[SI-LIST] Re: Electrostatic HiZ PCBs
- From: "Plesa, James T." <james.plesa@xxxxxxx>
- To: <bbolton@xxxxxx>, "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 19 Jul 2005 03:11:43 -0500
Bryce,
Buy rather than build would be good if possible, because this will
involve some craftmanship. If you're making more than a few or you can't
buy what you need, then you might start thinking of hard encapsulants.
You could inexpensively epoxy-pot your (hopefully few) high-Z discretes
in a mold, hand wired to the input leads of your amplifier. The rest of
the first-stage amplifier IC (sans input pins) could perhaps be
FR4-mounted and preferably epoxy-encapsulated in the same mold. This
approach comes to mind because even though you may find a suitable 10^17
ohm resistivity material, you will still probably need to encapsulate to
control surface moisture and contamination (unless you can otherwise
maintain a dry laboratory environment for the input network). So if your
input network is small and simple, and you're not making the
electrometers in high volume, you could avoid the teflon PCB. I mention
this possibility not because of teflon PCB cost, but because of the NRE
associated with finding a teflon-TCE-matched hard encapsulant; or a soft
(teflon-TCE-tolerant) encapsulant that maintains wator-vapor-seal over
humidity thermocycling, or another means of environmental protection.=20
There are many other good approaches. Parylene conformal coatings, for
instance, meet your resistivity requirement and have comparatively low
production cost. But there is a learning curve with parylene and it may
not be compatible with your full environmental requirement. The same is
true of the multiplicity of other approaches. So bulk epoxy
encapsulation of the high-Z network and of the low-Z portion of the
amplifier on FR-4 (the low-Z FR-4 network is epoxy-compatible in this
case) is not a bad starting point. A 20-3651 epoxy has 5^16 ohm-cm
resistivity, and comfortably satisfies what is eventually the critical
constraint for your ultra-high resistance application: adhesion (eg
environmental protection through the interfaces).=20
Note: I think it would be difficult to constrain FR-4 manufacture to
10^17, or to maintain 10^17 in FR-4 after manufacture without measures
as above, which would make the high-Z FR-4 concept moot. =20
Jim Plesa
Northrop Grumman Corporation =20
=20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of bbolton
Sent: Monday, July 18, 2005 1:03 PM
To: si-list
Subject: [SI-LIST] Electrostatic HiZ PCBs
Hello SI-LIST,
I am looking for advice on minimizing surface and resistive parasitic
leakages for a fempto-amp electrometer design.
At the moment, teflon PCBs look attractive due to their hydrophobic
(water
wicking) property and consistently
high volume resistivity (10^17 to 10^18) whereas FR4 is (10^10 to 10^17
Ohm). =20
It seems that if one could
constrain the fabrication of an FR4 PCB to achieve 10^17, that would be
preferred over teflon due to manufacturing difficulty.
How would I specify a very low loss (high volume resistivity) FR4 PCB?
Which layer materials or vendors (rogers or others for the dielectric
layers) would be used to specify such a PCB?
Does anyone know of a vendor who would produce a teflon PCB in the case
that I chose to go that route?
Any advice on hydrophobic coatings would be greatly appreciated. I know
that Humiseal makes this sort of product, but have not investigated the
dry resistivity of thier products.
This is an electrostatic analog signal integrity versus a low-Z digital
SI type of application. I'd be glad to know about other list-serves
that might be better for this sort of application.
If you can advise specific shielding or lessons-learned or good books /
articles on this topic, I'd be glad to know. It seems that these issues
might be addressed in capacitive sensing applications.
Best Regards,
Bryce Bolton
bbolton at vt dot edu
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