Crack initiation and growth during low cycle fatigue of Pb-Sn solder joints

The initiation and growth of fatigue cracks during low-cycle fatigue of Pb-Sn solder joints (60Sn-40Pb and 75Sn-25Pb) in shear at 300 K were followed employing two techniques: (a) surface examination by low-power optical microscopy and (b) liquid N₂ cracking of fatigued specimens. Cracks 50 to 100 μm long were first detected by the optical microscopy examination of the specimen surface at the beginning of Stage II of the load drop parameter φ vs. N curve. These cracks then grew as the specimen was cycled further into Stage II. SEM fractographs of fatigued specimens broken after immersion in liquid N₂ revealed that fatigue cracks initiated mainly at the external surface and at the internal voids and grew by cracking along all three of the common paths: eutectic colony boundaries, grain boundaries, and across the colonies, i.e., a transgranular path. The drop in load which occurred during the fatigue test was established to be proportional to the fatigue cracked area