nRF52840 - components on both layers

[Fran89]

Dear all,

I recently made a custom PCB using the aQFN73 package of the nRF52840 and, even though I respected the antenna constraints (but in 2 layers) I get no signal whatsoever (measured with spectrum analyzer) from the RF antenna. Unfortunately I cannot check if there is a short circuit between the ANT pin and VSS (2 ohm resistance between ANT and VSS measured with multimeter) so I am building a board using the BGA package this time (so I can reflow it myself if need be, with the aQFN it is impossible).

I am using Eagle and there are no reference designs using the BGA package.

Question 1) Can I copy the aQFN72 QIAA layout and replace it with the WLCSP CKAA package? If not, can you provide an Eagle layout for it, please?

Secondly, for size purposes, I would like to use a 4-layer design and put some components on the other side of the board.

Question 2) Is it possible to use a 2nd layer GND layer, routing on the 3rd layer and components on the 1st and 4th layers?

Since the BGA package requires buried vias, maybe it would be better to do the routing on the second layer with a keepout zone under the RF matching circuit, so that the vias of the BGA don't go through the ground layer?

Also regarding the other 3 layers, should the empty patches be filled with ground planes and connected through multiple vias to the 2nd-layer ground layer?

There is another post about this but the answer is not clear as "you are allowed to place components in the bottom layer" as I would require.

I would then later need to use a U.Fl. connector to use an external antenna, but I will make another post for that, after this one is solved.

Bests and thanks a lot in advance,

Fran89

[Andreas]

Hi,

 

The ANT pin is DC short to ground, so ~2Ohm resistance is as expected.

 

1) No, the QIAA variant pinout is completely different from the WLCSP ballout and the passives are on different sides (the die is flipped), you need to use the variant your layout is made for. We have some eagle libraries here, but they are not our official reference layouts and not regularly maintained. unfortunately the WLCSP variant of nRF52840 is not in there, and it probably will not be for some time.

2) You should have GND pour where possible in all layers, a single layer for ground is not optimal. A keepout in L2 is possible, but as mentioned you might instead make a local ground pour to maximize the amount of ground.

Components in the bottom layer is generally fine by us, provided the grounding is done well. You should try to keep the RF part in one layer though. Also check with your EMS that they can support this.

 

Best regards,

Andreas

[Fran89]

Thanks a lot.

I will try this and comment after the boards are built.