Lightweighting continues to be a key topic for the aerospace, avionics and defense industries as new metals and composites are being integrated into end products and assemblies with the goal of decreasing overall system weight. As technologies continue to evolve, with components often decreasing in size and increasing in complexity, the materials used to manufacture and protect the latest components and systems are also improving. Whether used in commercial or military aircraft, rockets, satellites, terrestrial or water vessels, or the latest in unmanned air, land and sea vehicles; systems within these industries must meet similar requirements – assemblies, components and electronics must be both lightweight and designed to withstand harsh operating conditions.
Aerospace, avionics and defense systems depend on the latest in electronics and communication technology, and reliability is crucial. In these industries, where lives hang in the balance, failure modes must be eliminated. Signal interference or failures due to corrosion, for example, are intolerable due to the nature of these systems and all that relies upon them. Components must be thoroughly shielded from their harsh operating environments and remain protected for the life of the system. Engineers must include this level of protection in their designs – without adding significant bulk, weight or anything that may interfere with system communication.
The goal of lightweighting is not just an issue of construction materials or shrinking the size and weight of actual devices and internal components – protection of devices and electronic assemblies must also be taken into account. Liquid conformal coatings, commonly applied by spraying, dipping or various dispensing technologies, have historically offered a certain level of protection to components in these industries, but as avionics continue to move to smaller, more complex components, these coatings add unnecessary weight and dimension to critical, lightweight electronic, optical and communication devices. Additionally, their application methods can result in uneven thickness, pooling and meniscus effects, leaving microscopic voids that greatly increase the propensity for component failure. Parylene coatings provide superior protection without adding significant mass to electronic assemblies.
Parylene Conformal Coatings
Parylene conformal coatings have provided rugged and reliable protection for a wide range of aerospace, avionics and defense applications for over 45 years. Listed on the QPL for MIL-I- 46058, the coatings are also recognized as meeting the requirements of IPC-CC- 830 and are RoHS compliant. Parylene films are optically clear, do not interfere with electrical, optical or RF signals, and have been shown to mitigate the growth of metallic whiskers in lead-free solder applications. Parylene coatings provide:
- Ultra-thin, conformal coating of all exposed surfaces;
- Excellent dielectric properties;
- Excellent moisture and chemical barrier properties;
- Thermal stability up to 350°C long-term;
- Superior ultraviolet stability.
Its application process is what differentiates the Parylenes from all other conformal coatings and is the reason they provide a truly protective barrier to electronics and devices, regardless of their size or complexity. Rather than being dispensed, sprayed, brushed or dipped, Parylene coatings are applied via vapor deposition. In this process, the parts to be coated are placed in a deposition chamber and a powdered raw material, known as dimer, is placed into the vaporizer at the opposite end of the deposition system. The dimer is heated, causing it to sublimate to a vapor, then heated again to break it into a monomeric gas. This gas is then transferred into an ambient temperature chamber where it spontaneously polymerizes onto the parts, forming the ultra-thin Parylene film. The Parylene process occurs in a closed system under a controlled vacuum, with the deposition chamber remaining at ambient temperature throughout the process. No solvents, catalysts or plasticizers are used in the coating process.
Parylene coatings are extremely lightweight, offering excellent pinhole-free barrier properties without adding dimension or significant mass to delicate components. Parylene is typically applied in thickness ranging from 500 angstroms to 75 microns. A 25 micron coating, for example, will have a dielectric capability in excess of 5,000 volts. No other coating material can be applied as thinly as Parylene and still provide the same level of protection.
Because there is no liquid phase in the deposition process, there are no subsequent meniscus, pooling or bridging effects as seen in the application of liquid coatings; thus dielectric properties are never compromised. The molecular “growth” of Parylene coatings also ensures not only an even, conformal coating at the thickness specified by the manufacturer, but because Parylene is formed from a gas, it also penetrates into every crevice, regardless of how seemingly inaccessible. This ensures complete encapsulation of the substrate without blocking small openings or vias.
As mentioned above, Parylene coatings offer a host of benefits to critical aerospace, avionics and defense applications. Following is a deeper look into a few of these beneficial properties:
- Smallest Molecular Structure: The ultra-thin and extremely small molecular structure of Parylene allows the coating to ingress deeper through open areas on the top or bottom of packages, regardless of the size or complexity of integrated devices. This complete encapsulation of the device components enables a high level of protection without adding significant weight or dimension to critical components.
- Lowest Dielectric Constant and Dissipation: Parylenes have an extremely low dielectric constant and dissipation factor, enabling the coatings to provide small, tight packages with dielectric insulation via a thin coating. It has been demonstrated that the voltage breakdown per unit thickness increases with decreasing Parylene film thickness.
- High Temperature Stability: Parylene is thermally stable in operating temperatures up to 350°C long-term and can withstand short-term exposures to 450°C. Parylene’s ability to survive these temperatures, while continuing to provide excellent moisture and dielectric barrier properties, is of great benefit to designs that must survive high heat environments.
- UV Stability: Unlike other conformal coating formulations, Parylene HT will not yellow or crack when exposed to ultraviolet light, offering measureable UV stability after more than 2,000 hours of an accelerated weathering, UV exposure test (ASTM G154). Its chemical structure provides protection from degradation and discoloration as a result of such exposure.
- No Outgassing: Since Parylene is applied in a vacuum, there are no hidden voids or incomplete coverage that may present pathways for failure when exposed to altitude.
Aerospace, Avionics and Defense Applications
Parylene coatings protect a wide array of systems and components throughout the aerospace, avionics and defense industries, including printed circuit boards (PCBs), complex circuit assemblies, wafers, sensors, LEDs and elastomeric components. These components, and more, are often subjected to hostile operating environments, including extreme temperature fluctuations, pressure changes, vibration and exposure to harsh chemicals and/or biological agents. Parylene not only provides protection for components operating in these environments, but does so while ensuring stable communication between systems and devices, e.g., aircraft, missiles, UAVs and UUVs, etc. Many applications within these industries benefit from the lightweight and reliable protection Parylene coatings offer, including:
- Flight data and recording systems;
- Communication systems between all vehicle types;
- Aircraft, UAV and UUV monitoring;
- Electrical power generation and distribution;
- Control monitoring systems;
- Fuel systems integrity;
- LEDs for interior, exterior and display lighting;
- Space vehicles, satellites and cameras.
Parylene coatings also play a role in protecting COTS (commercial off-the-shelf) components, which are not specifically designed for the demanding environments that many aerospace and defense applications must survive. As these components continue to be integrated into critical industry applications, Parylene provides protection to ensure their reliability.
The growth of the aerospace and defense industries, along with continued commercial investment in the space/satellite sector, will increase the demand for technologies that service these markets. Parylenes enable engineers to advance their designs due to the coatings’ lightweight, ultra-thin nature, while offering a host of beneficial properties that ensure the reliability of total systems and their components.
This article was written by Tim Seifert, Military & Avionics Market Manager, Specialty Coating Systems (Indianapolis, IN). For more information, Click Here .