The program involved the effect on joints when exposed to low-frequency thermal cycling and a definition of the metallurgical cracking mechanism. Four solder alloys 50Sn-50Pb, 63Sn-37Pb, 70Sn-30Pb, and 40Sn-60Pb were exposed to low-frequency thermal cycles. Sample soldered joints on printed circuit boards were installed in an environmental test chamber and subjected to thermal cycling. An exhaustive study was performed on the tested soldered joints to detect any metallurgical changes resulting from testing and to define the metallurgical mechanism of solder failure. Examinations consisted of bright-light microscopy and scanning electron microscopy on soldered fillet faces, joint cross-sections, and fractured surfaces.