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EMC Fundamentals

May 21, 2019

Electromagnetic Compatibility (EMC) is the state in which an electronic device can function properly in its intended environment without interference from electromagnetic or radio frequency waves from that environment.  Electromagnetic compatibility is achieved by reducing the interference below the level that disrupts the proper operation of the electronic system.  This compatibility is generally accomplished by product strategies for effective electrical system immunity using shielding, grounding, ground planes and absorber components.

PGC provides “enabling technology” for a host of new electronics applications utilizing high frequency circuits.  The PGC team supports development and manufactures components for control of Electro Magnetic Interference (EMI) and Radio Frequency Interference (RFI) at its Edina, Minnesota facility.   Controlling electronic interaction between different devices that are connected in a system, or those that are not intended to interact, helps pave the way to new innovations.

Electronic devices emit potentially harmful and disruptive electromagnetic emissions and are susceptible to interference from other electronic devices.   These emissions diminish product performance, and interfere with 5G communications infrastructure, for example.   Engineers and manufacturers must develop and produce electronically compatible products that

  1. Operate at the assigned frequency
  2. Do not interfere with other electronic devices
  3. Are not susceptible to interference from other electronic devices
  4. Comply with industry standards and regulations

Components that control EMI/RFI give our customers the ability to comply with EMC limits, therefore achieving FCC/CE compliance.  The Special Committee on Radio Interference (CISPR) of the International Electrotechnical Commission (IEC) sets limits to enable electronic systems to operate harmoniously within harsh radiated emission environments.

Both mechanical and electrical design aspects should be carefully considered in the selection of
EMC products. Mechanical considerations are significant because of physical dimensions and tolerances involved in construction of electronic equipment and systems. These factors
may seriously impact the electrical performance characteristics of EMI/RFI shielding products.  The designer must adequately consider the origin and methods of suppression of
EMI/RFI.

Here is a partial list of topics to consider during EMC system development.

Mechanical topics:

  1. Approximate coverage area (square inches) planned per unit
  2. EMC component thickness: Available space for EMC material thickness
  3. Ground plane connection available.
  4. Bus bar methods
  5. Needs to also seal environment, while serving as an electrically conductive component?
  6. Are the desired component solutions to be placed/applied at the board level, or the housing level, or within a conductive line/path?
  7. Material alloy: corrosive and metallurgical considerations/galvanic anodic index
  8. Materials compatibility
  9. Fastening/mounting methods
  10. Load/forces
  11. Nuclear, Biological, Chemical (NBC)
  12. Operating environment
  13. Solderability
  14. Space and weight considerations
  15. Static/dynamic relationship between mating components (relative motion)

Electrical topics:

  1. Desired level of attenuation performance
  2. Operating frequency

Other topics:

  1. Commercial or military
  2. Worldwide compliance
  3. Cost
  4. Cycle life
  5. Product safety
  6. Recyclability
  7. Component storage environment
  8. Flame retardancy requirements

Galvanic Corrosion:

EMC component failure can occur from corrosion damage that is induced when two dissimilar metals are coupled while in a corrosive electrolyte.  It is necessary to select metallic EMC component materials and finishes which inhibit corrosion, are compatible with the mating enclosure materials and are highly conductive.

To avoid corrosion, select metals as close together as possible in the galvanic spectrum.   The rate of component failure from corrosion depends on the electrochemical potential between two metals and the conditions under which contact is made.   Maximum galvanic activity occurs when dissimilar metals are exposed to salt atmosphere, fuels, chemicals and other liquids which may act as electrolytes. To minimize corrosion, all EMC component surfaces should be free of moisture.

EMC Component Options:

Component technologies should provide precise control of the level of EMI/RFI absorption, emission and reflectance at the component, subsystem or systems level.    The shielding effectiveness (SE) of a component is a measurement taken to determine the component’s ability to control EMI.    Here is a summary of component technologies:

EMI/RFI Gasketing

Conductive composites made of metal filled elastomeric materials used in applications such as electronic cabinets and cover plates to seal off stray EMI/RFI signals.  PGC’s versatility of materials provides assurance of protection against weather as well as electromagnetic interference.

EMI/RFI Shielding

An isolation barrier made of conductive and reflective metallic material.

EMI/RFI Shielding Tape with Conductive Adhesive

Tapes are an economical EMI/RFI shield solution for a variety of commercial uses. The tapes are available in copper, aluminum or tinned copper foil backed with highly conductive pressure sensitive adhesives.

Oriented Wire Shielding Gasket

A combination of silicone and conductive metal wire paths provide an environmental seal with EMI/RFI control. The two materials are combined to form a single material, which provides both weather and EMI/RFI shielding. The material is provided in solid or sponge silicone with either monel or aluminum wires for conductivity.

Knitted Wire Mesh Gasketing

Knitted wire mesh gasketing is available in various cross sections to satisfy the many different requirements encountered in shielding applications. The choice between rectangular, round, round with a fin, or a double round with fin depends on the enclosure or equipment that is being shielded.

Knitted Wire Mesh Tape

Knitted wire mesh tape is a double-layered strip of knitted wire mesh, providing an effective EMI/RFI shield for electronic cable assemblies. The knitted construction maximizes conformability and flexibility, while minimizing bulk and weight. Knitted mesh tape uses Sn/Cu/Fe or monel wire to provide good physical strength and shielding effectiveness.  Of the common materials used, monel has the best spring factors, while silver plated brass has the highest conductivity.

EMI/RFI Shielding Strip: Elastomer Core

Knitted wire mesh over an elastomeric core is ideal for low-pressure, high resiliency applications, such as gaskets. The mesh provides EMI/RFI shielding while the elastomer core provides low-pressure compressibility with an extremely high degree of resilience.

Molded Parts and O-rings

PGC provides both conductive and non-conductive materials in molded parts, in custom or standard shapes.   They can also be die-cut or digitally cut.  For example, O-rings are available in conductive and non-conductive materials.

This concludes an overview of several system topics.   PGC supports customer solutions with engineering advice and training, along with fast analytics.  This helps customers to identify their choices for each specific application, considering the system issues.