Conformal Coating RF Boards (Paralene Maybe?)

Has anyone used conformal coatings on their RF boards? If so, what were your frequencies min/max? Did you have filters and what were those topologies?

Has anyone used Paralene as a coating?

Thanks

well, depends on what you mean by a "board". If you have resonant elements like bandpass filters printed on the board, good luck with coating them! If you are just running very short traces of 50 ohm lines interconnecting chips, you might get away with it. And you COULD deliberately change the impedance of the lines to say 60 ohms, in the hope that after coating the conformal coat will capacitively load the lines and make them appear like 50 ohms again.

And yes, parylene due to its very thin layer, will have less effect on detuning the board.

so if you are willing to experiment with a very controlled coating process, you can get it to work. but you would need a good reason for the extra work, like the circuit will see a lot of salt fog or something like that.

you will have better luck with conventional microstrip rather than grounded coplanar line, since the later has tiny gaps that need to have tightly controlled capacitance.

I have typically done microwave assemblies which are then hermetically sealed, so I did not have any reason to conformal coat any RF. Now I am working on a set of RF PCBs which are going into a housing, but it is in no way hermetic. I won't be dealing with salt fog, but I will be dealing with atmospheric conditions and potentially humidity/frost.

I have some test boards that have lumped and hairpin filters which will be sent out to get Parylene coated soon, but those are VERY basic boards compared to the real hardware.

Do you know if there is a way to get around this? or could there be better solution?