What are EMC enclosures?
Things to consider when selecting materials for an EMC enclosure.
EMC enclosure design tips.
EMC enclosure designs are used to diminish the strength of interference
When a device is tagged as electromagnetically compliant, it indicates that the interference associated with it is within the levels specified by EMC standards. EMC-compliant devices are safe to operate in an electromagnetic environment and they do not interfere with the operation of nearby devices. Generally, to achieve the level of electromagnetic compatibility required for normal operation in an electromagnetically polluted environment, EMI filtering or equipment shielding is used. In this article, we will discuss what EMC enclosures are as well as offer some EMC enclosure design tips to consider.
EMC Enclosure Basics
An electromagnetic wave consists of an electric field and a magnetic field. The field intensity of the electric and magnetic fields determines the electromagnetic wave impedance. The electric fields are generated in high impedance voltage sourced circuits, whereas the magnetic fields are generated by low impedance current sourced circuits. To achieve electromagnetic compliance, it is critical to attenuate the strength of the electromagnetic fields.
To reduce the strength of interferences, EMI shields are utilized. EMI shields are EMC enclosures designed to be a shield between the emitter and the susceptor to diminish electromagnetic field strength. Think of them as a curtain placed between the EMI source and victim to mitigate the impact of radiated interferences by electromagnetic field attenuation.
The figure of merit that qualifies the ability of an EMI shield to attenuate the electromagnetic field is the shielding effectiveness. The shielding effectiveness is mathematically defined as the ratio of electromagnetic field strength before and after the placement of EMI shields and is expressed in decibel (dB).
Electromagnetic Field Attenuation
The attenuation (loss) of electromagnetic field strength with EMI shields is dependent on shield material properties such as thickness, permeability, conductivity, frequency of interference, and the distance between the EMI source and shield. An EMI shield establishes electromagnetic field attenuation through absorption, reflection, and re-reflection.
Absorption loss is dependent on shield thickness and the absorption coefficient of shield material. Reflection loss is influenced by the electromagnetic wave impedance and is inversely proportional to the intrinsic impedance of the EMI shield. The reflection happens at the air to metal shield boundary.
Re-reflection losses are highly dependent on absorption loss. Re-reflection occurs at the metal shield to air boundary.
EMC Enclosure Design
EMC design is an important parameter in the design of any electrical or electronic system. When device interferences are properly filtered and the grounding is perfect, the need for EMC enclosure designs is reduced. However, due to the unpredictable nature of EMI, it is smart to incorporate EMI shields or EMC enclosures in a design.
Selecting EMI Shield Materials
The selection of material for an EMI shield is important, as its physical and electrical properties are factors that influence shielding effectiveness. There are metallic as well as non-metallic enclosures available. Usually, to be used for EMI shielding, non-metallic materials undergo a process called metalizing. Some techniques used for metalizing non-metallic materials are electroless plating, vacuum deposition, conductive spray painting, arc spray, etc.
In any EMC enclosure design, there are some openings for harnessing wires, ventilation, or interfacing with upstream or downstream components. These openings or apertures are paths for electromagnetic coupling. It is better to avoid discontinuities in the EMI shield surface. In the case of providing holes or seams in the enclosure, sizing must be done carefully, as the electromagnetic coupling efficiency is dependent on the aperture size and wavelength (𝛌) of EMI. The general rule is to avoid openings larger than 𝛌/50 for microwave range products and 𝛌/20 for commercial products. Besides the opening dimensions, there are some other guidelines to follow when designing EMC enclosures.
- Minimize discontinuities in the design.
- Give sufficient bonding between the enclosures at seams and discontinuities.
- Use similar metals for bonding, which avoids galvanic corrosion.
- In the case of non-permanent bonding, ensure the fastening method exerts enough pressure to maintain contact.
- Use an EMI shielding gasket on uneven surfaces.
- Use the shielding gasket with the lowest thickness that offers the required strength.
- Make sure the enclosure surfaces that come in contact are clean and free from oil, dust, rust, and moisture.
In EMC enclosure designs, there are many small details to consider to increase shielding effectiveness. Carelessness in the material selection, aperture dimension, or gasket sizing can result in electromagnetic coupling instead of electromagnetic shielding. Cadence’s suite of design and analysis tools can help you in designing and validating EMI shields and enclosures.