EMI-shielding theory and applications
A clear story about shielding in various applications
Nowadays, every company in the electronics industry is faced with CE / EMI demands. The use of electronical devices is increasing, as is the exposure to a wide range of frequencies. Radiation and immunity have to be taken into account in early stages of the development of new products. In many cases EMI problems cannot be solved at the PCB level alone and instead, enclosures and cables have to be shielded as well.
Legal requirements and other reasons to apply shielding
Shielding is a fast way to comply with legal requirements issued by bodies like CE or FCC or to prevent electro-magnetic interference. Our shielding solutions are cost-effective, since they do not require time-consuming development before you can comply with EMI demands. Shielding is typically used in enclosures in order to isolate the electrical devices inside them from external influences and for cables in order to isolate wires from the environment through which the cable runs. If a company intends a product to have a quick market introduction but the emissions exceed the prescribed limits, easy-to-apply shieldings are a commonly used solution. Shieldings can be used for appliances with high radiation or sensitivity levels, or for products where these levels are not known in advance, like modular enclosures. Shieldings are used when dealing with sensitive measurements that can be affected by ambient fields as well. Shielding is evidently a wide-spread phenomenon and necessity in the electronics industry to meet today's emission standards.
Aspects to be taken into account in the design of shielded enclosures
Materials and corrosion
The cheapest materials for electromagnetic shielding are galvanized steel and aluminium. Today an estimated 90% of all cabinets and enclosures are made of these materials. For that reason we have developed a special material, 6800 series Amucor Shield. Amucor is compatible with both galvanized steel and aluminum. When the the material is used in a corrosive atmosphere, it is better to prevent it from making contact with stainless steel, brass, or an aluminum chromate layer (alocrom 1200).
Corrosion protection is very important for applications close to the sea (due to salt) and in outdoor applications. It is also important that the gasket material is compatible with the material of the structure/enclosure you are using. For free advice and direct support: call +31(0)78-6131366 or email [email protected].
Thickness of enclosure material
A thickness of 0.1 mm should be enough to efficiently shield from frequencies above 1 MHz. At lower frequencies, such as 30 kHz and less, one has to use materials with a good magnetic conductivity (as well as electric conductivity when dealing with eddy currents) and thicker material may be needed.
An EMP military bunker for example is constructed from material that is 6 mm thick. These bunkers shield frequencies of 10 kHz with an attenuation of 80 dB. If shielding against frequencies around 50 Hz is required to limit influences from a transformer (which can cause health risks or influence the operating system of machines), then using dense metal layers and a special material called Mu-ferro should be considered (see Magnetic shielding). For more information about shielding enclosures, see https://www.faradaycages.com.
Prevent openings in enclosures
Especially at frequencies above 5 kHz, it is important to prevent gaps in the enclosure. High frequencies ranging between 100 MHz and 40 GHz are very sensitive to small gaps in the enclosure. The higher the frequency, the more attention should be paid to prevent holes and gaps in the shield. That is where soft and flexible gaskets come into play. The gasket should not only provide high conductivity, but also make continuous electrical contact with the enclosure in conjuction with low compression force.
Distance between mountings, hinges and locks
We have designed special resilient gaskets such as Ultra Soft Shield and V-shape gaskets. These gaskets prevent any bending between the attachment points and near the hinges. They are a cost-effective solution since no fundamental changes to the enclosure have to be made and there is no need for extra fasteners.
To give you an idea of what kind of gasket would be suitable for your application, our specialists will be happy to receive a drawing (preferably including dimensions, quantity needed, materials of the enclosure and indication of stiffness of enclosure material). We have wide experience with all sorts of shielding applications and often see more than 50 designs on a daily basis. Our specialists can help you choose the right type of gasket for any application and their advice is free of charge.
We can produce our gaskets in any desired quantity, whether it is just one gasket or a thousand, and all gaskets can be made to your specifications.
You can send your drawing via fax (+31(0)78-614 9585) or by email to [email protected].
The conductive layer on the outside of the gasket has to be in the same galvanic range as the construction materials, in order to prevent galvanic corrosion which would undermine the electrical conduction within the structure. This would decrease the shielding performance. Commonly used criteria: no more than 0.3 Volts for harsh environments (salt spray / weathering) and no more than 0.5 Volts for benign environments (indoors, with only salt-free condensation).
To obtain a contact surface within the same galvanic range as the conductive covering of the gaskets, a Conductive tape with a conductive self-adhesive on the back can be applied. If the enclosure is to be painted, the tape can be provided with a masking tape of a slightly smaller width (the paint will cover the conductive tape along its edges, which inproves bonding and corrosion resistance) (Fig. 1).
Another way to avoid galvanic corrosion is by preventing corrosive environmental influences from reaching the EMI-shielding gasket, for example by means of a gasket that combines a water seal with EMI shielding (fig. 2).
Some manufacturers of EMI-shielding gaskets use layers that contain carbon on the outside of the gasket to prevent corrosion. Unfortunately, these carbon-layered gaskets are not galvanically compatible with many commonly used construction materials, which will lead to corrosion on the contact surfaces of the construction. EMI-shielding gaskets with a conductive layer of reinforced Amucor® foil, on the other hand, are compatible with materials like zinc-plated steel and aluminium and will thererfore prevent galvanic corrosion.
EMI and frequencies
Electromagnetic interference can be transferred by radiation and/or conduction. Conduction plays an important role with frequencies below 30 MHz. To prevent unwanted influences from lower frequencies, cables and enclosures have to be shielded with magnetically conductive materials. The lower the frequency, the thicker the shielding needs to be.
For high frequencies (HF shielding > 40 MHz), only a very thin layer of highly conductive material suffices.
Use gaskets to avoid gaps
The higher the frequency, the shorter the wavelength. This results in a decrease of tolerable gap dimensions as frequencies increase. In other words: doors, panels and other parts need to be connected electrically, on all sides (without gaps). The easiest way to do this is by means of our highly conductive EMI shielding gaskets. Most of these gaskets are self-adhesive for easy mounting.
To select the appropriate gasket, several aspects have to be taken into account:
- rigidity of the construction
- distance between the fixings
- construction materials used
- whether the construction has to be openened and closed frequently - fixed application vs. application with hinges
The stiffness of the gasket depends on the rigidity of the construction and the distance between the fixings. If the gasket is too stiff, a door, lid or panel will deflect which will cause gaps rather than prevent them (Fig. 3). Especially for doors, several kinds of gaskets have been developed which combine a very large compression range with low closure force and high conductivity. These gaskets can be used in most situations, without a need for changing the construction. The gasket selection diagram below may be helpful in determining the appropriate gasket material.
Shielding doors and lids
For shielding doors and lids, it is important to use gasket with the correct resilience. When the gasket is too rigid, the cover or door can become distorted, resulting in small gaps with the result that the enclosure loses its ability to shield from higher frequencies. Flawless contact between the door (and other apertures) and the rest of the enclosure is crucial in maintaining shielding integrity for higher frequencies.
Below are some examples of constructions with EMI gaskets. Each construction requires the right type of EMI gasket.
Displays / windows shielding
Not only connections between construction parts, but also displays and vent panels need to be shielded. Displays can be provided with a sputtered transparent conductive coating for HF shielding (>30 MHz, Fig. 4) or a fine metal wire mesh for high-quality lower-frequency shielding (Fig. 5). The transparent conductive coating is supplied on foil (easy to bend and stick onto a window), glass (standard window) or other materials (for heavy-duty purposes). For other windows and displays, including wire-meshed windows, click here. If you have existing displays/windows and you would like to have them coated with a sputtered conductive layer, please contact us and we will examine the possibilities.
The shielding of the displays has to make contact with the shielding of the enclosure to guarantee optimal attenuation. This can be done by means of a gasket or metal tape with conductive self-adhesive.
Shielding for ventilation openings
Vent panels are usually shielded with aluminium Honeycomb vents. These offer excellent shielding performance with minimal loss of airflow. The best shielding is achieved with so-called cross-cell honeycomb vents. These vents consist of two or more layers of aluminium honeycombs, rotated 90° (Fig. 6). Honeycombs are usually supplied with a rigid aluminium frame and a gasket of 2-5 mm for optimal contact with the construction.
To prevent radiation emission through power and signal cables, they need to be shielded or filtered. Shielding can be provided by covering the cables with shielding tubes or by wrapping conductive materials around them. A ready-made shielded cable would also do. A shielding tube consists of hollow braided metal wire, through which a cable or bundle of cables can be pulled in order to shield them. A wrapshield is a knitted-metal wire tape, which is wrapped around a cable or bundle of cables. It is easier to create side branches with the wrapping method than with shielding tubes. For an overview of all cable shielding solutions, click here.
The cable shielding always has to be properly connected to the shielding of the enclosure; otherwise the attenuation will be lower, and possibly even insufficient. For heavy-duty and military applications, shielded cable glands and specially designed cable entry systems are available. We have developed two different cable entry shields, one consisting of two fringed EMI gaskets at the top and bottom sides of a slot (4910 - Cable entry shield) and the other consisting of a contact plate to be mounted in front of a slot with holes to put cables through (4930 - High-performance cable entry shield). The hig- performance contact-plate entry shield can be made gastight and watertight. The plate system has higher performance, but with the fringe system it is easier to ad cables later on.
Please note that cables without a shielding jacket also have to be shielded outside the shielded enclosure to keep them from acting like antennas. Such a leak in the EMI shield can be prevented by installing a power or signal line filter or, if you are dealing with shorter cables leading to another shielded enclosure, by using a wrap shield or shielding tube.
What was said above about cables also applies to connectors. They also have to be shielded or filtered and they must make conductive contact with the enclosure. Connector gaskets can provide such connections easily. They consist of a 1-mm thick die-cut material, which can be manufactured easily according to customer specifications, with little additional tooling costs (Fig. 8). Standard sizes are available as well.
Shielding at circuit board level
Parts that cause interference can be packed in a folded shielding box or envelope (fig. 9) made of Mu-copper shielding foil with an insulation layer on the inside or with plastic studs to avoid short-circuits (Fig. 9). An alternative for the folded shielding box or envelope is a ready-made housing (see 1900 series EMI shielding housings/enclosures).
Shielding PCBs and shielding individual components can also be achieved by soldering vertically placed metal strips (e.g. Mu-copper or Mu-ferro) onto the PCB to create compartments. These compartments are closed by adding a lid of flexible die-cut shielding foil or by pressing a soft conductive foam sheet against the strips (Figs. 10/11). This option allows shielding many compartments with just a single cover.
If the radiation source or the EMI-sensitive component is known, shielding for the specific component(s) can be applied. The best way of shielding at the source is to shield only the interfering part or the sensitive part on the PCB with our specially developed 1500 series PCB shielding system.
Shielding of a complete space/room
For some applications, it is desirable to shield an entire room by covering the walls with metal foil like Mu-copper foil, for example in medical and military applications and rooms intended for executing very sensitive measurements. The technique may even be applied in forensic science, for example if a device needs to be blocked completely from communicating with the outside world while it is being examined.
For many applications an IP rating is required. However, the question i:s which IP rating? To help you to find which IP rating you need for the desired result, we have put all IP ratings in a clear table. Go to the IP ratings table.
Increasing clock speeds
Electromagnetic interference (EMI) is increasingly common as a result of higher clock speeds in today’s PCs and workstations.
This has forced the regulatory agencies to place limits on electromagnetic radiation produced by PCs and any electronic instrument that might use clocks and generate emissions.
Electromagnetic Interference Analysis
Almost any electrical transitions with sharp edges, such as clocks, data, address and control, produce electromagnetic radiation. As performance requirements increase, clock speeds have also increased. The transition edge, or in engineering terms, the slew rate, has become faster and faster as it has become more difficult to meet set up and hold time.
Clocks are no longer fed to only one or two devices on circuit boards. Rather, they are being distributed all over the circuit board. Also, increased memory requirements, and other loads on the clock lines, have significantly contributed to electromagnetic radiation.
Shielding is the most prevalent method used to decrease EMI.