A tensile stress developed in polycrystalline copper films during room-temperature aging was computed using a diffusion equation for excess vacancies migrating to the grain boundaries. This theory is based on an assumption that a free volume created by the arrival of excess vacancies at the grain boundaries of thin copper films is instantly eliminated and this action introduces a biaxial tensile stress in the plane of the film. The tensile stress was calculated as a function of aging time, grain size, and excess vacancy concentration and it was found that it could exceed the yield stress of copper. This result suggests that plastic deformation could occur in electrodeposited copper films during room-temperature aging, consistent with experimental observations. (c) 2007 The Electrochemical Society.