As many applications move from air cooling to liquid cooling, tubed cold plates are often a cost efficient and popular solution. Although many local shops are able to manufacture a simple tubed cold plate, a properly designed and manufactured cold plate based on application will result in significantly better performance and reliability. However, assessing the quality of a cold plate can be difficult if you do not know what to look for.
How Construction Affects Tube Cold Plate Reliability and Performance
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 and lower performance./p>
Next look at how the tubes are attached to the plate. For maximum thermal transfer, engineers should be careful designing cold plates utilizing epoxy between the tubing and the plate. Although epoxy can be used to keep the tube in place and keep thermal contact between the two components, it can also act as a thermal insulator when used incorrectly, such as being applied too thickly or not utilizing the correct epoxy.
Finally, examine the cooling surface of the plate. For demanding applications, direct contact between the tube and the component is best. This means that the tube surface must be flush with the aluminum plate. A common manufacturing technique is to put an over-sized tube into a channel and machine off or fly-cut 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.
A high performance option is described in patent US# 6853555 and used in Press-lock™ 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.
Most applications use a thermal interface material (TIM) between the component or board and the cold plate to help to minimize the gaps. A TIM should be as thin as possible, as the relatively high thermal resistance of the TIM greatly overshadows any conductivity improvements from having a smoother surface. Increasing the clamping force of the component or board to the cold plate can also help to offset a higher roughness, but it may increase the stress on the board or component. Clamping stress can also increase the impact of coefficient of thermal expansion (CTE) mismatches as the cold plate and component or board heat up.
To avoid galvanic corrosion, utilize the same materials, or materials with similar electrical potential, throughout your cooling loop. Ensure that the plumbing, connectors and other components do not introduce a reactive metal into the system.
Flattened top surface
Tube with joints
Skim-cut top surface
Tubes flush with surface of plate
Inserts over tube to create flat surface
In addition to these considerations, other factors of cold plate performance include: fittings, connectors, materials, fluids, channel patterns or number of passes, and the associated heat exchanger. Boyd offers a variety of options in standard sizes and custom and semi-custom designs.