The existence of voids in the solder joints has been a serious reliability problem in microelectronics. A comprehensive understanding of the formation mechanism of voids is the cornerstone to solve the problem. This study reports an unusual phenomenon of voiding in the Sn/Cu solder joints and the most unique finding is that the voids were formed at the solder side (the Sn/Cu6Sn5 interface) rather than at the Cu side (the Cu3Sn/Cu interface) due to Kirkendall effect. Cross-sectional and topographic examination of the Sn/Cu interface using a scanning electron microscopy clearly positioned the voids on the surface of the Cu6Sn5 grains and their grain boundaries (triple junctions). Gas chromatograph (GC)/flame ionization detector (FID) microanalysis was performed to rationally correlate the outgassing of the Cu electroplated substrate and the void formation at the Sn/Cu6Sn5 interface.