you think of printed circuits, probably what comes to mind is what is
really a photo-etched circuit board. In the early days of printed circuits
a lot of the circuits actually were 'printed' on a substrate of
some sort. A silkscreen process was often used for low resistance interconnects
and printed inductors, as well as for printed resistors on materials
like fiber, plastic, phenolic, and even (yikes) asbestos board. There
is still what is called a thick film screen printing process used today
primarily in military systems, but overwhelmingly the photo etching
process is used to generate circuit boards. In the era of vacuum tubes,
it was not uncommon to have grid biasing circuits printed directly on
the glass enclosure using a resistive paint and then trimming for the
target resistance by removing excess material. This article shows an
example of a circuit printed on a 6J6 twin-triode tube.
available vintage Radio
News articlesThanks to Terry
W. for providing this article.
By John T. Frye
Part 2. A discussion of the techniques and equipment
used in making printed circuits for home-built units.
of this article, appearing in the December 1949 issue, presented
the various methods by which these miniature circuits are produced commercially.
Now we are ready to roll up our sleeves and get down to the pleasurable
business of making our own printed circuits.
Fig. 1. Kit of printed circuit paints made by Microcircuits
Co. of New Buffalo, Michigan. The code oscillator (see circuit.
Fig. 6) is built from kit.
A quick review of all the methods of producing these circuits reveals
that the simple brushing of conductor and resistor paints onto a base
plate is the most practical way to start experimenting. Such a system
requires an absolute minimum of equipment; yet it produces results that
compare quite favorably with the much more complicated methods used
in mass production.
The first things we need are conductor and
resistor paints. There are two kinds of conductor paint in general use
- copper and silver. The copper paint is cheaper, but its resistance
increases with age to a terminal two to five ohms per inch. Silver paint,
on the other hand, will maintain its resistance of only a few tenths
of an ohm per inch for years. In all circuits except temporary experimental
ones, the silver paint is well worth its additional cost.
we need two or three different mixtures of resistor paint. More than
one degree of conductivity is needed if we are to be able to draw a
wide range of resistance values and still keep our resistors of reasonably
We shall also need a solvent material that can
be used to thin the paints when they become too thick through evaporation,
and to clean the brushes. In addition, we should have a good insulating
lacquer, the uses of which will be described later. Finally, we must
provide ourselves with base materials upon which we can draw the printed
We could prepare our own paints, but most of us do
not have the materials, facilities, nor inclination to do this. Fortunately,
it is not necessary as the paints can be purchased already prepared.
The E. I. duPont de Nemours Co., Inc., the Metaplast Co., Inc., and
the Acheson Colloids Corporation are among the companies that supply
these paints commercially, but it is doubtful if they would be interested
in supplying small quantities. The Microcircuits Company of New Buffalo,
Michigan, however will supply any quantity of needed materials. In fact,
they sell a "kit" of supplies for the beginning experimenter that is
illustrated in Fig. 1. This kit includes a bottle of copper paint, another
of silver paint, three resistor paints of different degrees of conductivity,
a bottle of lacquer, and a bottle of thinner and brush-cleaner.
Fig. 2. Two-stage amplifier painted
on the glass envelope of a twin-triode 6J6 tube.
Fig. 3. (Top row) Four-tube receiver printed on 3/32" Lucite
plate. 2" wide and 5" long. At left is stenciled silver wiring
with complete receiver at right. (Center) Four-tube receiver
printed on thin steatite plate. (Bottom row) Development of
a four-tube receiver on thin steatite plate 2" wide and 3" long.
The plate at left had paints applied with a brush except for
spiral inductances. The center one was stenciled. Leads from
the complete receiver at right connect to batteries and speaker.
All receivers use square law detector, two stages of pentode
amplification, and triode output.
Fig. 4. A base plate all ready for the painting of conductors.
Note that brass rivets are used as terminals and to form crossovers.
Fig. 5. Actual painting of resistors. Note how resistance lines
are lapped over the conductor lines and preparation of abutments.
Table 1. Resistance and wattage values for various sizes of
resistors painted with one kind of resistor paint. The intersection
of the width and length values gives the resistance value above
and the wattage rating below. For example, a resistor 1/4" wide
and 3/4" long has a resistance of 1500 ohms and a wall age rating
Table 2. Characteristics of some typical subminiature tubes
used in printed circuits.
Fig. 6. Circuit diagram of the code oscillator shown in the
photograph of Fig. 1.
Fig. 7. Five types of grid-modulated 132-144 mc. transmitters.
The two at the top left are painted on thin steatite cylinders
housing subminiature tubes. The one top center is printed on
the envelope of the 6K4 triode. The second from top right is
painted on the envelope of a T-2 subminiature type tube while
the circuit at top right is a flat-plate transmitter. The bottom
row (left) shows front and back views of a plate transmitter
with the completely assembled transmitter at the right.
A self-explanatory photograph of various printed circuit units.
Just about any material, if it is properly prepared, can be used as
a base upon which to paint the circuits. The only requirements are that
the material be rigid so that it will not flex and cause the lines painted
upon it to crack; that its surface be not too rough or porous; that
it be chemically inert; and that it have an insulated surface or one
capable of being insulated.
The paints do not stick too well
to glass unless the surface has been roughened by sand-blasting or etching;
but the experimenter can overcome this by applying a thin coat of lacquer
to the glass and then painting his circuits on this coating. Fig. 2
shows a complete two-stage amplifier circuit painted on the envelope
of a 6J6 tube.
Asbestos board, fiber-board, etc., can be used;
but if the surface of this porous material is not treated, absorption
will cause the characteristic values of resistors painted upon it to
be greatly changed. A sealing coat of lacquer will avoid this trouble.
The paint adheres very well to plastics because the solvent
used causes the surface of many of them to become slightly dissolved.
A heat-resisting coating on a metal base works quite well, for the metal
aids in carrying the heat away from the resistors. Two coats of lacquer
will provide such a coating.
No matter what base is used, the
cardinal principle is that the surface receiving the paints must be
absolutely clean. The slightest trace of grease, even that left by the
touch of a finger, will prevent the paint from making a good bond with
the base. Lacquered surfaces must not be touched. Glass, porcelain,
and plastics can first be cleaned with alcohol, gasoline, or carbon
tetrachloride, then washed with soapy water, and finally thoroughly
rinsed with clear water and dried.
Laying out the circuit should
be done with great care. Conducting lines should be as short and direct
as possible. "Cross-overs" of conducting lines should be held to a minimum.
Correct spaces should be left for resistors. Terminals should be provided
in the form of rivets. The possibility of inductive and capacitive effects
between adjacent conducting lines must be considered.
these details should be worked out with paper and pencil, and then the
complete diagram can be transferred to the base plate by means of carbon
paper. When actually drawing on the base with a pencil or drawing ink,
it must be remembered that these lines are conductors in themselves
and must not be left where they will connect parts of the finished circuit.
Fig. 4 shows a base plate of laminated paper and phenolic material
all ready for painting. Brass eyelets have been inserted at the proper
places to furnish terminals; the small brush and the silver conducting
paint are all ready to paint the conducting lines.
to brush on the paint, though, it is important to see that the paint
is very thoroughly mixed. Remember our "paint" is composed of metal
particles and a carrying solution. When undisturbed, the heavy particles
settle to the bottom and stay there. A line drawn with a brush dipped
into a bottle of this undisturbed paint would consist chiefly of non-conducting
thinner and be a very poor conductor.
Lengthy stirring will
mix the paint, but that makes it necessary to have the bottle uncorked
while doing the stirring. The thinner used is extremely volatile so
that the drying time will be short, and if the bottle is left un-stoppered
for any appreciable length of time, the paint becomes too thick to brush.
A temporary cover should always be set on top of the bottle of paint
except when the brush is inserted. A quick stirring followed by lengthy
shaking of the stoppered bottle is the best way to mix the paint, and
it should be kept mixed by repeated shakings during the painting process.
The conducting lines should be carefully drawn along the direction
of current flow, care being taken to make the lines of as nearly uniform
thickness and width as possible. The paint should be run up over the
edges of the eyelets to make sure that a good electrical connection
is obtained at these points. "Abutments," across which the resistors
will be bridged should be drawn in as shown in Fig. 5.
of the paint used will provide information on the drying time required.
In the case of the Microcircuits silver paint, the air-drying time is
one half hour to an hour. Copper and resistance paints take considerably
longer. The process can be speeded up by mild heating, not over 1500
F., in an oven or with an infrared lamp. Greater heat than this may
result in "bubbling" of the paint.
When the silver paint is
dry, you are ready to paint in the resistances. Painted resistors are
figured from a standard resistor one inch square, painted with a single
thickness of the given paint, and having a heat dissipation of ten watts.
As you know, the resistance of such a resistor will be directly proportional
to the length measured along the direction of current flow and inversely
proportional to the width and the thickness. The wattage rating varies
directly with the surface area.
Reductions in the dimensions
of this standard square will result in other resistors whose resistance
and wattage values are in accord with these laws. The resistance may
be higher or lower than that of the standard resistor, depending upon
whether the "width" or the "length" of the square was reduced. Table
1 shows the different values of resistance and wattage obtained when
a standard resistor of 500 ohms is reduced in 1/8" steps along its width
or length or both. In the same way, when you know the resistance of
a standard one-inch-square resistor painted with a particular resistance
paint, you can always estimate the dimensions of a needed resistor whose
value lies within the range of that particular paint.
should be drawn with a thick, even layer of paint that overlaps the
silver paint by at least 1/16" If the resistor is of uneven thickness,
the heat generated by the 12
R losses will be unequally distributed
and may cause the resistor to be damaged when operated near its maximum
AfAfter the resistors have dried for fifteen
minutes, they may be baked at up to 250° F. without danger of bubbling.
The cold resistance of a resistor that has been baked at 200° F.
is about one half that of the same resistor with normal air-drying;
so the method of drying should be taken into account when calculating
the size of resistors.
It is impossible to avoid having some
crossovers of conducting lines, and there are two ways of making them:
first, you can bridge one of the lines with a piece of paper that has
been soaked in an insulating varnish, shellac, or lacquer; and, after
it has thoroughly dried, you can paint the other line right over the
top of this bridge. The other method is to place a brass or copper rivet
on each side of the line to be crossed and then carry the crossing line
down through one of these rivets to the opposite side of the base plate,
along a conducting line to the other rivet, and then back up. This latter
scheme is more permanent and is less likely to cause trouble because
of any high capacitance between the crossed conductors.
in Part 1, small condensers can be painted by using silver-painted areas
on opposite sides of the base plate, especially when the plate is of
thin, high-dielectric material; but the experimenter will find it much
easier and more accurate to use the tiny disc-type ceramic condensers
by connecting them to the conducting lines at the points needed.
These condenser leads, as well as tube leads, transformer leads,
etc., can be connected to the painted lines by either soldering or painting.
When soldering, the painted surface is first sanded clean and a drop
of solder placed on it and flattened out; then the tinned conductor
is placed on this drop of solder, which is. re-melted just enough to
bond it to the conducting lead.
In order to get the "feel" of
drawing printed circuits, it is a good idea to reproduce in miniature
some simple electronic device such as the code oscillator shown in connection
with the Microcircuits kit; but the real field for these printed circuits
lies in those pieces of equipment where the reduction in bulk and weight
is of actual and not just "curiosity" value. Careful study of the accompanying
pictures of transmitters, receivers, and amplifiers will show the reader
what has been accomplished along these lines by expert technicians and
will provide him with many valuable pointers in regard to layout, printing
of inductors, tube-placement, etc.
While standard components
can be used with printed circuits, there is not much sense in using
a log-chain leash on a Pekingese or in employing a tube or transformer
that is five or six times as big as all of the rest of the circuit.
Subminiature tubes and the other tiny components used in hearing aids
make ideal companion units for printed circuits, and practically all
of them will be found advertised in the pages of RADIO & TELEVISION
NEWS. Table 2 gives subminiature tube characteristics that will be found
useful in designing printed circuits.
In conclusion, the writer
would like to say that he never could understand what pleasure the head-hunters
got out of shrinking the noggins of their enemies to the size of a human
fist; but he knows there is a very decided thrill in building equipment
in about one-tenth the space normally required.
Robert F.; "Design and Repair of Printed Circuits."
Cledo & Curtis, Roger W.; "Printed Circuit Techniques," National
Bureau of Standards, Circular 468.
Brunetti, Cledo & Khouri,
A. S.; "Printed Electronic Circuits," Electronics April 1946.
Pritikin, Nathan; Glass Products Co., Inc. Schwab, M.; "Printed
Electronic Circuits," Centralab Div.