M
ECHANICAL DESIGN TIPS FOR
EMI
SHIELDING
Nowadays every company is involved with CE / EMI demands. The use of electronics increases as well as the frequencies. Therefore radiation and immunity has to be taken into account in an early stage during the development of new products. In many cases EMI problems cannot be solved at PCB level, the
enclosures and cables will have to be shielded. When to use
shielding? Shielding is a fast way to comply with legal demands like CE or FCC or to prevent electro-magnetic interference. Since time-consuming development is not required,
shielding is cost-effective. Therefore
shielding is used for smaller production quantities or if a quick market introduction is needed. It is also used for appliances with high radiation or sensitivity levels or for products of which these levels are not known in advance, like modular
enclosures. Radiation and conduction Electro-magnetic interference can be transferred by radiation and conduction. Conduction plays an important role with frequencies below 10 MHz. To prevent this, cables and
enclosures have to be shielded with magnetically conductive materials. The lower the frequency, the thicker the
shielding needs to be. For high frequencies (HF
shielding, >40 MHz), only a very thin layer of highly conductive material is needed. Avoid gaps The higher the frequency, the shorter the wavelength. This means that when the frequency increases, the tolerable gap dimensions decrease. In other words: doors, panels and other parts need to be connected electrically on all sides. The easiest way to do this is with highly conductive EMI
shielding gaskets. Most of them are self-adhesive for easy mounting. To select the appropriate gasket, several aspects have to be taken into account: the rigidity of the construction, the distance between the fixings, the distance between them and the construction materials used. The allowed stiffness of the gasket depends on the rigidity of the construction and the distance between the fixings. If the gasket is too stiff, the door, lid or panel will deflect and gaps will be caused instead of prevented (fig. 1). Especially for doors several kinds of gaskets have been developed, which combine a very large compression range, low closure force and high conductivity. Many of them can be used in existing products, without changing the construction. The gasket selection diagram below is very helpful to determine the appropriate gasket material. fig. 1 - Deflection of parts due to excessive stiffness of the gasket
