The electromigration (EM) effect has been known to be atomic diffusion due to unbalanced electrostatic and electron-wind forces exerted on metal ions. Recent theoretical model on the mechanism were focused on the kinetics point of view. However, none of these models has coupled the EM effect and lattice stability. In this work, in situ current stressing experiments using synchrotron X-ray diffractometry and scanning electron microscopy with ab initio calculations based on density functional theory were performed for pure Cu strips. A new and novel mechanism of EM effect is revealed in this study, which enables the developments on new design rule for fine-pitch and high-power packaging.