Role of surface properties on the wettability of Sn–Pb–Bi solder alloys

Abstract

A statistical mechanical model has been used to investigate the surface properties of Sn–Pb, Bi–Sn and Bi–Pb liquid alloys. Our study suggests a decrease in the surface tension of Sn–Pb alloys with increasing Pb component. In contrast, the addition of Pb atoms increases the surface tension of Bi–Pb liquid alloys. However, Bi atoms cause the lowering of the surface tension of Bi–Sn alloys. The Bi–Sn and Bi–Pb systems exhibit segregation of Bi atoms to the surface, whereas in the case of Sn–Pb alloys, there is evidence for the enrichment of the surface with Pb atoms. For all the systems, the degree of segregation increases with lowering of the temperature. Our results for binary systems have been extended to a ternary (Sn–Pb–Bi) system with the emphasis on the influence of the Bi component on the surface tension and surface segregation of conventional solder alloys i.e. Sn 0.6Pb 0.4. Our study suggests a lowering of the surface tension of Sn 0.6Pb 0.4 alloys with the addition of Bi atoms. It is also observed that the surface of ternary alloys will be less enriched in Pb atoms in comparison with the conventional solder alloys, i.e. Sn 0.6Pb 0.4. The segregation of Bi atoms to the surface might inhibit the formation of a Pb layer near the intermetallic compound layer formed between Sn 0.6Pb 0.4 alloys and the Cu substrate, as observed in de-wetted solder joints.