Conformity Magazine- May 2008 - (Page 53) that screwing two surfaces together does not guarantee contact along their entire length. Contact will occur at the points of attachment and at most at the two highest points in between. In what I’ve called the “insulative” group of problems, we are dealing with unanticipated effects which interfere with the electrical contact of two parts that are mechanically connected. Unintentional insulation can break up contact just as surely if it were deliberate. Look for the following: • Improper masking of paint can lead to insulation of surfaces that were meant to be in contact. • Non-conductive anti-corrosion treatments may insulate the pieces from each other or from intervening gaskets. • Galvanic incompatibilities between dissimilar metals may introduce insulating surface corrosion. It is not uncommon for this to occur between gaskets and the portions of the shield they are meant to connect. When this happens, shielding effectiveness will degrade over time. Protective metal treatments are potentially problematic. The oxygen in our atmosphere is a very active chemical; commonly used metals want to oxidize. Aluminum forms a thick tenacious surface oxide after long exposure to moisture; steel simply rusts. Protective treatments keep this from happening, but by and large also form an insulating layer. The fact is, just about everything, even conductive finishes like nickel plating, form some sort of an oxide. The question is whether the protective layer is thin enough or brittle enough to scratch or part when light pressure mechanical contact is made. So, a coating may be effectively conductive or insulating depending on how thick the treatment is. For example, “yellow chromate” is conductive if lightly applied, Conductivity ~ 1 ohm/sq Comments At upper level of conductivity for effective shielding. Coating Nickel paint Copper, Copper alloy paint Electroless Coating Zinc arc spray Vacuum deposition ~1/4 – 1/2 ohm/sq. Capable of very good performance if properly applied and connected. very high very high varies widely Very high conductivity, excellent performance. Roughens surface; must be carefully applied. Not a noble metal. Very conductive. May not provide reliable contact if deposition too thin. Table 1: Characteristics of Common Surface Coatings R Magnetic Shielding That Works! MuShield is a small company that tackles big problems. Our team is a talented mix of engineers with extensive experience in the field of magnetic shielding. This enables us to build, test, and deliver a product that works within a reasonable time frame. Visit us online at www.mushield.com. EMI Shielding & Design Magnetic Shielding Materials Sheet Coil Seamless Tubing R MuMetal2 Permalloy R HyMU80 R Manufacturing Capabilities Heat Treatment Design Capabilities Performance Testing Production Capabilities Shielding Consultation Prototyping Custom Electronic Components Shielding The MuShield Company 9 Ricker Avenue Londonderry, NH 03053 Toll Free: 888.669.3539 Email: info@mushield.com mAy 2008 Conformity 53 http://www.mushield.com http://www.mushield.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.