The effect of pre-aging on electromigration is investigated in this study using flip-chip SnAg solder joints. The solder joints were pre-aged at C for h, h, h, h, h, and h, and then they were subjected to electromigration tests of. A at C. It was found that the average failure time increased about three times when the joints were pre-aged for h to h. But it decreased when the joints were over-aged. It is proposed that the major contributor to the prolonged failure time may be the densification of the nickel and copper under-bump metalli ation UBM and the solder due to the aging treatment. The pre-aging treatment at C may stabili e the microstructure of the solder. The vacancies in the solder were annihilated during the heat treatment, causing a slower diffusion rate. In addition, the UBM structure became denser after the pre-aging process. Thus, the denser UBM structure may lead to slower consumption rates of the nickel and copper layers, resulting in the enhancement of electromigration resistance.