Gold plating

Gilding is the process of covering metallic and non-metallic objects with gold, gold alloys and other decorative metal layers. 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 a chemical or electrochemical (electroplating) process. Plating refers to modern coating methods, such as the ones used in the electronics industry, whereas gilding is the decorative covering of an object with gold, which typically involve more traditional methods and much larger objects.

The traditional craft of gilding consists of applying metal leaf to workpieces, in contrast to coating them with metallic effect pigments (shell gold, "gold bronze"), which is part of the craft of painting. Chemical processes, with the exception of fire gilding, only developed in modern times.

Gold not only has a noble appearance, but is also one of the most corrosion-resistant metals. In its pure form, it is unsuitable for components and everyday objects because it is rare, expensive, and lacks strength. Coating readily available and everyday materials with a layer of gold has been valued since ancient times.

Functions

Historically, the main functions of gold coatings and platings are:

Decorative appearance

Valuable and prestigious appearance

Significance for cult activities and religion

Corrosion resistance

In modern times, the following functions were added:

For gold-plated electrical contacts and plugs, a small impurity layer resistance

Special properties for semiconductor technology, for example bondability

Base materials

The most important group of materials that are particularly suitable for gilding are metals and metal alloys, especially steel, stainless steel, zinc, brass, bronze, copper, silver, and many others. Non-metallic materials that can be gilded include porcelain, glass, ceramics, wood, paper, leather, plastic, and, in rare cases, even textiles.

Thanks to the latest technology, almost all organic and inorganic materials can now be permanently gold-plated. This is achieved using new electroplating processes.

Gold plating

Gold plating can be made of pure gold, but in most cases, a gold alloy suitable for the specific purpose is chosen as the plating material. Gold alloys vary in color: red gold, yellow gold, white gold, and rose gold. Green and blue shades are also possible. The colors are achieved by the following alloying elements: copper for red gold, silver, cadmium, and zinc for yellow to white gold, nickel for white gold, and indium for blue gold.

For technical applications, the coating's hardness and wear resistance are more important than its color. These properties are primarily enhanced by the alloying elements iron, cobalt, and nickel. Such a gold coating is called hard gold. The alloying elements are intended to improve hardness and wear resistance. The amount of gold saved, in contrast, is negligible.

Gold plating chemistry

There are five recognized classes of gold plating chemistry:

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 sulphite or chloride-based for gold and gold alloy plating

Miscellaneous

Jewellery

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

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 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 sulphide layer.

A layer of a suitable barrier metal, usually nickel, 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 resistance. It also reduces the impact of pores present in the gold layer.

Both the nickel and gold layers can be plated by electrolytic 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 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.

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.

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 and 85. The plating baths have to be kept free of contamination.

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.

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 cannot 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.

Soldering issues

Soldering gold-plated parts can be problematic as gold is soluble in solder. Solder which contains more than 4–5% 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% tin – 37% lead solder is used, no lead-gold compounds are formed, because gold preferentially reacts with tin, forming the AuSn

4 compound. Particles of AuSn

4 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, leading to mechanical failure of the joint, similar to the degradation of gold-aluminium bonds known as purple plague.

A 2–3 μm layer of gold dissolves completely within one second during typical wave soldering conditions. Layers of gold thinner than 0.5 μm (0.02 thou) 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.

Proceedings

The two basic gilding techniques are mechanical and electrochemical. Mechanical gilding is the oldest method; it involves flattening gold sheet. A distinction is made between bright and matte gilding, poliment gilding, oil gilding, mordent gilding, and reverse glass gilding. Electrochemical gilding, with the exception of fire gilding, emerged much later. Electrochemical gilding reached its peak with electroplating.

A gilding business is a craft business that primarily deals with the finishing of wood, metal or plastic surfaces.

Gold leaf

In painting, especially panel painting, the application of gold leaf to a substrate is also called gilding. This substrate can have various properties. In book illumination, the gold can be applied directly to the parchment or to a gold ground; this gold ground is most commonly found in panel painting. It is a primer (poliment gilding), which always consists of bolus and a binding agent.

For many centuries, high-quality works such as grilles, church furnishings, grave crosses, etc., have been accentuated with gold leaf or even fully gilded, initially by blacksmiths and later, at least since the Renaissance, by specialized metalsmiths. This process is usually carried out by the master metalsmith himself. The process involves weather-resistant oil gilding with mixture or application oil.

Electrochemical cementation

Cementation is based on the fact that when a copper sheet, for example, is immersed in a gold(III) chloride solution, the more noble gold ions are reduced and deposited on the surface of the copper, whereby copper is oxidized to copper(II) ions.

However, the copper surface must first be thoroughly cleaned or etched, and the solution must be slightly alkaline. After the gold has been deposited in a layer of a few micrometers, adhesion can be increased by heating the object to temperatures around 700 °C, which creates a diffusion zone between the copper and the gold layer.

The cementation process is already known from the metallurgy of the pre-Columbian Andean culture. To produce the gold chloride solution, gold foils were presumably dissolved in a hot solution of potassium aluminum sulfate, potassium nitrate, and sodium chloride, which, however, took several days. 

Galvanic processes

Most gold plating is applied using electroplating processes. This method has completely replaced many older techniques. The usually metallic objects are immersed in a gold electrolyte, and a gold coating is deposited by applying a direct current. The first patent for the deposition of gold from cyanide-containing baths was granted to George Richards Elkington and Henry Elkington in 1840.

In the electroplating process, gold(I) or gold(III) ions are cathodically reduced to elemental gold by electron absorption, usually from cyanide electrolytes at acidic, neutral, or alkaline pH values. By varying the temperature, voltage or current, and electrolysis time, layer thicknesses of 0.1 µm to 200 µm can be produced. These processes are used in electrical engineering for the gold plating of electrical contacts or in the surface treatment of electrical soldering surfaces on printed circuit boards. In these cases, the gold coating serves to prevent corrosion of the contact surfaces.

Electroplating processes are used in the areas of design, jewelry, and equipment. At the Pforzheim Goldsmith and Watchmaking School, training to become a master electroplater is available. 

Four different types of gold plating are used in the field of electronics: 

Soft gold plating in semiconductor technology. It is used to gold-plate the connection pads on semiconductor chips. The bond wires, usually made of gold, are connected to these connection pads and form the electrical connection between the semiconductor chip and the connection pins located on the outside of the chip housing. The Knoop hardness of the soft gold coating is in the range of 60–85.

Hard gold plating of electrical contacts. This has a Knoop hardness of 120–300 and a purity of just over 99%. The remaining components are small amounts of nickel or cobalt. For chemical reasons, this form of hard gold plating cannot be used in semiconductor technology for contacting semiconductor chips.

Hard gold plating of electrical contacts on printed circuit boards, as is common with PCB connectors. Hard gold plating is necessary because the contacts of PCB connectors are subject to greater mechanical stress than other areas of a printed circuit board.

Soft gold plating of the soldering surfaces on electrical circuit boards. This gold plating protects the copper soldering surfaces from oxidation during storage, allowing for the use of less aggressive fluxes during the soldering process. Only those areas of the circuit board that will contact electronic components during the subsequent manufacturing process are gold-plated; the rest of the board is covered with a passive solder mask. When the circuit board is populated with electrical components and the subsequent soldering process, the gold layer dissolves into the solder and loses its function.

During gold plating, only certain base materials can be gold-plated. For example, copper, which is used in electrical engineering due to its good electrical conductivity, cannot be permanently gold-plated directly because copper has a tendency to diffuse through the thin gold layer, adhere to the gold surface, and oxidize there. Multi-layer electroplating processes provide a solution, in which a thin layer of nickel is first electroplated onto the copper substrate, and only then is the nickel layer gold-plated. However, the additional nickel layer in the outer area results in poorer high-frequency properties of the cable due to the skin effect. 

Aluminium, a conductor material commonly used in electrical engineering alongside copper, tends to form the undesirable intermetallic compound AuAl 2 when it comes into contact with gold, for example in gold-plated switch contacts. This compound is also known as purple plague because of its typical purple colour.

Ceramic gilding

Using specially prepared solutions of gold salts and bonding agents such as rhodium(III) oxide, a metallic gold coating can be achieved on glass and ceramics. Depending on the parameters, the metal appears matte or shiny after firing. This process is used in dental preparation, but also for gilding ceramics and glass.

For gilding porcelain, gold is used, either precipitated from gold(III) chloride with oxalic acid or iron(II) sulfate and mixed with basic bismuth(III) nitrate as a flux. This gold must be polished after firing and is therefore called burnishing gold. Such items should not be used in microwave ovens, as the gold plating will be damaged.

Bright gilding (also known as oil gilding and Meissen gilding), on the other hand, produces a shiny surface directly. It is achieved by firing a solution of gold sulphur or fulminate gold in sulphur balsam, but is much less durable; simply passing it against the hair a few times will remove it, as if with a fine file.

Rolled gold plating

Gold plating (doublé) is based on the mechanical bonding of foreign metal and gold sheet under intense pressure. This process is used to produce semi-finished products, from which inexpensive gold jewelry is made. The rolled gold coating is permanent and very durable. A disadvantage of rolled gold plating is that the objects are not covered on all sides. Rolled gold doublé of a certain thickness is also a quality indicator.

Evaporation in vacuum

In a PVD or CVD process, the metal is deposited as gold vapor onto the workpiece to be coated. This allows plastics such as CDs and other sensitive materials to be coated with gold.

Rubbing gilding

This group of processes is considered historical and has largely been replaced by electroplating techniques. In particular, all processes that use mercury or amalgam are extremely harmful to health and pose a significant environmental burden.

Red gilding is created by dipping the hot piece into molten wax, green gilding with silver-containing gold amalgam. To matte the gilded objects, they are heated with a molten mixture of saltpetre, alum and table salt, which releases chlorine and dissolves gold. To avoid the disadvantages of fire gilding, the objects are also electroplated with mercury, then generously with gold and again with mercury, and then allowed to smoke. Copper uses more gold than tombac in this process, and the gilding on silver appears less vibrant. Gilded silver is called vermeil.

Iron and steel are amalgamated by boiling them with mercury, zinc, iron(II) sulfate, water and hydrochloric acid and then treated like tombac. In cold gilding on copper, brass, nickel silver and silver, gold scale (cloth soaked in gold(III) chloride and burned) is rubbed onto the bare metal using a finger or a slightly charred cork dipped in salt water and polished with bloodstone. It is much less durable than fire gilding on silver, but more beautiful than it and is therefore often used over very light fire gilding. In wet gilding, copper, brass, tombac, nickel silver, copper-plated steel and tinplate are dipped in a dilute gold chloride solution or in a boiling mixture of gold chloride and potassium carbonate, then rinsed, dried and polished. The resulting gilding is always only light.

For green gilding, silver nitrate is added to gold(III) chloride. For wet gilding of silver (also: Greek gilding), it is immersed in a solution of mercury(II) chloride and gold in nitric acid. Iron and steel are first copper-plated or, after etching with nitric acid, immersed in ethereal gold(III) chloride solution. This gilding, which can be strengthened by repeated immersion, adheres even more firmly to steel etched to a matt finish with nitric acid. Ether gilding is never permanent. For this reason, iron and steel are copper-plated and then the hot solution is used with potassium carbonate. The steel is also connected to zinc using a wire and immersed with the zinc in a solution of gold cyanide in potassium cyanide (potassium cyanide) and potassium thiocyanate.

Fire gilding

This ancient process was already used by the Egyptians. Despite the similarity in name, it has very little in common with hot-dip galvanizing. In fire gilding, a metal object, usually steel, non-ferrous metals, or silver, is covered with gold amalgam and then heated. The mercury from the amalgam evaporates, leaving only pure gold. The surface can then be polished with bloodstone, also known as hematite. Strict regulations under emissions and occupational health and safety laws must be met for the application of this process.

Gold-filled

Gold-filled refers to a process in which an outer layer of real (= solid) gold is bonded to a metal core made of brass, copper, or silver. The gold is rolled on using a special process and permanently bonded to the base metal at welding temperature.

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