Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!att!cbnews!military
From: voder!nsc!dtg.nsc.com!worden@decwrl.dec.com (Dennis Worden)
Newsgroups: sci.military
Subject: Re: Electronics in Desert Heat
Message-ID: <1990Sep27.031616.7450@cbnews.att.com>
Date: 27 Sep 90 03:16:16 GMT
References: <1990Sep20.022308.15355@cbnews.att.com>
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Organization: National Semiconductor, Santa Clara
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Approved: military@att.att.com



From: voder!nsc!dtg.nsc.com!worden@decwrl.dec.com (Dennis Worden)

In article <1990Sep20.022308.15355@cbnews.att.com>  writes:
>
>>From: rinne@mcnc.org (Glenn A. Rinne)
>>all components are spec'd to operate at a temperature of 125C (257F).
>>I'm assuming that the same  specificatiion applies to equipment.
>>Why, then, is equipment failing at the desert temperatures of
>>49C (120F)? ...
>
>In general, 125C is the spec for *chip* temperature.  That will be
>considerably higher than the temperature at the surface of the chip's
>package, which will, in turn, be considerably higher than the
>temperature of the air flowing over the package, which will be
>warmer than the outside air unless the package is right in front
>of the blowers.  And 49C will typically be a shade temperature, not
>a vehicle-out-in-sunlight temperature.
>
I don't know what military systems temperature requirements are, but for
IC's, the standard military temp range is -55 dec C to +125 deg C *PACKAGE
TEMP*.  This means that the actual die temp (also called junction temp)
is hotter yet, although hopefully not much in excess of 150 or 160 deg C.
Different device packages can be used to keep the die temp down while
maintaining package temp of 125.  Special cooling can be used to keep
specific IC's cooler than a package temp of 125 if the device doesn't 
meet spec, and the designer must use it in the system.  At that point,
it's the system designer's problem.

Another problem that I've seen is system manufacturer's buying commercial
grade product (its all the same right?) and then certifying the total system 
as mil-grade regardless of the fact that some of the component parts are not
spec'ed mil from the original manufacturer.  This often results in a system
that seems to work fine in the lab, but is actually quite marginal, and 
will probably fail in the field at some time.  I'm sure that there are other
ways equipment shows up in the field that doesn't meet the intended
-55 to +125 deg C range.  That is small consolation to the poor sucker
sitting in the hot Arabian desert.