Electrodeposition of Pb-free Sn alloys in pulsed current

A pulsed electrodeposition method is applied to the preparation of Pb-free Sn alloys solder bumps for flip-chip bonding with the aid of a photolithography. Sn–Ag alloy films with near eutectic compositions (Sn–3.5% Ag) were obtained using a pyrophosphate–iodide plating baths regardless under direct or pulsed current. The composition and the morphology of electrodeposits were examinated by SEM and X-ray photoelectron spectroscopy (XPS). The main results revealed that the organic additives affect the electrochemical reduction of tin–silver and the direct consequence on making Sn–Ag alloy is a decreased deposition rate. However, the addition of additives in the plating bath suppressed the dendritic tin–silver growth by adsorption on the deposited surface. Pulsed electrodeposition is shown to be an interesting approach to elaborate bumps with smooth and homogeneous surfaces.