12AX7
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Sulfuric acid and toner
Background: I am printing circuit boards using the toner transfer method. Circuit boards are, of course, copper, to which a resist is applied. The
unwanted stuff is dissolved, leaving useful tracks. Remove resist and assemble your circuit. The resist I've been using is simply ordinary laser
printer toner.
I've been printing on a less-than-ideal substrate of ordinary printer paper, which has a somewhat coarse grain. Therefore, when I soak and peel off
the paper, a lot of fibers remain stuck in the toner deposit. Bridges can form between tracks, preventing access by the etchant and failing boards.
So it's advantageous to remove the fibers first.
To do this, I've been employing sulfuric acid. The cellullose dissolves, leaving a clean, off-black deposit which resists etchant effectively without
obstructing it around the edges. Meanwhile, without an oxidizer, the copper is unaffected.
However, I've noticed it can dissolve if I leave it in the acid too long, or if I heat it too hot. Example:
This is the actual color of the affected deposit. A fresh toner deposit is almost black; this has whitened and thinned quite a bit. (Some has also
chipped off, as a result of weak bonding or excessive scraping.)
My question: what chemical reactions are taking place which could have such an effect on an otherwise inert material?
The acid is drain cleaner (the brownish stuff, I think 92%), diluted to about 70% (since I don't want to carbonize the cellulose, just loosen it), and
heated to maybe 50C for enhanced activity. Assume small quantities (<5%) of various contaminants, including sodium, chloride, iron, copper, etc.
As for the toner, I think it's little more than polystyrene and carbon black. After etching, acetone wipes it off quite easily (including material
that's been thusly affected).
Tim
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Taoiseach
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You could try tetramminecopper(II)hydroxide to dissolve the paper. It is known to dissolve cellulose, and it will certainly not affect the
copper/toner.
The easiest way to make it is to put some copper wire into conc. NH3 and bubble air trough it. The air will oxidize the copper to CuO, the latter
being soluble in NH3.
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watson.fawkes
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I have heard tell, but have not tried myself, that the clay-coated paper in glossy magazines make a good toner transfer medium, even through it's been
through an offset printer. Having said this, it occurs to me that a roll end of such paper might be had at a local print shop for free or cheap.
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12AX7
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Yes, I've tried glossy before, and it works quite well. The fibers are also finer, so what it leaves is hardly a problem. Photo paper too, if thick
and pricey.
I'm usually too lazy to remember to bring sheets, so I end up printing on what's already loaded. 
I've got plenty of CuO and 10% NH3 I can use, I'll give that a try next time. Any idea how long it will take? Will heating be of any use? (Man,
that could get *stinky*!)
Any thoughts on what the acid is doing?
Tim
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Heating & much stirring will certainly speed it up. I was suprised myself how fast copper dissolves in ammoniacal NH4NO3-solution, thus the
already oxidized copper should be a breeze to dissolve.
No idea what the acid is doing to your toner. I tried different methods to dissolve the paper myself, and even HCl attacked the toner. Quite possible
that this isn't a chemical effect at all. The acid might soak through microscopic holes in the toner where it hits a layer of CuO and dissolves it,
causing the toner to flake off. Also keep in mind that when you sand the PCB with fine grit sandpaper there will be a layer of Cu/CuO particles on top
of the board which then fuses with the toner, giving the acid something to attack.
Toner transfer I found very tricky because it depends on the right paper, toner, cleaning/sanding method, temperature, pressure etc. and you spend
hours figuring it all out. Here's a compromise I suggest: Simply stick with the quality you get and touch up the bad spots with an etch-resistant
marker. I make PCB boards by simply drilling tough the layout so I get the holes in the right place and then connect them with a marker. Looks ugly as
hell but works just fine 
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12AX7
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I am certain it's a chemical change. Although the toner gets thinner, it doesn't flake or spall off as far as I can tell, and it definitely turns
slightly whiter, like the resin binder is being chemically changed. Maybe it's sulfonated or something? Seems odd with some water around (even if
it's all protonated).
I've had fairly reliable results with toner transfer. I think the biggest problem is pressure and heat -- use lots of both. I use an iron cranked
all the way up (7, for linen and such).
Even on boards I've retouched, the toner seems to do the best, as improperly cured marker tends to dissolve (I've also found cooking the boards helps
cure the ink).
A recent example. I had to retouch a lot on this one, partly because I also oversoaked this one, but also because the solder side got lined up so
terribly with the holes...oops...
Here's an example of etching,
(You can see the paper fibers still stuck to this board, which was before I started dissolving the fibers as described above.)
I use CuCl2, HCl, and a dash of NaClO3 to keep things going fast. A 1 oz. board is etched through in about 5 minutes at ~50C.
Tim
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watson.fawkes
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I have exactly three thoughts, neither of which I have any
particular confidence in, since my knowledge of organic chemistry is so tiny. One, I'd look into whether the acid is depolymerizing the binder,
polystyrene as you mention. Two, if so, the resulting monomer might be reactive enough to condense with free carbon. Three, if there's no
condensation, you might be liberating free carbon in suspension. You can pretty easily test this by neutralizing and filtering. I mention these points
only to spur thinking in someone who knows more about this than I do.
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