Which Thermal Strap is Best for My Application?
With several strap types to choose from, it is essential to understand which material and configuration may make the most sense for your program, in light of the performance requirements, and the environmental/operational conditions. The table below outlines some of the most common applications, their operating temperatures, and the commonly used strap types:
Though the application and operating temperature are important, there are several additional factors to consider when identifying the ideal thermal strap material and configuration. Your program will likely also need to identify and weigh the following when determining the optimal strap in each situation:
|•RANGE OF MOTION
|•LOAD BEARING REQUIREMENTS
||•LIFE CYCLE BENDING / FLEXING
While most engineers are concerned with all—or a combination of—many of these factors, oftentimes, the most significant parameter (outside of the price), is the thermal conductance requirement. The second, and sometimes even more relevant factor to consider, is the deflection or stiffness requirement levied on the strap. The stiffness of each strap type can vary greatly depending on a number of factors, and your stiffness requirement may rule out certain strap materials to begin with.
At TAI, our experts are here to help identify, design, and manufacture the ideal thermal strap solution for your program, based on all of these factors.
Material Thermal Conductivity
Metallic Thermal Straps
Copper Rope / Cabling Straps (CuTS®)
Copper rope/cabling assemblies are either soldered, brazed, welded, or made via a swage ("cold press") process. It is important to note that straps which are sourced from third-party machine shops in the US & China, are frequently soldered or welded, and therefore present several performance, quality, and cleanliness issues. Soldering or welding straps can lead to 5-10x greater thermal contact resistance, and when heat is used to assemble straps, copper cables and foils will stiffen significantly, adding additional risk to expensive components and equipment in your system. Finally, when straps are simply welded or crimped in machine shops, they are often plagued by dimensional tolerance and surface flatness issues, and may be heavily contaminated with greases and cutting fluids.
When considering flexibility, durability, and performance, a copper cabled strap is the preferred, and most frequently used, in all industries/applications. They are the most durable of all heat strap products available, and are the ideal choice for applications operating at < 80K. CuTS® offer flexibility on all axes, and can handle exponentially greater loads and life cycle flexing than any other strap or material type.
Mass: Copper has a higher density than other materials, and in extremely mass-sensitive applications, a graphite strap may be your best option. It is important to note that aluminum foil straps are not always an ideal alternative to copper (when mass is a concern). Aluminum only offers a fraction of the conductivity of copper, and stacked foil straps must be designed into longer (and far thicker), S and U-shaped installation configurations, in order to provide flexibility on 2 of 3 axes, and match the thermal performance of our CuTS® products (negating the benefits of aluminum foil from the start).
Copper rope straps—even those made by TAI—can be stiff if multiple rows are incorporated into the design AND the cable length is less than 1.0 inch. At these shorter lengths, they still offer superior flexibility over stacked metallic foils, but they are not as flexible as single row or longer double row CuTS®.
Cross-sectional area: a cable (or braid), by its very nature, is not as densely-packaged as a stack of metallic sheets. As a result, cabled straps may not meet your thermal conductance requirement in certain volume-restricted applications.
Most engineers prefer to use copper cabled thermal straps because they offer the optimal combination of flexibility, durability, thermal performance, and vibration damping, at the most affordable price (when compared to high purity aluminum or carbon-based straps).
Volume-restricted applications (requiring high thermal performance), may benefit from a stacked metallic foil configuration. Cu foil straps—due to the increased density/cross-sectional area of the flexible portion of the material—can offer slightly improved thermal performance over cabled/rope straps (depending on the type of cabling/weave). In extremely mass-sensitive applications, a stacked aluminum foil strap may be a superior alternative but often sacrifices performance.
Stiffness: all foil straps are far stiffer (and on each axis) than equivalent copper cabled configurations. As a result, foil straps are designed in "S" and "U" shapes, in order to provide flexibility on the compression and lateral axes. However, this increases the length of the strap, which often negates the benefits of using foils to begin with. In fact, most engineers are able to substitute a much shorter copper rope or carbon strap resulting in reduced or equivalent mass, while offering equivalent—or improved—performance. Additionally, replacing a foil strap with a copper cabled configuration significantly reduces the price.
Many conventional assembly methods (brazing/soldering/welding), dramatically increase stiffness.
Foil straps typically cost 2-5x more than copper cabled thermal straps. Not only are the materials more expensive, but the assembly process is more complex and involves additional steps and costs (thus the exponentially higher price).
In many cases, foil straps are not the ideal solution. However, there are specific applications and environments in which they may offer benefits over a graphite or copper rope strap.
Graphite & Graphene Thermal Straps
There are multiple carbon-based thermal strap solutions to consider, and though graphite straps were initially used only for spaceflight applications operating between 230 - 400K, these materials offer unique benefits under nearly any operational or environmental condition and are now being incorporated into terrestrial and spaceflight cryogenic applications. Straps are either made using graphite fiber-based materials, or pyrolytic graphite and graphene foils and films (sheets). Each option offers a combination of mechanical and thermal performance, and financial costs to consider (and our graphite fiber, sheet and graphene foil are not to be confused with rigid Annealed Pyrolytic Graphite material, which is often used for structural components).
Graphite Fiber Thermal Straps are made using GraFlex; a bundled "toe" or rope of fibers with a material thermal conductivity of 810 W/(m-K). Fiber-based straps are more durable and lighter than carbon film/foil straps, and they offer lateral flexibility/deflection without needing to be installed in S-shaped installation configurations. The most notable attribute of graphite fiber straps is their high conductance to low mass ratio. The average GFTS® assembly is lighter than an equivalent carbon sheet strap, and just 1/5 - 1/10 the mass of a comparable copper rope strap. GFTS® products also have extensive spaceflight heritage and are used in programs such as NASA's ORION, JAXA's ASTRO-H, Boeing's CST-100 StarLiner, ESA's EnMap, and many others.
While GraFlex I and II are used to manufacture our GFTS® products, GraFlex III woven sheet is ideal for applications with extremely limited volumes, requiring flexibility on each axis. GraFlex rope and woven sheet material is also less expensive than PGS and Graphene Foils, as are GFTS® assembly costs.
GFTS® products—while more robust than graphite and graphene sheet/foil straps—are delicate, and more fragile than metallic straps.
Fiber-based strap assemblies provide a fraction of the performance of their foil/film-based counterparts.
GFTS® assemblies, like metallic foil straps, need to be designed and assembled into their installed configuration/shape, and do not offer an extensive range of motion on all axes (like a copper cabled strap).
While they offer flexibility on the lateral axis, GFTS® assemblies are best suited to applications requiring less than 1.0" deflection on each axis, and are stiffer on the vertical and compression axes than a PGS-based strap.
Graphite and Graphene Sheet/Film/Foil (PGL™& GTL™)
Graphite and Graphene sheet/foil straps offer the highest thermal performance of any of the strap products (above ~80K), ranging from 1,600 W/(m-K) - 1,840 W/(m-K). Their compact profiles make them ideal for volume-restricted applications, and though more fragile than metallic and fiber-based straps, they offer a unique combination of flexibility, low mass, and thermal performance.
Pyrolytic Graphite Thermal Links offer the highest thermal performance of any carbon-based strap at cryogenic operating temperatures (with performance peaking at 150K), and they are an effective replacement for aluminum foil thermal straps down to operating temperatures as low as 65K (and provide equivalent performance—at a lower mass—to OFHC copper thermal straps between 70 and 80K). Our Graphene Thermal Links/Straps offer the highest thermal performance at operating temperatures from 200K - 350K, though they are not as flexible as assemblies made with pyrolytic graphite sheet. Laboratory-verified tests demonstrate that graphene-based products can offer up to 4X the thermal performance of a comparable graphite fiber assemblies, and 20% better performance over equivalent pyrolytic graphite sheet assemblies.
All stacked pyrolytic graphite and graphene foils/sheets/films are fragile. These can be damaged if flexed on the lateral axis after installed (or in a straight configuration), or if improperly handled.
Carbon-based straps are not ideal at operating temperatures below ~60K. For these applications, TAI generally recommends OFHC copper-based straps (though for any application operating between 50K - 80K, we will review your requirements and make recommendations based on your specific case).
Graphite/Graphene Sheet/Foil straps are expensive. Graphite Fiber Strap products now sell for the same price as competing metallic foil straps, whereas carbon sheet-based products have somewhat higher material and assembly costs.
Affordable, High Quality Thermal Strap Solutions
Why do we offer several thermal strap options? Because no single product is ideal under all environmental and operational conditions. Each strap type offers a unique combination of thermal performance, flexibility, durability, vibration attenuation/damping, and mass, which customers must consider. Most importantly: while some materials may be ideal for your application, your budget may dictate which strap you ultimately choose. The TAI team is here to identify, design, and fabricate the best strap solution for your program, given your budget and requirements.
To learn more about our product offerings, download a catalog today, call or email us, or complete a Strap Questionnaire to get your inquiry started. Remember: all front end (pre-purchase order) design work is always free of charge, and our engineers and thermal strap experts are here to assist you at every step along the way!
Download Our Catalogs Today!
TAI provides on-site testing and analysis services here at our Boulder, CO facility (though we partner with an internationally renowned test facility for shock and vibe testing). From stiffness to thermal conductance, thermal cycling, shock & vibe, tensile strength measurements, and more, we have you covered! While we provide most testing services, our sister company, TAI, Inc., offers additional test services, and they will be releasing their own strap products and thermal management services in 2020.
TAI offers complimentary thermal assessments, providing mass and performance projections, schedule and pricing ROM's, and (when possible/if viable), Preliminary Trade Analysis of alternative aluminum and copper straps.