On Jun 3, 2008, at 2:12 PM, Alexander Thom wrote: > John Doty wrote: >> On Jun 3, 2008, at 10:11 AM, der Mouse wrote: >> >>>>> Console based processing would have to work as usual, perhaps >>>>> throwing a stderr / stdout warning that there might be out-dated >>>>> footprints for certain files. >>>> Unacceptable. In a large project, such warnings are lost in the >>>> spew. >>> Then surely the right thing to do is clean up the spew! >> >> Yeah, except nobody ever really achieves that with make. >> >>> Even if it >>> means a grep -v to throws out the commands reported by make...or >>> using >>> @ on the commands you don't want to see, if you can hack the >>> Makefile. >>> >>>> Some customers imagine hermetic packages improve reliability, >>> And, don't they...for some environments? >> >> Maybe if you're operating your circuits in high pressure steam. But >> in space? Nope. There's a widespread fervent *faith* that hermetic >> packages are more reliable, but actual experience with operation of >> electronics in space tell a different tale. > > Is it possible for you to elaborate on that? Can you point me to any > reference material on the subject?
There is none that I know of. But there is also none that relates the pseudo-reliability that aerospace R&QA "experts" deal in to actual reliability in the real world. Test data doesn't count, because you can get any answer you wish by choosing the tests. If you restrict design to hermetic parts you generally wind up engaging in a large number of poor engineering practices. Good engineering chooses technology to match the goals and requirements of the project. The aerospace approach bypasses this, choosing technology by arbitrary criteria that are often unrelated to any requirement. A well chosen part will have tight specifications, allowing the engineer to develop margins in the design to improve reliability. Available hermetic parts generally have relatively sloppy specifications, so margins suffer. An inexpensive part that meets tight specs can only be made by a manufacturer that has its processes under tight, effective control. It is reasonable to expect that this should correlate with reliability. Sloppy specs and high prices are not an encouraging sign here, but that's what you get with hermetic parts. Heat is a major cause of part unreliability. Available hermetic parts are generally less efficient and therefore run hotter than the best available parts. A major hazard in a space launch is shock and vibration. Hermetic parts are more susceptible to this than plastic encapsulated parts. They are also heavier, which means they stress the boards they are mounted on. One crazy idea is to build your prototypes with commercial parts and then switch to so-called "high reliability" parts for flight. Huh? In my experience, this almost always creates problems that, if you're lucky, you'll detect in testing. The "high reliability" parts don't behave predictably in real systems. Should that not make a thinking engineer suspicious? Given all of these problems, and given the complete lack of evidence that "high reliability" parts are really more reliable than parts procured from Radio Shack, what should a rational person conclude when confronted with the truth that the only kind of "reliability" that actually matters is whether *system* meets its *specific* *requirements*? How can anyone believe that torquing system design away from an implementation with low stresses and large margins improves reliability? Blind faith and ignorance... I know of two missions that flew mixtures of commercial parts and aerospace parts. On ALEXIS, the hermetic, rad-hard aerospace memories failed, while thousands of commercial parts lasted for a decade without failure. The story on HETE-2 was similar: the only electronic subsystem to fail in six years of operation was a GPS receiver build to aerospace standards, while most of the spacecraft and its instruments used ordinary commercial parts. In that case, the GPS receiver was a power hog and ran very hot: it seems likely to me that shortened its life, and I have no doubt that its inefficiency was due to a poor choice of parts constrained by inappropriate standards. > At my place of work we are severely > constrained by IC technology selection criteria as specified by a > *true > believer* in the ways of hermetic sealing. The burden of proof should be on him, not you. He's the one asking for you to abandon good requirements-based engineering practice. > >> >>> /~\ The ASCII der Mouse >>> \ / Ribbon Campaign >>> X Against HTML [EMAIL PROTECTED] >>> / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B >>> >>> >>> _______________________________________________ >>> geda-dev mailing list >>> [email protected] >>> http://www.seul.org/cgi-bin/mailman/listinfo/geda-dev >> >> John Doty Noqsi Aerospace, Ltd. >> http://www.noqsi.com/ >> [EMAIL PROTECTED] >> > >> ______________________________________________ >> geda-dev mailing list >> [email protected] >> http://www.seul.org/cgi-bin/mailman/listinfo/geda-dev >> > > Best regards > > Sandy Thom > > > _______________________________________________ > geda-dev mailing list > [email protected] > http://www.seul.org/cgi-bin/mailman/listinfo/geda-dev John Doty Noqsi Aerospace, Ltd. http://www.noqsi.com/ [EMAIL PROTECTED] _______________________________________________ geda-dev mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-dev
