Effects of Pd(P) thickness on the microstructural evolution between Sn-3Ag-0.5Cu and Ni(P)/Pd(P)/Au surface finish during the reflow process

The microstructural evolution between Sn-3Ag-0.5Cu (SAC305) solder and Ni(P)/Pd(P)/Au finish during the reflow process was investigated for various Pd(P) thicknesses (0 μm to 0.6 μm). The reflow process was carried out in a belt-conveying reflow oven with peak temperature of 260°C. In the early stages of the reflow process, the Pd(P) layer either dissolved or spalled in the form of (Pd,Ni)Sn₄ into the molten solder, leaving behind an Ni ₂SnP/Ni₃P bilayer on the Ni(P) layer. From the dissolution of the spalled (Pd,Ni)Sn₄ particles during the reflow process, the solubility of Pd in the molten SAC305 solder in the reflow process was estimated to be 0.18 wt.% to 0.25 wt.%. Regardless of the ratio of solder volume to pad opening size, the Ni₂SnP layer that formed in the early stage of reflow had a significant influence on the subsequent formation and growth of (Cu,Ni)₆Sn₅ at the solder interface. As the Ni ₂SnP layer became thicker with increasing Pd(P) thickness, the formation of (Cu,Ni)₆Sn₅ became increasingly sluggish and occurred only at locations where the Ni₂SnP layer was locally thin or discontinuous, leading to a discontinuous morphology of (Cu,Ni) ₆Sn₅. This was attributed to the Ni₂SnP layer that became an increasingly effective barrier to Ni diffusion with increasing thickness. Based on the experimental results, this study suggests detailed mechanisms underlying the effects of the Pd(P) thickness on the morphology and growth of the (Cu,Ni)₆Sn₅ formed during the reflow process.