I have been designing MCU based PCB that must be withstand hard EMI tests. This PCB includes ethernet transceivers, can transceivers, rs485 transceivers, II2 chips, 25MHZ oscillators, optoisolators and some inputs.PCB will be connected to metallic enclosure by screws. There isn't any RF antenna on my PCB. I need an expert device about shielding and grounding.
My plans and current situation are:
- There will be a outer border plane for chassis ground. It will be connected to metallic enclosure. And this chassis plane will be connected to ground by 22nF 1000V capacitor at one point (I will put spare capacitor connection for all edges but for first I will placed just 1) I used this method on my other PCB and it was very good for ESD.
- I'm planning to use board level shielding. Firstly I have been decided to cover MCU,oscillator and nearest coupling capacitors in 1 shield. Other Ethernet transceiver and capacitors will be covered by other shield.
My basic questions are :
- Need I faraday cage between chasis plane and ground plane ? Will It be helpfull to reduce any EMI ?
- How can I draw traces that must be connect any point where is out of shield? Is below figure proper? Can I draw traces just between the shield pins/pads. Won't it break the shielding? Or I have to use vias and draw trace on bottom layer?
Which components I must cover? My covering groups that I explained above is proper? Or I must cover all components in 1 shield. Or any other combinations ?
Where shield pads must be connect to ? Directly to the ground or chassis or anywhere?