Homebrew PCBs

If the solutions is translucent "emerald green" as opposed to opaque 
dark brown,green then the copper in solution is already completely 
ooxidized. The dark color appears when Cu(I) is present. Its very strong 
color and doesn't need much concentration to turn the solution dark. So 
I think your problem is low concentration of copper(II). If you want to 
etch with copper(II) chloride then I recommend S.G of at least 1.2. I 
run mine at 1.33. Etching at a SG of 1.17 as recommended in 
CuCl_ech.html is far too low I found. You *really* have to load the 
solution with copper. around 150g/L of dissolved Cu. The emerald green 
can get dark at these concentrations. You may need to put a drop on a 
glass sheet to see if its opaque or translucent (i.e does it copper(I) 
or not).

I haven't tried hydrogen peroxide, but from what I gather reading the 
results people have with HCl and H2O2 is that the peroxide is a lot more 
than just a replenisher for converting Cu(I) - > Cu(II). H2O2 must be 
speeding up the reaction a huge amount and also allowing HCl to 
indirectly oxidize copper metal. In the absence of H2O2 then the 
copper(II) chloride is all you have to etch, and its slow, probably 
around 50% to 30% the etch rate of fresh ferric chloride. Regeneration 
completion is determined by color of solution.

If etching in absence of H2O2 (as I assume you are now doing since all 
your H2O2 had been depleted of those few days) then HCl concentration 
does not effect etch rate significantly. The only requirement is the 
solution must be acidic at all times.

Adam

Phil wrote:

> I mixed up a batch of HCl and H2O2 and got good etching (which was
> mostly from the HCl).  There were some bubbles but no chlorine gas.  I
> etched 6 double sided boards that were on average 20x20 CM in the
> first session.  The etchant turned the well described emerald green. 
> 3 days later, I went to etch another board and got no (or r-e-a-l-l-y
> slow) progress.  I bubbled it for about 18 hrs total (with 2 airstones
> - lots of bubbles).  Still no etching.  
> 
> I got a hydrometer and the density is 1.075.  This is well below the
> 1.17 recommended by
> <http://users.rcn.com/rexa/Projects/CuCl_ech.html>.  From the HCl and
> H2O2 I put in, the s.g. should have been 1.029 so I assume that the
> difference, 0.046, is from the etched copper in the form of CuCl.  It
> appears I have too weak a cucl solution by about 1/4 but I'm not 100%
> sure about the chemistry here.  Everything I've read says that it
> doesn't take much copper at all so I'm a bit at a loss here.  I need
> to get pH measuring capability but the density is clearly wrong.
> 
> Any suggestions for fixing?  The page mentioned above recommends CuCl
> salts which I have no idea where to get and probably shouldn't have to.
> 
> I'm going to put a couple pieces of copper in over night to see if I
> can get more copper into solution but its not clear to me that will
> actually work.
> 
> 
> 
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