Aerospace Conformal Coatings: Acrylic vs Silicone vs Parylene

Conformal coatings made of acrylic, silicone, and Parylene polymers play a crucial role in diverse aerospace applications, pushing technologies to their limits. These coatings, applied to printed circuit boards (PCBs) and other electrical assemblies, help uphold device performance under challenging operational conditions. Factors like atmospheric changes, chemicals, humidity, mobile ion infiltration, moisture, temperature fluctuations, and excessive vibration can lead to corrosion, current leakage, dendritic or mold growth, RH-aging and thermal shock in electronic devices, potentially causing damage and malfunctions.

Acrylic (AR), silicone (SR) and Parylene (XY) coatings enhance the functional reliability of aerospace components, which continue to grow in complexity and decrease in dimension. In addition to PCBs, conformal coatings address the operational needs of:

• Elastomeric seals
• Light emitting diodes (LEDs)
• Microelectromechanical systems (MEMS)
• Semiconductor products
• Sensors, among many other assemblies critical to aerospace performance

Acrylic Conformal Coatings

Inexpensive and easy to apply by standard wet-methods — brush, dip (immersion), spray — acrylic coatings assure moisture protection for aerospace components, under specified performance conditions. They offer a significant operating temperature range of -65°C to +125°C, although they become ineffective at thermal levels exceeding 150°C, limiting aerospace uses.

More important, acrylic coatings meet approval standards for MIL-I-46058C, Insulating Compound (For Coating Printed Circuit Assemblies), which assigns performance requirements for coating military/aerospace components, including:

• Compatibility with substrate surface
• Curing time (under 4 hours for AR)
• Coating thickness — 1-3 mils
• Dielectric/insulation performance
• Film flexibility suitable for designated performance parameters
• Resistance to flame (self-extinguishing/on-burning), fungus, moisture, thermal shock

Q-resonance levels for AR aerospace coating fit within the standard’s 9-19% requirements; appropriate hydrolytic stability eliminates adhesion-loss/delamination, blistering, chalking, cracking, softening, tackiness or reversion to liquid state.

In addition to MIL-I-46058C, AR coatings also meet IPC-610 film thickness requirements and those stipulated by IPC-CC-830 (Qualification/Performance of Electrical Insulating Compound for Printed Wiring Assemblies) and the UL 746C Standard for Polymeric Materials, performance criteria/material property considerations for electromechanical aerospace/military uses.

As important for aerospace performance, acrylic coatings display humidity-resistance during component operation, minimizing internal moisture-development, with low glass-transition temperatures. Less suitable for such uses as MEMS-encapsulation, acrylic conformal films are best-applied as a secondary protection material, minimizing component condensation during aerospace-operation, and providing simplified repair/rework.

Aerospace Silicone Coatings

Silicone is also applied to substrates through liquid methods. For aerospace applications, its coating thickness is designated by MIL-I-46058C at 2-8 mils, with a Q-resonance between 8%-12%. One of its greatest aerospace-advantages is operational temperature range. Most silicone coatings will perform as designed between -55°C and +200°C, however, some are functional at 600°C, far exceeding the performance range of either acrylic or Parylene.

Silicone cures rapidly, resembling a very soft, durable rubber in appearance and performance. Thicker than acrylic/Parylene, silicone provides superior vibration dampening, thermal protection and defense against heavy impact. Silicone offers reliable UV-resistance, high dielectric strength, and good adhesion.
At the same time, thicker silicone films are unsuitable for MEMS/nano-technology common to aerospace assemblies. Very hydrophobic, with high moisture permeability, silicone can allow excess moisture-retention within PCBs, a source of component corrosion/metallization. Although repair is easy, it is not uncommon. Silicone’s durability against abrasion and solvent resistance are poor. Aerospace uses for high-profile, consistently active electronic components are limited. However, silicone shows major potential as a planarizing top-layer, generating added environmental protection for high-reliability applications and a mobile ion permeation barrier.

Parylene Coatings For Aerospace Applications

Parylene’s chemical vapor deposition (CVD) process offers aerospace industries a superior coating method compared to traditional liquid brush-on, dip and spray techniques. By infiltrating gaseous Parylene meticulously into assembly surfaces, layer by layer, it creates a genuinely conformal, ultra-thin coating.

To a greater extent than acrylics and silicones, Parylene reaches MIL-I-46058 conformal coating’s appearance/performance specifications for smooth, homogeneous, transparent, unpigmented conformal films free from blistering, bubbles, cracking, peeling. pinholes, whitish-spots and wrinkling. Parylene’s complete substrate coating conforms entirely to all device surfaces – flat, round, creviced or edged, while adding almost no weight to the covered device.

For type XY, MIL-I-46058C recommends a thickness of 0.5 to 2 mils, with a Q-resonance between 7%-11%. Lightweight, highly-durable Parylene’s truly conformal/pinhole-free coverage has no voids or gaps to disrupt PCB-protection/performance. It further provides:

• Thermal stability between 80°C — 350°C long-term (450°C short-term)
• Superior chemical/moisture/mobile-ion-permeation barrier properties
• High dielectric strength
• Low coefficient of friction
• Protection against excessive physical shock/vibration

Summary

Acrylic and silicone coatings lack the versatility of Parylene conformal coatings, yet they serve specific aerospace electronics needs. While silicone is less resistant to severe aerospace conditions compared to Parylene, it offers benefits for applications with low residual stress. Parylene surpasses acrylic and silicone coatings in aerospace use by boosting the longevity and efficiency of crucial PCBs, communication devices, power supplies, radar/detection gear and satellite electronics.