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January 2008

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 30 Jan 2008 06:59:05 -0600
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text/plain (111 lines)
Thank you (once again) Werner, Dave, and Vlad for educating me. Your
information is invaluable. 

-----Original Message-----
From: Igoshev, Vladimir [mailto:[log in to unmask]] 
Sent: Tuesday, January 29, 2008 3:46 PM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: RE: [TN] Microvoids immersion silver

Richard,

Kirkendall effect is not specific to the noble metals and it's has
nothing to do with the "Champaign" voids either :-) Here is a brief
description of the  effect:

"The Kirkendall effect is the migration of markers that occurs when
markers are placed at the interface between an alloy and a metal, and
the whole is heated to a temperature where diffusion is possible; the
markers will move towards the alloy region. For example, using
molybdenum as a marker between copper and brass (a copper-zinc alloy),
molybdenum atoms will migrate towards the brass. This is explained by
assuming that the zinc diffuses more rapidly than the copper, and thus
diffuses out of the alloy down its concentration gradient. Such a
process is impossible if the diffusion is by direct exchange of atoms."

Regards,

Vladimir

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, January 29, 2008 2:31 PM
To: [log in to unmask]
Subject: Re: [TN] Microvoids immersion silver

Werner and Don.
I was always under the impression that Kirkendahl voiding was a result
of the noble metal's (in this case, silver) nucleation through the SJ. I
always thought that this was a time-variable migration. I understand how
all of the process variables described in Werner's  response will affect
or support the eventual void formation, but I would not expect Gaby to
find anything in microsections taken immediately after soldering. Would
it not make more sense to thermally cycle the SJs in order to facilitate
the nucleation prior to microsectioning?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
Sent: Tuesday, January 29, 2008 12:35 PM
To: [log in to unmask]
Subject: Re: [TN] Microvoids immersion silver

Hi Gaby,
I take it what you are seeing are 'champagne voids.'
Voids like these are 'encouraged by rough topography copper, excessive
Ag thickness, some solder paste choices and rapid cool-down after
reflow. But as Donald Cullen says in his comprehensive paper, it
"requires the "Perfect Storm" 
of a rather contrived combination of parameters" to produce planar
micro-voiding.
Well Gaby, now you know it-you are "Perfect." ;-)



Werner



**************
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