Do Modern Circuit Boards Use Full or Partial Shielding Techniques?

[jeff1evesque]

Hello,

I was wondering if circuit boards today that encorporate shielding do so by covering the entire circuit with a conductor (like faradays cage), or if it can be done by simply coating the backside of the circuit with some conducting layer.

Jeffrey Levesque

 

[mgb_phys]

You typically cover the backside of a circuit board with a single layer - the ground plane. This helps with interference, although you can create problems if noise from one part of the circuit is coupled through the ground plane to another more sensitive part.

On the component side you can cover individual components or small areas with metal shields, normally for high frequency parts like RF tuners.

 

[jeff1evesque]

Are you familiar with faraday's cage? From my limited knowledge, I gather the reasoning that any enclosed conducting volume has zero charge within it (due to the electric field being zero along the equipotential surface, and thus the by Gauss law the enclosed charge being zero). Is faraday's cage (even gauss law) similar at all to the concept of these PCB's with their (one-sided) shield?

 

[mgb_phys]

That is correct, there is no field inside a sealed, grounded ideal conductor.
However in practice they don't work quite as well. It's difficult to be a perfect conductor at frequencies from 50Hz to 5GHz, at higher frequencies any small hole will allow some signal in (or out) and they don't shield magnetic fields at all. Additionally any current induced in part of the screen will generate a filed inside as will any sparks due to breaks in the conducting surface.
Then you have the problem of getting power and signal in/out of the enclosure.

Wrapping aliminium foil around a PCB won't magically make it immune to interference.

 

[jeff1evesque]

That is correct, there is no field inside a sealed, grounded ideal conductor.
However in practice they don't work quite as well. It's difficult to be a perfect conductor at frequencies from 50Hz to 5GHz, at higher frequencies any small hole will allow some signal in (or out) and they don't shield magnetic fields at all. Additionally any current induced in part of the screen will generate a filed inside as will any sparks due to breaks in the conducting surface.
Then you have the problem of getting power and signal in/out of the enclosure.

Wrapping aliminium foil around a PCB won't magically make it immune to interference.

I briefly talked to a professor today, and he said something about the plate is locally polarized, but the electric field doesn't really penetrate the shield. Do you know why the shield doesn't act like a parallel plate capacitor (in a small localized region)? Because if you have some charge outside the shield, then the electric field will cancel inside the shield, while the electric field from the applied charge will exist on the opposite side (only along the localized region).

 

[mgb_phys]

A shield is different from a capacitor, both sides are connected together.
If you have a break in the sheild then two parts of it can have different voltages and a field will exist between them.

 

[berkeman]

Jeff, I googled PCB Shielding, and got lots of good tutorial hits. Here's one:

http://mwrf.com/Articles/ArticleID/5430/5430.html

.

 

[jeff1evesque]

Thanks