System Level Gasket Considerations for RFI Susceptibility

System Level Gasket Considerations for RFI Susceptibility

RFI (Radio Frequency Interference), also known as EMI (Electromagnetic Interference), interrupts or interferes with the operation of electronic components, and electrical circuits. RFI or EMI may be introduced intentionally as in the case of electronic warfare, or unintentionally from an electronic or electrical device.

There are essentially two basic approaches for reducing or shielding electromagnetic emissions from a device or system. The first is shielding at the printed circuit board level utilizing proper design techniques. The second is to place the device or system in a shielded enclosure. This article will address shielding at the system level through use of a gasket.

Gaskets are used to maintain shielding effectiveness through proper seam treatment. It is, in fact, the unshielded seams that account for most of the RFI leakages in an enclosure. Of course, the shielding effectiveness of a seam is dependent upon the materials, contact pressure, and surface area. The solution to radiated problems in an enclosure is to make all the joints or seams of adjoining metal pieces continuous. If the adjoining metal pieces lack continuity, a radiating aperture for RF currents is created. Gaskets maintain conductive contact across the mating surfaces. The solution to radiated problems is, indeed, the gasket.

The majority of gasket applications involve two types of forces: compression and shear. When gaskets are installed under a flat cover panel in a compression configuration, the pressure preserves the shielding effectiveness of the seam. The alternative is a shear application wherein a flange or channel arrangement is maintained to preserve the shielding effectiveness.

Regardless of gasket type, there are several important factors that must be considered during the selection process. In addition to shielding effectiveness, one must take into account Radio Frequency (RF) impedance, material compatibility, gasket height, compression force, compressibility, and corrosion control.

The height or diameter of the gasket compensates for the joint unevenness of the mating surfaces, so that enough force can be applied, resulting in compressibility. Moreover, the difference between the minimum and maximum compressed gasket height should equal the joint unevenness.

Compression force– the force required to achieve maximum shielding effectiveness—is also essential, since the higher the pressure or compression force, the lower the impedance. A minimum closure force is recommended to obtain low surface contact resistivity and good shielding. Thus, a good joint seal needs to be a low surface contact resistivity, as well as a low gasket resistivity.

Corrosion is also a concern because it leads to the gasket material becoming insulative. This, in turn, reduces shielding effectiveness. Notably, there are two types of corrosion. The most common is galvanic corrosion due to contact between two dissimilar metals in the presence of moisture. The second type is electrolytic, and is due to current flow between two metals in the presence of an electrolyte.

In our next newsletter, we will address the various gasket materials available, as well as the cost considerations in good gasket design.

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