What is EMI Shielding?
EM shielding (electromagnetic shielding) is the practice of using conductive or magnetic materials to reduce incoming or outgoing electromagnetic fields in a space by blocking or guarding against the electromagnetic field.
Electronic devices are everywhere, and they’re only going to become even more ubiquitous. For example, we use our phones for work, entertainment, and even navigation.
However, more than that, electronic devices control our cars, guide planes in the sky, and even help keep sick people alive.
Imagine: if all these devices were to stop working suddenly. It’d be chaos!
While it sounds far-fetched that all of these gadgets would drop dead, the reality is that every one of them is sensitive to something that could potentially make them stop working: electromagnetic interference, or EMI.
In this post, I’m going to tell you all about EMI, and the means by which we protect our devices against it – EMI shielding.
What are EMI and RFI?
First, let’s look at the basics:
What exactly is electromagnetic interference?
The International Telecommunication Union describes electromagnetic interference in this manner:
“The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radio communication system, manifested by any performance degradation, misinterpretation, or loss of information which could be extracted in the absence of such unwanted energy.”
That’s a bit of a mouthful, but really, all it means is that EMI occurs when an external electromagnetic field negatively affects the operation of an electronic device.
EMI typically falls within the 1KHz to 10GHz frequency range, which covers the radio frequency segment of the electromagnetic spectrum.
When a source of EMI falls primarily within the radio frequency segment, it becomes known as radio frequency interference; or RFI.
Electromagnetic radiation, as its name suggests, is the coupling of magnetic and electric fields. Electric fields produce forces on electrons within conductors.
As electronic devices are mostly made of conductive material, they’re susceptible to forces produced by these fields.
Every single electrical device emits some amount of electromagnetic radiation. All of these devices are potential sources of EMI.
However, some types of devices are particularly strong sources of EMI, such as motors, radio transmitters and radar, televisions, and electric tools.
Other factors that may generate EMI include problems with power delivery and quality, such as power faults, electrical noise, and power outages.
As wireless devices become more and more popular, the potential for EMI is greatly increased. Cell towers emit broadband radiation at several frequencies, as do mobile phones.
2.4GHz Bluetooth signals are popular for wireless headphones, smartwatches, and in-car audio systems.
2.4GHz and 5GHz Wi-Fi are the leading mediums for Internet connectivity. And it gets worse:
On top of man-made sources, EMI can be caused by natural sources! These include lightning, solar storms, and the earth’s magnetic field.
To prevent the effects of interference, manufacturers use EMI shielding in the construction of electronic devices.
Let’s look at how it works.
Now, what exactly is EMI shielding?
EMI shielding is the use of conductive coatings or housings to absorb electromagnetic radiation before it can affect a protected device.
EMI shielding may protect a device from EMI, or it may prevent a device from producing EMI.
This kind of shielding has become very important lately; because manufacturers have started to use a lot of plastic in the housings of the devices they make.
Plastic is cheap and lightweight, which makes for a lot less bulk.
Sounds great for phones and laptops, right?
Unfortunately, because plastic is an electrical insulator, any EMI passes right through it and may affect the components within.
Let’s get this out of the way:
You can prevent EMI by merely keeping devices away from any potential sources of interference.
However, that’s not always going to be possible!
EMI shielding is a better, more reliable solution to protect our electronic devices from unwanted interference.
EMI RFI shielding techniques
There are many different kinds of EMI shielding, which are appropriate for different kinds of devices.
Each of these forms of shielding may be used by themselves, or they can be combined with other forms to maximize their effectiveness.
EMI shielding materials
Materials used for EMI shielding need to be electrically conductive, so they’re almost exclusively metallic.
Copper is a common material for EMI shielding due to its excellent conductivity, versatility, and the wide variety of alloys that can be used.
Nickel silver (which ironically has no silver in it, but is instead a copper-nickel alloy) is particularly desirable due to its corrosion resistance.
It can also be used for shields that will be soldered on, and applications where ferromagnetic materials aren’t allowed.
Galvanized steel, which is steel coated in zinc to reduce rust, is also a useful EMI shielding material, as it’s cheap, durable, and effective.
Aluminum can also be used for EMI shielding to significant effect.
It’s not ferromagnetic, so it can be used in rooms that can’t have magnetic materials, it’s very conductive, and it’s also very strong for its weight.
Graphite is the only non-metal that is electrically conductive. You’ll often find it in EMI shielding products that can’t have metal in them.
The simplest form of EMI shielding is a conductive metal housing that forms a continuous enclosure of shielding around a protected device.
The resultant continuous housing is known as a Faraday shield.
For heavy, static objects, metal shielding is an excellent way of attenuating any EMI that might make its way in.
Instead of a solid plate, a metal shield dotted with holes may be used, whether to reduce weight or allow light to pass through.
These holes must be smaller than the wavelength of undesirable electromagnetic radiation. A Faraday shield that is composed of plates with holes is called a Faraday cage.
EMI shielding gasket
One of the commonly used forms of EMI shielding is the gasket, a flexible enclosure that consists of metal foam or an EMI shielding mesh, conductive silicones, thermoplastic materials, and other EMI shielding materials.
Gaskets form a tight seal around devices which prevent any EMI from escaping.
Conductive gaskets as EMI shielding are considered lightweight and effective, but are vulnerable to mechanical stress such as compression, and may also be vulnerable to corrosion due to the environment.
EMI conductive coatings
For smaller devices like mobile phones, which would become too bulky with the application of a gasket or a large Faraday shield, an alternative exists in the form of EMI shielding coatings.
These apply to devices that are primarily encased in lightweight plastic.
Conductive coatings usually consist of metallic ink, which in turn is made of a compound material that contains a nonconductive carrier material and a mass of very small conductive metallic particles, usually copper, nickel, or silver.
The inside of the plastic housing is covered in this coating, whether through plating, spraying, or deposition techniques.
The result is an EM RFI shielding coating that doesn’t substantially add to the weight or thickness of the device.
EMI shielding paint
Similar to EMI conductive coatings, EMI shielding paint is composed of paint like acrylic, urethane, or epoxy, and a conductive pigment such as nickel, silver, or graphite.
EMI shielding paint is used in everything from satellite dishes to printed circuit boards.
Some kinds of EMI shielding paint may also protect against electrostatic discharge, which often occurs on factory floors as static electricity builds up.
EMI cable shielding
Cables that transmit power or electrical signals need to be free from EMI, lest data loss or “dirty” power occur.
To address this, these cables are wrapped in a multi-layer sheath that consists of an insulating outer layer and a conductive inner layer.
Cable shielding can be a spiraling, braided design, or a shielding material coated in a conductive layer, such as Mylar or aluminum foil EMI shielding.
EMI cable shielding is often used for audiovisual cables that are typically found in speakers and home theaters, and the power delivery cables used in factory machinery.
EMI shielding tape
RFI and EMI shielding tape consist of conductive metal foil, often with an adhesive layer. Examples include EMI copper foil shielding tape; and aluminum foil EMI shielding.
EMI shielding tape is quite flexible, both literally and regarding applications.
It can be used for shielding cabinets that contain sensitive equipment, to shield cables and power relays, and can even be applied to seams and openings in rooms that are designed to be EMI shielded, in order to seal the gaps.
EMI shielding theory and testing
The competence of any EMI shielding in a device is described by its ability to block incoming electromagnetic radiation.
One of the most critical measurable metrics is known as attenuation, expressed in decibels (dB).
Attenuation is determined by testing a device with and without shielding and comparing the intensity of EMI between both tests.
Attenuation is indicated in logarithmic decibels because EMI intensity drops off exponentially, not linearly, as the distance grows.
For example, a shielding of 70 dB is ten times stronger than one of 60 dB, 100 times stronger than 50 dB, and so on.
The methods used to measure EMI shielding effectiveness differ depending on the kind of shielding used, and the applications for the shielding.
Open field test
Appropriate for completed devices, the open field test is a simulated real-world test which takes place in, usually quite literally, an open field.
No large metallic obstructions or equipment are present in the field, and several antennas are placed at different distances from the device, while electrical noise and emissions are measured.
Coaxial line transmission test
Designed for testing planar materials, the coaxial line transmission test is conducted by applying an electromagnetic field to a shielded and an unshielded device.
The resultant generated voltage inside the devices is measured at different frequencies of EM radiation.
This voltage is compared between the two devices.
Shielded box test
Usually, for frequencies below 500 MHz, the shielded box test employs a sealed enclosure with an opening.
A shielding unit that is to be tested is placed over the opening, and all emissions inside and outside the box, both transmitted and received, are measured.
The ratio of the strength of the outside signals to the inside signals represents the effectiveness of the tested shielding unit.
Shielded room test
In situations where sensitive tests must be run without any outside interference, a shielded room test is appropriate.
The eponymous shielded room usually consists of two rooms separated by a wall, one containing sensors, and one with the device to be tested and the equipment used to test the device.
By isolating the tested device, testing equipment, and sensor arrays from external signals, the shielded room test allows for very precise measurement of EMI performance.
Shielded rooms often employ radio frequency anechoic chambers, which are analogous to acoustic anechoic chambers that reduce scattering and bouncing of signals around the room.
EMI shielding applications
Various EMI shielding products can be seen in day-to-day use.
Mobile devices require EMI shielding from themselves, as they contain several radio transmitters (3G/LTE, Bluetooth, Wi-Fi) operating in close proximity to microprocessors and display components.
Inside a mobile phone, metal shielding and interior coatings protect sensitive components from the transmitters.
They use a combination of a rigid metal plate for internal EMI shielding and a conductive coating for external EMI shielding.
Microwave ovens use a metal plate with small holes situated behind the door, in order to allow observation of food being cooked while preventing the escape of 2.45GHz microwave radiation.
Each of the holes is approximately 12 centimeters in diameter, which is slightly smaller than the wavelength of microwaves, preventing them from passing through while allowing light from the inside.
The rest of the oven is covered in metal.
Why is EMI shielding important?
We’ve gone almost two thousand words answering the question, “What is EMI shielding?”, so you must be wondering:
What really happens when sensitive devices are affected by EMI?
The answer is definitely nothing good.
Medical equipment, for one, is susceptible to EMI.
A good example is pacemakers, which very precisely regulate the contractions in a patient’s heart.
When a pacemaker is disrupted by interference, the patient could die!
Mobile phones have several highly sensitive radio transmitters that operate at different frequencies and broadcast at relatively low energies compared to background EMI.
This is sort of the equivalent of whispering a message in a room filled with loud voices.
Without proper shielding, a phone may be rendered inoperable with even mild background EMI.
Electric trains are severely affected by EMI; because the electronics and control systems in trains operate pretty much next to the high voltages and currents that are running through powered rails and switches.
Power quality issues in the train network may produce harmful EMI.
Aircraft are designed to be extremely well-shielded against EMI, because aviation electronics, or avionics, are essential to keeping the aircraft – well, in the air!
Interference may disrupt control surfaces or engine power, while navigation instruments may stop working when affected by EMI.
Aircraft contain several high-powered EMI sources such as onboard radar, communications transceivers, and massive engines.
High-altitude aircraft are also vulnerable to static discharge in clouds, and even lightning strikes. Moreover, on top of all these – literally:
Even observing the stars is affected by EMI!
Radio astronomy involves the observation of celestial bodies through their long-distance emissions of radio frequency radiation.
Because such radiation travels and degrades over such a long distance before it reaches the Earth, and is furthermore attenuated by passage through the atmosphere, a radio telescope on the ground must be strongly shielded against external RFI, or else it may pick up interference that drowns out the signals from space.
EMI is a big part of our lives, just like all of our electronic devices, and knowing how to protect against it is an increasingly important skill if we want to ensure that our devices always work when we need them to!
This is why EMI shielding is so necessary.
Have you ever experienced robot-like voices or noise in a phone call or a radio when you pass by electrical equipment?
What about static and buzzing in your home theater speakers?
Have your say in the comments if you’ve ever crossed paths with some nasty EMI!