Polyurethane Conformal Coatings: When To Consider An Alternative

Polyurethanes (urethane resin – UR) are polymeric, flexible, insulating, hard and chemically resistant conformal coatings that are used to protect electronic parts from chemical corrosion, oil, moisture, fungus and static discharges. Polyurethane coatings are supplied as single or two-component formulas. These coatings are suitable for printed circuit board applications such as sensors in the oil and gas industry, automotive, agriculture and common electronics. Polyurethane can be optically excited under UV-light and fluoresces therefore its inspection is also straightforward. Optically transparent coatings with a bit of tint are visually suitable for encapsulation purposes. Maximum service temperature for these conformal coatings (depending on the type) is ˂130°C.

Utilizing these coatings offers significant benefits due to the simple application process that does not necessitate costly tools. It can be easily applied via dipping, spraying and brushing techniques, and subsequently, the coating is cured under right conditions to reach its full potential as a protective layer. The curing can take from one hour to several days depending on the type of polymer being used, single component polymers can be cured in as little as three hours. The curing process is usually speed up by heating up the component. In general, the coating is applied at a thickness ranging form 0.001” to 0.005”.

Ensuring the cleanliness of the application surface is crucial. The surface must be free of moisture, dirt, grease and any other residues or contaminants. Contaminants may result in the failure of the urethane conformal coating under service conditions via different failure mechanisms such as delamination, blistering, de-wetting, bubbles, pinholes, cracking or by areas that are left exposed to the harsh conditions.

Polyurethane conformal coatings offer better chemical resistance than acrylic coatings. Because the mechanical hardness of polyurethane conformal coatings are second in the list after epoxy for applications that require high mechanical strength they are advantageous to use because they are easier to be reworked compared to the epoxy.

There are drawbacks to polyurethane coatings that require consideration. They are difficult to remove, therefore once applied there is a high risk that the electronic part will be damaged or become visually unattractive while trying to remove the coating. If the surface is not cleaned properly the layer might peel off. The curing process requires careful attention and patience as it can be time-consuming, potentially lasting a significant duration. When a quick cure time is needed, acrylic and silicone coatings are the go-to choices due to their faster curing process. Additionally, acrylic coatings offer easier reworkability compared to silicone coatings.

Common Standards Used for The Testing of Conformal Coatings

In order to compare different types of conformal coatings, a starting point for comparison is important. Standards set the rules for products under controlled test conditions. According to some common standards shown below, the selection process for the right type of coatings can be easier and more effective. According to the IPC-HDBK-830 (Guidelines for Design, Selection and Application of Conformal Coatings) a conformal coating may have several functions depending on the type of application[1]. In IPC-HDBK-830 the most common are listed as:

• To inhibit current leakage and short circuit due to humidity and contamination from service environment.
• To inhibit corrosion.
• To improve fatigue life of solder joints to leadless packages.
• To inhibit arcing, corona and St. Elmo’s Fire.
• To provide mechanical support for small parts that cannot be secured by mechanical means, to prevent damage due to mechanical shock and vibration.

Polyurethane, epoxy resin, acrylic resin, silicone and Parylene (XY) are the commonly used conformal coatings. While polyurethane coatings offer advantages as explained above they may not be the right coating under certain circumstances or there might be cases where a different coating type becomes more advantageous.

Processing

Polyurethane as mentioned above can be coated by spraying, dipping or brushing. Similarly, epoxy, acrylic and silicone can be applied through the same means. Parylene coating is a more time consuming method among all.

Rework and Removal

Polyurethanes have high chemical resistance. Removal of the coating requires the use of stripping agents. Acrylic coatings on the other hand can be easily dissolved in many organic solvents and can be selectively removed. Epoxies are impossible to remove chemically, and they can only be burned using a soldering iron. Parylene removal is also very difficult. In cases where reworking/removal of the sealant is required, acrylic is a better option compared to polyurethane.

UL 746E Standard for Polymeric Materials – Industrial Laminates, Filament Wound Tubing, Vulcanized Fibre, and Materials Used In Printed-Wiring Boards [5] ,UL94 V-0, V-1, V-2 Flammability Standard , MIL-I-46058C Amendment 7: Military Specification for Insulating Compound, Electrical (For Coating Printed Circuit Assemblies)

Acrylics (V1) and Silicones (V1) offer better flame resistivity compared to their polyurethane (V0) counterparts. Improper handling techniques might transmit an accumulated static charge through the board, damaging or destroying components therefore static protection is very important for PCBs. Polyurethane has good dielectric properties just like the other coating materials however the Parylene coatings offer better conformal coating without any air bubbles or pinholes therefore might be more advisable for applications with strict requirements. Polyurethane conformal coatings can withstand thermal shock cycles of -65°C to 125°C, Silicones (up to 600°C) and Parylene (Parylene HT^® of AF4 350 °C) on the other hand can show better performance under thermal cycles.

References:
[1]    “IPC-HDBK-830 – Guidelines for Design, Selection and Application of Conformal Coatings | Engineering360.” [Online]. Available: https://standards.globalspec.com/std/1636363/ipc-hdbk-830. [Accessed: 21-Jan-2020].
[2]     “(21) (PDF) Surface energy of a polyurethane as a function of film thickness,” ResearchGate. [Online]. Available: https://www.researchgate.net/publication/266318288_Surface_energy_of_a_polyurethane_as_a_function_of_film_thickness. [Accessed: 21-Jan-2020].
[3]     K. Grundke, S. Michel, and M. Osterhold, “Surface tension studies of additives in acrylic resin-based powder coatings using the Wilhelmy balance technique,” Prog. Org. Coat., vol. 39, no. 2, pp. 101–106, Nov. 2000, doi: 10.1016/S0300-9440(00)00129-6.
[4]     C. Chindam, A. Lakhtakia, and O. O. Awadelkarim, “Surface energy of Parylene C,” Mater. Lett., vol. 153, pp. 18–19, Aug. 2015, doi: 10.1016/j.matlet.2015.04.009.
[5]     “UL – 746E Standard for Polymeric Materials – Industrial Laminates, Filament Wound Tubing, Vulcanized Fibre, and Materials Used In Printed-Wiring Boards | Standards Catalog.” [Online]. Available: https://standardscatalog.ul.com/standards/en/standard_746e_6. [Accessed: 21-Jan-2020].