Comparing AR to UR Conformal Coating

Acrylic (AR) and polyurethane (UR) conformal coatings are widely known as commonly utilized materials for conformal coatings. As liquid coatings, both can be applied to substrates through a variety of methods:

• Brushing the substance onto the substrate surface
• Dipping properly masked components into vats of coating material
• Atomized/non-atomized spraying procedures, using either human or robotic labor

Although both coatings are applied using similar techniques and serve some of the same product end uses, it is crucial to note that they are not interchangeable. Significant differences must be taken into account before deciding which conformal coating is best suited for a specific project.

Basic Properties of AR and UR

Understanding the fundamental differences between AR and UR is essential to their effective use.

Acrylic
Protecting electronics from corrosion, dirt, dust, fungus, moisture, and thermal shocks is crucial. AR conformal films not only shield against these elements but also offer dielectric properties that withstand static/voltage discharge. With a dielectric strength of V 300/Mil., a dielectric constant of 2.5, and a dissipation factor of 0.01, ARs operate effectively in temperatures ranging from -65°C to +125°C. They are applied through liquid brush or spray methods, drying swiftly to a clear, salt-resistant finish with consistently high fluorescent levels.

AR coatings provide efficient protection and are easily applied, cleaned and removed, making them ideal for various component miniaturization and repair processes. Although they have poor solvent-resistance, they exhibit good resistance to acids and bases. Despite their moderate production costs, acrylics remain popular for simpler conformal coating applications. Certain AR coating types like HumiSeal® 1B31 meet the requirements of MIL-I-46058C and IPC-CC-830. Acrylic films usually range from 0.002 to 0.005 inches in thickness post-application, with curing times rarely exceeding 30 minutes.

Acrylic coatings are commonly utilized for moisture protection and conformally-coating printed circuit boards (PCBs). Their versatility and quick curing time make them a preferred option when rapid turnaround is essential. By combining protective properties with ease of application and removal, AR coatings play a key role in safeguarding electronics and supporting efficient repair and miniaturization processes.

Urethane
Exceptionally resistant to abrasion, mechanical wear and corrosion, UR’s durable surfaces also offer reduced reversion potential. The dielectric properties of UR promote miniaturization by insulating signal traces from closely situated circuits. With excellent moisture and chemical resistance, UR is effective in harsh chemical exposure environments. However, high levels of heat and vibration can damage UR conformal films and should be avoided.

UR coatings are particularly valuable for combating tin whiskers, which are crystalline, electrically-conductive tin structures that can develop on assemblies with tin surface finishes. Whiskers, though usually small, can lead to electrical short-circuits, especially for electroplated tin finishes. UR films are highly effective in preventing their development by creating a thick, strong coating that shields the tin surface from additional structures that could potentially cause a short-circuit.

Summary

In summary, both Acrylic (AR) and Polyurethane (UR) conformal coatings offer unique advantages and are essential for protecting electronics in various applications. While AR coatings excel in moisture protection and ease of application, UR coatings stand out for their resistance to abrasion and corrosion, making them ideal for harsh environments. Understanding the distinctive properties of each type is crucial in selecting the most suitable conformal coating for specific projects, ensuring optimal performance and longevity of electronic components.