The effect of pre-aging on the electromigration of flip-chip SnAg solder joints

Abstract

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 170°C for 1 h, 3 h, 5 h, 10 h, 25 h, and 50 h, and then they were subjected to electromigration tests of 0.9 A at 150°C. It was found that the average failure time increased about three times when the joints were pre-aged for 3 h to 25 h. But it decreased when the joints were overaged. It is proposed that the major contributor to the prolonged failure time may be the densification of the nickel and copper under-bump metallization (UBM) and the solder due to the aging treatment. The pre-aging treatment at 170°C may stabilize 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.