[SI-LIST] Re: Electrostatic HiZ PCBs

Bryce, unfortunately, this problem has been dealt with in the past and if 
you want to build an electrometer, you need to go with Teflon and very good 
circuit design and layout.  There are people who build these 
instruments.  If you are just building one or two, why not capitalize on 
their expertise and buy one?  To make this work, you will really want to 
put the board into a well-sealed, sic NEMA waterproof container, and place 
dessicant in the container with the board.

If you are going to try to use FR4 maybe you could get away with baking the 
board.  No matter what with FR-406 etc the substantial moisture content in 
the resin itself is still likely to eat your lunch.  You might get to where 
you need to be with a very thick board and lots of dessicant.

Steve.

At 02:02 PM 7/18/2005 -0400, bbolton wrote:
>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).
>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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