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Today we are kicking off a series of articles on how to select a heat sink for an OEM project. The principles are the same for overclockers building their own systems with one difference. In OEM projects, mechanical engineers usually (but not always) have a chance to simulate the design before hand and suggest changes to chassis and layout to help with airflow.
So, let’s get started with some basics. First, why is thermal management a challenge? There’s a few reasons and many of these are only getting worse if you consider them from the world of thermal engineering.
First on our list is higher frequency circuits. The International Technology Roadmap for Semiconductors notes that, “projected power density and junction-to-ambient thermal resistance for high-performance chips at the 14 nm generation are >100 W/cm2 and <0.2°C/W, respectively.” In other words, semiconductors are simply getting hotter as their clock speeds are increased.
Second, generally, semiconductors are being assembled into smaller packages. The packages are smaller, the circuits are denser and this combination means that they are warmer.
Third on our list of why thermal management is a (growing) challenge is low acoustic noise requirements. End users don’t want to be deaf just for using electronics. The result is many specifications that set a reasonable acoustic range for their equipment, often in the 100LFM to 400LFM range. This relatively low airflow is great for end users but creates a real challenge for mechanical engineers and systems integrators trying to create a solid system that meets end users needs and still operates at its optimal levels.
Fourth, circuit designers determine component placement. On the surface of this, this is how it should be. Electrical engineers have alot of pressure on them to reduce board latency and design for performance. While they often consider the thermal needs of the systems and circuits, it’s not their primary design point. For mechanical engineers that is what we do and so our challenge is the balancing act of working with EE’s to insure great placement, but also great airflow.
Fifth and finally, thermal management is a challenge because EMI shielding. Higher frequency components require better shielding and that shielding can restrict airflow.
When we pick this topic up next, we’ll cover why temperature is so important to manage. If you have any questions in the mean time about heat sinks or thermal management, contact us and lets see how can make your next project a success! Email us at ATS thermal [email protected] , call us at 781-769-2800 or visit our heat sink catalog at qats.com
Today we are excited to announce our new heat sink request tool on qats.com Just click on any Big Green Button like this one, anywhere you see it, to bring you to our new tool:
EVERY heat sink on our site is now proudly sporting our new Big Green Button with good reason. ATS’s team wants to make sure finding your best heat sink is as easy as possible, even for your most challenging thermal management application.
When you click on our Big Green Button, you’ll be brought to the tool. On it, you’ll see a web form:
The form helps guide you through a typical question process to determine key requirements necessary to get you the right heat sink. These include important facts such as:
Just a few short questions and we’ll get you going with a heat sink good for your application or at a minimum as a starting point for your research.
AND if you click on our Big Green Button to check out our new heat sink request tool we’ll send you a free white paper, “Air/Liquid Cooling”. This paper is filled with valuable information on the integration of air and liquid cooling and how to know when it’s time to make the switch.
In part 1 of our 2 part series on Hybrid Liquid/Air Cooling Systems and how you can use them to cool some of your toughest thermal challenges, we covered the air portion of our system, here in part 2 we’ll consider the liquid portion and how to integrate them.
Unless one is using a natural body of water for coolant, or operating in space, at the end of every cooling solution there is a liquid to air heat exchanger, when the generated heat is transferred.
Figure 2. Typical Liquid Cooling System
As shown in Figure 2, a conventional liquid cooling system consists of a cold plate, external plumbing, and a heat exchanger. The advantage of this type of system is to increase flexibility in packaging by allowing remote placement of the heat exchanger. Remote mounting does introduce disadvantages, as the external plumbing increases pressure drop throughout the system, which increases the required pumping power. The piping itself is a potential source of leakage at the plumbed junctions, as is the permeability of the piping system.
To appreciate the importance of spreading resistance, lets assume a high heat flux component generates 500 W/cm2 in a 10 x 10 mm package. The size of the copper heat sink used is 80 x 80 mm, with a base thickness of 5 mm. The spreading resistance alone for this case is 0.140C/W. Even if the thermal resistance of the heat sink is 0 (thermodynamically impossible), the temperature rise of the component above ambient is 700C. Considering an ambient of 500C, the above proposed heat sink will not cool the device adequately to prevent it from failure. The above example shows how important the spreading resistance is, especially in high heat flux applications.
There are many methods for reducing the thermal resistance. Among these methods are:
Use of a high conductivity material as the base plate of the heat sink to reduce thermal resistance. These materials include aluminum (k = 180 W/mK), copper (k = 380 W/mK), and CVD diamond (k = 2000 W/mK).
Using passive, high conductivity devices, like heat pipes, thermosyphons, or vapor chambers
Use of thermoelectric devices whose heat spreader structures consist simply of an electrically conductive heat sink with an applied external electric potential.
This induces a Thomson Effect, and provides heat transfer through the device. Of the above, the vapor chamber has been the most desirable method. Basically, a vapor chamber works like a heat pipe. The heat transfer to the base vaporizes the liquid and reaches the cold section of the chamber. The vapor condenses and returns back to the base with the help of the wick structure. But, even though the spreading resistance of a vapor chamber is theoretically appealing, it has been found that, under certain conditions, a solid copper spreader can have lower thermal spreading resistance [2,3,4].
Figure 3. Comparison of a Solid Copper Heat Sink and a Vapor Chamber
In order to alleviate spreading resistance issues, Advanced Thermal Solutions, Inc. (ATS) has developed a new technology, the Forced Thermal Spreader, or FTS [5]. A schematic picture of the FTS design is shown in Figure 4.
Figure 4. Structure of Advanced Thermal Solutions’ Forced Thermal Spreader
The FTS design is a combination of mini- and microchannels.
The heat transfer coefficient in the micro-channels is about 500,000 W/m20C. This high heat transfer coefficient creates a very small resistance between the heat source and the incoming liquid. The heat is then transferred to the bottom of the heat sink with the minifins attached to the top plate. Heat then transfers from the top plate to the ambient through the heat sink. The experimental test set up is shown in figure 5.
An experiment with an FTS was performed using an HFC-100 test equipment developed by ATS. The HFC-100 is a computerized data acquisition system capable of controlling up to 1KW of heat generated on a 1 cm2 simulated chip. This instrument is capable of ramping the heat with specified dwell times. The size of the FTS was 100 x 120 mm. Table 1 shows the experimental data from tests performed at power levels of 100, 200, and 300 W/cm2. The results show that the data is independent of the power. The experiment was conducted several times at each power level to ensure data repeatability.
Figure 5. Forced Thermal Spreader Test Setup
Table 1b: Experimental Data for the Thermal Resistance of the Forced Thermal Spreader
The thermal resistance from the FTS is around 0.1370C/W on average. The water flow rate is set as 1.0~1.2 L/min, and it was observed that increasing the flow rate beyond 0.3 L/min had no noticeable change in temperature. One very noticeable phenomenon is the interfacial resistance. Because the heat source is small, 1 cm2, this resistance value is significant even under best contact conditions. It is expected that this number would be much higher in a real device application. For proof, a second experiment was carried out. In this experiment the heat source was made part of the FTS, which eliminated the spreading resistance.
Table 2 shows the data for this case. The thermal resistance of the FTS is about 0.14 to 0.15oC/W.
Table 2. Experimental Data for the Thermal Resistance of the FTS, With No Interfacial Resistance
The importance of the above numbers shows itself when calculating the spreading resistance. For a 100mm x 120mm base size copper heat sink, and a 10mm x 10mm heat source, the spreading resistance is about 0.12 degree C/W. To achieve the total resistance of 0.14 degree C/W with a copper heat sink, we need a heat sink resistance of 0.02 degree C/W, which is not feasible using just air. To show this, we can look at the thermal resistance of a heat sink:
Where Cp is the fluid heat capacitance, h is heat transfer coefficient, and is the mass flow rate. Assuming a heat transfer coefficient of infinity (thermodynamically impossible):
To reach a resistance of 0.02oC/W, a velocity of 25 m/sec is required for a heat sink that is 100 mm wide and 20 mm high. In a typical systems environment, the h value is about 100-200 W/m2oC for very high speed flows. Assuming this heat sink has 65 fins at 1 mm spacing, the convective resistance will be around 0.02oC/W, but with an enormous amount of pressure drop of about 20 kpa (80H2O). This is clearly an impractical situation.
The data presented in this article shows that an effective hybrid system (liquid-assisted air cooling) has enormous capability for high heat flux applications. However the reader should not forget that the interfacial thermal resistance will always exist unless the interface is eliminated by integrating the cooling and the package systems. Equally important is the reliability of the cooling loops, as well as active control of the device functionality should the cooling system fail.
Got a question on part 1 or 2? Contact us and lets see how ATS’ thermal engineers can make your next project a success! Email us at [email protected] , call us at 781-769-2800 or visit our Design Services
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.
Posted in heat sink, Heat Spreaders, Liquid Cooling, thermal management
Tagged engineering, heat sink, liquid cooling, thermal management
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:
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 [email protected] , 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.
Posted in Heat Sinks, Heat Spreaders, Liquid Cooling, thermal management
Tagged engineering, heat sink, liquid cooling, thermal management