EMI Shielding Industry Information
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-channelled
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.