Conformity Magazine- May 2008 - (Page 54) when it will have a gold-tinted iridescent appearance, but will insulate the surface under modest contact if the parts are left in the tank until they are a deep uniform golden yellow. “Clear anodizing” of aluminum can be quite difficult to penetrate and leave the part difficult to make contact to. How can you tell if there is a problem? Coated or treated materials need to be checked for conductivity under light pressure, especially if the contact will be made with low pressure gaskets, such as those made of woven cloth over foam. The contact pressure one gets from gouging a pair of pointed test probes into the metal is very high, and may give misleading assurance of the contact that will be made through the coating in the assembled product. Check the likelihood of contact your coating will provide under the conditions of use with broad area pads made of soft conductive gasket material to simulate the actual installation. Another way to create an insulating layer that will lose contact is by galvanic action. Whenever two dissimilar metals contact each other in the presence of an electrolyte, a short circuited micro-battery is created. Since some moisture is always available, dissimilar metals won’t stay connected for very long. The galvanic scale positions materials in terms of the relative potential they will exhibit. Materials that are far apart on the scale will corrode; those that are closer together are “galvanically compatible.” It is unfortunate, but incompatible materials are commonly used, and practically unavoidable. Here’s a historical example from your author’s experience. Years ago, imported personal computers sometimes used zinc finished sheet metal chassis connected to a cover of similar material with beryllium copper spring fingers. These materials are far apart on the galvanic scale. After a fairly short time—days to weeks—the shielding performance deteriorated noticeably, and higher radiated emissions would be seen. Upon disassembly, a fine dark line of corrosion could be seen at the contact between the materials. The zinc, being less noble than the spring finger material, would corrode. In addition, the contact area was minimal, consisting of a line where the fingers curved against the case. When the case was flexed, or if it were disassembled and the surface cleaned, the shielding effectiveness would return to its original level. These problems have not entirely left us today, although they are usually less severe. A common combination of shielding/gasketing seen today involves aluminum panels and conductive cloth over foam gasketing. This provides a wide area contact, which is helpful, but the contact doesn’t provide a hermetic seal, and the conductive part of the gasket is made from cloth bearing fine threads of a galvanically incompatible metal such as silver, nickel, or copper. Sometimes products which have been stored in a humid environment need to be cleaned to return them to optimum shielding performance. Thickness For materials that are multiple skin depths in thickness, there is sharp attenuation of any electric and magnetic fields that The gasketing materials used are almost always at the noble are transmitted through the surface boundary. Every skin end of the scale, so they are incompatible with aluminum depth traversed means the field drops by a factor of 1/e, or 8.7 or zinc plated steel. The details of the gasket geometry can dB. Looking again at Figure 1, we can see that any structural make the situation worse by varying the size of the contact shield will be many skin depths thick in the RF range. At low area. Low area contacts—point or line—are going to be frequencies, from the 50/60 Hz mains frequency through the more susceptible to corrosion than those made over a broader multi-kHz frequencies used by CRT monitors and switching rectangular area. supplies, this may not be the case. And, thin, or somewhat less conductive coatings (i.e., conductive paints) may be electrically thin (less Type Comments than a skin depth or two thick) even well into the MHz radio frequency Gasket—Cloth over foam Conductive cloth over open cell foam. Very compressible. Wide variety of form factors (rectangular, P-shape, hinged, with/without range. adhesive, etc.). Related varieties include plated rubber foam. Wide contact area slows onset of galvanic problems. Gasket—Loaded rubber Includes very high-performance types; military heritage— combines hermetic seal with wide contact area; limited material compressibility (unless hollow extrusion); wide variety of extruded or cut shapes possible. Can be expensive, sometimes intolerant of rubbing contact. Wire and wire over foam types. Not currently popular in commercial applications. Relatively noble metal spring (e.g., beryllium copper, sometimes w/ nickel or tin plating), can lead to galvanic issues. High conductivity. Generally good tolerance to wiping contact on insertion along finger orientation. Relatively small area of contact—line or multiple points. Wide variety of shapes. Highly compressible. Continuous flat coil set in groove. Spring finger variation. Gasket—mesh Spring fingers – strip Spring coil Table 2: Properties of Common Types of RF Gaskets and Fingers 5 Conformity mAy 2008 “Thin” materials can still work by reflection, but when does thickness matter? Reflection works well enough by itself for far-field electromagnetic waves and for waves that are in the moderate near field. However, for magnetic sources (loop-like sources very much in the near field, where the E/H ratio is small), reflection is not a factor, and absorption—attenuation on transmission through the depth of the shield material—is the only mechanism available. Here, the amount of absorption is determined by how many skin depths thick the shield
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