Homebrew PCBs

Stefan Trethan wrote:
> On Sat, 12 Feb 2005 09:50:24 +1100, Adam Seychell  
> <a_seychell@...> wrote:
> 
> 
>>All references I've seen for hobby etching with CuCl2 is S.G < 1.2,
>>which in my experience found to be almost useless as an etchant.  If
>>H2O2 is present then etching rate is fast regardless of S.G even with no
>>CuCl2 present! It seems the etch rate is very sensitive to CuCl2
>>concentration if and only if the mechanism of regeneration is from
>>dissolved O2 alone. In other words people seem to be running
>>HCl+CuCl2+O2 at far lower concentration than what they should. So I'm
>>wondering who uses HCl+CuCl2+O2 and at what S.G and what are their
>>experiences ?
>>Adam
> 
> 
> 
> Yes, i understand that the same way. when i started with my etchant it  
> took long until the S.G. was high enough to allow etching without H2O2. I  
> don't know how much different the etch rate now is, between H2O2 added and  
> not added. It is har to say because i got my bubbler only a while ago, so  
> i don't have much experience with the much shorter times now. Also, i only  
> add enough H2O2 to achieve a color change, so it migh not be much "free"  
> after regenerating the bath.
> But remember, if the S.G. gets too high it stops working properly too.


That could be true, although I have not tested for upper limit for CuCl. 
It is possible that the etchant rate may be maximum when operating at 
saturation, i.e CuCl2 crystals settling on the bottom of the solution. 
In this case there is no "too high" S.G because it is limited by 
solubility.

I know saturation for NH3 + CuCSO4 + (NH4)2SO4 etchant I once 
experimented with did not show signs of etching slow down. This was a 
complete surprised to me since it seemed intuitive that saturation would 
not allow any additional dissolved copper and so cause the etching to 
stop. It was impressive to watch simultaneously action of ammonia copper 
sulfate crystals growing on the surface and etching of the plate. A 
continuous cycle of new crystals were being form and then breaking away 
falling to the bottom of the solution.

There are other etchants that can be run at saturation. The big problem 
here is sludge or solids build up, while having no real advantage in 
etch rate compared to operation of a solution near saturation. One 
example of an etchant is pH neutral saturated ammonia copper chloride. 
The overall reaction is

2Cu + O2 + 2H20 -> 2Cu(OH)2

The dissolved portion of copper is in the form of copper chloride 
complexed with ammonia. The solution consists of copper chloride and 
ammonia. All free ammonia is driven off by evaporation, so the solution 
doesn't stink. The Cu(OH)2 formed is a pale blue sludge that is very 
annoying and messy, and I suspect the reason why no one uses it. About 
the only good thing with this etchant is the regeneration chemicals are 
nothing but air and water, a truly cheap etchant.

Just an example of etching at saturation.



> I would only consider going with air oxygen alone if there is really no  
> way to get your hands on H2O2. A liter lasts years for my use, and it is  
> "instant". So if i need 5 boards at once i have no problems...


Yes, the rate of regeneration can be slow for a typical air bubbler in a 
vertical tank. Regeneration time after etching a single PCB should be 
less than one hour for a basic setup. The bulk of solution will tolerate 
the small increase in CuCl ( copper(I) chloride ) that results after 
etching. Thereafter the air bubbler continues to run until all CuCl is 
oxidized to CuCl2 as indicated by change in solution color. A more 
accurate way is to get a test tube and put a few drops in, then add some 
water until it goes pale blue color, and look for signs of white 
precipitate. The while solid formed is CuCl or copper (I) chloride. It 
falls out of solution because you've reduced the net Cl- ion 
concentration which is necessary for solubility of copper(I) chloride.

But as you say if H2O2 is readily available then that might be less 
hassle than leaving your air bubbler going for an extra hour after etching.

Adam