rinne@mcnc.org (Glenn A. Rinne) (09/18/90)
From: rinne@mcnc.org (Glenn A. Rinne) I heard on NPR (National Public Radio) that there have been significant electronic equipment problems plaguing our forces in Saudi Arabia. The report mentioned placing wet rags over transmitters to cool them and keep them operating. My meager experience with mil spec'd components reminds me that 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)? Granted, the temperature of the air flowing over the circuit boards does increase, but the largest increase I've ever seen is on the order of 20C (36F) for fan-cooled equipment. This would indicate that the maximum air temperature seen by the _downwind_ component would be [T_ambient + T_rise] 69C (156F). Am I missing something, or does the equipment not meet specification? [mod.note: A couple of possibilities: 1) If it's 120 in the desert, it will be hotter than that inside a closed vehicle. 2) If the temperature rise of a piece of electronics is 36F at room temperature, it will be greater than that at 120F ambient, as the cooling depends on the delta-T between the equipment and the environment. Still, 257F seems like a safe margin... - Bill ]
military@cbnews.att.com (William B. Thacker) (09/20/90)
From: att!utzoo!henry >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. Keeping electronics cool is a big problem in general. The added constraints of military equipment make it worse. There will always be a temptation to cut corners on cooling. The only way to be sure the stuff will work in the desert is to try it there. One particular thing that can cause trouble is design changes after initial testing. Often they aren't as thoroughly checked out as the original; if the stuff keeps on working well in its normal operational environment, that will be assumed to be good enough. For example, some proximity fuzes and such that had worked perfectly well in Vietnam started showing a very high failure rate in peacetime. The original specs had called for hermetically-sealed chip packages. Those are expensive and the market is small, so when production rates rose steeply for Vietnam, there was pressure to cut costs and use packages that could be had in larger quantities on short notice. They experimentally switched to commercial non-hermetic packages, and they worked fine. Until peace broke out, and the hardware started sitting on non-airconditioned shelves for years instead of going straight from the factory to combat. -- TCP/IP: handling tomorrow's loads today| Henry Spencer at U of Toronto Zoology OSI: handling yesterday's loads someday| henry@zoo.toronto.edu utzoo!henry
stevew@wyse.wyse.com (Steve Wilson x2580 dept303) (09/24/90)
From: stevew@wyse.wyse.com (Steve Wilson x2580 dept303) In article <1990Sep18.024659.21117@cbnews.att.com> rinne@mcnc.org (Glenn A. Rinne) writes: >I heard on NPR (National Public Radio) that there have been >significant electronic equipment problems plaguing our forces >in Saudi Arabia. The report mentioned placing wet rags over >transmitters to cool them and keep them operating. > >My meager experience with mil spec'd components reminds me that >all components are spec'd to operate at a temperature of 125C (257F). > stuff deleted... The real answer to your question is the gear they are doing this too, i.e. transmitters. A 100W xmitter is going to have a significant heat output that it has to dump to the external world. (let alone a 1kw or more xmitter which are just as likely..) Now put several of these into a single trailer and you have yourself a sauna. Chances are real good that the operating temparatures in these rooms is REAL hot, so the ambient air used to cool the equipment is even running 140-150F(obviously a guess...) Point here is that the transmitters are some of the largest heat generators you can find next to internal combustion engines. Steve Wilson, KA6S
worden@decwrl.dec.com (Dennis Worden) (09/27/90)
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.