One of the 2015 Hackaday Prize criteria is "“Wow” factor: is the entry innovative, is the build impressive?" Well, aside from the "Wow" of a DNA-based molecular diagnostic device, I hope this entry log is a little bit 'wow' on the build side too :)
PCB boards, stat!
Having got my (hopefully!) final schematic and PCB layout finalized, today it was time to fabricate my first PCB. Luckily, if I have messed it up, I can make new versions quickly using this technique :)
I used off-the-shelf single sided copper board. I gave them a light coat of black spray paint, waited 5 minutes, and a second light coat covered the copper I missed on the first pass. After 10 minutes, the boards were touch dry, and ready to use. I used a hacksaw to roughly cut out a section of board the right size for my design.
During the drying time, I prepared my PCB files. My board layout has the interface at the top, consisting of a reset and run button, and a RGB status LED. In the middle is the power input on the left, microcontroller in the middle and ISP programming port on the right. At the bottom is the heater, and the sample chamber location, marked by the grey square. Under it is the RGB sensor and UV excitation source for the fluorescent dye.
Making a PCB quickly requires just a few steps. The first is to hide all layer that should not be etched. I hide everything except for the top layer, pads and the dimension layer.
Then, I exported the image, at the highest resolution of 2400 DPI, and set the export to Monochrome. After that, use any graphics program to invert the black and white image, and you are ready to laser your boards.
Like most CO2 laser cutters, the one I used has an "engraving mode", that can mark acrylic, anodized aluminium and wood etc. with a black and white image. A quick test found that the engraving settings for acrylic were perfect for ablating the spray paint from the copper surface, allowing the exposed copper to be etched in the next step. It's import to mention that you can't cut the copper laser directly with a regular CO2 laser system. You can do it with a expensive pulsed laser system, but you won't find one of those at your local Hacker Space. Anyway, you just place your painted PCB in the machine, load up the files with the settings for acrylic engraving, and you have your mask ready in about 3 minutes!
After that, you do a regular PCB etching. I used a Ferric Chloride bath using the regular method you can find on google, and then removed the black spray paint mask with some solvent. Acetone removed spray paint very easily, so you won't need more than a few millilitres on some tissue paper. A quick soak and wipe, and you are finished!
If you have a few painted copper boards ready, you can go from the designs on your computer to a physical PCB board in about 5 minutes more than your etching time!