Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) are two sides the phenomena where electronic devices create and are affected by electromagnetic radiation. Often, the terms RFI and EMI are used interchangeably because radio waves are simply a subset of the electromagnetic spectrum. However, in practice, EMI generally refers to short range interference caused by high frequency emissions within the device itself, while RFI refers to longer wavelength interference from sources external to the device. EMI and RFI affect devices differently, but they are a related phenomenon and a common issue for today’s electronics.
EMI/RFI are a growing problem in the modern world. Today’s environment is filled with RFI. Radio, cell phone, and WIFI transmitters permeate space with signals. Solar activity and other sources from outer space also create significant radio wave noise. Additionally, as devices become smaller, they are increasingly vulnerable to EMI, especially when the distance between circuits are less than one wavelength. This creates a challenging environment for electrical engineers.
Electro-Magnetic Compliance (EMC) is a critical part of electronic design. EMC is achieved when a device is designed to be protected from external EMI/RFI, and does not significantly generate its own. Government bodies and industry organizations such as the Federal Communications Commission and Society of Automotive Engineers have written comprehensive laws and guidelines for EMC that electronic devices must meet before being sold. Achieving EMC is no small task.
Most EMC is achieved through good circuit design. Opposing magnetic fields cancel each other out, therefore circuits are designed so that the field from one part nullifies the field of another part. However, this does not eliminate all EMI/RFI. EMI/RFI shielding is commonly necessary to capture the residuals.
A basic example is shielded twisted pair wiring, where two wires are run in opposite directions and twisted together so that their electromagnetic fields cancel each other out. The twisted pair is then put into a metal tube that eliminates residual emissions.
The energy of an electromagnetic wave is reduced or “attenuated” when it passes through a conductive material. EMI/RFI shielding is a layer of conductive material. It may be designed to protect a device from its environment or components of a device from each other. In both cases, conductive paints provide effective solutions.
Metal enclosures inherently provide excellent EMI/RFI shielding, but most modern enclosures are plastic, offering no intrinsic protection. To achieve EMC, the inner surfaces of plastic enclosures are commonly coated with a conductive paint.
Board components are often shielded with metal caps, but with miniaturization there is not always room for one. However, a thin film of conductive paint can fit in tight places, and sometimes come to the rescue.
Attenuation is measured in decibels (dB) on a logarithmic scale. 10 dB of EMI/RFI Shielding will reduce the energy of the incident wave by a factor of 10. 20 dB will reduce it by a factor of 100; 30 dB by 1000, and so on.
It is important to note that the shielding effectiveness of all materials will differ depending on the wavelength of the radiation being shielded. A measurement of shielding effectiveness is only useful if the range of wavelengths for which that measurement is known.
MG Chemicals offers a range of conductive paints for EMI/RFI shielding and related applications.
Customers can choose between three chemistries:
- Acrylic is the most common. It is widely used on electronic enclosures, satellite dishes, and board level applications. It is easy to apply, durable, and adheres well to many surfaces.
- Water Based Urethane is the only choice in architectural applications because of its low VOC content. It is non-flammable, has no noxious vapors, and is not a dangerous good by air.
- Epoxy is used when extreme durability is needed. It offers mar and scratch resistance, very strong adhesion, extreme abrasion resistance, impact resistance, and strong chemical resistance.
Carbon is best for lowfrequency shielding,musical instruments, andgrounding.
Silver coated copper provides superior shielding athigher frequencies.
Nickel is suitablefor most device levelshielding applications. Itprovides good shieldingand excellent corrosionresistance
Silver offers the bestshielding and corrosionresistance. It is also thebest choice for board level shielding and missioncritical applications. It can beapplied very thin.
|Pigment System||Acrylic||Water Based Urethane||Epoxy|
|Silver Coated Copper||843AR||843WB||843ER|
Each coating base comes with its own tradeoffs depending on the application.
Each polymer system comes with its own adhesion strength depending on the substrate it is applied to.
|Acrylonitrile Butadiene Styrene (ABS)||Excellent||Excellent||Excellent|
|PolyVinyl Chloride (PVC)||Excellent||Excellent||Excellent|
|Nylon 66 (Polyamide)||Excellent||Excellent||Excellent|
|G-10 Fiberglass Epoxy||Excellent||Excellent||Excellent|
Each conductive filler comes with its own tradeoffs depending on the application.