[time-nuts] The ultraAtomic clock for home
kb8tq at n1k.org
Sat Apr 8 08:34:56 EDT 2017
> On Apr 7, 2017, at 10:10 PM, Charles Steinmetz <csteinmetz at 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
>> 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.
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