Electronics Protection - Summer 2017 - 9

Feature
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 (Fig. 3).
Why the Faraday Cage Principle for EMI Sheilding
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 3 meters distance while they are used within 15 cm).
Figure 3. Wire penetrates a shield
* Extra safety is desired for military use, e.g. for EMP (electro magnetic 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 / NEMP.
* Short circuit protection / prevention of sparks.
13. Identification systems like Radio Frequency Identification prevent RFID from making contact with the stations.
Several frequency ranges, lower the frequency are for longer distances.
* 125 kHz (Low Frequence).
* 13,56 MHz (High Frequence).
* 860 to 950 MHz (Ultra High Frequence).
* 2,45 GHz (Microwave).
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, etc.

How to Create Optimal EMI Sheidling

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 (Fig. 4)
Level II - The entire PCB is shielded by foil, wraps or a box (Fig. 5) or
the PCB and all the cables are connected inside the shielded box
Level III - Or the outer housing is shielded as well (Fig. 6).

Figure 4. (top) Shielding at the source,
Figure 5. (bottom) Shielding the PCB

Shielding at the Source

16. 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.

Figure 6. Shielding in the three levels. See Tips 16
through 24

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.

Holland continued on page 10
www.ElectronicsProtectionMagazine.com * Summer 2017

https://en.wikipedia.org/wiki/tempest_(codename) http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Summer 2017