Stabilisation of Cu6Sn5 by Ni in Sn-0.7Cu-0.05Ni lead-free solder alloys

Cu₆Sn₅ exists at least in two crystal structures with an allotropic transformation from monoclinic η'-Cu₆Sn₅ at temperatures lower than 186 °C to hexagonal η-Cu₆Sn₅. We recently discovered that the hexagonal structure of Cu₆Sn₅ in lead-free solder alloys with trace Ni additions is stable down to room temperature using high resolution TEM/ED/EDS. This report further confirm the phase stabilising effect of Ni by analysing samples of Cu₆Sn₅ extracted from a Sn-0.7wt%Cu-0.05wt%Ni lead-free solder alloy. Techniques used include X-ray diffraction, transmission electron microscopy and differential scanning calorimetry.