Influence of Pd Coating Thickness and Pd Content in Sn-Based Solders on Interfacial IMC Formation and Microstructural...

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.