A short reflection on direct laser PCB printing

I have been using a Lexmark E360d laser printer modified for depositing toner as an etch resist directly to PCB material for a while now. Details about the build have already been available on the project’s git repository, and this I’d like to focus on some comments on the actual usefulness of the project. PCB manufacture has been decreasing in its price, so I rarely use the ‘haxmark’ any more. It gets reactivated when I need a board fast and then it still does a decent job.

The general idea

The ultimate goal is to be able to etch the PCB as straight as possible from the printer. I use regular copper-clad PCB material without photo resist, so ordinary toner will have to be the etch resist. To deposit it, the laser printer has to modified: it should print on the PCB just the way it would on paper. Regular toner is really durable, so there won’t be any surprises when the PCB is submerged in chemicals.

From a mechanical point of view there are quite some modifications to be made. Because PCB material is a lot thicker and more rigid than paper, the printer’s feed mechanism won’t work as intended until large parts have been removed. Instead of being bent around internally, the PCB material is placed on a thin metal carrier which is pulled in at the front and ejected straight through a newly cut slot on the back. The optics and drum are still in place and the toner is directly transferred onto the copper.

Because a lot of the mechanics are missing, a lot of sensors are, too. Normally, the printer’s brains will get signals as the paper is passing through the internals and will stop everything when something unexpected happens to avoid damage. Thus, if the printer should actually print and not only produce paper jam errors, these signals need to be simulated. A small PCB with a microcontroller functions as a replacement for all the removed mechanics by feeding all expected signals to the printer’s main PCB.

Additionally, the printing process has to function without a fuser. It wouldn’t provide enough heat and certainly have problems with the thickness of PCB and carrier. The PCB exits the printer with the toner particles stuck on solely by electrostatic forces. Those are stronger than you might think (even blowing across the PCB isn’t a problem), but obviously not string enough for the etching process. A simple solution is to place the PCB in acetone vapour. That doesn’t degrade the printing quality at all but effects great durability as it ‘sets’ the toner particles by solvating it slightly.

And that’s pretty much all it takes! Etching works as it always does, but the process before it is greatly simplified.

The process step by step

I design my PCBs in KiCad, but the CAD program, of course, doesn’t really matter. These are my steps for etching a PCB from start to finish:

For double-sided PCBs I just do the whole process a second time after covering one of the copper surfaces with wide tape while etching. It would be possible to etch them in one go, but I didn’t bother (yet) to build a jig for holding the PCB in the acetone vapour so the previously printed side won’t be marred.

Pros and cons

This process isn’t perfect. If you are thinking about converting a printer for this job, my experiences might help you in your decision.

What works well:

Due to the direct printing, the resolution is a lot better than with UV exposure based resist.

Where there’s room for improvement:

Typical print failures due to uneven PCB material.

For my use case, the pros outweigh the cons. I still have all the stuff for UV exposure of photosensitive PCB material around but I haven’t used it since I built the ‘haxmark’.