The processes and the products of gold reduction in tetrachloroaurate electroless plating solutions

Electroless gold plating solution containing tetrachloroaurate and citrate ions together with urea is suggested that provides deposition of gold nonporous and adherent films ∼0.1 μm in thickness with a total surface area not less than 0.2 m2/l. The nature of gold reduction processes is studied and it is shown that in this solution gold is reduced by nickel (cementation process) and by citrate ions together with urea in a slight measure. The latter reduction proceeds on a substrate (gold plating on Ni-P and Ni-B sublayers) and in a solution bulk (formation of rather stable colloid solution). The quota of the cemented gold does not exceed 23-29 at.% compared to the whole gold quantity in films. It is shown that the accumulation of colloidal gold particles is accelerated by a metal of the substrate and coagulation is caused by nickel ions in a concentration ∼2.2×10−4 M. Gold plating can proceed in colloid solutions (containing ∼4×10−3 M gold in colloidal state) without a deterioration of the film quality.