[volt-nuts] The "averaging reference"

[Attila Kinali]

On Sat, 20 Dec 2014 22:17:57 +0100
Joel Setton <setton at free.fr> wrote:

> I wasn't aware of the degraded long-term drift performance in the 
> plastic packages, as compared to the metal can. I'm surprised they can't 
> protect the chip from package-induced effects!

Well, that's where physics strikes back... and economics.

None of the materials known to men are completely gas or water tight.
And an exchange of gas/water leads to change of physical properties or
even chemical reactions. AFIK the most gas-tight enclosure is a
metal can (afaik only hydrogen and helium diffuse trough a steel can,
surface does not readily react with most substances found in air, but
needs non-metalic isolation for the wires going in/out),followed by glas
packages (AFAIK water tight, but not completely gas tight. also can act as a
getter material if outside surface is clean and in vacuum), followed
by ceramics (little gas exchange with the inside, but porous, ie can
store gases/water on the surface).
Plastic packages are mainly one thing: cheap. Neither gas nor air-tight,
they even store a lot of chemical compounds from production within the
material, that slowly leaks out. Also they are quite hygroscopic, to
the extend that chips are backed out before soldering, in order to
prevent the vaporizing water from breaking the chip. Even small changes
in the composition of the plastic can change the pin-to-pin resistance
from 10M to 2M. For normal electronics this doesn't matter, but here...

So, yes, it is possible to have better packages than just plastic.
But it is not economical to keep these around for the one or two people
a year who actually need them. (well, they do it with space grade components,
but they charge you 1000 times the price of the commercial equivalent).



			Attila Kinali

-- 
I pity people who can't find laughter or at least some bit of amusement in
the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
superficial. It's a matter of joy in life.
			-- Sophie Scholl