Hints from Heloise (for the pcb etching household)

[mdh]

I just went through a somewhat troubled etch, and I thought I'd pass along a trick that occurred to me at work today, and I tested (well, sort of) when I got home.  Last night I tried to etch a Neovibe board before band practice.  I was kind of in a rush, and the only piece of copper clad I had that was big enough was double sided.  The reverse side got a little dirty in the process of ironing, so I tried to clean it up before putting it in the etchant.  I've been using sodium persulfate lately, and it seems that some steel wool might have found its way into the etchant bath.  I'm guessing here, but I think that's what happened, because what appeared to be rust started forming on the surface of the copper.  I heated the solution with a lamp, gave it time, but all I got was a nice brown coat of oxide building up on the copper.  Some copper had definitely gone into solution based on the color of the etchant, but the etch was not working.  I added some more sodium persulfate crystals, stirred and went to band practice.  When I got back, still no progress.  So, I let it sit overnight in the etchant (no heat), checked it in the morning, let it run all day.  Incidentally, I did some searching on here to see if I could get an idea of the problem, and I did come up with one thread in which someone experienced a similar mishap with an unknown etchant that had to be either sodium or ammonium persulfate.

It occurred to me that what I really needed to do was get that oxide layer off without mechanically destroying the etch resist (which had miraculously stayed solid through almost 24 hours of etchant immersion).  Hmm... well, an acid could do that.  I figured HCl would work, but I didn't have any laying around, so I broke out the vinegar (apple cider vinegar -- I was out of white).  I put the board in a bath of vinegar, and after about 30 minutes, there was a noticeable improvement -- the blank side (which had been face down) looked pink-orange, almost like a freshly polished board.  I flipped it over to clean the pattern side a bit better, and when it looked OK, I took it out, rinsed it, and etched in ferric chloride (which I absolutely hate, but I didn't want to throw good sodium persulfate after bad, so to speak).  The board etched like an absolute madman, way faster than I recall from my old ferric chloride days.

The reason I said that I sort of tested this trick is that if I'd really wanted to be sure that it worked, I would have cut the board in half, done the pickling treatment on one piece and not the other, and etched them at the same time.  Of course, then I wouldn't have a Neovibe board ready to populate.  So it's possible that the ferric chloride would have worked on the board straight out of the sodium persulfate bath.  I think I can live with the uncertainty.

So, if you get a stuck etch for some mysterious reason, pickle your board and try again!  And for extra mojo points, try balsamic vinegar