The Physics of a Foil Enclosure
A Faraday cage works by two mechanisms: reflection of the incident wave at the conductive surface, and absorption of the energy that penetrates. Shielding effectiveness is expressed as SE in decibels, the sum of reflection loss, absorption loss and a multiple reflection correction. Reflection loss dominates for a thin conductor in a low impedance or plane wave field, while absorption loss grows with thickness and frequency. For aluminium with a conductivity of about 3.77 x 107 S/m and a relative permeability of 1, both mechanisms are available from a material a few micrometres thick.
Skin Depth: Why Foil Thickness Matters Only at Low Frequency
Skin depth is the depth at which the current density has fallen to 1/e of its surface value, and it is given by the square root of 2 divided by the product of angular frequency, permeability and conductivity. For aluminium the resulting values are:
| Frequency | Skin depth in aluminium | Absorption loss for 12 micrometre foil |
|---|---|---|
| 10 kHz | about 0.8 mm | less than 1 dB |
| 1 MHz | about 82 micrometres | about 1 dB |
| 100 MHz | about 8 micrometres | about 13 dB |
| 1 GHz | about 2.6 micrometres | about 40 dB |
Absorption loss is estimated as 8.7 times the thickness divided by the skin depth, in decibels. The table shows the practical boundary: standard household foil around 10 to 15 micrometres thick behaves as an electrically thick shield from roughly 10 MHz upwards, but at mains and audio frequencies it is far thinner than one skin depth and most of the attenuation comes from reflection and from the grounding of the enclosure. This is why a foil wrap can block a mobile phone signal indoors and still fail badly as a low frequency magnetic shield.
Where Real Faraday Cages Leak
The dominant loss of shielding in a foil enclosure is not the foil but the discontinuities. Any aperture larger than roughly one twentieth of a wavelength acts as a slot antenna and leaks efficiently. At 1 GHz, one wavelength is about 300 mm, so a 15 mm gap is already a serious radiator. Other leakage paths are the seam overlap, which should be at least 10 mm and bonded or folded rather than merely butted; cable entries, which must be filtered or terminated to the enclosure wall; and the oxide film on aluminium, which grows to 2 to 4 nm immediately in air and produces a rectifying, high resistance contact unless the joint is mechanically bonded or assisted with a conductive compound.
Materials, Forms and Construction Practice
Aluminium foil, metallised film and conductive fabric all appear in shielding products. Foil gives the highest conductivity and the most stable long term performance, but it creases and punctures easily and creasing can crack a very thin layer. Metallised film is more flexible and resists flexing fatigue better, at the cost of lower conductivity. For packaging that must also control static discharge, the usual construction is an outer layer of a static dissipative polymer over a metal layer, and such bags are qualified to ANSI/ESD S541 for the protection of electrostatic discharge sensitive devices. Seams in a constructed enclosure are made by folding, by conductive adhesive tape or by welding, and the continuity of the joint should be verified by resistance measurement across the seam.
Testing Shielding Effectiveness
Material level shielding is measured by the flange coaxial transmission line method of ASTM D4935, which gives shielding effectiveness against a plane wave over a defined frequency range, typically 30 MHz to 1.5 GHz. Enclosure level performance is measured by the methods of IEEE 299, which uses calibrated antennas inside and outside the enclosure and reports attenuation at defined frequencies. Both methods report decibels, and a quoted figure is meaningless without the frequency range, the test method and the field type, because a foil cage that measures 60 dB at 900 MHz may measure only a few decibels at 50 Hz.
Limits and Common Misunderstandings
Three points are routinely misunderstood. First, a Faraday cage blocks electric fields and radio waves, but it does not block a static magnetic field. Second, aluminium foil does not shield by earthing alone; the enclosure must be electrically continuous, and a floating wrap will drain charge only if it is bonded to a defined potential. Third, wrapping a device that transmits its own signal will not silence it in every condition, because a foil layer close to an antenna detunes the antenna and can change the emission spectrum rather than simply absorbing it. Any shielding application that affects a certified product should be verified by measurement, not by assumption.
Frequently Asked Questions
Q: How thick must aluminum foil be to block a mobile phone signal?
A: A single layer of ordinary household foil around 10 to 15 micrometres thick is generally enough above a few hundred megahertz, but the result depends on how completely the item is wrapped. Coverage and seam overlap matter more than adding a second layer.
Q: Does aluminum foil block magnetic fields?
A: Only alternating magnetic fields, and only as far as its thickness approaches one skin depth. A static field is not attenuated, and at 50 or 60 Hz the skin depth in aluminium is of the order of 10 mm, far thicker than any foil.
Q: Why does a foil wrap sometimes fail to shield even though it looks sealed?
A: Gaps, creases and seams dominate. An aperture larger than about one twentieth of a wavelength radiates strongly, and the natural oxide film on aluminium creates a high resistance contact at seams unless the joint is bonded.
Q: Which standards are used to measure shielding effectiveness?
A: ASTM D4935 measures the shielding effectiveness of a flat material with a flange coaxial holder over a defined frequency range, and IEEE 299 defines methods for measuring the shielding effectiveness of an enclosure.
Q: What is the difference between a foil shielding bag and a metallised film bag?
A: A foil layer gives higher conductivity and more stable attenuation but creases and can crack. A metallised film is more flexible and survives repeated folding better, with lower conductivity. Bags for static sensitive devices are qualified to ANSI/ESD S541.
Q: Can aluminum foil be used to shield a whole room?
A: It can provide attenuation, but only if all six surfaces are continuous and every cable entry is filtered at the boundary. Practical room shielding uses rigid foil faced panels, bonded seams and filtered penetrations rather than loose foil.
