Thermal Management Solutions

EE202A, Fall 2003
S. Pouya Dormiani Tabatabaei
pouya@ee.ucla.edu

If you feel that I have missed or maligned any information provided on this page, please feel free to contact me with your criticisms and suggestions so I can update the site. Thank you in advance.

Thermal management solutions: Helping chips keep their cool

courtesy of icepak.com

A Brief Overview

Technologies discussed

Heat Sink and Fan Combinations

Thermal Vias

Heat spreaders

Heat pipes

Microjet Cooling Devices

References

A Brief Overview

Thermal management is a very broad topic of discussion in the sense that there are many different techniques used depending on the target platform. For example, the techniques used to cool an 8000 processor based computer that takes up several rooms in a facility is much different than those used on a notebook computer or an embedded device. In this web site, we will focus on only desktop, notebook and embedded devices. There are a vast number of techniques used to cool these systems at different levels. Cooling mechanisms embedded at the IC level, chip packaging techniques, package mounted peripherals, as well as system cooling mechanisms are used depending on the power dissipation of the system. This web site will not address the actual trends of power dissipation and thermal problems faced by modern devices, but rather, solutions to reduce their presence.

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Summary of thermal management technologies
This section is meant to briefly introduce current and future thermal management technologies. These solutions range from very complex to fairly simple ideas which have been engineered over time to meet the increasing demand of reducing chip and system temperatures. Bellow is a list of the technologies that are discussed, and a brief summary of the technologies they employ in achieving their goals.

Heat sink and fan combinations

This cooling method has been around for a relatively long time. It is simply a metallic based heat sink, which has a flat bottom surface which is mounted on top of the chip to be cooled. The top of the surface consists of many fin like structures which help dissipate the heat by providing a large surface area. There are situations in which the heat sinks are provided with an additional fan, which helps circulate a larger amount of air around the fins, and thus increases the rate at which the heat is dissipated from the heat sink. The figure below is an illustration of such a device.

courtesy of cooler master

These heat sinks are probably the cheapest solution to any thermal problem. They have been around for a very long time and come in all sorts of shapes and sizes. They are mostly used in desktop computers, but have found their way into many notebooks and even embedded devices in some cases. Their main advantage is their cost, and their disadvantages are requiring too much space, and being incapable of large scale heat removal. As the power dissipation of modern processors grow, these devices alone will not meet cooling demands.

Thermal vias

These are copper channels that run through the IC. These vias serve the purpose of moving the heat from the IC to a heat spreader (any large metallic surface, or heat sink -- namely something with a large surface area to dissipate heat rapidly). They are ideal because transferring heat through the IC takes up no additional space. Their primary advantage is that they can channel heat very well from the IC to another surface, rather than relying on the packaging of the IC to "pass the heat on".

Heat Spreaders

These are large films of copper or other materials which help distribute the heat of the system over very large areas. These devices have been used so far on some memory technologies and are also planned on being used inside notebook computers. Again, they are very nice in that they do not take up a large amount of space in a system, however, they are not reducing the core temperatureof the device under consideration by a great deal, but rather help dissipate system heat faster.

Heat Pipes

This must be one of the most interesting and innovative ideas I have run into while researching different thermal solutions. This technology has been around for a very long time in all sorts of cooling systems, but only lately has it begun to see the light as a viable solution for thermal management relating to electronics.

Microjet cooling devices

Another innovative idea that has been brought about by recent advances in MEMS technology. These "jets" are extremely small electronic "fans" that can work to cool the device, without requiring large volumes for primitive spinning structures via a motor.

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Heat Sink and fan combinations
Through the use of a large surface area as well as air flow, this technology addresses thermal management rather primitively. The results are a very fast time to market for the product and a small amount of cost in designing the thermal management for the technology. Usually, there is no custom design of heat sink and fan combinations. There are a vast variety already available in many sizes and specifications, each suited to a particular electronics package style, and power dissipation rating. The only real task for the designer is to discover which package provides enough thermal dissipation to prevent the IC from reaching threshold temperatures. Usually designers are not at all concerned with how cool the system is, as long as it does not affect its functionality. The following images demonstrate some very neat simulations using CFD (computational fluid dynamics) software, to simulate and model the heat flow in the structure (there are many packages available, e.g. Icepak and Coolit).

The above two diagrams show the actual heat sink as well as the heat sink thermal
distribution, obtained through CFD simulations (courtesy of Icepak).

Figure 1.

Figure 2.

Figure 3.

The above three figures show the flow of air and the relative thermal dissipation
relative to angle of the air flow. Figure 1,2, and 3 have the following relative
angles of airflow inclination: 0, 35, 45 (courtesy of flotherm.com).

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Thermal Vias
Thermal vias are metallic (copper) structures connected from the IC to a metallic heat spreader such as a heat sink. These structures are embedded within the packaging and the IC, and provide the benefit of passing the heat directly to the heat spreader rather than transferring the heat indirectly (and inefficiently) through the packaging of the IC. The best way to explain how this thermal management technique works is through a diagram which illustrates the construction of this technique.

The above 3. figures show the concept of a thermal via. As you can see, thevia, which connects directly
to the IC can rapidly transfer heat from the IC to the metal backing plate which in turn can dissipate heat
rapidly through its large surface area. Schematics courtesy of microsubstrates.com

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Heat Spreaders
This thermal management technology is very similar in nature to a heat sink. It once again utilizes surface area directly to dissipate thermal energy. The only real difference between heat spreaders and heat sinks is that heat spreaders have no fins, where as heat sinks are usually big bulky structures with some sort of fin like structure. Heat spreaders are relatively thin sheets of metal that usually try to utilize the "area" taken up by a component as a heat dissipation mechanism. For example, we can see the heat spreader in use on the following memory structure. The Heat spreader covers the entire DIMM area, resulting in much larger surface area than providing individual heat sinks for each of the ram chips, in addition to taking much less space.

The above image shows the use of heat spreaders on a memory structure.
Picture courtesy of ocztechnology.com

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Heat Pipes
Heat pipes are one of the most effective heat removal systems I encountered through my research. These devices use a pretty old technology (by today's standards) used previously in cooling systems. However, this technology has now been scaled to smaller sizes, and refined for use in electronic devices. There are many technical publications on heat pipes as well as several companies which have now commercialized the concept. This section gives a detailed view of what heat pipes are and how they function, as well as some diagrams that illustrate their effectiveness in thermal dissipation. I have overlooked any rigorous treatment of the technology and its optimizations, which is really immaterial to how the technology functions. There are references to technical papers for those people who demand a more formal treatment of the technology.

Heat pipes are exactly what they sound to be, they are pipes which channel the heat of a specific area to another. This is made possible through ducts that carry the heat with a very minimal change in temperature across the pipe [Gernert]. The following diagram illustrates how a heat pipe structure can be used to transfer heat.

The structure of the heat pipe. It is possible to see through this diagram that the use of
a heat sink like structure at the end of the heat pipe creates a temperature gradient
which in turn causes the flow of the temperature from the hotter to the colder region. This facilitates

the flow that enables heat movement. Courtesy of Thermacore International Inc. (thermacore.com).

The following is an image showing the thermal relief that the heat pipe provides to a heat source resembling a processor core. This is a very interesting image because it illustrates the drastically improved thermal state of the system with and without the heat pipe technology being present.

As evident in the picture to the very left, there is a very point like source of heat, which
resembles an IC core for example. Through the use of multiple heat pipe structures, the heat
is channeled to another area, where it is then cooled. The center diagram shows the temperatures
resulting from using multiple heat pipes to channel the heat to other parts of the structure. The figure to
the far right shows the resulting temperatures with the the use of a vapor chamber. A vapor chamber is
is essentially a 2-D heat pipe which facilitates heat transfer in a plane. [Thayer]

Heat pipes have long been used in large scale systems as well as communications systems. However, because of the very limited scaling of current thermal solutions (fan assisted heat sinks) to the exponential increase in chip power dissipation, there is a very demanding need for technologies that will accommodate very high power devices. Heat pipes are very likely to be the next wave of technology used in notebook and desktop computers. This technology has managed to perform reasonably well in multi-kilowatt systems, which leaves plenty of time in the near distant future to design cooling systems which can handle even higher loads. There are many products already available which use heat pipe technology. One such product was already shown at the top of this page (The heat sink shown above incorporates a heat pipe, which can be seen leaving and entering the heat sink on its side). Nearly all modern notebook computers include some form of heat pipe technology as a part of their thermal management solution [Ali]. Notebook computers have a very small amount of space available in their case, which makes heat sinks a very unattractive cooling solution. The figures below show how the heat pipe technology might be laid out in the chases of the notebook.

Figure 1. Figure 2.
Figure 1: A typical set of components incorporated in notebook computers that use heat pipes [Ali].
Figure 2: A system in which a thermal plane embedded in the back plane of the screen is used as the destination of the
heat channeled through heat pipe structures within the chases.

Heat pipes have some very good advantages to other thermal solutions. They take a relatively small amount of space and can be embedded efficiently into devices, in addition to their very effective operation. There is one draw back to heat pipes that limits their use in future designs. The heat pipe itself does not scale very well to small feature sizes. That is, the performance of the device is deteriorated through making the pipe too small. This will indeed hinder the use of this technology in the future, where devices become extremely small.

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Microjet Cooling Devices
There are different types of Microjet devices. These devices have been reduced in size through advancements in MEMS technology. Below is a diagram showing the structure of a microjet device.

Figure 1 Figure 2
Figure 1: This diagram shows the operation of the Microject device. The magnets attached to the membrane structure are
attracted toward the coils, much like a speaker system, and cause the magnets to load back and stretch the membrane. Upon release
of the magnet, the membrane recoils back to its resulting position (due to its elasticity) and pushes the air inside the cavity through
the orifice creating a jet of air.
Figure 2: This figure shows the mechanical setup of the Microjet device. As it can be seen, the structure is very similar to
a speaker cone.

Now, the question is.... So what? One might quickly judge that this is once again as primitive as blowing a fan on top of a heat sink structure. In all of the thermal management solutions described above, the fundamental idea has been to increase the surface area of the component to be cooled. This approach alone however is not very effective by itself. Heat transfers across gradients, and ultimately we would like to keep as big a gradient as we can between the two mediums (heat sink and surrounding air for e.g.). Through circulating the air surrounding the heat sink like structure, we can increase the temperature gradient by removing the already "hot" air around the structure with cooler air from the surroundings. The purpose of the fan is to blow away the hot air. Now that we know what we need a fan for, why do we like microjets? Well, its actually all a matter of size, the smaller we can scale the fan, the smaller volume which is required to hold the instrument. Through all the topics discussed, space has been the ultimate frontier. These microjet devices, can scale to very small levels which makes them an attractive technology for air circulation in cramped packages. The disadvantage to such devices, is their use of power. If you had noticed, many of the thermal solutions described have been passive, in that they do not require additional power to help dissipate thermal energy. Instruments such as fans and microjets are active, and require power which leaves them at a disadvantage to highly power conscious systems (for e.g. low power embedded devices).

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References

Nelson J. Gernert, "Cooling of Power Semiconductors in Cabinets", Thermacore International Inc. (www.thermacore.com).
John Thayer, "Analysis of a heat pipe assisted heat sink", Thermacore International Inc. (www.thermacore.com).
Andre Ali, "Advanced heat pipe thermal solutions for higher power notebook computers", Intel Corporation (www.intel.com); Robert DeHoff, Kevin Grubb, Thermacore Inc. 1999 (www.thermacore.com).
Microjet cooling devices for thermal management of electronics
Kercher, D.S.; Jeong-Bong Lee; Brand, O.; Allen, M.G.; Glezer, A.;
Components and Packaging Technologies, IEEE Transactions on [see also Components, Packaging and Manufacturing Technology, Part A: Packaging Technologies, IEEE Transactions on] , Volume: 26 Issue: 2 , June 2003
Page(s): 359 -366
Backside cooling solution for high power flip chip multi-chip modules
Patel, C.D.;
Electronic Components and Technology Conference, 1994. Proceedings., 44th , 1-4 May 1994
Page(s): 442 -449
Thermal management issues and evaluation of a novel, flexible substrate, 3-dimensional (3-D) packaging concept
Brown, W.D.; Malshe, A.P.; Railkar, T.A.; Lenihan, T.G.; Stone, J.W.; Sommers, W.T.; Schaper, L.W.;
Multichip Modules and High Density Packaging, 1998. Proceedings. 1998 7th International Conference on , 15-17 April 1998
Page(s): 135 -140
Development of a thermal management solution for a ruggedized Pentium based notebook computer
Rujano, J.R.; Cardenas, R.; Rahman, M.M.; Moreno, W.A.;
Thermal and Thermomechanical Phenomena in Electronic Systems, 1998. ITHERM '98. The Sixth Intersociety Conference on , 27-30 May 1998
Page(s): 8 -14
Jerome Toth, Robert DeHoff, Kevin Grubb, "Heat pipes, the silent way to manage desktop thermal problems", Thermacore International Inc. 1998