The Ammonium Persulphate bottle I bought from JayCar states that you dissolve 1 part of AS in 5 parts water.
But it does not give any info about how much AS is needed per square cm of copper.
Obviously, you need more for a larger PCB than a small one.
Does anyone know? Any assistance will be appreciated.  Happy New Year to all.

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Not sure what the optimum method is, but what has worked for me is make enough to cover the board to a depth of one or two cm and gently slosh back and forth until done. The amount needed in ml is the tank width x length x depth you fill it to, all in cm. You can reuse it a few times but it takes longer each time. Warming it to 50C helps.

Just use the 1:5 ratio.
Ignore the copper cm-squared.
Just have enough volume to cover the PCB as mentioned by phil99 above.

Ammonium Persulphate solution MUST be heated to work.
If you try to use it at less then about 70'C, it will not etch at all.
You will need some kind of heating bath.
70'C to about 90'C works well, anything hotter then that will cause extreme excitation of the fluid, and will probably etch away the pattern you want.
A digital thermometer is your friend here, as you can then keep track of the temperature of the water bath.

I used an old hotplate, with an old frypan 3/4 filled with water, and boil that up.
Put your etch bath into the hot water, and agitate/monitor.
It will NOT etch if not heated, but I said that before.

Used it LOTS of times for home-brew PCB's, and always worked fine.
MUCH cleaner and less staining then Ferric Chloride.
But now I get ALL my PCB's from PCB houses, so I don't really bother with etching at home anymore.

A cm depth of solution works ok for me, definitely 70 deg + is needed. Biggest hassle I had was sometimes buying an old pack that had gone off, if the pack is over two years old or so just chuck it out and get a new one. Once it is heated the solution decays so I usually use a fresh mix if I can.
The copper forms a black oxide as a halfway step as it etches, if you start getting persistent "shadows" the solution is getting iffy. The solution goes blue with the copper as  it etches as well - don't use the blue solution with a stainless steel utensil or you will copperplate it as a grotty stain. An old plastic tray works ok.
Edited 2021-12-31 21:00 by zeitfest