Abstract: In the process of electronic packaging, an intermetallic compounds layer is formed at the interface between solder and Cu substrate. The intermetallic compounds layer is mainly composed of Cu₆Sn₅ compounds. The size and morphology of the Cu₆Sn₅ intermetallic compounds have a notable influence on the reliability of solder joints. A series of Sn3.0Ag0.5Cu/Cu solder joints were prepared by reflow soldering method. Image-Pro Plus software was used to statistically analyze the size distribution and the thickness of the Cu₆Sn₅ grains. The results show that the average diameter of Cu₆Sn₅ grains is proportional to t^0.38, where t is the reflow time. The mean thickness of the interfacial compounds layer is proportional to t^0.32. With the increase of reflow time, the growth rate of interfacial compound slows down, and Cu₆Sn₅ grain size distribution becomes more uniform. The size distribution of Cu₆Sn₅ in samples with long reflow time is basically consistent with the theoretical curve of FRD model, while for samples with short reflow time, the grain size distribution deviates from the FRD theory. The statistical results show that the grain size with the highest frequency is less than the average value. The growth mechanism of interfacial Cu₆Sn₅ grains was discussed, and the effect of reflow time on the growth way of Cu₆Sn₅ grains was analyzed.