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

PCB Design

Recommend gEDA. Project template with CNC Mill and gerber export scripts available on github.

Design Rules

Recommend at least the following tolerances:

6 mil (0.15mm) feature size (traces and spacing)
34 mil (0.85mm) hole size with 50 mil (1.25mm) annular ring size - fits most through-hole components, standardization on one size obviates manual milling bit swapping

For comparison, oshpark offers:

The minimum specs for 2 layer orders are 6 mil traces with 6 mil spacing, and 13 mil drills with 7 mil annular rings.

Simple, consisting of:

Photosensitive copper clad board. If not already available, more can be made by laminating dry film photoresist to bare copper clad board.
Via/hole drilling, outline milling, etc. Best done automatically on CNC mill.
Photomask. Printed on transparencies at high-resolution. Some laser printers may require double layer photomasks. Alternatively, <445nm lasers may be able to selectively expose the photoresist.
UV exposure. Sunlight or high-brightness CFL bulbs can expose the photoresist in ~30 minutes.
Development. Use a foam brush with the appropriate developer solution from the chemicals shelf.
Etching. Recommend a sponge with ferric chloride to remove exposed copper. This is quicker and less wasteful than the "bath" process.
Striping. Sodium hydroxide completely removes photoresist. May need additional UV exposure.
Solder mask. Optional. Laminate two dry film photoresist sheets onto the PCB, apply photomask, expose, and develop. Alternatively, laser cut a negative solder mask from blue tape, and apply high-temperature spraypaint.

For large boards with hundreds of components, reflow can be quicker than manual soldering.

Laser cut solder paste stencil from blue tape. Apply stencil to PCB.
Add smooth, paper thin layer of solder paste with a razor.
Remove stencil.
Place components on the appropriate pads.
Carefully melt the solder with the hot-air reflow tool. Hold the hot-air tool close to the components at 400C and maximum airflow, watch carefully, and progressively move the tool across the board as solder melts.

HacDC has the requisite resources:

Laser printer (Phaser740, use dual layer transparency masks, monochrome only, color causes streaks).
Transparencies.
Positive photosensitive copper clad board.
Negative photosensitive copper clad board.
Negative dry film photoresist.
Bare copper clad board, various types and thicknesses.
Various photodeveloper chemicals. Use the positive developer solution and foam brush for positive resists, sodium carbonate (washing soda) for negative resists.
Lots of copper dissolving etchant, various types, some good, some bad.
CNC Mill.

Chemically developed and etched PCB. Photomask remaining.

Photomask stripped, bare copper PCB electrically tested.

Aligned reverse side.

This category has only the following subcategory.