UV-Cure Touchup and Parylene Coating
Posted by Sean Horn
Friday, March 25, 2016 7:39
@ 7:39 AM
A photochemical process used to preserve conformal coatings, adhesives, and inks, UV curing generates a variety of value-added properties in comparison to conventional curing techniques. Applying high-intensity UV light to dry (cure) coatings or other substances, UV curing can provide instant results, increasing production speed while reducing the need for and number of typical set-up and clean-up processes. Lowered operating costs and increased production capacity are further advantages of UV curing for many coating materials and processes.
In these cases, the consequent superior bonding between coating and substrate is environmentally friendly, saving energy without need for emissions’ controls. The diminished incidence of process/product rejection offers the additional benefits of:
- better adhesion and bond strength,
- durable yet elastic coating surfaces, and
- enhanced resistance to abrasion/surface scratching,
- with improved protection against exposure to chemicals and solvents.
Procedurally, the photochemical reaction essential to the UV curing process mixes liquid monomers and oligomers with minute traces of photoinitiators, which are subsequently exposed to UV energy. In the UV curing process, ultraviolet light interacts with specially formulated chemistries to cure coatings more rapidly than possible with traditional methodologies. The photoinitiators absorb the UV energy from the process light source, either arc light or laser light. The resultant chemical reaction converts liquid coating formulation into a stable, cured film in a matter of seconds.
Parylene and UV Light
Despite its many applications and assets as a conformal coating, the overall resistance of most parylene varieties to UV radiation is limited. While it remains stable indoors, most formulations of parylene are not recommended for long term use outdoors where exposure to direct sunlight is a condition of the operating environment.
That is, the UV stability of parylenes C and N seldom exceeds 100 hours; parylene AF-4 is superior in this respect, providing UV-protection for 2,000 hours or more, when tested according to the prevailing ASTM G 154 standards. This level of performance also exceeds competing conformal coatings produced by acrylic, epoxy, silicone and urethane.
After parylene has been deposited, if there were any masking areas, the masking materials are removed. It is fairly common for the parylene film around the de-masking area to be minimally damaged by the de-masking process. This can result in “fingers” along the edges of the parylene masking line or even large tears. Sometimes, it is critical that the edges that were created be sealed to prevent any possibility of moisture or other chemicals penetrating the side edges.
These edges or other imperfections can be repaired via the application of a liquid conformal coating. We often use urethane conformal coating, as it most closely exhibits the properties of parylene. Sometimes our customers will request that we use a UV curable liquid touchup material. However, for the reasons outlined above, we do not recommend using any UV curing mechanism in concert with parylene coating, as it can severely degrade the film.
While efficient for its purposes, this approach is largely incompatible with parylene’s chemical vapor deposition (CVD) process, severely limiting its suitability for use with parylene.
To learn more about the effects that UV light has on parylene, download our whitepaper: