gradient, causing faster growth of the grain than the grain with c-axis perpendicular to the temperature gradient; the growth rate difference of the two types of grains becomes more pronounced under high temperature gradient. It is revealed that the faster and preferential growth of this type of intermetallic grain is attributed to the higher thermomigration induced diffusion flux and accompanying faster atomic interdiffusion process, especially near the solder/intermetallic interface.