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Design007-Aug2020

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78 DESIGN007 MAGAZINE I AUGUST 2020 2. Why would you need to mask some components/areas of the board? Some areas of the board, such as test points, may need to be masked to avoid the applica- tion of coating to enable functional testing after coating. Other areas may need to be masked to prevent obscuring labels—for example, essentially wherever the application of coat- ing is undesirable from a form, fit, or function point of view. In the case of unsealed connec- tors, switches, etc., the main issue is the coat- ing "wicking" up and coating the connector mating surfaces, thus reducing the contact or potentially insulating the mating surface, pre- venting either the form, fit, or function of the connector or switch. 3. Can you please explain the "orange peel effect" in more detail? Where does it come from, why does it happen, and can it cause board failure or have other detrimental effects? Orange peel is a cosmetic defect in which the appearance of the coating surface can appear matted and mottled under the correct lighting conditions, resembling the peel of an orange (hence the name). However, orange peel is poorly understood and often used incorrectly to describe a host of other cosmetic and func- tional defects. True orange peel is primarily an issue with coatings that dry by solvent evaporation. Dur- ing the drying of a solvent-borne coating film, the solvent at the surface evaporates causing differences in temperature, surface tension, solvent concentration, and density within the film. To balance the newly-formed thermody- namic non-equilibrium, currents occur in the coating film. These currents produce eddies in the drying layer, a phenomenon known as the formation of Bénard cells. The surface ten- sion is higher at the edges of the cells than at their centres and coating material flows from regions of lower surface tension to regions of higher surface tension. The resulting uneven- ness in the surface dries into the coating film. This produces an irregular surface as the coat- ing shows marked texture resembling the peel of an orange under normal lighting conditions. Under UV light, however, the coating will normally appear homogenous as long as the thickness is sufficient to cause correct fluo- rescence. Orange peel is exacerbated in coat- ings that dry quickly, especially when applied thickly. Since this effect is primarily cosmetic in nature and doesn't really affect the long-term protection offered by the coating, it has been removed as a defect from various inspection guidelines. Applying the coating in the correct thickness range and replacing very fast-drying solvents in the formulation with slower drying solvents will largely eliminate these issues. Sometimes, the formation of tiny bubbles of micro-foam in the surface of the coating can cause a matting effect and be confused with orange peel, but these can usually be seen under magnification, especially under UV light and are usually the result of a change in spray pressure or a change in the curing pro- file. These tiny bubbles can usually be ignored as a cosmetic finish, unless they are bridging conductor spacings or exposing metal surfaces. Any instance where the material does not fluo- resce continuously is not orange peel and is likely to be de-wetting due to contamination. 4. What is the difference between a critical and non-critical area of a board? Typically, during design and development, engineers will evaluate the robustness of their board design and prototypes. They will deter- mine that certain areas of the board (or com- ponents) are critical to performance and more susceptible to failure than other areas and will concentrate their coating or protection strate- gies on these critical areas. These will often be denoted as "must-coat" areas. Areas of the board that must not be coated—including con- nectors, switches, test points, or any other area that will affect form, fit, or function— will be denoted as "must not coat" areas. The remaining areas will preferably be marked up as "don't care" areas of the board. This helps simplify the coating of the assembly by focus-

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