New Advances in

Printed Circuits

United States Department of Commerce

National Bureau of Standards

Miscellaneous Publication 192
Printed Circuit Techniques

. . . Printed circuits have emerged from purely laboratory experiments to

become one of the most practical new ideas of mass production of

electronic devices. Although many of the techniques were known and

used long ago, printed circuits as we understand them today represent a comparatively recent accomplishment. Printed Circuit Techniques

presents a comprehensive treatment of the subject.

. . . The scope of the book is indicated by the following chapters:

Painting Die-Stamping

Spraying Dusting

Chemical Deposition Performance

Vacuum Processes Applications

. . . The book consists of ten chapters, totaling forty-three large, two-column

pages. It is adequately illustrated with twenty-one halftones, eighteen

line cuts, and five tables.

. . . Available from the Superintendent of Documents, U. S. Government

Printing Office, Washington 25, D. C. Price: 25 cents a copy.
UNITED STATES DEPARTMENT OF COMMERCE • Charles Sawyer, Secretary

NATIONAL BUREAU OF STANDARDS • E. U. Condon, Director

New Advances in

Printed Circuits

Proceedings of the First Technical Symposium

on Printed Circuits held October 15, 1947, in

Washington, D. C, under the Sponsorship of the

Aeronautical Board and Technical Direction of the

National Bureau of Standards

Miscellaneous Publication 192

Issued November 22, 1948

For sale by the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C.

Price 40 cents
PREFACE

The wide interest shown in printed circuits by Government, scientific

and industrial organizations throughout the country, since the first release

of information by the National Bureau of Standards in February 1946, increased to the point where it was found desirable to call a general technical

symposium for the purpose of bringing together all interested persons and

affording a free and open presentation of the status, applications and

limitations of the art.

The symposium was sponsored by the Electronic Components Subcommittee of the Aircraft Radio and Electronics Committee of the Aeronautical Board with the cooperation and under the technical direction of the National Bureau of Standards. The meeting was held in the Department of

Interior Auditorium, Washington, D. C, on October 15, 1947. Twenty

speakers and an audience of over 700 persons reviewed the many methods

of printing circuits. New techniques were described in detail, many examples of new approaches to the method of printing circuits were demonstrated, and a number of samples were exhibited by manufacturers.

Since the symposium did not treat, and hence these proceedings do not

include, all details of printed circuits, the reader is referred to National

Bureau of Standards Circular 468, "Printed Circuit Techniques" for a complete treatment of printed circuit techniques.

E. U. Condon, Director.
FOREWORD

The authors of this volume are men who were either responsible for or played an

important role in the developments described. It was beyond the scope of the symposium to attempt to cover completely all the various processes, applications, and other

matters related to printed circuits. Several of the principal methods have been treated in

detail ; others have only been introduced, with the thought of stimulating further interest

and experimentation in the. processes by laboratories throughout the country.

The symposium was opened by Mr. R. J. Framme, Chairman of the Electronic Components Subcommittee. The general interest of the Armed Services in printed circuits was

indicated by the introductory remarks of General F. L. Ankenbrandt, ACO, Headquarters USAF and Captain Frank Akers, USN, Bureau of Aeronautics. General Ankenbrandt said that:

The Electronic Components Subcommittee of the Aeronautical Board has noted that

new electronic developments concerning printed circuits are particularly adaptable to

aeronautical needs. The reasons are quite easy to see : You have the inherent advantages

of extremely small 'size, light weight, simple fabrication, extremely durable materials

and low costs. This technical symposium of all interested persons of Government and industry has been arranged to provide opportunities for the exchange of ideas and to promote rapid dissemination of information on printed circuit techniques . . . Printed circuitry came into its own during the war, and being pushed very rapidly, quite a number

of highly classified applications were developed and actually put into use. The Military

has been extremely aware of the wide application of printed circuits to industry as a

whole and consequently has been very glad to declassify the techniques developed during the war. These techniques are for the use of industry and the world as a whole for

the advancement of the electronics art. Naturally, the Military have their own specialized applications. It takes very little imagination to see where printed circuitry will fit into aircraft electronics equipment. We want the lightest equipment possible. Someone

once said, "We want electronics equipment that takes up no space, weighs nothing, but

will do everything". Perhaps we almost have this in the form of printed circuits.

Captain Akers pointed out that

We (the Navy) are becoming concerned over the increase in weight and bulk in our

equipments, particularly in aircraft design. Today with the modern high performance

airplane, electronics is a must if we are to get the benefit of the tremendous performance

available. A new Navy jet fighter has so much of its space taken up by large air ducts

necessary to feed its jet engines that we are unable to install tactical communication

equipment without repackaging it . . . We have made electronics equipment smaller, have

made it lighter and have made it work. We have miniature tubes, are bringing out subminiature tubes, potted circuits and printed circuits, the greatest weight and space saver

of them all. We need to carry this further to handle the more complex problems and to

better control the characteristics under varying environment. The purpose of this symposium is to bring you up to date on the progress and particularly to make all and the

latest information available to equipment manufacturers.

Attention is called to the summary of data obtained from the questionnaires filled out

by the persons attending the symposium. This information, which follows the formal

papers, suggests the potential possibilities and applications of printed circuits.

Cledo Brunetti, Editor.

in
Electronic Components Subcommittee

of the

Aircraft Radio and Electronics Committee of the Aeronautical Board

Mr. R. J. Framme, Chairman, Air Materiel Command, Wright Field, Ohio.

MR. Harry Mulkey, Air Materiel Command, Pentagon Bldg., Washington

25, D. C.

MR. Alvin Saltzman, AMC, Watson Laboratories, Red Bank, N. J. Mr. A. W. Rogers, Signal Corps Engineering Laboratories, Fort Monmouth, N. J. Mr. A. E. Zdobysz, Bureau of Aeronautics, Navy Department, Washington

25, D. C.

Mr. J. N. Hall, Bureau of Ships, Navy Department, Washington 25, D. C.

Mr. F. M. Gager, Naval Research Laboratory, Anacostia, Washington 25,

District of Columbia.

Dr. Cledo Brunetti, National Bureau of Standards, Washington 25, D. C.

Mr. Lew Sieck, Secretary, Aeronautical Board Secretariat.

The symposium was presided over by Dr. Cledo Brunetti. The proceedings were prepared under the editorship of Dr. Brunetti with the assistance

of D. S. Hoynes and W. J. Cronin of the National Bureau of Standards.

Assistance in planning the symposium was also rendered by Dr. R. W.

Curtis of the Bureau's staff.

IV
CONTENTS

Page

Preface II Foreword Ill

1. Status of printed electronic circuits 1

2. Centralab printed circuit techniques 10

3. Conductive silver preparations 15

4. Printed resistors 17

5. Metal films and their application to the fabrication of resistors

and printed circuits 25

6. Trends in military communication and the applicability of

printed circuits 27

7. Vitreous enamel dielectric products 30

8.- Printed electronic components on glass 34

9. Electrical circuits on plastics and other nonconductors 36

10. Imprinted circuit inlays 41

11. Spraying techniques for producing electronic circuits 42

12. Physical aspects of printed circuit conductors 44

13. Mechanization of electrical wiring 46

14. Die-stamped wiring 48

15. Printed circuit production 51

16. Prefabricated components for printed circuit application 54

17. Typical commercial applications 61

Two stage printed circuit amplifiers 61

Printed circuits in transmitters and receivers 61

First application of the printed circuit to a hearing aid 62

Mycalex 410 base plates for printed circuits 64

Printed inductors 64

Herlec bulplates 65

18. General discussion 66

Summary 66

Questions and answers 68

Summary of questionnaires 72

1. Status of Printed Electronic Circuits1

Printed circuits have emerged from purely

laboratory experiments to become one of the

most fascinating new ideas of mass production

of electronic devices in many years. Although

some of the techniques employed in the practice were known and used long ago the printing of complete electronic circuits is a comparatively recent accomplishment. Interest has

increased steadily since February 1946, when

the military approved the release of the development which had played an important part

in the design of the miniature radio proximity

fuze for the trench mortar shell. Printed electronic components and subassemblies have been

manufactured in large quantities, and the first

commercial electronic set designed for and produced by the printed process, a hearing aid, has

been placed on the market.

Over the past 2 years the National Bureau

of Standards has received an unprecedented

demand for technical information on the subject from other Government agencies, from industry and scientific institutions. Because of

this interest, a thorough study of the techniques

was undertaken. This study, to which valuable

contributions were made by many industrial

and scientific organizations, revealed a large

number of possible methods for condensing the

size of electronic assemblies, for mechanization

of electronic wiring and for reducing the wiring essentially to two dimensions.2 These methods are simply different ways of

reproducing a circuit design upon a surface.

Since printing may be defined as "The act of

reproducing a design upon a surface by any

process," the methods have come to be classified as printing techniques. The Bureau's

studies unfolded so many methods of printing

circuits that it would take much of the remainder of the symposium merely to introduce each

properly. Listed below are twenty-six individual methods, each distinct from the other.

Combinations of these methods have also either

been used or proposed but will not be described

at this time. The methods are distinguished

principally by the way in which the conductors,

resistors, inductors, capacitors, or combinations

of them, are applied. Inductors for high frequency circuits are usually printed in the same

operation as the conductors. Electronic tubes

1 By Cledo Brunetti, Chief, Engineering Electronics Section, National Bureau of Standards.

2 Many of the technical details of printed circuits presented herein have been abstracted from a circular entitled "Printed Circuit Techniques" prepared by the National Bureau of Standards. Further in- formation regarding any of the processes may be obtained by consulting the circular, available from the Superintendent of Documents,

U. S. Government Printing Office, Washington 25, D, Q.i price 25

cents.

are attached by hand or machine soldering, or by solder-dipping. Other components such as transformers, electrolytic capacitors and speakers are attached by conventional methods. Unless otherwise mentioned, both conductor and

resistor paints are applied in the same manner.

The following is a partial list of all the

known methods, described by their principal

features

:

1. Painting conductors and resistors with a

brush, and soldering on tiny disk capacitors;

2. Stenciling conductors and resistors in

place using suitable paints (this is often re- ferred to as the stenciled-screen process) ; 3. Stenciling or painting the circuit onto a base plate which1 incorporates vitreous dielectric capacitors within itself

;

4. Stenciling conductors and capacitors in

the same operation, using silver paint and a base plate of high dielectric constant;

5. Stenciling the conductors on a phenolic

base plate prior to curing, and curing phenolic

and paint simultaneously

;

6. Printing conductors using silver paint and

ordinary newspaper printing press techniques

;

7. Application of resistor and conductor

paints with special ruling or fountain pens,

brushes or pencils;

8. Application of paints with ordinary rubber, metal or plastic stamps

;

9. Application of paints, using steel or copper plate engraving techniques;

10. Dipping the stencil-coated surface into a suitable paint;

11. Spraying resistor and conductor paints

through masks or stencils;

12. Spraying molten metal conductors

through a stencil

;

13. Spraying molten metal onto a premolded

panel, then machining or grinding to bring out

the circuit pattern

;

14. Die casting or pouring metal into preformed grooves in an insulating surface

;

15. Spraying or pouring chemical silvering

and reducing solutions onto a stencil-coated

panel to deposit metal films in the form of the

desired circuit. With proper choice of metallic

salts, it is possible to produce printed circuits

in colors

;

16. Imprinting a catalyst onto an insulating

panel and depositing copper in the desired circuit pattern;

17. Evaporating metal films on panels

through a stencil in a vacuum;

18. Evaporating a metal film completely over

the base plate and spraying off the metal with

Printed Circuit Techniques 1
an abrasive through a circuit-defining stencil;

19. Cathode-sputtering metal films on panels through a stencil in a vacuum;

20. Cracking hydrocarbons at high temperature onto a stencil-coated surface to produce

resistors

;

21. Die-stamping circuit wiring from a metal

plate and soldering or riveting to a panel

;

22. Die-stamping circuit wiring out of foil or thin metal sheet and either pressing into an

adhesive coated surface in one operation or attaching later with adhesive;

23. Die-stamping metal foil onto a panel

using a hot-die which imprints a metal pattern

of the circuit into the surface;

24. Dusting carbon or metallic powders onto

a tacky surface through a stencil and either

firing or electroplating;

25. Applying conducting powders to a sur- face in the proper circuit pattern by electrostatic means, then flash firing the powder;

26. Making up decalcomanias with the metallic circuit pattern, then applying to the sur- face and removing the decalcomania by firing.

These methods may be divided into six principal classes: painting, spraying, chemical

deposition, vacuum processes, die-stamping,

and dusting.

Painting

Metallic paints for conductors, inductors and

shields are made by mixing metallic powder

with a liquid binder to hold the particles together and a solvent to control the viscosity.

Resistance paints are made in similar fashion

using either carbon or metallic powders. The

circuit is painted on the surface by brush, stencil, printing process or any of the other methods outlined above. Tiny disk capacitors and

subminiature tubes are added to complete the

electronic unit. If desired, the capacitors may

be incorporated directly within the base plate

itself by a vitreous dielectric process.

Since the techniques of applying paints to

most types of surfaces are to be treated in

many of the other papers on the program, only

the status of the art will be treated here. It

is possible to purchase paints for conductors

ready-mixed and applicable to almost any solid

surface. Resistor paints are not readily procured although the ingredients are available

commercially. Like most commercial formulas

which have been developed at considerable cost

to the organization selling the product, most of

the resistor paint formulas are justifiably re- garded as trade secrets. Some insight as to the

3 See tables 1 to 3, pp. 6 to 8. "Printed Circuit Techniques," Superintendent of Documents, XT. S. Government Printing Office, Washington 25, D. C.

composition of paints has been attained as the

result of limited experiments at the National

Bureau of Standards.3 Paints for printed circuits are made up of se- lected combinations of constituents, the most

important of which are the pigment or conducting material, the binder which holds the pigment on to the surface, and the solvent, which

adjusts the viscosity. In those cases where it

is not possible to heat the base plate to high

temperature (as, for example, when a thermoplastic base plate is used), a reducing agent is

substituted for the solvent. Such a paint for

conductors could, for example, be made of silver

oxide, linseed oil and a reducing agent such as hydrazine sulfate. The filler is used only in

resistors, when needed to increase the resistance value. Protective coatings are applied to

both resistors and conductors for protection

against atmospheric conditions and mechanical

abuse.

It was found at the National Bureau of

Standards that the preparation of conductor

paints is not an unusually difficult task although

careful attention to the quantity and quality

of the ingredients is necessary if good adhesion

to the base plate and high conductivity is desired. The table shows formulas for use on

base plate materials ranging from ceramics to

the thermoplastics.

In the present state of the art, it is not feasible to present a set of resistor paint formulations which one may use without special attention in the laboratory. A paint formula

which is successful for one experimenter may

not work well for another because of the manner in which the ingredients are mixed, the

quality of the ingredients, the amount of evaporation of solvent prior to application, or any

one of other small but important factors.

So far as is known, there is no single paint

formula which can be used to print resistors

from one ohm to several hundred megohms. To

cover the complete range, it is necessary to use

several paint mixes and vary the number of

applications as well as the dimensions of the

printed resistor.

Current knowledge points to the use of

carbon-blacks of relatively small particle size

for resistor paints, those of particle diameter

in the range of 20 to 50 millimicrons. The

carbon should have its surface impurities, principally oxygen, removed by calcining, that is

by heating to a temperature of approximately

1,050° C for 4 hr., preferably in an inert atmosphere such as nitrogen. After calcining,

the carbon is dispersed in the binder by ball

milling. The composition, size and density of

the balls, the speed of the mill and milling time

are all important factors in this operation.

Much also remains to be learned about the

2 Circulars of the National Bureau of Standards
factors contributing to noise in resistors. Noise

appears to be a function of particle size; the

finer the carbon, usually the less the noise.

This is perhaps due partly to the fact that

smaller particles present more contacts. A good

deal of experimentation, including X-ray and

electron microscope studies, is now underway

seeking to clarify the effect on resistor performance of carbon particle size and shape,

particle arrangement in solution, and other factors.

A conventional printing press may be used

for printing conductors on a base plate. This

press, of course, may be operated by hand or power. In such a press, the soft rollers first

pass over the ink plate which is coated with

conducting ink. On the return motion, they

sweep over the block which carries the metal

pattern of the circuit to be printed and coat

it with a layer of ink. In the final step, the

carrier bearing the insulated base plate on

which the circuit is to be printed presses this

plate firmly against the inked block thus impressing the desired pattern on the base plate.

Units of this type may print a layer of silver

paint 0.002 or 0.003 in. thick. To increase the

conductance, the printing operation may be

repeated.

Spraying

The spraying of conducting films on insulating surfaces, like the spraying of ordinary lacquers and paints, not only has popular appeal

but is fairly easy to adapt to production line

practice. A successful development of one

spraying technique will be presented later in

the program, hence the subject will be treated

only in general terms here. Either molten metal

or paint may be sprayed onto an insulating

surface through a stencil using a suitable spray

gun. In some processes, metals in the form of

wire, powder or solutions are supplied to the

gun and sprayed directly on the surface

through stencils to form the conductors. The

same operation also fastens in place resistors,

capacitors and other electronic components

which have previously been set either within

or under the panel with their leads making

junctions with the conductors. Conductor and

resistor paints may also be sprayed. In spraying resistors, the electrical values may be controlled by controlling the speed of the conveyor

belt which carries the work under the spray

gun as well as by regulating the flow of material

from the gun.

Chemical spraying is possible, using a spray

gun with two openings, one ejecting a silvering

solution and the other a reducing liquid. Another method which accomplishes the same rePrinted Circuit Techniques

793811—48—2

suits as spraying is a process wherein the

system of conductors is die cast into the desired

pattern on an insulating base. Recesses for the

conductors are molded into the base plate and

an alloy chosen for the conductors which ex- pands on cooling thus holding them firmly

within the base. Helical resistors for electric

heaters are made by setting up a metal spray

gun on the carriage of a lathe and spraying

a helix on a ceramic tube, using the threadcutting mechanism of the lathe. The spray gun

beam is defined by a suitable aperture.

A novel method adaptable to electronic wiring involves " spraying-off" the metal from a

metal-plated plastic to leave the desired circuit

wiring. A plastic or other insulator having on

its surface a thin evaporated coating of metal

such as silver or copper has a stencil applied

over the surface by a photographic process.

When the surface is exposed to a spray of abrasive material, the metal is removed from all

parts not covered by the stencil. With this

method, circuit wiring including resistors and

inductors may be printed with a dimensional

tolerance of ±0.0002 in. The process may be

used to trace out contacting segments and other

related components of electrical systems such

as radio sonde elements.

Chemical Deposition

This process involves the well known methods of silvering mirrors. A stencil is placed

over the base plate and a silvering solution

poured on it. Standard silvering solutions such

as silver nitrate mixed in ammonium hydroxide

may be prepared which, with the addition of a reducing agent, serve to precipitate metallic

silver onto the insulating surface. Thin metallic films of relatively low conductance are formed. The films cannot be soldered to directly. They may be built up by repeating the

silvering process as often as desired or, more

practically, by electroplating.

Not only silver films but those of copper,

nickel, gold, iron and other metals and alloys,

such as silver-copper, may be deposited on insulating surfaces by chemical methods. An interesting variation is offered by the possibility

of selecting the metallic salts so that metal films

of different colors are deposited, thus allowing

the printing of colored electronic circuits. Circuits of different colors may be used for identifying different sections in a multisection unit,

for classifying as to frequency and volume

ranges and other uses. Usually, however, such

metallic salts produce high resistance films and

as such may be used to produce resistors of

limited wattage.

3
Vacuum Process

The vacuum techniques employed to produce

metallic layers of nonmetallic surfaces which

may be adapted to electronic wiring are those

of cathode sputtering and evaporation. In the

sputtering process, the metal to be volatilized

is made the cathode and the material to be

coated, the anode. After evacuation, a high

voltage is applied between them. Metal emitted

from the cathode is attracted to the plate by

maintaining the plate at positive potential. In

the evaporation process, the metal is heated in

a vacuum to a temperature at which it evaporates onto the work located nearby. After the

base plate is coated, the stencil is peeled or burned off leaving the desired circuit wiring.

The more practical of the two methods at

present appears to be the evaporation process.

A simple procedure is to place the metal in a

vessel and heat it to vaporizing temperature

by means of either a flame or induction heating. The plates to be coated may be placed

upside down on a supporting grid over the vessel. The thin films formed, as in the case of

the chemical deposition methods, may serve

either as resistors or, when plated, as conductors. Accurately defined areas may be coated

by the evaporation process thus improving the

uniformity of the resistors.

Die-Stamping

There are several methods of producing electronic circuits by the die-stamping process. In

one, a wiring grid is punched out of 1/16-in.

copper plate and silver plated. The grid is then

placed over an array of projecting lugs attached

to various electrical components. It is soldered

to all of the lugs in a single automatic operation. The parts of the grid not desired are

clipped out and the remainder forms the complete wiring of a telephone set. In another

method, circuit wiring is punched out of metal

foil and attached to one or both sides of an

insulating panel.

Radio set manufacturers are now employing

spiral loop antennas die-stamped from copper

or aluminum sheet a few thousandths of an

inch thick. One design is formed by feeding an

automatic punch press with a composition or

plastic panel having a metal sheet over it. The

sheet is coated on one side with a thermoplastic cement. The press has a vertical reciprocating steel die with the antenna pattern on

its face. In a single stroke the die cuts the

metal sheet and attaches it to the panel. The

result is a combined antenna and back or housing for a receiver. The process has been ex- tended to stamping out conductors as well.

The hot stamping process used in the marking of leather and plastic materials lends itself

to the mechanization of electronic circuit manufacture. In this method, a hot die engraved

with the pattern of the conductors, including

inductors, is pressed onto the plastic with a thin

sheet of gold, silver or other conducting foil

between the hot die and the plastic. The foil

adheres to the plastic where the pressure and

heat from the die have been applied, and can

be brushed away at other places, leaving a pattern of conductors. Samples produced with gold

foil have proven satisfactory. The resistors

may be applied in the same way using resistor

material deposited on a film of plastic previous

to the hot stamping operation. Since foils as

thick as 0.002 in. may be used, very good electrical properties are obtained, particularly with

inductors made by this method. Other components to complete the electronic circuit may

be added by riveting, soldering or spot welding.

Dusting

In the dusting techniques, metallic powders

with or without a binder are dusted onto a sur- face in a wiring pattern and fired. The powder

may be held to the surface by coating the latter

with an adhesive through a circuit-defining

stencil. Metal adheres to the surface in the

desired circuit pattern and fuzes strongly to

it on firing.

In one process a suitable bonding material is

selected, such as shellac, wax or any of the

synthetic resins. The material is dissolved in

alcohol or benzine and a thin layer of the solution is sprayed or painted onto the surface. A

stencil bearing the circuit pattern is placed over

it and leafed silver powder dusted on. The unit

is then subjected to a temperature which drives

off the bonding material and fuses the metal to

the plate. A variation is to apply the bonding

material instead of the paint through the stencil. The powder is sprinkled on after the stencil ii> removed and while the bonding surface

remains somewhat tacky.

An electrophotographic method has been developed to hold the powder to the surface in the

proper pattern prior to flashing. It is applicable to any of the usual nonconducting surfaces,

including paper. The surface is first coated

with photoconductive material, then placed

under an electrostatic charging device. The

electrostatic field produces a charge on the sur- face coating. Exposure to light through a positive photograph of the electronic circuit re- moves the charge from that portion of the

photoconductive material illuminated and leaves

an electrostatic image of the circuit. A mixture

Circulars of the National Bureau of Standards

met

min
of leafed silver powder and a binder dusted

onto the surface adheres only to the charged

image. Flashing with a flame produces a pattern of thin silver films, completing the wiring.

Applications

Printed circuits have now been applied to the

design of audio, radio and VHF amplifiers, detectors, oscillators, trigger circuits, hearing

aids, electrical instruments, radio sets, meteorological instruments, electronic ordnance equipment, remote control equipment, radar, subminiature radio transmitters, receivers and

transceivers and even toys and games. Several

of these applications will be described in the

papers to follow.

Amplifiers, special electronic sets and small

radio transmitters and receivers made in the

Bureau's laboratories, have shown performance

qualities comparable to equipment built along

conventional lines, as well as improved miniaturization and ruggedness. Complete circuits

have been printed not only on flat surfaces but

on cylinders surrounding a radio tube and on

the tube envelope itself.

Observance of good electronic wiring practice is as essential to the successful design of

printed circuits as it is in standard circuits.,

In printed circuits the parts are usually placed

very close to each other ; hence the lay-out must

be such that the components do not affect each

other adversely while the circuit is in operation.

In most applications the circuit arrangement

may be chosen in any convenient manner. It

will be found convenient and economical to lay

out the painted circuit in such a way as to keep

the length of leads to a minimum and to avoid

cross-overs. Cross-overs are handled by going

through the base plate and continuing on the

opposite side, by going over the edge, or by ce- menting or spraying a thin layer of insulating

material over the lead crossed.

If the design permits, some advantage may

be gained by placing the low values of resistance on one side of the plate and the higher

values on the other. This reduces the number

of paint applications per face required to produce the requisite number and range of resistors. High values of resistance may be painted

in a small space by zigzagging the lines in any

of the several variations used to denote resistors

in conventional wiring diagrams. If resistors

of large power capacity are needed, they may

be painted on the inside of the cabinet housing

the set. The resistance may also be divided in

two or more parts each placed on a separate

wall to dissipate the heat better and further

increase the power rating.

A significant advantage of printed circuits

is that once manufactured they are less likely

to be tampered with, for tampering is readily

detected. A manufacturer who places a guaranteed instrument or set on the market can

readily determine if the circuit has been altered. Transmitters designed for a fixed frequency and set at the factory are less liable

to be altered by the user. This is an advantage

which may be of some value in the regulation

of the use of personal radio telephones in the

Citizens Communication Band.

A large variety of subminiature tubes are

available which are applicable to the design of

practically every type of low power electronic

circuit (fig. 1.1B). These include many types

of triodes for amplifiers, oscillators (including

UHF types), electrometers, gas filled thyratrons, phototubes, diodes, tetrodes, twin tetrodes, diode-pentodes, converters and several

different kinds of pentodes. The subminiature

tubes have very low filament drain (10 to 200

m.a. at 1.5 to 6.0 volts). They work well as

voltage or (limited) power amplifiers. Their

power output varies from a few milliwatts to

almost one watt. Triodes of general purpose

and UHF types are available with amplification factors of 20 to 60 and mutual conductances up to 5,500 micromhos. At 500 megacycles some of them deliver as much as 700

milliwatts of output. RF pentodes have mutual

conductances up to 5,000 micromhos and plate

resistances from 0.1 to 3.0 megohms.

A complete two stage amplifier painted on

the envelope of a miniature 6J6 tube requires

only connection to a power supply to operate.

The circuit wiring is applied with a stencil

wrapped around the tube. For circuits on tube

envelopes, paints are used that do not require

baking at extremely high temperatures to drive

out binder and solvents. In this way tube performance is not deteriorated by gases which

may be released from its metal parts by the

heat. The circuit may be applied to the tube

envelope either before or after the tube elements are mounted in place. A glass tube was

employed in the unit, although a metal tube

might have been used after first coating the

metal envelope with a layer of lacquer or other

insulating material. Leads from the circuit to

the tube prongs have been painted on with a

brush. Wires may also be soldered to points

directly on the tube envelope, ribbon type leads

usually being employed.

It is generally desirable to avoid lead crossovers. When this is impossible, cross-overs on

glass may be made by painting a thin layer of

insulating lacquer over the lead and, after drying, painting the cross-over lead over the lacquer. Another method is to place or cement

Printed Circuit Techniques 5
Figure 1.1

A, Examples of electronic circuits printed on steatite plates by the stenciled-screen process. To minimize size, subminiature tubes are employed.

B, top row, five types of subminiature 132-144 mc radio transmitters, utilizing printed circuit techniques. All types are grid modulated

and require only connection to a microphone and battery to operate. The oscillator circuits of the two units at the left are printed on the outer surface of a thin steatite cylinder housing the subminiature tube. The circuit of the unit at center is painted on the glass envelope of a 6K4 subminiature triode % in. in diameter and 1% in. long. The transmission second from the right is painted on the glass envelope of a T-2 tube measuring % in. in diameter and 1 in. in length. The circuit of the transmitter at the extreme right is painted on a 3/32 in. steatite plate, 1% in. wide and the same in length. Bottom row, developmental stages of a steatite plate transmitter. The plate at the left carries three silvered spiral inductors and a

single high dielectric constant ceramic capacitor. The reverse side of the plate (center) shows the silver wiring, three (black r-ectangular)

resistors, and four circular ceramic capacitors. Next is the complete transmitter with subminiature tube and battery plug-in added.

a thin insulated strip such as Scotch tape over

the lead and run a foil strip or ribbon over it. The cross-over ribbon is connected to the circuit by a drop of silver paint or solder at its

ends.

A three stage amplifier printed by one manufacturer as a unit suitable for a hearing aid has

a gain of 10,000. Included are a miniature volume control and specially designed clips to

hold the subminiature tubes. It was printed

on a ceramic plate by the stenciled-screen

process.

Manufacturers have also placed on the market a variety of printed coupling circuits in

which the dielectric material for the capacitors

is the base plate itself. The base plate is made

of material having a high dielectric constant.

Conductors and capacitors are printed in the

same stenciling operation. The result is an

unusually compact unit. Even when entirely

coated with a protective plastic cover, some of

the units are only 0.06 in. thick. A diode filter

circuit consisting of a resistor and two capacitors is 0.19 in. wide and 0.5 in. long. Other

units such as audio coupling circuits and a-c/d-c

radio subassemblies consisting of three resistors and three capacitors are 0.5 in. wide

and 1.0 in. long.

A number of radio transmitters and receiv- ers (figs. 1.1A, 1.2A), produced by the printed

circuit technique have been designed to operate in the Government short-wave band of 132

to 144 megacycles. These examples illustrate

only a few of the wide number of possible applications of printed circuits. Silver and car- bon paints were used to make the sets. The five types of transmitters (fig. 1.1B, top

row) are single-tube grid-modulated units,

which require only connection to modulator and

battery to operate. In the two units at the left,

the oscillator circuit is printed on the outer sur- face of a thin steatite cylinder. The tube is

inserted within the cylinder and the combination wired to a battery plug. The unit at the

top center is a transmitter with the circuit

painted on the envelope of the subminiature

tube, a 6K4. It was made by first wrapping a

stencil of the inductor pattern around the tube

using masking tape. The glass envelope was

then etched in fumes of hydrofluoric acid.4 The

conducting paint was applied to the etched sur- face and allowed to dry in air. To improve the

Q of the inductor, it was silver plated in a

silver-cyanide bath.5 The grid-leak resistor was

4 Caution should be exercised when etching with hydrofluoric acid not to seriously weaken the glass envelope; also, in general, when

oven-firing tubes for baking on printed circuits, temperatures should be kept well below the softening point of the glass.

5 Where strong adhesion is desired, it has been found advantageous

to copper plate over the initial painted inductors prior to silver plating. A simple copper-sulfate bath may be used.

6 Circulars of the National Bureau of Standards
Figure 1.2

A, Top row, four-tube radio receiver printed on a 3/32 in. lucite plate, 2 in. wide and 5 in. long. The silver circuit wiring (applied

through a stencil) is shown on the plate at the left, with the completed receiver at the right. Battery and speaker are omitted. Center row,

four-tube radio receiver unit printed on thin steatite plate. All receivers have four stages, consisting of an input stage of square law detec- tion followed by two stages of pentode amplification, and a triode output stage feeding a permanent-magnet type speaker. Bottom row, two

developmental stages of a four-tube radio receiver printed on a thin steatite plate, 2 in. wide and 3 in. long. The plate at the left shows a complete circuit wiring (less tubes and capacitors) applied free hand with a camels-hair brush except for the spiral inductors. Wiring on

the center plate was applied with a squeegee through silk screen stencils. The leads from the complete receiver at the right are for a bat- tery and speaker.

B, Two-tube wrist transmitter for personal use, complete with batteries.

applied with carbon paint and dried under an

infrared lamp. The addition of a tiny highdielectric ceramic capacitor completed the circuit on the tube envelope.

The circuit for the unit second from the right

(fig. 1.1B, top row) is painted on the glass en- velope of a tube measuring approximately *4

in. in diameter and 1 in. in length. The silver

inductors were applied with a ruling pen

mounted on a lathe with the tube held in the

chuck and rotated by hand. Both tube and circuit have been coated with a thin layer of plastic resin to protect against rough handling and

humidity.

The circuit of the transmitter at the right

(fig. 1.1B) was stenciled on a 3/32-in. steatite

plate l!/2-in. wide and the same in length. The

development of the flat-plate transmitter (fig.

1.1B, bottom) shows three spiral inductors and

a 50 micromicrofarad coupling capacitor on one

side and the remainder of the circuit wiring

including three resistors (the dark rectangles)

and four capacitors on the reverse side. One of

the resistors serves as a blocking resistor for

testing the unit. Wiring of the units was completed by soldering the subminiature tubes and

leads for the antenna, batteries, and microphone directly to the silver wiring on the plate.

A complete radio transmitter for personal

use has been assembled in a small lucite case. This small transmitter (fig. 1.2B), employs a

grid-modulated oscillator printed on a steatite

cylinder housing a subminiature triode. A crystal microphone with a single stage of pentode

amplification is used to grid-modulate the oscillator. Hearing aid batteries are used to

power the set. The complete unit is included

within a plastic housing 21/& by 2% by % i n- Two of the receiver units (fig. 1.2A), are on

steatite plates 2 by 3 in. and 2 by 5 in. (bottom

right and center, respectively) while the third

is on a 2 by 5 in. lucite plate. They employ a square-law detector stage followed by two

stages of pentode amplification and a triode

output stage feeding the loud speaker. The

input tuning is broad so as to allow reception

over the complete band of 132 to 144 megacycles. All but the unit in the lower left corner

were made by the stenciled-screen process. The

circuit of the other, with the exception of the

spiral inductor, was painted on with a camel's

hair brush. The spiral inductors have all been

silver plated. As silver plating is relatively

easy, it was found convenient to plate all wiring on the base in the same operation.

Miniature microphones, speakers, and batPrinted Circuit Techniques 7
teries complete the transmitting and receiving

units. The units also operate satisfactorily with

standard large size microphones or speakers.

The transmitter is plugged into a power pack,

while the standard size carbon microphone with

matching transformer is plugged into the other

end. This arrangement has been used together

with the 2- by 3-in. receiver mounted on the

10-in. console speaker with sufficient power to

operate the speaker so that it could be heard

throughout a fair sized auditorium. Personal

transceivers incorporating printed circuits are being engineered for commercial use. One

manufacturer has designed them for the proposed Citizens Communication Band, 460-470

megacycles.

The ease of replacing defective printed subassemblies in an installation introduces new

possibilities in manufacture and maintenance,

particularly applicable to complex equipment

and to rural and foreign markets where maintenance is a difficult problem. This advantage

is realized bv the use of printed plug-in subassemblies. Principal units of a set can be re- moved, tested and replaced in the same manner

as tubes are handled. It should be useful in areas where skilled repair men are not available and in applications where it is necessary to

do trouble shooting under different conditions.

With all major subassemblies wired in plug-in

fashion, the repair man can replace all the

subassemblies in the set. taking the old units

back to the shop for checking. Such subassemblies have been encased in a special casting

resin developed at the Bureau, useful at freouencies up to and bevond the VHF range. It

is thus protected against manual abuse and atmospheric conditions.

Performance

Conductors

Sufficient data on the performance of printed

circuits have been accumulated to show that

thev behave in a manner similar to conventional circuits. The current-carrving capacity

of the usual printed conductor is more than

sufficient for all currents used in low-power

plectronic circuits. For example, a silver con- ductor in. wide and about 0.0005 in. thick

fired on steatite did not fuse until the current

reached 18 amperes, while another 1/16 in.

wide carried 8 amperes for 9 minutes before

fusing. Other data and curves of loading characteristics of printed conductors may be had

from the publication, Printed Circuit Techniques, referred to above.

On plastic bases, where firing of the paint

is not possible, the printed leads have a higher

resistance and less current carrying capacity.

A lead 0.08 in. wide and 0.005 in. thick withstood only 0.5 ampere before the plastic base

softened and the silver peeled off. Since heating tends to loosen the bond between the deposited metal and the plastic base, an experimental determination of the current carrying

capacity should be made for each particular

case. Lower and more consistent values of re- sistance and increased current carrying ca- pacity are to be had simply by increasing the

number of coats of paint or by electroplating.

Resistors

Among the principal factors affecting the

power dissipated by a resistor are the paint

mixture, the base material on which it is

printed and the surface area. The composition of the paint determines the maximum temperature to which the resistor may safely be

raised; the dimensions and properties of the

base material and the area of the resistor are

the principal factors which determine the rate

at which heat is conducted away. The close

contact of the printed resistor with the base

material in the case of glass or ceramic, prevents local heating and gives the resistor better power dissipation than might at first be

expected. Resistors painted on plastics tend

to loosen from the base material on heating,

hence must be operated at lower power levels.

While no standard method of rating printed

resistors for power dissipation has yet been

established sufficient data are available for a

preliminary evaluation for carbon resistors on

steatite. Typical results of an intermittent load

test on 100,000 ohm carbon resistors 0.002 in.

thick and 0.038 sq in. area (0.1 in. by 0.38 in.)

painted on steatite were obtained by operating

the resistors for 200 hours at loads of 0.25,

0.50 and 1 watt respectively. As a control, commercial fixed composition 0.25-watt and 0.50-

watt carbon resistors were also subjected to

the same loads. The curves show clearly that

printed resistors perform very well compared

to fixed composition resistors. Although the re- sistance of the printed resistors decreases 3 to

5 percent it soon stabilizes at a constant value.

Typical results of another determination of

power dissipation were obtained by increasing

the current through two 1,500 ohm resistors

(one printed and one 0.25-watt fixed composition resistor), until they failed. The current

was increased in small steps and allowed to

stabilize at each value before going to the next.

Both resistors withstood 20 milliamperes (0.6

watt) before any effective change in resistance

took place.- Further increase in current caused

both to increase in resistance rapidly, peaking

at approximately 37 milliamperes and then

8 Circulars of the National Bureau of Standards
decreasing. It is important to note that the

printed resistor opened on excess current

whereas the fixed composition resistor decreased in value. The opening of the printed

resistor under excess load may be a desirable

property, as it will not sustain heavy overload

currents with the consequent damaging of

other parts of the circuit.

Since the size of the printed resistors is not

standard, it is not practical to specify their

power rating in terms of watts per resistor.

The rating must be specified as watts per unit

of resistor area. In the first of the two tests

reported above, an area of 0.038 sq in. dissipated 1 watt, giving a dissipation factor of 26

watts per sq in. while in the second test an area

of 0.023 sq in. dissipated 0.6 watt giving the

same dissipation factor. This factor is considered representative of average performance.

The selection of a good resistance formula

requires careful attention to the character,

quality and quantity of the ingredients. Examples of variations in behavior due to different

formulas are more fully described in the Bureau

publication, Printed Circuit Techniques. In

brief, these indicate the possibilities of using

the response characteristics of particular formulas to compensate for negative temperature

response caused by other elements in the circuit, or of using the resistance-temperature

characteristics as a basis of a temperatureindicating element in such a device as the radio

sonde.

Inductors

Inductors having thin metallic lines on a

ceramic form show very small variations in inductance with temperature. The fused-on coating being thin and somewhat elastic does not

tear away from the ceramic surface when subjected to extreme temperature cycling. This

is true even though the thermal expansion coefficient of the metal is greater than that of

the ceramic. For all practical purposes, a combination of metal on ceramic behaves as though

the expansion were due to the ceramic alone.

The high values of Q required for oscillator

tank circuits can be obtained by electroplating over the printed inductor. A spiral inductor

made of silver lines 0.0003 in. thick printed on

steatite had a Q of 25. Silver-plating to a

thickness of 0.001 in. increased the Q to 125.

Silver inductors painted on fused quartz were

also developed during the war for the Signal

Corps. These inductors, spirals on a flat surface, had a Q of 80 after firing. The Q was

increased to between 150 and 200 by electroplating. Where inductors are printed on glass or ceramic tubes and the conductor built up

by electroplating to a thick layer, Q's of 175

to 200 are readily obtained. The effective Q

of inductors painted on tube envelopes is ac- tually lower than this because of the loading

effect of the metal parts of the vacuum tube

located inside the inductor.

In special cases, the Q of a solenoidal inductor

on a ceramic form has been increased by grinding away the ceramic material between the conductors, leaving practically an air core inductor

which is supported by a ceramic material having a low positive coefficient of thermal expansion. When used in an oscillator in combination with a capacitor having a small negative

temperature coefficient, a frequency stability

approaching that of quartz crystals was obtained.

The lowest frequency for which inductors

may be printed is determined by the printing

area available. In a limited area, the inductance may be increased by printing the inductors

in multiple layers using a layer of insulation

between them. The usefulness of this method

is limited principally by the distributed ca- pacity and the desired Q of the inductor.

The maximum inductance available in the

usual size of plane spiral inductors without

magnetic core material is of the order of 60

microhenries, usually limiting their use to frequencies above 0.5 megacycle. The feasibility

of increasing the inductance by coating printed

inductors with an electrically insulated magnetic paint is now being investigated at the

Bureau.

The multiple layer idea mentioned above need

not be restricted to inductors. Several circuits

may be printed on the same plate, one above

the other, by interposing a layer of lacquer or

other insulation between them. The proximity

of the circuits to each other must be taken into

account in laying out the design so that undesirable couplings are avoided.

Printed Circuit Techniques 9
2. Centralab Printed Circuit Techniques6

An important question at this moment is

"What do the basic everyday circuits look like

when printed?" Both simple and complex circuits have been reduced in size and complexity

of construction by the techniques (fig. 2.1),

how used in manufacturing at Centralab. The

lay-out in the left corner of the upper row

of figures represents a simple RC coupler. Various more complicated coupling and filter circuits can be made using two or more of the

units. The lay-out in the right hand corner of

the top row includes two capacitors and one

resistor and can be used as a diode filter. An

important point to note is that the ceramic

plate serves as a base for the circuit and re- sistors and as the dielectric for the capacitors.

The illustration in the middle of the row represents an audio interstage coupler which we call

the Couplate. The figures on the bottom row

represent, on the left, a mechanical detail drawing of a 3-stage audio amplifier shown with

tubes in place, and a photograph of obverse and

reverse views of the unit; on the right a schematic drawing of the amplifier circuit.

Note that the Couplate and the amplifier emc By A. S. Khouri, Centralab Division of Globe-Union, Inc., Milwaukee, Wis.

ploy a steatite plate as a base for the circuit and resistors, and that the capacitors are thin, silvered disks of high dielectric cera- mic soldered flat against the plate. These four

items illustrate the two basic methods we employ at Centralab. Each has its advantages. It

is important to note, however, that when a

high dielectric constant plate is used for the

circuit and resistor base as well as for the

dielectric for the capacitors, a high degree

of sometimes undesired coupling of components

presents a problem. Where this coupling can- not be minimized to a negligible effect by

proper spacing, lay-out and other expedients,

it becomes necessary to use the method illustrated with the Couplate and amplifier, that is,

attaching wafer-thin ceramic capacitor disks

separately to a steatite plate having a low K

instead of a high K.

Together with a general idea of the appearance of some commercial applications of printed

circuits, we will attempt to answer another

question arising in your mind, that is, "How do

we manufacture printed circuits at Centralab?"

Starting with a schematic drawing of the circuit, the shape and size of a base plate is determined, and a layout drawing is made (fig.

LAYOUT OF R.C. COUPLER "KOPEC" LAYOUT OF INTERSTAGE COUPLHIG-"COUPLATE" LAYOUT OF DIODE FIITER-"FILPEC"

THREE STAGE AMPLIFIES "AMPEC

Figure 2.1. Examples of techniques used in printed circuits.

Circulars of the National Bureau of Standards
2.2). The lay-out drawing is accurately made

to enlarged scale and photographed. From the

lay-out photographs, which are reduced to actual size, screens are made which are used to

apply the circuitry and resistors, as shown in

the upper right corner of the photo. A silver

paint used for the circuitry is fired at approximately 1,400° F. to bond it to the ceramic and

to render it conductive. For the resistors, a

carbon-resin dispersion is used which is baked

at a moderate temperature to stabilize it

against the effects of mechanical abrasion and

humidity. After resistors are applied, wire

leads and capacitors, if needed, are soldered to

the plate to complete the assembly. A phenolic

coating is used to provide insulation and additional protection against humidity and abrasion.

For a simple RC circuit (fig. 2.3), a high K

base is used as the dielectric for the capacitor.

Silver and resistor screening are done in the

same manner as illustrated before. However,

it is unnecessary to attach external capacitors

since they are included as an integral part of

the unit in this design. If this circuit had been

more complex and intercoupling of components

had to be reduced to a minimum, then it would

have been preferable to use a steatite base with

wafer-type capacitors attached separately. In

either case, the ceramics used for the base plate

and the capacitors are dense bodies, impervious

to moisture, and have excellent dielectric properties.

How these ceramics are made is another

question, since it is a science all its own. In

producing either steatite or high K ceramic,

careful laboratory control precedes all operations such as mixing, molding, and firing.

k COUPLATE" CIRCUI SILVER SCREEN PATTER

i°o?

SILVER SCREENING "COUPLATES'

COMPLETEDi "COUPLi

$

1 <B> ! l?YA OOl-OOl

fl 2 3 4 WW ft *

HIHf 1

Figure 2.2. Steps used in manufacturing the "Couplate."

Printed Circuit Techniques

793811—48—3

11
DIODE "FILPEC" CIRCUIT SILVER SCREEN PATTERN

—^

A^WV

1

I

OlOfifc LOAD FILTER

SILVER SCREENING "FILPECS"

» - » — » 3; » i «»:

V.I

COMPLETED DIODE LOAD "FILPEC"

Figure 2.3. Steps in the manufacture of a simple RC circuit utilizing a high K ceramic base.

SENSITIZING

Figure 2.4. Steps used in preparing a screen stencil.

Circulars of the National Bureau of Standards
Table 2.1.

Electrical characteristics of printed resistors

Example: 1 Megohm Resistor, 1/4 in. X 5/64 in.

Huiyi l (.1 i ty Load life Noise Voltage

coefficient

Duration and type of test

Change immediately following

96 hours at 90%-95% relaI • 1_ * J • i _ J j AO /~1 tive humidity, and 40 G.

+11%

1,000 hr. intermittent (l 1^

hr. on, % hr. off). Total power dissipated 0.5 watt. — —4.7% _ 4 db over equivalent composition type resistor. 0.007% per volt.

Permanent change following

test —1% -

The screens used to deposit the circuit pattern and the resistors are an important part

of printed circuit equipment, and their manufacture requires considerable skill, especially

for small and intricate lay-outs such as (fig.

2.4). Either silk or steel mesh screen is used.

The choice depends on first cost, expected life,

and quality of work. After a photosensitive

emulsion is applied to the surface of the screen

it is exposed to light against the photographic

positive of the circuit lay-out. After washing

the unexposed sections of the screen it is

checked for accuracy and is then ready for use. In order to deposit the circuit pattern or the

resistors, a small amount of paint is poured

over the screen and a rubber squeegee moved

over the surface of the screen forces paint

through the pattern openings onto the cera- mic plate below.

Let us examine each of the four types of circuit components that can be made by our

printed circuit technique in order to determine

what these printed circuits can do and how

they perform.

The conductors of "wires" are silver nor- mally applied in widths from 0.010 in. to 0.060

in. and approximately 0.003 in. thick. A 1-in.

length of conductor 0.030 in. wide will have a

resistance of approximately 0.1 ohm. Humidity,

load life, noise, and voltage coefficient data for a 1 megohm resistor !/£ in. long and 5/64 in.

wide may be seen in table 2.1.

The ceramic dielectrics used for the printed

circuit capacitors have a number of important

characteristics. Of first importance is the simple parallel plate construction which allows the

capacitors to be made wafer thin so that they

can be mounted flat against the printed circuit

plate, retaining the two dimensional nature of

the circuit and resistors. Typical temperature

coefficient and ageing curves are also shown

(fig. 2.5).

Inductances are applied either as spiral con- ductors on a flat surface, the maximum inductance practical to obtain in this manner being

approximately 0.1 microhenry with a Q of 150, or as parallel lines having lower inductance

but higher Q. In general, resonant circuits can

be handled over the range from 25 to 509 Mc.

It is important to note, since the ceramics used

Printed Circuit Techniques

have a temperature coefficient of linear expansion one-half or less than that of good metallic

conductors, that an inductance which is bonded

to the ceramic and constrained to expand with

the ceramic base will have at least half the temperature coefficient of inductance of a similar,

form-wound inductance. Also, this type of inductance has greater stability when exposed

to repeated temperature cyclings than it is possible to obtain with either air or form-wound

types.

Wherever it is desired to miniaturize a circuit, printed circuit techniques should be considered. For instance, it is possible to make a

printed circuit having 15 capacitors and 15 resistors, or more, a total of 30 components, and

associated wiring, on a plate % sq in. in area

and 1/16 in. thick. Uniformity in performance

and appearance of printed circuits produced in

quantity is an important advantage to con-

-*0° -40° -M° -10° * 10° 10° »° W)° 40° «° TO* *0°

-

-60° -JS° -»* "35° -S0° -10° 10° 20* M° W° S0° W° 70° 80°

TEMPERATVAE IN DECREES CENTIGRADE

I i 15 10 IS 20 10 W M IM 200 300 SOD 1001

2

' i <* 5 10 IS « jo so loo it* joc sou l«w g ACE IN DAYS Figure 2.5. Important characteristics of ceramic dielectrics used for printed circuit capacitors.

A, typical temperature-capacity curve; B, typical aging: curve.

13
Wmn

mmWEBmm

Figure 2.6. Three-stage miniature amplifier showing various uses.

sider. Since all components are integrally

bonded to the base plate, there can be no movement of parts relative to one another due to

vibration or shock. Because of the dense nature

of steatite and the ceramics used for capacitors,

printed circuits are impervious to the effects of

high humidity, and consequently, they should

be useful in equipment destined for tropical

use.Circuits which can be treated as a unit lend

themselves ideally to printed circuit techniques.

In this case the circuit can be molded or potted

as a unit section and provided with plug con- nections to the remainder of the circuit. Servicing them becomes a matter of locating the

faulty section and plugging in a new unit.

A single three-stage miniature amplifier,

mass produced at low cost, can be used in vari- ous applications. It can be used wherever a

small "packaged amplifier" is needed such as

in a stethoscope, hearing aid, pocket signal

chaser, or pocket transceiver (fig. 2.6).

There is no question of greater importance

to the prospective commercial user of printed

circuits than that of cost. At the present time

printed circuits are competitive with and sometimes cheaper than circuits using standard

components, providing, the intangible considerations such as savings in labor, wiring mistakes, purchasing, inventory and stock handling, are given their proper weight. The arithmetic illustrating the economics involved for

the Couplate, a single component with 4 leads

replacing 4 separate components with 8 leads,

is shown in simple tabular form (table 2.2). A

similar analysis can be made for any of the

other printed circuit items discussed in this

paper.

Table 2.2.

Printed electronic circuit costs By comparative ratio computed from averaged cost analyses

supplied by Couplate users ORDINARY COMPONENTS COUPLATE EQUIVALENT

Index Index

1. 4 Separate components 9.0 1. Multiunit component- 10.0

2. 8 Soldered joints 2.0 2. 4 Soldered joints 1.0

3. Wiring mistakes 1.5 3. Wiring mistakes 0.75

4. Purchasing _ 1.0 4. Purchasing 0.5

5. Inventory, storage, 5. Inventory, storage, and stock handling 1.0 and stock handling. 0.5

Total 14.5 Total 12.75

14 Circulars of the National Bureau of Standards
3. Conductive Silver Preparations7

Before describing the uses of the various sil- ver preparations suitable for printed circuits,

it might be wise to briefly outline their compositions and properties. There are available for

general use two distinct types of preparations,

namely: The air-set or low temperature bake

compositions and the fired-on compositions.

The air-set or low temperature bake compositions are intended primarily for application

to phenolics and practically all other plastics, as well as paper, wood, leather, and cloth, or other base materials which can be subjected to

temperatures not exceeding 200° F, or merely

air dried, to obtain the necessary bond of the

conductive film to the material to which it is

applied.

These compositions are paints consisting of

a binder, a thinner, and a conductive silver

powder which is thoroughly dispersed in the

system by roll milling, ball milling, or some

similar method. When applied in a thin film,

even though the conductive metal particles are

surrounded by an organic binder, it is possible

to obtain coatings having fairly low electrical

resistance. When this type composition is air

dried or baked, the thinner is evaporated and

the binder becomes somewhat hard, or is set,

thereby bonding the particles of silver together

and also to the material to which it has been

applied. The form of binder and thinner incorporated in the system determines whether

it is to be air dried or whether it is to be baked

to obtain the greatest adherence and lowest

electrical resistance. Materials which are baked

on result in coatings which will have somewhat

lower electrical resistance and harder, more

scratch resistant, surfaces.

The fired-on compositions require firing at

temperatures ranging from 850° to 1,300° F

and are intended for application to ceramic

base materials, such as steatite, porcelain, titan-i

ates, and glass.

The fired-on compositions or preparations

are similar to the air-set or low-bake temperature preparations, except that in addition to

silver powder, binder, and thinner, a ceramic

flux, which is no more than a low melting, finely

divided glass, is also incorporated in the system.

When the thin deposited film of composition

is subjected to the elevated temperature of the

firing operation, several changes take place as

the temperature increases. First, the volatile

thinners evaporate and leave the system. Next,

the binder, present merely to produce a hard,

7 By W. V. Pattern, ceramic division, E. I. duPont de Nemours and

Co., Inc., Electrochemicals Department, Perth Amboy, N. J.

unfired or dry film, burns out as the temperature of the furnace is increased. Finally, the

flux, or the low-melting powdered glass, fuses,

and after the peak of the firing cycle is reached

and the temperature of the furnace is decreased, the fine particles of silver are found

to be firmly imbedded in the fused flux which,

in turn, is firmly bonded to the base material.

Whenever the base material permits, the use

of fired-on type of coatings are recommended.

It can readily be seen that the adherence of the

fired-on type is far superior to the adherence

obtained when the air dry or low temperature

bake preparations are used. The fired-on compositions result in a metallic film which is about

90 to 95 percent pure silver, while the air-dry or baking coatings when dry, or after baking,

result in a film which is made up, to a great

extent, of more or less nonconductive, organic

binders. The fired-on materials have much

greater electrical conductivity, usually desirable and sometimes a necessity.

It is sometimes desirable to put a thin film

of electroplated copper over the conductive

silver film. In this case, it is necessary to use a silver composition which has been formulated so as to withstand the effects of the acid

copper bath.

One might wonder why a metal as costly as

silver is used in these formulations, when

other much cheaper metallic powders, such as

copper, aluminum, etc., are available in a similar powder form which would be compatible

with the same binders and thinners, and are almost as conductive as silver. The use of sil- ver can be appreciated when it is realized that

almost all films formed with powdered metals

oxidize readily and are quickly attacked in con- taminated atmospheres. Silver is one of the

few metals that has the faculty of being highly

conductive in several of its forms, including

oxide or sulfide. Copper and aluminum, unfortunately, do not have this quality. The

oxides and sulfides of these metals are very

poor conductors, more or less eliminating their use in conductive coatings.

Both the air-dry or low-temperature bake

and the fired-on type conductive coatings are formulated to be applied to the base material

by any one of the conventional means of application. The preparations may be applied by

dipping, brushing, spraying, or the squeegee

or silk screen stencil process. The size and

shape of the object to be coated and the area

to be covered will determine the method to be

used. Whenever possible, we recommend the

squeegee process. It has been proven that when

Printed Circuit Techniques 15
this method of deposition is used, a uniform

thickness is obtained, and the coating of a specific area may be accomplished with less equipment and at a greater speed. Also, since no

material is lost due to overspray, it is more

economical. This method also lends itself to

mechanization.

To make possible these various means of application, it is necessary to vary the vehicle incorporated in the composition, so as to result

in finished preparations having the proper viscosity and drying time for any specific use. After the silver composition has been deposited on the base material by the desired

means, one of the most important steps in this

metallizing process is encountered. If the material is not dried thoroughly or baked at a

high enough temperature for the required

length of time, the full bonding properties of

the organic binder are not obtained and the

adherence of the metallic coating to the base

material will be poor. In addition to this, some

oils or similar materials may remain on the

surface of the film and satisfactory electroplating cannot be accomplished. The reverse

of this condition is also harmful. Drying or baking at higher temperatures than recommended will usually cause some of the organic

binder to be burned out resulting in a poor

bond.

To obtain best results with the fired-on

preparations on steatite or porcelain, both

glazed and unglazed, the recommended firing

temperature and cycle must be adhered to. It

has been found that in most cases a peak temperature of 1,220° to 1,300° F, using a cycle of

6 hours to reach this peak, will give the utmost

in regard to bond of the coating to the ceramic.

For this firing operation a periodic furnace

is usually used, but a conventional type decorating lehr or a continuous belt type furnace can

be used with excellent result's. In all cases, cycle

and temperature must be regulated according

to the kiln load and size of objects, as well as

other operating conditions.

On glass, a firing temperature of 1,000° to

1,050° F, for soft glass and 1,100° to 1,200°

F, for Pyrex or other heat resistant glass is

recommended. Best results can be obtained

with the usual glass firing lehr, but a periodic

furnace may be used. The firing time will depend upon the equipment used, kiln load, size

and shape of object and annealing time re- quired for the particular type glass used.

For some applications such as on glass, steatite and other ceramics, it is necessary to solder leads or contacts to the silvered surface. It

is possible to solder directly to the fired silver

surface, but if the surface of the coating is first

burnished slightly, it will be found that the

solder will more readily wet the silver.

When using the air-set or low-temperature

bake type silver preparations in printed circuit

work it is usually necessary to copper plate the

dried coating so that the desired conductivity

may be obtained. When using the fired-on coatings, it is sometimes desirable to copper plate,

especially if the particular design of the circuit requires that many soldered connections

be made to the fired film. In this case, the copper plating serves as a protective coating and

prevents injury or rupture of the connection

by the heat of the soldering operation in con- junction with the action of the solder.

Up to this point, facts concerning the composition, properties, and method of applying

the various silver preparations have been mentioned. The various possible uses and advantages offered by these materials in printed

circuit work will now be briefly outlined. Because of the versatility and flexibility of con- ductive silver preparations, mixtures are now

available which make possible the application

of printed circuits to practically any base material by any desired means. This fact permits

the use of such inexpensive materials as fiberboard, plastics of all kinds, as well as the various ceramic materials. Hence, it is possible to use this means to wire (in addition to radio

receivers and transmitters) electrical toys,

games, musical instruments, switchboards, electrical measuring instruments, and any other

unit whose components are usually connected

by the conventional wire and soldered connections.

There are those who are somewhat con- cerned as to the use of silver, because of the

cost of this precious metal. It has been determined that the cost of the conductive silver

preparation is somewhat less than that of the

copper wire and solder it would replace for the

wiring of a typical circuit. The saving in labor

and time should also be considered in the overall cost comparison. It should also be remembered that each printed circuit is an exact

duplicate of the master pattern and there would

be no rejects due to human error. In most

cases this would result in further savings as

it would not be necessary to have as rigid inspection facilities as is required for conventional wiring. It will also serve as a means of

forever eliminating poorly soldered joints and

connections.

Although the economy of printed circuits

is one of its most appealing features, there are many others which should not be overlooked

;

foremost among these is the high rate of production which is possible. By using a suitable

conductive silver preparation in conjunction

16 Circulars of the National Bureau of Standards
with the squeegee application process, with only

slight mechanization, it is possible to perform

the principal circuit wiring at the rate of at

least 50 units per minute with an active area

coverage of approximately 4 square feet per

troy ounce of silver composition.

Of equal importance is the exceedingly small

size of the unit. Because of the extreme thin- ness of the silver lines, it is not necessary to

confine the circuit to the two sides of one base,

but it is possible to build up layers of the individual units into one elaborate circuit or series

of circuits.

At the present time, work is progressing on

the use of conductive silver preparations in

forming inductances and tuning devices on

small flat plates, vacuum tubes, and other bases.

When these are perfected, it will be possible

to eliminate all wiring and other conventional

components.

The du Pont company is continuously car- rying on the development of new formulations

and the improvement of existing silver preparations. In addition, technical service facilities are available to manufacturers desiring assistance in setting up equipment and selecting the

proper silver preparations for printed circuit

work and related applications.

4. Printed Resistors 8

The interest aroused by printed circuit development has assumed such proportions that

a more detailed consideration of its component

elements is warranted. One of the primary circuit elements actually printed is the resistor,

whose importance is emphasized by the many

difficult requirements it must meet. While in

appearance it is simplicity itself, some of its

behavior patterns are extremely complex and

unorthodox. This type of resistor has a considerable history in back of it and production

methods and product have been developed to a

high degree of perfection.

It is proposed first to review briefly the more

modern methods for their mass production and

then discuss a few of their more important

characteristics.

Definition

The subject matter under discussion should

be clearly defined. The phrase printed resistors

( 1 ) indicates a method for producing resistors,

namely, by a printing process, and (2) implies

that the resistor is a film—since any printing

process produces a mark which is a film. However, there are many ways of obtaining a film

resistor—for example, evaporation, spraying,

printing, dipping, pyrolysis, painting, etc. We

will therefore employ the term printed resistor

in the usual sense as a film resistor applied

by any means.

There are many different types of films:

(1) Metallic films such as are produced by

evaporation of metals, (2) films made from

metallic compounds, such as phosphates, (3)

carbon films produced by pyrolysis, (4) films

in which the conductor is graphite or carbon

mixed in a resinous binder. It is the last type

of film resistor which is the subject of this

paper.

Description

Graphitic or carbon resistance films have

been and are being applied commercially on a

large scale to various types of insulating bases

such as glass, ceramic, paper and phenolic

sheets. The supporting bases may be cylindrical or flat. Typical of the applications of such

films is their use as resistance elements for low

and medium power composition resistors and

for volume controls. Although the commercial

objective up to now has been to produce at

any one time large numbers of separate individual resistors of a given value, the methods

developed lend themselves to production of a number of resistors of different values on a

single base.

Resistance Films

A brief review of some modern techniques

in commercial practice for printing resistor

films on various types of bases, and a few illustrations of the types of films so produced will

reveal potentialities for use in printed circuits

on a large scale.9

8 By Jesse Marsten, vice president, and Alexander L. Pugh, Jr., research engineer; International Resistance Co., Philadelphia, Pa.

9 EditorsNote: See Operating Characteristics of Film Resistors by Jesse Marsten and Alexander L. Pugh, Jr. Tele-Tech, March

1948, p, 46, for other applications.

Printed Circuit Techniques 17
Resistance Films on Glass

Printed resistance films are applied to clear

glass rod or tubing in a process in which the

glass rod or tube is drawn continuously on a machine. At it is drawn, a liquid resistive film

is applied on the outside of the glass base. As

with all films of this character, the resistivity

of the film in the liquid phase is extremely high,

but as the coated glass tube or rod continues

on its journey through the machine, it is con- verted to a tough, hard film of the desired

resistivity. This film is continuously measured

and its tolerance indicated on a limit bridge as

the filament completes its passage through the

machine resulting in a black-coated glass resistor. This printed resistor or film is used as

the element of the so-called "filament" type

composition resistors. This filament is then

cut to various lengths, terminals are applied,

and the assembly molded in phenolic.

The initial charge of glass from which the

tube is drawn lasts for a period of 2 to 4 hours.

The speed at which this resistor film is applied

is 2 in. per second. For the average size composition resistor this is equivalent to four re- sistor elements per second. The film is made

commercially in values varying between 100

ohms per inch to 100 megohms per inch. Films

have been made as high as 1,000 megohms per

inch. The thickness of the film depends upon

resistance value and varies from 0.0004 in. for

the low resistance value to 0.0002 in. for the

very high values. The method of application of

the film is such that a high degree of uniformity in the thickness, and therefore resistance, of the film is obtained. The important

point to note is that the entire operation is

mechanical and continuous and the end resistance value of every section of the rod or tube

is measured during the manufacturing process.

Any or all of the control factors which determine resistance value can be adjusted during

the process without stopping the machine. If

the resistance value wanders out of tolerance,

adjustments are made and the resistance is

brought back in tolerance as shown by an indicating meter. The end resistance value of the

completed and assembled resistor is essentially

controlled while the film is applied. Typical

characteristics of printed resistors on glass are temperature coefficients at 0° to 100° C of —0.02 percent per °C in the 1,000 ohm range

to —0.05 percent per °C in the 10 megohm

range; temperature coefficients at 0° to —55°

C of —0.02 percent per ° C in 1,000 ohm range,

to 0.03 percent per ° C in the 10 megohm range

;

voltage coefficients of —0.004 percent per volt

in the 1,000 ohm range, —0.035 percent per

volt in the 10 megohm range; noise level at

rated loads of 0.25 microvolts per volt in the

1,000 ohm range and 3.5 microvolts per volt

in the 10 megohm range.

Resistance Films on Ceramic

Printed resistance films are also applied to

ceramic bodies. Unlike the film on glass each

resistor is handled individually. The film is

applied in a spiral line on the ceramic tube.

Low resistance terminations are applied at the

ends. The resistors are then cured to obtain

the desired resistivity. The film is also applied

as a solid coating for high frequency applications. These resistors are frequently used immersed in oil or water, which imposes a severe requirement on the adhesion characteristics of

the film to ceramic. The adhesion characteristics of this film to ceramic are sufficiently

tenacious to meet this service condition. Typical characteristics are voltage coefficients of —0.002 percent to —0.04 percent per volt per

in. and temperature coefficient of —0.005 percent to —0.075 percent per ° C.

Linear Resistance Film on Phenolic Sheet

Many applications require resistors having

unusual shapes. To meet such requirements a

uniform resistance film is applied to phenolic

sheets and these sheets processed to meet the

specific needs. In this operation phenolic sheets

4 to 5 feet long and of the appropriate width

and thickness, are fed by conveyor to a mechanism which deposits a given quantity of the

resistive solution on the sheet. The printed

sheet goes through its curing stages to produce

a hard durable film having a predetermined re- sistance value which varies linearly with length

in any direction. Resistance elements are punched out in a die to meet the particular mechanical and electrical specifications required.

Some of the different shapes currently produced are used as attenuators for wave guides,

strain gages, etc. Printed concentric disk re- sistors used as coaxial line terminations are produced from a printed sheet such as previously described. Silver annular terminals are sprayed on the disks.

Phenolic sheet with printed resistance film

is also made with silver conductive terminations, applied simultaneously in one operation

with the resistance film or separately by spraying. Individual printed resistors with silver

conductive end terminals are then made by

punching out elements to size. As in the case

of applying a film to glass, the operation is a

continuous one. The application of resistance

material being a mechanical one, a high degree

of uniformity in resistance value is possible.

A resistance element punched out of a strip

l!/2 in. wide in any position or direction will

not vary more than ±25 percent from the

18 Circulars of the National Bureau of Standards
mean. Closer tolerances may be obtained by

additional processing. The rate at which the

sheet is printed is one inch per second. The

thickness of the film varies between approximately 0.0004 in. to 0.0012 in. depending upon

resistance values, the lower resistance films

being thicker. Using as a standard a unit 1

in. long and 1/16 in. wide, resistance values

can be produced between approximately 500

ohms and 10 megohms.

Nonlinear Resistance Film on Phenolic Sheet

There are some applications where resistance

elements are required whose resistance is not a

linear function of length. The most notable

illustration of this is that of the tone control

and volume control in radio receivers. A phenolic strip, conveyor-fed, on which four con- tiguous resistance films are continuously and

simultaneously printed, each film having a different specific resistivity is used for this purpose. A horseshoe shaped resistance element

punched from this strip would show a nonlinear resistance-rotation curve. By suitable

choice of calibrated resistance inks and by

appropriate design of printing system, a re- sistance film obeying almost any resistancerotation law can be produced.

Typical performance characteristics of

printed resistor films on phenolic sheet are

voltage coefficients of —0.004 percent per volt

per inch to —0.02 percent per volt per inch

and temperature coefficients of 0.015 percent to

0.1 percent per ° C from 0° to 100° C and 0.06

percent to 0.3 percent per C from 0° to

—50° C.

The resistor film produced is hard enough to

withstand the wearing action of a smooth sliding contactor exerting 4 to 6 grams pressure

for 50,000 cycles of operation without damage

to its surface or without changing resistance

value more than 15 percent.

Printed Tape Resistor

Resistance film has also been printed on a

flexible base Cellophane tape about V2 in- wide

and 0.001 in. thick is fed into a printer from a

reel containing several hundred feet of tape.

The resistance material is printed in a line

which may vary in width from 1/32 in. to %

in. depending upon the requirements. Wider

bands may also be applied. As with the filament on glass, the entire operation is a con- inuous unbroken chain, feeding tape into

printer, printing resistance film, curing of film,

measuring of final resistance value and rereeling. The speed of travel through the printer

is 1 inch per second. Resistance values capable

of being printed vary from 500 ohms to 1.0

Printed Circuit Techniques

793811—48—4

megohm per inch. The film is mechanically

strong. It adheres tenaciously to the tape and

will not flake off. It has flexibility and can

stand a 160° bend without any cracking or

checking.

Application To Printed Circuits

Multiple Printed Resistors

The mass production techniques for printing

resistor films that have been described are in

everyday commercial use. It is a short step

to apply these techniques to printed circuits by

printing multiple resistance films, which will

yield simultaneously a number of different

valued resistors on one base. A phenolic sheet,

conveyor-fed to a printer, on which are printed

five independent resistance lines illustrates this

(fig. 4.1A). Such lines may be equal in width

or vary as determined by electrical requirements. From these strips, by suitable design

of punch and die, a bank of resistors can be

punched out and all interconnections may be

applied as shown. Size, shape, number of independent resistors, and interconnections can

be varied at will, within the limitations of the

printing device and the dimensions of the sheet.

Punched Printed Circuits

Complete resistor complements of circuits

with associated wiring can be printed and

punched in one operation ready for use. The

resistor complement of a particular two-stage

amplifier consists of five units, three 5-megohm

units and two 2-megohm units. The entire re- sistor system and interconnections can be produced from a resistor strip (fig. 4. IB). The

resistance film is printed together with the sil- ver conducting terminations by methods previously described. By use of dies a printed

element with all the interconnections is obtained with holes to which the other circuit

elements can be eyeletted or rivetted. Provision can also be made in proper designed openings to enclose other components such as

capacitors or tubes.

Discussion

The printed resistor under discussion is a

member of the general class of so-called "composition resistors." By and large, all these resistors, whether of the solid body or film type, are made in the same general way although

there are wide variations in materials, formulations, details of procedure, etc. Resistors of

this class are made by preparing a homogeneous

mix of a conductor, generally graphitic or car- bonaceous, or both, with a resinous binder and

19
Figure 4.1. A, Phenolic sheet with five independent resistance lines. B, Punched-printed resistor complements on phenolic base.

sometimes with filler. In the case of the solid

resistor the binder is a resinous molding powder. In the case of the printed film resistor, the

resin is in the form of a varnish. In both cases a curing operation is essential to harden the

mass or film. Prior to the final polymerizing

operation the mixtures have infinite or very

high resistance. This operation, involving temperatures or pressure, or both, shrinks and

fuses the resistive composition into a tough

hard body or film. The resulting structure then

consists of conducting particles held together

by a dielectric binder. Resistance values are

controlled by modifying the ratio of dielectric

to conductor. The higher this ratio, the higher

the resistance value. The conducting particles

make contact with each other under the pres- sure that exists in the structure. Since most

of the resistivity of the device is a summation

of the contact resistances between particles,

whatever alters the contact pressure will affect

the resistivity. Many of the unorthodox performance characteristics of this type of resistor are a direct result of the structure of this device.

Required Characteristics

The characteristics that are usually desired

in a resistor can be secured individually but

some properties appear to be conflicting when

they are wanted simultaneously. For example

:

The size for a given power rating might be

reduced but the noise, voltage coefficient, temperature rise, or some other property might

then be affected. For this reason all resistors are compromises. To be generally acceptable

printed resistors should meet the following mechanical requirements: (1) Adhesion of film to

base should be good; (2) film should be hard

and resistant to abrasion; (3) film should not

be brittle; (4) film should be flexible; (5)

thermal characteristics of film must not deviate

too greatly from that of the base.

All these requirements are dictated by the

obvious consideration that unless they are met,

normal use conditions such as handling, and

environmental factors such as humidity and

temperature, will result in injury of the film

with resultant deterioration of the resistor.

Most of these requirements are so obvious as to

be axiomatic. However, a brief discussion of

some of them is in order.

The requirements of hardness and nonbrittleness are dictated by use conditions. Storage

of parts, handling in storerooms and on factory

assembly lines subject these units to considerable abuse. The films must be tough enough to

withstand such usage. Even if the film is protected by a varnish coating, this coating must

then be capable of meeting these requirements.

Resistance of abrasion is essential in those

cases where sliding contacts are used on the

printed resistor, otherwise material is removed,

resulting in major alteration of resistance

value and deterioration of the film.

Thermal characteristics become important

when the resistor is subjected to variations in

temperature. If the expansion coefficients of

the supporting base and film vary too widely,

differential expansion takes place. If the temperature change is large the film may break

or lift from the base. This can be minimized

or even completely eliminated by selection of

materials which adhere tenaciously to the base.

Resistors should also meet the following electrical requirements : ( 1 ) The resistance should

20 Circulars of the National Bureau of Standards
not change appreciably on the shelf; (2) humidity should not affect the film; (3) the film

should not show polarization effects; (4) temperature coefficient should be low; (5) voltage

coefficient should be low, i.e., resistance should

be insensitive to voltage changes; (6) must be

capable of withstanding wide variations of temperature; (7) have good high frequency characteristics; (8) operate at its rated load indefinitely without important changes in resistance values; (9) withstand vibration; (10)

have low noise level; and (11) withstand effects

of salt water spray.

When all of these are met, there is the last,

but important, production requirement that it

should be possible to produce this printed resistor economically in a wide range of resistance values.

Properties of Printed Resistors

The properties of a printed resistor are a composite of two factors, (1) the conducting

film and (2) the supporting base. The properties of the conducting film in turn depend primarily upon its ingredients, their processing,

and the structure of the film. The supporting

base assumes a much greater importance in

determining the end properties of the resistor

than is normally assigned to it. A few of the

more important properties will be discussed.

Power Rating and Temperature Rise

In the last analysis the rating of devices like

the printed resistor is determined by long time

tests. That safe rating is usually applied which

insures satisfactory performance under the

rated load. Obviously the temperature limitations of the resistor components are determining factors in this rating. It is therefore desirable that the temperature rise be kept as low

as possible. If it can be made low enough the

rating might be increased, or the resistor could

be operated at higher ambient temperatures,

which latter factor is becoming more important

as equipment becomes smaller in size. In this

connection, the supporting base plays a very

important part.

This is shown in data (fig. 4.2) obtained in

the course of a cooperative development program for printing resistors on vitreous dielectric bodies, conducted by the Remington Arms

Co. and the International Resistance Co. Comparison was made between phenolic sheet, steatite, and the Remington Arms Co.'s vitreous

dielectric, with and without printed silver inlay.

The resistor dimensions were 0.040 by 0.5 in.

printed on supporting bases having a projected area of 1 sq in., except in the case of the

90

80

60

10

J—

/

/laminated

/bakelite 0.032" thick

^/VITREOUS

/DIELECTRIC

^0.077" THICK

STEATITE y,

0.077" THICK /

r^ao6o"

^^VITREOUS

DIELECTRIC

(ITH SILVER INLAY

THICK (096 SQ IN}

0.1 0.2 0.3 0.4 05 0.6 0.7 POWER — WATTS

Figure 4.2. Temperature rise vs. power for printed

resistors on various base materials.

Projected area 1 sq. in.; resistors 0.040 in. x % in.

vitreous dielectric with silver inlay, which was

0.56 sq in.

The differences shown are due primarily to

the differences in the thermal conductivity of

the base materials. As is to be expected, the

lower thermal heat conductivity of phenolic

results in higher temperature rises than with

ceramic. The other curves show the large differences that exist in thermal conductivity of

different types of ceramics.

It should be pointed out that despite the

higher temperature rise for phenolic sheets,

they have proved to be a very satisfactory base

for numerous reasons. The ratings and temperature requirements which would normally

be assigned to a resistor of these dimensions,

namely, 0.1 to 0.25 watt, are well within the

temperature and life limitations of these materials. Also many applications require resistors carrying negligible power. In addition,

the adhesion of printed film to phenolic is re- markably good. Another factor of importance

is that the thermal properties of a phenolic are very close to those of the film. Finally, the use of phenolic lends itself readily to mass

production methods.

The effect of the silver inlay in increasing

the heat conductivity of the vitreous dielectric

Printed Circuit Techniques 21
u

< : C

UJ

0.2

Hi

01 02 03 0.4 05 06 07

POWER — WATTS

Figure 4.3. Temperature rise vs. power on different

projected areas of base.

Steatite 0.150 in. thick; resistors 0.040 in. x % in.

is shown by the greatly reduced slope of its

curve compared to that without the silver inlay.

The silver inlay reduces the temperature rise

to less than one-half that of the plain dielectric.

Since the area of this plate is approximately

one-half that of the others, the comparative improvement is even greater than that shown in

the curve. The area of the supporting base affects the

temperature rise to a great degree (fig. 4.3).

As, is to be expected, enlarged base area reduces

temperature, but here we run into the law of

diminishing returns when dealing with one

resistor. The benefit of the larger area is realized when multiple resistors are applied to the

base.

Temperature Coefficient

The temperature coefficient of this class of

resistors is not a constant factor. It varies

with resistance value, and for a given resistance value it varies with temperature. The

coefficient may even change sign at some point

on the temperature scale. This anomalous behavior is due to the peculiar structure of the

resistor. We are not dealing with a pure conductor, which has a positive coefficient, or a semiconductor, which has a negative coefficient.

We are dealing with a conductor, (graphite or carbon, or both) which may have negative or

positive coefficients, mixed in varying proportions with insulation. The supporting base is

also affected by temperature. Temperature

changes affect all these components differently.

Stresses are set up by even small differences in

coefficient of expansion between these various

components. Also, contraction and expansion

may affect the contact pressure between con- ducting particles nonlinearly for the same temi

12 <

to 10

in

ui

a.

o

z

uiu

a.£

2

-4

RESISTANCE CURVE NO.

1000 OHMS

10000 OHMS

.100 MEGOHMS

1.00 MEGOHMS

10.0 MEGOHMS

-60 -20 20 40 TEMPERATURE IN DEGREES C Figure 4.4. Temperature characteristics of resistance

films of different values on phenolic base.

perature increment at different temperatures.

The temperature coefficient of the conducting

material itself—namely, graphite or carbon—is

not the dominant factor in establishing the

temperature characteristics of the resistor.

Rather it is the result of all the stresses set up

by differential expansion, which affects the contact pressure between conducting particles,

which pressure determines the ultimate resistance and therefore temperature coefficient. As

a result it is necessary to speak of temperature

characteristics over a wide range of temperatures rather than temperature coefficient.

The temperature characteristics of a resistance film of different values on phenolic sheet

show that the coefficient is negative but greater

at lower than at higher temperatures (fig. 4.4)

.

Also, the coefficient is greater for the higher

resistance values.

The temperature characteristics of a given

resistance material on phenolic and two different ceramic bodies are almost identical (fig.

4.5), indicating the supporting base in this case does not affect the characteristic appreciably.

High Frequency Characteristics

It is well known that the apparent a.c. re- sistance of composition resistors shows a drastic fall-off from the d.c. value as frequency in- creases. This is more particularly true of high

resistance values. This effect, known as the

"Boella effect," named after its discoverer in

Italy, was further investigated in England

where G. 0. W. Howe proposed a theory ex- plaining this behavior. He assumed that the

resistor simulated a transmission line of half

the length of the resistor, with uniformly dis22 Circulars of the National Bureau of Standards

P
U 10 o

-50 -25 25 50 75 100 125 TEMPERATURE IN DEGREES C Figure 4.5. Temperature characteristic of a given re- sistance material on phenolic and two different cera- mic bodies. — laminated phenolic base, - - - - ceramic "S" base, — . — . —

ceramic "D" base.

tributed capacitance. In this theory he derived

R£ a universal relationship between — and fR , R

R£ in which the ratio of — is constant for equal

R

products of R and f. (R r is the apparent h.f.

resistance at frequency f ; R equals d.c. resistance.) The fall-off in resistance value with

increasing frequency is therefore primarily due

to shunting effect of the distributed capacitance

in the resistor. Many observations confirm this

theory generally, although Salzburg and Miller,

in a paper published in April 1939 in the RCA

Review, show considerable departure at frequencies above 200 megacycles. More recently

this subject has been studied by R. F. Field of

the General Radio Co. In an unpublished paper,

he introduced the additional consideration of

dielectric loss in the distributed and lumped

capacitances in Howe's artificial line, which

would increase still further the fall-off in apparent a.c. resistance. The Boella effect in composition resistors of all types, including printed

films, is a direct result of the resistor structure. Large numbers of conducting particles

interspersed with large amount of dielectric

produce capacitances and its concomitant dielectric losses which act as shunts at high frequencies. Minimum dielectric is therefore the

condition for better frequency characteristics.

As a direct result of this condition, the

geometry of the resistor influences the high

frequency characteristics. Consider two resistors of equal length, one having twice the crosssection of the other. From a given resistance

value, the smaller unit would require resistance

material with the lower specific resistivity.

This means less dielectric resulting in better

0l_—L_——I I—U I

I L_U 1 1 L_l_l 1 1 L_LJ

01 O.I I 10 100 f

xR oc ( MEGACYCLES x MEGOHMS)

Figure 4.6. Frequency characteristics of different types

of resistors.

(1) film type 1% in. long in ceramic body; (2) film type % in. long in ceramic body; (3) film type 5/7 in. long in molded bakelite; (4) solid rod in molded bakelite.

frequency characteristics. The condition for

minimum dielectric is therefore minimum cross

section of resistor. Similarly, if the area of

both resistors is constant, the condition for

minimum dielectric is maximum length. For

best high frequency characteristics, the ratio

of area to length of the resistor should be a minimum.

The printed film resistor most nearly meets

both requirements established for minimum

Boella effect. Its cross section is extremely

small since it is a film. The film volume also

being small, the dielectric mass is very low.

The most comprehensive set of high frequency measurements on composition and film

type resistors have been made by D. T. Drake,

Massachusetts Institute of Technology, published in Unclassified Report 520; and R. F.

Field, previously mentioned. Curves representing the average performance of a number of

resistors of each type (fig. 4.6) have been taken

from the latter. Field states that these curves

have been smoothed out and extrapolated well

beyond the actual observations on the basis of

the best present information. The curves are

self-explanatory and also indicate the superior

performance of the longer unit. Although the

third curve from the top is also a film resistor,

its characteristics are poor because of its en- casement in molding material with large losses.

For comparative purposes the h.f. characteristics of a solid type composition resistor is

shown.

Voltage Coefficient

The resistance value of printed resistors is

not independent of applied voltage, i.e., it does

not strictly obey Ohm's law, and decreases with

Printed Circuit Techniques 23
„ ->0.05 z S o

Z

v * "ill -0.05

IS 2

o u u

0.0

-0.05

'A LONG

'osire—

COMP0 » 'if

1 I— -

P- 1

i/i" LONG COMP0SI1XW V COMPOSITION V

1

< 6 i

1 I 1 2 ' i ) ! e 1 ,

OHMS MEG RESISTANCE

1/2 WATT RESISTOR l/8* DIA. x 3/8" LONG

uj

ui<» oo

z >o

3

e —A;

I WATT RESISTOR \/< DIA. » 3/4" LONG

A . 6

!

'

i () : < i i i i > i i ! t (> E 1000 10000 OHMS

1.0 MEGOHMS10.0

RESISTANCE

Figure 4.7. A, Variations of voltage coefficient with resistance value, length

of resistance path, and composition of film. B, Variation of noise in printed resistors of different composition, length and resistance

values. A, B, and C are different types of compositions.

increasing voltage. This characteristic, called

the "Voltage Coefficient," is a function of: (1)

Composition, i.e., materials used in film; (2)

resistance value; for a given type of composition and size resistor, the coefficient decreases

with decreasing resistance value; (3) length of

resistor; for a given type of composition and

resistance value, the coefficient decreases with

increasing length.

This is illustrated in measurements (fig.

4.7A) taken on resistors made with two formulas of resistance material and on two different lengths. Lower resistance values in a film

resistor, for a given type of composition, implies less insulating material in the film.

Greater length of resistor, for a given value,

implies lower resistance per unit length and

therefore less insulating material. The variation required in each of these factors to produce minimum voltage coefficient is in the direction to reduce the amount of the dielectric

required.

The explanation for this is largely in the observed relationship between contact resistance

between carbon particles and voltage across

contacts. The contact resistance between two

carbon particles is an inverse function of voltage. Since most of the resistance resides in

the contacts, anything which reduces the voltage across the contact of conducting particles

will reduce the voltage coefficient. Increasing

the number of conducting particles is the equivalent of reduction of the amount of dielectric

in the resistor, which is the condition for minimum voltage coefficient.

Noise

Noise measurements were taken substantially as outlined in JAN-R-11 specifications

on printed film resistors on glass (fig. 4.7B),

with variations in composition, length of resistor, and resistance value. The noise variation pattern is similar to the pattern of voltage coefficient and high frequency characteristic. All other factors being the same, noise

decreases with ( 1 ) increase of resistor length,

(2) decrease in voltage applied, (3) decrease

in resistance value and (4) a favorable change

in materials, as shown by the curves. Christensen and Pearson reported in the Bell

System Technical Journal for April 1936 that

they failed to detect any noise in a solid carbon

filament other than that due to thermal agitation. They concluded that most of the noise

in carbon composition resistors came from the

contacts between carbon particles, and that

this was a function of voltage. They demonstrated that increasing the number of contacts

would decrease the over-all noise. Empirical

results with composition and film-type resistors

confirm their conclusions.

In the type of resistor under discussion re- sistance is increased by adding dielectric

binder. This is equivalent to reducing the contact density or the number of contacts per unit

volume, and increasing the resistance of the

contacts. For a given current through the re- sistor or a given power in the resistor, this results in greater voltage drop across each contact, which produces greater noise. As in the

high frequency and voltage characteristic the

condition for minimum noise is minimum dielectric in the resistor.

Many of the other characteristics of the

printed resistor can be similarly explained on

the basis of the composition and physical structure of the resistor. Once the materials, compositions, and physical shape of the resistor are

established, these characteristics will repeat

themselves uniformly and the resistor behavior

can be predicted.

Conclusion

There are available methods for large scale

production of printed resistors that can meet

the difficult requirements imposed on them. The

application of these methods to printed circuit

work is one of adapting these methods to particular requirements.

The characteristics of these resistors are well

known. The conditions required to produce satisfactory properties are known and by a proper

choice of materials and processing most of these

characteristics can be obtained.

24 Circulars of the National Bureau of Standards
5. Metal Films and Their Applications to the Fabrication

of Resistors and Printed Circuits 10

Constant research and development of components in electronics have helped tremendously

not only in stabilizing over-all circuit parameters but in reduction of size. One such notable

ij advancement was in the field of subminiature

jj receiver and transmitter design, made possible

by the development of the proximity fuze which

contributed much toward the winning of the war.Modern subminiature vacuum tubes and

printed circuit techniques are now being used

commercially on a large scale. However, the

printed circuit can be adapted to many uses where the present stage of the art does not

suffice. Thus, if the entire printed circuit could

be made inorganic in nature, in order to withstand abuse at exceedingly low or high temperatures, then the scope of its use could be vastly

enlarged.

Let us center our interest primarily on the

resistive components ordinarily employed. Prior

art discloses that organic bonds used in the

fabrication of the conventional carbon resistors

have a rather limited temperature range and,

therefore, limit the ambient temperature under

which the resistor will operate satisfactorily.

It is thought, therefore, that if the same proccesses now used in the fabrication of "Nobleloy" metal film resistors could be adapted to

the printed circuit process, a superior product

would, no doubt, be the result.

Before attempting to describe a tentative

metal film resistor process applicable to the

present-day printed circuit technique, consider

some of the processes now employed in the

manufacture of "Nobleloy" X-type resistors

(fig. 5.1).

The ceramic form upon which the metal is

to be precipitated is composed of either pure

magnesium silicate or a combination of magnesium and aluminum silicate. The former

material is preferred since experiment has

shown that better control of the finished product may be expected. The ceramic tube or rod is then sprayed with a solution of an es- pecially prepared metallo-organosol of palladium, the concentration of which is carefully

controlled in order to yield a unit of a given

predetermined resistance value.

The metallo-organosol solution may be prepared by either of two methods—the precipitation or the fusion method. We have chosen the

precipitation method because the reaction can

be more carefully controlled. Partially hydro10 By J. W. Jira, executive vice president, Continental Carbon,

Inc., Cleveland, Ohio.

genated abietic acid is reacted with sodium

hydroxide to form sodium resinate. The resinate thus formed is reacted with an aqueous

solution of palladium chloride under very carefully controlled conditions, forming a precipitate of palladium resinate. The precipitate thus

formed is washed very carefully of residue and

force-dried. The dehydrated resinate is then

dissolved in a balanced organic solvent of very

low surface tension, forming the metalloorganosol of palladium. The above prepared

solution is a very good insulator and will show

no conduction prior to processing.

The processing of the coated tubes or rods

consists of subjecting the units to a pyrochemical process in which the resinate is caused to

revert back to the pure metal and to precipitate

very slowly and evenly throughout the surface

area of the ceramic form. The carbonaceous

residue formed during this process is made to

Figure 5.1. "Nobleloy" resistors in various stages of

manufacture.

Top to bottom: Plain magnesium silicate tube; tube sprayed with

metallo-organosol; metallized film after firing silver end contacts

to film; vitreous enamel coating; wire terminals soldered to silver contacts; spiralled resistor to obtain required value; completed unit.

Printed Circuit Techniques 25
combine with atmospheric oxygen forming car- bon monoxide which, in turn, is oxidized to

form carbon dioxide gas, which is easily re- moved. The affinity of the metal film to the

ceramic form is such that mechanical grinding

is the only means by which it may be effectively

removed.

Electrical contact to the resistive element is

made by coating each end with a silver enamel,

which, after firing, produces a very tough

metal surface to which wire loads are soldered.

Prior to the soldering operation, however,

comes the vitreous enameling process, upon

which the success of the metal film resistor

hinges. This operation not only protects the

metal film from mechanical abrasion but also

from atmospheric influences, thus causing the

unit to display an ultrastable shelf life or, in

other words, excellent permanency in ohmic

resistance. Also, the vitrified coating greatly

increases the wattage rating by permitting

higher surface temperatures, normally not possible when employing organic coatings.

The use of a vitreous enamel over the metallic

film appeared simple enough from the outset,

but as time went on insurmountable difficulties

appeared. The standard enamels were not low

enough in fusion; their high alkalinity would

attack the film at temperatures of fusion and

thus completely change its electrical characteristics ; crazing and spalling when under load

would cause cracks to appear in the fused coat

and thus permit entrance of outside influences

and cause the resistance value to drift. A special research program by Continental's engi- neers produced an enamel that solved the above

difficulties. A roller coat method of applying

the enamel to the metal film was also devised

and is in use at the present time.

After firing, the resistors are fitted with wire

leads which are soldered thereto. The units are then assorted to values, such as 1, 10, 100, 1,000,

10,000, 50,000 and 100,000 ohms. From these

basic resistance values, any conceivable value

from 1 ohm to 100 megohms, within tolerances

of plus or minus 1 percent may then be obtained

by the spiralling process. The spiralled resistor

is then complete and ready for use in circuits

of most exacting requirements. Units of vari- ous resistance values were set aside for resistance stability study. After 60,000 hours (which

is equivalent to 7 years) elapsed time, the

X-type "Nobleloy" metal film resistors were

all within 0.1 percent of their initial value.

The conventional carbon-type resistor is designed to dissipate approximately 1 watt per

square inch. The "Nobleloy" type resistor, however, with its unique metal film construction,

will safely dissipate 4 watts per sq in. This

design factor of 4 watts per sq in. is utilized

in the industrial high-wattage type resistors

26

where resistance tolerances are standardized

j between plus or minus 5 and 10 percent. For | precision application, however, a design factor I of 2 watts per sq in. is used. This procedure is < followed in order to keep the effects of tern- I

perature coefficient at a relatively low value. 1 The average temperature coefficient of resist- I ance pertaining to the "Nobleloy" X-type metal

J film resistors varies between the limits of \ —0.0002 to —0.0005 which, in other words, is

equivalent to approximately —0.035 percent

per deg. C. Recently, new alloy-type metal

films have been developed for the fabrication

of ultraprecision-type resistors. Test data indicates these units to have a temperature coefficient approximating that of the well-known

alloy, manganin.

Let us now turn our attention again to the

printed circuit and see how the metal film re- sistor and interconnecting circuits could be incorporated therein.

The first procedure would be to provide two

masks. The first of these would provide for

the necessary wiring and interconnected inductors. The second mask would provide for

the necessary resistors. The required capacitors would be affixed in their respective places

after printing.

The first mask, carrying the wiring and inductors, would be fixed into place on a ceramic

plate. A special metallo-organosol alloy solution would then be sprayed into the openings

of the mask and permitted to air-dry. The

second mask would then be affixed in place and

an organosol capable of yielding high resistance

metal films would be sprayed into the openings.

The same process would be repeated on the re- verse side of the ceramic plate to provide for

the necessary network.

The entire ceramic plate thus processed

would be fired in order to metallize the circuit, i The same mask employed in spraying on the

resistor components could be similarly used in

spraying on a film of vitreous enamel over the

resistor surface and then retired. The whole

process, in the author's estimation, should not

take more than 30 minutes to complete.

Since a thorough investigation of the possibilities of using the metallo-organosol process

pertaining to the printed circuit has not been

made, it is recognized that changes in the procedure described will have to be made. In the

first place, the resistivity of metals akin to | palladium is within the range of 9 to 11 X 10"6 ohms centimeter, whereas that for elemental : carbon is approximately 3,500 X 10 6 ohms cen- timeter at 0° C. In the second place, in the fabrication of metal film resistors using the

ceramic rod or tube, one may resort to the

process of spiralling in order to adjust the unit

accurately to the resistance value required.

Circulars of the National Bureau of Standards
This, however, cannot be done in the case of

the printed circuit. Therefore, other procedures should be devised in order to attain the

same end.

The metallic film could be deposited in the

form of a noninductive grid, which would in- crease the film length and, consequently, its

resistance value. In order to adjust the metal

film resistor thus formed, care should be exercised in that the initial value of resistance thus

received should necessarily be a certain percentage above the resistance value required and

then trimmed down to within its tolerance

range. This could easily be accomplished by

superimposing, or tracking, a metal of low resistivity upon the deposited grid, thus permitting the initial resistance value to recede as

required. The capacitors and tubes would then

be affixed into their respective places, thus completing the process.

6. Trends in Military Communication and the Applicability

of Printed Circuits 1

1

This paper is presented in two parts with

two objectives in mind: First and principally,

to acquaint the reader with military communication equipment trends, especially as they affect component development; and second, to

consider the applicability of printed circuits in

furthering certain of these trends.

The most important of these communication

trends, insofar as components, materials, or techniques are concerned, is reduction in equipment size, even in the face of increased complexity. The earliest military "walkie-talkie"

was a simple super-regenerative transceiver

employing only two tubes which occupied 650

cu. in. The replacement for this set was an FM

transceiver employing 18 tubes, which occupied

575 cu. in. The latest version of the "walkietalkie", currently under development, will employ 16 tubes and occupy approximately 125 cu.

in. Advances in miniaturizing equipment have

thus made important strides in incorporating

eight times as many tubes and their associated

circuits in one-fifth the space. Tactical applications of such equipment, however, requires even

further reductions in size and weight. This can

only be accomplished if reduction in the physical size of components continues, new techniques are developed, or major circuit simplifications are accomplished.

The term "miniaturization" has generally

been associated only with extremely small

equipment. In military communications, however, miniaturization is not limited to subminiature construction. It also includes the reduction

in size of 60-line switchboards and even 5-kw

transmitters to permit their use as mobile

equipment.

One component problem inherent in miniaturization, whether it is a subminiature unit or

a 5-kw transmitter, is that of heat dissipation.

In reducing the size of equipment, whether this

11 By B. V. Blom, Signal Corps engineering laboratories, Fort Monmouth, N. J.

reduction is accomplished by a reduction in size

of the components or in the evolution of a new

technique, it will be a major achievement if the

efficiency can be maintained. These smaller

components or units will therefore have to dissipate as much heat as their larger prototypes.

This may very well be the limiting factor in the

degree of miniaturization that can be obtained.

Miniaturization, especially subminiature construction, also means the crowding of components into less and less space. This again can

only result in higher ambient temperatures.

A second development trend is that toward

unitized construction. The earliest examples of

such construction stressed unitization of design

almost entirely from the maintenance standpoint. Subsequent steps in this trend were

directed toward reduction in size and improvement in performance. This type of construction has finally evolved into plug-in hermetically

sealed stages or subassemblies employing

soldered-in subminiature tubes.

From the component viewpoint, this hermetically-sealed type of construction offers one

big advantage in that it seals off components

from the high humidity effects present in most

areas. It also, however, imposes strict requirements on the stability of the components used

in these assemblies due to changes in temperature, age, and particularly shock or vibration,

especially those components used as frequency

determining elements. This is quite evident

when one stops to realize that in a hermetically

sealed IF stage, for instance, adjustments are not possible once the unit is sealed unless the

seal is broken or some relatively large and complicated mechanism is used.

During the past war, due to the pressure of

getting equipment into the field, equipment was

primarily designed for particular applications.

The trend at the present time is to develop universal equipment having sufficient flexibility to

permit adaptation to a number of applications.

Printed Circuit Techniques

793811—48—5

27
I

While this general heading of increased flexibility covers a broad field, two items of particular interest are increased frequency cover- age and increased number of preset channels.

Increased frequency coverage requires tuned

circuits with increased timing ranges. Tuning

ranges of 1 to 6 mc, 5 to 30 mc, 20 to 70 mc,

and 50 to 500 mc, are typical examples. Components and techniques are needed which will

permit accomplishment of these tuning ranges

in an acceptable size.

Equipment developed during the past war,

while covering a fairly large frequency band,

provided only from 1 to 10 preset channels.

This relatively small number of presets permitted the use of a crystal for each preset. The

trend at the present time is to make all channels immediately available to the operator. This

will be equivalent to providing from 80 to 1,000 or more preset channels and will require the use of crystal saver or reference crystal type

circuits or of self-contolled oscillators having

essentially crystal stability. From the viewpoint of increased flexibility, the self-controlled

oscillator seems to have most to offer. The development of an oscillator with stabilities of

crystal order, however, imposes extremely se- vere requirements on the stability of the components used in these oscillator circuits. For

example, in one particular case, an oscillator,

which is being designed to cover the 20 to 70

mc band, has a capacity of 25 ^f and an inductance 0.4 /xh at 50 mc. In order to achieve

the required stability, this inductance and ca- pacity must be maintained, under all operating

conditions, within two parts per million per

degree centigrade.

Reliability has always been one of the primary requirements for military communication

equipment. Its importance has been stressed

time and time again, and much has been

achieved in the development of reliable components for military usage. Certain trends have

developed of late which, however, put even

more stringent requirements on reliability of

operation. In the past, equipment failure has

meant, at most, the loss of one communication

channel. With the advent of multichannel radio

relay equipment, failure of some components

of a radio relay system can result in the loss of

24 channels of communication. This figure may

rise to as large as 100 channels in the near

future. The increased use of components which

are common to a number or all channels of

multichannel equipment can result in material

savings in size and weight. For instance, one

such scheme under consideration is the use of

electronic multiplexing in telephone carrier

equipment, using one oscillator common to all

channels. Obviously, since in this application

failure of the common oscillator will put all

channels out of operation, the advantages of obtaining miniaturization by using common circuits have to be balanced against reliability of

operation and it can only be employed to its

fullest extent when the reliability of compon- |

ents so warrant.

Multichannel radio-relay systems are also I tending toward unattended operation of equipment installed at remote locations. Such operation imposes the very ultimate in reliability on ; components. This type of operation also raises

another interesting problem. Whenever life-:

figures have been quoted in the past, they have

been average life-figures based on a relatively

large number of failures. If a life-figure of

10,000 hours was quoted, it meant that the j average unit or some 70 or 80 percent of the

units would operate 10,000 hours before fail- ; ure. In unattended operation, this average lifefigure does not have much meaning. The information that is actually required is the

guaranteed life of say 99.99 percent of the components. Whether this guaranteed life is 2

months or 12 months is relatively unimportant

as compared to a definite knowledge of the

guaranteed life. If this guaranteed life were

only 1 month, it would simply mean that this

unattended station would have to be visited and

this component replaced every month.

All previous equipment development trends

have imposed increasingly complex requirements on components. There is one trend, however, that actually makes the problem easier.

That is the trend toward lower operating voltages. Vacuum tubes are available today, or will

be available shortly, which in most applications

will give practically the same measure of performance as older type tubes but which operate

with only 1/3 to 1/10 the plate and screen voltages as their prototypes. One series of these

low voltage tubes is designed to operate with

24 volts on the filament, screen, and plate.

Radio communication equipment has been ; moving higher and higher into the radio frequency spectrum Prior to World War II, 65

mc was the highest frequency used for communication purposes. Today, we have equipment operating at 5,000 mc, employing 65 mc

intermediate frequencies. No one knows what

the upper frequency limit may be, in the future.

Military communication equipment must be

capable of working over increasingly wide temperature ranges. Initially, consideration was

only given to the temperature ranges encount- ', ered in the temperate zone. Warfare in the

African desert and the South Pacific jungles

resulted in increasing the operating ambient

temperature requirements to 145° F. The

strategic value of the north polar regions in an

aircraft or guided missile war are now introducing cold temperature requirements as low

28 Circulars of the National Bureau of Standards
as —80° F for storage and —70° F for operation. This combination of events has thus re- sulted in an equipment operating range of —70° to +145° F. Within this temperature

range, rapid temperature changes of up to 60°

I F can be expected. In addition to these general

temperature requirements, the advent of high

speed aircraft with high skin temperatures are

1 introducing requirements for components which

will operate at 200° C.

Still another trend is that toward the inI creased utilization and importance of communi-

! cations in modern warfare. The importance of

I radio as a means of communication increased

l from the position of almost a curiosity in World

1 War I to that of a major communication source

in World War II. Even with the tremendous

i increase in communication facilities made avail1 able by this increased use of radio in the past

' war, requirements for wire communication facilities also increased to the point where multichannel carrier telephony equipment and teletype equipment were required to handle the

wire traffic. This increased demand for communication facilities will make mass-production

capabilities one of the major design considerations for all future military equipment.

In summarizing, the following equipment

design trends are to be noted : ( 1 ) Miniaturization (reduction in size), (2) unitized conI struction (hermetically sealed plug-in assemIblies), (3) increased frequency coverage, (4)

increased number of preset channels, (5) increased realiability, (6) lower operating voltages, (7) operation at higher frequencies, (8)

operation over extreme climatic conditions, and

(9) increased mass-production capabilities.

The applicability of printed circuits in furthering these development trends seems to offer

' much in furthering the last of the design trends

< summarized—that of increased mass-production capabilities. During the past war the two

primary shortages were in strategic materials

(that is, materials which were not available or in short supply in this country such as rubber, tin, mica, quartz, etc.) and manpower.

Strategic materials can be stock-piled or substitutes or synthetics can be devised. Manpower

cannot be stock-piled and the only substitute

, is machine power. The fact that the producj tion of printed circuits is a machine process is

therefore of primary military importance.

Being a machine process, the technique offers a number of advantages and also imposes some

new problems. Mechanized production of radio

equipment will result in more uniform production, reduction in inspection time and cost, and

reduction in line rejects. Quality control prob-

! lems and stocking problems will be considerably

reduced since resistors need not be stocked in

their multiplicity of values and wattages, but

will be mixed in the plant from the basic ingredients. This also applies to capacitors which

will be purchased in sheets and then stamped

out to the required capacity. A multiplicity of

wire sizes and types will be replaced by a silver

paste. One of the problems that will arise, however, will be the fabrication of accurate components. For example, 5 percent tolerance re- sistors are now obtained by a process of selection after fabrication, and accurate inductances

are obtained by adjustment after winding.

Neither of these processes is possible where the

circuit is printed directly on a form. Industry

has a long record, however, of overcoming such

obstacles whenever the goal is worth the effort.

Most early publications on printed circuits

placed primary emphasis on the reduction in

size possible with this technique. While the

application of printed circuits to military communication equipment may result in some re- duction in size, in the opinion of the writer, the

degree of miniaturization will not be startling.

A three-stage audio amplifier was built at the

Signal Corps engineering laboratories using

conventional subminiature technique and comparing in size with a commercial three-stage

printed audio amplifier of a certain manufacturer. From this comparison it was apparent

that even with the most advantageous use of

space, the printed circuit amplifier could not

be made substantially smaller than that employing conventional miniaturization techniques12 In connection with the trend toward unitized

construction, printed circuits offer many interesting possibilities. The inherent type of construction used in printed circuits is such as to

lend itself very readily to its employment as

plug-in subassemblies. While the hermetic sealing of these units may be a more difficult problem, the sealing of the unit by means of a protective coating such as Selectron might be an

acceptable solution. With respect to the stability requirements imposed by the sealing of

these units, the use of printed circuits seem to

offer some decided advantages over conventional techniques, especially with regard to immunity to shock and vibration. The very nature

of printed circuits also lends itself admirably

to the application of the technique of plating

coils on highly stable forms such as fused

quartz, a technique which holds much promise

with regard to temperature stability.

In general, to the best of the writer's knowledge, printed circuits (as differentiated from

12 Editors note: The author points out the very real misconception that in all eases printed circuits will lead to greater miniaturization than is possible by other means. In many cases, using newly

developed subminiature electronic components, a device may be made as small as a printed circuit unit by using standard miniaturization techniques. Examples do exist, however, as pointed out

in the general discussion, where printed circuits allow miniaturization which cannot be duplicated by other methods.

Printed Circuit Techniques 29
plated or printed components) in their present

stage of development, have been applied primarily to high frequency fixed-tuned or untuned

circuits which do not nave critical resistance,

capacity or inductance requirements. This limitation on the number of applications to which

printed circuits have been applied may be due

in part to the relatively poor stability of high

K ceramic capacitors, to the accuracy with

which resistors can be printed and to the inability to adjust the resonant frequency of

printed circuits. With these limitations, the

application of this technique to "high-performance" Signal Corps communication equipment

must necessarily be restricted almost exclusively to audio amplifier circuits. No doubt

these problems are being studied and solutions

may be at hand which will permit their more

widespread use. Circuit applications such as | intermediate frequency amplifiers, multivibrators, etc., seem to be admirably suited for

the application of printed circuits if and when

these limitations are eliminated.13 To summarize, the printed circuit technique

has been developed to where its practicability

has been established, basic advantages have

been proven, direct applications have been

found, and many interesting possibilities have

been envisioned. Most of the applications of

printed circuits, insofar as a furthering of military communication equipment development

trends is concerned, fall in the category of

future possibilities. It is hoped that this paper

will assist in speeding up the day that these

interesting possibilities will become accomplished facts.

7. Vitreous Enamel Dielectric Products 14

During the past war the critical shortage of

mica resulted in the Signal Corps' sponsorship

of research on alternate materials, particularly

for use in electrical capacitors. As part of

this program, a research contract was placed

with the electrochemicals department of du

Pont in their plant in which raw materials are produced for the ceramics industry. This contract, still in force, in modified form resulted in

the development of materials and a process for

making vitreous enamel dielectric capacitors

with electrical characteristics comparable to

silver mica capacitors.

The inherent high quality of this product

plus the fact that the process is admirably

adaptable to the production of units involving

printed circuits resulted in a development program by du Pont and the Remington Arms

Co., a du Pont affiliate, to evaluate the commercial possibilities. Consequently, we are currently producing samples of single unit capacitors similar to conventional products and

"specialty" units involving printed circuits. The

process and typical products will be described.

Process

The process is based in the laying down by

spraying of layers of vitreous dielectric en- amel alternating with layers of conductive sil- ver paste deposited by silk screen or squeegee

printing.

The requirements on the silver paste have

been discussed by Mr. Patton in another paper.

The enamel is of suitable consistency for spraying and in general consists of 15 to 25 mole

percent of silica, 3 to 11 mole percent of alkali

oxides, 5 to 11 mole percent of alkali fluorides,

15 to 25 mole percent of lead oxide and 11 to

30 mole percent of other bivalent metal oxides.

The basic unit of the process is the build-up

machine in which the desired structure is built

up on 9-by-12-in. steel conveyor plates. The

steel plates are loaded onto a conveyor which

is arranged in a rectangle. The conveyor carries the plates into a paint spray booth where

the enamel is applied from a spray gun, which

sprays downward onto the horizontal surface of

the plates.

The plates, after receipt of the sprayed en- amel, are carried by the conveyor through an

infrared drying tunnel in which the enamel

dries to a consistency which we refer to as "cheesy." For the next step, the plates are momentarily halted at a station where the desired silver pattern is squeegeed on them. This

pattern may contain as many electrodes as can

be drawn within the area of the steel plate and

the configuration of each of these electrodes

may be anything that can be printed by the

of a;

are

13 Editors note: See other opinions on these points in general

discussion. Printed circuits have been applied successfully to the design of two-way personal trans-ceivers of the tunable type meeting F.C.C. frequency-stability requirements (see paper by Al Gross),

to the design of intermediate frequency amplifiers, and to circuits such as multivibrators, etc. Limitations on high K ceramic capacitors are gradually being overcome by continued research; however,

printed circuits are not limited to this type of capacitor as the fol- lowing paper indicates. Other papers in this series describe accurate

resistors for printed circuit applications.

14 By C. I. Bradford, technical department, Remington Arms Co.,

Inc., Bridgeport, Conn.; S. A. McNeight, electrochemicals department, E. I.du Pont de Nemours and Co., Inc., Perth Amboy, N. J.; and B. L. Weller, technical department, Remington Arms Co., Inc., Bridgeport, Conn.

jiy

is o

silvi

irei

The

is a

var

30 Circulars of the National Bureau of Standards
Figure 7.1. Microphotograph of cross section of a

vitreous dielectric capacitor.

i squeegee printing technique. After receiving

the silver pattern the plates continue along

the conveyor to the spray booth.

This spray-dry-silver cycle is repeated until

the desired structure has been obtained. On

leaving the machine, the plates pass through

a mechanism which cuts the build-up into units

of appropriate size and shape. Next, the plates are baked to remove some of the organic mati ter, to free the.enamel-silver structure from the

steel plate, and to impart to each cut unit sufficient rigidity to allow it to be handled.

Depending upon the relation between the sil- ver patterns and the size of the cut unit, silver

will be exposed at specified points on the edges.

Any desired connections between different lay- rs of exposed silver may be made by applying,

silver paste to these edges. The units are then

fired for about 12 hours to produce a monolithic structure typical of vitrified ceramic

articles. The electrodes at the top do not extend

to the sides since they form another capacitor.

The fired unit (fig. 7.1) at this stage of the

process is a usable capacitor to which, in ordinary practice, leads are attached and a finish

is applied.

Product Description

This process can be used to produce a wide

variety of capacitors with dimensions limited

only by economic considerations. The present

equipment is capable of making pieces with

maximum dimensions of 6 by 8 in. It is possible to cut the single units as small as 0.3 in.,

and the most economic thickness is of the

order of y8 in.

The simplest form which these units can

take are those incorporating single capacitors.

The particular sizes and dimensions are governed by product design considerations. The

electrodes are terminated along the edges of the

capacitor and connections are made to the end

silver. The product has sufficient mechanical

strength to make it unnecessary to enclose it

in rigid plastic, and an organic finish is adequate to give electrical insulation. These units

can be supplied in capacitances up to 5,000 /xfif ; however, the relative economics favor the ca- pacitances of less than 1,000 /x/if,

Printed Circuit Techniques

Figure 7.2. Arrangement of internal electrodes of four

capacitor, S-3 unit.

The production technique can easily be ex- tended to include numerous capacitors incorporated in one unit. This one unit contains two

capacitors which have a common terminal at

one edge and separate terminals at the other.

This particular arrangement contains an electrostatic shield between the separate plates

which is connected to the common terminal.

A similar application to two capacitors in one

unit may be used as the tuning capacitors for

i-f transformers. In this case a hole through

the chip is provided for mounting the i-f coils or making the core accessible for timing.

Although there is no limitation to the combinations of capacitors which can be built into a single unit (fig. 7.2), practical considerations

usually make it undesirable to extend the combinations beyond about four to six capacitors.

Capacitors between electrodes 5 and 6, 2 and 6,

1 and 2, and 3 and 4 are formed with multiple

dielectric thicknesses although only single lay- ers are illustrated here. Connections to these

units are made with the tabs that are shown.

Proper design reduces unwanted capacitances

below the limits allowed from circuit considerations. In a completed unit of this type, the six

terminals have been made with end-silver. A

hole through the center of the unit is supplied

as in the two-capacitor unit. External silver

may be arranged to receive resistors on the

surface of the ceramic. The International Resistance Corp. is cooperating with us in this

program and has developed resistors with characteristics comparable with the best conventional commercial units. A four-capacitor, oneresistor unit for incorporation in the base of a final i-f transformer has been designed in

cooperation with the F. W. Sickles Co. It con31
1

1

1

Figure 7.3. Two stage amplifier unit incorporating

four capacitors as a part of the base plate.

tains the two tuning capacitors and a twocapacitor single resistor diode filter.

A somewhat different arrangement for use

as a diode filter incorporates two capacitors and

four resistors in one unit. This is an experimental assembly used to investigate the practicability of supplying eyelet terminals in the

ceramic for special applications.

These techniques can be extended to accommodate an infinite number of combinations of

capacitors, resistors, and inductors in complete

circuits.15 Since conductors may be printed

either internally or externally, the arrangements need not be limited to one plane (fig.

7.3). This block contains all circuit elements

and interconnections of a two-stage amplifier.

A cable connector unit illustrates the versatility of the process. The cable connector unit,

about 11/2 m- in diameter, contains four capacitors—one from each of the outer four terminals

to the common central terminal. A high-voltage

unit, about 1/10 in. thick, contains floating

electrodes spaced 1/100 in. apart and will withstand operation at 20 kilovolts.

Electrical Characteristics

Of primary importance to the engineer who

is interested in taking advantage of this process

are the electrical characteristics of the materials used. The ceramic dielectric has a loss

factor of the same order as mica and the properties of the silver are such that high electrical

conductivity is obtained. Incorporation of this

silver and the ceramic into a monolithic block

adds the further characteristic of high stability.

To demonstrate how these properties are used to good advantage, the following characteristics of typical capacitors are given:

15 Editors note: See "Printed Vitreous Enamel Components" by

C. I. Bradford, B. L. Weller, and S. A. McNeight, Electronics, De- cember 1947, p. 106 for added details.

Capacity Range

At present, capacitors are made in 500-volt \ ratings only and result in units with approxij mately 0.02 [d in. 2 Typical sizes are 0.2 in. by I 0.25 in. by 0.1 in. for 50 ^f and 0.375 in. by I 0.425 in. by 0.11 in. for 330 ^f. The smallest I sizes obtainable are restricted only by lead ca- pacitances and handling limitations.

Capacity Tolerance

Since it is possible to adjust both the electrode areas and the dielectric thickness while j producing these capacitors, good control of

capacitance yield is possible. With appropriate

sorting, capacitance to any tolerance can be I obtained.

Capacity vs. Frequency

There is relatively small change in capacitance with frequency up through 20 megacycles

(fig. 7.4). At about 50 megacycles inductive

effects increase the apparent capacitance by

about 10 percent. (Series inductance is of the \ order of 0.005 to 0.01 mH.)

Loss vs. Frequency

The fact that the dissipation factor is of the

order of 2 X 10-3 at frequencies as high as 100

megacycles (fig. 7.4) is of particular interest

to those who are working with FM or television. These data were taken for us by Mr.

R. F. Field of the General Radio Co.

Capacity Drift

The capacity drift as defined in JAN-C-5 is

0.02 percent. This value places the drift within

the rigorous limitations of the F characteristic

and demonstrates the high stability of the unit.

Temperature Coefficient

A typical graph (fig. 7.5) of capacity variation with temperature shows the units to have

a positive temperature coefficient of about 105

ppm per deg C. The capacitors are usable up

to 125° C with little change in temperature

coefficient.

Loss vs. Temperature

The inorganic nature of these units makes

them basically immune to temperature (fig.

7.5). From practical considerations the maximum temperature is considered to be 125° C.

Insulation Resistance

The leakage resistance of these units is high

and of the order of 10* to 106 megohms for

32 Circulars of the National Bureau of Standards
(X/ACC /.O/CC /O.KC /OO.KC /OOOXC /O A*C /OOMC

capacitances up to a few thousand micromicrofarads. Proper finishing insures that this

high resistance is maintained in spite of high

humidity. Considerable data on the performance characteristics of this product are available at the Components and Materials Branch,

Signal Corps Engineering Laboratory, Fort

Monmouth, N. J.

Mechanical Features

The mechanical characteristics of units made

by this technique have been touched on briefly

in this paper. They can be summarized as follows : The integral construction of silver and cera- mic results in high mechanical strength approximately comparable to steatite. Although

the material is brittle, it is adequately shock

resistant to withstand normal handling. Leads

attached directly to the block without further

support can meet the pull and bend tests in

common use. A unique feature of the construction is the

high thermal conductivity of the blocks. Since

the most economical designs contain a minimum of unused ceramic, the silver is distributed

throughout the entire volume and the resulting

thermal conductivity is excellent. This feature

makes the unit relatively immune from heatshock, and soldering of the leads directly to

the end silver can be accomplished.

The units can be finished in any appropriate

manner to meet particular applications. In

general, it has been found that organic lacquers

or waxes are sufficient.

Figure 7.5. Graphs showing capacity vs. temperature

and loss vs. temperature of vitreous dielectric capacitors.

Printed Circuit Techniques 33
Conclusion Acknowledgments

In conclusion, the vitreous enamel process

appears to offer attractive possibilities for the

production of printed circuit subassemblies and

the production of individual components be- cause (1) the capacitor elements possess high

quality electrical characteristics to an outstanding degree; (2) the mechanical properties of

the basic capacitor units are superior to most

of the components in general use in the communication-electronics fields; and (3) the process possesses an inherent flexibility which

makes practical the combining into single compact units of several high quality capacitors

with other circuit elements including their interconnecting leads by the printed circuit technique.

The authors of this paper acknowledge the

contributions made to this development by Dr.

C. Robertson, Dr. K. H. Ballard, and Dr. A. J. Deyrup of the technical division of the du Pont

electrochemicals department and E. H. Schmidt

of the du Pont engineering department. The

contributions and cooperation of the members

of the Signal Corps engineering laboratories is

also acknowledged. The advice and assistance

of Harold A. Wheeler, our consultant, and the

contribution of T. G. Owen of Remington in the

production of samples are also appreciated.

8. Printed Electronic Components on Glass 16

It is desired to limit the discussion of this

subject to those components for electronic circuits which have been reduced to commercial

practice by Corning Glass Works, rather than

to extend it to developments which are yet to

be thoroughly demonstrated. Separate circuit

components, such as inductances and capacitors

of several types, are now commercially available while various combinations of such units are yet in their development stages.

Glass, the base material of these components, is generally characterized by good mechanical and electrical properties. It is hard,

nonabsorbent, and cannot be permanently deformed. It is much lower in thermal expansion

than metals, except those of the Invar type, so that dimensional changes with temperature are

small. Glasses are regularly produced for which

power-factor values are comparable with the

best solid insulating materials available. Values

of dielectric constant lie generally within the

range of 4 to 9, which is satisfactory for most

types of electronic applications, although it

does not approach values obtainable in the titania ceramics. Except for surface resistivity,

glass is unaffected by moisture, and nonhygroscopic coatings are available which reduce

greatly this effect of moisture. Glass also has

excellent nontracking properties, so that the

surface is unaffected by surface leakage or adjacent discharges. Because of these properties,

16 By H. S. Craumer, Corning Glass Works, Corning, N, Y.

o4

glass is an ideal insulating material for electronic use. Glass is readily available in standard

forms such as plates, tubes, and cylinders, and,

when design considerations necessitate it, other

special forms may be considered.

During the recent war period, extensive development work was carried out on the formation of metallized films and coatings on glass.

This included the development of new techniques and the improvement of existing techniques. Films of one general type are formed

by a vacuum metallizing process. They are of

relatively high resistance and are of a permanent nature. The metal is deposited in a thin

uniform film so that its resistance is relatively

independent of frequency to high frequencies

of the order of 100 Mc, and with low reflection

of power loss at microwave frequencies. Values

of d.c. resistivity for these particular films are

readily obtainable within the range of 10 to

1,000 ohms per square, with a low positive coefficient of resistivity with temperature. The

dissipating capacity of these films depends to a considerable extent on the size and shape of

the glass part to which it is applied. Values

of 7 watts per sq. in. with a temperature rise

of 170° C have been attained on small glass

strips mounted in still air.

Metal coatings of low resistivity can be

formed by firing metals such as silver into the

glass surface which bonds them firmly to the

glass. In many cases this silver coating is cov- ered with a second coating of electroplated copCirculars of the National Bureau of Standards
per, and then tinned to facilitate soldering.

Such coatings can be produced to accurate dimensions and in a variety of patterns, to form

circuits, conducting areas, etc., to which structural parts or terminal leads may be soldered

without resorting to complicated techniques.

The joint made by soldering to these coatings

can be made vacuum tight so that it may be

used for purposes of hermetically sealing.

The bond strength between the coating and

the glass is roughly 2,000 lb. per sq in. and

its resistivity when copper plated is approximately 1.8 X 10-3 ohms per square.

The current trend toward high frequencies

for home receivers such as the 88-108 Mc band

for FM, and frequencies of over 200 Mc for

television, has created a need for capacitors

and inductances of somewhat different characteristics than for those used in equipment operating at frequencies of about 1 Mc. A trimmer capacitor (fig. 8.1A) now being produced

by Corning Glass Works illustrates this. Its

size can be better appreciated by realizing that

the thread of the adjusting screw is 4-40. In

this capacitor the glass performs several functions: (1) Insulation between the two conductors; (2) mechanical support for the bushing

on the panel at one end, and the terminal on

the other; (3) a base material for the metallized surface acting as one plate of the capacitor; and (4) the dielectric material between the

two conductors. In this case the conducting

surface on the glass consists of a "fired-on" sil- ver coating, copper plated and tinned. The

assembly of the various elements of the unit

is made by soldering.

The use of an Invar rotor, together with the

glass, which is stable both electrically and mechanically, results in a trimmer capacitor with

a low coefficient of drift with temperature. Its

coaxial construction and design give it excellent frequency characteristics. The largest

trimmer of the present line has a range of 1 to

12 fijif. These capacitors are available in two types

of mounting, one with a metal bushing and

nut, and the other with a snap-on, or push-on

mounting which reduces assembly time in set

manufacture. The smallest capacitor in the

line has a range of 0.3 to 3 ^f. The difference

in the maximum capacitance ratings is obtained

by adjusting the length of the metallized sur- face on the glass tube. A third intermediate

size has a rating range from 1 to 8 fifd. Inductors are also made utilizing the principle of metallized surfaces in the form of a

spiral metallized ribbon applied to the outer

surface of a glass tube, so that the conducting

surface approaches the conditions of a theoretical current sheet. The advantages of this

Figure 8.1. A, 1 to 12 uuf midget trimmer capacitor with push-on mount.

B, "Fixed-tuned" or "trimmer" inductance with powered iron core

of standard 0.20 in. diameter.

type of construction are readily discernable.

The metal being firmly bonded to the glass, re- mains in intimate contact with it, and dimensional changes with temperature are determined by the properties of the glass, which has

a much lower coefficient of expansion than

metals used as conductors. Dimensional tol- erances from piece to piece, both as to diameter

and coil pitch, can be controlled within close

limits in manufacture. Inductors of this type

may be used for permeability tuning in the

88-108 Mc. FM band. Three inductors are normally used in such a receiver, one in the an- tenna circuit, and one each in the RF and the

oscillator stages. These inductors can be

mounted conveniently by the use of a conventional grommet on one end. The metallized sur- face is dipped in water-protective lacquer to

make it weather resistant. This dip does not

interfere with ease of soldering to the metallized surface. Connections and taps may be

soldered to any point on the spiral ribbon.

Quality control checks on this inductor in production measure an average Q-value of 180

at 100 Mc. The temperature coefficient of the

Printed Circuit Techniques 35
inductance at 100 Mc is of the order of 7 parts

per million per degree centigrade, and is always

positive.

Another inductor used for second subharmonic operation of the FM band has a pitch

of 8V2 turns per inch, as contrasted to the 4

t.p.i. for the first inductor shown. Fifteen turns

per inch is normally considered a maximum,

because a finer pitch complicates the termination problem, and tends to lower the Q-value,

because of the increased d-c resistance of the

turns.

Both inductors are designed for use with

powdered iron cores with a standard diameter

of 0.20 in. The glass form of the first inductor

shown has a wall thickness of 0.035 in., while

for the second it is 0.070 in. These wall thicknesses, which have been standardized, permit

ample frequency spread and facilitate tracking

of the various stages of the circuit.

A "fixed tuned" inductor (fig. 8. IB), or it

might perhaps be termed a "trimmer," has an

application in those cases where the 88-108

Mc FM band is variable capacitor tuned to

facilitate tracking. It can also be used for fixed

frequency circuits. This particular inductor

has a rating of 0.07 to 0.1 /.Ji and uses the

standard glass form and powdered iron core mentioned previously. This unit is mounted

and locked in the panel in a manner similar to

that already described for the midget trimmer

capacitors in the first part of the discussion.

A 4-40 screw attached to the powdered iron core permits fine adjustment of the inductance.

These inductors are also supplied with the pushon mounting. Since powdered iron cores are

also available with a diameter of % in., inductors can be made suitable for this size of core

also.

The glass used for the forms of these inductors is a low expansion glass of good electrical

properties. It is considerably lower in dielectric

constant than that of the glass used for the

trimmer capacitors.

Although the components discussed here are

of relatively recent development, experience has

indicated that the principle of metallized glass

applied to them represents an eminently satisfactory solution to many of the problems involved in designing radio equipment for the

higher frequencies. It is obvious that these

principles can be expanded to still other single

components and in combining one or more components into a single unit. Specific discussion

of such developments may best be left for some

future occasion, when it is hoped that a more

concrete and complete contribution to this

phase of the subject can be presented.

9. Electrical Circuits on Plastics and Other Nonconductors 17

Metals may be applied to plastics by cementing, by stamping, by inlaying, etc., and these

methods are both simple and relatively cheap.

Metal coatings may also be applied to plastics

and other nonconductors by a variety of other

methods, but these are usually more expensive

so that they are used for special requirements

such as electrical circuits, thin metal coatings,

and decorative effects. In the second group of

methods the metal at some stage of the process

is in a finely divided form, e.g., powder, atoms,

ions, etc. For simplicity, these methods will be

termed metal plating and include metal evaporation, metal spraying, metal dusting and

painting, and chemical reactions, with or without electroplating.

Metal plating is used to apply circuits to non-

"Contribution from Metaplast Co. Inc., New York, N. Y., by

Charles E. Marlies, associate professor of chemical engineering,

The City College, New York, N. Y.

conductors for applications in which intricate

shapes of conductor and insulator are necessary

or in which thin layers of metal are required.

Examples are shielded housings, Faraday

shields, condensers, direction finding loops, airplane antenna masts, high frequency wave

guides, slip rings, special commutators, piezoelectric crystals, coils, connector circuits in

electrical devices, etc.

Methods of Metal Plating

A wide variety of methods has been reported

for the coating of nonconductors with metals.

Some of them have proven unsatisfactory or of

limited application because of high cost, poor

adherence, porosity, or poor mechanical properties. All the methods involve a number of

steps and some require special and expensive

equipment. Extreme cleanliness and care are

36 Circulars of the National Bureau of Standards
an indispensable requirement of success in

these methods.

The methods of plating metals on nonconductors may be classified into metal spraying,

vacuum deposition, metal dusting and painting,

and chemical reaction. After applying the

metal coating by one of these methods, the coating may be electroplated to enhance conductivity, increase strength, extend wear resistance, increase corrosion resistance, or even

improve appearance.

Metal Spraying

The metal in the form of wire or powder is

fed into a special oxy-gas gun and blown on

the surface being coated. A low fusing metal

must be used on thermoplastics so that the

plastic will not be burned or softened. If other

metals are desired, they can be sprayed or

plated over the initial coat. Metal spraying

finds its best use in the production of relatively

thick layers of metal.

Vacuum Deposition

There are two of these methods : metal evaporation and cathode sputtering. They are usually

relatively expensive and find their best use for

special cases, e.g., the exact duplication of the

underlying surface as for sound recording, light

reflection (especially first surface mirrors),

etc., or for the production of a film of a metal

which cannot be applied by chemical reaction,

especially a film of aluminum. Both methods

involve the use of vacuum chambers to house

the article and carry out the operation. Highly

plasticized materials require longer times and

higher pumping speeds owing to the volatilization of the plasticizer.

The surface of the material must be cleaned

very carefully for good and uniform adhesion.

In metal evaporation the metal is contained in a small crucible or is in the form of a filament

and is heated causing the evaporation of metal

atoms which condense on the article and all

other cool surfaces in the chamber. A vacuum

of 0.001 mm Hg or better is required. In cathode sputtering the material to be coated is

placed on or near a metal surface which is

made the anode and the metal to be deposited

is made the cathode. Under the influence of a

high voltage, 2,000 volts or more, metal atoms

in the form of ions leave the cathode and deposit on the article. A vacuum of 1 to 0.01

mm Hg is best.

Vacuum deposition methods are usually slow

and tedious compared to other methods, because

the chamber must be charged and evacuated,

the metal deposited, and finally the vacuum

must be broken and the chamber emptied.

Dusting and Painting

The application of a binder and a conducting

material is a very old procedure. Originally

wax was used as binder and graphite as con- ducting material. This combination is still ex- tensively used for electrotyping and the production of molds to be stripped, but cannot be

used if any degree of adherence of an electroplated metal coating to the nonconductor is re- quired. This method involves using a wax

object or coating the object with warm liquid

wax, allowing to cool and dusting with graphite

or spraying with a graphite suspension.

Graphite is employed because it is attracted by

the wax in preference to water. An alternative

method is to use a mixture of graphite and a

suitable binder. A disadvantage of graphite

is relatively low conductivity. One method of

increasing the conductivity of graphite coats is

to dust with iron powder and then immerse in a copper sulfate solution ; the iron displaces the

copper from solution and the copper plates out

on and covers the graphite. The low conductivity of graphite makes it useful for the production of resistors in printed circuits.

Greater conductivity is obtained by using a metal powder. This may be applied by coating

the object with a tacky material and dusting

or brushing with a metal powder. However, a

more convenient method is to use a bronzing

mixture, consisting of lacquer or varnish con- taining a relatively large amount of metal powder. This is a one-step method requiring simple

equipment. The conductive paint may be applied by brushing, dipping, spraying or printing.

Regular lacquer spray equipment is entirely

satisfactory for spraying the paint. A suction

feed with the container attached to the gun is

best because the constant motion during spraying keeps the paint well stirred.

The size, shape, and surface character of the

metal particles are extremely important in determining covering power of the paint and the

conductivity of the final film. Small size and

flat shape are most desirable. Note, however,

that aluminum leaf pigment used in aluminum

paints is unsuitable, because the surface of the

metal particles is composed of aluminum oxide

so thin as to be transparent but nonetheless a nonconductor.

Although relatively expensive compared to

copper and other base metals, silver has a number of advantages over other metal powders.

Thus, silver has the highest conductivity of all

metals. Copper and other base metals form

metal salts with the organic acids in the medium and cake in the container. The corrosion

products of copper and other base metals which

may form as the film dries are nonconductors,

Printed Circuit Techniques 37
whereas, all corrosion products of silver (silver

sulfide, silver chloride, etc.), are conductors.

Because of these advantages silver may be used

with a higher ratio of binder to metal powder

than can any other metal, and conductive silver

paints are commercially available which are

stable indefinitely, which is not the case with

any other conductive metal paint.

Silver has a further advantage over base

metals when used on ceramics and similar materials which can be fired. Base metals oxidize

during firing, thus losing conductivity. A

binder is used which fluxes with the ceramic

on firing. The conductivity of a fired silver

paint is considerably greater than that of an

air-dried paint.

Chemical Reaction

The methods already described may be

termed physical in that the metal exists as such

before application to the nonconductor. A sec- ond class, which may be termed chemical, involves using a chemical compound which is

reduced to metal by chemical reaction.

Metal may be deposited from unstable gas- eous compounds under appropriate conditions,

e.g., nickel from warm nickel carbonyl. However, this method has not been found feasible

for coating nonconductors because of equipment needed, temperature requirements, poisonous nature of compounds, cost, etc. High temperature reduction is widely used

for applying the noble metals to ceramics and

similar materials which can be fired. A compound of the metal is used in solution or sus- pension in a suitable medium, often organic or containing an organic binder. Reduction takes

place by dissociation of the compound at high

temperature, assisted by chemical reduction by

the organic material if present. This method

is not suitable for applying a layer of appreciable thickness or high conductivity, but is excellent for applying a thin mirror-like film. It

is widely used for the decoration of glassware

and ceramics with gold or one of the platinum

metals, and for the production of resistors in

printed circuits.

Many metals may be deposited from metallic

compounds in solution at ordinary temperatures using a suitable reducing agent. Not only

is this method applicable to the noble metals,

but also to many base metals. A large number

of formulas for such reduction have been patented or otherwise published.

The production of silver films by chemical

reduction is the method most widely used today

for coating nonconductors with metals. When

properly applied on a clean, smooth surface a

mirror-like coating of silver can be obtained.

However, the major application for this film

is as a conductive coat for electroplating. In

this process the plastic is sensitized with a so- lution whose main ingredient is stannous chloride and then silvered with a mixture composed

of freshly mixed ammoniacal silver nitrate and

a reducing solution. Small objects such as disks are handled in quantity by immersion in the

silvering solution. Large objects are usually

coated by spraying with a special gun in which

the silvering solution and reducer are mixed.

Many special variations in composition of solutions and in handling have been worked out for

particular materials and particular uses. A number of "trick" variations of the silver

reduction method have been reported, e.g., immersing phenolic plastics in ammoniacal silver

solution without use of a reducing solution, the

formaldehyde in the plastic causing reduction

of the silver. These variations have been found

uncertain in application, usually producing

coats of low conductivity.

Production of Designs

The most important industrial application of

metal plating a design on a nonconductor is for use as an electrical circuit. Metal designs are

also put on plastics for decorative purposes.

Four types of methods are used to produce a design : stencil, stop-off methods, regular printing processes, and complete coverage of the

nonconductor with metal followed by removal

of selected portions of the metal coating.

A stencil is satisfactory when metal spray,

vacuum deposition, or metal paint spray are

used, because there is little chance of the con- ductive coat running under the edge of the

stencil.

Stop-off methods must be used when applying

metal paint by dipping or brushing or when

depositing the metal film from aqueous solution, in order to prevent spreading of the coat.

Either masking tape or stripping lacquer is

used.

The most convenient method of applying

a design is to use metal paint with regular

printing processes, of which the screen process

has usually been found the most suitable.

Screens for printing the coating must be of

high quality. The screen itself should be of

fine weave for uniformity of printed pattern

and sharp edge of line. The blocking material

must be resistant to attack by the solvents in

the paint. Printed circuits in electrical devices are usually made in this manner.

This method involves more steps than any of

the three mentioned previously. However, it is

more versatile so that it finds many applications where the others fail. Removal of the

selected portions of the metal coating may be

by chemical or mechanical means. Mechanical

38 Circulars of the National Bureau of Standards
Figure 9.1. A, Rotating commutator with intricate commutating surfaces plated into grooves. B, Flat commutator with three connecting circuits. means is more common, especially if the metal

coating is thick. Thus the commutators (fig.

9.1A) were made by machining or molding the

circuits in the plastic, metal plating to a depth

greater than the greatest depth of the molded

design, and machining or grinding to a level

surface. Another method is complete coverage

of the surface, then removal of selected portions

by abrasive blast through a stencil or other

stop-off.

Electroplating

As stated earlier it is frequently necessary

to electroplate the initial metal coating to improve electrical, mechanical or other properties.

In such cases the initial conductive coating is

usually applied with conducting silver paint or by the silver reduction method because of su- perior conductivity. Electroplating is more

common on plastics than on ceramics, because

firing cannot be used to increase conductivity

as is the case with silver paint on ceramics.

Low conductivity is a great disadvantage be- cause a nonuniform electroplated coating is produced, the plating starting at the cathode contact and gradually spreading. This effect may

be partially overcome by making a number of

fine wire contacts prior to electroplating. If

the conductivity is too low the conductive layer

sometimes strips before the plating has spread

to the entire surface. The conductivity of silver

paint coatings is considerably enhanced by

burnishing with a dry lintless cloth, probably

owing to removal of surface binder and to improved particle contact.

The suitability of a conductive coat for electroplating may be tested with an ohmmeter.

Some of the above processes can be used several times to give second or third coats of conductive material, if a specimen is found not

sufficiently conductive.

The electroplating process itself follows

standard practice with minor variations. Thus,

since the conductive coat is usually very thin, a very high current density cannot be used

initially on a large piece unless a number of

contacts are made for distribution of the current. Hot baths cannot be used because they

soften some plastics, distort others, and in any

case introduce expansion problems which les- sen adhesion. Furthermore, since the conductive coat is usually porous, highly acid or alkaline plating baths cannot be used on all plastics. The usual barrel plating methods may be

used for small objects.

Electrical circuit designs are often electroplated to increase conductivity, and decorative

designs are electroplated to improve the metallic finish. Naturally, all parts of the design

must be in the electroplating circuit, which is

best attained with a completely interconnected

design. If the final design is to consist of several nonconnected circuits, sections of the

electroplated design can be cut out later at appropriate places. Otherwise, separate electroplating contacts must be made to each circuit

in the design.

Electrical Connections

Obviously, firm connections must exist in

every portion of an electrical circuit. Perhaps

the greatest drawback of the methods described

in this paper lies in the difficulty of securing as- sured connections in all parts of the circuit,

especially connections suitable for low voltages.

The problem may be divided into two parts

connections within the circuit and connections

to outside leads.

The major drawback to securing perfect internal connections is that the conducting material is finely divided at some stage of its existence, often in the presence of organic matter

or other insulating material. As a consequence,

there may be poor contacts between some of the

particles, not only decreasing conductivity, but

what is far worse, yielding conductivities which

Printed Circuit Techniques 39
vary from one article to the next or which in

any one article decrease with time. This problem is minimized by using multiple layers of

conductive coating. This problem can only be

eliminated by electroplating the circuit.

Connections to outside leads are a major

problem and must be given adequate consideration when designing the circuit. The problem

is considerably simplified if the circuit is electroplated. However, even with electroplated

circuits, soldering usually cannot be employed

if the nonconductor is a plastic but may be used

with ceramics. One satisfactory method of

bringing in leads is to insert metal contacts

beforehand at appropriate places in the nonconductor and applying the design on top. This

method has been used in producing commutators. Another satisfactory method of bringing in leads is to have each circuit terminate

in an enlarged area located over a hole in the

nonconductor, so that a nut and screw can be

inserted for holding the external lead and

tightening the connection (fig. 9. IB). For

piezoelectric crystals spring pressure on the

faces is sufficient, the spring serving as part

of the mount. Obviously, if the circuit is to be

electroplated, the final leads should be made

first, so that they will be electroplated firmly

in place.

Adhesion of Metal Layers

For some applications of metal plated nonconductors, the metal layer is stripped, e.g., electroforming and electrotyping. The electrotyping industry in which copper is plated on

wax and other nonconductors is the largest and

oldest application of the deposition of metals

on nonconductors. However, for most applications of metal plated nonconductors, the metal

layer remains on the material, so that good adhesion is desirable. For some industrial uses,

excellent adhesion under extreme temperature

and humidity variation is essential.

In most metal-nonmetal combinations, there are one or more intermediate layers between

metal and the nonmetal. The most common intermediate layer is a conductive coat such as

graphite, bronze powder, chemically reduced

silver, etc., which must be applied before electroplating. Other intermediate layers sometimes used are wax, varnish, etc. These intermediate layers complicate the situation considerably. Naturally, unless they adhere both to

the metal and the nonmetal, the strength of

the bond will be low.

Maximum adhesion between two surfaces

such as metal and nonconductor is dependent

on the formation of an interlayer which is

stronger than at least one of the materials

joined, i.e., on attempting to separate the object as in tension or shear, the failure will be

within one of the materials and not at the interlayer. Therefore, the following conditions must

be met: (1) There must be no weak interlay- ers such as grease, dirt, etc., due to improper

cleaning or preparation; (2) there must be no

weak interlayers such as nonadhering powder

due to improper application of conductive layer or improper plating; (3) all intermediate lay- ers must be joined to neighboring layers by

bonds stronger than the cohesive strength of

the weaker material of the two to be joined.

These bonds at the interlayer may be mechanical, due to the interlocking of rough surfaces, or may be valence forces, due to specific attraction between the two materials.

Due to the greatly different chemical natures

of metals and nonmetals, there is little "natural" adhesion between the two. For this reason,

roughening of the surface is widely used, both

to increase surface area and to provide mechanical interlocks. But even in such case, it

is difficult to obtain good adhesion if the metal

is to have appreciable thickness (0.001 in. or more) because of the different coefficients of

expansion of metals and nonconductors.

The coefficients of expansion of metals range

from 0.5 to 3 X 10 5 per deg C, the highest

values being those of cadmium, lead and zinc.

The very useful metals, copper, nickel, and silver have values about 1.5 X 10-5

, and chromium

has the value of 0.8 X 10~5

. The coefficients of

expansion of plastic range from 2 to 20 X 10-5 per deg C, the lowest values being attained by

only a few of the phenolformaldehdye types.

Polystyrene, one of the most useful of the plastics for electrical purposes, has a coefficient of

expansion about 7 X 10-5

. The coefficients of

expansion of glasses and ceramics range from

0.1 to 0.8 X 10-5

. If the metal film thickness is 0.001 in. or

less, the metal expands and contracts with the

basis material, whether ceramic or plastic,

probably due to insufficient strength of the

metal or presence of pores or fissures. For

such thicknesses, adhesion of metal films is

usually satisfactory.

If the metal film thickness is 0.001 in. or greater the metal expands and contracts with

its own expansion coefficient, so that it frequently becomes detached from the basis material. In such case special precautions must be

taken in the design of the object so as to provide mechanical interlocks by means of indentations, grooves, and reentrant curves. Usually,

if the metal film thickness must be greater than

0.001 in., it is best to make it at least 0.003 in.

Within the range 0.001 to 0.003 in., the metal

film strength and stiffness are so low that blistering and splitting of the metal may take

place during temperature change.

40 Circulars of the National Bureau of Standards
10. Imprinted Circuit Inlays 18

The advantages in applying printed circuits

to the design and manufacture of electronic

equipment, such as radios, telephonic devices,

hearing aids, transceivers, etc., have been

clearly emphasized. Through the use of printed

circuits in lieu of customary methods of as- sembly by means of a multiplicity of wires,

soldered joints, etc., the manufacturer can now

take advantage of a great saving in manhours

and eliminate wiring errors in the basic grid

lay-out.

The method of printing circuits described

herein will be referred to as the "imprinted

circuit inlay." In this method all of the circuit

conductors are cut out of a solid sheet of silver as a single grid and firmly attached to a dielectric panel (fig. 10.1) by fusing it to and imprinting it into the dielectric material in a single operation. The process is fully mechanical.

Present experiments indicate that in the near

future we shall be able to produce a printed

circuit which will also include resistors. The

main problem in this respect is the development

of suitable resistive materials for the varied

ranges of values that will cover the requirements of any standard circuit.

Through the use of properly designed steel

embossing and cutting edge dies, every detail of the entire schematic is cut out of a

sheet of solid silver ranging in thickness from

0.001 to 0.003 in. The possibility of wiring

errors is thus nonexistent.

A separate steel die is employed for each

side of the dielectric base plate. This solves the

wire-crossover problem. With one portion of

the basic grid imprinted on each side of the

base plate, eyelets, rivets, or other suitable

Figure 10.1. Basic wiring grid and inductor imprinted on plastic base.

18 By J. M. Ternes, Altair Machinery Corp., New York, N. Y.

Printed Circuit Techniques

means may be used to form the connections between them.

The dies are mounted on the heater plates

of an embossing press. These plates slide in

and out of the press, thus allowing a quicker

change-over from one circuit design to another.

In mounting the dies, care is taken to properly

align the dies to give equalized pressure in

relation to the center of the die-layout and of

the heating plate. The heater plates with dies

properly mounted are heated in the press by

means of built-in electric heating units. Controlled heat is an important factor in giving the

proper indentation within a given time limit. As

mentioned, the design of the die allows cutting

and imprinting in one operation. After the

silver is cut to shape it is fused to the dielectric material of the base material through use

of a suitable sizing which liquefies under heat.

Another important factor of our method is

the pressure applied to the cutting and embossing dies; this is accomplished through use of

joint-knuckle type stamping and embossing

presses, which have a high-working pressure

yet allow a smooth dwell pressure. The correct

amount of pressure can easily be regulated by

hand control and the opening of the press can

be quickly set for various thicknesses of material within given limits. For testing and

short runs a 15-ton hand-operated embossing

press seems ideal. Power operated presses for

long production runs are readily available. For

short runs the silver is hand fed in sheet form

;

for long production runs the conductive material may be fed automatically from rolls.

The use of the proper type of press, the application of the proper amount of pressure to

Figure 10.2. Front view of a two-tube receiver with

tubes and standard resistors added.

41
the dies, the smooth dwell under pressure, and

the controlled heat will result in an imprinted

circuit inlay thoroughly embedded into and

fused to the dielectric material of the base.

The sizings employed to fuse the conductors

to the dielectric vary according to the thermal

cycles used to imprint the conductors.

Several tests have been conducted on sample

assemblies (fig. 10.2). Current capacity tests

were conducted for 4 hr, starting at 1 amp and

advancing i/o amP approximately every 5 min

up to 9 amp. Nine amperes of current was held

for 1 hr, then 10 amp for i/2 hr, 11 amp for

!/2 hr, 12.5 amp for 14 hr, and finally 13 amp

for a y<i hr duration. The test was discontinued

at this point since this was the maximum output of the power supply. It is obvious that

the average electronic circuit does not have

operating currents of this magnitude so there

was no point in continuing this test. The inductor in the circuit has a value of approximately 0.15 microhenrys and resonated at 140

megacycles when shunted by 10 \x\xi. with a Q

of 150.

Although hot-die stamping is only one of the

many methods of printing circuits, we can

produce a highly satisfactory imprinted circuit inlay on many types of plastics, phenolics,

wood, fiberboard, etc., and we believe the

method has many advantages.

11. Spraying Techniques for Producing Electronic Circuits19

"Spraywiring," a process for the rapid production of electrical circuits by metallizing, is

one of the recently developed means for eliminating hand wiring in many applications. Instead of improving the method of installing

conventional wiring, this process actually re- places drawn wire with an efficient conductor

which is mechanically formed by mass production methods. In spraywiring, heated granular

metal is sprayed through stencils into channels

in dielectric chassis, thereby forming the basic

wiring of the devices being produced. The

process consists of three principal operations

the preparation of stencils, the insertion of

eyelets and mounting of components, and sandblasting and metallizing.

In the first step a stencil is prepared in exact

conformity with the basic wiring design to be

produced, and is applied as a mask to the dielectric panel or chassis. A special "Scotch" masking tape has been developed by the Minnesota

Mining and Mfg. Co. for use as a stencil material with this process. This type is lightly adhesive on the side applied to the panel, and has

a surface sufficiently hard to withstand sandblasting and metallizing. Stencils made of this

tape can be die-cut in continuous strips, and

can be produced economically by firms equipped

for such work.

Tape stencils are used only once, after which

they are collected for salvage of the surplus

metal deposited. When this type of stencil is

used, it is advisable to allow a minimum separation of about 1/16 in. between conductors.

Good results have been obtained experimentally with metal stencils faced with rubber

coatings. These stencils are semipermanent;

sandblasting cleans off the metal deposited during each spraying. These stencils are especially

19 By Gordon Johnson, Spraywire Laboratories, Inc., Minneapolis, Minn.

suitable for the production of small, complex

panels.

Metal eyelets, inserted in the panel prior to

sandblasting, form terminals for the connection of component lead wires to the sprayed

elements of the circuit. These eyelets are positioned so as to be metallized into permanent

connection with the conductive strips when the

latter are sprayed in. Eyelets are also used as connections between sprayed conductors on opposite sides of the same panel.

Standard sandblasting equipment is used to

form grooves in the panel (fig. 11.1) coinciding

with the stencil openings after the stencil is

applied. The depth of these grooves must be

at least 0.010 in. thick. The action of the abrasive forms grooves which are slightly recessed,

and this characteristic contributes to the adherence of the sprayed conductors to the panel.

The metallizing technique and equipment are

those of the Schoop process, familiar uses of

which are the building up of worn bearing sur- faces and the application of protective coatings

of metal. The equipment is made by the Metallizing Engineering Co., Inc. of Long Island

City. The metallizing gun (fig. 11.2) operates

on acetylene, propane, or any commercial gas,

and can be operated manually or mounted in a

tool-post holder for automatic operation. A

heavy duty gun known as the Metco Y Gun,

uses 3/16 in. diameter and smaller wire, and

can double the spraying speed of other guns.

Panels are sprayed at a distance of 6 inches

from the gun. Two passes are made (or the

panel moves before two guns in sequence), at

the rate of 1 foot per second. An average of

3.5 mils of metal is deposited on each pass,

thus building up a thickness of 7 mils suitable

for most applications. This technique of spraying will provide deposits of the proper porosity,

and will insure that the base material is not

42 Circulars of the National Bureau of Standards
Figure 11.1. Three steps in the production of a SPRAYWIRE circuit. At the left is a panel with stencil applied, prior to sand-blasting and metallizing. In the center is a panel which has been metallized, with the stencil still in place. At the right is a completed amplifier embedded in the bottom of the chassis. (Courtesy of Minnesota Mining

and Mfg. Co.)

damaged by burning. Sprayed copper has a

hardness of F78 to F79, and its specific gravity

is 7.535. Due to their porosity, sprayed copper

conductors are 10 percent higher in resistance

than comparable drawn wire.

With the stencils now in use the practical

minimum width of a sprayed conductor is about

1/16 in. A width of % in. or more, however,

is preferable from a mechanical standpoint.

Stencil openings of less than 1/16 in. retard

the uniform deposit of metal in the grooves.

Above these minimums, sprayed conductors

can be made as wide as may be required depending on the current they are to carry.

There may be some question as to the cur- rent carrying capacities of Spraywire in applications which require heavier currents than

radio receivers. The thickness of 7 mils used

in radio, with widths of % to 14 in. produces

conductors unable to withstand heavy currents.

When circuits require heavier current carrying

capacities, deeper grooves and thicker deposits

of metal must be provided. Suitable grooves

can be formed in plastics by molding.

The dimension of conductors which is or- dinarily varied in order to match wire sizes

is the width. Corresponding wire sizes are determined by use of a table showing the conversion of square inches to circular mils, and

adding 10 percent for porosity. In tests of

current carrying capacity, a copper Spraywire

conductor 7 mils thick and Yq in. wide carried a current of 17 amp for several hours. When

the current was increased to 20 amp, the con- ductor failed after 5 min. This conductor was

therefore the equivalent of a No. 23 copper

wire. Another sprayed copper conductor, 7

mils thick and 1/16 in. wide, carried 8 amp for

several hours. This conductor failed after 10

min when the current was increased to 10 amp.

Figure 11.2. Metallizing a panel.

This illustrated a laboratory operation, and not a production set- up. Automatic handling equipment would be used in mass production.

It was, therefore, the equivalent of a No. 27

copper wire. These tests were repeated several

times with identical results.

The bonding between sprayed metal and suitable base materials is excellent, even under

vibration and shock. Sprayed metal adheres

to rough or porous surfaces, and the sandblasting used with the process insures that the

grooves into which the conductors will be

sprayed are sufficiently roughened to provide a very strong bond. The heating of some base

materials which are not sandblasted will also

provide for strong bonding with the sprayed

metal.

Forty-two alloys are prepared in wire form

for use with metallizing equipment; these embrace all of the commonly used conductors, including those employed for high resistance

Printed Circuit Techniques 43
(such as Nichrome). The wide variety of base

materials which can embody Spraywiring includes plastics, wood, ceramics, glass, and other

insulators. Many types of plastics are suitable,

including the general purpose resin phenolics

and the polystyrenes of high dielectric strength.

Plastic chassis can be premolded to desired

shapes; the use of flexible stencils makes it

possible to mask surfaces which are not flat

and spray circuits onto them.

The most extensive research to date with the

Spraywire process has been in experimental

application to radio receivers. It is believed

that a radio, with its different amplitudes of

voltages, a-c/d-c, and radio frequencies, provides an excellent proving ground for this

process.

A 5-tube a-c/d-c receiver incorporating

"Spraywiring" was found to compare favorably with any standard receiver of comparable

size now on the market. In the customary tests

made in a shielded room, the receiver was lined

up for peak performance in all stages, and a

standard loop antenna was employed. A calibrated signal generator and a precision output

meter were used to measure sensitivity and

performance. Its sensitivity was 18 microvolts

for 500 milliwatts output through the entire

broadcast frequency spectrum. Distributed ca- pacities are reduced because of the thin character of the surfaces of sprayed conductors

which are adjacent or in close proximity. No

difficulties with stray oscillations were encountered. Shake tests up to 10 G's at 50 cycles

failed to break or loosen any Spraywire connections.

One of the very important advantages possible with this process is the ease and simplicity

with which devices made by this means can be

serviced. The elimination of nests of wiring

makes components readily accessible.

Extensive information on comparative costs

must of necessity be derived from wider production experience than is now available for

study. However, the speed of producing panels irrespective of the intricacies of their wiring

lay-outs, the use of cheap materials made possible, the reduction of rejections for faulty

wiring, and the numerous advantages of re- placing manual work by an automatic process

all point to the probabilities of greatly lowered

costs. The Spraywire process is a potentially

important contribution to the science of production.

12. Physical Aspects of Printed Circuit Conductors 20

I should like to make a few remarks about

the structure of thin layers and what happens

to layers, which are used in printed circuits as

conductors, during the process of aging or during their lifetime.

Apparently, most of the conductors which

are used have a granular nature. This granular nature is amply demonstrated by the fact

that a silver conductor produced by the silk- screen process has a conductivity before firing

up to about 30 percent of the conductivity of

an equal amount of bulk silver. This indicates

that, besides the grain insulation produced by

the incorporated binder we have also some

other phenomena, such as separation of the

grains, which interferes with the conductivity.

Now, after firing, the conductivity of the same

silver layer is about 80 percent of a similar

bulk conductor. There are data that have been

supplied by Mr. Henry of Dr. Brunetti's staff,

and I am using them as examples of the behavior of such granular conductors.

This behavior makes one think immediately

of the behavior of other granular materials,

and in particular the materials used in powder

metallurgy. In powder metallurgy, too, one

begins with fine grains. These fine grains are compressed and after the compression they are

fired and subjected to further mechanical treatment. During that process the resistance

changes as well as the density. At my request

Mr. Henry made a few measurements on the

density of the thin silver layers which I previously mentioned. The density of the unfired

layer with 30 percent conductivity was 40 percent of the density of the bulk material. After

firing, the density was raised to 60 percent of

the bulk silver which indicates that even when

most of the binder is removed, there is still a

rather porous layer.

We must ask, therefore, what happens to

such a layer after aging. A similar question

that can be asked is: What happens to very

thin layers produced by other processes. If a

metal is evaporated in a vacuum, the resulting

thin layer is by no means a continuous one, but

resembles the layer produced by the silk screen process in that it is inherently unstable and is

subject to irreversible changes with temperature. If such a layer is examined under an

electron microscope, it is found to be composed

of extremely fine grains. The following figures

20 By L. Marton, National Bureau of Standards.

44 Circulars of the National Bureau of Standards I':
will indicate the granular nature of such very

thin layers and how the conductivity anomalies

can be explained by the appearance of the layer.

At very high magnifications the individual

grains of an antimony layer (fig. 12.1 A) are

revealed to be about one millionth of an inch

[ in diameter. Note the voids between the indiI vidual grains of the layer and that the grains

i sometimes touch at only one point. The total

j conductivity is produced by the bridges formed

between the individual grains. A layer of

{ aluminum with approximately the same degree

of magnification (fig. 12. IB) shows a similar

structure. The scale in the figure is 1 micron

* (one 25 thousandths of an inch) . The large

I particle is a dust particle. A similar case is an

evaporated layer of gold. Although the individual grain size may change, the conductivity

anomaly is given by the fact that the individual

grains do not form a uniform layer but touch

each other only at a few points where bridges

are formed.

Now we ask what happens physically when

we have such a complicated structure. Obviously, when we evaporate a metal, the metallic atoms first reach the substratum (the support) and condense on it. After condensation,

however, these atoms will not remain in the

same place. I mean by that, that although the

underlaying layers are more or less firmly

bound, the ton layer of atoms will not be bound

completely. All layers are bound to the extent

that the atoms cannot reevanorate unless the

temperature is high enough. But even at room

temperature or below, such metals as gold or even the most refractive metals, will still show

a marked mobility of the uppermost atoms in

the plane of the top layer. That mobility creates

certain rearrangements of the atoms on the

surface and that rearrangement is known under

the name of recrystallization. I would like to

discuss quite briefly by using table 12.1 how

that recrystallization depends on different factors.

Table 12.1 indicates how, in powder metallurgy, the effect of compression and of heat

treatment change both the resistivity and the

density of materials. In very close analogy to

what happens in a silver layer you see that the

first sintering of a tungsten ingot produces

a density of 12. I do not have any data about

the resistivity for both the sintered and the

formed ingot, but in the ingot formed at 3,400°

C the density raises to 17. Then the mechanical drawing of the wire increases the density

considerably until we reach the highest figure

of 19.23 at 1 millimeter diameter. At that

point, or even before, resistivity is lowered to

5.45 in arbitrary units. If we go further, something else happens and both the density and

Figure 12.1.

A, Evaporated layer of antimony on collodion. The bright spot on

the micrograph is a hole in the collodion film; the dark spot is a

particle of dust. B, Aluminum evaporated on collodion film. Note the structure of the aluminum surface, the shadow cast by the large dust particle

in the center, and the aluminum top of the particle when the particle is thin enough to be transparent.

Table 12.1.

Tungsten (after Smithells)

Diameter in mm Density Resistivity

Sintered ingot, 900°C. 12 Formed ingot, 3,400°C. 17.5 Drawn wire 5.6 X 5.6 17.0 6.13 Do 5.0 18.45 5.58 Do 4.0 19.00 5.48 Do - . _ 3.0 19.20 5.45 Do 1.0 19.23 5.58 Do 0.1 19.21 6.55 Do 0.03 18.95 7.27

Printed Circuit Techniques 45
resistivity will show reverse changes. The rea- son for that is that as we go to a certain degree

of compression we close the voids in the compressed metal, but when we go beyond a certain

degree of mechanical treatment we break up

larger crystals into smaller ones and there the

process is reversed. There is an obvious optimum somewhere.

In evaporated layers we have to investigate

not only the type of material used, but also the

nature of the substratum. The mobility of the

atoms is not only dependent on the binding

forces between the atoms of the same material,

but also on the binding forces between the

metal atoms and the atoms of the substratum.

It is, therefore, necessary to distinguish between three classes. These three classes have

been published in a paper by Picard and Duffendack in The Journal of Applied Physics,

1943. In this paper they make use of a theory

by Lennard-Jones which was published in the

Transactions of the Faraday Society, 1932. The

following rough classification distinguishes between different recrystallization phenomena:

Class A—Surfaces which recrystallize with

increasing temperature to form agglomerates

separated by interstices which are much nar- rower than the diameter of the aggregates.

Class B—Same as class A but interstices are

of same order of magnitude as diameter of

particles.

Class C—Surfaces recrystallize to form relatively isolated crystals.

Table 12.2 indicates the behavior of a few

metals with different substrata by using the

classifications A, B, and C. It shows very

clearly that we need to know not only what

temperature treatment is inflicted on the metal,

but also exactly what substratum we are placing it on. A proper combination of the two

will achieve better results than a search for

better materials alone. Also, I would like to

remark that all this indicates that there may

be other methods for achieving an aging, particularly in cases of materials which cannot

be fired at high temperature. I wonder if, on

close analogy to the powder metallurgy method,

it would not be worthwhile investigating to see

if unfired metallic layers could not be aged

faster by compression methods (hydraulic pres- sure or other) than can be done by temperature treatment alone.

Table 12.2.Recrystallization of thin layers of metals on different substrata

Metal

Aluminum

Cadmium

Copper

Gold

Magnesium

Zinc

Substrate

f Collodion

) Glass

f Collodion _

\ Glass

L Copper

f Collodion

) Glass

f Collodion

( Glass

1 Collodion

) Glass

f Collodion _

I Glass

] Aluminum

[ Copper

Class

A

A

C

C

BA

A

A

A

A

A

C

C

B

B

13. Mechanization of Electrical Wiring21

Over the past several years there has been a

steady trend from electrical controls to accomplishing the same service by electronic means.

In a great many instances greater accuracy

could be obtained by electronic means plus the

very attractive feature of flexibility. Early ex- periences in this direction revealed that the advantages to be gained from electronic design

were sometimes outweighed by an increased

cost in production. Accordingly, MinneapolisHoneywell undertook an investigation of new

means for mechanization of electrical wiring to

reduce this cost. A large proportion of the re- search that has been done to date on mechanization of electrical wiring has been done on its

application to military equipment. This application puts a premium upon small size and

weight. In many other applications size and

weight are not the predominating require21 By Ralph T. Squier, assistant director of research, MinneapolisHoneywell Regulator Co., Minneapolis, Minn.

ments. In such cases the mechanization of electric wiring is being used because it is eco- nomically desirable. After consideration of

several methods, three appeared to be most

feasible. These were (1) circuits stenciled on

steatite, a development which was available

commercially; (2) stamped wiring from a metal plate; and (3) sprayed metal wiring onto

a premolded panel.

All of these methods have some inherent advantages : (1) the errors of wiring are reduced

to practically zero as far as the wiring of the

circuit is concerned (any error that shows up

is caused only by faulty components such as'

transformers, relays, etc.) ; (2) the speed of

wiring on the assembly line may be increased

50 to 75 percent depending on the type of circuit used and the kind of mechanization used;

(3) lead wires may be prespaced in circuits

where feedback, pickup, and other interactions

affected by the relationship of the lead wires

46 Circulars of the National Bureau of Standards P
dictate the fidelity of the instrument; (4)

mechanized wiring automatically solves the

problem of properly protecting a wired circuit

from vibration, etc., since it is unnecessary to

elaborately cable and tie the connecting wires;

and (5) the problem of insulation break-down

is eliminated because it is possible to locate

leads in a desired manner.

The use of complete interstage coupling circuits (fig. 13.1) purchased from Centralab has

reduced cost by 25 percent. The possibility of

the elimination of error can readily be seen in

this instance.

A complete amplifier (fig. 13. 2A) has been

made using the stamped wiring technique believed to be first used by Kellogg Switchboard

& Supply Co., Chicago, 111. A single flat stamping or grid made of silver plated 0.025 in. brass

contains all connecting leads plus the necessary

links to support the grid as one piece. To ac- complish connection to the grid, all component

terminals are brought to one common plane.

The grid, which fits in only one position, is

laid down on the terminals and all terminals

soldered to it. The final step is the clipping

away of those parts of the grid which were

used only as supports and are not to be used as

electrical connectors. This is done with a special instrument that cuts all supporting bars

at once. This amplifier is in regular production and the techniques shown have helped in

making it a more economically attractive item.

An amplifier with sprayed wire leads (fig.

13. 2B) is still in the research stage. Grooves

were made in the base, first by milling and later

by molding, into which the metal was sprayed.

It was found that the metal impinging upon

the lands between the grooves could be easily

wiped off. There were, however, some problems

that immediately presented themselves concerni Z 3 4 5 |

I f

i

Figure 13.1. Interstage coupling unit—stenciled and

conventional.

ing spacing, width, depth, bottom surface, and

shape of the groove.

Certain minimum separations between leads

must be maintained in electronic circuits in

order to eliminate electrical leakage. Using

a groove width of 1/16 in., even with miniature tubes having closely-spaced pins, resulting

separations were within these limits. This

groove width is more or less arbitrary, being

wide enough to allow ample room for spraying

to the bottom of the groove and to provide sufficiently high conductivity while leaving adequate spacing between leads.

A series of tests were conducted to determine

optimum groove depth for adhesion of the copper to the base. This depth seems to be quite

critical and should be maintained at 0.040

±0.005 in.

Measurement made on the bonding of the

copper to the bottom of a molded plastic groove

shows conclusively that some sort of roughening is desirable. Longitudinal and diagonal

ridges gave much better results than did a

smooth surface. To determine the bonding of

Figure 13.2. A, Stamped wiring applied to an amplifier. B, Bottom view of amplifier showing sprayed wiring on plastic panel.

Printed Circuit Techniques 47
the copper to the panel, a small circular hole

of the same width as the grooves was drilled

through the panel prior to spraying. A steel

pin was inserted through this hole flush with

the bottom of the groove. After spraying, this

pin was removed and one of slightly smaller

diameter inserted in the hole and the force re- quired to dislodge the copper conductor was

measured. The average of these forces for

smooth grooves was 100 gm and for roughened

grooves, 600 gm. The groove must have a sharp

edge at its top to automatically cut the lead off

from the flashing at the time of deposit.

After spraying, the plastic and copper are painted with a mixture of Durez and varnish

which further improves the adherence of the

copper to the plastic.

Sprayed copper leads in grooves on a plastic

base were subjected to the following conditions

without detectable change in electrical or physical characteristics: (1) Temperature variation

in 10 cycles from —65° F to +160° F; (2)

pressure variation from normal atmospheric to

that prevailing at 50,000 ft; and (3) vibration

in a 0.020 in. diameter circle inclined at 45°

for 1 hour each at 10, 30, and 50 cycles per

second.

Amplifiers constructed with sprayed copper

leads in grooves on a plastic base have been

subjected to all the conditions noted above and

to extremes of humidity and have operated

under moderate conditions for months with no

change whatever that could be attributed to

deterioration of the sprayed copper leads.

14. Die Stamped Wiring22

For years engineers have worked to develop

something resembling packaged wiring. Early

electrical and electronic equipment used rigid

bus-bar that was difficult to handle and harder

still to keep in place during shipment. This

soon gave way to flexible wiring and cabling

techniques, but even the latter required many

individually soldered joints. During the war

—' N.

_v *-

—/ \

_/ v.

„y

1 1 *

f * *

t \

S || I 4 ;

* * <

j \ 1

, * \ « a

T

1 - BSta|s 1 f M

Figure 14.1. Development of stamped wiring panel

(center) from schematic diagram (top). (Bottom)

the same circuit using a standard die.

printed circuits came into use. Since then cast

conductors somewhat reminiscent of processes

tried in this country back in the 1920's, have

been introduced in England.

A number of new packaged wiring ideas are

in the experimental stage. Some are well along

in development. One such idea involves stamped

wiring, originated by A. W. Franklin, president of the Franklin Airloop Corp. of New

York, which lends itself to mass production

methods since a basic wiring package can be

turned out for manufacturers of many types of

electronic equipment. Alterations in the basic

package are readily made by means of dies, so that 90 percent of the wiring within the

average device can be stamped out. Furthermore, most component parts may be connected

to the wiring in one operation by dip or induction soldering. Substantial savings in material,

in alinement and testing, as well as in assembly,

seem likely.

Basic Idea

Basically, stamped wiring consists of a thin

sheet of insulation with a series of parallel

conductors running in a horizontal direction

on one side and a series of vertical conductors

on the other side. Interconnection between

horizontal and vertical conductors is accomplished by punching through the insulation intervening between such conductors and then

joining them by means of an eyelet or pin. A

method of interconnection which requires

neither form of fastening, just pressure and

heat, is also under development.

A stamped wired panel may be developed

from any schematic wiring diagram (fig. 14.1).

22 By W. W. MacDonald, managing editor—"Electronics." Presented by J. B. Straughn, Franklin Airloop Corp., New York, N. Y.

48 Circulms of the National Bureau of Standards h
8 Figure 14.2. A, Stamped wiring system for use with a simple welding process or resistance soldering method to provide contacts between horizontal and vertical conductors. B, Stamped wiring panel including sockets, for five tube superheterodyne receiver. The top illustration shows the circuit with dots

indicating connection points and arcs indicating

crossover. The center illustration shows

stamped wiring on a Bakelite panel, approximately 1/16 in. thick—horizontal conductors

stamped on one side and vertical conductors

stamped on the other side. The lower illustration is the same circuit developed from a standard die which would stamp a series of horizontal conductors on one side, vertical conductors on the other, in grid formation. Eyelets

through the panel make the electrical connections indicated on the schematic. Slots perforated completely through the panel open the

connections where necessary. Flexible dies

could be made to perforate the holes for the

eyelets or pin connections and slots for the

open connections. It can readily be seen from

these illustrations that any electrical circuit

can be developed from one of these panels.

A further development (fig. 14.2A) ) indicates a simpler and less costly arrangement of

making connections. Having holes and the slots

perforated in the insulated panel before the

stamped wiring is superimposed permits electrical contact to be made by a simple welding

process or resistance soldering method. This

eliminates the use of eyelets as the vertical and

horizontal conductors are drawn together

through holes in the insulating panel and soldered together. The illustration gives an exaggerated view as to depression required for connection points. In actual practice, the conductors would be drawn approximately 1/32 in.

through the prefabricated holes on the insulated

board. Such a depression can be formed at the

same time that the wiring is stamped. The copper used for the stamped circuit is approximately 0.005 in. thick and can be varied between 0.002 in. and 0.010 in. depending on the

conductivity required. The copper can be tinned

for electrical resistance soldering. Tinning

would not be necessary for spot welding.

Where connection to a single horizontal or a single vertical conductor is desired, without

interconnection, an eyelet or pin may be

punched into the selected conductor at a point

which causes it to miss metal on the reverse

side. Where breakup of a single horizontal

conductor or a single vertical conductor into

several horizontal or vertical conductors is re- quired, this may be accomplished by the simple

process of cutting the conductors at one or more points along their length or height.

Electronic equipment circuit diagrams consist essentially of horizontal and vertical lines,

with cross-overs and interconnections. Stamped

wiring consists of horizontal and vertical conductors, with the insulation between them constituting an inherent cross-over, and eyelets,

rivets or some other type of fastening providing interconnection. Thus it is readily possible

for an engineer-draftsman to make the transition from schematic to stamped wiring. Location of eyelets or pins, points at which conductors should be cut, and placement of parts is

also planned at this time.

The type of fastenings used for interconnection of conductors, whether eyelets, pins, or the

conductors themselves are used as terminals

for component parts, and whether short con- ductors are stamped out that way or produced

subsequently by stripping away unused metal

from longer conductors depends upon what the

equipment assembler wishes to buy, and upon

what the supplier of packaged wiring ultimately finds it most desirable to deliver. Developmental work is still proceeding at a pace

that suggests that such details will soon be

standardized.

Stamped Wiring Deck

Completely wired panels (fig. 14. 2B) for

radios including sockets for tubes and IF transformers have been made using stamped wiring.

Printed Circuit Techniques 49
Note that a socket contact was developed having tubular soldering tails instead of the usual

standard flat type wrap-around contacts. Thus,

for radio or electronic devices, all that would

be necessary to complete the device would be

to drop small components to the wiring deck

through hoppers or fixtures. The bent wire

terminals of components would fit into the hollow pins of the contact tails or tubular pins and

be completely soldered in one dip-soldering operation. This would be a similiar operation to

that now performed in soldering tube socket

bases. In this way, the number of individually

soldered points in a radio receiving set would

be reduced from approximately 150 to the

comparatively few required for soldering main

components such as tuning capacitor, volume

control and speaker.

It might be well to point out at this time

that it would be advantageous to use small flat

printed components as part of the circuit. A

combination of stamped wiring for the main

part of the circuit and printed circuits embodying components would be ideal in many devices.

Performance

Performance tests are being made on equipment using stamped wiring. At this writing it

appears that little or no circuit modification is

required where it is to be employed. Developmental radio receivers perform quite as well

as conventionally wired sets with respect to

sensitivity and selectivity. There is reason to

believe that the fixed nature of the wiring, plus

the fact that necessarily careful planning of

both wiring and placement of parts, may make

it possible to operate tubes nearer the spill- over or hot point in production models, with

resultant improved performance.

Alinement of circuits in production should

be materially simpler than where conventional

wiring is used, since wiring stamped out by a

die will not vary from set to set. This factor

should prove of particular interest to manufacturers of television equipment.

Television

Development work on stamped wiring now

embraces television receivers in which approximately 500 soldered joints are required. By

using a combination of stamped wiring and

stamped inductance the development work on

a push-button switch covering the 13 television

channels is now practically completed. Instead

of winding 30 individual inductance coils, 3

parallel transmission lines (each carrying 13

channels) and the wiring for r-f mixer and

oscillator tubes is stamped on an insulated

panel in one operation.

Method of Manufacture

The method used for stamping the wired circuits of a complete receiver embodying stamped 1 wiring and the Franklin Airloop is the same

J as that used for stamping the antenna or in- I ductance coils. The stamped wiring decks can

|

be produced at the same rate as a machine pro1

during AIRLOOPS. This machine produces,

AIRLOOPS at a rate of 20 per minute and is

'

especially designed so that the copper is automatically fed through the machine while the

insulated panels, stacked up in a hopper, are

inserted under a hot die so that on each cycle

of operation a complete wiring panel circuit is

stamped. The scrap copper is ejected at the

other end of the machine.

Sheets of single-X 1/16 in. Bakelite punching

stock, similar to that used in the manufacture j of wafer-type sockets, are sheared to 3 X 9

1

inch size. A roll of 0.005 in. pure electrolytictype oxygen-hydrogen-free copper, tinned on

both sides, is coated on one side with U. S.

Rubber's Kotol thermoplastic cement.

Insulation and copper are fed to a 150-ton

Standard automatic toggle press, containing

a shearing and forming die. When the press

is operated the die cuts the copper into con1

ductors 5/32 in. wide, with equal spacing between conductors and presses their edges and

ends 3/1,000 in. into the insulation. The die

is heated electrically to 230° F and softens the

insulator sufficiently to facilitate locking the

conductors securely in place. The heat simultaneously sets the thermoplastic cement so that

the conductors are both mechanically locked

and cemented to the insulation.

The deck next goes to a punch press, where

all holes for eyelets and pins are knocked out

in a single operation. The holes in this particular case are 96/1,000 in. in diameter and take

pins similar to those used in the manufacture

of octal tube bases. In still another press, eyelets and pins flow from hoppers through feeder

tubes to deck holes, as in the manufacture of

tube sockets, and are clinched in place. Heating by conduction may be used to sweat con- ductor and eyelet and/or pin tinning together.

The stamped wiring deck is now complete and

ready to receive component parts.

Cost

Cost determination must wait until a sufficient number of units embodying stamped wiring are manufactured to permit accurate cost

accounting, since material, and alinement and

test labor, as well as assembly labor, is involved.

Then too, die cost will vary depending upon

the size of the deck required, upon the relative

complication of the wiring to be stamped, and

50 Circulars of the National Bureau of Standards
upon the volume achieved by supplier and as- sembler.

Substantial savings should be possible. One

clue is the fact that Franklin believes it will

be possible to supply stamped wiring decks for

5-tube table model radios, with tube sockets

built in and ready to receive component parts,

for about double present cost of sockets.

Die Stamped Cable Wiring

A more recent development (fig. 14.3) , makes

it possible to replace conventional wire cables

with a die-stamped wire conductor panel of

various lengths, ending in terminals for making soldered connections to other parts of a

switchboard or communication panel. While

the conductors for the main part of the cable

can be reduced in size to give possibly 50 per

in., they could also be branched off from either

end in terminals or conductors of increased

width. This is accomplished by ending the

main part of the cable die in an acute angle

(30° in this particular case) which makes the

terminals or branch conductors twice the width

of the conductors in the main part of the cable.

Thus complete panels can be formed leading

conductors directly to other points of connection.

The new method of wiring may be used in

the following applications: dashboards and

panel boards for automobiles, airplanes and

ships; flat type disk commutators; multi-pole

switches; hearing aids; annunciator boards;

intercommunication units ; television r-f coils

and i-f transformers ; FM i-f and discriminator

coils ; and electronic circuits combining wiring,

inductances and capacitors.

—< •—.013 AIRSPACE

Figure 14.3. Die-stamped wire panel for replacing

conventional cables.

An interesting development is the fact that

the stamped wiring method can be applied to a

molding process which permits all of these devices to be made on molded Bakelite or lowloss materials, such as polystyrene, etc. The

process is especially adaptable to metal insert

molding as one sheet of metal inserted into a mold can be cut and formed into a large number of smaller inserts.

15. Printed Circuit Production

The techniques employed by the Kenyon Instrument Co., Inc., differ from others described

in this symposium principally in that the starting point is an insulating base material that

is completely covered with a conductive layer.

Unwanted portions of these conductive layers are progressively removed until only the desired surface pattern remains. The end results

do not differ markedly from those obtainable

by any of the previously described methods.

While we do not wish to claim any outstanding

advantages either in economics or performance,

there are two features that merit special consideration. These are the dimensional precision

with which the "wiring" may be laid down and

the flexibility of the process in its ability to

reproduce conductive patterns upon a wide

range of sizes, shapes, and materials. These

advantages are made possible through our use

of photographic and optical techniques.

The problem of applying conductive coatings

to insulators is a matter of first concern. In

our case, which involves "all over" coating

prior to production of the pattern, the problem

is somewhat simplified and some methods of

application not readily applicable to other

processes of circuit production become available for our use.

23 By Clifton M. Tuttle, vice president in charge of research and

engineering. Kenyon Instrument Co., Inc., Huntington, Long

Island, N. Y.

-' Editor's note: Since the receipt of this paper a new de- velopment involving the use of commercially available films as the

stencil matrix has been brought to our attention by Kenyon. In

this process a stencil is formed photochemically, the finely divided conductor suspended in a solvent is applied by spraying, and the conductive pattern is then finally transferred from the commercial film to a plastic or possibly ceramic base. This method will be

described in the forthcoming technical journals.

Printed Circuit Techniques 51
The Kenyon company has conducted a systematic survey into the commercially available

application techniques and has devised some

others which may be new. Some of the more

significant results are presented in table 15.1.

Circuit Patterns On Metallized

Insulators

Given an insulator whose surface is covered

by a conductive coating of uniform adherence,

surface quality, and chemical nature suitable

for the certain end product, the task of removing the unwanted portions so as leave the desired wiring networks may be approached in

two distinct ways:

One of these is mechanical—by sand blasting. The other is chemical—by acidic or basic

etching. Either method is practical and both

are capable of producing precise results. Which

method to use depends upon several factors

which will become apparent as this discussion

progresses. In some cases, a combination of

both techniques is indicated.

For either method to become operative it is

first necessary to produce upon the metallized

surface a stencil pattern that will act as a resist

for the sand blasting or the chemical etching.

Other papers have discussed various mechanically applied stencils. Our techniques employ

photographic resists as stencils because of the

dimensional accuracy of the pattern, the versatility of application to any contour and size of

surface, versatility in changing from one pattern to another, and the dimensional accuracy

of registration of the pattern with respect to

location on the piece.

Practically, whether the photographic stencil

is used as a sand blast or chemical resist, the

material must have certain chemical and physical attributes. The material must be in solution

and possessed of a viscosity allowing it to be

evenly spread over the surface. Its photochemical nature must be such that it may be ren- dered either soluble or insoluble in some re- agent by the action of light or ultraviolet radiation.

Without going into the chemistry of such

materials in any detail, it is possible to list in

tabular form the types and attributes of a few of the more useful materials. The maTable 15.1.

Properties of various conductive coatings

Application

process

Vacuum evaporation

Sprayed metal

Low melting point

alloy (flowed on)

Silver or gold

precipitation

Graphite

Deposition

Electro-chemical

deposition

Conductive

Material

fNi-Cr .

I Ni-Cr _

I Ni-Cr _

{ Ni-Cr -

I Ni-Cr .

I Ag

[Ag ____

f Zn

I Zn

! Zn

I Al [Al

f Zn-Pb-I

•! Zn-Pb-I

I Zn-Pb-I

(Ag

| Au

(C

I C

c

r cu

I Ag — 1 Cr

I Ni

Base

Material

Bakelite

Bakelite Glass Polystyrene

Steatite Bakelite Glass

Steatite Ground Glass

Bakelite Glass

Steatite

Bakelite

Glass Lucite

Bakelite Bakelite

Vinylite

Polystyrene

Cellulose Acetate Any of above coated with any metal but

Al

Adherence

Excellent do

do Fair

do

do

do

Good

Poor

Fair Good

Excellent

Excellent do Poor

Fail- Fair

Excellent dodo

Excellent do

do

do

Surface

Quality

Mirrorlike __do__

__do__

—doPorous

Mirrorlike —doPorous —do— —do—

__ do— —do— —do— —do— —do—

Mirrorlike —doSmooth

__ do—

__do—

1 Smooth

(• after

I buffing

Uniformity

of thickness

Percent

11

11

1

1

1

2525

25

2525

2525

25

1010

2.5

2.5

2.5

55

5.5

Thickness

In. 0.00001

.00005

.00001

.00001

.00001

.00001

.00001

.002

.002

.002

.002

.002

.001

.001

.001

.0001

.0001

.0005

.0005 .0005

up to 0.002

.002 .002

Resistance Ohm-Cm

2.6 x 10-1 5 x 10-2 2.5 x 10- 1 2.5 x 10-1 5 x 10-1 1.2 x 10-1

1.1 x 10-1

5 x 10 2 5 x 10- 2 5 x 10-2 2 x 10-1 2 x 10-1

1.0

1.0 1.0

8 x 10-5 3 x 10-4

2 x 102 2 x 102 2 x 102

Note—The following is given as a brief explanation of column 1: "Vacuum evaporation" refers to the high vacuum technique now made comparatively inexpensive by quantity production methods. "Sprayed metal" refers to the application of molten metals either from wire or powder.

"Low melting point alloy" refers to materials similar to Woods metal, especially those having a high indium content. These platings

are, of course, useful only in situations where the ambient temperatures are below the melting point. "Silver or gold precipitation" refers to the technique of mixing two sprayed-on solutions, one containing metal halide salts and the second containing a reducing agent. "Graphite deposition" crystalline carbon is precipitated from solution by newly developed techniques. "Electrochemical deposition" refers to the well-known plating technique using a metallic salt as an electrolyte. Column 4—"Adherence" needs an apology in that it is qualitative rather than quantitative. "Excellent" means the coating cannot be

scratched off with the finger nails, lifted off by Scotch tape, or rubbed-off with vigorous treatment with a pencil eraser. "Good" means that

the coating fails in the last two "tests". "Fair" means that it fails on the third. "Poor" means that it fails on all three. A yet unpublished paper describes an apparatus that might well yield significant quantitative tests. Column 5—'"Surface quality" is also qualitative. The standard instruments of surface specification such as the Brush Analyzer are probably too refined for the job.

52 Circulars of the National Bureau of Standards
terials in table 15.2, with the exception of

number 1, are all of a nature such that the

latent image produced by exposure becomes

insoluble in the exposed areas but is dissolved

and removed in the unexposed areas when subjected to the reagents in the second column.

The exposures listed in the last column are only

rough approximations. The values are based

upon a visual illumination value with the radiation being produced by an H4 mercury capillary lamp. Actually the most photographically

effective part of the radiation lies in the ultraviolet region and this is not well specified by

the meter-candle unit.

Table 15.2.Photomechanical resists

Material

Experimental

photosensitized colloidBichromated

gelatin treated with

glycerine

Shellac based

cold top

enamel

Bichromated

glue

Experimental

cold top

enamel

Solvent

5% NHiOH in 130° F. H2 Q .

H:0

C2 H5 0H with dye

H2

Higher alcohols -

Use

Sandblast resist.

Sandblast resist.

Nitric acid resist.

Nitric acid resist.

fAlkali resist;

-I mild acid resist; [electroplating resist.

Approximate

exposure

in meter

candle sec- onds to produce

latent image

6.3 xlO5

2x10°

1.7 x 10s

1.8 x 108

1.9 x 10"

In producing circuit patterns on the metallized insulators the following steps are carried

out: (1) The part is covered with a thin

(0.0004 in.) layer of asphalt; (2) the photosensitive resist is applied, (materials 1 or 2 of

table 20.2 are used if the metal layer is of a nature best removed by sand blasting) ; (3) the

part is exposed through a black and white photographic transparency bearing an image of the

circuit pattern; (4) the whole part is immersed

in the appropriate solvent leaving the resist

layer untouched in the areas to be protected

;

(5) the part is dried and subjected to a blast of

fine abrasive, i.e. 1,300 mesh silicon carbide,

until the metal layer has been removed (this

process takes from 5 seconds to a minute depending upon the layer-thickness, adherence,

and air pressure of the blast) ; (6) the resist

layer remaining over the conducting pattern is

removed. Use of the chemical etching technique eliminates step 1 of the foregoing, substitutes a different solvent in step 4, and substitutes a chemical bath for step 5.

On the whole the chemical method proves

somewhat more economical and convenient than

the sand blast process, and it leaves the base

material unabraded and extremely smooth. Its

universal use is, unfortunately, limited by the

fact that many of the conductive layers that

show the best adherence to the base material

are the least affected by the oxidizing reagents

such as nitric acid.

Most of these layers such as nickel and chromium are readily attacked and removed by the

halogen acids HC1, HF1, HBr, and HI. For

these acids, we have not yet devised a satisfactory photosensitive resist and have found none

on the commercial market. This situation, however, does not prevent the use of the chemical

method though it does require an additional

step in the preparation of the conductive layers.

In dealing with one of the strongly adherent

layers such as an evaporated coating of chromium and nickel, a thin coating of silver is

deposited either by electroplating or by evaporation. We then have a material that can be

treated by the standard chemical etching process so that the desired pattern can be produced

in the silver layer superposed on the base metal.

The whole part may now be treated by one

of the halogen acids. Since silver is unaffected

by this treatment, the silver image acts as a

protective resist for the underlying base metal

and the final desired result of a circuit pattern

in the strongly adherent nickel or chromium

layer is achieved.

Advantages of the Photographic

Technique

Final results of a high order of precision are

possible. It is possible to reproduce with very

little loss, most of the detail of which the modern fine grain photographic emulsion used as

the printing negative are capable. The end re- sult is perhaps best expressed in the customary

photographic terms of resolving power.

Starting with a chrome nickel evaporated

coating on glass, we have been able with no

difficulty to reproduce a line pattern on grating

having eighty lines to the millimeter. Probably

this is not the limit, though with thicker metallic coatings—of the order of 0.001 inch—it is

doubtful whether more than one-tenth of this

resolving power could be achieved.

Optical Printing

On flat surfaces, good definition with resultant high resolving power can be achieved by

contact printing. The photographic transparencies can be made either on Eastman Kodalith

film or on process glass plates. A vacuum printPrinted Circuit Techniques 53
ing frame is desirable to assure good contact.

When the part is cylindrical or of some

shape other than flat, optical printing is preferable and sometimes necessary. An optical

printer suitable for the job of printing on the

cylindrical surface of a tube envelope consists

of a slit source of light, a high quality projection objective, a means for moving the negative

uniformly past the slit, and a means for rotating the tube with uniform surface speed. The

speed of the negative traverse is so adjusted

that the optical image of the pattern moves at

the same rate as the sensitized surface of the

tube so that there is no relative movement between the image and the tube.

Acknowledgment

A large part of the work of this company in

studying these techniques has been made possible through a development contract with the

Squier Laboratory of the Signal Corps. The

author wishes to thank members of the Squier

Laboratory staff and in particular Mr. F. K.

Priebe for his many helpful suggestions

throughout the course of this work.

16. Prefabricated Components for Printed Circuit

Application25

The production of electronic and radio equipment is today almost entirely an assembly

operation with the fabrication of components

being done in plants which are completely independent of the equipment manufacturer in

organization and ownership. Even when equipment manufacturers design and engineer special components, production is frequently subcontracted to the component manufacturers. In

the early days of the radio industry, however,

it was necessary for radio manufacturers to

fabricate many of their components. There

were no large scale specialists in the manufacture of such items as resistors, coils, condensers, and the like. The development of mass

production methods in the industry was accompanied by, and indeed was made possible by,

the separation of equipment manufacture from

component manufacture.

The major consequence of the type of specialization which has evolved is the production

of very large numbers of standard components

at low cost and with continued improvement in

quality. The amount of know-how required for

the manufacture of components has become

quite formidable. Equipment manufacturers no

longer possess the organization, the personnel,

the know-how, or the facilities for manufacturing components. Even if they did, their own

requirements are usually not large enough to

permit a production volume which results in a

cost as low as that charged by the large scale

manufacturers of components such as resistors,

capacitors, and coils.

This situation is not substantially altered by

the use of printed circuit techniques. The mass

production of printed components, such as re25 By M. U. Cohen and Lewis Balamuth, Balco Research Laboratories, Newark, N. J.

sistors, of adequate quality to required tolerances is a difficult art. Highly specialized

knowledge and experience are still prerequisites. It must therefore seem to equipment

manufacturers that the introduction of printed

circuit methods into their plants would be a throw-back to the primitive era of the 1920's.

Nevertheless, there is a great deal of interest

in printed circuits because of several outstanding possible advantages26

, particularly compactness, lower wiring and inspection costs, and

simplified production systems. If the radio

manufacturer found it possible to use these advantages without forcing upon him the new and

serious problem of component fabrication, then

printed circuit procedures would undoubtedly

secure widespread adoption.

One way of solving this problem is by prefabricating components designed for subsequent insertion into printed circuits. The two

most important specifications for such components are miniature size and thermal stability.

The first requirement is obvious, and the second

is necessary for two reasons. In the first place,

the compactness of printed circuits automatically results in high ambient temperatures of

operation. The heat from the electronic tubes

alone is frequently sufficient to heat all the

other elements to temperatures far above the

limits ordinarily set for their satisfactory operation. Secondly, in order to avoid any loss

in compactness and to secure the full production advantages of printed circuits, prefabricated components must be directly attached to

the printed conductors without the intervention

of wire leads. Therefore, such components

must be stable at soldering or welding temperatures.

26 C. Brunetti and R. W. Curtis, Printed Circuit Techniques, N. B. S. Circular 468 (1947).

54 Circulars of the National Bureau of Standards
With the support of the Signal Corps, which

has its own requirements for miniature components stable under wide extremes of temperature, the Balco Research Laboratories has for

some time been developing resistors, capacitors,

and inductors to meet the military demands for

high thermal stability and miniature dimensions. The novel materials and fabrication

methods employed in meeting these requirements may also be utilized in the manufacture

of prefabricated components which are very

well adapted to printed circuit assembly.

The components which we have developed

further lend themselves to belt-line circuit as- sembly operations which in many respects

promise to be even simpler than those currently

in use for standard circuit assemblies in that

they may be made almost automatic. Thus, the

manufacture of electronic equipment can continue to be an assembly process and the advantages of printed circuit techniques can be

obtained without the necessity for learning the

difficult art of component manufacture.

Balcohm Resistors

"Balcohm" resistors (fig. 16.1), employ a

nonmetallic and noninductive resistive element

which has high thermal stability. Although

most of the experimental units have had a

resistance in the range from about 100 ohms

to 5 megohms, we have prepared resistors

whose value is as low as a few ohms and as high as several hundred megohms or more.

Standard commercial tolerances may be maintained. Because of its high stability, this type

of resistor is expected to have significant application in the precision resistor field, that is,

where 1 or 2 percent tolerance components are

installed and expected to maintain their values

over long periods of time.

The resistor composition is generally applied

as a very thin coating to an insulating support.

This support may be made of glass, fused

quartz, ceramic, mica, or other material which

has the requisite insulation resistance and

thermal stability. The body may be of practically any shape or size, and coating methods

Figure 16.1. Reading down (center): Commercial xk watt resistor; Balcohm hermetically sealed axial lead resis- tor; Balcohm hermetically sealed radial lead resistor; resistors with body ends tinned for direct soldering.

Left, printed circuit with two resistors and one film capacitor soldered direct to printed wiring. Right, strip resistor soldered to printed

wiring. Disk resistor for coaxial application.

i Printed Circuit Techniques 55
have been developed for the fabrication of tubular, flat strip, disk, and other types of resistors.

Miniaturization in size may be carried to any

desired extent, resistors having been constructed whose total volume is 0.0001 cubic

inches. Indeed, coating methods have been developed which permit the construction of two

or more resistors on a single miniature form.

Thus the interior and exterior surfaces of a

tubular form or the opposite faces of a flat

strip may carry two independent resistive films,

with separate leads or terminals for both resistors.

Tubular resistors may be internally coated,

and result in an insulated body resistor of excellent space factor. Where desired for special

applications such resistors may be hermetically

sealed without requiring any increase in size

because the insulating form itself serves as the

sealed envelope to protect the resistive film.

For inserting Balcohm resistors into standard circuits radial or axial leads are provided.

Pin type and cap terminals may also be used

for hermetic sealing and for mounting into

jacks or fuse clips. For printed circuit applications we have developed a lead-less resistor

whose ends are prepared for direct soldering

to printed wiring.

Thermal Stability and Power Rating

For the purpose of comparing the thermal

stability of Balcohm resistors with high quality

commercial composition resistors, it may be

useful to recall that the JAN-R-11 specification

covering the latter provides for (a) full power

rating at ambient temperatures up to only 40°

C ; (b) continuous derating as the ambient temperature increases, so that at 85° C (185° F),

the power rating is 1/4 that of the full rating;

and (c) a resistor body temperature limit of

135° C (275° F). In addition, the high temperature (85° C) load-life test for JAN-R-11

composition resistors permits a 10 percent

change after 200 hours of intermittent operation under derated load.

In the light of these specifications it is interesting to compare the behavior of two groups

of Balcohm resistors tested under much more

extreme conditions by the Signal Corps engineering laboratories. These groups comprised

six 100,000-ohm tubular units and six 2,000-

ohm tubular units, whose size was about 1/2

of the Jan-R-lli/2 watt unit. These resistors

were run at 14 watt intermittent load at an

ambient temperature of 150° C (302° F).

After 200 hours of operation under these conditions the average change of the 100,000-ohm

units was less than ±2 percent (2.6 percent

maximum change) , and the average change of

the 2,000-ohm units was ±0.3 percent (0.6

percent maximum change).

We have made some attempt to establish iiotl power rating and operating temperature speci- llesii fications for these resistors, but results such I may as the following make this difficult. A resistor lita of JAN-R-11 one watt dimensions was tested |:liei under load at a series of ambient temperatures 9 that ranging from 20° to 200° C (68° to 392° F)

J with power dissipation ranging from to 12

j watts. The resistor was held for 1 hour at each

temperature level at a given power, then the f temperature was increased to a new level and ind the resistor operated again for one hour at nil the same power. After a run at the highest am- que bient temperature (200° C), the cycle was re- not peated with the power rating increased in W

steps of 1 watt, until a final test was run at 12 tit watts. Thus the final test at 12 watts was on a tioi 1 watt size resistor which had had 1 hour each §1 of operation at power levels of 0, 1, 2, 3, 4, 5, ma 6, 7, 8, 9, 10, 11, and 12 watts at 20°, 100°, jif 150°, and 200° C. The resistance at the start tin of this test was 194 ohms, and at the end of the n

test it was 182 ohms, a change of 6 percent.

Because of their extraordinary stability at iy high ambient temperature and high surface re! temperature of operation, it is not yet possible

to assign definite power ratings to the various ti| sizes and styles of Balcohm resistors, since n present standards never contemplated either tr such thermal characteristics or the miniature vi dimensions now achievable. F The temperature coefficient of resistance of m Balcohm resistors is approximately 0.1 percent pi per C (0.05 per cent per F) from room tl temperature to at least 200° C (392° F) . m

Direet Solderability

One of the potentially important applications 1; of Balcohm resistors to the printed circuit field \ depends on the fact that these resistors may i be furnished with metallized or tinned ter- ! minals which are integral with the body of the

resistors and which may be heated to ordinary

soldering temperatures without deterioration

of the resistor. When wire leads are soldered

to these terminals, or when the resistor body

terminals are soldered directly to a printed or stamped metal conductor, the resistance change

is less than 1 to 2 percent and thus negligible

in the majority of circuit applications. The insulated tube or strip resistors may also serve

directly as "cross-over" elements in printed

circuit wiring.

This makes it possible for the equipment

manufacturer to print conductors according to I his wiring diagram, which is a simple matter, : and then to solder resistors selected from his

stock shelves directly into place without being

concerned about "printing" his resistors. It is

our opinion that this general method of approach will overcome the most serious obstacle

56 Circulars of the National Bureau of Standards
to the widespread adoption of printed circuit

designs and techniques, since the manufacturer

may purchase his "printed resistors," rather

than be required to fabricate them. He will

then be able to concentrate in his own field,

that of design, assembly, and sales.

Balcoil Inductors

The problem of manufacturing highly stable

inductors is a perennial one which has become

more acute with the trend toward higher frequency ranges. Most inductors are unstable

not only with respect to temperature variation

but they also age badly, that is they vary with

time even under ordinary conditions of operation. These secular fluctuations result from the

dimensional instability of ordinary coil form

materials. The mechanical stresses set up by

differences between the thermal properties of

the wire and coil form also cause the wire on

wound solenoids to shift slightly. Furthermore

in many cases the dimensional changes induced

by temperature variation may never be wholly

restored or may show considerable lag.

The application of the printed wiring principle offers the possibility of completely eliminating secular changes and significantly increasing the thermal stability of inductors provided that the coil form is properly selected.

Fused quartz has the lowest coefficient of thermal expansion of any known substance and

probably the lowest thermal hysteresis. Thus,

the dimensions of a fused quartz form are not

only changed very little by temperature variation, but they also revert very closely and

quickly to original dimensions at any particular temperature. But in order to take full advantage of fused quartz as a coil form material,

it is necessary to wind a metallic conductor in

such a way that the dimensions of the winding

are strictly determined by the coil form at all

operating temperatures. By the use of previously described printed wiring techniques and

others which we have developed, we have been

able to construct inductors of this description

in the form of solenoids and spirals (fig. 16.2)

.

These methods present advantages in that they

make certain types of novel designs possible.

For example, the inductors may be terminated

in standard sized pins to fit into sockets or pin

jacks or they may be soldered directly into a

circuit. They may also be hermetically sealed

and shielded in a variety of ways. High silica

and electrical glasses and ceramics may be

also used as form material.

A design with interesting possibilities is an

internally wound solenoid ; i.e., one in which the

conductor is firmly bonded to the interior sur- face of the cylindrical form rather than the

exterior. Hermetically sealed leads may then

Figure 16.2. Flat spiral and solenoid printed inductors on fused quartz and glass forms.

The two bottom solenoids are printed on the interior surface of

the coil form and the units are then hermetically sealed.

be provided at both ends of the coil or both

leads may be brought to one end and the whole

form sealed off. Evacuation or filling with dry

air or hydrogen may precede the sealing-off

operation.

Q Values

The Q of inductors of this general description depends in the first instance on structural

and design factors common to all inductors

whether wire wound or metallized. Among

these factors are the dielectric properties of

the coil form, wire size, turn-width and turnspacing ratios, geometric shape, the ratio of

such dimensions as length and diameter, etc. In addition to these factors, the Q of metallized

inductors depends to a large extent on the

nature of the metallizing process used. Thus,

while the size of the conductor is one factor

which determines Q in ordinary coils, two inductors of the same design and the same con- ductor metal and thickness prepared by metalTable 16.1.

Q values of balcoil inductors

Inductor L

uh

1 mc/sec Q

Flat spiral 0.7 35 200 Solenoid .8 20 150 Flat spiral 1.3 20 150 Flat spiral 1.4 25 125 Solenoid 6.5 10 135

Printed Circuit Techniques 57
lizing in different ways may have very different Q values, because of differences in quality

of the metal film. The Q values of Balcoil inductors may be as low as desired, and where

necessary may go up to 200, design permitting.

Typical values are shown in table 16.1.

Thermal Stability

The problem of specifying the stability of

inductors as a function of temperature is complicated by the presence of other factors that

enter into the measurement of inductance, such

as the distributed capacitance and resistance of

the coil. We have preferred to proceed by rec- ognizing that the great majority of applications for highly stable inductors involve their use in frequency determining circuits such as

oscillator tank coils or tuning coils. Therefore

we have attempted to determine the thermal

stability of an inductor by making it the frequency determining element in an otherwise

highly stable oscillator and then measuring the

shift in frequency of the oscillator as the temperature of the test coil alone is varied. Our oscillator uses the transitron circuit originally

described by Brunetti'- 7

, modified for stable operation up to 20 Mc. The test coil is thermally

isolated from the oscillator. In the temperature

range from 25° C to 150° C, preliminary measurements indicate inductor stabilities (expressed in terms of frequency shift) of 5 to 20

parts per million per ° C (3 to 11 ppm/° F)

.

Fabrication Tolerance

In connection with the high performance

quality inherent in inductors of this type, it is

of interest to note that they can also be fabricated with considerable precision with respect

to both physical dimensions and inductance

values. If necessary, physical dimensions including mounting hole or terminal spacings

may be held within a few thousandths of an

inch. Inductance values may be regularly held

within a tolerance of ±2 percent, and ±1 percent or less is achievable.

Thermal Shock Behavior

Despite frequently voiced doubts as to the

thermal shock resistance of metallized coils be- cause of the disparity in expansion coefficients

of the form and the metallized conductor, Balcoil inductors are highly resistant to thermal

shock. In many cases we have found it possible to eliminate coil form fracture or winding separation in thermal shock tests run from

—75° to 300° C (—103° to 572° F) and from

300° to —75° C. The inductors also possess

highly satisfactory mechanical ruggedness, es- pecially when compared with wire wound components.

27 Brunetti. The Transition Oscillator, Proc. IRE 27, 88 (1939).

Balcap Capacitors

The possibility of fabricating miniature ca- pacitors of high dielectric materials such as titania and the titanates has been recognized for

some time and such units are commercially

available. However, the inability to produce

thin films of good quality by standard ceramic

techniques has made it impossible to fully exploit the potential value of materials with high

dielectric constants. The capacitance of a parallel plate capacitor is C = KA/t, where C is

capacity, K is dielectric constant, A is the electrode area and t is thickness of the dielectric.

For a given area, C is a function of K/t. In the

case of oiled paper films used in capacitors, the

dielectric constant is, say, about 5 and the thick- ness about 0.001 in., and therefore K/t is about

5,000. In order that a ceramic film with a

dielectric constant of 100 should occupy an

area equivalent to that of 0.001 in. oiled paper,

its thickness must be 0.020 in. To secure any

real advantage, its thickness must be much

less. Conversely, for a thickness of 0.030 in.,

which is frequently the practical lower limit

with ordinary ceramic fabrication methods, the

material must have a dielectric constant con- siderably in excess of 150 in order to obtain a decided advantage over oiled paper.

To overcome this handicap and to secure space advantages, ceramics with very high

dielectric constants have been developed. However, these have been found to be useless for

many purposes both because of their erratic

behavior and because of their ferroelectric

properties which result in excessively large

changes in dielectric constant as the ambient

temperature is varied.

Materials and techniques of fabrication

which promise to overcome these difficulties are in process of development in our laboratory.

Capacitors have been produced with dielectric

films varying in thickness from 0.001 in. to

0.010 in. and with a dielectric constant of about

90. The power factors may be between 0.0002

and 0.001, that is, capacitor Q's are between

1,000 and 5,000. The temperature coefficient

of capacity is about 0.05 percent per ° C and is

uniform between room temperature and 300°

C.

While the thinnest films have so far been produced only on an experimental basis, films as

thin as 0.005 in. are believed to be already

practicable from a production standpoint.

Dielectric films of this thickness are easily

handled and may be stacked and fabricated

into condensers by simple techniques.

Dielectric Constant and Power Factor

Some preliminary investigations have been

made on the variation of dielectric constant

58 Circulars of the National Bureau of Standards
and power factor of Balcap films with temperature. These measurements have been carried

up to 300° C for dielectric constant and up to

250° C for dielectric loss. As may be imagined,

precise measurements at such temperatures involved several difficult problems. Since the

methods by which these difficulties may be

overcome may be of considerable interest, full

details will be subsequently published. It should

be noted parenthetically, however, that the so- lution of these problems was considerably facilitated by the use of printed circuit wiring

techniques, and the extension of such techniques

to the construction of special units such as coTable 16.2. — Temperature

variation of dielectric con- stant and power factor

Temperature K PF

° C

40 89.0 0.001 80

' 87.5 .0007 120 85.6 .0008 160 83.7 .0008 200 81.5 .0006 250 79.7 .0009 280 78.85 300 78.0

axial lines. The temperature coefficient of

dielectric constant (fig. 16.3) is about 0.05 percent per °C (0.03 percent per °F) and is approximately uniform up to at least 300° C as shown by table 16.2. The power factor data in

table 16.2 demonstrates that low losses are maintained at least up to 250° C. (These power

factor data include losses in the measuring line

;

only a few of the measured values are shown in

the table).

Breakdown Voltage

Ceramic dielectrics of very thin cross section

have generally been considered to be impractical not only because of the difficulty of fabrication, but also because of their dielectric

strength is low and erratic. The presence of

voids, pinholes, or microscopic impurities

(which are covered in thick sections) produce

regions of high dielectric stress in thin sections

which are fatal to capacitor quality. Films prepared by our methods appear relatively free

from this defect, and excellent breakdown

strengths are maintained up to very high temperatures. Thus a series of measurements were

made on 27 different samples approximately

0.006 in. thick at temperatures ranging from

100

I

COgo

ko 120 160 200 2U0 280

TEMPERATURE (°C)

Figure 16.3. Variation of dielectric constant with temperature of Balco dielectric films.

Printed Circuit Techniques

320

59
room temperature up to 520° C (968° F). The

break-down voltage in all cases was greater

than 1,300 volts and this value or better is

maintained even at the highest temperatures,

almost a dull red heat, as shown in table 16.3.

While previously proposed methods of application of printed circuit techniques appear

to result in freezing design and reducing the

opportunity for individuality on the part of the

equipment manufacturer, the components described here will permit retaining the advantages of flexible design and circuit modifications which are desirable features of classical assembly techniques. This is made possible

by the high temperature stability of our components which permit one to eliminate leads

and solder the components directly into the

circuit wiring.

Assembly-line processes based on these considerations may be conceived as follows. A

moving belt has placed upon it at the first station, the base plate for the printed circuit

wiring. Prewired die-stamped or metal-sprayed

circuit plates may be used. Alternatively, the

wiring may be printed while on the belt, and

then heat-treated in a conveyor furnace if metallizing paints are employed.

Table 16.3. —Breakdown

voltage for Balcap con- densers as a function of

temperature

(Average thickness=0.006 in.: 27 samples)

Temperature Breakdown voltage

(d.c. volts)

° C

20 1,340 110 1.840 200 1.560 300 1.650 400 1,820 450 1.570 473 1.740 520 1,780

Electrodes

When working with dielectric films possessing such high dielectric constants and low

power factors, the method of electrode application is of great importance if the qualities of

the resulting capacitor are not to be degraded

below those which are inherent in the dielectric

film itself. Satisfactory electrodes may be applied by firing on silver paints, vacuum evaporation, vacuum sputtering, or any of the other

standard methods for working with mica dielectrics. Techniques have also been developed

for stacking such films so that structures

analagous to mica condensers result.

Thermal Shock Behavior

The thermal and mechanical shock resistance

of these film capacitors is very good. Soldering

irons can be used on pretinned electrodes without fear of shattering the film. We have even

found it possible in some cases to fasten leads

to these film capacitors by spot welding techniques. These features obviously will permit

the direct soldering of these components into

printed circuit wiring.

Assembly Line Techniques

It has been evident that much of the interest

of equipment manufacturers in printed circuit

techniques has been in the opportunities which

they provide for production simplification and

assembly cost reduction, factors which are at

least as important as compactness. We wish

to point out that the prefabricated components

described above make it possible to utilize these

opportunities to a very large extent, without

introducing the problems of fabricating components such as resistors on the assembly line.

The prefabricated resistors, the capacitors,

and where applicable, the inductors are supplied to the equipment manufacturer with terminals already tinned, and are stored in individual hoppers mounted over the belt. At the

second station, either the entire set of components is dropped onto the wired plate, or if too

complex an arrangement is required, this placement may be done in several steps. Correct,

automatic positioning may be obtained by providing pregrooved and recessed plates, or indented strips in the case of die-cut wiring, so that the components do not shift. Another simple method of positioning is to provide a thin

stencil jig held in position over the plate, with

openings cut to receive the components and re- tain them in their correct place relative to the

wiring. The unit with components in position

is then passed under one or more heated plat- ens which are then brought down on the circuit plate. The components are thus soldered

to the circuit wiring without the use of highly

skilled solderers. Another soldering method

that is extremely effective in our experience is

induction soldering. By the use of a high frequency induction furnace we have found it possible to solder our components directly to either

ordinary or printed circuit wiring at high

speed.

Tubes and other components which do not

lend themselves to assembly by hopper feed

may then be inserted in the ordinary way, and

the circuit assembly thus completed.

An additional advance that promises to be

significant is based on our finding that it is

possible to metallize and firmly bond conductors to a ceramic or glass plate without treating the whole plate in a high temperature furnace. This has been achieved by induction heating and other electrical methods. The treatment

60 Circulars of the National Bureau of Standards
cycles are very rapid because the most massive

part of the unit, the insulating base plate, is

not heated very much. The components may

also be soldered at the same time, and indeed

because of the cementing action of metallizing

paints it may eventually prove to eliminate

much of the soldering. Considerations of this

nature, however, soon move into realms where

assembly speeds become absolutely fantastic,

at least on paper.

Meanwhile, it is to be hoped that the availability of the prefabricated, highly stable components and the simpler assembly techniques

described above will be of help to equipment

manufacturers in more fully utilizing the

miniaturization, production simplification, and

assembly cost reduction made possible by

printed circuit wiring.

Acknowledgment

In the fabrication and test of the components described above we wish to acknowledge

the help of our laboratory staff including J. A.

Brady, D. Buskin, A. Hershler, M. Pope, S. A.

Thompson, A. U. Koppel, A. Weinstein, under

the supervision of S. Eisenberger.

Further thanks are due to the United States

Signal Corps which aided this work financially.

17. Typical Commercial Applications

Two Stage Printed Circuit Amplifiers 28

A brief description of a printed two-stage

amplifier with a filter circuit, which incorporates several variations from the type of printed

circuits previously discussed, is of interest.

This amplifier was tooled for use in the Citi- zens Communication band transceiver under

development by the Citizens Radio Corp. of

Cleveland. One of the engineering models of

this amplifier measures 2% in. long, 2 in. wide,

and with the tubes approximately % in. thick.

The amplifier has eight capacitors and nine resistors.

One of the variations on our amplifier is the

use of tubular ceramic capacitors rather than

the flat variety. These tubular capacitors may

be laid on the surface of the printed ceramic

base and soldered directly to the silver conductors. Another variation would be to place these

capacitors in slots in the ceramic base so that

the entire surface of the base will be flush.

Due to the difficulties sometimes encountered

in mounting tubes and affixing terminal lead

wires to the ceramic base, we have developed

another variation from standard printed circuit techniques. This method consists of countersinking the hole to which the lead will be

soldered, from both sides of the base. By silvering the entire surface of the countersunk hole

and by placing a drop of solder on either side

of this hole, a rivet of solder is formed on the

lead which is strong, durable, and highly re- sistant to vibration. A production model of

the amplifier (fig. 17.1), illustrates two of these

variations from standard procedure.

28 By G. G. Hornung, Stupakoff Ceramic & Mfg. Co., Latrobe, Pa.

Printed Circuits in Transmitters

and Receivers29

Printed circuit techniques are here to stay,

for they are a boon to the radio and electronic

industry. From our experience in applying

printed and associated components to transmitters and receivers, the economy and compactness of these circuits is without parallel.

Wiring by printed circuit techniques, reduces

cost and bulk of wiring, simplifies design and

lends itself to rugged compact construction. Inventory is reduced and the need for testing of

individual components is eliminated. This simplification of subassembly and assembly operaFigure 17.1. Production model of two-stage printed

circuit amplifier.

Nine printed resistors are on the reverse side of the plate.

By AI Gross, president, Citizens Radio Corp., Cleveland, Ohio.

Printed Circuit Techniques 61
tion, reduces cost of labor and overhead and

affords a tremendous increase in production.

In the standard methods of wiring, terminal

boards and mountings are required in various

parts of the chassis, whereas, in the printed circuit techniques the insulation serves as terminal

board, socket mount, and chassis and sometimes

as a dielectric medium for built in capacitors.

Our adaptations of printed circuits are applied both to audio and radio frequency circuits. In our transceiver audio circuits, the

application of a printed circuit eliminates 26

separate components and the inspection, forming, cutting, and other preparatory operations.

In applying a printed circuit to radio frequency applications great care must be exercised in the design and layout of the circuit particularly when dealing with very short wave

lengths. At these wave lengths, the dielectric

used to support the printed design which carries radio frequency currents becomes a deciding factor in the efficiency and operation of the

over-all circuit. The presently known dielectrics such as ceramics, quartz, glass, certain

phenolics and plastics, are excellent mediums

for supporting these very high frequency circuits. In our transceiver, the radio frequency

circuit and several associated components are supported by a ceramic plate. This circuit

should be supported by a quartz plate. The

rigidity that is offered by this arrangement

tremendously increases the frequency stability

and makes it difficult to tamper with the circuit elements. It also reduces the possibility

of circuit misalinement. In our transceiver,

the use of this plate increases the speed with

which the radio frequency unit is assembled.

It also eliminates three components, one which

requires brazing to the chassis, and another

which is a wire configuration that is die formed.

Once the circuit is adjusted and in place, it will

withstand a certain amount of rough handling.

This means that the equipment using printed

circuits and components will be much more

rugged than conventional design. Testing be^

comes a simple procedure since there is so lit- tle to adjust.

Since our transceivers will be used in various

parts of the country, the use of printed circuits and components greatly increase resistance to temperature and humidity effects. Enhancing this property still more, a complete

printed layout can be made practically impervious to temperature and humidity by coating

the entire unit with plastic protective material.

It may be desirable to cast the entire unit in a

styrene or plastic resin to minimize the effect

of mechanical shock. Once cast, the unit is also

impervious to submersion in water.

The development of printed circuits is not

new; as a matter of fact, the application of

metallic coating to ceramic dates back to the

days of the early Roman Empire, when pottery

and other objects of art were trimmed and

covered with silver and gold foils. It was later

discovered that by firing the object of art, the

foil would fuse with the ceramic or pottery

and form a permanent bond.

Our "Ultraphone" transceiver incorporates

printed circuits and the associated refinements

making it an effective and practical two-way

Personal Citizen's Radio transmitter and re- ceiver.

First Application of the Printed

Circuit Technique To A Hearing Aid30

Late in 1946 the Allen-Howe Electronics

Corp. of Peabody, Mass., began to consider the

possibilities of applying the printed-circuit

technique to a hearing aid.

It was felt that there were certain technical

limitations still to be mastered before it would

be possible to produce a quiet efficient hearing

aid circuit. Investigation of available data on

the printed-circuit technique indicated, however, that the application was feasible and the

first circuit designs were attempted by the

Allen-Howe Electronics Corp. in cooperation

with Centralab, division of Globe-Union, Inc.,

in June 1947.

Variations in circuit and layout designs were

reduced to four types on two basic circuits.

After complete testing and further investigation a circuit (fig. 17. 2A) with suitable gainfrequency characteristics was decided upon as

the basic circuit for the production model (fig.

17.2B).

In designing the hearing aid it was found

possible to incorporate the steatite plate as a

structural part of the chassis unit, a factor

which makes for extreme simplicity in assembly. The chassis consists of only four basic

units. These include the steatite printed-circuit

panel, the transformer panel, the control panel

and the battery panel. All of these units are

held together by four bolts located at each cor- ner of the printed circuit panel.

A great deal of study was necessary to make

possible the proper lay-out of controls and output plug, since hearing aid design requires that

volume and tone controls, as well as the output

cord connections, be placed at the top of the

instrument so that they are easily accessible

when in the pocket. In spite of the high-gain

circuit required, severe feedback problems were

finally mastered.

30 By Leon Strauss, Jr., president, and Verche Strauss, vice president, Allen-Howe Electronics Corp., Peabody, Mass.

62 Circulars of the National Bureau of Standards
C6 OOlnf T3

CKSIBr

EXTERNAL VOLUME CONTROL

3.0 M«g. 100 500

Figure 17.2.

A, Schematic diagram of the printed circuit amplifier used in the "Solo-Pak" hearing aid. Pak" hearing aid.

1000 FREQUENCY (CPS) B

5000 10000

B, Gain-frequency characteristic of the "Solo-

* «

B

Figure 17.3.

A, Front and back views of the printed circuit amplifier shown in varying stages of completion. B, Printed circuit amplifier in lowerleft hand corner as compared with the 173 items used in making up a conventional amplifier of similar electrical design.

The printed circuit in various stages of completion (fig. 17.3A) beginning with the blank

steatite plate and ending with the completed

unit shows the arrangement of components and

the manufacturing steps involved. A more obvious advantage of the printed circuit technique is the replacement of the 173 items used

in a conventional amplifier of similar electrical

design by a single printed circuit plate (fig.

17.3B).

Tests have shown this amplifier to have su- perior power-gain characteristics over the best

of the conventional hearing aid circuits. With

1 millivolt input, the circuit delivers an output

of 1.3 volts. With the same input, the best of

15 conventional hearing aid amplifiers tested

delivered not more than 1 volt output. This

advantage is understandable because the maze

of components wired together and crammed in

a small space in the ordinary hearing aid introduces "coupling" between output and input

that has a degenerating effect. Conversely, the

clean arrangement of printed wiring and the

scientific placement of capacitors, resistors, and

other components made possible by the printed

circuit technique reduces "coupling" to the

absolute minimum.

The degree of miniaturization attained

through the printed circuit technique has made

it possible to use some of the space saved in

furnishing greater battery life. The instrument utilizes the Mallory RMB-4 Mercury 'A'

Printed Circuit Techniques 63
cell which provides over 80 hours of operational life as compared to 25 hours available

in the conventional hearing aid.

The 'B' battery utilized is the Eveready

22l/2-volt, type 412E Minimax. This battery

furnishes approximately 300 hours of life.

With the advent of small batteries developed

during the war the entire hearing aid industry

has followed a trend toward miniaturization

through combining into one unit the battery

pack and amplifier unit. This trend has necessitated the use of increasingly smaller microphones which are particularly insensitive to

low frequencies and tend to peak up at the high

frequencies. This factor usually decreases the

quality of reproduction obtainable in current

one-piece hearing aids. The additional gain obtainable through use of the printed circuit technique over standard methods has made it possible in the Allen-Howe SOLO-PAK to electrically compensate for the microphone deficiencies and still obtain an output that is greater

than can be obtained in the average instrument.

This particular advantage was found to be an

unlooked for dividend in the production of the

hearing aid using a printed circuit.

One of the greatest difficulties encountered

in the use of the conventional hearing aid has

been the constant necessity for repairs in the

field. These repairs are caused primarily by

physical weakness of tiny solder connections,

physical and electrical weakness of small paper

condensers and damage caused by humidity.

With the advent of the printed circuit in a hearing aid all of these factors have been reduced

to a negligible quantity. The degree of miniaturization obtained through this new technique

has made possible, in addition to better tonal

qualities and greater power, the inclusion of

additionally long life batteries for more eco- nomical operation in a unit only slightly larger

than a package of cigarettes, believed to be

the smallest hearing aid available.

The case is formed with removable back sec- tion from pure aluminum sheet type 2S—V2

hard. It is finished with a high polish and

anodized to give it a hard, smooth aluminum

oxide coating which does not scratch or mar

easily and reduces acoustic case noise to a

minimum.

It is believed particularly significant that the

hearing aid was the first such "consumer

product" application of the printed circuit technique developed by the Bureau of Standards

because it dramatically demonstrates how ex- tremely practical and efficient this technique

can be even on such an item of relatively low

quantity production as a hearing aid. It has

been conclusively proven to us that this technique is far superior to older methods from the

point of view of performance, cost, ease of

production, product dependability and miniaturization.

Mycalex 410 Base Plates for

Printed Circuits31

Mycalex 410 is a molded glass bonded mica

material having extremely low loss characteristics in the high frequency range as well as other desirable electrical properties. It is an

approved ceramic of grade L-4 under Joint

Army-Navy Specification JAN-I-10.

Potentially, Mycalex 410 offers many advantages in the printed circuit field. Outstanding is the fact that since it can be molded, intricate shapes are entirely feasible at low cost.

The coefficient of thermal expansion is in the

range of metals and very closely matches that

of soft steel. Silver and other inserts for contact purposes can be molded directly into the

material without fear that they will ever loosen

in service. The combination of inserts molded

into the base material with any desired contour

of the part, adds a degree of flexibility to

printed circuit design which may prove of great

importance in this field as the art progresses.

In a simple form of Mycalex plate (fig. 17.4)

,

it will be noted from the back view that the

inserts may be used as contact prongs for a

plug-in type of assembly. The use of these inserts can be extended to many other types of

applications.

4$t

Figure 17.4. Printed plug-in unit on a molded Mycalex

base plate.

Printed Inductors 32

This paper describes a simple adjustable tuning element recently developed by General

Ceramics & Steatite Corp. It is a stable inductor having an adjustable member which varies

the distributed capacitance.

31 By Felix L. Yerzley, director of research and engineering,

Mycalex Corp. of America, Clifton, N. J.

32 By C. L. Snyder, vice president in charge of sales, General Ceramics & Steatite Corp, Keasbey, N. J.

64 Circulars of the National Bureau of Standards
The device consists of a pair of spiral inductors of silver deposited on spiral ridges pressed

on both sides of a steatite plate. They are

joined by a common connection through a hole

in the plate at their centers. When viewed from

one side their paths are opposite causing the

magnetic fields to aid. A wafer made ot material having a high dielectric constant is

mounted on one side of the plate on a pivot

(fig. 17.5). Deposited on a semicircular segment of the wafer on the side not bearing on

the steatite is a group of singly connected con- ducting bands which make contact with the

pivot. An exterior terminal of one of the

spirals is also connected to the wafer pivot. In

the sample the wafer conductor is connected

to the terminal of the spiral on the wafer side

of the steatite plate. A larger tuning range

may be had by connecting the pivot to the other

terminal of the inductor. Both the steatite plate

and the wafer are ground flat over their mating surfaces.

When the wafer is turned so that its conductors are away from the spiral, the distributed

capacitance is slightly greater than without

the wafer. It is possible to omit the unsilvered

half of the wafer or relieve it so that it does

not bear on the inductor and remove the slight

capacitance caused by its presence. When the

conducting bands on the wafer are over the inductor, the distributed capacitance is greatly

increased for two reasons: First, the bands

appear to the electric field as a uniform plate

because the high dielectric constant of the

material on which they are deposited prevents

the formation of appreciable gradients in the

space between them. Second, the high dielectric also prevents much of a gradient through

the wafer so that to the electric field of the

spiral there will appear to be a conducting plate

very close to the coil. The magnetic field will

be little disturbed by the presence of the wafer

conductors because no closed loops are formed.

Therefore, the introduction of capacitance does

not decrease inductance and a uniformly high

Q is maintained.

Stable characteristics are, of course, obtained

from the ceramic construction. In addition,

economical large-scale production with close

tolerances can be achieved because both cera- mic pieces can be pressed on high-speed machines. The formation of the spirals during

the pressing operation and a grinding operation after firing insure uniformity.

Herlec Bulplates 33

The Herlec type of printed circuits are units

of two or more capacitors with printed con33 By G. Milton Ehlers, Herlec Corp., Milwaukee, Wis.

I 2 3 Figure 17.5. Drawing showing the construction of the

adjustable resonance circuit.

(1) Back view; (2) side view; (3) front view.

Figuure 17.6. Herlec "Bulplates," showing construction.

The unit at the right is coated with a thermosetting material.

nections with or without resistors, all of which

are placed directly onto the ceramic dielectric

plate. This means that the supporting ceramic

plate for the printed circuits, resistors, and etc.,

also forms the dielectric of the printed capacitor and results in the fewest number of soldered

connections. The only soldered connections are

those of the wired leads for external connections. The Bulplates (fig. 17.6) with leads for

external connections are usually covered with a

coating of superstrong thermosetting material

which adds strength to the units and acts as a positive anchorage for the leads throughout

the handling and soldering operations when

assembled into electronic circuits. This mechanically strengthening thermosetting coating

also electrically insulates the Bulplates from

any parts that they might touch and with a

wax impregnation serves as an excellent protection against high humidity. The physical

size and the dielectric constant of the Bulplates

Printed Circuit Techniques 65
S0B5

VC G, B- p a 2

B- B + Figure 17.7. Schematic diagram showing the capacitors

included, in the "Bulplate" of figure 17.6.

are determined by the allotted space and the

electrical capacitance requirements in each specific case. By the proper choice of the ceramic

dielectric in the plates, Bulplates can be successfully used in both the audio and RF circuits.

Where space and cost are individually or collectively of major consideration, Herlec recommends a Bulplate circuit (fig. 17.7) consisting

of capacitors, printed wiring, and leads only ; omitting printed resistors. At the present state

of the art body type of resistors are manufactured automatically in extremely large numbers

at prices less than the value of the capacitor

area required by a printed resistor. The sav- ings are, therefore, greater with a Bulplate

in which the space required for a printed resistor is converted into a capacitor and the space

saving is also greater because most of the ca- pacitors replaced by Bulplate ceramic capacitors are larger than the present day molded

resistors.

Bulplate capacitors have all the well-recog- |[p

nized permanence and high quality of ceramic:!

dielectric capacitors. In the Bulplate-type of I

circuit there are fewer solder connections in Lj the circuits than in cases where the capacitors \\l are soldered into position which reflects itself

into a more trouble-free circuit and a definite

saving in the assembly costs. Additional benefits are the savings in space and weight and the

simplification of ordering and inventory and

stock problems.

An example which is used to express the'

tangible savings in money, weight, and space ; includes four capacitors of the audio coupling , networks (fig. 17.6) of a typical a-c/d-c table \ model circuit.

The following approximate figures picture a j typical case of the tangible savings and benefits of the above Bulplate type of circuit versus

the conventional type with standard component

parts

:

Bulplates Conventional Savings

per M per M per M

Weight .... 4.4 lbs. 30.4 lbs. 26.0 lbs. Volume .... 31.0 cu. in. 388.0 cu. in. 357.0 cu. in. !

With savings of two solder joints at each

the total savings is upwards of 15 percent to

the set manufacturer.

The above figures are approximate because

of varying labor rates and capacitor sizes and

prices throughout the United States but can | be considered balanced well enough to indicate

that at the present state of the art of printed

circuits definite savings in costs and space can

be achieved with printed circuits by the proper

choice of the component parts and sections of

the circuit which are replaced with Bulplates.

18. General Discussion

Summary34

It will be of value to sum a few facts from

the papers presented here today. We may

conclude that the conductors of an electronic

circuit may readily be printed by any one of a large number of successful methods. Many

of these methods, described in detail, have been

proven in practice on production lines; others

have just been started. A principal item requiring further attention to achieve over-all perfection in printing circuits is the development

of improved methods of printing resistors

34 Dr. Cledo Brunetti, National Bureau of Standards.

without the requirements of expert knowledge

or experience.

Not all the components of an electronic cir- . cuit may be printed. The practice has been \ adapted principally to conductors, resistors,

capacitors, inductors, shields and antennas as 1 well as some miscellaneous electronic accessories. We have not printed a vacuum tube,

but there are ideas for incorporating a vacuum

tube within a flat ceramic sheet. We mentioned

one of them in the paper "Printed Circuit

Techniques," which I referred to this morning.

The paint stenciling and the spraying processes are the ones which have been used with

66 Circulars of the National Bureau of Standards
greatest success in making complete electronic

circuits today. Most of the printed processes

are adaptable to manufacture of practically all

of the ordinary low power electronic devices.

The printed circuit technique may be used at

high as well as low frequencies.

The problem of tolerances can no doubt be

overcome. As soon as such a problem is turned

over to industry, American ingenuity finds the answer. Close tolerances have been obtained

in high production as some of the men showed

you today.

A properly designed printed circuit offers

size reduction comparable to the best of standard miniature electronics practice and in certain cases affords a degree of miniaturization

unobtainable by other means. The subminiature transmitters and the amplifiers in which

the circuit is painted on the tube envelope are examples of miniaturization unattainable by

other means. Other examples are the tiny commercial filter and interstage units printed on

high dielectric base plates which are now on

the market and were described earlier. A pi

filter made in this way, consisting of two ca- pacitors and a resistor, has a total volume of

less than six-thousandths of a cubic inch. A

complex audio coupling circuit occupies a volume of less than three-hundredths of a cubic

inch. These are items which are on the commercial market and hence the production problems have been overcome.

Just how much space saving may be realized

depends on the application. Standard electronic components are now available in such

miniature size that complete amplifiers may

be built into volumes of less than one cubic

inch using standard methods. In the printed

electronic circuit, most of the volume is occupied by the base material. By providing thin- ner base materials or better yet by applying

the wiring to an insulated outer or inner surface already present in the assembly such as

the tubes themselves or part of a plastic cabinet,

an additional reduction in volume occupied by

the wiring may be had. Someone has suggested

that radio manufacturers simply print the circuit on the inside of the cabinet and use the

cabinet for storing magazines.

While definite advantages are realized when

printed circuits are produced in large quantities, it is also possible to use some of the techniques, such as the painting process, for making a few tailored sets. The advent in the market of a new printed hearing aid proved that

printed circuits are feasible in moderate quantity production.

A manufacturer does not need to equip his

plant to produce sets that are printed in every

electrical detail to take advantage of printed

circuits. Some have introduced the process of

printing only a subassembly or an interstage

network of a complex set. Others have printed

only the conductors, and have used standard

resistors and capacitors for the remainder of

the circuit.

The radio I showed you in a slide this morning, being engineered by a Florida concern, incorporates the standard interstage coupling

networks available on the market today and

illustrates one method of introducing printed

circuits. The interstage coupling units cost

less than the standard components plus the cost

of wiring and of handling components.

None of the sponsors of the symposium are

in the printed circuit business. We are trying

to help you evaluate both the advantages and

disadvantages of printed circuits. That is why

production quantities and prices, ordinarily not

treated in a symposium of this kind, have been

mentioned.

Another measure of what is possible has been

provided by the development in England of a

completely automatic apparatus for wiring

panels. It is a spraying-milling technique designed for the automatic manufacture of small

a-c/d-c radios. A plastic plate is used with

molded indentations for conductors, inductors,

capacitors and mountings for other components. The plate is fed into an automatic machine which sand-blasts both sides, sprays the

surfaces with zinc, mills the surfaces to remove

the surplus metal, tests the resulting circuit,

sprays on graphite resistors through stencils,

inserts tube sockets and miscellaneous small

hardware, tests the unit again and applies a

protective coating over the panel, all at the

rate of 20 seconds per panel. Each operation

is controlled separately by electronic circuits

and operates only on the arrival of a panel.

Should two successive panels be rejected at any

point on the line, all previous operations are stopped until a personal inspection is made.

All panels beyond that point are continued on

to completion. In a single radio set the need

for hand assembly of 30 components was eliminated and 80 hand-soldered connections were

said to be avoided.

Experienced judgment leads to the conclusion that the new techniques are practical ; that

they promise size reduction comparable to the

best of standard miniature electronics practice

and in certain cases, afford a degree of miniaturization unobtainable by other means. Conventional production methods cannot easily

compete with printed circuits from the standpoint of uniformity and minimum required inspection since each of the latter units is a

duplication of a master pattern.

Manufacturers evaluating the advantages of

printed circuits often compare them with present production costs based on years of engiPrinted Circuit Techniques 67
t

neering design and production experience. The

initial printed assemblies which have come on

the market have in general been found to cost

no more, and in most cases less, than present

costs of conventional methods of production.

Remembering the pattern new developments

usually follow, one can reasonably expect that

if the costs of the new techniques today do not

exceed those attained by years of practice with

the conventional methods, it should not be long

before printed techniques will reflect considerable savings in both engineering and production. When the initial production difficulties are inevitably mastered, printed circuits should

provide substantial reduction of assembly, inspection, purchasing and stocking costs and

problems. , , J

Patents

Before concluding, I would like to say a final

word regarding the patent situation, although

not too much can be said in explicit terms.

Many of the techniques are adaptations of

processes patented long ago and on which patents may have expired. Much of the technical

information is classed as standard knowledge

of the art and is unpatentable. However, cer- tain patents have been applied for by industrial organizations and also by the Government. Because of the large backlog of work

in the Patent Office, it is not expected that final

decisions on these applications will be reached

early. It is believed that most of the patents in

process relate principally to specific processes

and applications. Patents applied for by the

Government may ultimately be made available

to industry on a nonexclusive basis without

charge. However, concerns planning to use

printed circuits commercially are advised to

check the patent situation in the same manner

as would be employed in adapting any manufacturing process.

Questions and Answers

Question

:

In regard to the British method Dr. Brunetti

mentioned in his summary on printed circuits,

I have a question and would also like to inform

the audience that the subject was written up

quite thoroughly in the January-February 1947

issue of the British Institute of Radio Engineers (vol. 7, No. 1). Among other things in

their article was mentioned the ability to produce metal film resistors on these chassis plates

that can dissipate as much as 10 or 20 watts.

Have we been able to get anywhere near that?

Answer

:

Yes, resistors printed on steatite have been

manufactured in this country, with a dissipa68

tion of about 25 watts per sq in. We tried

but unfortunately were not able to get one of

the Sargrove chassis here in time for this;

meeting. They appear to have a workable process which would merit looking into since the

machine is completely automatic and 20 secondsi

per panel is a very short time for assembling.

The paper would be worthwhile reading since'

it contains many good ideas on how to produce'

printed elements of a set. Has anyone in the

audience seen one of the completed receivers

made by this process?—Dr. Brunetti.

I have just returned from a trip to England

where I visited E. K. Cole who does a great

deal of Bakelite molding in that country. They

are molding the plates used in the Sargrove

process and told me they were running into

some difficulties, although these were not insurmountable. As the article indicates, the

capacitors are very thin sections of the Bakelite molding. In the process of molding the

plate it is difficult to maintain the tolerances

required for the set. The other difficulty is the

danger of the Bakelite cracking at that point.

I have not seen the completed receiver.—Mr.

D. I. Cooper. (Editor's Note—Samples of this

receiver may be seen by addressing Lord Pentland, Director, American British Technology,

Inc., 57 Park Avenue, New York 16, N. Y.)

Question

:

What is the highest capacitance and greatest

thickness of the disk-type ceramic capacitors

achieved up to this time?

Answer

:

Capacitors have been manufactured in the

ranges of 7 to 10,000 ^f. They have a thick- ness in the range of approximately 20 to 40

mils.

Question

:

What problems would one run into in applying printed techniques to wide band video amplifiers?

Answer

:

There might be some problems due to the

effects of distributed capacitance of the system

at the higher frequencies. The Bureau of

Standards has some experience with amplifiers

in the VHF range and has obtained satisfactory gain-frequency characteristics. The losses

in the material on which the circuit is printed

may cause a loss of gain at high video frequencies. Rather than try to achieve miniaturization

in the first step in a printed unit, it would be

better to print the circuit in a convenient size

and achieve standard performance first, then

miniaturize the set. The Bureau of Standards

is tackling the problem in two directions. They

are trying to achieve good electronic performCirculars of the National Bureau of Standards

W

re i

JBl

ill

mve

18 I

St
ance in smaller packages by using conventional

subminiature techniques as well as by the

printed circuit method. Both methods appear

very promising.

Question

:

How does the capacitance vary with temperature in ceramic capacitors with a dielectric constant of 4,400?

Answer

:

Standard performance tests on ceramic ca- pacitors show that the capacitance-temperature

curve has a sort of resonance effect. The ca- pacitance increases with temperature up to a

point and then decreases again. A dielectric

constant of 4,400 seems to be rather high, but

fairly constant performance across the temperature range has been achieved with dielectric constants up to 1,200.

Question

:

What voltage ratings are possible in the

ceramic dielectric capacitors?

Answer

:

That depends upon the bodies concerned and

the dielectric constant of those bodies. The

higher the dielectric constant the lower the

voltage rating will be. For a material with a

dielectric constant of 90, a 10-mil sample breaks

down somewhere between 7,000 and 10,000

volts on a flash test. With high K materials

of the same thickness, breakdown is on the

order of 1,600 to 2,200 volts. It has been found

that at the peak dielectric constant breakdown

is more probable than at either end of the

range, or in other words at the Curie point.

Question

:

What has been done in applying printed resistors to flexible bases?

Answer

:

In one application a printed resistor material

has been used in a phonograph pick-up. A

small cantilever beam is made up with a printed

resistor band on its sides so that the needle

flexes the beam, thereby varying the resist- ance. 35 A current is passed through the resistor

and the varying voltage across the resistor is

used as the pick-up signal. Also, Mr. Marsten

of International Resistance Co., described a

cellophane base tape with resistance material

printed on it. He stated that this tape resistor can be bent 160° without breaking or cracking

the resistance film.

Question

:

Can savings in procurement and distribution

be executed in using printed circuits in field

equipment?

35 See "Printed Circuit Techniques" loc. cit.

Answer

:

Savings in distribution and procurement in

field equipment should be possible especially

when modifications are made in equipment that

has been made obsolete by new designs.

Question

:

What will happen when a change of design

has taken place in a unit and the stock of re- placement units is depleted for older equipment? Will this mean that old equipment must

necessarily be scrapped?

Answer

:

If a stock should run down it would be a simple matter to run off a new stock. The machinery and equipment involved is not usually

very complex and since a few stencils and dies

can be stored rather easily, a stock could be

replenished whenever necessary.

Question

:

What is the maximum or the best tolerance

which has been achieved in the production of

printed resistors and are there any automatic

adjusting processes for making printed re- sistors on the line?

Answer

:

It has been found expedient to have very

careful controls in existence in all stages of

resistor processing. The economics of the problem more or less necessitates that in establishing large quantity production. Careful control

is maintained over the processes at all times

to insure high quality of the product. It is better to add a little additional cost to the controls

over a process and particularly in the application of resistors to a plate so that a predetermined value is obtained, than it is to adjust

the resistor after it has been applied. However, the value of a resistor can be adjusted

after it is applied, in two ways : one is simply

by scraping off or erasing36 a little of the re- sistance material if the value is too low; the

other is by painting over a resistor if its value

is too high. In considering the components discussed in my paper earlier, where only two or three components in a set have been sectionalized, a certain number of these units continually keep going back to the beginning of the

line for reprocessing. In other words, the yield

might be 80 percent, and the 20 percent that

do not conform to tolerances must go back to

the beginning of the line to begin over again.

Mr. A. S. Khouri.

Question

:

Has anything been done in connection with

the magnetic paints mentioned earlier in the

day?

36 Editor's note: The standard glass-brush eraser works very

well in this application.

Printed Circuit Techniques 69
Answer

:

Not much information can be given since

most of this work has been in the planning

stage. Preliminary checks made at the National Bureau of Standards indicate that the

inductance can be increased by using magnetic

paint, but with lowered Q's as a result. This

however, indicates that more experience is

needed in making magnetic paints. It would

be highly desirable to obtain a powdered magnetic material and completely surround the

particles with an electrical non-conductor so that it may be painted over an inductor. The

problem is amenable to solution.

Question

:

Has anyone contemplated making resistors

by spraying nichrome?

Answer

:

We have done some work with nichrome in

the field of radiant heating. Nichrome has

been sprayed on plate glass but since this work

has just started not much information is available at present. The process does work, however, and nichrome elements have been produced that should prove useful.—Mr. Gordon

Johnson.

Question

:

One of the speakers has indicated that although a considerable saving of space has been

achieved in his firm's application of printed

circuit production, costs were higher than with

the use of conventional wiring. Is this generally true?

Answer

:

Many organizations have found printed circuits more expensive in their preliminary ex- perimentation with the process. It is only after

the process has been placed into smooth production that one can evaluate the real cost.

Information received from manufacturers show

a substantial saving in cost of the final product

even in limited production quantities.

(This section consists of answers to questions

taken from the questionnaires submitted at the

time of the symposium as well as questions

which have subsequently been raised.)

Question

:

What type of solder is recommended for circuits printed on steatite?

Answer

:

A solder with a 2 to 4 percent silver added

to saturate against further absorption of silver

is recommended. Ordinary solder, however, can

be used without too much difficulty.

70

Question

:

What effect will potting have on the high

ambient temperatures usually associated with

miniaturized circuits?

Answer

:

Potting will increase the working temperatures, hence components must be of higher

quality in order to withstand the higher re- sultant ambients.

Question

:

Is the general saving in cost using printed

circuits due primarily to saving in simplified

stocking and handling problems or are there

other important savings possible?

Answer

:

Savings are not restricted to stocking and

handling but may be reflected from fewer wiring mistakes, fewer soldered connections, simplified testing procedures, and added production

possible when the final design of a unit has

been decided upon.

Question

:

Is there any difficulty in obtaining reliable,

noise-free connections in printed circuits?

Answer

:

Little difficulty has been encountered in this

matter. In those circuits made by firing the

silver paint onto a ceramic, external leads may

be soldered directly onto the conductors with

good mechanical and electrical stability. Good

connections can be obtained on plastics by the use of mechanically rigid eyelets, rivets, or clamps fastened to the printed circuit base plate

before applying the conductor system by any

one of the various methods described. These

eyelets or rivets can then in turn be soldered.

Question

:

Up to this time it seems that most of the

printed circuit applications have been limited

to miniature devices involving subminiature

tubes. Since the cost of subminiature tubes is

more than the others, I am wondering if it would be entirely practical to use standard

tubes in conjunction with printed circuits or are the current carrying requirements too

high?

Answer

:

Printed circuits are not limited to subminiature low-drain tubes, as may be seen from at

least two of the papers delivered here today.

Actually, a printed silver conductor 0.04 in.

wide and about 1 mil thick, fired on steatite is

capable of carrying up to 8 amperes before

opening. This is more than ample for the av- erage electronic circuit but if a greater current

Circulars of the National Bureau of Standards

arrj

lies

1113:11

tl'ril;

Jiiei

Is

bly

Pi

loml w

jftl

aim

It
carrying capacity should be necessary it is a Answer:

simple matter to increase the width or thick- ness of the conductor.

Question

:

It seems that printed circuits require increased designing time and that this is a serious

draw-back. Is this necessarily true?

Answer

:

In some cases it may be necessary to spend

more time in the lay-out of a unit but this is

mainly due to the process being comparatively

new. As experience is gained the time neces- sary in the design of a printed circuit unit

will be less than that required for a unit of

conventional design. In any event the advantages in high speed production possible

with printed circuits greatly outweigh any increased time spent in the design.

Question

:

Is there any difficulty involved in the assembly of equipment using printed circuits with

the addition of standard components?

Answer

:

Printed circuits adapt themselves well to

combination with standard components. A sav- ing in assembly time may be expected because

of the increased simplicity and fewer soldered

connections which are needed to complete the

circuit.

Question

:

How does the use of silver preparations for

printing circuits compare with the cost of or- dinary wire?

Answer

:

Information gathered from various manufacturers using silver preparations find the cost

about the same as that of tinned-copper wire.

If the cost of solder is taken into consideration

the total cost of materials alone will be less

when printed circuits are used.

Question

:

Can printed circuits be protected adequately

against humidity?

Answer

:

Printed circuits lend themselves well to protection against the effects of humidity. They

may easily be coated with a resin thereby making them practically impervious to moisture.

Low-loss resins such as the NBS casting resin

prove very satisfactory for this purpose up

through the VHF range.

Question

:

Will thermal shock tend to loosen fired-on

conductors after prolonged periods of time?

Fired-on conductors, although they may have

a different coefficient of expansion compared to

the base plate on which they have been deposited, do not tend to loosen upon thermal shock

if not too thick. A conductor assumes approximately the coefficient of expansion of the baseplate material since it is fused on, thereby becoming an integral part of the base plate. This

should be a useful property in producing highlystable inductors since the coefficients of expansion of ceramics, glasses, and quartz are lower

than the metal of which inductors are usually

made.

Question

:

Will printed circuits make the testing of completed assemblies more difficult than presentday practices?

Answer

:

Testing of printed circuits should be easier

than with conventional circuits since printed

circuits lend themselves to unitization. Individual units can be tested much the same as vacuum tubes as each unit is a replica of the

master design. One manufacturer employs a completely automatic console for testing

printed circuits.

Question

:

How many manufacturers are planning to use printed circuits in hearing aids?

Answer

:

Two manufacturers already have placed

printed circuit hearing aids on the market ; several more are planning similar units.

Question

:

How complex have printed circuit units been

made to date?

Answer

:

In one commercial product, 45 individual

parts have been replaced by a printed unit;

another unit replaces 173 standard items.

Question

:

What manufacturers produce conductor

paints ? Answer

:

Conductor paints are being manufactured

by E. I. duPont de Nemours and Co., Inc., Wilmington, Del ; Metaplast Co., New York, N. Y.

;

and Sauereisen Cements Co., Pittsburgh, Pa. A

number of other companies have expressed interest in manufacturing paints suitable for

conductor systems.

Printed Circuit Techniques 71
Question

:

Are any manufacturers contemplating the

production of printed circuit transmitters and

receivers ? Answer

:

A small transceiver, in which both the r-f

and a-f sections will incorporate printed circuits, is now being manufactured by one firm.

Several other manufacturers are contemplating

production of equipment utilizing printed circuits.

Summary of Questionnaires

Of the estimated 700 persons attending the

Printed Circuit Symposium, 527 submitted

answered questionnaires. From these questionnaires, prepared by the printed circuit committee, much information may be had on the

outlook for printed circuits in both Government and industry. The pertinent questions

were as follows:

6. Do you believe that another symposium

on PC would be desirable?

In how many months? Where?

7. Do you believe PC will achieve miniaturization?

8. Do you believe PC will result in efficient

design?

9. Do you believe the substitution of PC will

reduce costs?

11. What is your firm's interest in PC work?

(check applicable boxes)

Planning, in near future.

Actively engaged.

General interest.

12. In what phase of PC work is your firm

interested?

Research or development of PC.

Manufacturing PC assemblies.

Using PC assemblies.

13. What in particular would you like to see PC technique applied to?

Describe.

14. What outstanding design feature have you

(your firm) accomplished in the application of PC technique? (Check applicable boxes)

Miniaturization. Low cost production.

Efficient design.

Describe.

15. What difficulties do you anticipate in the use of PC techniques?

The results of these questionnaires are re- corded in the following table 1. The questionnaires have been grouped according to industry,,

Government, and other interested parties.

Table 1.Percentage replying in the affirmative tod

the question asked

Question

Total (approximately 500

replies

Industry

(approximately 300

replies

fioVPVTl - \j yj v ci 1

1

( approxi - mately 150

repl ies

Miscel- laneous

( jxpp l'oxi- m:i i pi v 9ft vm n 1 i p«

Percent Percent Percent Percent

97 96 99 95

7 _ 98 97 98 100

8 _ 95 95 95 100 85 83 86 93 11(a) 1 38 37 45 55

11 (b) 20 23 16 25 11(c) _ 55 60 55 55 12(a) 1 50 53 51 75 12(b) 35 50 14 30 12(c) . __ 60 60 72 55 14(a) 13 16 10 20 14(b) 7 11 1 10 14(c) 10 14 3 10

1 These items total greater than 100 percent since some indi- viduals checked more than one of the three.

It will be noted that the response from industry and Government was approximately the

same and hence in evaluating the results, one

can deal almost entirely with the totals rather

than the break-down. Considering each of the

questions individually

:

(6) Ninety-seven percent felt that another

symposium on printed circuits would be desirable. Of these the majority preferred the symposium to be held within 6 to 12 months and

preferably in Washington, with New York sec- ond, Chicago third, and remainder distributed

between various cities in the East, Mid-west,

West Coast and the South.

(7, 8) Ninety-five to ninety-eight percent of

those present believed that printed circuits

would result both in miniaturization and in

efficient design.

(9) Eighty-five percent felt that the substitution of printed circuits for standard methods of producing electronic circuits would re- duce costs.

(11) It was possible to answer this question

more than once, such as for example if a plant

was not only actively engaged in doing printed

circuit work but also was planning to expand

their activities into other directions in the near

future. Hence, the percentages of replies will

total greater than 100 percent as some of the

individuals checked more than one of the three

parts of this question. It is interesting to note

that as many as 38 percent are planning to use printed circuits in the near future. Twenty

percent are already engaged in this development and 55 percent showed general interest.

(12) As high as 50 percent of the organizations represented were interested in research

or development, 35 percent were interested in

manufacturing and 60 percent were interested

in applying printed circuits to their products.

72 Circulars of the National Bureau of Standards
The figure of 35 percent for those interested in

manufacturing is an average which should perhaps be resolved by saying 50 percent of the

300 replies received from industry showed interest in manufacturing electronic assemblies

whereas only 14 percent of the Government replies indicated this interest.

(14) It was surprising to note that although

printed circuits are considered something new,

as many as 13 percent of the 527 replies showed

that their organizations had already accomplished miniaturization through the use of

printed circuits, while 7 percent had already

accomplished low cost production and 10 percent had already accomplished efficient design.

The first public release of information on

printed circuits was made by the National Bureau of Standards in February 1946. The above

figures are even more significant when it is

considered that a large part of the audience

came principally to learn about printed circuits for the first time.

The answers to question (13), were of such

wide variety that it was found necessary to

group them in broader categories. The question was not answered in every instance by

those submitting the questionnaires, however,

sufficient data were gathered to show definitely

where the main interest in the application of

printed circuits is and should give a good indication as to where one may expect to find

printed circuits used in the future. These categories are listed below in the order of interest

expressed. The magnitude of interest as reflected in the number of replies is shown in

parentheses following the categories. Note the

interest expressed in the application of printed

circuits to the manufacture of home type AM

and FM radio receivers.

1.

3.

4.

5.

6.7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

Home receiving equipment (AM

and FM) (35)

Aircraft radar and navigation

equipment (25)

Television receivers (24)

Radar equipment (general) (22)

Aircraft communication equipment (22)

Military communication equipment (19)

Audio frequency amplifiers (18)

Test equipment and instruments (17)

Citizens radio and transceiving

equipment (17)

Transmitting equipment (14)

Hearing aids (13)

Frequency control equipment (11)

Special circuits (sync, sweep, trigger, etc.) (10)

Ordnance equipment (8)

Switch Boards and panels (6)

Telemetering equipment (5)

Industrial electronic controls (4)

Recording equipment (4)

Computing equipment (3)

Power supplies (3)

Question 15 inquired as to what difficulties

were anticipated in the use of printed circuit

techniques. Typical difficulties expected in the use of printed circuits were question of cost, the

effects of high capacitances in wiring, choice of

base materials, replacement of defective units,

instability of disk-type high dielectric capacitors, the effect of mechanical and thermal

shock, standardization of techniques and materials, tolerances of components, and difficulties in making external connections. This subject is dealt with further in the general discussion.

S. GOVERNMENT PRINTING OFFICE —793811

is Printed Circuit Techniques 73
I

RESEARCH

DEVELOPMENT

STANDARDS

TESTING

PHYSICS

MATHEMATICS

CHEMISTRY

ENGINEERING

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