BY Alan Hardy
In previous decades, almost all military devices were custom designed for their specific applications. Today, while some are still designed this way, directives such as RoHS, as well as growing budget constraints, are pushing the use of commercial off-the-shelf (COTS) components, devices, and electronic assemblies for military use.
Although these COTS products are not designed for military applications, they still may be going into very rugged environments. What might work perfectly in a consumer device might need added protection in a fighter aircraft.
Financial considerations show that the use of COTS electronics for military applications will continue growing, so designers should protect them from heat, moisture, and radiation. The challenge is how to protect devices originally designed for non-military use without additional enclosures.
Many types of conformal coatings are available, but not all are suitable for this type of protection and many assemblies that are now reduced to the nano-scale, simply cannot be protected by standard dip, spray, and brush-on coating methods.
Parylene is a mil-spec approved conformal coating which has been in use for years in military and aerospace applications for custom devices, and can enhance the integrity of COTS devices without adding high cost.
There are several formulations of Parylene available, including a new high-temperature, UV-stable variant. Parylene offers a conformal coating alternative that is lightweight and provides 100 percent protection for the full device life-regardless of how and where it is used.
Parylene N, C, and HT are listed on the Defense Supply Center Columbus Qualified Parts List (QPL) for MIL-I-46058 and are also recognized as meeting the requirements of IPC-CC-830. Parylene HT provides similar capabilities as the otherParylenes but just goes a step beyond, offering an alternative solution for aerospace engineers who are working on instrumentation that requires added protection for enhanced reliability.
While all Parylene formulations have a small molecular structure, Parylene HT is the smallest of all. Because of this, it can penetrate extremely small areas, providing complete coverage without compromising operational capabilities. It also provides long-term thermal stability up to 350 degrees Celsius (short term up to 450 C).
Many properties are similar among Parylene variants. All Parylenes provide excellent barriers for moisture and chemical and biological agents, and because they are applied via a vapor disposition process, where the coating molecules basically grow to an ultra-thin barrier completely covering all components, they create minimal added weight.
Parylenes actually strengthen delicate leads and connections by 10x. This lends durability wherever vibration is present. Additionally, Parylene coatings are RoHS compliant and have been shown to suppress the formation of metallic whiskers in lead-free solder applications.
As a result ofParylene's vacuum-deposition process, there is no potential for the trapping of air bubbles underneath or in the coating. Other industry-standard coatings that are sprayed, dipped, or brush applied have the potential to trap air bubbles in or underneath the coating. When this occurs and the coating is exposed to high altitudes or space, air bubbles could open and expose the circuit to the environment, and possibly cause a short circuit.
Optimal adhesion of Parylene to a wide variety of substrates, including metal, plastic, elastomer, glass, and paper, is commonly achieved by a treatment with A-174 silane prior to Parylene coating. New forms of pre-treatment are available today that improve adhesion of Parylene coatings to newer materials. Additionally, this new method has demonstrated improved stability at elevated temperatures, making it an excellent adhesion promotion tool for harsh-environment applications. This enables Parylene conformal coatings to protect an even wider range of materials that may come into play when using COTS components for complex military technologies.
When configuring instrumentation incorporating MEMS and other multiple-layer circuit devices for rugged applications, such as those seen in cockpit, engine management, and control systems, thermal stability is critical. A good example of this is found in using industrial-grade SENSORS that are coated with Parylene. This enables the SENSORS to perform true to their design and functions in standard industrial applications, as well as in military applications.
Rapid changes in non-controlled atmospheres, such as aircraft wing and tail applications, often are too rugged for standard circuit card assemblies. Protecting the assembly with a Parylene coating allows the same circuit card assembly to work properly as it moves from sea level to all operating altitudes.
The low dielectric constant and dissipation factor well suits the increasing use of RF and other forms of wireless devices entering into control-to-human and control-to-control cross-device signal interface. These low dielectric constant and dissipation features allow Parylene coatings to provide dielectric insulation for true and undistorted signals.
These properties are valuable wherever electrical and fiber-optic interconnect systems are used, particularly in control and communication of armaments, radar systems, and other applications where prevention of failure is critical.
With increased use of LEDs in aircraft lighting, Parylene can give the same long-term protection from UV degradation. This would be applicable for any COTS component used in navigation lights, cabin lighting, or landing gear lighting. It is also a valuable coating to enhance the life of components used for optical windows and in radiation detectors.
Protecting devices in space applications can cover virtually everything from wiring and cabling assemblies and camera components to power supplies.
Electronics in communicationsand key circuit card assemblies used in any rocket application are launched at sea level (ambient temperature) and, in a matter of seconds, are into high altitudes (subzero temperature). One of the foremost benefits of Parylene protection of all devices that need to withstand the atmospheric, temperature, and pressure changes of a launch is that Parylene adds extra protection from condensation that results from extreme temperature changes. It also restricts outgassing.
Additionally, rocket launchescause vibration, which could break wire bonds. Parylene actually strengthens delicate wire bonds and connections by 10x to ensure continued reliability of the connection.
Finally, Parylenes do not exhibit changes in mechanical properties with changes in temperature, as do other materials. In oxygen-free atmospheres or in the vacuum of space, the continuous service temperature projections exceed 200 Cfor Parylenes N and C. Parylene HT has the ability to resist thermaloxidation up to 450 C (short term) in both oxygen and oxygen-free atmospheres.
Alan Hardy is military market manager at Specialty Coating Systems in Indianapolis. Specialty Coating Systems can be found online at www.scscoatings.com.