EMI compliance: Choosing the right shielding and gasketing
Compliance to EMI regulations is essential in today’s global market and applies to almost any electronic/electrical device. Also, almost every country in the world now requires meeting not just EMI emissions standards, but also immunity requirements. There are essentially two basic approaches for reducing or shielding electromagnetic emissions from a device or system as well as improving its immunity performance. 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 discusses available shielding products for use at both the printed circuit board level (board level shielding) and at the system level (gasketing).
Board Level Shielding (BLS)
A board level shield can be viewed as a five-sided can. Available in a variety of sizes and heights, board level shielding (BLS) is placed around the component or circuit on the printed circuit board that needs to be shielded. (See Figure 1).
BLS is used to attenuate the amount of electromagnetic energy propagating between the source and a receptor to acceptable levels. When designing and manufacturing BLS, the following elements need to be considered in relation to shielding effectiveness:
Near-field effects: Many complications occur when the shield is in the near-field of the source. Shielding performance will be impacted by the frequency of the source, the field configuration, position of the source, and the parasitic or distributed inductances and capacitances. In other words, the approach now becomes a “coupling” problem and should no longer be considered a radiated problem. So, even with accounting for the apertures in a shield, calculating or estimating shielding effectiveness of a shielded enclosure or box could still fall short of approximation. The coupling of the source to the shield, the effect of mutual coupling between elements, effect of the shield termination, and grounding technique need to be accounted for.
Layout and hole considerations: The effectiveness of BLS is highly dependent on the proper design of the printed circuit board mounting area. Normally, the sixth side of this “box” will be a ground plane on the board. The number and the spacing of vias and/or traces running from this shielded area to other board components can affect the effectiveness of BLS. What may occur is that the designer forgets about the various noise escape paths through the interlayer traces, vias, pads, and holes.
With higher frequencies and shorter wavelengths, the size and number of holes are becoming issues along with thermal effects. However, this concern is tempered by the near-field effect. Capacitive and inductive coupling are more significant than aperture size for shielding.
Resonances: Another issue with higher frequencies is resonance effect. Its coupling is a consequence of self-resonance of various structures. These structures behave as cavity resonators. A 2-inch by ½-inch enclosure resonates at a first order mode of around 12 GHz. Even weak coupling at these extremely high frequencies can induce strong oscillations that can then couple to any other point in the enclosure.
Thermal management: As devices become faster in frequency, they generate more heat. Hence, thermal management is also a design factor. Thermal management can be achieved through the use of thermal pads and heat sink. Companies like Orbel can assist with various design options that may be available.
Gaskets are used to maintain shielding effectiveness through proper seam treatment. It is the effect of these seams and discontinuities, in general, that accounts for most of the leakages in an enclosure design. The shielding effectiveness of a seam is dependent upon the materials, contact pressure, and surface area. Gaskets maintain conductive contact across mating surfaces. A solution to radiated problems is found by making all the joints or seams of adjoining metal pieces continuous. If there is no continuity between metal pieces, a radiating aperture for RF currents is created. This is where gasket material can be used. These conductive surfaces must be cleaned of any insulating finish. Although close-spaced fasteners (approximately 25mm or 1”) could be used alone, gaskets are preferred in order to reduce the number of fasteners and compensate for mechanical variations or joint unevenness.
Chosen based on specific shielding effectiveness requirements, application atmosphere, and spatial specifications, both beryllium copper gaskets and metalized fabric gaskets can be used to ensure maximum EMI compliance:
Beryllium copper gaskets: Beryllium copper gaskets offer the highest level of attenuation over the widest frequency range and are useable in both compression and shear type applications. Solid fingers have greater cross-sectional area, hence higher conductivity. In addition, the finger shape has the characteristics of an interconnecting ground plane with a large contact area. The inductance will therefore be low as well. The movement of the finger shape also provides a “wiping” action that aids in penetrating or removing any oxide buildup in the contact area. They are very forgiving to compression, meaning that it is very difficult to over-compress them causing compression set or breakage. (See Figure 2).
Metalized fabric gaskets: These types of gaskets are composed of conductive fabric material over foam. The conductivity can be very low and hence offer very high attenuation. The amount of attenuation is determined by the level or amount and matrix of the conductive particles used, and the compression force. These gaskets come in different styles and shapes (i.e., hollow core, D-shaped, etc.) that allow various compression ranges down to low values. (See Figure 3).