By Matt Sisti and Dr. Alan Ruffini
Sirius Technology, Inc.
Oriskany, NY

Electroless Nickel (EN) deposits are classi-fied as a “Functional Class” of coatings used primarily to enhance the surface performance properties of a variety of substrates. The majority of applications require that the coating provide maximum protection against corrosion and abrasive wear resulting in an extension in the useful life of the component. Because of the unique deposit properties and uniformity of the resultant film, many other engineered applications have emerged within the electronics industry. Methods to improve solderability, enhance thermomagnetic stability and effectively replace Hot Air Solder Leveling (HASL) with electroless nickel/immersion gold on printed circuit boards have been well documented in the literature.

The majority of EN films used commercially are deposited from solutions formulated with sodium hypophosphite as the reducing agent. This results in nickel films that are alloyed with phosphorus in ranges between 1–12 weight percent.

The mechanical properties of NiP (ENP) deposits can be further enhanced not only by the codeposition of inert particles such as Teflon®, silicon carbide or boron nitride but also by alloying with a third element, forming a ternary alloy of NiPX, where X can be copper, tungsten, molybdenum or tin depending on the particular formulation.

Electroless Nickel Boron (NiB) alloys are also well cited in the literature, although they are less commercially viable than the NiP alloys. The films are generated using either sodium borohydride or dimethylaminoborane as the reducing agent and can range in boron content from 1–5 weight percent. NiB films are typically used in the electronics industry where low resistivity coatings are required and also find use in industrial applications when extreme wear and increased coating hardness is specified.

Optimum performance of the mechanical and physical properties of the EN films will result only if the coating is free of microdefects. It is well documented that deposit phosphorus content plays an important role in determining how well the coating will perform. Equally important is the presence of these microdefects including micropores, nodules, ductility and intrinsic stress. All can contribute to premature performance failure and do not necessarily correlate to the phosphorus content of the bulk deposit. Interestingly, they appear to be associated more with the deposition mechanism itself, primarily the diffusion characteristics of the plating solution and the related catalytic activity of the growing film during the plating process. Ultimately, this has been found to have the greatest impact on composition, microstructure and performance of the resultant film.

The Autocatalytic Deposition Process
The continuous deposition process for chemical reduction of nickel ions in an aqueous solution, using a reducing agent, is termed autocatalytic. The most common reducing agent is sodium hypophosphite and for the purpose of brevity will be referenced throughout this section. The plating process is heterogeneous since the reactants, namely nickel ions and sodium hypophosphite are evenly distributed throughout the plating solution. Since the reaction does not occur in the bulk of the plating solution but rather at the interface of the solution and the catalytic surface, two conditions must be fulfilled for the reaction to occur:

First, the interface must be a catalytically active site for the oxidation of hypophosphite to occur, readily adsorbing the hydrogen produced and secondly, the reactants must easily migrate to this site for the reaction to continue and self propagate.

The requisite migration of the dissolved ions to the catalytic surface can occur either by diffusion or solution agitation (convection). Ions are charged species that are constantly moving throughout the plating solution to maintain an even distribution. However, because charged species tend to adhere to stationary solid surfaces and there is a depletion of ions at the plating interface a concentration gradient is established and diffusion will take over. This part of the solution is called the diffusion zone and is defined as follows:

• Diffusion of reactants to the catalytic surface.
• Adsorption of the reactants onto the catalytic surface.
• Reaction on the catalytic surface
• Desorption of by-products from the catalytic surface.
• Diffusion of by-products away from the catalytic surface.

An important point to note is that all species do not diffuse linearly to the catalytic surface. Additives such as heavy metal stabilizers, divalent sulfur compounds, heavy metal brightners, usually present in ppm quantities, diffuse non-linearly to the catalytic surface. Controlling both the concentration and type of additive used in the formulation of the chemistry is a critical component in optimizing the performance of the plated ENP film.

Figure 1: Deposit uniformity. Electroplated nickel is on the left and EN on right and the deposit uniformity of EN is superior

Key Physical and Mechanical Properties

The Microstructure and Composition of ENP Films
One of the distinct advantages of the electroless nickel deposition process is the ability to produce an alloy of nickel and phosphorus in varying composition. Depending on the formulation and the operation of the chemistry the film compositions can vary from 2–13 weight percent phosphorus. This variation in alloy content has a significant effect on the deposit microstructure and performance characteristics and offers flexibility to well informed platers and engineers who can take full advantage of these differences.

Electrodeposited nickel has a purity of greater than 99% and is highly crystalline. On the other hand, electroless nickel deposits that contain more than 10.5% phosphorus appear to be amorphous, i.e., lacking crystal structure. EN deposits with less than seven weight percent phosphorus have a clear microcrystalline structure (2–6 nm grain size) and film properties are distinctly different. Some studies have found that higher phosphorus deposits (above 10.5 weight percent phosphorus) may not be truly amorphous but rather a mixture of microcrystalline and amorphous phases.

The degree of amorphous character can be altered for a given high phosphorus formulation by the addition of additives that affect the growth process of the film. Deposits with a high degree of amorphous composition are free of grain boundaries, which typically act as sites for intergranular corrosion commonly encountered in crystalline deposits.

Deposit Uniformity
A significant advantage of the electroless nickel process is the ability to produce deposits with uniform thickness on parts with complex geometries and shapes. Since this is a chemical reaction, any catalytic surface exposed to the plating solution will plate uniformly, provided it meets the criteria established a few paragraphs earlier. The current density effects typically associated with electroplating are not a factor, therefore sharp edges, deep recesses and blind holes are readily plated to uniform thickness with electroless nickel chemistry. Many applications for electroless nickel exist today because it is often the only way to plate certain components. The difference in deposit uniformity is illustrated in Figure 1.

Link To Graphic Figure 2: Effect of EN deposit composition on electrical resistivity. Figure 3: Effect of EN deposit composition on magnetic properties

Surprisingly, the degree of uniformity can vary on edges, threads, small holes or deep recesses where exchange of fresh solution may be difficult. This can also occur under conditions with excessive bath agitation, especially in the presence of heavy metals. This thickness variation may be controlled by optimizing solution dynamics and/or by controlling the concentration of certain additives formulated into the EN plating bath.

Melting Point
Unlike electrolytically deposited nickel, electroless nickel deposits do not have a precise melting point but rather have a melting range. Pure nickel has a melting point of 1,455°C, however as the phosphorus content is increased within the film, the deposit begins to soften at lower temperatures and continues to soften until it eventually melts. The melting range decreases linearly as the phosphorus levels increase. The eutectic or lowest melting point for NiP alloys is 880°C and occurs at a deposit phosphorus content of 11% by weight.

Electrical Resistivity
The electrical resistivity of electroless nickel alloys is higher than that of pure nickel. High-purity nickel has a specific resistivity of 7.8 × 10-6 ohm-cm. Increasing the phosphorus content increases the electrical resistivity of the film. (Figure 3.) Values range from 30-100 × 10-6 ohm-cm. Heat treatment of the ENP film can affect resistivity. At temperatures as low as 150°C, resistivity will decrease due to release of physically absorbed hydrogen. At temperatures greater than 250°C, a similar marked decrease will occur as a result of phosphorus migration and the structural transformation to nickel phosphide.

Magnetic Properties
The single largest application for electroless nickel is as a sub layer for computer memory discs. To meet the requirements for this application the coating must remain non-magnetic even after one-hour bake cycles of 250–320°C. This can be achieved only with high phosphorus alloys (> 10.5% P). The non-magnetic property of these high phosphorus films is the most important physical characteristic. Figure 4 depicts the effect phosphorus content has on ferromagnetic properties.

Link To Graphic Figure 4: Effect of EN deposit composition on corrosion resistance in 45% sodium hydroxide at 40°C. Figure 5: Effect of EN deposit composition on corrosion resistance in 75% phosphoric acid at 40°C.

Not all high-phosphorus alloys will maintain the same level of thermomagnetic stability. The deposit performance is also dependent on the chemical formulation and the solution dynamics during the plating process. Reducing the onset and the rate of crystallization, minimizing the volume fraction of microcrystallinity and maintaining a homogeneous grain structure at higher bake temperatures are key requirements for optimization of the films thermomagnetic performance.

Corrosion Resistance
The most widespread application of electroless nickel technology is to provide superior corrosion protection in a multitude of corrosive environments. Since the ENP coating is more noble than steel and aluminum, it protects the substrate by providing a pore free barrier coating against the corrosive environment.

The nature of the corrosive environment and the deposits and resistance to chemical attack are also important criteria when selecting a particular EN coating for optimum performance. High-phosphorus coatings (10-12%) are more readily attacked in strongly alkaline media than low-phosphorus deposits (1-3%), however they perform significantly better in acidic media. This is illustrated in the Figures 4 and 5.

Optimizing the performance of EN coatings for maximum corrosion protection requires that the coating be continuous and free of any microporosity, roughness, nodules and inhomogenieties within the microstructures. Since the microstructure of high-phosphorus EN films are amorphous they are essentially free of the grain boundaries that could potentially serve as sites for corrosion. The absence of such phase boundaries and the ability of the high phosphorus EN film to form a passive film on its surface, make it an excellent choice for protection on aluminum and steel substrates in highly corrosive, acidic environments.

The phosphorus content alone however is not enough to maximize the coatings' corrosion resistance in a given environment. Factors that will affect the corrosion performance of ENP films are:

1. The distribution of phosphorus throughout the coating
2. The volume fraction of micro-crystallinity within the film
3. Maintaining a uniform grain structure, minimizing the formation of phase boundaries and co-deposited impurities
4. Avoiding high temperature post treatment that can produce micro-cracked deposits
5. Substrate affects such as alloying constituents surface roughness and forming operations
6. Selecting the optimum pretreatment for the substrate type
7. Understanding the nature of the corrosive environment and
8. Operation and maintenance of the solution during the plating process.

It is important to note the condition of the substrate prior to plating in order to determine the optimum pretreatment cycle and required deposit thickness. For example, machined substrates alloyed with sulfur and/or lead are particularly vulnerable to a high degree of porosity, since these alloying constituents are catalytic poisons to the deposition process. Substrates with high roughness values require thicker deposits to provide adequate corrosion protection. This is primarily due to the formation of nodules on the roughened surfaces during the initial stages of deposition, resulting in microporosity of the ENP film.

Increasing the deposit thickness can minimize this effect. Smooth substrates require a thickness of 25 microns to obtain a film with minimal porosity whereas roughened surfaces can require up to three times the thickness to provide similar corrosion protection.

The use of neutral salt spray, in accordance with the ASTM B 117 specification, for the measurement of corrosion performance is well documented in the literature and is still the most widely accepted method for coating evaluation. The results of neutral salt spray exposure, phosphorus content and deposit thickness are summarized in Figure 6.

Deposit Hardness
Deposit hardness is one of the key tribological properties of ENP coating technology. Factors that affect hardness are the film composition (%P), the heat treatment temperature and the heat treatment time. Typical microhardness values for the as plated ENP deposits range from 500– 720 VHN (Figure 7). This is in contrast to electrolytically deposited nickel, which has typical values of 150-400VHN.

Heat treatment of ENP deposits will significantly increase the microhardness. Figure 8 illustrates the effective temperature range and time required to attain specific values.

Link To Graphic Figure 7: Effect of EN deposit composition on plated microhardness. Figure 8: Effect of different heat treatment periods on hardness of a high phosphorus EN deposit.

In cases where the substrate cannot withstand the temperature requirements to achieve maximum hardness, low phosphorus EN is often recommended. The increase in microhardness for ENP films is attributed to the phase transformations that take place during the heat cycle to form nickel metal and nickel phosphide. Some volume shrinkage and cracking of the film will occur and must be taken into account if the application requires corrosion protection.

Wear Resistance
Electroless nickel phosphorus coatings are specified for a wide variety of engineered applications related to wear resistance. This is mainly attributed to the fact that the coatings not only have high hardness and intrinsic lubricity but also afford excellent corrosion resistance and deposit uniformity. The wear properties of ENP films can also be enhanced through the codeposition of inert particles such as PTFE, silicon carbide or boron nitride, forming a composite coating.

Figure 9: Taber wear resistance of various coatings.

The abrasive wear of ENP films is typically measured by applying a mechanical action of an abrasive, rotating wheel on the plated surface and measuring the weight loss (in milligrams) of the coating at intervals of 1,000 cycles. This test is referred to as the Taber Wear Index (TWI) and is perhaps the most frequently used procedure to evaluate the wear characteristics of ENP films. Typical weight loss results for various deposits are illustrated in Figure 9.

The “as plated” low phosphorus deposits tend to withstand abrasive wear better than the high phosphorus coatings, largely attributed to the higher hardness values. However, after heat treatment the trends remain the same even though the deposit hardness is similar, suggesting that other factors, such as deposit composition, play a role in abrasive wear resistance.

Adhesive wear is defined as the removal of material between mating surfaces and measures the films' ability to resist galling, welding or seizing. If the mating surfaces are both electroless nickel, optimum performance is achieved if the surfaces in contact have dissimilar properties, such as phosphorus content or hardness. The adhesive wear resistance of ENP films improves with increasing phosphorus content.

It is important to consider as many variables as possible when evaluating an ENP coating for tribological applications. Factors such as hardness, phosphorus content, the presence of corrosive liqiuids or gases, the temperature, the nature of the mating surface, the degree of lubrication and the characteristics of the substrate are critical to optimum performance.

Applications
Electroless nickel gained market acceptance over the past 50 years through a combination of trial and errors, excellent marketing initiatives and commitment from many plating shop pioneers willing to take on a fledgling technology. Today, electroless nickel offers the engineering community a technology that is as reliable as it is diverse in meeting various application challenges. An approximate breakdown of worldwide applications for electroless nickel is detailed on the previous page.

Automotive

Figure 10. differential pinion shafts, EN plated

Applications for use in this industry take full advantage of the many benefits electroless nickel has to offer. Deposit properties such as uniformity, corrosion resistance, lubricity and wear resistance are all reasons why the use of EN for automotive applications continues to grow. Historically, a cost sensitive market segment, auto maker, have embraced the use low cost materials plated with a thin film of electroless nickel to meet strict requirements in an economical fashion. The well documented use of electroless nickel on zinc die cast carburetors in Brazil is an example of this. Fuel injection systems, aluminum fuel filters and bleed valves are a few other applications that take full advantage of the deposit’s corrosion resistance.

Differential pinion shafts (Figure 10) and a variety of pins and washers are plated in large quantities due to the coatings lubricity, wear resistance and anti-galling properties. Brake pistons are plated in bright, medium phosphorus electroless nickel and heat treated to increase hardness and wear resistance. High deposit luster is desired and actually improves the surface finish of the piston resulting in reduced friction (Figure 11). Cast iron slip yokes are plated with EN to eliminate noise associated with galling.

Figure 11. High-luster reduces friction

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Future applications will most likely include the use of EN composites such as PTFE, boron nitride and silicon carbide. The use and growth of fuel cell technology in automobiles shows promise and electroless nickel could play a role. A challenge EN has faced recently is the End-of Life Vehicle (ELV) directive that bans the use of various toxic metals often found in EN deposits. Recent technological advances appear to have overcome this.

Aerospace
Design engineers have found the combination of electroless nickel’s functional properties to be very appealing for aerospace use. Due to the obvious reliability issues associated with this market segment, a long term, thorough evaluation of electroless nickel has been ongoing and has made for slow progress. Successful testing and application for a number of years resulted in a better understanding of when and how to specify electroless nickel. Its use on engine assemblies, servo valves, landing gear, turbine blades and the like have found widespread use. Unlike hard chrome, compressive stressed high phosphorus deposits do not significantly reduce the fatigue strength of these critical components. For this reason and reasons stated above, its continued application appears secure.

Electronics
Use of electroless nickel for electronic applications continues to grow and is clearly the most diverse market segment. Magnetic properties, corrosion resistance and solderability are properties that have contributed the most to its success. A large application involves the use of high phosphorus deposits on polished aluminum substrates for magnetic data storage on computers (Figure 12). Uniformity, non-magnetic character and reliably smooth, defect free nature lend to its continued use. Over the past 15 years this application for EN has been threatened a number of times by emerging technology. In each case, EN has proven up to the challenge and maintained its position as the most reliable and cost effective technology.

Heat sinks, semiconductor packages, and battery components are examples of other high-volume electroless nickel electronic applications. A variety of aluminum and zinc die cast connectors are plated with EN to enhance corrosion and wear resistance. Uniformity, electrical conductivity and solderability are other important properties for this application. Typical electronic parts are shown in Figure 13.

An area of continued interest and growth is the plating of microwave components. For complex shapes and deep recessed areas electroless nickel is ideally suited to act as a corrosion resistant barrier film on top of aluminum and beneath the silver or tin electroplated topcoat.

Although certain technical barriers must be overcome, the use of medium phosphorus electroless nickel beneath immersion gold on circuit boards shows promise. Commonly referred to as the ENIG process, it offers improved shelf life in humid conditions and maintains excellent solderability over periods of long storage.

Chemical Processing
Requirements for this application often include the need to maintain product purity in addition to the typical needs for uniformity and corrosion resistance. Selecting the proper electroless nickel is often critical to successful application. Recent studies have found phosphorus content plays an important role in the overall performance of the EN deposit in a specific environment. Pumps, valves and flanges are typically plated with 50–100 microns of electroless nickel for very severe applications.

Oil and Gas Industry
A proving ground for electroless nickel over the past 25 years, the success of electroless nickel for these applications is well documented. Ball valves, heat exchangers, pumps, etc. fabricated from less expensive materials and plated with high phosphorus EN has greatly contributed to the success. Corrosion resistance in harsh environments and resistance to erosive wear as well as its uniformity will allow EN to maintain its dominant position in this market segment.

Other Applications
Food handling, mold protection, foundry tooling, plating on non-conductors and the printing industry are a few other applications where electroless nickel finds significant use. Textile applications take advantage of excellent wear resistance and lubricity characteristics. Composite coatings such as EN/boron nitride have found increased use in applications that require extending service life. Due to its chameleon-like nature, with properties and performance characteristics that can be selected or easily fine tuned, electroless nickel is poised to maintain its lofty position as a reliable engineered finish.

Specifications
An important factor in the success of electroless nickel today is the effort put into developing reliable specifications. Regardless of the source, be it a MIL spec, ASTM, AMS or internal, the purpose of these specifications is to provide a consistent and dependable method to apply and test electroless nickel deposits.

Specifications typically contain:

Type or types of EN deposit
Applicable Specs
Coating Classifications
Service Conditions
Heat Treatment
Sampling/Testing

The most widely used specification is ASTM B733. It is an excellent specification that fills many of the voids other specs left vacant. Others in use include AMS 2404, AMS 2405 and MIL-C-26074. The latter spec was retired but reinstated by the Department of the Navy in1998 and may now be used for procurement purposes by the government.

References
Reidel, W., “Electroless Nickel Plating,” 1991, Finishing Publications Ltd., U.K. and ASM International, Metals Park, Ohio, U.S.A.
Parkinson, R., “Properties and Applications of Electroless Nickel,”1997, Nickel Development Institute.
Gawrilov, G.G., “Chemical Nickel Plating,” 1979, Porticullis Press, Redhill, England.
Bayes, M. “The Physical Properties of Electroless Nickel Coatings”, 1995, Proceedings EN 95 Conference, Cincinnati, Ohio.
Ruffini, A.J. and Weil, K. “A Mechanistic Approach Toward Improving the Performance Characteristics of Electroless Nickel Phosphorus Films,” 1997, Proceedings EN 97 Conference, Cincinnati, Ohio.
Totlani, M.K. “Corrosion Control with Electroless Nickel Coatings,” Vol.1, No.3, 1992, Transactions of the Metal Finishers Association of India.
Tomlinson, W.J. and Mayor, J.P. “Formation, Microstructure, Surface Roughness, and Porosity of Electroless Nickel Coatings,” Vol.4, No.3, 1988, Surface Engineering.
Tracy, Robert and Colaruotolo, Dr. J, “Corrosion Performance and Economics of EN Coatings in Chemical Process Environments,” Corrosion 1986, Paper#22.