GRAIN-BOUNDARY SLIDING IN SURFACE MOUNT SOLDERS DURING THERMAL CYCLING

This article reports the growth of fatigue cracks in near-eutectic surface mount solder joints during cyclic, thermal displacement-controlled loading. Ceramic leadless chip carriers (LLCC's) and small outline J-leaded (SOJ) packages undergo thermal displacement-induced fatigue with a frequency of 1/h. Two ranges of temperature are utilized: -36-125-degrees-C and -20-75-degrees-C. Surfaces and cross sections of the solder joints are inspected using scanning electron microscopy. Individual grains (colonies of lamellae or globulae) are found to slide relative to one another during thermal cycling. Cracks initiate at sites where the boundaries between colonies intersect the outer surface of the solder fillet. In the case of the LLCC solder joints, cracks also initiate beneath the chip carrier. These cracks rapidly reach the bulk portion of the fillet, where they are observed to prefer travel along interfaces between phases.