Gold plating

For other uses, seeGold plating (disambiguation).

Gold platingis a method of depositing a thin layer ofgoldonto the surface of another metal, most oftencopperorsilver(to makesilver-gilt), by a chemical or electrochemical (electroplating) process.Platingrefers to modern coating methods, such as the ones used in theelectronics industry,whereasgildingis the decorative covering of an object with gold, which typically involve more traditional methods and much larger objects.

Gold-plated aluminium cover onVoyagerspace craft that protects a gold-platedSounds of Earthrecord

A gold plated desktopStirling engine

A gold platedDMC DeLorean—one of five known examples to have been plated—on display at theNational Automobile MuseuminReno, Nevada

Gold plating chemistry

There are five recognized classes of gold plating chemistry:

1. Alkaline goldcyanide,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 sulphite or chloride-based for gold and gold alloy plating
5. Miscellaneous

Jewellery

Gold plating of silver is used in the manufacture ofjewellery.The thickness of gold plating on jewellery is noted in microns (or micro-meters). The microns of thickness determines how long the gold plating lasts with usage. The jewellery industry denotes different qualities of gold plating in the following terminology

1. Gold flashed / Gold washed - gold layer thickness less than 0.5 micron
2. Gold plated - gold layer thickness greater than or equal to 0.5 micron
3. Heavy gold plated / Vermeil - gold layer thickness greater than or equal to 2.5 micron

Gold plated silver jewellery can still tarnish as the silver atoms diffuse into the gold layer, causing slow gradual fading of its color and eventually causingtarnishingof the surface. This process may take months and even years, depending on the thickness of the gold layer. Abarrier metallayer is used to counter this effect; these can be nickel or rhodium. 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.

Space applications

Gold, applied by evaporated methods or electroplating, has been specified by NASA to thermally control spacecraft instruments, due to its 99.4% reflectivity in infrared wavelengths.

Electronics

Gold-plated electrical connectors

Gold plating is often used in electronics, to provide acorrosion-resistant electrically conductive layer oncopper,typically inelectrical connectorsandprinted circuit boards.

With direct gold-on-copper plating, the copperatomstend to diffuse through the gold layer, causingtarnishingof its surface and formation of anoxideand/orsulphidelayer.

A layer of a suitablebarrier metal,usuallynickel,is often deposited on the copper substrate before the gold plating. The layer of nickel provides mechanical backing for the gold layer, improving itswearresistance. It also reduces the impact of pores present in the gold layer.

Both the nickel and gold layers can be plated byelectrolyticorelectrolessprocesses. There are many factors to consider in selection of either electrolytic or electroless plating methods. These include what the deposit will be used for, configuration of the part, materials compatibility and cost of processing. In different applications, electrolytic or electroless plating can have cost advantages.

At higher frequencies, theskin effectmay 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.

Gold plating may lead to formation of goldwhiskers.

Wire bonding between gold plated contacts and aluminium wires or between aluminium contacts and gold wires under certain conditions develops a brittle layer ofgold-aluminium intermetallics,known aspurple plague.

Types

There are several types of gold plating used in the electronics industry:

• Soft, pure gold platingis used in thesemiconductor industry.The gold layer is easily soldered andwire bonded.ItsKnoop hardnessranges between 60 and 85. The plating baths have to be kept free of contamination.
• Soft, pure goldis deposited from specialelectrolytes.Entireprinted circuit boardscan be plated. This technology can be used for depositing layers suitable for wire bonding.
• Bright hard gold on contacts,with Knoop hardness between 120–300 and purity of 99.7–99.9% gold. Often contains a small amount ofnickeland/orcobalt;these elements interfere with die bonding, therefore the plating baths cannot be used for semiconductors.
• Bright hard gold onprinted circuit boardtabsis deposited using lower concentration of gold in the baths. Usually contains nickel and/or cobalt as well.Edge connectorsare often made by controlled-depth immersion of only the edge of the boards.

Soldering issues

Gold-plated printed circuit board

Solderinggold-plated parts can be problematic as gold is soluble insolder.Solder which contains more than 4–5% gold can become brittle. The joint surface is dull-looking.

Gold reacts with bothtinandleadin their liquid state, forming brittleintermetallics.Wheneutectic63% tin – 37% lead solder is used, no lead-gold compounds are formed, because gold preferentially reacts with tin, forming theAuSn
₄compound. Particles ofAuSn
₄disperse in the solder matrix, forming preferentialcleavageplanes, 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 causeKirkendall effect,leading to mechanical failure of the joint, similar to the degradation of gold-aluminium bonds known aspurple plague.

A 2–3 μm layer of gold dissolves completely within one second during typicalwave solderingconditions. Layers of gold thinner than 0.5 μm (0.02thou) 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 platingcontains phosphorus. Nickel with more than 8% phosphorus is not solderable.Electrodepositednickel may containnickel hydroxide.An acid bath is required to remove thepassivationlayer before applying the gold layer; improper cleaning leads to a nickel surface difficult to solder. A strongerfluxcan help, as it aids dissolving the oxide deposits.Carbonis another nickel contaminant that hinders solderability.

See also

• Daniel Davis Jr.- inventor who was first to do gold plating as a business
• Gold-filled jewellery

External links

• Media related toGold platingat Wikimedia Commons