Failure mechanism of solder interconnections under thermal cycling conditions

Increasing miniaturization, power densities and internal heat dissipation of novel electronic packages have made their solder interconnections more vulnerable to failures. To improve the reliability of electronic devices the underlying physical failure mechanisms of solder interconnections must be clarified in detail in order to find means to control, or even prevent, the development of failures. Therefore, the evolution of microstructures and the development of failures in Sn-rich lead-free solder interconnections were investigated by employing methods of orientation imaging microscopy: cross-polarized light imaging and electron backscatter diffraction. The as-solidified microstructures of the SnAgCu solder interconnections (composed of a few large Sn colonies) were observed to undergo a notable change of microstructures at the strain/stress concentration regions before cracking. The investigations of microstructures indicate that the change of microstructures take place at two different stages in the course of thermal cycling: 1.) a gradual formation of low angle tilt grain boundaries caused by a rotations of small volumes of the as-solidified microstructures around the [100] and [110] axes. It is suggested that these boundaries are formed by recovery, i.e. the boundaries are a consequence of the rearrangement of dislocations by polygonization. 2.) In subsequent stages the microstructures in the strain concentration regions transformed into a more or less equiaxed grain structure by recrystallization. It is evident that cracking of solder interconnections under thermomechanical loadings is enhanced by the recrystallization, because the network of high-angle grain boundaries extending through the interconnections provide favorable paths for cracks to propagate intergranularly.