I'll put my 2 cents worth in, just in case you aren't yet satisfied.
     We ran into this phenomenon a few years ago & from a scientific point 
     of view, it is pretty cool.  Unfortunately it also causes scrap.
     
     Here is the explanation.  I'm afraid that I am forced to use chemistry 
     terminology & hope it isn't too technical:
     
     The corrosion (attack) and etching of a metal is, by definition, an 
     electrochemical process.  That is, the copper being attacked must lose 
     electrons in order to change form from a solid metal, with a valence 
     number of zero, to a liquid salt in solution with a higher valence 
     number (which is positively charged).  Copper, for instance, changes 
     from neutral metal (0) to +2 valence.  It then reacts with sulfuric 
     acid in the etching solution and makes copper sulfate (the "salt").
     
     The galvanic attack occurs when two metals are contacting each other 
     and they are not equally susceptible to this attack.  In our case, the 
     gold is inert but the copper is in a solution designed to attack it.  
     Think of it like a battery:  two dissimilar metals in an electrolyte 
     (a conductive solution).  One of the metals in the battery is made to 
     be attacked when the metals are connected across the battery terminals 
     and the electrons start flowing.
     
     Galvanic corrosion is an accelerated attack.  The galvanic series of 
     metals has gold at the "protected" end and copper up on the scale, 
     which is therefore corroded.  Also, there is an "area effect" at work 
     since the size of the gold finger is much larger than the exposed 
     copper (under the overhang of the solder mask).  This concentrates the 
     flow of electrons on the relatively small copper area which then loses 
     thickness VERY fast (100 to 1000 times faster than if the areas were 
     equal).
     
     Matt Byrne


______________________________ Reply Separator _________________________________
Subject: copper gold galvanic during OSP application
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    9/18/96 07:46 PM


     Does anybody have any information or experience with the above issue?
     
*************************************************************************** 
* TechNet mail list is provided as a service by IPC using SmartList v3.05 * 
*************************************************************************** 
* To unsubscribe from this list at any time, send a message to:           * 
* [log in to unmask] with <subject: unsubscribe> and no text.        * 
***************************************************************************
     

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************