Gold plating is a commonly used term to describe a thin gold coating on an object. In fact, gold can be deposited from solution by three similar processes. These are:

• Electroplating where an electric current passes through a solution containing gold ions resulting in gold metal being deposited onto the cathode (negative electrode).
• Electroless plating does not require electricity but a chemical reducing agent is used to convert gold ions to metal.
• Immersion plating where gold ions from solution are displaced by dissolution of a more reactive metal.

These three processes are used for different applications and give coatings with quite different properties. Gold electroplating is used for making contacts and connectors and so is the biggest use of gold in electronics.

There are many types of gold plating bath available commercially. Most of these are based on cyanide. Non-cyanide plating baths are also available but tend to be less stable and so their use is limited. However some advantages are found with non-cyanide baths apart from the obvious health and environmental benefits. Electroplating is always lower cost than electroless or immersion plating as fewer chemicals are consumed.

One limitation with electroplating is that there must an electrical connection made to the areas to be plated. Isolated areas can not be electroplated but alternative processes have been developed. An alternative to electroplated gold is electroless gold, where a chemical reducing agent is used to convert gold ions to metal.

Printed circuit boards (PCBs) can also be protected during storage before populating with components with a layer of electroless nickel followed by a thin coating, so called ‘immersion gold’. Immersion gold is deposited by a displacement reaction in which gold in solution is deposited and nickel from the substrate dissolves. The thin gold layers on electroless nickel used on PCBs are suitable for maintaining a solderable surface while the boards are in storage and this can be for at least a year in dry and clean conditions. These coatings are much too thin, however, for contacts or connectors, as the underlying nickel will corrode in humid conditions and insulating corrosion products will form on top of the gold.