Applying Conformal Coatings
Various coating techniques can be used, depending on the volume of parts to coat, and the capabilities of your organization.
For small repairs and low-volume jobs, brushing is an ideal technique as it requires no preparation. It is very precise and requires no expertise. Dipping is also ideal for low-volume work, reliable coating penetration under components and generally fast processing, but it may require intensive masking and can be messy.
For high-volume projects, spray technique becomes important, and is ideal when rapid turnaround, enhanced edge-tip coverage and better film uniformity are required.
Spraying can be accomplished using a simple pneumatic spray gun in a dedicated fume booth. However, this technique requires some expertise and may require masking The spray process can be streamlined by using automated equipment of the sort made by PVA, Nordson Asymtek and Specialty Coating Systems. These systems provide speed and precision to maximize throughput and minimize rework.
Cure – There are three types of conformal coatings classified by their curing mechanisms: thermoplastic, thermoset and UV-curable. Thermoplastic materials include acrylics and silicone/acrylic hybrid coatings. They cure by physical drying as solvent simply evaporates, leaving the coating film. These coatings can be cured either under ambient conditions or by using heat. Thermoplastics are materials that undergo a chemical reaction as part of their curing mechanism. Like thermoplastics, they can typically be cured under ambient conditions. However, to achieve optimum properties and full cure, these coatings require elevated temperatures. UV-curable materials are acrylates and methacrylates that cure when exposed to certain wavelengths of UV light.
Quality Control – The main consideration when inspecting conformal coatings is coverage. This can be done quickly and easily using a black lamp. Simply place the coated board under a black lamp and check for areas which fluoresce blue.