Electromigration-induced failures in sputtered TiN/Ti/AlSiCu/TiON/Ti interconnects with aluminum (Al) reflow treatment have been studied in terms of aluminum microstructure and stress relaxation between the AlSiCu and TiON barrier metal. The sputtered interconnects with Al reflow treatment have an electromigration lifetime three times longer than the sputtered interconnects without Al reflow treatment. Furthermore, electromigration-induced voids in sputtered interconnects without Al reflow treatment are generated at the interface between the AlSiCu and TiON. Voids in sputtered interconnects with Al reflow treatment are seldom generated at the interface between the AlSiCu and TiON. The difference in electromigration performance between the two metallizations cannot be explained merely by the <111> orientations of aluminum crystallites and the grain size of the aluminum. We conclude that the most effective factor in electromigration performance is the stress relaxation between AlSiCu and TiON during reflow annealing. The negligible residual compressive strain has little influence on the electromigration resistance of sputtered interconnects with Al reflow treatment. On the contrary, the residual compressive strain lowers the electromigration resistance in sputtered interconnects without Al reflow treatment.