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

  • From: Doug Smith <doug@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Sun, 22 Aug 2010 20:56:02 -0700

Doug

On 8/22/10 5:56 PM, Mike Brown wrote:
> Steve, many years ago (45+), Milgo Electronics used G10 boards with a
> bake and a wax-like impregnation to achieve 1e12 ohms surface R, for
> boards used in a high-performance analog computer.  Special cleaning was
> also involved.  I was only peripherally involved, so I don't know the
> process details, and memory may mislead after so long.  Point being it
> can be done with enough attention to detail.
>
> I suspect that there are better materials for such an application today,
> but I've never had to achieve anything like that kind of performance in
> my recent experience.
>
> Regards
>
> Mike
> - - - - -
> Steven Waldstein wrote:
>
>> 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
>>
>>
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