EMI Shielding

About EMI Shielding and EMI Filter, Including Electromagnetic Shielding, EMF Protection, EMI Coating, EMI Enclosures, EMI Filters, EMI Gaskets, EMI Shields, Magnetic Shielding, Magnetic Shields, Mu Metal, RF Absorbers & RFI Shielding.

EMI shielding is an enclosure, coating or transmitted signal which protects electronic equipment from ambient electromagnetic interference. EMI, or electromagnetic interference, is the magnetic field which is created by electrical currents; EMI can travel along wires, electrical circuits and conductors, while radio frequency interference (RFI) is radiated electromagnetic "noise" which travels through the air as radio waves. Both EMI and RFI cause the same effect. EMI enclosures, EMI coating, EMI gaskets and EMI filters are used as magnetic shielding and RFI shielding to guard sensitive electronic equipment. EMI and RFI are created, by widely varying degrees, by everything which conducts electrical energy, including electrical wires, electrical appliances, computers and all electronic equipment. Signal-bearing electronic devices such as mobile phones give off RFI. Normal electrical currents are disrupted when moderate to high levels of EMI or RFI are introduced; un-channeled electromagnetic signals and radio frequency waves can act as "noise", magnetically altering normal flow of electrical currents and impeding the flow of electrical energy. EMI shields, magnetic shields and RF absorbers can be found in medical devices, computers, mobile phones, stereo systems, television sets, windows, vents, fabrics, foils and even walls.

With circuit boards and CPUs, most problematic EMI comes from electromagnetic cross-talk between different components on the same board as EMI travels across wires and conductive materials. Shielding sensitive electronic equipment from electromagnetic interference (and shielding high EMI-emitting equipment from contaminating other equipment) can be achieved by enclosing equipment in a conductive EMI absorbing material (EMI enclosure) or by coating equipment or equipment enclosures with EMI coating. EMI enclosures and shields are also called magnetic shields, and may be made from metals with high magnetic permeability, which is the ability to absorb magnetic energy. Metals such as nickel, copper, steel and aluminum are commonly used, although the industry standard magnetic shielding material is Mu-Metal®, an alloy of nickel, iron, copper and molybdenum. Many industries depend on EMI shielding products to protect communication systems, such as hospitals, telecommunication, stage production, sound production and many others.

EMI enclosures, or "faraday cages" surround the equipment which needs to be shielded, absorbing harmful EMI and RFI. EMI enclosures are generally constructed from aluminum, stainless steel, nickel or copper alloys. Enclosures and shields built to block EMI need not be solid; perforated metals are often sufficient, as long as the surface's holes are smaller and closer together than the electromagnetic waves it is meant to block. Microwave doors are a good example of this. EMI coating allows for more flexible applications, as it may be spray-coated to the inside of plastic housings or the outsides of wires and regular enclosures. "Metal inks" containing copper, nickel or chromium alloys may be sprayed on to virtually any housing or material to create the effect of EMI shielding. EMI gaskets create more robust EMI and RFI protection by preventing the leakage of EMI in or out of an enclosure through seams or gaps. Gaskets are made from electrically conductive rubber materials and are often combined with EMI shielding metals. As electronic equipment advances and electromagnetic and radio frequencies become higher and stronger, EMI gaskets are used for more robust protection.

An alternative to enclosures, coatings and gaskets is the EMI filter, a passive electronic device which suppresses the electromagnetic interference generated by surrounding electronics. EMI filters are used to block electromagnetic interference which is conducted through power lines and signals (not RFI). Usually attached or integrated into the power source or switch which is to be shielded, EMI filters shunt high EMI frequencies, allowing normal low frequencies to pass; as with all types of EMI shielding, EMI filters work to both block incoming EMI and outgoing EMI. EMI filters can be an effective way to manage the EMI created on a specific power source or power line, although EMI shielding such as coating, enclosures and gaskets are still necessary to provide full protection from RFI, or electromagnetic interference conducted by radio waves.

EMI Enclosures Image Provided by Compac Development Corporation	EMI Gaskets Image Provided by Spira Manufacturing Corp.

EMI Enclosures Image Provided by United Western Enterprises, Inc.

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EMI Shielding Types

• Conductive coatings, also known as "EMI coatings," are used to coat plastics to prevent EMI and ensure electromagnetic compatibility (EMC). Conductive coatings, which are becoming more common as the use of plastic in the electronic industry increases, provide good adherence and resistance to environmental factors.
• Electromagnetic shielding is a barrier which protects electronic equipment from ambient electromagnetic interference.
• EMF protection requires the use of magnetic shielding materials or products to absorb or redirect electromagnetic fields (EMF) from interfering with sensitive devices as well as contact with people.
• EMI coating minimizes electromagnetic interference and electrostatic discharge.
• EMI enclosures are areas from which EMI/RFI is blocked. EMI enclosures include electronic enclosures, chambers, walk-in enclosures and enclosed rooms.
• EMI ferrites consist of beads, blocks and bars of metal oxides that protect electronic components by blocking absorption of the interfering waves.
  
• EMI filters, or "EMI/RFI filters," are used to filter unwanted EMI and RFI signals from circuits and power lines. EMI filtering often utilizes low pass filtering, a process in which high-frequency RFI is attenuated while low frequencies are allowed passage through the filter.
  
• EMI gaskets are manufactured from materials such as foam, fabric, metal or wire mesh and shield gaps between electronic components.
• EMI shields are materials or products used to redirect or absorb electromagnetic interference in order to protect sensitive electrical devices.
• Magnetic shielding protects equipment from magnetic fields.
• Magnetic shields are materials or products that protect electronic devices from interference from magnetic fields or electromagnetic fields (EMF). 
• Mu Metal is a nickel-iron alloy that is used to protect sensitive devices from magnetic fields that would cause magnetic interference. 
• RF absorbers also referred to as radar absorbers or microwave absorbers, protect equipment from radio frequency interference (RFI) by absorbing the magnetic energy before it can reach the equipment.
• RFI shielding protects equipment from interference from radio waves.

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EMI Shielding Terms

Anechoic Chamber - A room in which no acoustical reflections or echoes exist. The floor, walls and ceilings of these rooms are lined with a metallic substance to prevent the passage of electromagnetic waves.
 
Attenuation - An electrical signal's reduction in concentration as the signal passes through media. Attenuation is measured in decibels.
 
Canted Coil - A round-wire spring with inclining (canted) elliptical coils that, when compressed, deflect independently. Whenever any part of the coil is deflected, the whole spring responds, allowing consistent loading at each point of contact.
 
CE Marking - A mandatory mark for about 70% of the products sold on the European Free Trade Association (EFTA) and European Union (EU) markets. CE Marking is sometimes called "CE Mark" and referred to as the "Trade Passport to Europe " for non-EU products.
 
Double Shielded Enclosure - A shielded enclosure containing inner and outer walls isolated from one another, except at the point of filter penetration
 
Electromagnetic Compatibility (EMC) - The ability of electronic equipment to be operated without EMI.

Electrostatic Discharge (ESD) - The release of electrical energy usually caused by the shift of electrons from one object to another as the objects come into contact with and separate from one another. ESD is also known as triboelectric charge.

ESD Shielding - Shielding that prevents the failure of electronic equipment from interference associated with electrostatic discharge.

Emission - The release from electronic equipment of electromagnetic waves that have the potential to create EMI.
 
Filtering - The removal of certain unwanted signal components through attenuation while allowing the passage of the remaining signal components.
 
Impedance - The complete opposition offered by a circuit to the flow of current at a certain frequency that is a combination of resistance (R) and reactance (X). Impedance is measured in ohms; the quality of the conductor is better the lower the ohmic value.
 
Insertion Loss - Measurement reflecting filtering ability. Insertion loss represents the difference in power reception prior to and after the use of a filter.
 
Radiation - The movement of electromagnetic energy in the form of particles, rays or waves.
 
Radio Frequency - Low frequencies of electromagnetic radiation used for communications.
 
RFI (Radio Frequency Interference) - Interference in a specific frequency range used for radio communication. Unlike EMI, RFI is not conducted through circuits or power lines, but through "free" air space as radio waves. EMI and RFI are often used interchangeably; the term EMI/RFI shielding covers all interference caused by electromagnetic radiation.
 
Shielding Effectiveness - The ability of a shield component to prevent the passage of electromagnetic radiation. Shielding effectiveness is expressed as a ratio of electromagnetic signals with and without the shield component.
 
Skin Effect - The propensity of high frequency current to travel close to the outside of an electric conductor rather than through its cross section. At high frequencies, the effective resistance of a wire is increased by skin effect.
 
Slot Antenna - A radiating element that is created by a slot in the conducting surface or in a waveguide's wall.
 
Vacuum Deposition - In a vacuum, the condensation of thin material coatings on cool surfaces.
 
Waveguide - Media that controls the passage of electromagnetic energy and signals.

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