Cooling device for an electronic component

A cooling device for cooling an electronic component. The device has an enclosure adapted to contain a liquid coolant. The enclosure has an internal channel system comprising a cavity adjacent the electronic component, a first group of arborizing channels adapted to carry the liquid coolant away from the cavity, a second group of arborizing channels adapted to carry the liquid coolant to the cavity, and a plenum fluidically connecting the first and seconds groups of arborizing channels. Each group of arborizing channels has a parent branch and multiple successive sets of daughter branches with successively smaller cross-sectional areas, wherein the sum of the cross-sectional areas of the daughter branches of any set is approximately the same as that of its parent branch. Distal sets of the daughter branches are most distant from the cavity, fluidically connected to the plenum, and have the smallest cross-sectional areas of the daughter branches.

BACKGROUND OF THE INVENTION

The present invention generally relates to cooling systems for electronic components. More particularly, this invention relates to a sealed cooling device with enhanced thermal management capabilities.

Cooling of electronic devices has become increasingly challenging as electronics have evolved. As manufacturing processes are constantly refined, the migration to smaller design processes and the incumbent reduction in operating voltage has not kept pace with the increased complexity of faster integrated circuits (ICs). Increasing number of transistors in combination with increasing operating frequencies has resulted in higher numbers of switching events over time per device. As a result, within the same market space and price range, ICs are becoming more and more sophisticated and power-hungry with every generation.

Compared to earlier generations, the implementation of smaller design processes has allowed the integration of more electronic building blocks such as transistors and capacitors on the same footprint. Consequently, area power densities have increased, resulting in smaller dies dissipating higher thermal load. As a result, formerly sufficient, passive heat spreaders and coolers often do not provide adequate cooling. While sophisticated fin designs and powerful fans increase the active surface area useable for offloading thermal energy to the environment, even extremely well designed coolers are hitting inherent limitations. In particular, significant limitations stem from the bottleneck of limited heat conductivity of the materials used, and specifically the fact that passive heat transfer throughout a solid structure is limited by the coefficient of thermal conductivity (CTE) of the material and the cross sectional area of the structure.

In a two-dimensional heat spreader of uniform thickness, the amount of thermal energy decreases as a square function of the distance from the source, where the thermal conductance coefficient of the material and the cross sectional area define the slope of the decrease. Therefore, even the most highly conductive material will not be able to maintain an even temperature distribution across the entire surface of the cooling device. Any gradient, on the other hand, will cause a decrease in cooling efficiency since the temperature difference (ΔT) between the cooler's surface and the environment is the primary limiting factor for thermal dissipation to the surrounding.

In view of the above, it is desired that coolers transfer heat from a heat source as quickly and efficiently as possible to optimize cooling efficiency for the heat source. In combustion engines, liquid cooling has become the method of choice, using the fact that a liquid (e.g., water) is taking up thermal energy and subsequently being pumped to a remote radiator where it releases the absorbed heat. In electronic devices, liquid cooling is still only marginally accepted for reasons that include the inherent risk of spills, cost overhead, and complexity of the installation, which involves routing of tubing and installation of radiators. Alternatively, some self-contained liquid cooling devices have been proposed and marketed.

Four primary factors defining the efficacy of a liquid cooling device are the uptake of heat by the cooling fluid at the heat source, the transport rate of the fluid away from the heat source, the offloading of heat to the solid components of the cooler, and finally the dissipation rate of heat into the environment. The exchange of heat between the heat source and the cooling fluid mainly depends on the surface area of the heat source that is exposed to the fluid in a direction normal to the plane of the heat source, for example, a semiconductor die. The exchange of heat between the fluid and the cooling device largely depends on the routing of the flow of the coolant within the device. If the channels are too wide, laminar flow can cause a decrease in efficacy of heat exchange between the fluid and the device. Therefore, it is desirable to have a capillary system to achieve an optimal surface to volume ratio. Such capillary systems have been referred to as microchannel systems.

One often overlooked problem with a nondescript microchannel system is that the hydraulics are poorly defined. If a pump simply pumps the fluid through the interstitial space without further routing in the form of macrochannels, then the centrifugal movement of the fluid can easily interfere with the centripetal flow that recycles the fluid back to the pump. A number of workaround possibilities have been proposed, among which is the separation of the centrifugal and the centripetal fluid movements into two individual planes that are separated by a septum. In other words, centrifugal flow of the fluid may occur within a lower plane whereas the centripetal “suction” of the fluid back to the pump may occur in an upper layer. This separates the centrifugal from the centripetal channel and as a consequence the pump does not have to work against itself.

A natural occurrence of such a “counter-flow” system is known as rete mirabile or wonder mesh in biomedical sciences. A relevant example in this context is the micro-vascularization in the feet of aquatic birds where, within less than one centimeter, the blood temperature drops in the arterial path from about 38° C. to the outside temperature, and then warms back up to body temperature in the venous path. The temperature change or rather exchange between the two paths occurs in an entirely passive manner, that is, without the addition of any extra energy. In other words, having liquids flow in opposite direction through adjacent capillary networks or microchannels can create the effect of a very efficient thermal isolation between two points. However, in the case of a cooling apparatus, such an effect is highly undesirable since heat would be trapped at its source despite all fluid movement. Consequently, it appears highly disadvantageous in a heat exchange apparatus to have flow and counter flow in immediate proximity, especially if the septum between outgoing and incoming channels is thermally conductive.

An online publication by C. Hammerschmidt, “IBM Technology Keeps Future Chips Cool,” EE Times Online, http://www.eetimes.com/news/semi/showArticle.jhtml;jsessionid=N3LUY22LJ4EB MQSNDLSCKHA?articleID=193402569 (visited 16 Nov. 2006), describes an approach by IBM using direct jet impingement. This technology uses water jets sprayed with several tens of thousands of micro-nozzles onto an integrated circuit as the primary cooling technique in combination with a tree-like branched return architecture also referred to as hierarchical channel system.

The use of microchannels for coolant fluids has been known for some time, as evidenced by U.S. Pat. No. 4,450,472 to Tuckerman et al. The preferred embodiment featured in this patent integrates microchannels into the die of a microchip to be cooled and coolant chambers. U.S. Pat. No. 5,801,442 also describes a similar approach. Still other approaches have focused on the combined use of coolant phase change (condensation) and microchannels, an example of which is U.S. Pat. No. 6,812,563. U.S. Pat. No. 6,934,154 describes a similar two-phase approach including an enhanced interface between an IC die and a heatspreader based on a flip-chip design and the use of a thermal interface material. U.S. Pat. Nos. 6,991,024, 6,942,018, and 6,785,134 describe electroosmotic pump mechanisms and vertical channels for increased heat transfer efficiencies. Variations of microchannel designs include vertical stacking of different orientational channel blocks as described in U.S. Pat. No. 6,675,875, flexible microchannel designs using patterned polyimide sheets as described in U.S. Pat. No. 6,904,966, and integrated heating/cooling pads for thermal regulation as described in U.S. Pat. No. 6,692,700.

The use of a mesh to increase surface contact area with a cooling fluid has also been proposed. For example, meshes have been employed as condensers in evaporative cooling systems. Meshes have also been employed in the context of increasing contact with a cooling fluid used to cool semiconductor devices, for example, as described in U.S. Pat. No. 5,719,444 to Tilton et al. Commonly-assigned U.S. Pat. No. 7,219,715 to Popovich, the contents of which are incorporated herein by reference, describes an alternative approach using a mesh or woven screen that is between and bonded to two foils that define a flow cavity. With this approach, the interstices between the warp and weft strands of the mesh, as well as the gaps between the strands and the bordering foils, allow the passage of a cooling fluid, providing direct contact with the fluid for heat absorption and transfer heat through the bonding contacts with the foils. Both Tilton et al. and Popovich describe the semiconductor device being immersed in the cooling fluid.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a cooling device that uses spatial and thermal separation between centrifugal and centripetal pathways in a channel system containing a fluid coolant. The device is particularly well suited for cooling heat sources from which heat can be conducted in a generally radial outward direction, as in the case of semiconductor devices.

The cooling device generally includes an enclosure adapted to contain a liquid coolant and comprising oppositely-disposed first and second walls. A surface portion of the first wall is adapted to attach the electronic component thereto. The enclosure has an internal channel system that includes a cavity adjacent the surface portion of the first wall, a first group of arborizing channels adapted to carry the liquid coolant away from the cavity, a second group of arborizing channels adapted to carry the liquid coolant to the cavity, and a plenum fluidically connecting the first and seconds groups of arborizing channels. Each of the first and second groups of arborizing channels has a parent branch and multiple successive sets of daughter branches with successively smaller cross-sectional areas, and the sum of the cross-sectional areas of the daughter branches of any set is approximately the same as that of the parent branch thereof. A distal set of the daughter branches of each of the first and second groups of arborizing channels is most distant from the cavity, fluidically connected to the plenum, and has the smallest cross-sectional areas of the daughter branches. Finally, a pump provides for movement of the liquid coolant through the first and second groups of arborizing channels.

The cooling device as recited above targets the immediate removal of a liquid heated by a heat source away from the heat source, and its distribution throughout a hierarchical channel system in the device. Outbound and inbound groups of channels are spatially separated in order to avoid thermal exchange between the groups of channels that would undesirably create a rete mirabile effect. Throughout the channel system, a substantially uniform cross sectional area of each generation of branches is maintained, thereby avoiding bottlenecks that would negatively impact the efficacy of the pump used to circulate the fluid and result in flow asymmetries.

In view of the above, advantages of the present invention include rapid removal of heat from a heat source, separation of cooler (inbound) and hotter (outbound) fluid channels, fractal arborization of channels for efficient area coverage, and maintenance of overall channel cross-sectional area through all generations of branching. The invention also offers the capability of a cost-effective cooling solution because of the capability of employing inexpensive materials, and good scalability into large-scale cooling devices.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a schematic overview of a channel system12defined in a cooling plate10adapted for cooling a semiconductor device14. The location of the device14relative to the channel system12is superimposed inFIG. 1, though it is to be understood that the device14is not required to be located within the channel system12. Instead, the device14may be limited to contact with an outer wall16(FIG. 2) of the cooling plate10that encloses the channel system12and separates the channel system12from the device14, or may be partially inserted into the channel system12through the wall16(FIG. 3). The channel system12is delimited by the outer wall16and a base wall46(FIGS. 2 and 3), as well as structures38A-38D,48, and50between the outer and base walls16and46, as discussed in more detail below. The channel system12can be defined in the base wall46of the plate10, such as by etching the base wall46, or can be patterned in a layer sandwiched between the outer and base walls16and46. Other construction and fabrication approaches could be foreseeably used, and are within the scope of this invention.

The channel system12is shown inFIG. 1as having a central cavity18aligned with the device14. The cavity18is fluidically connected to two primary outbound branches20and two primary inbound branches22. The flow directions within these branches20and22are identified with arrows, which point radially outward (outbound from the device14) or radially inward (inbound toward the device14), respectively. Each outbound and inbound branch20and22is fluidically connected to a set of smaller daughter branches24and26, which in turn are fluidically connected to progressively smaller sets of daughter branches28,30,32, and34. The smallest sets of daughter branches32and34are fluidically connected to an outer plenum36that surrounds the branches20,22,24,26,28,32, and34, and in turn is surrounded and enclosed by a peripheral wall48of the plate10. As a result, each outbound branch20is fluidically connected to the plenum36via eight different outbound flow routes, and each inbound branch22is fluidically connected to the plenum36via eight different inbound flow routes. Furthermore, the channel system12is divided into distinct and spatially separated groups, namely, the outbound (centrifugal) branches20and their respective sets of daughter branches24,28, and32and the inbound (centripetal) branches22and their respective sets daughter branches26,30, and34are all spatially separated from each other.

The parent branches20and22and their daughter branches24,26,28,30,32, and34are defined by a pattern of structures50within a circular-shaped interior region of the plate10. The multiple outbound flow routes formed by each branch20and its corresponding daughter branches24,28and32and the multiple inbound flow routes formed by each branch22and its corresponding daughter branches26,30and34can be readily devised to be of approximately equal length, since their radially outermost (distal) extents are at their junctions with the plenum36, and these junctions are equidistant from the cavity18as a result of the circular shape of the interior region containing the structures50. The pattern of structures50is represented as being symmetrical about at least two axes (a first axis passing through two oppositely-disposed radial walls38A and38C, and a second axis passing through two oppositely-disposed radial walls38B and38D), though such symmetry is not a requirement.

The plenum36is made up of four quadrants36A,36B,36C, and36D separated by the four radial walls38A,38B,38C, and38D, respectively. The plenum36(or at least each of its quadrants36A,36B,36C, and36D) contains a mesh40that defines what are termed herein microchannels. Fluid flow within each plenum quadrant36A,36B,36C, and36D is guided by spacers42. Based on the fluid flow directions indicated for the outbound and inbound branches20and22and the locations of the radial walls38A,38B,38C, and38D, the general flow direction through the microchannels of the quadrants36A and36C is in a counterclockwise direction and the general flow direction through the microchannels of the remaining quadrants36B and36D is in the clockwise direction, as indicated by the arrows superimposed within these quadrants36A-D.

As represented inFIGS. 2 and 3, the mesh40within the plenum36preferably has essentially the same thickness as the height of the plenum36. The peaks projecting from both sides of the mesh40are preferably bonded, such as by soldering or brazing, to the walls16and46of the plate10to establish a highly-conductive thermal contact between the mesh40and walls16and46. Bonding also serves to cross-link the walls16and46, which resists any shearing forces to which the walls16and46might be subjected and contributes additional mechanical stability and rigidity to the plate10. The warp and weft strands of the mesh40form interstices that are more or less freely penetrable by any fluid, yet define tortuous paths that avoid laminar flow conditions within the plenum36that would reduce the heat transfer rate between the cooling fluid, the walls16and46, and the mesh40.

The geometry of the channel system12shown inFIG. 1is generally two-dimensional, with a single-point heat source defined by the semiconductor device14, and an outer periphery towards which heat is conducted through the plate10. Analogous to two-dimensional flow of fluid from a single-point source, the flow of thermal energy from the device14is a square function of the dissipated power with the distance from the device14. In other words, as one doubles the distance from the device14, the area that can or needs to be serviced increases by a factor of four. The present invention provides an effective balance between this characteristic of two-dimensional thermal and fluid flow by the use of a technology called arborization, and using fractal algorithms. For a given increase in distance from the central cavity18(and therefore the device14aligned therewith), each parent branch20and22, each successive set of daughter branches24and26, and each successive daughter branches28and30splits into multiple daughter branches (hence, defining an arborization pattern), and the combined cross-sectional area of each set of daughter branches24,26,28,30,32, and34is approximately equal to the cross-sectional area of its respective parent branch20or22. In the example ofFIG. 1, each branch20splits into two daughter branches24each having roughly half the cross-sectional area of its parent branch20, each branch24splits into two daughter branches28each having roughly half the cross-sectional area of each branch24, and each branch28splits into two daughter branches32each having roughly half the cross-sectional area of each branch28. In this way, the total cross-sectional area within each inbound and outbound group of channels remains constant, regardless of the distance from the central cavity18, such that a constant flow rate is maintained throughout the system12at constant pressure. Even if the combined cross-sectional areas for a given set of daughter branches do not precisely follow this rule, the channel system12design will still provide for substantially even flow throughout the system12without imposing bottlenecks and consequent pressure gradients that negatively impact the efficacy of a pump used to circulate a fluid flowing through the system12.

In view of the above, the cooling plate10contains a hierarchical channel system12of multiple arborizing flow routes, defining an arborization pattern at whose outer periphery is the plenum36. The plenum36provides a globally defined flow direction within each plenum quadrant36A-D that transitions from the centrifugal (outbound) flow routes to the centripetal (inbound) flow routes, and the mesh40within the plenum36defines a microchannel system that optimizes the transfer of thermal energy from the fluid within the plenum36to the walls16and46of the plate10. Because the plenum36is divided into fluidically separated sections36A-D in the manner shown inFIG. 1, each individual quadrant36A-D is fluidically connected to one-half of the distal set32of daughter branches of one of the outbound groups of channels20,24,28and32, and to one-half of the distal set34of daughter branches of one of the inbound groups of channels22,26,30, and34.

From the foregoing, it should be understood that the number of groups of channels and the number of branches within each group is not critical, as long as multiple arborizing flow routes are defined and a substantially constant flow rate is maintained throughout the system12at substantially constant pressure.

The cooling plate10described above is well suited as a fluid cooling device for electronic components with high power densities. The hierarchical channel system12of the plate10routes a cooling fluid in a generally radial outward direction from the device14to the periphery of the cooling plate10, and subsequently collects the fluid from the periphery and returns the fluid to the device14. In the embodiment shown inFIG. 1, the centrifugal and centripetal groups of arborized channels within the channel system12are sufficiently spatially separated to the point where there is insubstantial thermal exchange between the heated outbound fluid (flowing in the centrifugal branches20,24,28, and32) and the cooled inbound fluid (flowing in the centripetal branches22,26,30, and34). The arborization pattern of the channel system12further allows the distribution of the heated fluid over a larger surface area than would be possible through a simple stub leading into the microchannel network within the plenum36.

Within the immediate vicinity of the device14, the material of the plate10is generally sufficient to conduct heat away from the device14, and therefore does not require (though may be provided with) microchannels similar to that provided by the mesh40within the plenum36. As noted above, with increasing distance from the device14, the arborization pattern of the channel system12becomes more pronounced to maintain essentially the same flow cross-sectional area. The surface areas of the walls16and46enclosing the branches20,22,24,26,28,30,32, and34act as the primary heat exchange interfaces between the plate10and the surrounding environment. Because the effective cross-sectional area of any given branch20or22and its daughter branches does not change, the velocity of fluid flow therethrough remains substantially constant and the fluid flows relatively quickly to the plenum36and its microchannels, which act as a secondary heat exchange interface with the surrounding environment.

The cooling fluid is preferably pumped through the channel system12with a pump52, such as a centrifugal pump, connected to either the central cavity18or the plenum36. The choice of pump is primarily dependent on the specific application since pressure and noise requirements need to be taken into consideration. In a closed hydraulic system such as the channel system12of the invention, any pumping or positive displacement of a fluid will be equal to the suction of the inbound path. Positive displacement is generally considered more efficient than suction, suggesting that the pump52is preferably coupled to the channel system12so that cooling fluid is drawn from the inbound branches22and discharged to the outbound branches20.

If a positive displacement pump is coupled to the channel system12to discharge fluid to the outbound branches20, the tortuosity of the outbound branches20and/or any or all of their daughter branches24,28, and32can be increased by, for example, inserting a mesh (not shown). The cross-sectional area of any higher order branch24,28, or32containing a mesh should be appropriately increased to compensate for the mesh, so that the sum of the microchannels created by the mesh interstices will match the cross-sectional area of the lower order branches20,24, and28from which the fluid flows. The desirability of adding or omitting a mesh in the outbound branches20,24,28, and32will depend on the specific design and application.

The inbound branches22and their daughter branches26,30, and34are primarily for collection of the cooled fluid from the plenum36. Within the higher order branches26,30, and34, a certain amount of heat exchange with the walls16and46is not only possible but also desirable for increased surface utilization of the cooling plate10. In the proximity of the semiconductor device14, however, it is advantageous to avoid excessive heat exchange with the walls16and46in order to maintain the lowest possible temperature of the coolant until it reaches the immediate area of the semiconductor device14.

As generally known in the art, suitable coolant fluids include liquids such as water, mineral spirits/oils, alcohols, and fluorocarbonate derivatives, though various other fluids could also be used, including air, vapor, etc., depending on the required temperature range of operation. For example, in extremely cold environments, a fluid with lower viscosity is a better choice than in extremely hot environments. Various other parameters for choosing a cooling fluid exist and are well known, and therefore will not be discussed in any further detail here.

The embodiment of the cooling plate10shown inFIGS. 1 and 2is self-contained and hermetically sealed to allow easy mounting of a wide variety of heat sources. Though a loss in thermal transfer is generally incurred where a heat source is hermetically sealed from the coolant used to cool the heat source, the greatest challenge of thermal management is more often the dissipation of heat from the cooling device into the surrounding environment rather than heat transfer from the heat source to the cooling device. Moreover, the current invention allows the device14to be attached directly to the outer wall16as shown inFIG. 2, as well as the integration of a heat slug (not shown) into or on the surface of the outer wall16. In the latter case, the slug may directly contact the semiconductor device14and wall16, and provide sufficient thermal capacitance to buffer transient temperature spikes of the semiconductor device14. Transfer of thermal energy from the heat slug (or any other structure in contact with the semiconductor device14) to the wall16can be augmented by a thermally conductive mesh between the slug and wall16. The inclusion of a mesh at the contact area between the semiconductor device14and the wall16also avoids the potential for localized boiling of the cooling fluid at the interior surface of the wall16opposite the device14, which could result in greatly reduced cooling efficacy of the cooling plate10.

As an alternative toFIG. 2,FIG. 3shows that it is possible to partially immerse the semiconductor device14in the cooling fluid through an opening in that portion of the outer wall16enclosing the central cavity18. For better heat uptake, a screen or mesh may be placed within the cavity18and in contact with the semiconductor device14.

Because the wall16of the cooling plate10only plays a supportive role in transferring heat away from the device14, the CTE of the material(s) used to form the cooling plate10, its walls16and46, and its internal structures50is less important than in cooling structures that rely solely on passive heat transfer. As such, a wider variety of materials could be used to form the cooling plate10and its components. Moreover, because the cooling plate10is hollow, the total amount of material used is substantially lower than in a comparable solid structure, resulting in reduced material costs for manufacturing the plate10. A related issue is the mechanical stability of the cooling plate10. Hollow structures generally exhibit only a minor reduction in rigidity as compared to a solid body of the same dimensions. The rigidity of the plate10is promoted as a result of the mesh40being bonded to both walls16and46. Consequently, the cooling plate10can be much lighter but yet nearly as strong and rigid as a solid heat spreader of comparable size. The outer surfaces of both walls16and46can be outfitted with fins (54inFIG. 3) in order to increase the effective surface area of the plate10and, thus, facilitate offloading of the heat to the surrounding environment.

While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the functions of the components of the cooling plate10could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, the plate10and its components could differ in appearance and construction from the embodiments shown in the Figures, and appropriate materials could be substituted for those noted. Therefore, the scope of the invention is to be limited only by the following claims.