Microstructural evolution during thermomechanical fatigue of 62Sn-36Pb-2Ag and 60Sn-40Pb solder joints

An initial study was performed to examine the effect of adding a small amount of silver on the thermomechanical fatigue behavior of near-eutectic Sn-Pb solders. The solder studies were 60Sn-40Pb and 62Sn-36Pb-2Ag. For both solders, as the strain rate decreased the lifetime increased, because the dislocation substructure anneals more rapidly than it work-hardens, which tends to minimize subsequent heterogeneous recrystallization and growth. The addition of silver resulted in the formation of large Ag₃Sn intermetallic whiskers in the solder joints. The whiskers were too large to be benefactors in thermomechanical fatigue, but neither were they so brittle as to decrease the fatigue life. From a microstructural viewpoint, it does not appear that the addition of silver has any significant effect on the thermomechanical fatigue behavior