What Is EMI Shielding?
EMI (electromagnetic interference) shielding is a design feature used to prevent unwanted noise or improper operation of electrical systems. EMI shielding can be accomplished in many ways, using many different materials, to prevent interference in many different component systems. Regardless of how it is accomplished, EMI shielding is critical in sensitive test equipment, military installations, automotive systems, medical equipment, and even consumer products. Failure of electrical systems in any of these goods can result in incorrect output, damage to internal componentry, or failure to operate at all. Achieving proper EMI shielding is the focus of many industry specifications, including CISPR 32, IEC 60601-1-2, MIL-STD-464 and others.
Types of EMI
EMI stems from two separate phenomena: magnetic fields and electric fields. When these fields interact with electrical systems in unplanned ways, they cause EMI. Britannica defines magnetic fields as “vector field[s]…in which magnetic forces are observable”. For example, when a rare-earth magnet gets close enough to a piece of iron to attract it, its magnetic field is exerting a force on the iron. Earth’s magnetic field is another common example – it generates the force that makes compasses point north. Magnetic fields can also be created by high current electrical lines. Electric fields are similar to magnetic fields, but are created by voltage differences between two points in space. Some examples of sources of electric fields are WiFi routers, cell phone towers, and microwave ovens.
How Does EMI Shielding Work?
When electric fields encounter a conductive material, they are:
1) Reflected away from the material or
2) Travel along the material instead of passing through it
This is the main concept behind many EMI shields – they simply redirect the interference away from the sensitive electronic components. One of the critical things to remember about EMI shielding is that any holes in the material can allow some electric field to pass through. Some examples of different types of EMI shielding materials are wire mesh, metallized cloth and solid metal foil.
Magnetic fields shielding works in a different way. Instead of a magnetic field passing through the shield, the field is redirected through the material, protecting the device behind the shield.
What Materials Can Be Used For EMI Shielding?
EMI shielding requires different approaches depending on whether you are protecting from an electric field or magnetic field. Electric fields can be shielded by nearly any conductive material; a thin sheet of aluminum is often used in electronics to achieve robust shielding. Metal mesh (like chicken wire), metallized cloth, and some carbon- or metal-filled plastics can also provide varied levels of protection from electric fields. Magnetic fields are more difficult to shield, as they require the use of materials which are thicker and have high magnetic permeability. Permeability is a characteristic of magnetic materials, such as iron. One easy way to avoid magnetic field interference is to simply move your electrical component further away, because magnetic fields become less powerful quickly as they become further away.
Examples of EMI Applications
1) Signal Wires
When wires are carrying low voltage signals from one electronic component to the next, EMI can result in dropped, altered, or untriggered signals being transmitted. Many of these wires are also very small, with no built-in shielding to protect from external fields that can cause interference. By using a wire with built-in shielding or adding an external shield around the signal wire, disruptions from EMI can be reduced or eliminated.
2) Printed Circuit Boards (PCBs)
Almost all electronics have, at minimum, a small circuit board inside which is susceptible to EMI. Similar to signal wires, components on circuit boards can be triggered or even destroyed by unwanted electromagnetic fields. By including a conductive enclosure or other shielding component around the PCB, damage and improper operation caused by radiated fields can be minimized.
Why is EMI Shielding Important?
As devices get smaller and lighter, and the use of powerful radio frequencies, WiFi, and other communication signals (all electric fields!) become more common, EMI shielding becomes critical to ensure that devices operate in the intended way. Even natural sources of EMI (space radiation, Earth’s fields, decaying elements) must be considered for sensitive electrical systems. When the electrical components you use need to work just right, EMI shielding cannot be ignored – whether it’s your home stereo or medical equipment used near a radar array.