Conformal Coating: How Important Is Cleaning Before Coating?

From Manncorp

For the best possible adhesion, manufacturers should be cleaning boards before applying conformal coating—but it’s not just about adhesion.

Conformal coating can have a huge impact on reliability, which is (ironically) one of the reasons conformal coating is applied in the first place. Many assemblers think that they are protecting the assembly when they coat it, but if residues and contamination are present on the surface, they’ve just sealed in their potential reliability problems.

Another common mistake is the thinking that if you’re using no-clean flux, there’s no need to clean. In addition to no-clean residues affecting adhesion, there will be other contaminants on the board and components—fingerprint oils, metallic salts, mold release agents, particulates, tape residue, halides and more. All of these contaminants get trapped under the coating—along with the moisture that’s inherent in the board itself. The coating then acts like a greenhouse, creating a humid environment in which susceptible residues can become reactive.

This situation leads to electro-migration, dendritic growth and corrosion under the coating as well as possible breakdown of the coating itself, all of which will have a negative impact on reliability in the long run.

Some assemblers have actually found that they’ve been able to improve reliability by washing instead of coating, compared to coating without cleaning.

If your product will have a short expected life or is not going to be exposed to harsh temperatures, corrosives or humidity, the cost of pre-washing before conformal coating may not be worth it (or may be exchanged for the cost of washing instead of coating), but the more critical the reliability and expected life are, the greater the problem coating on unwashed assemblies poses.

At the very least, cleaning the circuit assembly, whether it’s prior to coating or in lieu of coating, takes residue-created reliability issues off the table.

Manncorp offers a number of cost-effective PCB cleaning/defluxing systems, including zero-discharge systems, which require neither a drain nor costly and time-consuming permits and wastewater treatment measures.

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