In the past, My milled PCBs corroded quickly. I began coating them with nail polish. I liked this method because I could be precise and not paint the soldered components in case I had to fix or hack the boards later. Spray on conformal coating is what Neil from FabAcademy recommended and he’s right that Nail Polish isn’t designed for circuits. But the overall results are fine, colors are abundant, and it’s like $1 a bottle rather than $20 a can of conformal coating.
While I see lots of folks online using UV curable liquid soldermask, or dry soldermask film, however the liquid is made of some pretty interesting chemicals. I also don’t want to figure out a way to UV cure the dry film such that I can get the pads uncovered on the PCB.
I found one guy who came up with a technique that was quick, easy, and cheap. Colored lacquer is cheap and readily available at hobby stores for less than $10. It also protects the traces from corrosion and comes in a variety of colors. With a quick spray (like conformal coating) I can paint the entire PCB. The benefit here is the second step. Instead of trying to figure out how to UV cure a dry film and then remove material from the pads, I simply throw the design into the old CO2 laser and burn off the lacquer on the pad areas. To get the files for this I went into EagleCAD’s layers and used the old technique of exporting bitmaps of the dimension+top and separately the dimensions + modified tStop. (This is my AtTiny412 General purpose Blinky board if you are curious. Detailed info about building and using it here).
I changed the fill style to solid by clicking on the tStop layer, then the Change button.
Along with the Top+Dimensions layers the design is complete.
You can tweak the colors in good old Gimp Like we “used-t’do” and throw it on the laser. At this point I handed the files over to Tom Dubick of Charlotte Latin School FabLab fame and he cut out a jig to hold the PCB in place and etched the layer of lacquer away.
I found the Lacquer quite soft and for smaller boards it might be just as quick to scrape it off the pad areas with a razor knife. Scraping actually was a more complete methods of removal as well. In the future maybe a milling step would work for this.
The lacquer will still melt when enough head is applied with the soldering iron (and you do NOT want to breathe that in…) but it does somewhat act like a solder resist when dealing with the tiny pins on the boards we make in the FabLab.
I think the better idea is to slap some kapton tape on the PCB, then laser it off. Kapton seems to be commonly used with lasers even in industry and is made of essentially Carbon, Oxygen, Nitrogen, and Hydrogen (ignoring the adhesive) and according to this MSDS “At temperatures above 400 degrees C the major off-gasses are carbon dioxide and carbon monoxide”. It should be safe to do.