Investigation on interfacial compound growth kinetics in Sn-0.7Cu/Cu solder joint and mechanism analysis: Experiments and molecular dynamics simulations

Abstract

Herein, the interfacial compound growth kinetics and corresponding mechanisms in Sn-0.7Cu/Cu solder joint at aging stage were investigated using both experiments and molecular dynamics simulations. The research results indicate that, the growth rates of interfacial IMCs (Cu6Sn5 IMC and Cu3Sn IMC) are affected by the aging temperature severely, the higher aging temperature leads to the larger growth kinetics for both Cu6Sn5 IMC and Cu3Sn IMC. At aging stage, Cu substrate continuously supplies Cu3Sn IMC with Cu atoms. Those Cu atoms diffuse across Cu3Sn IMC to the Cu3Sn/Cu6Sn5 interface and react with Sn atoms diffused from the Sn-based solder to form new Cu3Sn grains growing towards Cu6Sn5 IMC, and the main crystal orientations are (100), (42–1), and (3−10). As the grain boundary diffusion rate of Cu atoms in Cu3Sn IMC determines the CuSn reaction rate (new Cu3Sn grains formation rate) on Cu3Sn/Cu6Sn5 interface, the growth of Cu3Sn IMC is driven by grain boundary diffusion, which is manifested by a growth index n near to 0.3. Meanwhile, the rod-like Cu6Sn5 IMC preferentially grows into Sn-based solder attributed to the large number of Cu atoms provided by Cu3Sn IMC, which can diffuse to the top of Cu6Sn5 IMC and reacts with Sn-based solder. While when Cu6Sn5 IMC is thick enough, Cu atoms cannot reach to the top of Cu6Sn5 IMC anymore, but diffuse laterally within the Cu6Sn5 grains, resulting in the widening of scallop-like Cu6Sn5 IMCs. As the thickening of Cu6Sn5 IMC is caused by the growth of the existing Cu6Sn5 grains, the diffusion of Cu and Sn atoms in Cu6Sn5 grains plays a decisive role. Therefore, the growth of Cu6Sn5 IMC is driven by bulk diffusion, which is manifested by a growth index n near to 0.5.