Patent Description:
Many supercomputing applications employ super conducting circuits that are predominantly implemented in integrated circuits. These integrated circuits often perform best when operating in a cryogenic environment that is maintained at or near cryogenic temperatures, which may extend down to or below <NUM> (four) Kelvin (K).

In a cryogenic environment, the circuit boards on which the integrated circuits reside generally operate in a medium to high vacuum to avoid convective heat leakage and various gasses condensing on the surface of the circuit boards and the integrated circuits residing thereon. This leaves conduction as the main method of removing heat from the integrated circuits. The heat generated from the integrated circuits can be transmitted through the solder connections (i.e. a ball grid array) to a circuit board and then to a heat sink, which is made of aluminum, copper, or like material that is highly thermally conductive.

Unfortunately, the materials used for the integrated circuits, the circuit boards, and the heatsinks are different and have widely varying coefficients of thermal expansion (CTEs). The thermal contraction and expansion associated with cycling the system between <NUM> to <NUM> are extreme and vary from material to material. As a result, the integrated circuits may be damaged and/or break loose from the circuit boards to which they are attached. The expansion and contraction of the heatsinks at different rates than the circuit board may fracture the circuit boards under compressive and/or tensile stresses as well as break the thermal bond between the circuit boards and heat sinks. Any damage to the integrated circuits, circuit boards, or electrical connections therebetween leads to failure of the overall system. Further, a failure in the thermal bond between the circuit board and the heat sink may lead to overheating and failure of the integrated circuits and/or the circuit boards. <CIT> relates to a portable computing device with thermal management. A method of thermally managing a computing device is provided where the computing device includes a housing that has a wall adapted to contact a body part of a user, a circuit board in the housing, and a semiconductor chip coupled to the circuit board. The method includes placing a first heat spreader in thermal contact with the semiconductor chip and the circuit board but separated from the wall by a gap. The circuit board is a semiconductor chip package substrate or virtually any other type of printed circuit board. <CIT> relates to a thermal clamp apparatus for electronic systems. An electronic device includes an outer case generally defining an internal volume, a circuit board positioned within the internal volume and having a first surface and a second surface, one or more active components mounted on the first surface of the circuit board, and a thermal management system configured to provide cooling for the one or more active components. The thermal management system further includes a first heat spreader in thermal contact with at least one active component of the one or more active components, a second heat spreader in thermal contact with the second surface of the circuit board, a thermal carrier coupled to each of the first heat spreader and the second heat spreader to remove thermal energy therefrom, and a heat exchanger coupled to the thermal carriers to receive thermal energy therefrom and dissipate the thermal energy, wherein one thermal carrier is routed between the first heat spreader and the heat exchanger and the other thermal carrier is routed between the second heat spreader and the heat exchanger. <CIT> relates to a thermally conductive shelf. An electronic module chassis assembly is provided for operation with a conduction cooled electronic module. The chassis assembly includes a thermally conductive shelf extending between and in fluid communication with first and second sidewalls of the chassis and an expansion mechanism positioned adjacent to the shelf, where the expansion mechanism operates to expand the shelf, where at least a portion of a top surface of the shelf contacts a first electronic module, and where at least a portion of a bottom surface of the shelf contacts a second electronic module. The chassis first and second side walls can define a fluid passage, where the first sidewall includes a fluid inlet and the second sidewall includes a fluid outlet. Also, a fluid source can be coupled to the fluid inlet of the first sidewall. The inlet can receive a conductive fluid that passes through the first sidewall to the thermally conductive shelf, and the conductive fluid can flow to the second side wall, where the conductive fluid can exhaust through the fluid outlet.

It is the object of the present invention to reduce thermal stress between integrated circuits and a circuit board.

Disclosed is a cooling assembly for circuit boards. In one embodiment, the assembly includes a circuit board that is thermally and physically coupled to a heat spreader by a thermal interface. In one configuration, the circuit board is formed from a semiconductor material and includes a first board surface on which integrated circuits are mounted and a second board surface opposite the first board surface. The heat spreader is formed from a thermally conductive material and includes a plurality of vanes that are spaced apart from one another. The thermal interface is coupled between at least one area of the second board surface of the circuit board and a contact area of each of the plurality of vanes. Heat generated by the integrated circuits is conducted from at least one integrated circuit to the plurality of vanes of the heat spreader through the circuit board and the thermal interface.

In certain embodiments, thermal interface is at least <NUM>%, <NUM>%, or <NUM>% indium, wherein alloys of indium are also applicable, and/or the heat spreader is at least <NUM>% aluminum. The thermal interface may be a single layer or multiple layers. If the thermal interface includes multiple layers, one layer may be of indium, zinc, copper, or the like, with a purity of at least <NUM>% or <NUM>%. The overall thickness of the thermal interface may be less than <NUM> - <NUM> or <NUM> - <NUM> thick. The integrated circuit and the circuit board may be formed from silicon, silicon carbide, or the like, and have a thickness of <NUM> - <NUM>. Further, the contact areas for each of the plurality of vanes may be the only portions of the thermal interface in contact with the heat spreader. The thermal interface may take various forms or shapes, including that of an elongated strip that resides exclusively within a central portion of the second board surface extending along a first axis, and has a length that is at least ten times the width.

In one embodiment, the above assembly is formed as follows. A circuit board is initially provided. The circuit board is formed from a semiconductor material and has a first board surface on which integrated circuits are mounted and a second board surface opposite the first board surface. A first thermal interface subsection is provided on a central portion of the second board surface of the circuit board.

Next, a heat spreader formed from of a thermally conductive material is provided. The heat spreader has a plurality of vanes that are spaced apart from one another by a plurality of slots. A second thermal interface subsection is formed on the heat spreader. The first thermal interface subsection is then bonded to the second thermal interface subsection to form a thermal interface, such that the circuit board is physically and thermally coupled to the heat spreader via the thermal interface. Heat generated by the integrated circuits is conducted from the integrated circuits to the plurality of vanes of the heat spreader through the circuit board and the thermal interface.

In one embodiment, providing the second thermal interface subsection on the heat spreader includes: forming an interface subsection precursor that resides on the plurality of vanes and spans the plurality of slots of the heat spreader, and removing the portions of the interface subsection precursor that reside over the plurality of slots to form the second thermal interface subsection. The first thermal interface subsection and the second thermal interface subsection may be formed from a cold bondable material, such as indium or alloys thereof. Bonding the first thermal interface subsection to the second thermal interface subsection may include pressing the first thermal interface subsection against the second thermal interface subsection to bond the first thermal interface subsection to the second thermal interface subsection.

For the following description, attention is directed to <FIG>. With particular reference to <FIG>, a thermosiphon cooling chassis <NUM>, which is referred to hereinafter as simply a cooling chassis <NUM>, is used to cool numerous blade assemblies <NUM>, according to an exemplary embodiment. The blade assemblies <NUM> are mounted in parallel with one another in a stacked fashion within the cooling chassis <NUM>. The cooling chassis <NUM> may be formed from a continuous machined assembly and is characterized by opposing chassis walls <NUM>. The side walls <NUM> can be divided into numerous chassis segments <NUM>. Within each chassis wall <NUM>, a return channel <NUM> is formed to effectively provide a conduit that extends through the chassis segments <NUM>.

A supply conduit <NUM> extends along the stack of the blade assemblies <NUM> and connects with the return channels <NUM> at the bottom of the chassis walls <NUM> via a conduit coupling <NUM>. In the illustrated example, the conduit coupling <NUM> is a T-shaped coupling that provides fluid communication between the supply conduit <NUM> and both return channels <NUM> of the two chassis walls <NUM>. With particular reference to <FIG>, the supply conduit <NUM> and the return channels <NUM> are in fluid communication with a phase separator <NUM>. The phase separator <NUM> is tubular and provides an internal separation chamber <NUM>, which is in fluid communication with each of the return channels <NUM> via return conduits <NUM> and the supply conduit <NUM>.

In this embodiment, the supply conduit <NUM> is in fluid communication with a bottom portion of the phase separator <NUM> and does not extend into the separation chamber <NUM>. The return conduits <NUM>, which are in fluid communication with the return channels <NUM>, extend into the separation chamber <NUM> and terminate at a level above that of the opening leading to the supply conduit <NUM>. In other embodiments, the supply conduit <NUM> may extend into the separation chamber <NUM>; however, the return conduits <NUM> may, but need not, rise to a level above that of the supply conduit <NUM>. The reason for this configuration is provided further below.

The blade assemblies <NUM> include a circuit board <NUM> with numerous integrated circuits <NUM> and a heat spreader <NUM>, which is illustrated in <FIG>. Each blade assembly <NUM> is mechanically and thermally coupled to the chassis walls <NUM> via the heat spreader <NUM>. While details are provided further below, at a high level, the circuit board <NUM> is thermally coupled to the heat spreader <NUM>, which is thermally coupled to the chassis segments <NUM> of the chassis walls <NUM>. Heat generated by the integrated circuits <NUM> flows through the circuit board <NUM> to the heat spreader <NUM>, which facilitates heat transfer to the chassis segments <NUM> of the chassis wall <NUM>. The heat transferred to the chassis walls <NUM> is removed by circulating a low temperature fluid in a gas, liquid, or combination of gas and liquid states.

In one embodiment, liquid helium is provided in a bottom portion of the separation chamber <NUM> of the phase separator <NUM> and allowed to flow in a liquid state down the supply conduit <NUM>. As the liquid helium passes through the conduit coupling <NUM> into the respective return channels <NUM> of the chassis walls <NUM>, heat is transferred to the liquid helium from the chassis walls <NUM>. The heat causes the liquid helium to develop bubbles, which effectively decreases the density of the liquid helium and causes the liquid helium in the return channels <NUM> to become less dense than the liquid helium in the supply conduit <NUM>. As a result, the liquid helium will circulate through the system by flowing down the supply conduit <NUM> and up the return conduit. The bubbles separate from the liquid helium in the separation chamber <NUM>, and as a result, the liquid helium pools in the lower portions of the separation chamber <NUM> before flowing downward through the supply conduit. Circulation of the helium through the supply conduit <NUM>, the return channels <NUM>, and the separation chamber <NUM> may occur naturally as long as the temperature of the helium is maintained at a proper level or facilitated through an additional pump, which is not illustrated. While helium is used in this example, other cooling fluids such as nitrogen, neon, oxygen and the like, may be employed.

<FIG> illustrates an enlarged view of a single blade assembly <NUM> mounted between respective pairs of chassis segments <NUM>. Opposing edges of the heat spreader <NUM> are clamped between the respective pairs of chassis segments <NUM>, wherein a locking mechanism <NUM> is used to both hold adjacent chassis segments <NUM> against one another as well as hold the heat spreader <NUM> in place. The opposing edges of the heat spreader <NUM> extend past the corresponding edges of the circuit board <NUM> to releasably engage each pair of chassis segments <NUM>.

<FIG> is an exploded isometric view of a blade assembly <NUM>. In this embodiment, circuit boards <NUM> are mounted along both sides of the heat spreader <NUM>. Further, the heat spreader <NUM> in this embodiment is characterized by a first bar <NUM>, a second bar <NUM>, and numerous vanes <NUM> that extend from the first bar <NUM> to the second bar <NUM>. As illustrated, the vanes <NUM> are substantially in parallel with one another and separated by thin slots <NUM>; however other configurations are envisioned.

The circuit boards <NUM> have opposing surfaces that will be generally referred to as a first surface and a second surface. The first surface is populated with the integrated circuits <NUM> and the second surface is the surface opposite that of the first surface. As illustrated in <FIG>, the second surface of the circuit board <NUM> includes a first thermal interface subsection <NUM> that aligns with a second thermal interface subsection <NUM>, which is formed on the heat spreader <NUM>. The second thermal interface subsection <NUM> may have multiple components, which are formed on the vanes <NUM> of the heat spreader <NUM>.

As will be described in further detail below, when the circuit board <NUM> is placed on the heat spreader <NUM>, the first thermal interface subsection <NUM> on the circuit board <NUM> will come into direct contact with the components of the second thermal interface subsection <NUM> on the heat spreader <NUM>. The first thermal interface subsection <NUM> and the components of the second thermal interface subsection <NUM> join together to form an overall thermal interface <NUM> (not shown in <FIG>), which provides the primary thermal conduction path between the circuit board <NUM> and the vanes <NUM> of the heat spreader <NUM>.

The first and second thermal interface subsections <NUM>, <NUM> may be formed of the same or different materials, and may be thermally coupled in a variety of ways. A particularly effective technique is to form the first and second thermal interface subsections <NUM>, <NUM> out of the same material, and then bond the first and second thermal interface subsections <NUM>, <NUM> to one another to form an integrated thermal interface <NUM>. A particularly effective material for the first and second thermal interface subsections <NUM>, <NUM> is indium or an alloy thereof. In certain embodiments, the thermal interface is at least <NUM>%, <NUM>%, or <NUM>% of indium. The thermal interface may be a single layer or multiple layers. If the thermal interface includes multiple layers, one layer may be of indium, zinc, copper, or the like, with a purity of at least <NUM>% or <NUM>%. The other layers may include, but are not limited to zinc, copper, and the like. The overall thickness of the thermal interface may be less than <NUM> - <NUM> or <NUM> - <NUM> thick. The integrated circuit and the circuit board may be formed from silicon, silicon carbide or the like, and have a thickness of <NUM> - <NUM>.

As illustrated in <FIG> and <FIG>, edge interface strips <NUM> of indium or other highly conductive materials, such as thermally conductive epoxies and the like, may be used to provide efficient thermal coupling of the first and second bars <NUM>, <NUM> of the heat spreader <NUM> to the chassis segments <NUM>. For clarity, <FIG> does not illustrate a second circuit board <NUM> being coupled to the bottom side of the heat spreader <NUM>.

<FIG> illustrates a cross section of a blade assembly <NUM>, wherein circuit boards <NUM> are mounted on both sides of the heat spreader <NUM>. Each circuit board <NUM> is thermally and physically coupled to a respective surface of the heat spreader <NUM> via the thermal interface <NUM>. In one embodiment, the thermal interface <NUM> resides within a central portion <NUM> of both the circuit boards <NUM> and the heat spreader <NUM>. Further, the thermal interface <NUM> may provide only a single thermal contact area between the circuit board <NUM> and any one of the vanes <NUM> of the heat spreader <NUM>. In other words, a thermal interface <NUM> only couples a single contact area on the surface of each vane <NUM> to the corresponding circuit board <NUM>. The thermal interface <NUM> is configured such that the contact area is relatively small to minimize the negative effects of expansion and contraction of the circuit boards <NUM> and the heat spreader <NUM> due to changing temperatures. In essence, a centrally located thermal interface <NUM> absorbs the stresses associated with the different rates of expansion and contraction of the circuit boards <NUM> and the heat spreaders <NUM>. While the thermal interface <NUM> may take various configurations, a narrow elongated strip that runs across the vanes <NUM> of the heat spreader <NUM> is particularly beneficial and inherently provides a single, relatively small contact area for each of the vanes <NUM>. In one embodiment, the vanes <NUM> are substantially parallel to one another, and the thermal interface <NUM> runs substantially orthogonally to the vanes <NUM> and resides within a central portion <NUM> of the various vanes <NUM>. As defined herein, a central portion <NUM> of either the heat spreader <NUM> or the circuit board <NUM> is considered to be confined within a central <NUM>% (or smaller portion) of the respective component relative to one or two orthogonal axes. In other embodiments, the central portion <NUM> may be further limited to a central <NUM>%, <NUM>%, <NUM>% or <NUM>%.

The solid lines with periodic arrows represent the heat conduction path for the blade assembly <NUM>. In particular, heat generated by the respective integrated circuits <NUM> flows into the circuit boards <NUM> and toward the corresponding thermal interfaces <NUM>. The heat flows through the thermal interfaces <NUM> and then along the vanes <NUM> of the heat spreader <NUM> toward the outer edges of the heat spreader <NUM>. At this point, the heat will flow into the chassis segments <NUM> and be dissipated into the cooling fluid flowing through the return channels <NUM> of the respective chassis walls <NUM>.

A benefit of maintaining a single contact area on each vane <NUM> of the heat spreader <NUM> by the thermal interface <NUM> is to avoid confining a segment of the circuit board <NUM> between two points on a vane <NUM> of the heat spreader <NUM>. Confining a portion of the circuit board <NUM> between two contact areas of a vane <NUM>, especially if there is considerable distance between the two contact areas, will inherently subject that portion of the circuit board <NUM> to compressive and/or tensile stresses, which may result in crushing a portion of the circuit board <NUM> and/or pulling a portion of the circuit board <NUM> apart.

The following describes an exemplary process for fabricating a blade assembly <NUM>. With reference to <FIG>, the back side of a circuit board <NUM> is illustrated, wherein the dashed lines define the area on which a strip of indium will be formed to provide the first thermal interface subsection <NUM> of the thermal interface <NUM>. Notably, the circuit board <NUM> may have various compositions. In one embodiment, the circuit board <NUM> is a wafer board formed from a traditional semiconductor material, such as silicon, silicon carbide, silicon germanium, and the like. While not limited thereto, wafer board-based circuit boards <NUM> typically range in thickness from <NUM> to <NUM>. The examples described below assume that the circuit board <NUM> is formed from silicon, and as such, is a silicon wafer board. Further, certain embodiments employ integrated circuits <NUM> that are formed from the same semiconductor material as the circuit board <NUM>. Accordingly, both the integrated circuits <NUM> and the circuit board <NUM> may be silicon based in certain embodiments. Forming the integrated circuits <NUM> and the circuit board <NUM> from the same semiconductor material maintains a common coefficient of thermal expansion (CTE) between the integrated circuits <NUM> and the circuit board <NUM> and maintains structural integrity as temperatures change because the integrated circuits <NUM> and the circuit board <NUM> expand and contract at the same rates. While indium is a particularly beneficial material used for the thermal interface <NUM>, other materials such as thermally conductive epoxies and the like are envisioned.

As illustrated in <FIG>, the indium strip that forms the first thermal interface subsection <NUM> is provided in the central portion <NUM> of the circuit board <NUM> relative to the Y axis. As illustrated, the first thermal interface subsection <NUM> extends along the entire length of the circuit board <NUM> along the X axis. The indium or other material being used for the first thermal interface subsection <NUM> may be applied using traditional deposition and etching processes, electroplating, tinning, or the like. The first thermal interface subsection <NUM> typically has a thickness between about <NUM> - <NUM> and <NUM> - <NUM>; however, thicknesses outside of this range are applicable.

Notably, the first thermal interface subsection <NUM> may take on any number of shapes in addition to the elongated and thin strip illustrated in <FIG>. However, employing a single strip that resides entirely within the central portion <NUM> of the circuit board <NUM> provides the most stress relief with respect to thermal expansion and contraction, as will be explained in more detail below.

<FIG> illustrate the preparation of the heat spreader <NUM> according to one embodiment. The heat spreader <NUM> is formed from a highly thermally conductive material, such as aluminum, copper, or the like. High purity aluminum is particularly beneficial given its superb ability to conduct heat. 4N (<NUM>% pure), 4N6 (<NUM>% pure), 5N5 (<NUM>% pure), 6N (<NUM>% pure), or higher purity aluminum is particularly beneficial. The heat spreader <NUM> typically has a thickness between about <NUM> and <NUM>; however, thicknesses outside of this range are applicable.

<FIG> illustrates a heat spreader precursor <NUM>'. To initiate the process of providing the second thermal interface subsection <NUM> on the heat spreader precursor <NUM>', a strip of indium, thermal epoxy, or like material is provided across the vanes <NUM> in the X direction. The strip of indium is located in a central portion <NUM> of the heat spreader precursor <NUM>' and will generally align with as well as mirror the shape and size of the first thermal interface subsection <NUM> that was formed on the circuit board <NUM> in <FIG>. With reference to <FIG>, the strip of indium is referred to as a thermal interface subsection precursor <NUM>' and may be formed as a continuous strip that resides on top of the vanes <NUM>. Forming the thermal interface subsection precursor <NUM>' on the heat spreader <NUM> may include deoxidizing at least the area of the heat spreader <NUM> on which the thermal interface subsection precursor <NUM>' is applied and then brushing liquid indium on the heat spreader precursor <NUM>' to form the thermal interface subsection precursor <NUM>'. A second process may include a multi-step process wherein the area on which the thermal interface subsection precursor <NUM>' is to be formed is initially zincated to form a layer of zinc. The zinc layer is then flashed with copper to form a thin layer of copper of the zinc layer. Finally, the zinc layer is electroplated with indium to form an indium layer. The overall thermal interface subsection precursor <NUM>' typically has a thickness between about <NUM> and <NUM>; however, thicknesses outside of this range are applicable.

Next, the thermal interface subsection precursor <NUM>' is diced at each slot <NUM> to effectively break the thermal interface subsection precursor <NUM>' into segmented components to form the second thermal interface subsection <NUM> and form the slots <NUM> in the heat spreader precursor <NUM>' to form the heat spreader <NUM> shown in <FIG>. Multiple vanes <NUM> extend between the first and second bars <NUM> and <NUM>. As illustrated, the vanes <NUM> are linear and substantially parallel to one another. The vanes <NUM> are separated by the slots <NUM>. The thermal interface precursor <NUM>' may be diced with a water jet. Other mechanical or chemical etching, material removal, or the like techniques may be employed to dice, slice, or otherwise separate the thermal interface subsection precursor <NUM>' into the segmented components of the second thermal interface subsection <NUM>.

Each component of the second thermal interface subsection <NUM> resides directly on a corresponding vane <NUM>, as illustrated in <FIG>. As such, each component of the second thermal interface subsection <NUM> is spaced apart from the others. With continued reference to <FIG>, the segments of the second thermal interface subsection <NUM> correspond to the contact patch that the thermal interface <NUM> will provide for each vane <NUM>.

In one embodiment, the circuit board <NUM> is approximately <NUM> X <NUM>, and the indium-based thermal interface <NUM> is approximately <NUM> X <NUM>. The thermal interface <NUM> may be linear or non-linear and take on any variety of shapes. In certain embodiments, the thermal interface <NUM> is an elongated strip that has length of at least <NUM> times, <NUM> times, or <NUM> times its width. The contact patches of the thermal interface <NUM> to each of the vanes <NUM> are approximately <NUM> X <NUM>. These dimensions are merely exemplary for one embodiment; however, it is particularly beneficial for the contact patches to be equal to or less than <NUM> X <NUM>, <NUM> X <NUM>, or <NUM> X <NUM>. Maintaining smaller contact patches allows the thermal interface <NUM> to better absorb stress of the heat spreader <NUM> expanding and contracting at a much higher rate than the circuit board <NUM>.

<FIG> and <FIG> illustrate assembly of the blade assembly <NUM> in an embodiment wherein a circuit board <NUM> on both sides of the heat spreader <NUM>. Initially, the circuit boards <NUM> are aligned with the respective sides of the heat spreader <NUM>, such that the first thermal interface subsections <NUM> of the circuit boards <NUM> align with the second thermal interface subsections <NUM> of the heat spreader <NUM>. The circuit boards <NUM> are moved into contact with the respective sides of the heat spreader <NUM>, such that the first thermal interface subsection and the second thermal interface subsection <NUM>, <NUM>, which are formed from numerous individual components, contact each other. The first and second thermal interface subsections <NUM>, <NUM> are then bonded together.

In an exemplary process, the first thermal interface subsection <NUM> and the second thermal interface subsection <NUM> are bonded together using a cold bonding process. The cold bonding process is particularly applicable for applications wherein the first thermal interface subsection <NUM> and the second thermal interface subsection <NUM> employ indium, a thermal epoxy, or the like. The process simply includes applying pressure to the circuit boards <NUM> and/or the heat spreader <NUM> such that the first thermal interface subsection <NUM> and the second thermal interface subsection <NUM> are pressed against each other at or near room temperatures, such that bonding occurs between the first thermal interface subsection <NUM> and the second thermal interface subsection <NUM> to form the composite thermal interface <NUM>. Other bonding techniques may be employed and may require heat, additional materials or layers, or the like. However, maintaining a uniform, highly thermally conductive material as the thermal interface <NUM> tends to provide the least thermal resistance. The resulting thermal interface <NUM> typically has a thickness between about <NUM> and <NUM>; however, thicknesses outside of this range are applicable.

<FIG> illustrate the thermal gradient across a <NUM> thick silicon circuit board <NUM>, wherein the contact patches for the thermal interface <NUM> are approximately <NUM> X <NUM>. The thermal interface <NUM> consists essentially of indium, and the heat spreader <NUM> is made of 5N5 aluminum (at least <NUM>% Al) and is approximately <NUM> thick.

Turning now to <FIG>, an alternative blade assembly <NUM> is illustrated. In this not claimed example, a solid heat spreader <NUM>, as opposed to the heat spreader <NUM> with vanes <NUM> and slots <NUM> resides at the center of the blade assembly <NUM>. A first circuit board <NUM> with integrated circuits <NUM> is mounted over a top surface of the heat spreader <NUM>, wherein a first heat preform <NUM> is sandwiched between the top surface of the heat spreader <NUM> and the first circuit board <NUM>. Similarly, a second circuit board <NUM> resides over a bottom surface of the heat spreader <NUM>, wherein a second heat preform <NUM> resides between the bottom surface of the heat spreader <NUM> and the second circuit board <NUM>.

Notably, the surfaces of the first and second circuit boards <NUM>, <NUM> that include the integrated circuits <NUM> face away from the heat spreader <NUM>. Accordingly, the integrated circuits <NUM> of the first circuit board <NUM> are on a top surface of the first circuit board <NUM>. The integrated circuits <NUM> of the second circuit board <NUM> are on a bottom surface of the second circuit board <NUM>. Further, a first spring sheet <NUM> is placed over the top surface of the first circuit board <NUM> such that the first spring sheet <NUM> rests against the top surface of the integrated circuits <NUM>. Similarly, a second spring sheet <NUM> rests against the integrated circuits <NUM> of the second circuit board <NUM>. In a further example, the heat spreader <NUM> is formed from a high purity aluminum, like the heat spreader <NUM> in the prior embodiments. As noted above, the heat spreader <NUM> is a solid sheet of aluminum that does not include vanes, slots, holes, or the like in the illustrated example. Such features may be provided in the heat spreader <NUM>, but may reduce cooling efficiencies. Further, the heat spreader <NUM> may be formed from copper, high purity aluminum, or other highly thermally conductive material. The first and second heat preforms <NUM>, <NUM> are formed from indium in one example, but other highly thermally conductive material may be employed. The first and second spring sheets <NUM>, <NUM> are formed from sheets of beryllium copper (BeCu) in the illustrated example, but may also be formed from stainless or carbon steel, or like highly conductive material.

In this example, there is direct conduction cooling through the back surfaces (those closest to the heat spreader <NUM>) of the circuit boards <NUM>, <NUM>. The first and second heat preforms <NUM>, <NUM> are unbonded, and provide thermal conductance between the first and second circuit boards <NUM>, <NUM> and the respective surfaces of the heat spreader <NUM>. The first and second spring sheets <NUM>, <NUM> function to provide a compressive load between the first and second circuit boards <NUM>, <NUM>, the first and second heat preforms <NUM>, <NUM>, and the heat spreader <NUM>. This design is compatible with the clam shell configuration required by the cooling chassis <NUM> and helps to eliminate the thermal gradient across the first and second circuit boards <NUM>, <NUM>.

Claim 1:
An apparatus comprising:
at least one integrated circuit (<NUM>) formed from a semiconductor material;
a circuit board (<NUM>) formed from the same semiconductor material as the integrated circuit and comprising a first board surface on which the at least one integrated circuit is mounted and a second board surface opposite the first board surface;
a heat spreader (<NUM>) of a thermally conductive material and comprising a plurality of vanes (<NUM>) that are spaced apart from one another; and
a thermal interface (<NUM>) coupled between at least one area of the second board surface of the circuit board and a contact area of each of the plurality of vanes, wherein heat generated by the at least one integrated circuit is conducted from the at least one integrated circuit to the plurality of vanes of the heat spreader through the circuit board and the thermal interface, and wherein the thermal interface is an elongated strip that resides exclusively within a central portion (<NUM>) of the second board surface that extends along a first axis.