Tin pest in lead-free solders? Fundamental studies on the effect of impurities on phase transformation kinetics

Tin is the main component of contemporary lead-free solders. Under equilibrium conditions, tin exists as either a tetragonal β-Sn) or cubic (α-Sn) allotrope at temperatures above and below approximately 13°C, respectively. The transformation from β to α, often referred to as 'tin-pest' is poorly understood, despite the phenomena being documented over 100 years ago. Possibly because the high lead content in traditional solder alloys offers some protection against 'tin-pest', this transformation has not been a priority in solder-related research. With the transition to lead-free solder alloys and the availability of modern analytical techniques there is both an increased need and opportunity to better understand the transformation. This research was conducted to investigate the phase-transformation kinetics of the β to α transformations in high-purity powdered tin using variable temperature synchrotron XRD analysis and electron microscopy. From the results, time-temperature-transformation (TTT) diagrams were developed, with the intention of being used as a baseline for examining the effects of composition in current generation lead-free solders. The effects of pre-exisiting α-Sn and the impurity elements of Pb, Cu, Si and Ge on the β to α transformation kinetics were measured. α-Sn accelerates transformation kinetics, while all other elements tested delayed the transformation.