We use Ferric Chloride etchant at ~55deg C + bubler with reverse plating process
A note one Ferric Chloride: You can't ever clean this stuff up it will stain everything. It is easy to make (google) easy to get in most places, none toxic before you add copper and 2 liters will yield you over 400 10x15cm 1oz DS boards with our method. This stuff will corrode everything metal in its vicinity. We took all the electrical stuff apart (power supply air pump etc...) and coated it with oil, works great just don't do it with high speed electronics. Varnish or Polyurethane should work as well. Electrical connectors have to be replaced on a regular basis.
Disposal for us is a problem since there is no one, in this country that will take it. So for now we dry it out and store it until we come up with a proper disposal.
If you got the concentration/temp just right you can etch 1oz in 5 minutes using the standard method, it's tricky without measuring tools. With the reverse plating method it does not really matter, always seems to take 3 minutes.
This photoresist we use can take about 10 minutes in the etch tank before it starts to degrade, you can push it a bit with some tricks but unless your etching 6oz copper boards it should never take that long.
You'll need a voltage and current controlled power supply; 10A @ 10-30V works nicely.
So the reverse plating process works by striping negative electrons from the copper (Anode) thereby breaking the bond. The copper ions then travel to the Cathode where electrons are supplied.
The Cathode consist of just a plain old scrap PCB connected to Negative. The Cathode since it gets plated thicker and thicker over time should not degrade but it does, the Ferric Chloride etching fast then the plating takes place?, Maybe a Nobel metal would be a better choice? (Don’t stick lead or other heavy metal in the tank, disposal problem). The Anode, the PCB to be processed, is connected to Positive, both sides. And if you’re getting confused by the logic that Positive has a lack of current carriers, electrons, with a negative charge, it's just old convention.
One important consideration is that you do not want the current density per area to go to high, the FR4 will burn and fine details get messed up. 10A for a DS 10x15cm board @15% exposure is fine. When the copper starts disappearing and the area starts decreasing the current has to decrease as well. We do that by setting the power supply to constant voltage at which 10A current flows at the start of the process. This will keep the current/unit area at reasonable levels.
So when everythign is ready stick the both Andode (the PCB) and Cathode (the scrap copper) in the etching tank and connect it up to the power supply.
Ajust the Power supply, constant voltage at which there is 10A flowing, around 8-15V, notice it is set to constant volage.
About 2 minutes in the process over 95% of the copper is gone and there is not enough left to too make contact and the reverse plating process halts, the current drops to 2A. You can barly see the thin remaining copper in the picture, makred red, this the etchen can remove in one minute.
Scrap the reist off with a knife blade and little sand paper. Quicker and easyer then using chemicals, but if you like there are chemicals available.And you get this:
So that is it for etching, stay tune for part 3 Solder Mask.