Recently, we are working in class on a variation of the Rock and Roll Speakers from Fashioning Technology. Rather than using perfboard for the circuit, we’ll be burning our own circuit board. The chips are through-hole LM386’s, but I don’t think it is realistic to have the students drill 8 aligned holes on the circuit board at this point in the course. Jockeying for time on the drill press would create a logjam in the project. There is an excellent primer on printed circuit board etching in MAKE, Volume 02. After thinking this through a bit, I came up with an idea to turn the through-hole component into a smd component. The technique is a bit like the design of the Broadcast Your Podcast FM transmitter circuit, which just has you solder the components together in pools of solder on chips of board. However, in the BYP project, there is no custom board, the blank copper clad board is cut with a utility knife to create the various connecting points.
When the chips finally arrived from Electronic Goldmine, I looked up the datasheet for the LM386 to get the measurements. In Open Office Draw, I drew out a design that would match up with the pins. With the help of Pat, who is doing an independent study on CNC tools this year, I sent the file to the machine with the vinyl cutter. He cut the file, then we weeded it to see if it matched the chip. The file matched the pin locations of the chip, so we made a few more iterations to get the process down and the layout right. When we got it right, we cut three copies of the file for boardmaking.
The vinyl cutter can be used in two ways for this project: In the first technique, you can cut out the image in sheet copper with adhesive. This is then weeded to reveal just the circuit traces. When this is transferred to a substrate, you can solder the components directly to the copper traces. The second, more time consuming technique is to use the vinyl as a resist for PCB etching. Again, you weed the image, leaving just the parts where you want the copper to remain on the board. Scuff up the surface of the copper clad board (I used sandpaper, but normally would use a plastic kitchen scrubbie). Next, attach the vinyl with the trace image. Transfer tape or masking tape will help keep the relationships accurate. In the ferric chloride, the exposed copper is removed. The label on the bottle suggested adding a bit of hot water to the mix, which speeds up the chemical reaction. All I had was double sided board, so it probably used up a bit more of the chemical capacity than needed. Agitation helps keep the etch even. I have heard that an acceptable substitute for ferric chloride is muriatic acid, which is easier to find at most hardware stores in the cement section.
When all the excess copper was removed, I pulled off the sticker, and found a nicely burned board with sharp definitions. Bending out the pins on the chip gave plenty of surface area to solder to the traces. After tinning the areas where the component would go and laying the chip down on top of the tinned areas, it was easy to tack it down into place. The next step will be to solder the rest of the circuit components to the board.
The etching took a bit longer than would be ideal with a group of students, but the results of this process turned out way better than the toner transfer technique that I have used in the past. There were many variables in the toner transfer process relating the the amount and timing of heat application. When over etched, the toner transferred images just burned right off. With the vinyl, the copper beneath the sticker is very well protected from the chemical action.
If you want to try out this technique, you should be able to use the image file used to cut the vinyl. Feel free to modify the
as well. You could probably take the image file to a local sign shop and have them cut it for you. You may also find that the image works well with toner transfer. Try it out and let us know what you make!
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