|Fig. 1 - example of moisture permeation|
Conformal coating on PCBs (Printed Circuit Boards) is a must for proper protection of your investment with an LED Sign (or any digital signage system components exposed to the elements). Conformal coating is a thin polymeric layer which “conforms” to the exposed face of the printed circuit board and its components. Conformal coating acts as an insulator, protecting the circuitry and components against shorts and contact with moisture and other contaminants. It also provides mechanical protection from vibration and thermal shock.
Conformally Coated PCBs aren’t enough, the type of coating used and how it’s applied is just as important. Improper conformal coating can cause as many issues if improperly selected and applied. Selecting the appropriate coating and application reduces the risk of failure.
Moisture permeability has to be considered when choosing the right coating compound. Coatings are porous, so water and other materials permeate to varying degrees and cause failures. In Figure 1, solvent-based urethane allowed enough moisture permeation which caused corrosion underneath the coating. Some types of coatings are more permeable to moisture than others.
Though moisture permeation causes issues, permeability to moisture can also be an advantage. A coating which allows a board to “breath” creates a low static moisture level and trapped moisture can be a problem; one standard recommends baking a PCB for a minimum of 4 hours at 93+/- 5.5 ºC before conformally coating. Manufacturers which skip the cooling step or choose the wrong compound can expose your investment to serious issues.
Major issues to watch for with conformal coating compounds:
Shelf Life of the compound is often overlooked; a manufacturer can have the right coating but end up using product which is no longer at its prime. How often is the conformal coating used/replaced? Is the shelf-life of the product taken into consideration and old coatings replaced periodically to ensure optimal performance?
Uncured coating can result from the inability for a coating to obtain the optimum properties at the manufacturer’s specified time and temperature. Coatings can be inhibited by contaminants such as organic acids from flux residues. In the case of natural latex (a common masking agent), the residue alkali can reduce curing of certain catalytic-type coatings.
Adherance to the surface is critical for a coating. Since most coatings are liquids, the coating must be thick enough that it remains on the PCB surface as a uniform film but not too thick or have too high a surface tension to cause voids (liquid coating application is commonly referred to as “wetting”). This wetting is influenced by changes in the composition of the coating and compatibility with the PCB surfaces (solder masking, board or component surfaces, potting compounds, etc.). Wetting results in “spotting” and areas on the PCB which are flat/dull compared to the rest of the board/coated material.
When you’re looking at manufacturers of LED Message Centers, ensure they pay attention to the details. Many will claim to do conformal coating, but what steps have they taken to qualify the coating is done properly? Don’t accept a “standard” answer, dig below the surface to expose the truth.
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