This article provides a complete guide to EMI and EMIF shielding.
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EMI shielding is a technique of creating a barrier that prevents leakage of strong electromagnetic fields that can interfere with sensitive devices and signals. They can be installed to isolate the electromagnetic field source or as an enclosure of the device that needs protection. Electromagnetic interference, or radio frequency interference (RFI), is a problem for most electronics since it can decrease the performance of the circuit or even cause it to fail. Electronics deal with small voltages and currents that an electromagnetic field can easily disrupt.
Electromagnetic interference (EMI) is the coupling of signals from one system to another. There are three components to creating an EMI: the source, path, and receiver. The two systems are the source and the receiver. The source is generally the external circuit or phenomenon that creates the disturbance, which can be naturally occurring (lightning, auroras, cosmic microwave background, and solar flares) or artificial (cellular networks, AM/FM radio waves, power transmission lines, measurement and control devices). The receiver, or the victim, is the sensitive signal or device in which its output signal is distorted by the interference. The path is where signal coupling occurs, which can be through four modes.
EMI can be classified according to the duration of the interference. Types of interference are continuous and pulse interference. A continuous interference is a type of EMI where the source continuously emits the unwanted signal. Continuous interference is generally characterized as low energy and low frequency. Continuous interference includes radio frequencies, electromagnetic field leaks from industrial equipment, power transmission lines, etc. On the other hand, pulse, intermittent, or transient interference is an EMI that occurs only in a short duration. The definition of its duration varies from each application, but the usual is less than a period of one AC cycle (1/60 Hz = 16.67 milliseconds). Pulse interference is characterized by high bursts of energy, which can be repetitive or random events. Repetitive is usually artificial, making it predictable in terms of amplitude and duration. Random events can be artificial and naturally occurring, such as lightning strikes, power surges, electrostatic discharge, and so forth.
Another classification of EMI is by the length of the wave bandwidth of the disturbance, which can be narrowband or broadband. The definition of the two depends on the bandwidth of the signal on the receiver, termed the resolution bandwidth. A narrowband disturbance has a bandwidth of less than or equal to the receiver, while a broadband disturbance has greater bandwidth.
Every day, new innovations are flooding the commercial market with new conveniences that are designed to make life easier. This wide array of electronics has the potential to damage and interfere with each other unless they are shielded properly. EMI shielding aims to isolate the energy of a device so that it does not affect what is around it and blocks external energy from attacking. Every new electronic device emits some amount of electromagnetic energy and has zero resistance to EMI.
With an understanding of the nature of EMI, it is clear that it can affect its surroundings in various ways. It can affect electronics that are in contact (conducted EMI), in close proximity without contact (capacitive and magnetic EMI), and even over large distances (radiated EMI). Along with the progress of the information age, the increased usage of electronics for data processing and communication creates considerable pollution to the electromagnetic wave spectrum, on top of the other disturbances caused by electrical transmission and distribution systems and natural phenomena such as lightning strikes and solar flares. Below are some effects of EMI.
International organizations developed electromagnetic compatibility (EMC) standards to address these problems. EMC is the property or characteristic of equipment to operate correctly in an electromagnetic environment without generating or transmitting electromagnetic energy to other equipment. International EMC standards are stipulated by the International Special Committee on Radio Interference (CISPR), a part of the International Electrotechnical Commission (IEC), and the International Organization for Standardization (ISO). In the United States, EMC standards are managed by the Federal Communications Commission (FCC).
There are many machines available to produce EMI (Electromagnetic Interference) shielding, which are important in today's society as they enable the manufacturing of components and devices that provide protection against electromagnetic interference, ensuring reliable and safe operation of electronic systems in industries such as telecommunications, aerospace, medical devices, and automotive. Below, we study many leading brands of machines available in the United States and Canada used to create EMI shielding materials, along with specific models and their unique abilities, features, or components:
Features: Chomerics offers manufacturing equipment for producing EMI shielding materials, such as conductive gaskets, films, and coatings. Their equipment includes various machinery, such as extruders, calenders, coaters, and curing systems and allows for precise control over material thickness, conductivity, and other properties. It supports different manufacturing processes, including extrusion, coating, and lamination. Chomerics' equipment is designed for high-volume production and customization of EMI shielding materials.
Features: Schaffner Group provides manufacturing systems for producing EMI shielding solutions, including filters, contact strips, and ferrites. Their systems include machinery for metal stamping, injection molding, and assembly. These systems offer precise tooling and production capabilities for high-quality EMI shielding components and incorporate automation features for efficient and cost-effective manufacturing processes. Schaffner Group's manufacturing systems are designed to meet industry standards and customer-specific requirements.
Features: Integrated Microwave Corporation specializes in equipment for producing EMI/RFI shielding materials, such as conductive foams and absorbers. Their production equipment includes laminating machines, die-cutting systems, and custom tooling solutions. This equipment enables precise fabrication and customization of EMI shielding materials to meet specific requirements and supports various manufacturing processes, including lamination, cutting, and shaping. IMC's production equipment ensures high-quality EMI/RFI shielding materials with consistent performance.
Features: JEMIC Shielding Technology offers many machines for manufacturing EMI shielding materials, including conductive fabrics, tapes, and gaskets. Their machines include coating systems, cutting machines, and sewing machines. Their machines allow for precise coating, cutting, and assembly of EMI shielding materials. They offer flexibility in material selection, thickness, and configuration and are designed for efficient production and customization of EMI shielding products.
Features: Leader Tech Inc. provides equipment for manufacturing EMI shielding materials, such as conductive elastomers, vents, and thermal pads. Their equipment includes mixing and dispensing systems, molding presses, and curing ovens. Their equipment enables precise material formulation, molding, and curing processes for consistent EMI shielding performance. It also supports customization options, including material properties, shapes, and sizes. Leader Tech's manufacturing equipment ensures high-quality EMI shielding materials with reliable performance.
Please note that specific model availability and features may vary, so it is advisable to contact the manufacturers or their authorized distributors for the most up-to-date information on the models that suit your requirements.
EMI shielding is one of the methods of achieving EMC aside from grounding, filtering, and bonding techniques. This involves creating enclosures with the appropriate material, structure, and form to alter the path of most undesired electromagnetic waves coming into or out of the equipment. The path is altered by absorbing or reflecting the electromagnetic wave through conductive or ferromagnetic materials.
An electromagnetic wave consists of an electric component and a magnetic component. Both of them travel at the same frequency and are perpendicular to each other. A conductive material blocks the electric components, while a material with high magnetic permeability blocks the magnetic components. Since a component of an electromagnetic wave cannot exist without the other, it is enough to protect one component. When it comes to EMI shielding, there are different mechanisms involved to filter out each. Enumerated below are the three mechanisms of EMI shielding.
The main mechanism of EMI shielding is reflection, which attenuates the electric component of the EMI. In order to achieve EMI reflection, the material must have mobile charge carriers. This means the material used for shielding must be conductive. The incoming electromagnetic wave interacts with the mobile charge carriers present in the conductive shield. This interaction causes the charges to flow and redistribute along the conductor creating an opposing electromagnetic field. The electromagnetic field generated by the redistribution of charges cancels out the external magnetic field. In this mechanism, the higher the conductivity of the material, the better the shielding characteristics.
The problem with this mechanism is that a discontinuity on the enclosure that is larger than the wavelength of the external electromagnetic field will defeat its shielding properties. Thus, the sizes of holes and openings in the enclosure design are minimized. However, this is not possible for higher electromagnetic wave frequencies. The only way to counter this in high-frequency EMI is through the use of filtering devices.
Another problem is the skin effect, which is seen in AC circuits. When AC flows through the conductor, the charges accumulate at the surface or the top-most layers of the conductor, increasing the current density in that area. The inner section is used less, which lowers the conductivity and ultimately, the performance of the shield. This effect is highly evident in high-frequency electromagnetic waves. A solution for this is to increase the conductor's surface area, thereby increasing the effective conducting cross-section. Another solution is by electroplating the surface with a highly conductive material at the surface such as silver.
This is the secondary mechanism of EMI shielding which acts on the magnetic component of the EMI. To achieve EMI absorption, the material must have electric and magnetic dipoles. These are materials with high dielectric constant and high magnetic permeability. In the presence of an external magnetic field, the magnetic field lines are cut since they tend to travel through the material. An enclosure with this property absorbs the magnetic ane electric field lines by creating a pathway within itself. However, a problem in using these materials is that they do not have high conductivity. Thus, they are less efficient in protecting from the electric component of the electromagnetic wave.
Part of the absorption mechanism is weakening of the incoming electromagnetic wave through eddy currents. This is observed when the electromagnetic wave is oscillating at a high frequency, which induces currents within the conductor. The eddy currents create their magnetic field that opposes the external magnetic field. Materials with high electric conductivity create stronger eddy currents.
This is another mechanism observed in composite materials with large interfacial areas or surfaces with porous structures. Shielding is achieved by having multiple reflecting boundaries for reflecting the electromagnetic wave. This results in the scattering of electromagnetic waves.
The previous chapter discussed the mechanism of EMI shielding and the desired properties of materials necessary to achieve it. Two main properties achieve EMI shielding: electrical conductivity and magnetic permeability. Below are some of the materials that demonstrate these properties.
Carbon steel alloys such as mild carbon steel and ferritic stainless steel, and iron-nickel alloys such as Mu-metal, Permalloy, and Supermalloy are common materials used for magnetic shielding. The most popular is Mu-metal which has a relative permeability of 100,000 at 1kHz.
Graphitic foam is made from petroleum, coat tar, or synthetic pitch and is more expensive to produce. Due to the expense of production, graphitic foam is in limited supply but is widely used for EMI shields. Used as an EMI shield, it is capable of shielding radio frequencies from 100 Mhz up to 20 GHz without any metal component.
Foam is an ideal material for EMI shielding due to its flexibility and adaptability. These characteristics have made it a popular choice for EMI shields. Additionally, foam lasts longer in extreme and harsh conditions, weighs very little, and does not rust or corrode. Foam is the most environmentally friendly of the many materials used to make EMI shields.
EMI shielding can be considered the most cost-effective method of EMC compatibility since it decreases the use of intra-equipment devices to manage undesired signals. Achieving EMC through shielding depends on two factors: the structural or form design and the materials used. A simple EMI shield design is a Faraday cage made of conductive materials such as copper. The EMI shield design depends on the characteristics of the electromagnetic environment within which the equipment must reliably function. Below are some design considerations in constructing an EMI shield.
EMI shielding components have varying forms depending on the application. Below are the common EMI shields seen in the market.
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