Electronics Protection - Summer 2017 - 8

Feature

Shielding Tips and Tricks
Holland Shielding Systems BV
Shielding radiated emission and susceptibility of electronics components can be done in many ways. Sometimes it is
possible to achieve the same goal in 10 different ways. But what is the most economical manner and which has the
longest lifespan? In this article we have 100 shielding tips and tricks that can help you make the right choice.

Principle of Shielding

1. The principle of shielding is creating a conductive layer completely surrounding the object you want to shield. This
was invented by Michael Faraday and this system is known as a Faraday Cage.
2. Ideally, the shielding layer will be made up of conductive sheets or layers of metal that are connected by means
of welding or soldering, without any interruptions. The shield is perfect when there is no difference in conductivity
between the used materials. When dealing with frequencies below
30 MHz, the metal thickness affects shielding effectiveness. We also
offer a range of shielding methods for plastic enclosures. A complete
absence of interruptions is not a realistic goal, since the Faraday cage
will have to be opened from time to time so electronics, equipment
or people can be moved in or out. Openings are also needed for displays, ventilation, cooling, power supply, signals etcetera.
3. Shielding works in both directions: (Fig. 1) items inside the shielded
room are shielded from outside influences.
4. The quality of the cage is expressed as the ratio of the field strength
in Volts/meter (V/m) inside the cage and outside the cage.

Figure 1. Shielding works in both directions

5. It is common practice to present field strength figures in a logarithmic scale.
6. The reduction depends on the frequency in Hz. Each frequency
has a wavelength in meters. For example 100 MHz = 100.000 Hz =
3 meter. For a better explanation, see Fig. 2.

Wave

Figure 2. The reduction depends on the frequency

7. A wave is a combination of electric field and magnetic fields. A electromagnetic wave is composed of a magnetic part depending on the electric
current (Ampere), and an electrical section, depending on the electrical
voltage (volts) (Fig. 3). Near the source (near-field) the magnetic part is
dominant. At a greater distance, the electrical part and the magnetic part
are present in a fixed ratio (far field).
8. The material thickness determines which frequencies are blocked from
penetrating into or out of the cage. For low frequencies like 10 kHz (generally the near-field/magnetic fields), a mild steel layer of 6 mm is needed to
Figure 3. Wavelength vs. Frequency
achieve a reduction of 80 dB, but a frequency of 30 MHz can be shielded
by copper foil that is only 0.03 mm thick. For higher frequencies in the GHz
area the mechanical strength of the used shielding material will generally specify the thickness of the shield.
9. For very low frequencies and DC, where the magnetic field is dominant, besides thick layers also special materials
like Mu-metal and Mu-ferro alloys are needed. In addition, combinations of multiple layers are required to get sufficient shielding performance.
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Table of Contents for the Digital Edition of Electronics Protection - Summer 2017