I got together my temperature sensor and my hot plate and my glass container and some water and figured out where on the hotplate temp dial I need to be to get the desired water temperature. I then used label paper sticker and marked it off and wrote 118f on it which is the average temperature of that spot on the dial. It has about 15 degrees variance I think up or down. Good enough.

Next, I mixed up a batch of water and PCB etchant (Ammonium Persulfate crystals and water mixture) using a postal scale to get the ratios right. I mixed it into the bottom third of a wet ones baby wipes container. It was 0.8oz total. Then I submerged the etchant mix into the warm glass basin of heated water to bring it up to temp. I noticed the etchant wet ones container wanted to tip over so I used scotch tape to tape it off in all 4 directions to the glass container and pull it under water a bit too in order to maximize temperature transfer from the hotter outside water into the etchant container contents to warm it efficiently. I skipped the automated agitator entirely opting to just stir the mix with a hot glue stick manually.

All in all, it did etch within the 5 to 8 minutes range and I think etched even faster when the temperature was closer to 130f - closer to 5 minutes then maybe even 4 minutes I didn't time it. It was lightning fast compared to room temperature etchant which took 2-3 hours to etch a single board. So indeed, the instructions that said room temperature is fine were wrong. All that leads to is undercutting.

Anyways, with the stirring and the warm temps my results were significantly improved and almost every board was usable. I produced 8 or 9 in total. Enough to do at least 2 motors and part of a 3rd at least. So plenty for now. I had still a tiny bit of undercutting on the fine traces, but rarely enough to break their continuity. I will make the traces slightly wider in Photoshop for future prints with the expectation that around 15% of the outer edges will be removed so we print wider to compensate and then the result will be the width we wanted in the first place after the undercutting has done its thing. This way the undercutting is worked with and non-detrimental. That's my planned workaround for it.

In the rare instance of pinholes randomly in the center of pads or traces or a tiny open circuit on a thin trace, I am confident that a coating of solder over all of those areas will correct that. These little holes and whatnot are microscopically small so very easy to solder bridge as needed. I think for the most part my boards are almost all fully usable as is. I'm sure my methods will improve as I continue to tweak and iterate techniques over time - but the fact that we can already produce working circuits at this extremely fine detail and miniaturization level is already very encouraging. Any improvements from here is just icing on the cake IMO.

My next step will be to solder on the half bridge IC chips using the method shown in this video - a method I think looks very impressive and sound and hopefully not too hard to replicate on my end: