Thermodynamic Assessment of Sn-Co-Cu Lead-Free Solder Alloy

To select a lead-free solder system, factors such as eutectic/peritectic point, electron negativity, abundance, cost, toxicity of elements, world production capacity, segregation during solidification, possibility to form low melting phases with Pb, among others must be carefully considered. On the basis of thorough analysis of binary phase diagrams of Sn-X-systems (X represents other elements) and the properties of the element X, the Sn-Co-Cu eutectic ternary alloy system has been chosen as a new lead-free solder candidate. In order to find the eutectic point for the Sn-Co-Cu system, the Sn-Co binary system was thoroughly assessed with CALPHAD (CALculation of PHAse Diagram) methods. The ternary phase diagram of Sn-Co-Cu system was extrapolated with the assessed thermodynamic parameters of Sn-Co, Sn-Cu, and Co-Cu system. The eutectic point for L–Sn₂Co+(Sn)+Cu₆Sn₅ is 224.4°C and 0.37%Co and 0.68%Cu and 98.95%Sn.