Calculation of Phase Equilibria in Candidate Solder Alloys

Abstract

Abstract New solder alloys are being developed for electronic assemblies as replacements for traditional Pb-containing solder materials as a result of international pressure to remove Pb from the waste stream. In the transition period from Pb-containing to Pb-free solder alloys, component leads and copper interconnections on circuit board will continue to be pre-tinned with a Pb-containing solder to improve solderability. Phase equilibria information is needed for the evaluation of candidate Pb-free solder alloys in terms of their melting behavior and of the possible effects of Pb-contamination on manufacturing and reliability. The CALPHAD method of phase equilibria calculation using thermodynamic descriptions from databases provides a powerful tool for obtaining such information. The reliability of the calculation, however, depends on the quality of the thermodynamic descriptions of the individual phases. Criteria for the evaluation of these descriptions and the available thermodynamic databases are discussed. The effect of Pb-contamination on four solder alloys is analyzed.