Which Thermal Strap is Best for My Application?
With multiple products 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
At TAI, our experts are here to help identify, design, and manufacture the ideal thermal solution for your program, based on all of these factors.
Material Thermal Conductivity
Copper Rope / Cabling Thermal Straps (CuTS®)
Copper rope/cabling straps are either soldered, brazed, welded, or made via a swage ("cold press") process. However, a cold press, heat-free assembly method is the most efficient and preserves the flexibility of the conductive materials used in a thermal strap. Soldering or welding copper straps can lead to 5-10x greater thermal contact resistance; significantly reducing thermal performance. Further, when heat is used to assemble straps, copper cables and foils will stiffen significantly, increasing the risk of vibration transmission and damage to sensitive equipment.
When considering flexibility, durability, and performance, a copper cabled strap is the preferred, and most frequently used, in all industries and applications. They are the most durable of all heat strap products available, and are the ideal choice for cryogenic applications. 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 conductive strap materials, and in extremely mass-sensitive applications, a graphite strap may be your best option. It is important to note that while aluminum is less dense than copper, aluminum straps are not always an ideal alternative (when mass is a concern). Aluminum offers a fraction of the conductivity of copper, and stacked foil straps must be designed into longer (and thicker), S and U-shaped installation configurations, in order to provide flexibility on 2 of 3 axes, and match the thermal performance of a copper cabled thermal strap.
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, cables continue to offer superior flexibility over stacked metallic foils, but the increased stiffness of the assembly is noticeable.
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.
Volume-restricted applications (requiring high thermal performance), may benefit from a stacked metallic foil configuration. However, there are a number of trade offs to consider:
Stiffness: all metallic foil thermal straps are 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 negates the benefits of using foils to begin with. In fact, most engineers are able to substitute a much shorter copper rope or graphite fiber or sheet strap when considering a foil configuration. This results 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 (thus, the 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 Straps
There are multiple carbon-based strap solutions to consider in the industry. Though graphite straps were initially used only for spaceflight applications operating between 230 - 400K, graphite and graphene offer unique benefits under nearly any operational or environmental conditions 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, thermal performance, and financial costs to consider (and 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 with 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 and deflection without needing to be installed in S-shaped configurations. The most notable attributes of graphite fiber straps is their high conductance to low mass ratio, and their unparalleled ability to attenuate and absorb vibration. 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—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 Thermal Straps (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. Though PGS and graphene straps are 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). They are an effective replacement for aluminum foil 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 offer the highest thermal performance at operating temperatures from 200K - 350K, though graphene is not as flexible as pyrolytic graphite sheet.
All stacked pyrolytic graphite and graphene foils/sheets/films are fragile. These can be damaged if flexed on the lateral axis if improperly handled or used.
Carbon-based straps are not ideal at operating temperatures below ~60K, unless the goal is to use them as a flexible thermal switch.
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 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.
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 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.