What is conformal paint?

Process considerations

Conformal coatings protect circuit boards and components from moisture, dust and chemicals. If the coating material is not cured properly, shrinkage, orange peeling, solvent entrainment, or bubble formation may occur - stressing the board. To avoid these failures, it is important to follow the curing practice that best suits your application.

Room temperature curing

Room temperature curing is a slow process that can take hours or even days to complete. In this method, solvent-based coatings are placed at room temperature to allow the solvent to evaporate. Heat can be applied to further promote evaporation. It is important to allow the solvent to evaporate completely, otherwise premature surface curing can trap the solvent within the coating. The presence of solvents in the coating prevents the cross-linking and eventual curing of the material. The composition of different solvent materials can change the evaporation rate. Solvents such as toluene and xylene evaporate quickly, while others, such as butyl acetate, evaporate at a slower rate.

A drying rack can be used to store the coated board while it cures. When the solvent evaporates, proper ventilation is required for the health of the operator. During curing, the coating thickness decreases as the material leaves the board solvent, resulting in a dry coating thickness that is less than the wet coating thickness.

thermosetting

Heat curing takes less time than room temperature curing. In some cases, heat acts as the primary curing mechanism. Heat can also be used as a secondary mechanism, paired with another curing method to speed up the curing process. For example, room temperature curing can be combined with thermal curing to accelerate the evaporation of solvents. With the UV curing process, heat can be used to cure fluids hidden under UV light. This curing method can be used for solvent-based and 100% solid materials.

Nordson ASYMTEK offers infrared/convection ovens with programmable heating zones, electric conveyors and ventilation. Four different conveyor lengths are available. The system features downdraft ventilation to safely ventilate volatile organic compounds (VOCs) and a programmable heating zone that allows you to create custom cure curves to meet fluid manufacturer specifications and parameters.

Ultraviolet curing

Ultraviolet (UV) curing is common in polyurethane materials. On the spectrum, UV-A and UV-C lamps have different wavelengths - providing surface and deeper penetration curing. Typically, UV curing requires UV-A and UV-C light to fully cure the coating. UV-A operates at a wavelength of about 365 nm. In contrast, ultraviolet UV-C light operates at a wavelength of 254nm. The surface and subsurface of the coated plate can be cured using a combination of UV-A and UV-C light.

Curing with ultraviolet light is limited by light and deep penetration. Areas blocked by higher components (shadows) and materials below the components require secondary mechanisms to cure, as they are not exposed to light. Similarly, thicker coatings with limited UV penetration depend on secondary curing mechanisms - moisture or heat.

Other parameters that affect the UV curing curve include duration, type of light, and intensity. Different light sources also provide different curing effects. Mercury or H bulbs are commonly used for curing materials. Irons and LED bulbs are also available alternatives.

Moisture curing

Moisture curing requires the presence of humidity, and the degree of humidity will affect the curing time. Moisture curing occurs on first contact, at the surface, and then pushes inward. Moisture-cured materials require additional consideration during application. To reduce material clogging in the fluid system, it is recommended to use clean, dry air or nitrogen to apply reservoir pressure.