Influence of UBM metallurgy on Sn textures and EM failure

Influence of UBM and substrate metallurgy on electromigraiton is studied. It is found that SnAg interconnects with Cu UBM and CuOSP substrate finish tend to have single or few grains, while the same SnAg interconnect with Ni UBM and NiAu substrate finish tend to have interlaced twining textures. The twining in Ni/Ni metallurgy randomize the grain orientation to a certain degree, so that no fast diffusion paths exists between the cathode and anode, which could lead to the existence of the critical length observed in EM experiments. With a single grain texture, Cu/Cu metallurgy is more prone to electromigration degradation when the c-axis of the Sn grain is aligned with the current direction, which could be an explanation for the lack of an EM critical current. Morphology of EM induced voids sectioned with FIB, showed that voids are formed by out diffusion of fast diffusors such as Ni and Cu from the intermetallic, leaving behind voids and Sn rich material.