Thermal degradation of sputtered Al/Ni(V)/Cu-UBM in Pb-free flip chip solder joints connected to substrate with different surface finishes

The thermal stability of flip chip solder joints made with Al/Ni(V)/Cu-UBM and SAC-405 solder bumps on substrates with either electroless Ni(P)-immersion gold (ENIG) or Cu surface finish (Cu-SOP) was determined at 170C. On ENIG, the resistance changed by more than one order of magnitude after 400 hours of high temperature storage, whereas on Cu-SOP, no change in resistance was observed up to 2400 hours of aging. The detailed electrical characterization of the solder joints was supplemented with time dependent physical characterization (microstructure and materials composition) of the joint using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), focused ion beam microscopy (FIB), and transmission electron microscopy (TEM). The microstructural characterization clearly showed that electrical degradation of the ENIG devices was a direct result of the conversion and interfacial oxidation of the as deposited Ni(V)-barrier UBM layer into a porous layer that contained a web like fine grain V₃Sn IMC. This conversion was driven by the Au from the ENIG surface finish. The results were similar to that reported by the authors for eutectic PbSn solder bumps, the exception being that for ENIG surface finish the resistance stability of SAC solder was 25x greater than eutectic that for PnSn solder.