Homebrew PCBs

At 4:16 am ((PST)), Sat Jan 20, 2007 Stefan Trethan wrote:

>The BBC just had a piece about the trial of those would-be bombers.
>[snip]

Moral: remember to ask for a discount on quantity ;-)

>It may still be possible to buy small quantities of H2O2 for PCB etching
>at those shops, especially if one is not afraid of questions and some
>interest.

On Friday last I went into our local chemists and checked with the
pharmacist whether she stocked hydrochloric acid (which I had
bought from her some long time ago) and hydrogen peroxide.

The answer was yes in both cases, though she called the HCl
"Spirits of Salts" - which is how it is labelled. The H2O2 was
stocked in 2 strengths: 6% (20vol) & 9% (30vol). A bit weak
for regenerating CuCl, but perfectly adequate for HCl-H2O2
etching.

I wasn't asked "Would you like acetone with that?" ;-)

I did this research in delayed response to someone's
claims of not being able to buy chemicals in the UK,
which he has repeated despite my reply pointing out
that one can (and yes, IPA is still available from Maplin,
too, shelf stock last time I looked).

Might I also urge people to identify the active ingredient
and its _concentration_. Most of us easily can look up the
etymology of "muriatic acid" and such like common names,
but we can't read the label on _your_ bottle of chemical to
know the concentration - you have to tell us.

[A recent recommendation to prepare PCB with a 30sec
pre-etch  in HCl-H2O2, which could have stripped most
of the copper at the strengths some have available. ;-)
Please remember that people using your advice may not
have the benefit of your experience.

Another post identified soda ash with washing soda -
although both are sodium carbonate, IIIRC those pretty
crystals of washing soda contain ten molecules of water
of  crystallization - so a 1% solution of washing soda
is a lot weaker than a 1% solution of sodium carbonate.]

Whereas hydrochloric acid seems usually to be supplied in
the mid-30% range near or at its maximum concentration,
H2O2 is usually diluted. Pharmacy shelf strength as a
general bleach, disinfectant and and sterilizer is 3, 6 or 9%,
beauty parlour strength seems to be around 30% and
anything more is industrial going on rocket fuel strength
  - and could incinerate you.

Also remember that H2O2 is de-stabilized by metal ions
and other particulate matter, so (a) it isn't a good idea
to store it in another bottle than it was supplied in and
(b) don't expect to part-used HCl-H2O2 to have any
shelf life at all.