Placement of nrf5340 and AS7038 AFE

Hi,

It should be OK having a component on the opposite side of the board, but there are a couple of things to keep in mind:

• The ground pad/pins under the SoC must have a good connection to a internal ground layer, this require the use of  blind vias under the SoC .
• There must be a continuous ground layer under the entire RF path, from the ground connection under the SoC to the antenna feed point.
• If the layer directly under the RF transmission line is used as the reference ground layer the width of the transmission line must be calculated for it to have the correct impedance of 50 Ohm

Best regards,

Bendik

Is it okay to put via on RF trace? We are planning to route the track from 4 layer rigid (top layer) to a 2 layer flex PCB.

It is not recommended to route the RF transmission line through a via.  If the via is not properly impedance matched to the transmission line there will be loss of TX and RX signal strength, which will lower the maximum range.
Tools like Saturn PCB Desing Toolkit can be used to calculate the via impedance. This will help reducing this loss, but it will not completely fix the problem.

It is not recommended to route the RF transmission line through a via.  If the via is not properly impedance matched to the transmission line there will be loss of TX and RX signal strength, which will lower the maximum range.
Tools like Saturn PCB Desing Toolkit can be used to calculate the via impedance. This will help reducing this loss, but it will not completely fix the problem.