Copper electroplating is the process of electroplating a layer of copper onto the surface of a metal object. Copper is used both as a standalone coating and as an undercoat onto which other metals are subsequently plated. [1] The copper layer can be decorative, provide corrosion resistance, increase electrical and thermal conductivity, or improve the adhesion of additional deposits to the substrate. [2] [3]
Copper electroplating takes place in an electrolytic cell using electrolysis. As with all plating processes, the part to be plated must be cleaned before depositing metal to remove soils, grease, oxides, and defects. [4] [5] After precleaning, the part is immersed in the cell's aqueous electrolyte solution and functions as the cathode. A copper anode is also immersed in the solution. During plating, a direct electric current is applied to the cell which causes the copper in the anode to dissolve into the electrolyte through oxidation, losing electrons and ionizing into copper cations. The copper cations form a coordination complex with salts present in the electrolyte, after which they are transported from the anode to the cathode. At the cathode, the copper ions are reduced to metallic copper by gaining electrons. This causes a thin, solid, metallic copper film to deposit onto the surface of the part.
The anodes can be either simple copper slabs or titanium or steel baskets filled with copper nuggets or balls. [6] The anodes may be placed in anode bags, which are typically made of polypropylene or another fabric and are used to contain insoluble particles that flake off the anode and prevent them from contaminating the plating bath. [2] [7]
Copper electroplating baths can be used to plate either a strike or flash coating, which is a thin highly-adherent initial layer that is plated with additional layers of metal and that serves to improve adhesion of the subsequent layers to the underlying substrate, or a thicker coating of copper that may serve as the finish layer or as a standalone coating. [5]
There are a variety of different electrolyte chemistries that can be used for copper electroplating, but most can be broadly characterized into five general categories based on the complexing agent: [2] [6]
Alkaline cyanide baths have historically been one of the most commonly-used plating chemistries for copper electrodeposition. [5] [8] Cyanide copper baths typically provide high covering and throwing power, allowing uniform and complete coverage of the substrate, but often plate at lower current efficiency. [2] They produce a metal finish favored for its diffusion blocking character. Diffusion blocking is used to improve the long term adherence of different metals, e.g. chromium and steel. It is also used to prevent the second material from diffusing into the substrate.
Cyanide baths contain cuprous cyanide as the source of copper(I) ions, sodium or potassium cyanide as a source of free cyanide that complexes with cuprous cyanide to render it soluble, and sodium or potassium hydroxide for increased conductivity and pH control. [9] Baths may also contain Rochelle salts and sodium or potassium carbonate, as well as a variety of proprietary additives. [2] Cyanide copper baths can be used as low-efficiency strike-only baths, medium-efficiency strike-plate baths, and high efficiency plating baths. [6]
| Chemical Name | Formula | Strike [6] | Strike-plate [6] | High-efficiency plate [6] | |||
|---|---|---|---|---|---|---|---|
| Sodium | Potassium | Sodium | Potassium | Sodium | Potassium | ||
| Copper(I) cyanide | CuCN | 30 g/L | 30 g/L | 42 g/L | 42 g/L | 75 g/L | 60 g/L |
| Sodium or potassium cyanide | NaCN or KCN | 48 g/L | 58.5 g/L | 51.9 g/L | 66.6 g/L | 97.5 g/L | 102 g/L |
| Sodium or potassium hydroxide | NaOH or KOH | 3.75–7.5 g/L | 3.75–7.5 g/L | Control to pH 10.2–10.5 | 15 g/L | 15 g/L | |
| Rochelle salt s | KNaC4H4O6·4H2O | 30 g/L | 30 g/L | 60 g/L | 60 g/L | 45 g/L | 45 g/L |
| Sodium or potassium carbonate | Na2CO3 or K2CO3 | 15 g/L | 15 g/L | 30 g/L | 30 g/L | 15 g/L | 15 g/L |
Commercial platers typically use a copper cyanide solution, which retains a high concentration of copper. However, the presence of free cyanide in the baths makes them dangerous due to the highly toxic nature of cyanide. This creates both health hazards as well as issues with waste disposal. [6]
Due to safety concerns surrounding the use of cyanide-based plating chemistry, alkaline copper plating baths that do not contain cyanide have been developed. However, they generally see only limited use compared with the more common cyanide-based alkaline chemistry. [2]
Acid copper sulfate electrolytes are relatively simple solutions of copper sulfate and sulfuric acid that are cheaper and easier to maintain and control than cyanide copper electrolytes. [2] Compared to cyanide baths, they provide higher current efficiency and allow for higher current density and thus faster plating rates, but they usually have less throwing power, although high-throw variations exist. [2] Additionally, they cannot be used to plate directly onto less-noble metals such as steel or zinc without first applying a cyanide-based strike or other barrier layer, otherwise the acid in the bath will cause an immersion coating to form that will compromise adhesion. [6] Due to this phenomenon as well as the lower throwing power, acid sulfate baths are not usually used as strike baths. [2]
Along with alkaline cyanide, acid copper baths are among the most commonly-used copper plating electrolytes, [10] with industrial applications that include decorative plating, electroforming, rotogravure, and printed circuit board and semiconductor fabrication. [6] [11]
Acid sulfate baths contain cupric sulfate as the source of copper(II) ions; sulfuric acid to increase bath conductivity, ensure copper salt solubility, decrease anode and cathode polarization, and increase throwing power; and a source of chloride ions such as hydrochloric acid or sodium chloride, which helps reduce anode polarization and prevents striated deposits from forming. [6] Most baths also contain a variety of organic additives to help refine the grain structure, improve ductility, and brighten the deposit. [12] Variations of the acid copper electrolyte include general-purpose baths, high-throw baths, and high-speed baths. The high-throw and high-speed baths are used when greater throwing power and faster plating rates are required, including for printed circuit board fabrication where high throw is required to plate the low-current-density areas in the through holes. [2]
| Chemical Name | Formula | Bath concentration [2] | ||
|---|---|---|---|---|
| General-purpose [2] | High-throw [2] | High-speed [2] | ||
| Copper(II) sulfate | CuSO4 | 190–250 g/L | 60–90 g/L | 80–135 g/L |
| Sulfuric acid | H2SO4 | 45–90 g/L | 150–225 g/L | 185–260 g/L |
| Chloride ion | Cl− | 20–150 ppm | 30–80 ppm | 40–80 ppm |
| Additives | Varies | Varies | ||
Various common and proprietary additives have been developed for acid copper electrolytes to help improve throwing and leveling power, brighten the finish, control hardness and ductility, and impart other desired properties to the deposit. Historical formulations dating to the mid-20th century often used thiourea and molasses, while other formulations used various gums, carbohydrates, and sulfonic acids. [13] [8]
For semiconductor and printed circuit board applications, acid copper baths use additives that facilitate plating in high-aspect-ratio vias and through holes. Such additives can be grouped into three categories: [14]
Without these additives, copper will preferentially deposit on the surface near the top of the vias instead of inside the vias due to the lower local current density inside the vias, leading to top-down via filling and undesirable voids. The suppressor inhibits plating near the top of the via and the surface, while the brightener accelerates plating near the bottom of the via. The leveler helps prevent buildup at the via opening and creates a smoother surface finish. [14] [15]
Copper fluoroborate baths are similar to acid sulfate baths, but they use fluoroborate as the anion rather than sulfate. [6] Copper fluoroborate is much more soluble than copper sulfate, which allows one to dissolve larger quantities of copper salt into the bath, enabling much higher current densities than what is possible in copper sulfate baths. Their main use is for high-speed plating where high current densities are required. Drawbacks to the fluoroborate chemistry include lower throwing power than acid sulfate baths, higher cost to operate, and greater safety hazards and waste treatment concerns. [2]
Acid fluoroborate baths contain cupric tetrafluoroborate and fluoroboric acid. Boric acid is typically added to the bath to prevent hydrolysis of the fluoroborate ions, which generates free fluoride in the bath. Unlike acid sulfate baths, fluoroborate baths usually do not contain organic additives. [6]
| Chemical Name | Formula | Bath concentration [6] | ||
|---|---|---|---|---|
| High concentration | Low concentration | |||
| Copper(II) tetrafluoroborate | Cu(BF4)2 | 459 g/L | 225 g/L | |
| Fluoroboric acid | HBF4 | 40.5 g/L | 15 g/L | |
Pyrophosphate copper plating baths possess gentler chemistry compared to the toxic alkaline cyanide baths and the corrosive acid copper baths, operating at mildly alkaline pH and utilizing relatively non-toxic pyrophosphate compounds. While pyrophosphate electrolytes are easier to waste treat than alkaline cyanide and acid plating baths, they are more difficult to maintain and control. Pyrophosphate baths offer high throwing power and produce bright, ductile deposits, making them particularly useful for printed circuit board fabrication where high throw is required for plating high-aspect-ratio through holes. [2] [16]
Pyrophosphate baths contain cupric pyrophosphate as a source of copper(II) ions, potassium pyrophosphate as a source of free pyrophosphate that increases bath conductivity and helps with anode dissolution, ammonia for increased anode dissolution and deposit grain refinement, and a source of nitrate ions such as potassium or ammonium nitrate to decrease cathode polarization and increase the maximum allowed current density. When the bath is made up, the copper pyrophosphate and potassium pyrophosphate react to form a complex, [K6Cu(P2O7)2], which dissociates to form the Cu(P2O7)26− anion from which copper deposits. Variations of the pyrophosphate electrolyte include general-purpose baths, strike baths, and printed circuit baths. Printed circuit baths typically contain organic additives to improve ductility and throwing power. [2] [6]
In pyrophosphate baths, orthophosphate ions are formed from the hydrolysis of pyrophosphate and tend to build up in the electrolyte over time, which presents maintenance challenges. Orthophosphate ions decrease bath throwing power and deposit ductility at concentrations above 40–60 g/L, and they lead to lower solution conductivity, banded deposits, and lower bright current density range at concentrations beyond 100 g/L. Orthophosphate is removed from the bath by either doing partial bails and dilutions or by completely dumping and remaking the bath. [6]
 | This section needs additional citations for verification .(August 2022) |
It is important to control the current to produce the smoothest copper surface possible. With a higher current, hydrogen bubbles will form on the item to be plated, leaving surface imperfections. Often various other chemicals are added to improve plating uniformity and brightness. These additives can be anything from dish soap to proprietary compounds. Without some form of additive, it is almost impossible to obtain a smooth plated surface.
The surface formed always needs to be polished to achieve a shine. As formed it has a matte luster.
Excluding the continuous strip plating industry, copper is the second most commonly-plated metal after nickel. [6] Copper electroplating offers a number of advantages over other plating processes, including low metal cost, high-conductivity and high-ductility bright finish, and high plating efficiency. The process has a variety of both decorative and engineering applications.
Decorative copper electroplating takes advantage of the high levelling power of copper bath formulations that produce bright deposits, the ability of copper to cover defects in the base metal, and the softness of copper that makes it easy to buff and polish for a glossy finish. While copper may be used as the final decorative surface layer, it is usually subsequently plated with other metals that are more resistant to wear or tarnish such as chromium, nickel, or gold; in this case, the brightness of the copper undercoat enhances the appearance of the subsequent finish layer. [5] Products that utilize decorative copper plating include automotive trim, furniture, door and cabinet handles, light fixtures, kitchen utensils, other household goods, and apparel. [9] [17]
Copper plating is also used for minting currency. [18] [19]
Copper electroplating sees widespread usage in the manufacture of electrical and electronic devices, owing to copper's high electrical conductivity – it is the second-most electrically conductive metal after silver. [20] Copper is electroplated onto printed circuit boards to add metal to the through holes and fabricate the board's conductive circuit traces. This is done either through a subtractive process where copper is plated as a blanket unpatterned layer that is subsequently etched with a patterned mask to form the desired circuitry (panel plating), or through an additive or semi-additive process where a patterned mask that exposes the desired circuitry is applied to the board followed by copper plating onto the unmasked circuit areas (pattern plating). [12] The semiconductor industry uses the damascene process to pattern-plate copper into vias and trenches of interconnects for metallization. [21] Copper is also used to plate steel wire for electrical cabling applications. [22]
As a soft metal, copper is also malleable and so has the inherent flexibility to maintain adhesion even if a substrate is subject to being bent and manipulated post plating. When electroplated, copper provides a smooth and even coverage which therefore provides an excellent base for additional coating or plating processes. Corrosion resistance is another advantage to copper. Although copper is not as effective at resisting corrosion as nickel and so is commonly used as a base layer for nickel if enhanced corrosion protection is needed; typically the case for materials that are required to work in marine and subsea environments. Lastly, copper has anti-bacterial properties and so is used in some medical applications. [23]
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outcome of a particular chemical change, or vice versa. These reactions involve electrons moving via an electronically-conducting phase between electrodes separated by an ionically conducting and electronically insulating electrolyte.
In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity".
Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode of an electrolytic cell; the electrolyte is a solution of a salt of the metal to be coated; and the anode is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
Chrome plating is a technique of electroplating a thin layer of chromium onto a metal object. A chrome plated part is called chrome, or is said to have been chromed. The chromium layer can be decorative, provide corrosion resistance, facilitate cleaning, or increase surface hardness. Sometimes, a less expensive substitute for chrome such as nickel may be used for aesthetic purposes.
Electropolishing, also known as electrochemical polishing, anodic polishing, or electrolytic polishing, is an electrochemical process that removes material from a metallic workpiece, reducing the surface roughness by levelling micro-peaks and valleys, improving the surface finish. Electropolishing is often compared to, but distinctly different from, electrochemical machining. It is used to polish, passivate, and deburr metal parts. It is often described as the reverse of electroplating. It may be used in lieu of abrasive fine polishing in microstructural preparation.
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its British inventor, Francis Thomas Bacon, is one of the most developed fuel cell technologies. Alkaline fuel cells consume hydrogen and pure oxygen, to produce potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70%.
Plating is a finishing process in which a metal is deposited on a 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 improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.
Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.
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The Daniell cell is a type of electrochemical cell invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consists of a copper pot filled with a copper (II) sulfate solution, in which is immersed an unglazed earthenware container filled with sulfuric acid and a zinc electrode. He was searching for a way to eliminate the hydrogen bubble problem found in the voltaic pile, and his solution was to use a second electrolyte to consume the hydrogen produced by the first. Zinc sulfate may be substituted for the sulfuric acid. The Daniell cell was a great improvement over the existing technology used in the early days of battery development. A later variant of the Daniell cell called the gravity cell or crowfoot cell was invented in the 1860s by a Frenchman named Callaud and became a popular choice for electrical telegraphy.
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Batteries provided the primary source of electricity before the development of electric generators and electrical grids around the end of the 19th century. Successive improvements in battery technology facilitated major electrical advances, from early scientific studies to the rise of telegraphs and telephones, eventually leading to portable computers, mobile phones, electric cars, and many other electrical devices.
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Nickel electroplating is a technique of electroplating a thin layer of nickel onto a metal object. The nickel layer can be decorative, provide corrosion resistance, wear resistance, or used to build up worn or undersized parts for salvage purposes.
The IsaKidd Technology is a copper electrorefining and electrowinning technology that was developed independently by Copper Refineries Proprietary Limited (“CRL”), a Townsville, Queensland subsidiary of MIM Holdings Limited, and at the Falconbridge Limited (“Falconbridge”) now-dismantled Kidd Creek refinery that was at Timmins, Ontario. It is based around the use of reusable cathode starter sheets for copper electrorefining and the automated stripping of the deposited “cathode copper” from them.
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