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by Pat F. Mentone
Computers are a billion
dollar business, and there is no doubt that the industry
will continue to grow as we advance through the 21st century.
With this in mind, platers may be thinking of throwing their
hat into the ring and beginning to perform electronics finishing.
However, there are many areas that are different in electronics
finishing that finishers should know about. One area that
is important to look at is rectifiers.
A Little History
In industries such as printed circuit
board and semiconductors, output from rectifiers must be
precisely controlled. As electronic components have become
smaller and smaller, the use of electroplating concepts
to manufacture the components and interconnects between
these devices has become significantly more important. Historically,
the standard in the circuit board industry was to use one
large rectifier for all of the plating tanks on an automated
line. For example one 1,000 amp SCR rectifier could be used
for three tanks with four panels being plated in each tank.
The amount of copper (metal traditionally used in electronics
finishing) plated on each panel would be the same as long
as the resistance to each panel and to each anode was the
same. Plating companies quickly found out that this never
happened because of the difficulty in getting good electrical
contact to each anode bar, anode basket, flight bar and
to each panel on the flight bar. As the power requirements
changed and the plating requirements continued to become
more sophisticated, people started to specify outputs with
the ripple less than 5 percent, so chokes and capacitors
were used to smooth the output. The next evolutionary step
was the use of one rectifier per tank. This idea helped,
but there was still the problem of varying contact resistance
to the baskets and panels.
The use of dual rectification was
implemented to solve this problem. Dual rectification means
using two rectifiers in each tank that are hooked up to
the same cathodic panel. Each rectifier is also connected
to the anodes on opposite sides of the panel. Using a separate
rectifier for each side of the panel was the solution that
helped solve the problem of varying contact.
A switch mode DC rectifier is the
primary type used for this dual rectification. Dynatronix,
a rectifier manufacturer based in Amery, WI, introduced
switch mode plating power supplies in 1980. The advantage
of using a switch mode supply is that the rectifier has
a much smaller footprint than the equivalent "powerstat"
or SCR style supply. It also provides little ripple and
current regulation that is less than 1 percent of full scale.
The switch mode supplies changes the frequency of the incoming
power to approximately 40,000 hertz and then does the rectification.
Using a higher frequency means that a much smaller transformer
can be used, providing most of the space and weight savings.
The Next Step
In using the dual rectification process,
the amount of copper on each side of the panel was easily
controllable, but the problem of controlling the amount
of copper inside the holes was still an issue. These holes
become smaller as the number of metal layers through the
hole increased. For example, to obtain 1 mil coating in
the center of the hole, the typical amount of copper on
the surface of the panel ended up being two to three times
more than was needed. In addition, the plating times increased
up to two to three hours.
Solving this problem was the next
step that the industry had to overcome and was accomplished
by installing pulse periodic reverse (PPR) rectifiers. By
using PPR, which periodically reverses the rectifier output
in one millisecond, and special chemistry, the proper amount
of copper is plated in the hole rather than wasting excess
copper on the surface of the board. In using PPR techniques,
the results are increased throughput and decreased costs.
What's Next?
As the industry continues to expand
and evolve, companies have worked to develop products to
meet these changes, including Dynatronix. The company has
developed a complete line of programmable rectifiers --
typically 10 to 20 volts rectifiers with currents ranging
from a few milliamps up to and beyond 1,000 amps units -
to be used in the semiconductor and printed circuit board
industries. By incorporating a microprocessor into each
rectifier, the output of the power supply can be DC, pulse
or PPR. These rectifiers have three different control options.
They can be controlled manually from the front touch-screen
panel, from a PLC, or - by the preferred method of control
- from a host computer either through a serial port connection
or through and Ethernet connection.
The critical parameters to control
are the type of regulation (constant current, constant voltage
or crossover), pulse "on" time, "off"
time, cathodic plating time, reverse time, and amplitudes
of the forward and reverse currents. Typically, all of the
timing settings are in milliseconds and the rise time of
the pulse is between five and 50 microseconds depending
on the amplitude. With the large array of parameters, there
is almost an infinite variety of settings possible.
Using the onboard microprocessor along
with the host computer will allow the user to write a rectifier
program (commonly referred to as a recipe), and then download
the recipe to the rectifier. The rectifier can then feed
the operation data back to the PC for data logging and further
process analysis. Within the recipe, the operator can set
the mode of regulation, the output amplitude, the pulse
timing, and the amount of time the rectifier is to run these
conditions. The operator can then construct a second set
of conditions and so on, only limited by the storage capacity
of the PC. For every different part plated, the operator
can scan in a part number, download a part specific recipe
to the rectifier, and start the process to plate the part.
During the plating cycle, the rectifier will store current
and voltage readings and periodically send them back to
the PC at whatever time interval the user selects. All of
these technology advances are allowing users to plate printed
circuit boards and semiconductors more efficiently and easily.
Summing it Up
The printed circuit board and semiconductor
industries are ever-expanding and changing. With the advances
made from dual rectification to PPR to computer-driven rectifiers,
there is no telling what is on the horizon for the future
of electronics finishing. Companies like Dynatronix, however,
will always be striving to make this type of finishing a
little easier.
About the Author
Pat F. Mentone PhD, CEF-SE has
30 years of experience in the plating and electroforming
of electronic components. He has been a consultant for the
last 15 years specializing in pulse plating and selective
plating. He is the inventor or co-inventor of 5 patents.
He holds a PhD in inorganic chemistry from the University
of Minnesota. He can be contacted at pmentone@attbi.com.
This article was reprinted
with permission from the January 2002 issue of Finisher's
Management.
