[SI-LIST] Re: PCB materials, solders, and components for VERY HIGH impedance applications

  • From: "o. laney" <olaney@xxxxxxxx>
  • To: swldstn@xxxxxxxxx
  • Date: Mon, 23 Aug 2010 13:16:14 -0700

For a really, really hi-Z circuit, discard the PCB paradigm.  Use PTFE
standoffs to hold a wire in air, and a flying connection to the
transistor or IC lead.  This connection should also be in air rather than
involving a PCB pad. Same trick at the input connector.  Put a shroud
over this path to prevent resistance lowering dust accumulation and
electrostatic charging from air circulation.  Make it from plastic if
capacitance has to be minimized but if so use a high resistivity
anti-static formulation lest the shroud itself become charged.

Orin Laney

On Sat, 21 Aug 2010 17:18:19 -0400 Steven Waldstein <swldstn@xxxxxxxxx>
writes:
> To all,
> 
> I'm wondering if anyone has experience with PCB boards for very high
> impedance application (i.e > 100 GOhm). I'm learning about Isolation
> Resistance on capacitors and realizing that surface mount capacitors
> might only offer ~ 1e9 ohms of isolation. I'm now beginning to think
> about PCB materials as well. What kinds of leakage is there on FR-4?
> Do I need to worry about this as well. I also have been told that 
> the
> right solders and fluxes need to be used as well along with good
> baking to ensure all trapped moister, which can lower the impedance,
> is used as well. Anyone have any experience with these as well and 
> can
> recommend the proper types. Any and all help is appreciated.
> 
> The application is a sensor that has a very low capacitance for AC
> coupling, a very high impedance input is used to ensure a very low
> highpass corner.
> 
> Thanks in advance for any help
> 
> Steve
> -- 
> Steve Waldstein
> E-mail: swldstn@xxxxxxxxx
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