Influence of Pd Coating Thickness and Pd Content in Sn-Based Solders on Interfacial IMC Formation and Microstructural Evolution in Solder/Ni Joints

Interfacial reactions between Sn-based solders and Au/Pd/Ni metallization were investigated at 260 °C, with particular emphasis on the effects of Pd and Sn thicknesses. Au/Pd/Ni substrates with Pd layers of approximately 70 nm, 200 nm, and 1 µm were reacted with Sn layers of about 50, 20, and 10 µm. Additionally, Sn-Pd and Sn-3Ag-Pd solders containing 0.1-1 wt.% Pd were reacted with Ni substrates. In the Sn/Au/Pd/Ni reactions, rapid dissolution of the Pd layer and partial Ni dissolution at the early stage promoted the formation of large amounts of faceted (Pd,Ni)Sn4. With increasing reaction time, continuous Ni diffusion enriched the interfacial region, leading to the nucleation and growth of Ni3Sn4. Once the Ni solubility limit in (Pd,Ni)Sn4 was exceeded, this phase gradually transformed into the thermodynamically more stable Ni3Sn4. In addition to phase evolution, Pd was found to significantly influence the interfacial grain morphology. Minor Pd additions enhanced the Ni3Sn4 nucleation, resulting in refined and columnar grains. In the Sn-Pd/Ni reactions, low Pd contents led to the rapid replacement of (Pd,Ni)Sn4 by Ni3Sn4, whereas higher Pd contents significantly enhanced the stability and interfacial retention of (Pd,Ni)Sn4. These results reveal that increasing Pd thickness or Pd content in the solder significantly enhances the stability of (Pd,Ni)Sn4, whereas reducing Sn thickness markedly accelerates interfacial reactions and phase transformation. The experimental observations can be consistently interpreted using a local interfacial equilibrium hypothesis based on the Sn-Pd-Ni phase diagram.