Sn-Ag-Cu based alloys are well known as an important series of lead-free solders. Gallium has been found to be beneficial for the melting point of the Sn-Ag-Cu solders. In the present work, interfacial reactions between Sn-3.0Ag-0.5Cu-xGa (x = 0, 0.1, 0.5, 1.5 in wt.%) solders and Cu substrates have been examined. The solder and substrate couples were annealed at 80°C for 2, 4, 6, 12, 18, and 24 days, respectively. Various layers of intermetallic compounds have been observed to form using scanning electron microscopy. It has been found that only CuSn phase forms when Ga additions are 0, 0.1 and 0.5 in wt.%; but CuSn and CuGa form if Ga addition reaches 1.5 wt.%. Thicknesses of these intermetallic compounds have been measured at different annealing stages in order to study the growth kinetics. The growth of all the intermetallic compounds follows the parabolic rule. [ABSTRACT FROM AUTHOR]