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As Harvey said, it is to put the Oxygen back into the etchant without diluting it too much. It is much easier to dilute 30% H2O2 than it is to concentrate the 3% solution. Adding sufficient O2 with the 3% stuff adds a lot of water that is not necessary for the chemistry. Using the 30% H2O2, you can add as much or as little water as necessary to keep the chemistry where it belongs.
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The goal is controlled etching, and too much water defeats that. (I'm not a fan of bubbling to put the O2 back in - too much drag-out into the atmosphere.) Donald. -- *Plain Text* email -- it's an accessibility issue () no proprietary attachments; no html mail /\ <> On 2019-03-23 3:24 p.m., Jim Higgins wrote:
Received from Harvey White at 3/23/2019 07:08 PM UTC:On Sat, 23 Mar 2019 14:33:48 -0400, Mark wrote:I absolutely don't understand the desire some have to complete etching in "seconds." As Mark says above, "it wasn't worth the problems."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. |
