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72 DESIGN007 MAGAZINE I OCTOBER 2022 board, which can also cause the coating to become too thick and crack or it can leave pockets under the coating where solvent can gather and cause corrosion. Tall components can bring their own challenges by creating shadowed or hard to reach areas. It is there- fore sensible to avoid placing tall components in direct proximity to "must-not-coat" areas of the PCB. Larger spacing between such areas can help to achieve a cleaner coating picture. If a larger gap is not maintained, coating may splatter onto "must-not-coat" areas. Preparation the Key to Success To truly minimize the risk of defects and fail- ure, cleaning prior to coating needs to be a top priority, otherwise potential residues on the substrate will have a critical impact on coating performance. Without cleaning, residues may interfere with curing, leading to poor adhesion of the coating to the substrate and trapping conductive/ionic materials under the coating. Without a pre-coat cleaning regimen, corro- sive residues bridging the PCB's conducting tracks can cause failures over time. It is of key importance that the PCB is fully dry following the cleaning process to ensure that any resid- ual solvents and moisture are removed prior to the coating and curing procedures. Humidity below the coating can lead to corrosion. Resid- ual solvent on the board can cause bubbles or cracking during the curing process. Apply the Coating Well Applying a coating correctly is probably the biggest factor for coating success. You may have purchased the best coating known to the industry, but if it isn't applied well, it simply will not deliver sufficient protection. Attainment of the "right" coating applica- tion is just as important as selecting the right material. Top Tips 1. Check the coating thickness that's required. Do not apply the coating in a thicker layer than is recommended. If the coating is applied too thickly, solvent entrapment can occur, causing a weakness for external influences to attack the board. 2. Check the level of coverage. Gaps in the coating coverage can leave areas of com- ponents exposed to humidity or corrosive gasses, providing an entry point to metal surfaces of components. 3. Consider the adhesion of the coating to the board and its components. Contami- nation or processing residues on the board can lead to an insufficient wetting of the coating. Even where the coating is showing a good level of adhesion, the contaminant may not show the same good adhesion to the substrate, potentially leading to delamination. Remember: e typically successful appli- cation method would ensure that each board receives a suitable coating coverage on all required metal surfaces, at a sufficient thick- ness to provide protection against the environ- ment. ese requirements will change with different board designs and environments and will need to be tested and verified ahead of production. Coating Selection ere are many coating options to choose from, such as an acrylic, a polyurethane, a silicone, a UV cure, or a two-part system. e design of the board may or may not affect the choice of coating material, but it will definitely Tall components can bring their own challenges by creating shadowed or hard to reach areas.