So I ran into some issues trying to make the DIY flex PCB for my integrated half bridge IC chip.  This chip has extremely fine 0.4mm pin spacing so the PCB has to be insanely accurate.  My previous discrete components BLDC motor controller variant enabled me to create much more crude and less dialed in flex PCBs and things still worked.  But with this flex PCB it has to be very dialed in and with very high execution precision.  This is no joke.  The first issue is that my laser printer does not adhere well to the pcb transfer paper I bought on amazon.  It prints on it with some of the toner showing up mirrored a inch or so away from where the print originally lands onto the PCB transfer paper.  

This means the ink isn't setting onto the transfer paper enough and is coming off onto the fuser roller or something and that is corrupting the fuser roller.  This can destroy my printer's performance over time and cause improper fusing onto all prints going forward even for normal office use which means addresses I print on envelopes are smearing off while in transit and envelopes are being lost in the mail system for me.  This is VERY VERY bad.  So I had to ditch using this transfer paper.

Thankfully chatgpt recommended using glossy magazine paper and so I gave that a try.  I used Psychology Today magazine paper and the print went onto there PERFECTLY.  I used 600 dpi setting, heavy as paper type.  I prepped my copper flex PCB blank (Pyralux) with 400 grit sandpaper followed by alcohol prep pad wipes.  Next, I laminated the glossy magazine paper print onto my copper flex PCB blank (Pyralux) with my laminator a few passes.  Next I soaked the magazine and flex PCB sandwich in 110F water for around 30 minutes which turned the glossy magazine paper mushy/pulpy.  I then rubbed the paper repeatedly with my fingers working from the outside edges and was able to roll it off gradually and gently.  It came off in two layers.  It leaves a bit of pulp residue behind on the pads but that is ok it doesn't affect the etching process later you can leave that.  And with this method I got the cleanest traces on there EVER.

But when I went to etch this clean PCB with toner in place, things fell apart again.  I used room temperature water with my Ammonium Persulfate crystal and water mixture.  So it was 68F water.  I did not agitate the etchant much.  The etching took about 2.5-3 hours!  That is horrible etching speed.  The larger copper planes took their dear time to evaporate and meanwhile the finer traces had undercutting so bad that the entire copper under the toner etched away and evaporated and the toner came off having nothing to stick to anymore and whose sections were lost.  The total etching time is supposed to be no more than 5-15 minutes.  3 hours is totally unacceptable.  I found out from chatgpt that the reason it took forever was I failed to heat the etchant to 110-120F and I failed to agitate the mix (stir or vibrate or w/e helps).  I also learned from chatgpt that for every 10F increase in temperature of the etchant, the etching time cuts in half.  So a increase to 110F will mean the etching time should come down to the 5 minute range pretty likely if I also agitate well.  The instructions on the container of etchant crystals said room temperature and no agitation is fine.  THEY WERE WRONG for SURE on that.

So to address perfecting the etching process I plan to get a AC hot plate with temperature adjust which I already own - one for like pans or kettles cooking/heating.  I also determined that the easiest way to agitate would be to put the etchant and PCB in a small container and create a apparatus that lifts and lowers one side of the container in a rhythmic way so that it rocks the etchant back and forth across the PCB.  Below is my simple apparatus design for that.  The advantage of this approach is its free if you have the very few parts needed.  I can power it with my lab power supply.  The n20 gear motor is like $1.  Super easy to make.  Can handle my 15g of etchant sloshing easily.  Simple to make.  Does not have anything going INSIDE the acid which would then be spinning and crashing into the PCB and possibly causing issues there.  We want just a bear minimum amount of etchant batch per PCB job and so even a spinning stir rod with magnet setup would be hitting the PCB and tossing it around violently etc I didn't want that and didn't want acid on that and having to fish it out and clean it of acid.  Prefer nothing going into the acid but the PCB itself.  So rocking the whole container makes sense for this and resolves that problem.

The rocker apparatus consists of a n20 gearmotor ($1 on aliexpress), a little wheel (paper and superglue composite wheel), string, a little block to get the motor up higher in placement than the etchant container.  As the n20 gearmotor rotates it lifts the string, raising up one end of the etchant container.  As it reaches 180 degrees of rotation (6 o'clock) it has lowered the etchant container back down.  It repeats this raising and lowering of the container over and over in a cyclic pattern which will cause the etchant solution to slosh back and forth over the PCB improving the rate of the chemical etching reaction and moving copper ions away from the PCB surface being etched more efficiently.  No pwm motor controller is needed I don't think since you can change the rock speed by changing voltage setting on the lab power supply and/or changing radius of the wheel that is turning and doing the lifting and lowering action.