This project is mainly about my new build process, which I used to create a single sided PCB for a rebuild of the Capslocker project from last year. Instead of using a laser printer to create an iron-on resist pattern, I'll cover a whole PCB in resist and then use a laser cutter to etch off the resist.
First, I created a new design in Eagle. I wanted this version of the Capslocker to be a little easier to program and adjust, so I added a potentiometer and a 6-pin AVR ISP header. It makes the device a bit bigger, but really makes it easy to modify the code.
Next, I lightly spray painted some single sided copper clad PCB material from Jameco. The best paint I've found is Rust-Oleum "Painter's Touch" flat black spray paint. It dries in about 20 minutes.
I set up the spray painted copper clad on the laser table and focused it as precisely as possible. A sharp beam is essential to getting a precision resist mask. Then I printed-to-file using a postscript driver, imported that file into Corel, converted to a 1200x1200 DPI bitmap, and inverted. I set the laser to 50% speed and 40% power, raster mode, bottom-up etching. It can be a good idea to print two copies since the second pass cleans up paint residue a lot, but it's not required.
It's easy to remove most of the residue using water and a paper towel. I used to use alcohol because it removed the residue almost instantly, but I found that it also eroded the paint mask too much. Water alone doesn't get enough of the residue off, so you need a very mild abrasive polish. The best I've found for the purpose is ordinary toothpaste, although I forgot to bring it to Techshop and used some plastic polish instead. This also works fine, but it has a wax which needs to be removed with a grease-cutting soap before you attempt to etch the copper.
Now it's time to etch! I'm using ferric chloride solution from Jameco because it's easily available and relatively safe to transport. There isn't a permanent etch tank at Techshop or I'd probably use cupric chloride.
I didn't take photos during the solder process, but here's the Capslocker ready for annoyance. It's difficult to keep solder from creeping up unmasked traces, so I went ahead and tinned them all. I glued a piece of plastic to bring the overall connector thickness to about 2mm, which fits well in most USB ports.
And here's a comparison between the old and new Caplockers. The old one is definitely a lot smaller, but reprogramming it requires special adapters or soldering wires directly to the IC. The new Capslocker also includes zener diodes to be compatible with a lot more computer USB ports.
I hope you've enjoyed this glimpse into my current proto process, which couldn't exist without Techshop (or $10,000 for a laser cutter). Laser toner transfers are definitely more accessible to most people, and you could easily do this project with that technique. However, I'm getting more repeatable results from the laser etch process, though I think the minimum feature size is actually a bit coarser than toner transfers. It works for larger SMD components, and is perfect for quick through-hole PCBs.
Submitted by Garrett on Wed, 04/01/2009 - 23:47.