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Glossary
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Bath Considerations
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Equipment Considerations
Chuck
Van Horn
Manager
Semi/Microelectronics Ethone-OMI
Inc.
West Haven, CT
Conventional
pulse plating can simply be defined as metal deposition
by pulsed electrolysis. Explanation in the simplest form
is using interrupted D.C. current to electroplate parts.
This is accomplished with a series of pulses of D.C. current,
of equal amplitude and duration in the same direction, separated
by periods of zero current. The pulse rate (frequency) and
ON and OFF times (duty cycle) are controllable to meet the
needs of a given application. The shape of the pulsed current
is generally thought of as shown in Fig.
1A. An oscilloscope should be used to reveal how well
the equipment controls the output. (See Fig. 1B)
This method
of plating has gained acceptance in a number of metal finishing
industries, especially the electronics industry. With the
advent of solid state pulse plating power supplies, the
art has been taken out of the process. The amount of time
the current is off and the amount of time the current is
on are set directly on the digital thumb wheel switches
or on units with the software, programmed directly prior.
There are two different modes of operation possible; constant
current or constant voltage pulses. Figure 2 illustrates the constant current
mode of operation. The tops of the current pulses are kept
flat by allowing the voltage to vary during the pulses ON
time. The situation is different in the constant voltage
mode illustrated in Figure 3. The tops of the voltage pulses
are kept flat by varying the current.
Because of the shape of the current
pulse in this mode, the peak current is not useful to control
the plating rate. An ampere-minute controller is needed
to accurately control the plating thickness.
The advantages
of pulse plating vary from one user to another. However,
the most common are:
- Pulsing
the current produces dense fine-grained deposits and in
some cases such as gold plating, less gold is needed to
meet an end-use specification.
- The
variation in thickness from one part to the next is reduced
considerably.
- Plating
speeds can normally be increased. The current efficiency
in most instances is better than conventional D.C. plating.
- The
need for organic additives in most cases is reduced by
50 to 60%.
- The properties from the pulsed
coating is summarized as follows:
a. The coating is free from dendritic growth even if
devoid of additives.
b. The coating has fine crystalline structures.
c. The coating is smooth.
d. The coating is nearly free of pinholes.
e. The current efficiency, in most instances, is better
than D.C. plating.
The disadvantages, although minimal
are:
- In most
cases the cost of a pulse rectifier are much greater than
a D.C. unit. It is a highly regulated and sophisticated
design that costs more to manufacture.
- The
technology requires one to think and plan ahead with a
series of various procedures to follow to obtain the best
results.
- For
the chemical manufacturer., the requirement for additives
is reduced.
- To take best advantage of
the pulse capabilities, one must optimize the mechanics
of the plating equipment design before applying the pulse
unit. Areas of use for the pulse rectifiers are:
a. Reel to reel plating for speeds, distribution, and
reasons mentioned prior.
b. Rack plating for the prior statements.
c. Gold (both pure and alloy), nickel, silver, copper,
chrome, tin/lead, palladium and anodizing are all areas
of use.
d. In some select cases pulse is being used in etching,
cleaning, and electroforming.
The theory behind pulse plating
is simple. The cathode film is kept as rich in metal ions
as possible and as low in impurities as possible. During the
period when the current is ON, the metal ions next to the
cathode are depleted and a layer which is rich in water molecules
is left.
During the portion of the cycle when
the current is OFF, the metal ions from the bulk of the
plating solution diffuse into the layer next to the cathode.
Then the process is repeated again. Also during the time
the current is OFF, gas bubbles, and impurities which have
been absorbed on the cathode have a change to desorb.
Typical
ON times are from 0.1 to 9.9 milliseconds and typical OFF
times from 1 to 99 milliseconds. If an ammeter is inserted
into a plating circuit which uses pulsed current it would
respond to the average current. In order to have the same
plating rate using pulsed current as with D.C., the average
current must be the same. Either the peak current, ON time
or OFF time can be adjusted. By carefully picking the plating
parameters the physical and chemical properties of the deposit
can be very precisely controlled.
One of
the most dramatic advances in modern electroplating has
been the recent use of microprocessor-controlled modulation
of applied direct current to improve the electrodeposition
process. The method has found application across a broad
spectrum of the industry for both precious and non-precious
metal plating. It is being used in real-to-reel selective
plating, in automatic tab platers, on barrel lines, in still
plating, electroforming, anodizing, electrocleaning, electro-polishing
and machining and, most recently, has been adapted by the
semiconductor bump and wafer plating technologies.
Results
obtained with this sophisticated power control include (in
addition to greatly increased plating speeds) improved distribution,
lower deposit stress, finer grain structure, increased ductility,
improved adhesion, increased micro-throwing power, reduced
hydrogen embrittlement, and a markedly decreased need for
additives.
A careful
selection of a select few marketed series of modulated D.C.
power supplies embodies the most advanced electronic circuitry
to control output patterns with extreme precision.
Simply
speaking, a high quality unit will superimpose periodic
reverse on high frequency pulse. The power pattern that
results, however, is quite complex with a wide range of
profiles. The output, a series of pulses with controllable
amplitude, frequency, duration and polarity, has an influence
on the deposition characteristics of any solution which
is far different from that of conventional pulse or periodic
reverse. By "tuning" or shaping the output power pattern
to a given plating application, the rate of deposition and
the character of the deposit can be enhanced dramatically.
In periodic
reverse plating, the polarity of a constant D.C. output
is switched back and forth in a regular pattern. Figure 4A depicts the ideal output; Figure 4B shows the actual output from a
slow-response control unit.
The sharpens
of the output current pattern as revealed by a scope depends
upon the degree of ripple in the rectifier output and the
quickness of response in the internal switching circuitry
of the controller. Quality units produce extremely sharp
square wave patterns when seen on a scope as shown in Figures 3 and 4.
Figure 5 illustrates the wave form of the
Forward (cathodic) and Reverse (anodic) output of a quality
unit.
The duration
of the current in each direction, called the Forward and
Reverse envelopes, can be individually controlled from 0.1
millisecond to 99.99 seconds. (The zero current delay of
less than 0.1 millisecond indicated in the diagram between
Forward and Reverse is a design feature of high quality
units to prevent transistor failure due to "shoot through".)
The simple,
square-wave in Figure 3
is a precisely controlled periodic reverse output upon which
pulse frequencies are then superimposed. Within each envelope
a square-wave pulse is generated as depicted in Figure 6. The frequency and the duration
of the pulses are set independently for the forward and
reverse envelopes. Frequency range is from 10 to 9,999 Hertz.
Duty cycle settings in percentages determine ON and OFF
for each pulse.
On some
quality models, forward and reverse amplitude can be controlled
individually as illustrated in Figure 7. This permits, for example, a higher
current density in the reverse (deplating) state than in
the forward (plating) stage - highly desirable for some
applications. For a more complete explanation of output
control with specific manufactured units, refer to the operational
manuals supplied by the manufacturer.
In order
to avoid confusion, a very condensed "Glossary of Terms"
follows:
Cathodic, Anodic
Used to describe current direction
- i.e., Cathodic indicates flow is in normal (plating) direction;
Anodic indicates flow is in reverse (deplating) direction.
Forward, Reverse
Used interchangeably
with Cathodic and Anodic to indicate direction of current.
In normal operation of a reversing pulse unit, current direction
alternates in a controllable forward and reverse pattern.
Envelope
The time span during which current
may flow in only one direction. The time spans of the Forward
Envelope and the Reverse Envelope are set individually.
Pulse
Train
A regularly interrupted current flow
in either cathodic or anodic direction. A pulse Train exits
within the envelope.
Pulse
The individual interval in a pulse
train, consisting of an "ON and OFF" period.
Pulse
Rate
The number of times the current is
switched on in a given period of time, usually a second.
Duty
Cycle
The ratio of time an individual pulse
is ON compared to the total pulse time (ON and OFF). For
an example: 5 m/sec ON and 5 m/sec OFF is a 50% duty cycle;
4 ON and 1 OFF is an 80% duty cycle, etc. (Note that if
the duty cycle is set for 100%, there is no OFF time. The
current will be on for the duration of the envelope and
there will be no pulse or frequency.)
Frequency
The pulse rate expressed as Hertz
units, e.g., 100 Hertz = 100 pulses per second.
Pulse
Width
The time span of the ON portion
of a pulse. Pulse width is a function of both frequency
and duty cycle. For example: a 1,000 Hertz pulse with a
duty cycle of 50% has a pulse width of 0.5 milliseconds.
Bath
Considerations
With the changes that take place in
the tank when a modulated periodic reverse pulse is impressed
on the electrolyte, changes in the other operating conditions
or even in the formulation may be required. Generally speaking,
better results are obtained with simpler, rather than sophisticated,
formulations.
The polarization imposed by the power
pattern on the bath reduces, or even eliminates, the need
for some addition agents. Typical formulations used in pulse
plating:
NICKEL:
Nickel Sulfate
Boric Acid
Temperature
pH
Anodes
Organic Additives |
reel-to-reel with insoluble anodes, Watts type
650 g/L
50 g/L
60° C
3 to 4
Platinized Niobium
None |
Note: When using soluble
nickel anodes and reversing pulse modes, the need for an
anode activator such as chloride is not required as the
reversing current keeps the anode active and soluble.
PURE GOLD
Potassium Citrate
Citric Acid
Potassium Phosphate
Boric Acid
Gold Metal
Temperature
pH
Anodes |
150 g/L
15 g/L
26 g/L
72 g/1
8.2 g/1
140° F
3.5 to 4.0
Platinized titanium |
HARD
GOLD
Citric Acid
Potassium Citrate
Cobalt
Gold
pH
Temperature |
65 g/L
50 g/L
0.5 to 0.6
g/L
8.2 g/L
3.8 to 4.0
90 to 100° F |
With pulse, you have a higher voltage
than with D.C. plating. As voltage favors the deposition
of the alloying agent, one must analyze the deposit to determine
if an adjustment to the amount of cobalt in solution must
be made. In most cases, the reduction of available cobalt
or any alloying agent must be reduced to obtain the desired
hardness etc.
In many cases, additives can actually
inhibit the effectiveness of the power pattern. Large molecule
additives do not respond as they do under conventional power.
In a high frequency pulse field, their molecular size is
a disadvantage. Small molecule organic or inorganic will
generally function well as additives. In many cases, brighteners
can be reduced as much as 90% without diminishing the brightness
of the deposit because of the improved grain structures.
(If brightener level is not reduced, longer pulses - i.e.,
lower frequency and/or higher duty cycle, may be required.)
It is vital that the conductivity
of the electrolyte be maintained at a high level to allow
the peak pulse current to be completely effective. If the
conductivity is not high enough, an excess in voltage will
be required to attain the desired peak current. Such peaks
are power-inefficient and less effective.
Note that anode-to-cathode ratios
are rarely the same as for conventional power applications.
Generally speaking, in acid or alkaline non-chelating formulations,
the anode area should be reduced. In a cyanide or other
chelating formulations, the reverse is generally the case,
and a greater anode area is required.
Temperature and agitation conditions
may also have to be altered from normal for modulated power
pattern plating. Unfortunately, no general rule applies;
each application has its own requirements.
Equipment Considerations
One factor which should always be
checked when planning a change in power is the tank electrical
contact system. Although perfectly suitable for conventional
plating, some anode an/or cathode contacts may present unwanted
resistance to high frequency peak currents. Overlooking
this factor may prevent the realization of the true benefits
of a modulated power supply.
The major consideration, of course,
is the power system itself. Existing rectifiers may or may
not be suitable for use with modulated periodic reverse
or direct pulse units. A high voltage, quick response rectifier
is required, and the lower the ripple the more precise and
predictable the output.
Although pulse units are available
for use with existing power supplies, the models with self-contained
rectifiers specifically designed for this function give
greater assurance that full benefit of the control system
will be realized.
Quality pulse units with self contained
power may be operated in either a constant average current
or constant voltage mode. The significance of this option
is illustrated graphically in Figure 8.
Figure 8A is a depiction of a pulse train
with a 50% duty cycle. The average current delivered
is 50% of the peak value, represented by the dotted
line.
Figure 8B illustrates the effect of reducing
the duty cycle to 25% when in a constant voltage mode. The
peak current remains the same, but the average current changes
directly with the duty cycle, in this case dropping to half
its former value. Note that the current density of the pulsed
current will remain the same, but twice as much real time
will be required to deliver the same ampere-minutes of current.
Figure 8C shows the effect of reducing
the duty cycle from 50% to 25% when operating in constant
average current mode. In this case, the peak current changes
inversely to the duty cycle, increasing in value to maintain
the same average current delivered as before but in shorter
pulses.
Figure 9 illustrates what should be apparent;
that a change in frequency, although it, too, changes the
pulse width, does not effect either peak or average current
regardless of output mode.
There is one other consideration that
must be made, and which, unfortunately, is occasionally
overlooked in "sizing up" the unit required. Unlike conventional
plating rectifiers which are rated by average current capacity
(ignoring the ripple), modulated periodic reverse pulse
units are normally rated by their peak current capacity.
Since both peak and average current values are intrinsic
to modulated power pattern plating, both output capacities
must be considered. Depending upon the internal circuitry
of the unit, the average current output capacity of some
makes may be as low as 25% or 30% of the peak capacity.
With that
low value for average current, the rated peak current output
would be attained even at average current capacity only
if a duty cycle as low as 25% or 30% was used. Attempting
to push average current up would drastically shorten the
life of the unit. Since experience has shown that effective
duty cycles are rarely less than 50% most units are designed
to deliver an average current capacity of 50 to 60% of the
peak current capacity rating. However, any desired duty
cycle may be used or specified, keeping in mind that the
average current is the percentage (duty cycle) of the peak
rating.
References
1. H.Y. Chen, J. Electrochem
Society., 11W 551 (1971)
2. R.J. Tedeschi, Metal Finishing, Nov.49 (1971)
3. J. Padden, Pwr, Inc., J. Lochet, C. Vanhorn, Vanguard
West, Inc. - "Improvement of electrodeposition through modulated
D.C. power patterns" (1981)
For further information, Contact Chuck
VanHorn c/o
Enthone-OMI, Inc.
P.O. Box 1900
New Haven, CT 06508
Marketing Services Dept
Tel: (203) 799-4907
Fax: (203) 799-1513
