CHARACTERIZATION OF SOLVUS IN TERNARY AND QUATERNARY BISMUTH CONTAINING SOLDER ALLOYS

摘要

Compliance with RoHS 2 requirements has necessitated the development of high reliability alternatives to SAC305 lead free solder. Many of the solders being considered for this role contain bismuth, and as such characterizing the behaviour of this alloying element is vital to understanding the evolution of microstructure in these candidate solders. One of the features that makes Bi so attractive for high reliability solder applications is the balance of solid solution strengthening and precipitation hardening it provides in a beta-tin matrix. Therefore, a particular point of interest is the solvus temperature - the point above which Bi will dissolve into the tin matrix to form a solid solution, and below which Bi will precipitate out of the matrix. To this end, the solvus temperature of a number of Bi containing ternary and quaternary solders was determined using a novel differential scanning calorimetry (DSC) method. This value was validated using classic metallographic techniques applied to samples aged both above and below the experimentally determined solvus temperature, and the observed microstructure was characterized.