l finally got to etch a board in the more concentrated solution and it works very well. Etching was noticeably faster. the solution started off a lovely emerald green and did get darker as the copper dissolved and the amount of Cu(1) in the solution increased. I wasn't using a very large volume of solution so I had to add some more to keep it going but if finished up very well. I've got it all back in the bottle now with the bubbler going and I expect it to all return to the nice green color.
My interpretation of how this all works is as follows:
The etching process coverts both the green Cu(2) and the solid elemental Cu into black Cu(1) which is then oxidised later back to Cu(2) but at a slower rate than the etch. This means that it's important to start off with enough Cu(2) in the solution to deal with all the copper you need to remove. The solution becomes progressively darker as the etch progresses and this might cause the etch to slow down as the amount of Cu(2) falls unless there is an excess present to begin with. I imagine that commercial operations use high volumes of solution and circulate it between the etch tank and a bubbling tank. Converting the black Cu(1) to green Cu(2) uses oxygen either from the air or from H2O2 and that will gradually make the solution more alkaline so small amounts of HCl need to added to counteract that and provide more Cl- ions to match the Cu dissolved into the solution from the PCB.
I'm retired now but? I used to work at a place where there was a chemical lab that employed some every highly educated people with doctorates and years of experience. However the lab was there for other reasons and dealt with entirely different processes and functions? than this. Nobody could help with an understanding of CuCl2 etching or even swimming pool chemistry until I found discussions here!
Morris
