Study of Flux/E-GaIn Interfacial Tension and Wetting Behavior of Fluxes

The interfacial tension between flux and liquid metal is a crucial parameter during the soldering process, but is often ignored. In this work, based on the pendant-drop method, the effect of the electric field on the interfacial tension of a GaIn alloy in flux was studied. The results show that the interfacial tension was reduced by eutetic GaIn (E-GaIn) adsorbing R·COO⁻ ions in the rosin-ethanol solution. When the metal droplet was applied to the positive electrode, a significant further reduction in the interfacial tension was achieved. Based on the top-view sessile drop method, the wetting behavior of rosin-ethanol flux and orthophosphoric acid aqueous solution on Al, Cu, and 430ss substrates was studied, in which the former flux is a quasi-inert wetting system, and the latter is a typical reactive wetting system, which can be described by a reaction-limited spreading model. On this basis, we used orthophosphoric acid flux as an auxiliary and the E-GaIn alloy as a filler, and the Cu-Cu joint was successfully implemented under conditions close to room temperature, thus providing the possibility for low-temperature (or even room-temperature) joining with high-temperature operation.