Introduction

Industrial gold plating: Many chemical and physical characteristics are shared by gold and silver. With melting temperatures of 1,065°C and 961°C, respectively, both are soft, malleable, and ductile materials. In all proportions, both crystallise in face-centred cubic structures and form stable alloys with each other. Both are resistant to most common acids, except nitric acid, which is easily attacked by silver, and aqua regia, which is easily attacked by gold. Alkali cyanides form compounds with both of them. Both have been sought since ancient times due to their relative scarcity. They have been around for a long time, and they’re still prized for ornamental, monetary, and utility purposes.

Industrial gold plating

Gold plating and its precursors, fire gilding and leafing were virtually purely ornamental in their early applications. Because of its chemical inertness, low and stable contact resistance, conductivity, and resistance to arcing, gold found uses in the electrical and electronics sectors in the latter part of the nineteenth and early twentieth century. Because of its high cost, it has always been critical to deposit gold just where and at the thickness required by the application. As a result, early on, selective gold deposition techniques were created, and they are still being refined.

The majority of gold plating and alloy plating is done with solutions containing gold as a soluble cyanide complex. Gold forms monovalent (MAu(CN)2) or trivalent (MAu(CN)4) complexes with alkali cyanides, where M is an alkali metal or, in certain instances, an ammonium ion. As initially designed, gold plating solutions had an excess of free alkali cyanide, which functioned as a part of the electrolyte. This immediately set the pH of the solution to around 10 to 12, and because cyanide is very surface active and challenging to clean, it also created staining issues.

Monovalent alkali gold cyanides were discovered to be stable in water at pH levels as low as 3.2 about 1950. In addition, trivalent alkali gold cyanides are nearly pH-insensitive. This revelation paved the way for the creation of custom gold plating electrolytes that were devoid of excess cyanide and could operate over a wide pH range, as well as numerous additions for brightening, hardness, and other purposes.

Colour-flash golds are designed to provide thin coatings of a certain colour as a final touch to previously prepared work. Gold is usually included in modest levels (0.5-1.5 g/L) in colour-flash solutions. The typical electrolyte is potassium gold cyanide, KAu(CN)2, which contains a tiny quantity of free KCN (0–7.5 g/L) and additions such as KAg(CN)2, Cu(CN)2, and K2Ni(CN)4 in appropriate proportions to create the desired colour.

The solutions are heated to 50–70°C, and plating is generally done for 10 to 12 seconds at 4 to 6 V.  Colour-flash golds usually don’t have any brighteners, and they’re not abrasion-resistant since the deposits are so thin (0.025–0.075 m) and heavily alloyed. Therefore, prior to the final colour flash, a coating of abrasion-resistant hard gold should be applied.

The first commercially effective gold brightening agent was silver. Large quantities of free cyanide (90-120 g/L) were employed in early formulations of this system and comparable formulations using antimony or tin, which produced staining issues and attacked printed circuit board laminates. These solutions have been reformulated with electrolytes based on phosphate, and free cyanide decreased to 0-7.5 g/L, comparable to those used in colour-flash baths. Silver deposits with a silver concentration of 4 to 9% are highly resistant to sliding friction and are still utilised in slidewire and rotary-switch applications.

Neutral golds are an electrolyte used to deposit highly pure, soft deposits for electrical and electronic applications. There is no such thing as free cyanide. Instead, phosphates, phosphonates, and the salts of different organic acids are used as electrolytes. Most neutral gold solutions have a pH of around 6 to 7. However, it’s usual to raise it to prevent immersion deposition or lower it to avoid photoresist disintegration.

Similarly, the amounts of individual components can be increased to achieve better overall conductivity for barrel applications or decreased to provide more fluidity for efficient pumping. Excellent solderability and wire-bonding capacity are often needed for deposits made from neutral gold solutions, both of which are typically associated with softness and high purity.

The discovery that alkali-gold cyanides like KAu(CN)2 were stable in solutions without free cyanide at pH values as low as 3.2 benefited acid hard-gold systems immediately. It is feasible to integrate transition metals such as cobalt, nickel, and iron into alloyed gold deposits that are hard, brilliant, ductile, and, with proper fluxing, sinterable by operating in the pH range of 4 to 5. Even when plating in current density levels, deposit compositions of 99.7% gold or greater can be maintained, and both composition and physical characteristics can be preserved.

Acid hard golds were quickly adopted as the preferred finish for detachable electrical connections. Acid hard-gold plating methods have been extensively developed for use at extremely high current densities, homogeneous deposit distribution, and compatibility with various selective plating devices. The majority of today’s electrolytes are made up of weak organic acids and their salts.

A strike solution is when the ratio of crystallite nucleation to grain development is increased by using higher-than-normal current densities from a solution with a tiny quantity of metal. Keeping the current efficiency low and the deposition period short, the risk of dendritic development, or burning, is reduced. The end product is a fine-grained, thin layer with good adherence and homogeneous dispersion. Before heavy deposition from a standard plating solution, strike coatings are frequently employed.

The alkali gold (III) cyanide MAu(CN)4 is formed when chloroauric acid, HAu(Cl)4, reacts with alkali cyanide, where M is sodium or potassium. As previously stated, these organisms are stable at pH levels as low as 0. This makes it possible to create gold-strike solutions that may activate and adhere to stainless steel. Jewellery plating has also employed trivalent gold-cyanide solutions to create exceptionally brilliant, adherent coatings of around 5-micron thickness. Electrolytes used in these procedures generally have a pH range of 2.5 to 3.0. However, most organic acids are only weakly ionised below pH 2.5, necessitating the addition of inorganic sulphates or chlorides to increase conductivity.

Silver plating

While gold’s complex cyanides are stable at pH levels low enough to enable the use of electrolytes without free cyanide, silver’s complex cyanide (which only forms the monovalent species) is unstable below neutrality hydrolysing to liberate insoluble AgCN. This necessitates the presence of at least some free cyanide in the system and a minimum operating pH of 8.0 to 8.5 for cyanide silver plating solutions. Several variants within the broad class of cyanide silvers are optimised for anode corrosion, deposit brightness, or plating speed.

Typical general-purpose cyanide silver-plating solutions contain around 90 to 120 g free alkali cyanide per litre and about 25 to 40 g metallic silver per litre, added as the equivalent alkali silver cyanide or as AgCN. The usage of sodium cyanide was once common, but because of the greater solubility of most potassium salts, potassium cyanide has mostly replaced it.

Carbonate develops gradually in alkaline cyanide systems due to hydrolysis, and at concentrations more than roughly 90 g/L, occasionally less, it obstructs anode dissolving and produces deposit roughness. In a silver bath, free cyanide serves numerous purposes. It erodes the anodes, solubilises the silver, and acts as an electrolyte. The cyanide ion, like gold, is very surface-active and necessitates thorough washing after plating. 

Antimony, bismuth, selenium, and sulfur-based compounds are among the brightening agents for silver. The majority of them are exclusive. Sulfur-bearing materials, in particular, are frequently complicated, resulting from reactions between different organic molecules and carbon disulfide, sodium thiosulfate, or other reagents. As a result, they are frequently referred to as “organic” brightening agents, yet the active activity is nearly always derived from sulphur. For cyanide silver solutions, brightener systems nearly usually comprise both the primary brightener and a surface-active ingredient, the role of which is unknown.

Only two of the numerous soluble non-cyanide forms of silver have found commercial success in electroplating. Organosilver complexes, the most well-known of which is the succinimide complex, and alkaline thiosulfate, succinimide is added as an anode activator. The necessity to keep the anodes active and film-free as a source of silver for replenishment constrains the design of electrolytes for non-cyanide silver plating.

This can be done using nitrate, lactate, or sulfamate, and numerous combinations have been employed. Also, regardless of whether or not the plating solution is utilised, succinimide hydrolyses slowly at the working pH (usually about 7.5-9) and must be supplied regularly. Because the hydrolysis process consumes hydroxyl ions, the pH of the solution decreases slowly and must be adjusted periodically.