Patent Application
20140202665
The present application relates generally to thermal management. It finds particular application in conjunction with cooling heat-producing bodies, such as integrated circuits, and will be described with particular reference thereto. However, it is to be appreciated that the present application is also amenable to other like applications.
Thermal management for electronics packaging is an active area of research given the increasing demands for high power density and three-dimensional (3D) integrated circuit (IC) architectures. With reference to
Most approaches for electronics gaseous working fluid thermal management are focused on back-side thermal management. An example of such an approach is described in U.S. Patent Application Publication No. 2005/0280162 to Mok et al. which describes an integrated vapor chamber thermal interposer on the backside of an IC. However, increasingly, 3D architectures (e.g., with heat fluxes approaching 100 watts per square centimeter (W/cm2)) have made this approach insufficient.
3D die stacks have severe problems with hotspots on each of the layers. Spreading this heat is a major challenge since the typical combination of solder balls, (relatively) low thermal conductivity filler and copper-filled through silicon vias (TSVs) does not provide a sufficiently conductive thermal path. Further, spreading the heat only partially solves the problem as the heat still needs to be dissipated to the environment.
One proposed solution is described in U.S. Patent Application Publication No. 2010/0044856 to SRI-JAYANTHA et al. SRI-JAYANTHA et al. modifies the active side or lateral thermal path to improve heat transfer away from the 3D IC using an integrated thermal interposer that utilizes either an additional spreader layer on the backside or a very complex micro channel cooler. This former realization demonstrates fairly modest thermal improvement, while the latter entails considerable complexity in terms of fluidic sealing and pumping.
Another proposed solution is to use thermoelectric thermal bump, examples of which are shown in
Yet another proposed solution is thin film evaporation. It uses a phase change process such as boiling, but mitigates some of the well-known shortcomings of boiling. Specifically, thin film evaporation mitigates the amount of superheat required to initiate boiling, the unpredictability of boiling nucleation sites, handling of combined vapor-liquid flow after boiling (in a flow system) and the critical heat flux (CHF) in which the hot surface dries out if the heat flux is too high.
The foregoing is described in Ohadi et al., “Ultra—Thin Film Evaporation (UTF)—Application to Emerging Technologies in Cooling of Microelectronics”, Microscale Heat Transfer Fundametals and Applications, pages 321-338 (2005):
The second reason can be clarified using the relationship between heat transferred, thermal resistance and temperature drop, and the definition of the heat transfer coefficient. The heat transfer coefficient R can be defined by Q*R=DT, where Q is heat transferred in watts (W), R is in degrees Celsius per watt (C/W) and DT is temperature drop in degrees Celsius (C). A high thermal resistance means less heat is transferred and there is a large temperature gradient across the material of interest. The thermal resistance and heat transfer coefficient R are related by the relationship R=L/(kA) for conduction, where L is the conduction path length, A is the heat transfer area, and k is the thermal conductivity.
Additional heat dissipation due to evaporation is what makes thin film evaporation particularly compelling. The amount of heat Q that can be removed at the vapor-liquid interface is Q=m(ΔHv), where m is the mass flow of evaporating liquid and (ΔHv) is the latent heat of vaporization of the refrigerant. Taking a 2 centimeter (cm)×2 cm die, as typical of integrated circuit packages, and a thermal density of 25 W/cm2, the heat to transfer is 100 W. Using water as a heat transfer fluid, the latent heat is 2260 joules per gram (J/g), so the amount of mass flux is 25/2260=0.01 grams per second (g/s) or 10 microliters per second (μL/s) of fluid must be evaporated to dissipate this much heat.
In view of this, it is clear that large heat transfer area A and small conduction path length L provide a small conduction resistance through the thin film, which increases the amount of heat transferred to the phase change interface. A challenge in thin film evaporator design is feeding the thin film with enough material to match the evaporation rate for high heat fluxes. One solution is to employ electrohydrodynamics (EHD) polarization pumping to draw a thin film of dielectric liquid along a hot surface. However, this solution suffers from orientation dependence and the resulting film is not especially thin. Even so, this solution has been shown to be able to transfer heat fluxes of up to 40 W/cm2.
It is well-known in the heat pipe community that a significant fraction of heat transfer in the evaporator section of heat pipes occurs in the thin film region where conductive losses are low and evaporation rates are highest. As such, there has been significant work on using wicking structures to maximize the thin film region. A realization of such work combines the application of an actively pumped microchannel cooler with a porous membrane for evaporation. The evaporation rate is augmented with air jet impingement to further improve the heat transfer. This realization demonstrates the ability to dissipate a heat flux of 500 W/cm2. However, this realization requires external infrastructure for pumping and a relatively large thin film. Roughly 85% of the heat transfer is due to the forced convection in the microchannel.
The present application provides new and improved methods and systems which overcome the above-referenced challenges.
In accordance with one aspect of the present application, a heat dissipation device to provide thermal spreading and cooling for a heat-producing body is provided. A thin film evaporator in thermal communication with the heat-producing body removes heat from the heat-producing body using a working fluid. A heat pipe integrated with the thin film evaporator, and extending from the thin film evaporator, dissipates heat removed by the thin film evaporator to the external environment of the heat dissipation device. A pumping element at least one of: 1) pumps working fluid to the thin film evaporator; and 2) augments transfer of working fluid to the thin film evaporator.
In accordance with another aspect of the present application, a heat dissipation method to provide thermal spreading and cooling for a heat-producing body is provided. By a thin film evaporator in thermal communication with the heat-producing body, heat from the heat-producing body is removed using a working fluid. By a heat pipe integrated with the thin film evaporator and extending from the thin film evaporator, heat removed by the thin film evaporator is dissipated to the external environment of the heat dissipation device. By a pumping element, at least one of: 1) working fluid is pumped to the thin film evaporator; and 2) transfer of working fluid to the thin film evaporator is augmented.
In accordance with another aspect of the present application, a heat dissipation device to provide thermal spreading and cooling for a heat-producing body is provided. A sealed housing includes a fluid reservoir of working fluid in liquid phase and a vapor chamber, the heat-producing body thermally coupled to an external surface of the sealed housing. A thin film evaporator is within the sealed housing and in thermal communication with an internal surface of the sealed housing adjacent the external surface. The thin film evaporator receives working fluid in liquid phase from the fluid reservoir and vaporizes the received working fluid to working fluid in gaseous phase using heat from the heat-producing device. A heat pipe within the sealed housing transfers the working fluid in gaseous phase away from the thin film evaporator, condenses the working fluid in gaseous phase to liquid phase, and returning the condensed working fluid to the fluid reservoir. A pumping element at least one of: 1) pumps working fluid to the thin film evaporator; and 2) augments transfer of working fluid to the thin film evaporator.
This present application combines an actively driven thin film evaporator for spreading heat with an integrated planar heat pipe extended surface for heat sinking. The thin film evaporator allows for a high rate of heat removal to remove hot spots, and the integrated planar heat pipe transports heat from the thin film evaporator to an extended surface for dissipation to the environment or sinking to an interposer layer.
With reference to
The heat dissipation device 10 includes a sealed housing 14, which is constructed of a thermally conductive material. The thermally conductive material can, for example, include one or more of copper, copper foil, copper alloys, aluminum, aluminum alloys, polyimides, metals, and the like. The sealed housing 14 can be flexible and seals in a working fluid 16 (
An external surface 18 of an interface portion 20 of the sealed housing 14 thermally contacts the heat-producing body 12. For example, the external surface 18 can directly contact the heat-producing body 12. As another example, the external surface 18 can indirectly contact the heat-producing body 12 by way of a substrate upon which the heat-producing body 12 rests or a thermal interface material intermediate the heat-producing body 12 and the external surface 18. Typically, the interface portion 20 is formed from copper, copper foil, copper alloys, aluminum, or aluminum alloys, but other materials are contemplated.
Extending away from the interface portion 20, the sealed housing 14 further includes one or more extended portions or fins 22 (two as illustrated). As discussed hereafter, the extended portions 22 are used to convey heat into the external environment, typically by convection, or to sink heat to an interposer layer. The extended portions 22 are typically formed from a flexible polyimide or metallic substrate to allow the extended portions 22 to be shaped into a desired form factor after manufacture, but other materials are contemplated.
With reference to
The fluid reservoir 26 holds working fluid 16 in the liquid phase (i.e., liquid working fluid 32), and the vapor chamber 24 holds working fluid 16 in the gaseous phase (i.e., gaseous working fluid 34 shown by the arrows in the vapor chamber 24). The working fluid 16 can include, for example, water, Freon, acetone, alcohol, and the like. As described hereafter, the working fluid 16 is employed to transfer heat away from the heat-producing body 12 through the sealed housing 14 using thin film evaporation, where the extended portions 22 act as heat pipes. In this way, heat fluxes of 10 to 1000 Watts per square centimeter (W/cm2) or more can be managed.
The heat dissipation device 10 includes a thin film evaporator 36 for evaporating liquid working fluid 32 from the fluid reservoir 26 into gaseous working fluid 34 with heat from the heat-producing body 12. Typically, the thin film evaporator 36 is actively driven to ensure sufficient transfer of working fluid 16 to cool the heat-producing body 12, as discussed above, but it can also be passive. For example, the thin film evaporator 36 can be actively driven when the heat-producing body 12 is producing heat exceeding a predetermined threshold and passively driven when the heat-producing body 12 is producing heat less than the predetermined threshold.
The thin film evaporator 36 includes an evaporator wick 38 in thermal contact with the internal surface 28 of the interface portion 20 of the sealed housing 14 and within the vapor chamber 24. Typically, the surface area of the evaporator wick 38 in contact with the internal surface 28 is approximately (i.e., +/−5%) equal to, or greater than, the surface area of the heat-producing device 12 in contact with the external surface 18. The evaporator wick 38 receives liquid working fluid 32 from the fluid reservoir 26 and disperses the liquid working fluid 32 substantially uniformly on the internal surface 28 of the interface portion 20 of the sealed housing 14 to form a thin layer 40 of liquid working fluid 32. Suitably, the evaporator wick 38 is engineered to maximize the extent of capillary wicking and the area of the thin layer 40.
The heat dissipating device 10, particularly the thermal coupling with the heat-producing body 12, is designed to allow sufficient transfer of heat to the thin layer 40 of liquid working fluid 32 to cool the heat-producing body 12. The heat transferred from the heat-producing body 12 to the thin layer 40 of liquid working fluid 32 is dictated by the heat transfer coefficient R for conduction, where R=L/(kA), L is the conduction path length, A is the heat transfer area, and k is the thermal conductivity. The greater the heat transfer coefficient R, the less transfer of heat. Hence, the greater the area of the thin layer 40 of liquid working fluid 32 and the thermal conductivity of the material intermediate the heat-producing body 12 and the thin layer 40 of liquid working fluid 32, the greater the heat transfer. Similarly, the less the conduction path length, the greater the heat transfer.
A feed conduit 42 of the sealed housing 14 extends between the fluid reservoir 26 and the evaporator wick 38 to provide liquid working fluid 32 to the evaporator wick 38 from the fluid reservoir 26. The evaporator wick 38 draws liquid working fluid 32 from the fluid reservoir 26 by way of the feed conduit 42 using capillary action. This capillary action also serves to disperse the liquid working fluid 32 on the internal surface 28 of the interface portion 20 of the sealed housing 14. The greater the dispersion of liquid working fluid 32, the greater the transfer of heat from the heat-producing body 12. Additional feed channels are also contemplated to improve the transfer of liquid working fluid 32 to the evaporator wick 38.
One or more synthetic jets 44 (two as illustrated) within the sealed housing 14 can be employed to improve the transfer of liquid working fluid 32 to gaseous working fluid 34 by removing gaseous working fluid 34 from the vapor chamber 24 and cooling the evaporator wick 38 to allow greater dispersion of the liquid working fluid 32 before evaporation. Typically, the synthetic jets 44 include a plurality of synthetic jets arranged in a grid or other two-dimensional arrangement to cool, and/or remove gaseous working fluid 34 from, the whole of the evaporator wick 38. Power is provided to the synthetic jets 44 by way of corresponding wires 46 and power sources 48.
The synthetic jets 44 create a series of vortex rings of gaseous working fluid 34 in the vapor chamber 24 using corresponding orifices 50 and corresponding oscillating actuators 52. The axes of the vortex rings are suitably perpendicular to the internal surface 28. Suitably, the oscillating actuators 52 are piezoelectric actuators (illustrated), but other oscillating actuators are contemplated. While any configuration of the synthetic jets 44 is contemplated, the oscillating actuators 52 of the synthetic jets 44 typically oscillate corresponding diaphragms along the axes of the vortex rings. The oscillating actuators 52 can be the diaphragms (e.g., piezoelectric diaphragms), as illustrated, or merely oscillate the corresponding diaphragms.
The orifices 50 typically include corresponding open ends 54 through which the vortex rings enter the vapor chamber 24 from the orifices 50. The oscillating actuators 52 can be, for example, positioned within the orifices 50 to push vapor within the orifices 50 out the open ends 54. The orifices 50 can further include additional corresponding open ends 56 opposite the open ends 54 through which the vortex rings enter the vapor chamber 24 from the orifices 50. The oscillating actuators 52 can then be, for example, positioned at the additional open ends 54 to create the vortex rings using, for example, diaphragms spanning the additional open ends 54.
The oscillating actuators 52 can also be employed to pump liquid working fluid 32 to the evaporator wick 38 by way of the feed conduit 42 to ensure that sufficient liquid working fluid 32 is provided to the evaporator wick 38 to prevent dry out of the thin layer 40 of liquid working fluid 32. Typically, the oscillating actuators 52 are out of plane (i.e., oscillate perpendicular to the direction flow of the liquid working fluid 32). In such instances, it's important to ensure that the liquid working fluid 32 can only flow in the direction of the feed conduit 42. Other approaches to pumping the liquid working fluid 32 can also be employed.
One approach to pump liquid working fluid 32 using the oscillating actuators 52 is to employ corresponding diaphragms with the oscillating actuators 52. As noted above, the oscillating actuators 52 can be the diaphragms (illustrated) or merely oscillate the corresponding diaphragms. In such an approach, the diaphragms partially define the wall of the fluid reservoir 26 and oscillate in and out of the fluid reservoir 26. Typically, the oscillations are perpendicular to the thin film evaporator 36 and the flow of liquid working fluid 32. As the diaphragms moves in to the fluid reservoir 26, the diaphragms pump liquid working fluid 32. As the diaphragms move out of the fluid reservoir 26, the diaphragms creates the above described vortex rings.
With specific reference to
While the thin film evaporator 36 employs the evaporator wick 38 for receiving and dispersing the liquid working fluid 32, other approaches for receiving and dispersing the liquid working fluid 32 can be employed. For example, a wickless approach or an electrohydrodynamics (EHD) polarization pumping in conjunction with an electrically conductive wick can be employed. As another example, the synthetic jets 44 can spray the liquid working fluid 32, as described above, on to the internal surface 28 to create the thin layer 40 of liquid working fluid 32 without the evaporator wick 38. As another example, the thin film evaporator 36 can work without the synthetic jets 44, but optionally with the oscillating actuators 52 pumping liquid working fluid 32 as described above.
The thin film evaporator 36 and/or the synthetic jets 44 need to be designed to transfer and disperse a sufficient amount of liquid working fluid 32 to remove the heat transferred by the heat-producing body 12 to the thin layer 40 of liquid working fluid 32. The amount of heat Q that can be removed at the thin layer 40 of liquid working fluid 32 is Q=m(ΔHv), where m is the flow of evaporating liquid work fluid 32 and (ΔHv) is the latent heat of vaporization of the working fluid 16. Hence, thin film evaporator 36 and the synthetic jets 44 are designed around this equation.
As the thin layer 40 of liquid working fluid 32 evaporates, the gaseous working fluid 34 is transported to the extended portions 22, typically to the distal ends 30 of the extended portions 22, by way of the vapor chamber 24. The synthetic jets 44 facilitate transport of the gaseous working 34 fluid to the extended portions 22 by pushing the gaseous working fluid 34 to the extended portions 22. Within the extended portions 22, the gaseous working fluid 34 dissipates and condenses back into liquid working fluid 32.
Adjacent the vapor chamber 24, each extended portion 22 includes a return wick 60 at least extending from the corresponding distal ends 30 to the fluid reservoir 26 and typically lining the extended portion 22. The return wicks 60 capture working fluid 16 as it condenses back to liquid and return it to the fluid reservoir 26, typically using capillary action. In this way, the extended portions 22 can be viewed as planar heat pipes.
The design of the return wicks 60 is important to the successful operation of the heat dissipation device 10. The flow of working fluid 16 through the return wicks 60 must be sufficient to complete the working fluid recirculation loop (shown by the arrows). The return wicks 60 are multi-layer wicks with engineered hydrophobic condensation surfaces and a sub-layer of feed channels that return the liquid working fluid 32 to the fluid reservoir 26. With reference to
Different approaches to returning the liquid working fluid 32 to the fluid reservoir 26 can also be employed. For example, a wickless approach or an electrohydrodynamics (EHD) polarization pumping in conjunction with an electrically conductive wick can be employed.
The coupling of a heat pipe to a thin film evaporator, as described above, is to be contrasted with conventional thermal packaging arrangement in which the heat sink is connected to a spreader by way of a thermal interface material (See
With reference to
As illustrated, the evaporator wick 38 receives liquid working fluid 32 using capillary action and/or the synthetic jets 44 from the fluid reservoir 26. Using this liquid working fluid 32, the evaporator wick 38 creates the thin layer 40 (not shown in
Within the extended portions 22, the gaseous working fluid 34 cools and condenses back to liquid working fluid 32. This liquid working fluid 32 can collect at corresponding capture reservoirs 68 at the distal ends 30 of the extended portions 22 and/or be collected by the return wicks 60. The return wicks 60 return liquid working fluid 32 collected thereby and/or from the capture reservoirs 68 to the fluid reservoir 26, typically using capillary action. In this way, the working fluid 34 follows a fluid recirculation loop, which is shown by the arrows.
In some embodiments, additional or alternative approaches for removing gaseous working fluid from the interface portion can be employed. For example impinging jets can be employed. Further, in some embodiments, additional or alternative approaches to spreading heat in the extended portions 22 can be employed. For example, although less efficient than the preferred realization, the extended portions 22 can include pulsating heat pipes (PHPs). Such embodiments employing pulsating heat pipes would be limited by the conduction contact area between the PHPs and the vapor chamber.
With reference to
Referring to
The heat exchangers 82 receive gaseous working fluid 34 from the vapor chamber 24. Within the heat exchangers 82, the heat from the gaseous working fluid 34 is absorbed by the PHPs 80, which transfer the absorbed heat to the distal ends 30 of the extended portions 22 for dissipation to the external environment. As the heat is absorbed by the PHPs 80, the gaseous working fluid 34 condenses back to liquid working fluid 32 and is returned to the fluid reservoir 26.
With reference to
“Drop-wise condensation” is generally desired since it gives higher heat fluxes. This is encouraged by coating and/or encapsulating the portions of the PHPs 80 extending into the heat exchangers 82 with hydrophobic material 86. For example, a thin layer of Polytetrafluoroethylene (PTFE), such as that found on a nonstick cooking pan, can coat these portions of the PHPs 80. As illustrated, the PHPs 80 are encapsulated in hydrophobic material 86 and cause droplets 88 of liquid working fluid 32 to form and fall into the capture reservoirs 82.
To return the condensed working fluid 32 to the fluid reservoir 26, each of the extended portions 22 includes a return wick 90 extending from the corresponding capture reservoir 84 to the fluid reservoir 26. The return wick 90 uses capillary action as described above to transfer the liquid working fluid 32 in the captured.
It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
