The initiation and growth of fatigue cracks during low-cyclefatigue of Pb-Sn solder joints (60Sn-40Pb and 75Sn-25Pb) in shear at 300K were followed employing two techniques: (a) surface examination bylow-power optical microscopy and (b) liquid N2 cracking offatigued specimens. Cracks 50 to 100 μm long were first detected bythe optical microscopy examination of the specimen surface at thebeginning 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 liquidN2 revealed that fatigue cracks initiated mainly at theexternal surface and at the internal voids and grew by cracking alongall three of the common paths: eutectic colony boundaries, grainboundaries, and across the colonies, i.e., a transgranular path. Thedrop in load which occurred during the fatigue test was established tobe proportional to the fatigue cracked area
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