Coverage in Undesired Areas
Conformal coatings cover the substrate to which they are applied. However, it's also possible for them to end up coating parts of the substrate that aren't supposed to be covered. Sometimes, this happens because of inaccurate spraying. Other times, it happens because of the nature of the application -- dipping, for instance, is not good at selectively coating components. Parylene coatings are uniquely problematic because Parylene coats anywhere that air can touch, even inside microscopic openings.
Over-coverage issues are manageable. Before applying the coating, substrates can be masked off so that certain areas won't be covered. In the event that the masking isn't applied or fails, most coatings can be removed though reworking, although it can be time-consuming and expensive to do. Parylene and epoxy coatings are both harder to rework than others, making accurate masking particularly important.
Uneven Coating and Adhesion
Uneven coating or adhesion issues can come in a range of different types and from many different causes.
- Orange Peel. When a coated surface has a pebbly appearance -- like an orange peel -- it usually comes from the viscosity or quantity of the coating applied. Applying a coating that is less thick can help it to self-level. You may also need to adjust the spray gun's position or pressure for better performance. Allowing more time for the solvents in the coating to evaporate before applying elevated heat-curing processes can also help.
- Capillary flow. When coating flows away from some parts of the substrate to others, you end up with an uneven coating. Typically, applying less coating or making it thicker can help to mitigate this problem.
- Delamination and de-wetting. When coating peels away from the board or simply doesn't stick to it, it's usually a sign that there's a contaminant on the board that prevents the coating from adhering to it. Cleaning the board before coating can usually eliminate this problem.
Not every coating works with every material. But you can improve things by varying the chemical properties of the coating or the way that it is applied. For instance, single-part epoxy coatings are frequently cured with heat. When you need to coat a thermally-sensitive component, another coating or a non-heat cured two-part epoxy coating would be more appropriate. Silicone is frequently applied in thicker coats than other materials, making it unsuitable for small components or for spaces with tight tolerances. Parylene's mechanical adhesion technique limits its ability to bond to many metals, including stainless steel and gold. Pre-treating items that are designated for Parylene coating with A-174 silane can help to solve this problem and give Parylene excellent adhesion.
Many conformal coatings form cracks, usually during the curing and drying process. These cracks happen because areas of the coating dry and change shape before other areas, placing stress on the overall coating. Typically, slowing down the curing process by reducing the temperature and waiting longer to apply heat can help to mitigate this problem. This can also happen if too much coating is deposited, leading to uneven drying where some parts of the coating pull on others as they dry. When cracks happen after the drying process has finished, it's usually due to the coating being a poor match for the substrate's operating temperature. Selecting a coating with a higher temperature range can eliminate this problem.
Applying a conformal coating isn't a simple process. Choosing the right coating for the material, applying it correctly, and curing it requires experience and skill. Contact Diamond-MT to learn how our 25 years of experience can help your project succeed.