The other day, one of our junior engineers told me enthusiastically, "I solved that oscillation problem on the new circuit. I just put down the probe on that sensitive node I suspected, and the oscillation went away." I said, that's great, but how many picofarads do you think that is? He replied, "I don't know, how many?" I told him to measure it on the Impedance Bridge.
He came back in a few minutes, kind of glum, because the probe had 3.2 pF, and he knew he didn't have room to fit in that much capacitance on the chip, which was already rather crowded. I said, "Oh, don't feel bad, maybe it only needs less than 3 puffs. Try a probe with less capacitance." He went out and measured every kind of probe that we had ever bought, and they were all kind of gross, 2 or 3 or 4 puffs. I said, "No problem, try this prober with a new arm that we just made up." We had fabricated the lever arm out of small strips of copper-clad, with insulation provided by peeling the copper off the glass-epoxy material. He went over and measured 0.22 pF. Then he dropped this probe tip on his circuit and it turned out that even 0.22pF was enough to stop the oscillation. Because he had room to fit in that much capacitance, he was in pretty good shape.
The moral of this story has nothing to do with oscillations, but rather about copper-clad, printed-circuit-board material - glass-epoxy board material. Now, I sort of take this stuff for granted, but I realized that this rather magical stuff is extremely useful. Without it, we'd have lots of problems. I always wanted to write a story about copper-clad for Pop 'Tronics, or one of the other popular electronics magazines for hobbyists, but now that I think of it, this is a better place to write the story. I mean, we engineers can bluff a lot and pretend we know more than most technicians, but if we get to Crunch Time needing to produce a miracle, it's nice to know how to solve problems that even the smart technicians can't. And if we weren't aware of all the good things you can do with copper-clad, maybe we couldn't envision the kind of little fixture we will need to pull off that miracle. I'll try to list several examples of useful, valuable things you can do with it.
First, I'll mention that here around Silicon Valley, several places sell the board to hobbyists, over-the-counter, for about 1 cent per square inch. So, if you want to a make a 6-sided box about 4 in. x 4 in. x 4 in., that will cost you less than a buck - not too bad. Of course, we're talking about the conventional 1/16-in.-thick glass-epoxy material. If you shop around, though, you can get the 1/32 in. and the 1/8 in. and whatever else you need.
Next, I must admit that the tools and techniques for working with it aren't very obvious. If you try to cut up a slab of copper-clad with a saw, you'll probably dull the blade pretty fast. But heavy shears or tin-snips do quite well. Metal nibblers (I'm delighted to say they're available at every Radio Shack) are priceless. In our lab we have a big shear; it can cut precision lines across a big sheet. However, you may not need one of those.
Thermal approaches are also valuable. A good hot iron (with a little solder to help the heat transfer) can easily remove a strip of copper, leaving some pretty high-grade insulator. In the first example, a strip of epoxy board 1/2 in. wide by 4 in. long, with a stripped area just 1/2 in. square, had only 0.2 pF. If we had tried to get low capacitance, we could have made the unclad area 1/4 in. wide by 1 in. long and then drilled holes in it, getting the capacitance down below 0.1 pF. Of course, before you peel off an area of foil, you need to cut with a good knife or saw to define the edges.
Then, of course, when you want to join two sheets together, you need a good hot soldering iron with a decent size of tip, 1/4 in. or bigger (those cute little ones with the skinny tip don't get the foil hot enough fast enough, which tends to cause delamination when you try to soak the heat in there). You can put little dabs of solder along a seam to make a mechanically strong joint. If you want to make something air-tight and water-tight, you need a good continuous bead of solder. It's possible, but a lot of work is required.
Okay - what can you make with copper-clad? You can make BOXES of just about any shape and size. Inherently, these boxes give you good electrostatic shielding, which i s an added bonus. Just remember that the copper gives you no magnetic shielding: If you put one of these boxes near a transformer, such as on a soldering iron, the magnetic flux at 60 and 120 Hz comes booming on in and can (temporarily) ruin a quiet breadboard. If you need shielding from magnetic flux, add some iron, or push the offending transformer far away.
You can make any kind of boxes - square, L-shaped, multi-shielded with cute little compartments and walls, and holes in the walls, and feedthroughs, connectors, etc., etc. These boxes can be extremely strong if you put on a cover that bolts on tight, and/or solder them with heavy seams. You can make 3-sided boxes, or 5-sided boxes. You can bolt on real hinges, or make poor-man's hinges out of copper wire.
You can peel off strips of copper to make zones where a slab of copper is insulated. You can add heat sinks, fins, or any kind of connector. One of my all-time favorite discoveries was that you can use a metal nibbler to cut slots in the side of a sheet of copper-clad, and then slide 5-way binding posts into the slots. They look neat and sit secure; no drilling is required. You can use some of these techniques to get quick results, saving a huge amount of time compared to conventional metal-working techniques.
The next major thing you can do is make structural beams - brackets, levers, cantilevers, I-beams, L-brackets, spacers, shims, push-rods, flying buttresses - just about any kind of levers or beams or supports. Cut first, solder second, drill as needed - it's an awfully creative medium to work in. The technicians in our lab all look at each others' mechanical designs and say, "That's neat." Now that reminds me of some even wilder arrangements that will occasionally be useful.
I was recently helping a neighbor with a serious Meccano set project, and I realized that when I was a kid playing with my Erector Set, I built all kinds of structures. It was a lot of fun, but if I could have played with an equivalent amount of copper-clad and a soldering iron, I could have invented some marvelous machines and structures, just before I burned the house down.
Other things I like to build with copper-clad are breadboards and circuits. When people discard the 1/8-in.- or 1/4-in.-wide strips that they cut off the edge of a sheet with the shear, I scoop them up and save them and use them for little ground buses and power buses. When you solder them to cross-braces (which have insulating stripes peeled off), they're quite rigid and rugged, and very neat for op amps or logic designs.
I'm also compelled to state that some of the cleanest, lowest-leakage (sub-picoampere) layouts in the world use the air over a piece of copper-clad as the insulator. You can buy a clean polyimide board or you can get teflon pc boards or Teflon stand-offs. But plain old air above a crummy piece of copper-clad is just as good an insulator, and usually better.
Another trick I like to use with copper-clad, for a quick-and-dirty applications circuit, is to use a 16-pin wirewrap DIP socket for a 14-pin IC. I take the two pins on one end, and one of the other pins that will be grounded, and solder them to the copper-clad ground plane. All of the other pins I bend up at varying angles, for ease of soldering. I tack a couple of capacitors to the ground plane to use as power-supply bypasses. Then I tack some power-supply wires on them, and I have a breadboard in about 2 minutes, all ready for me to slap in the resistors and other components.
Now, I think you readers ought to know, I don't just sit at home on an evening and type out these ideas, then shove them into print. First I type out a good draft and make 30 copies and show them to my friends. At this point of the story, I threw copies to my Brain Trust, and invited them to show me some more things you can do with copper-clad.
Dennis Monticelli pointed out that when you peel the copper off and file down the tip, you can make a non-metallic screwdriver or a non-magnetic tool for adjusting RF circuits. Fran Hoffart explained how he uses copper-clad for shims and spacers. And at the last minute, I recalled a little framework I had made, to hold up a 35-mm slide in front of my camera, so I could take a photograph of the slide's image and thus make my own copies of slides.
In conclusion, there are almost an infinite number of things you can do with copper-clad, pc-board materials, and I wouldn't mind hearing your neat ideas, too.
Comments invited! / RAP
Robert A. Pease / Engineer
Originally published in Electronic Design, September 26, 1991.
RAP Update: Some prices are up. Halted Specialties (Santa Clara, Calif., 408-732-1573) still sells hobby-grade copper-clad in 6 " x 15 " sheets, but the price is 2 cents per square inch. And Electrical Insulator Supply (Fremont, Calif., 510-490-5855) sells 3' X 4'sheets for about $42, or 2.5 cents per square inch.
