Mounting PCB onto metal housing

[buyan]

Dear All,

Normally, we use screws to mount PCB to metal housing. I would like to know whether the screws' position will affect the RF performance of the PCB. Or will the position affect the grounding of the PCB? thanks for your help.


rgds
buyan

 

[biff44]

Well, two thoughts:

If you just drop a pcb onto a metal surface, because the pcb is warped (sometimes a lot due to soldering processes) it does not lie flat. If a cavity forms between the pcb and the metal, then you can have a resonator that does very weird things at certain frequencies. So you need some screws in the center to break up larger areas with a "grounding post" to short out those resonances. Obviously, the higher the frequency you have, the tougher it is to stop those effects.

Secondly, you need to have an excellent contact between the ground plane on the back of the board and the metal surface at EACH RF CONNECTOR. This is important because, while on the board Faraday's law says most of the ground currents will stay on the top surface of the ground plane, at the connector interface the ground currents are allowed to loop around over the groundplane edge to the back side (where you do not want them to go). So if you make an excellent contact at those points to the metal surface, and assuming the connectors are also connected to the metal surface, then the ground current goes straight from the board to metal to connector shell and you have little inductance/radiation/resonance problem. If I have the room, I like to put a screw on either side of each connector interface (2 per connector) holding the pcb down.

 

[buyan]

biff44, thank you so much.

I know that the grounding is very important in RF circuit. Someone told me the distance between screws should be wavelength/10. I actually have no idea why there is such a distance constraint. For ideal case, the PCB should stay on the metal surface perfectly. So I conclude that the more screws, the better the contact between PCB and metal surface.

Hope someone can give me some advice.

buyan