Tubed and Submerged-Fin Cold Plates in Electronics Thermal Management

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)

Many of today’s electronic devices need the performance of liquid cooling to meet the thermal demands of certain hot components. Liquid cold plates are common cooling systems in high power lasers, fuel cells, battery coolers, motor drives, medical equipment, avionics and other high-power, high-heat flux applications.

Cold Plates
Figure 1. A Custom liquid cold plate design by D6 Industries. [1]

Cold plates provide localized cooling by transferring heat from a device to a liquid that flows to a remote heat exchanger and dissipates into either the ambient or to another liquid in a secondary cooling system. Component heat flows by conduction through a thermal interface material and the metal plate to the metal tubing. Then it flows by convection from the internal surface of the fluid path material into the flowing coolant.

A cold plate in electronics cooling is often an aluminum block with an embedded, coolant-filled metal tube. Another common cold plate type is made with metal shells that are brazed or friction-welded together and filled with a liquid coolant.  On the inside, the metal shells have integral cooling fins that are submerged in the coolant.

Tubed Cold Plates

Embedded tube designs are the simplest version of cold plate cooling devices. They feature a continuous tube set into grooves in a metal plate, and are often bonded in place with thermal epoxy. The flowing coolant moves heat from the component away from the cold plate to a heat exchanger, where it is cooled before being pumped back into the plate. 

A common example of a tubed cold plate features an aluminum plate with an exposed copper tube. The tubes can be routed in different pathways to optimize the thermal performance.

The tubing can be continuous or constructed from straight tubes connected by soldered joints, though joints may increase the potential for leakage.

Figure 2. A Tubed cold plate consists of copper or stainless-steel tubing pressed into a metal plate. [2]

This design can provide a cost-effective thermal solution for component cooling where the heat load is low-to-moderate. Tubed cold plates ensure minimum thermal resistance between the power device and the cold plate by placing the coolant tube in direct contact with the power device’s base. Direct contact reduces the number of thermal interfaces between device and fluid, thus increasing performance for the application.

A variant of this design features a thermal epoxy completely applied over the pressed in tubing and flush with the metal plate surface. These are sometimes called buried tube liquid cold plates. This provides a gap-free thermal interface between the tube and the plate. The epoxy layer protects from any leakage from the metal tube. Another key feature is that that fully buried tube is not exposed to the outside environment.

Figure 3. A buried tube cold plate’s metal tube is covered with a conductive epoxy layer. [3]

The choice of liquid coolant affects thermal performance as well. Choosing the right coolant depends to a great extent on the tube material. Copper tubes are compatible with water and most other common coolants, while stainless steel tubes can be used with deionized water or corrosive fluids.

One cold plate OEM offers a proprietary technology with a tube locking system and pressing techniques that ensure the tube is flush with the plate surface, providing good thermal contact with the component being cooled. This manufacturing method eliminates the need for thermal epoxy between the tube and plate which improves thermal performance. [4]

Submerged Fin Cold Plates

Another type of cold plate is an all-metal construction with brazed or friction welded internal fin field.

Figure 4. Standard, liquid coolant-containing metal cold plate [5]

The integral, internal fins increase the surface area that contacts the fluid and enhances heat transfer. Fin shape and fin density affect the performance of heat exchangers and cold plates. By their geometry, the fins also create turbulence, which minimizes the fluid boundary layer and further reduces thermal resistance.

One high-performance version features tightly packed aluminum pin fins that create turbulence with low flow rate values, resulting in high thermal performance with low pressure drop. In this design, the high density of the internal fins increases the heat transfer area without adding bulk to the cold plate assembly. [6]

Figure 5. Close-spaced pin fins with complex geometry create turbulence with low flow rate values inside submerged fin cold plates. [6]

In most high-performance applications, fins are made of copper or aluminum. Aluminum fins are preferred in aircraft electronic liquid cooling applications due to their lighter weight. Copper fins are mostly used in applications where weight is not an important factor, but compatibility with other cooling loop materials is.

For submerged-fin cold plates, many different fin geometries can be tested to find the best improvement in performance. Some of the most commonly used are louvered, lanced offset, straight, and wavy fins.

Figure 6. Fin designs for submerged-fin cold plates. Clockwise from top: louvered, lanced offset, wavy, and straight fins. [7]

With cooling requirements increasing in many areas of electronics, engineers are turning to liquid cooling to replace air cooling. Lower cost, safer liquid cooling systems have also spurred the trend to liquid cooling.

The prime example is the cold plate – relatively simple in design, affordable, available in alternative versions, and extremely customizable. Cold plates should be considered wherever thermal performance above air cooling is needed.

References:

  1. https://d6industries.com/portfolio/custom-designs-liquid-cold-plate-hydroblock/
  2. https://www.lytron.com/Cold-Plates
  3. http://www.wakefield-vette.com/products/liquid-cooling/liquid-cold-plates/standard-liquid-cold-plates.aspx
  4. https://www.lytron.com/Tools-and-Technical-Reference/Application-Notes/Assessing-the-Quality-of-a-Tubed-Cold-Plate
  5. https://www.qats.com/Products/Liquid-Cooling/Cold-Plates
  6. http://www.cooltech.it/products/liquid-cold-plates/
  7. https://www.lytron.com/Tools-and-Technical-Reference/Application-Notes/Fins-for-Cooling-Success

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com. To register for Qpedia and to get access to its archives, visit https://www.qats.com/Qpedia-Thermal-eMagazine.


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