Calculation of phase equilibria in candidate solder alloys

New solder alloys are being developed for electronic assemblies as replacem ents for traditional Pb-containing solder materials as a result of internat ional pressure to remove Pb from the waste stream. In the transition period from Pb-containing to Pb-free solder alloys, component leads and copper in terconnections on circuit board will continue to be pre-tinned with a Pb-co ntaining solder to improve solderability. Phase equilibria information is n eeded for the evaluation of candidate Pb-free solder alloys in terms of the ir melting behavior and of the possible effects of Pb-contamination on manu facturing and reliability. The CALPHAD method of phase equilibria calculati on using thermodynamic descriptions from databases provides a powerful tool for obtaining such information. The reliability of the calculation, howeve r, depends on the quality of the thermodynamic descriptions of the individu al phases. Criteria for the evaluation of these descriptions and the availa ble thermodynamic databases are discussed. The effect of Pb-contamination o n four solder alloys is analyzed.