What are EMI gaskets?
As discussed in the previous article, EMI Shielding materials are used to protect electronic circuits from electromagnetic interference, which is to ensure the exact operation of the device and protect human health from negative impact caused by electromagnetic waves. The method of shielding is to use an conductive material to produce an enclosure that shield components or circuit boards. A discontinuity on the enclosure that is larger than the wavelength of the external electromagnetic field will reduce shielding effectiveness. Therefore, one of the first requirements when using the electrical shielding material is to create an enclosure that is completely continuous.
EMI gaskets are used to provide a continous conductive path along seams and other openings in an electronic enclosure. Based on their geometry and materials, shielding gaskets can be basically categorized as pre-form conductive elastomers and form-in-place gaskets.
Considerations to choose the suitable EMI gasket
Regardless of the gasket type, the important factors to be considered when choosing a gasket are:
- EMI impedance
- Shielding effectiveness
- Material compatibility
- Compression forces
- Compressibility
- Compression range
- Compression set
- Environment sealing
When selecting gasketing materials, they should be galvanically compatible with the metallic coating to minimize corrosion. Generally, gaskets are subject to minimum and maximum pressure limits to achieve a proper EMI seal. The greater the pressure applied to the gasketed joint, the better the apparent environmental and EMI seal. However, if the pressure exceeds the maximum pressure limit of the gasket, permanent damage to the gasket can occur. This damage may decrease pressure across the seam and degrade the environmental and EMI shielding characteristics.
Generally, the higher the conformability and conductivity, the higher the shielding effectiveness. Surface conductivity of both the gasket and the mating surfaces is the single most important characteristic that makes the gasketed seam effective. Meaning, the resistance between the flange and gasket should be as low as possible.
Common types of EMI gaskets
Form-in-place gaskets
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FIP EMI gaskets are among the highest performance EMI shielding materials. Typical applications consist of dispensing FIP beads onto metal or plastic substrates. This shielding solution is ideal for modern densely populated electronics packaging. Form-in-place gaskets provide excellent electrical contact to mating conductive surfaces, including printed circuit board traces for cavity-to-cavity isolation.
FIP EMI gaskets can be an ideal choice thanks to its ability to be dispensed in smaller bead sizes than most traditional EMI shielding gasket solutions. FIP EMI gaskets show excellent adhesion on prepared surfaces such as machined metals, cast housings and electrically conductive plastics. Because FIP EMI gaskets are robotically dispensed, they are high accurate and able to follow very complex geometries. Form-in-place gaskets offer a total cost savings in the form of reduced raw materials, labor or assembly time.
However, it should be noted that FIP EMI gaskets may not be the optimal choice for larger areas such as machined covers that can accommodate a gasket groove. Besides, you must also consider carefully if the enclosure requires submersion or durable weather sealing. In that case, FIP gaskets can be paired with additional sealing gaskets for enhanced weatherproofing
Pre-form conductive elastomer gaskets
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Conductive elastomer gaskets deliver great performance. In formulating conductive elastomer, a resilent polymer is combined with highly conductive rigid particles. The resilent polymer is usually silicone, fluorosilicone or EPDM with the good level of chemical resistance. Compounds can be molded or extruded shapes, sheet stock, custom extrudeed, or die-cut shapes to meet a variety of applications
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Conductive elastomer gasket can conform to flange surface irregularities providing low interface resistance between the mating electronic enclosure flanges, while simultaneously providing moisture, pressure, and environmental sealing. The shielding effectiveness of conductive polymers ranges from 60 dB to 120dB. Variations in their shielding effectiveness are related to the compression force applied and the amount of conductive filler loading.
Form-in-place gasket vs pre-form gasket
From the information above, it is easy to derive the basic pros and cons of the above gasket materials, then base on that to choose the right one for your particular application. FIPGs take the advantages of automation and ability to follow complex geometry. However, it seems to be a more costly method. When using FIP gaskets, it is necessary to pay attention to the curing time of the material, as it is initially dispensed in liquid form. Die cut gaskets are simpler to use but cause waste material in the die cut process.
Factors related to the end product, current production process and technical requirements must be considered in order to choose the right material. The right one for your application could be FIP gasket, die-cut gasket, or a combination of both.
If you are not sure which material is right for your application, Prostech’s engineers are here to help
