Hi > On Apr 7, 2017, at 10:10 PM, Charles Steinmetz <csteinm...@yandex.com> wrote: > > Jim wrote: > >> Charles wrote: > >>> [blob over wire bond construction] >>> is also extremely unreliable, particularly WRT environmental effects >>> such as temperature changes, humidity, and atmospheric pollutants. >>> In my view, it is unsuitable for use in anything but dirt cheap, purely >>> disposable devices like greeting-card audio players and disposable >>> cameras. > >> Interestingly enough it *is* used in space flight hardware. It is much >> less expensive, lighter weight and easier to inspect than thick film >> hybrids and similar schemes. > > Very interesting. > >> I suspect that there is a wide variation in the material you blob on >> there and so forth. > > No doubt. I suspect also that space flight hardware doesn't use blobs on > plain FR4. While one problem with the blob technique is the permeability of > the blob material, another is the permeability of the substrate -- and FR4 is > pretty bad in this regard.
Unless you are building a thick film on ceramic, or a thin film on glass, the rest of the likely substrates are pretty permeable. > > It would not surprise me to find that space-qualified blob material is very > different from consumer-grade blob material, and is actually *more* expensive > than using consumer-grade packaged die would be (which would, of course, > defeat the purpose of using it for consumer circuits). > > I suppose in the vacuum of space permeability to gasses and humidity may be > less of a problem than it is in Earth's atmosphere, so the blob may need to > be the primary means to prevent ingress of gasses and humidity only from the > time of construction until launch. The other feature it provides is vibration protection for the wire bonds during launch. One would *hope* the device is stored in a low humidity package or dry box for the time (possibly years) between manufacture and launch. > > Makers of space flight hardware can also afford to spend more for materials > with similar coefficients of thermal expansion than makers of consumer > devices can. As long as the interface materials (mounting cement and die coat) are a bit elastic, you can get some pretty good thermo cycle performance out of the normal mismatches. Bob > > Best regards, > > Charles > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
