Wetting Kinetics of Molten 60Sn40Pb on a Ag-0.33 wt.% Pt Thick Film Conductor

The effects of time (10-6000 s) and temperature (194°-265°C) on the wetting of a Ag-0.33 wt percent Pt thick film (10 μm) conductor substrate by molten 60Sn40Pb were investigated using the sessile drop method. The wetting occurred in three stages. In Stage I the solid alloy piece melted and spread rapidly to form a spherical cap. The spreading slowed down appreciably in Stage II but increased again in Stage III, the time exponent in Stage III being of the order of 0.1–0.25, which was 2–3 times that in Stage II. In general, the contact angle θ and the time exponent decreased with temperature. The activation energy for the effect of temperature on the contact angle in Stages II and III and on the transition time t_c between these stages was approximately 7 kcal/mole. This value was concluded to represent the diffusion of Sn atoms over the halo surface as a precursor to the formation of the intermetallic compound Ag₃Sn at the molten solder/substrate interface.