Cu6Sn5 is a common intermetallic compound formed during electrical packaging. It has an allotropic transformation from the low-temperature monoclinic η’-Cu6Sn5 to high-temperature hexagonal η-Cu6Sn5 at equilibrium temperature 186 °C. In this research, the effects of this allotropic transformation and Ni addition on the thermal expansion of η’- and/or η-Cu6Sn5 were characterized using synchrotron x-ray diffraction and dilatometry. A volume expansion during the monoclinic to hexagonal transformation was found. The addition of Ni was found to decrease the undesirable thermal expansion by stabilizing the hexagonal Cu6Sn5 at temperatures below 186 °C and reducing the overall thermal expansion of Cu6Sn5.