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Re: Conformal coating of shielded RF modules

I have an IoT device that consists of a PCB that goes inside a watertight enclosure. The device will operate under constant water, since it will be installed below sprinklers. For an added layer of protection, mainly due to possible human errors when closing the case, I want to add a silicone conformal coating to protect against moisture or small amounts of water that may leak inside. However, my PCB uses a Microchip RN2903 LoRa module, which has an RF metallic shielding as shown below:

rn2903 module

As you can see, the module has three small holes on the top, as well as vertical gaps in each corner. My main question is: If I completely cover the holes in the metal shield, could the trapped air inside the shield then cause problems to the module? Maybe due to the pressure of temperature change? The inside temperature of the device is about 15 Celsius at night and 40 Celsius in the day. Then again, the whole inside of the device would have these temperature changes, not just the RF module, so maybe it would be fine?

If you have any ideas, or suggestions on conformal coating modules with an RF shield, please let me know!

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You mustn't have coating inside the can, or on the RF traces to the connector.

Air has a lower \$\varepsilon_\text{r}\$ than coating material. The physics of controlled impedance RF traces depend on the surrounding medium having an \$\varepsilon_\text{r}\$ much lower than that of the board material. Things might simply stop working if you coat such traces!

At 433 MHz such effects are likely to be benign. However, if the module uses e.g. MEMS oscillators, then that might quickly become a problem. It feels like the three holes in the module are there on purpose.

I'd definitely contact Microchip about this. Conformal coating is certainly something their application engineers have dealt with before.

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    \$\begingroup\$ Thank you for your comment. I had read about this, but went ahead regardless and tested modules with and without the conformal coating, could not see any significant differences in SNR reaching my LoRa gateway (about 5 km away). So, I think in that sense it's not much of a problem for me. I will contact Microchip, thank you for the suggestion! \$\endgroup\$
    – Dan Alvarez
    Commented Mar 10, 2022 at 15:49
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    \$\begingroup\$ "Air has a higher εr than coting material" Opposite! Air is 1. Conformal coating is around 2. \$\endgroup\$
    – winny
    Commented Mar 10, 2022 at 16:04
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    \$\begingroup\$ @winny darn, that's what I meant to write (otherwise the "substrate >> surrounding medium" makes little sense) \$\endgroup\$
    – Marcus Müller
    Commented Mar 10, 2022 at 16:30

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