Cavity plate and jet nozzle assemblies for use in cooling an electronic module, and methods of fabrication thereof

[0002] The present invention relates to heat transfer mechanisms, and more particularly, to heat transfer mechanisms and cooling assemblies for removing heat generated by an electronic circuit module.

[0003] The efficient extraction of heat from electronic circuit modules for very large scale integrated circuit packages has presented a significant limitation on the design and use of such electronic modules. The power consumed in the integrated circuits generates heat which must in turn be removed from the package. Lacking an efficient heat transfer mechanism, the speed, reliability and power capabilities of the electronic circuit modules are limited. As the density of circuitry within very large scale integrated circuit chips has increased, the need for improved heat extraction has become even more acute since more densely packed chips tend to have a higher need for heat dissipation per unit area. It is also known that runaway thermal conditions and excessive heat generated by chips is a leading cause for failure of chip devices. Furthermore, it is anticipated that demand for heat removal from these devices will continue to increase indefinitely. Accordingly, it is seen that there is a significant need to continue to further improve upon cooling mechanisms for electronic devices.

[0004] Briefly summarized, the present invention comprises in one aspect a cooling assembly for an electronic module. The cooling assembly includes a thermally conductive cavity plate and at least one jet nozzle. The cavity plate has a first main surface and a second main surface with at least one blind hole formed in the second main surface extending towards the first main surface. Each blind hole has a lower surface and a side surface. The lower surface and the side surface connect at a non-orthogonal angle to facilitate the flow of fluid therealong. Each jet nozzle is sized to reside within a respective blind hole of the cavity plate. Fluid is introduced into the cavity plate through the at least one jet nozzle, and impinges upon the lower surfaces of the respective blind hole and flows outward through space defined between the side surface of the blind hole and the side surface of the jet nozzle. The non-orthogonal angle between the lower surface and the side surface of the blind hole facilitate fluid flow, after impinging upon the lower surface, outward through the space defined between the side surface of the blind hole and the side surface of the jet nozzle.

[0005] In another aspect, the present invention comprises a cooling assembly for an electronic module, wherein the cooling assembly includes a thermally conductive cavity plate and at least one jet nozzle. The cavity plate has a first main surface and a second main surface with at least one blind hole formed in the second main surface extending towards the first main surface. Each blind hole has a lower surface and side surface. Each jet nozzle is sized to reside within a respective blind hole of the cavity plate. Fluid introduced into the cavity plate through a jet nozzle impinges upon the lower surface of the respective blind hole and flows outward through a space defined between the side surface of the blind hole and a side surface of the jet nozzle. At least one jet nozzle includes a side surface having a plurality of radial channels disposed therein. The space defined between the side surface of the blind hole and the side surface of this jet nozzle includes the plurality of radial channels. Fluid delivered through the jet nozzle impinges upon the lower surface of the respective blind hole and flows outward from the blind hole at least partially through the plurality of radial channels disposed within the jet nozzle.

[0006] In still another aspect, a method of fabricating a cooling assembly for an electronic module is presented. The method includes: providing a thermally conductive cavity plate having a first main surface and second main surface with a plurality of blind holes formed in the second main surface extending towards the first main surface, wherein each blind hole has a lower surface and a side surface; providing a plurality of jet nozzles, at least one jet nozzle including radially extending pedestals on a side surface thereof which are sized to physically contact the sidewall of the respective blind hole when the jet nozzle is disposed at least partially therein; and interference fitting the at least one jet nozzle into a respective blind hole, wherein fluid introduced into the cavity plate through the jet nozzle impinges upon the lower surface of the blind hole and flows outward through the space defined between the side surface of the blind hole and the side surface of the jet nozzle, wherein the radially extending pedestals physically contacting the side surface of the blind hole define the size of the space between the side surface of the blind hole and the side surface of the jet nozzle.

[0007] In a further aspect, the invention comprises a method of fabricating a cooling assembly for an electronic module. This method includes: providing a thermally conductive cavity plate having a first main surface and a second main surface with a plurality of blind holes formed in the second main surface extending toward the first main surface, wherein each blind hole has a lower surface and a side surface, and radially inwardly extending fins disposed along the side surface; providing a plurality of jet nozzles, each jet nozzle including radially inwardly extending fins on a side surface surface configured to interdigitate with the inwardly extending fins on the side surface of a respective blind hole when the jet nozzle is disposed within the blind hole, with a gap being defined between the interdigitated fins; and disposing each jet nozzle of the plurality of jet nozzles within the respective blind hole, wherein fluid introduced into the cavity plate through a jet nozzle impinges upon the lower surface of the respective blind hole and flows outward through the gap defined between the interdigitated fins of the side surfaces of the blind hole and the jet nozzle.

[0008] To restate, various enhanced cavity plate and jet nozzle assemblies are disclosed herein to facilitate the removal of heat from a structure, such an electronic circuit module. These assemblies enhance the ability to remove a large amount of heat with a low temperature difference. Further, the assemblies presented provide a low coolant pressure drop, and are compact and modular in design.

[0009] Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention.

[0010] The above-described objects, advantages and features of the present invention, as well as others, will be more readily understood from the following detailed description of certain preferred embodiments of the invention, when considered in conjunction with the accompanying drawings in which:

[0011] FIG. 1 illustrates a schematic of a hybrid air—closed loop coolant system to employ a cooling plate assembly in accordance with the principles of the present invention;

[0012] FIG. 2 is a cross-sectional elevational view of one embodiment of a jet cavity plate with a jet nozzle disposed within a blind hole formed therein, and depicting the basic fluid flow concept of a jet cavity cold plate;

[0013] FIGS. 3A-3D depict various cross-sectional elevational embodiments of jet nozzles and blind hole cavity plates formed in accordance with one aspect of the present invention;

[0014] FIG. 4A is a perspective view of one embodiment of a jet nozzle having multiple radially extending pedestals disposed along a side surface thereof in accordance with another aspect of the present invention;

[0015] FIG. 4B is an elevational view of the jet nozzle embodiment of FIG. 4A;

[0016] FIG. 4C is a top plan view of the jet nozzle embodiment of FIG. 4A;

[0017] FIG. 4D is a top plan view of the jet nozzle embodiment of FIG. 4A interference fit within a blind hole of a cavity plate in accordance with the principles of the present invention;

[0018] FIG. 5 is an exploded view of one embodiment of a cooling plate assembly employing a plurality of jet nozzles and a cavity plate in accordance with the principles of the present invention;

[0019] FIG. 6 is a plan view of the assembled cavity plate and jet nozzles of the cooling assembly of FIG. 5 in accordance with the principles of the present invention;

[0020] FIG. 7 is a cross-sectional elevational view of one embodiment of the assembled cooling assembly of FIG. 5;

[0021] FIG. 8 is a top plan view of an alternative embodiment of a jet nozzle in accordance with another aspect of the present invention;

[0022] FIG. 9A is a top plan view of still another embodiment of a jet nozzle in accordance with a further aspect of the present invention;

[0023] FIG. 9B is a plan view of one embodiment of a blind hole in a cavity plate for use with the jet nozzle embodiment of FIG. 9A in accordance with the principles of the present invention;

[0024] FIG. 9C is a top plan view of the jet nozzle of FIG. 9A disposed within the blind hole of FIG. 9B showing the outwardly projecting fins of the jet nozzle interdigitated with the inwardly projecting fins of the cavity plate in accordance with one aspect of the present invention;

[0025] FIG. 10A is a cross-sectional elevational view of one embodiment of the cavity plate and blind hole depicted in FIG. 9B taken along line B—B;

[0026] FIG. 10B is a cross-sectional elevational view of an alternate embodiment of the blind hole and cavity plate of FIG. 9B taken along line B—B;

[0027] FIG. 11 is a top plan of a further embodiment of a jet nozzle and blind hole cavity plate in accordance with the principles of the present invention, wherein multiple physical contacts are provided between the interdigitated fins of the jet nozzle and the blind hole cavity; and

[0028] FIG. 12 is a top plan view of an alternate embodiment of a jet nozzle and blind hole cavity in accordance with the principles of the present invention, wherein spacing between the interdigitated fins of the jet nozzle and the blind hole cavity wall is maintained by multiple interference fit key structures disposed therebetween.

[0029] As noted above, computers continue to increase both in speed and function. Dissipated heat increases accordingly, even for complementary metal oxide semiconductor (CMOS) circuitry that was once considered a low power technology. The limits of existing cooling medium for large server processors are quickly being approached using conventional direct air cooling technologies (air cooled heat sinks) as heat dissipation rises. One possible extension to direct air cooling is a hybrid/closed loop coolant system 10 such as depicted in FIG. 1. With this system, heat is transferred from an electronic module 12 to the coolant (for example, water) via a cold plate 14 which is thermally coupled to the module 12. Coolant is pumped 16 through an air/coolant heat exchanger 18 for dissipation of the heat to the surrounding environment. The present invention is directed in one aspect to an enhanced cold plate 14 for use within a cooling structure, such as depicted in FIG. 1. Those skilled in the art, however, will note that the present invention can be applied to other cooling applications, and work with a multitude of coolant fluids. Circulating water systems or the evaporator/cold head of a refrigeration system are examples.

[0030] Disclosed herein are novel cooling assemblies, and methods of manufacture thereof, for high thermal performance cold plates in which a fluid impinges upon lower surfaces of cavities within a plate in thermal contact with an electronics module. In addition to the heat that is transferred to the fluid at the lower surface of the cavity, heat is also transferred across the cavity walls by virtue of a very small annular gap formed between each cavity wall and the fluid impinging jet nozzle. Heat is also conducted into the jet nozzle, in certain embodiments, through physical contact of each jet nozzle to the side wall of the respective cavity, thus enhancing the heat transfer into the body from an electronic module.

[0031] A jet cavity cold plate includes a plurality of cavities fabricated within a relatively high thermal conductivity material. FIG. 2 depicts the basic concept for a single cavity 20 within a jet cavity cold plate 22. A jet nozzle 24 is shown partially disposed within jet cavity 20. Cavity 20 is referred to herein as a blind hole, and extends from a first main surface of the cavity plate into the plate towards a second main surface, i.e., the surface receiving the heat flux in FIG. 2. Water, or any other suitable coolant, is directed down the center of nozzle 24 and impinged onto the lower surface 21 of blind hole 20. The jet effluent then flows outward and up a very thin annulus formed between the nozzle 24 and the blind hole sidewall 23. After leaving the annulus, the effluent is directed out of the cold plate, for example, through an outlet plenum (not shown). Very high heat transfer coefficients are established on the cavity surfaces thus making the structure a very low thermal resistance cold plate.

[0032] Although the basic concept defining the function of the above-summarized jet cavity cold plate has been established, specific enhancements are disclosed herein to further improve thermal performance. In FIG. 2, applicants recognize that there exists a “dead” fluid zone where lower surface 21 of the blind hole 20 meets side surface 23 of the blind hole. The result of this orthogonal intersection is a relatively poor heat transfer to the coolant in this region. The heat transfer characteristics can be improved by providing curvature to the region as shown in FIG. 3A. Even greater enhancement may be realized by providing a matching curvature to the nozzle. See, for example, FIGS. 3B-3D.

[0033] In FIG. 3A, a blind hole 30 is defined within a cavity plate 32, and is shown with a jet nozzle 34a. In this embodiment, lower surface 31 of blind hole 30 connects to side surface 33 thereof via a curved portion 35.

[0034] In FIG. 3B, jet nozzle 34bhas a side surface and lower surface interface which is also slightly curved or angled 37 to further facilitate the transfer of fluid injected into the blind hole 30 outward. In FIG. 3C, jet nozzle 34chas a curved lower surface 39, which again facilitates the removal of fluid from the blind hole after impinging upon the lower surface of the blind hole. In FIG. 3D, the lower surface and side surface of the jet nozzle 34dare configured to conform to the lower surface and sidewall surface, respectively, of blind hole 30 in cavity plate 32. In this embodiment, a uniform spacing is established between the nozzle and the blind hole surface walls.

[0035] The heat transfer that takes place within the annulus between the blind hole sidewall and the nozzle surface is inversely proportional to the annulus width between the two surfaces. It is therefore preferable to make the annulus width as small as practical, preferably on the order of 0.1 mm (i.e., 0.004″). However, if this dimension is to vary by as much as 0.001″, the average heat transfer coefficient and annulus could change by 25%. Thus, the annulus should be formed with very tight tolerance in its characteristic dimension. This can be accomplished by creating an interference fit between the blind hole sidewalls and individual jet nozzles which are to be disposed therein in accordance with the present invention.

[0036] When two members are inserted into one another (for example, a shaft into a hole), and there is no resulting clearance between them, an interference fit is said to be made. The result is a tight joint which has a finite pressure between the two members where the common surfaces contact. It is this pressure that “holds” the pieces together.

[0037] By way of example, interference fits can be made in two ways. First, they can be made simply by forcing two members together. This method is usually termed a forced fit or a press fit. The second method, typically referred to as an expansion fit, utilizes the principle of thermal expansion. For example, the part to be inserted is reduced in temperature thus compacting (or shrinking) the member. Sometimes, the receiving member is heated to correspondingly expand the hole. The members are then fitted together and allowed to return to room temperature. In doing so, the inner part expands and the outer part contracts. The result is an interference fit with a significantly greater contact pressure that can be achieved by press fitting alone.

[0038] FIGS. 4A-4D illustrate how an annulus may be defined when a jet nozzle is interference fitted within a blind hole formed in a cavity plate. In this embodiment, the jet nozzle 40 has multiple radially extending pedestals 42 disposed along a side surface thereof. These pedestals 42 are sized and configured to physically contact with the inner side surface 43 of a blind hole in a cavity plate 44 (see FIG. 4D).

[0039] In one fabrication embodiment of nozzle 40, the jet nozzle's outer side surface can be altered to remove material to a prescribed depth (e.g., 1 mm over all but a small percentage, for example 15-20%) of the nozzle surface area. The alteration can be accomplished through mechanical or chemical means. An example of a mechanical process would be a skiving operation, and an example of a chemical operation would be an etching process. The resulting tolerance on the annuli would be less than 10%.

[0040] In addition to defining a narrow annuli using the above fabrication/assembly process, the high pressure contact between the nozzles and the blind hole surface walls will also serve as thermal conduits for heat to pass into the jet nozzles, i.e., providing the nozzles are made of a relatively high thermal conductivity material (e.g., copper). Without heat passage into the nozzles, only the outer surfaces of the annuli contribute to heat transfer. The inner surfaces of the annuli can thus contribute to heat transfer when the nozzles are thermally connected to the blind hole sidewall. Additionally, by establishing a heat flow on both walls, the Nusselt number which relates to the heat transfer taking place between the walls and the fluid is increased. However, the effectiveness of heat transfer from the inner surface is lower because, while thermally connected, the conduction path is still relatively high in thermal resistance. This conduction resistance may be reduced by increasing the area of contact between the nozzle and the blind hole sidewall, but it will come at the expense of reducing the annular surface area for heat transfer into the fluid. FIGS. 8-12 depict various jet nozzle configurations designed to further facilitate heat transfer into the coolant.

[0041] Before discussing these configurations, however, FIG. 5-7 depict one embodiment of a completed cooling assembly in accordance with the principles of the present invention. As shown in FIG. 5, the assembly includes a cavity plate 50, which is machined with one or more blind holes that may or may not have a surface curvature at the lower portion of the cavity in accordance with FIGS. 3A-3D. One or more individual jet nozzles 52 are sized and configured to interference fit inside respective blind holes of cavity plate 50. FIG. 6 illustrates a plan view of one embodiment of a cavity plate/jet nozzle configuration which comprises an interstitial array. Alternatively, cavity patterns could be formed within the cavity plate in any desired configuration, such as an in-line array. Arrays may also be fashioned with varying diameter cavities. Once each nozzle is interference fitted into the respective blind hole, a jet nozzle support plate 54 having openings 56 is brought down over the upper section of each of the plurality of nozzles. This support plate 54 has a through-hole pattern that matches the cavity/nozzle pattern. The upper section of each jet nozzle as a stepped diameter change in this embodiment which facilitates the placement of the jet nozzle support plate. Solder or braze ring pre-forms 58 are then placed over the tips of the jet nozzles and the subassembly is taken through an appropriate solder/braze reflow process. This operation provides the needed isolation between the inlet and outlet reservoirs, which are formed by an inlet plenum plate 60 and an outlet plenum plate 62. Plates 60 and 62 are designed to produce the inlet and outlet plenums when assembled as shown in FIG. 5. A cross-section of one embodiment of the final assembly is shown in FIG. 7. Not shown in FIGS. 5-7 is the perimeter sealing which would be required between the stacked plates. One way to seal the assembly is to concurrently braze or solder the plates in place. The plates may also be sealed in a mechanical compressive fashion by bolting the plates together at the periphery and incorporating conventional compression seals, for example, O-ring, C-ring, gasket, etc.

[0042] An additional assembly alternative to solder/brazing the upper sections of the nozzles to the jet nozzle support plate would be to use an elastomer or epoxy to create the isolating joint. Either compound can be applied as a liquid to the annulus formed by the nozzle and jet nozzle support plate and then cured to a solid with appropriate time and temperature processing. This alternative allows more flexibility in choosing a less dense material for the jet nozzle support plate. Furthermore, since the outlet and inlet plenums do not contribute to the transfer of heat, the plenums may be fashioned from a less dense material to reduce weight, such as a light-weight metal or a plastic.

[0043] Additional thermal enhancements may be realized by the formation of a jet nozzle structure which has a plurality of radial microchannels as shown in FIG. 8. In this top down embodiment, jet nozzle 80 has a central channel 82 and an outlet orifice 84 through which coolant passes to impinge upon a lower surface of a blind hole (not shown). Jet nozzle 80 further includes a plurality of closely space, radially extending fins 86 which define therebetween radial microchannels 87. In one fabrication embodiment, the outer surface of jet nozzle 80 undergoes a chemical etching process that produces the radial microchannels with a width on the order of 0.1 mm at an aspect radio (depth/width) of 5-10. Additionally, the number of channels, plus the spacing of the channels, may be chosen such that the sum of the channel widths roughly equals a third of the outer circumference of the nozzle. Both the nozzle/cavity surface contact (i.e., assuming interference fit) and the surface area for heat transfer to the coolant fluid is dramatically increased in this embodiment over that of a more conventional annulus configuration such as depicted in FIG. 2. The former serves to maximize the effective heat transfer of the latter.

[0044] Further, it should be noted that the surface curvature enhancements discussed above in connection with FIGS. 3A-3D are independent of the annulus formation and its enhancements such as described in FIGS. 4-8. Thus, the use of radial microchannels as presented in FIG. 8, for example, could be incorporated together with one or more of the nozzle/cavity embodiments of FIGS. 3A-3D, or used separately.

[0045] FIGS. 9A-12 present further jet nozzle and cavity plate embodiments which, when assembled, provide significantly more surface area for heat transfer (e.g., 5-10×) compared with a conventional structure such as depicted in FIG. 2, while still maintaining the small spacing necessary for high heat transfer coefficients on the opposing surfaces. In the embodiments of FIGS. 9A-12, the jet nozzle and blind hole of the cavity plate have fins or arms which extend radially and are interdigitated. For example, FIGS. 9A & 9B depict one embodiment of a jet nozzle 90 and a corresponding cavity plate 92. As shown, jet nozzle 90 has a plurality of radially outwardly extending fins 94; and the side surface defining blind hole 93 has a plurality of radially inwardly extending fins 95. When assembled as shown in FIG. 9C, fins 94 on jet nozzle 90 are disposed between fins 95 extending into blind hole 93 of the cavity plate. In other words, the fins of the cavity plate wall are interdigitated with the fins of the jet nozzle. This results in a narrow gap between fins which defines channels 91 for coolant flow. Depending upon the design, the surface area for heat transfer can be increased by a factor of up to 10 times the surface area of a more conventional jet cavity assembly. The geometry depicted in FIGS. 9A-9C represents a 7.1×increase in surface area for heat transfer over a conventional design such as depicted in FIG. 2.

[0046] The cavity plate structure of FIG. 9B can be formed, for example, by etching or skiving to remove material within the blind hole to define fins 95. By way of example, one technique is to start with a relatively thick plate (for example, 15-25 mm), drill through holes, and skive the sidewalls to form the fins. The finished plate 92acan then be brazed, soldered or diffusion bonded to a base plate 97 as shown in FIG. 10A (which is a cross-section of plate 92 taken along line B—B in FIG. 9B) to create the final cavity configuration. An alternative manufacturing approach is to pattern relatively thin plates (e.g., 1-1.5 mm) using etching or stamping techniques, stamp the plates, and then braze, solder or diffusion bond the plates to form a laminated final structure 92bas shown in FIG. 10B. As with 10A, the final structure is completed with a base plate 97 which is bonded to the laminated plates 99 defining cavity plate 92b.

[0047] Positioning a jet nozzle within a blind hole cavity so that fins interdigitate and the channels for coolant flow are properly defined can be accomplished in a variety of ways. For example, if the jet nozzle is metal, then providing a press or shrink fit with the nozzle 102 contacting the cavity 100 wall at multiple locations 104 as shown in FIG. 11 will position the insert correctly, plus provide good thermal contact between the blind hole cavity plate and the jet nozzle so that the nozzle surfaces can contribute to the overall heat transfer.

[0048] As an alternative, a press/shrink fit with a keying structure 106 such as shown in FIG. 12 could also be used to properly position a nozzle 112 within a blind hole cavity plate 110 so as to define a uniform spacing 114 between the interdigitated fins.

[0049] To summarize, numerous concepts are presented herein for an enhanced jet nozzle/cavity plate assembly. For example, in accordance with the present invention, jet nozzles are individually placed into the cavity plate and then subsequently secured to a jet nozzle support plate. This allows the formation of annuli between the cavity sidewall and the jet nozzle on the order of 0.002″-0.004″ with a tolerance of +/−10% or better. In addition, curvature at the base of a blind hole cavity and/or the base of the jet nozzle facilitates elimination of the conventional “dead” fluid region where the lower surface of the blind hole meets the side surface. By matching the jet nozzle surface curvature to that of the blind hole lower surface and side surface, the surface area within the cavity/nozzle annulus is increased. Further, a nozzle with radial microchannels can be provided to maximize the conductive coupling between a cavity wall and jet nozzle, while also significantly increasing the surface area for heat transfer and maintaining a high heat transfer coefficient commensurate with fluid flows in very narrow channels. Additional concepts are also disclosed and claimed herein.

[0050] While the invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.