Aerospace Conformal Coatings
Posted by Sean Horn
Friday, February 9, 2018 8:05
@ 8:05 AM
Conformal coatings composed of acrylic, silicone and parylene polymer materials are valuable for a wide range of aerospace applications, which can push technologies to their limits. Applied to printed circuit boards (PCBs) and related electrical assemblies, conformal coatings maintain device performance through difficult operational conditions. The presence of atmospheric variation, chemicals, humidity, mobile ion permeation, moisture, temperature fluctuation, or excessive vibration can generate:
- current leakage,
- RH-aging or
- thermal shock
within the electronic device that can engender damage and dysfunction.
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 and
- sensors, among many other assemblies critical to aerospace performance.
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 – + 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, regarding:
- compatibility with substrate surface,
- curing time (under 4 hours for AR),
- coating thickness — 0.002, +/- 0.001 inch,
- dielectric/insulation performance,
- film flexibility suitable for designated performance parameters, and
- 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.
Silicone is also applied to substrates through liquid methods. For aerospace, its coating thickness is designated by MIL-I-46058C at 0.005, +0.003 inches, with a Q-resonance between 8%-12%. One of its greatest aerospace-advantages is operational temperature-range. Most SR coatings will perform as designed between -55°C — +200°C, however, some are functional at 600°C, far exceeding the performance range of either acrylic or parylene.
SR 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. SR offers reliable UV-resistance, high dielectric strength, and good adhesion.
At the same time, thicker SR 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. SR’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.
XY’s chemical vapor deposition (CVD) application methodology provides a more reliable coating for aerospace purposes than standard liquid brush-on, dip, and spray coating methods used for AR/SR. CVD applies gaseous parylene deep within assembly surfaces, molecule-by-molecule, generating an authentically conformal, micro-thin coating.
To a greater extent than AR/SR, 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 0.0006, +0.0001 inches, 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,
- ultraviolet protection,
- exceptional adaptability for MEMS/nano-technologies, and
- protection against excessive physical shock/vibration, as well as
- incursion of in-flight particulates of questionable origin.
Neither acrylic nor silicone display the versatility of parylene conformal coatings, but remain useful for specified military electronics purposes. Compared to parylene, silicone displays lower resistance to harsh aerospace environments, but provides working advantages for low residual stress applications. Superior to AR/SR liquid coatings, parylene’s aerospace applications enhance lifespan and performance of mission critical PCBs, communication devices, power supplies, radar/detection equipment, and satellite electronics.