Gold- and Palladium-Induced Embrittlement Phenomenon in Microbumps with Au/Pd(P)/Ni(P) Metallization Pads

Abstract

In this study, the microstructural evolution of Sn-3Ag-0.5Cu/Au/Pd(P)/Ni(P) microbumps during isothermal aging was examined by electron probe microanalysis and scanning electron microscopy equipped with electron backscatter diffraction. The Au/Pd(P)/Ni(P) trilayer was deposited on 85-μm (opening diameter) Cu pads with a thickness combination of 0.15 μm/0.2 μm/8 μm. The reaction of the Au/Pd(P) dual layer with the molten solder yielded plenty of AuSn₄ and PdSn₄ grains in the solder matrix after reflow, which has never been observed previously in the literature. At the interface, discontinuous, facet-type Cu₆Sn₅ [or (Cu,Ni)₆Sn₅] nucleated. Interestingly, the Cu₆Sn₅ transformed into layered Ni₃Sn₄ after subsequent aging for 250 h at 180°C. Moreover, the AuSn₄ and PdSn₄ agglomerated with each other and resettled to the region neighboring the interface. This configuration changed significantly when a pronounced corrosion occurred in the as-plated Ni(P) metallization pad. In the corroded Ni(P) case, Cu₆Sn₅ rarely appeared at the interface immediately after reflow, but a large quantity of PdSn₄ formed in the solder region near the corroded Ni(P) layer. AuSn₄ in this case distributed uniformly throughout the entire solder joint. Isothermal aging treatment drove the PdSn₄ and AuSn₄ to deposit onto the corroded Ni(P) surface, where they merged with each other as a dense layer of (Au_0.61Pd_0.30Ni_0.09)Sn₄ (250 h). The formation of the (Au,Pd,Ni)Sn₄ layer over the corroded Ni(P) substrate provided a crack initiation site, deteriorating the mechanical reliability of the solder joint. The results of this study show that appropriate deposition of Au/Pd(P)/Ni(P) is very important for the thermal/mechanical reliability of microbumps.