Comparison of electromigration performance for Pb-free solders and surface finishes with Ni UBM

A series of electromigration (EM) experiments were undertaken to evaluate the time to failure performance of solder joints comprised of Sn-Ag and Sn-Cu alloys in combination with three solderable surface finishes, Cu, Ni-Au and Ni-Cu. The opposing pad structure in the solder joints was the same in all experiments and was comprised of a layered structure, simulating Ni based under - bump - metallurgies (UBM) for controlled collapse chip connection (C4). As anticipated the Sn grain size was large with the typical solder joint containing only a few grains. In all experiments, reported here, the electron current exited the pad with the surface finish under evaluation and passed into the solder. Two failure modes were identified. The manifested failure mode depended on the orientation of the c-axis of the larger Sn grains in the solder joint with respect to the applied current direction. When the c-axis is not closely aligned with the current direction, cavitation at solder-IMC interface leads to electrical failure. A more rapid failure mode occurred when the c-axis was closely aligned with the current direction. With this alignment the interfacial IMC structures were swept away by rapid diffusive processes from the pad surface and the pad material was quickly consumed. Interfacial void formation leads to rapid failure in this mode. The Sn-Ag solder appeared to demonstrate greater microstructural stability. But, clearly the best EM performance was seen with the addition of significant levels of Cu to the Sn-Ag alloy. This alloy modification showed the best EM lifetime in combination with a Ni pad structure.