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November 1999

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Subject:
From:
David Whalley <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 11 Nov 1999 10:28:23 +0000
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>Hi technos, we have some problems with 1812 (and also with 1210) capacitors
>on FR4 because cracks at the corner. Is there any size limit for SMT cap's
>on FR4 (may be different classed for different application temperatures)?
>Thanks, Bernhard, Zürich

Bernhard,

I ran some tests over 10 years ago where we did -55 to +125 cycling tests
of (mostly) 1206 and 1812 chip components wave-soldered onto FR4. The 1206s
survived 2000 cycles with little evidence of degredation, but the 1812s had
severe cracks in the solder fillets after only 1000 cycles. The test regime
may or may not have much to do with your own product use environment, but
the results do show that things rapidly get worse for larger components.

I also had some TCE measurements performed at the UK National Physical
Laboratory on 4 different types of capacitor dielectric material, which
although all supplied by the same manufacturer, covered most of their
commonly used range of dielectrics. The variation in TCE was surprisingly
small - only 8.1 to 9.1 ppm/C (20-100C), so the results are likely to be
similar for different capacitor types.

The fact that narrower pads are often favoured for the larger capacitors
suggests that their ability to survive the stresses of life on a PCB is
pretty marginal and in the 60's it was shown that without yielding in the
solder that chip capacitors on alumina wouldn't stand a chance - one of
my worries about the prospect of a switch to lead free is that "harder"
solders might result in a dramatic rise in the incidence of component
failures and force the abandonment of the larger chip sizes, even in less
severe operating environments. Do any other technetters have a view on
this?

David Whalley

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