Void formation in lead-free solder joints, away from the joint interface, has been observed after solid-state aging. These voids are attached to intermetallic precipitates in the solder matrix, especially to those that are adjacent to the layered intermetallic at the joint interface. Two potential void formation mechanisms are discussed. The mechanism proposed to describe void formation is that a flux of vacancies is created due to volume contraction during solid-state reaction. The ripening process among the intermetallics also assists this process. Using the suggested mechanisms, the void size was estimated. This phenomenon differs from the classical Kirkendall void formation because it is a nonequilibrium state of void formation and stress generation.