EMI Shielding Materials

by William D. Kimmel, P.E.

and Daryl D. Gerke, P.E.

Kimmel Gerke Associates, Ltd.

We are often questionead about what kind of shielding material is needed for an enclosure. This is especially true in the military world, where the mechanical engineer is concerned about other important factors, including weight, strength, corrosion, etc. In most cases, the answer is, use whatever you want – the material is usually not important.

The fact is that most metals are such good conductors that the shielding effectiveness of the enclosure is determined by factors other than conductivity of the material, specifically openings and penetrations.

But there are cases where the shielding material becomes an important factor. In particular if the shielding must extend to low (power line) frequencies, then ferromagnetic materials must often be used. Let’s start with a brief look at shielding mechanisms, then take a look at the most common cases of shielding material failure.

Reflection and Absorption

There are two modes of shielding, reflection and absorption – total attenuation is the sum of the two effects.

Reflection is primarily a surface phenomenon, depending on an impedance mismatch between the incident wave and the barrier. This is the same phenomena as a mirror, albeit at a different part of the electromagnetic spectrum. A plane wave with an impedance of 377 ohms encounters a low impedance at the surface of a good conductor, and most of the energy reflects. At low frequencies, the conductivity is high, but skin effect squeezes the currents to the surface, reducing effective conductivity, and resulting in lower reflection.

For shields near the source, however, the impedance may be much different than that of a plane wave. The impedance for an electric field is high, so that shielding effectiveness (SE) is even higher than that of a plane wave. Thus, we see that shielding electric fields from power sources is almost trivial. The big problem is low impedance waves (magnetic fields), originating largely from windings from motors and inductors, but also from power distribution. These low impedance waves reflect very poorly, giving negligible SE at power frequencies.

Absorption is primarily a volume phenomenon, like insulation in a building – the thicker the better – or, more accurately, the more skin depths the better. At high frequencies, where the skin depth is very small,the losses become higher, so that absorption dominates over reflection. Absorption is a function of conductivity and permeability but is not dependent on wave impedance. Thus, permeable materials are good absorbers. Permeability is also a strong function of frequency, decreasing to near unity at 1MHz for most metals.

For reference, we usually expect to see commercial shielding effective requirements between 30 and 60dB, military SE between 60 and 90dB, and special cases SE above 90dB and sometimes even above 120dB. Conductive coatings will typically produce upwards of 60dB of SE, good enough for commercial applications and even for some military applications. Essentially all metals will produce 90dB of shielding and the metals most commonly used for enclosures will generally produce 120dB of SE.

Low Frequency Magnetic Fields

Having armed ourselves with the basic requirements for shielding, we can now turn to the most common shielding failure, which is for low frequency magnetic fields. AC power is the most common source of low frequency magnetic fields As mentioned above, reflection attenuation for magnetic fields is poor, which leaves the task to absorption.

Some years ago, we had occasion to do some magnetic field shielding effectiveness tests and discovered that the commercial grade shield room went transparent to magnetic fields below about 10kHz.

In another case, we encountered a shielding effectiveness with a rack-mounted power supply. The power supply had been enclosed in a steel enclosure, but had been converted to an aluminum enclosure to reduce weight in a military application. Unfortunately, this created a low frequency emission problem that couldn’t be fixed with an aluminum housing.

As mentioned, absorption depends on thickness, especially permeable materials. Sheet metal is not thick enough – you need plate steel. For power distribution, thinwall (EMT) provides but little attenuation – rigid conduit will give about 20dB at 60Hz.

Materials with higher permeability are more effective at shielding, but are very prone to saturation and cannot tolerate high magnetic fields.

Thus, we see that shielding is not easy to achieve, so take all possible steps to reduce the field, including balancing or canceling magnetic fields and increasing the distance from source to receptor.

Very Thin Shields

The second case is where the shield is very thin. This situation is usually not significant even for thin metal coatings of a few mils. Skin depth actually becomes noticeable at power frequencies and decreases to a few mils at 1MHz. So at frequencies above about 1MHz, the thickness of the shielding material is not terribly relevant. This is the situation for most conductive coatings.

The notable exception is for very thin coatings, as with optical coatings (most common is Indium-Tin-Oxide, or ITO). The layer is of the order of 10nm (nanometers) – so thin that the resistivity is high at any frequency, limiting SE. Lower resistivities are perfectly feasible, at the expense of visual transmissivity – if you want see through the panel, you will have to settle for less shielding effectiveness.

Even so, thin coatings provide adequate shielding for many purposes. Some years ago, we used ITO for CRT shielding on a TEMPEST program – it was not quite adequate but, then, the requirements were very stringent.

If ITO is not adequate, you can go to a silver coating, which is a significantly better conductor but lots more expensive. Conductive screen gives significantly better shielding effectiveness, at the expense of degrading the visual resolution.

Regardless of material, the coating must be conductively bonded to the enclosure all around the perimeter.

Very High Attenuation

The last, and least common, case of shielding failure is where very high attenuation, typically more than 120dB, is needed. In terms of materials needed, copper will pretty much give this much attenuation (low frequency magnetic fields excepted) all across the RF spectrum. At low frequencies, reflection is exceedingly effective, and at higher frequencies, absorption is very effective – the minimum attenuation will be in the 100MHz range, depending on thickness of material.

Considering that most military shielding requirements are about 80dB, we see that we have lots of headroom, no matter what material we use.

If you do have high shielding requirements, your material selection now becomes important. Copper is the best conductor of the affordable materials, and would be the first choice. If this is not enough, your next move is to use a double shielding layer – a box within a box. In effect, this is what shield rooms do, with two conductive layers sandwiching a dielectric layer.

You can be flexible with your shielding geometry. If your entire housing requires high attenuation, then the double shield approach mentioned above is your only effective choice. But in many cases, you will have mixed shielding requirements – modest shielding for some of the hardware and high shielding for other hardware. In this case, you can consider a shield surrounding only the highest shield needs (either very noisy or very sensitive), and another shield over the entire electronics. Thus, you might put a 60dB shield over a portion of the electronics and another 60dB shield over the entire electronics.

Summary

For most shielding needs, just about any shielding material will do the job, so your efforts should be directed at plugging the gaps – seams and penetrations will dominate. In practice, you need to do a very good job of plugging the seams and penetrations before the shielding effectiveness of the material is even measurable.

But there are cases where shielding materials are not up to the task, specifically low frequency magnetic fields, very thin coatings and very high shielding needs. When these cases exist, you need to pay particular attention to the solutions at the start, or you won’t get there.