Ken,  We do some pretty intense thermal screening of our product and we also
test while the product is being thermal cycled.  You should thermal cycle at
least 10 times for infant mortality and vibe the product, especially SMT.  A
reliability testing profile is very specific to your product and its
intended use.  You should have a reliability expert evaluate your product
and operational environment. Then try a testing profile on a selected lot of
product and track the lot to determine if the reliability testing worked.
There is no doubt that thermal testing contributes to product quality.
Testing can pay huge dividends in terms of product integrity and field
support.
I know my response is very general, hope it helps.  Good Luck!

-----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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