Effects of alloying elements on the interfacial segregation of bismuth in tin-based solders

Sn-Bi solders are potential candidates for replacing traditional lead-based solders in electronic packaging due to their low melting points, superior wettability, and mechanical properties. The grain coarsening due to the interfacial Bi segregation is a vital issue that affects the reliability of Sn-Bi solder joints. Through first-principles calculations, this study systematically revealed the effects of different alloying elements on the interfacial segregation of Bi towards Sn(100) surface. Bi tends to segregate to the top-surface of Sn due to the formation of vacancies on Sn surface. However, doping alloying elements into the Sn-Bi solders could effectively suppress the interfacial Bi segregation. The presence of elements such as Pd, Pt, and Au located at the adjacent surface layer of Bi could inhibit the Bi precipitation, by apparently increasing the segregation energies of Bi to the Sn surface. The results can be interpreted by the enhanced bond orders and charge transfer between Bi, alloying elements, and their neighboring Sn atoms. This study could provide valuable insights into the design of multi-component Sn-Bi solders with minor alloying elements doping in the future.