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Gold plating is a method of depositing a thin layer of
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
onto the surface of another metal, most often
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
or
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
(to make
silver-gilt
Silver-gilt or gilded/gilt silver, sometimes known in American English by the French term vermeil, is silver (either pure or sterling) which has been gilded with gold. Most large objects made in goldsmithing that appear to be gold are actually ...
), by chemical or electrochemical
plating
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improv ...
. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see
gilding
Gilding is a decorative technique for applying a very thin coating of gold over solid surfaces such as metal (most common), wood, porcelain, or stone. A gilded object is also described as "gilt". Where metal is gilded, the metal below was tradi ...
.
Types
There are several types of gold plating used in the electronics industry:
/ref>
* ''Soft, pure gold plating'' is used in the semiconductor industry
The semiconductor industry is the aggregate of companies engaged in the design and fabrication of semiconductors and semiconductor devices, such as transistors and integrated circuits. It formed around 1960, once the fabrication of semiconduct ...
. The gold layer is easily soldered and wire bonded. Its Knoop hardness
The Knoop hardness test is a microhardness test – a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed i ...
ranges between 60 and 85. The plating baths have to be kept free of contamination.
* ''Soft, pure gold'' is deposited from special electrolytes
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon di ...
. Entire printed circuit boards
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich struct ...
can 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 of nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
and/or cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
; these elements interfere with die bonding, therefore the plating baths cannot be used for semiconductors.
* ''Bright hard gold on printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
tabs'' is deposited using lower concentration of gold in the baths. Usually contains nickel and/or cobalt as well. Edge connector
An edge connector is the portion of a printed circuit board (PCB) consisting of traces leading to the edge of the board that are intended to plug into a matching socket. The edge connector is a money-saving device because it only requires a sing ...
s are often made by controlled-depth immersion of only the edge of the boards.
Gold plating chemistry
There are five recognized classes of gold plating chemistry:
# Alkaline gold cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
, 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 sulphite or chloride-based for gold and gold alloy plating
# Miscellaneous
Jewelry
Gold plating of silver is used in the manufacture of jewelry
Jewellery ( UK) or jewelry (U.S.) consists of decorative items worn for personal adornment, such as brooches, rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a western ...
. 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
# Gold flashed / Gold washed - gold layer thickness less than 0.5 micron
# Gold plated - gold layer thickness greater than or equal to 0.5 micron
# 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 causing tarnishing of the surface. This process may take months and even years, depending on the thickness of the gold layer. A barrier metal A diffusion barrier is a thin layer (usually micrometres thick) of metal usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other..
Adhesion of a plated metal layer to it ...
layer 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.
Infrared reflectivity
Gold, applied by evaporated methods or electroplating, has been specified by NASA to thermally control spacecraft instruments, due to its 99% reflectivity in infrared wavelengths.
Electronics
Gold plating is often used in electronics, to provide a corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
-resistant electrically conductive layer on copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
, typically in electrical connector
Components of an electrical circuit are electrically connected if an electric current can run between them through an electrical conductor. An electrical connector is an electromechanical device used to create an electrical connection between ...
s and printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
s.
With direct gold-on-copper plating, the copper atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, and ...
s tend to diffuse through the gold layer, causing tarnish
Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in ...
ing of its surface and formation of an oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
and/or sulphide layer.
A layer of a suitable barrier metal A diffusion barrier is a thin layer (usually micrometres thick) of metal usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other..
Adhesion of a plated metal layer to it ...
, usually nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
, is often deposited on the copper substrate before the gold plating. The layer of nickel provides mechanical backing for the gold layer, improving its wear
Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology.
Wear in m ...
resistance. It also reduces the impact of pores present in the gold layer.
Both the nickel and gold layers can be plated by electrolytic
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
or electroless processes. 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, the skin effect
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the co ...
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.
Gold plating may lead to formation of gold whiskers
Vibrissae (; singular: vibrissa; ), more generally called Whiskers, are a type of stiff, functional hair used by mammals to touch, sense their environment. These hairs are finely specialised for this purpose, whereas other types of hair are c ...
.
Wire bonding between gold plated contacts and aluminium wires or between aluminium contacts and gold wires under certain conditions develops a brittle layer of gold-aluminium intermetallics, known as purple plague
Purple is any of a variety of colors with hue between red and blue. In the RGB color model used in computer and television screens, purples are produced by mixing red and blue light. In the RYB color model historically used by painters, p ...
.
Soldering issues
Soldering
Soldering (; ) is a process in which two or more items are joined by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involv ...
gold-plated parts can be problematic as gold is soluble in solder
Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...
. Solder which contains more than 4–5% gold can become brittle. The joint surface is dull-looking.
Gold reacts with both tin
Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal.
Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
and lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
in their liquid state, forming brittle intermetallics
An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elemen ...
. When eutectic 63% tin – 37% lead solder is used, no lead-gold compounds are formed, because gold preferentially reacts with tin, forming the compound. Particles of 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 effect The Kirkendall effect is the motion of the interface between two metals that occurs as a consequence of the difference in diffusion rates of the metal atoms. The effect can be observed for example by placing insoluble markers at the interface betwee ...
, leading to mechanical failure of the joint, similar to the degradation of gold-aluminium bonds known as purple plague
Purple is any of a variety of colors with hue between red and blue. In the RGB color model used in computer and television screens, purples are produced by mixing red and blue light. In the RYB color model historically used by painters, p ...
.
A 2–3 µm layer of gold dissolves completely within one second during typical wave soldering
Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the ci ...
conditions. Layers of gold thinner than 0.5 µm (0.02 thou
The word ''thou'' is a second-person singular pronoun in English. It is now largely archaic, having been replaced in most contexts by the word ''you'', although it remains in use in parts of Northern England and in Scots (). ''Thou'' is the ...
) 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
Electroless nickel-phosphorus plating is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containi ...
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
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
can help, as it aids dissolving the oxide deposits. Carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
is another nickel contaminant that hinders solderability.
See also
* Daniel Davis Jr. - inventor who was first to do gold plating as a business
* Gold filled jewelry
Gold-filled jewelry is jewelry composed of a solid layer of gold (typically constituting at least 5% of the item's total weight) mechanically bonded to a base of either sterling silver or some base metal. The related terms "rolled gold plate" an ...
References
External links
*
{{DEFAULTSORT:Gold Plating
Electronics manufacturing
Coatings
Jewellery
Metal plating
de:Vergolden#Galvanische Verfahren