Reverse Etching Question (or how do I prevent my acid from boiling?)

[Kearns892]

So I tried a reverse etch (mostly white space on the transfer) the other day. I had a really great transfer with few problem areas. I moved the enclosure under a light to look for any holes in the transfer and covered up the few areas with problems with nail polish and tape. Originally I had the acid in a warm water bath (my usual process for pcb etching), but the acid starting getting really hot so I removed the enclosure from the bath, but the acid solution kept getting hotter until it reached a rolling boil. I quickly removed the enclosure from the solution. There was a shallow etch, but the heat had damaged the toner and some of my tape creating some pitting. Also, some of the bubbles from the solution attached and solidified to the enclosure which resulted in uneven etching beneath these covered areas.

Since my toner was pretty much destroyed, I tried to make do with the shallow etch, but it wasn't deep enough for me to paint the base layer and sand off the top layer without damaging the recessed area. Since the etch was shallow I sanded the whole thing flat and hope to try again. Any suggestions for taming the reaction when using Ferric Chloride on a reverse etch? I searched and came up with this thread: http://www.diystompboxes.com/smfforum/index.php?topic=91376.0 from it it sounds like diluting the solution may help or make the reaction even more exothermic.

Has anyone else had this happen?

[mistahead]

Sounds like a runaway chemical reaction, ice bathing the whole thing might slow it down...

[Jopn]

I'm getting ready to do my first enclosure etch this week, and I read that it might be necessary to pull the enclosure and rinse under cool water a few times during the process if it gets too hot.

[deadastronaut]

well, ive had it fizzzzzz a lot with etching a large area,with a very strong solution  but not to the point of destroying the etch/toner..

sounds like you may need to dilute it more...

usually i need to warm it a little, but yours sounds like it needs cooling...

[Kearns892]

Thanks for the tips. I think I'll try a cooling it and pulling the enclosure out of the tank for a rinse from time to time.

For reference, I'm using the Radio Shack brand etchant

[TG Flatline]

well, ive had it fizzzzzz a lot with etching a large area,with a very strong solution  but not to the point of destroying the etch/toner..

sounds like you may need to dilute it more...

usually i need to warm it a little, but yours sounds like it needs cooling...

Same here. How long are you leaving it in for?

[CodeMonk]

Have a look here:
http://www.diystompboxes.com/smfforum/index.php?topic=91376.0

[Kearns892]

I came across that thread in my initial search, but it still didn't give me a workable solution. I tried again today. The transfer was excellent and I touched it up. I put the enclosure in ferric chloride that had already been used to etch boards in hope that would slow the reaction. I still had some fizz, but no boiling. I took the enclosure out of the etchant after about 3 minutes and there was already considerable relief. Gunk had built up on the enclosure face and I used a tooth brush to brush some of that off and then put some water on the surface of the enclosure to rinse away the rest, but the water seemed to react with what was left on the enclosure and after only a few seconds, the water-residue combo had eaten through most of the toner on the enclosure. The etch wasn't deep enough to be usable and I couldn't put it back in at this point.

Any one else had this problem? The thread that CodeMonk linked to mentions adding water creating HCL. Perhaps this is what I was experiencing, but why don't more people have problems with this?

[tubegeek]

why don't more people have problems with this?

My guess is because your work involves exposing much more metal to the etchant than is typical for PCB's or "positive" etches. So you may be in a rarer circumstance.

Don't take the brown acid, that much I know.

[mistahead]

More surface exposure means FASTER and MORE reaction, acid + metal = heat and some (often flammable at low flash temps) fumes.

If adding water to the solution increases the amount of hydrogen available to bond to byproduct from the acid solution and become HCL you end up with two acids rights? One reacting and resulting in another which reacts (albeit a little differently).

In your second attempt I see we got reaction without as much thermal runaway, so concentration is clearly a factor. Your description of water becoming reactive to the toner after being added to the toner and residue to me points a bit towards a secondary reaction. I just had a look at Ferric Chloride as a chemical, not a big deep investigation but a look...

I'd start by diluting it down more as a starting point, grab some low grade HCL from a hardware store and test it with your toner, ice bath to prevent thermally melting anything... copper oxide will also be produced in lower amounts... hmmm.

[Arcane Analog]

The key to reverse etching is to use room temperature FeCl3. As you have discovered the exposure will heat up the soulution on its own.  Pull the box and check it often to ensure you are not eating into unwanted areas and to let the FeCl3 'chill' for a moment.

The only time I ever rely on heating FeCl3 is to etch PCBs that have a thick copper side.