Polarographic Study on the Smoothing of Sn-Ag Alloy Film Electrodeposited from Pyrophosphate-Iodide Bath

Abstract

Effects of organic additives on the surface morphology of electrodeposited Sn-Ag alloy for use as Pb-free soldering were investigated. Pyrophosphate-iodide bath was used as a basic Sn-Ag alloy electroplating bath. Polyethyleneglycol (mean molecular weight = 600 : PEG600) and formaldehyde were used as additives. Fine-grained and smooth electrodeposit was obtained from the bath containing both PEG600 and formaldehyde. Electrochemical behavior of tin pyrophosphate complex ion, silver iodide complex ion, PEG600, formaldehyde, and the combinations of them was examined by polarography. Formaldehyde decomposed reductively at about −1.5 V vs. saturated calomel electrode (SCE) on the mercury electrode and had no inhibitory effect on the reduction of tin pyrophosphate complex ion and silver iodide complex ion. PEG600 had a strong inhibitory effect on the reduction of tin pyrophosphate complex ion. As a result, the tin deposition (or tin-silver alloy codeposition) potential was shifted from −0.9 V to −1.6 V vs. SCE which is more negative than the potential of reductive decomposition of formaldehyde. It is suggested that the process of the reductive decomposition of formaldehyde on the cathode surface on which Sn-Ag alloy are being electrodeposited related to the smoothing of Sn-Ag alloy films.