A 90 dB material does not make a 90 dB box

Solid 0.5 mm aluminum offers well over 90 dB of shielding effectiveness at 1 GHz as a material. The enclosures built from it routinely measure 20–30 dB at the same frequency. The missing 60+ dB did not leak through the metal — it leaked through the seams, apertures, and cable penetrations. Enclosure shielding is not a material-selection problem; it is a discontinuity-management problem, and this issue derives it from first principles.

The full SE equation

Total shielding effectiveness of a barrier is the sum of three mechanisms (Schelkunoff):

SE_total = A + R + B (dB)

A — Absorption loss: SE_A = 8.69·(t/δ), where t is wall thickness and δ is skin depth.
R — Reflection loss: the impedance mismatch between the wave and the metal; large at low frequency for electric fields, small for magnetic fields.
B — Multiple-reflection correction: a negative term, significant only when t < δ.

The governing material parameter is skin depth:

δ = √(2ρ / (ω·µ)) = √(ρ / (π·f·µ))

For copper (ρ = 1.68×10⁻⁸ Ω·m, µ = µ₀): δ ≈ 2.1 µm at 1 GHz, 66 µm at 1 MHz. Any wall many skin depths thick has effectively infinite absorption loss — which is precisely why the material is never the limit. The limit is everywhere the material is interrupted.

The real limit: seam transfer impedance

A seam is two metal faces that must carry shield current across the joint. The current flowing in the shield wall must cross the seam; the seam's transfer impedance Z_seam (Ω) converts that current into a voltage that drives leakage. Leakage rises with:

Slot length — a seam of length L radiates per SE_aperture = 20·log₁₀(λ/2L). At 1 GHz, a 15 mm gap caps SE at ~20 dB no matter how thick the wall.
Contact spacing — fasteners or contact points spaced d apart leave the seam between them free to radiate; keep d ≤ λ/20 at the highest frequency of concern.
Contact resistance — corrosion, oxide, paint, or compression loss raises Z_seam directly.

Aperture rule of thumb: many small holes beat one big hole. N holes of diameter d have far less leakage than one hole of area N·(πd²/4), because each small hole is deep below waveguide cutoff f_c = c/2w. This is why honeycomb vents work.

⚠️Conductive paint and compression set will betray you

Conductive paint degrades with handling and thermal cycling; its surface resistance can rise 10× over service life, adding 20 dB to Z_seam. EMI gaskets fail by compression set — an elastomer held compressed loses its restoring force, contact pressure drops, Z_seam climbs, and SE silently decays. Specify gasket compression to the manufacturer's working range (typically 10–30% deflection) and design the joint stiffness so it stays there over temperature and life.

Gasket selection physics

A gasket restores conductivity across a seam. It must (1) make low-resistance contact, (2) maintain contact pressure over life and temperature, and (3) survive the galvanic environment. Galvanic compatibility matters: an aluminum enclosure with a pure-silver-filled gasket forms a galvanic couple that corrodes the aluminum; match the gasket filler to the flange plating. Common choices: Monel or tin-plated mesh for steel/tin flanges; nickel-graphite-filled elastomer for aluminum.

Board-level shields: the source-level stackup

The most efficient place to shield is the smallest enclosure — a board-level can over the noisy component. A board-level shield is a layered system:

The board-level shield's SE is governed by the same physics as the box: the wall is never the limit; the fence-to-ground solder joints are. A continuous soldered fence at ≤ λ/20 pitch, over a via-stitched ground pour, with a 360° gasketed cable entry, delivers 30–45 dB from 100 MHz to 6 GHz — enough to convert most CISPR 32 / MIL-STD-461H RE102 failures into passes at the source.

Worked example and material economics

A board fails RE102 (now extended to 40 GHz coverage for select classes under MIL-STD-461H, released April 17, 2026) at 2.4 GHz. A POCONS custom two-piece shield — stamped nickel-silver frame soldered to the ground pour, removable lid — placed over the RF front end gives 38 dB SE at 2.4 GHz. Pass with 16 dB margin.

A note on material cost in 2026: with copper at $13,335/t, tin at $51,613/t, and nickel at $18,985/t, board-level shield stock is dominated by the base metal and its plating. Tin's price makes plating thickness a real cost lever — specify 2–5 µm tin for solderability and galvanic match, not more. Tinned cold-rolled steel is the value option; nickel-silver is the premium choice where formability and corrosion resistance justify it.

POCONS connection

POCONS USA (San Diego) designs and sells custom board-level EMI shields, clips, and stampings, with products manufactured in Korea (IATF 16949). A shield designed to your component footprint, with the fence pitch, cable-entry gland, and plating chosen for your frequency and galvanic environment, is the difference between a 25 dB generic can and a 40 dB engineered solution. The physics in this issue is the spec sheet for that conversation.

One thing

The metal is never the limit — the seams are. SE_total = A + R + B tells you the wall is fine; transfer impedance and aperture length tell you the truth. Solder the fence at ≤ λ/20, stitch the ground pour, terminate cables 360°, and keep the gasket in its compression range for life.