Thermal Management

Electronics used in military and aerospace applications are becoming increasingly smaller, more powerful, and more complex, making them much hotter. As cockpit microelectronics take on more functionality, beyond communication and sensing, thermal management can no longer be an afterthought. Design engineers understand that electronics in most military/aerospace applications tend to generate excessive heat, and that thermal management issues need addressed at the very beginning of the design process.

By replacing traditional mechanical, hydraulic, and pneumatic systems with sophisticated electronics, new applications are making themselves known. From aircraft, ground vehicles, ships, and satellites to battlefield soldiers, everything has an electronics focus. Powerful avionics, fly-by-wire, electronic warfare, communications, and radar systems are driving electronics development, and are ultimately generating more heat. Reliability is always a critical factor in these applications and failure due to overheating is not acceptable.

In military/aerospace applications, size, weight, and power (SWaP) considerations drive everything in the design process. As more powerful electronics and microelectronics systems require squeezing into a cockpit, thermal management needs addressed from system, to board, to device. New thermal interface materials can manage thermal issues locally, at the device, with liquid cooling, spray cooling, forced air-cooling, even fuel cooling, seeing use in removing excess heat through the package and off the platform – out of the vehicle and into the ambient.

Earlier Entry, the Better
Leaving thermal management as an afterthought is no longer possible with advanced avionics. Thermal management requires addressing early in design at the box, board, and device level. Design engineers routinely use computational fluid dynamics (CFD) and finite element analysis (FEA) tools to perform upfront analysis of thermal designs.

Under investigation and development are new materials with low coefficients of thermal expansion (CTEs), low density, and high thermal conductivity. The materials allow design engineers to create better designs with more complete thermal management, starting at the device level.

New thermal interface materials (TIMs) manage, block, and dissipate heat in microelectronics for mil/aero.

The rule of thumb with electronic devices is that every 10°C increase in temperature at the chip junction, where the device meets the substrate, cuts the life of the device by 50%. New materials are available to help manage, block, and dissipate heat in mil/aero electronics applications as thermal interface materials (TIMs). They range from improved thermal grease to the latest polymer matrix composites, metal matrix composites, carbon composites, insulating papers, and solder formulations.

Thermal interface materials (TIMs) are essentially the material between the device/heat source and the heat sink. They occupy the space between filling the voids between two imperfect mating surfaces, and replace air, a poor thermal conductor, with a much better thermal conductor.

To manage heat, effectively, design engineers working in mil/aero applications must use TIMs with very high thermal conductivity, low CTE to minimize thermal stress that can affect reliability and performance, and low density. Significant design compromises and reduced cooling efficiency can result from using material with high CTEs. A mismatch in CTEs between the PCB substrate and the TIM can cause thermal stress, warping, and failure. High density TIMs can create thermal impedance instead of thermal conductance.

TIMs need to match the CTE of the material to which they will attach: printed circuit board (PCB), ceramic substrate, and semiconductor. In order to reach the goals of high thermal conductivity with low CTE, many new materials are under investigation.

• Polymer matrix composites (PMCs) – different types of carbon fibers combined with a variety of thermosetting and thermoplastic resins, including epoxy, cyanate ester, liquid crystal, nylon, polycarbonate, ABS, PBT, and polyphenylene sulfide
• Metal matrix composites (MMCs) – silicon carbide particle reinforced aluminum, beryilia particle-reinforced beryilium, carbon fiber-reinforced aluminum, copper-tungsten, copper molybdenum, aluminum silicon, and Invar silver
• Carbon/carbon composites (CCCs) – carbon nanofibers, vapor grown carbon fibers, nano-graphene platelets, pyrolitic graphite, and other carbon/carbon mixes.

Recent research has uncovered several promising applications using nanotechnology, such as carbon nanotubes and graphene nanocomposites. In addition, work with high thermal conductivity graphite foams used as heat sinks has also offered promise for aircraft applications.

All these new materials promise the following advantages in managing heat at the device level, and perhaps eliminating the need for heat pipes and other thermal dissipation components.

Selecting the Appropriate TIM
Thermal interface materials come in a wide variety with different thermal impedance and thermal conductivities, different gap filling capabilities, compressibility, temperature ranges, and ease of application.

Phase Change Materials
Phase change materials (PCMs) are solids at room temperature but change to liquid once the excess heat of a device pushes the material past its melting point. Typically composed of a coating of phase change compound on an aluminum or polyimide substrate, coating of new PCMs can be directly onto a release liner without using a substrate. This improves performance by creating a better flow when the PCM is in the liquid stage, and better gap and void filling. The interface is thinner without the substrate, resulting in more efficient heat transfer.

Thermal Grease
The traditional interface material in electronics is thermal grease. Available in silicone or non-silicone varieties, thermal grease provides low thermal resistance and excellent gap filling, a thin bond line, easy application which may include automated dispensing, and low cost. Conversely, it is messy, and can pump-out or run during application and use, leaving new voids.

Gap Fillers
Gap fillers are available in pad or liquid form, which easily conform to the dimensional discrepancies, and their compliancy reduces component stress. Pad versions can blanket multiple components of different sizes and act as a common heat spreader.

Thermally Conductive Adhesives
Adhesives provide unique options for thermal management. They are often the best choice where components do not require connection by mechanical attachment, or where the micro-movement of substrates requires adhesion for a component to maintain contact with the substrate. These often find use with semiconductor packages as an interface between a chip and a heat spreader. Thermally conductive adhesives are usable as:

• Interface Pads – Conformable adhesive pads that are easy to handle and provide high conductivity
• Liquids – Usually epoxies, ultra-thin bond line and easy integration into manufacturing dispensing equipment
• Pressure-Sensitive Adhesive (PSA) Tapes – High mechanical strength plus good surface wetting and excellent shock absorption.

Thermal Gels
Gels are paste-like materials that perform like grease but with reduced pump-out. They exhibit good gap filling characteristics and compressibility.

Selecting the right type of TIM for your application requires assessing the advantages and disadvantages in each type and matching the characteristics to your specific requirements.

Experienced Materials Supplier
It has never been more important for a design engineer working in military/aerospace applications to work with a materials converting partner who has expertise in thermal management and adhesives.

From identification and selection of the appropriate materials and adhesives, to slitting, layering, laminating, precision die-cutting, and packaging of the finished product, an experienced converter can provide the design, prototyping, testing, and manufacturing knowledge required for success.

New thermal interface materials (TIMs) manage, block, and dissipate heat in microelectronics for mil/aero.

When designing in mil/aero applications, finding the right adhesives and materials is often a process of elimination. The more knowledge of how much heat the component generates, its place within the overall product, and other thermal management details, the shorter the process of selecting and matching appropriate adhesives and materials.

A materials converter provides:

• Precision die-cutting, multi-layer laminating, and slitting to tight tolerances
• Access to a range of thermal management solutions
• Testing capabilities.

An experienced converter can select from servo driven rotary die-cutting, CNC die-cutting, laser cutting, and water jet cutting to meet the complex specifications of thermal management for electronic components. For example, a servo-driven rotary die-cutter can maintain tight tolerances ranging from 0.015" to ±0.005" at speeds up to 500fpm, and is ideal for the complex, multi-layer die-cutting, and lamination that a thermal interface pad or tape may require.

For complex foam tape converting, water jet technology provides clean edges with no distortion. Laser cutting, kiss-cutting, slitting, and laminating also sees use in converting applications.

With the selection of a grease or liquid TIM, the converter can provide a plan for easy integration into the manufacturer’s process with dispensing recommendations and solutions.

A converter with a fully equipped test laboratory can ensure that customer materials meet designed-in specifications before they move to the factory floor, often eliminating the need to test materials at the customer’s facility. A complete test lab offers:

• Accurate and precise part dimension measurement and verification
• Adhesive/release liner testing to determine converting properties and high-speed application characteristics
• Material strength measured to ensure that material meets application requirements
• Static shear testing to measure the cohesive strength of the adhesive to withstand a fixed load over time
• Material weight measurement to determine adhesive coating weight
• Microscopic imaging to determine differences between adhesive and material over time
• Dielectric testing to determine a material’s electrical insulation properties
• and much more.

Fabrico
Kennesaw, GA
www.fabrico.com