Which Flexible Thermal Link / Strap is Best for My Application?
With several strap products to choose from, it is important to understand which material and configuration may make the most sense for your program, given your performance requirements, and the environmental conditions/operational environment. While our experts work with every customer to determine the best strap material and configuration to meet your requirements and budget (and all we provide all front end design and consultation services free of charge), the table below outlines some of the most common requirements to consider when identifying the ideal thermal strap material and configuration:
|RANGE OF MOTION
|LOAD BEARING REQUIREMENTS
||LIFE CYCLE BENDING / FLEXING
While all customers are concerned with a combination of many of these factors, perhaps the most significant (outside of your budget and the program price), is the thermal conductance requirement (and how that relates to the operating temperature, the dimensions of the source and sink, and the available volume we have to work with).
For applications operating below 100K, there is no substitute for OFHC copper. OFHC copper cabling straps offer the greatest flexibility and highest thermal performance, at a fraction of the cost of any other metallic or carbon-based strap configuration (foil/film/sheet). For applications between 100K - 450K+, TAI will work with you to determine the ideal strap configuration and material, based on your budget, and the other factors listed above.
Pictured (Right): 99.99% Au and Electroless Ni-Plated Custom Cryocooler Series Straps.
Thermal Strap Material Thermal Conductivity
Depending on who makes your straps, copper rope (cabling / braid) assemblies are either soldered/brazed/welded, or made using proprietary cold press (solderless) processes (by TAI). It is important to note that outsourced straps from other suppliers are welded, and thus, far less flexible.
Copper Rope / Cabling Straps (CuTS®)
When it comes to flexibility, durability, and performance, the OFHC copper rope strap is the preferred, and most frequently used, in all industries/applications. They are the most durable of all thermal strap products available, and are the ideal choice for applications operating at <80K. They offer flexibility on all axes, and can handle exponentially greater loads and life cycle bending/flexing (TAI's CuTS® assemblies utilizing our OFHC UltraFlex™ Copper Cabling, have been flex tested to 1,000,000 cycles).
Most customers prefer to use CuTS® for cryocoolers, space applications, electronics cooling, and cold laboratory instruments, because they offer the best combination of high flexibility, durability, thermal performance, vibration damping, and extremely low cost.
- Mass: Copper is heavy, and in extremely mass-sensitive applications, a graphite strap may be your only option, especially in those requiring a high conductance to low mass ratio, or those above 80K. It is important to note that aluminum foil straps are not an ideal alternative to copper (when mass is a concern). Aluminum is far less conductive than copper, and stacked foil straps need to be designed into longer (and far thicker), S and U-shapes, in order to provide flexibility on 2 of 3 axes, and match the thermal performance of OFHC cu (negating the benefits of using aluminum to begin with).
- Copper rope straps—even those made by TAI—can be stiff if multiple rows are incorporated into the design AND the braid length is below 1". At these shorter lengths, they still offer improved flexibility over stacked metallic foils, but they are not as flexible as single row or longer double row CuTS®.
Graphite straps offer the highest thermal performance of any of the strap products (above cryogenic temperatures); ranging from 800 W/(m-K) - 2,000 W/(m-K). TAI's pyrolytic graphite sheet links / straps (PGL™), offer the highest thermal performance in the industry; with over 2x the material conductance of our graphite fiber straps, and >30% performance over any competing graphite or graphene sheet thermal strap. Graphite straps are also the lightest thermal links commercially-available; weighing in at just 1/4 - 1/10 the mass of all other strap types (making them ideal for many spacecraft / satellite applications).
- Unlike Woven Graphite Fiber Sheets (TAI's GraFlex III), stacked pyrolytic graphite and multilayered graphene foils/sheets/films are extremely fragile. These can be damaged easily—something as simple as improperly removing the material or strap assembly from its plastic bag can cause the sheets to tear. Graphite rope/GFTS® assemblies are far more robust, but they too, are not as durable as metallic straps.
- Graphite straps are not ideal at cryogenic temperatures. Performance drops quickly and dramatically below 100K, and bottoms out just below 50K (temperatures typically associated with cryocoolers, cryomodules, dilution refrigerators, and other equipment operating in the mK, or 4K to 40K range). This makes OFHC copper or 5N aluminum more attractive (really), below 150K.
- Graphite straps are expensive. While TAI's GFTS® and PGL™ products now sell for the same price as competing metallic foil straps, they are still more expensive than CuTS®, and can also include NRE/design fees (in a prototype program), that are higher than metallic straps. However, TAI's graphite / carbon straps are the most affordable carbon based straps commercially-available.
Stacked Metallic Foil Straps (Copper and Aluminum)
Volume-restricted applications (requiring high thermal performance), may sometimes benefit from a stacked foil configuration (especially if the operating temp is between 50K - 80K). Cu foil straps—due to the increased density/cross sectional area of the flexible portion of the strap—can offer slightly improved thermal performance over rope straps/CuTS® (depending on the type of cabling/weave). In extremely mass-sensitive applications, a stacked aluminum foil strap may be a better alternative, but you often sacrifice performance. These straps are usually made via welding, or pressing foils, and then brazing the assembly.
- Stiffness: Depending on the thickness and length, foil straps are often extremely stiff on each axis. As a result; they are typically designed into "S" and "U" shapes, to give them any flexibility at all (on the x and z axes). However, doing so increases the length of the strap, which often negates the benefits of using foils to begin with. For example: many of our customers are able to substitute a much shorter copper rope or carbon strap (with a gentle L or U shape), resulting in reduced or equivalent mass, while offering equivalent (or improved) performance. Further, substituting a cu rope strap (for an al or cu foil), often saves our customers thousands of dollars per unit (see below).
- The assembly methods (brazing/soldering/welding) cause increased stiffness.
- Foil straps typically cost 2-5x more than our CuTS®. 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 most 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.