# Patent

## Title
Heat sink structure with a vapor-permeable membrane for two-phase cooling

## Abstract

A heat sink, and cooled electronic structure and cooled electronics apparatus utilizing the heat sink are provided. The heat sink is fabricated of a thermally conductive structure which includes one or more coolant-carrying channels coupled to facilitate the flow of coolant through the coolant-carrying channel(s). The heat sink further includes a membrane associated with the coolant-carrying channel(s). The membrane includes at least one vapor-permeable region, which overlies a portion of the coolant-carrying channel(s) and facilitates removal of vapor from the coolant-carrying channel(s), and at least one orifice coupled to inject coolant onto at least one surface of the coolant-carrying channel(s) intermediate opposite ends of the channel(s).

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. application Ser. No. 13/189,597 entitled “HEAT SINK STRUCTURE WITH A VAPOR-PERMEABLE MEMBRANE FOR TWO-PHASE COOLING”, filed Jul. 25, 2011, which published Jan. 31, 2013, as U.S. Patent Publication No. 2013/0027878 A1, and which is hereby incorporated herein by reference in its entirety.


BACKGROUND
The power dissipation of integrated circuit chips, and the modules containing the chips, continues to increase in order to achieve increases in processor performance. This trend poses a cooling challenge at both the module and system level. Increased airflow rates are needed to effectively cool high power modules and to limit the temperature of the air that is exhausted into the computer center.
In many large server applications, processors along with their associated electronics (e.g., memory, disk drives, power supplies, etc.) are packaged in removable node configurations stacked within a rack or frame. In other cases, the electronics may be in fixed locations within the rack or frame. Typically, the components are cooled by air moving in parallel airflow paths, usually front-to-back, impelled by one or more air moving devices (e.g., fans or blowers). In some cases it may be possible to handle increased power dissipation within a single node by providing greater airflow, through the use of a more powerful air moving device or by increasing the rotational speed (i.e., RPMs) of an existing air moving device. However, this approach is becoming problematic at the rack level in the context of a computer installation (i.e., data center).
The sensible heat load carried by the air exiting the rack is stressing the ability of the room air-conditioning to effectively handle the load. This is especially true for large installations with “server farms” or large banks of computer racks close together. In such installations, liquid cooling (e.g., water cooling) is an attractive technology to manage the higher heat fluxes. The liquid absorbs the heat dissipated by the components/modules in an efficient manner. Typically, the heat is ultimately transferred from the liquid to an outside environment, whether air or other liquid coolant.
BRIEF SUMMARY
In one aspect, provided herein is a heat sink comprising a thermally conductive structure, with at least one coolant-carrying channel, and a membrane structure associated with the at least one coolant-carrying channel. The membrane structure includes at least one vapor-permeable region. At least a portion of the at least one vapor-permeable region overlies a portion of the at least one coolant-carrying channel and facilitates removal of vapor from the at least one coolant-carrying channel. The membrane structure further includes at least one orifice coupled to inject coolant onto at least one surface of the at least one coolant-carrying channel intermediate ends of the at least one coolant-carrying channel. The heat sink further includes: at least one vapor transport channel in fluid communication with the at least one vapor-permeable region of the membrane structure, the at least one vapor transport channel facilitating exhausting of vapor egressing from the at least one vapor-permeable region of the membrane structure from the at least one coolant-carrying channel; at least one coolant exhaust channel in fluid communication with the at least one coolant-carrying channel and facilitating exhausting of coolant from the at least one coolant-carrying channel; and a coolant exhaust outlet port coupled in fluid communication with the at least one coolant exhaust channel, and a vapor outlet port coupled in fluid communication with the at least one vapor transport channel for separately discharging coolant exhaust from the at least one coolant exhaust channel and vapor from the at least one vapor transport channel.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered part of the claimed invention.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
One or more aspects of the present invention are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is an elevational view of one embodiment of a cooled electronics rack comprising one or more heat-generating electronic components, and employing one or more heat sinks, in accordance with one or more aspects of the present invention;
FIG. 2 is a schematic of one embodiment of an electronic subsystem or node of an electronics rack, wherein an electronic component and associated heat sink are cooled by system coolant provided by one or more modular cooling units disposed within the electronics rack, in accordance with one or more aspects of the present invention;
FIG. 3 is a schematic of one embodiment of a modular cooling unit for a cooled electronics rack such as depicted in FIGS. 1 & 2, in accordance with one or more aspects of the present invention;
FIG. 4 is a plan view of one embodiment of an electronic subsystem layout illustrating multiple heat sinks cooling multiple electronic components of the electronic subsystem, in accordance with one or more aspects of the present invention;
FIG. 5A is a cross-sectional elevational view of one embodiment of a cooled electronic structure comprising a heat-generating electronic component and a heat sink with a vapor-permeable membrane, and taken along lines 5A-5A in FIGS. 5B & 5D, in accordance with one or more aspects of the present invention;
FIG. 5B is a cross-sectional plan view of the cooled electronic structure of FIG. 5A, taken along line 5B-5B thereof, in accordance with one or more aspects of the present invention;
FIG. 5C is a cross-sectional plan view of the cooled electronic structure of FIG. 5A, taken along line 5C-5C thereof, in accordance with one or more aspects of the present invention;
FIG. 5D is a cross-sectional plan view of the cooled electronic structure of FIG. 5A, taken along line 5D-5D thereof, in accordance with one or more aspects of the present invention;
FIG. 6 is a schematic one embodiment of a cooled electronics apparatus comprising an electronic subsystem or node with multiple heat sinks and illustrating rack-level vapor separation and condensing, in accordance with one or more aspects of the present invention;
FIG. 7 depicts a more detailed embodiment of the cooled electronics apparatus of FIG. 6, illustrating rack-level vapor separation and condensing, in accordance with one or more aspects of the present invention;
FIG. 8A is a cross-sectional elevational view of another embodiment of a cooled electronic structure, and taken along lines 8A-8A in FIGS. 8B & 8D, in accordance with one or more aspects of the present invention;
FIG. 8B is a cross-sectional plan view of the cooled electronic structure of FIG. 8A, taken along line 8B-8B thereof, in accordance with one or more aspects of the present invention;
FIG. 8C is a cross-sectional plan view of the cooled electronic structure of FIG. 8A, taken along the line 8C-8C thereof, in accordance with one or more aspects of the present invention;
FIG. 8D is a cross-section plan view of the cooled electronic structure of FIG. 8A, taken along line 8D-8D thereof, in accordance with one or more aspects of the present invention;
FIG. 9 is a schematic of one embodiment of a cooled electronics apparatus employing node-level merging of vapor and coolant exhaust, and rack-level vapor separation and condensing, in accordance with one or more aspects of the present invention;
FIG. 10 is a schematic view of an alternate embodiment of a cooled electronics apparatus comprising multiple cooled electronic structures and rack-level merging of vapor and coolant exhaust, as well as rack-level vapor separation and condensing, in accordance with one or more aspects of the present invention;
FIG. 11 is a more detailed schematic view of the cooled electronics apparatus of FIG. 10, in accordance with one or more aspects of the present invention;
FIG. 12A is a cross-sectional elevational view of another embodiment of a cooled electronic structure comprising a heat-generating electronic component and a heat sink with a vapor-permeable membrane, and taken along line 12A-12A in FIG. 12B, in accordance with one or more aspects of the present invention;
FIG. 12B is a cross-sectional plan view of the cooled electronic structure of FIG. 12A, taken along line 12B-12B thereof, in accordance with one or more aspects of the present invention; and
FIG. 12C is a cross-sectional plan view of the cooled electronic structure of FIG. 12A, taken along line 12C-12C thereof, in accordance with one or more aspects of the present invention.



DETAILED DESCRIPTION
As used herein, the terms “electronics rack” and “rack unit” are used interchangeably, and unless otherwise specified include any housing, frame, rack, compartment, blade server system, etc., having one or more heat generating components of a computer system or electronics system, and may be, for example, a stand alone computer processor having high, mid or low end processing capability. In one embodiment, an electronics rack may comprise a portion of an electronic system, a single electronic system or multiple electronic systems, for example, in one or more sub-housings, blades, books, drawers, nodes, compartments, etc., having one or more heat-generating electronic components disposed therein. An electronic system(s) within an electronics rack may be movable or fixed relative to the electronics rack, with rack-mounted electronic drawers and blades of a blade center system being two examples of electronic systems (or subsystems) of an electronics rack to be cooled.
“Electronic component” refers to any heat generating electronic component of, for example, a computer system or other electronic system requiring cooling. By way of example, an electronic component may comprise one or more integrated circuit dies, chips, modules and/or other heat-generating electronic devices to be cooled, such as one or more processors, memory modules and/or memory support structures. Further, as used herein, the terms “heat sink” and “coolant cooled heat sink” refer to thermally conductive structures having one or more channels (or passageways) form therein or passing therethrough, which facilitate the flow of coolant through the structure. One example, the coolant carrying channels comprise microchannels having a hydraulic diameter of 1.0 mm or less, for example, in the range of approximately 0.1 mm to 0.5 mm.
As used herein, “liquid-to-liquid heat exchanger” may comprise, for example, two or more coolant flow paths, formed of thermally conductive tubings (such as copper or other tubing) in thermal or mechanical contact with each other. Size, configuration and construction of the liquid-to-liquid heat exchanger can vary without departing from the scope of the invention disclosed herein. Further, “data center” refers to a computer installation containing one or more electronics racks to be cooled. As a specific example, a data center may include one or more rows of rack-mounted computing units, such as server units.
One example of facility coolant and system coolant is water. However, the concepts disclosed herein are readily adapted to use with other types of coolant on the facility side and/or on the system side. For example, one or more of the coolants may comprise a brine, a dielectric liquid, a fluorocarbon liquid, a liquid metal, or other similar coolant, or refrigerant, while still maintaining the advantages and unique features of the present invention.
Reference is made below to the drawings (which are not drawn to scale for ease of understanding), wherein the same reference numbers used throughout different figures designate the same or similar components.
FIG. 1 depicts one embodiment of a liquid-cooled electronics rack 100 which employs a liquid-based cooling system. In one embodiment, liquid-cooled electronics rack 100 comprises a plurality of electronic subsystems or nodes 110, which may comprise processor or server nodes, as well as a disk enclosure structure 111. In this example, a bulk power assembly 120 is disposed at an upper portion of liquid-cooled electronics rack 100, and two modular cooling units (MCUs) 130 are disposed in a lower portion of the liquid-cooled electronics rack. In the embodiments described herein, the coolant is assumed to be water or an aqueous-based solution, again, by way of example only.
In addition to MCUs 130, the cooling system includes a system water supply manifold 131, a system water return manifold 132, and manifold-to-node fluid connect hoses 133 coupling system water supply manifold 131 to electronics structures 110, 111 and node-to-manifold fluid connect hoses 134 coupling the individual electronics subsystems 110, 111 to system water return manifold 132. Each MCU 130 is in fluid communication with system water supply manifold 131 via a respective system water supply hose 135, and each MCU 130 is in fluid communication with system water return manifold 132 via a respective system water return hose 136.
As illustrated, heat load of the electronic structures is transferred from the system water to cooler facility water supplied by facility water supply line 140 and facility water return line 141 disposed, in the illustrated embodiment, in the space between a raised floor 145 and a base floor 165.
FIG. 2 schematically illustrates operation of the cooling system of FIG. 1, wherein a liquid-cooled heat sink 200 is shown coupled to an electronic component 201 of an electronic subsystem 110 within the electronics rack 100. Heat is removed from electronic component 201 via the system coolant circulated via pump 220 through heat sink 200 within the system coolant loop defined by liquid-to-liquid heat exchanger 221 of modular cooling unit 130, lines 222, 223 and heat sink 200. The system coolant loop and modular cooling unit are designed to provide coolant of a controlled temperature and pressure, as well as controlled chemistry and cleanliness to the electronic component(s). Furthermore, the system coolant is physically separate from the less controlled facility coolant in lines 140, 141, to which heat is ultimately transferred.
FIG. 3 depicts a more detailed embodiment of a modular cooling unit 130, in accordance with an aspect of the present invention. As shown in FIG. 3, modular cooling unit 130 includes a facility coolant loop wherein building chilled, facility coolant is supplied 310 and passes through a control valve 320 driven by a motor 325. Valve 320 determines an amount of facility coolant to be passed through liquid-to-liquid heat exchanger 221, with a portion of the facility coolant possibly being returned directly via a bypass orifice 335. The modular cooling unit further includes a system coolant loop with a reservoir tank 340 from which system coolant is pumped, either by pump 350 or pump 351, into the heat exchanger 221 for conditioning and output thereof, as cooled system coolant to the associated rack unit to be cooled. The cooled system coolant is supplied to the system supply manifold and system return manifold of the liquid-cooled electronics rack via the system water supply hose 135 and system water return hose 136.
FIG. 4 depicts one embodiment of an electronic subsystem 110 layout wherein one or more air moving devices 411 provide forced air flow 415 to cool multiple devices 412 within electronic subsystem 110. Cool air is taken in through a front 431 and exhausted out a back 433 of the drawer. The multiple devices to be cooled include multiple processor modules to which coolant-cooled heat sinks 420 (of a cooling system) are coupled, as well as multiple arrays of memory modules 430 (e.g., dual in-line memory modules (DIMMs)) and multiple rows of memory support modules 432 (e.g., DIMM control modules) to which air-cooled heat sinks are coupled. In the embodiment illustrated, memory modules 430 and the memory support modules 432 are partially arrayed near front 431 of electronic subsystem 110, and partially arrayed near back 433 of electronic subsystem 110. Also, in the embodiment of FIG. 4, memory modules 430 and memory support modules 432 are cooled by air flow 415 across the electronic subsystem.
The illustrated liquid-based cooling system further includes multiple coolant-carrying tubes connected to and in fluid communication with coolant-cooled heat sinks 420. The coolant-carrying tubes comprise sets of coolant-carrying tubes, with each set including (for example) a coolant supply tube 440, a bridge tube 441 and a coolant return tube 442. In this example, each set of tubes provides liquid coolant to a series-connected pair of heat sinks 420 (coupled to a pair of processor modules). Coolant flows into a first heat sink of each pair via the coolant supply tube 440 and from the first heat sink to a second heat sink of the pair via bridge tube or line 441, which may or may not be thermally conductive. From the second heat sink of the pair, coolant is returned through the respective coolant return tube 442. In an alternate implementation, tubing is provided for separately passing coolant in parallel through the heat sinks of the electronic subsystem.
In one embodiment, the above-described cooling system can be employed with single-phase liquid-cooling. However, such a system requires a large liquid flow rate, and correspondingly large, high-power pumps, to avoid the liquid boiling and minimize sensible heating of the fluid as it absorbs the heat dissipated. The flow rate and pump power required may be reduced by an order of magnitude by leveraging the large, latent heat of vaporization, allowing the liquid to boil. Flow boiling enjoys high heat transfer coefficients, which can facilitate reducing the junction-to-ambient module thermal resistance, and can couple the module temperature to the boiling point of the coolant (or working fluid), resulting in better temperature uniformity.
However, flow boiling in the confined flow geometries of small heat sink channels, and small impingement jets in the heat sink, result in a detrimental rise in pressure due to bubble nucleation, bubble growth and advection of the vapor phase. The rise in pressure shifts saturation conditions, delaying the onset of boiling, and also results in the development of flow instabilities and flow maldistribution at the heat sink and node level, which can lead to premature liquid dryout. These issues have made flow boiling microstructures difficult to implement.
As used herein, a “microchannel”, “micro-jet” or “microstructure” refers to a structure having a characteristic dimension less than 1.0 mm, for example, of approximately 0.5 mm or less. In one implementation, the microchannel has a hydraulic diameter of approximately 100 microns, and the jet channel (or jet orifice) has a diameter less than 100 microns. In the implementations described herein, the jet orifice diameter is assumed to be less than the microchannel width, since the jet orifice injects coolant into the microchannel(s) of the heat sink.
Disclosed hereinbelow are various heat sink structures which combine local jet impingement of coolant (through jet nozzles (or jet orifices)) with local vapor removal via a porous, vapor-permeable membrane, which minimizes the various challenges encountered during flow boiling in microstructures. The microchannels provide a larger heat transfer area and improved thermal performance as compared to larger, conventional channels, and by incorporating a vapor-permeable membrane within the heat sink structure, vapor generated within the microchannels can escape the confined microchannel geometry directly into a separate vapor transport channel/plenum. This local removal of vapor provides several advantages, including: a reduced two-phase flow pressure drop and a reduced required pumping power for circulating coolant through the heat sink structure(s); a lower and more uniform coolant saturation temperature within the heat sink structure; an improved heat transfer coefficient and reduced heat sink thermal resistance due to phase change; improved wetting and improved jet impingement; and a reduced possibility of flow instabilities which might lead to premature dryout within the heat sink.
The separated vapor can be reintroduced into the coolant exhaust from the cooling microchannels within the heat sink structure itself, or at a node level within an electronics rack comprising the heat sink structure. Alternatively, the vapor may be piped directly to a rack-level manifold, as explained further below. Secondary, buoyancy-driven vapor separation occurs in the rack manifold, with vapor rising to a condenser disposed in the upper portion of the rack unit. The vapor is then condensed back to liquid, which rejoins the liquid coolant returning to the modular cooling unit, where the liquid can be cooled and pumped back to the nodes of the electronics rack. In one embodiment, the coolant flowing through the heat sink structures comprises water, and the membrane is a porous, hydrophobic membrane. Further, in one embodiment, the membrane may be modified to have a spatially-varying porosity and stiffness, which allows for both the injection of fluid, through jet orifices provided in rigid portions of the membrane, and local removal of vapor generated within the microchannels. Alternatively, a plate mask could be associated with the vapor-permeable region of the membrane to define a multilayer structure, which comprises one or more coolant injection regions and one or more vapor removal regions from the microchannels. Note that in the embodiments described herein, the membrane, or the membrane and plate mask structure, overlie and form part of the coolant-carrying channels so as to be exposed to vapor within the coolant-carrying channels of the heat sink. For example, in one embodiment, the membrane forms a top portion of each of the coolant-carrying channels of the heat sink.
FIGS. 5A-5D depict one embodiment of a cooled electronic structure, generally denoted 500, in accordance with one or more aspects of the present invention. Cooled electronic structure 500 includes, in this embodiment, an electronic component 510, such as an electronic module, mounted to a printed circuit board 501 with an associated back plate 502 (for example, a metal back plate). A heat sink 520 is mechanically coupled via securing mechanisms 505 to back plate 502 of printed circuit board 501, which provide a compressive load forcing heat sink 520 in good thermal contact with electronic component 510. Electronic component 510 includes, in this embodiment, an integrated circuit chip 511 connected to a chip carrier or substrate 513 via, for example, a first plurality of solder ball connections 512. Similarly, substrate 513 is electrically connected to printed circuit board 501 via, for example, a second plurality of solder ball connections 514. A thermally conductive cap 516 is interfaced to integrated circuit chip 511 via a first thermal interface material 515, such as a silicone-based paste, grease, or pad, or epoxy or solder. A second thermal interface material 517 facilitates thermal interfacing of cap 516 to heat sink 520.
In this embodiment, heat sink 520 comprises a multilayer heat sink with a heat sink base 521, a membrane structure 523 and a heat sink cap 526, which are respectively depicted in cross-sectional plan view in FIGS. 5B-5D. Unless otherwise indicated, referring collectively to FIGS. 5A-5D, heat sink base 521 comprises one or more coolant-carrying channels 522, each of which may comprise a microchannel structure, such as described above. Note that five coolant-carrying microchannels are depicted in FIG. 5B, by way of example only. More or less coolant-carrying channels may be defined within the heat sink base, as desired. Heat from the electronic component is rejected to coolant within the coolant-carrying channels in the heat sink base. Two-phase cooling of the heat-generating electronic component is achieved by at least partial vaporization of the coolant (i.e., working fluid) within the one or more coolant-carrying channels of the heat sink.
As illustrated in FIGS. 5A & 5C, various regions of the coolant-carrying channels are capped by at least one vapor-permeable region 524 of membrane structure 523. As illustrated in FIGS. 5A & 5D, disposed over these regions are vapor transport channels 525 formed in heat sink cap 526. Thus, localized venting of vapor directly from the coolant-carrying channels, across the vapor-permeable membrane into the vapor transport channels is provided within the heat sink. In one embodiment, membrane 523 is modified to include, in addition to at least one vapor-permeable region 524, at least one vapor-impermeable region 528. In one embodiment, the at least one vapor-impermeable region 528 comprises a plurality of parallel-extending digits that are interdigitated with a plurality of vapor-permeable areas of the at least one vapor-permeable region 524, as illustrated in FIG. 5C. The vapor-impermeable digits extend substantially transverse to the coolant-carrying channels 522.
In the embodiment depicted, at least one orifice 550 is provided in each of the vapor-impermeable digits where extending over a respective coolant-carrying channel. Coolant is introduced into the coolant-carrying channels through orifices 550 via liquid coolant delivery channels 527 in fluid communication with a liquid coolant inlet 530 of heat sink 520. Coolant exhaust is discharged via coolant exhaust channels 529 extending through an opening in membrane 523 into heat sink cap 526. Coolant exhaust channels 529 are in fluid communication with a coolant exhaust outlet port 532 of heat sink 520. In this embodiment, the vapor transfer channel 525 vent within the heat sink into coolant exhaust channel 529, as illustrated in the plan view of FIG. 5D. Note that in this embodiment, the orifices 550 in the vapor-impermeable digits of the membrane are jet orifices, which provide jet impingement of coolant into the respective coolant-carrying channels of the heat sink. Note also that, in this embodiment, a single liquid coolant inlet port and a single coolant exhaust outlet port are provided in the heat sink.
As illustrated in FIGS. 5B & 5D, heat sink base 521 and heat sink cap 526 are configured to accommodate an O-ring 540 to seal coolant within the heat sink. Coolant and vapor are additionally sealed within the heat sink by vapor-impermeable region 528, which is provided to extend around the perimeter of the membrane, that is, where held by the heat sink base and heat sink cap as illustrated in FIG. 5A.
In one embodiment, the heat sink base and heat sink cap are fabricated of a metal material, such as copper, the coolant comprises water, and the membrane is a porous hydrophobic membrane, such as a vapor-permeable PTFE or polypropylene material, such as the membranes available, for example, from Sterlitech Corp., of Kent, Wash., USA, or Sumitomo Electric Interconnect Products, Inc., of San Marcos, Calif., USA.
FIG. 6 depicts one embodiment of a rack-level cooling apparatus comprising multiple heat sink structures, such as depicted in FIGS. 5A-5D. In this embodiment, two heat sink structures 520 are illustrated within an electronic subsystem 610, such as a node of an electronics rack 600. The cooling apparatus includes a modular cooling unit 620, such as described above. Modular cooling unit 620 includes a liquid-to-liquid heat exchanger 621 and a reservoir with an associated pump 622 for providing cooled liquid coolant via a coolant supply manifold 623 and node-level supply lines 624, 625 to the coolant inlet ports of the respective heat sinks 520. In the embodiment of FIGS. 5A-5D, the vented vapor is combined within the heat sink with the coolant exhaust so that a single coolant exhaust line 626, 627 extends from each heat sink 520. These coolant exhaust lines 626, 627 are coupled in fluid communication (in this embodiment) at the node level into a single node-level coolant exhaust line 628, which is coupled in fluid communication with a phase separation manifold 630 of the rack unit. Phase separation manifold 630 comprises a buoyancy-driven phase separator, with the coolant exhaust comprising (in one mode of operation) both vapor and liquid. Vapor within the manifold rises to a vapor condenser 640 disposed in an upper region of the electronics rack 600. In one embodiment, the condenser is liquid-cooled 641, for example, via a facility coolant. The resultant condensate is returned via a condensate return line 642 to the liquid coolant return line 631 coupling the phase separation manifold 630 to the modular cooling unit 620, to repeat the process.
FIG. 7 depicts a more detailed embodiment of the cooling apparatus and electronics rack of FIG. 6. In this embodiment, heat exchanger 621 of modular cooling unit 620 is shown to comprise a liquid-to-liquid heat exchanger, with a facility coolant loop 700 providing facility coolant to the liquid-to-liquid heat exchanger, as well as to vapor-condenser 640. In the embodiment of FIG. 7, the phase separation manifold 630 is shown to comprise an elongate, vertically-oriented structure, such as a long tube with a relatively large internal diameter. Multiple nodes 610 are also illustrated in FIG. 7, with each node receiving liquid coolant via coolant supply manifold 623, and rejecting (in one embodiment) two-phase coolant exhaust via node-level coolant exhaust line 628 to phase separation manifold 630.
Referring collectively to FIGS. 5A-7, operationally, at low heat fluxes, coolant impinges on the coolant-carrying channel surfaces of the heat sink base and flows down the coolant-carrying channels as a single-phase liquid to the coolant exhaust plenum at either end of the channels. The liquid-impermeable nature of the vapor-permeable membrane stops the liquid from leaking from the coolant-carrying channels through the pores of the membrane into the vapor transport channels in the heat sink cap. The liquid impingement has a higher heat transfer coefficient, and the relatively shorter flow lengths facilitate reducing flow pressure drop, and may maintain better temperature uniformity compared with coolant delivery parallel to the heated surface. The liquid flows to the external cooling apparatus (as shown in FIGS. 6 & 7), where it drops down the phase separation manifold to the modular cooling unit. Within the modular cooling unit, the heated coolant is cooled by the heat exchanger, with heat being rejected to the facility coolant passing through the heat exchanger. The cooled liquid coolant is then pumped back to the nodes of the electronics rack, and in particular, to flow through the heat sinks, in a manner such as described above.
At higher heat fluxes, a portion of the impinging coolant vaporizes within the coolant-carrying channels, with a liquid and vapor mixture flowing down the length of the channels. However, the vapor phase may also egress through the vapor-permeable region(s) of the membrane into the vapor transport channels of the heat sink cap, leaving a relatively liquid-rich coolant exhaust flowing in the coolant channels. This local removal of the vapor helps maintain a high heat transfer coefficient, reduces the pressure drop, and reduces dryout within the heat sink. The separated vapor can then be reintroduced into the coolant exhaust (e.g., a two-phase exhaust mixture) exiting from the edges of the heat sink base, through the large openings in the membrane, to the heat sink cap (see FIGS. 5A-5D). The reintroduction of the vapor in the heat sink cap does not significantly add to the pressure drop, due to the larger length scales of the channels in the heat sink cap. Doing so also simplifies plumbing external to the heat sink, as shown schematically in FIG. 6. The two-phase coolant effluent then flows to the separation manifold of the electronics rack, where the liquid drops down to the modular cooling unit, and the vapor rises to the vapor condenser disposed in the upper region of the electronics rack. Within the vapor condenser, the vapor is condensed, for example, with the assistance of facility chilled water passing through the vapor condenser. The condensed liquid then flows down the rack to be merged with the liquid drip from the phase separation manifold, and enters the modular cooling unit to be chilled and pumped back to the nodes to repeat the process.
FIGS. 8A-8D depict an alternate embodiment of a cooled electronics structure, generally denoted 800, in accordance with one or more aspects of the present invention. Cooled electronic structure 800 is similar to cooled electronic structure 500 of FIGS. 5A-5D, except the layers that make up heat sink 801 of FIGS. 8A-8D are modified from the layers that make up heat sink 520 (see FIG. 5A).
Specifically, as shown in FIG. 8A, cooled electronic structure 800 includes, in this embodiment, electronic component 510, such as an electronic module, mounted to printed circuit board 501, with an associated back plate 502. Heat sink 801 is mechanically coupled via securing mechanisms 505 to back plate 502 of printed circuit board 501, which provide compressive loading of heat sink 801 to electronic component 510. Electronic component 510 includes, in this embodiment, integrated circuit chip 511 connected to chip carrier or substrate 513 via a first plurality of solder ball connections 512. Substrate 513 is electrically connected to printed circuit board 501 via a second plurality of solder ball connections 514. A thermally conductive cap 516 is interfaced to integrated circuit chip 511 via first thermal interface material 515, and to heat sink 801 via second interface material 517, which may be the same or different interface materials.
Heat sink 801 is again a multilayer heat sink with a heat sink base 810, a membrane structure 820, and a heat sink cap 830, which are respectively depicted in cross-sectional plan view in FIGS. 8B-8D. Referring collectively to FIGS. 8A-8D, heat sink base 810 comprises one or more coolant-carrying channels 812, each of which may comprise a microchannel structure, such as described above. In operation, heat from the electronic component is rejected to coolant within the coolant-carrying channels in the heat sink base 810, causing boiling of the coolant.
As illustrated in FIGS. 8A & 8C, various regions of the coolant-carrying channels are capped by at least one vapor-permeable region 821 of membrane 820. As illustrated in FIGS. 8A & 8D, disposed over these regions are vapor transport channels 831 formed in heat sink cap 830. Thus, localized venting of vapor 833 directly from the coolant-carrying channels, across the vapor-permeable membrane into the vapor transport channels is provided within the heat sink. In one embodiment, membrane 820 is modified to include, in addition to the at least one vapor-permeable region 821, at least one vapor-impermeable region 822. The at least one vapor-impermeable region 822 comprises a plurality of parallel-extending digits that are interdigitated with a plurality of vapor-permeable areas of the at least one vapor-permeable region 821, as illustrated in FIG. 8C. The vapor-impermeable digits extend substantially transverse to the coolant-carrying channels 812.
In the embodiment depicted, at least one orifice 860 is provided in each of the vapor-impermeable digits where extending over a respective coolant-carrying channel. Coolant 834 is introduced into the coolant-carrying channels through orifices 860 via liquid coolant delivery channels 832, which as illustrated in FIG. 8D, are interdigitated with the vapor transport channels 831 within the heat sink cap 830. Liquid coolant delivery channels 832 are in fluid communication with a liquid coolant inlet port 840 of heat sink 801. Coolant exhaust is discharged via coolant exhaust channels 814 through a coolant exhaust outlet port 842. In this embodiment, the vapor transport channels 831 vent vapor from the heat sink through a vapor outlet port 841, as illustrated in FIG. 8D.
As with the cooled electronic structure embodiment of FIGS. 5A-5D, heat sink base 810 and heat sink cap 830 are configured to accommodate, in this embodiment, an O-ring 850 to seal coolant and vapor within the heat sink. Coolant and vapor are additionally sealed within the heat sink by the vapor-impermeable region 822 defined around the perimeter of the membrane 820, that is, where held by the heat sink base and heat sink cap, as illustrated in FIG. 8A.
In one embodiment, the heat sink base and heat sink cap are fabricated of a metal material, such as copper, the coolant comprises water, and the membrane is a vapor-permeable, liquid-impermeable membrane (except for the jet orifices), such as a vapor-permeable PTFE or polypropylene material.
FIG. 9 depicts one embodiment of a rack-level cooling apparatus comprising multiple heat sink structures, such as depicted in FIGS. 8A-8D. In this embodiment, two heat sink structures 801 are illustrated within an electronic subsystem 910, such as a node of an electronics rack 900. The cooling apparatus includes a modular cooling unit 620, such as described above. Modular cooling unit 620 includes a liquid-to-liquid heat exchanger 621 and a reservoir with an associated pump 622 for providing cooled liquid coolant via a coolant supply manifold 623 and node-level supply lines 624, 625 to the coolant inlet ports of the respective heat sinks 801. In the embodiment of FIGS. 8A-8D, the vented vapor and the coolant exhaust are discharged separately via, for example, coolant exhaust lines 911, 912 extending from each heat sink 801. In the embodiment depicted, these exhaust lines are merged within the electronic subsystem or node 910 into a single, two-phase coolant outlet line 913, which is coupled in fluid communication with phase separation manifold 630. Phase separation manifold 630 comprises a buoyancy-driven phase separator, with the coolant exhaust comprising (in one mode of operation) both vapor and liquid. Vapor within the manifold rises to vapor condenser 640 disposed in the upper region of electronics rack 900. In one embodiment, the condenser is liquid-cooled 641, for example, via facility coolant. The resultant condensate is returned to the modular cooling unit 620 to repeat the process.
One advantage of the heat sink design of FIGS. 8A-9 over the heat sink design of FIGS. 5A-7 is that the vapor reintroduction into the coolant stream is at the node level, which reduces the risk of vapor channel flooding by the coolant exhaust.
FIG. 10 depicts another embodiment of a rack-level cooling apparatus comprising multiple heat sink structures, such as depicted in FIGS. 8A-8D. In this embodiment, two heat sink structures 801 are again illustrated within an electronic subsystem 1010, such as a node of an electronics rack 1000. The cooling apparatus includes a modular cooling unit 620, such as described above. Modular cooling unit 620 includes a liquid-to-liquid heat exchanger 621 and a reservoir with an associated pump 622 for providing cooled liquid coolant via a coolant supply manifold 623 and node-level supply lines 624, 625 to the coolant inlet ports of the respective heat sinks 801. In the embodiment of FIG. 10, the vapor outlet ports of the heat sinks are coupled to vapor vent lines 1011, which are connected in fluid communication at the node level into a single vapor vent outlet line 1012, which is also connected in fluid communication with phase separation manifold 630. Similarly, the coolant exhaust outlet ports of the heat sinks 801 are connected to respective coolant exhaust lines 1013, which are merged within the node into a single coolant exhaust outlet line 1014 that is coupled in fluid communication with phase separation manifold 630. As noted above, phase separation manifold 630 is a buoyancy-driven phase separator, with the coolant exhaust comprising (in one mode of operation) both vapor and liquid. Vapor within the manifold rises to the vapor condenser 640 disposed in the upper region of electronics rack 1000. In one embodiment, the condenser is liquid-cooled 641, for example, via facility coolant. The resultant condensate is returned to the modular cooling unit 620 to repeat the process.
FIG. 11 depicts a more detailed embodiment of the cooling apparatus and electronics rack of FIG. 10. In this embodiment, heat exchanger 621 of modular cooling unit 620 is shown to comprise a liquid-to-liquid heat exchanger, with the facility coolant loop 700 providing facility coolant to the liquid-to-liquid heat exchanger, as well as to the vapor condenser 640. In the embodiment of FIG. 11, the phase separation manifold 630 is shown to comprise an elongate, vertically-oriented structure, such as a long tube with a relatively large internal diameter. Multiple nodes 1010 are illustrated in FIG. 11, with each node receiving liquid coolant via coolant supply manifold 623, and rejecting vapor via vapor outlet line 1012, and coolant via coolant exhaust outlet line 1014 to phase separation manifold 630. Vapor within the manifold rises to vapor condenser 640 disposed in an upper region of electronics rack 1000. In one embodiment, the condenser is liquid-cooled, for example, via facility coolant flowing through facility coolant loop 700. The resultant condensate is returned via a condensate return line 642 to the liquid coolant return line 631 coupling phase separation manifold 630 to modular cooling unit 620, to repeat the process.
FIGS. 12A-12C depict another embodiment of a cooled electronic structure, in accordance with one or more aspects of the present invention. The cooled electronic structure of FIGS. 12A-12C is similar to cooled electronic structure 500 of FIGS. 5A-5D, except that the single-layer membrane 523 of FIGS. 5A-5D is replaced by a multilayer structure comprising (in one embodiment) a vapor-permeable membrane 1210 disposed between two masking plates 1220, as illustrated in FIG. 12A.
Specifically, as shown in FIG. 12A, the cooled electronic structure includes, in this embodiment, electronic component 510, such as an electronic module, mounted to a printed circuit board 501, with an associated back plate 502. Heat sink 1200 is mechanically coupled via securing mechanisms 505 to back plate 502 of printed circuit board 501, which provide compressive loading of heat sink 1200 to electronic component 510. Electronic component 510 includes, in this embodiment, integrated circuit chip 511 connected to chip carrier or substrate 513 via a first plurality of solder ball connections 512. Substrate 513 is electrically connected to printed circuit board 501 via a second plurality of solder ball connections 514. A thermally conductive cap 516 is interfaced to integrated circuit chip 511 via first thermal interface material 515, and to the heat sink 1200 via second thermal interface material 517, which may be the same or different interface materials.
Heat sink 1200 is again a multilayer heat sink, with a heat sink base 521, a multilayer membrane structure comprising masking plates 1220, and vapor-permeable membrane 1210, and a heat sink cap 526. By way of example, embodiments of masking plate 1220 and vapor-permeable membrane 1210 are respectively depicted in cross-sectional plan view in FIGS. 12B & 12C.
Referring collectively to FIGS. 12A-12C, heat sink base 521 comprises one or more coolant-carrying channels 522, each of which may comprise a microchannel structure, such as described above. In operation, heat from the electronic component is rejected to coolant within the coolant-carrying channels in the heat sink base 521, causing (in one mode) boiling of the coolant.
As illustrated in FIGS. 12A-12C, various regions of the coolant-carrying channels 522 are capped by vapor-permeable membrane 1210, which is positioned between masking plates 1220 and exposed to the coolant-carrying channels via open regions 1222 in masking plates 1220. These exposed regions of vapor-permeable membrane 1210 align to vapor transfer channels 525, which vent vapor egressing from the coolant-carrying channels, as explained above.
Jet orifices or nozzles are defined in the multi-layer membrane structure via aligned through-holes 1221 in masking plates 1220, and through-holes 1211 in vapor-permeable membrane 1210. As explained above, these jet orifices inject coolant from liquid coolant delivery channels 527 into the coolant-carrying channels 522 in heat sink base 521.
In one embodiment, masking plates 1220 comprise metal masking plates, which may be epoxied, soldered or press-fitted to heat sink base 521 and heat sink cap 526. Additionally, masking plates 1220 may be epoxied to the vapor-permeable membrane 1210 for better sealing. Note also that the open regions 1222 in the masking plate 1220 exposed to the coolant-carrying channels 522 operate as vapor traps, where vapor collects between the channels and the membrane. This further facilitates egress of the vapor across the membrane into the vapor transport channels 525. Note further, that in the depicted multilayer membrane structure embodiment, the vapor-permeable membrane of FIGS. 12A & 12C need not have a vapor-impermeable region, such as in the embodiments of FIGS. 5A-11, described above. Note also that other multilayer membrane structure embodiments may alternatively be employed with a heat sink structure as described herein. For example, a single masking plate could be employed with the vapor-permeable membrane, if desired.
Those skilled in the art will note from the above discussion that the heat sink structures described herein include a heat sink base which comprises one or more coolant-carrying channels. In one embodiment, these coolant-carrying channels have sub-millimeter hydraulic diameters, and also are referred to herein as “microchannels”. Such small channels help increase the surface area, as well as the single-phase heat transfer coefficient of the coolant within the channels. The channels can be made via chemical etching or mechanical methods, such as skiving or end-milling. In one embodiment, the heat sink is fabricated of copper, due to its high heat transfer coefficient and relatively simple machineability. However, other materials, such as aluminum and silicon are also suitable, though may have disadvantages in terms of thermal conductivity, fragility and machineability.
The second layer of the heat sink comprises a vapor-permeable membrane, such as a porous, hydrophobic membrane, in the case where the coolant comprises water. Examples of micro/nano-porous, natively hydrophobic membranes include polypropylene, PTFE, and nylon. Natively hydrophilic materials, such as porous glass, porous silicon, porous aluminum and porous organic materials could also be used, but require a liquid-phobic coating to prevent liquid from leaking into the vapor channels. The porous membrane is prepared such that the regions with the nozzles or orifices, as well as the edges of the membrane, are hardened and non-porous to provide better nozzle definition as well as edge sealing. The membrane can be patterned using a variety of techniques, such as hot press (wherein a heated master is pressed onto the porous membrane to plastically deform it and close the pores in the desired regions), laminating with a non-porous material (one example of which is laminated porous PTFE, where the laminate is made of non-porous polypropylene), or epoxy/photoresist infiltration (where epoxy could be used to selectively close the pores and provide additional mechanical stiffness in desired regions).
In an alternate embodiment, the second layer of the heat sink might comprise a multilayer membrane structure, for example, such as depicted in FIGS. 12A-12C, and described above. In such a multilayer structure, the membrane may be a vapor-permeable membrane, for instance, a porous, hydrophobic membrane, in the case where the coolant comprises water. Additionally, the masking plate may be fabricated of various vapor-impermeable materials, with metal being one example.
The third layer of the heat sink, that is, the heat sink cap, comprises relatively larger liquid and vapor channels which help distribute the fluid from and to the inlet and outlet ports of the heat sink. In order to minimize the pressure drop in these channels, the hydraulic diameter is maintained relatively large. A large hydraulic diameter also reduces the pressure drop when the vented vapor is reintroduced to the coolant effluent (which may be a two-phase effluent) at the heat sink level. The heat sink cap can be made of copper or aluminum or any other material with a similar coefficient of thermal expansion (CTE) as that of the heat sink base to avoid excessive thermal stresses developing.
The coolant (or working fluid) should be compatible with the selected membrane, thus requiring specific fluid/membrane combinations. Examples, of coolants (or working fluids) include: water at sub-ambient pressures, dielectric fluids at atmospheric pressure, and refrigerants at higher pressures.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises”, “has”, “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of a device that “comprises”, “has”, “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but may also be configured in ways that are not listed.
The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention.

## Claims

1. An apparatus comprising:
a heat sink for cooling at least one electronic component with a coolant, without the coolant directly contacting the at least one electronic component, the heat sink comprising:
a thermally conductive structure comprising at least one coolant-carrying channel defined therein, the at least one coolant-carrying channel comprising a first channel end and a second channel end;
a coolant inlet port;
a membrane structure associated with the at least one coolant-carrying channel, the membrane structure comprising at least one vapor-permeable region, at least a portion of the at least one vapor-permeable region overlaying a portion of the at least one coolant-carrying channel and facilitating removal of vapor from the at least one coolant-carrying channel, and the membrane structure further comprising at least one coolant inlet orifice aligned over the at least one coolant-carrying channel, intermediate the first channel end and the second channel end of the at least one coolant-carrying channel, and coupled in fluid communication with the coolant inlet port to inject coolant onto at least one surface of the at least one coolant-carrying channel of the thermally conductive structure intermediate the first channel end and the second channel end of the at least one coolant-carrying channel;
at least one vapor transport channel in fluid communication with the at least one vapor-permeable region of the membrane structure, the at least one vapor transport channel facilitating exhausting of vapor egressing across the at least one vapor-permeable region of the membrane structure from the at least one coolant-carrying channel;
a first coolant exhaust channel in fluid communication with the at least one coolant-carrying channel at the first channel end thereof and a second coolant exhaust channel in fluid communication with the at least one coolant-carrying channel at the second channel end thereof, the first coolant exhaust channel and the second coolant exhaust channel facilitating exhausting of coolant from the at least one coolant-carrying channel; and
a coolant exhaust outlet port coupled in fluid communication with the first and second coolant exhaust channels, and a vapor outlet port coupled in fluid communication with the at least one vapor transport channel for separately discharging coolant exhaust from the first and second coolant exhaust channels and vapor from the at least one vapor transport channel.

2. The apparatus of claim 1, wherein the membrane structure comprises a liquid-impermeable membrane and the at least one coolant-carrying channel comprises at least one coolant-carrying microchannel having a characteristic dimension less than 1.0 mm.

3. The apparatus of claim 1, wherein the membrane structure further comprises at least one vapor-impermeable region, the at least one vapor-impermeable region comprising the at least one coolant inlet orifice coupled to inject coolant into the at least one coolant-carrying channel.

4. A heat sink comprising:
a thermally conductive structure comprising at least one coolant-carrying channel;
a membrane structure associated with the at least one coolant-carrying channel, the membrane structure comprising at least one vapor-permeable region, at least a portion of the at least one vapor-permeable region overlaying a portion of the at least one coolant-carrying channel and facilitating removal of vapor from the at least one coolant-carrying channel, and the membrane structure further comprising at least one orifice coupled to inject coolant onto at least one surface of the at least one coolant-carrying channel intermediate ends of the at least one coolant-carrying channel;
at least one vapor transport channel in fluid communication with the at least one vapor-permeable region of the membrane structure, the at least one vapor transport channel facilitating exhausting of vapor egressing across the at least one vapor-permeable region of the membrane structure from the at least one coolant-carrying channel;
at least one coolant exhaust channel in fluid communication with the at least one coolant-carrying channel and facilitating exhausting of coolant from the at least one coolant-carrying channel;
a coolant exhaust outlet port coupled in fluid communication with the at least one coolant exhaust channel, and a vapor outlet port coupled in fluid communication with the at least one vapor transport channel for separately discharging coolant exhaust from the at least one coolant exhaust channel and vapor from the at least one vapor transport channel; and
wherein the membrane structure further comprises at least one vapor-impermeable region, the at least one vapor-impermeable region comprising multiple vapor-impermeable digits, at least one vapor-impermeable digit comprising the at least one orifice coupled to inject coolant into the at least one coolant-carrying channel, the multiple vapor-impermeable digits being interdigitated with multiple vapor-permeable areas of the at least one vapor-permeable region.

5. The heat sink of claim 4, wherein the thermally conductive structure comprises multiple coolant-carrying channels arranged substantially parallel to each other, and wherein the multiple vapor-impermeable digits extend transverse to the multiple coolant-carrying channels, and each vapor-impermeable digit of the membrane comprises multiple orifices, each orifice of the multiple orifices being coupled to inject coolant into a respective coolant-carrying channel of the multiple coolant-carrying channels.

6. The apparatus of claim 1, wherein coolant within the at least one coolant-carrying channel comprises water, and wherein the membrane structure comprises a hydrophobic membrane.