This paper studied the effects of thermal aging conditions on interfacial IMC layer of Sn0.7Cu and Sn0.7Cu0.05Ni solders joint. Solder joint was formed using the reflow soldering method and were thermal aged for 24 and 120 h at ambient temperature, 85 0C and 150 0C respectively. The characterization emphases on the microstructure evaluation of interfacial intermetallic compound (IMC) layer during thermal aging using optical microscopy (OM) and Image-J software. The study found that adding 0.05 wt.% Ni to Sn0.7Cu solder changes the properties of the interfacial IMC layer. Sn0.7Cu0.05Ni reduced the interfacial IMC thickness with thinner and more refined IMC layer compared to Sn0.7Cu at the solder/substrate interface. Additionally, the presence of Ni significantly improves wettability by decreasing the wetting angle and stabilizes the IMC structure, preventing the formation of brittle Cu3Sn layers and more suitable for high temperature applications.