Solving Signal Interference Challenges in Aerospace and Defense Electronics
As the complexity of signal challenges in aerospace and defense technology increases, design engineers need to creatively combine materials to solve multiple problems at once. Comolding, which involves combining two uncured elastomers with different electrical properties into one homogeneous material or molding elastomers with metal, fabric or composite structures, is an innovative way to address these challenges.
There are many ways to use comolding in aerospace and defense applications. For example:
Comolding of elastomeric absorbers to conductive elastomers.
Molding low dielectric or conductive layers to an absorber or to an absorbing layer.
Molding nonconductive layers to conductive layers.
Molding conductive elastomers directly to metal housings or shielding.
Cocuring absorbing elastomers within prepreg composite structures.
Calendaring elastomers for molding with conductive or abrasion resistant fabrics.
To illustrate how comolding can help design engineers cut down on signal interference and improve signal integrity in aerospace and defense electronics, here are some examples from our work in the industry:
- Filling a shifting gap: In this instance, we needed to create a ground plane to improve the performance of a flexible elastomeric absorber. Reflections off a conductive surface create a reflected wave and a surface wave. To optimize absorber performance, it’s important to put in place a conductive substrate under the absorber to set up the surface wave, which can then be attenuated by the absorber. Without this conductive substrate, energy will flow through the absorber and create interference. There are several ways to achieve this result (from embedding mesh to bonding conductive metal foil onto the back of an absorber). However, a complicating factor in this situation was that our signal-attenuating solution had to also fill a gap. And since this gap was on an airframe structure, it expanded and compressed with movement and temperature changes. So, rigid conductive surfaces wouldn’t work. Instead, we comolded a ground plane to the absorber, creating a rubber (and, by nature, flexible) material that filled the gap as it expanded and closed, eliminated edge scatter, and effectively absorbed signal.
- Suppressing signal noise from parallel circuits: When an engineer needs to suppress high frequency noise on a circuit board, an absorber is usually the solution. However, in this instance, we had to suppress noise from parallel circuit traces. And with parallel circuit traces, an absorber can impede signal integrity. The solution we devised here was to comold a low dielectric material to the absorber, putting this dielectric layer between the absorber and the circuit traces. With this comolded solution, the differential mode of the signal is unaffected while the common mode is suppressed. The result in this case was improved signal integrity and the suppression of signal interference.
- Creating a corrosion-resistant window seal: We needed to create a conductive seal for a window on a ship. However, conductive gaskets often consist of metal fillers, which corrode in seawater environments. So, making our seal corrosion-resistant was our top priority. Leveraging our knowledge of galvanic compatibility and experience with comolding, we created a gasket with a conductive layer and a nonconductive, absorbing layer. Essentially, we made the inside of the window seal conductive and the outside nonconductive and corrosion resistant. This nonconductive layer protected the conductive portion of the seal from corrosion.
- Conductive gaskets that can solve multiple problems: Electrically conducive elastomers are an excellent way to provide shielding and environmental protection. In some cases, one or more of the shielded surfaces move during the operation of the part, for example, sliding doors or opening and closing of doors. It also may be on the external surface of the application and need to attenuate currents running along the surface of the vehicle. And of course, the abrasion of moving surfaces needs to be resolved.
Some of the unique solutions involve comolding absorber and conductive elastomer; embedding conductive fabric into nonconductive elastomers and unique geometries using FEA analysis to allow the closing action to minimize stress and strain into the part. Wherever abrasion is a factor, fabrics like Nomex and Dacron can be molded into the outside of the gasket. These fabrics can greatly increase tear and tensile strength of the gasket as well as provide excellent resistance to abrasion of the outside elastomer surface. Finally, the fabrics can improve the coefficient of friction of the mating surfaces, thus ensuring longer gasket life.
As illustrated by these examples, the key prerequisites for successful comolding are:
Detailed knowledge of materials and their performance under various conditions. For example, use of fluoro-elastomers can provide temperature resistance to 475°F as well as excellent fluid resistance. Like silicones, fluoro-elastomer softness and good compression properties make them an excellent choice for gaskets. Microwave absorbing versions are also available along with significant experience comolding absorbers. This is an elastomer of choice for many aircraft applications.
Another prerequisite is sophisticated manufacturing capabilities to ensure each comolded solution has the right mix of materials to meet the precise specifications of different aerospace and defense applications. This includes tooling and layup procedures to produce manufacturable parts.
The last prerequisite is sophisticated FEA analysis tools for both mechanical and electrical analyses to ensure initial designs can meet the requirements before the tooling is produced.
To ensure the reliable operation of aerospace and defense electronics, it’s crucial that design engineers leverage multifunctional solutions to address increasingly complex signal challenges. Comolding is a key enabler of these integrated and innovative packages.
This article was written by Rick Johnson, Aerospace and Defense Director, Laird R&F Products, a DuPont business (Carlsbad, CA). For more information, go here .