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Design007-July2019

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50 DESIGN007 MAGAZINE I JULY 2019 allow dip coating to be explored as a potential alternative methodology, speeding up appli- cation times, and reducing costs. Also, avoid large arrays of discrete components, which can pose a huge coating challenge due to the high levels of capillary forces present. The net re- sult is often areas of no coverage/protection on the board as well as areas of excessive thick- ness prone to stress-cracking, delamination, and other coating defects. Similarly, tall com- ponents present challenges of their own by the creation of shadowed or hard-to-reach areas. Splashing is another associated problem. The trick is to avoid placing tall components next to must-coat components to avoid this eventu- ality. 2. Be Mindful of the Processes That Can Impact the Coating The designer should ideally be aware of what kinds of manufacturing practices may oc- cur following the application and cure of the coating because other materials, such as ther- mal greases/putties and rework/repair chem- icals, can all have an impact on the integri- ty 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 compat- ible can have a detrimental effect on the over- all performance of the coating. 3. Consider the Importance of Pre-coat Cleaning It is worth bearing in mind that choosing the most appropriate conformal coating is merely the first stage in the protective process. The condition of the assembly before coating is an often-overlooked part of the protective pro- cess. The designer should always consider the process steps leading up to conformal coating, and a key element of this is cleaning. The over- all cleanliness of the substrate or the potential presence of residues on the substrate (such as no-clean flux residues) can have a critical im- pact on coating performance. If the substrate is not adequately clean, the residues present may interfere with the curing mechanism, lead to poor adhesion of the coating to the substrate, and trap conductive/ionic materials under the coating. Without meticulous attention to prepara- tion or pre-coat cleaning regimes, corrosive residues bridging the PCB's conducting tracks can cause failures over time. Whilst the coat- ing may delay failure for many years, at some point, failure is still inevitable. Contaminants present on the surface before conformal coat- ing will be sealed in by the operation and may cause long-term problems. Such contaminants might include fingerprints, flux residues and moisture, and other atmospheric pollutants. Boards should always be cleaned and dried before conformal coating to obtain optimum long-term performance. Even when using so- called no-clean fluxes, cleaning boards before coating will usually improve long-term perfor- mance and reliability. 4. Create a Buffer to Make Coverage Easier for Production Conformal coatings are usually liquid when applied and will flow with a combination of gravity and the capillary forces present. Con- formal coating materials will simply go where they are placed with some naturally occurring wetting/spreading to a greater or lesser extent, which is a function of the material's surface tension, thixotropic index, and drying time. Most materials will tend to slump away from sharp edges of the component, leads, and sol- der joints due to gravity. This behaviour can be made worse by longer dry times and also by baking if the initial viscosity drop is greater than the increase due to solvent evaporation. Understanding and controlling this behav- iour, and its effect on the conformal coating coverage, will be key to the performance of coated assemblies operating in harsh environ- ments. Whether the process specifies mask- ing or reliance upon the process of selective conformal coating, a production team will be greatly relieved if you leave a buffer (or a don't-care area) of at least 3 mm clear between the areas that must be coated and those that

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