I have two guesses Re: fuming with high copper levels.
     
     Guess 1: ORP reading becomes less sensitive as copper levels get 
     higher, requiring more Cu(I) to trip ORP adder point.  This may 
     encourage a slow control response, allowing overdosing of oxidizer.
     
     Guess 2: Conductivity controllers are set up on a narrow response to 
     conductance in a very specific mixture where the only significant 
     variable is HCl.  High Cu may add to the conductivity and cause acid 
     to fail to add.  This will definitely throw the system out of balance, 
     either causing overadd of oxidizer (liberating gas when some HCl is 
     added) or some other phenomenon.

     I may be able to be more complete with more detailed operation info.

        Marshall Gurian
        Coates ASI
        e-mail:  [log in to unmask]        
           
______________________________ Reply Separator _________________________________
Subject: FAB:Cupric Chloride Etchant
Author:  [log in to unmask] at internet
Date:    8/13/96 9:33 AM


     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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