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February 2000

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 12 Feb 2000 15:11:49 EST
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Hi Jason,
The "large grain structure, yielding a low fatigue resistance" is a red
herring, since the grain structure of solder is unstable and coarsens with
time and temperature. True, if you fatigue cycle fine grained solder in an
accelerated test you get about twice the life as compared to coarse grained
solder, but for actual product this is immaterial since within a years time
the initial fine grained solder is virtually indistinguishable from initially
coarse grained solder.
You ask: "will the invar layers help with TCE mismatch issues and/or board
warpage". The CTE (not TCE--coefficient of thermal expansion) mismatch is not
the issue in solder joint reliability, it is the thermal expansion mismatch.
That is not quite the same thing for real product (it is for temperature
cycling in a chamber).
And the answer is : It depends. It depends on the type of components
(materials) you are using, whether the run much hotter than the PCB, and
whether they are attached with compliant leads.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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