The electromigration characteristics of A1 films deposited onto low dielectric polyimide and CVD-Si02 have been investigated. The polyimides used in this study were DuPont PI-2734 and Toray BG 2480. 500 ㎚ Al-0.5%Cu- l%Si thin films were deposited onto polyimides and LPCVD-SiO₂ by DC magnetron sputtering. Al lines were tested at constant voltage, at a temperature of 225℃ and a current density of 1.85 MA/㎠. The electromigration reliability of Al lines onto polyimides was worse than that onto SiO₂. Joule heating was the main cause of the degradation of electromigration reliability in the Al lines deposited onto polyimides. Since the thermal conductivity of polyimide is about one order lower than that of SiO₂, joule heating was more significant in Al/polyimide during electromigration test and the temperature increase of A1 lines on polyimide decreased the electromigration life.