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October 1999

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Subject:
From:
TPE Engineering TechNet <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 8 Oct 1999 12:59:25 -0700
Content-Type:
text/plain
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text/plain (98 lines)
Here's a reply by Mike Barmuta which I believe will answer
your question.

Joe: This galvanic reaction is known as bimetallic corrosion.
It takes place
when dissimilar metals are in contact with each other in the
presence of an
electrolyte. In your case the copper is acting as an anode
and being
deplated away in the microetch.

Here are some things to consider that will help.

1. Minimize the difference in surface area between the Au
and Cu features.
Think of it as anode to cathode ratio. Make the anode(Cu)
bigger than the
cathode(Au). Don't run small Cu traces into large Au pads if
possible.

2. Minimize the time/concentration/temp of the microetch. In
your earlier
posting you stated you where trying to remove 60u". That is
a lot of Cu.
What is it you are trying to prep the surface for?
At one time in our OSP line we ran our peroxysulfuric
micoetch at excessive
immersion times based on vendor recommendations.  We
had similar etchout
experiences, as you are seeing, on Cu/Au boards. We now
run this process
using 30 sec. Vs the original 4 mins. in the micoetch. This
removes about
5u" of Cu based on the chemistry concentration and temp
for our microetch
bath. This has eliminated the severe etchout problem and
has not affected
board finish or solderabiluty quality. You may want to
consider a similar
approach.

3. Whenever possible cover exposed Cu and Cu/Au interface
areas with
soldermask. This will help prevent localized etchout.


Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett Wa.

425-356-6076

-----Original Message-----
From: Joe Aronson [mailto:[log in to unmask]]
Sent: Wednesday, August 18, 1999 7:34 AM
To: [log in to unmask]
Subject: [TN] GALVANIC ETCHING MECHANISM


ALL:

Has anyone out there experimented with etches to see
wether a more/less
aggressive etch produces a change in the degree of galvanic
etching?  It
sounds to me like the main limitation in the mechanism of
galvanic etching
is going to be the diffusion of the oxidizer to the more noble
metal.  Is
this the case?? If there is a known rate determining step
please let me
know.  I may e able to use it to fight this effect.  Thanks.


Joe Aronson
Wet Process Engineer
JMSPI

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