Kinetics of wetting Ag and Cu substrates by molten 60Sn40Pb

The time and temperature dependence of wetting Cu and Ag substrates by molten 60Sn40Pb drops was determined using several fluxes. Four stages were identified. In Stage I, the solder melts and develops into a spherical cap, which spreads rapidly to the quasi-equilibrium stage II, wherein the contact angle θ changes only slightly with time. This is followed by stage III, where6 decreases with a time exponent of ≈0.2, leading ultimately to stage IV, where θ again changes only little with time. An intermetallic compound was observed at the liquid/solid inter-face throughout stages II to IV. The flux influenced the magnitude of θ and the small time dependence in stage II. For a nonactivated rosin flux, the temperature dependence of θ_II yielded an apparent activation energy Q^II _a = 2 to 3 kcal/mole for all substrates, including pretinned Cu. It is speculated that the driving force for the decrease in θ during stage III may result from a decrease in the free energy of the system by diffusion, with a corresponding change in one or more of the interfacial tensions. Pretinning Cu, which formed the intermetallic compound Cu3Sn on the surface, had a significant effect on the time dependence of θ, the effect in stage II being relatively greater than in stage III.