Work of adhesion and reactive wetting in SnPb/Cu,Ni and SnBi/Cu,Ni soldering systems

Low contact angles in reactive wetting are often used as an indicator for strong solder connections. However, from a physical point of view, the so-called work of adhesion is the more justified measure, for which quantitative data are rare. Here, we present measurements for SnPb and SnBi solders on Cu and Ni base metallization. Surface tension and wetting angles are determined by the sessile drop method over the whole composition range of the solders. Provided comparable measurement conditions, the Young-Dupre equation is applied to determine interfacial energies and the work of adhesion. They reveal a characteristic dependence on the intermetallics formed in contact with the liquid solder, clearly supporting the concept of Eustathopoulos which assumes wetting angles are always in contact with the intermetallic of highest possible Sn content. The concentration dependence of the work of adhesion indicates significant segregation of Sn to the liquid-solid interface which significantly affects the bonding strength.