Printed Circuits Are Here to Stay
November 1959 Radio-Electronics

November 1959 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

I was very surprised to find an example of a rather significant flexible printed circuit substrate in this "Printed Circuits Are Here to Stay," article appearing in a 1959 issue of Radio-Electronics magazine. That era was when much debate, particularly amongst service people, was occurring regarding whether printed circuits would be a welcome replacement for the previous point-to-point wiring method of connecting components and cables. Proponents appreciated the neatness and compactness, while opponents didn't like the lack of robustness and intolerance to heating and pulling off mounted components. What was even more interesting was the mention of a "Persister" element, that was made by Ramo-Wooldridge - the "RW" part of "TRW" (Thompson Ramo Wooldridge, which merged in 1958). A Web search on persister turns up only biological cells which resist antimicrobial treatments. This computer persister worked at near absolute zero as a superconductor, and could switch in "10 millimicroseconds" or in modern lingo 10 nanoseconds - not bad for 1959!

Printed Circuits Are Here to Stay

By Allan Lytel

Resistors, capacitors, switches - all printed on circuit boards that may be shaped to suit the job - give electronic designers a new dimension to work in

From a modest beginning just a few years back, the uses for printed circuits have grown until today when they are found in almost every type of electronic instrument. They are used in satellites and missiles, in radio and TV receivers, and in computers and industrial control systems.

These circuits take many forms and many are unusual. Stromberg-Carlson uses a book type construction, shown in Fig. 1, where the "pages" of the book are printed-circuit boards. They are interconnected by flat multiconductor cables so the unit opens up for servicing and folds flat when in use. Saunders Associates uses a flexible circuit which can be applied to curved surfaces. (see Fig. 2), Centralab made a four-transistor amplifier (Fig. 3) in a package only a little over an inch long. Only the input, output, volume control and battery are external.

Other types of printed circuits include ceramics for high-temperature use, plated circuits for resistance to corrosion and wear, and miniaturized printed circuits including built-in transistors.

Flush switches with very long operating lives at lower cost are in production. They are plated with metals of greater wearing qualities than copper. Connectors make it possible to remove circuit boards from equipment for testing and repair. Connectors are usually molded and have from 6 to 44 connecting terminals on a single strip. Contacts are made of phosphor bronze or beryllium copper. Some are gold plated to prevent corrosion.

Cables for use with printed-circuit connectors are available. These are ribbons of cable made from many flat copper conductors. Typical construction encases the conductors in transparent polyester insulation.

Circuit packaging includes stacks of ceramic wafers, single laminated plastic plug-in boards, and sandwiches of two single-faced boards with the components in between.

Computer Memories

Large-scale data processing machines use electronic memory devices with small magnetic cores for one form of data storage.

Printed circuits are appearing in some new forms for memory units in digital computers. Ramo-Wooldridge has experimental printed units for high-speed low-temperature use. Fig. 4 shows one of these units. It is about four times the size of the production models. Known as a Persister, this printed memory unit operates at temperatures of nearly absolute zero or -459.6° F and because of its superconductivity at extremely low temperatures, it can perform computer switching operations in 10 millimicroseconds or 1/100 microsecond.

Also, MIT has shown that it is possible to use printed wiring to connect the small magnetic cores now used for memories. This interesting and highly specialized type of printed circuit has been developed at the MIT Lincoln Laboratory as a three-dimensional circuit of etched wiring which goes through holes in ferrite cores of memory planes. The TX-2 computer stores 2.5 million binary digits in a 256 by 256 by 38 stack of ferrite cores. The standard 64 by 64 core modular plane requires four insulated wires which are threaded through each of 4,096 cores, having 0.050-inch diameter holes.

The new technique uses collimated light to produce, in a single exposure, the image of the complete wiring for a memory plane. With collimated light means the pattern mask does not have to be in direct contact with the sensitized laminate to produce a sharp image. The cores are mounted in holes in the base laminate. The entire board is then coated with a plastic to hold the cores in place. The board is then coated with copper, by electroplating. Wiring of the cores was arranged to avoid crossovers. The finished wiring pattern extends through each of the cores and connects to another pattern on the other face. The method is much faster than current production methods for memory planes.

Printed Switches

Computer switches are only one type of switching. For lower speed switching, printed switches are used. These may be ordinary printed wiring or flush wiring. There are cost savings in complicated patterns made of printed wiring as compared to other, older methods.

Switches, commutators and slip rings, especially in complex patterns, are a good example of this. Many switches that would be impractical by any other method can be made economically, even in small quantities, using printed-circuit techniques. The circuit itself can often be combined with the switch contacts for still greater savings. Silver, gold or nickel-rhodium platings provide low contact resistance and long life. With careful design, a life of many millions of cycles is being obtained from printed-circuit switches.

Fig. 5 illustrates a complex multiple group of switch patterns with a plug connector having leads from the circuit switch segments. In regular printed wiring there are dead segments between the contacts to prevent bouncing.

In flush circuits the etched wiring is forced into the laminate base after heating the base material. Since the conductors and the base are in the same plane, the moving contact can pass over the switch plate without bouncing. Aerovox has developed a method of producing flush circuits using silver. Copper-foil switches are often plated to extend their useful life.

A new type of switch is the HEP (Hartsock Etched Plate) (Allison Laboratories, Inc.). It is made of a printed circuit, shown at the top of Fig. 6, and a contact board shown beneath. As this contact board is rotated the spring contacts, by their circular motion, do the switching. A cam is used to rotate this contact plate and guide pins control the board movement.

Switches of this type offer simplified switching techniques for many applications. In one example, with silver contacts touching rhodium plating, there was no appreciable wear after 110,000 cycles of operation. Since only the contact plate is moved, very little force is required for switching.

Circuits on Ceramic

Ceramic circuits have long been used for R-C networks. Printing resistors and capacitors on ceramics goes back to the early beginnings of printed circuits.

Ceramic-based circuits were the first type in wide use. The proximity fuse, which was a miniature transmitter and receiver mounted inside an artillery shell, called for mass production of highly reliable circuits. For this purpose, the National Bureau of Standards developed the printed circuit using a ceramic base. Silver paint was used as the conducting medium and it had to be fired at between 900° and 1,400° to burn off the binder and fuse the silver into a highly conductive metallic pattern.

Because of this high temperature, only a ceramic could be used as a base. Steatite is a dense ceramic material that has great strength and hardness. It also has excellent electrical properties which are not affected by high temperature and humidity. The powder is molded or pressed into wafers with the required holes and notches. After firing, at temperatures up to 2,400°F, the silver conductors are applied and the wafer is again fired (this time at a lower temperature) to fix the liquid silver into solid metallic conductors.

Electronic circuits printed on ceramic boards are finding increasing acceptance with the growing use of printed wiring board chassis. Replacing many components with one part saves solder joints and simplifies and reduces the size of circuit boards. Also, one part instead of several parts reduces the number of automatic insertion machines resulting in more flexibility in changing from one receiver to another and a reduction in capital investment for the manufacturer.

In each design, circuit response is made to duplicate as exactly as possible the response obtained from separate components. Important stray capacitances are carefully reduced to an absolute minimum. The values and tolerances of the separate components and a schematic showing the function of the circuit in the overall unit give our engineers data for a mutually acceptable design.

Reproducibility in production is assured by using screen printing. Several patterns are reproduced from the master negatives used for the original samples. Fig. 7 shows two circuits used in radio receivers. One is a diode-pentode coupler and the other a diode-triode coupler. Television receivers have other printed ceramic circuits. Some are shown in Fig. 8 (a horizontal afc circuit, a sync takeoff and a vertical integrator). Many other circuits are also available.

A natural extension of ceramic-based circuits with printed wiring, capacitors and resistors is to add active elements to make a complete amplifier. Centralab has combined the elements with a transistor. They end up with a single-stage amplifier about the size of a pencil eraser. A group of these are used for a hearing aid as in Fig. 9.

Circuits with ceramic bases are also being used to extend electronics to high-temperature operation. Some of these special boards are shown in Fig. 10. Welding is used to connect the wiring and components on boards for high-temperature applications.

Posted April 5, 2022