Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
ELECTROLESS NICKEL PLATING - WHERE IS IT GOING
Will electroless nickel plating continue in to the future? If it continues, will it grow? What will electroless nickel be like in the next 20 + years? These and other questions will be discussed along with factors that could change the use of electroless nickel plating in the future. Electroless nickel plating has become a practical and popular finish since its beginnings in the Bureau of Standards laboratory in 1944. There, Abner Brenner and Grace Riddel discovered nickel plating without external electrical current. They were astute enough to recognize the potential and thus developed and patented the first electroless plating processes in 1946.
Electroless Nickel Phosphorus
There are many variations of “electroless nickel phosphorus” (EN-P) alloys. It is not just one alloy or one finish, but many. The deposit characteristics vary widely with the amount of phosphorus co deposited in the alloy. For example, as-plated hardness can vary between 490 Knoop hardness number (100 gm load) for high phosphorus containing alloys (10 -11%P) to760 Knoop hardness number (100g) for low phosphorus (3-4%) containing alloys. For deposits below 1% the deposit becomes much less hard. The EN deposits can be heat-treated to increase hardness, or annealed (another heat treatment process at higher temperatures) to soften the deposit. Low phosphorus deposits heat treat to higher hardness numbers than high phosphorus deposits. Maximum hardness is obtained at somewhat different temperatures depending on the phosphorus content. On the other hand, high P alloys tend to have better chemical resistance and corrosion resistance in acid corrosive environments than the low P alloys. These alloys have found many useful applications. Low phosphorus plating solutions are a recent development. As suppliers improve their products, more and more applications will be found. I believe that low phosphorus alloys will be used in an increasing number of applications. Low phosphorus alloy plating formulations that do not contain sulfur, or other materials that would detract from chemical and corrosion resistance will find the largest market in the future. I further believe that electroless nickel phosphorus of all phosphorus contents will increase in usage. Education of design and application engineers in the virtues of electroless nickel is one of the keys to growth of EN.
It would be nice to have these variables and the consequences spelled out in the form of specifications corresponding to the various characteristics of the EN-P deposits. Porosity would have to be specified for corrosion protection applications. Resistance to chemical attack should be separated from corrosion protection of coatings. Corrosion protection is dependent on a continuous coating with no porosity.
Bright Electroless nickel
Most electroless nickel solutions will deposit semi-bright to bright naturally. Matt or dull deposits are most often the result of something bad in the solution. Many impurities, metallic or organic can cause dull deposits. Stabilizers are used in most formulations will also brighten the deposit as well as stabilize the solution to prevent extraneous plating on particles, tanks, heaters and will prevent spontaneous decomposition of the plating solution. Sulfur containing stabilizers will lower the chemical resistance and corrosion resistance and increase porosity in the deposit. Many other stabilizers will brighten and stabilize without sacrificing corrosion protection. Note: Nickel-P is not sacrificial to most metals onto which it is deposited. Therefore, for EN-P to protect, it must be pore free. Note also, that most of the porosity in EN-P deposits have their source in the basis metal and not necessarily a characteristic of the EN deposit.
A plating solution made of the purest constituents available (AR grade or better) will be semi-bright without stabilizers, but will not be practical due to its instability. (Experimentally, EN solutions filtered continuously through sub micron filters and at a high rate of filtration remain stable without stabilizers) The solutions for memory disc production have only a fraction of the stabilizers that solutions for general use have.
I predict that there will be a growing use of electroless nickel phosphorus plating in the years to come.
Electroless nickel boron
Electroless nickel boron processes will find limited additional applications. Nickel boron deposits with less than 1% boron have applications in electronic devices. Low electrical resistance (less than tin-lead) and the ability to plate isolated circuit elements, even very fine pitch, with close lines or pads. The multi purpose surface of nickel boron deposits will be receptive to wire bonding, soldering brazing and die bonding. The very high cost of the reducing agent, dimethylamine borane, prevents its wide use. The cost per mil-square foot is about 8 times that of electroless nickel phosphorus. Electroless nickel solutions using Sodium borohydride for its reducing agent is much less costly than the solutions using DMAB. The deposit contains 3-8% boron. The deposits are as hard as 1% boron alloys, making this process suitable for hard wear resistant requirements. The solution operates at pH of 13 or higher. It cannot be used to plate directly onto aluminum. The use of an electroless nickel strike makes it possible to use Nickel-boron plating from the borohydride solutions. Some new applications have been found recently.
Poly alloys (alloys with three or more alloying constituents) will find increasing use in the future. It is possible to include a number of alloying constituents into electroless nickel phosphorus and nickel boron alloys to form poly alloys that have unique deposit characteristics. New uses for these special alloys will be found. Examples of applications could be substitutes for chrome plating using electroless nickel-tungsten phosphorus and corresponding boron containing alloys have chrome like deposits for industrial applications as chrome comes under increasing pressure form environmental concerns. Molybdenum, iron, copper and some precious metals and rare earth metals could find applications in electronic and industrial components.
I believe the use of composite coatings produced by electroless nickel processes will grow significantly.
With present day technology, particulate matter of many descriptions can be successfully included in electroless nickel deposits. (co deposited). Presently, hard particles such as silicon carbide, diamonds, and aluminum oxide are fairly common. In addition, other hard materials that can be deposited are chromium carbide, boron carbide, tungsten carbide, boron nitride, glass, silicates, and insoluble sulfide of various metals fluorides of various metals. Lubricating particles that are being used today are PTFE, CFx, Molybdenum disulfide and some other plastics. There are numerous other materials that can be included in the deposits, table I. The co deposition particulate materials is made possible by the use of “particulate matter stabilizers” that alter the zeta potential of the particles causing them to be attracted to the surface being plated. They also stabilize the plating solution by preventing plate out on the particles. The particle size is usually between 1-4 microns with some at 6 microns. Plastic and ceramic particles are from 4-8 microns with some at up to 10 microns. Particle densities of 18-25% by volume are commonly used. Densities of up to 40% are possible. A one gram sample of 1.0 micron diamond particles, for example, contains 310,000,000,000 particles. The area is about 800 times the normal bath loading. Poly crystalline diamonds are conductive, thus can plate out causing gross instability in the bath, therefore, the need for particulate matter stabilizers.
Applications of co deposited particles are many. More obvious are to provide a hard wear resistant surface, and to provide a lubricating wear resistant surface. The choice between these two depends on the type pf wear to be encountered and the mating surface. Other applications less know are improved corrosion resistance, light absorbing, phosphorescence light emitting, altered appearance, heat transfer and others.
Heat transfer is an interesting application. For example, diamond films have outstanding heat conductivity (probable better than any other known substance). The films are now formed by various types of vapor deposition processes. The primary use is for heat sinks in the electronics industry. It is now possible to co deposit the type of diamonds that produce the high thermal conductivity into electroless nickel for use on heat sinks. Diamond co deposited in EN provided up to 19.6% better than electroless nickel alone.
Light-emitting particles can be co deposited with EN. These coatings appear normal under ordinary light, but when viewed under ultra-violet light, the coating emits a distinctive, brightly colored light.(2) Phosphorescent coatings can also be produced by co depositing luminescent particles. Phosphorescence is defined as the ability to continue to emit light after the slight source is removed for from .0001 seconds to 100 sec. Applications include parts plated with a light emitting coating then over coated with another usual coating. When wear takes place, excessive wear can be detected by inspection using ultraviolet light. To prevent counterfeiting parts one of many different light-emitting particles can be co deposited giving proof of the genuine parts. For original or OEM parts platers could be authorized to plate coatings that emit different colors. This is also useful for part identification such as for fasteners that are similar in shape and size.
Soap Box Advice
If platers make their customers aware of the numerous characteristics and virtues of the various EN plating processes, the use of electroless nickel in all its forms would increase. Educating engineers, designers, and purchasing people would move the use of electroless nickel forward. Specifications and standards that include the many variations of EN-P and EN alloys would further help usage of EN. Delineating the characteristics of the various deposits and how to achieve these characteristics would be helpful. Characteristics such as hardness, wear resistance, as tested by various means, corrosion resistance and protection, magnetics (are different for various P contents) electrical resistivity or conductivity data for each type of alloy, internal stress resulting from the deposit alone, thermal coefficient of expansion for the different alloy deposits, tensile strength, elongation, yield, thermal conductivity, modulus of elasticity and perhaps other useful engineering data would help carry the message. There is much of this in the literature, but not all alloy groups and variations are published. There is much to do. Each plater must think in terms of marketing, and service to help increase the use of plated products.
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