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The authors found inhibition of cracking in the interfacial Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 phase, avoiding stresses induced by a volumetric change that would otherwise occur on phase transformation.