Just to clarify things, the housing that this assembly is installed in is a
two part housing.  One side houses the SMT assembly and the other side is a
microelectronic hybrid, which eventually gets hermetically sealed.  The
extended burn-in (air) at 125 C is for the hybrid requirements of MIL 883
and 38534.  We're actually doing 48 hours pre-seal and 112 hours post seal.
The electrical engineers want to keep the SMT assembly in the housing during
the hybrid burn-in because the whole thing was tested and tuned this way.  I
guess their afraid that removing the assembly prior to burn-in then
replacing it might screw up their performance.  I just had a gut feeling
that subjecting this board to 125C for a total of 160 hours isn't a good
thing.

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


        -----Original Message-----
        From:   [log in to unmask]
[SMTP:[log in to unmask]]
        Sent:   Wednesday, April 24, 2002 7:20 PM
        To:     [log in to unmask]; [log in to unmask]
        Subject:        Re: [TN] TEMP EXTREME


        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
        [log in to unmask]




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

        Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please
respond
               to "Marsico, James" <[log in to unmask]>

        Sent by:    TechNet <[log in to unmask]>


        To:    [log in to unmask]
        cc:

        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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