Conformal coatings frequently play a critical role in protecting medical, electronics, defense, aerospace, LED, and automotive applications from contaminants and other potentially hazardous substances. But they're not infallible.
If the preparation, application, and drying processes aren't properly managed, for example, conformal coating defects could develop that compromise the coating and, ultimately, the end product. Luckily, most common conformal coating defects can be avoided.
Here, we review six common conformal coating failure mechanisms:
- Capillary Flow. This common conformal coating defect occurs when capillary action pulls the coating material away from some areas and onto others, yielding an inconsistent finish. At best, the resulting conformal coating is uneven and patchy; at worst, the substrate is left completely uncoated. Factors that can impact capillary flow of conformal coatings include coatings that are too thin, an over-application of coating, or an attempt to coat a surface that is too cool or that is contaminated, among others. Because of this vulnerability to contaminants, cleaning the surface of the product prior to coating is crucial to achieving the desired end result, as is correctly applying the optimal coating for a given application.
- Cracking. Cracking, another failure mechanism of conformal coating, is often attributed to temperatures that are too high during the curing or drying processes. However, cracking of the conformal coating can also be caused by particularly low temperatures or by the application of a coating that is too thick. Strategies for avoiding this defect include reducing coating thickness to manage the coefficient of thermal expansion and paying strict attention to temperature control.
- De-wetting. Contamination can prevent a conformal coating solution from evenly sticking to and "wetting" the substrate. In the event of such a problem, areas of the product remain uncoated, exposing the substrate to further contamination and coating failure. Cleanliness is the key to avoiding this conformal coating defect. But the solution—should the problem occur—is to strip the affected area and manually re-coat it through an exacting re-work process.
- Delamination. Sometimes, everything in the coating process goes right except that the coating just doesn't end up sticking to the board. However, proper adhesion of conformal coatings is critical to their success. Corners can be particularly vulnerable to delamination because the coating lifts slightly and not only leaves an unprotected area, but also leaves an opening for the coating to peel back even further. As with most other conformal coating defects, cleanliness plays a role in preventing delamination. This failure mechanism, however, can also be caused by moisture, selection of the wrong coating material for a given surface, or curing issues.
- Orange Peel. A dull conformal coating that features a rocky, textured appearance is described as having an "orange peel" defect. Usually, this failure mechanism arises when the coating is too thick and can't level itself out or when the coating is applied via spray methods from too great a distance. It can also be caused by a problem in the curing process. Orange peel conformal coating defects can be remedied by spraying from the correct distance, thinning out the coating material, fixing the spray air pressure, or adjusting the curing temperature and process.
- Bubbles, Pinholes, and Foam. If air gets trapped in the liquid coating material, conformal coating defects such as bubbles, pinholes, or even a foamy surface can be an unfortunate by-product. Bubbles can get trapped in the coating material or can even form after the coating is applied if it doesn't dry properly. Usually, the defects are caused by problems in the coating method, although viscosity and curing temperatures can also play a role.
Conformal coating defects can be caused by a range of factors. However, cleanliness of the product, careful conformal coating selection, and knowledge of the process can help to mitigate the potential for these problems to arise.