Heat Pipe Thermal Ground Planes
with CTE Matching Increase Efficiency of Power-Generating Components
Advances in embedded technology have created a demand for higher performance, which in turn means more heat generation than ever before. One way to address this is with a heat pipe Thermal Ground Plane (TGP) directly attached to the electronic components. Effectively dealing with the heat allows use in high-performance applications, including those in military and aerospace industries.
Advances in electronics technology – including miniaturization, complexity, and the necessity for seamless integration with other existing systems – have created a demand for higher performance in increasingly smaller spaces. This higher performance also means the generation of more heat than ever before, and many electrical systems are pushing the limits of thermal management technology in cooling electronic components and allowing them to run more efficiently. In addition, solutions developed for the military and telecommunications industries must meet stringent volume requirements while at the same time perform under harsh temperatures that could range from -40 °C to +100 °C.
Effectively controlling heat in electronic components leads to new developments in high-performance applications, including those in the military, aerospace, telecommunications, and power generation industries. For example, more-electric-aircraft systems feature highly integrated electrical networks in avionics and aerospace applications. Thermal management is the key to their successful operation. An efficient way to address the challenges of thermal management in small, high-performance applications is a heat pipe Thermal Ground Plane (TGP).
Conventional Cooling Methods
Traditionally, solid metallic conductors and heat pipes have been used to cool electronic components. Other conventional alternatives include compartmentalized systems in which solid conduction and heat pipes are used within modules. Liquid cooling systems – in which pumps circulate a fluid that absorbs heat and transfers it into air – have been used as well. As electronic component capacity increases, further thermal advances that work in concert with the traditional thermal control approaches are needed.
In many conventional systems, electronic devices are mounted on copper alloy substrates using soft or hard die attach. But these copper alloy substrates, although inexpensive and reliable, possess thermal conductivity that is about half that of copper (~200 W/m•K) however they are closer in Coefficient of Thermal Expansion (CTE) to various semiconductor materials than more traditional thermal management materials such as pure copper and aluminum. Being based on conduction in a solid, the distance that these materials can transport heat with a given temperature difference is very limited; in addition, these solid materials may add significant mass to the overall system. Using a solid conductor like diamond can improve thermal conductivity and CTE matching, but it is very expensive. The best approach is a replacement material that has a high thermal conductivity and a matching CTE at a reasonable cost.
Heat Pipe Thermal Ground Planes
Heat pipes typically have three components: a vacuum-tight, sealed containment shell or vessel, working fluid, and a capillary wick structure. The wick allows the structure to develop the capillary action for the liquid returning from the condenser to the evaporator.
Heat pipes have been successfully used in various types of electronic systems for decades, for example, in highly efficient radar electronics and power supply cooling for next-generation naval destroyer radar systems. Heat pipes are inexpensive, reliable, and widely used in many electronic systems; because they have no moving parts, heat pipes are used in systems where expected system life is 20 years or more. They provide effective heat removal over long distances, as well as bendability, flexibility, routability, and reliable performance under demanding conditions such as high-g spin, up to 10-g, and stand up well to shock/vibration and freeze/thaw (1600 temperature cycles from -40˚C to +90˚C) requirements typical of military and aerospace applications. To increase customization, fluid and casing materials can also be bent and formed to fit certain custom applications.
A heat pipe TGP directly attaches to the electronic components. TGPs are thin, flat heat pipes, typically 1mm to 3mm thick, to which the electronic devices can be mounted directly, minimizing thermal interfaces. The heat pipe, a two-phase cooling device, has a small quantity of working fluid inside and the heat is absorbed by vaporizing that fluid. For most electronics cooling applications, water is the most commonly used working fluid; other fluids can be used in applications requiring operation at unusually hot or cold temperatures. A typical TGP would usually require less than 2grams of fluid. The heat pipe transfers the heat to a condenser region, where the condenser releases heat to a cooling medium such as air or a circulated liquid at a lower heat flux.
Benefits of the Heat Pipe TGP
The heat pipe TGP uses all the same reliable components as the construction of a regular heat pipe. It also employs a sintered powder metal wick structure that allows the heat pipe to provide the highest heat flux capability along with the greatest degree of freeze/thaw tolerance and insensitivity to gravitational orientation. In addition, the thin, flat structure makes an ideal substrate for mounting electronic devices in packages where vertical space is highly constrained.
The heat pipe TGP represents a proven two-phase cooling approach, where the benefits include very high effective thermal conductivity (500 W/m•K to 2,000 W/m•K or more, depending on the size of the TGP), extreme reliability, and no moving parts, electrodes, or need for external power, allowing the attached device to run cooler as a result of improved heat spreading. A reduction in junction temperature of 20-30°C through the introduction of a TGP heat spreader versus conduction in low-CTE materials is not uncommon. Because the TGP moves heat through the flow of evaporated working fluid, heat can be transported longer distances with less temperature difference than is possible in a solid conductor; this is the effect that makes the TGP a superior heat transport/heat spreading technology. Generally, the larger the TGP is, i.e. the longer the heat transport distance, the greater the effective thermal conductivity. For a solid material, the thermal conductivity is a fixed, intrinsic property, so transporting more thermal energy or transporting the same energy a longer distance requires greater temperature difference, which is undesirable for electronic systems.
Depending on the application, creating a heat pipe TGP with a matching CTE can improve cooling while increasing reliability through reduced junction temperature and reduced thermal stress in the bonding material between the TGP and semiconductor. In addition, the TGP substrate can be tailored to more closely match various semiconductor materials, including silicon (Si), silicon carbide (SiC), gallium arsenide (GaAs), and gallium nitride (GaN). Heat pipe TGP structures can be effective up to 350 W/cm2. New electronic systems utilizing the TGP-based thermal solution can run at a higher power density than previous systems based on thermal conduction in solid materials without increased weight or complexity.
Bright Future for Demanding Applications
In addition to using heat pipe TGPs for newly designed electrical components and device packages, designers that use heat pipe TGPs can improve existing electronic components that need to fit in tightly constrained spaces such as small hand held devices, avionics and laptop computers. Insertion of TGPs into existing systems can overcome the constraints of size and power that designers currently face while simultaneously reducing the operating temperature of the electronic components, or improving the performance of the current thermal management system for handling increased thermal dissipation. In essence, the cost savings is in the extension of the current product life without having to significantly re-engineer it for increased power.
Heat pipe TGPs, based on proven and reliable two-phase cooling technology, can be constructed using a variety of materials and in a range of sizes to surpass many thermal management challenges in various industries and applications. The thin, flat form factor of the TGP makes it ideal for directly mounting electronic devices for improved thermal dissipation and reduced interface resistance. From there, design engineers can thermally control electronic components for increasingly demanding military, avionic, and aerospace applications – including satellite radiator panels, target acquisition systems, remote wing electronics, and navigational avionics – more efficiently than with traditional cooling methods.
Nelson J. Gernert and has primarily worked on research and development contracts related to heat transfer and heat pipe technology in military, aerospace and commercial applications. He has published more than 50 technical papers and holds eight U.S. patents.
Dr. Mark North completed his Ph.D. in Mechanical Engineering at Cornell University. He has more than 20 years of experience with heat pipe technology. He is an Associate Editor of the Journal of Thermal Science Engineering and Applications and a member of the ASME K-16 committee. He has published more than 40 technical papers and holds six U.S. patents.
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