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August 1999

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Subject:
From:
Craig Hillman <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 5 Aug 1999 05:00:00 EST
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text/plain
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text/plain (120 lines)
Ken,

        6 thermal cycles over the expected operating range should not create
more defects than it identifies. If it does, you may want to review
the construction of your product or review the promised temperature
ranges over which it supposedly operates.
        I would also recommend contacting a reliability expert. Your ESS
should be tailored towards the defects you are trying to screen. Is
it poor solder joints? Defective components? Board defects? Why
are you holding for 2 hours during thermal cycling? You should be
able to answer these questions before you begin ESS. You should
also perform functionality testing before and after ESS and have some
sort of in-situ event detector attached during ESS to detect
intermittents.
        If you have anymore questions, please feel free to contact me by
email.

Best Regards,
Craig



> -----Original Message-----
> From: Chafin, Ken G. [mailto:[log in to unmask]]
> Sent: Thursday, August 05, 1999 3:58 PM
> To: [log in to unmask]
> Subject: [TN] ESS Screening
>
>
> A proposal has been made that all our recently designed surface mount
> printed circuit board assemblies be subjected to the following thermal
> cycling process:
>
> 1) Heat chamber to +71 degrees C as rapidly as chamber allows (but not to
> exceed 15 degrees C per minute).
>
> 2) Dwell at +71 degrees C for 2 hours.
>
> 3) Ramp temperature down as rapidly as chamber permits to -40 degrees C (but
> not to exceed 15 degrees C per minute).
>
> 4) Dwell at -40 degrees C for 2 hours.
>
> 5) Ramp temperature from -40 degrees C to +71 degrees C as rapidly as
> chamber allows (but not to exceed 15 degrees C per minute).
>
> 6) Conduct the above +71>-40>+71 thermal cycle 6 times.
>
> 7) Retest PCB assemblies.
>
> Note: No power or signal is applied to units during thermal cycling.
>
> View #1:  The thermal cycling described above will tend to screen out those
> assemblies with defective components and/or workmanship faults--including
> those involving solder connections.  It will help identify likely infant
> mortalities and assemblies likely to fail prematurely in field operation.
>
> View #2:  The thermal cycling described above is likely to create more
> defects than it identifies.  The cost of performing this screening process
> outweighs its benefits.
>
>
> Our electronic equipment is generally designed and qualified to operate from
> -40 degrees C to +70 degrees C.
>
> I would appreciate any references to personal experience or literature which
> would help determine whether View #1 or View #2 is more credible.
>
> Thanks for any replies.
>
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>
Dr. Craig Hillman
CALCE Electronic Products and Systems Consortium
University of Maryland
College Park, MD  20742
(301)-405-5316
[log in to unmask]

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