Effect of Ni on phase stability and thermal expansion of Cu6−xNixSn5 (X = 0, 0.5, 1, 1.5 and 2)

The crystallography of the Cu6Sn5 intermetallic that forms at the solder–substrate interface in many soldering operations can be influenced by Ni additions. It has been established that when Ni is present at 5–9 at%, the high-temperature hexagonal Cu6Sn5 (η) does not transition to the low-temperature monoclinic Cu6Sn5 (η′) at the equilibrium temperature of 186 °C. In fact, the hexagonal phase remains stable from room temperature to 250 °C. This paper shows the stabilising effect of Ni exists in the range of 4.6–17.2 at% Ni in stoichiometric samples, over the larger temperature range of −100 to 250 °C using synchrotron X-ray diffraction. The results are also used in combination with dilatometry experiments and show that Ni decreases the magnitude of thermal expansion, and prevents the discontinuity in expansion that occurs with the polymorphic transformation.