In the continuous pursuit of miniaturization, multifunction and high-reliability of electronic products and devices, the packing density has been increasing and the dimension of solder joints has been scaling down. In electronic packaging, during the soldering process being employed to Sn-based solders, an intermetallic compound (IMC) layer is formed between molten solder and pad (or under bump metallization, UBM), whose morphology and thickness as well as growth kinetics play an important role in controlling the service performance of the solder joints, in particular for solder interconnects with the decreasing size where the interfacial IMC layer takes up a high volume fraction in the solder joint. Thus, characterizing the morphology change and growth kinetics of interfacial IMC layer is very important to optimize the soldering process and evaluate the reliability of solder interconnects. In this study, a multi-phase-field model is applied to intensively account for the effect of grain boundary diffusion coefficient (D_{GB}) and IMC/liquid interfacial energy {\sigma }_{\eta L} on the morphology evolution and and growth kinetics of IMC. The simulation results show that Cu₆Sn₅ grains grow up and contact with each other exhibiting a scallop-like morphology which can be influenced by both the grain boundary diffusion coefficient and IMC/liquid interfacial energy. The IMC growth process exhibits three stages, including the initial stage associated with Cu₆Sn₅ grain broadening followed by the transition stage characterized by scallop shape formation and the last normal growth stage dominated by IMC layer thickening and concurrent Cu₆Sn₅ grain coarsening. It is also found that the IMC layer thickness increases with grain boundary diffusion coefficient but decreases with IMC/liquid interfacial energy, while the scallop average width decreases with grain boundary diffusion coefficient and increases with IMC/liquid interfacial energy. The relationships between IMC layer thickness/width and reaction time can be well fitted by an exponential growth law, in which the large grain boundary diffusion coefficient combined with {\sigma }_{GB}=2{\sigma }_{\eta L} (where {\sigma }_{GB} is the grain boundary energy) can produce precise growth exponent closing to that in the ideal solid-liquid interface reaction.