Evolution of CuSn intermetallics between molten SnAgCu solder and Cu substrate

The evolution of SnCu intermetallic compounds (IMCs) between a molten SnAgCu alloy and the Cu under-bump metallization (UBM) throughout reflow is presented based on interruption of soldering reactions in experiments by removing the liquid solder from the substrate. This allows to capture and visualize interfacial reactants at arbitrary moments of the soldering process, and to gain an insight into their formation characteristics. The results show that the interfacial Cu6Sn5/Cu3Sn structure is formed at an early stage of reflow and is maintained throughout the process. Based on the experiments, formation mechanisms of interfacial CuSn IMCs are discussed.