Radio & Television News Article
Most people today under 30 years old have probably never seen the mechanics
or electronics inside their many personal devices. Everything is so
miniaturized and optimized that if something does go wrong, there is
little chance of the owner repairing it. Instead, the phone, television,
stereo, microwave oven, whatever, gets thrown away and a relatively
cheap (compared to paying for a repair) replacement is purchased (or
stolen). Besides, if the item was more than two years old, it was on
the verge of obsolescence anyway.
Up until around the early
to mid 1980s you had a fair chance of being able to repair an electronic
circuit if trouble arose because at least with commercial products printed
circuit boards (PCBs) were usually 1- or 2-sided and the components
still had leads protruding from the sides of the packages. A $10 Radio
Shack soldering iron and some solder wick was sufficient to remove and
replace just about any failed component. Home brew PCBs could be made
to nearly the same quality as commercial versions using a resist ink
pen (basically a Magic Marker) and a dish of ferric chloride etchant
liquid. A drill press helped with making holes for the component leads,
but a hand drill would get the job done. No more, though. If you are
resourceful enough to get your cellphone or camera open without destroying
it, you will find a very neatly laid out, extremely high density PCB
with parts so small you might wonder how they could work at all. Forget
servicing the thing with a soldering iron and a pair of pliers - you
will need at least a hot air wand, a magnifier, tweezers, and, of course,
electrostatic discharge (ESD) preventative gear.
In 1949 when
this article appeared in Radio & Television News, printed
circuits were just coming onto the scene. Bakelite, steatite, and ceramic
substrates were typically used at the time. Some processes were already
using printed resistors and small-value inductors via silkscreening
Part 2. A discussion of the techniques and equipment
used in making printed circuits for home-built units (January
See all available
vintage Radio News
Thanks to Terry W. for providing
Part I. A review of printed circuit techniques. To be concluded next
month with on article on how the experimenter can apply, in a simplified
form, printed circuits to home constructed units.
By John T.
A very loud bang announced to the electronic world early in 1945 that
printed circuits had moved from the experimental to the practical stage,
for it was at that time that the National Bureau of Standards, working
closely with the Centralab-Division of the Globe Union Company, began
mass production on the tiny radio proximity fuse for mortar shells:
a fuse incorporating a complex electronic circuit "printed" on a thin
steatite plate 1 3/4" long by 1 1/4" wide!
This typical group, only a few of the many commercially built
units already produced, is an example of how Centralab's printed
circuit audio amplifier has been received by the industry.
Since that time,
the printed circuit has thrust its tentacles into every portion of the
electronic field; and it has miraculously shrunk everything it touched.
Hearing aid amplifiers, complete with batteries, that are smaller than
a cigarette package; personal radios that can be cradled in the palm
of the hand; radio and television subassemblies occupying only one-tenth
the space needed for conventional assemblies and requiring one-half
as many soldered connections for installation: these are but a few of
the achievements of this new process, and the surface has barely been
scratched. Every day sees new applications of this method by which space
is saved, weight is reduced, assembly is simplified, and cost is cut.
Every electronic worker is certain to come in contact with printed
circuits in increasing number, and it is the purpose of this article
to prepare him for that contact by making him familiar with the various
methods and techniques by which these circuits are produced commercially
and then showing him how he can develop and experiment with his own
First, it should be clearly understood that
the term "printed circuit" covers any reproduction of an electrical
circuit upon an insulating surface by any process. Essentially it changes
a bulky three-dimensional array of electrical parts and conductors into
a compact and very nearly two-dimensional arrangement. An example best
shows how this is done:
Suppose we want to build the complete interstage coupling circuit shown
in Fig. 2. First, let us redraw our diagram on a tiny plate of steatite
approximately 1" x 3/4". If. Then let us carefully trace out the heavy
lines with a small brush which we have dipped into a "paint" made by
mixing fine particles of silver together with a liquid binder to hold
the particles together and a solvent used to make the mixture thin enough
Fig. 1. The "Couplate" unit. It contains a complete
interstage coupling circuit.
Fig. 2. Diagram of "Couplate." Finished unit measures 1-1/16
x 13/16 x 3/16 in.
Fig. 3. These individual operations show the method used in
preparing a silk screen.
Fig. 4. Silk-screen printing. Paint is forced through the open
mesh of the screen. After the screen is removed. the surface
of the base plate is found to be printed with an exact, sharp-edged,
uniformly thick design of the required conductor circuit. A
second stencil can then be used to print the resistors in their
Fig. 5. Front and rear views of one of the many hearing-aid
amplifiers that are printed on ceramic plates.
Fig. 6. A high temperature oven is used for firing a group of
printed circuits. (Note lack of hand and eye protection)
Fig. 7. Partially completed electronic circuits printed on steatite
plates and cylinders by the silk-screen process. Light lines
are silver conductors and inductors; dark rectangles are resistors;
circular disks are ceramic condensers.
Fig. 8. Illustrating the evolution of an audio plate-to-grid
Next, suppose we have several different solutions
of finely powdered graphite or lamp-black, a resin binder, and a solvent.
We can experiment with these until we find just the right combination
of mixture, thickness, and length of line needed to produce resistances
equal to R1 and R2; and then we carefully paint
in these resistance lines at the proper points between the silver conducting
lines already drawn. Then we place our little plate in an oven and raise
the temperature to the point where our lines of paint will be "fired"
directly to the ceramic base, adhering to it with a tensile strength
of 3000 pounds to the square inch. Finally we solder tiny ceramic condensers
of the proper values across the gaps representing the condensers, and
then we attach flexible leads to our silver paint at points 1, 2, 3,
and 4. The result is a "printed circuit" that will perform exactly the
same as one using conventional components, but our printed sub-assembly
will be no bigger than a postage stamp and require only four soldered
connections to be made by the radio assembly-line operator. A commercial
version of just such a printed circuit is shown in Fig. 1.
a manual process, while pointing up the difference between printed and
conventional circuits, obviously could not be adapted to mass production.
Various stenciling methods are the answer to producing more uniform
circuits at higher speed, and the silkscreen process is one of the most
In this system, a fine-meshed silk screen is tightly
stretched on a wooden frame and covered with a photosensitive material
that becomes insoluble when exposed to strong ultraviolet light. A photographic-positive
mask of the exact shape of the required conducting circuit is placed
on top of the screen, which is then exposed to the rays from an ultraviolet
lamp. Finally, the portions of the film protected by the mask are washed
away in cold water, leaving a stencil of the conductor design to be
printed. All four of these steps are clearly illustrated in Fig. 3.
This finished stencil is held securely against the base plate
to be printed; and the circuits can be printed on practically any insulating
material, or even on conducting material that has been coated with a
non-conducting film, such as lacquer, and a quantity of silver paint
is placed at one end of the screen. A neoprene bar, or "squeegee," is
moved across the top surface, forcing the paint ahead of it and down
through the open mesh of the design, as is shown in Fig. 4. When the
screen is removed, the surface of the plate is found to be printed with
an exact, sharp-edged, uniformly-thick design of the required conductor
circuit. A second stencil can be used to print the resistors in their
proper places. The paint is fired to the base exactly as was done before.
This process is shown in Fig. 6. In Fig. 7 are displayed base plates
at various stages of completion.
Brushing and stenciling with
a silk screen are not the only ways in which the conducting and resistor
paints are applied. For example, a decalcomania, .on which the circuit
is printed on a thin flexible film that can be transferred to the final
surface, is useful in applying the circuits to cylindrical or irregularly-shaped
objects. The film is removed by firing.
Most standard printing
processes are also used. As a single example, the required design can
be raised on the face of a rubber stamp, and this stamp can be pressed
first on a pad of conducting ink and then on the surface to be printed.
Plating of this printed design will increase its conductance if necessary.
In the same way, other printing processes such as engraving, lithographing,
and intaglio are also employed.
You old-timers who used to draw
your own grid-leaks with a lead pencil were using a form of printed
circuits that still may have possibilities. Pencils having "leads" of
varying degrees of conductivity, or pens filled with conducting inks
are being used experimentally. With such devices an experimental circuit
could be drawn and constructed ready for testing all at one and the
Condensers can be painted, too, by employing
silver disks painted on opposite sides of the base plate so that the
plate material becomes the dielectric. If the plate is constructed of
high-dielectric material, condensers of reasonable capacity can be obtained
by this method; otherwise, miniature thin-disk ceramic condensers are
often employed by soldering them with a low temperature solder directly
to a silvered area on the base.
Printed inductors are also used,
especially in the low-inductance values. Spiral forms are used on flat
bases, although the more conventional forms can be used when the circuit
is printed on the tube envelope or a cylindrical base plate as is shown
in Fig. 9. The inductance of a spiral conductor can be increased by
covering it with an insulating layer of lacquer and then painting another
spiral right on top of it and connecting the two in series, painting
another spiral on top of that, etc. The distributed capacity and the
Q of the circuit required are the limiting factors to the usefulness
of this method.
Placing a layer of magnetic paint, made of a
colloidal suspension of powdered magnetic material, both beneath and
above the spiral conductor, with insulating layers serving to protect
the turns of the inductance from shorting. will also increase the inductance.
The spraying of conducting films on insulated surfaces is another
method of printing circuits. The same paints can be used in paint spray
guns as for the stenciled-screen process; or molten streams of metal
can be sprayed through locating stencils. Guns are available in which
the metal to be sprayed is fed into the gun in the form of a wire, where
it is heated to the melting point by a hydrogen-acetylene or other flame.
Compressed air is used to atomize the molten metal and to drive it on
to the work. This molten material can .be sprayed on wood, Bakelite,
plastic, and even ceramic surfaces.
One popular method employs
a plastic base plate. This plate is sandblasted through a mask so that
shallow grooves are cut where the conductors are needed. These grooves
are sprayed full of molten metal, after which the surface can be milled,
leaving conducting lines that are flush with the surface of the plastic
Still another scheme uses an insulated base plate
with a thin evaporated coating of conducting metal. This is covered
with a photosensitive film and exposed to light through a mask. The
film is developed so that the portions exposed to light are removed,
and the remaining portions, outlining the desired circuit, resist an
abrasive spray so that the protected portions beneath remain intact
while the rest of the metallic coating is cut away by the sand blast.
Another method of producing "printed circuits" is by chemical
deposition. This method is not used much on a commercial basis because
of the very thin layers deposited and other technical difficulties,
but it consists essentially of depositing a thin silver coating on a
masked surface by the same chemical methods that are used in silvering
mirrors. Increased conductivity can be secured by repeated silvering
or by plating.
Cathode sputtering and evaporation are two other
processes for depositing the metallic film. In the former, the material
to be deposited is used as a cathode and the masked base plate is used
as the plate of a temporary vacuum tube. The "plate" is maintained at
a high positive potential with respect to the cathode, and the latter
is raised to a volatizing temperature. The metal particles emitted by
the cathode are attracted to and deposited on the base plate through
the stencil openings.
The evaporation process is the same except
that the plate is not maintained at a high positive potential. The cathode
material is simply heated in the vacuum until it vaporizes on to the
work. This permits the use of non-metallic as well as metallic base
plates. In neither case is the film deposited thick enough to be used
for conductors, but this can be overcome by plating.
technician is very familiar with one form of printed circuit: the die-stamped
loop antenna. This is produced by placing a thin sheet of copper on
top of a composition or bakelite panel with a layer of thermoplastic
cement between. This sandwich is placed in a punch press, and at one
stroke the metal is cut into a helix and is bonded to the panel.
Dusting is the final major method of printing circuits. This consists
of depositing a layer of metallic dust on a base plate along the lines
where conductors or resistors are required and then raising the temperature
sufficiently to drive off the bonding material and to fuse the metal
particles together and to the plate. The entire plate can be covered
with an adhesive material and the dust applied through a stencil, or
the adhesive material can be applied through the stencil and then the
whole plate subjected to dusting, with the same results.
9. Two complete high-frequency transmitters ready to be connected to
a power supply. The one printed on the glass envelope of the 6J4 tube
operates on 136 mc.; that printed on the ceramic cylinder surrounding
the subminiature triode operates on a frequency of 116 mc. Both transmitters
are intended for grid modulation.
While an attempt
has been made to touch on all of the methods ordinarily used for printing
circuits, the new industry is advancing so rapidly that one cannot be
sure how long this will hold true. Very recently, for example, the Glass
Products Company of Chicago announced a new process, "Micro-screening,"
which they claim has several advantages over the silk-screen methods.
Unfortunately, because of current patent proceedings, details of this
new method are not available.
Several illustrations are given
to show the wide variety of devices to which printed circuits are applied.
For a more detailed discussion of the various methods discussed in this
article, the author recommends the purchase, for 25c, of "Printed Circuit
Techniques," by Cledo Brunetti and Roger W. Curtis. This National Bureau
of Standards Circular 468 can be obtained from the Superintendent of
Documents, U. S. Government Printing Office, Washington. D. C. An excellent
group of references for further reading will be found in the back of
Part 2 of this article will be concerned solely
with explaining and illustrating how the experimenter can design and
construct his own printed circuits with materials easily obtainable.
(To be continued)
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