Determination of the minimum Ni concentration to prevent the η to η4+1 polymorphic transformation of stoichiometric Cu6Sn5

The minimum Ni concentration required to prevent the η to η4+1 polymorphic transformation of stoichiometric Cu6Sn5 was found to be as low as 1 at.%. This is crucial in lead-free solder joints, especially in the ball grid array packages, in which only limited Ni can be supplied by micrometer-size solder balls. The Ni stabilization effect is present over a large temperature range with no phase transformation occurring between −120 and 250 °C for the timeframes used in this study.