Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver (to make silver-gilt), by chemical or electrochemical means.
Gold plating is often used in electronics, to provide a corrosion-resistant electrically conductive layer on copper, typically in electrical connectors and printed circuit boards. With direct gold-on-copper plating, the copper atoms have the tendency to diffuse through the gold layer, causing tarnishing of its surface and formation of an oxide/sulfide layer. A layer of a suitable barrier metal, usually nickel, has therefore to be deposited on the copper substrate, forming a copper-nickel-gold sandwich.
There are several types of gold plating used in the electronics industry
- Soft, pure gold plating is used in the semiconductor industry. The gold layer is easily soldered and wire bonded. Its Knoop hardness ranges between 60-85. The plating baths have to be kept free of contamination.
- Bright hard gold on contacts, with Knoop hardness between 120-300 and purity of 99.7-99.9% Au. Often contains a small amount of nickel and/or cobalt; these elements interfere with die bonding, therefore the plating baths can't be used for semiconductors.
- Bright hard gold on printed circuit board tabs is deposited using lower concentration of gold in the baths. Usually contains nickel and/or cobalt as well. Edge connectors are often made by controlled-depth immersion of only the edge of the boards.
- Soft, pure gold is deposited from special electrolytes. Entire printed circuit boards can be plated. This technology can be used for depositing layers suitable for wire bonding.
Chemistry of Gold Plating
There are five recognized classes of gold plating chemistries:
- Alkaline gold cyanide, for gold and gold alloy plating
- Neutral gold cyanide, for high-purity plating
- Acid gold plating for bright hard gold and gold alloy plating
- Non-cyanide, generally sulfite based for gold and gold alloy plating