As electronics become faster and more powerful, thermal management solutions must evolve to deal with the increasing heat loads. Simply increasing the size of a heat sink, or adding a fan, was once enough to provide the required increased performance. But, while air cooling remains the dominant method of thermal management in the electronics industry, there are applications where traditional air cooling is not sufficient. These are bound to increase in frequency in the near future. This week we have a two part series on Hybrid Liquid/Air Cooling Systems and how you can use them to cool some of your toughest thermal challenges.
Today, liquid cooling is being used in a steadily increasing number of thermal applications. Desktops, servers, and even laptops are all potential products for such cooling methods. The attractiveness of liquid is its density and specific heat over air (Table 1). However, these material properties can be misleading if compared side by side.
Table 1: Physical Properties of Air and Water[1]
In electronics, there is no basis for comparison between air and liquid for cooling. The term liquid cooling is itself misleading, as air is the final coolant in nearly all applications. The role of the liquid is not as a coolant, but as an active thermal transport vehicle. The main benefit from using liquid is the reduced thermal resistance from the heat source to the air cooled system peripheries. This is due to forced convection replacing pure conduction as the heat transport method, where the heat is delivered to the convective surfaces.
In general, all electronics cooling systems can be divided into three important components as shown in Figure 1:
- Interface
- Heat spreading
- Ambient heat exchanger
Figure 1: Typical Electronics Cooling System used in Thermal Management
The Interface refers to the junction between the component and heat sink or cold-plate. This resistance is typically minimized with a high performance grease or phase-change material, and is the same in both liquid and air cooling systems.
The spreading resistance in a thermal solution can be described as the transport of heat from the component to the cooling surfaces that are in contact with the ambient air. This is the only part of a cooling system that greatly differs from air to liquid cooling. With a typical air cooled heat sink, the thermal spreading is done at the base of the heat sink through pure conduction. When using liquid, the spreading is done by the movement of the liquid in a loop from the cold-plate to heat exchanger by mass transport, i.e. coolant.
The final part of an electronics cooling system is the ambient heat exchanger. For air cooling, this part is the heat sink fins, and for liquid cooling is the radiator fins. Both systems work in the same way, by using extended surfaces (fins) to transfer heat into the ambient air through convection.
In part 2 we’ll cover the liquid part of our article and more about integrating for a best in class solution!
Got a question on part 2 already or maybe part 1 from today? Contact us and lets see how ATS thermal engineers can make your next project a success! Email us at ats-hq@qats.com , call us at 781-769-2800 or visit our Design Services
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References:
1. Soul, C., The Benefits of Liquid Cooling over Air Cooling for Power Electronics, www.icepak.com/prod/icepak/solutions/articles/iceart19.htm
2. Sauciu, I., Chrysler, G., Mahajan, R., Spreading in the Heat Sink Base: Phase Change Systems or Solid Metals?, IEEE Transactionson Components and Packaging Technologies, Vol 23, No.4., 2002.
3. Jeung, S., Quantitative Thermal Performance Evaluation of a Cost-effective Vapor Chamber Heat Sink Containing a Metaletched Microwick Structure for Advanced Microprocessor Cooling, Sensors and Actuators, A: Physical Volume 121, Issue 2, 2005.
4. Wei, J., Cha, A., Copeland, D., Measurement of Vapor Chamber Performance, IEEE SEMI-THERM Symposium, 2003.
5. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on a Hybrid Liquid/Air Cooling System, IEEE, Semiconductor Thermal Measurement
and Management Symposium 2006.
