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At 5/1/2022 11:30 UTC Morris Odell wrote:

>Hi all,
>
>I've been using the regenerated copper chloride method for etching PCBs for
>years and have had very good results. One problem I do have though is
>getting adequate supplies of hydrogen peroxide which I use to give the
>solution a boost while etching a board.


If your solution needs a boost during etching, perhaps the volume of solution
used is too small. Or perhaps I'm misunderstanding and all you're looking for
is overall faster etching than you would get with a decently large volume
that doesn't get appreciably weaker as etching progresses. If the latter, I'd
tend to be more patient before resorting to a peroxide boost containing
silver nitrate... which will react with the solution to give insoluble silver
chloride which may be difficult to remove without boiling the solution for a
while to clump the silver chloride, then filtering. It's not much silver, but
it may look like a lot of precipitate if it remains suspended.

If you can buy 35% peroxide, you should also be able to get it without
silver... tho maybe not in a pool store.


>The local pharmacies only sell it in 100 ml bottles of 6% and even then they
>keep it behind the counter and give you the third degree when you ask for
>it. I thought I could get larger volumes from a hairdressing supply place
>but it turned out to mixed into a gel.


If pharmacies keep it behind the counter, I'd guess you don't live in the
USA. Here the typical peroxide that some use for wound cleaning (bad idea
despite such use by many) is 3%. The 6% strength is better suited to
bleaching hair... it's way too strong for application to skin and definitely
to wounds... where it was never a very good idea anyhow.


>However I was at the swimming pool shop yesterday and was surprised to find
>1 litre bottles of "chlorine free pool sanitizer" that contains 35% H2O2 and
>0.33 g/L of silver nitrate which is 1.9 millimoles/litre. My question is,
>can this be used instead of pure H2O2? Should I be concerned that the AgNO3
>would interfere with the etch or have a detrimental effect on the stock
>CuCl2 solution that I have built up over the years especially as it would
>accumulate?


Silver will accumulate, but it will do so in the form of a precipitate that
you will hope settles to the bottom of the container... but it might not...
or at least slowly. Silver nitrate will react immediately with the chloride
in the solution to form insoluble silver chloride, which I wouldn't want in
my etch tank... just because it's going to be "messy" - at least for my
definition of messy. It's will form a precipitate that probably won't settle
readily unless the solution is boiled to "clump" the precipitate. If you can
buy 35% peroxide, you should be able to also buy it without silver. Any pool
stores selling the stuff you're describing are catering to folks who don't
want chlorine in their pools and thus don't have problems with precipitation
in their pools. Both peroxide and silver nitrate are powerful germicides. I'm
wondering if this is used as the normal disinfectant, or as a shock
treatment, but none of that is really important for your purposes.


>Any advice from someone who knows more chemistry than I can remember from
>half a century ago would be most welcome.


I majored in chemistry the same half century ago that you mention... but
having been retired a long time I've forgotten a lot of my more specific
chemistry. From what I recall I'd tend to steer clear of silver in PCB
etching solution... not because silver will plate onto the board as someone
said... but because it will precipitate as silver chloride immediately - well
before it has any real chance to plate out. And the precipitate has a pretty
good chance of being annoying if it doesn't settle so you can get rid of it
by decanting periodically. Boiling will make it settle far better, but that
seems like a pain in the backside as well as likely to result in rapid loss
of any remaining peroxide... with loss not being a big problem, but *rapid*
loss could be if the solution suddenly foams up.

I've never had any problem etching using a proper strength well rejuvenated
(by air bubbling) cupric chloride solution. Lots of air bubbling - even to
great excess - also has the benefit of removing some of the water that gets
added if you regularly add a bit of 3% peroxide... tho I've never thought
that was necessary either. More than a touch of peroxide might even tend to
cause some undercutting, which could matter if the traces are thin to begin
with.

All in all I recommend at most very minimal added peroxide when etching and
no added silver.

Good luck.

JimH