In the last 20 years the applications for electroless nickel have grown considerably due to its deposit properties as well as its ability to plate uniformly over all surfaces in contact with the plating solution. The properties of electroless nickel originate from its mechanism of deposition, alloy content and deposit structure.
Physical Properties of Electroless Nickel
Deposit uniformity and deposition rate. Electroless nickel deposition is a chemical reduction process whereby any catalytic surface in contact with the plating solution is coated uniformly regardless of part geometry. An adequate replenishment of chemical components from the bulk solution to the substrate surface must be ensured.
This differs from electrolytic deposition where thickness depends on current density distribution over the part surface. With this, thicker deposits build up on outside corners and edges (high current density) and thin to no plating in deep recesses (low current density). The thickness of the electroless nickel deposit depends solely on the deposition rate and the length of time the part is immersed in the solution.
The type of electroless nickel solution as well as the deposition rate will vary from supplier to supplier. Deposition rates can be grouped as in Table I.
Plating rates will decrease with solution age as chemical by-products buildup in the solution.
Deposit structure. The microstructure of an electroless nickel-phosphorus deposit is strongly dependent upon the alloy content of the deposit. Phosphorus levels can vary from one to 14 pct by weight with most commercial baths, ranging from three to 12 pct by weight. At low phosphorus levels, (< seven pct by weight) the electroless nickel deposit is microcrystalline, consisting of many small grains, approximately two to six nm in size. As the amount of alloyed phosphorus increases, the microstructure changes to a mixture of amorphous and microcrystalline phases and finally to a totally amorphous phase (>10 pct by weight).
The density of electroless nickel-phosphorus alloys depends upon the phosphorus content as illustrated in Figure 1.
As the phosphorus content increases, the density of the deposit decreases because of phosphorus in the nickel lattice.
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| 1. Phosphorus Content (pct w/w) |
Melting point. The melting range of electroless nickel deposits also varies with phosphorus content, decreasing with increasing phosphorus levels. Electroless nickel does not have a melting point, but rather a melting range. Pure nickel has a melting point of 1445C. Low-phosphorus electroless nickel melts at approximately 1300C, while mid- and high-phosphorus electroless nickel melts at approximately 890C.
Coefficient of thermal expansion. Reported values of the coefficient of thermal expansion for electroless nickel range from 12 × 10-6 C-1 to 14.5 × 10-6 C-1.
Electrical properties. The electrical resistivities of electroless nickel alloys are listed in Table II. Alloying elements such as phosphorus as well as the presence of amorphous phases increase the electrical resistivity of the deposit.
Magnetic properties. One of the most important applications of electroless nickel films is in the data storage industry. This is due primarily to its corrosion protection, hardness, polishability and magnetic characteristics. At high phosphorus levels, electroless nickel deposits are non-magnetic. This is a prerequisite for an undercoat to the subsequently deposited cobalt storage media. As the phosphorus content decreases, electroless nickel deposits show increasing magnetization. Table III summarizes the coercivities of the various deposits.
Solderability/Weldability. An important aspect of electroless nickel to the electronics industry is its solderability. All electroless nickel deposits are solderable provided the soldering conditions are matched to the condition of the particular electroless nickel deposit. Lower-phosphorus electroless nickel is more easily solderable immediately after plating than higher-phosphorus electroless nickel. However, this advantage disappears after 12 - 24 hrs. At this point, the ease of solderability depends upon the characteristics of the passive layer that forms on the surface of the electroless nickel deposit. Those deposits plated from baths containing heavy metal and sulfur-bearing brighteners and stabilizers, such as most commercial low- and mid-phosphorus electroless nickel systems, form a thicker tenacious passive layer than those that do not, such as most high-phosphorus electroless nickel systems. High-phosphorus electroless nickel systems tend to be more solderable in aged deposits.
Other important factors influencing the solderability of electroless nickel deposits include residual contamination left on the surface after plating and storage conditions after drying. Surface contamination and exposure to environments containing sulfur dioxide, chlorine, high humidity and high ambient temperatures will detrimentally affect solderability. Therefore, it is imperative that parts be thoroughly rinsed in clean DI water, dried and stored in a cool, dry atmosphere, preferably nitrogen.
Corrosion resistance. One of the primary uses of electroless nickel in engineering applications is enhanced corrosion protection of critical components. Because electroless nickel is cathodic to most common metals in most environments, its corrosion protection is not afforded through sacrificial mechanisms. Corrosion protection occurs through encapsulation of the substrate. EN's low porosity and resistance to many chemicals and atmospheric conditions make it ideal for this application.
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| Aluminum Memory disk plated with 0.4 mil of high-phosphorus electroless nickel. |
The corrosion resistance of various electroless nickel deposits on steel as measured by the ASTM B117 salt spray test is summarized in Table IV.
Although electroless nickel is a corrosion resistant coating, there are many factors that affect its ability to protect a substrate. These include: 1) Composition, structure and surface finish of the substrate; 2) Pretreatment; 3) Deposit properties; 4) Deposit thickness; 5) Post plating processes, such as heat treatment or passivation; and 6) Corrosive nature of the environment to which the substrate will be exposed.
The following generalized statements can be made regarding the effects of various factors on the corrosion resistance of electroless nickel.
- The rougher the surface, the lower the corrosion protection.
- The more porous the substrate material, such as in castings, the lower the
corrosion protection.
- The use of strong acids prior to electroless nickel plating lowers corrosion protection.
- The use of sulfur-stabilized baths
introduces deposit porosity that decreases protection.
- The higher the phosphorus content, the better the corrosion resistance.
- Corrosion protection decreases as bath age increases.
- Post-plate treatments such as low temperature baking, chromating, increase protection.
- Heat treatment for hardness decreases protection.
A complete understanding of the process from material selection through post-plate treatment will result in realistic expectations and optimal performance.
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| Aluminum Connectors plated with 1.0 mil of mid-phosphorus electroless nickel. |
Hardness. The hardness of an electroless nickel deposit is inversely related to its phosphorus content. As the phosphorus content increases, the as plated hardness decreases. In all cases, electroless nickel deposits can be hardened through heat treatment that causes the formation and precipitation of nickel phosphide. Typical heat treatment conditions for full hardness are 400C for one hr in an inert atmosphere. If there is no access to an inert atmosphere oven, and discoloration is objectionable, or there is a need to harden without affecting the hardness of the substrate, full hardness can be obtained at lower temperatures with longer times. Hardness as plated and after heat treatment are summarized in Table V and temperature vs. time in Table VI.
Wear properties. Electroless nickel coatings have good wear resistance because of their high hardness and natural lubricity. This, coupled with the uniformity of the electroless nickel deposit, makes it an ideal wear surface in many sliding-wear applications. Relatively soft substrates with poor abrasion resistance, like aluminum, can be given a hard, wear-resistant surface with electroless nickel. (Table VII) Electroless nickel has also found widespread use in anti-galling applications where the use of certain desirable materials could not otherwise be used due to their mutual solubility and propensity to gall and seize. A deposit's hardness can be increased through heat treatment to further enhance wear properties, rivaling the wear properties of chrome.
Many different substrates. Electroless nickel can be applied with excellent adhesion to many different substrates: Steels, including leaded and resulfurized; Cast iron; Stainless steels; Aluminum; Copper, bronze and brass; Non-conductors (ceramics, plastics); Powdered or sintered metals; and Magnesium, beryllium and titanium.
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| Zinc Die Cast connector plated with copper and then 0.5 mil of mid-phosphorus electroless nickel. |
Electroless Nickel Specifications
As electroless nickel technology continues to grow and different types of electroless nickel deposits are invented, the specification that allows a designer to specify exactly the type of coating wanted for a certain application is needed. All electroless nickels are not the same and do not offer the same deposit properties. Therefore, it is important that the correct electroless nickel is specified for each application.
The purpose of a specification is to enable purchasers to effectively order the proper coatings for their components. It also assists the plater in successfully plating a given component (and gives him assurance he has done so). It is a contract between the plater and manufacturer.
There are a number of major specifications for electroless nickel today. The ones most cited in the United States are as shown in Table VIII.
There is, however, a number of engineering specifications for electroless nickel coatings developed by major corporations that do not follow the above specifications.
Aerospace Material Specifications AMS 2404 and AMS 2405.AMS 2404 was originally issued in 1957 and AMS 2405 in 1965. They remain essentially the same documents today. The broad range of phosphorus contents and composite coatings readily available today were not commercially available when these specifications were written. AMS 2404 does not specify phosphorus content even though it is common knowledge today that deposits with different phosphorus contents have different physical deposit properties. AMS 2405 is a "low-phosphorus" specification that specifies less than eight pct by weight phosphorus (this is considered mid-phosphorus by today's standards). Even though these specifications contain many valid points, these documents have not changed as technology has changed. Therefore, the ASTM specifications that are regularly reviewed and modified are gaining greater importance.
American Society of Testing Materials
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| Mid-Phosphorus electroless nickel plated to 0.1 mil and then followed by gold on a printed circuit board. |
ASTM B656 and ASTM B733. Actually, ASTM B656 is not a specification, but a guide for obtaining a superior quality electroless nickel coating. The target audience for this piece is the plater running the plating line, the chemist who assists in maintaining operating solutions and the quality assurance person who needs to insure that the coating produced by the plater meets the customer's specifications. There are some valuable tips on bath operation and control contained within this document. It is currently undergoing revision to reflect the most modern technology available today.
ASTM's electroless nickel specification document B733 breaks down suggested coating thicknesses by service conditions, depending on the severity of the exposure in which the coating is expected to perform. ASTM B733 also contains three coating types that dictate which performance and quality tests are to be performed. Of the specifications available today, this one contains the most relevant and up-to-date information. However, the new revised version of B733 released July 1997 contains five different deposit alloy types broken down by phosphorus weight percentage. This will allow a designer to correctly specify the right coating for a particular job. This new revision of B733 was written in conjunction with an ISO document that will essentially be identical in nature. The key points that need to be discussed between the plater and the purchaser ordering the component to be coated include the following:
- Alloy of part to be plated;
- Part manufacturing history;
- Deposit alloy type;
- Service condition;
- Heat treatment class;
- Significant surfaces and surfaces not to be plated indicated on a drawing;
- Peening, if required;
- Stress relief, class of heat treatment before plating;
- Hydrogen embrittlement relief after plating;
- Supplemental requirements: abrasive wear, corrosion resistance, electrical resistivity,
solderability, porosity and microhardness;
- Specific packaging/handling requirements; and
- Requirements for sampling.
This new revision to ASTM B733 will have the following effects in today's marketplace:
- More job shops will install several types of electroless nickel
- There will be more high- and low-phosphorus electroless nickel ordered (and less mid-phosphorus)
- Suppliers will develop advanced versions of both high- and low-phosphorus baths.
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| Steel automotive brake pistons plated with 0.5 mil mid-phosphorus electroless nickel for wear resistance. |
Military Specifications
MIL-C-26074E dated October 30, 1990 has been reinstated by the Naval Sea Systems Command, Department of the Navy, and may be used, once again, for acquisition. This reinstatement occurred February 25, 1998.
Applications for Electroless Nickel
Electronics. This market segment is by far the largest and most diverse of all the areas where electroless nickel is used. The major component plated is the aluminum memory disk that is found in many of the computer storage devices. It consumes a huge amount of electroless nickel. The high-phosphorus electroless nickel used in this application has to be exceptionally smooth and non-magnetic to serve as a base for the magnetic layers.
Another large electronic application is coating both aluminum and zinc connectors. The coating serves to provide corrosion protection, wear resistance, and is especially important because of the complex shape of these components. These are for both commercial and military uses. Sometimes a topcoat of cadmium is used, however electroless nickel is used increasingly as a stand-alone coating.
Components for semiconductor packages, ceramic micro-devices, microwave devices, air bearings, battery components, heat sinks and electroless plating of plastics for EMI shielding are all large consumers of electroless nickel. New applications in the electronics sector include floor grates for semiconductor manufacturing facilities and electroless nickel/gold for circuit boards.
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| Wheel Cylinders plated with 0.6 mil of high-phosphorus electroless nickel for corrosion resistance. |
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| Electroless nickel PTFE composite coatings are used to provide excellent lubricity in plastic injection mold applications. |
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| Steel Printer shafts plated with 0.2 mil of high-phosphorus electroless nickel for wear and corrosion resistance. |
Automotive. This market segment is growing rapidly and is driven by the need for higher quality and superior performance components and by the public demand for longer warranty vehicles. Brake pistons, wheel cylinders, caliper pins, steering column yokes, pinion shafts, air bag components and transmission parts are all plated in large volumes with electroless nickel. Electroless nickel is also commonly used as the first conductive coating and then coated with copper/nickel/chrome in many decorative applications such as interior hardware and aluminum wheels.
Oil, Gas and Chemical Process. The corrosion and wear resistance and the chemical and electrochemical stability of electroless nickel are critical to the performance of
the coating in this market segment. Housings, flanges, pipes, pumps, ball valve bodies and connector sections are commonly plated with thicknesses of up to 100 microns in the most severe environments. Low-phosphorus electroless nickel has shown promise and performed the best of all electroless nickel types in some of the most severe applications involving exposure to strong alkalis even at elevated temperatures and flow rates.
General Industrial (business equipment, textile, printing and mold applications). Shafts for ink-jet printers, heddles for weaving machines, rolls for the printing of newsprint and mold cavities where superior release properties are paramount are just a few of the myriad of applications in this general segment.
Food Process. No coating has blanket USDA approval for use in the food process industry, but electroless nickel (most often high-phosphorus type) is used for a variety of applications, including evaporators in ice machines, bearings, gears, conveyors and chains. Electroless nickel coatings have replaced expensive stainless steel (or have been used to coat stainless steel), because foodstuffs do not tend to stick readily to them. Also electroless-nickel-coated components are not as receptive to stress corrosion cracking as stainless steels (many cleaning compounds use chlorides).
Aerospace. Compressor and stator components from jet engines have used electroless nickel successfully for many years. In many cases the components coated are either high-nickel-alloy materials or titanium. Also, drive trains and landing gear components are coated with electroless nickel. Extensive testing needs to be done in this area for electroless nickel to gain widespread acceptance. PFD
1 Oersted: the cgs electromagnetic unit of magnetic
intensity equal to the intensity of a magnetic field in a vacuum in which a unit magnetic pole experiences a mechanical force of
one dyne in the direction of the field.
REFERENCES
- Wolfgang Riedel, Electroless Nickel Plating, 1991, Finishing Publications Ltd., Great Britain.
- AMS 2404, Electroless Nickel Plating, Society of Automotive Engineers, Warrendale, PA.
- AMS 2405, Electroless Nickel Plating, Society of Automotive Engineers, Warrendale, PA.
- ASTM B656, Standard Guide for Autocatalytic Nickel-Phosphorus Deposition on Metals for Engineering Use, ASTM, West Conshohocken, PA.
- ASTM B733, Standard Specification for Autocatalytic Nickel-Phosphorus Coatings on Metals, ASTM, West Conshohocken, PA.
- Milt Stevenson, Jr., Anoplate Corp. and Kurt Weamer, Belmont Plating,
Electroless Nickel Specifications: Today's Prenuptial Agreements for Surface Finishers? EN '93, Products Finishing, Cincinnati, OH.
- Glen Mallory, et al.,
Electroless Nickel Plating: Fundamentals and Applications, 1990, American Electroplaters and Surface Finishers Society, Orlando, FL.
- G.G. Gawrilov, Chemical Nickel Plating, 1979, Portcullis Press, Redhill, England.
