Contact angle and reactive wetting in the SnPb/Cu system

Wetting with a low contact angle is a prerequisite for reliable solder connections. However, the contact angle is only an indirect measure of adhesion energy. To quantify this energy, we measured wetting angles of solder droplets as well as surface tension of SnPb solders under systematic variation of composition. From the data, the effective interface energy is evaluated. Although surface tension and wetting angle depend continuously on solder composition, the adhesion tension reveals distinguished plateaus which are related to different reaction products.