[SI-LIST] Re: Potting/encapsulation/conformal coating and SI

One other thing to deal with if you intend on using conformal coating or 
potting  on your PCB assembly is to make absolutely sure your PCB is 
absolutely DRY and free of contaminants prior to applying the coating 
lest you want bad things to happen to the circuitry some time in the 
future.  A pre-coating bake is usually recommended to ensure a dry board 
prior to coating.

As others have mentioned, some coatings can be lossy at RF frequencies. 
An option to consider is called selective coating. This allows only 
selected portions of the circuitry to be coated while allowing  other 
areas to be left uncoated.

-Ray



VVLP wrote:

>I've been involved in the trials and tribulations of potting small
>transmitters operating at 900MHz. We used a silicon based potting
>compound which was not that lossy and had a reasonably controllable Er
>which (from memory)was about 2. This had a significant impact on microstrip
>that obviously had to be designed with the layer of potting rather
>than air above it. It is quite soft and has little mechanical impact on
>components.
>The major problems for us were in the board / potting interface where
>sometimes the material wouldn't cure properly - but that's a different
>story.
>
>Regards
>Dave
>  
>
.
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