In a message dated 02/04/99 21:44:38, you wrote:
>Would it be so severe in normal ambient cycle Werner ?
>Even 90'C span over 24 hours seems (likely mistakenly?) pretty mild ?
>I came across this as well ;
>is there any average daily span ? in say average Alaskan warehouse ?
>Could you elaborate a bit also on that 20'C cross line significance I noted
>in the 785 standard ?
>Could it be said storage forms equal part of the product life & warranty
>considerations ?
>Paul Klasek
>http://www.resmed.com.

Hi Paul,
Well on the real ambient cycle I have of course no idea, but long-term storage
at  -50C would imply some more exotic locale.  AT&T did some study many years
ago for their outside-plant equipment and came up with a dailt delta-T of 35C
as beeing representative of its customer base. What that means for Alaska or
Spizbergen or Kamtschatka I do not know.
In IPC-SM-785 the 20C demarcation temperature comes from the fact that above
this temperature you have creep-fatigue low-cycle fatigue (LCF) dominant and
below you have creep no longer dominant and solder behaving more like a
structural metal including the build-up of stresses on loading. This has been
shown by work of Roger Wild at IBM and Peter Hall at Bell Labs; thus, cycling
through this temperature (it is of course not exact) means you are confounding
two (and perhaps more) different damage mechanisms. This makes correlation
difficult if not impossible and that is why the old MIL-cycle of -55 to + 125C
is not a good accelerated reliability test.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]

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