Conformity Magazine- May 2008 - (Page 49) any of today’s complex electronics depend at least in part on shielding to provide the EMC performance mandated by regulatory and quality considerations. Shields act as barriers to electric and magnetic fields, keeping unwanted signals in (emissions containment) or out (immunity). Most engineers take it as an almost unshakable axiom of engineering faith that a conductive surrounding will provide adequate shielding protection. While shields can be very effective, designers will get the most out of them when some key issues are kept in mind. Shielding In A Nutshell There are a handful of factors which interact to decide whether a shield will perform as intended: 1. 2. 3. 4. Conductivity (and to a lesser extent, permeability) of the shield Continuity and connectivity of the different pieces of the shield Thickness (important for low-frequency magnetic field applications, but not at RF frequencies) Treatment of I/O and power leads which enter and exit the shield enclosure M In this article we’ll use these categories to organize our discussion of the major issues affecting shielding performance. Conductivity, connectivity, and shield thickness interact to determine a shield’s effectiveness. The shielding or filtering of I/O leads determines whether the anticipated shield performance will be maintained or compromised. First, let’s not overlook the most important point: topology. If a shield doesn’t completely enclose the product it is protecting, it won’t work very well, if at all. A bunch of conductive pieces that are not designed to form a more or less continuous enclosing surface are not going to provide reliable, wide-frequency shielding. How Shields Work Shields work by two primary methods. The first is reflection, which occurs as an electromagnetic disturbance is incident on the shield, while the second is absorption as the fields attempt to traverse the shield. At RF (say MHz frequencies and above), both are extremely effective. The ultimate shielding effectiveness of an unflawed metal shield of reasonable thickness (i. e., a seamless box of sheet metal thick enough to supporting its own weight) is in the vicinity of 100 dB to 200 dB, which is more than adequate for commercial applications. Even thin conductive layers, such as those provided by conductive paint or electroless plating, are capable of very mAy 2008 Conformity 9 http://www.magnetic-shield.com http://www.magnetic-shield.com
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