Generally, the critical factor in applying conformal coating is not the number of coats applied. Factors that can influence the performance of the coating are:
- The total coating thickness. Too thick and the coating may crack in the long term. Too thin and it will have a less effective performance
- The defects created when applying the coating layer. Again, thickness is the overriding influence and applying too thick a coating can cause issues such as bubble entrapment in the film
That said it is best to avoid creating laminating layers of coating with too much time between coating layers.
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There are several factors that can affect adhesion conformal coating. These include the cleanliness of the PCB, the coating material / PCB and component surface compatibility and how well the coating is cured.
Cleanliness of the Circuit Board
Of the three factors considered here, the cleanliness of the PCB appears to be the most important for local area adhesion problems where the coating either does not wet or is poorly adhered. The cleanliness of the board can be affected by many factors like the laminate and component cleanliness , process contaminants added such as flux and cleaning residues and particulates from the surrounding production area. This can be very critical specifically in a no clean process and care has to be taken here.
The Conformal Coating Material / Laminate Surface Compatibility
This could be considered as the single most important factor if wide spread de-lamination of the coating is seen. If the coating comes off like a sheet of plastic then it is probably due to the surface energy of the laminate being incompatible with the coating applied even though technically the laminate is clean. Measuring the surface energy of the circuit board with dyne pens and consulting with the material supplier can quickly identify this issue.
This can be important when multiple layers are being applied and the degree of cure can affect the adhesion between the layers. The best advice is follow the manufacturers recommendations and this should not be an issue.
So why is adhesion important for the conformal coating?
Well first of all no one wants to have a conformal coated PCB that looks bad because the coating is de-laminating or blistering. But, ultimately the critical reason is that the coating is put on to protect the circuit board and if it isn’t adhered then it could affect the functionality and reliability of the board. Therefore, it is crucial to ensure that the coating has good enough adhesion to provide an adequate level of protection to the circuit whilst in operation. Also, a conformal coating seals in contamination as well as keeping it out. If there is ionic contamination below the surface then this can quickly lead to long term reliability issues with the conformal coating failing due to cleanliness issues which will need to be addressed.
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Yes, Using approximately 5% solids content version of the conformal coating you are using.
To reduce the conformal coating down to a suitable primer-like material I would mix a ratio of 1:5 (coating: thinner) or more depending on the solids content of the conformal coating. The primer can then be applied and leaves a ~5um layer on the surface. It doesn’t necessarily need to be baked as it will cure in 5minutes and you can add the second layer on top.
This thin layer of conformal coating increases the surface energy on the PCB so that the main conformal coating layer can next be applied. This is useful in cases of low surface energy solder resists.
For further information on conformal coating processing click Conformal Coating Defects, Finishing & Repair FAQs