Effect of twin grain boundary on the diffusion of Cu in bulk β-Sn

Abstract

For Cu is the dominant diffusing species in the Cu/Sn diffusion couple, the diffusion behaviors of Cu in the β-Sn grain and at the twin grain boundary (TGB) have been studied in this paper by combining the first-principles calculation and molecular dynamics simulation, which has a significant impact on solder joint reliability. The results show that the energy barrier for Cu bulk diffusion in β-Sn depends on the diffusion path, ranging from 0.01 eV to 0.33 eV. Among the three possible diffusion paths at the (101) TGB, the primary diffusion path is along the [010] direction of the TGB plane. However, this diffusion path along the TGB still presents a relatively high energy barrier of 0.26 eV compared to the energy barrier for bulk diffusion. Furthermore, due to the segregation of Cu atoms at the β-Sn TGB and the inhibition effect of multiple energy barriers for diffusion crossing the TGB, long-range diffusion of Cu across the TGB between the two β-Sn bulk grains is difficult. As a result, the (101) TGB actually inhibits the diffusion of Cu in β-Sn. It can be concluded that increasing the number of TGBs inside a Cu/Sn solder joint can effectively suppress the interdiffusion between Cu and Sn, which will prevent the growth of Cu-Sn intermetallic compounds and improve the reliability of the solder joint.