The effect of additives (i.e. complexing agents) to organic acid baths in obtaining solder plating films at 0.8 to 0.9 of Sn/(Sn+Pb)-molar fraction {0.6 to 0.9 of Sn/(Sn+Pb)-weight fraction} by displacement plating was investigated by physicochemical methods. An effective plating film was obtained from a basic bath composed of metal salts of Sn and Pb, an organic acid, a first complexing agent (thiourea, abbreviated as TU) and a surface active agent (laurylpyridinium chloride), and the molar fraction of Sn/(Sn+Pb) was 0.50 {the weight fraction of Sn/(Sn+Pb) was 0.36}. It was found that the effective first complexing agent in the compounds containing sulfar had a C=S group such as TU and its derivatives. The formation of Sn whiskers on the plating film was depressed by adding a second complexing agent (glycine) to the bath. The working efficiency (deposition rate and lifetime) of baths with glycine was similar or superior to those without glycine.
