Relationship Between the Grain Orientation and the Electromigration Reliability of Electronic Packaging Interconnects

Various studies of the electromigration (EM) reliability of solder interconnects have suggested that early EM failure may have different failure kinetics depending on the grain orientation of Sn within the solder bump. Since Sn has a body-centered tetragonal (BCT) crystal structure, it has anisotropic properties, having a fast EM diffusing axis, along [001] grain orientation due to a relatively larger diffusion path. In order to study a relationship between early EM failure and Sn grain orientation, both early failed and late failed samples from accelerated EM tests are characterized with scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Unfortunately, SEM does not show distinctive differences between early and late failure features, but EBSD analysis shows a distinctive feature, which is an alignment of the c-axis with the EM direction. The early failed samples have a high degree of c-axis alignment with the EM direction, while the late failed samples have less or no degree of c-axis alignment. The failure analysis suggests that the presence of c-axis along the EM direction near the void nucleation area accelerates the EM failure kinetics.