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22 DESIGN007 MAGAZINE I SEPTEMBER 2021 a conformal coating is as important, if not more important, than the selection of the right mate- rial. I can't stress this enough, because even the best coating, when applied poorly, may not provide the level of protection required or that it is capable of. Significant elements to consider include the thickness the coating is applied, the level of coverage achieved and how well the coating adheres to the board and its components. It is imperative to choose the best method for applying the coating. ere are numerous methods to apply coat- ings, which include hand applications such as brush, aerosol and atomised spray, and machine applications such as dip and selective coating. When applied correctly they form a thin film that conforms to the contours of the PCB and its components, as well as the exposed metal of component leads and solder joints. Typi- cally, conformal coatings are applied in the 25–250-micron range. e application method chosen for a par- ticular assembly will depend upon which exist- ing equipment is available to the manufacturer, the coating processes in use, the takt time (the average time interval between the start of pro- duction of one unit and the start of the next), and the design of the assembly. is includes those areas of the circuit, which must be coated and those which must not, connectors, switches and so forth. In addition, it is incredibly helpful to define don't care areas, which may or may not be coated. e best application method will ensure that each board coated receives good coverage on all required metal surfaces at a sufficient thickness to afford protection against the environment. ese requirements will change from board design to board design, and environment to environment, and invari- ably they need to be tested and verified ahead of the production run. Coating Thickness Achieving the correct coating thickness is essential; bear in mind that if the coating is too thick it can lead to entrapment of solvents which can result in blow-out bubbles, delami- nation, or lack of adhesion. 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. Conformal coatings should not be applied at thicknesses greater than necessary and should never exceed the maximum thickness they were designed to be applied at. Is it Necessary to Clean Prior to Coating? e potential presence of residues on the substrate can have a critical impact on coating performance. If the substrate is not clean, the residues present may interfere with the curing mechanism, lead to poor adhesion of the coat- ing to the substrate, and/or trap conductive/ ionic materials under the coating. Without meticulous attention to preparation or pre-coat cleaning regimes, corrosive residues bridging the PCB's conducting tracks can cause failures over time. It is important to test the reliability of no-clean flux residues from your assembly process with your coating process, to ensure compatibility and adequate performance in your design. What Will the PCB Board Layout Be? e design of the board should include con- sideration of the placement of components that should not be coated. Selective spray equipment or the application of a peelable coating mask can be used to help avoid such areas. Alternatively, gel materials can be used to form a "dam" to contain the coating and avoid capillary effects transferring material to unwanted areas, such as connectors. Another aspect to consider is how close together the components are. If there is not a sufficient gap between the components, it can lead to the coating bridging the gap rather than correctly conforming to the board. is can have sev- eral effects including the coating building up to become too thick and crack, or it can leave pockets under the coating where solvent can gather and cause corrosion.