The in situ intermetallic compound (IMC) growth in Cu pillar/Sn bumps was investigated by isothermal annealing at 120°C, 150°C, and 180°C using an in situ scanning electron microscope. Only the Cu 6 Sn 5 phase formed at the interface between the Cu pillar and Sn during the reflow process. The Cu 3 Sn phase formed and grew at the interfaces between the Cu pillar and Cu 6 Sn 5 with increased annealing time. Total (Cu 6 Sn 5  + Cu 3 Sn) IMC thickness increased linearly with the square root of annealing time. The growth slopes of total IMC decreased after 240 h at 150°C and 60 h at 180°C, due to the fact that the Cu 6 Sn 5 phase transforms to the Cu 3 Sn phase when all of the remaining Sn phase in the Cu pillar bump is completely exhausted. The complete consumption time of the Sn phase at 180°C was shorter than that at 150°C. The apparent activation energy for total IMC growth was determined to be 0.57 eV.