Cleanliness: The Key to Conformal Coating Success

The quality of a conformal coating application is directly related to the cleanliness of the substrate being coated. Clean substrates coat well; contaminated ones do not. The only way to manage this problem is to inspect and clean electronic components before the coating process.

Ionic and Non-ionic Contaminants

Contaminants come in two broad categories: ionic and non-ionic. Ionic contaminants break down into separate molecules inside the coating. These contaminants 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 coating corrosion or the formation of small, vertical imperfections called dendrites.

Non-ionic contaminants do not cause boards to short circuit. They are usually effective as dielectrics, but they can prevent coatings from adhering to the board, similar to a masking compound. They also provide a place for foreign matter and debris to adhere. These contaminants generally appear as organic compounds like grease, oil or lotion, although rosin and silicone are also non-ionic.

Testing the Substrate

To ensure that components are clean before coating, it is recommended to pre-test them for contamination. In some cases, customers can perform a quick test by applying water or alcohol to any visible contaminants. Water dissolves ionic contaminants, while alcohol dissolves non-ionic. It is important to test ionic and non-ionic contaminants separately.

Ionic substrates are tested with the ROSE (Resistivity of Solvent Extract) method, which works by measuring a given solution’s conductivity. Once conductivity has been established, the solution is used to essentially wash the substrate being tested. After the wash is finished, the solution is then 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 glass slides with an aluminum coating. After the solvent evaporates, the board gets rinsed onto the slide again. After several cycles, usually six, the tester looks for any residue on the slide; if any appears, it’s a sign of non-ionic contamination. Finally, 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 the type of contaminant.

Cleaning the Substrate

Organic and other non-ionic contaminants can be removed by solvents and surfactants. Rigorous cleaning is typically 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 ionic compounds in the water onto the substrate.

Fixing Problems

Once a circuit board has been coated, some issues can be repaired. If the entire coating is poor, submersion stripping equipment can remove coatings in a multi-step process. Alternatively, it is possible to cut, abrade, dissolve or burn away local areas where the coating did not properly adhere. Once the area or the board is free of this coating, it can be cleaned and recoated.