With the development of electronic products towards further miniaturization, multifunction and high-reliability, the packaging density has been increasing and the dimension of solder joints has been scaling down. In electronic packaging, during the soldering process of Sn/Cu system, an intermetallic compound (IMC) layer is formed at the interface between the molten solder and pad (substrate), the interfacial microstructure plays an important role in the reliability of solder interconnects. Generally, during the reflow soldering and subsequent aging process, a large number of Kirkendall voids may form at the Cu/Cu₃Sn interface and in the Cu₃Sn layer. The existence of Kirkendall voids may increase the potential for brittle interfacial fracture of solder interconnects and reduce the thermal conductivity. Thus, characterization of formation and growth of Kirkendall voids is very important for the evaluation of performance and reliability of solder interconnects. In this work, the formation and growth of Kirkendall voids at the Cu/Cu₃Sn interface and in the Cu₃Sn layer of Sn/Cu solder system have been investigated by means of phase field crystal modeling. The growth mechanism of Kirkendall voids was analyzed. The effects of thickness of Cu₃Sn layer and impurity particles in the Cu₃Sn layer on the growth of Kirkendall voids were discussed. Phase field simulation results show that the growth of Kirkendall voids exhibits four stages during the thermal aging, including the formation of atomic mismatch areas at the Cu/Cu₃Sn interface, the rapid growth of the atomic mismatch areas leading to the formation of Kirkendall voids, the growth of Kirkendall voids and the subsequent coalescence of Kirkendall voids. Kirkendall voids nucleate preferentially at the Cu/Cu₃Sn interface and their sizes increase with the aging time, and the coalescence of the voids can be observed obviously in the later stage of thermal aging. It has also been shown that the increase of the Cu₃Sn layer thickness and the amount of impurity particles lead to an increase in both number and size of Kirkendall voids, as well as an increased growth exponent; and the number of Kirkendall voids increases initially and then decreases with the aging time.