The Role of Recrystallization in the Failure of SnAgCu Solder Interconnections Under Thermomechanical Loading

The chip scale packaged/ball grid array component boards were thermally cycled according to the IEC 68-2-14N standard (+125 degrees C/-45 degrees C, 15 min dwells, 5 min ramps). The as-solidified microstructures of Sn-rich solder interconnections were composed of relatively few, typically from two to five, large tin colonies distinguished by high-angle boundaries. However, during the thermal cycling tests the as-solidified microstructures transformed gradually into more or less equiaxed grain structure by recrystallization. It is suggested that cracking of solder interconnections under thermomechanical loadings is enhanced by the recrystallization, because the network of newly formed grain boundaries extending through the interconnections provide favorable paths for cracks to propagate intergranularly. The incubation time of recrystallization was about 50% of the average cycles-to-failure. The decrease of stored energy in high-stacking fault energy metals such as Sn takes place very effective by the recovery. Therefore, microstructures recrystallize only under restricted loading conditions: dynamic loading condition where strain hardening is more effective than recovery. Therefore, the experimentally observable transformation of the microstructures by recrystallization enables us, in principle, to correlate the field use loading conditions with those produced in accelerated reliability tests. Furthermore, the recrystallization provides the means to incorporate the effects of microstructural evolution into the lifetime prediction models.