Electrochemical Deposition of Gold-Tin Alloy from Propyleneglycol Electrolyte

Stable and simple in composition nonaqueous propyleneglycol (PG) electrolyte containing tetracyanoaurate(III) and tin(IV) chloride pentahydrate in mole ratio 1:14 has been developed for the electrochemical deposition of gold–tin alloy with the elemental, phase composition and microstructure acceptable for soldering in electronics. This electrolyte provides deposition of Au–Sn coatings at a rate of 2.4 mg/cm²·h (1.8 µm/h accounting the alloy composition) and to obtain alloys which consist of Au₅Sn and AuSn crystalline intermetallics containing 14–67 at.% of tin (optimal 39 at.%) in total that corresponds to the composition of eutectic applicable as a solder. Coatings have a very uniform tightly packed and fine-grained microstructure and are composed of spherical grains 0.1–1.0 µm in diameter and average size of 0.2 µm. The addition of water or full change of PG on water fastens the rate of coating deposition and stipulates the formation of coatings with mush less tin content, more coarse-grained microstructure and predominance of gold or gold-enriched intermetallics in their phase composition. The effect of the electrolyte composition and the regime of its work on the elemental and phase composition of coatings, their microstructure and the rate of deposition have been determined. The role of PG in the process of mutual gold and tin electro-reduction and intermetallic compounds crystallization has been found out.