X-ray microscopy was employed to investigate void nucleation and propagation during electromigration in solder joints. The shape of the voids at various stages can be clearly observed. The voids became irregular when they propagated to deplete the contact opening. Growth velocity at the early stage was found to be 1.3 μmh under 6.5× 103 A cm2 at 150 °C, and it decreased to 0.3 μmh at later stages. Formation of intermetallic compound (IMC) and compositional changes at the interface of solder/IMC on the chip side were attributed to the retarded growth rate at later stages.