Potting circuits using the nRF9151

Question

Is it advisable to POT circuits using the nRF9151? If so, please advise of any precautions. The IOT product that I'm designing requires to be intrinsically safe. As such, potting will be necessary.

JONATHAN LL · Answer

The nRF 9151 the SiP is encapsulated under the Microshield. Potting the SiP itself should not be a problem, provided there is no chemical incompatibility between the potting compound and the plating (Ni/Cu) or as long as there is not unwanted added mechanical stress during curing of the potting that could cause the SiP to warp or cause micro fractures in the soldering around the SiP or for its passive components. The RF performance will not be affected unless you also pot the external RF front-end and antenna. You can make up for some of this loss by taking into account the potting material when determining RF transmission line dimensions (for 50 Ohm) or when tuning the antenna, but some antenna efficiency is probably lost due to the compound. It is recommended that you keep the antenna and one side of the ground plane outside the compound if possible. Potting is not ideal for RF performance. Performance might be degraded enough that it does not comply with operator/etc. requirements to EIRP and possibly more. Using an external antenna might workaround this. You should also keep in mind that the compound might hamper the thermal dissipation, causing the SiP to overheat, probably not a problem with good thermal PCB design. We do not have any guidelines for potting unfortunately. There is RF transparent potting but we don't provide a list of options or alternatives, you will have to work together with the provider to figure out what works best for your use case. But we have seen other customers use EPO-TEK 320 for some of our other products, but we don't have data on how it impacts the passive components. So I cant provide any guarantees there. Regards, Jonathan