Circuit boards form an integral part of all electronic and electrical devices. A key aspect of circuitry involves the flow of current and other electromagnetic signals critical in electronics functioning. Because of this, it becomes prudent to consider and incorporate the principle of EMI shielding during the circuitry design phase. After all, you must avoid incurring the cost-associated losses of having to redevelop your circuit board upon malfunctions.
An Overview of EMI Shielding
EMI stands for electromagnetic interference and forms a key aspect that needs consideration when developing circuit boards. It helps in ensuring the proper functionality of the different electronic components and the circuitry as a unit. EMI shielding thus comes as an instrumental technique of developing a barrier to prevent leakages when it comes to heavy-duty electromagnetic fields. Such leakages interfere with crucial and sensitive signals besides devices.
Installation of electromagnetic shields helps isolate the sources of electromagnetic fields besides acting as the perfect protective enclosure for sensitive devices. EMI or radio frequency interference often becomes a problem for electronics as it limits the performance or leads to failures of electronic circuits.
Electronic devices and equipment handle small currents and voltages in their operations, and these get disrupted easily by electromagnetic fields.
EMI or Electromagnetic Interference
It represents the signal coupling process from a system to the next. Electromagnetic interference gets composed of three components in the receiver, the path, and the source. Its two systems feature the receiver and the source. EMI’s source comes as the phenomenon or external electronic circuit that generates disturbance. It can naturally occur, for instance, as lightning cosmic microwave setting, solar flares, etc. It can prove artificial in FM/AM radio waves, cellular networks, control, and measurement devices, besides power transmission lines.
The receiver, on the other hand, also inferred as the victim, implies to the device or sensitive signal whose output signal gets distorted by interference. Universally, the path refers to where the signal coupling happens and can happen through one of four modes.
The Four Modes of Signal Coupling
Conducted EMI
It arises from the conductive path located between two electronic circuits that can permit the flow of stray currents or signals. The conducted electromagnetic interference can get classified as differential- or common-modes. Common-mode features the unidirectional flow of the stray current from the two systems via a grounding connection. It also serves as the common return path. Conversely, the differential mode allows for the opposite flow of the undesired current between the two systems. It happens through the power supply lines besides proving autonomous from the ground.
Radiated EMI
It propagates through open spaces between the receiver and the source. The source emits the electromagnetic wave that, in turn, gets unintentionally transmitted to the electronic circuit. Most conductors like circuit board traces or cables can function as antennae, receiving or transmitting external electromagnetic waves.
Capacitive EMI
It occurs between conductors (two) within a system with proximity to each other – mostly a wavelength-apart. The tiny space then generates parasitic capacitance in which the electric charge gets stored and transmitted by charge differentials. Charge differentials get created from the emission of electric fields by conductors. As a result, parasitic capacitance proves a transfer pathway for stray signals.
Magnetic EMI
It is similar to capacitive coupling that ensues at close proximities. The transfer of signal happens via the creation of current through another conductor (electromagnetic induction). It becomes a possibility when the current within the initial conductor oscillates or changes.
Types of Electromagnetic Interference (EMI)
Period of Interference
EMI gets classified based on the period of interference. As a result, the existing types come as either pulse or continuous interference.
Continuous Interference. Continuous interference represents a situation where the EMI source consistently emits the undesired signal. It gets characterized as low frequency and low energy. Examples of continuous interference include electromagnetic field leakages from industrial machines, radio frequencies, and power transmission lines.
Pulse Interference. Intermittent, transient, or pulse interference occurs in short durations. Its period or duration definition varies with application, though the standard definition details a single AC cycle (mostly 16.67 milliseconds). Transient interference has a characteristic high energy burst that can prove random or repetitive. Repetitive predominantly comes artificial and thus predictable regarding their duration and amplitude. On the other hand, random events prove naturally occurring or artificial. It can include power surges, lightning strikes, electrostatic discharge, etc.
Wave Bandwidth Length
It is also another essential classification of electromagnetic interference and can come as either broadband or narrowband. However, the definition of either type proves dependent on the signal bandwidth of the receiver (resolution bandwidth).
Narrowband Disturbance entails a bandwidth not exceeding that of the receiver, whereas a broadband disturbance includes that that exceeds that of the receiver.
Significance of EMI Shielding
Electronic shielding becomes a key aspect every designer or engineer needs to take into consideration when developing circuits. Comprehension of the electromagnetic interference nature makes it obvious its potential effects on its immediate surrounding. It can impact electronics both in contact or conducted EMI, near and devoid of contact (magnetic and capacitive EMI), and over longer distances as with radiated EMI.
Advancement in the present information age, coupled with increased usage of electronics for communication and data processing, has created considerable pollution of the electromagnetic wave spectrum. All this applies, including other disturbances that arise from electrical distribution and transmission systems besides natural occurrences like solar flares or lightning strikes.
Effects of EMI
- Distorted or jammed signals that communication devices receive
- Absolute damage or failure of the electronic circuit
- Abrupt power outages, EFT or fast electrical transitions, and power fluctuations
- Electric burns and shocks
- Decreased performance and shelf-life of electronic systems
- A probable ignition source
Addressing the problems above has necessitated the intervention of global organizations that have developed EMC or electromagnetic compatibility standards. EMC implies the characteristic or property of specific equipment to function properly in an electromagnetic setting. It should also work without creating or conveying electromagnetic energy to supplementary equipment.
Global EMC standards get stipulated by CISPR, which is part of IEC (International Electro-technical Commission, and ISO.
EMI Shielding Mechanism
Apart from filtering, bonding, and grounding approaches, EMI shielding represents the other viable technique of realizing EMC. It entails the creation of enclosures using the right EMI shielding materials, form, and structure to change the course of undesired or unwanted electromagnetic waves (either out of or into the equipment). The path gets altered through absorption or reflection of the electromagnetic waves, which happens through ferromagnetic or conductive materials.
Electromagnetic waves have two primary constituents – the magnetic component and the electric component. Both of the constituents travel at a similar frequency and come perpendicular to one another. As a shielding PCB or any other electronic circuit mechanism, the conductive material shields the electric components, whereas the magnetic parts get shielded with a high magnetic permeability attribute. However, by protecting one electromagnetic wave component, you protect the other by default as they exist in complementarity.
EMI shielding has three different mechanisms in reflection, absorption, and multiple reflections.
Reflection
It represents the main EMI shielding mechanism. Reflection tempers the electric part of the electromagnetic interference. However, to realize EMI reflection, it becomes necessary to have the material equipped with mobile charge carriers. It implies that the RF shielding material deployed for shielding needs to have conductive properties. Interaction between the mobile charge carriers and the electromagnetic waves within the conductive shield necessitates the redistribution of charges and the creation of the opposing electromagnetic field.
The generated electromagnetic field from the charge redistribution neutralizes the outer magnetic field. It is a mechanism where the higher the material conductivity, the better the shielding attributes. One of the key problems with the mechanism entails the discontinuity aspect. If the discontinuity within the enclosure proves larger than the wavelength of the outer electromagnetic field, then the shielding properties become null; therefore, the enclosure design needs to have the hole sizes minimized. While reducing the hole size proves impossible for higher electromagnetic waves, you can counter it using filtering devices.
Another challenge comes in skin effect, primarily in alternate current circuits (AC circuits). It features the accumulation of charges at the conductor’s surface upon the flow of alternate current. The result leads to an increased density of current within this particular area. The conductor’s inner section gets utilized less, which lowers conductivity and, by extension, the shielding effect. Such an effect becomes more pronounced in electromagnetic waves of high-frequency. Therefore, the solution lies in increasing the conductor’s surface area, which in turn increases the effective cross-section of the conducting area. Another solution lies in electroplating the top layer or surface with a conductive material like copper or silver.
Absorption
It acts as a secondary approach or mechanism of achieving EMI shielding, especially on the EMI’s magnetic component. For you to realize EMI absorption, the material needs to have magnetic and electric dipoles. Such materials possess high magnetic permeability and dielectric constants. Whenever an outer magnetic field proves present, the field lines (magnetic) get cut as they tend to travel through the material. Having an enclosure possessing this attribute will absorb the magnetic field line through a self-perpetuated pathway within. However, a common problem encountered when using the materials entails the absence of high conductivity. Consequently, they become less efficient in shielding or protecting electrical components.
A key element of the absorption mechanism entails the weakening of the inbound electromagnetic waves by eddy currents. It becomes apparent whenever the electromagnetic eave oscillates at high frequencies. In turn, it induces the currents within the conductor. Eddy currents from their magnetic field oppose the outer magnetic field. It also holds that materials possessing electric conductivity develop stronger or sturdier eddy currents.
Multiple Reflections
It represents another observed mechanism predominant in compound materials having vast interfacial spots or surfaces complete with absorbent structures. RF shielding occurs through numerous reflecting boundaries that reflect the electromagnetic waves. It thus results in the electromagnetic waves getting scattered.
Materials Used in EMI Shielding
Proper deliberations about EMI shielding materials need comprehension of the two primary EMI shielding properties in magnetic permeability and electrical conductivity. Good riddance, we have already established and canvassed them in the previous section. However, for such materials, lodging an inquiry at RayMing PCB and Assembly can also prove helpful. The company will offer solid solutions for your electromagnetic interference shielding needs.
So what materials can get utilized for EMI shielding purposes?
Metals
It represents one of the obvious choices as a cost-effective and straightforward EMI shielding material. The intrinsic nature and properties of metal like magnetic permeability, electrical conductivity, ductility, and strength make it a perfect EMI shielding material option. Silver represents the most suitable material in terms of efficiency as an electric field attenuator. It is due to its excellent corrosion resistance and electrical conductivity properties. However, silver also costs a lot compared to other types of EMI shield materials. Therefore, silver finds its application primarily as a surface coating or an alloy component through electroplating.
If you consider a balance of shielding efficiency and the cost of using the material, aluminum and copper become standard. For instance, copper’s electrical conductivity almost equates to silver’s, while aluminum falls a further 40% less. Carbon steel (as alloys) like ferritic stainless or carbon steel acts as perfect materials for electromagnetic shielding. Besides this, iron-nickel alloys like Mu-metal, Supermalloy, and Permalloy also apply as magnetic shield materials. However, the most common and popular comes in the form of Mu-metal because of its relative permeability.
Carbon Allotropes
It represents a group containing carbon forms like exfoliated Graphene, graphite, carbon nanotubes, and carbon fibers. Such allotropes get deployed as fillers for composites of EMI shield materials. It is effective as a material because of its intrinsic conductivity and strength. Additionally, they primarily operate via a multiple reflection approach of shielding.
Exfoliated graphite has widespread use as an EMI shielding gasket owing to its flexibility and capacity to flow on the surface anomalies of the sealing surfaces. All the allotropes come as porous structures, which then promote the absorption of electromagnetic interference. Carbon fibers, carbon nanotubes, and Graphene act as fillers because of their enormous aspect ratio. The fillers get embedded in ceramics, polymers, metals, and cement to develop rigid structures. However, carbon nanotubes and Graphene mainly apply for high-frequency shield applications because of their smaller dimension ratio and skin depth. Consequently, this makes the materials better conductors compared to metals at the gigahertz range.
ICPs or Intrinsically Conducting Polymers
It represents a set of special polymers capable of conducting electricity without extra conducting materials getting incorporated. ICPs prove popular because of their process-ability and light nature. Intrinsically conducting polymers conduct electricity because of the presence of conjugated bonds between atoms. As a result, it allows the delocalized or loose electrons to function as mobile chargers. It is also possible to modify the current or electricity conducting property of ICPs. Such modification happens either through de-doing and doping.
ICPs are still an evolving concept and thus remain under development despite their use. It faces challenges when it comes to chemical and mechanical stability. Despite this, they stay favored as the go-to components for composites, especially those containing carbon filaments and nanoparticles.
Design of EMI Shielding
Electromagnetic interference or EMI shielding is one of the most affordable EMC compatibility methods. It decreases the need for intra-equipment devices when it comes to managing undesired signals. Achieving EMC via the shielding route depends on the materials used and the structural design. A modest design for an EMI shield comes as a faraday cage created out of conductive materials. It depends on the electromagnetic environment attributes that the equipment has to function reliably. So what are some of the design considerations to take into account during the design phase?
Structural Design
- Discontinuities should be at a minimum to regulate the radiated EMI leakages.
- Enclosures need adequate bonding on all seams besides creating discontinuities. It ensures a consistent conductive surface.
- Bond similar metals to eradicate incidences of galvanic corrosion.
- Deploy an EMI shielding gasket for uneven surfaces. Electromagnetic interference shielding gaskets prove popular for enclosures possessing retractable panels, covers, or drawers. The EMI gaskets plug the gaps to offer continuous electrical contacts among surfaces. An electromagnetic interference shielding gasket needs to feature the following attributes: corrosion resistance, high conductivity, and high strength, resilience, and toughness.
- If permanent bonding becomes impossible, ensure that the selected fastening approach exerts sufficient pressure to retain contact.
- The worst electrical bond determines the effectiveness of the enclosure shield.
- Shielding gaskets need to possess the least possible thickness and be devoid of any compromises with their strength.
- Compress the shielding gasket with the required pressure as its effectiveness only enhances up to a specific limit.
- The enclosure’s mating surfaces must prove free from any contaminant like moisture, oil, dirt, and rust.
- Cable penetration tends to degrade the shield integrity. Because of this, it becomes pivotal to use the appropriate filters whenever the conductors penetrate the shield. Filters permit the creation of deliberate discontinuities within the enclosure. It accomplishes this by allowing the desired current or signals to flow while subduing undesired noise. Filters comprise electronic components like capacitors, inductors, resistors, etc., generating preferred impedance discontinuities.
- Shielded cables become pivotal for signal lines that penetrate the shielded enclosures. Such cables also come grounded to the enclosure’s outer shield.
- The openings should be small to avoid lessening the efficiency of shielding. Because of this, the opening needs to prove smaller compared to its operating wavelength.
- In instances where you cannot minimize the sizes of the holes, use shield screens.
Material Selection
RF shielding materials picked depend on the strengths of the magnetic and electrical components of the magnetic field (electromagnetic)
Low-frequency electronic circuits get characterized by magnetic field-providing currents. On the other hand, high-frequency circuits get characterized by electric field-providing voltages.
Plenty of materials that also prove ideal for the construction of enclosures give shielding properties from electrical fields. It can entail copper, silver, and aluminum. The principal shielding mechanism entails signal reflection and not absorption.
Shielding, especially against magnetic fields, needs high magnetic permeability materials. One of the critical materials includes iron and the Mu-metal. Here, absorption features get predominantly compared to reflection.
Forms of Electromagnetic Interference Shields
Electromagnetic interference (EMI) shielding comes in varied forms based on the form of application. Some of the common types in the market include the following.
Solid enclosures. It comes as a metallic case with adequate rigidity to support and contain the electronic device. Support comes as a frame, and the capacity to bar electromagnetic waves out of or into the system serves two primary functions. The enclosure has to get grounded too.
Screen and wire mesh. It represents a shielding material possessing discontinuities or penetration that prove less than the anticipated EMI wavelength. The wire mesh and the screen function similarly to an enclosure, though with the extra advantage of permitting ventilation. It is a trait that becomes helpful when the electronic device generates plenty of heat. Further, deploying a wire mesh forms a translucent feature that can become helpful for see-through displays and enclosures.
Discontinuities have to come small. Therefore, the manufacturing process needs to happen through a high-resolution process like photochemical printing and etching.
O-rings and Gaskets
It always becomes vital for the enclosure to prove continuous devoid of penetrations to realize the electromagnetic interference shielding effect fully. However, it never proves the case in most instances as enclosures need to access the housed components. Because of this, the continuity challenge with detachable enclosure parts gets solved with O-rings and shielding gaskets. It absorbs electromagnetic interference besides proving elastic and flexible as the typical sealing material.
Cable shielding comes as tapes to insulate the instrument cable or power conductor.
Coatings can come in the form of spraying, painting, electroplating, and dispensing. It predominantly gets deployed in lightweight applications.
Final Remarks
EMI shielding is vital to ensure electronic circuits function correctly in their respective applications. As a circuit board designer or engineer, understanding the various aspects of EMI shielding, as detailed in the article, will help in your PCB and other circuit board development processes.
