The Influence of a Superimposed Alternating Current on the Structure and Porosity of Gold Electrodeposits

Abstract

The structure and porosity of gold electrodeposits produced by conventional direct current plating and by plating with an asymmetric square wave alternating current have been studied. Simple electrolytes free from any gddation agents or alloying elements were used as well as proprietary alloy gold solutions. It was found that the porosity determined by a 24 hour sulphur dioxide test was reduced hii tactore which increased the activity of the gold ions in the electrolyte such as higher temperature and increased concentration of gold salts in the electrolyte. When applied to simple solutions the use of alternating current was found to reduce porosity, and at the same time the structure of the deposit was modified. The change was particularly noticeable at 20°C when the direct current structure of loose enuica laths became more uniform and compact when alternating current was used. The reverse was true for commercial alloy solutions. Porosity was increased by the use of alternating current and the structure reverted from a solid compact form with direct current to a less uniform and more porous type. This was due to the fact that the alloying element was preferentially removed during the anodic period. Individual scree were studied using the scanning electron microscope, and two types of porosity were found. Large pores over 0.5uM diameter were usually associated with some form of substrate defect such as embedded particles of polishing compound, Smaller pores which could not be resolved by the scanning electron microscope were detected by the sulphur dioxide porosity test. These pores arise from the failure of adjacent crystals to coalesce during the growth of the electrodeposit and may be termed crystallographic porosity.