Many electronic components and circuit boards have a conformal coating applied as protection against humidity, corrosion, heat, fungal growth, dust, etc. These coatings may also help maintain circuit integrity and component characteristics. “Conformal” simply means that the coating conforms to the landscape of the PCB and the shape of the components.
Conformal coatings may have defects, though, and it is important to recognize whether or not these are critical enough to affect circuit functions. In other words, is it actually a defect or within the coating process parameters (a process indicator). Customers receiving many products from a supplier may need some understanding of coatings to make decisions regarding ongoing production or sales, and they may need to meet certain industry standards. Single-product customers may wish to recognize defects for warranty purposes rather than having a product fail in the future. There are some basic observations that can help in either case.
Most coating defects stem from a prior cleaning process—either the final result or the complete lack of cleaning–or from conditions during the application process. Fewer cases result from other reasons such as the wrong choice of coating, but they do occur. Examples of defects and process indicators include cracking, flaking, voids and bubbles, trapped materials, lack of adhesion, and discoloration. A few of these are discussed here.
Air bubbles are very common in any type of coating (painting a wall, for example). If these occur in a non-critical area of a PCB, there may not be a functional problem with the board. However, these may lead to deterioration of the conformal coating over time, and they may indicate a problem with the coating or cleaning process. If these bubbles are in a critical area, they may not provide the protection that was expected.
A coating that doesn’t level out and adhere to surfaces may simply trap air, and then bubbles are embedded as they try to escape through the hardening surface. Applying coating with a brush may also produce bubbles within the surface of the coating. The most common source of trapped gases, however, is from solvents or contaminants that have not vaporized and escaped before the coating dries. These might be on the board before the coating is applied, but just as often it is the solvent within the coating compound that did not dissipate before the coating hardened. Other factors that can lead to trapped bubbles are the coating thickness and the speed of curing.
Dewetting and Beading
Wetting is the term used to describe the spreading and penetration of a liquid over a surface, and it is the most important characteristic of a conformal coating. Dewetting, or beading in general, is the condition where the coating compound beads up in areas where the surface is resistant to spreading.
A very likely reason for dewetting is contamination of the PCB or component surface. These contaminants include process oils, flux residues, and fingerprints. Many of these are already on the components during assembly or are introduced during soldering, but all of them should be removed as part of a cleaning process prior to coating.
One lesser known reason for dewetting is the surface energy of the component or PCB in relation to the properties of the coating. This can be loosely translated as ‘surface tension’. The surface energy of a liquid is usually expressed in ‘dynes’ (units of force). Basically, the target surface must have a higher surface energy than the applied coating to allow for proper wetting and to avoid beading. An example outside of electronics is waxing a car. The surface energy of a vehicle changes with the applied wax, so water beads up instead of spreading out (dewetting vs. wetting). This is illustrated, below.
Wrinkles, Waves, and Orange Peels
Textures within a conformal coating usually indicate a process flaw. Reasons for these surface patterns include thick coatings that do not level properly, solvents that flash (dissipate) too quickly, incorrect spray angles, and incorrent coating timings. These may prove to be merely cosmetic (non-critical) flaws, but even if they do no constitute a product rejection, they do suggest that the coating process must be modified. Examples are shown below.
Two other common coating defects are delamination and ‘fisheyes’. Delamination is an area of coating that starts separating (lifting) from the substrate, and it may not become noticeable until after the parts are in use. Like dewetting, delamination may be caused by surface contamination, but also from an improper drying time or coating thickness.
Fisheyes are similar to common air bubbles, but the area has cratered or collapsed into a particular shape that resembles a fish eye. These are often caused by a contaminant at a single point, and this is common with inadequate filtration within an air-powered spray application.
These previous examples are only a few of the most common anomalies in conformal coatings, and the reasons listed here are only touching the surface (pun intended). More resources are available from manufacturers regarding their individual coating products or for complete guidelines on conformal coating applications.
Conformal coating products can be found in the Coating, Grease, Repair category [ Conformal Coating ] or by searching for relevant categories with the keyword ‘conformal’ from the Digi-key home page: [click here https://www.digikey.com/ ].
Chemtronics: CH_PCB cleaning-coating booklet (3).pdf PCB Cleaning + Conformal Coating Guide
Chemtronics Chemtronics Coating Webinar_082020 (3).pdf Conformal Coating Defects: When Things Go Horribly Wrong
Chemtronics: (YouTube Video) [ Chemtronics Conformal Coating Defects Webinar ]
HumiSeal: PCB Surface Energy (Blog) [ HumiSeal Conformal Coating ]
Digi-key Forum Post [ Conformal Coating Processing Tips for Samtec Connectors ]
For Convenience, full URLs are posted below:
Coating, Grease, Repair ‘Conformal’ Keyword
Chemtronics Conformal Coating Defects Webinar
HumiSeal PCB Surface Energy (Blog)
Conformal Coating Processing Tips for Samtec Connectors
Conformal Coating Processing Tips for Samtec Connectors