Utilizing Advanced Materials to Ensure High Performance and High Reliability for UAVs

Aerial drones are rapidly becoming integral to modern society, dominating headlines in combat tactics and finding widespread use across various industries. From 2020 to 2030, the global drone market is anticipated to grow at a compound annual growth rate (CAGR)  of 20%, with much of this expansion taking place in the segments of logistic drones, enterprise drones, and defense drones.

Advancements in drone technology accelerate the need to meet strict demands of lightweighting, electronics thermal management, and electromagnetic interference (EMI) shielding to ensure uncompromised signal integrity.

Types of Drones and Their Growing Applications

Before we talk about some engineering solutions to thermal management and EMI shielding challenges, let’s explore the scope of drones we’ll cover in this article. When we say drones, we’re referring to unmanned aerial vehicles (UAVs), which are aircraft that are meant to be operated remotely or without a human pilot on board. And while many of the examples we give will refer to drone applications, it’s important to note that the products we discuss can and are used in other drone-adjacent remote or aerial applications. This includes commercial aircraft, defense aircraft, electric aircraft, and even ground-based drone defense technology.

Drones come in nearly every shape, size, and price range. They can be as small as a bumblebee or as large as a small passenger jet, and they can cost anywhere from $10 to hundreds of millions of dollars. Their propulsion systems can be electric motors, gas-powered heat engines, and even jet engines, while propeller types include fixed wing and rotary wing. And, while vertical takeoff and landing (VTOL) and short takeoff and landing (STOL) are not exclusive features of drones, they are common in many types of commercial and defense drones.

Commercial drones are used for non-defense or non-military applications, such as for recreational or industrial purposes. You’ve likely seen drone footage used for the latest Hollywood blockbuster or in a nature documentary or even experienced drone light displays at sporting events or holiday celebrations. Commercial applications have driven a 25% CAGR in drone usage over the last decade.

From an industrial standpoint, drones are used in a variety of applications. Drones offer improved vision and sensors for agriculture and forestry surveying as well as wildlife tracking. Contractors and civil engineering firms are using drones to inspect difficult-to-reach or dangerous locations such as infrastructure and construction sites. Some drone manufacturing companies are marketing their technology for public safety, touting their benefits for fire inspection, police operation, search and rescue, and even crowd control.

And we can’t forget about logistics drones that are used for delivery and fulfillment. Around the world, we’re seeing more and more small-scale trials with delivery drones for packages. Drones also play a vital role in getting critical equipment and supplies like medicine to remote locations that may otherwise be difficult to reach. Drones are playing a major part in our lives, even if they aren’t always visible or obvious. From Washington to Botswana, from Detroit to Japan, from Hollywood to India, drones are being used for all kinds of purposes and making headlines every day.

Differences Between Commercial and Defense Drones

While some of the technology utilized for EMI shielding and thermal interface materials is common to both defense and commercial applications, there are some notable differences between these classes of drones.

Many defense drones have a high degree of autonomy as well as interoperability, meaning they need to be able to communicate with other military systems. Commercial drones have varying levels of autonomy and interoperability, and it’s important to note that those requirements tend to be much more application-specific. For example, a light show is one situation where perfectly synchronized drones that operate autonomously and in communication with the base terminal and the surrounding drones would be required.

Longevity and reliability often vary as well. Defense drones are expected to operate with minimal maintenance for years or decades. They must work continuously for hours or days at a time while potentially carrying hundreds or thousands of pounds of payload equipment and flying at lightning-fast speeds.

On the other hand, commercial drones often have relatively light payloads, if any at all, and use lower-power propulsion systems to operate for shorter periods. Most commercial drones don’t have a mission-critical reliability need, except for those utilized for public safety and rescue operations. Recreational drones may need more frequent battery changes and repairs to motors or propellers.

The security and regulatory requirements around each drone type are different as well. Commercial drones are usually only required to meet some U.S. Federal Aviation Administration (FAA) restrictions around flight locations and heights, as well as U.S. Federal Communications Commission (FCC) regulations around wireless communication. The requirements for defense drones are much more strict. Defense drones must meet many military standards, such as  MIL‑STD-461 for EMI shielding of electronics, in order to provide resistance against interception, jamming, and cyber threats.

When we refer to the advanced technology within drones, we are not only referring to their propulsion and communication modules but also their advanced sensors. Lidar, radar, laser, and ultrasonic sensors are used for collision avoidance and precision positioning when paired with location control GPS sensors and stabilization or orientation modules. Advanced flight analytics, such as time of flight sensors, can give operators details about how the drone is performing relative to environmental conditions and can be used to enhance future flights.

Additional sensors are needed if the drone is meant to do a specific job, such as videography or imaging. Cameras, chemical detection, thermal sensors, and hyperspectral sensors are just a few examples. It’s important to note that some of this technology can also collect data internally, process the inputs, and respond automatically or communicate in real-time to the operator. Drones do a tremendous amount of data processing, which is the primary reason they need high levels of EMI shielding and thermal management.

EMI Shielding Solutions for Drone Applications

Now that we’ve provided a brief introduction to drone technologies and requirements, let’s dive into how one can shield drones from radiated susceptibility as well as radiated emissions.

An important note is that all devices have different needs for EMI shielding to make sure that nearby electronics are not impacting their performance. The right combination of EMI shielding and thermal interface materials will vary by device and application to provide device-level or component-level protection from unwanted electromagnetic radiation.

Conductive Elastomer Gaskets

One of the most commonly used and versatile solutions for system-level EMI shielding is a conductive elastomer gasket. Conductive elastomers consist of a base polymer such as silicone, a fluorosilicone, or an ethylene propylene diene monomer (EPDM) that gives the material its flexibility and structure. This base polymer is then embedded with metallic particles such as silver-plated aluminum, nickel-plated aluminum, silver-plated copper, nickel-plated graphite, and others that give the gasket its electrically conductive properties.

The specific particles and binders each lend themselves to different benefits based on the design requirements. For example, fluorosilicones will be used where the gasket may come into contact with harsh chemicals or washdown fluids. A silver-plated aluminum particle will provide very high conductivity, shielding, and galvanic corrosion resistance against aluminum substrates that are exposed to moisture and salt bog.

Conductive elastomers can be extruded into a gasket that sits in a groove or molded into a flat sheet and then die-cut into very intricate shapes, such as those that would be suitable for a connector for grounding. They can provide the advantage of being an EMI shield as well as an environmental seal, cutting down on the number of seals or gaskets required. They can also be developed as co-extruded parts where there is a durable non-conductive gasket permanently bonded to a conductive gasket for an even higher level of galvanic corrosion resistance.

Conductive elastomers can also come in form-in-place formats where a very thin bead of conductive gasketing is robotically applied onto a thin wall for cavity-to-cavity isolation and precise shielding within electronic enclosures.

Conductive Heat-Shrinkable Polyolefin Tubing

One product that has seen particular use in drone applications is an electrically conductive heat-shrinkable polyolefin tubing. The tubing and boots get their conductivity courtesy of a flexible conductive coating filled with either silver or silver-plated copper particles. The tubing has a 2:1 shrink rate, the same as standard shrink tubing, but it offers significant weight reduction compared to braided cable shielding or shielding cable wrap while giving the added benefit of water sealing.

Conductive Coatings and Sealants

Electrically conductive coatings are often applied via airbrushing onto metal or plastic substrates to provide EMI shielding, an intentional ground path, or a corrosion-resistant and conductive surface for mating against conductive elastomer gaskets. Conductive sealant and gap fillers are applied using a caulking gun directly from the packaging tube or unique applicator and are used as gap fillers at the seams of conductive enclosures. Sealants and conductive gap fillers are designed to be painted, sanded, or smoothed so they can provide the optimal surface finish and then integrated with other sealing or esthetic components of an airframe. Some things to consider when working with materials are working life, times, and masking or fixturing for accurate application.

Conductive Plastics

Injection-molded conductive plastic parts are made from engineered polymers that incorporate a conductive powder or fiber into the pellet blend. The pellets are then molded into complex shapes that provide the physical benefits of plastics while adding the advantages of an electrically conductive housing. Conductive plastic parts have a lower density than aluminum for when light weighting is important and provide significant time and cost savings of having to machine metal housings or covers for electronics protection. The final part can incorporate embedded hardware such as captive fasteners and minimize secondary manufacturing practices while holding similar tolerances as machined parts.

Overall, the advantages of using conductive plastics are weight reduction, RF absorption, corrosion resistance, good shielding effectiveness, and suitability for harsh environments. These plastics are ideal for moderate to high volumes, and while they do provide many benefits, some considerations are the initial cost for the injection molding tooling, upfront design time, minimum wall thickness, fluid exposure, and the color options that are available.

Conductive Foam Gaskets

Conductive fabric-over-foam and conductive foam solution applications were developed mainly for high-volume, cost-sensitive, low-compression force applications like consumer electronic devices. The foams used in these gaskets are often urethane or silicone, where higher temperature limits of up to 125 C are required. Conductive fibers or fabrics are used to provide electrical continuity and shielding ability. These materials are often used as a grounding gasket on board-level shields or as a connector gasket that’s needed to provide low contact resistance.

There are many advantages to using these gaskets, and one important one is that they are soft with a very low compression force. Additionally, they are lightweight and low density, typically low cost, and work well as a dust seal. Hundreds of standard parts and profiles are available, and tooling for custom parts is a relatively inexpensive option compared to other solutions. One drawback is that foam-based gaskets are typically not recommended for water or moisture sealing.

Board-Level Shielding

While most shielding products are used at the enclosure level, precision-stamped metal shield cans are used to shield components at the board level and give individual component-level attenuation. Board shields come in an infinite number of shapes and sizes with all kinds of board mating styles and precision features. RF broadband absorbers can be added to the shields to give extra RF absorption.

The pros of board shields are that they’re low cost and highly customizable with a lot of design options, and they can be integrated into automated assemblies. Additionally, they can be made of several materials and packaged in tape and reel formats, as well as assembled by pick-and-place machines. Aluminum is an increasingly common material for precision board shielding as it has the added benefit of excellent thermal conductivity, serving as a shield and a heat sink. While the upfront tooling cost is a drawback, the low unit cost can certainly make up for that over the course of a high-volume program.

Thermal Interface Materials for Drone Applications

Thermal Gap Filler Pads

Thermal gap filler pads or, simply, gap pads are designed to be soft to reduce component stress when creating an interface between heat-generating components and heat-dissipating surfaces. This conformability helps with vibration dampening and gives the gap pads a large compression range to take up assembly or manufacturing tolerances.

Nearly all gap pads are NASA E595 outgassing certified, meaning they’re approved for use in vacuum, space, and high-altitude applications. Gap pads are traditionally manufactured in sheets and can be cut into any shape or size. While common thicknesses range from 0.25 mm up to about 5.0 mm, gap pads can be made in much larger thicknesses as well. One of the advantages of gap pads is their ease of application, as they can simply be peeled off a protective liner and applied onto a heat sink or electronic component.

Thermal Gels

Thermal gels, also known as dispensable gap filler gels, are one-component, fully-cured dispensable thermal interface materials. A single-component material is advantageous because it requires no mixing or additional curing after dispensing onto a substrate. Thermal gels have very different physical properties from those of gap pads, providing some added benefits. These materials can be easily dispensed to meet various tolerance ranges or gap heights without requiring an additional part number in your bill of materials.

While gap pads have a typical minimum thickness of about 0.010” or 0.25 mm, gels can be dispensed in bond lines as thin as about 0.002” or 50 microns and up to well over half an inch on the thicker side. This means significantly increased thermal performance at thinner bond lines as the material can wet out and make effective contact between surfaces. Other benefits include very low compression forces, even lower than those of the already soft gap pads, thus reducing the force on underlying components. They also tend to have a lower density than pads, further reducing weight.

Meeting the Needs of Drone Applications

As you can see, there are many tools available to ensure heat management and EMI shielding for drones, and many more innovations are on the horizon. Current advancements are focused on higher thermal conductivity, higher flow rate, lower compression force, and higher reliability products to keep up with the needs of higher power connectivity equipment. This includes silicone and non-silicone solutions for gap pads and gels, as well as additions to thermal grease, phase change material, and even two-component material product families.

On the EMI shielding side, current research is directed toward new elastomer solutions, such as unique form-and-place materials and RF-absorbing solutions. The industry is not only developing new products but ensuring that these products are augmented with supporting information, such as high-frequency shielding data up to 115 GHz for EMI shielding products and environmental reliability data. Enhanced reliability testing capabilities aim to better align with customer requirements so that products perform reliably and consistently over the entire lifetime of the device.

Finally, remember that there are easy steps to reduce significant weight and ensure reliability in any environment. Lightweighting products such as conductive heat string tubing and plastics can provide up to 75 percent weight reduction while maintaining an important level of EMI shielding and RF absorption. Conductive foams and some thermal gels allow you to take advantage of light weight solutions while providing grounding or excellent heat transfer, respectively. These are all important considerations to keep drones flying safely and reliably.