US20130056176A1

US20130056176A1 - Heat Exchanger with Controlled Coefficient of Thermal Expansion

Info

Publication number: US20130056176A1
Application number: US13/596,020
Authority: US
Inventors: Javier A. Valenzuela
Original assignee: Mikros Manufacturing Inc
Current assignee: Mikros Manufacturing Inc
Priority date: 2011-08-26
Filing date: 2012-08-27
Publication date: 2013-03-07

Abstract

The heat exchanger with a controlled coefficient of thermal expansion (CTE) includes a low CTE thermal expansion control member operatively connected to a heat transfer member in order to restrain the lateral thermal expansion of the heat transfer member during use. The thermal expansion control member is placed outside the heat transfer path, so the thermal expansion control member can be made of a low thermal conductivity material without increasing the thermal resistance of the heat exchanger. The CTE of the thermal expansion control member is lower than that of the heat transfer member, so as to constrain lateral thermal expansion of the heat transfer member during operation. The difference between the CTE of the device being cooled and the heat transfer member, i.e. the CTE mismatch, is reduced over conventional heat exchangers by restraining the heat transfer member during operation.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/528,073, filed Aug. 26, 2011 and entitled “Heat Exchanger with Controlled Coefficient of Thermal Expansion”, the entire contents of the application being incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates generally to an apparatus for cooling a heat producing device, whose coefficient of thermal expansion is substantially lower than that of metals. More specifically, this invention relates to a heat exchanger whose coefficient of thermal expansion can be constrained to be similar to that of the device being cooled.

BACKGROUND

The use of heat exchangers for cooling a range of heat producing devices is known in the art. Heat exchangers are used to transfer heat from the heat-producing device to a coolant, either a liquid, or a gas. Liquid cooled heat exchangers have flow passages disposed in a heat transfer member to distribute the coolant over the area to be cooled. The passages can range from simple tubes to micro-channels, depending on required heat transfer capacity of the heat exchanger.
A typical application of liquid cooled heat exchangers is in the cooling of power electronic components, such as IGBTs and power FETs. FIG. 1 is a schematic of a conventional prior art approach to liquid cooling of an electronic component, namely die 11. As shown, die 11 is soldered (“s1”) to a direct bonded copper (DBC) substrate 13. The DBC 13 includes a ceramic layer 13 a with a layer of copper 13 b, 13 c bonded to either side. Copper layer 13 b includes a pattern etched thereon to form the electrical connection to the die while the other copper layer 13 c is soldered (“s2”) to a heat spreader 15. The heat spreader 15 distributes the heat over an area approximately 5 to 8 times larger than the die 11, and is mechanically attached to an external heat exchanger 17. Care is taken to minimize the gap between heat spreader 15 and heat exchanger 17 in order to minimize the interface thermal resistance. Thermal grease 19 is often placed between the heat spreader 15 and the heat exchanger 17 to further reduce the thermal resistance of the interface.
Many heat exchangers and heat spreaders are made of copper or aluminum, and for such heat exchangers the coefficient of thermal expansion (CTE) is generally above about 17 ppm/° C. which is substantially higher than that of the die. For example, copper has a CTE of 17 ppm/° C. and Aluminum has a CTE of 24 ppm/° C.; whereas the electronic devices being cooled (for example IGBT's, solar cells, and the like) have a CTE in the range of about 3-6 ppm/° C. (for example silicon 3.2 ppm/° C., germanium 5.8 ppm/° C.). Thus, the device being cooled and the heat exchanger have substantially different coefficients of thermal expansion (CTE), i.e. a CTE mismatch, which results in undesirable thermal stress.
In order to address the issue of CTE mismatch, prior art applications utilize a copper-ceramic-copper (direct bonded copper or DBC) substrate to attach the device being cooled to the substrate. Typically the DBC substrate will have a CTE between 5-8 ppm/° C., an intermediary value between the device being cooled and the substrate. The DBC is also used to provide electrical isolation. Prior art devices further utilize a layer of solder to add compliance in order to accommodate the CTE mismatch. Providing the DBC and solder helps to accommodate the CTE mismatch, with the DBC reducing the overall range of the mismatch and the solder providing compliance, but there remains a mismatch and the DBC and solder layers add significant thermal resistance. In addition, while the solder can plastically deform repeatedly without cracking during use, the solder will eventually fatigue and fail, the amount of time to failure being dependent upon the operating conditions. For operating conditions with high temperature excursions fewer cycles will cause failure of the solder joint, while lower temperature excursions will allow for more operating cycles. As will be appreciated, the solder is chosen primarily for its mechanical properties, and indium based solder is commonly utilized because it deforms easily.
The trend in electronics packaging toward smaller, more powerful components results in the need for the heat exchangers to operate at ever increasing heat fluxes. The prior art cooling device illustrated in FIG. 1 is normally used with devices that dissipate no more than about 100 W/cm². For cooling higher heat flux devices, such as high-power electronic devices dissipating about 300 W/cm², or solid-state laser diodes, which dissipate heat at a rate of about 500-1000 W/cm², heat exchangers integrated into the device package are generally employed. In the integrated heat exchangers, the heat spreader also functions as the heat exchanger heat transfer member, and has flow passages or fins to transfer heat to the coolant. By integrating the heat exchanger into the package, the interface resistance of the mechanical attachment between the heat spreader and the heat exchanger is eliminated. In order to handle these large heat fluxes without exceeding the maximum operating temperature of the electronic component, it is desirable for the heat transfer member to be constructed out of a high thermal conductivity material such as copper or aluminum.
Other prior art applications that use integrated heat exchangers reduce the CTE mismatch by utilizing material for the heat exchanger other than copper or aluminum. For example, low CTE refractory metals, such as molybdenum (CTE=7 ppm/° C.) or tungsten (CTE=6 ppm/° C.), or metal-ceramic composites, such as aluminum silicon carbide (AlSiC; CTE=8-15 ppm/° C.) have been utilized in place of copper. The use of these materials reduce the CTE mismatch, but at the expense of increased thermal resistance and cost. The thermal conductivity of these materials is in the range of about 140-200 W/m-K which is significantly lower than copper at about 400 W/m-K. The price of refractory metals is several times that of copper.
In practice, most prior art cooling devices only allow for the device being cooled to operate at about half the maximum rated current. This is because while the device to be cooled may have the capability to run at a certain maximum current, prior art heat exchangers lack the ability to adequately remove heat, resulting in overheating of the device if run near its maximum current capability.

SUMMARY

In order to increase the power handling capacity of the heat exchanger and improve operating performance, the thermal resistance should be reduced while also lowering thermal stress that can lead to failure of the joint between the device being cooled and the heat exchanger. This is achieved in the present application by restraining the thermal expansion of the heat transfer member to reduce thermal stress and by removing the restraining member from the heat transfer path to reduce thermal resistance. The difference between the CTE of the device being cooled and the heat transfer member, i.e. the CTE mismatch, is reduced over the prior art by restraining the heat transfer member during operation, thus effectively reducing the coefficient of thermal expansion of the heat exchanger.
The heat exchanger includes a thermal expansion control member operatively connected to a surface of the heat transfer member to restrain the thermal expansion of the heat transfer member during use. The thermal expansion control member is placed outside the heat transfer path, so the thermal expansion control member can be made of a low thermal conductivity material without compromising thermal performance of the heat exchanger. The CTE of the thermal expansion control member is lower than that of the heat transfer member, so as to constrain lateral thermal expansion of the heat transfer member during operation.
In one embodiment, the manifold is part of the thermal expansion control member. As such, the manifold is also made of the low CTE material and is generally rigid.
In one embodiment, a bow compensation member is provided to balance thermal stresses on the heat exchanger and reduce heat exchanger bow that can result from the difference in CTE between the thermal expansion control member and the heat transfer member. The bow compensation plate may be made of the same material as the heat transfer member and mounted to the opposite side of the thermal expansion control member in order to reduce any bowing that may occur during operation by balancing the thermal stresses.
In another embodiment, the heat exchanger includes a symmetrical construction with heat transfer members making up both the top and bottom faces of the heat exchanger. In this embodiment, the thermal expansion control member is disposed in between the two heat transfer members. Heat transfer is symmetric and bowing in the heat exchanger is controlled by the symmetry of the heat transfer members, and a separate bow compensation member is, therefore, not required.
In yet another embodiment, the manifold is formed as a separate member from the thermal expansion control member. In this embodiment the manifold may also be made of a compliant material to accommodate the different thermal expansion between the manifold and the portion of the heat exchanger that has the controlled CTE. In this embodiment, because the thermal expansion control member does not include the manifold, it is generally thinner than other embodiments. With a thinner thermal expansion control member, the need for a bow compensation member becomes more important as bowing from temperature gradients in the heat exchanger is more pronounced.
In all the embodiments, a thermal expansion control member that is outside of the heat transfer path is provided to reduce thermal stresses without adding thermal resistance. Because thermal stresses are reduced, the thickness of the solder can also be reduced, and a less ductile but higher thermal conductivity solder can be utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention.

FIG. 1 is a schematic of a conventional prior art liquid cooled heat exchanger for cooling an electronic component;

FIG. 2 is a schematic, perspective view of a first embodiment of a heat exchanger having a controlled coefficient of thermal expansion in combination with a device to be cooled according to the present description;

FIG. 3 is a schematic, perspective view of the first embodiment of the heat exchanger of FIG. 2 without the device to be cooled;

FIG. 4 is an exploded view of the heat exchanger of FIG. 3;

FIG. 5 is a schematic, perspective view of a second embodiment of a heat exchanger having a controlled coefficient of thermal expansion including a bow compensation member;

FIG. 6 is an exploded view of the heat exchanger of FIG. 5;

FIG. 7 is a schematic, perspective view of a third embodiment of a heat exchanger having a controlled coefficient of thermal expansion and a symmetrical construction;

FIG. 8 is an exploded view of the heat exchanger of FIG. 7;

FIG. 9 is a schematic, perspective view of a fourth embodiment of a heat exchanger having a controlled coefficient of thermal expansion and a compliant manifold;

FIG. 10 is an exploded view of the heat exchanger of FIG. 9;

FIG. 11 is a schematic, perspective view of a fifth embodiment of a heat exchanger having a controlled coefficient of thermal expansion including a compliant gasket and a rigid manifold; and

FIG. 12 is an exploded view of the heat exchanger of FIG. 11.

DETAILED DESCRIPTION

Referring initially to FIGS. 2-4, a first embodiment of a heat exchanger 10 having a controlled coefficient of thermal expansion (CTE) is illustrated. In the present description, the coefficient of thermal expansion is used in the conventional manner to mean the fractional increase in the length per unit rise in temperature of a material. The heat exchanger 10 has a thermal expansion control member 12 including a restraining plate 14 and manifold 16 in the present embodiment, and also includes a heat transfer member 18 for transferring heat to a coolant, and a cover plate 20, as illustrated. As shown in FIG. 2, a device to be cooled, such as die 11 can be mounted to a DBC substrate 13 and to the cover plate 20. In use, heat is transferred to, and/or from the heat exchanger 10 over the portion of the heat transfer member 18 that includes channels 22, as described in more detail below. The function of distributing and collecting the fluid to the channels 22 is achieved by the manifold 16 while the function of transferring the heat is achieved by the heat transfer member 18. Manifold 16 is not disposed in the path between the device to be cooled 11 and the heat transfer member 18 (the heat transfer path) and, therefore, it is not in the heat transfer path. In the present embodiment, the manifold 16 is part of the thermal expansion control member 12, including restraining plate 14, which is also not in the heat transfer path. By placing the thermal expansion control member including manifold 16 and restraining plate 14 outside of the heat transfer path, thermal stresses can be reduced without adding thermal resistance.
The function of the manifold 16 is to distribute and collect the fluid over a heat transfer surface of the heat transfer member 18. In the present embodiment, the manifold 16 may have an interdigitated design to promote uniform heat transfer capacity over the heat transfer surface, as is known in the art. The manifold 16 includes an inlet port 26 through which fluid enters the manifold and an outlet port 28 through which fluid exits the manifold. Other designs for the manifold may be utilized as would be known to those of skill in the art, provided, that the manifold is made of a low CTE material in the present exemplary embodiment.
Because the thermal expansion control member 12 is not in the heat transfer path, it can be made of a low thermal conductivity material, for example low CTE nickel alloys such as Invar® (generically 64FeNi, extra pure grades having CTE as low as 0.62-0.65 ppm/° C.), Kovar® (a nickel-cobalt ferrous alloy designed to be compatible with the thermal expansion characteristics of borosilicate glass, CTE of about 5.9 ppm/° C.) and Alloy 42 (FeNi42, CTE of 5.3 ppm/° C., which matches Germanium), or any other material with a low CTE. As described herein a material with a low CTE is below about 8 ppm/° C. In the present embodiment, the thermal expansion control member 12 includes both restraining plate 14 and manifold 16, which are made of the same low CTE material, but the two may be made from different low CTE materials, if desired.
In contrast, the heat transfer member 18 is made of a high thermal conductivity material, for example copper (CTE 16.7 ppm/° C.) in order to transfer heat from the device to be cooled with a low thermal resistance. In the present embodiment, heat transfer member 18 includes one or more layers 24, each having a plurality of non-linear, winding micro or mini-channels 22 formed therein as described in pending U.S. patent application Ser. Nos. 12/188,859 and 13/115,956, which are incorporated by reference herein in their entirety. Other channel configurations may be utilized, for example linear channels, channels having different geometries, and/or different dimensions, including those that are not micro-channels, as would be known to those of skill in the art.
Heat transfer member 18 is secured to the thermal expansion control member 12 in a known manner, for example by bonding. In the present embodiment, the thickness of the heat transfer member 18 “T_h” is very thin, approximately 1 mm, and includes a plurality of channels 22 which form voids in the heat transfer member. In contrast, the thickness of the thermal expansion control member 12, i.e. the combination of restraining plate 14 and manifold 16 “T_m”, is in the range of approximately 5-10 mm in the present embodiment. The restraining plate 14 is also largely solid. The resulting stiffness of the thermal expansion control member 12 is greater as compared to the stiffness of the heat transfer member 18, which helps constrain lateral expansion of the heat transfer member 18 during use. As a result the CTE of the heat exchanger can be made closer to that of the thermal expansion control member 12 than to that of the heat transfer member 18. The proportions and the stiffness of the heat transfer member 18 and the thermal expansion control member 12 can be varied to get an acceptable CTE for a particular application. Having a lower CTE mismatch reduces thermal stress and can prolong the life of the device incorporating the heat exchanger. Reduced thermal stress also allows for a thinner layer of solder, as compliance from the solder becomes less important the lower the CTE mismatch. As such, the solder can be chosen for its thermal conductivity properties instead of its mechanical properties.
Referring now to FIGS. 5-6, a second embodiment of a heat exchanger having a controlled CTE is illustrated. In this embodiment, the same or similar elements as the previous embodiment is labeled with the same reference numbers, preceded with the numeral “1”. Heat exchanger 110 also includes a thermal expansion control member 112 having a restraining plate 114 and manifold 116, and further includes a heat transfer member 118, a cover plate 120, and a bow compensation member 130. The bow compensation member 130 helps balance the thermal stresses on the heat exchanger 110 which can result in thermal distortion, i.e. bowing of the heat exchanger. The bow compensation member 130 may be made of the same material as the heat transfer member 118, and is secured to the opposite side of the thermal expansion control member 112 in order to reduce any bowing that may occur during operation by balancing the thermal stresses. Alternatively, the bow compensation member 130 may be made of a different material than the heat transfer member 118, depending upon whether more or less compensation may be required to help alleviate bowing. In either case, the bow compensation member 130 will have a CTE that is higher than that of the thermal expansion control member 112.
Referring now to FIGS. 7-8, a third embodiment of a heat exchanger having a controlled CTE is illustrated. In this embodiment, the same or similar elements as the previous embodiments are labeled with the same reference numbers, preceded with the numeral “2”. Heat exchanger 210 also includes a thermal expansion control member 212, a heat transfer member 218, a cover plate 220, and further includes a symmetrical construction including a second heat transfer member 218 b. In this embodiment, the thermal expansion control member 212 is disposed in between the two heat transfer members 218 and 218 b and includes a second low CTE restraining plate 214 b in addition to the first low CTE restraining plate 214 and manifold 216. By providing a heat transfer member 218, 218 b secured on either side of the thermal expansion control member 212, which includes two low CTE restraining plates 214, 214 b, the thermal stresses are symmetric and bowing in heat exchanger 210 is controlled. The symmetry of this embodiment and the provision of a single manifold 216 for two heat transfer members 218, 218 b provides a compact configuration that is symmetrical in both material and temperature distribution.
Referring now to FIGS. 9-10, a fourth embodiment of a heat exchanger having a controlled CTE is illustrated. In this embodiment, the same or similar elements as the previous embodiments are labeled with the same reference numbers, preceded with the numeral “3”. Heat exchanger 310 also includes a thermal expansion control member 312 which in the present embodiment is a low CTE restraining plate 314. The heat exchanger 310 further includes a bow compensation member 330, a heat transfer member 318, a cover plate 320, and a compliant manifold 337. As with the previous embodiments, the manifold 337 distributes and collects the fluid over a heat transfer surface of the heat transfer member 318. However, the manifold 337 is not part of the thermal expansion control member 312, and does not operate to constrain the heat transfer member 318 in the present embodiment. As with the previous embodiments, the manifold 337 is also is not in the heat transfer path. Thus, manifold 337 is only responsible for flow distribution, not mechanical constraint or heat dissipation in the present embodiment. Because the manifold is not made of a low CTE expansion material, and the thermal expansion control member 312 only includes restraining plate 314, a bow compensation member 330 is also provided in the present embodiment in order to reduce any bowing that may occur during operation by balancing the thermal stresses. As illustrated in FIGS. 9 and 10, the bow compensation member 330 may be disposed between the compliant manifold 337 and the thermal expansion control member 312, i.e. CTE restraining plate 314 in the present embodiment.
The manifold 337 is compliant, i.e. not rigid, such that the manifold can stretch during use to accommodate the differential thermal expansion between the manifold and the controlled CTE portion while continuing to distribute and collect fluid over the heat transfer surface. In order to provide flexibility to the manifold 337, the manifold may be made from a polymeric or elastomeric material (for example a rubber, silicone, urethane, etc) or another type of compliant material as would be known to those of skill in the art. Referring now to FIGS. 11-12, a fourth embodiment of a heat exchanger having a controlled CTE is illustrated. In this embodiment, the same or similar elements as the previous embodiments are labeled with the same reference numbers, preceded with the numeral “4”. Heat exchanger 410 also includes a thermal expansion control member 412 which in the present embodiment is a low CTE restraining plate 414. The heat exchanger 410 further includes a bow compensation member 430, a heat transfer member 418, a cover plate 420, and a compliant gasket 438 disposed between the bow compensation member 430 and a rigid manifold 440. This embodiment is essentially the same as the embodiment of FIGS. 9-10, except the compliant manifold has been replaced with a compliant gasket 438 supported by a rigid manifold 440. The compliant gasket 438 operates to accommodate the differential thermal expansion between the manifold and the controlled CTE portion of the heat exchanger while the rigid manifold 440 operates to distribute and collect fluid over the heat transfer surface, in the present embodiment. As illustrated in FIGS. 11 and 12, the bow compensation member 430 may be disposed between the compliant gasket 438 and the thermal expansion control member 412, i.e. CTE restraining plate 414 in the present embodiment.
In all of the exemplary embodiments described herein, the thermal expansion control member is operatively connected to a surface of the heat transfer member to restrain the thermal expansion of the heat transfer member during use. The thermal expansion control member is placed outside the heat transfer path, so the thermal expansion control member can be made of a low thermal conductivity material without compromising the thermal performance of the heat exchanger. The CTE of the thermal expansion control member is lower than that of the heat transfer member, so as to constrain lateral thermal expansion of the heat transfer member during operation. The difference between the CTE of the device being cooled and the heat transfer member, i.e. the CTE mismatch, is reduced over the prior art by restraining the heat transfer member during operation, thus effectively reducing the coefficient of thermal expansion of the heat exchanger.
While various embodiments of the invention have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
For example, the dimensions, geometric shapes, and materials disclosed herein may be varied, as would be known to those of skill in the art. More specifically, the stiffness of the heat transfer member relative to that of the thermal expansion control member can be varied depending upon the application. Likewise, the material utilized for the thermal expansion control member and/or the heat transfer member may also be varied, depending upon the application. The heat transfer member and manifold may take any of a variety of forms, as would also be known to those of skill in the art. In addition, the heat exchanger while illustrated as parallel flow could also be configured as a normal flow heat exchanger. Therefore, the above description should not be construed as limiting, but merely as exemplifications of preferred embodiments. Those skilled in the art will envision other modifications within the scope, spirit and intent of the invention.

Claims

1. A heat exchanger for cooling a heat producing device, the heat exchanger comprising:

a heat transfer member having a first surface operatively connected to the heat producing device, a second surface opposite the first surface, at least the first surface of the heat transfer member having a coefficient of thermal expansion larger than the coefficient of thermal expansion of the heat producing device;

a thermal expansion control member operatively connected to the second surface of the heat transfer member such that the thermal expansion control member is outside of a heat transfer path disposed between the heat transfer member and the heat producing device, the thermal expansion control member having a coefficient of thermal expansion sufficiently lower than that of the heat transfer member so as to constrain lateral thermal expansion of the heat transfer member during use; and

wherein during operation of the heat producing device, the thermal expansion control member constrains the lateral thermal expansion of the heat transfer member so that the heat transfer member expands a similar amount as the expansion of the heat producing device, allowing a rigid connection between the heat producing device and the heat transfer member without inducing excessive thermal stresses in the heat producing device or at the interface between the heat producing device and the heat transfer member.

2. The heat exchanger according to claim 1, wherein the coefficient of thermal expansion for the heat producing device is in the range of about 3-6 ppm.

3. The heat exchanger according to claim 2, wherein the coefficient of thermal expansion for the heat transfer member is above about 15 ppm.

4. The heat exchanger according to claim 1, wherein the thermal expansion control member includes a restraining plate.

5. The heat exchanger according to claim 4, wherein the thermal expansion control member further includes a manifold, the manifold being constructed and arranged to distribute and collect fluid over a heat transfer surface of the heat transfer member.

6. The heat exchanger according to claim 5, further comprising a bow compensation member.

7. The heat exchanger according to claim 6, wherein the bow compensation member is disposed adjacent the manifold on a side opposite that of the restraining plate.

8. The heat exchanger according to claim 4, further including a compliant manifold constructed and arranged to distribute and collect fluid over a heat transfer surface of the heat transfer member and a bow compensation member disposed between the compliant manifold and the thermal expansion control member.

9. The heat exchanger according to claim 4, further including a rigid manifold constructed and arranged to distribute and collect fluid over a heat transfer surface of the heat transfer member, a compliant gasket disposed between the rigid manifold and the thermal expansion control member, and further including a bow compensation member disposed between the compliant gasket and the thermal expansion control member.

10. The heat exchanger according to claim 5, wherein the thermal expansion control member further includes a second restraining plate and wherein the manifold is disposed between the restraining plate and the second restraining plate.