barcode scanner asp.net mvc DESIGN FOR CONFORMAL COATING in Software

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DESIGN FOR CONFORMAL COATING
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When designing a conformal coating system into the product, the designer needs to consider a number of questions:
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Is coating required for this product, or should it be omitted for cost savings or other reasons What material properties are most important for this application, and which materials will provide the best balance What areas should not be coated
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Is Conformal Coating Required One of the basic questions to consider is whether or not to coat the particular assembly. Generally this will be a trade-off between reliability and cost. 41.7.1.1 Why Use It Customers may flow down requirements for conformal coating for industries where it is commonly used such as military or aerospace electronics. The following are some of the reasons to consider the use of conformal coating:
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The end products are used in humid or condensing atmospheres, such as electronics boxes for military and shipboard applications. It should be noted that although conformal coatings are moisture resistant, they are not waterproof, and should not be used for items that will see extended or continuously wet conditions. For those applications, use a watertight enclosure and conformal coat the boards inside. Conformal coating will protect against condensation, such as when the product is brought from a cold environment into a warm humid one. Clean conformal coating does not support mold and fungus growth. The end products can become contaminated by dirt and debris. Conformal coating is especially useful in space applications, since loose debris will float readily from place to place inside the electronics box. Conformal coating greatly improves the board s capability to withstand high voltage, especially at altitude or low pressure, since an electrical arc now has to travel through two layers of insulation rather than just jumping from one trace to its neighbor. Conformal coating protects against tin whiskers. Pure tin plating has the tendency to recrystallize and extrude long (>1 mm) whiskers of pure tin, which can short to neighboring part leads and cause short circuits. Conformal coating provides protection against short circuits caused by tin whiskers. Even if a tin whisker can force its way through a layer of coating on the way out, it cannot force its way back through the coating on the neighboring lead without bending and buckling.
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CONFORMAL COATING
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Conformal coating provides mechanical support for components during shock and vibration. Components weighing more that 1/4 oz. per lead should be supported by other means as well, such as staking with a bead of adhesive along the side. Lastly, conformal coating may be made opaque, and thus provide a level of security. A thick opaque epoxy coating will prevent the casual observer from determining what is under the coating.
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41.7.1.2 Why Not Use Coating Most of the reasons for not coating on an assembly stem from the recurring cost of cleaning, masking, demasking, touch-up, etc. The target cost of the product may not support the extra cost incurred by the coating, and if reduced reliability of the uncoated assembly is still acceptable, the rational decision is not to coat the assembly. Since the coating is a dielectric material, it changes the impedance of high-speed traces on the surface of the PWB. Boards with microstrip are usually not coated for this reason. Radio frequency (RF) PWBs for severe environments can still be coated if the traces are buried microstrip or stripline, which are unaffected by the additional coating on the board surface.
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Desirable Material Properties When selecting a coating, the entire suite of material properties must be considered in order to choose the best coating for the application. Unfortunately there is no one size fits all solution, but many different coatings, each suited to different applications, as shown in Table 41.1. The following are some of the properties to consider:
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Ease of application and producability The intended production environment may determine the cure process and application process that is best for the job, and will narrow the field of alternatives. Compatibility The coating must not be inhibited from curing by any material on the completed assembly. Often the best way to address this is to ask the manufacturer and try out the coating in question on a sample of the materials that you are going to use. Moisture resistance Dielectric constant For RF and high-speed circuits, a material with a low dielectric constant will change circuit performance less than a material with a higher one. Dielectric strength For high-voltage application, materials with a high dielectric strength should be used. For a valid comparison, tests must be performed on samples of equivalent thickness, since thin samples will break down at a higher volt/mil than thick samples of the same material. Insulation resistance Also known as resistivity, insulation resistance is of concern in highvoltage or high-impedance circuits, where the resistance in the circuit is very high (>10M ohm) and is significant compared to the resistance of the coating and PWB laminate. For these applications, choose a coating with high (>10^15 ohm-cm) resistivity and excellent moisture resistance. Hardness For products that need to withstand cold temperatures < 10 C, a soft coating is recommended so that at cold temperatures the coating will not exert excessive force on fragile components (see Fig. 41.7). For products that need good abrasion resistance and protection from external handling damage, a harder coating must be used. Glass transition temperature (Tg) For assemblies that need to withstand cold environments, select a coating that either has a Tg below the coldest temperature required or a Tg of 25 C and above. Coatings that have a low Tg in the range of operation will suddenly get hard at cold temperatures and impose great stress on the surrounding components.
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