Thermomechanical fatigue of solder joints: a new comprehensive test method

A comprehensive method for testing solder joints under conditions of thermomechanical fatigue is presented. The method involves simultaneous imposition of temperature cycles and strain on discrete solder joints in a shear orientation. The stress, microstructure, and number of cycles to failure were monitored. Cycles to failure were determined by a continuous electrical detection method. 60Sn-40Pb and 40Sn-40In-20Pb solder joints were tested using the proposed method at 20% shear strain. The 60Sn-40Pb alloy has a shorter fatigue lifetime than 40Sn-40In-20Pb. This is attributed to heterogeneous coarsening strain in a small area of the 60Sn-40Pb microstructure. In contrast, the 40Sn-40In-20Pb microstructure becomes refined. The heterogeneous coarsening also results in cyclic softening in 60Sn-Pb, which was not observed in 40Sn-40In-20Pb. Failures initiated within the coarsened band in 60Sn-40Pb at Sn-Sn grain boundaries or phase boundaries. In contrast, failures initiated at the surface of 40Sn-40In-20Pb joints and propagated through both phases of the microstructure.>