Does gold electroplate tarnish?
Introduction
The annual gold consumption of technology up to 326t in 2024. which implies high speed development of semiconductor production. On the other hand, electronic industry as a core of it require special connecting electronics. It also proved the importance of gold electroplating technique meet the demands of appliances electrical electronics manufacturing industry trends. Gold with excellent properties of thermal expansion and thermal conductance. Most importantly, ductile fracture and abrasion resistance are key properties, giving high performance in electrical contacts[1]. Gold is less likely to promote the formation of insulating layer. In addition,gold remains a preferred material for integrated circuit wire bonding due to its excellent bonding characteristics and process reliability. However, the ability of resistance gold tarnish is not obvious. Which will tarnish with time passing by. In this paper, electroplated gold, dose gold plated tarnish and precaution methods will be introduce.
What is electroplated gold?
Gold electroplated is a way of electroplating, adopting this way can produce a thick forms of gold, in either positive or negative form, from a electrical conductivity of an object.
In electronic industry, electroplated gold can be divided into soft gold and hard gold. Soft gold is commonly applied as a surface coating to facilitate the bonding of gold or aluminum conductor in traditional semiconductor devices assembly on circuit boards, particularly after precision surface preparation processes in metal etching and silkscreening. Hard gold is regarded as connector for electrical devices and resistance to mechanical wear as well as low electrical contact resistance[2]. For golden layer, it is to be noticed that a maximum thickness is 20 µin. (0.5 µm). Gold electroplating is widely applied to various substrates, including copper, nickel,ceramic PCB pads, and electrical connectors, to improve corrosion resistance, electrical conductivity resistivity, and surface reliability. gold-plated surfaces also contribute to improved Soldering performance, especially in high-density interconnect structures and multilayer ceramic assemblies. In addition, electroplated layers are often integrated with manufacturing steps such as precision metal patterning, die cut technologies, and aluminum laser welding to support the fabrication of high-reliability electronic modules and thermal management substrates
Does Gold Electroplating Tarnish?
Pure gold also can be regarded as soft gold, with low hardness(about 40-60vhn)[3].
Which widely use for bonding. There is no oxide layer exist lead to excellent tarnish resistance. where intimate contact between gold surface, combined with high atomic diffusion coefficient, enables rapid joining without the need for elevated temperature or flux. The same chemically inert that facilitates thermal compression bonding also explains why pure gold coatings exhibit minimal tarnish during service.
Hard gold, a gold content of 50 at.% corresponds to approximately 15.6–18 carats, depending on the copper-to-silver ratio. Below this threshold, alloys are more prone to tarnishing due to the presence of reactive base metals. By contrast, 22-carat gold alloys contain about 91.7 wt.% gold, which is well above the critical gold concentration required for corrosion resistance alloy and tarnish resistance.
Electroplating Gold in Electronics
The development of electronic devices and circuits further improve machining. In order to meet demands of fine patterns and component density. The key issues are fix connection and bump formation. These issues rely on electro plating technology. A number of circuit boards select gold as plated edge connectors, enhancing its wear resistance,electrical conductivity, material corrosion resistance, via plating of gold. For microwave communication systems, Sputtering is used to metallize the boards, while the tracks are created using photolithographic patterning followed by gold electroplating[4]. Owing to chemical properties of infrared reflectance and tarnish resistance, controlling heat transfer and radiation. Gold also remains stable and shows minimal sublimation in the high-vacuum environment of space.Advancements in selective plating and pulse electroplating have further enhanced precision and uniformity distribution, enabling finer pitch designs and greater electronic components integrated circuits. These improvements support the miniaturization trend in modern electronics, maintaining signal integrity and device reliability.
Main Causes of Tarnishing in Gold Electroplating
In gold–copper–silver alloy systems usually apply to industrial connectors. To overcome drawback of permanent contact closure, harden metal gold electrodeposits emerge. Metals like nickle, iron and cobalt cause metal and alloys codeposition, employ as lubricant, meanwhile, The inclusion of these materials significantly changes the deposit properties, leading to increased hardness and improved wear resistance.
Oxidation and corrosion will influence on alloying elements inside gold, it used to enhance its gold hardness. while pasting a thin layer of gold on another layer of metal, the underlying metal reach to gold layer and finally contaminate the surface. Then lead to corrode or oxidize.
The gold plating process can decide coating porosity. The majority of crystallites growing from nuclei coalesce fully, whereas a small number do not achieve complete coalescence. With the adding thickness of layer. The gap is more likely exist permanently, causing void production. Grain boundaries are regions of elevated defect density and disorder relative to the rest of the material. The thickness of gold coating decide the these grain and void growth, covering the the surface brightness.
Temperature and humidity are harmful to discoloration of coating protective. It reveal coating do not work as protective film and oxygen molecule access to the metal surface. Coating exposure to temperature and humility condition will discolor.
How to Improve Tarnish Resistance
Catalyst like semimetal can produce more growth point to accelerate deposition. It shows ionic species can be absorb. The effect is particularly pronounced in sulphite gold deposit due to the co-deposition of transition metal, including cobalt, nickel, and iron. While these elements improve brightness, they are also critical factors in gold coatings used for connector and ceramic PCB contacts.
Controlled rinsing after gold deposits reduces material loss from gold plating solution carryover. This is generally achieved by employing multiple static rinse baths before flowing water is introduced. These baths are equipped with recovery systems to reclaim gold dissolve[4]. Final hot and cold ultrapure water rinses ensure removal of residual salts from the surface deposits.
The temperature of hard gold deposition cannot up to 250°C. Since the alloying elements would decomposes at high temperature. Pure gold can withstand temperature excess 500°C, plating on substrate surface.
Tarnish Tests
Tarnish tests are commonly used to compare the resistance of metals and alloys to discoloration. However, many standard test methods focus mainly on ranking results and give little explanation of the chemical or electrochemical processes involved.
Liquid Phase Tests
Liquid phase tarnish tests usually involve exposing samples to aqueous sodium sulphide solutions. A major limitation of these tests is poor reproducibility. Sodium sulphide solutions are unstable in air and oxidize rapidly, causing the sulphide concentration to decrease significantly within a few hours. In addition, both precious metals and non-noble alloying elements can accelerate this oxidizing process. As a result, the test conditions change continuously during the experiment, making the results difficult to control and compare reliably.
Gas Phase Tests
Gas phase tests expose samples to low concentrations of sulphur-containing gases, such as hydrogen sulphide. These tests avoid rapid depletion of the tarnishing agent, but the chemical reactions involved are more complex. Hydrogen sulfide can be partially oxidized to sulphur dioxide or sulphur trioxide, especially in humid condition. These oxides dissolve in the thin moisture layer present on metal surface, forming an increasingly acidic film. Unlike natural environments, test systems usually lack alkaline species that would neutralize this acidity, which can influence the tarnishing behaviour.
In both test types, tarnishing is not governed by a single reaction. Local pH, surface potential, and catalytic effects can change during the test, allowing different chemical and electrochemical plating process to occur at different stages. Therefore, tarnish test results should be interpreted with caution and not assumed to represent a simple or mechanism uniform.
Conclusion
In summary, gold electroplating remains an essential surface finishing technologies in electronics, but its performance depends strongly on engineering design process and application environment. A clear understanding of tarnishing mechanisms is key to achieving reliable, high-performance gold-plated components.
