Category:Photolithography PCB Fab
When the 10mil isolation paths offered by CNC Milling are insufficient, photolithography becomes the PCB fabrication method of choice.
 Large Boards
For large boards, note the following:
- Foam backing will be needed to press the PCB evenly against a photomask. Otherwise, acrylic (and similar) frames may bubble under pressure, allowing the photomask to 'pop-up' in the middle.
- During chemical processing, alternate between photodeveloper and etchant. Proceed with etchant only when all exposed regions are changing color. Otherwise, problems with ultrathin photoresist residue may remain.
- Avoid placing UV light sources in extreme proximity to the board. Doing so may unevenly expose some regions.
 PCB Design
 Design Rules
Recommend at least the following tolerances:
- 6 mil (0.15mm) feature size (traces and spacing)
- 34 mil (0.85mm) hole size with 8 mil (0.21mm) annular ring size (50mil/1.25mm outside diameter) - fits most through-hole components, standardization on one size obviates manual milling bit swapping
- 25 mil minimum hole size - tested for milldrilling
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. Inkjet printers, >600dpi, do well, given true inkjet transparencies. 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.
- Carefully repeat above two steps as necessary to isolate all traces.
- 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).
- 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.
Many thanks to:
- Dan Barlow
This category has only the following subcategory.
- [×] Photolithography Supplies (empty)