Received from Harvey White at 3/23/2019 07:08 PM UTC:
On Sat, 23 Mar 2019 14:33:48 -0400, Mark wrote:
High concentrations of H2O2 result in VERY exothermic reactions. Etching takes place in seconds. Lots of heat. Lots of acrid fumes. Do outside or with a good hood. Have water available to put your board in to stop the etching. I did a lot with those high mixtures but decided it wasn't worth the problems and went back to 3% peroxide. I usually use a sponge with a small quantity of etchant. Takes a minute or two but is very controllable and uses only a fraction of the volume, making disposal much easier. I don't make a lot of boards, so it's not worth keeping vats of chemicals around.
The "top up the solution" is meant to add enough H2O2 to restore the solution to good operation, but NOT to replace a lot of water. You want to maintain the overall concentration. The goal (I think) is to maintain the same concentration you get with 3%, but not end up diluting the overall mixture. You do lose some to evaporation, etc.
Harvey
I absolutely don't understand the desire some have to complete etching in "seconds." As Mark says above, "it wasn't worth the problems."
And Harvey is right... if you regenerate with H2O2 (and you'll also need some HCl eventually) you may find the volume of your solution growing and growing. OTOH, if you regenerate with bubbling, and some HCl eventually same as with H2O2 regeneration, you may actually find you have to maintain the etchant volume by adding water due to water loss from evaporation while bubbling. That's if you turn on the bubbler and forget about it.
Jim H
