Electroless (autocatalytic) plating

Electroless (autocatalytic) plating

ELECTROLESS (AUTOCATALYTIC) PLATING by James R. Henry Wear-Cote International, Rock Is~and, III. Electroless plating refers to the autocatalytic or ch...

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ELECTROLESS (AUTOCATALYTIC) PLATING by James R. Henry Wear-Cote International, Rock Is~and, III. Electroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions plated to a base substrate. The process differs from immersion plating in that deposition of the metal is autocatalytic or continuous.

THE ELECTROLESS BATH Components of the electroless bath include an aqueous solution of metal ions, reducing agent(s), complexing agent(s), and bath stabilizer(s) operating in a specific metal ion concentration, temperature, and pH range. Unlike conventional electroplating, no electrical current is required for deposition. The electroless bath provides a deposit that follows all contours of the substrate exactly, without building up at the edges and corners. A sharp edge receives the same thickness of deposit as does a blind hole. The base substrate being plated must be catalytic in nature. A properly prepared workpiece provides a catalyzed surface and, once introduced into the electroless solution, a uniform deposition begins. Minute amounts of the electroless metal (i.e., nickel, copper, etc.) itself will catalyze the reaction, so the deposition is autocatalytic after the original surfaces are coated. Electroless deposition then continues, provided that the metal ion and reducing agent are replenished. If air or evolved gas, however, are trapped in a blind hole or downward facing cavity, this will prevent electroless deposition in these areas. In electroless plating, metal ions are reduced to metal by the action of chemical reducing agents, which are simply electron donors. The metal ions are eleclron acceptors, which react with electron donors. The catalyst is the workpiece or metallic surface, which accelerates the electroless chemical reaction allowing oxidation of the reducing agent. During electroless nickel deposition, byproducts of the reduction, orthophosphite or borate and hydrogen ions, as well as dissolved metals from the substrate, accumulate in the solution. These can affect the performance of the plating bath. As nickel is reduced, orthophosphite ions (HPO3 2-) accumulate in the solution and at some point interfere with the reaction. As the concentration of orthophosphite increases, there is usua!ly a small decrease in the deposition rate and a small increase in the phosphorus content of the deposit. Ultimately, the accumulation of orthophosphite in the plating solution results in the precipitation of nickel phosphite, causing rough deposits and/or spontaneous decomposition. The metal ion and reducer concentration must be monitored and controlled closely in order to maintain proper ratios, as well as the overall chemical balance of the plating bath. The electroless plating deposition rate is controlled by temperature, pH, and metal ion/reducer concentration. Each of the particular plating reactions has optimum ranges at which the bath should be operated (Table I). A complexing agent(s), also known as a chelator, acts as a buffer to help control pH and maintain control over the "free" metal salt ions available to the solution, thus allowing solution stability. The stabilizer(s) acts as a catalytic inhibitor, retarding potential spontaneous decomposition of the electroless bath. Few stabilizers are used in excess of 10 ppm, because an electroless bath has a maximum tolerance to a given stabilizer. The complexing agent(s) and stabilizer(s) determine the composition and brightness of the deposit. Excessive use of stabilization material(s) can result in a depletion of plating rate and bath life including poor metallurgical deposit properties. Trace impurities and organic contamination (i.e., degreasing solvents, oil residues, mold releases) in the plating bath will affect deposit properties and appearance. Foreign inorganic

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o

T a b l e I. T y p i c a l

Electroless Bath Acid nickel

Plating Bath Components and Operating Parameters

Temperature

pH

77-93°C (170 200*F)

4.4-5.2 (medium P) (high P)

Deposition Rate/hr

Metal Salt(s)

Reducing Agent(s)

12.7-25.4/am (0.5-1 rail)

Nickel sulfate Nickel chloride

Sodium hypophosphite Sodium borohydride Dimethylamhle borane (DMAB)

Citric acid Sodium citrate Succinic acid Proprionic acid Glycolic acid Sodium acetate

Citric acid Sodium citrate Lactic acid Glycolic acid Sodium acetate Sodium pyrophosphate Rochelle salt EDTA Ammonium hydroxide Pyridium-3-sulfonic acid Potassium tartrate Quadrdi

6.0-6.5 (low P)

Alkaline nickel

26-95°C (79-205°F)

8.5-14.0

10-12.7 p,m (0.4-0.5 mil)

Nickel sulfate Nickel chloride

Sodium borohydride Sodium hypophosphite DMAB Hydrazine

Copper

26-70°C (79-158°F)

9.0-13.0

1.7-5 /am (0.04-0.3 nail)

Formate Formaldehyde DMAB Sodimn hypophosphite

Gold

65-88°C (149-1900F)

10.0-13.0

2-5 txm (0.08-0.2 rail)

Palladium

45-73°C (113 165"F)

8.0-12.0

2-5 /xm (0.08-0.2 rail)

Cobalt

85-95°C (185-203°F)

9.~11.0

2.5-10 ~m (0.1-0,4 rail)

Copper sdilate Copper acetate Copper carbonate Copper formate Copper nitrate Gold cyanide Gold chloride Potassium aurate Palladium chloride Palladium bromide Cobalt chloride Cobalt sulfate

Complexing Agent(s) or Chelators

Stabilizer(s)

pH Adjustment

Fluoride compounds Heavy metal salts Thiourea Thioorganic colnpounds (i.e., 2-mercaptobenzothiazole, MBT) Oxy anions (i.e., iodates) Thiourea Heavy metal salts Thioorgadic compotmds Triethanolamine Thallium salts Selenitml salts Thiodiglycolic MBT Thiourea Sodium cyanide Vanadium pentoxide Potassium ferrocyanide

Ammonium hydroxide Sulfuric acid

Ammonium hydroxide Sulfuric acid Sodium hydroxide

Hydrochloric acid Sulfuric acid Sodium hydroxide Potassium hydroxide

DMAB Sodium hypophosphite Potassium borohydride Potassium cyanoborohydride

Sodium phosphate Potassium citrate Sodium borate Potassium tartrate EDTA

Alkali metal cyanide Alkali hydrogen fluoride Acetylacetone

Potassium hydroxide Phosphoric acid Sulfuric acid

Sodium hypophosphite DMAB Triethylamine borane

Ammonia Melhylamine EDTA

Ammonium hydroxide Hydrochloric acid

DMAB Sodium hypophosphite

Sodium citrate Citric acid Ammonium chloride Succinic acid

Thioorganic compounds Organic cyanides Thiourea Thiocyanates Urea Thioorganic compounds

Ammonium hydroxide Sodium hydroxide

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Table II. Alkaline Electroless NickelPhosphorus Bath Nickel sulfate Sodium hypophosphite Sodium pyrophosphate Triethanolamine pH Temperature

30 g/L 30 g/L 60 g/L 100 ml/L 10.0 30-35°C (86-95°F)

ions (i.e., heavy metals) can have an equal effect. Improper balance and control will cause deposit roughness, porosity, changes in final color, foreign inclusions, and poor adhesion.

ELECTROLESS NICKEL The most widely used engineering form of electroless plating is, by far, electroless nickel. Electroless nickel offers unique deposit properties including uniformity of deposit in deep recesses, bores, and blind holes. Most commercial deposition is done with an acid phosphorus bath owing to its unique physical characteristics, including excellent corrosion, wear and abrasion resistance, ductility, lubricity, solderability, electrical properties, and high hardness. Electroless nickel baths may consist of four types: 1. Alkaline, nickel-phosphorus. 2. Acid, nickel-phosphorus. a) 1-4% P (low phosphorus) b) 5-9% P (medium phosphorus) c) 10-13% P (high phosphorus) 3. Alkaline, nickel-boron. 4. Acid, nickel-boron. The chemical reducing agent most commonly used is sodium hypophosphite (NaH2POa); others include sodium borohydride (NaBH4), or an aminoborane such as n-dimethylamine borane (DMAB) [(CH3)2NHBH3]. Typical reactions for a hypophosphite reduced bath are as follows: HzPO 2 - + H 2 0 ~ H + + HPO32

+ 2H

Ni 2 + ÷ 2H --> Ni + 2H + H2PO 2

+ H ~ H 2 0 + OH

(1) (2)

+ P

H2PO 2 - + H20 --> H + + HPO32 - -t- H 2

(3)

(4)

Alkaline nickel-phosphorus deposits are generally reduced by sodium hypophosphite. These alkaline baths can be formulated at low temperatures for plating on plastics. Deposits provide good solderability for the electronics industry, and operating energy costs are reduced due to some solutions' low operating temperatures; however, less corrosion protection, lower adhesion to steel, and difficulty in processing aluminum due to high pH values are drawbacks. One such bath consists of the components shown in Table II. An example of a high-temperature, alkaline, electroless nickel-phosphorus bath is given in Table IlL Acid nickel-phosphorus deposits normally consist of 87-94% nickel and 6-13% phosphorus, operating at 77-930C (171-200°F), with a pH of 4.4-5.2. Low phosphorus

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Table III. High-Temperature, Alkaline Electroless Nickel-Phosphorus Bath Nickel sulfate Sodium citrate Ammonium chloride Sodium hypophosphite pH Temperature

33 g/L 84 g/L 50 g/L 17 g/L 9.5 85°C (185°F)

electroless nickel baths contain 1-4% phosphorus and normally operate at 80-82°C (176-180°F), with a pH of 6.0-6.5. The reducing agent is commonly sodium hypophosphite. The resultant deposit melting point is 890°C (I,635°F) for 8-9% phosphorus baths and will vary dependent on the amount of phosphorus alloyed in the deposit. The pH of the solution is the controlling factor affecting the phosphorus content of the deposit. The higher the pH, the lower the phosphorus content, resulting in deposit property changes. Lower phosphoruscontaining deposits (i.e., 1-3%) typically have less corrosion resistance than 10% alloys. Low phosphorus deposits do have good corrosion protection against alkaline solutions such as sodium hydroxide. Also, deposits containing phosphorus in excess of 8.0% are typically nonmagnetic. When the pH drops below 4.0, subsequent nickel deposition virtually stops. As-deposited nickel-phosphorus hardness is 500-600 Vickers hardness number (VHN), and maximum values of 1,000 VHN may be realized by post-heat-treatment of the coating at a temperature of 399°C (750°F) for 1 hour. The temperature is a dominant factor in determining the final deposit hardness. Careful consideration should be given to the choice of temperature in order not to affect structural changes of the base substrate. Additionally, low temperatures are used (116°C/240°F) to relieve any hydrogen embrittlement that may be produced from pretreatment cycles or subsequent electroless nickel deposition. postbaking of the deposit produces marked structural changes in hardness and in wear and abrasion resistance. Depending upon the temperature, bath composition, and phosphorus content, this postbake cycle will totally change the initial amorphous structure, resulting in nickel phosphide precipitation creating a very hard matrix. Complete precipitation of nickel phosphides does not occur at temperatures significantly below 399°C (750°F). In general, deposits with 9.0% phosphorus and above tend to produce lower as-deposited hardness values, but give slightly higher hardness when post-heat-treated. The coating will discolor above 250°C (482°F) in an air atmosphere. Prevention of coating discoloration can be accomplished in a vacuum, inert, or reducing atmosphere oven. Physical properties affected by the post-heat-treatment include increasing hardness, magnetism, adhesion, tensile strength, and electrical conductivity while decreasing ductility, electrical resistivity, and corrosion resistance. Thickness of the nickel-phosphorus deposit generally ranges from 2.5 to 250 /xm (0.1-10.0 mil). Deposits less than 2.5 /xm and greater than 625 /Lm are currently and successfully being performed. The latter being typical of repair or salvage applications. Thickness measurements can be carried out with electromagnetic devices (eddy current), micrometers, coulometrics, beta biackscatter, and X-ray fluorescence. Table IV gives an example of an acid hypophosphite-reduced bath. Table IV. Acid Hypophosphite-Reduced Electroless Nickel Bath Nickel sulfate Sodium acetate Sodium hypophosphite Lead acetate pH Temperature

28 g/L 17 g/L 24 g/L 0.0015 g/L 4.4-4.6 82-88°C (180-190°F)

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Table V. Sodium Borohydride-Reduced Electroless Nickel Bath Nickel chloride Sodium hydroxide Ethylenediamine, 98% Sodium borohydride Thallium nitrate pH Temperature

31 g/L 42 g/L 52 g/L 1.2 g/L 0.022 g/L >i3 93-95°C (20~205°F)

Alkaline nickel-boron solutions utilize the powerful reducing agent, sodium borohydride, to produce a deposit containing 5-6% boron and 94-95% nickel by weight. These highly alkaline solutions operate at a pH of 12.0-14.0 and temperatures of 90-95°C (t 95-2050F). These baths tend to be less stable because of their high alkalinity, and bath decomposition may occur if the pH falls below 12.0. Complexing agents such as ethylenediamine are used to prevent precipitation of nickel hydroxide. As-deposited hardness values of 650 to 750 VHN are typical. After post-heat-treatment at 399°C (750°F) for 1 hour, values of 1,200 VHN can be produced. The melting point 0f borohydride-reduced deposits is 1,080°C (1,975°F). Table V gives an example of a sodium borohydride-reduced electroless nickel bath. Acid nickel-boron varies from 0.1 to 4% boron by weight depending on the bath formulation. The boron content of electroless nickel is reduced by DMAB. Bath parameters include a pH of 4.8-7.5, with an operating temperature range of 65-77°C (149-171°F). DMAB-reduced deposits have a very high melting temperature of 1,350°C (2,460°F). Baths containing less than 1% boron have excellent sotderability, brazing, and good ultrasonic (wire) bonding characteristics. A typical DMAl3-reduced bath is gJven in Table VI.

ELECTROLESS COPPER Electroless copper deposits are generally applied before electroplating on plastics and Other nonconductors, providing a conductive base for subsequent plating. These include acrylonitrile butadiene styrene (ABS), polystyrene, modified potyphenylene oxide, polyvinyl chloride (PVC), Noryl, polyethylene, polysulfone, structural foam, epoxy, and ceramics. In such applications, usually a thin deposit (0.127 /xm; 0.05 mil) is applied, followed by an additional decorative or protective thickness of copper, nickel, or gold deposited electrolyticalty or etectrolessly. The electrotess copper in such applications provides good life in corrosive atmospheric and/or environmental exposures. Automotive, appliance, printed wiring boards, molded interconnect devices, plastic composite connectors, multichip modules, and EMI/RFI shielding of other electronic devices represent major markets for etectrotess copper. In through-hole plating of printed wiring boards; the use of electroless copper has eliminated the need for an electrodeposited flash and provides excellent electrical conductivity in these hard-to-reach areas. In the pretreatment of circuit boards, the most common method involves an acidic aqueous solution of stannous chloride (SnC12) and palladium chloride (PdC12) immersion for Table VI. Dimethylamine Borane-Reduced Electroless Nickel Bath Nickel sulfate Sodium acetate n-Dimethylarnineborane (DMAB) Lead acetate pH Temperature

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25 g/L 15 g/L 4 g/L 0.002 g/L 5.9 26°C (78°F)

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This book is a translation of the original German book on the same subject, but includes a new chapter on environmental considerations, which provides an overview of regulations and disposal options in the U.S. The basics of adhesion between metals and plastics are discussed, followed by a chapter on engineering for metallizing plastics. Quality assurance and plant equipment are also considered. Send Orders to:

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Table VII. Formaldehyde-Reduced Electroless Copper Bath Copper salt as Cu2+ RochelIe salt Formaldehyde as HCHO Sodium hydroxide 2-Mercaptobenzothiazole (MBT) pH Temperature

1.8 g/L 25 g/L I0 g/L 5 g/L < 2 g/L 12.0 25°C (77°F)

subsequent deposition of the electroless copper. Many proprietary activators are available in which these solutions can be used separately or together at room temperature. Palladium drag into the electroless copper bath can cause solution decomposition instantly. The pH of an electroless copper bath will influence the brightness of the copper deposit. Usually a value above 12.0 is preferred. A dark deposit may indicate low bath alkalinity and contain cuprous oxide. The plating rate is equally influenced by pH. In formaldehyde-reduced baths a value of 12.0-13.0 is generally best. Stability of the bath and pH are critical. A high pH value (14.0) results in poor solution stability and reduces the bath life. Below 9.5, solution stability is good; however, deposition slows or ceases. The principal components of the electroless copper bath (copper, formaldehyde, and caustic) must be kept within specification through replenishment. Other bath chemical components will remain within recommended ranges. Complexing agents and stabilizer levels occasionally need independent control. Other key operating parameters include temperature, air agitation, filtration, and circulation. Various common reducing agents have been suggested, however, the best known reducing agent for electroless copper baths is formaldehyde. The complexing agent (i.e., Rochelle salt) serves to complex the copper ion to prevent solution precipitation and has an effect on deposition rates as well as the quality of the deposit. These conventional baths are stable, have plating rates of 1-5/xm or 0.04-0.2 mil/hr, and operate in an alkaline solution (pH 10.0-13.0). An example of a formaldehyde-reduced electxoless copper bath is provided in Table VII. Recent formulations allow for alkanol amines such as quadrol-reduced baths. These high build [>10/xm/hr >0.4 mil/hr)] or heavy deposition baths operate at a lower pH without the use of formaldehyde. High build baths generally are more expensive and exhibit less stability but do not have harmful formaldehyde vapors given off during subsequent solution make up, heating, and deposition. These baths can deposit enough low stress copper to eliminate the need for an electrolytic flash. Quadrol is totally miscible with water and thus is resistant to many conventional waste treatment procedures.

ELECTROLESS GOLD There is a growing need in the electronics industry for selective plating to conserve plating costs and to allow the electronics engineer freedom for circuit design improvement.

Table VIII. Electroless Gold Bath Gold hydrochloride trihydrate Sodium potassium tartrate Dimethylamine borane Sodium cyanide pH (adjusted with NaOH) Temperature

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0.01 M 0.014 M 0.013 M 400.0 mg/L 13.0 60°C (140°F)

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Electroless Nickel Plating by W. Riedel 311 pages $130.00 This book is a translation of the German Edition updated through 1989. It contains 17 chapters covering all aspects from pretreatment to post-treatment including brief discussions of the market size, stripping, specifications, and waste treatment. The history and chemistry of electroless plating solution are covered in more detail as are set-up, operations, and applications; however, the heart of this volume is concerned with the structure and properties of deposits, which make up nearly one-third of this volume. A valuable reference for finishers. Send Orders to: M E T A L FINISHING 650 Avenue of the Americas, N e w York, NY 10011 For faster service, call (212) 633-3199 or FAX your order to (212) 633-3140 All book orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS to addresses in the U,S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped express to all other countries.

Table IX. Electroless Palladium Bath Palladium chloride 10 g/L Rochelle salt 19 g/L Ethylenediamine 25.6 g/L Cool solution to 20°C (68°F) and then add: Sodium hypophosphite 4.1 g/L pH (adjusted with HCI) 8.5 g/L Temperature 68-73°C(155-165°F) Many electronic components, today are difficult to gold plate by electrolytic means. Thus, electroless gold is currently being used in the fabrication of semiconductor devices, connector tabs, chips, and other metallized ceramics. Most commercially available electroless gold deposits are produced first by plating a thin deposit of immersion gold, followed by electroless gold plating. There are a few true autocatalytic gold processes available with 99.99% purity. Table VIII gives an example of an electroless gold bath. Electroless gold can successfully be applied to Kovar, nickel, nickel alloys, electroless nickel, copper, copper alloys, electroless copper, and metallized ceramics. Electroless gold can be deposited onto already present thin electrodeposited gold to give added strength.

ELECTROLESS PALLADIUM Electroless palladium deposits are ductile and ideal for contacts undergoing flexing (i.e., printed circuit board end connectors and electronic switch contacts). The deposit has also been used as a less expensive replacement for gold, providing tarnish resistance and solderability. Electroless palladium has been used to replace rhodium for wear applications. Using specific bath components, the deposit can be hard and bond to electroless nickel with a bond strength greater than the tensile strength of the palladium plate itself. Metals such as stainless steel and nickel can be plated directly. Copper, brass, and other copper alloys require an electroless nickel preplate. The electroless nickel preplate can be either from a hypophosphite- or boron-reduced bath. Table IX gives an example of an electroless palladium (hypophosphite-reduced) bath.

ELECTROLESS COBALT Thin electroless cobalt deposits have use in the electronics industry on magnetic memory discs and storage devices primarily for their magnetic properties. Table X gives an example of an electroless cobalt bath.

COMPOSITES AND POLYALLOYS The uniform dispersion of micron or submicron particles in an electroless composite deposit will enhance the lubricity and the wear and/or abrasion resistance over base substrates Table X. Electroless Cobalt Bath Cobalt chloride Sodium hypophosphite Sodium citrate Ammonium chloride pH Temperature

418

30 g/L 20 g/L 35 g/L 50 g/L 9.5 95°C (203°F)

and conventional electroless deposits. Composites containing fluorinated carbon (CFx), fluoropolymers (PTFE), natural and synthetic (polycrystalline) diamonds, ceramics, chromium carbide, silicon carbide, and aluminum oxide have been codeposited. Most commercial deposition occurs with an acid electroless nickel bath owing to the unique physical characteristics available to the final codeposit. The reducing agent used may be either a hypophosphite or boron complex. For Lamellar solids, starting materials are naturally occurring elemental forms like coke or graphite. Fluorinated carbon (CFx) is produced by reacting coke with elemental fluorine. The thermal stability of the CF Xclass of solid lubricants is higher than PTFE, allowing the CF x composite to be postbaked for maximum hardness (1,100 VHN). The CF× composite exhibits high wear resistance coupled with a low coefficient of friction. The inclusion of these finely divided particles within an electroless matrix (15-25% by volume) involves the need to maintain uniform dispersion of the occluded material during metal deposition. Specialized equipment is required andpart size, configuration, and deposit thickness are limited. Deposition rates will vary, depending upon the type of electroless bath utilized. The surface morphology of the particle used (i.e., type, size, and distribution in the matrix) will greatly influence the final codeposit properties and composition. The coefficient of friction and wear resistance of the composite are related to particle size and concentration in the electroless bath. Applications include food processing equipment, military components, molds for rubber and plastic components, fasteners, precision instrument parts, mating components, drills, gauge blocks, tape recording heads, guides for computers, and textile machine components. Due to the resultant matrix surface topography (when using diamonds or silicon carbide, for example), the final surface roughness must be considered. Special postplate surface finish operations must be employed to regain the required rms (microinch) finish. In severe abrasion applications involving high pressure foundry molding, it has been noted that the softer electroless nickel matrix wears first, exposing harder silicon carbide particles, which create poor drawability of the resin/binder from the mold. Polyalloys have been developed to produce deposits having three or four elements with specific coating properties. These include applications where unique chemical and high temperature resistance or electrical, magnetic, or nonmagnetic properties are requirements. The use of nickel-cobalt-iron-phosphorus polyalloys produce magnetic (for memory) properties. Other polyalloys include nickel-iron-phosphorus, nickel-cobalt-phosphorus, nickelphosphorus-boron, nickel-iron-boron, nickel-tungsten-phosphorus, nickel-molybdenumboron, nickel-tungsten-tin-phosphorus, and nickel-copper-phosphorus. The final selection is dependent upon the final application and the economics of achieving the results required. Electroless composites and polyalloys have made unique contributions to various engineering applications. Extensive field testing is ongoing to gain experience for proper applications, inclusions and sizes, plus proper electroless bath operating parameters for these new forms of electroless plating.

WASTE TREATMENT The electroless bath has limited life due to the formation of reaction byproducts. For example, in acid electroless nickel (hypophosphite-reduced) baths, the added accumulation or concentration of orthophosphite (HPO3 2-) in the solution will eventually decrease the plating rate and deposit quality, requiring bath disposal. Also, the chelators and stabilizers make it difficult to reduce the electroless metal content by alkaline precipitation. Regulations regarding effluent discharge vary globally and with respect to local POTW limits. In the United States, electroless metal legal discharge limits of 1 ppm or below are common for nickel and copper effluents. Conventional precipitation to form metal hydroxide or sulfide sludge through continuous or batch treatment involves a series of pH adjustment steps to convert dissolved metals into

419

solids for dewatering and hazardous disposal. Emphasis must be placed on waste minimization as the first step in reducing waste treatment. Examples include ion exchange, reverse osmosis, and electrowinning or electrolytic recovery, which electroplates the spent bath into nickel or copper metal onto special cathodes helping to reduce the amount of sulfide or hydroxide hazardous sludge eventually created. The resultant plated metal produced can be reclaimed as scrap metal. Other waste minimization methods include using steel wool to plate out the electroless bath prior to further waste treatment.

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