By definition, “electroless plating” is metal deposition by a controlled chemical reaction. In contrast to an electroplating solution, electroless nickel (EN) solutions require no external source of current to plate. EN baths utilize a chemical reducing agent built into the bath. The process provides a continuous build-up of deposit, since the metal being plated is itself a catalyst for the plating reaction. This is why electroless nickel is also known as autocatalytic nickel plating.

Electroless Nickel Family
There is a large family of EN coatings, typically defined by their alloy. They all share several properties, for example, a high degree of deposit uniformity regardless of part geometry. Due to the unique combination of deposit properties, applications for electroless nickel can be found in virtually every industry:

• Excellent corrosion protection
• Superior wear resistance
• Uniform deposit regardless of part geometry
• Hard deposits as plated, which can be heat treated to increase hardness
• Plates on catalyzed non-conductors, such as plastic
• Solderable deposit
• Can change the magnetic properties of a part
• Diffusion barrier
• Ideal for salvage of worn or mis-machined parts.

Figure 1 shows the primary uses for EN parts by property.

Nickel Phosphorus. The majority of electroless nickel plating is done using electroless nickel phosphorus (Ni-P) systems. These deposits give a low coefficient of friction and are anti-galling. They have superior as-plated hardness, and the deposits can be further hardened by post-plating heat treatment processes. These deposits have excellent corrosion performance in many types of environments.

Nickel boron alloys are widely used in electronic and aerospace applications. The deposits provide high electrical conductivity, low contact resistance, excellent as-plated hardness, a high melting range, and outstanding wear resistance, and are easily soldered or brazed.

Composite Coatings. EN coatings can contain co-deposited soft particles, such as polytetrafluoroethane (PTFE), or hard particles, for example silicon carbide. EN/PTFE deposits provide a coating with a very low coefficient of friction. Co-deposited hard particles provide improved wear resistance.

Ternary Alloy Coatings. Also called poly-alloys, these deposits contain more than two elements. An example is nickel phosphorous tungsten, which provides a very hard coating.

Electroless Nickel Bath Chemistry
Nickel deposition by hypophosphite is usually represented by the following reactions:

1. NiSO₄ + H₂ O –› Ni² + SO₄ ² - + H₂ O
2. NaH₂ PO₂ + H₂ O –› Na+ + H₂ PO₂ - + H₂ O
3. Ni² + + H₂ PO₂ - + H₂ O –› Ni⁰ + H₂ PO₃ - + 2H+
4. H₂ PO₂ - + H₂ O (CATALYST)–› H₂ PO₃ - + H₂

Typically, an electroless nickel plating bath consists of a source of soluble nickel ions, a reducing agent, complexors, neutralizers/buffers, stabilizers, and (in some systems) brighteners.

Figure 1. Primary uses for EN deposits.

Nickel Source. In EN plating, the metal source is a soluble nickel salt. The choice of what salt to use is based on solubility, purity, compatibility, and price. Nickel sulfate is the most widely used nickel salt, but processes using nickel chloride, nickel sulfamate, nickel acetate, and nickel hypophosphite are commercially available.

The reducing agent replaces the rectifier used in electroplating. The most widely used reducing agents are sodium hypophosphite [NaH2PO2∙H2O], sodium borohydride [NaBH4], and dimethylamine borane [CH3NHBH3].

Complexing agents keep the nickel in a stable complex until needed for plating. The choice of complexors determines the deposit alloy, and thus the performance characteristics. For example, in hypophosphite-reduced systems stronger complexors are used in high-phosphorus systems, while weaker nickel complexes favor lower-phosphorus deposits.

Neutralizers/Buffers. While plating, an EN bath will generate hydrogen, both as a gas and as ionic hydrogen. This will lower the pH of the solution. Buffers can be used to minimize pH swings in the plating bath, but in order to maintain the process at the correct pH the excess acidity must be neutralized. Typical neutralizers are ammonium hydroxide, carbonates, or sodium or potassium hydroxide.

In North America, ammonium hydroxide is currently the most commonly used neutralizer. However, due to its objectionable smell and its negative impact on waste treatment, more EN applicators are switching to alternatives. Currently methodologies and proprietary chemical additives exist which allow EN platers to utilize sodium hydroxide as a viable alternative for pH adjustment.

Stabilizers control the plating reaction. Without these catalytic poisons, the reaction may be uncontrolled. Stabilizers are divided into two basic categories, metallic and organic. Historically, lead compounds have been frequently used as metallic stabilizers.

Brighteners. Many EN systems use a brightener to enhance deposit appearance. Brighteners can be either metallic or organic compounds. Cadmium compounds are commonly used.

Figure 2. Taber wear of EN (CS-10 wheel, 1000 gram load).

Environmental Drivers
Recently there has been a major shift in electroless nickel technology, due primarily to European environmental legislation. Among the most influential legislation are the RoHS, WEEE, and ELV directives. The Restriction of Hazardous Substances (RoHS, 2002/95/EC) and the Waste Electrical and Electronic Equipment (WEEE, 2002/96/EC) directives are intended to promote the reuse, recycling and recovery of electrical components. The European End of Life Vehicle Directive (ELV, 2000/53/EC) is aimed at waste minimization through recycling, thereby eliminating hazardous waste from landfills.

The RoHS directive limits the maximum concentration value for lead, mercury, hexavalent chromium, polybrominated biphenyl or polybrominated diphenyl ether to 0.1% by weight (1,000 ppm), and cadmium to 0.01% by weight, in homogeneous materials. A plated layer is considered a homogeneous material.

Most existing lead-stabilized, hypophosphite-reduced electroless nickel systems should provide a deposit containing less than 0.1% by weight (1,000 ppm) lead. Deposits from conventional cadmium-brightened systems would almost certainly contain more than 0.01% (100 ppm) of cadmium. In all probability, existing lead-stabilized, hypophosphite-reduced systems which do not contain cadmium are RoHS- and ELV-compliant, while cadmium-brightened processes would likely be noncompliant. While many existing lead-stabilized baths that do not contain cadmium are probably compliant, it is very possible that future legislation and/or OEM specifications will mandate the total elimination of lead and cadmium from electroless nickel deposits. Virtually all major electroless nickel suppliers offer chemistries containing no lead or cadmium.

Deposit Properties
Since electroless nickel is a broad family of coatings, the deposit characteristics will vary according to process. Some deposit attributes of different processes are shown in Table II.

Corrosion Protection. An EN deposit normally protects the substrate by acting as a barrier coating, and offers excellent corrosion protection provided there is total encapsulation of the substrate.

As with any barrier coating, the protection EN offers depends on initial surface quality. Smooth, pore-free surfaces tend to perform better than rougher, more porous substrates. Assigning absolute figures for the corrosion protection of electroless nickel deposits can be misleading, since most problems can typically be traced back to substrate porosity or improper pretreatment resulting in deposit porosity. In most applications high-phosphorus deposits generally provide best corrosion protection.

Post-plating operations also can impact corrosion performance. High-temperature heat treatments for hardness (>400°C) can crack the coating, compromising the barrier coating and lowering its effectiveness. Conversely, adding a supplemental coating, such as oils, waxes, and lacquers can help. On certain substrates, such as aluminum, the corrosion performance can be enhanced by using a post-plate passivation treatment.

Corrosion resistance of a coating can be described as how well the deposit resists attack. Electroless nickel coatings offer good corrosion resistance in many harsh environments. In most environments, high-phosphorus deposits provide the best resistance to chemical attack, but low-phosphorous deposits often show advantages in alkaline environments.

Wear Resistance. Electroless nickel coatings provide excellent resistance to most types of wear, both in the as-plated condition as well as at maximum hardness after heat treating. Figure 2 compare various types of EN in Taber Abraser tests.

Microhardness. Hardness values for EN should be measured in Knoop or Vickers, since surface readings such as Rockwell (RC) are not accurate. Typically the electroless nickel deposit is too thin for reliable surface testing, resulting in the reading being influenced by the substrate. Lower-phosphorus deposits are typically harder than higher-phosphorus deposits as-plated. Deposits can be heat treated to improve hardness, both by creating a more crystalline nickel deposit and by forming nickel phosphide in the deposit.

Deposit Properties by Phosphorus Content. The properties of electroless nickel phosphorus deposits vary based on the percentage of phosphorus. Below are some general guidelines of the effect of phosphorus content in the deposit on physical properties:

• Hardness increases as % P decreases
• Wear resistance generally increases as % P decreases
• Electrical conductivity increases as % P decreases
• Solderability increases as % P decreases
• Melting range increases as % P decreases
• Corrosion resistance generally decreases as % P decreases
• Corrosion protection typically decreases slightly as % P decreases
• Ductility is generally highest at less than 2% P and at greater than 10% P
• Deposits become more magnetic as % P decreases.

Control of EN Baths
Since EN relies on a chemical reduction reaction, control of the process is critical in obtaining optimum results. A typical EN bath is more sensitive to operating conditions than an electroplating bath, and care must be taken to control the process within relatively tight parameters to achieve the best
performance.

Bath age is typically tracked by metal turnovers (MTOs). In a 1-liter plating bath operating at 6 grams per liter (g/l) of nickel metal, one MTO occurs for each 6 g of nickel added back to the system. As the plating reaction proceeds, byproducts will form and will eventually degrade the performance of the plating solution and the deposit. In a sodium hypophosphite-reduced bath using nickel sulfate, the byproducts include sulfate, sodium, and orthophosphite. About 45 to 60 g/l of reaction byproducts are formed every MTO. Depending on criteria, EN baths will normally last from 4 to 10 MTOs before their performance degrades beyond acceptable limits and the bath must be discarded. Bath age can be determined by maintaining accurate replenishment records, analyzing orthophosphite concentration, or measuring bath specific gravity.

Extending Bath Life. There are methods available to extend the useful bath life. One method entails reducing the amount of byproducts generated by using an alternate nickel source, such as nickel hypophosphite or nickel acetate, both of which would eliminate sulfate and dramatically reduce the amount of sodium generated. These systems work very well, but the downside is the higher cost of the nickel salt. There are also purification methods available, such as “bleed & feed”, precipitation, and
electrodialysis.

Several years ago a new method to extend bath life was developed. A typical hypophosphite-reduced EN bath running at 6 g/l of nickel would contain about 120 g/l of dissolved solids at make-up, depending on the bath formula. Each MTO of operation adds around 45 to 60 g/l of dissolved solids. Reducing the amount of dissolved solids present in a new solution will result in a system able to hold more reaction byproducts. Operating at lower metal concentrations has been used for years as one method of reducing the total dissolved solids in a new plating bath. In a plating bath designed to run at 3 g/l nickel, solids on make-up are reduced to about 75 g/l, about 45 g/l less than a bath operating at 6 g/L. Reduction of dissolved solids on make-up allows the solution to hold more reaction byproducts, resulting in a bath life extension of one-half to one MTO. There is also less drag-out, less nickel misting, and potentially lower waste treatment costs.

Bath concentration is typically tested by analysis of the nickel, since the titration for nickel is a relatively quick, easy test. Analytical frequency depends on bath loading and plating rate. If replenishment additions of greater than 10% activity are routinely being added, the frequency of analysis should be increased. The ideal operation is a steady-state condition, adding replenishment chemistry at the same rate it is consumed by plating. The better the control of the process, the better the process will perform.

Reducer concentration should be checked, generally once every MTO. Reducer should be consumed at a rate proportional to the nickel, but different operational variables, such as bath concentration, tank loading, agitation method, and amount of idle time at operational temperature can impact the amount of reducer consumed in the tank.

Operating temperature is the primary factor in determining plating rate. Low temperatures provide less energy to the chemical deposition reaction and result in a lower plating rate. Very high temperatures can make the bath too active, possibly resulting in plate-out and general bath instability. Automatic temperature controllers, frequently calibrated, are strongly recommended.

Operational pH. Aside from bath formulation, operating pH is the most influential factor on deposit phosphorus content. Typically, higher pH ranges give lower % P in the deposit, while lower pH values give higher % P in the deposit. The pH of an electroless nickel plating bath should be checked every time a nickel titration is performed.

Bath Volume. Maintaining the operating level of the plating bath is a critical and often overlooked control factor. Consider a plating tank 50 inches deep. At the 50-inch level the solution is at 100% activity, and the bath is chemically balanced. A load is plated and consumes 10% of the bath chemistry. During plating, the solution level of the tank dropped 5 inches, or 10%. When the evaporated bath is analyzed, it would show 6 g/L of nickel metal. However, the bath is not in balance. Specifically, the stabilizers would be low, and the ratio of chelates in the bath would be higher than normal. The bath is now unbalanced, and the low stabilizers could lead to bath instability. PFD