The Circuit Designer's Companion - diagramas.diagram...

Electromagnetic compatibility 2818.5.2 SeamsAn electromagnetic shield is normally made from several panels joined together atseams. Unfortunately, when two sheets are joined the electrical conductivity across thejoint is imperfect. This may be because of distortion, so that surfaces do not mateperfectly, or because of painting, anodising or corrosion, so that an insulating layer ispresent on one or both metal surfaces (cf section 1.1.3). Consequently, the shieldingeffectiveness is reduced by seams just as much as it is by apertures. The reduction ofshielding effectiveness depending on the longest dimension of the aperture appliesequally to a non-conductive length of seam. The problem is especially serious forhinged front panels, doors and removable hatches that form part of a screened enclosure(Figure 8.12(a)). The penalty of poor contact is mitigated to some extent if theconductive sheets overlap, since this forms a capacitor which provides a partial currentpath at high frequencies. There are a number of other design options you can take toimprove the shielding effectiveness at the seams:• ensure that supposedly conductive surfaces remain conductive: don’t paintor anodise them; alochrome is a suitable conductive finish for aluminium.• maximise the area of overlap of two joined sheets. This can be done bylapped or flanged joints.• where you are using screwed or riveted fastenings, space them as closely aspossible. A good rule is no farther apart than λ/20, where λ is thewavelength at the highest frequency of interest.(a)fasteners make electrical contactddNo electrical contact across butt-joint poor contact along seam between fastenersDimension "d" determines shielding effectiveness as in Fig. 8.11(b) conductive gasketmating surfacegasket makesgood contactover entiremating area(c) beryllium copper finger stockFigure 8.12 Improving conductivity of seams and joints