In recent years, the electronic packaging technologies have become more diversified and sophisticated, and among these technologies the interfacial metallurgical reaction between the solder and metallic substrate is significantly different from each other. However, in contrast to plenty of studies about the microstructure evolution in solder joints prepared by the conventional soldering processes, such as wave soldering and reflow soldering in surface mount technology (SMT), relatively little is known about the solder's melting feature, interfacial mass transition and metallurgical reaction as well as dissolution behavior of the under bump metallization (UBM) in the micro－scale solder joints manufactured by using the novel soldering technologies, for example, the local melting reflow process. In this study, the differential scanning calorimetry (DSC) experimental approach was employed to simulate the local melting reflow soldering process, and the melting behavior of ball grid array (BGA) structure Sn－3.0Ag－0.5Cu/Cu joints, the interfacial reaction and intermetallic compound (IMC) growth behavior of the joints during the liquid isothermal aging at the temperatures near or above the solder's melting point were studied systematically. Results showed that only a part of the solder matrix adjacent to the interface of the joints melted during the liquid isothermal aging at the solder's melting point of 217 ℃. Then, a slight increase of the aging temperature from 217 ℃ up to 218 ℃ has a significant influence on the melting status of the solder matrix and the Cu substrate consumption during isothermal aging process, that is, all the eutectic phases and partial β－Sn phase in the solder matrix melted, and the proportion of the Cu substrate dissolved into the Sn matrix in the total consumption of Cu substrate also increased greatly. However, the thickness of the interfacial Cu₆Sn₅ layer and total Cu－Sn IMC layer had a slight decrease due to the fact that Cu substrate rapidly dissolved into the solder matrix and resulted in less supply of Cu in the growth of interfacial IMC layers. When the aging temperature was increased to 230 ℃, the content of Cu in the solder matrix nearly reached saturation, and the thickness of interfacial Cu－Sn IMC layers also reached the highest value. The results of interfacial IMCs growth kinetics show that the growth of the interfacial Cu₆Sn₅ and Cu₃Sn is mainly controlled by grain boundary diffusion and bulk diffusion, respectively; and the grain boundary grooving and grain coarsening can also influence the growth kinetics of the interfacial Cu－Sn IMCs.