Electroless nickel plating

Electroless nickel plating (EN) is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metal or plastic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO₂H₂·H₂O) which reacts with the metal ions to deposit metal. The alloys with different percentage of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of alloys depend on the percentage of phosphorus.

Overview

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Electroless nickel plated parts

Electroless nickel plating is an auto-catalytic reaction used to deposit a coating of nickel on a substrate. Unlike electroplating, it is not necessary to pass an electric current through the solution to form a deposit. This plating technique is to prevent corrosion and wear. EN techniques can also be used to manufacture composite coatings by suspending powder in the bath. Electroless nickel plating has several advantages versus electroplating. Free from flux-density and power supply issues, it provides an even deposit regardless of workpiece geometry, and with the proper pre-plate catalyst, can deposit on non-conductive surfaces.

Historical overview

The EN plating of metallic nickel from aqueous solution in the presence of hypophosphite was first noted as a chemical accident by Wurtz in 1844. In 1911, Roux reported that metal was inevitably precipitated in the powder form; however these works were not in practical applications. In its early stage, progress in the field remained slow until World War II. In 1946, Brenner and Riddell developed a process for plating the inner walls of tubes with nickel-tungsten alloy, derived from the citrate based bath using an insoluble anode, which brought out the unusual reducing properties of hypophosphite. The U.S. Patent Office says that the patent it issued in 1950 differed from the earlier patent in that Roux reaction was spontaneous and complete, while the Brenner and Riddell process was a controlled catalytic process so that deposition occurred only on catalytic surfaces immersed in the bath. Brenner later wrote that his patent was an accidental discovery similar to the work of Wurtz and Roux, but said that he took out a patent to protect U.S. government rights. In fact, a declassified U.S. Army technical report written in 1963 goes on extensively about Wurtz and Roux work, and gives more of the discovery credit to them than to Brenner. This plating process was attributed to the action of chemical reduction of Ni ions. During the 1954-59 period, Gutzeit at GATC (General American Transportation Corporation) worked on full scale development of electroless plating by chemical reduction alone, as an alternate process to conventional electroplating. Initially, the co-deposition of particles was carried out for electrodepositing Ni-Cr by Odekerken, during the year of 1966. In that study, in an intermediate layer, finely powdered particles like aluminum oxide, polyvinyl chloride (PVC) resin were distributed within a metallic matrix. A layer in the coating is composite but other parts of the coating are not. The first commercial application of their work used the electroless Ni-SiC coatings on the wankel internal combustion engine and another commercial composite incorporating polytetrafluoroethylene (Ni-P-PTFE) was co-deposited, during the year of 1981. However, the co-deposition of diamond and PTFE particles was more difficult, than the composites incorporating Al2O3 or SiC. The feasibility to incorporate the fine second phase particles, in submicron to nano size, within a metal/alloy matrix has initiated a new generation of composite coatings.

Pretreatment

Before performing electroless nickel plating, the material to be plated must be cleaned by a series of chemicals, this is known as the pre-treatment process. Failure to remove unwanted "soils" from the part's surface result in poor plating. Each pre-treatment chemical must be followed by water rinsing (normally two to three times) to remove chemicals that may adhere to the surface. De-greasing removes oils from surfaces, whereas acid cleaning removes scaling.

Activation is done with a weak acid etch, or nickel strike or, in the case of non-metallic substrate, a proprietary solution. After the plating process, plated materials must be finished with an anti-oxidation or anti-tarnish chemical such as trisodium phosphate or chromate, followed by water rinsing to prevent staining. The rinse object must then be completely dried or baked to obtain the full hardness of the plating film.

The pre-treatment required for the deposition of nickel and cobalt on a non-conductive surface usually consists of an initial surface preparation to render the substrate hydrophilic. Following this initial step, the surface is activated by a solution of a noble metal, e.g., palladium chloride. Silver nitrate is also used for activating ABS and other plastics. Electroless bath formation varies with the activator. The substrate is now ready for nickel deposition.

Advantages and disadvantages

Advantages include:

1. Does not use electrical power.
2. Even coating on parts surface can be achieved.
3. No sophisticated jigs or racks are required.
4. There is flexibility in plating volume and thickness.
5. The process can plate recesses and blind holes with stable thickness.
6. Chemical replenishment can be monitored automatically.
7. Complex filtration method is not required
8. Matte, Semi Bright or Bright finishes can be obtained.

Disadvantages include:

1. Lifespan of chemicals is limited.
2. Waste treatment cost is high due to the speedy chemical renewal.

Each type of electroless nickel also has particular advantages depending on the application and type of nickel alloy.

Types

Low phosphorus electroless nickel

Low phosphorus treatment is applied for deposits with hardness up to 60 Rockwell C. This type offers a very uniform thickness inside complex configurations as well as outside, which often eliminates grinding after plating. It is also excellent for corrosion resistance in alkaline environments.

Medium phosphorus electroless nickel

Medium phosphorus electroless nickel(MPEN) is referred to the nickel-phosphorus alloy deposited by electroless/autocatalytic process in which the resulting alloy consists of medium levels of phosphorus; the definition of medium levels is different in sources of different branches of technology(decorative, industrial, ...). the range accepted as medium levels can be (percent by weight) 4-7 (decorative purpose), 6-9 (industrial sources), or 4-10 (Electronic applications). The EN bath is typically composed of (a.) Nickel source [nickel sulfate ], (b.) Reducing agent [sodium hypophosphite], (c.) Complexing agent; which are necessary to increase phosphite solubility and also to slow down the reaction speed in order to prevent the white-out phenomena but are not co-deposited into the resulting alloy [carboxylic acids or amines], (d.) Stabilizers; which slow down the reduction by co-deposition with the nickel [lead or sulphur or organics], (e.)Buffers [most complexing agents perform double duty as buffers], (f.) Brighteners; mostly co-deposited with nickel and usually can double as stabilizers [cadmium or certain organics], (g.) Surfactants; which reduce surface tension in order reduce pitting and staining [SLS or almost any other surfactant] and (h.) Accelerators; which are added overcome the slow plating rate imparted by complexing agents and usually are co-deposited and can cause discoloration of the deposit [sulfur compounds]. Medium phosphorus treatment has a high speed deposit rate and offers bright and semi-bright options for cosmetic particularization. The processing is very stable, used often for Slurry Disposal Industries. This is the most common type of electroless nickel applied.

High phosphorus electroless nickel

High Phosphorus electroless nickel offers high corrosion resistance, making it ideal for industry standards requiring protection from highly corrosive acidic environments such as oil drilling and coal mining. With microhardness ranging up to 600 VPN, this type ensures very little surface porosity where pit-free plating is required and is not prone to staining. Deposits are non-magnetic when phosphorus content is greater than 11.2%.

Applications

form of electroless nickel plating produces a nickel phosphorus alloy coating. The phosphorus content in electroless nickel coatings can range from 2% to 13%. It is commonly used in engineering coating applications where wear resistance, hardness and corrosion protection are required. Applications include oil field valves, rotors, drive shafts, paper handling equipment, fuel rails, optical surfaces for diamond turning, door knobs, kitchen utensils, bathroom fixtures, electrical/mechanical tools and office equipment.  It is also commonly used as a coating in electronics printed circuit board manufacturing, typically with an overlay of gold to prevent corrosion.  This process is known as electroless nickel immersion gold.

Due to the high hardness of the coating it can be used to salvage worn parts. Coatings of 25 to 100 micrometres can be applied and machined back to final dimensions. Its uniform deposition profile mean it can be applied to complex components not readily suited to other hard wearing coatings like hard chromium.

It is also used extensively in the manufacture of hard disk drives, as a way of providing an atomically smooth coating to the aluminium disks, the magnetic layers are then deposited on top of this film, usually by sputtering and finishing with protective carbon and lubrication layers; these final two layers protect the underlying magnetic layer (media layer) from damage should the read / write head lose its cushion of air and contact the surface.

Its use in the automotive industry for wear resistance has increased significantly, however it is important to recognise that only End of Life Vehicles Directive or RoHS compliant process types (free from heavy metal stabilizers) may be used for these applications.

Standards

• AMS-2404
• AMS-C-26074
• ASTM B-733
• ASTM-B-656
• MIL-DTL-32119

See also

• Electroplating
• Nickel electroplating
• Nucleation - (Electroless plating solutions are unstable, but the difficulty of nucleation means they will only act on a metal surface.)
• Electroless nickel