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Assessing the Quality of a Tubed Cold Plate

As many applications move from air cooling to liquid cooling, tubed cold plates are often the technology of choice. Their growing presence begs the question "What are the significant criteria for selecting a high quality tubed cold plate?" Most machine shops are able to produce a tubed cold plate. However, a well designed cold plate results in a product with noticeable and significant advantages in both quality and performance. Figure 1 shows tubed cold plates from three different manufacturers: Can you tell which cold plate is likely to have the best performance or be the most reliable?

Using the information presented in this article, the quality of a tubed cold plate can be assessed through quick observation.

 Fig. 1. Tubed cold plates from three different manufacturers

Fig. 1. Tubed cold plates from three different manufacturers

High reliability is important, as a coolant leak can be catastrophic in a liquid cooling system. First examine the tubes; they can reveal a lot about the reliability of the cold plate. A cold plate that uses continuous tubing is inherently more reliable than one constructed from straight tubes connected by soldered joints, as any joint increases the potential for leakage.

Look also at the quality of the bends. If the tube bending is not carried out carefully, the tube can be deformed. While this does not affect the reliability, it changes the cross-sectional profile of the tube, which can result in increased pressure drop.

 Fig. 2. Look closely at the bends for an indication of reliability and performance

Fig. 2. Look closely at the bends for an indication of reliability and performance

Next look at how the tubes are attached to the plate. For maximum thermal transfer, tubed cold plates should be designed without epoxy between the tubing and the plate. Where epoxy is used between the tube and the plate, the epoxy serves the dual purpose of holding the tube into the plate and making thermal contact between the two components. However, if applied too thickly, it can be counterproductive and act as a thermal insulator.

Finally, examine the cooling surface of the plate. For demanding applications, direct contact cooling between the tube and the component is desired. This means that the tube surface has to be flush with the aluminum plate. A common manufacturing technique is to put an over-sized tube into a channel and machine off the top. However, this costly technique, known as skim cutting, creates a tube section of varying thickness that affects structural integrity and performance.

Another alternative is to embed the tube below the surface of the cold plate, and add copper inserts to level the surface. This technique is both expensive and limits performance - the cooling tube is further from the components being cooled, and the additional layer of epoxy required between the tube and the insert further decreases performance.

 Fig. 3. Differing methods of holding the tube in place and ensuring a flat mounting surface.

Fig. 3. Differing methods of holding the tube in place and ensuring a flat mounting surface.

A high performance option is described in Lytron's patent US# 6853555 and used in Lytron's Press-lockTM CP12 and CP15 cold plates. Careful design of the locking feature and proprietary pressing techniques ensure that the tube is flush with the plate surface, providing good thermal contact with the component being cooled. This manufacturing method also ensures good metal-to-metal contact between the tube and the plate, guaranteeing excellent thermal performance and eliminating the need for epoxy. Figure 4 demonstrates the performance advantage of Press-Lock technology compared to cold plates manufactured using epoxy.

 Fig. 4. Performance comparison of an epoxy-free cold plate manufactured using Lytron's Press-lock technology to other cold plate designs.

Fig. 4. Performance comparison of an epoxy-free cold plate manufactured using Lytron's Press-lock technology to other cold plate designs.

To avoid galvanic corrosion, we highly recommend using the same materials, or materials with similar electrical potential, throughout your cooling loop. You should ensure that the plumbing, connectors and other components do not introduce a reactive metal into the system.

Using the same materials throughout your circuit does not mean that you have to sacrifice performance. Lytron offers high performance heat exchangers and cold plates with aluminum, copper and stainless steel fluid paths.

Summary:

Reliable
Continuous tube
Flattened top surface
Unreliable
Tube with joints
Skim-cut top surface
High Performance
Epoxy-free construction
Tubes flush with surface of plate
Lower Performance
Epoxy between tube and plate
Inserts over tube to create flat surface

When assessing the quality of a tubed cold plate, the most important considerations are continuous tubing, an epoxy-free construction, and an overall appearance that demonstrates design expertise and careful manufacturing. Other factors that you may also want to bear in mind are the choice of fittings and the selection of sizes. Many manufacturers offer a handful of standard sizes, which are usually considerably less expensive than custom designs. If you are likely to need a custom cold plate rather than an off-the-shelf design, it is wise to select a manufacturer that can offer full custom design capabilities, including thermal modeling and analysis, rather than one who simply offers 'build-to-print' cold plates.