About EMI Shielding and EMI Filter, Including Electromagnetic Shielding, EMI
Coating,
EMI Enclosures, EMI
Filters, EMI Gaskets, Magnetic
Shielding & 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 shielding and EMI filters 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.
Types of EMI Shielding
- ,
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.
- minimizes electromagnetic interference and electrostatic discharge.
- are areas from which EMI/RFI is blocked. EMI enclosures
include electronic enclosures, chambers, walk-in enclosures and enclosed
rooms.
- consist of beads, blocks and bars of metal oxides that
protect electronic components by blocking absorption of the interfering
waves.
- ,
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.
- are manufactured from materials such as foam, fabric,
metal or wire mesh and shield gaps between electronic components.
- protects equipment from magnetic fields.
- protects equipment from interference from radio waves.
EMI Shielding Terms
- 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.
- An electrical signal's reduction in concentration
as the signal passes through media. Attenuation is measured in decibels.
- 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.
- 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.
- A shielded enclosure containing inner
and outer walls isolated from one another, except at the point of filter
penetration
- The
ability of electronic equipment to be operated without EMI.
- 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.
- Shielding
that prevents the failure of electronic equipment from interference associated
with electrostatic discharge.
- The
release from electronic equipment of electromagnetic waves that have
the potential to create EMI.
- The removal of certain unwanted signal components through
attenuation while allowing the passage of the remaining signal components.
- 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.
- Measurement reflecting filtering ability. Insertion
loss represents the difference in power reception prior to and after
the use of a filter.
- The movement of electromagnetic energy in the form
of particles, rays or waves.
- Low frequencies of electromagnetic radiation
used for communications.
- 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.
- 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.
- 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.
- A radiating element that is created by a slot in
the conducting surface or in a waveguide's wall.
- In a vacuum, the condensation of thin material
coatings on cool surfaces.
- Media that controls
the passage of electromagnetic energy and signals.