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
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 silk-screening techniques.
Part 2. A discussion of the techniques and equipment used in
making printed circuits for home-built units (January
Thanks to Terry W. for providing this article.
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. Frye
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
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 printed circuits.
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 to brush.
Fig. 1. The "Couplate" unit. It contains a complete interstage
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 proper location.
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.
Such 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 successful.
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
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 same operation!
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 base plate.
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.
The radio 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 this booklet.
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)
Posted May 25, 2013