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May 2007

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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:
Fri, 25 May 2007 14:25:14 -0500
Content-Type:
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I appreciate all of your comments on the history of Au embrittlement
versus the supposed lack of the same history for Ag embrittlement, etc. 
However, lets take the facts for what we have here with this particular
case (Steve's boards). The question is whether soldering a
gold-over-alloy-42 component termination (the component lead) to a
circuit board with IAg finish using tin/lead solder (right?) could have
resulted in a type of alloy where the combination of the gold and
silver, and the slower dissolution rate of the nickel, and the fact that
the particular components that failed are known to get hot enough to
burn your finger could have led to a brittle fracture failure. It may
not have been any single one of these variables, but a combination of
all of them. I could not find out in all of the emails if the solder was
63/37 or SAC. If it was SAC305, than factor in a much more brittle alloy
with another, additional, small amount of silver.

1. I believe that there is quite a bit of evidence related to the gold
embrittlement issues, I hope we all agree on that.
2. We all know there are issues with silver migration/nucleation. There
are good papers on that, beginning with Kirkendall's, and many since.
3. We all know the issues with soldering to alloy 42.
4. We know the alloy 42 leadframe is not compliant.
5. We know that the component is getting very hot, probably expanding at
a much faster rate than the circuit board underneath it during initial
powerup. So there is probably a pretty good CTE delta to deal with.

Therefore the question is, can a combination of noble metals cause some
similar embrittlement issues, compounded by having to form the IMF with
the nickel. And is this, in turn, exacerbated by the fact that there are
certain compliance issues with the component leadframe?
And the fact that in this particular design, the components get very
hot.

Does this not sound like a recipe for disaster? Sure it does. Steve's
hunch about the heat and the fact that the leads are gold and the board
is silver rings very true.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: Friday, May 25, 2007 1:50 PM
To: [log in to unmask]
Subject: Re: [TN] Silver Embrittlement?

Were the issues with silver plated component issues back i Dave,

Were the issues with silver plated component issues back in the SMART
conference era diagnosed as solder joint embrittlement by silver because
of too thick a plating or were they termination failures on the silver
plated components.  In the RF world silver plating is liked by the
design community because of its electrical properties.  However, solder
assembly engineers always have difficulty trying to make reliable solder
joints to components that have upwards of 15 to 20 microns of silver
plating.  When you look at the dissolution rate of silver into solder
you realize very quickly that when you try to solder to silver plating
this thick you wind up making a solder joint to the plating and not to
the base metal.  So the question really is that when a solder joint made
to thick silver fails why did it fail?  Was it due to silver
embrittlement or was it because you made a solder joint to silver rather
than a base metal.  If all of the silver isn't dissolved then if you
evaluate the silver content across the termination interface you see
that the silver content in the solder may be a couple of percent but
when you reach the silver plating side of the termination the silver
content is at 100%.  One needs to make sure that an analysis of the
failure mode is done in order to classify solder joint failures.  I for
one do not believe that silver embrittlement is worst than gold
embrittlement.  Certainly I've seen many more solder interconnection
failures on joints made to thick silver plating than I have to ENIG or
even thick 1 micron of gold plating but I've never seen any data
anywhere that shows solder joints made to silver those thickness is 0.05
to 1 micron is any more brittle that solder joints made to 0.05 to 1
micron of gold. 

Regards,
George
George M. Wenger
Andrew Corporation Wireless Network Solutions Senior Principal FMA /
Reliability Engineer 40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [Office]  (732) 309-8964 [Cell] -----Original
Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Friday, May 25, 2007 12:51 PM
To: [log in to unmask]
Subject: Re: [TN] Silver Embrittlement?

Hi folks! Just a comment. Some of Werner's comments of Ag embrittlement
are based on some ceramic components with silver plated finishes which
resulted in severe solder joint issues on a series of investigations
back in the SMART conference era. The IPC-4553 Immersion Ag finish group
is in the final stages of finishing up an investigation where Immersion
Ag surface finish was applied to test vehicles in both 1X and 3X
thicknesses plus combine with SnPb and Pbfree soldering processes. The
goal of the testing is to characterizes and determine if silver
embrittlement of solder joint can be created under practical
applications. The thermal cycle testing is complete and 100+ cross
sections are undergoing SEM elemental mapping analysis. The report will
be completed this year - keep an eye on the IPC-4553 group activities.

Dave Hillman
Rockwell Collins
[log in to unmask]




Werner Engelmaier <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
05/25/2007 08:12 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
[log in to unmask]


To
[log in to unmask]
cc

Subject
Re: [TN] Silver Embrittlement?






Hi Lee,
Au- and Ag-embrittlements are certainly documented?the combined effect
is 
simply my extension of the phenomena.

Werner



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