The wetting of metal surfaces by molten solder is usually considered to be driven solely by an interfacial energy imbalance. The effect of chemical reactions on the wetting process is neglected, although the growth of an intermetallic layer in the wetted interface is commonly observed. In this work, the energy release during the incremental advance of a spreading solder droplet due to the interfacial energy imbalance and the formation of the intermetallic layer is calculated. The free energy of formation, ΔG, of the intermetallic layer is shown to be an important driving force for solder wetting.

This approach to wetting has been applied to three systems, Cu-Sn, Cu-Sb and Cu-Cd. Liquid Sn, Sb and Cd react with solid Cu to form Cu6Sn5 (η), Cu2Sb (γ) and CuCd3 (ε), respectively. The free energy of formation, ΔG, for these intermetallic compounds is unknown experimentally, but can be calculated from the phase diagrams and other solution data using classical thermodynamics. These thermochemical calculations yield ΔG(η) = 465–3.09T for Cu-Sn, ΔG(γ) = −2500+0.54T for Cu-Sb and ΔG(ε) = −825+0.44T for Cu-Cd (cal/mole). These relations were evaluated at the respective melting temperatures and compared with the interfacial energy exchange. In all three cases the ΔG contribution was approximately two orders of magnitude larger than the interfacial energy exchange making it the dominant driving force for wetting kinetics.