Look in Coombs, Handbook of Printed Circuits 3 rd edition on page 14.15 at 
paragraph 14.4.3  for the chemical reactions.  I think you will find that 
the amount of copper is not as important as the state of the copper, if 
there is no more cuprous ions to be converted to cupric then the chemistry 
you are still adding is there to react. Or if  you are really putting 
through the volume you could be overwhelming your cooling system allowing 
the temperature to rise and liberating gasses from solution.  In short you 
have excess chlorate and excess hydrochloric acid and should anticipate 
chlorine gas and hydrogen chloride gas as a minimum as a result.  If the 
solution is bright green the correct response is in fact to add copper clad 
material.  This converts some of the cupric back to cuprous and that 
regenerates knocking down the excess chemistry.

 ----------
From: [log in to unmask]
To: [log in to unmask]
Subject: FAB:Cupric Chloride Etchant
Date: Tuesday, August 13, 1996 11:01AM

     We etch inner layers using Cupric Chloride Etchant. We control the 
     chemistry with ORP and Conductivity probes.  The ORP adds Sodium 
     Chlorate and the Conductivity controls HCl adds.        We have 
experienced gas generation (chlorine or HCl fumes) when both      the ORP 
and Conductivity are in control.  The gas/fume release occurs      when the 
copper gets as high as 33 - 35 ounces per gallon.
        Standard procedure is to feed copper into the system when a chemical 
     out of balance situation results in gas generation.  Clearly in this 
     case this would be the wrong thing to do.
        Can anyone explain the chemical reaction behind a release of 
gas/fumes      when the copper is high?

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