Development of high melting point, environmentally friendly solders, using the calphad approach

An attempt has been made using the CALPHAD approach via Thermo-Calc to explore the various possible chemical compositions that adhere to the melting criterion i.e. 270-350 degrees C, required to replace the traditionally used high lead content solders for first level packaging applications. Various ternary combinations were extrapolated iteratively from well assessed binary systems using the COST 531 (version 2) and SSOL2 thermodynamic databases. The ternary combinations that met the solidification requirement were optimized for a narrow solidification range. Taking advantage of the CALPHAD approach meaning that Scheil's equation could now be applied to multicomponent alloys in a more efficient way than the traditional; the optimized ternary compositions were subjected to non-equilibrium solidification simulations. Effort has also been made to predict the phase fractions during solidification of promising solder alloy candidates. The ternary combinations that satisfied the primary solidification requirement were scrutinized taking into account the commercial interests i.e. availability, cost-effectiveness, recyclability and toxicity issues. Technical issues like manufacturability and surface tension have also been considered. Special focus has been given to toxicity related issues since the main ideology of looking for an alternative to high lead containing solders is not related to technical issues but due to environmental concerns.