Hi Jim! The solder joint microstructure is going to experience pretty
extensive recrystalization under the 112 hours at 125C soak.  I haven't a
clue on what the overall impact will be on solder joint reliability but it
is certain you are losing some of the operation life of the assemblies.
Many Class 3 products withstand 2000 hours of thermal cycling from -55C to
+125C failure free. Those assemblies spend a total of 500 hours at the 125C
extreme (assuming 15 minute dwells). However, there is a difference between
thermal cycling and thermal soaking - is your use environment going to be
more thermal cycle or thermal soak?

Dave Hillman
Rockwell Collins
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"Marsico, James" <[log in to unmask]>@ipc.org> on 04/24/2002 09:33:21
AM

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Subject:    [TN] TEMP EXTREME


Hello Technet:

I think I may have asked this question a while ago (or something similar)
but couldn't find anything in the archives.  I have a surface mount
assembly
that gets installed into a top assembly housing, with covers.  The test
guys
want to subject this assembly to 125 degrees C for 112 hours.  Assuming
that
all of the components are rated for this temperature, is there anything
else
that says this isn't a good idea?  I'm thinking about the solder joints,
maybe decreased reliability due to excessive grain growth?  How about
acrylic conformal coating?  The PWB is polyimide, so this shouldn't be an
issue.  Anything else?

Thanks again...

Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879

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