Inhibiting Cracking of Interfacial Cu₆Sn₅ by Ni Additions to Sn-based Lead-free Solders

Abstract

The authors found inhibition of cracking in the interfacial Cu₆Sn₅ intermetallic when Ni containing Sn–0.7Cu alloy was used for soldering. It is thought that this crack inhibition occurred due to the stabilisation of the high temperature hexagonal Cu₆Sn₅ phase through the presence of Ni from a Sn–0.7Cu–0.05Ni solder alloy. To explore the mechanisms associated with the differences in joint integrity, in-situ synchrotron X-ray diffraction (XRD) at the Australian Synchrotron was conducted in the temperature range of 25 to 200°C. The results show that Ni stabilises a high-temperature allotrope of the Cu₆Sn₅ phase, avoiding stresses induced by a volumetric change that would otherwise occur on phase transformation.