Gold plating

  Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical means.

Mechanical or chemical affixing of thin gold foils onto the surface of objects is instead known as gilding.

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Types of gold plating

There are several types of gold plating used in the electronics industry:^[1]

• Soft, pure gold plating is used in 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:

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 based for gold and gold alloy plating
5. Miscellaneous

Jewelry

Gold plating of silver is used in the manufacture of jewelry. 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.

Electronics

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 tend to diffuse through the gold layer, causing tarnishing of its surface and formation of an oxide and/or sulfide layer.

A layer of a suitable barrier metal, usually nickel, is usually deposited on the copper substrate before the gold plating. The layer of nickel provides mechanical backing for the gold layer, improving its wear resistance. It also reduces the impact of pores present in the gold layer.

Both the nickel and gold layers are usually deposited by electroplating. Electroless plating is used as well, however the deposited layer is unsuitable for long-term corrosion protection nor for wire bonding, as the resulting layer is typically only 97% pure and thin (0.5-0.75 µm). As the deposit is based on displacement of some of the copper, electroless nickel plating may be unsuitable for boards with very fine traces.

At higher frequencies, the skin effect may cause higher losses due to higher electrical resistance of nickel; a nickel-plated trace can have its useful length shortened three times in the 1 GHz band in comparison with the non-plated one. Selective plating is used, depositing the nickel and gold layers only on areas where it is required and does not cause the detrimental side effects.^[2]

Gold plating may lead to formation of gold whiskers.

Soldering issues

Soldering gold-plated parts can be tricky.

Gold is soluble in solder. Solder which contains more than 5% gold can become brittle. The joint surface is dull-looking.

Gold reacts with both tin and lead in their liquid state, forming brittle intermetallics. When eutectic 63% Sn - 37% Pb solder is used, no lead-gold compounds are formed, because gold preferentially reacts with tin, forming the AuSn₄ compound. Particles of AuSn₄ disperse in the solder matrix, forming preferential cleavage planes, significantly lowering the mechanical strength and therefore reliability of the resulting solder joints.

If the gold layer does not completely dissolve into the solder, then slow intermetallic reactions can proceed in the solid state as the tin and gold atoms cross-migrate. Intermetallics have poor electrical conductivity and low strength. The ongoing intermetallic reactions also cause Kirkendall voiding, leading to mechanical failure of the joint, similar to the degradation of gold-aluminum bonds known as purple plague.

A 2-3 µm layer of gold dissolves completely within one second during typical wave soldering conditions. [1] Layers of gold thinner than 0.5 µm (20 microinches) also dissolve completely into the solder, exposing the underlying metal (usually nickel) to the solder. Impurities in the nickel layer can prevent the solder from bonding to it. Electroless nickel plating contains phosphorus. Nickel with more than 8% phosphorus is not solderable. Electrodeposited nickel may contain nickel hydroxide. An acid bath is required to remove the passivation layer before applying the gold layer; improper cleaning leads to a nickel surface difficult to solder. A stronger flux can help, as it aids dissolving the oxide deposits. Carbon is another nickel contaminant that hinders solderability.

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