Patent Application
20130056176
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.
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.
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
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.
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.
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.
Referring initially to
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 “Th” 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 “Tm”, 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
Referring now to
Referring now to
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
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 61528073 | Aug 2011 | US |
