Assembly Planbook - April 2008 - (Page 55) machine can identify specific products on the line and automatically adjust process parameters, such as conveyor width, coating speed and dispense pattern. Vacuum deposition is a unique coating process, in which the coating grows on the substrate one molecule at a time. The devices to be coated are placed in one vacuum chamber, and a powdered raw material, known as a dimer, is placed in another. The dimer is heated, causing it to sublimate into a monomeric vapor. This vapor is then transferred into the first chamber, where it condenses onto the parts, forming a thin film. No solvents, catalysts or plasticizers are required. Choosing a Coating There are many variables to consider when choosing a conformal coating. No single coating is right for every application. Before contacting vendors, engineers should ask themselves the following questions: What are you protecting your electronics from? What temperature range will your electronics need to survive? What is the desired life of the product? How will the coating be applied? How important is ease of processing and curing? Are you coating one product or a mix of products? Can the design of the product be modified to facilitate the coating process? Are the electrical, chemical and mechanical attributes of the coating compatible with all parts of the assembly or will masking be required? Does the coating need to be biocompatible? How fast does the material cure? Can the material be reworked? Does the material present safety or disposal concerns? Adhesion is the most important feature of any coating material. If the material does not adhere well, moisture can penetrate the substrate. Several factors affect adhesion, including incompatibility with the substrate and component materials, and residues from flux and cleaning solutions. Lead-free electronics assemblies can present a special challenge to conformal coating. That’s because lead-free solder reflows at much higher temperatures than tin-lead solder. As a result, a variety of ionic contaminants tend to leach out from the assembly. These electrically conductive residues come from cleaners, etchants, plating, fluxes, human contact and the components themselves. To prevent product failure, these contaminants must be cleaned or they will be sealed to the board during coating. A You can learn more about conformal coating with a click! www.assemblymag.com Series Torque Measurement Systems Introducing the “Torque Cube ” TM Torque CubeTM Series Torque Testers ■ For use with hand tools or power tools ■ User friendly menus and programming ■ Highly featured and portable Also Available: UET/MTM Series Torque Testers ATDA Series Torque Analyzers UET Series Torque Testers Power Tools ■ Assembly Systems ■ Torque Measurement A IMCO ■ (800) 852-1368 ■ w w w.aimco-glob al.co m www.assemblymag.com April 2008 / ASSE M B LY 55 ASB11074Aimco.indd 1 10/15/07 10:35:57 AM http://www.assemblymag.com http://ww.aimco-global.com http://ww.aimco-global.com http://www.assemblymag.com
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