Wow – I just had an etch fail dramatically. I don’t think this has ever happened before – certainly not since my process stabilized with presensitized board (long before I started with cupric chloride). I suspect it was just left in the etchant too long, but can’t say that for sure.
I was doing several things at the same time, and was using a timer for the etching (as well as UV exposures). As is my custom, I set a timer for 10 minutes with the expectation of flipping the board vertically to correct for routinely uneven etching (less at the top). When I checked on the board after 10 minutes, nothing was etched thru (no bare board), so I reset the timer (and probably didn’t flip the board). I don’t know what I reset it for -possibly another 10 minutes. There might have been another reset – maybe for another 5 minutes? I vaguely remember checking it at one point and noticing a very large area (~1/2 of the board) not etched – presumably copper colored. That particular board (the recording preamp for the latest small box) has a large area of copper by design. Of course, it should have been covered by resist, so presumably the resist was gone by that time.
A couple of things were slightly different about that board. I had exposed and developed the board the night before I etched it. I almost always do that immediately before etching. Might dried resist be more fragile? Another thing different from usual is that since I was doing several boards, I used a little more developer than usual, but kept it and reused it for multiple boards. The later boards didn’t develop nearly as quickly as usual. I eventually dumped the old (blue) developer and poured some fresh. Might sitting in the stale/used developer have softened the remaining resist so it didn’t stand up to the etch bath?
Later today I made a new copy of that board – and used old developer. When it didn’t develop right, I got spooked, dumped out the old stuff and poured some fresh. It developed OK – and then etched just fine.
I also etched another board I’d exposed and developed the day before, and that one came out fine, too. The etchant is dirty (5+ g/l Cu+1) today, but I’ve etched a couple of other boards today, and while etch time is up from ~20 min to ~30 min, I’ve seen no other evidence of resist failing. So about all I can guess is that I kept putting the board back in the etch and lost track of how long it was. Bummer not having closure on this.