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From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: Low leakage parts
Date: 7 Jan 2003 09:29:36 -0800
References: <email@example.com> <8npR9.16063$jM5.firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 7 Jan 2003 17:29:36 GMT
"Phil Allison" wrote in message news:...
> ** Isn't polystyrene even better ?
(than Polypropylene)...Well, not according to the graphs I have. But
since the graphs are so understated, the reality may be something
different. I do have some 0.01uF polystyrenes at home, but they are a
big no-no for reliability and have been "outlawed" in our production
for many years. Teflon would be another one to have a look at, as
would fused quartz (though we probably won't find any 0.1uF fused
quartz caps kicking around). But my goal in my simple experiment was
to see what the caps we actually use looked like, not necessarily to
find the very best (lowest-leakage) possible cap.
> * I *did* attempt to make pre-amps for condenser mics once ( 25 years
> ago) and could not source the essential 1000 Mohm resistors in small
> quantities so I resorted to drawing the component, zig zag and all, on the
> PCB with a fountain pen filled with resistive ink.
> I got my 1G resistors OK but the PCB leakage was in the same order
> order of magnitude if the weather was even slighly damp.
> Any clear coating I applied over the area to solve this ate the
> resistor. Very frustrating.
I have a small catalog around here somewhere that lists high-value SMT
resistors; it was from at least 10 years ago (may have tossed it by
now!) Anyway, http://www.irctt.com/pdf_files/HR.pdf shows values in
0805 size up to 50 gigohms.
My experience with modern FR4 pc material, at least from the vendors
we use, is that it's very low conductance IF it's been properly
handled...run through a controlled no-clean process or perhaps
thoroughly cleaned after processing. When a very high resistance
between two nodes is required, it's possible to slot the board; you
may then have to guard around one or more of the separated nodes as
well. (It can be quite a challenge to make the guard track the node
you're guarding closely enough to do the job you want. With 1E10 ohms
resistance, 10uV gets you 1fA of leakage.) Some of the boards we use
have a little "diving board" area, a peninsula if you will, with a
slot routed on three sides. With light weight, small components, it's
possible to make an area like that which won't give too much trouble
in vibration. Of course, as Win notes, you generally don't need to
isolate very many nodes, and a single teflon standoff or feedthrough
may do the trick. They just don't fit well into 100% SMT processes.
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