Normally and highly accelerated electromigration tests on Al-Cu lines of different widths are compared. It is shown that the use of the Black equation gives different extrapolated results depending on the range of stress conditions considered. It is concluded that for extrapolations it is safe to use the Black equation only in the case of normally accelerated stress conditions (temperatures in the range 150–240°C and current densities between 1 and 2 MA/cm2) and when the extracted value for the current density acceleration factor is 2.