US20070034360A1

US20070034360A1 - High performance cooling assembly for electronics

Info

Publication number: US20070034360A1
Application number: US11/404,388
Authority: US
Inventors: Jack Hall
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-06-08
Filing date: 2006-04-14
Publication date: 2007-02-15
Also published as: WO2006133429A2; WO2006133429A3

Abstract

An assembly for high performance cooling of electronics is described that includes a container for heat transfer liquid in which complex electronic assemblies are immersed. The electronics are sealed inside the liquid container such that an electrical connector protrudes to the exterior of the container. A thermally conductive plate is made part of the liquid filled container assembly such that a portion of the plate is in contact with the liquid and a portion of the plate protrudes from or forms part of the exterior of the container.

Description

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/688,641 filed on Jun. 9, 2005, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of cooling systems for electronic circuit boards.

BACKGROUND OF THE INVENTION

The following discussion is provided solely to assist the understanding of the reader, and does not constitute an admission that any of the information discussed or references cited constitute prior art to the present invention.
Heat dissipation is an important operational aspect of electronic assemblies. As performance and density of electronics increases, the devices typically produce more heat. Fans are widely used to provide cooling in electronics enclosures, but certain configurations of electronic circuits are difficult to cool sufficiently solely with forced air. Air has low density and poor thermal conductivity compared to other materials. To cool dense, high heat generating electronic assemblies, a high volume of air must be moved past the electronics to maintain optimal or even acceptable operating temperatures. Moving high volumes of air typically generates high levels of noise and turbulence.
Electronic assemblies are cooled more efficiently when the electronics are in direct contact with a substance having good thermal conductance properties, such as a dielectric liquid. Being denser than air, a dielectric fluid is a superior conducting medium and is conformal, able to be in contact with all surfaces of an electronic assembly. On the other hand, use of liquid with electronic assemblies is problematic because of the potential for spillage and leakage.
Liquids have been employed in electronics cooling for some time. Liquid cooling of electronics has generally been done in one of three ways. (1) Liquid is contained in tubes and substrates that are proximal or in contact with electronic devices. (2) Liquid is sprayed, or misted onto electronics and collected in a drip tray for reuse. This technique may be combined with air cooling. (3) Electronics are submerged in liquid and the liquid is circulated via tubing to a heat exchanger and back again. The first technique may use a variety of heat transfer liquids, while the later two techniques require use of a dielectric, inert liquid.
Several patents describe the use of cooling liquids indirectly by isolating a heat transfer liquid within tubes and substrates in order to prevent wetting of the electronics. In U.S. Pat. No. 5,343,358 by Hilbrink, electronic components and heat pipes are assembled on a circuit board. The circuit boards are attached to a backplane via electronic connectors and the heat pipes are connected via collets. The collets are used to connect the board mounted heat pipes to a fluid circulating system. In this invention heat transfer is limited to individual electronic component surfaces that are in contact with the heat pipes. While this solution may work for microprocessors and other flat components; it is impractical where heat generating components of odd shapes and/or a large number of components are present. In this invention the electronic devices to be cooled are not engulfed by the cooling fluid and therefore the heat flow path is limited. Further, the system is prone to leaks due to the numerous fluid connections.
Similarly, U.S. Pat. No. 5,177,666 by Bland provides a cooling rack that circulates cooling fluid to conductive pads that are fitted closely to heat generating components on a circuit board. The pads are connected to the rack by use of fluid connectors. The heat flow path is limited to the specific components that are in contact with the pads, and the system is prone to leaks due to the numerous fluid fittings.
In U.S. Pat. No. 5,057,968 by Morrison, a configuration is described as a cooling system for printed circuit boards that has guide rails with serpentine cooling passages inside. The serpentine passages contain a liquid that is regulated to produce phase change, or boiling of the liquid. There are circuit board mounted heat pipes that are clamped to the guide rails, and heat migrates from the hot components, through the heat pipes, to the metal guide rail. The fluid in this configuration does not engulf the electronic components and so the thermal pathway is limited to the surfaces in contact. The fluid is contained within channels and is circulated from the rack to an external heat exchanger. Because of the numerous fluid couplings, this design is prone to leakage.
U.S. Pat. No. 4,962,444 by Niggemann describes a cold chassis with impingement attached circuit boards. The circuit boards are constructed with electronic components mounted to an aluminum plate. A clamping mechanism is used to attach the aluminum plates to the cold chassis. The cold chassis utilizes internal cooling fluid as a means of dissipating the heat from the chassis. As described this invention relies on an aluminum plate to conduct the heat away. The plate is not conformal; it does not engulf each electronic device as a fluid does, and so heat transfer is limited.
In U.S. Pat. No. 6,616,469 by Goodwin, an electronic component is cooled by means of a substrate containing a channel for liquid to flow through. The substrate is pressed against the electronic component to be cooled and heat is conducted through the substrate and into the fluid. The cooling channel is connected to a fluid valve that controls fluid flow in and out of the substrate. The design provides connectors for electrical and fluid coupling. In this invention electronic components of different sizes and shapes are difficult to accommodate. Only the component surface in contact with the substrate is cooled and therefore heat transfer is limited. The tubes, valves and fluid connections described in this invention are prone to leakage.
U.S. Pat. No. 5,323,292 by Brzezinski describes a container made with a heat sink on the outside. A metallic membrane sealed to the inside of the heat sink is pressed against the components inside the container. Between the membrane and the heat sink, a pressurized dielectric fluid forces the membrane against the electronic devices. Dielectric fluid is therefore not in direct contact with the electronic devices. This invention can accommodate slight variations in device planarity but is impractical for electronic assemblies containing a wide range of component shapes and circuit board configurations. Further, the components are not engulfed in a heat transfer medium and the invention provides a heat flow path from only one surface of each component. The invention is therefore only partially efficient as a cooling technique.
In U.S. Pat. No. 6,744,136 by Dubhashi, a heat generating electronic device such as a semiconductor die, a diode, transistor or thyristor is suspended inside an enclosure filled with heat transfer liquid. The enclosure that houses the fluid has a wall that is thermally conductive and is sealed to the remainder of the enclosure. The patent describes electrodes that protrude through the enclosure and are sealed to prevent fluid leakage. The invention is an improvement over the epoxy encapsulation method typically used to package semiconductor devices, however, it is not suitable for larger assemblies of components such as a circuit board. There is no accommodation for expansion and contraction of the fluid that occurs with larger volumes of fluid.
U.S. Pat. No. 4,312,012 by Frieser and U.S. Pat. No. 3,741,292 by Aakalu, et al, describe an enclosure containing a heat generating electronic device partially filled with inert liquid. The invention relies on nucleate boiling of the liquid to transfer heat to the enclosure walls and the walls are configured to promote condensation of the liquid. In this configuration the fluid is not circulated and ultimately relies on air flow to remove heat from the ambient exterior of the package.
In U.S. Pat. No. 4,590,538 by Cray, heat generating circuit boards are immersed in a coolant liquid. The liquid is pumped to a heat exchanger, cooled and pumped back to the vessel containing the circuit boards. This technique is similar to U.S. Pat. No. 4,302,793 by Rohner. In the Rohner patent a circuit board is suspended inside a fluid filled tank and a circulating pump is used to move the liquid into heat exchanger and back again. Systems that use a pumping device to circulate cooling liquid are difficult to seal because of the numerous connections between hoses, fittings, gaskets and pumps.

SUMMARY OF THE INVENTION

The present invention provides a high performance method for the cooling of complex, heat generating circuit boards and other complex electronic assemblies, using a dielectric liquid as a heat conducting medium, without necessitating the use of complex apparatus for circulating cooling liquid. This is accomplished using a container enclosing the entire electronic assembly such that the assembly is submerged in the dielectric liquid. A thermally conductive plate is in contact with the fluid and the plate extends beyond the exterior wall of the container. The plate can be attached to a thermal pathway such as a heat pipe that is connected to a heat exchanger. By cooling the heat pipe and consequently cooling the conductive plate, the fluid inside the container becomes cooler through diffusion, and consequently the electronic components are cooled. Electrical connection between the electronic assembly within the container and other external electronic devices is made through a sealed electrical connector. The protruding connector may be attached to a mating connector on either a printed wiring board or electrical wiring cable.
The invention provides a number of distinct advantages, including for example, the following:

- a) Electronic assemblies are contained within an enclosure sealed in a manner that provides a deterrent to anyone attempting to tamper with these devices.
- b) The sealed liquid filled containers are easy to remove and replace, without risk of spillage.
- c) The sealed container can be assembled and serviced in a factory environment where proper tools, equipment and trained personnel are present.
- d) Electronic assemblies of varying sizes and shapes can be accommodated.
- e) Expansion and contraction of the fluid is accommodated by means of a flexible surface such as diaphragm or an expandable surface of the enclosure, permitting a large volume of fluid to be used.
- f) The dielectric fluid used as a thermal transfer medium is sealed within the enclosure and does not circulate outside of the enclosure. The absence of fluid couplings, tubing, valves and pumps makes this invention more reliable, mitigating the issues of spillage and leakage.
- g) The absence of fluid couplings, tubing, valves and pumps makes this invention more economical than fluid circulating inventions due to the reduced number of parts.
- h) A wide range of cooling devices may be used in this invention including refrigerators, heat exchangers, evaporators, and heat sinks.
- i) When compared to conventional fan cooling, this invention has a lower noise potential.

Thus, in a first aspect the invention provides a liquid cooled electronics circuit board that includes a circuit board containing a plurality of heat generating components, a sealed enclosure or container surrounding the circuit board and containing a heat transfer, dielectric liquid such that the circuit board is at least partially (and preferably fully) submerged in that liquid. The enclosure also has a fill port, at least one heat conductive plate that is in contact with the liquid and protrudes to the exterior of the enclosure, and a thermal expansion compensating surface. An electrical connector extends from the circuit board to the exterior of the enclosure.
In particular embodiments the circuit board is installed in a computer system; the heat generating components on the circuit board together generate 20-200 watts, or 50-150, 50-100, 20-40, 40-60, 60-80, 80-100, 150-200 wafts; the circuit board includes at least 10, 20, 30, 40, 50, or even more heat generating components; the container contains at least 10, 20, 30, 40, 50, 75, 100, 200, or 500 ml or is in a volume range defined by taking any two different values from those listed as the endpoints; thermal expansion of the liquid volume inside the enclosure is accommodated by a flexible diaphragm (a thermal expansion compensating surface) that is in contact with the liquid on one side and ambient atmosphere on the other side, such as a low curvature, concave or essentially flat wall of the container or a flexible diaphragm; the thermal expansion compensating surface expands at least 1, 2, 3, 4, or 5 ml upon heating of thermally conductive liquid within the container with a heat rise of 20 degrees C.; the thermal expansion compensating surface expands at least 1, 2, 3, 4, or 5 ml with a pressure differential across the surface of no more than 70 g/cm²; the thermally conductive liquid expands approximately 1% of the volume of the liquid for every 10° C. rise in temperature.
In certain embodiments, the heat conductive plate is adjacent to the connector; the connector penetrates through the heat conductive plate: the heat conductive plate is distal from the connector (e.g., in or on a surface of the container opposite from the surface in which the connector is located); the liquid is a fluorocarbon, an oil, or distilled water; the enclosure can be opened or removed allowing access to components on the circuit board, and sealably replaced; the fill port is configured to allow extraction and replacement of the liquid; the fill port comprises an orifice that can be breached by a nozzle and is self-sealing when the nozzle is retracted; the fill port includes an elastomeric material with a re-sealable aperture through which a liquid fill and/or removal nozzle is inserted; the heat conductive plate is thermally coupled with a heat transfer component such as a heat sink, heat pipe, or heat exchanger; the heat transfer component is held against the heat conductive plate with a clamp such as an over-center clamp; the thermal coupling is accomplished through a thermal interface material such as a heat conductive elastomeric material, a phase change material, an aligned fiber material, a thermal gel, or a thermal grease.
In particular embodiments, the walls of the container are formed of a thermoplastic or a thermoset plastic; the walls of the container comprise, consist essentially of, or consist of a material selected from the group consisting of polycarbonate, acrylic, and ABS plastic; the walls of the container comprise, consist essentially of, or consist of a metal, e.g., steel, aluminum (which can be an alloy), or copper (which can be an alloy); the thermally conductive plate includes at least 80% copper, aluminum, or heat conductive ceramic; the thermally conductive plate averages at least 1, 2, 3, 4, or 5 mm thick; the contact portion of the thermally conductive plate averages at least 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 mm thick.
A related aspect concerns an electronics cooling assembly that includes a sealed or sealable container that has a fill port allowing filling of the container with a heat transfer, non-electrically conducting liquid after the container is sealed, at least one thermally conductive plate that is (or will be) in contact with the liquid inside the container and protrudes to the exterior of the container, and a thermal expansion compensating surface. At least one circuit board is or can be mounted within the container such that it is (or will be) substantially submerged in the liquid. In many cases, the assembly includes an electrical connector that has or is designed to have interior connections with the circuit board, and extends to the exterior of the container. As indicated, the sealed container is substantially filled with the heat transfer, non-electrically conducting liquid.
In particular embodiments, the container is as described for the preceding aspect.
In certain embodiments, the assembly includes a plurality of circuit boards, e.g., 2, 3, 4 or more circuit boards; the container encloses a power supply, e.g., on a circuit board; the walls of the container are formed of a thermoplastic or a thermoset plastic; the walls of the container comprise, consist essentially of, or consist of a material selected from the group consisting of polycarbonate, acrylic, and ABS plastic; the walls of the container comprise, consist essentially of, or consist of a metal, e.g., steel, aluminum (which can be an alloy), or copper (which can be an alloy); the fill port comprises an orifice that can be breached by a nozzle and is self-sealing when the nozzle is retracted; the thermally conductive plate comprises at least 80% copper, aluminum, or heat conductive ceramic.
The circuit boards or electronics cooling assemblies are particularly applicable to computers. Thus, another related aspect concerns a computer system that includes at least one (commonly a plurality, e.g., at least 2, 3, 4, 5, 10, or more) of such liquid cooled circuit boards or cooling assembles as described in the aspects above. In some case, the computer system includes a circulating coolant system for removing and/or dissipating heat from the exterior of the heat conductive plate in the container.
Likewise, in another aspect the invention provides a method for cooling a complex electronics assembly (e.g., a circuit board or boards) by enclosing the complex electronic assembly in a liquid submersion cooling assembly (e.g., using a liquid cooled circuit board as described herein) as described above. The method can also include thermally coupling the thermally conductive plate of the cooling assembly with another heat transfer device, e.g., a heat pipe, heat sink, pumped liquid cooling system, and the like.
In certain embodiments, the other heat transfer device is mechanically attached against the thermally conducting plate, e.g., using one or more over-center clamps, screws, or clips; the thermal coupling involves a thermally conductive compliant material, such as an elastomeric material, compressed between the plate and the other heat transfer device.
Thus, in another aspect, the invention concerns an electronic assembly heat transfer device that includes a thermally conductive plate that conducts heat from an electronic assembly (e.g., as described in an aspect above), and a thermally conductive device thermally coupled with the thermally conductive plate through a compliant, thermally conductive material between the plate and the thermally conductive device. The compliant material may, for example, be an elastomeric material. The device can include a connector located adjacent to or penetrating through the thermally conductive plate, e.g., such that the thermal pathway from the plate to the thermally conductive device is established concurrently with mating of the connector with a corresponding external connector.
In particular embodiments, the plate and/or thermally conductive device and/or compliant material are as described herein and/or the mechanism for attaching the plate and thermally conductive device together are as described herein.
Similarly, in a related aspect the invention concerns a cooling assembly for heat generating electronics that includes a thermally conductive plate which conducts heat from the heat generating electronics (e.g., in a cooling device or liquid cooled circuit board as described above), a second thermally conductive element attached to a surface of the thermally conductive plate using a means for providing a low thermal resistance connection between the conductive plate and second heat transfer element. For example, the means for providing a low thermal resistance connection may involve a high thermal conductance elastomer and an over-center clamping mechanism. The assembly can include a first electrical connector mounted adjacent to or penetrating the thermally conductive plate. This first electrical connector may be engaged with a matching external connector by the same action by which the low thermal resistance connection between the conductive plate and the second heat transfer element is established (e.g., attaching the plate also engages the electrical connectors).
In certain embodiments, the cooling assembly also includes an electronic connector mounted on the second thermally conductive element.
In a further related aspect, the invention concerns a method for thermally coupling a thermally conductive plate that conducts heat from an electronic assembly with a thermally conductive device, by mechanically compressing a thermally conductive compliant material between the thermally conductive plate and the thermally conductive device. As indicated above, the mechanical compression can utilize an elastomeric material and/or one or more over-center clamps. In certain advantageous applications, a connector is located adjacent to or penetrating the thermally conductive plate. In particular cases, the connector location is such that creation of the thermal pathway by attaching the thermally conductive plate to the thermally conductive device concurrently establishes electrical conduction pathway through the connector with a second connector (which may be mounted in or on the thermally conductive device).
The term “circuit board” is used to refer to a board used in electronic devices that are made from an insulating material and contain electronic components that are interconnected to form a circuit or group of circuits that perform a specific function. Typically the board includes a printed metal pattern which serves as interconnections in the electrical circuit. Other circuit components are typically soldered to the board.
As used herein, the term “dielectric” means that the referenced material has zero or near zero electrical conductivity. In particular the term “dielectric liquid” refers to a material which is liquid over the intended operating range of a particular electronic system (e.g., over the range of 20-150 degrees C.).
The term “electrical connector” or “electronics connector” is used to refer to a substantially rigid device in which electrical conductors (e.g., wires and/or printed circuit conductors) are terminated and which is constructed for replaceable connection with a mating connector such that an electrical pathway is established across the pair of connectors. In most cases, for use in the present invention such a connector will connect with at least 5 conductors and commonly more, e.g., at least 10, 15, 20, 40, or more conductors. Such connectors may be in various formats, e.g., rectangular, circular, edgeboard, and strip connectors.
In the present context, the term “fill port” refers to a structure that includes a penetration channel from the exterior to the interior of a container through which liquid can be introduced and which is sealable after such liquid introduction, e.g., includes a valve or elastomeric seal. For example, a hollow needle or nozzle can be used to pass through a sealable channel in an elastomeric material, liquid is introduced through the hollow needle, and the channel is sealed upon removal of the needle (e.g., automatically or by mechanical compression.
As used herein, the term “heat transfer liquid” means a liquid that has a thermal conductivity of at least 0.05 W·m⁻¹·K⁻¹, or in some cases at least 0.06, 0.07, 0.10, 0.15, 0.2, 0.3, or 0.4 W·m⁻¹·K⁻¹.
The terms “heat conductive” and “thermally conductive” as used in connection with the heat conductive plate means that the plate material has a thermal conductivity of at least 1.0 W·m⁻¹·K⁻¹, more typically at least 10 W·m⁻¹·K⁻¹, and in many cases at least 20, 30, or 40 W·m⁻¹·K⁻¹(e.g., bronze, heat conductive plastics), and often at least 100 or even 200 W·m⁻¹·K⁻¹, for example, copper, brass, or aluminum.
The term “heat generating devices” is used in the present context to refer to electronic devices that produce substantial heat under normal operation, such that the device or a portion thereof will experience a temperature rise in still air at 1 atm of at least 10 degrees C. under normal operating conditions from an unpowered temperature of 21 degrees C.
In connection with the present invention, the term “liquid cooled” means that the temperature of particular heat producing devices is reduced or controlled by contact with a liquid that functions as part of a heat conducting pathway in which heat from the devices is dissipated distal from the source.
In the context of the present invention, the term “sealed container” refers to a device having a hollow interior such that a chemically compatible liquid in that hollow interior will not leak or otherwise escape during normal operation with the container in any orientation.
The term “substantially submerged” as used in connection with electronic assemblies enclosed in the present devices means that the components in the assembly that generate at least 80% of the heat are within the liquid in the container. In many cases, at least 90% or even 100% of the heat generating components are within the liquid.
As used in reference to the present liquid-filled containers and assemblies, the phrase “thermal expansion compensating surface” refers to a section of the container or attached structure that accommodates thermal expansion of the enclosed liquid by flexing and/or stretching, thereby creating increased internal volume. Such surface may, for example, be wall of the container or portion thereof that can expand outward, e.g., a wall or wall section having low curvature (i.e., flat or nearly flat, advantageously with a slight inward curvature or concavity). An alternative involves an expansion diaphragm sealed to the container that is exposed to liquid pressure on one face and ambient exterior pressure on the other face. Other structures that accommodate thermal expansion of the liquid can also be used.
Additional embodiments will be apparent from the Detailed Description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings provided are for purpose of illustration and description and not limit the scope of the invention.
FIG. 1 is a perspective right-side view of a liquid filled container with a circuit board assembly inside. The circuit board assembly includes multiple electronic components, and the container is filled with an electrically non-conductive fluid.
FIG. 2 is a lateral cross section showing the upper and lower housings of the container ultrasonically welded together and a fill port fused to one of the container walls.
FIG. 3 is a section drawing showing a hook, a snap, and a screw, as examples of attachment features that can be used to suspend a circuit board assembly in position within the container.
FIG. 4 is a cross section through a container showing the thermally conductive plate of a container and a diaphragm exposed to ambient air on one side and to the dielectric liquid in the interior of the container on the other.
FIG. 5 is a cross section drawing showing a board mounted connector that is sealed with a compliant gasket.
FIG. 6 is a cross section drawing showing a circuit board edge connector protruding beyond the container envelope and sealed with a compliant gasket.
FIG. 7 is a cross section drawing showing a thermally conductive plate that protrudes through the container envelope, and is sealed with a compliant gasket.
FIG. 8 is a cross section drawing showing a thermally conductive plate fused into the wall of the container.
FIG. 9 is a perspective view of the liquid filled container showing a simplified thermally conductive plate with lateral mounting flanges.
FIG. 10 is a perspective view of the liquid filled container showing a thermally conductive plate with lateral mounting flanges and cooling fins.
FIG. 11 is a perspective view showing a liquid filled container with a thermally conductive plate attached to a heat pipe having a clamping mechanism holding the plate against the surface of the heat pipe.
FIG. 12 is a side view of a clamping mechanism which has an over-center mechanism mated to a heat pipe.
FIG. 13 is a perspective view of the back of the heat pipe, an attached heat exchanger, and a printed wiring board backplane.
FIG. 14 is a cross sectional view of a container showing the liquid-filled enclosure, a circuit board, a thermally conductive plate, a heat pipe, and a printed wiring board.
FIG. 15 is a perspective drawing of the cooling assembly showing the general direction of heat flow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides advantageous devices and methods for cooling complex and often rather large, electronics assemblies. In many cases, the electronic assemblies are or include circuit boards. As explained herein, the devices utilize thermally conductive, dielectric liquid-filled containers in which circuit boards or other complex electronic devices are mounted. The heat generated by components on the circuit board or other complex device is conducted through the liquid and through a thermally conductive plate that extends from the interior of the container in contact with the liquid to the exterior of the container, thereby providing an efficient heat conduction pathway. From the exterior surface of the plate, the heat can be conducted or dissipated away from the container.
Additional advantageous features of such devices relate to compensating for thermal expansion of the large volume of liquid in the container, the mechanisms used to thermally couple the thermally conductive plate with further thermal conductors, and the connectors used to electrically connect the complex electronic device inside the container with other electronic components or devices.
Heat Conduction Components
As pointed out above, the present devices are directed to providing a quiet and simple but effective method for cooling complex electronic devices. In many cases, the complex device is a complete circuit board, or a set of mounted circuit boards, that includes a number of heat generating components. In many cases the circuit boards include a large number of such heat generating components.
In the absence of an effective cooling mechanism, heat build-up is often a problem, contributing to shortened operating lifetimes and reduced performance. Forced air cooling (e.g., fans) has frequently been used, but has limitations due to the relatively low heat capacity and thermal conductivity of air. The result is that boards generating large amounts of heat are not effectively cooled, thereby adversely affecting component lifetimes and limiting performance. To address such limitations, and as discussed in the Background, a number of techniques have been used for cooling using liquids, but generally those techniques have limitations of complexity, leak potential, and/or uneven cooling, or are directed to cooling of individual devices (e.g.,
The present invention addresses all of those limitations or concerns. In particular, effective and even cooling is provided by using a thermally conductive, but electrically non-conductive liquid in direct contact with the circuit board or other complex electronic device. This liquid provides an efficient pathway conducting heat away from the vulnerable electronics. The direct contact between the thermally conductive liquid and the circuit board or other complex device is achieved by using a container that encloses the circuit board and is filled with the liquid.
Any of a number of heat conducting dielectric liquids can be used, such as halogenated hydrocarbon compounds, especially fluorinated compounds (e.g., GALDEN® PFPE from Solvay Solexis S.A. and FLUORINER™ liquids from 3M Corporation). Other compounds can also be used, such as oils, and distilled water. In most cases, the oils or halogenated compounds will be preferred.
To maintain the temperature gradient so that heat is efficiently conducted away from the circuit board, the container includes a thermally conductive plate that is in contact with the liquid on the interior of the container and extends to the exterior of the container. Such thermally conductive plate can be configured in many different ways, so long as it provides an efficient heat flow pathway from the interior of the container to the exterior. While the term “plate” is used to refer to this component, it will not necessarily have parallel planar surfaces. Thus, in many cases, the plate will have a planar exterior surface, but can also have other shapes. In come cases, the interior surface of the plate will be planar, e.g., to allow maximum clearance for the electronic components, but can have other shapes, e.g., to increase surface area allowing greater heat conduction into the plate and thus to the exterior. For example, the interior surface of the plate can be ribbed or can include other shape projections.
Likewise, in many cases the plate is a thermally conductive element mounted in a container wall such that a unitary material extends from the interior to the exterior across one wall of the container. However, other constructions can be used. In some cases, the plate will include a thermally conducting portion of the wall in thermal contact (e.g., bonded to such as using solder or heat conductive glue) with an externally attached thermally conducting material that provides a conduction path away from the container wall and may also provide stiffness and/or mounting for further heat conduction components. In such cases, the “plate” includes both the first heat conductive element and the portion of container wall to which it is attached.
The plate can also extend into more than one wall of the container. For example, a plate may be formed in a U-shape such that it covers at least part of three contiguous walls, an L-shape that covers at least part of two contiguous walls, cup-shaped such that it covers at least part of five contiguous walls, and the like.
The plate may be present in any desired surface of the container. However, in certain advantageous configurations, the container has an electrical connector that penetrates one surface (can also be referred to as a face or wall) of the container (or has electrical conductors that penetrate one surface and terminate in the connector) and the plate is located such that the contact surface of the plate (i.e., the plate surface that contacts the further heat conducting device) is on the same face of the container. In these configurations, the assembly can be designed such that electrical connection and the thermal pathway are created at essentially the same time, that is, as the electrical connector is mated with its corresponding connector, the exterior contact surface of the plate is brought into contact with the further heat conducting device. Such contact and/or the electrical connection can be positively created and/or stabilized by fastening the assembly into place with mechanical fasteners such as screws, clamps, clips, slide latches, and the like.
The exterior surface of the conductive plate of the container assembly is usually attached to (or formed with) another heat transfer device (secondary heat transfer device) such as a heat pipe, heat sink, or other such devices. The secondary heat transfer device can be attached using a mechanical fastener(s), e.g., a clamping mechanism, a screw device, or a slide latch. The thermal pathway across the interface between the exterior plate surface and the secondary heat transfer device can be facilitated by using a conforming material between the two surfaces (or on at least one of the surfaces). Use of such conforming material allows a more uniform connection to be made across the areas of the surfaces, thus allowing more uniform and/or efficient heat transfer. Such conforming materials can be soft metals, but can also advantageously be heat conductive elastomeric materials and other such termally conductive materials. Such elastomeric materials are beneficial because their inherent resiliency assists in creating and maintaining thermal contact, e.g., throughout multiple cycles of heating and cooling, as well as with mechanical disturbances such as accidental bumping.
The secondary heat transfer device provides a thermal pathway to a heat dissipation medium, e.g., to a heat exchanger. The heat exchanger or other thermal dissipation medium cools the thermal pathway, the attached thermally conductive plate, the liquid inside the container, and the heat generating electronics within.
In this high performance cooling system, the enclosed heat generating devices will be cooled by conduction and/or convection of heat through the various materials and sections of the heat transfer path. When the heat generating devices have reached their steady operating state, and the heat exchanger has reached its steady operating state, the transfer of heat will be almost constant and can be expressed mathematically. Heat transfer through the heat flow path is limited by the sum of the boundary contact resistance between sections of the path, plus the thermal resistance of each section. The sum of thermal resistance (R) of the system can be approximated by using the formula:
R=K1+K2+K3+K4+K5+K6+K7
Where each K=

- the resistance across boundary conditions between respective sections (K1,K3,K5,K7), and
- the thermal resistance across each material section (K2,K4,K6).
  With the sum of the thermal resistances (R) derived, the total heat loss (or heat transfer) of the system can be approximated as:
  Q=−(T ₁ −T ₂)/R
- Where
- Q=approximate heat dissipated, (W/h)
- T₁=Temperature at the heat source, (° C.)
- T₂=Temperature at the heat exchanger, (° C.)
- R=sum of the thermal resistance, (m²K/W)

The present invention contrasts with certain techniques in which cooling devices are constructed around simple electronic components such as transistors and the like. In such constructs, the liquid volumes are so small that thermal expansion is minimal. Further, the amount of heat generated by the enclosed transistors and the like are relatively small, so that simple air flow over a thermally conductive surface of the enclosure is generally sufficient. Still further, the constructs are configured with leads or conductors from the devices passing directly to the exterior of the enclosure.
Operation and Exemplary Devices
In typical configurations of the present devices, the complex electronic assemblies to be cooled (e.g., circuit boards), the heat transfer plate(s) and thermal expansion diaphragm (when present) are attached to the inside of the container parts. The container parts are joined, sealed and filled with dielectric liquid (e.g., through a fill port).
One or more such liquid filled containers are fastened to a heat transfer element such as a heat pipe, using a suitable fastening mechanism. In certain embodiments, as the container is fastened to the heat pipe, the internal electrical connector is simultaneously mated to an external electrical connector. The external electrical connector may be a wired connector or a circuit board mounted connector.
The electrical system including circuit boards, heat exchanger (when present), and power supply, are connected to an external power source that will supply necessary operating power. When power is turned on to the system, heat generating components begin to generate heat. If a heat exchanger is part of the system, it begins cooling the heat transfer system. The heat exchanger is connected to the thermally conductive plate, creating an efficient heat flow path from inside the liquid filled container through the thermally conductive plate to an external heat transfer element(s), which can be further coupled to a heat exchanger or other heat dissipation device. The effect is that heat is conducted from the heat generating components to be dissipated in the surroundings, resulting in cooling of the electronic components suspended within the liquid.
In some cases, for the purpose of providing the end-user with a finished product, the fluid filled container, electrical backplane, electrical wiring, heat pipe, heat exchanger power supply, and associated electronics, may be enclosed within a cabinet or other outer housing or structure. The present invention includes such products, e.g., computers or computer systems, which include the present cooling devices.
As pointed out in the Summary above, the present invention provides a number of advantages over conventional electronics systems cooling devices. These include:

- a) Liquid is contained within a sealed enclosure and is not pumped out of the container and back again. This eliminates hoses, fittings, seals and pumps, thus reducing cost and complexity. The invention is therefore a more reliable system.
- b) The sealed enclosure acts as a deterrent to unauthorized persons attempting to tamper with the electronics contained within, while still being easy to remove and replace. Usually defective modules will be serviced in a factory environment.
- c) Due to the reduced number of parts, the system is also more economical to build than circulated coolant systems.
- d) Electronic assemblies of varying sizes, shapes, and heat generating levels can be placed inside the enclosure.
- e) Expansion and contraction of the fluid is accommodated by means of a flexible diaphragm or surface, permitting a large volume of fluid to be used
- f) When compared to conventional fan cooling, this invention has a lower noise potential. As a result, systems incorporating the present devices are more acceptable in general office and home environments.

The present invention is further illustrated by the drawings, which are not intended to limit the scope of the invention.
As shown in perspective view in FIG. 1, a circuit board assembly 2 is positioned inside a container 1. Discrete electronic components 3 are hard wired (soldered) to the circuit board. The container is sealed and filled with a dielectric liquid in internal volume 4. Thermal expansion of the liquid is compensated by flexible diaphragm 10, i.e., a thermal expansion compensating surface. The assembly includes a heat transfer plate 15 covering one end and extended partially up two opposing walls of the container.
It is preferred that factory servicing of the container is necessary instead of allowing user access. Tampering with the electronics inside the container and accidental spillage of dielectric fluid by an unauthorized person is thereby deterred. In preferred embodiments, the container is sealed ultrasonically along a seam 5 as illustrated in FIG. 2, without common fasteners that would allow an unauthorized person to open the container. The container is designed such that it can easily be opened at the factory by personnel using custom built fixtures to drain and open the container.
A number of electrically inert liquids will work in this invention including fluorocarbons, oil, and distilled water. Individual formulations of these insulating liquids have different physical properties. If the heat transfer liquid has a high dielectric constant, then device connector sockets may be used reliably. If the heat transfer liquid has a relatively low dielectric constant then solder connections are recommended.
The size of the container will generally depend upon the size of the electronic assembly to be contained. The recommended operating temperature of the electronic devices in the assembly determines the amount of heat the system will be designed to dissipate. The specific dimensions and configuration of the high performance cooling assembly described herein are, therefore, determined by the specific electronic components to be cooled.
FIG. 2 shows a self-sealing fill port 6 fused into one wall of a container housing. After the housing is sealed, a fill nozzle is inserted into the fill port and dielectric liquid is pumped into the container. When the nozzle is withdrawn the fill port closes. As pointed out above, the housing is constructed using overlapping seam sections 5 which are ultrasonically welded along the overlapping seam. During such a filling operation, air or other gas displaced from the container can be vented, e.g., through a separate passage in the nozzle.
The incorporation of exemplary holders for holding the circuit board inside a container is shown in FIG. 3. The supporting features extend from opposing wall of the container (the top and bottom in FIG. 3). Illustrated examples of supporting features that can reliably hold a circuit board assembly in position with a container include hooks 7, snaps 8, and screws 9.
The container is made of a relatively rigid material suitable to contain the liquid (although the container material can deform to some extent). The container material may be thermally and/or electrically conductive or non-conductive. On average, typical dielectric liquids increase in volume approximately 1% for every 10° C. rise in temperature due to thermal expansion of the liquid. To accommodate increases in volume, the container is fitted with a thermal expansion compensating surface. As shown in cross-section in FIG. 4, such compensating surface can be provided by a diaphragm 10. In this configuration, the diaphragm is made of a compliant material and is in contact with the heat transfer liquid on the interior side 4 and surrounding atmosphere on the exterior 11. As the fluid expands the diaphragm stretches to accommodate the increased volume, and upon cooling the liquid volume decreases and the diaphragm returns to its original position. Also illustrated is an exemplary positioning of a U-shape thermally conductive member 15 encasing or forming one end of the container, with the plate fused or bonded with container wall 1. In such configurations, the plate may be edge-bonded, e.g., welded, with the container wall, such that the plate forms both the wall and the thermally conductive plate in that area, or the member may be bonded to a face or the container wall such that the wall in that area together with the U-shaped member form a thermally conductive plate which conducts heat from the liquid inside the container to the exterior.
In order to connect the circuit board or boards within a container assembly with external electronic components and devices, a connector extending from the interior to the exterior of the container is used. One exemplary configuration is shown in FIG. 5, where a connector 12 is provided so that electric current can flow between the internal electronic assembly 2 and electronic devices that are external to the container. The connector is sealed against the container, preventing liquid from leaking out of the container using gasket 13 made of compressible material which creates a seal between the connector and the container wall 1.
An alternative is shown in FIG. 6, where the edge of circuit board 2 (i.e., an edge connector) is shown protruding through the container wall 1. A gasket 14 made of compressible material is used to form a seal between the connector and the container wall 1.
An alternative configuration of thermally conductive plate as compared to that shown in FIGS. 1 and 4 is illustrated in FIG. 7, where thermally conductive plate 15 protrudes into the liquid filled space inside the container 2 proximal to circuit board 1 and extends to the exterior of the container. The conductive plate 15 can, for example, be sealed with a compressible material 16 or fused or otherwise bonded directly to the container such that no liquid will leak out.
Such a alternative configuration is illustrated in FIG. 8, where thermally conductive plate 17 (having a generally T-shaped cross-section) penetrates through the wall of container I such that the “head” of the T is on the exterior of the container, and the “leg” of the T is in contact with the liquid in the interior of the container and in proximity to the circuit board 2. The plate is fused or otherwise bonded with the container wall to prevent leaks.
The thermally conductive plate can be configured according to the application. FIG. 9 shows a simple U-shaped plate 15 which wraps around the connector end of the container 2 and that has flanges 18 at each side that are designed to allow the plate to be clamped to another heat transfer element such as a heat pipe. The edge connector portion of circuit board 2 extends through the container wall and through the plate. In most cases, clamping of the plate to another heat transfer element also positively engages the edge connector into a matching external connector.
Where low to moderate heat loads are to be dissipated, a heat sink may be sufficient for heat dissipation (with or without forced air flow) depending on heat load and installation environment. For example, as shown in FIG. 10, a thermally conductive plate 15 that extends into the heat transfer liquid and also extends beyond the exterior of the container walls 1 may be fashioned in the shape of a heat sink, with additional fins 19 added to its configuration to make a heat sink. This configuration allows the thermally conductive plate to be cooled by free or forced convection. The fins may be formed as part of the plate, or may be formed separately and attached to the plate (e.g., glued, soldered, clamped, screwed, and the like).
Where higher heat loads are to be dissipated, the thermally conductive plate can, for example, be attached to another heat transfer element such as a heat pipe or a heat sink, typically by means of a mechanical fastening system. In the exemplary configuration shown in the top perspective view of FIG. 11, the container 1 with thermally conductive plate 15 is fastened to a heat pipe 20 utilizing a clamping mechanism 21. The heat pipe conducts heat to heat exchanger 24 for dissipation.
An over-center type of clamping mechanism as shown in FIG. 12 can be used as the clamping mechanism (as shown in FIG. 11) and provides ease of operation. The clamping mechanism has a lever 22 that moves an opposable jaw 23. As used in the manner shown in FIG. 11, the clamp forces the thermally conductive plate firmly against the other heat transfer element 20 (for example a heat pipe), providing efficient heat transfer between the plate 15 and the heat pipe 20. Parts comprising the clamping mechanism can advantageously be made of thermally conductive material and mounted on the heat pipe 20 providing additional heat flow through the clamping mechanism 21 into the heat pipe.
FIG. 13 shows a bottom perspective view of an assembly as in FIG. 11, and shows the opposite side of the heat pipe 20. As indicated for FIG. 11, located on the heat pipe is a heat exchanger 24 that acts to cool the heat pipe 20.
An assembly as shown in FIG. 13 is shown in cross-section in FIG. 14, where the heat pipe 20 acts as a thermal transfer pathway for heat between the liquid-filled container 1 via thermally conductive plate 15, and the heat exchanger apparatus 24. Several types of heat exchanger apparatus may be employed to cool the heat pipe, including, for example, heat sinks, refrigerant coils and evaporative liquid systems. The heat exchanger choice is usually based upon the level of cooling required and the space available per a given electronics assembly. Circuit board assembly 2 contained inside each fluid filled container 1 connects electrically to other, external electrical devices through one or more electrical connectors (e.g., circuit board edge connectors) that protrude through the container. More than one electrical connector may be required depending on the requirements of the circuits being enclosed. The connector may be mated directly into a circuit board back plane 25 as shown in FIG. 14 or may be mated to a wire terminated, cable connector. In the illustrated configuration, heat flows from the components on the internal circuit board, into the surrounding liquid in the interior 4, into the thermally conductive plate 15, and into the heat pipe 20 for dissipation externally. Heat flow at the interface between thermally conductive plate and the heat pipe can be enhanced by providing a compliant (e.g., soft), thermally conductive layer of material 26 which increases the contact area between the plate surface and the heat pipe surface (or other thermal conductor surface). Such a material can be a compliant material, e.g., a soft gasket, elastomer, or grease with low thermal resistance (i.e., thermal grease). Use of such materials can substantially reduce thermal resistivity at the interface.
When power is turned on, the enclosed electronic devices generate heat and after a short period of time the heat is transferred to adjacent materials through conduction (and internal fluid convection). The electronic devices and adjacent materials reach thermal equilibrium and the maximum amount of heat transfer is achieved. Heat will travel through each section of the thermally conductive system as shown generally by the arrows in FIG. 15
All patents and other references cited in the specification are indicative of the level of skill of those skilled in the art to which the invention pertains, and are incorporated by reference in their entireties, including any tables and figures, to the same extent as if each reference had been incorporated by reference in its entirety individually.
One skilled in the art would readily appreciate that the present invention is well adapted to obtain the ends and advantages mentioned, as well as those inherent therein. The methods, variances, and compositions described herein as presently representative of preferred embodiments are exemplary and are not intended as limitations on the scope of the invention. Changes therein and other uses will occur to those skilled in the art, which are encompassed within the spirit of the invention, are defined by the scope of the claims.
It will be readily apparent to one skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention. For example, variations can be made to the shape of the fluid filled container can have other shapes than those illustrated (such as spherical, pyramidal, asymmetric, etc.); the method of leak proofing the enclosure may use gaskets, sealing compounds, etc.; the conductive plate may attached to the heat pipe with screw fasteners, slide latches, and other clamping mechanisms; etc. Thus, such additional embodiments are within the scope of the present invention and the following claims.
The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein. Thus, for example, in each instance herein any of the terms “comprising”, “consisting essentially of” and “consisting of” may be replaced with either of the other two terms. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.
In addition, where features or aspects of the invention are described in terms of Markush groups or other grouping of alternatives, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group or other group.
Also, unless indicated to the contrary, where various numerical values or value range endpoints are provided for embodiments, additional embodiments are described by taking any 2 different values as the endpoints of a range or by taking two different range endpoints from specified ranges as the endpoints of an additional range. Such ranges are also within the scope of the described invention.
Thus, additional embodiments are within the scope of the invention and within the following claims.

Claims

1. A liquid cooled electronics circuit board, comprising

a circuit board comprising a plurality of heat generating components;

a sealed container surrounding said circuit board and containing a heat transfer, dielectric liquid such that said circuit board is substantially submerged in said liquid, wherein said container comprises a fill port, at least one heat conductive plate that is in contact with said liquid and protrudes to the exterior of said container, and a thermal expansion compensating surface; and

an electrical connector connecting to said circuit board and extending to the exterior of said container.

2. The liquid cooled electronics circuit board of claim 1, wherein said circuit board is installed in a computer system.

3. The liquid cooled electronics circuit board of claim 1, wherein the volume of said liquid is at least 50 ml.

4. The liquid cooled electronics circuit board of claim 1, wherein said heat generating components together generate 20-150 watts.

5. The liquid cooled electronics circuit board of claim 1, wherein said heat generating components together generate 50-100 watts.

6. The liquid cooled electronics circuit board of claim 1, wherein thermal expansion volume of said liquid within said container is accommodated by a flexible diaphragm that is in contact with the liquid on one side and ambient atmosphere on the other side.

7. The liquid cooled electronics circuit board of claim 6, wherein said flexible diaphragm is a low curvature, concave wall of said container.

8. The liquid cooled electronics circuit board of claim 6, wherein said flexible diaphragm is a circular disk diaphragm.

9. The liquid cooled electronics circuit board of claim 1, wherein said heat conductive plate is adjacent said connector.

10. The liquid cooled electronics circuit board of claim 1, wherein said heat conductive plate is distal from said connector.

11. The liquid cooled electronics circuit board of claim 1, wherein said liquid is a fluorocarbon or an oil.

12. The liquid cooled electronics circuit board of claim 1, wherein said container can be opened or removed allowing access to components on said circuit board, and sealably replaced.

13. The liquid cooled electronics circuit board of claim 1, wherein said fill port is configured to allow extraction and replacement of said liquid.

14. The liquid cooled electronics circuit board of claim 1, wherein said heat conductive plate is thermally coupled with a heat sink.

15. The liquid cooled electronics circuit board of claim 14, wherein the thermal coupling is accomplished through a heat conductive elastomeric material.

16. An electronics cooling assembly, comprising

a sealed container substantially filled with a heat transfer, non-electrically conducting liquid and comprising a fill port allowing filling of said container with said liquid after sealing, at least one conductive plate that is in contact with the liquid inside the container, and protrudes to the exterior of the container, and a thermal expansion compensating surface;

at least one circuit board mounted within said container substantially submerged in said liquid; and

an electrical connector connecting to said circuit board and extending to the exterior of the container.

17. The electronics cooling assembly of claim 16, wherein the walls of said container consist essentially of a material selected from the group consisting of polycarbonate, acrylic, and ABS plastic.

18. The electronics cooling assembly of claim 16, wherein said fill port comprises an orifice that can be breached by a nozzle and is self-sealing when the nozzle is retracted.

19. The electronics cooling assembly of claim 16, wherein said heat conductive plate comprises at least 80% copper, aluminum, or heat conductive ceramic.

20. The electronics cooling assembly of claim 16, wherein thermal expansion volume of said liquid within said container is accommodated by a flexible diaphragm that is in contact with the liquid on one side and ambient atmosphere on the other side.

21. The electronics cooling assembly of claim 20, wherein said flexible diaphragm is a low curvature, concave wall of said container.

22. The electronics cooling assembly of claim 20, wherein said flexible diaphragm is a circular disk diaphragm.

23. The electronics cooling assembly of claim 16, wherein said liquid is a fluorocarbon or an oil.

24. A computer system comprising

a plurality of electronics cooling assemblies, wherein each said cooling assembly comprises

a circuit board mounted within said container substantially submerged in said liquid; and

an electrical connector connecting to said circuit board that protrudes to the exterior of said container.

25. A method for cooling a complex electronics assembly, comprising

enclosing said complex electronic assembly in a sealed container substantially filled with a heat transfer, non-electrically conducting liquid and comprising a fill port allowing filling of said container with said liquid after sealing, at least one conductive plate that is in contact with the liquid inside the container, and protrudes to the exterior of the container, a thermal expansion compensating surface, and an electrical connector connecting to said circuit board and extending to the exterior of said container.

26. The method of claim 25, further comprising thermally coupling said conductive plate with a heat transfer device.

27. A method for thermally coupling a thermally conductive plate that conducts heat from an electronic assembly, with a thermally conductive device, comprising

mechanically compressing a thermally conductive compliant material between said thermally conductive plate and said thermally conductive device, wherein an electronic connector connected with said electronic assembly is mounted adjacent to or penetrates said thermally conductive plate.

28. The method of claim 27, wherein said mechanical compression is performed using at least one over-center clamp.

29. A cooling assembly for heat generating electronics, comprising

a first thermally conductive plate which conducts heat from said heat generating electronics;

a second thermally conductive element attached to a surface of said thermally conductive plate using a means for providing a low thermal resistance connection between the conductive plate and second heat transfer element, and

a first electronics connector adjacent to or penetrating said first thermally conductive plate which connects said heat generating electronics with separate electronic components.

30. The cooling assembly of claim 29, wherein said means for providing a low thermal resistance connection comprises a high thermal conductance elastomer and an over-center clamping mechanism.

31. The cooling assembly of claim 29, further comprising a second electronic connector mounted on said second thermally conductive element which connects with said first electronic connector.