20 Shielding Tips and Tricks

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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. There are also 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 etc.

3. Shielding works in both directions: items inside the shielded room are shielded from outside influences, and vice versa.

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.

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.

7. A wave is a combination of electric field and magnetic fields. An electromagnetic wave is composed of a magnetic part depending on the electric current (Ampere), and an electrical section, depending on the electrical voltage (volts). 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 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.

10. When a wire penetrates a shield that is not completely connected to the shield, it will work as an antenna and thus reduce the shielding performance of the cage. This is especially the case at higher frequencies.

Why the Faraday Cage Principle for EMI Shielding?

11. Circumstances in which EMI shielding has to be implemented:

  • When a product has to meet government standards like CE or FCC which regulate immunity and compatibility of products
  • The regulations do not cover the requirements of daily practice (e.g. medical instruments are tested at three meters distance while they are used within 15 cm)
  • Extra safety is desired for military use, e.g. for EMP (electromagnetic pulses)
  • One wants to create increased levels of shielding for TEMPEST requirements, so that there is no risk of spying – see https://en. wikipedia.org/wiki/Tempest_(codename)
  • Sensitive instruments or equipment are to be protected from interfering or harmful frequencies
  • Rules for sensitive measuring and weight equipment like balances and petrol-delivery materials have to be met

12. Other aspects related to shielding

  • Regulations regarding ESD (electrostatic discharge)
  • Regulations regarding ATEX (explosion safety)
  • Lightning protection / EMP/ HEMP / NEM
  • Short circuit protection / prevention of sparks

13. Identification systems like RFID (Radio Frequency Identification) prevent RFID from making contact with the stations Several frequency ranges, lower the frequency are for longer distances:

  • 25 kHz (Low Frequency, LF)
  • 13,56 MHz (High Frequency, HF
  • 860 to 950 MHz (Ultra High Frequency, UHF)
  • 2,45 GHz (Microwave, MW)

14. Medical / personal protection

  • Shielding certain frequencies can prevent illness caused by high radiation levels. To this end there is personal protection in the form of clothing, hats, gloves, stockings, sleeping bags, tents and so on.

How to Create Optimal EMI Shielding

15. In general, a shield consisting of more layers or zones is cheaper to produce than a shield made out of one high-performance layer. It is easy to create three zones:

  • Level I: The component on the PCB is shielded by a can. Shielding at the source
  • Level II: The entire PCB is shielded by foil, wraps or a box or the PCB and all the cables connected to it are inside the shielded box
  • Level III: Or the outer housing is shielded as well

Shielding at the Source

16. Source. Shielding at the source is usually the most cost-effective solution. Generally speaking, the source of unwanted radiation can be produced by one or more components or interconnections on the PCB. Application of a shielding can will reduce it directly at the source.

17. Clip mounting. Shielding cans are mounted onto the PCB with SMD clips, which come in several sizes. After the reflow, the can (a cover with walls attached) is placed into the clips and can subsequently be removed for adjustments.

18. Pin mounting. There are also systems with pins for though holes or covers with integrated pins that can be soldered directly onto the PCB.

19. Shield layout. Cooling holes can be made in the cover or steps to prevent short circuits with the tracks on the PCB. Covers can also consist of a fixed part on the PCB (fence) and a separate cover which is clipped on to this fence.

20. Covering the entire PCB. Another option is covering the entire PCB in shielding material. This can be achieved either by means of a small housing, custom-made to exactly the right shape, or by simply wrapping or sticking material around the PCB. Foils, textiles, stretch material, and wrapshields, cut to the appropriate shape, are easy to apply. Since it is always important to prevent short circuits, all materials can be provided with insulation layers.

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