[sdiy] Etching PCBs with non-toxic chemicals

[David G. Dixon]

> > It seems that one can etch PC boards with vinegar, hydrogen 
> peroxide, 
> > and salt. I've kind of sworn off etching my own boards, but this 
> > discovery tickles me enough that I might have to try it.
> 
> I use hydrogen peroxide and hydrochloric (a.k.a. muriatic) 
> acid.  Same basic reaction, and it works a treat.  I've been 
> using the same bottle of solution for two years.  
> Occasionally, each time I use it, I add a teaspoon of 30% 
> hydrogen peroxide, and occasionally I throw in a dollop of acid.

I went to the weblink provided in the previous post, and read the
explanation by the guy's sister, who apparently has a PhD in chemistry.
Well, too bad she didn't have a PhD in metallurgy, because she completely
misunderstood the role of sodium chloride in that system.

The etchant is hydrogen peroxide (H2O2), vinegar or dilute acetic acid
(CH3COOH) and sodium chloride (NaCl).  The sodium (Na+) and acetate
(CH3COO-) do nothing in this system, and basically form a solution of sodium
acetate, which is inert and useless.  Vinegar is just a convenient and
harmless household acid.  However, the chloride is playing a key role: it is
stabilizing the cuprous ion, Cu+.  You see, the main oxidant in this system
is not actually the peroxide, as supposed.  It is in fact the cupric ion,
Cu2+.  However, for the cupric ion to act as an oxidant, its reduced form,
cuprous Cu+, must be stable.  However, cuprous is unstable in most aqueous
solutions.  The chloride ion acts here as a complexing agent, which combines
with cuprous to form the dichlorocuprate(I) complex, CuCl2(-).  This is a
very stable ion and serves to allow the existence of cuprous in solution.
Since cuprous is stable, the following oxidation reaction (called
"disproportionation") can occur:

Cu2+ + Cu ==> 2Cu+

This reaction normally could not occur in the absence of chloride (or some
other complexing agent -- there are many).  The peroxide serves to reoxidize
the cuprous back to cupric.  Oxygen from the air would do the same thing,
but the etching occurs much faster with peroxide.  (One other mistake our
PhD chemist made was to refer to hydrogen peroxide as a "weakly oxidizing
agent."  It is, in fact, one of the strongest oxidizing agents available.
The only reason why household peroxide is weak is because it is diluted to
only 3% in water.  Getting even one drop of concentrated (30% or 50%) H2O2
on your skin is a very painful experience.)

The vinegar serves only to provide acid protons, H+, which are required to
combine with the peroxide to form water during oxidation of cuprous:

H2O2 + 2H+ + 2e- ==> 2 H2O

2Cu+ ==> 2 Cu2+ + 2e-

Without the protons, the peroxide cannot act as an oxidizing agent, and will
instead simply decompose to oxygen and water:

2H2O2 ==> 2H2O + O2(g)

This happens anyway, which is why H2O2 forms bubbles.  If this occurs in
your etching solution, then it implies that you have too much H2O2.  You
will probably also etch too fast under those conditions, and your traces
will take on a somewhat ragged appearance as they will etch from the edges a
little bit.  This probably is not too much of a problem with dilute
peroxide, but I sometimes over-etch with my concentrated stuff.

Thus endeth today's chemistry lesson.