# Why is it bad to have unconnected metal areas around PCBs/enclosures for EMI?

I've heard that it is generally bad to have unconnected metal around PCBs due to both safety reasons of charge buildup, but also EMI in the sense that these metal pieces can act as antennas and re radiate to induce coupling into traces.

It seems that when it comes to the solution, it is to have a good low impedance path from the metal to the ground so noise is shunted.

As I try to relate this to my 'small' antenna knowledge, I would have thought that for the signal from the 'antenna' to be properly guided requires an equal impedance to the antenna itself - or else the incoming waves will be re-radiated (like the passives on a yagi.) My logic tells me this trace connecting the radom metal should be somehow matched to the metal and then be matched to ground. I know most noise covers a range of frequencies, hence I am assuming this unkown impedance of the metal to the 'feedpoint' (the connection to ground,) would be imposible to find as it is different for different frequencies (of which you want to sink all to ground)so I don't see why these signals are not just reflected. That is, why does terminating the floating metal even do anything?

In response to feedback, I have supplied a diagram below to help explain my confusion. I am aware that there are many things missing to actually prevent EMC/EMI but I am specifically wondering about the act of supressing noise incident on the metal connector (in this case.)

Diagram of shield with low impedance to ground reference:

Secondly, I see it's common to place a capacitor in series with any metal shield and the ground. Surely this does not solve the chance of any buildup of static. What's the point of the capacitor?

• A bit crudely: For the metal to radiate "on its own" it needs the incoming waves to change the metals potential. The metal can not build up any potential from incoming waves if it's firmly connected to earth. Commented Jul 19 at 16:32
• @MrGerber I would suggest avoiding terms like "earth" in EMI context: an AM broadcast antenna director is by all definitions "firmly connected to earth", yet it radiates extremely efficiently. Earth is a non sequitur at best, and confusing at worst. Or consider the case of a 1mH, 1mΩ inductor tying the metal area in question to safety ground (earth): it's "firmly connected", in some sense, right? Yet clearly it can have almost any AC voltage on it, even from a fairly modest-impedance source. Commented Jul 19 at 17:37
• George: the middle paragraph sounds contradictory; I can't make heads or tails of what you're getting at. Perhaps you could rephrase it, or add a diagram to support your point? Commented Jul 19 at 17:41
• @TimWilliams I agree I agree. I simply mirrored the language used by OP. But you're correct of course! Also why I caveated with "a bit crudely". Commented Jul 19 at 19:00
• @TimWilliams Hopefully i have cleared the question, thanks for reading Commented Jul 20 at 12:16

Two things, in regards to the drawing:

1. There cannot be a "low impedance connection for all frequencies" that also looks like the drawing. A wire or trace has inductance, the metal part has capacitance to the plane, and the resulting slot between metal parts has resonant frequencies; together, a variety of resonant frequencies results, making a leaky connection (there is poor attenuation overall, and there are myriad pass bands). (If nothing else, realize that at optical frequencies, you can see through the gaps!) Below all these resonances, there is asymptotic attenuation, topping out at whatever the DC resistance dictates.

2. To an incident wave, mismatch is intended. Can't mismatch more than a ground plane!* The surest way to make an extension of the ground plane is to make a solid connection to the metal part, so that it isn't a part at all! Short of welding it on, any way to keep gaps small will suffice up to a cutoff frequency, hence EMI fingers, gasket, etc. Bolted lap joints covered with conductive tape are also a common choice.

*Well, a metal plane might be a 99.something% effective reflector; there are dielectric mirrors, albeit over modest bandwidths (enough to reflect white light, say) that are 99.999something%, very close to 100%. Close enough that, if you seemingly pinch off a container lined with em, it doesn't become dark due to total internal reflection, oh no... that light still finds a path out!

Typically we only need performance over a modest range of frequencies, say DC to low GHz (most electronics), or mid GHz, or across the 100s of THz (optical shielding), and minimizing transmission is the goal, not necessarily maximizing reflection: absorption is also an option in many cases.

Examples of use cases:

• EMI gaskets/fingers to close metal enclosures around commercial equipment (DC to ~GHz)
• Anti-resonant gaps to seal waveguides, microwave ovens, etc. (λ/2 long contact gap) -- only needs to be good enough for the frequency in question, lower frequencies excluded by below-cutoff waveguide, higher frequencies ignored by other waveguide components (mode-specific coupling loop, filtering at receiver)
• Absorptive materials, anywhere from radar to visible (black): reduces transmission by absorbing instead; whereas reflective surfaces can't be sealed perfectly and accidental internal reflection paths, waveguides, etc. are likely to be created.

Another strategy, by shift of perspective:

Consider a solid block of metal. Stick a circuit down in the middle of it (somehow). It's not communicating with anything, right? Complete short circuit in all directions, surrounded by reference plane, the perfect shield.

Drill a hole into the block, and run a single wire out from the circuit: voila, coaxial cable. This gives you have a port, a point in space through which waves flow in two directions. Nothing gets in or out except for this connection; bandwidth extends as far as this remains an effective waveguide (but for signal quality reasons, we might be wise to filter out or absorb non-TEM00 frequencies, i.e., so the coax still acts like a coax as we usually think of it), and nothing else gets in or out.

Suppose you mill away most of one side, leaving it thin, but still solid. Now it's a sheet metal panel. At frequencies above several skin depths, isolation is still very good (100s dB?), but below there, there is some transmission.

Suppose you cut a slot in the panel. Now there's a visible path in, and at low frequencies, a slot antenna. For frequencies below cutoff (λ/2), it looks like an inductive loop, and has an asymptotic response (down to DC, where the maximum attenuation depends on the resistance around the slot). Above cutoff, there are resonances for all odd multiples, and eventually waveguide modes, then mode breakup as it becomes multiple wavelengths across, relative to the actual width (kerf of the cut) and depth (panel thickness) of the slot.

When we eventually carve this block down to a narrow wire connected to a plate, we've introduced so many slot modes that high frequencies are just wide open (overlapping passbands), and only the lowest-frequency cutoffs remain, including the asymptotic inductive response until DC. Which can also be described in a bottom-up fashion as the panel (with its resonant modes, and capacitance to the main plane) and wire link creating an LC resonator, and other things.

The housing stops EMI via currents that flow in the metal, producing a field that, from an external point of view, cancels the field from your board. If you block that current with a gap, there is no cancellation.

Antennas don't have to be matched in impedance to their drive circuitry. They are most efficient when matched, but EMI can still be a problem with an inefficient antenna.

A conductor that penetrates the shield directs energy through the hole in the shield it traverses (that is, after all, the point of having such a conductor). Coaxial cable extends the shield, keeping this energy inside the cable. That, of course, depends on keeping the cable shield intact at the far end.

For non-coaxial connections, pay attention to the current. Bypassing EMI current to internal "ground" with a capacitor as close as practical to the shield is effective. Balanced currents on external connections don't couple much to external fields. Put plenty of "ground" return wires in external multiconductor cables to encourage the return current to flow close to the outgoing current.

A separate connection to earth ground is only helpful if some external connection is close to the earth and has an inadequate current return path.