Gold Plating Baths
Gold plating baths are aqueous solutions with optimised concentrations of specified salts and promoters, each of which is designed for different metals and alloys, for precision electroplating. In this article, the various constituents of gold plating baths, their functions, and the physical and chemical properties of the baths are discussed.
Gold Chloride
Gold chloride is the main constituent of most gold plating baths. It contains gold in trivalent form and acts as the source for metallic gold ion release during the electroplating process. Additionally, it modifies the plating process by optimising the morphology and grain size of the deposit. The concentration of gold chloride varies depending on the desired outcome; usually between 1-25 grams/I.
Additives
Organic Additives
The purpose of organic additives is to stabilise the bath so that it can be stored and used over time. The exact type and concentration of organic additive is dependent on the type of gold plating, but common organic additives include:
Surfactants, which are added to reduce interfacial tension between the electrode and the liquid,
Chelates, which are used to prevent the deposit from discolouring,
Brighteners, which are used to balance the deposit surface and improve the appearance of the deposit,
and Levelers, which are used to reduce porosity and achieve a smooth deposit.
Inorganic Additives
Inorganic additives are added to the gold plating bath to improve its characteristics, such as structural integrity, throwing power, and deposition rate. Common inorganic additives include:
Nitrate ions, which are used to increase the throwing power of the bath,
Chloride ions, which are used to stabilise the Trivalent gold ions,
Sulfate ions, which are used to reduce the surface tension between the bath and the electrode, and
Acids, which are used to adjust the pH of the bath.
Additive Concentration
The concentration of each additive base on the type of gold plating being performed and the desired outcome. Generally, it is recommended that the concentration of each additive be between 0.2-1.0%, but depending on the application, the concentration could be higher or lower.
Bath Temperature
The temperature of the gold plating bath is another important factor determining the success of the gold plating process. Generally, gold plating baths are best performed at temperatures between 25-35°C. At higher temperatures, the bath is more likely to be unstable and be prone to electrolysis, which can cause the bath’s components to break down and create defects in the finish.
Other Factors
Apart from the components and temperature of the gold plating bath, there are other factors that can affect the outcome of the plating process, such as the voltage, current, electrode composition, and cathode entry rate. All of these factors should be carefully monitored and adjusted as needed in order to achieve the desired results.
Conclusion
Gold plating baths are essential to the gold plating process, as they provide a solution containing all the required elements and additives to ensure a successful gold plating outcome. By following the correct procedure and optimising the components and temperatures of the gold plating bath, one can ensure consistent and reliable results every time.
