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Thermomigration in Sn58Bi solder joints at low ambient temperature

This paper investigated the individual effects of thermomigration behavior in Cu/Sn58Bi/Cu solder joints by applying a pure thermal gradient field at an ambient temperature of 20°C. To separate electromigration behavior, a novel apparatus was used to carry out the thermomigration experiment. The thermal finite-element simulation results showed that a sufficient thermal gradient existed in the Cu/Sn58Bi/Cu solder joint during the experiment. After loading a constant thermal gradient for over 100 hours, an obvious phase redistribution phenomenon occurred in the Sn58Bi solder layer. It was found that Bi atoms in the Sn58Bi solder layer had migrated toward the lower temperature side. Meanwhile, Bi-rich phase coarsening was detected in the Sn58Bi solder layer, and with the loading time increased, the Bi-rich phases went on coarsening. After thermomigration for 200 hours, a thin Bi-rich layer formed at the cold side of the Sn58Bi solder layer. Moreover, Kirkendall voids were produced in the intermetallic compound layers, with their number at the hot sides larger than that at the cold side. Compared with the aging test samples, Bi-rich phase aggregation only appeared at the cold side of the solder joint in thermomigration samples while the aggregation appeared at both sides in the 200h-aged samples, which further emphasized the great effects of thermomigration in the Sn58Bi solder joints.