Electroless plating, also known as chemical plating or autocatalytic plating, is a class of industrial chemical processes that create

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

coatings on various materials by

autocatalytic A single chemical reaction is said to be autocatalytic if one of the reaction products is also a catalyst for the same or a coupled reaction.Steinfeld J.I., Francisco J.S. and Hase W.L. ''Chemical Kinetics and Dynamics'' (2nd ed., Prentice-Hall 199 ...

chemical reduction 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 a d ...

of metal

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 convent ...

s in a liquid bath. This class is contrasted with

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 ...

processes, where the reduction is achieved by an externally generated

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...

. The main technical advantage of electroless plating is that it creates an even layer of metal regardless of the geometry of the surface—in contrast to electroplating, which suffers from uneven current density due to the effect of substrate shape on the

electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...

of the bath. Moreover, electroless plating can be applied to non-

conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gene ...

surfaces and parts of the object that cannot be connected to the current source. Electroless plating has many industrial applications, from merely decorative to the prevention of corrosion and wear. It can even be used to coat individual grains of powdered materials, with arbitrarily small size. It can also be used to create

composite Composite or compositing may refer to: Materials * Composite material, a material that is made from several different substances ** Metal matrix composite, composed of metal and other parts ** Cermet, a composite of ceramic and metallic materials ...

coatings, by suspending suitable powders in the bath.

History

The earliest electroless plating process can be considered to be Tollen's reaction, that deposited a uniform metallic silver layer on glass and other substrates. It was extensively used for

silvering Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often silver, the term is used for the application of any reflective metal. Process Mos ...

mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...

s. The first electroless plating process to compete with electroplating was nickel-phosphorus, using

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...

salts and

hypophosphite Phosphinates or hypophosphites are a class of phosphorus compounds conceptually based on the structure of hypophosphorous acid. IUPAC prefers the term phosphinate in all cases, however in practice hypophosphite is usually used to describe inorganic ...

as both a reducing agent and a source of phosphorus. The reaction had been discovered in 1844 by

Charles Adolphe Wurtz Charles Adolphe Wurtz (; 26 November 181710 May 1884) was an Alsatian French chemist. He is best remembered for his decades-long advocacy for the atomic theory and for ideas about the structures of chemical compounds, against the skeptical opinio ...

, and had been patented in 1914 as a metal-plating method by François Auguste Roux of L'Aluminium Français. However, Roux's invention did not seem to receive much commercial use. In 1946 the process was accidentally re-discovered by Abner Brenner and Grace E. Riddell of the

National Bureau of Standards The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...

. They presented their discovery at the 1946 Convention of the American Electroplaters' Society (AES); a year later, at the same conference they proposed the term "electroless" for the process and described optimized bath formulations, that resulted in a patent.

General process

The general reaction of electroless plating is : + + Z where M represents the metal, the reducing agent, and Z its oxidized byproducts (which may be liquids, solids, or gases). In order for the metal to be deposited as a uniform solid coating on the intended surface, rather than a precipitate through the solution, the reaction must require a

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

that is either the substrate itself or is applied to it beforehand. In fact, the reaction must be

, so that it can continue after the substrate has been coated by the metal. Electroless nickel plating uses

salts as the metal cation source and either

(or a

borohydride Borohydride refers to the anion , which is also called tetrahydroborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for example cyanoborohydride or cyanotrihyd ...

-like compound) as the reducer. A byproduct of the reaction is elemental

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

(or

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

) which is incorporated in the coating.

Tollens' reagent Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nit ...

is a classical example of electroless silver plating. For other metals like gold and copper, the reducing agent is typically a low-molecular-weight

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

Main processes

* nickel-phosphorus ** nickel-gold (ENIG, ENEPIG) * nickel-boron (NiB) *

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...

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 pinkis ...

References

Charles R. Shipley Jr. (1984):
Historical highlights of electroless plating
. ''Plating and Surface Finishing'', volume 71, issue 6, pages 24-27. G. O. Mallory and J. B. Hajdu, editors (1990): ''Electroless plating: fundamentals and applications''. 539 pages. François Auguste Roux (1914):
Process of producing metallic deposits
. US Patent 1207218. Granted 1916-12-05, assigned to L'Aluminium Français, expired on 1933-12-05. Abner Brenner and Grace E. Riddel (1946):
Nickel plating on steel by chemical reduction
. ''Journal of Research of the National Bureau of Standards'', volume 37, pages 31–34 Abner Brenner and Grace E. Riddel (1946): ''Proc. 33rd Annual Convention of the American Electroplaters' Society'' page 23. Abner Brenner and Grace E. Riddel(1947): ''Proc. 34th Annual Convention of the American Electroplaters' Society'', page 156. Abner Brenner and Grace E. Riddel (1950): "Nickel plating by chemical reduction". US Patent 2532283. Granted on 1950-12-05, expired on 1967-12-05. Abner Brenner (1954): ''Metal Finishing'', volume 52, issue 11, page 68. Abner Brenner (1954): ''Metal Finishing'', volume 52, issue 12, page 61. W. T. Ferrar, D. F. O'Brien, A. Warshawsky, and C. L. Voycheck (1988): "Metalization of lipid vesicles via electroless plating". ''Journal of the American Chemical Association'', volume 110, issue 1, pages 288–289. Bernhard Tollens (1882): " rl=https://babel.hathitrust.org/cgi/pt?id=uiug.30112025692838;view=1up;seq=535 Ueber ammon-alkalische Silberlösung als Reagens auf Aldehyd ("On an ammonical alkaline silver solution as a reagent for aldehydes"). ''Berichte der Deutschen Chemischen Gesellschaft'', volume 15, issue 2, pages 1635–1639. {{doi, 10.1002/cber.18820150243 Metal plating