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Design007-Jan2022

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JANUARY 2022 I DESIGN007 MAGAZINE 91 5. What other processes can impact the coating? Flux residues from "no-clean" solder pastes and selective soldering or wave soldering can significantly impact the ability of the coating to adhere and perform its function of protect- ing the assembly. Compatibility studies should always be performed prior to the implementa- tion of coating over a "no-clean" process and care must be taken to ensure the soldering pro- cesses remain under tight controls. e designer should ideally be aware of what kinds of manufacturing practices may occur following the application and cure of the coat- ing, as other materials such as thermal greases/ putties and rework/repair chemicals can all have an impact on the integrity and overall performance of a coating. Also, when selecting adhesives for assemblies, care should always be taken to ensure that they are compatible with the selected coating materials and processes. Adhesives that are not compatible can have a detrimental effect on the overall performance of the coating. Having started the conversation about the importance of making sound early-stage design decisions, it is important to under- stand "what affects what" on the surface of the board to ensure successful conformal coat- ing is achieved. Implementing these lessons will prevent potential production disasters not only with conformal coatings, but also in other areas of production as well. In my next column, I will be covering more coating tips and tricks. DESIGN007 Phil Kinner is the global busi- ness and technical director of conformal coatings at Electro- lube. To read past columns or contact Kinner, click here. Down- load your free copy of Electrol- ube's book, The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments, and watch the micro webinar series "Coatings Uncoated!" ence with form, fit or function. e coating stripes deposited are usually in the 8–15mm 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 positional accuracy, material fluid dynamics and component topography, 2-3 mm is usually as close to keep-out areas as anyone would be comfortable coating for a repeatable process. So, must-coat and keep- out areas within 2–3 mm of each other pres- ent a problem and, again, dispensing will be required, another process step which kills cycle time. 4. How to improve coating flow For many coatings, it is difficult to limit the flow of coating from the application site to adja- cent sites, as the coating will have some level of capillary flow (oen referred to as wicking), taking the coating to unanticipated areas. is brings us back to the importance of avoiding putting components which must not be coated close to components which must be coated. 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. Coating flow is also important on low stand- off devices, such as BGAs or CSPs, which may have microvias under them. Coating can wick under the parts and down the vias to the oppo- site side of the assembly. To avoid this prob- lem, the vias under low standoff components can be tented with solder mask or have the vias filled with solder or a suitable via-fill material. In addition, the presence of conformal coating beneath BGAs, for example, can significantly reduce the reliability of the solder joints. e coating process should be designed to aim to minimise coating flow beneath BGAs or use an underfill material to improve the mechanical reliability of these devices.

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