The quality of a conformal coating job is directly related to the cleanliness of the substrate that is being coated. Clean substrates coat well, and contaminated ones don't. The only way to manage the problem is to inspect and clean the board or other item before applying the coating -- once it's coated, it's essentially too late.
Ionic vs. Non-ionic Contaminants
Contaminants come in two broad categories, each of which has different impacts. Ionic contaminants break down into separate molecules inside the coating. Among other properties, they can make the coating into a conductor, potentially impacting the reliability and operation of coated printed circuit boards or other electronic components. Ionic contaminants can also lead to the coating corroding or forming small vertical imperfections called dendrites. While many ionic contaminants are by-products of manufacturing, sweat from workers is also ionic due to the dissolved salt in it.
Non-ionic contaminants don't short out your printed circuit boards. In fact, they're usually effective as dielectrics. What they do is prevent coatings from adhering to the board -- similar to a masking compound. On the other hand, they can also provide a place for foreign matter and debris to stick to. They're generally organic compounds like greases, oils or hand lotion, although rosin and silicone are also non-ionic.
Testing the Substrate
Unless you're absolutely sure that the components that you deliver for coating are completely clean, it's wise to test it for contamination before coating it. Both ionic and non-ionic contaminants are tested for separately. In some cases you can do a quick-and-dirty test by applying water or alcohol to a contaminant that you see -- water dissolves ionic contaminants, but only alcohol will dissolve a non-ionic one.
Ionic substrates are tested through the resistivity of solvent extract method. The ROSE method works by measuring how conductive a given solution is. Once the conductivity has been established, the solution is used to wash the substrate that is to be tested. After the wash is finished, the solution gets retested. If the board is contaminated with ionic compounds, the solution's resistivity will go down and its conductivity will go up.
Two different tests are used to find and identify non-ionic contaminants. First, the item gets rinsed with a solution containing acetonitrile onto special glass slides with an aluminum coating. After the solvent evaporates, the board gets rinsed onto the slide again. After a number of cycles -- usually six -- the tester looks for any residue on the slide. If any appears, it's a sign of non-ionic contamination. The slide then gets analyzed under a microscope. Next, the slide goes through a spectroscopic process known as Fourier transform infrared (FTIR). A special FTIR microscope analyzes the residue's spectrum under infrared to identify what type of contaminant it actually is.
Cleaning the Substrate
Organic and other non-ionic contaminants can be removed by solvents and surfactants. Typically, a rigorous cleaning is enough to eliminate the problem. Ionic contaminants can frequently be removed with water, although water that is not adequately pure can compound the problem by depositing the ionic compounds that are in the water onto the substrate.
Once an item is coated, some coating issues can be repaired. If the entire coating is bad, submersion stripping equipment can remove all of the coating in a multi-step process, letting you start again from the beginning. Alternately, you can cut, abrade, dissolve or burn away local areas where the coating did not properly adhere. Once the area or the board is free of coating, you can then clean it and re-coat it. Bear in mind that urethane, epoxy and Parylene coatings can be hard to remove and rework, so it's particularly important to get them right the first time.
Diamond-MT can work with you to ensure that your product gets coated correctly. We'll work with you to test and clean your products before coating so that reworking doesn't become an issue.