The formation of interfacial η′-Cu6Sn5 in Sn–0.7Cu/Cu solder joints at different aging temperatures was studied using x-ray diffraction (XRD). The time-temperature-formation curve was obtained and is discussed based on the phase transformation from existing η-Cu6Sn5 and interfacial reaction at temperatures below 186 °C. A minimum formation time was observed in the temperature range of 135–150 °C.