Your RF Cafe
|December 1949 Radio & TV News|
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged.
Most people today under 30 years old have probably never seen the mechanics or electronics inside their many personal devices. Everything is so miniaturized and optimized that if something does go wrong, there is little chance of the owner repairing it. Instead, the phone, television, stereo, microwave oven, whatever, gets thrown away and a relatively cheap (compared to paying for a repair) replacement is purchased (or stolen). Besides, if the item was more than two years old, it was on the verge of obsolescence anyway.
Up until around the early to mid 1980s you had a fair chance of being able to repair an electronic circuit if trouble arose because at least with commercial products printed circuit boards (PCBs) were usually 1- or 2-sided and the components still had leads protruding from the sides of the packages. A $10 Radio Shack soldering iron and some solder wick was sufficient to remove and replace just about any failed component. Home brew PCBs could be made to nearly the same quality as commercial versions using a resist ink pen (basically a Magic Marker) and a dish of ferric chloride etchant liquid. A drill press helped with making holes for the component leads, but a hand drill would get the job done. No more, though. If you are resourceful enough to get your cellphone or camera open without destroying it, you will find a very neatly laid out, extremely high density PCB with parts so small you might wonder how they could work at all. Forget servicing the thing with a soldering iron and a pair of pliers - you will need at least a hot air wand, a magnifier, tweezers, and, of course, electrostatic discharge (ESD) preventative gear.
In 1949 when this article appeared in Radio & Television News, printed circuits were just coming onto the scene. Bakelite, steatite, and ceramic substrates were typically used at the time. Some processes were already using printed resistors and small-value inductors via silk-screening techniques.
Part 2. A discussion of the techniques and equipment used in making printed circuits for home-built units (January 1950).
Thanks to Terry W. for providing this article.
Part I. A review of printed circuit techniques. To be concluded next month with on article on how the experimenter can apply, in a simplified form, printed circuits to home constructed units.
By John T. Frye
This typical group, only a few of the many commercially built units already produced, is an example of how Centralab's printed circuit audio amplifier has been received by the industry.
Since that time, the printed circuit has thrust its tentacles into every portion of the electronic field; and it has miraculously shrunk everything it touched. Hearing aid amplifiers, complete with batteries, that are smaller than a cigarette package; personal radios that can be cradled in the palm of the hand; radio and television subassemblies occupying only one-tenth the space needed for conventional assemblies and requiring one-half as many soldered connections for installation: these are but a few of the achievements of this new process, and the surface has barely been scratched. Every day sees new applications of this method by which space is saved, weight is reduced, assembly is simplified, and cost is cut.
Every electronic worker is certain to come in contact with printed circuits in increasing number, and it is the purpose of this article to prepare him for that contact by making him familiar with the various methods and techniques by which these circuits are produced commercially and then showing him how he can develop and experiment with his own printed circuits.
First, it should be clearly understood that the term "printed circuit" covers any reproduction of an electrical circuit upon an insulating surface by any process. Essentially it changes a bulky three-dimensional array of electrical parts and conductors into a compact and very nearly two-dimensional arrangement. An example best shows how this is done:
Next, suppose we have several different solutions of finely powdered graphite or lamp-black, a resin binder, and a solvent. We can experiment with these until we find just the right combination of mixture, thickness, and length of line needed to produce resistances equal to R1 and R2; and then we carefully paint in these resistance lines at the proper points between the silver conducting lines already drawn. Then we place our little plate in an oven and raise the temperature to the point where our lines of paint will be "fired" directly to the ceramic base, adhering to it with a tensile strength of 3000 pounds to the square inch. Finally we solder tiny ceramic condensers of the proper values across the gaps representing the condensers, and then we attach flexible leads to our silver paint at points 1, 2, 3, and 4. The result is a "printed circuit" that will perform exactly the same as one using conventional components, but our printed sub-assembly will be no bigger than a postage stamp and require only four soldered connections to be made by the radio assembly-line operator. A commercial version of just such a printed circuit is shown in Fig. 1.
Such a manual process, while pointing up the difference between printed and conventional circuits, obviously could not be adapted to mass production. Various stenciling methods are the answer to producing more uniform circuits at higher speed, and the silkscreen process is one of the most successful.
In this system, a fine-meshed silk screen is tightly stretched on a wooden frame and covered with a photosensitive material that becomes insoluble when exposed to strong ultraviolet light. A photographic-positive mask of the exact shape of the required conducting circuit is placed on top of the screen, which is then exposed to the rays from an ultraviolet lamp. Finally, the portions of the film protected by the mask are washed away in cold water, leaving a stencil of the conductor design to be printed. All four of these steps are clearly illustrated in Fig. 3.
This finished stencil is held securely against the base plate to be printed; and the circuits can be printed on practically any insulating material, or even on conducting material that has been coated with a non-conducting film, such as lacquer, and a quantity of silver paint is placed at one end of the screen. A neoprene bar, or "squeegee," is moved across the top surface, forcing the paint ahead of it and down through the open mesh of the design, as is shown in Fig. 4. When the screen is removed, the surface of the plate is found to be printed with an exact, sharp-edged, uniformly-thick design of the required conductor circuit. A second stencil can be used to print the resistors in their proper places. The paint is fired to the base exactly as was done before. This process is shown in Fig. 6. In Fig. 7 are displayed base plates at various stages of completion.
Brushing and stenciling with a silk screen are not the only ways in which the conducting and resistor paints are applied. For example, a decalcomania, .on which the circuit is printed on a thin flexible film that can be transferred to the final surface, is useful in applying the circuits to cylindrical or irregularly-shaped objects. The film is removed by firing.
Most standard printing processes are also used. As a single example, the required design can be raised on the face of a rubber stamp, and this stamp can be pressed first on a pad of conducting ink and then on the surface to be printed. Plating of this printed design will increase its conductance if necessary. In the same way, other printing processes such as engraving, lithographing, and intaglio are also employed.
You old-timers who used to draw your own grid-leaks with a lead pencil were using a form of printed circuits that still may have possibilities. Pencils having "leads" of varying degrees of conductivity, or pens filled with conducting inks are being used experimentally. With such devices an experimental circuit could be drawn and constructed ready for testing all at one and the same operation!
Condensers can be painted, too, by employing silver disks painted on opposite sides of the base plate so that the plate material becomes the dielectric. If the plate is constructed of high-dielectric material, condensers of reasonable capacity can be obtained by this method; otherwise, miniature thin-disk ceramic condensers are often employed by soldering them with a low temperature solder directly to a silvered area on the base.
Printed inductors are also used, especially in the low-inductance values. Spiral forms are used on flat bases, although the more conventional forms can be used when the circuit is printed on the tube envelope or a cylindrical base plate as is shown in Fig. 9. The inductance of a spiral conductor can be increased by covering it with an insulating layer of lacquer and then painting another spiral right on top of it and connecting the two in series, painting another spiral on top of that, etc. The distributed capacity and the Q of the circuit required are the limiting factors to the usefulness of this method.
Placing a layer of magnetic paint, made of a colloidal suspension of powdered magnetic material, both beneath and above the spiral conductor, with insulating layers serving to protect the turns of the inductance from shorting. will also increase the inductance.
The spraying of conducting films on insulated surfaces is another method of printing circuits. The same paints can be used in paint spray guns as for the stenciled-screen process; or molten streams of metal can be sprayed through locating stencils. Guns are available in which the metal to be sprayed is fed into the gun in the form of a wire, where it is heated to the melting point by a hydrogen-acetylene or other flame. Compressed air is used to atomize the molten metal and to drive it on to the work. This molten material can .be sprayed on wood, Bakelite, plastic, and even ceramic surfaces.
One popular method employs a plastic base plate. This plate is sandblasted through a mask so that shallow grooves are cut where the conductors are needed. These grooves are sprayed full of molten metal, after which the surface can be milled, leaving conducting lines that are flush with the surface of the plastic base plate.
Still another scheme uses an insulated base plate with a thin evaporated coating of conducting metal. This is covered with a photosensitive film and exposed to light through a mask. The film is developed so that the portions exposed to light are removed, and the remaining portions, outlining the desired circuit, resist an abrasive spray so that the protected portions beneath remain intact while the rest of the metallic coating is cut away by the sand blast.
Another method of producing "printed circuits" is by chemical deposition. This method is not used much on a commercial basis because of the very thin layers deposited and other technical difficulties, but it consists essentially of depositing a thin silver coating on a masked surface by the same chemical methods that are used in silvering mirrors. Increased conductivity can be secured by repeated silvering or by plating.
Cathode sputtering and evaporation are two other processes for depositing the metallic film. In the former, the material to be deposited is used as a cathode and the masked base plate is used as the plate of a temporary vacuum tube. The "plate" is maintained at a high positive potential with respect to the cathode, and the latter is raised to a volatizing temperature. The metal particles emitted by the cathode are attracted to and deposited on the base plate through the stencil openings.
The evaporation process is the same except that the plate is not maintained at a high positive potential. The cathode material is simply heated in the vacuum until it vaporizes on to the work. This permits the use of non-metallic as well as metallic base plates. In neither case is the film deposited thick enough to be used for conductors, but this can be overcome by plating.
The radio technician is very familiar with one form of printed circuit: the die-stamped loop antenna. This is produced by placing a thin sheet of copper on top of a composition or bakelite panel with a layer of thermoplastic cement between. This sandwich is placed in a punch press, and at one stroke the metal is cut into a helix and is bonded to the panel.
Dusting is the final major method of printing circuits. This consists of depositing a layer of metallic dust on a base plate along the lines where conductors or resistors are required and then raising the temperature sufficiently to drive off the bonding material and to fuse the metal particles together and to the plate. The entire plate can be covered with an adhesive material and the dust applied through a stencil, or the adhesive material can be applied through the stencil and then the whole plate subjected to dusting, with the same results.
While an attempt has been made to touch on all of the methods ordinarily used for printing circuits, the new industry is advancing so rapidly that one cannot be sure how long this will hold true. Very recently, for example, the Glass Products Company of Chicago announced a new process, "Micro-screening," which they claim has several advantages over the silk-screen methods. Unfortunately, because of current patent proceedings, details of this new method are not available.
Several illustrations are given to show the wide variety of devices to which printed circuits are applied. For a more detailed discussion of the various methods discussed in this article, the author recommends the purchase, for 25c, of "Printed Circuit Techniques," by Cledo Brunetti and Roger W. Curtis. This National Bureau of Standards Circular 468 can be obtained from the Superintendent of Documents, U. S. Government Printing Office, Washington. D. C. An excellent group of references for further reading will be found in the back of this booklet.
Part 2 of this article will be concerned solely with explaining and illustrating how the experimenter can design and construct his own printed circuits with materials easily obtainable. (To be continued)
Posted May 25, 2013