EMC Shielding Effectiveness
Shielding effectiveness (SE) quantifies how much an enclosure or barrier reduces the electromagnetic field at a point. It is expressed in dB as the ratio of the field without the shield to the field with it. Good shielding is essential for meeting regulatory limits (FCC Part 15, CE/ETSI), preventing interference between circuits, and protecting sensitive receivers from external interference.
Shielding Mechanisms
A metallic shield attenuates fields by three mechanisms:
- Reflection (R): At the air-metal interface, impedance mismatch reflects waves. Significant even for thin shields. Dominant at low frequencies for electric fields.
- Absorption (A): Energy is dissipated as the wave propagates through the metal. A = 20·log₁₀(e) × t/δ = 8.686 × t/δ (dB), where t is shield thickness and δ is skin depth. Increases with frequency.
- Multiple reflections (B): For thin shields (t < δ), waves bounce between the two surfaces, reducing the net absorption. B is negative and usually negligible when A > 15 dB.
Shielding Effectiveness vs Frequency
| Frequency | δ (Cu) | A (1mm Cu) | R (plane wave) | Total SE (1 mm Cu) |
|---|---|---|---|---|
| 1 kHz | 2.1 mm | 4 dB | 132 dB | ~136 dB |
| 100 kHz | 0.21 mm | 40 dB | 112 dB | ~152 dB |
| 1 MHz | 66 µm | 131 dB | 102 dB | >200 dB (limited by seams) |
| 1 GHz | 2.1 µm | >400 dB | 62 dB | Limited by apertures |
Above ~10 MHz, even a very thin copper sheet is electromagnetically opaque — the limitation is apertures (holes, slots, seams), not the metal itself.
Aperture Leakage
Holes and slots in shielding enclosures are the dominant leakage path at high frequencies. A rectangular aperture of length L acts as a half-wave resonant slot antenna at f = c/(2L). Below resonance, SE decreases by 20 dB/decade as frequency increases:
Practical consequences:
- A 30 cm slot (ventilation opening) resonates at 500 MHz — completely transparent above that.
- A 3 mm slot resonates at 50 GHz — fine for cellular but useless at mmWave.
- Replace single large holes with many small holes of the same total area: N holes of diameter d instead of one hole of diameter D=d√N gives ~20·log₁₀(√N) more SE.
- Honeycomb waveguide below cutoff arrays provide ventilation with >100 dB SE.
Seams and Joints
The weakest point of most shields is where panels join. A seam with periodic fasteners (screws every L mm) acts like an array of slots of length L. For good HF shielding, seams must be:
- Soldered or welded for maximum conductivity
- EMI gaskets (beryllium copper finger stock, wire mesh, conductive foam) for removable panels
- Screws spaced to λ/10 at the highest frequency of concern
- Conductive paint or plating on plastic enclosures (provides 30–50 dB SE)
Cable Shield Transfer Impedance
Cable shields leak energy through their transfer impedance Z_T (Ω/m) — the ratio of voltage induced on the inner conductor to the current flowing on the outer shield:
| Cable type | Z_T @ 10 MHz | SE @ 10 MHz |
|---|---|---|
| RG-58 (single braid) | 10–50 mΩ/m | 50–60 dB |
| RG-214 (double braid) | 1–5 mΩ/m | 70–80 dB |
| Semi-rigid coax | < 0.1 mΩ/m | > 100 dB |
| Foil + braid | 2–10 mΩ/m | 60–75 dB |
Always connect cable shields at both ends for RF shielding. Single-end grounding (for audio hum prevention) leaves the shield as an antenna above a few kHz.
PCB Shielding Cans
Stamped metal shielding cans solder directly to a PCB ground pour to isolate sensitive circuits (RF front-ends, oscillators). Key design rules: no gaps > λ/20 at the highest frequency; soldered continuously or with fences of vias spaced < λ/20; include a test access point (small hole or removable lid) for production testing.