When magnetic lines of flux encounter high permeability material, the magnetic forces are both absorbed by the material and redirected away from their target.
The most effective shields are constructed as enclosures such as boxes or cylinders with cap ends. The field follows the line of the enclosure so an enclosed shape keeps stray fields from finding gaps that could cause unintended interference.
Most magnetic fields are man-made. They are found in solenoids, bar magnets and some motors and transformers. Magnetic fields are used in creative ways to create sound, microscopic images and record resonance images through MRI technology. In some cases, however, the fields interfere with sensitive electronic equipment and shielding is necessary.
Electromagnetic Interference (EMI) is electromagnetic energy that has an adverse effect on the performance of electrical/electronic equipment by creating undesirable responses or complete operational failure.
Electromagnetic Compatibility (EMC) is the ability of electrical or electronic equipment/systems to function in the intended operating environment without causing or experiencing performance degradation due to unintentional EMI.
EMI can be a problem for designers of many products. If we use mobile phones as an example, designers have to incorporate RFI shielding to stop unwanted RF emissions as well as to prevent other electrical equipment from interfering with the mobile phone operation. Designers may find that densely packed electronic assemblies may have internal components that interfere with each other, requiring electromagnetic shielding. When the EMI includes low frequency radiation, magnetic shielding is essential to assure proper operation of the electronic equipment.
There is no known material that blocks magnetic fields without it being attracted to the magnetic force. Magnetic fields can only be redirected, not created or removed. To do this, high-permeability shielding alloys are used. The magnetic field lines are strongly attracted into the shielding material.
Other than the fact that products that do not comply cannot be marked legally in the EC, more disruptive manifestations may exist that could affect the very manner in which we lead our lives. These encompass health and safety issues, the security of data processing and the functioning of vital electronic equipment. ABS brake systems, engine management systems, telecommunications and data transfer plus the security of both commercial and military data could all be readily compromised without adequate screening.
Other issues such as the dangers to health due to the emissions from mobile telephones and computer monitors plus ongoing research into the effects of living in close proximity to high voltage overhead power lines, all form part of the global concept of EMC. Some aspects can be easily controlled but others will continue to remain unresolved for years to come. The doomsday scenario of EMP (Electromagnetic Pulse) resulting from a high altitude nuclear explosion would certainly lead to the almost total loss of commercial communications and data processing plus much of the non-hardened military network, whilst at the same time avoiding the mass destruction of the infrastructure necessary for future use, possibly by the instigator of the blast.
In other words, potential chaos with no communications, services, power, media such as TV or radio and the loss of many forms of transport.
In all cases there has to be a source and a victim for a path to exist thereby permitting a radiated or conducted coupling. Electromagnetic Interference (EMI), results from the operation of electrical or electronic devices involving rapidly changing voltage or current levels, which cause the generation of electromagnetic energy at discrete frequencies and over frequency bands.
Discounting natural sources such as atmospherics or lightning, conduction and radiation emitting sources include:
In simple terms, welding the equipment into a seamless steel box could solve most EMI shielding problems. The problems that this would lead to are obvious - access would be impossible, power unavailable, visibility would be zero and the equipment would rapidly overheat.
We have, therefore, to permit suitable penetrations of the shield to allow cables to pass in and out, to provide airflow and to facilitate the viewing of LED/LCD or CRT displays.
Enclosures for equipment are commonly made from steel, aluminium and, with increasing popularity due to its aesthetic design capabilities, plastic. Plastic enclosures should be coated with a metalised compound applied by vacuum deposition, painting or sputter coating. Alternatively, a lining of aluminium or copper foil can provide excellent shielding effectiveness.
In brief, the metal (or metalised) enclosure will, in most instances, require gasketing to seal mating surfaces and to provide the necessary low impedance electrical bond, attenuating vent panels for airflow and EMI shielded windows to prevent the whole 'open' area where a display is mounted from representing an unacceptable gap in the shielding.
Inspection panels and doors can be fitted with 'soft-closure' gaskets or with conductive finger strips (CFS) which also provide high EMI shielding effectiveness with long service life and ease of closure.
Connectors for cable entry should similarly be fitted with gaskets where necessary, conductive cable glands to connect with screened cables if these are incorporated in the specification.
Radio Frequency Interference (RFI) is considered as part of the EMI spectrum, with interference signals being within the radio frequency (RF) range. This term was once used interchangeably with EMI. Conducted RFI is most often found in the low frequency range of several kHz to 30MHz. Radiated RFI is most often found in the frequency range from 30MHz to 10GHz.
Radio frequency (RF) shielding is required when it is necessary to block high frequency – 100kHz and above – interference fields. These shields typically use copper, aluminium, galvanised steel, or conductive rubber, plastic or paints. These materials work at high frequencies by means of their high conductivity, and little or no magnetic permeability. Magnetic shields use their high permeability to attract magnetic fields and divert the magnetic energy through themselves. With proper construction, magnetic shielding alloys have the ability to function as broadband shields, shielding both RF and magnetic interference fields.
Very often, the design of the equipment enclosure will dramatically limit the choice of gasket materials. Some enclosures are lightweight aluminium or even plastic with very little potential for withstanding distortion during compression of the EMI shielding gasket. Normally, the more robust the enclosure is the wider the range of materials that can be employed for gasketing. Similarly, wide and well reinforced flanges permit higher compressive forces particularly where limit stops are fitted.
Possibly the most difficult configuration for conventional EMI shielding exists where a 'knife-edge' return on a door or cover is to mate with a conductive gasket, in addition to providing an environmental seal to IP55 [PDF] or higher. If the enclosure can be modified to feature an additional return edge or flat where the knife-edge originally finished, the problem is easily solved but this is not always possible and is usually costly.
Where electrodes of two different metals are immersed in an electrolyte and externally connected by a conductive media, a 'battery' is effectively created whereby electrons will flow from one electrode to the other. According to the potential difference between the two metals, one electrode will become the donor electrode and will gradually be eroded.
The potential difference, or PD, of various metals commonly used in enclosures and shielding materials, and expressed in volts, is indicated in the table which follows based on the potentials against saturated calomel electrode in sea water:
|Zinc die-casting alloy||-1.10|
|Zinc plating on steel chromate passivated||-1.05|
|Cadium plating on steel||-0.80|
|Aluminium, wrought, cast Al Magnesium alloy||-0.75|
|Iron and steel: not corrosion resisting||-0.70|
|Iron and steel: corrosion resisting, 12%Cr||-0.45|
|Tin-plating on steel||-0.45|
|Chromium plating on Nickel-plated steel||-0.45|
|Iron and steel, corrosion resisting, High||-0.35|
|Copper and its alloys||-0.25|
|Nickel-copper alloys, incl. Mone||-0.25|
|Carbon (colloidal graphite in acetone)||+0.10|
The criteria most commonly employed to define a 'threat' is the likelihood of the equipment to be subjected to exposure to conditions of dampness or condensation through to wetting with salt water, salt mist/spray or the weather.
Where exposure is likely, the general rule of thumb regarding PD is as follows:
If the equipment is hermetically sealed and any exposed gasket edges are 'elastomer protected', i.e. with dual or twin gasket configuration there would then be no restriction. Some combinations of metals, such as Silver (PD 0) and Aluminium (PD -0.75), have inherent problems. In these circumstances, gaskets should be totally enclosed or of 'twinstrip' construction.
Quite simply, the best route to take is to utilise printed circuit board (PCB) shielding cans. These comprise a board-mounted 'fence' with a demountable cover or lid. The cover can be supplied with ventilation holes and cable-entry points if required. The fences can be supplied with several standard pin spacings and with or without stand-offs. Using PCB shielding can considerably reduce radiation and susceptibility problems, thereby making the overall EMI shielding of equipment more cost-effective.