EMI Shielding

About EMI Shielding and EMI Filter, Including EMI Coating, EMI Enclosures, EMI Filters, EMI Gaskets, Magnetic Shielding and RFI Shielding.

EMI shielding and EMI filter reflects, absorbs or redirects electromagnetic waves to prevent interference from affecting sensitive equipment. EMI is any electrical signal that disturbs the function of a circuit, cable or other electronic component. EMI can originate from other equipment or from the affected component itself. The number of electromagnetic signals has increased with the ever growing use of computers, mobile phones and other electronic equipment. The EMI caused by these signals necessitates protective materials to avoid diminished product performance or product failure. EMI shielding and EMI filters can be found in medical devices, computers, mobile phones, stereo systems, television sets, windows, vents, fabrics, foils and even walls.
 
There are many forms of EMI shielding. Sheet metal provides excellent shielding effectiveness, environmental reliability and abrasion/scratch resistance. However, its design versatility is poor. Conductive paint provides good to very good shielding effectiveness, good design versatility, fair to good environmental reliability and poor to good abrasion/scratch resistance. Electroless plating gives excellent shielding effectiveness and is highly resistant to abrasion. Its design versatility is good to excellent and is quite environmentally reliable. Conductive plastics have excellent resistance to abrasion, good shield effectiveness and environmental reliability and fair design versatility. Vacuum metallization provides very good shielding performance and excellent electrical continuity at the surface and has virtually no effect on the environment. Molding cosmetics are possible with select coatings. The disadvantages of vacuum metallization include line-of-sight operation, which limits part complexity, and limited sources of supply. Spring gaskets provide long-term durability and reliability, high shielding effectiveness, consistent shielding despite surface irregularities and tolerance variations and are easy to assemble and install.
 
EMI/RFI gaskets are the most effective, simplest and most inexpensive method of shielding available, because they prevent the occurrence of slot antennas. Ferrite designs are made with solid and split beads arranged in versatile geometries and with many types of material compounds, and they offer many installation alternatives. Copper alloy gaskets are among the strongest gaskets available and come in a wide selection of profile shapes and sizes to meet any specification. Canted spring gaskets have superior shielding ability, excellent spring properties and compact configurations and are reliable. EMI gaskets eventually lose flexibility from compression set or thermal cycling and undergo torquing that will produce unwanted variations. Even conductive elastomers possibly the highest performing class of EMI gaskets, will deform under load over time, due to their low elastic limit. Some gaskets have closely aligned coils that maintain constant metal-to-metal contact to counteract the negative effects of regular wear and tear. High performance EMI/RFI gaskets are utilized in the following industries: aerospace, defense electronics, medical electronics, interconnections, automotive electronic and any other industry utilizing sensitive equipment.
 
There are also a variety of EMI coatings available. Commonly used conductive coatings are aluminum, copper, silver and gold and can meet up to one ohm per square inch resistance. Reflective coating is typically used on plastic or glass. This type of coating is used on such things as light bulbs and aircraft navigational lamps to dramatically increase light output. Reflective heat shield coatings can shield and reflect all electromagnetic, heat or infrared waves. This process is commonly used on light fixtures to prevent housing from overheating and increase light output but can also be used on satellite components and everything in between. Magnesium fluoride is applied to glass to produce an extremely hard, anti-reflective coating, providing an average of less than 2% reflection. Dielectric coatings are non-conductive thin films used as insulators or protective, scratch-resistant coatings that are applied to substrates. Metals, plastics or glass are the most commonly used, though other materials such as aluminum oxide and silicone oxides are also used, depending on the application.

EMI Shielding and EMI Filter Images Provided by SpiraTM Manufacturing Corporation

EMI Shielding and EMI Filter Image Provided by Compac Development Corporation

Types of EMI Shielding

• Conductive coatings, also known as "EMI coatings," are used to coat plastics to prevent EMI and ensure electromagnetic compatibility (EMC). Conductive coatings, which are becoming more common as the use of plastic in the electronic industry increases, provide good adherence and resistance to environmental factors.
  
• EMI coating minimizes electromagnetic interference and electrostatic discharge.
  
• EMI enclosures are areas from which EMI/RFI is blocked. EMI enclosures include electronic enclosures, chambers, walk-in enclosures and enclosed rooms.
  
• EMI ferrites consist of beads, blocks and bars of metal oxides that protect electronic components by blocking absorption of the interfering waves.
  
• EMI filters, or "EMI/RFI filters," are used to filter unwanted EMI and RFI signals from circuits and power lines. EMI filtering often utilizes low pass filtering, a process in which high-frequency RFI is attenuated while low frequencies are allowed passage through the filter.
  
• EMI gaskets are manufactured from materials such as foam, fabric, metal or wire mesh and shield gaps between electronic components.
  
• Magnetic shielding protects equipment from magnetic fields.
  
• RFI shielding protects equipment from interference from radio waves.

EMI Shielding Terms

Anechoic Chamber – A room in which no acoustical reflections or echoes exist. The floor, walls and ceilings of these rooms are lined with a metallic substance to prevent the passage of electromagnetic waves.
 
Attenuation – An electrical signal’s reduction in concentration as the signal passes through media. Attenuation is measured in decibels.
 
Canted Coil – A round-wire spring with inclining (canted) elliptical coils that, when compressed, deflect independently. Whenever any part of the coil is deflected, the whole spring responds, allowing consistent loading at each point of contact.
 
CE Marking – A mandatory mark for about 70% of the products sold on the European Free Trade Association (EFTA) and European Union (EU) markets. CE Marking is sometimes called “CE Mark” and referred to as the “Trade Passport to Europe ” for non-EU products.
 
Double Shielded Enclosure – A shielded enclosure containing inner and outer walls isolated from one another, except at the point of filter penetration
 
Electromagnetic Compatibility (EMC) – The ability of electronic equipment to be operated without EMI.

Electrostatic Discharge (ESD) – The release of electrical energy usually caused by the shift of electrons from one object to another as the objects come into contact with and separate from one another. ESD is also known as triboelectric charge.

ESD Shielding – Shielding that prevents the failure of electronic equipment from interference associated with electrostatic discharge.

Emission – The release from electronic equipment of electromagnetic waves that have the potential to create EMI.
 
Filtering – The removal of certain unwanted signal components through attenuation while allowing the passage of the remaining signal components.
 
Impedance – The complete opposition offered by a circuit to the flow of current at a certain frequency that is a combination of resistance (R) and reactance (X). Impedance is measured in ohms; the quality of the conductor is better the lower the ohmic value.
 
Insertion Loss – Measurement reflecting filtering ability. Insertion loss represents the difference in power reception prior to and after the use of a filter.
 
Radiation – The movement of electromagnetic energy in the form of particles, rays or waves.
 
Radio Frequency – Low frequencies of electromagnetic radiation used for communications.
 
RFI (Radio Frequency Interference) – Interference in a specific frequency range used for radio communication. EMI and RFI are often used interchangeably; the term EMI/RFI shielding covers all interference caused by electromagnetic radiation.
 
Shielding Effectiveness – The ability of a shield component to prevent the passage of electromagnetic radiation. Shielding effectiveness is expressed as a ratio of electromagnetic signals with and without the shield component.
 
Skin Effect – The propensity of high frequency current to travel close to the outside of an electric conductor rather than through its cross section. At high frequencies, the effective resistance of a wire is increased by skin effect.
 
Slot Antenna – A radiating element that is created by a slot in the conducting surface or in a waveguide’s wall.
 
Vacuum Deposition – In a vacuum, the condensation of thin material coatings on cool surfaces.
 
Waveguide – Media that controls the passage of electromagnetic energy and signals.