nRF52840 Layout

[jpelo]

Figure 222 of the OP spec states "important: no components in bottom layer". Are you recommending that we keep the bottom side clear under/near the nRF52840? This feels a bit onerous in terms of small/space constrained implementations. Can you expand on this note?

Also, in terms of the thru-vias in the exposed GND pad is there any characterization of lost performance if these vias are reduced, exchanged for uvias to the next layer, or removed and connected on the top sheet?

Thanks.

[FormerMember]

To have a stable antenna impedance, ground is very important, because ground is the reference for the antenna. The larger and more solid the the ground layer is, the more stable the antenna impedance will be. Therefore, we recommend to not have any components in the ground layer.

Note: In the reference layout, the ground layer is placed in layer 4. But it will work fine to have the ground layer in layer 2 as well. Having the ground layer in layer 2 will make the design more flexible, because there doesn't need to be any keep-out areas in layer 3 and 4.