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64 The PCB Design Magazine • July 2016 single selective-coating process with a solvent- based acrylic material is a recipe for a disaster, likely to result in excessive bubble formation, film shrinkage, coating delamination and addi- tional stress on components. The result is poor- er protection, rather than an improved overall level of circuit protection. Aiming for a uniform 30-50 microns and focusing on achieving per- fect coverage at each application is a much bet- ter approach to improving the protection of electronic circuits. Achieving the correct coating thickness is important; bear in mind that if the coating is too thick it can lead to entrapment of solvents in areas where the coating does not fully cure. Similarly, it can cause the coating to crack as it cures or as the result of changes in temperature, or due to mechanical shock and vibration. Problem: Liquid conformal coatings are subject to strong capillary forces from low standoff components such as passives, as well as BGA, QFP and QFN style packages. If a conformal coating is applied too thick- ly or at too low a viscosity, the material can be sucked underneath components, leading to a non-uniform fluorescence (some coatings con- tain a fluorescent dye that allows blacklight inspection of the PCB after coating to ensure complete and uniform coverage), as well as po- tential issues with package reliability. If the ma- terial takes too long to dry, the same phenom- ena can be seen. Some solvent-based conformal coatings can be difficult to inspect by fluorescence when the coating thickness is 20 microns or less. The temptation is to apply more material, which can exacerbate the component reliability issue due to coefficient of thermal expansion (CTE) mismatches in the Z direction. Increasing the viscosity and the rate at which the material builds viscosity can help to improve uniformity. While it may involve some double handling or the inclusion of an additional process, an al- ternative approach is to apply a thin "primer" coat layer. This will improve the ability of the coating to cover sharp, vertical edges of compo- nents and reduce the likelihood of bubble en- trapment and capillary force effects during the main coating layer application. Problem: Ambient temperature and its effect on the viscosity of liquid conformal coatings. Some factories can have a 10-15°C tem- perature differential between mid-summer and mid-winter. Flow-cups are often used to control the dilution of material prior to use. Material blends with the same flow time in winter may be very different in their make-up to the summer blend, leading to a different set of process parameters, thickness, coverage, degree of uniformity achieved and so on. The best way to overcome this problem is to ensure that temperature measurement is included along with flow-time in the viscosity monitor - ing process. Problem: Using "off-spec" thinners can lead to incompatibility issues with conformal coatings. A recent customer experienced a problem with his conformal coating material appear- ance changing from clear/transparent to yellow hazy/cloudy, with a wrinkly and "de-wetted" finish when dried. Upon investigation it trans- pired that, in wishing to reduce his overall pro- cess costs, the customer had resorted to the use of paint thinner purchased from a local hard- ware store. The paint thinner was not compat- ible with the conformal coating formulation, a fact that came to light initially with the forma- tion of a cloudy solution, and which was fur- ther suggested by the appearance of pin holes, confirmed by the wrinkled finish. Indeed, with- in 30 minutes of mixing the solution, it formed two layers! In summary, while it might be tempting to use shortcuts either to reduce costs or to speed production, there will inevitably be a higher price to pay. Know the limitations and/or spe- cial properties of the materials you use to coat electronic assemblies and abide by the correct procedures. PCBDESIGN Phil Kinner is technical director for Electrolube's Conformal Coatings Division. WHEN COATINGS GO WRONG