My Cupric Chloride Etchant Experiment

Encouraged by promise of not having to dispose of etchant bearing environmentally harmful copper salts, I decided to try the great green cupric chloride etchant.  I started my batch with 8 oz 31% HCl and 4 oz of 3% H2O2.  It etched the first board very cleanly in roughly the same time as my usual heated ammonium persulfate.  The solution was a light green after that first 3″x3″ 40% etched board.  There were lots of bubbles in the jar when I put it away – presumably H2O2 decomposing.  I left the lid loose.  I etched that first board completely outdoors.  When the wind blew toward me, the fumes were unpleasant unto choking.

When I tried another board a day or two later, it didn’t do much.  I was using a bent up piece of (insulated!) copper wire that hooked around the corners as a handle so I could agitate the board easily.  I heated the etchant up, and still not much.  (Some color change of the copper, but with continuous agitation, it didn’t feel like much was happening.  I added ~2.5 oz H2O2 – and got lots of action.  Immediate bubbles on the board, and more color change.  I started outdoors, but it was kind of cold and I ended up back in the kitchen.

Resisting the urge to only change one variable at a time, I also put an aquarium air stone in the etch pan (connected to a small aquarium pump).  I tried holding the board (which was copper side down – not my usual, but probably a good idea) horizontal, barely out of the etchant, directly above the stream of bubbles.  That looked like it would be a pretty effective technique, with continuous wash of bubbles across the board.  By centering the stream on the board and adjusting the tilt of the board carefully, I could get bubbles about uniformly escaping around all 4 sides of the board.  I did heat it some, too.  Surprisingly, fumes didn’t seem to be much of a problem.

The bubbles might have been too aggressive, as the resist seemed worn away at the edges of some traces.  The board came out pretty good, but with edges not as crisp as the first board or as with my usual AP.

The next board was bad.  I used the bubble technique quite a bit (at least at first).  It didn’t seem to be doing as much, and I heated it more.  It took a long time, and by the time it was done, the resist was completely gone!  I’m sure the traces were at least slightly compromised, though they looked at least acceptable.  The etchant has been getting darker and darker green.  I’m afraid at least some of the color is from the green resist that’s now part of the soup.  I was watching for signs of blue rather than green – indication of significant (undesirable) Cu+1, but the solution remained green.  The aeration might have been part of that.

It felt like the etchant wasn’t working by itself – presumably not enough Cu+2.  Adding more oxidizer helped a lot, but I’m afraid of diluting the batch with too much H2O2.

I left the aeration running overnight, and wound about 6g of stranded copper wire around the air hose so it was in the stream of bubbles.  The goal was to get more copper in solution, with the hope that then the Cu+2 will be enough of an oxidizer that the etchant will start working as steady state.

Amount of copper in the soup:  I’ve now done 4 boards 2.3″x2.9″ with about 40% of the copper removed.  With the 1 oz copper board I’m using, that’s an ounce of copper per square foot.  So my boards were 4×2.3×2.9 => 27 in².  At 40%, that’s 10.6 in², or 10.6/144=>0.074 oz, or ~2g.  If the wire all dissolves, that will increase the Cu concentration by about 4 times.

In both the reference article on this etchant, and this Chemcut paper there is indication that optimum etch times are with ~120-170 g Cu per liter!  I have less than a liter, but even when the wire all dissolves I’ll be more than a factor of 10 down from that!  Reading replies to articles about CuCl2 etchant, many comment that it was great initially, but then didn’t work.  I can believe there’s this significant barrier of getting enough copper into solution that people doing one or two boards will run into.  I guess if I hope to have something that works, I’d better put a LOT more wire into it!

I’m really starting to wonder whether this stuff is appropriate for hobbyist use.  If I etch 20 boards per year – say 2×3″ and 50% etch, that’s only about 10g of copper per year.  But I have to start with more than 10 times that for the etchant to work well!  I think my expectations were set inappropriately by the lure of starting with H2O2 as the oxidizer.  For onesy-twosy etching, it’s going to take a LONG time before you can etch without adding oxidizer.  And by that time the etchant will be so dilute with the water from the H2O2 that it still won’t work.  Maybe ammonium persulfate isn’t so bad after all…

I guess the good news is that if I could ever get a batch that worked, the chemistry shouldn’t drift very much with the amount of etching I do.  Aerating it to keep the Cu+1 concentration down (that concentration has significant effect on etch speed) is probably all I’d need to do, at least for several years.

Update 5/27/12:  After discovering that the original host of Adam Seychelle’s paper on “Etching with Air Regenerated Acid Cupric Chloride” was no longer available on its original host (at http://members.optusnet.com.au/%7Eeseychell/PCB/etching_CuCl/index.html), I put a copy of the paper here.  I think I’ve updated all the links in this thread to point to my local copy.  Many thanks to Jim Harvey for noticing that the original was gone and especially for finding a copy elsewhere!