Electromigration study of various layer-structured aluminum conductors with three widths revealed that lifetime (defined as a 1% resistance increase) depends on two critical parameters : the grain size/conductor width ratio and the grain orientation. These two parameters work independently and monotonically. Investigation of the number and size of voids formed by electromigration showed that lifetime is proportional to the square of the number of voids. This relationship is independent of linewidth and current density.