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We examined the effects of layers of intermetallic compound (IMC) Ni 3 Sn 4 and (Cu,Ni) 6 Sn 5 formed at the solder/Ni interface, on the cross-interactions between Pd and Ni during solid-state aging and reflow soldering. Two types of diffusion couples, Pd/Sn/Ni and Pd/Sn–Cu/Ni, were aged at 150 °C to study the solid-state interactions. In contrast to the Pd/Sn/Ni couples in which a Ni 3 Sn 4 layer formed at the Ni interface, the Pd/Sn–Cu/Ni couple where a (Cu,Ni) 6 Sn 5 layer formed at the Ni interface exhibited no significant interaction between Pd and Ni. The (Cu,Ni) 6 Sn 5 layer acted as an effective barrier against Ni diffusion and thus inhibited the resettlement of (Pd,Ni)Sn 4 onto the Ni interface. For the interaction during reflow, Sn–3.5Ag and Sn–3.0Ag–0.5Cu solder balls were isothermally reflowed on an electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) surface finish at 250 °C, and the dissolution kinetics of the (Pd,Ni)Sn 4 particles converted from the 0.2-μm-thick Pd-finish layer were examined. The spalled (Pd,Ni)Sn 4 particles very quickly dissolved into the molten solder when the IMC layer formed on the Ni substrate was (Cu,Ni) 6 Sn 5 rather than Ni 3 Sn 4 . The dependence of the dissolution kinetics of the spalled (Pd,Ni)Sn 4 particles on the IMC layers was rationalized on the basis of a Sn–Ni–Pd isotherm at 250 °C. The present study suggests that the formation of a dense (Cu,Ni) 6 Sn 5 layer at the solder/Ni interface can effectively alleviate the (Pd,Ni)Sn 4 -related degradation of solder joint reliability. |
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