What Is Involved in Gold Electroplating?

Electroplated gold plating has strong corrosion resistance, good electrical conductivity, easy welding, high temperature resistance, and a certain abrasion resistance (such as hard gold doped with a small amount of other elements), good resistance to discoloration, and at the same time, gold alloy plating has more A variety of shades, and gold plating on silver prevents discoloration. And the coating has good ductility and is easy to polish, so it is often used as decorative coating, such as plating jewelry, watch parts, art, etc .; it is also widely used in precision instruments and meters, printed boards, integrated circuits, electronic tubes, electrical contacts, etc. Parameter performance Long-term stable parts plating. But because gold is expensive, its applications are limited.

Electroplated gold began in 1838 with the invention of cyanide gold plating, mainly used for decoration. In the 1940s, the electronics industry developed, and the price of gold skyrocketed, mostly using thin gold plating. In order to further save gold, brush gold plating (ie, selective gold plating) appeared in the 1960s, and pulse gold plating and laser gold plating appeared in the 1980s. In 1950, the stability of potassium cyanide was found in the presence of organic acids, and neutral and weakly acidic gold plating solutions appeared. In the late 1960s, cyanide-free gold plating was also used, especially sulfite plating. .

Electroplated gold has a history of more than 200 years. The gold plating layer has a golden yellow appearance, has good chemical stability, discoloration resistance, electrical conductivity, corrosion resistance, and oxidation resistance. At the same time, it has good solderability, low contact resistance, and excellent thermocompression bonding properties, making it electroplated gold. The coating can be used as both a decorative coating and a functional and protective coating. Therefore, electroplated gold is widely used in jewelry, clocks, crafts and electronics, instruments, meters, aviation, aerospace and other industrial fields. Most of the electroplating gold processes used at home and abroad contain cyanide. Traditional cyanide gold plating solutions are stable and reliable, high current efficiency, good dispersing ability and covering ability, fine and shiny plating crystals, and good bonding force. However, cyanide is harmful to the environment and human body. With the increase of environmental protection requirements, the plating solution is developing in the direction of cyanide-free and environmental protection. It is the general trend that cyanide-free gold plating process replaces the cyanide-containing process. ^[1]

Cyanide-free electroplated gold-cobalt alloys Sandstorm and Washton measured the cathodic polarization curves of electroplated gold-cobalt alloys in 1979. The concentrations of gold and cobalt ions in the solution, plating The effects of liquid pH, current density, and agitation on the efficiency of the cathode and the cobalt content in the coating. The co-deposition of gold and cobalt can significantly increase the hardness of the gold coating. The microhardness of the electroplated pure gold coating is about 70 HV, while the microhardness of the gold alloy coating obtained by using a gold-cobalt plating solution can reach 130 HV. Gold-cobalt alloy plating is mainly used for wear-resistant parts such as integrated circuit electrical contacts and printed circuit boards. The following types of cyanide-free electroplating gold-cobalt alloy plating solutions that have been put into production or under development are: halide plating solution, thiosulfate plating solution, thiomalate plating solution, sulfite plating solution, pyrophosphoric acid Salt plating solution, etc. ^[2]

The gold plating layer is mainly plated on the nickel plating layer. The nickel plating layer (low-stress nickel, semi-bright nickel, bright nickel, chemical nickel) is 3 to 8.9 m. As a barrier layer between gold, copper, and iron, the main role is to prevent gold and copper. , Iron diffuses with each other. The brightness and leveling of the underplating layer have a significant effect on improving the brightness of the thin gold layer. Gold can also be plated on copper, brass and other substrates, but after long-term use, copper will diffuse to the gold plating and lose its role. For steel, copper, silver and their alloy substrates, gold plating is a cathodic coating. The porosity of the coating affects its protective performance. Depending on the base material of the pre-plated part, the gold plating process is slightly different. Copper and copper alloys need to be plated bright nickel flash gold plated gold plated; copper and cyanide plated on iron and iron alloy substrates bright nickel plated flash gold plated gold or dark nickel plated bright nickel plated flash gold plated Gold plating; gold plating on stainless steel requires activation treatment, and then flash gold plating after rapid water washing; for gold plating on nickel or high-nickel alloys, flash nickel plating must be quickly washed and flash gold plating. The purpose of flash gold plating is to make the combination of gold plating and the substrate good. Often, an acid solution is used, which is washed with distilled water and then gold-plated. Flash gold plating is particularly important for gold-plated layers with a thickness of more than 5 m. After gold plating, it should be thoroughly washed with pure water or hot pure water to eliminate residual salts on the surface of the coating and maintain a long-lasting gloss of the coating. Thin gold plating requires anti-tarnishing treatment. The anti-tarnishing treatment is usually used to close the pores of the gold plating layer to prevent the corrosion products from spreading to the surface due to the corrosion of the bottom layer of gold, which will cause the gold plating layer to change color. Anti-discoloration treatment can draw on anti-discoloration treatment. It can be used for chemical passivation, electrolytic passivation, spray (immersion) or electrophoretic organic protective film.