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Constructing PC Boards
September 1972 Popular Electronics

September 1972 Popular Electronics

September 1972 Popular Electronics Cover - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

Today if you need a printed circuit board (PCB) for prototyping, there is a good chance you will look up a quick-turn company like those found on the PCB Directory website like Bittelle, San Francisco Circuits, or a host of other providers. Costs can be a little as $40 to $50 for three, 3" x 3", 2-layer PCBs and can be delivered in a week or less. A 4-layer PCB of the same size would cost somewhere around $100. Considering how much circuitry can be squeezed into a 9 inch2 board these days, board with surface mounted components on both sides, that's a lot of board for a little money. If you are really in a hurry for your boards, the fabrication companies offer various levels of expedited service for more $$$. There are some people who for one reason or another still prefer to make their own PCBs. For them, this article from Popular Electronics magazine might be a good resource for how to go about it. All the necessary materials like ferric chloride and copper clad substrate material can be purchased from Digi-Key, Allied Electronics, Newark Electronics, and others. Of course you can still use good old-fashioned Vector Board (yep, it's still around) for point-to-point wiring.

Useful Technique to Make Professional-Like Printed Circuits

Constructing PC Boards, September 1972 Popular Electronics - RF CafeBy Robert A. Sullivan & Robert S. Brodsky

Making your own printed circuit board has never been a particularly easy job; but the rewards of doing so are great. You save time and money; and you get a real feeling of satisfaction from having accomplished the task. Not, of course that it's all that difficult. A number of kits for making PC boards are available commercially-varying as to the degree of complexity involved in the technique and the quality of the final result.

It is not always necessary, however, to have a complicated layout to make a good PC board. Described here is a technique that combines many of the best features of different kit manufacturers. The non-camera photographic technique (generally agreed upon as the best approach) requires a mini-mum investment in equipment and is virtually" goof-proof."

The procedure involves five basic steps: laying out the etching guide, making a film positive of the layout, making a negative of the positive, printing the negative pattern on a sensitized PC board blank, and etching and drilling the board. The procedure is much less complicated than it sounds and you will be able to make a commercial-quality board on your first try.

Basic Materials Needed.

Reproduction of printed page shows how patterns are presented in magazine - RF Cafe

Fig. 1 - Reproduction of printed page shows how patterns are presented in magazine, making the first step easy.

Actual size foil pattern - RF Cafe

Actual size foil pattern (above*) and component installation (left) for a 16-terminal tester. If you need only 14 terminals, two end circuits are removed and two leads taken from 16-lead cable.

1/2" centers with the two rows 1 3/4" apart. Drill the lamp holes just large enough for a press fit with the lamp assembly. Determine which way the display is to be observed and make a conspicuously large dot at the number 1 lamp (pin). You can also outline the lamps with an IC layout, being sure to include the notch between pin 1 and pin 14. Use permanent black ink.

With the lamps installed, solder one end of each lamp to the common ground solder pad on the foil pattern. Then, being very careful, solder the other leads to their respective solder pads that are connected to the transistors.

* Note: Not actual size on this web page.

Before you can begin making a printed circuit board, you must have on hand a few basic items. Table I lists the most important. In addition, not listed in Table I but a basic necessity is a supply of sensitized PC board blanks (see Table II). There are three types available. Epoxy fiberglass has the best electrical and mechanical properties, but it is also the most expensive by a wide margin. Polyester boards can yield electrical properties as good as the epoxy-fiberglass type, but they tend to warp - a difficulty which can be overcome by storing them flat. The phenolic board is the least expensive. It is adequate for all but the most critical projects. Its tendency to chip during drilling (which applies to the polyester board to a lesser degree) can be circumvented by careful and patient work.

For almost all projects, a 1/16"-thick board blank with 1-ounce copper cladding on one side will suffice. Buy fairly large board blanks, which cost less per square inch.

A photo reversing kit (such as the one from Kepro) is needed to make the film negative. The kit generates a total of 480 square inches of film negative for about $7.00. Film and developer are available separately. (The developer supplied with the photo reversing kit is expensive, however 70% methyl - NOT propyl-alcohol works just as well and costs considerably less.) Very carefully read and follow all instructions which you will find on a sheet packed with the film.

You will also need a solution for developing the exposed PC beard, Purchase this in gallon quantities if you plan on doing a lot of PC work.

Etchant is used to remove the unwanted copper from the developed board blank. This solution contains ferric chloride which permanently stains virtually anything it touches and corrodes most metals. Handle it with care. Again, buy by the gallon.

Layout materials come next. They include self-adhering black dots and black tape, dry-transfer decals which can be used to title and number component locations on the board (an option you can do without if you are on a tight budget), and sheet acetate for the layout base. For the latter, select clear, untreated acetate in a medium or heavy weight.

Kepro recommends that you use a No.2 photoflood lamp for exposing the negative and board blank, but if you cannot find this item, a standard 150-watt reflector lamp, available at any drug store, can be substituted. Add to your shopping list two 9" X 5 1/2" X 2" Pyrex dishes, to be used for developing and etching the board, and a small plastic funnel for replacing reusable chemicals in their containers.

Filter paper is an option which can pay for itself in the long run. Use it to periodically filter the accumulation of photoresist out of the developer. Cotton swabs with long wooden handles are a must. They are required for developing the film negative, and the handles can be used to lift the board out of the etchant.

A black ink designed for touching up photographic film can be used to correct layout errors on the film negative. And for errors caught before the board is etched, you should have on hand a supply of rub-on resist and a bottle of paint-on resist for corrections.

Finally, you will need No's. 60 and 67, 1/16", and 1/8" drills, all of them high-speed types. The No. 67 drill is particularly useful for drilling IC lead holes.

Step-By-Step Procedure.

Table I, Materials Required - RF Cafe

Table I - Materials Required

* Catalog numbers followed by (K) are available from Kepro Circuit Systems, Inc., 3630 Scarlet Oak Blvd., St. Louis, MO 63122; those followed by (B) are available from Bishop Graphics Inc., 7300 Radford Ave., North Hollywood, CA 91605. Items are also available from distributors such as Allied Electronics and Newark Electronics.

You will most likely be interested, at least at first, in duplicating an actual size PC board from a magazine etching guide. In this case, the hard job of laying out the conductor pattern (see Fig. 1) has been done for you. Lay a piece of acetate sheet over the etching guide and fix it in place with masking tape or staples. You are now ready to make the film positive.

Begin your layout by pasting solder pads on the acetate, matching the dot sizes reasonably close as shown in Fig. 2. After all the pads are in place, put a piece of paper over the layout and use the back of a spoon to burnish them down.

Typical pattern on plastic sheet - RF Cafe

Fig. 2 -  Typical pattern on plastic sheet. Remaining lines must be added.

Ngative is made by photographic process - RF Cafe

Fig. 3 - At the left, the lines and border needed for Fig. 2 have been added to the plastic sheet. Then negative is made by photographic process (right).

Remove the exposed emulsion with a swab and developer solution - RF Cafe

Fig. 4. Remove the exposed emulsion with a swab and developer solution.

Next, use the black crepe tape to interconnect the pads according to the published layout, again matching widths. It is also a good idea to make a border with the tape (see Fig. 3) to assist in trimming the board after it is etched. When positioning the tape strips, allow 1/16" of width for each 5 amperes of current they must handle and 1/32" minimum spacing between strips. Burnish down the strips.

Remove the acetate positive from the layout and very carefully compare the two for accuracy. A mistake now is difficult to correct later. This done, turn off all fluorescent lighting, if any, in your work area. Place a lamp equipped with a 15-watt bulb and shade (to diffuse the light) 8 feet or more away from the work area. Turn on the lamp and extinguish all other lighting in the room. From now until you finish developing the board, this is the only lighting there should be in your work area.

Table II, Board Blanks - RF Cafe

Table II - Board Blanks

* Kepro Circuit Systems, Inc., 3630 Blvd., St. Louis, MO 63122.

Note: All are 1/16" thick, clad on one side only with 1-ounce copper with photosensitized resist coating. 

Open the reversing film container and remove the instruction sheet packed inside. Carefully read the instructions provided. Then, remove the reversing film and cut off enough to make your negative. Immediately return the rest of the film to its light-tight container and seal the container with masking tape.

Place the reversing film, emulsion side down on a sheet of clear glass. Over this, place and center your positive. Complete the sandwich with another sheet of glass. If you are using the No.2 photoflood lamp, expose the negative as instructed. But if your exposing medium is a 150-watt reflector lamp, exposure time will be about 3 1/2 minutes at a distance of 12 inches.

After exposing the film for the recommended time, shut off the exposing lamp. Next, use a swab and the film developer to remove all unwanted emulsion as shown in Fig. 4. Move the swab back and forth, using enough developer to keep the film wet. In a few seconds, the unwanted green coating will begin to dissolve. Continue swabbing until all of the exposed emulsion has dissolved; wash and hang the negative up to dry.

If your board blank must be trimmed to size before being exposed, sandwich it between two sheets of heavy, opaque paper and seal the edges with masking tape. Then use a nibbling tool to trim the blank to size. Immediately seal with masking tape the exposed edges of the portion to be saved and return it to its light-tight envelope.

Now, sandwich the board (copper side up) and film negative between the two sheets of glass, and expose the board according to Kepro's instructions for the No.2, or for 3 1/2 minutes at 12" for the reflector lamp.

Pour board developer to a depth of 1/2" into both Pyrex dishes. Handling it carefully only by edges, place the board into the developer and gently rock the dish back and forth for 2 minutes. Still handling it by its edges, remove the board from the dish and lay it on a protected flat surface where the developer on it can evaporate undisturbed. Meanwhile, pour the used developer into a non-plastic (preferably glass) container labeled "used developer." Thoroughly rinse the dish.

When the board is completely dry, inspect it to see if the developer has done its work. You should be able to see clearly the photo-resist pattern on the copper. If necessary, place the board into the dish containing the clean developer and rock the dish for 45 seconds. Remove the board and again allow it to dry undisturbed. You can now turn on the regular lighting in the room. When it has dried completely, carefully inspect the board's resist pattern. Repair any messy or incomplete areas with rub-on or paint-on resist.

The board is now ready to be etched. To do this, pour etch ant to a depth of 1/2" for small and 3/4"-1" for medium to large boards into a Pyrex dish. Float the board, copper side down, in the resist. It will take about an hour for the etchant to completely remove the unwanted copper, but you should check the progress every 10 or 15 minutes. Do not rinse the board until the entire etching process is complete. When the etching process is complete, however, thoroughly rinse the etchant off under running water. Then remove the resist with fine steel wool and follow up with a cleaning in soapy water.

Do not try to economize on etchant. Once used, it should be discarded by pouring it in a slow, lazy stream down the drain with plenty of running water. Let the water continue to run for about 15 seconds after all the etchant is gone and the dish has been thoroughly cleaned. The final steps in fabricating your PC board are drilling component lead and mounting holes and trimming to final size. If desired, you can rub onto the component side of the board dry-transfer legends to identify component locations.

 

 

Posted November 5, 2019

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