Investigation of reactions between lead/tin solder and palladium surface finishes

Abstract The role of palladium as a surface finish for the electronics industry was investigated. Of particular interest were the metallurgical phases that form during and following soldering operations of palladium thin films. The formation of PdSn4 intermetallics on composite substrates (gold/palladium/nickel) with varying palladium chemistry and layer thickness were soldered at 220 °C and observed using scanning electron microscope combined with energy dispersive X-ray (SEM/EDX) analysis. Thin layers of palladium approximately 1 μm thick, combined with a fast dissolution rate also resulted in the formation of Ni3Sn4 intermetallics. The role of phosphorus in palladium thin films was also investigated. Its presence was found to increase the solubility of palladium in molten solder producing intermetallic crystals that are confined to the bulk. A thin gold layer, used to protect the palladium was found to dissolve rapidly into the solder and to redeposit, forming a thin layer just outside the PdSn4 intermetallic structure.