Patent Application
20080225489
This invention is concerned with techniques for thermal management of electronic devices and more particularly with high heat flux cooling technology for microelectronic systems.
Both the performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment, with a reduction in the temperature corresponding to an exponential increase in the reliability and life expectancy of the device. Therefore, long life and reliable performance of a component may be achieved by effectively controlling the device operating temperature within the design limits for the device. One of the primary devices employed for heat dissipation in microelectronic systems is a heat sink, which absorbs and dissipates heat from a microelectronic device using thermal contact, either direct or radiant. The heat sink is typically a metal structure in contact with the electronic component's hot surface, though in most cases a thin thermal interface material mediates between the two surfaces. Microprocessors and power handling semiconductors are examples of electronics that need a heat sink to reduce their temperature through increased thermal mass and heat dissipation, primarily by conduction and convection and, to a lesser extent, by radiation.
Heat sinks function by efficiently transferring thermal energy from an object at a relatively high temperature to a second object that is at a relatively lower temperature and that has a much greater heat capacity. The goal is to effect a rapid transfer of thermal energy that quickly brings the high temperature object into thermal equilibrium with the low temperature object. Efficient functioning of a heat sink relies on the transfer of thermal energy from the first object to the heat sink at a high rate and from the heat sink to the second object. The high thermal conductivity of the heat sink material, combined with its large surface area (often provided by an array of comb or fin like protrusions), results in the rapid transfer of thermal energy to the surrounding cooler air. Fluids (such as refrigerated coolants) and thermally efficient interface materials can ensure good transfer of thermal energy to the heat sink. Similarly, a fan may improve the transfer of thermal energy from the heat sink to the air.
Heat sink performance, by mechanisms including free convection, forced convection, and liquid cooling, is a function of material, geometry, and the overall surface heat transfer coefficient. Generally, forced convection heat sink thermal performance is improved by increasing the thermal conductivity of the heat sink materials, increasing the surface area (usually by adding extended surfaces, such as fins or foamed metal) and by increasing the overall area heat transfer coefficient (usually by increasing the fluid velocity, by adding fans, coolant pumps, etc.). In addition, heat sinks may be constructed of multiple components exhibiting desirable characteristics, such as phase change materials, which can store a great deal of energy due to their heat of fusion.
When the microelectronic device is substantially smaller than the base plate of a heat sink, there is an additional thermal resistance, called the spreading resistance, which needs to be considered. Performance figures generally assume that the heat to be removed is evenly distributed over the entire base area of the heat sink and thus do not account for the additional temperature rise caused by a smaller heat source. This spreading resistance could typically be 5 to 30% of the total heat sink resistance.
Heat pipes are another useful tool that in the thermal management of microelectronics. A heat pipe can transport large quantities of heat between hot and cold regions with a very small difference in temperature. A typical heat pipe consists of a sealed hollow tube made of a thermoconductive metal such as copper or aluminum. The pipe contains a relatively small quantity of a working fluid, such as water, ethanol or mercury, with a remainder of the pipe being filled with the vapor phase of the working fluid. The advantage of heat pipes is their great efficiency in transferring heat.
The demands made on the thermal management of microelectronic systems are increasing with smaller form factors, elevated power requirements and increased bandwidth being established for next generation electronic systems. High power density, wide bandgap technology, for example, exhibits an extremely high heat flux at the device level. In addition, composite structures have low thermal mass and are not effective conductors of heat to heat sinks. The design of low cost COTS (commercial off the shelf) electronics frequently increases heat dissipation, and modern electronics is often packaged with multiple heat sources located close together. Some systems have local hot spots in particular areas, which induce thermal stress and create performance degrading issues.
These changes are resulting in an increase in the average power density, as well as higher localized power density (hot spots). As a result, the dissipation power density (waste heat flux) of electronic devices has reached several kwatts/cm2 at the chip level and is projected to grow much higher in future devices. Management of such power densities is beyond the capability of traditional cooling techniques, such as a fan blowing air through a heat sink. Indeed, these power densities even exceed the performance limits of more advanced heat removal techniques, such as a liquid coolant flowing through a cold plate. A common practice to address heat spreading issues is to adopt highly conductive bulk materials or to incorporate a heat pipe as the heat spreader. These approaches, however, involve heavy components, the thermal conductivity may be too low, mechanical strength can be a limiting factor, and the heat flux may be too low. Consequently, some new electronic devices are reaching the point of being thermally limited. As a result, without higher performance thermal management systems, such devices may be hampered by being forced to operate at part of their duty cycle or at a lower power level.
Improvements are needed to increase the heat transfer coefficient, as well as to reduce the spreading resistance, primarily in the base of the heat sink. Advanced high heat flux liquid cooling technologies, based on phase change heat transfer, are needed to satisfy requirements for compact, light weight, low cost, and reliable thermal management systems.
A heat spreader for transferring heat from a heat source to a heat sink, using a phase change coolant, includes microporous wicks for supporting flows of the coolant in the liquid phase, via capillary action, within the spreader from proximate the heat sink to proximate the source and macroporous wicks for supporting flows of the coolant, in the liquid and vapor phase, within the spreader from proximate the source to proximate the heat sink.
The microporous wicks may be microporous nanotube wicks, while the heat spreader may be configured for positioning between a substantially planar surface of the heat source and a substantially planar surface of the heat sink, with the nanotube wicks oriented substantially perpendicular to the planar surfaces, substantially parallel to the planar surfaces, or both substantially perpendicular and substantially planar to the surfaces.
The microporous nanotube wicks may, in a particular embodiment, be microporous acid treated carbon nanotube wicks. The heat spreader may further include support structure for positioning the spreader between the heat source and the heat sink, the macroporous wicks being passageways extending through the support structure. The support structure may be silicon support structure.
In more particular embodiments, the effective size of the microporous wicks is between approximately 10 nm and approximately 1,000 nm in radius, while the macroporous wicks may be sized between approximately 1 um and approximately 500 um in radius.
Advantageously, the microporous wicks, the macroporous wicks, and the coolant of the heat spreader are configured to remove substantially all of the heat generated by the heat source, thereby maintaining the heat source at a constant temperature. The heat source will typically be a microelectronic device.
The invention also encompasses a heat spreader with a plurality of cells, each cell including at least one microporous wick for supporting flows of the coolant in the liquid phase, via capillary action, within the spreader from proximate the heat sink to proximate the source and at least one macroporous wick for supporting flows of the coolant, in the liquid and vapor phase, within the spreader from proximate the source to proximate the heat sink.
In a particular embodiment, each cell is hexagonal in cross section.
A method of transferring heat from a heat source to a heat sink, using a phase change coolant, includes, according to the invention, providing a plurality of microporous wicks for supporting flows of the coolant in the liquid phase, via capillary action, within the spreader from proximate the heat sink to proximate the source, allowing the liquid coolant to absorb heat from the heat source via vaporization, providing macroporous wicks for supporting flows of the coolant, in the liquid and vapor phase, within the spreader from proximate the source to proximate the heat sink, and allowing the vaporized coolant to condense to the liquid phase via proximity to the heat sink.
A microelectronic system, according to the invention, includes a microelectronic device, a heat sink, and a heat spreader for transferring heat from a heat source to a heat sink using a phase change coolant, the heat spreader including microporous wicks for supporting flows of the coolant in the liquid phase, via capillary action, within the spreader from proximate the heat sink to proximate the source and macroporous wicks for supporting flows of the coolant, in the liquid and vapor phase, within the spreader from proximate the source to proximate the heat sink.
As depicted by
In addition, the cavity includes multiple macroporous wicks, such as, for example, the wicks 130, 132, and 134, to support flows of the coolant, in both the liquid and vapor phases, including liquid/vapor mixtures, from the source to the heat sink.
In one embodiment, the microporous wicks are microporous nanotube wicks and, in particular, may be microporous carbon nanotube wicks. Carbon nanotube wicks are typically individually grown in the spreader in areas near the heat source or attached to the macrowicks in such areas. Moreover, as depicted in
The nanotube wicks may be oriented substantially perpendicular to the planar surfaces, as depicted by the wicks 118, 120, and 122, or the wicks may be oriented substantially parallel to the planar surfaces, as depicted by the wicks 124, 126, and 128. Alternatively, the wicks may include, as in the embodiment depicted in
In more particular embodiments of the heat spreader, the effective pore size of the microporous wicks is very small, with a high flow resistance, and will range between approximately 10 nm and 1,000 nm in radius. This provides a high capillary pressure for liquid pumping. Microporous nanotube wicks, when grown on an internal surface of the heat spreader, will typically range in height from approximately 100 to 2,000 microns. The microwicks will preferably be connected to the macrowicks to provide a continuous supply route for liquid coolant. When the microwicks are attached to the macrowicks, the microwicks will typically range in height from 1 to 1,000 microns. The pore size of the macroporous wicks will range between approximately 1 and 500 microns.
The heat spreader may include, in addition, support structure for positioning the spreader between substantially planar surfaces of the heat source and the heat sink. This embodiment is depicted in
Each cell made of silicon or metal materials may include, in one approach to fabrication, an upper piece and a lower piece, symmetrical in geometry. Both the upper and lower pieces could be gold bonded, then reinforced by epoxy poured into a pre-etched cavity. The heat spreader structure can be, for example, a non-metallic material, such as silicon, SiC or SiNa, or a metallic material, such as copper, aluminum or silver. For a non-metallic structure, the fabrication process would typically use a dry or wet etch MEMS (microelectromechanical system) process. For a metallic structure, fabrication process would typically employ the sintering of metal particles.
The macroporous wicks establish passageways that extend through the cellular support structure in a direction substantially parallel to the planar surfaces. Although the scale of
As shown in
Only a very small amount of liquid coolant is needed, to cover the wick structure. The cavity is primarily occupied by saturated coolant vapor. The macroparticles incorporate relatively large pores, to reduce pressure losses in the liquid flow attributable to viscosity, while the microwicks generate large capillary forces to circulate the liquid coolant within the spreader, without the need for an external pump.
The phase change involves the absorption and release of a large amount of latent heat at the evaporation and condensation regions of the spreader. With the proper sizing of components, this allows the heat spreader of this invention to operate with no net rise in temperature. This mechanism, which is the cornerstone of modern heat pipe technology, is very efficient for heat transfer. The incorporation of nanotechnology in this invention allows heat pipe technology to advance to a new level of performance and to be integrated into a multifunctional structural material, making possible a significant increase in the thermal mass of composite structures.
The preferred embodiments of this invention have been illustrated and described above. Modifications and additional embodiments, however, will undoubtedly be apparent to those skilled in the art. Furthermore, equivalent elements may be substituted for those illustrated and described herein, parts or connections might be reversed or otherwise interchanged, and certain features of the invention may be utilized independently of other features. Consequently, the exemplary embodiments should be considered illustrative, rather than inclusive, while the appended claims are more indicative of the full scope of the invention.
This application claims the benefit under 35 USC 119(e) of U.S. Provisional Patent Application No. 60/854,007, filed Oct. 23, 2006.
The United States Government has rights in this invention pursuant to a contract awarded by the Defense Advanced Research Projects Agency.
| Number | Date | Country | |
|---|---|---|---|
| 60854007 | Oct 2006 | US |
