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Surface current concerns

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Hello,

I was wondering if anyone could point me in the right direction for finding some quantitative analysis done regarding surface currents as a function of relative humidity.

My concern stemmed from a circuit design which uses a 10 M-ohm resistor in humidity levels up at 90%. Will surface current dominate with a 10 M-ohm resistor at those high humidity levels? I would rather redesign the circuit than add a conformal coating to the PCB to keep production costs low which is why I'm looking for some actual data.

Many thanks for any information.
 

Without knowing the size of the resistor, working voltages likelyhood of condensation and PCB construction, it is difficult to answer this query. I would build it and test it over a boiling pan of water!
Frank
 
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10 M-ohm
0603 SMD package
90% RH, non-condensing
< 5V working voltages

I'm not sure what to tell you about the PCB, the board will be professionally constructed, assembled and cleaned.
 

If the board has been throughly cleaned, you don't need to fear surface currents at 90% humidity. "Dominating surface currents" would be a fatal condition in my opinion, for high performance instruments a variation of 1% (< 1 Gohm surface resistance) or even less would be considered critical.

The problem is however to assure long term cleanness, also it may be difficult to assure condensation free operation. Thus conformal coating isn't a bad idea. A thorough analysis for critical points, where high impedance nodes come near to supply voltage nodes is also suggested.
 
Thank you, both of you have provided some good insight and ideas for testing.

It may be difficult to maintain cleanliness over a long period of time. The PCB will be housed inside an enclosure with fans, so maybe I have no option to avoid conformal coating not only for this circuit in particular but to ensure proper operation of the rest of circuitry as well.
 

A partial manual coating of critical circuit parts can work as well, although it doesn't look that professional.
 

Put a plastic/cardboard dust cover over the PCB to shield it from the dust. Poke it away from the main air flow. Because you are using SMD resistors, I would engineer the track layout to minimise the effects of surface leakage on the PCB, you could even try "guarding" this part of the circuitry.
Frank
 
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    FvM

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I would engineer the track layout to minimise the effects of surface leakage on the PCB, you could even try "guarding" this part of the circuitry.
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Good point. Guarding can in fact drain PCB surface currents. It's a standard technique in high impedance cicrcuit design. The surface current on resistor chip or IC package surfaces can't be reduced however and demands for circuit encapsulation. In practice, both techniques should be combined.
 

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