When the 10mil isolation paths offered by CNC Milling are insufficient, photolithography becomes the PCB fabrication method of choice.

The process is simple, consisting of: 1) Dry film photoresist, laminated onto copper clad board by laminator. 2) UV exposure. Can be done by low-power UV laser strapped to CNC mill or high-resolution inkjet printed transparencies and various random light sources (eg. sunlight, CFLs, etc). 3) Development. Strong basic solution (sodium carbonate for negative resists, sodium hydroxide + sodium silicate for positive resists) chemically removes exposed resist. 4) Etching. Acid solution removes exposed copper. 5) Striping. Sodium hydroxide completely removes photoresist residue. Probably unnecessary and/or excessive. 6) Solder mask. Optional. Repeat steps 1-3 and bake in oven at 220F for 10 minutes. Alternatively, laser cut a negative solder mask from blue tape, and apply high-temperature spraypaint. 7) Via/hole drilling, punching, etc. Easy to do with the thin variety of copper clad board. Alternatively, can be done automatically on CNC mill. If using printed transparency photomask, CNC milling should be done first to ease alignment.

For surface mount projects, solder paste stencils are the easiest assembly technique, and can be made from blue tape on HacDC's laser cutter.

HacDC has all the required materials. The chemical process has been thoroughly tested, and a multilayer photomask has produced slightly better than 10 mil resolution. A more capable photomask printer is strongly expected to enable 'professional' 6 mil or better resolution.

Reference

http://www.instructables.com/id/Killer-PCBs/ http://www.mgchemicals.com/downloads/pdf/negativePrototyping.pdf