The interfacial reaction and joint strength of the Pb-free flip chip solder joints were investigated. Microstructural investigations were conducted using scanning electron microscopy (SEM), and phase analysis of the intermetallic compound (IMC) was carried out using energy dispersive spectrometer (EDS). Cu6Sn5 IMC layer was formed at the as-reflowed state, and Cu3Sn was additionally settled at the interface between the Cu6Sn5 and Cu under bump metallurgy (UBM). Growth of the IMC layers followed the parabolic law, while the IMC layer growth was not highly affecting the mechanical properties of the solder joints. The shear force of the solder joints decreased because of the microstructural coarsening within the solder region.