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78 DESIGN007 MAGAZINE I NOVEMBER 2019 Coating Cycle Time Coating cycle time is very important to the balance of the production line, so what kills coating quality and cycle time on a typical se- lective coating line? Selective coating machines deposit a stripe of coating material that can be programmed to stop and start on demand. Stripes of coating can be deposited to create a coating pattern, avoiding areas such as switch- es, connectors, and test points, which must not be coated to prevent interference with form, fit, or function. The coating stripes deposited are usually in the 8–15 mm range for optimum accuracy and minimizing overspray and splashing. When the area to be coated is less than 8 mm in width, then it is necessary to utilise a dispensing step, which is a cycle time killer. Due to the combination of machine X/Y po- sitional accuracy, material fluid dynamics, and component topography, 2–3 mm is usual- ly as close to keep-out areas as anyone would be comfortable coating for a repeatable pro- cess. In summary, must-coat and keep-out ar- eas positioned within 2–3 mm of each other present a problem. Dispensing is also required, representing another process step adding to cy- cle time. Coating Flow In manufacturing, for many coatings, it is difficult to limit the flow of coating from the application site to adjacent sites. Most coatings will have some level of capillary flow (often re- ferred to as wicking), taking the coating to un- anticipated areas. Therefore, components that must be coated should not be placed in prox- imity to components that must not be coated. Coating flow is also an important consider- ation on low-standoff bottom-terminated de- vices, such as BGAs or QFNs, which may also have microvias under them. Coating can wick under the parts and then down the vias to the opposite side of the assembly. To avoid this problem, the vias under low standoff compo- nents can be tented with solder mask or have the vias filled with solder or suitable via-fill material. Of course, since conformal coating can reduce the solder joint lifetime of bottom- terminated devices, deliberately coating be- neath these devices should be avoided. Keep in mind that liquid coatings will flow some- what during the coating application process, so placing a keep-out zone at the base of a tall component, such as an electrolytic capacitor, will create a lot of problems in manufacturing as the coating flows down the tall part into the keep-out zone. Engineering drawings or quality control doc- uments should be very specific on what coat- ing anomalies are defects, requiring disposi- tion, and those that are primarily cosmetic. For many years in the industry, anything that looked different in the coating was considered a defect, requiring aesthetically perfect assem- blies. While possible, such requirements often drive up the cost of coating and often have no impact on assembly reliability. Designers should be aware that coating the edges of an assembly is not considered as a value-add for most applications. Most design standards do not allow inner layer circuit trac- es to be closer than 0.63 mm from the edge of the board. The act of routing the board edg- es results in smearing the epoxy resin over the cut glass fibers or the reinforcement ma- terial, sealing off the inner layers. A designer might specify coating board edges where such sealing does not occur, such as with V-scored, punched, or sheared edges. Understanding the dynamics of what affects what on the surface of the board during the ear- ly stages of design will go a long way in help- ing you achieve a successful conformal coating operation. This will also help avoid potential disasters in other areas of production. DESIGN007 Phil Kinner is the global business and technical director of confor- mal coatings at Electrolube. To read past columns or contact Kinner, click here. Kinner is also the author of The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments. Visit I-007eBooks.com to download this and other free, educational titles.