Thermodynamic prediction of interface phases at Cu/solder joints

A thermodynamic method to predict the intermetallic compound which forms first at the substrate/solder interface during the soldering process has been suggested through calculations of metastable phase equilibria between the substrate and the liquid solder and by comparison of the driving forces of formation of individual intermetallic compound phases. It has been applied to the interfacial reaction between Cu substrate and Sn-Ag, Sn-Zn eutectic solders. The prediction from thermodynamic calculations was in good agreement with observed experimental results. The solid-state growth behavior of compound phases formed at the interface of Cu/Sn-Zn and Cu/Sn-Ag eutectic solder joints was explained and a schematic diffusion path suggested through calculated ternary phase diagrams.