John - We had problems with very low leakage current photomultiplier tubes
(on the order of a few femto-amps). No matter how we cleaned/treated the
PCB there was always enough cinductivity (on the surfaces, I guess) to
swamp out the PMT signal. Our solution was to put the critical components
on teflon standoffs swaged into the PCB. The final assembly was washed with
de-ionized water as a final rinse. On earlier circuits with less sensitive
PMT's cleaning and conformal coating was sufficient. I don't know if this
helps you with your customer but We've hit are times when extreme cleaning
is necessary.

Ray Klein

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