Effects of Ni₃Sn₄ and (Cu,Ni)₆Sn ₅ intermetallic layers on cross-interaction between Pd and Ni in solder joints

We examined the effects of layers of intermetallic compound (IMC) Ni ₃Sn₄ 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 solidstate 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.