Solid-State Diffusion of Bi in Sn: Effects of β-Sn Grain Orientation

Abstract

Bismuth (Bi) is an alloying element being considered for inclusion in lead-free solder alloys to improve microstructure, properties, and reliability. It has been determined that aging a Bi-bearing alloy will preserve its strength, whereas the strength of SAC alloys is degraded. Examination of microstructure reveals that, over time, homogenization of Bi in the tin (Sn) matrix via solid-state diffusion occurs, which leads to stabilization of mechanical properties. In this study, the effect of Sn grain orientation on the diffusivity of Bi was analyzed. Sn was solidified slowly to produce coarse grains typical of grain morphologies in solder joints. Bi was subsequently sputtered onto the Sn, and samples underwent annealing at 125°C for 24 h. Electron probe microanalysis (EPMA) was utilized to collect compositional data, and diffusivities were extracted for several grain orientations. It was determined that the diffusivity of Bi in Sn has a low anisotropy ratio, with a difference in diffusivity of around an order of magnitude between Sn samples oriented with the concentration gradient perpendicular to the ‘c’ axis and those oriented parallel to the ‘c’ axis. As a result, low angle grain boundaries in the Sn samples can have a profound effect on the diffusivity and confound the orientation relationship.