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Applications in electronics continue
to dominate
but other industries are beginning to benefit.
Pulse plating is a method of depositing
metal on a substrate using interrupted direct current (dc).
These pulses are often employed at a rate of 500 to 10,000
times per second. They favor the initiation of grain nuclei
and greatly increase the number of grains per unit area.
The intended result is a finer grained deposit with better
characteristics and properties than conventionally plated
coatings.
The electronics industry has been
and continues to be the major user of pulse plating. In
fact, pulse plating has become a requirement in many cases
where the process and/or product specifications are highly
restrictive and sophisticated.
We will discuss some of the principles,
practices, and applications of pulse plating, then conclude
with a forecast of what the technique might hold for the
future. We will begin with a discussion of equipment and
wave shapes.
Pulsing Equipment
Early equipment for pulse plating
consisted of a dc rectifier and chopper circuit, which provided
low-frequency pulses with little regard for pulse frequency,
rise-and-fall times, and regulation. Semiconductor regulators
were added to the dc rectifier, resulting in a pulse power
supply that offered switching frequencies up to 10 Hz with
fast rise-and-fall times. In recent years, pulse power supplies
have been changed to offer complex waveform capabilities
(e.g., pulse reversing, pulse-on-pulse, and duplex pulses).
Single direction pulse power
supplies are referred to as
unipolartypes.
They have been the workhorse
of pulse platers and represent about 90 to 95 percent of
those in the market. They are available as add-on switches
(converters) to be used with a separate dc rectifier or
as an integrated supply with the dc portion and the switching
circuits contained in one unit. The latter type is the most
common arrangement.
Converter pulse
systems usually feature saturated or low-voltage-loss switches.
They depend on the dc rectifier to provide current or voltage
regulation. One disadvantage of this approach is that any
ripple on the dc output is passed through the switch to
the plating bath. When a current regulated dc rectifier
is used, its output voltage will rise during the pulse off-time,
charging the rectifier filter capacitors and yielding high-current
spikes at the start of the on-time of the pulse.
SCR or thyristor-controlled dc power
supplies often will not operate well with switching applications.
At low output voltage or current, the SCRs are operating
at a very low phase angle and the ac ripple component is
very high.
Integrated pulse
power supplies consist of a dc rectifiers and pulse switch
circuits in one unit. They are more commonly used than those
with an add-on switch to the dc rectifier. Integrated supplies
offer the advantage of being able to better control the
amplitude and ripple of the output current. Also, the dc
portion of the supply is designed to be compatible with
pulse switching techniques. These systems offer current
or voltage regulation and are usually more readily applied
to plating because of their ease of operation.
Wave Shapes
Pulse power supplies are offered
with two basic wave shapes: sine
and square
wave. The modified sine-wave
system uses the power-line frequency of 50 to 60 Hz half
wave or 100 to 120 Hz full wave as its time base. The output
pulse usually has a fast turn-on time and a slow sinusoidal
turn-off time. Its advantage is lower cost by comparison
with square-wave systems. However, because lower frequencies
have not shown the benefits of higher ones, the modified
sine-wave system has not found wide acceptance for pulse
applications.
Square-wave pulse power supplies
feature a time base that is independent of the power-line
frequency. They operate over a wide frequency range up to
and often exceeding 10 kHz. These systems are successfully
applied to both precious and base metals for electronic,
microelectronic, and decorative applications. The wide operating
parameters allow the user the most flexibility in determining
the optimum operating parameters for a specific application.
Power supplies that deliver
complex pulse
wave shapes are also being applied to many processes. Pulse-reversing
units feature a train of pulses in the cathodic direction
followed by a train in the anodic direction. Some manufacturers
offer equipment that allows independent control of forward
and reverse pulse timing and amplitude. Converter systems
for pulse reversing usually offer the same pulse amplitude
in both directions and independent timing to allow the user
to vary the average current.
Pulse-on-pulse systems
offer complex wave shapes that have pulses at one amplitude
riding on top of those of a lower amplitude riding on top
of those of a lower amplitude. Duplex
systems feature a burst
of pulses at one level followed by a burst at another, all
in one direction. Pulse-on-pulse and duplex systems are
used mainly for research.
Because of the vast variety of plating
processes and products, manufacturers of pulse power supplies
have had to offer a great deal of flexibility in their equipment.
Some companies offer custom-designed systems or standard
equipment that can be adapted to the user's requirement.
These custom-designed products
usually command a higher price than standard equipment.
However, customized products offer the customer a piece
of equipment that is often a better answer for his needs.
Also, these custom-made units sometimes evolveinto
new standard products that can be offered at a more attractive
price. Multiple-output pulse platers are an excellent example
of this. They allow precision control of the current to
each cathode for improved part-to-part thickness uniformity.
Application Overview
Now let's outline some of the current
and prospective uses of pulse plating:
Connectors:Pulse
plating is being used to a large degree for plating nickel
and gold on electronic connectors and switch contacts. Some
contacts are barrel plated with gold over the entire surface.
But the cost of gold and concerns for minimizing its use
have led manufacturer to develop selective plating methods
for applying the gold only on the contact areas. Stripe
plating of nickel and gold has been very successful with
pulse deposition.
A reduction in stress of the pulse-plated
deposits has allowed manufacturers to stamp and form contacts
after plating. The economic gains have far surpassed the
relatively high cost of pulse power supplies. A pulse cycle
of 1.0 millisecond on and off is typical for depositing
cobalt-and nickel-hardened gold.
Lead frames:manufacturers
of semiconductor lead frames are using pulse plating to
increase the reliability of wire bonds and to enhance deposition
speed. This has been accomplished through the use of proprietary
high-speed gold and silver plating solutions specially formulated
for pulse deposition. Peak voltages in the range of 40 V
are required to deliver the high peak amperes that are necessary.
Fine patterns:The
microelectronics industry has recognized the advantage of
pulse plating for high-density circuitry. A report of Missel
et al on the square profile of pulse-plated circuit paths
describes the process and advantages.
Typically, dc plating results in
mushroom-shaped deposits, which limit the proximity of one
line trace to another in fine pattern plating. With the
use of pulse technology, circuit traces can be positioned
closer together without shorting one another. Companies
that manufacture high-density circuits have been able to
increase the number of circuits on a given surface dimension.
Increased circuit density in thin-film magnetic heads for
computer disc drives allows for greater magnetic strength
of the head.
SAW Technology:Pulse
plating is being studied for use in Surface Acoustic Wave
(SAW) technology, which is employed in the manufacture of
high-frequency (10 MHz to 1 Ghz) oscillators, filters and
resonators for cable television, satellite communications,
modems and radar applications.
Electroforming:
This is a very important application for pulse power supplies.
Nakamura reported that pulse deposition could be used to
produce stronger electroformed copper and nickel parts with
thinner walls and lighter weight. Also, companies are using
pulse deposition to electroform nickel venturi valves for
cryogenic applications, where it is very important to maintain
exact replication of the machined aluminum mandrel.
Moreover, pulse power is being used
in the electroforming of diamond cutoff wheels for the semiconductor
industry. Improved properties of the pulsed nickel deposit
result in better wear characteristics and heat transfer
away from the cutting edge. Optics manufacturers are using
pulsing techniques to electroform exacting molds for contact
lenses,. The nickel deposits exhibit little or no stress.
Molds for light-reflecting products (e.g., reflectors for
cars and bicycles) are also being pulse electroformed. Reflectivity
is said to be uniform over the full surface of the reflector.
Finally, pulse electroformed nickel
for large machined parts has improved machine ability. Pulsed
current has reduced hydrogen embrittlement and treeing of
the deposit. Reduced pitting has resulted in less scrap
being generated during the machining operation.
Etching:Most
applications of pulse power supplies are those where metal
is being deposited. However, they are also being used to
etch fine patterns in high-temperature metal alloys. Pulse
etching has proven to be far superior to alternative methods
in some cases. The capability to etch very sharp corners
and straight walls in deep crevices by comparison with conventional
etching methods has yielded products that meet exacting
design requirements.
Waveguides:A
smooth surface topography in high-frequency waveguides is
important for the reduction of radio-frequency losses in
transmission. Pulse-plated gold deposits exhibit reduced
surface roughness for improved waveguide performance.
Decorative
Work:Many high technology jewelry manufacturers are
using pulse techniques to apply gold, rhodium and silver.
Some decorative platers report that pulse plating allows
better deposition into recessed areas of complex shapes
while minimizing overplating at high-current-density areas.
Circuit Boards:Some
PC board companies have been using pulse plating for tin-lead
alloys and copper for a number of years. They report being
able to maintain a very consistent tin-lead alloy content
over the entire surface of a panel (4 to 5 ft). The resulting
deposits are said to exhibit excellent characteristics with
regard to infrared solder reflow.
Copper deposition using pulse-reversing
techniques was shown by Hall et al. To offer significant
improvements in through-hole plating and elongation and
thermal properties of the deposit. Ratios of up to 10:1
in board thickness to hole diameter are said to be possible.
With the increased demands on circuit
board makers to produce boards with smaller holes and closer
line widths and spaces, it would appear that pulse plating
will become an increasingly important method. The challenge
for rectifier manufacturers is to offer the PC industry
power supplies that can deliver the large currents required
for copper plating, and at an economical price.
Electroless
Nickel:The application of pulsed current to Electroless
nickel has been shown by Mallory and Lloyd to increase the
deposition rate by a factor of several times while yielding
deposits with physical properties similar to those of conventionally
applied EN. This is an unconventional application of pulse
power supplies, though perhaps it may become a significant
one. R&D in this field could result in pulse plating
becoming a very large and important part of the "Electroless"
process.
A Forecast
Many companies have been using pulse
plating primarily to obtain an improvement in deposit properties
as a result of grain refinement. Others claim improvements
in throwing power and thickness distribution, but the superior
metallurgical properties by comparison with conventional
dc plating are the major benefit. Nonetheless, it should
be pointed out that pulse plating is not a panacea and that
it is still an evolving technology.
Finishing and manufacturing engineers
are continually requesting plating solutions that offer
specific deposition results for stated pulse operating parameters.
Solution formulators are being called upon to answer these
needs Contrary to the theory that pulse plating eliminates
the need for solution additives, we feel that baths developed
for pulse deposition will continue to use additives in order
to achieve optimum and repeatable results.
There is also the possibility that
pulse power will become a commonly used tool for other electrochemical
processes (e.g., electrodischarge machining, electro etching,
and electro cleaning). Electrical current can be defined
and controlled more easily than can the chemical makeup
of the solutions employed for these operations. This marriage
of chemistry and the highly predictable and controllable
parameters of electrical current could will result in future
electrochemical processes that are easier to use and maintain
and that deliver improved results.
The electronics industry continually
pushes process technology to new horizons. The dynamics
of this industry results in manufacturing processes that
often experience a very short life. The process that was
good yesterday may not even be used today. In the extreme,
processes are sometimes outdated before they are even put
to use.
This technological change is evidenced
by new manufacturing methods for circuit boards, CaAs wafers,
and microelectronic components. Greater demands to place
more and more electronic circuits on a given substrate may
result in line widths and spacings between lines that cannot
be achieved with dc plating or even by altering solution
chemistry.
Pulse power supplies will be changing
continually to take advantage of new electronics technology
- e.g., microprocessor control, smaller components, and
greater current-handling semiconductors. However, the real
future of pulse power supplies lies not in the equipment
technology but in the sophisticated processes that require
the use of pulse power to achieve the desired results.
Finally, it is worth noting that
the AESF has become increasingly active in this area. Its
Third International Pulse Plating Symposium is scheduled
for October 28-29 in Washington, DC, and authors are nearing
completion of a society-sponsored book on the subject.
Yes, the future of
pulse plating is now!
References
1. L. Missel, P. Duke and T. Montelbano,
Semiconductor International (Feb 1980).
2. Report on AESF First International Pulse Plating Symp.,
Plat & Surf. Fin. 66, 37 (June 1979); also see Osamu
Nakamura, "Application of Pulse Plating Technique to
Copper and Nickel Electroforming," available from Dynatronix,
Amery WI.
3. W.F. Hall and A.R. Chaudhuri, Proc. AES 10th Plat,
in the Electronics Industry Symp. (1983).
4. G.O. Mallory and V.A. Lloyd, Proc. AES 71st An. Tech.
Conf., Session B (1984).
About the Author
Norman M. Osero is president
of Dynatronix, Inc. Amery, WI 54001. He has been active
in the development, manufacture, and marketing of pulse
plating equipment since 1971. Mr. Osero is a member of the
AESF Upper Midwest Branch and holds a degree in electronics
from North-central Technical Institute, Wausau WI.
