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From [log in to unmask] Mon Sep 23 17: |
46:50 1996 |
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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?
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