Sn/Co solid/solid interfacial reactions

Interfacial reactions at the Sn/Co joints reacted at temperatures from 150 to 200 °C are examined using reaction couple techniques. Only one reaction phase, CoSn3 phase, is formed. The reaction layer is very thick comparing with those in most solid/solid couples. For example, it is 325 μm in the couple reacted at 180 °C for 35 days. The reaction layer grows thicker with longer reaction time and higher reaction temperatures. Indentation mark experiments confirm that Sn is the primary diffusion element, and Co is nearly immobile. The rate of interfacial reaction is reaction-controlled in the early stage and becomes diffusion-controlled when the layer thickness reaches a critical value. Based on the thickness measurements and a diffusion model of the Sn/Co interfacial reactions, the reaction constants, diffusion constants and critical thickness of the reaction layer are determined. The critical thickness is about 87 μm and has no significant temperature dependence.