Effect of Ni on phase stability and thermal expansion of Cu _6-xNi _xSn ₅ (X = 0, 0.5, 1, 1.5 and 2)

The crystallography of the Cu ₆Sn ₅ 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 Cu ₆Sn ₅ (η) does not transition to the low-temperature monoclinic Cu ₆Sn ₅ (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.