Conformal Coatings and UV Trace
Conformal coatings provide exceptional protection for printed circuit boards (PCBs) and similar electrical assemblies, through a wide variety of operating circumstances, safeguarding their chemical, electrical, and/or mechanical properties.
The reliability of coatings varies according to the coating material and conditions of use. Most conformal coatings require specialized processing and inspection procedures if PCBs are to be used in direct exposure to UV light. Invisible to the naked eye, UV radiation’s wave length registers between of 100-400 nanometers (nm). The addition of UV trace to coatings is commonly used in liquid conformal coatings for inspection purposes, but has significant drawbacks for parylene use.
Unsuitably of UV Treatments for Parylene Conformal Coatings
Parylene has repeatedly demonstrated superior utility in comparison to most competitive coatings – including acrylic, epoxy, silicone and urethane -- surpassing their performance as a substrate covering for biological, consumer, industrial, medical and military systems, in terms of:
- adaptability to component shape,
- dielectric strength,
- durable, flexible component protection,
- insulating properties,
- pinhole-free uniform coating, and
- resistance to chemicals and solvents.
In addition, parylene coatings can be applied in ultra-thin layers, compared to competitors’, providing significant protection at thickness levels are generally undetected on the final product.
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.
UV Trace and Parylene
UV trace is used during the inspection stage of most liquid conformal coating processing. However, in the case of parylene coatings, care must be taken for the following reasons:
- The UV trace additive is not part of the parylene dimer and must be added prior to coating.
- The molecular structure of UV fluorescent materials is larger than that of parylene molecules.
- Parylene materials retain the ability to penetrate masking, even where those of UV fluorescents do not.
- Thus, cursory inspection may not detect UV material within the coating-free zone.
- The component may pass inspection, authorized and cleared for functions it is designed for, even as these flaws go undetected.
This is a problem for application of parylene coatings, in comparison to competitive conformal types, when a UV tracer is added. For instance, while urethane (UR), acrylic (AR) and silicone (SR) typically include a uniform fluorescing agent in their chemistry, parylene does not. During UV light inspection, UR, AR and SR coatings fluoresce brightly and clearly. They plainly indicate the precise location of the trace within the coating, where it has and has not been applied.
Under these circumstances, it is wrong to presume the addition of a UV florescent to the parylene dimer correctly indicates whether or not the coating has infiltrated any coating free zones.
- The UV trace tends to randomly splatter onto PCBs or other assemblies during its deposition process.
- Uneven disbursement of the UV trace can cause haphazard streaking of the material in some instances.
- Other PCBs may receive no UV trace at all, regardless of the care taken to alleviate these possibilities.
Passing the component through inspection based on these unreliable outcomes can not only lead to component malfunction, but also potentially dangerous outcomes, where the component is relied on for reliable performance through ruggedized, or similarly specialized, operating conditions.
UV Curing and Parylene
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.
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” of parylene 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.
Applying UV trace and touching up with UV cured material as components of parylene conformal coatings for LED appliances is not recommended.
Of paramount importance is understanding that the UV trace is not part of the original parylene dimer. Because of this reality, combining the two materials changes the composition of the deposited conformal coating, tainting the purity of the parylene. The consequent parylene-UV trace hybrid may pass inspection, but will typically not be any more resistant to UV light than it would otherwise have been. At the same time, the conductive, insulating and protective properties basic to the use of parylene conformal coatings will be diminished because of the introduction of UV trace contaminants.
Absolutely no data supports any improved properties of this hybrid film and, as a result of the introduction of the contaminant, it is highly likely that UV trace will ultimately degrade the parylene's performance, potentially in a serious way. Nevertheless, the current evidence shows that, despite the limited capacity of most parylene types – C, D, N – to provide dependable, longer-term protection for LEDs exposed to UV sources, parylene coatings have been devised that are useful for external LED use. As with UV trace, UV curable coating is a bad choice for combination for any of the numerous applications where parylene is the preferred conformal coating material; thus, it should not be used for touch up with parylene.
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