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Conformal Coating RF Filters

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Jimster55

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Which coating works best for UHF RF boards? I'm trying to minimize detuning of the filter. The PCBs will be placed inside a housing / enclosure, which has a few drip holes at the bottom (so not air tight) and there is no power applied (a passive impedance matching with passive filters for a receive only antenna) to the circuit. Is conformal coating even necessary or recommended?
 

I've not had any problems with a thin coat of Humiseal 1B31 over filters up to a few GHz. If it is a high Q filter it might shift it a bit, but you should be able to compensate for that during development, and if the conformal coat does cause a noticeable shift that will be nothing compared to the effects of any condensation.
 

There are many acrylic-based coatings (like the humiseal recommendation) which apply a very light coat and have minimal or no electrical effects.

Please note however, that acrylic coats should be used where the degree of required environmental protection is low, i.e. normal condensation.
For more aggressive environments, i.e. coastal areas with high humidity and salinity, and certainly automotive environments, a better silicone or urethane-based coating is required.
 

my experience is...the thinest is the best.
Since you said UHF...just make sure the layer goes on very thin.
Microwave filters would be many times harder to do, since the higher frequencies make stray capacitance have a big effect
 

Hi,

I'm not very experienced with this.
But once I had an RF application designed by an RF enthusiast.
He said that two component coatings are not useful for sensitive RF circuits like filters.
They cause huge thermal drift in dielectric behaviour.
Sadly I'm not sure what he used instead. Wax like? Thermoplast? Other single component coating?

Klaus
 

well, if you are talking about a production job. probably the thinnest coating would be Parylene.
 

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