How do I know if my conformal coating and the process is compatible with my printed circuit board and its components?
There are many different ways the conformal coating can interact with the circuit board in a bad way. However, they can split into two types of interaction; direct and indirect.
Direct interaction is when it occurs during or just after application of the conformal coating in the drying / curing stage.Again, this can be split down into two areas. Damage to the circuit board and parts is the first. Typical examples include solvent based coatings “melting” a plastic component sufficiently to cause integrity issues or the coating removing the inks marking components thereby rendering the component unidentifiable by inspection. The second is immediate deterioration of the performance of the coating or the masking materials themselves. An example include issues such as inhibition of the coating to cure or dry due to materials on the circuit board with certain coatings like heat cure conformal coatings or the circuit board creating conformal coating defects such as de-wetting 0r delamination. Another example is the coating attacking the masking materials which leads to harmful residues being let behind which then could lead to an indirect interaction.
Indirect interaction is where the coating or masking material has interacted with the circuit board materials and causes long term reliability issues. It could be stated that defect formation can also lead to this but what we are specifically talking about is a chemical reaction with the materials on the PCB that leads in the long term to field failures. A good example of this is lead free soldering materials which appear to be susceptible to interactions with coatings which lead to effects such as electrochemical migration and field failures.
So, how do you test to see if the coating is compatible?
Its fairly obvious to state but a series of tests is required to check the interactions. An examination of the component list and datasheets can quickly identify likely candidates which could be damaged by the coating and its solvents. You can also check to see if the cure schedule will damage the components. Will heating the PCB for 2 hours at 90C cause any failures? Other possible tests include over-exposure experiments of the individual components to the coating and its solvents to see if they are damaged or absorb the solvents and increase in weight (submersion in thinners for a few minutes is likely to be worse than the coating application.
The next stage is to apply the coating to the PCB and see how the coating dries and cures. If defects like lack of adhesion or de-wetting occur then you may need to look at either bringing in cleaning or improving the cleaning process. Also, you can now check to see if the coating fully cures on the board?
Finally, you have the reliability test stage of the process. Techniques like Surface Insulation Resistance testing, thermal cycling and accelerated aging can shed light on the long term reliability of the product. If the product corrodes or fails after testing then you know you have an issue and you can examine directly if the coating actually improved the reliability of the circuit board or whether it contributed to its downfall.
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