Electrochemical Quartz Crystal Microbalance Studies of Gold Oxidation in Cyanide Baths

Abstract

As a means of understanding pulsed reverse plating of gold from cyanide baths, the anodic gold oxidation has been characterized by electrochemical methods, with supplemental information from a quartz crystal microbalance. Dissolution into electrolytes containing excess cyanide and into a gold-cyanide plating bath was compared. Results suggest that in the initial moments after the cathodic portion the dissolution proceeds with a mechanism involving excess cyanide. At longer times, a more complex mechanism leading to an adsorbed AuCN layer on the gold surface prevails. These results may provide guidance in developing plating waveforms for deposited hard gold with desirable properties.The electrodeposition of cobalt hard gold is widely used in connector applications. 1 A major impediment to reducing gold film thickness is the corrosion of the underlying substrate, which is impacted by the porosity of the gold layer. 2 Both the morphology and the hydrogen evolution reaction have significant influence on porosity. 3 Pulsed reverse plating 4-14 can be expected to impact both hydrogen evolution and morphology. A reverse current in cobalt hard gold electroplating may dissolve deposited gold and may oxidize hydrogen produced during the cathodic portion of the waveform. Whether the dissolution of gold and oxidation of hydrogen can be realized depends on the type of plating bath and the plating conditions adapted.A three-step oxidation mechanism for gold in the presence of cyanide has been proposed 15-19However, the potentials at which these reactions occur 18 and the extent of other gold oxidation products, such as trivalent gold formation, 16 remain unresolved. Most studies have been conducted in alkaline solutions, where free cyanide ions are abundant. In weakly acidic solutions, cyanide is associated with hydrogen ions or gold. In reversed pulse plating, the amount of near-surface excess cyanide is changed after the cathodic current is applied, and the oxidation of gold under these conditions has not been fully addressed.With an eye toward optimization of pulsed reverse plating parameters, we present a study of gold oxidation. Experiments are conducted in a KCN solution and in a plating bath. Electrochemical methods and a quartz crystal microbalance ͑QCM͒ ͑Ref. 20-25͒ are employed. In a subsequent paper, we report on the impact of reversed plating parameters on the deposit microstructure and also the porosity. ExperimentalSolutions from a commercial plating bath, Metalor Gold, are used in the study. This Metalor Gold bath contains 0.04 M KAu͑CN͒ 2 , cobalt additives ͑approximately 0.008 M in solution, resulting in around 0.5 wt % of Co in the electrodeposits͒, and supporting electrolyte including conducting salt ͑a mixture of organic acid salt and potassium oxalate monohydrate͒, acid salt ͑citrate based͒, and brightener ͑mixture of potassium hydroxide and aromatic compounds͒. Several solutions, A, B, and a series of C, containing cyanide were employed in this study ͑Table I͒. Estimates of concentrations of cyanide speci...