Conformity Magazine- May 2008 - (Page 52) dries. For this reason, it is important that conductive paints be well stirred during the application process, to ensure that the particle to organic chemical ratio remains at the designed level. There are several alternatives to paint when a conductive coating is required. Among them are: • electroless plating • zinc arc spray • vacuum deposition • laminates Electroless plating is a technique whereby a thin layer (of the order of 100 microns) of pure metal is electrically plated onto the plastic. Typical metals used are nickel, or copper and First, consider the resistance of a block of material of resistivity r that is some length “L” and has cross-sectional area A. nickel in combination. Electroless plating results in a coating of very high conductivity. Zinc arc spray involves spraying a hot metal alloy, primarily composed of zinc, onto the surface to be shielded. The metal is fed into a flame or arc, melted, and blown onto the plastic. The resulting coating is of very high conductivity, but it is somewhat rough and may affect dimensional tolerances. In vacuum deposition, a heated metal is deposited on the plastic in a vacuum chamber. Vacuum deposition isn’t as commonly used for shielding as the other techniques. It can provide adequate performance, but only if the deposition is thick enough to provide a film that is thick enough and durable enough to make reliable contact with. Some vacuum depositions are not thick enough to do this, particularly if aluminum is used as the deposited material. Table 1 contains a summary of the major coating methods and their main characteristics. Continuity/Connectivity Problems arise when the conductivity provided by the main shield materials is not put to proper use to create a continuous overall shield. Continuity is our second key factor. It can be compromised in many ways. These can be grouped into “mechanical design” and “unintentional insulating” factors. The mechanical issues are primarily topological—dealing with the shape or fit of the various pieces which comprise the shielding enclosure. • There may be deliberate gaps and slots in the shielding for ventilation which are “electrically large” – i.e., significant fractions of a wavelength at some of the radiated frequencies. Sometimes these gaps are due to an inadequate number of fasteners between panels, or between panels and an underlying frame or cabinet. • The pieces and panels that compose the shield surface may not mate mechanically. This may be through design error (e.g. two panels which fold over but do not touch—as in an “overbite”), or because the components bow slightly under mechanical stress when assembled The resistance in ohms is given by : Next, apply this formula to a square block composed of a thin layer. If the thickness of the layer is “t”, and the length along each side is “L”, the cross-sectional area A will be equal to the quantity (L*t). Applying the formula used above gives the resistance of the square block as: • Gaskets, if used, may not be properly dimensioned. Every gasket has a working dimension range and pressure. Too little, and the designed gap won’t be filled mechanically or electrically. Too much, and the gasket will be too deformed to work on subsequent installations. It is well known that a slot or gap can be excited by fields and radiate in a manner similar to that of a wire of the same length. (See references at end of article). The longest dimension of such a gap governs the frequency above which shielding will be compromised. Deliberate large holes and slots are becoming less common as designers become more aware of the need to shield. Unintentional gaps, due to poor tolerances or incompatible finishes are still common. Remember also The resulting value for the resistivity of the surface layer is often referred to as having the units of “ohms per square” because it is the same for a square of any area. Figure 2 52 Conformity mAy 2008
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