Copper electroplating is the process of
electroplating
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 ...
a layer of
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
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. The copper layer can be decorative, provide corrosion resistance, increase electrical and thermal conductivity, or improve the adhesion of additional deposits to the substrate.
Overview
Copper electroplating takes place in an
electrolytic cell
An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would not otherwise occur. The external energy source is a voltage applied between the cell′s two electrod ...
using
electrolysis
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 n ...
. As with all plating processes, the part to be plated must be cleaned before depositing metal to remove soils, grease, oxides, and defects.
After precleaning, the part is immersed in the cell's
aqueous
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
electrolyte
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
solution and functions as the
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
. A copper
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
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
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
, losing
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
s and
ionizing into copper
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s. The copper cations form a
coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. M ...
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.
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.
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.
Types of plating chemistries
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:
# Alkaline
cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
# Alkaline non-cyanide
# Acid
sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
# Acid
fluoroborate
#
Pyrophosphate
Alkaline cyanide
Alkaline cyanide baths have historically been one of the most commonly-used plating chemistries for copper electrodeposition.
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.
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
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
or
potassium cyanide
Potassium cyanide is a compound with the formula KCN. This colorless crystalline salt, similar in appearance to sugar, is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications inc ...
as a source of free cyanide that complexes with cuprous cyanide to render it soluble, and
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
or
potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash.
Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...
for increased conductivity and pH control.
Baths may also contain
Rochelle salts and
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
or
potassium carbonate, as well as a variety of proprietary additives.
Cyanide copper baths can be used as low-efficiency strike-only baths, medium-efficiency strike-plate baths, and high efficiency plating baths.
Bath composition
Operating conditions
* Temperature: 24-66 °C (strike); 40-55 °C (strike-plate); 60-71 °C (high-efficiency)
* Cathode current density: 0.5-4.0 A/dm
2 (strike); 1.0-1.5 A/dm
2 (strike-plate); 8.6 A/dm
2 (high-efficiency)
* Current efficiency: 30-60% (strike); 30-50% (strike-plate); 90-99% (high-efficiency);
* pH: >11.0
Toxicity
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.
Alkaline non-cyanide
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.
Acid sulfate
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.
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.
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
Immersion may refer to:
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* "Immersion", a 2012 story by Aliette de Bodard
* ''Immersion'', a French comic book series by Léo Quievreux
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* ''Immersion'' (film), a 2021 ...
to form that will compromise adhesion.
Due to this phenomenon as well as the lower throwing power, acid sulfate baths are not usually used as strike baths.
Along with alkaline cyanide, acid copper baths are among the most commonly-used copper plating electrolytes, with industrial applications that include decorative plating,
electroforming,
rotogravure
Rotogravure (or gravure for short) is a type of intaglio printing process, which involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a cylinder because, like offset printing and flexography, it ...
, and
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich str ...
and semiconductor fabrication.
Acid sulfate baths contain
cupric sulfate as the source of copper(II) ions;
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
to increase bath conductivity, ensure copper salt solubility, decrease anode and cathode polarization, and increase throwing power; and a source of
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...
ions such as
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...
or
sodium chloride
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
, which helps reduce anode polarization and prevents striated deposits from forming.
Most baths also contain a variety of organic additives to help refine the grain structure, improve ductility, and brighten the deposit.
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.
Bath composition
Operating conditions
* Temperature: Usually ambient,
although some baths may operate as high as 43 °C
* Cathode current density: 2–20 A/dm
2 (general purpose); 1.5–5 A/dm
2 (high throw); 5–20 A/dm
2 (high speed)
* Current efficiency: 100%
Additives
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
Thiourea () is an organosulfur compound with the formula and the structure . It is structurally similar to urea (), except that the oxygen atom is replaced by a sulfur atom (as implied by the ''thio-'' prefix); however, the properties of urea a ...
and molasses, while other formulations used various gums, carbohydrates, and
sulfonic acid
In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide. As a substituent, it is k ...
s.
Acid fluoroborate
Copper fluoroborate baths are similar to acid sulfate baths, but they use fluoroborate as the anion rather than sulfate.
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.
Acid fluoroborate baths contain
cupric tetrafluoroborate and
fluoroboric acid
Fluoroboric acid or tetrafluoroboric acid (archaically, fluoboric acid) is an inorganic compound with the chemical formula +BF4−], where H+ represents the solvated proton. The solvent can be any suitably Lewis-basic entity. For instance, in w ...
.
Boric acid
Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolve ...
is typically added to the bath to prevent
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
of the fluoroborate ions, which generates free
fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ty ...
in the bath. Unlike acid sulfate baths, fluoroborate baths usually do not contain organic additives.
Bath composition
Operating conditions
* Temperature: 18-66 °C
* Cathode current density: 13-38 A/dm
2 (high concentration); 8-13 A/dm
2 (low concentration)
* pH: 0.2-0.6 (high concentration); 1.0-1.7 (low concentration)
Pyrophosphate
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.
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
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
for increased anode dissolution and deposit grain refinement, and a source of
nitrate
Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolu ...
ions such as
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
or
ammonium nitrate
Ammonium nitrate is a chemical compound with the chemical formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates. It is ...
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,
6Cu(P2O7)2">6Cu(P2O7)2 which dissociates to form the Cu(P
2O
7)
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.
In pyrophosphate baths,
orthophosphate
A phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is bonded to four oxygen (O) atoms, one of them through a double bond, arranged as the corners of a tetrahedron. ...
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.
Current control
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.
Applications
Excluding the continuous strip plating industry, copper is the second most commonly-plated metal after nickel.
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 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.
Products that utilize decorative copper plating include automotive trim, furniture, door and cabinet handles, light fixtures, kitchen utensils, other household goods, and apparel.
Copper plating is also used for
minting currency.
Engineering applications
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.
Copper is electroplated onto
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich str ...
s to add metal to the
through holes
A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
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).
The
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...
industry uses the damascene process to pattern-plate copper into vias and trenches of
interconnects for metallization.
Copper is also used to plate steel wire for electrical cabling applications.
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.
See also
*
Electroless copper plating Electroless copper plating is a chemical process that deposits an even layer of copper on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing copper salts and a reducing ...
*
Electroplating
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 ...
References
External links
* (responsibly)
{{DEFAULTSORT:Copper Electroplating
Electroplating
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 ...
Metal plating