Effects of Ni3Sn4 and (Cu,Ni)6Sn5 intermetallic layers on cross-interaction between Pd and Ni in solder joints

We examined the effects of layers of intermetallic compound (IMC) Ni3Sn4 and (Cu,Ni)6Sn5 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 Ni3Sn4 layer formed at the Ni interface, the Pd/Sn-Cu/Ni couple where a (Cu,Ni)6Sn5 layer formed at the Ni interface exhibited no significant interaction between Pd and Ni. The (Cu,Ni)6Sn5 layer acted as an effective barrier against Ni diffusion and thus inhibited the resettlement of (Pd,Ni)Sn4 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)Sn4 particles converted from the 0.2-μm-thick Pd-finish layer were examined. The spalled (Pd,Ni)Sn4 particles very quickly dissolved into the molten solder when the IMC layer formed on the Ni substrate was (Cu,Ni)6Sn5 rather than Ni3Sn4. The dependence of the dissolution kinetics of the spalled (Pd,Ni)Sn4 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)6Sn5 layer at the solder/Ni interface can effectively alleviate the (Pd,Ni)Sn4-related degradation of solder joint reliability.