pcb-printer: Double-sided printing

project, hardware, pcb, electronics, process, infrastructure

Challenge: Get top and bottom layer to align
Improved process draft:
1. Prepare layout:
  - Eventually panelize small PCBs for better yield, check http://www.neufeld.newton.ks.us/electronics/?page_id=81
  - Put four markers arranged in a rectangle outside of actual PCB area
    - Markers are a 1mm diameter circular dot with a small hole surrounded by a thin 3mm diameter circle on both top and bottom layer
  - Here's an example:
2. Before printing, make sure that *no* layer has features reaching beyond the markers
  - Otherwise, the markers will shift slightly when mirroring the layout during printing
  - The tNames/bNames or tValue/bValue text is a likely candidate for this
  - Fix: Smash the offending parts, delete the text
3. Tape a piece of paper to the bottom of the tray, print the top layer to it
  - We need the markers for later
4. Cut the PCB into size
  - Saves etching solution and time
  - It might be a good idea to file away the burrs (DE: Grat), they'll wreck cloth and paper during cleaning and priming
5. Clean and prime the PCB, put it into the tray so that it covers the print on paper, print the top layer, cure ink as usual
6. Drill two 1.5-2mm holes through the exact center of two of the markers
  - Copper is hard to drill, don't use your precious PCB drills for this
  - Try to make this drill as precise as possible, your layer alignment will depend on it
  - What worked for me: Prime the drill with a _precise_ punch mark (I used a large screw and a hammer), position the drill without power, then start drilling
7. Remove the copper burr (DE: Grat) on the other side!
  - Don't forget this, or the print head will scratch across the burr and smear ink!
  - I used a larger drill that I simply turned by hand
8. Clean & prime the still-empty PCB side
9. Put the PCB on the tray and align it so that the marker dots are in the exact center of the holes you drilled
  - Hence the circular shapes – the human eye can see deviations from a circular shape pretty well
  - If your holes came out slightly misplaced, use the two most precise holes for this and take into account their misalignment
10. Fix the PCB in place
  - The primer will prevent some tapes from sticking properly, maybe double sided tape to the rescue
  - Or just tape the PCB in place before priming and prime later
11. Print the bottom layer (mirrored!) and cross your fingers
  - The outer circles of the markers can be compared against the drill holes to verify alignment
12. If alignment is „good enough“, cure ink and etch