====== pcb-printer: Double-sided printing ======
{{tag>project hardware pcb electronics process infrastructure}}

  * Challenge: Get top and bottom layer to align
  * Improved process draft:
    - 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: {{http://dojoe.net/shack/markerexample.png}}
    - 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 
    - Tape a piece of paper to the bottom of the tray, print the top layer to it
      * We need the markers for later
    - 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
    - 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
    - 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
    - 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
    - Clean & prime the still-empty PCB side
    - 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
    - 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
    - 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
    - If alignment is "good enough", cure ink and etch