The electromigration (EM) resistance of interconnections manufactured by electroplating was investigated from the viewpoint of temperature and diffusion paths in the polycrystalline materials. The crystallinity of the interconnections was evaluated by image quality (IQ) value obtained from electron back-scatter diffraction analysis. The degradation process under the EM test was observed by using the IQ value. The degradation of the interconnections was dominated by the diffusion of component atoms along random grain boundaries with low crystallinity. It was also found that the electrical resistance of the interconnections varied drastically depending on the crystallinity of the material, and thus, the maximum temperature in the interconnections caused by Joule heating during the EM loading also varied drastically because of the change of the resistance. Since the diffusion constant of the component atoms is accelerated by not only the current density but also temperature, the lifetime of the interconnections under the EM loading is a strong function of the crystallinity of the interconnections. It is necessary, therefore, to evaluate and control the crystallinity of the interconnections quantitatively using IQ value to assure their long-term reliability.