Why do we need to put PCB inside a (metal) box?

[ericz]

Putting PCB inside a box

Hi,

I wanted to know why do we need to put a PCB inside a Metal (or any other conducting material ) Box ?


Is the PCB's ground should be shorted to the Box so that the Box also acts as a ground?

If the Box is grounded to the PCB's ground, EMI that goes on the box making the metal box noisy and so the PCB's ground will be noisier?

I will apprichiate if anyone could help me and give me some material where can I learn such things.

Best Regards,
eric

 

[IanP]

Re: Putting PCB inside a box

Shielding can be anything from using a coaxial or shielded cable, to a sealed conductive chamber for circuit isolation. Shielding serves a reciprocal purpose: it protects the circuit in it by shielding from outside noise or unwanted signals; and conversely, it contains its own signals and thus protects the outside world from interference of its own making. Shielding is mostly used to block electrostatic or "E" fields (Faraday shield). However, if ferrous metal (tempered Mu Metal works best for magnetic fields) is used then both electrostatic and some level of magnetic shielding is accomplished. This is especially useful where open frame transformers or unshielded coils are used and would otherwise exchange signals by mutual inductance.
When is a Shield a Shield?
One important requirement for a shield to be effective, is that there must be no currents flowing through the shield itself. This is best accomplished by connecting the reference or common, at only one point on the shield, thus preventing any flow of current. The reason for this, is that any current flowing in the shield material itself can produce secondary fields on the other side of the shielding material and thereby reducing the effectiveness of the shield. An extreme case of this might be a shielded cable, whose shield has a different potential at each end, and the resulting current flow in the shield, inducing unwanted noise into the center or shielded conductors. (In this situation one might find a remedy by disconnecting one or the other ends of the cable. However, this may not prove satisfactory in certain environments, and may require a "Guard" potential, which is some compromise potential.)

Regards,
IanP

 

[biff44]

Re: Putting PCB inside a box

A metal box is sometimes used to either stop unwanted radiation from a circuit, or to shield a circuit from picking up unwanted noise )either RF or low frequency hum).

For RF shielding, it is pretty important that the board ground plane be attached to the box at every point that a signal is going thru the box wall. This explains the extensive use of coaxial connectors, that insure the circuit ground is attached to the box ground at the connector. If you tried, for instance, to run a coaxial cable thru a metal box wall without making a grounding connection, you might as well throw the box away, since the outside of the coaxial cable will be a re-radiating antenna now.

The whole concept revolves around the idea of a Faraday Shield. If you have an unbroken metal box around something, there is no way for ground currents to get from the inside surface to the outside surface of the box, so there is no re-radiation from currents floating around on the outside surface of the box.

Now most metal boxes are copper/aluminum/steel/nickle, and are there to stop electromagnetic fields. These boxes have little or no effect on magnetic fields (like 60 Hz hum from a big transformer or motor). To stop magnetic noise, you need a mu metal box.

 

[springf2000]

Re: Putting PCB inside a box

when every part of the circuit need isolation, then a metal box can help you to do this!

 

[mudrik]

Re: Putting PCB inside a box

Metallic Box is a killer of electromagnetic radiation (waves and fields).
It might not be a massive box to protect your circuit or to prevent
radiation from your circuit. You can also use boxes made by 'wires'.
As long as the distance of the wires smaller than the wavelength,
the wave penetration is small.

However, you must be aware, that the box does not make additional
resonances, that can destruct your circuit.

regards

 

[ericz]

Re: Putting PCB inside a box

Is the ground inside the box should be connected to the metal box surface at one point only or at every point where there is a signal going outside of the box?

If EMI is covering the metal box - where does the EMI vanishes? is he transformed into current which goes to the ground and doesn't disturb the signal?

Thanks.