Electroplated copper thin films are indispensable for the interconnections in the advanced electronic products, such as TSV (trough silicon via) structures, fine bumps, and thin-film interconnections in various devices and interposers. However, it has been reported that both electrical and mechanical properties of the films vary drastically comparing with those of conventional bulk copper. The main reason for the variation can be attributed to the fluctuation of the crystallinity of grains and grain boundaries in the films. Porous or sparse grain boundaries cause the increase in electrical resistivity and the embrittlement of the films. Thus, the thermal conductivity of the electroplated copper thin films should be varied drastically depending on their micro texture based on Wiedemann-Franz law. Since copper interconnections are used for not only electrical conductor but also thermal heat conductor, it is important to clarify the relationship between the crystallinity and thermal properties of the films. In this study, the local distributions of the crystallinity and physical properties were investigated experimentally. As the result of the temperature distribution due to local Joule heating along an interconnection, it was suggested that the variation in the quality of the grain boundaries in the electroplated copper thin-films caused the non-uniformity of the resistivity and thus, Joule heating in the thin films. In this study, the effect of the seed layer material on the thermal properties of the electroplated copper thin film was investigated. When a Ru seed layer was deposited as a buffer layer between the electroplated copper thin film and the Ta diffusion barrier layer, both the crystallinity and uniformity of grain boundaries in the electroplated copper films were improved since lattice mismatch between copper and the seed layer metal was decreased. The improvement of the crystallinity increased the long-term reliability of the interconnections under the loads of electromigration and stress-induced migration.