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

摘要

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 N/sub 2/ cracking of fatigued specimens. Cracks 50 to 100 mu m long were first detected by the optical microscopy examination of the specimen surface at the beginning of Stage II of the load drop parameter phi 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 N/sub 2/ 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.