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December 1997

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Subject:
From:
Guenter Grossmann <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Mon, 1 Dec 1997 09:57:07 +0200
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Hi Sven-Eric

It is a matter of fact that the solder joints of TSOP's are less reliable
than the ones of packages with high standoff since it is the length of the
leads which absorbs most of the deformation due to the CTE mismatch of
component and PCB. Long leads deform easier than short leads. Additionally
if the leadframes of TSOP's are made of Alloy 42 than the leads are stiff
compared to copper leads. Using copper leads instead of Alloy 42 eases the
problem to a certain extend.

However, I believe it is important to consider the environment the product
is used in. Namely the temperature swing, the temperature extremes the
temperature gradient and the reliability goal of the solder joints ( As
Werner already mentioned, there is nothing wrong with TSOP's per se ). In
my opinion the reliability of these solder joints has to be estimated for
the application their used in, since even a material combination that is
potentially harmful might lead to a reliability of the solder joints that
is well within the requirements for a given stress.
Let's say one uses TSOP's on FR 4 in an office environment 20- 30deg. C and
the equippment sees a temperature swing of 40deg.C with a ramp of 1 deg.
C/min. When switched on and off once a day. I don't see a point why not
using TSOP's unless the thing has to live 100 years.

Reliability testing does not always mean to find the evil part in an
assembly corrupting the reliability of the whole equippment but also to
find out whether a technology that is absolutely disastrous for some
applications but has advantages ( price, space ) may be used in an
application with moderate stress. Therefore I think it doesn't make sense
to use data from tests with extreme testconditions to estimate the
reliability for a much less severe application without checking whether the
testconditions are applicable to the real stress.

Besides that's the work Werner and I are living of so it's got to be important.


Best regards

Guenter Grossmann
Swiss Federal Institute of Technology
Reliability Laboratory

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