US20050274487A1

US20050274487A1 - Method and apparatus for reducing thermal resistance in a vertical heat sink assembly

Info

Publication number: US20050274487A1
Application number: US10/855,046
Authority: US
Inventors: Gary Goth
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2004-05-27
Filing date: 2004-05-27
Publication date: 2005-12-15

Abstract

A method and apparatus for an electronic package includes a substrate; a heat source component operably coupled to the substrate, and in direct contact with and electrically connected to a top surface of the substrate; a heat sink assembly in thermal communication with the substrate. The heat sink assembly includes a plurality of distinct vapor chambers, each containing a heat transfer fluid configured to evaporate on a wall in thermal contact with a back surface of the heat source component and condense on an opposing wall defining an exterior wall defining the vapor chambers. Each of the plurality of distinct vapor chambers are serially aligned having facing sidewalls defining each relative to contiguous vapor chambers and at least one of the plurality of distinct vapor chambers includes a lower sidewall defining one distinct vapor chamber substantially aligned with a bottom defining the heat source component such that a bottom portion defining the one distinct vapor chamber is substantially aligned with a bottom portion of the heat source component.

Description

BACKGROUND OF THE INVENTION

The present invention relates to dissipating heat generated by integrated circuit (IC) modules, and a method of constructing such devices. In particular, the present disclosure relates to a method and apparatus for eliminating a dry out condition of a heat transfer or cooling fluid in a vertical heat sink assembly configured to dissipate heat generated by integrated circuit modules.
As is known, operating electronic devices produce heat. This heat should be removed from the devices in order to maintain device junction temperatures within desirable limits: failure to remove the heat thus produced results in increased device temperatures, potentially leading to thermal runaway conditions. Several trends in the electronics industry have combined to increase the importance of thermal management, including heat removal for electronic devices, including technologies where thermal management has traditionally been less of a concern, such as CMOS. In particular, the need for faster and more densely packed circuits has had a direct impact on the importance of thermal management. First, power dissipation, and therefore heat production, increases as the device operating frequencies increase. Second, increased operating frequencies may be possible at lower device junction temperatures. Finally, as more and more devices are packed onto a single chip, power density (Watts/cm²) increases, resulting in the need to remove more power from a given size chip or module. These trends have combined to create applications where it is no longer desirable to remove the heat from modern devices solely by traditional air cooling methods, such as by using traditional air cooled heat sinks.
For example, with the advent of multichip modules (MCMs), containing multiple integrated circuit (IC) chips each having many thousands of circuit elements, it has become possible to pack great numbers of electronic components together within a very small volume. As is well known, ICs generate significant amounts of heat during the course of their normal operation. Since most semiconductor or other solid state devices are sensitive to excessive temperatures, a solution to the problem of the generation of heat by IC chips in close proximity to one another in MCMs is of continuing concern to the industry.
A conventional approach to cooling components in electronic systems in which devices contained in MCMs are placed on printed circuit/wire boards or cards is to direct a stream of cooling air across the modules with the addition of heat sinks attached to the module to enhance the effectiveness of the airflow.
Limitation in the cooling capacity of the simple airflow/heat sink approach to cooling has led to the use of another technique, which is a more advanced approach to cooling of card-mounted MCMs. This technique utilizes heat pipe technology. Heat pipes per se are of course, well known and heat pipes in the form of vapor chambers are becoming common. In the related art, there are also teachings of heat pipes/vapor chambers for dissipating the heat generated by electronic components mounted on cards.
One approach includes using a cooling fluid or heat transfer fluid in a vapor chamber heat sink. A vapor chamber base enables heat sinks to perform better as the thermal resistance to spreading the heat in the base is reduced. The heat is removed from one side of the base in thermal communication with a heat source by evaporation of the heat transfer fluid and travels rapidly in a gaseous state until it condenses on a fin side of the base. In this manner, the heat is transferred from the base to the fins for subsequent conduction to convectively cooled fins extending from the base.
However, vapor chamber technology has several limitations when applied to MCMs. One limitation is that the above described heat transfer mechanism can fail if inadequate heat transfer fluid or cooling fluid is present on the evaporator surface of the base near the heat source. This is often the case when the vapor chamber is positioned vertically such that gravity causes the returning or condensed cooling fluid to accumulate at a lower area of the vertically oriented vapor chamber. For applications where the heat source is centrally located with respect to the vertically oriented heat sink, dry out conditions are often created near the heat source.
For the foregoing reasons, therefore, for an efficiently cooled electronic module or MCM that employs vapor chamber cooling. In particular, there is a need in the art for a method and apparatus of providing a vertically oriented vapor chamber and corresponding heat source to be cooled with a fluid coolant, while simultaneously eliminating dry out conditions near the heat source.

SUMMARY OF THE INVENTION

One embodiment is an electronic package includes a substrate; a heat source component operably coupled to the substrate, and in direct contact with and electrically connected to a top surface of the substrate; a heat sink assembly in thermal communication with the substrate. The heat sink assembly includes a plurality of distinct vapor chambers, each containing a heat transfer fluid configured to evaporate on a wall in thermal contact with a back surface of the heat source component and condense on an opposing wall defining an exterior wall defining the vapor chambers. Each of the plurality of distinct vapor chambers are serially aligned having facing sidewalls defining each relative to contiguous vapor chambers and at least one of the plurality of distinct vapor chambers includes a lower sidewall defining one distinct vapor chamber substantially aligned with a bottom defining the heat source component such that a bottom portion defining the one distinct vapor chamber is substantially aligned with a bottom portion of the heat source component.
Another embodiment is a method for lowering a thermal resistance of a vertically oriented heat sink assembly to dissipate heat from a heat source component. The method includes configuring a heat sink assembly with a plurality of distinct vapor chambers, each of the distinct vapor chambers containing a heat transfer fluid configured to evaporate on a wall in thermal contact with a back surface of the heat source component and condense on an opposing wall defining an exterior wall of the heat sink assembly; and configuring each of the plurality of distinct vapor chambers to be serially aligned having facing sidewalls defining each relative to contiguous vapor chambers and at least one of the plurality of distinct vapor chambers includes a lower sidewall defining one distinct vapor chamber substantially aligned with a bottom defining the heat source component such that a bottom portion defining the one distinct vapor chamber is substantially aligned with a bottom portion of the heat source component.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:
FIG. 1 depicts a perspective view of a partially populated central electronics complex (CEC) illustrating an exposed vertically oriented MCM and a vertically oriented heat sink assembly disposed over another MCM;
FIG. 2 depicts a partial exploded cross section view of the vertically oriented heat sink assembly disposed over a MCM of FIG. 1;
FIG. 3 depicts a partial exploded cross section view of an exemplary embodiment of a vertically oriented heat sink assembly having vapor chambers integrated into a base of a heat sink disposed over a MCM of FIG. 1;
FIG. 4 depicts a schematic side view of a prior art vertically oriented heat sink assembly having a single vapor chamber in thermal communication with a centrally located heat source;
FIG. 5 depicts a schematic side view of a vertically oriented heat sink assembly having two distinct vapor chambers in thermal communication with the heat source of FIG. 4 in accordance with an exemplary embodiment of the present disclosure; and
FIG. 6 depicts a schematic side view of a vertically oriented heat sink assembly having two distinct vapor chambers in thermal communication with the heat source of FIG. 5 in accordance with an alternative exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure will now be described in more detail by way of example with reference to the embodiments shown in the accompanying figures. It should be kept in mind that the following described embodiments are only presented by way of example and should not be construed as limiting the inventive concept to any particular physical configuration.
Further, if used and unless otherwise stated, the terms “upper”, “lower”, “front”, “back”, “over”, “under”, and similar such terms are not to be construed as limiting the invention to a particular orientation. Instead, these terms are used only on a relative basis.
For the purposes of the present disclosure, the terms printed circuit board (PCB) and printed wire board (PWB) are equivalent terms. The terms “in contact” and “contacting” indicate mechanical and thermal contact
FIG. 1 illustrates a so-called central electronics complex 10 (CEC) of a computer system. The CEC 10 is comprised of an enclosure (such as a cage 12), a backplane or midplane 14 as illustrated, and a circuit board or daughter card, such as a blade or node 16 having two processor multi-chip modules (MCM) 17, and a corresponding vapor chamber heat sink assembly 20 disposed over each MCM 17 (only one shown of each for sake of clarity), 256 GB memory on 16 cards (not shown), an input/output (I/O) card 18, and a control multiplexer card (not shown), for example, attachable to the backplane 14. Air inlets 22 are shown at a back of CEC 10 and air flow across fins of heat sink assembly 20 and out air exhausts 24 is generally shown with arrows 26.
FIG. 2 is a partial cross-sectional view of an exemplary embodiment of a multichip module (MCM) mounted on a PCB having a lid in accordance with the present disclosure. It will be recognized by one skilled in the pertinent art that a dual chip module (DCM) is also contemplated for use in the present disclosure. In FIG. 2, MCM 100 includes a substrate 102 having a multiplicity of components 105 mounted thereto, each component having a front surface 110 and a back surface 115. MCM 100 is mounted to a PCB 120 by a multiplicity of solder balls 125. Substrate 102 may be a single or multi-level substrate and may be ceramic, fiberglass or polymer based. MCM 100 also includes a lid 130. Lid 130 is mounted to substrate 102 by lid support 132 connecting the periphery of lid 130 to the periphery of substrate 102. Lid support 132 may be fabricated from the same material as lid 130 and may be integral with the lid. Alternatively, lid support 132 may be fabricated from a material different from that of lid 130. Lid support 132 may provide a hermetic seal between lid 130 and substrate 102.
Lid 130 includes a lower wall 135 having an outer surface 140, an upper wall 145 having an outer surface 150 and sidewalls 155 defining a vapor chamber 160. It will be noted that opposing sidewalls 155 defining vapor chamber 160 are shown closer together than with respect to FIG. 5 for sake of clarity in describing chamber 160. Moreover, FIG. 2 is shown with a single vapor chamber 160, wherein two or more vapor chambers 160 are included in an exemplary embodiment depicted in FIG. 5 in accordance with the present disclosure.
Vapor chamber 160 contains a heat transfer fluid such as, inter alia, water, freon or glycol. Front sides 110 of components 105 are electrically connected to a top surface 165 of substrate 102. Components 105 may be flip chip, wire-bonded or soldered to substrate 102. A thermal transfer medium 170 is in contact with back surfaces 115 of components 105 and outer surface 140 of lower wall 135 of lid 130 to enable thermal contact, mechanical restraint and pressure support over the contacting region. Thermal transfer medium 170 enables heat generated by the operation of components 105 to be efficiently transferred to lid 130.
Because of the excellent heat transfer capability afforded to lid 130 by vapor chamber 160, the lid may be fabricated from many different materials including but not limited to metals such as aluminum, copper, nickel, gold or Invar and other materials such as plastics, ceramics and composites. Because of the wide range of materials available, lid 130 may fabricated from a material having a CTE matched to (between about 25% to 700% of the coefficient of thermal expansion) substrate 102 or from the same material as the substrate. For example, if MCM 100 is a HyperBGA® International Business Machine Corp., Armonk, N.Y., in which substrate 102 is a polytetraflouroethylene (PTFE) based material having a CTE of about 10-12 ppm/° C., then lid 130 may be fabricated from an aluminum-silicon carbide composite having a CTE of about 10 ppm/° C.
Thermal transfer medium 170 may include a thermal adhesive, thermal grease, thermal-conductive pads, phase change or other materials known in the art.
A heat sink 180 having a plurality of horizontal fins 182 (see also FIG. 1) is in thermal communication with outer surface 150 of lid 130. Heat sink 180 is shown remove from outer surface 150 for sake of clarity. Heat sink 180 may be formed from aluminum, copper, beryllium, white metal or any other suitable material with high heat conductivity. Furthermore, it will be recognized by one skilled in the pertinent art that heat sink 180 may be fabricated from a material having a CTE matched to (between about 25% to 700%) the CTE of lid 130. Moreover, it will be recognized that although heat sink 180 and lid 130 are shown as separable parts, heat sink 180 may be integrated with lid 130 in a single integral part.
In an exemplary embodiment referring to FIG. 3, vapor chamber 160 is integrated in a base defining heat sink 180 while lid 130 is a solid substrate. It will be recognized by one skilled in the pertinent art that having vapor chamber 160 integrated in a base of the heat sink versus lid 130, enables the vapor chambers to spread the heat well beyond the confines of the cap of a module (e.g., lid 130). In an exemplary embodiment depicted in FIG. 3, a width of the heat sink vapor chambers are about twice the width of lid 130. In either case, it is noted that the vapor chamber heat sink assembly 20 of FIG. 2 includes a combination of heat sink 180 and lid 130 or a base of heat sink 180 having vapor chamber 160 integrated therewith, as in FIG. 3.
While MCM 100 has been illustrated in FIGS. 1 and 2 and described above as a ball grid array (BGA) module, MCM 100 may be pin grid array (PGA) module. Instead of solder balls 125 (see FIG. 2), Land Grid Array (LGA) connections between substrate 102 and PCB 120 are also contemplated. Since LGA connections are asperity contact connections, generally some degree of pressure must be maintained on the connection to ensure good electrical conductivity. Therefore, flanges 184 defining ends of heat sink 180 may be used accept a mechanical fastener and engage substrate 120 to provide the necessary pressure to ensure suitable electrical conductivity.
Referring now to FIG. 4, a vertically oriented heat sink 180 is in thermal communication with a prior art evaporator 200 having a corresponding vertically oriented single vapor chamber 260, which is in turn in thermal communication with a heat source 210. Heat source 210 is centrally located with respect to a bottom surface 240 defining a length of evaporator 200. It will be recognized that heat source 210 may be MCM 100 as described above. In many such evaporators, a heat transfer fluid generally indicated at 220 such as, inter alia, water, has a tendency to accumulate at a bottom of vapor chamber 260 generally indicated at 224 because of gravity acting on transfer fluid 220. The result is inadequate liquid to evaporate heat load proximate heat source 210 and evaporator 200 proximate the heat source 240 dries out as the heat therefrom is conducted further towards the water accumulation at 224, reducing thermal performance of the assembly. The thermal performance is reduced by the increase in thermal resistance due to an increase in path length for the heat to travel to heat sink 180 because of the dried out section local to the centrally located heat source 210.
Referring now to FIG. 5, vertically oriented heat sink 180 is illustrated in thermal communication with an exemplary embodiment of an evaporator 300 defining at least two distinct vapor chambers 360, which is in turn in thermal communication with heat source 210. Heat source 210 is substantially centrally located with respect to a bottom surface 340 defining a length of evaporator 300 as described with respect to heat source 210 in FIG. 3, however, heat source 210 may be aligned anywhere along a length defining bottom surface 340. It will be recognized that heat source 210 may be MCM 100 as described above. Since heat transfer fluid 220 has the tendency to accumulate at a bottom of vertically oriented vapor chamber 260 in FIG. 4, vapor chamber 360 in FIG. 5 includes an upper vapor chamber 361 and a lower vapor chamber 362 separated from upper chamber 361 via a horizontal barrier 370 therebetween. Barrier 370 is configured to prevent condensed heat transfer fluid 220 from being pulled by gravity past heat source 210 into vapor chamber 362. Barrier 370 extends from bottom surface 340 to upper surface 350, substantially normal to both.
In an exemplary embodiment as illustrated, barrier 370 is a horizontal solid section separating vapor chamber 360 into two distinct chambers, 361, 362. The larger upper vapor chamber 361 will have an ample supply of heat transfer fluid available near heat source 210 in spite of gravity acting thereon in vertical mount applications. Even though the lower vapor chamber 362 still works against gravity, lower vapor chamber 362 still provides some added cooling. Vapor chambers 361 and 362 together outperform a single vapor chamber in most vertical applications because adequate liquid is available and local to heat source 210 to evaporate heat load proximate heat source 210. Heat from heat source 210 is less prone to dry out vapor chamber 361 since a bottom of vapor chamber 361 is substantially aligned with heat source 210. In particular, a bottom of heat source 210 substantially coincides with a bottom of vapor chamber 361 where heat transfer fluid 220 would tend to accumulate due to gravity. Thus, the heat transfer path is less likely to increase because vapor chamber 361 insures that the local evaporator area is wet, thus lowering thermal resistance of heat transfer to heat sink 180.
The thermal performance is increased by lowering thermal resistance due to a decreased path length for the heat to travel to heat sink 180 because of eliminating a dried out section local to the centrally located heat source 210.
In an exemplary embodiment, upper vapor chamber 361 is configured having a bottom portion thereof substantially aligned or alternatively, not extending much past heat source 210 insuring that the evaporator area local to the heat source 210 is wet with condensed heat transfer fluid 220. The lower vapor chamber 362 works against gravity over a region similar to that described with respect to the single vapor chamber 260 in FIG. 3, but still provides some added cooling. The mean path for the heat to travel from heat source 210 to find condensed heat transfer fluid is reduced by two or more separate vapor chambers defining evaporator 300, thus lowering the thermal resistance to heat sink 180. Although, upper vapor chamber 361 has been described as being larger, e.g., longer relative to vertical, than lower vapor chamber 362, vapor chambers may be configured having substantially the same length or lower chamber 362 may be longer than upper vapor chamber 361.
For example, FIG. 6 depicts lower vapor chamber 362 configured longer than upper chamber 361. In this case, it will be noted that heat source 210 is then more efficiently disposed towards an upper portion defining a length of bottom surface 340.
Thus, an efficiently cooled IC, such as a MCM, that employs vapor chamber cooling with a plurality of separate vapor chambers in thermal communication with a vertically oriented heat sink assembly while minimizing dry out conditions and reducing thermal resistance has been described.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention is not to be limited to the particular embodiment disclosed as the best or only mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

Claims

1. An electronic package comprising:

a substrate;

a heat source component operably coupled to said substrate, said component in direct contact with and electrically connected to a first surface of said substrate;

a heat sink assembly in thermal communication with an opposite second surface of said substrate, said heat sink assembly including a plurality of discrete vapor chambers vertically aligned with respect to each other, each of said plurality of discrete vapor chambers containing a heat transfer fluid in thermal contact with a back surface of said heat source component, said heat transfer fluid configured to evaporate from a bottom portion defining each of said plurality of discrete vapor chambers and condense on an upper surface defining an top portion of each of said vapor chambers of said heat sink assembly; and

wherein said each of said plurality of discrete vapor chambers are serially aligned having facing lower and upper surfaces defined by at least one wall therebetween corresponding to contiguous vapor chambers, a lower surface defining one of said plurality of discrete vapor chambers being substantially aligned with a bottom defining said heat source component such that the lower surface defining said one of said plurality of discrete vapor chambers is substantially aligned with a bottom portion of said heat source component.

2. The electronic package of claim 1, wherein said heat sink assembly includes a heat sink in thermal communication with an outer surface defining a lid attached to said substrate.

3. The electronic package of claim 2, wherein said heat sink is integrally formed with said lid.

4. The electronic package of claim 2, wherein said plurality of discrete vapor champers are separated by a horizontal barrier extending from said lower surface to said upper surface and defining contiguous vapor chambers.

5. The electronic package of claim 1, wherein said plurality of vapor chambers includes an upper vapor chamber and a lower vapor chamber.

6. The electronic package of claim 5, wherein when said heat source component is substantially centrally located relative to a length defining said heat sink assembly, said upper vapor chamber is longer than said lower vapor chamber.

7. The electronic package of claim 5, wherein when said heat source component is located substantially above a central location relative to a length defining said heat sink assembly, said lower vapor chamber is longer than said upper vapor chamber.

8. The electronic package of claim 1, wherein said package is selected from the group consisting of ball grid array modules, pin grid array modules, land grid array modules and HyperBGA® modules.

9. The electronic package of claim 2, wherein said lid is formed from material selected from the group consisting of aluminum, copper, Invar, gold, silver, nickel, aluminum-silicon carbide, plastics, ceramics and composites.

10. The electronic package of claim 1, wherein said substrate includes material selected from the group consisting of ceramics, fiberglass, polytetraflouroethylene, and polymers.

11. The electronic package of claim 1, wherein a solid thermal transfer medium is in direct contact with a back surface of each heat source component and an outer surface of a lower wall of said heat sink assembly.

12. The electronic package of claim 1, wherein said heat source component is one of a DCM and a MCM.

13. A method for lowering a thermal resistance of a vertically oriented heat sink assembly to dissipate heat from a heat source component, the method comprising:

configuring a heat sink assembly with a plurality of discrete vapor chambers, each of said plurality of discrete vapor chambers containing a heat transfer fluid in thermal contact with a back surface of the heat source-component, said heat transfer fluid configured to evaporate from a bottom portion defining, each of said plurality of discrete vapor chambers and condense on an upper surface a top portion of each of said vapor chambers of said heat sink assembly; and

configuring said each of said plurality of discrete vapor chambers to be serially, vertically aligned having facing lower and upper surfaces defined by at least one wall therebetween corresponding to contiguous vapor chambers, a lower surface defining one of said plurality of discrete vapor chambers being substantially aligned with a bottom defining the heat source component such that the lower surface defining said one discrete vapor chamber is substantially aligned with a bottom portion of the heat source component.

14. The method of claim 13, further comprising:

disposing a heat sink in thermal communication with an outer surface defining lid attached to said substrate.

15. The method of claim 14, further comprising:

integrally forming said heat sink with said lid.

16. The method of claim 14, further comprising:

separating said plurality of discrete vapor champers with a horizontal barrier extending from said lower surface to said upper surface defining contiguous vapor chambers.

17. The method of claim 13, wherein said plurality of vapor chambers includes an upper vapor chamber and a lower vapor chamber.

18. The method of claim 17, further comprising:

locating the heat source component in a central location relative to a length defining said heat sink assembly, wherein said upper vapor chamber is longer than said lower vapor chamber.

19. The method of claim 17, further comprising:

locating the heat source component substantially above a central location relative to a length defining said heat sink assembly, wherein said lower vapor chamber is longer than said upper vapor chamber.

20. The method of claim 13, further comprising:

disposing a solid thermal transfer medium in direct contact with a back surface of each heat source component and an outer surface defining said heat sink assembly.

21. The method of claim 13, wherein the heat source component is one of a DCM and a MCM.