Martensitic Transformation in Sn-Rich SnIn Solder Joints

Abstract

The Sn-rich portion of the Sn-In phase diagram includes three phases: diamond cubic α-Sn, tetragonal β-Sn, and simple hexagonal γ-InSn. Despite their very different symmetries, the three phases have relatively simple crystallographic relations, which are described and illustrated. The γ-phase is stable at high temperature, and transforms to β-phase on cooling. Quenching leads to a martensitic transformation when the In content is below about 13 wt.%. The M _s temperature decreases linearly with In content, lying between ambient temperature and liquid-nitrogen temperature (77 K) for Sn-(9–11)In. The microstructure of the martensitic β-phase consists of single-variant “blocks” with two variants alternating in packets, as in the “dislocated martensite” structure of steel. This microstructure is preferred since it lowers the elastic energy, given the dyadic form of the γ → β transformation strain. A pronounced martensitic transformation is induced by deformation at temperatures slightly above the M _s, particularly in Sn-9In, and the resulting transformation-induced plasticity (TRIP) leads to a significant increase in overall ductility of the joint. Deformation-induced martensite is also produced during the creep of Sn-9In just above the M _s (110°C), leading to a low stress exponent (n ≈ 2) for steady-state creep at intermediate stresses with a large strain to failure. This observation is significant scientifically since it is the first case known to us in which significant TRIP has been found in high-temperature creep.