Electroless-plating

Electroless nickel (EN) plating is a chemical reduction process that depends upon the catalytic reduction process of nickel ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of nickel metal without the use of electrical energy. Electroless plating processes are widely used in industry to meet the end-use functional requirements and are only rarely used for decorative purposes. 

In a true electroless plating process, reduction of metal ions occurs only on the surface of a catalytic substrate in contact with the plating solution. Once the catalytic substrate is covered by the deposited metal, the plating continues because the deposited metal is also catalytic. 

Nickel deposits have unique magnetic properties, except deposits containing more than 8 percent phosphorus are essentially nonmagnetic. In Ni-P coatings, phosphorus is present as supersaturated solution in fine microcrystalline solid solution, bordering on amorphous or liquid-like (glass-like) metastable structure, and this phosphorus is 

A second generation of EN plating has been developed by codepositing micrometer-sized particles of silicon carbide with the nickel, thereby creating an extremely wear- and corrosion-resistant coating. In this composite coating, the nickel alloy matrix provides corrosion resistance while the silicon carbide particles add wear resistance. 

The growth of electroless plating is directiy traceable to (/) the discovery that some alloys produced by electroless deposition, notably nickel phosphoms, have unique properties (2) the growth of the electronics industry, especially the development of printed circuits (see ELECTRONIC COATINGS Integrated circuits) and (3) the large-scale introduction of plastics into everyday life. 

Modem electroless plating began in 1944 with the rediscovery that hypophosphite could bring about nickel deposition (7,8). Subsequent work led to the first patents on commercially usable electroless nickel solutions. Although these solutions were very usefiil for coating metals, they could not be used on most plastics because the operating temperature was 90—100°C. The first electroless nickel solution capable of wide use on plastics was introduced in 1966 (9). This solution was usable at room temperature and was extremely stable (see Nickel and nickel alloys). 

Electroless copper solutions underwent similar development during the same period (10). Early printed circuit boards used mechanically attached eyelets to provide electrical conductivity between the copper sheathing laminated on two sides of a plastic board. Electroless copper plating provided a less expensive, better conductive path, allowing much greater numbers and smaller sizes of holes. Later, electroless coppers even replaced the laminated bulk copper sheathing in the semiadditive and additive processes (see Copper). 

The theory and practice of electroless plating parallels that of electrolytic plating. 

The simplest electroplating baths consist of a solution of a soluble metal salt. Electrons are supplied to the conductive metal surface, where electron transfer to and reduction of the dissolved metal ions occur. Such simple electroplating baths are rarely satisfactory, and additives are required to control conductivity, pH, crystal structure, throwing power, and other conditions. 

It is important to distinguish between three types of electrochemical process for the deposition of metals (1) electroplating (2) immersion plating and (3) electroless plating (Table 8.6). They differ in the nature of their anodic reaction. Considering the simple case of discharge of a metal ion, the common cathodic 

Property Electroplating Electroless deposition Immersion plating 

Driving force Power supply Autocatalytic redox reaction Chemical displacement 

Site of cathode reaction Substrate (work-piece) Substrate (work-piece) which must have a catalytic surface Substrate (workpiece) which must remain partially exposed 

Site of anode reaction Separate anode Substrate (workpiece) Substrate (workpiece) which dissolves 

Reactions (12.43) and (12.44) take place simultaneously, but in practice only about one-third of the hypophosphite is utilised as in (12.44). The plating rate depends very much on temperature (which is often raised to near boil), and on pH (the rate at pH 5 is five times the rate at pH 3.5). The nickel deposit is less porous than that obtained by conventional electroplating methods and plating thickness is not affected by the shape of the article. Electroless plating has the great advantage that surfaces inside inaccessible cavities receive an even deposit of the metal. 

The initial deposit is amorphous and like a metallic glass (Chapter 8.2), and it usually contains 7-12% of P [42]. It is believed that this phosphorus may arise from the reduction of hypophosphite by nascent hydrogen absorbed on the nickel surface (12.45). 

Heat treatment of the amorphous deposit above about 240°C results in the formation of some crystalline nickel phosphide, NijP, and a consequent increase in hardness. Some metal phosphides may be present before heat treatment. 

The electroless deposition technique has a special application in the chromium plating of plastics, particularly for automobiles. A primary nickel coating is nsed to obtain the conducting layer necessary for the subsequent electrodeposition of chrominm or other metals. A common practice is to coat the electroless Ni first with Cu, then with more Ni and finally with very thin Cr, all by electroplating techniques. 

Metals which apparently cannot be nickel-coated from hypophosphite solntions are Zn, Cd, Pb, Sb, Bi, Sn, Mo and W. Small amounts of these metals in the electroless plating solntion can stop the deposition of nickel on to other metal surfaces. 

Metal-plated plastics find use in the automotive industry, hardware, plumbing fixtures, knobs, and electronic applications (95). 

In order to metallize a polymer surface, electroless plating can be applied. This process typically consists of a pretreatment process in order to improve the adhesion. In the second step a surface seeding of the electroless catalyst is done. Wet chemical methods of pretreatment are using strong acids such as chromic acid, sulfuric acid and acidified potassium permanganate in order to achieve a surface modification of the polymers (96). 

The presence of chromium may impose serious environmental problems, because of the known toxicity of Cr6+. For this reason attempts have been undertaken to develop methods that work without using chromium compounds (95). 

Alternatively, for an ABS polymer, a photocatalytic reaction can be applied as a pretreatment method prior to electroless plating. Unlike the conventional wet chemical method, this method can improve the adhesion strength without severe morphological changes (96). The pretreatment method uses a photocatalytic reaction in a TiC 2 dispersed solution. 

The photocatalytic reaction occurs by the formation of an electron-hole pair in a semiconducting material when the photon energy exceeds the band gap. Thus the photogenerated holes react readily with water and hydroxyl ions adsorbed in forming hydroxyl radicals. The hydroxyl radicals in turn act as oxidizing agents (97,98). 

See Batteries Electroanalytical techniques Electrochemical processing Electroseparations. 

Nickel can be deposited from NiCl2 using sodium hypophosphite as the reducing agent and controlling the pH with NH4CI and sodium citrate acting as buffers to maintain a pH of 9-10 at 85-90°C. 

HCHO as the reducing agent. Care should be taken as HCHO is a known carcinogen [36], 

Suzuki et al [37] compared electroless Ni and Cu coated carbon fiber reinforced with Al, fabricated by the centrifugal pressure infiltration method and found the average bending strength of the Cu plated Al composite (fiber Ff 33%) was 491 MPa whilst the Ni coated (fiber Ff 37.6%) was 278 MPa. 

Huang and Pai [38] determined the optimum conditions for the electroless plating of Ni on carbon fibers for the EMI shielding of ENCF/ABS composites. 

Nickel-boron coatings have excellent resistance to wear and abrasion, but because they are not completely amorphous they have reduced resistance to corrosive envirorunents. Furthermore, they are much more costly than nickel-phosphorus coatings. 

As deposited, the microhardness of electroless nickel-phosphorus coating is about 5(X) to 600 HVN (48-50 HRC), equivalent to many hardened steels. After precipitation hardening, hardness values as high as 1100 HVN are reported, which is equivalent to commercial hard-chromium 

Electroless nickel coatings can be easily soldered and are used in electronic applications to facilitate soldering of light metals such as aluminum. Electroless nickel is often used as a barrier coating to be effective, the deposit must be free of pores and defects. In the as-deposited amorphous state, the coating corrosion resistance is excellent (Table 12), and in many environments is superior to that of pure nickel or chromium alloys. However, after heat treatment the corrosion resistance can deteriorate. 

Metallic particles such as chromium can be introduced into a metal plating electrolyte (for example, nickel and cobalt), and the deposited composite can be subsequently heat treated to form high-temperature oxidation-resistant alloys. MCrAlY composites have been made by depositing 10 p,m CrAlY powder in a cobalt or nickel matrix. Heat treatment bonds 

EnviroMwiM Temperature Corrosion rate Electroless nickel- Electroless pbO( Aonis(a) nidcd-b(Mmi(b)  

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