Guest Editorial
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Don Baudrand, Don Baudrand Consulting, e-mail:donwb@tscnet.com
Editorial
ELECTROLESS NICKEL PLATING - WHERE IS IT GOING
Introduction
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
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.
Composite Deposits
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.