On 4/7/17 7:10 PM, Charles Steinmetz 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.
We fly a fair amount of FR4 - sure, we might do some coupons or get
source traceability. And it depends on the mission - a billion dollar
mission to Europa is different than others.
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).
Not necessarily - the market for "custom hi-rel" stuff is getting
smaller every day and a lot of times it just isn't available at all. You
might want to choose a material with the right properties, but stuff
that's made in large volumes tends to be pretty consistent - a mass
market product can't have a huge dead on arrival rate. I'd say
automotive applications probably have the most stringent, yet cost
sensitive, requirements.
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.
But that is a fairly long time - launch delays are pretty common.
It's not unusual for something to be launched 5-6 years after being
built (and, of course, spares for mission 1 get used on mission 2,
coming along behind).
Sure, we're not doing salt spray tests or condensing humidity - but most
space electronics sits in a regular old airconditioned room for years.
(The days of ashtrays built into the test console are long gone)
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.
True enough - but then even for cheap commercial stuff, the CTEs are
published. It's pretty easy to get the right materials. (barring buying
your raw materials on eBay from unknown vendors, but then who knows what
you're getting.. it's one step from a guy in an alley saying "hey, I've
got some nice 2 part epoxy here, in the original package, fell off the
back of a truck, I can let you have it for a good price, as long as it's
cash"
The real issue with CTE these days is large ceramic packages (e.g. those
1000+ pin BGA/CGA packages) vs the boards (whether FR-4 or polyimide) -
given that we're not using anywhere near 1000 pins, a few dozen bond
wires on a the die seems like a great idea.
ANd bonding the die to the board is a lot better thermal transmission wise
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