Gold plating
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This article deals with the technique. For the economic term, see the article Gold-plating.
Gold plating is a method of depositing a thin layer of gold on the surface of other metal, most often copper or silver, by chemical or electrochemical means. Mechanical or chemical affixing of thin gold foils on the surface of objects is instead known as gilding.
Also, in Software development or Time Management in general, Gold plating refers to continuing to work on a project or task well past the point where the extra effort is adding some value. After having met the requirements, the developer works on further enhancing the product, thinking the end user would be much more delighted to see the additional or enhanced features in the product, than what the user asked or expected for. The user might be disappointed at this, and the extra effort by the developer might go futile. This context is referred to as Gold plating.
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/ 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 in comparison with the non-plated one. Selective gold plating is therefore used, depositing the nickel and gold layers only on areas where it is required and where it does not cause the detrimental side effects. [1]
Gold plating may lead to formation of gold whiskers.
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[edit] Soldering issues
Soldering to gold-plated parts can be tricky. Gold is soluble in solder; an alloy of solder which contains more than 5% of gold can become brittle and the joint surface is dull-looking. If the gold layer does not completely dissolve in the solder, it may be pulled off the underlying surface later as the tin and gold atoms cross-migrate and undergo Kirkendall voiding, leading into a mechanical failure of the joint; a similar process on the gold-aluminum interface is known as purple plague. Thin layers of gold (under 20 mil) dissolve in the solder, exposing the underlying metal (usually nickel) to the solder.
The nickel layer may be contaminated with impurities preventing proper bonding of the solder. An electroless-plated nickel may contain codeposited phosphorus; more than 8% of phosphorus make the nickel layer not solderable. Electrodeposited nickel may contain nickel hydroxide. An acid bath is required to remove the passivation layer before applying the gold layer; failure to do so properly leads to a nickel surface difficult to solder. A stronger flux can be helpful here, as it aids dissolving the oxide deposits. Carbon is another nickel contaminant that hinders solderability.
Gold easily reacts with both tin and lead in their liquid state, forming brittle intermetallics; it preferably reacts with tin, forming the AuSn4 compound. When the eutectic 60% Sn - 40% Pb solder is used, no lead-gold compounds are formed. The particles of AuSn4, dispersed in the solder matrix, form preferential cleavage planes, significantly lowering the mechanical strength and therefore reliability of the resulting solder joints.
A 2-3 µm thick layer of gold is easily dissolved within a second during typical wave soldering conditions. [2]
[edit] Types of gold plating
There are several types of gold plating used in electronics industry: [3]
- 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. The 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.
[edit] 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
- Miscellaneous
[edit] Jewelry
Gold plating of silver is used in jewelry. However, like copper, silver atoms undergo diffusion through 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, while migrating through gold as well, is much slower in this process, and it is usually further plated with nickel. Therefore a gold-plated silver is usually a sandwich of silver substrate with layers of copper, nickel, and gold deposited on top of it. [4]
