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August 1999

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Subject:
From:
Paul Klasek <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 6 Aug 1999 09:43:44 +1000
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Ken, this is LESS than burn in .
"Storing" units for 2h parked on 71'C WITH NO POWER ?
Sorry if 'm missing something : it's a mystery to me ,
I don't see it working for any of the views :
both are incredible (insufficient):

Not that it would do much harm (6x) ; but what good is it supposed to be ?
What makes me really curious is the proposals +71'C ;
71 is A very specific No. ; do you follow some validated/calculated model ?

We have here a similar "dilemma" ; when some of our factions do try to
reinvent wheel by constructing internal models with little understanding of
specialized solder statistics going into modern reliability standards
nowadays (resulting in reduced pads for caps for example);
whereas I'd say simple sticking to relevant IPC standards will keep us
without drama quite sufficiently .

Trouble with test constructions is they have to be proven as valid ;
than the question is how (time consuming) .
If you could explain the logic of the 71'C phenomenon ; we'd appreciate it
(it may be an interesting subject after all)

Hate to agree with Werner, as we used to perform vibration assessments
(regardless of TH or SMD's):
sorts out infants and design in most instances .

See yo'

Paul Klasek
httP://www.resmed.com

-----Original Message-----
From: Chafin, Ken G. [mailto:[log in to unmask]]
Sent: Friday, 6 August 1999 5:58
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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