[SI-LIST] Re: Potting/encapsulation/conformal coating and SI
- From: VVLP <davew@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 5 Oct 2004 14:56:27 +0100
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
Tuesday, October 5, 2004, 1:35:13 PM, you wrote:
> You also would want to choose a potting material that doesn't exhibit
> undesirable characteristics (i.e., loss) at the frequencies of interest.
> I know zilch (nothing) about the electrical characteristics of potting
> materials, so I don't know whether there are lossy compounds that you need
> to avoid. Certainly anything that is even slightly conductive, or sprinkled
> with fine metal particles, would be bad. This sounds like something where
> the electrical engineer should help specify the potting material and make
> sure someone can't change the formula later, not realizing the electrical
> impact it could have.
> I have seen L-C circuits that were potted. Because it's in such close
> proximity to the coil's windings, it probably affects their self-capacitance
> as well, so final tuning should take that into account.
> Regards,
> Andy
>> Be sure to evaluate what the coating will do to the impedance of your
>> PCB traces. If you intend to coat the board, design to account for that
>> effect. Adding conformal coating to a properly operating board _may_
>> cause improper operation of the circuitry if you are depending on a
>> particular impedance.
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