Wettability of Liquid Tin on Solid Copper

For obtaining good joints by soldering, a wettability of liquid metal on the solid metal surface, related to solder ability, is one of the most important factors, and the wettability is accelerated by using a suitable flux and atmosphere. In atmosphere soldering the flow of molten solder will be retarded by oxide films on base metals formed by a small amount of oxygen in atmosphere.In this study a wetting phenomenon of liquid tin on solid copper was clarified in relation to soldering, and the effects of additional elements to pure tin and oxide films on the base metal were mainly examined as the basic factors affecting the wettability. Li, Na and P were selected as additional elements to pure tin for making soldering more efficient. For the wettability test of an oxidized copper base metal, the reaction furnace which keeps the oxygen partial pressure in its atmosphere above the dissociation pressure of Cu2O was employed to preserve oxide films at the experimental temperature.It was clarified in this experiment that the activated tin alloys containing Li, Na and P as additional elements showed better wettability, and the effect of P addition was excellent. The spreading area of liquid tin decreases owing to the presence of oxide films, and the spreading area on oxidized copper with Cu2O (600 Å in thickness) was approximately 50% of that on pure copper at 340°C.