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 plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.
Gold plating types
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% gold. 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.
Gold plating chemistry
There are five recognized classes of gold plating chemistry:
1.Alkaline gold cyanide, for gold and gold alloy plating
2.Neutral gold cyanide, for high-purity plating
3.Acid gold plating for bright hard gold and gold alloy plating
4.Non-cyanide, generally sulfite or chloride-based for gold and gold alloy plating
Gold plating of silver is used in the manufacture of jewelery. Like copper, silver atoms diffuse into the gold layer, causing slow gradual fading of its color and eventually causing tarnishing of the surface. This process may take months and even years, depending on the thickness of the gold layer. A barrier metal layer is used to counter this effect. Copper, which also migrates into gold, does so more slowly than silver. The copper is usually further plated with nickel. A gold-plated silver article is usually a silver substrate with layers of copper, nickel, and gold deposited on top of it.