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Conformal Coating Mechanisms

Mitigating Curing Issues

Process Considerations
Conformal coating protects boards and components from exposure to moisture, dust, and chemicals. If the coating material is not cured properly, shrinkage, orange peel effect, solvent entrapment, or the formation of bubbles can occur – introducing stress to the board. To avoid these failures, it is important to follow the curing practice that best fits your application.

Room Temperature Cure

Room temperature curing is a slow process that can take hours or even days to finalize. In this method, solvent-based coatings sit 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 fully evaporate, otherwise premature curing at the surface can trap the solvent inside the coating. The presence of solvent in the coating prevents crosslinking and final curing of the material. Different solvent compositions in the material can change the evaporation rate. Solvents like Toluene and Xylene evaporate quickly, while others, like Butyl Acetate evaporate at a slower rate.

Drying racks can be used to store coated boards while they cure. Proper ventilation is required for operator health when solvents are evaporating. During the cure process, coating thicknesses reduce as solvent materials leave the board – resulting in a dry coating thickness that is less than the wet coating thickness.

Heat Cure

Heat curing takes less time than room temperature curing. In some instances, heat operates as a primary cure mechanism. Heat can also be used as a secondary mechanism that is paired with another cure method to accelerate the curing process. For example, room temperature curing can be combined with heat curing to speed up the evaporation of solvents. With UV-cure processes, heat can be used to cure the fluid hidden from UV rays. This cure method can be used for both solvent based and 100% solid materials. 

Nordson ASYMTEK offers IR/Convection ovens with programmable heating zones, motorized conveyors, and ventilation. Four different conveyor lengths are available. The system features down-draft ventilation to ventilate Volatile Organic Compounds (VOCs) in a safe manner, and programmable heating zones that allow you to create customized curing profiles to meet fluid manufacturer specifications.

Ultraviolet Cure
Ultraviolet (UV) curing is common with urethane materials. On the light spectrum, UV-A and UV-C lights have different wavelengths – offering surface and deeper penetration curing. Typically, UV curing requires both UV-A and UV-C light to fully cure the coating. UV-A operates at a wavelength of approximately 365 nm. Ultraviolet UV-C light, by comparison, operates at a wavelength of 254 nm. The surface and subsurface of coated boards can be cured using a combination of UV-A and UV-C light.

Curing with UV is limited by light exposure and depth penetration. Areas blocked by taller components (shadowing) and material underneath components require a secondary mechanism to cure because they are not exposed to light. Similarly, thicker coatings with limited UV penetration depend on a secondary cure mechanism – either moisture or heat.

Other parameters affecting UV cure profile include duration, type of light, and intensity. Different light sources also provide different curing results. A Mercury or H bulb is commonly used to cure materials. Iron and LED bulbs are also available alternatives.

Moisture Cure

Moisture curing requires the presence of humidity, and the degree of humidity affects the curing time. Moisture curing occurs at first contact, at the surface, and then progresses inward.  Moisture cure materials require additional consideration during the application process. To reduce material clogging in the fluid system, the use of clean, dry air or nitrogen is recommended for reservoir pressures.

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