If the gold is low porosity than it is impervious to nitric acid.  
     However, if the interface between the nickel/gold plating and the 
     non-plated copper trace is exposed to the nitric acid you have a 
     potential for an accelerated etching of the copper at the interface to 
     occur.  We perform electrolytic nickel/gold plating as a service and 
     have to deal with this problem from time to time.  The accelerated 
     etch rate is caused by a galvanic potential created between the copper 
     surface:acid solution:gold surface system.  Our theory is that the 
     less noble copper acts as an anode while the gold surface somehow 
     catalyzes the reduction of some species in solution, probably hydrogen 
     ions.
     
     The galvanic etch rate is unpredictable because it varies with the 
     surface area of the gold as well as all the physical and chemical 
     variables of the solution and equipment.  You have to determine the 
     maximum dwell time empirically.
     
     I hope this helps.
     
     
     Michael Holan
     Process Engineering
     Continental Circuits of Texas
     512.246.5612


______________________________ Reply Separator _________________________________
Subject: FAB: Nitric Acid Strip
Author:  [log in to unmask] at INTERNET
Date:    5/7/97 7:06 PM


     Address,
     
     Is hard/soft gold impervious to a Nitric Acid Strip, if so, how many 
     passes are typically allowed (assuming a gold tab area)?
     
     John Gulley
     
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