The cross-interactions in the Ni/Sn/Cu diffusion couples with an electroless palladium surface finish

Four diffusion couples, Ni/Pd/Sn, Ni/Pd/Sn/Cu, Cu/Pd/Sn, and Cu/Pd/Sn/Ni, were prepared by hot pressing and the interfacial reactions in these multi-layer structures aged at 200 °C were investigated. The Pd layer (0.4 and 2 μm) was deposited on the Ni and Cu substrates using electroless plating. The (Pd,Ni)Sn4 phase formed in the Ni/Pd/Sn diffusion couple grew thicker and dissolved with more Ni atoms upon increasing the aging time even though the Pd layer was completely consumed. By contrast, the PdSn4 phase formed in the Cu/Pd/Sn diffusion couple exhibited negligible growth with very limited Cu dissolution after the Pd layer was completely consumed. In the Ni/Pd/Sn/Cu and Cu/Pd/Sn/Ni diffusion couples, the (Cu,Ni)6Sn5 phase was formed on the Ni side as a result of the cross-interaction caused by the Cu diffusion from the Cu side to the Ni side. With a thicker Pd layer (2 μm), a thicker (Pd,Ni)Sn4 (or PdSn4) phase was formed in these two diffusion couples and was found to behave as a diffusion barrier. The effect of cross-interaction due to the Cu diffusion became weaker and as a result the (Cu,Ni)6Sn5 phase formed on the Ni side was gradually transformed into the (Ni,Cu)3Sn4 phase.

► Cross-interactions occur in the Ni/Pd/Sn/Cu and Cu/Pd/Sn/Ni diffusion couples.
► The Cu atoms diffuse to the Ni side to form the (Cu,Ni)6Sn5 phase there.
► A thicker Pd layer results in a thicker (Pd,Ni)Sn4 phase at the interface.
► The (Pd,Ni)Sn4 phase behaves as a barrier retarding the diffusion of Cu.
► Without Cu supply, the (Cu,Ni)6Sn5 phase is transformed into the (Ni,Cu)3Sn4 phase.