The Design and Layout of Printed Circuit Patterns, November 1952 Radio News

This article describing the "Auto-Sembly" technique for fully automated processing of electronic assemblies appeared in the 1954 issue of Radio & Television News magazine. Auto-Sembly was developed by the U.S. Army Signal Corps, and might have first appeared in print in the 1951 issue of Electronics* magazine. Single-sided printed circuit boards (PCBs) with components mounted on the far side were hot-dipped in a solder bath. All components were through-hole at the time since surface mount was not in the picture yet. The large mass (weight) and relatively low adhesion strength of copper foil to the substrates would not reliably hold the components in place under even normal use. PCBs were just entering the electronics market, and as with many new technologies was enthusiastically embraced and encouraged by proponents, or vehemently shunned by opponents. Given that transistors had only been invented four years earlier, PCBs of the era incorporated vacuum tube sockets (tubes were rarely soldered in except for high reliability applications). The term "tantalytic" appears here, which while I of course know it referred to tantalum electrolytic capacitors, it was new to me. It might have been a General Electric trademarked word.

Complete mechanization of electronic equipment production depends on printed circuit and Auto-Sembly techniques.

Several techniques of printed circuit fabrication are available that are compatible with the Auto-Sembly system of circuit fabrication. Among these are the etched foil process, the pressed powder technique, and the die stamping method.

Reviewing briefly the elements of the Auto-Sembly system, there is the insulating base chassis bearing the circuitry, with conventional components in their proper position on the blank side, and all component leads passing through perforations to the circuit side of the chassis. The application of a suitable soldering flux, solder dipping, and the removal of excess component leads complete the assembly. The assembly thus fabricated may then be treated with a protective coating, encapsulated or otherwise packaged, depending on its ultimate application.

The layout of the circuitry is essentially the same regardless of the final process used to form the conductors. In the following discussion, the etched foil process will be the particular method considered. The layout of a prefabricated circuit in one plane (card type structure) with miniaturization and rapid assembly of components as prime objectives will be discussed first.

With the limitation of layout in one plane being considered, the optimum chassis area is that area occupied solely by the components required by the given circuit. Factors limiting the approach to this minimum area are:

1. Physical position of the components as dictated by the electrical requirements of the circuit

Fig. 2 - Five-stage experimental amplifier fabricated by the Auto-Sembly process from right-angle copper foil-plastic laminate.

Fig. 3 - Assembly details of three-tube, six-stage video receiver with delay line.

Fig. 4 - Experimental live-tube a.c.-d.c. amplifier using printed circuit techniques, showing miniature tube socket adapters. Another illustration of the Auto-Sembly system.

Fig. 5 - Circuit of a 10-light decade counter. See Fig. 12 for the printed circuit layout.

Fig. 6 - Percent power factor vs. frequency for several insulating materials used as a base for printed circuits.

Fig. 7 - Temperature rise vs. current for various widths of 0.00135" copper foil on 1/16" XXXP phenolic laminate.

Fig. 8 - Experimental layout board used as an aid in transcribing electrical schematics to printed circuit patterns.

Fig. 9 - Experimental decade counter assembly showing component placement.

Fig. 10 - Some of the component terminations used in the Auto-Sembly process.

Fig. 11 - Experimental "long-lines" oscillator tuned by metal disc.

Fig. 12 - The circuit of Fig. 5 prepared for a printed circuit layout.

Good electrical design may in some in-stances be a factor in limiting this approach, causing the designer to locate a component in an electrically correct position but one which is wasteful of space. Space factors of 50% may be readily achieved while an 80% utilization of available area may require careful planning and precise positioning of components. Experience usually enables the designer to resolve these conflicting aspects in order to develop a circuit that is both compact and electrically correct, and to position components geometrically in order to obtain an optimum space factor. Each specific circuit requires individual treatment and only general principles of design may be indicated. Circuits with RC networks usually fall into certain repetitive pat-terns that the designer will apply as required by the particular circuit under consideration.

Spacewise, the conductor widths and separation are of secondary importance, as their dimensions are usually small compared to the dimensions of the components used. The spacing of the conductors should not be less than 1/32" if possible and each termination for a component should have at least a 3/32"-diameter circle or the equivalent for a terminal area. These dimensions assure proper solder pickup and the forming of a meniscus of solder at the terminal points to join the component leads securely to the prefabricated circuit.

Small eyelets can be used in terminal locations that may require component replacement, such as tubes when tube sockets are not employed. These eyelets permit repeated soldering operations without any possibility of delamination of the conductor pattern due to the application of excess heat. Conductor widths of 1/32" or larger are recommended although widths of 1/64" have been successfully solder-dipped. Wider lines may be employed to obtain in-creased adhesion of the pattern to the base material provided that over-all economy of space is not affected and no adverse electrical effects are caused.

In general, good electrical design principles which apply to conventional design are equally important in pre-fabricated circuitry, for example, the separation of the input and output circuits of amplifiers for the prevention of unwanted feedback. The shielding of a conductor requires a different treatment in a prefabricated circuit than in conventional wiring. A method which provides a degree of isolation is the placing of the conducting lines or areas which are at ground potential on either side of the conductor to be shielded. It should be pointed out that excessively large ground areas in certain sensitive circuits may introduce unwanted coupling between portions of the circuit due to circulating currents in the ground area. To reduce this possibility, ground returns should be made as close to one point as the pattern layout will permit. Although the capacity between two adjacent conductors is a function of the distance between them and also a function of the dielectric constant of the base material, it may be of interest here to indicate the order of values.

Etched foil conductors of 0.00135" thickness on XXXP phenolic with a separation of 1/32" have a capacity of approximately 1 μμfd. per linear inch. Other design factors, such as the current a specific conductor may be required to carry, will in some instances determine the width of conductors or the thickness of the conducting foil employed. Continuous current carrying capacity of a conductor of copper foil based on a 40°C rise in temperature for a conductor 0.00135" thick and 1/32" wide is 3.5 amperes; a conductor of similar width, 0.003" thick, has a capacity of 5.5 amperes. Conductor resistance may be a design consideration in some circuits. The specific application of the circuit may impose certain electrical and physical requirements on the base material. Manufacturers of metal foil-plastic laminates have provided the designer with many base materials having characteristics suitable for various applications. Phenolics in various grades, Teflon, glass fabrics impregnated with melamine, silicone, epoxy resins and polyesters provide a wide range of electrical and temperature characteristics (Fig. 6). Copper and aluminum are two of the commonly used foils available laminated to these base materials.

The conversion or transcribing of the electrical schematic to a wiring pattern in one plane with no crossed conductors requires an entirely different approach than conventional wiring .techniques. Components are used to bridge conductors and interconnect portions of the circuitry. Where the circuit pattern would be made unduly complicated in avoiding a "crossover," a staple-like wire conductor may be used to bridge the printed circuit conductors. This is applied from the blank side of the printed circuit chassis in the same manner as a component. Redesign of the circuit pattern may in some instances eliminate the necessity for its use.

As an aid to the final pattern layout, it is often desirable to redraft the electrical schematic and to rearrange the circuit elements in a form that will be more adaptable for transcribing to a printed circuit pattern. In this redrafting, advantage should be taken of all arrangements of components to avoid as many crossed conductors as possible. From the redrafted schematic, the actual development of the pattern may be initiated. The usual circuit contains electron tubes, and the physical location of the tube or socket terminals fixes originating points for the evaluation of the complete pattern. If the designer has certain dimensional specifications to meet, these may alter his approach to the over-all design.

To facilitate the layout and drafting of printed circuit patterns, a template may be used permitting accurate location of terminal points for the most commonly used components. This device employs outlines of the exact dimensions of the components with properly positioned holes to locate the points for the component leads. The layout template is of a clear, transparent plastic material and follows the general principles of templates now available for drafting electrical and chemical symbols, etc. This transparent template permits the designer to visualize the area the component will occupy on the blank side of the printed chassis and allows for the positioning of adjacent components to best advantage. The holes located at the ends of the outlines, designated on- the template as fixed capacitors and resistors, provide for the accurate location of these component lead terminations on the conductor pattern.

Layout work as discussed thus far has been restricted to actual size. The layout template has been designed for this approach. Special components not indicated on the layout template may have their lead terminations located by use of the actual components to determine dimensions. Layout of all the components may be accomplished by this method if so desired. It may also prove convenient at times to use the actual components in preliminary planning of the layout in order to achieve a general over-all view of the space requirements, and then employ the layout template for dimensional accuracy. As some designers prefer to layout original drafts two or three times actual size, a template to this scale may be used. After completion of the final draft of the pattern, photographic reduction is employed to prepare the pattern in its final form. In the experience of designers within the miniaturization group at the Signal Corps Engineering Laboratories, it has been found advantageous to layout the circuitry to actual size in the preliminary draft. Patterns of subminiature circuits requiring a greater degree of accuracy may be laid out in this manner to obtain close spacing and optimum arrangement of components. This pattern may be reproduced three or four times the original size to guide the draftsmen in preparing an accurately dimensioned drawing of the pattern to this scale. Advantage may be taken of the enlarged drawing to include lettering, terminal designations or other circuit details, before reduction to the required size. Precision of conductor size and spacing is achieved through this photographic reduction.

Mention should be made of another aid to circuit layout. This device is a perforated transparent plastic board having coordinate lines . ruled on it which form 1/4" squares (Fig. 8). The perforations are located at the intersections. The scale used is four times actual size and flat paper mockups of the components are utilized, the mock-ups also being four times actual size.

Conducting lines are simulated by hook-up or enamel wire that is easily bent and will retain its shape. Component mockups are held to the board by the insertion of the wires through the mock-up terminals and into the perforations in the board. The ruled lines aid in producing a symmetrical layout. This layout board readily enables the designer to arrange and rearrange the position of these mockups in order to achieve an optimum space factor. Upon completion of the design, a copy is drawn by a draftsman in preparation for the final processing.

Another system of layout may be used which permits a direct transition from the original layout to a photographic transparency. This is accomplished by using black cellophane adhesive tape of the proper width to simulate 1/32"-wide conductors four times actual size, and by placing the tape on a white base plate - preferably of plate glass. Component mockups (four times actual size) are used to assist in the layout, The designer then proceeds to layout the circuit in the usual manner in accordance with the procedure outlined previously. Upon completion of the circuit layout, the mockups are removed and the base plate bearing the black tape is photographed. Reduction to actual size is accomplished by this photographic step. The contrast between the black conductor lines and the white background permits direct photography without loss of detail.

In many instances it has been found necessary to employ decking of the one-plane card type structures in order to attain an optimum form factor as related to the over-all assembly, or to fulfill other specified dimensional requirements. An assembly fabricated for a specific application requiring a cylindrical configuration employing decked printed circuit discs is shown in Fig. 1. An exploded view of the assembly is also included. The decking principle has in many cases alleviated fabrication difficulties encountered by conventional techniques where dimensional limitations were imposed. Decks with circuitry on one or both sides may be used for this purpose with eyelets as interface connectors. This decking principle may be used, regardless of circuit complexity, and still retain the solder dip feature. It may be convenient for maintenance purposes to provide such integrated decks with small pin and jack assemblies in order to permit a rapid connect-and-disconnect feature without sacrificing space. Layout of the decks is essentially similar in all details to the one-plane card type structure, with the exception that provisions are made to have portions of the circuitry drop through to the next and succeeding decks either by extended component leads or pins, as shown in Fig. 3. A minimum of inter deck connectors, such as the signal and power terminations, should be used. An exception to this may occur where a better distribution of components would be obtained by placing components on succeeding decks and providing inter-deck connectors. The usual practice has been to place complete sub-circuits on one deck. For example, the r.f. section of a superheterodyne receiver circuit would occupy the first deck, the mixer and oscillator section the second deck, the i.f. section the third deck, and the second detector and audio section the fourth deck.

Circuits have been processed successfully on curved and right-angle surfaces, complete with components, to conform to the contours of the over-all assembly. In laying out circuits requiring special configurations, the designer should follow the same procedure as outlined for laying out circuits on one plane. An insulating chassis formed to provide a right-angle surface offers a convenient means of mounting tubes in a horizontal plane without the use of a right-angle socket. The conventional 8-pin subminiature tube socket may be placed on the vertical portion of the right-angle insulating chassis so that the socket terminations protrude through the proper perforations in the vertical portion to connect to the continuous conductor pattern. Solder dipping is carried out in the usual manner except for two immersions in the solder bath, one to solder the horizontal portion of the circuitry and another to solder the vertical portion. See Fig. 2.

To illustrate the conversion of a typical circuit from the electrical schematic to the prefabricated wiring pattern, a decade counter circuit is shown in Fig. 5. This same circuit as transcribed to a printed circuit pattern is shown in Fig. 12. The layout of the conductor pattern progressed from left to right, originating at the input terminal, as in the schematic. Components were positioned as close together as possible. The non-critical nature of the circuit permitted this positioning of components without introducing any unwanted coupling between portions of the circuit. The many components employed in this circuit provided ample opportunities to arrange the circuitry without any resort to wire conductors for bridging. Due to the over-all design requirements, the tubes were mounted on a heat shield above the other components, and the flying tube leads were terminated at points on the prefabricated pattern. The indicating lamps are located in a vertical position along the edge of the prefabricated conductor pattern. Over-all requirements for a flat rectangular package to be stacked with identical units indicated the use of a single card type structure rather than a decked assembly. Fig. 9 shows the components in place on the etched foil-plastic laminate card.

The only requirement for a component to be applicable to the Auto-Sembly process is that it have extended wire-like terminations to pass through the printed circuit chassis and permit dip soldering to the prefabricated circuit conductors. All the standard JAN components having the requisite terminations are therefore available to the designer, as well as the various networks of resistors and capacitors marketed as "printed circuit" assemblies. The degree of miniaturization possible is limited primarily by the size of the required components for a specific circuit. Some components have terminal hardware that can be modified to permit their acceptance into a printed circuit.

Connectors (plugs and receptacles) are available for the design of plug-in printed circuit assemblies, permitting integration of subassemblies into a complete functioning item. An adapter socket, recently developed, accepts protruding hollow pins that are swaged and soldered to the conductor pattern, and allows the use of miniature, octal and noval type tubes. (See Fig. 4). Subminiature tube sockets, such as the Cinch experimental socket with extended base pins, are used in the decking (stacking) of assemblies where it is desired to extend the tube leads through two or more decks of printed circuitry. Right-angle subminiature tube sockets permit the designer to mount subminiature tubes flat against the printed circuit chassis. Several miniature tube sockets compatible with "Auto-Sembly" have recently been developed, some of which are commercially available. New types of connectors, sockets and switches compatible with the Auto-Sembly process are currently under development by the Signal Corps.

Hi-K ceramic capacitors are available in capacities and dimensions that lend themselves to subminiature assemblies. Paper dielectric capacitors, with and without molded phenolic cases, are also utilized in printed circuit designs; where the circuit requires the use of an electrolytic capacitor, the type BBR provides a choice of sizes and values. Tantalytic capacitors, both the polarized and non polarized types, also provide high values in small volume. Subminiature silvered mica capacitors have dimensions that permit their use in subminiature design.

The majority of fixed resistor requirements for miniaturization are met by the lower wattage ratings of the carbon composition type of fixed resistor. Standard resistors, including the type BTR resistors, adapt themselves to miniaturized assemblies. Small finger-tip type potentiometers are now avail-able that have stiff right-angle terminals directly applicable to printed circuits. For audio frequency applications subminiature transformers, while not having stiff terminations, are components of small size and weight suitable for miniature equipment. The many miniature and subminiature tubes now available provide the designer of subminiature equipment with a wide selection of electron tube characteristics to meet design requirements.

Some mention should be made of printed components. Printed circuit inductors may be designed either in the conventional flat spiral or in square or rectangular shapes. Size limitations usually restrict these inductors to the higher frequencies. Some variation in effective inductance may be obtained by the use of a tuning disc of conductive or magnetic material placed over the printed inductor. Tuning is accomplished by varying the distance between the disc and the inductor. Long line oscillators (Fig. 11) have also been successfully fabricated on various low-loss base materials. Tuning may be achieved by the variable metallic disc method or a movable shorting bar arrangement. Fixed capacitors formed by interleaved conductors are possible when capacity requirements are low. Capacitors using the base material as the dielectric can be formed when double-faced metal-clad base materials are employed. The low value of capacity, high area consumption, and relatively high losses inherent in such capacitors restrict their application in this form.

In conclusion, it should be noted that the layout and design of equipment utilizing printed circuit techniques requires not only familiarity with electrical design principles but some imagination and ingenuity in the application of these principles. With experience, the designer should develop skill and dexterity in laying out critical and complex circuitry. This experience will enable him to determine quickly the correct and most rapid approach to take in transcribing an electrical schematic to a prefabricated conductor pattern.

The Design and Layout of Printed Circuit Patterns November 1952 Radio & Television News

The Design and Layout of Printed Circuit Patterns
November 1952 Radio & Television News