Precursor film of tin-based active solder wetting on ceramics

Abstract

The phenomenon of the precursor film in a metal-ceramics wetting system was investigated using tin-based active solder (active element: Ti, Zr, Nb, V, Hf or Ta, the third element Ni, Cu, Ag, In or Al) wetting on the ceramics (sialon, mullite, barium titanate alumina and ZTA-SiC). The results show that the formation of a precursor film in the wetting system is dependent on the following factors. (1) The active metal: the presence of titanium, zirconium or hafnium in the solders induced the formation of a precursor film, but niobium, vanadium and tantalum did not. (2) Temperature: a precursor film will not form unless the critical wetting temperature is reached, while a weak film will form during a second wetting at higher temperature. (3) Ceramics: under the same wetting conditions for Sn-4Ti solder, a precursor film forms on the surface of sialon, ZTA-SiC, and red alumina, but not on white alumina, mullite or barium titanate. (4) The third element; small amounts of nickel (1-3 at %), copper (5 at %), or silver (5 at %) in Sn-5 at % Ti solder will enhance the precursor film. On the other hand, small amounts of Al (5 at %) will completely inhibit the occurrence of precursor film. SEM observation reveals the precursor film to be mainly composed of a continuous film with segregated active metal and some small tin islands on the film. Its thickness is several micrometres, similar to that of the interfacial reaction layer between the solder and the ceramic. Two early theories for the formation of a precursor film, surface diffusion and evaporation-condensation, cannot explain the above phenomenon very well. A new model of rapid absorption then film overflow is proposed here for the first time and some problems with the model are also discussed.