EMI/EMC Academy ~30 min Interactive

Shielding & Enclosure Techniques

A shield is only as good as its worst seam. A solid metal box gives 100+ dB — but one unbonded slot the length of a business card can throw most of that away. Shielding is really the art of managing apertures.

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Learning objectives. Explain shielding effectiveness (reflection + absorption); apply the "longest-slot" aperture rule; use gaskets/contact, conductive coatings and correct shield grounding; and estimate an aperture's SE with the built-in calculator.

Shielding effectiveness (SE) is the attenuation a barrier adds, in dB. It has two parts: reflection (impedance mismatch at the metal surface — dominant for electric fields and higher frequencies) and absorption (loss inside the material — grows with thickness and frequency). For a solid conductor the total is huge; in practice, real enclosures are limited not by the metal but by their openings and seams.

Magnetic-field shielding at low frequency is the hard case — reflection is weak, so you need thickness and high-permeability material. Most product EMI, though, is higher-frequency electric/plane-wave energy, where thin conductive walls are very effective if apertures are controlled.

Leakage depends on a slot's longest dimension, not its area. A long thin slot leaks like an antenna; the same area as a round hole leaks far less. A slot radiates strongly once its length approaches λ/2. The approximate SE of a single slot is:

SE (dB) ≈ 20 · log₁₀( λ / 2L )
L = longest slot dimension; λ = wavelength at the frequency of concern. SE → 0 dB when L = λ/2 (the slot resonates and leaks freely).

The design consequence: break one long seam into many short bonded segments. Ten short slots leak far less than one long one of the same total length. This is why lids get screws/clips every few centimetres rather than only at the corners.

Common mistake: a beautiful shielded box ruined by a long unbonded display bezel, ventilation slot, or seam. Also remember: any cable penetrating the shield without filtering/bonding carries energy straight through the wall — see Module 8.

Enter the longest slot dimension and the frequency of concern.

Gaskets & contact. Conductive gaskets, finger stock and close fastener spacing keep seams electrically continuous. The goal is many low-impedance contact points so no slot approaches λ/2.
Coatings & grounding. Plastic enclosures need conductive coating/paint to shield at all. A shield only works if it's bonded — an ungrounded shield can re-radiate. Bond it to chassis with a short, wide, low- inductance connection.
Bench trick: use conductive copper tape to temporarily close a suspected leaky seam, then re-measure with a near-field probe or antenna. A big drop confirms the seam was the leak — then design in a gasket.
Key takeaways. SE is limited by apertures, not metal. Keep every slot well under λ/2, bond seams with many contacts, coat plastics, and always ground the shield. But the most common shield breach is a cable — which is Module 8.
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