WO2006046022A1

WO2006046022A1 - Heat sink

Info

Publication number: WO2006046022A1
Application number: PCT/GB2005/004112
Authority: WO
Inventors: Robert Clive Mullins
Original assignee: Mch Technology Limited
Priority date: 2004-10-25
Filing date: 2005-10-25
Publication date: 2006-05-04
Also published as: GB0423678D0; GB2419463A

Abstract

A heat sink for a semiconductor device is disclosed. The heat sink comprises a body of a heat-conducting material, which includes a three-dimensional array of interconnected chambers arranged so that fluid may pass through the body via the chambers. The body further includes a channel for a cooling medium, the channel passing through, but being closed off from, the array of interconnected chambers. The heat sink may be used, for example, in a liquid cooling system for a microprocessor.

Description

Heat Sink

The present invention relates to a heat sink. Preferred examples relate to a heat sink for a semiconductor chip such as an integrated circuit, logic device, memory chip or microprocessor.

Modern microprocessors and other semiconductor devices are becoming increasingly complex. The greater complexity of such devices leads to increased heat generation, creating a need for efficient cooling solutions. Complex microprocessors can also generate substantial electromagnetic radiation which can interfere with nearby equipment, so that some form of electromagnetic shielding may also be needed.

Cooling arrangements for modern processors used in personal computers (PCs) often include a heat sink mounted on top of the processor and consisting of a series of parallel metal fins on a metal base which is attached to the top surface of the processor chip, often by way of a heat- conducting intermediary such as a thin layer of thermal paste. Furthermore, a fan is often provided fixed to the top of the heat sink for providing a flow of air through the fins of the heat sink.

As the complexity of processors increases, the size of the heat sink needed also increases, which can be a problem particularly in small form- factor PC cases and laptop computers. This problem is exacerbated where in addition to the cooling system some form of electromagnetic shielding is also required to reduce the amount of electromagnetic radiation escaping the computer. The present invention seeks to alleviate some of these problems.

Accordingly, in a first aspect of the invention, there is provided a heat sink for a semiconductor device, comprising a body of a heat-conducting material, the body comprising: a three-dimensional array of interconnected chambers arranged so that fluid may pass through the body via the chambers; and a channel for a cooling medium, the channel passing through, but being closed off from, the array of interconnected chambers.

In this way, a more effective heat sink can be provided which can combine separate cooling flows. A heat sink with a greater surface area exposed to a cooling fluid, such as air, can be provided, and cooling efficiency can thereby be improved.

The array of interconnected chambers is preferably a regular array. The chambers are preferably regularly spaced and preferably of substantially equal size. The body preferably comprises at least 100 chambers, more preferably at least 500 chambers, more preferably at least 1000 chambers. In some examples, the body may comprise 2000 or more chambers. Preferably, the array of chambers comprises multiple aligned chambers in at least two, and preferably in each, of its three dimensions. The outermost chambers are preferably open to the exterior of the heat sink.

The invention may also be used in heat exchanging devices other than heat sinks for semiconductor devices. Accordingly, in a further aspect of the invention, there is provided a heat extracting device comprising a body of a heat-conducting material, the body comprising: a three-dimensional array of interconnected chambers arranged so that fluid may pass through the body via the chambers; and a channel for a cooling medium, the channel passing through, but being closed off from, the array of interconnected chambers so as to allow different cooling media to flow separately through the chambers and the channel. The heat extracting device may have any of the features of the heat sink described herein.

In a further aspect, the invention provides a method of producing a heat sink for a semiconductor device, comprising: forming a heat sink ^"body of a heat-conducting material, the body comprising a three-dimensional array of interconnected chambers arranged to allow fluid to pass through the body via the chambers; and forming within the body a channel for a cooling medium, the channel passing through, but being closed off from, the array of interconnected chambers.

In a further aspect of the invention, there is provided a method of producing a heat sink for a semiconductor device, comprising: forming a heat sink body of a heat-conducting material, the body comprising a three- dimensional array of interconnected chambers arranged to allow fluid to pass through the body via the chambers; and wherein the body is formed using a process including selectively fusing particles of the heat-conducting material. Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic illustration of a conventional heat sink arrangement;

Figure 2 is a schematic illustration of a heat sink with a body having a three-dimensional mesh or lattice structure; Figures 3A to 3G show examples of structures for use in the heat sink;

Figure 4 is a schematic illustration of an alternative heat sink arrangement;

Figure 5 is a cross-sectional schematic illustration of a heat sink having a channel for liquid coolant; and Figure 6 is a schematic of a device for use in a selective laser remelting process.

A conventional heat sink arrangement is shown schematically in Figure

1. A microprocessor 10 is mounted on a circuit board 12. A heat sink 14 consisting of parallel fins is attached to the top of the processor. Between the processor 10 and the heat sink 14, a layer of heat-conducting material 18, such as thermal paste, is provided.

A fan 16 is fixed to the top of the heat sink 14. The heat sink and fan are held in place by clips (not shown), and the fan is connected to the circuit board 12 for the supply of power. Figure 2 schematically illustrates an alternative heat sink arrangement.

Here, the conventional heat sink is replaced by a heat sink 20 having a body of a fine three-dimensional lattice or mesh structure. The mesh or lattice structure consists of interconnected metal strands of a material having good thermal properties. Examples of suitable materials include copper, gold, silver, platinum, lead, tin, carbon graphite or bismuth. The metal strands form a plurality of small interconnected chambers. The chambers may, for example, be octahedral or diamond-shaped. A variety of other suitable shapes may also be used, for example tetrahedra, dodecahedra, icosahedra or cubes. The metal strands thus typically form the edges of the polyhedra, with the term "chambers" here preferably referring to the interior spaces defined by those edges. Typically, the faces of the polyhedra forming the chambers are open to the adjacent chambers, thus connecting the chambers. The interconnected chambers allow cooling fluid, such as air, to pass through the heat sink via the chambers. To increase the volume of air that can pass through the heat sink, the chambers may be arranged to provide straight-line paths through the heat sink in one or more directions.

Figures 3A to 3G show examples of structures for use in the heat sink. These are meant to illustrate only the structure itself and not its shape or dimensions.

Figure 3A shows a structure having diamond-shaped chambers. Figures 3B, 3C and 3D respectively show top, front and side views of the structure of Figure 3A.

Figures 3E and 3F show perspective views of a similar but slightly denser structure, and Figure 3G shows a cross-section through such a structure.

The shape and size of the heat sink is determined by the heat output from the device and the size of the device.

As an example, the heat sink may have diamond-shaped chamber structures having lattice sides (or chamber edges) of between 1 mm and 2mm length, for example 1.5mm length, with the metal strands forming the chambers having a thickness of 0.2mm; and the lattice may consist of 20

(width) by 20 (depth) by 5 (height) individual chamber structures. Smaller structures may also be used, for example of micrometer or even nanometre scale. Such small structures can be formed using the laser remelting process described below.

Optionally, the heat sink body having a lattice or mesh structure may be provided on a solid base, such as a copper plate. Alternatively, a number of separate solid metal pads may be provided at the base of the heat sink, preferably corresponding to the areas of the chip where larger amounts of heat are generated. The base or pads may be made of any suitable material, including the materials mentioned above, and for ease of manufacture may be of the same material as the lattice itself. A fan 16 is optionally provided attached to the top of the heat sink to draw (or blow) air through the heat sink (alternatively, a fan may be mounted on a side of the heat sink). Because the mesh or lattice structure is open to all sides, air is drawn through the heat sink from all directions, which can increase the efficiency of the heat sink. In the conventional heat sink consisting of parallel fins (as shown in Figure 1 ), air can only pass through the heat sink in a direction parallel to the fins (that is to say, into and out of the page).

Heat is conducted from the processor into the metal structure of the heat sink, optionally via an intermediary layer such as thermal paste. The heat is then transferred to the air passing through the chambers (either by way of forced air flow, typically using a fan, or by convection). Due to its lattice or mesh structure, the heat-conducting metal (or other material) of the heat sink has a comparatively large surface area exposed to the air. This can improve heat transfer to the air.

Figure 4 shows a further heat sink arrangement. Here, the base of the heat sink 30 extends beyond the chip 10, and includes a recess 32 for accommodating the chip. In this way, the chip is entirely encased by the heat sink. A heat-conducting plane may additionally be provided as a lower layer in the circuit board (for example in the form of a thermal plane or ground plane), in which case the heat sink 30 may at specific locations be in contact with the heat-conducting plane, for example by way of protrusions extending through apertures in the top layer of the circuit board. This may further aid heat dissipation. A fan may be provided as described above. The microprocessor itself may comprise heat-conducting terminals connected to thermal planes within the processor, in which case the heat sink may be connected directly to those heat terminals, which are preferably provided on the top surface of the processor.

Heat sinks with the mesh / lattice structure described above may further serve as electromagnetic radiation shields to reduce the amount of radiation escaping the processor. The metal structures in the heat sink serve as wave-guides or attenuators for electromagnetic radiation emitted by the processor. A heat sink as shown in Figure 4 may provide improved shielding since it completely encases the processor chip, thereby reducing radiation escaping towards the sides of the chip.

The heat sink as described therefore provides a combined heat dispersion and radiation shielding device. Combining the two functions in a single device can provide cost, weight and space savings. The latter can be particularly important in small-form factor PCs, laptops or other small devices.

To ensure a sufficient volume of heat-conductive material (typically metal) is present in the heat sink to dissipate the heat generated by the processor 10, it is preferable to provide a very fine lattice or mesh having a small chamber size.

A structure with the required properties may be created using a technique known as selective laser remelting. This technique is similar to stereolithography methods using laser-hardening of liquid resins in that the structure is built up layer-wise. This is achieved by selectively remelting portions of a layer of fine metal powder using a laser. The process is illustrated in Figure 6.

The process uses a laser remelting device 70 comprising a building chamber 72 filled with an inert gas, in which a three-dimensional structure 92 is built up on a moveable platform 74. The device comprises a laser 82, and a scanner 84 for directing laser beam 86 through laser window 88 onto a central area 94 of the building chamber 72. The scanner 84 moves the laser beam across area 94 in two dimensions.

In use, a quantity of metal powder 90 is added to the building chamber

72. Levelling mechanism 76 ensures that a level layer of metal powder is provided over the central area 94 above moveable platform 74. The laser then traces a shape in the layer of powder corresponding to a layer of the structure being built, thereby melting portions of the powder. The melted portions then cool and solidify. A solid layer of the structure is thereby produced. The moveable platform 74 is then lowered, and a new layer of powder is applied to central area 94, from which the next layer of the structure is then made.

The above-described process is suitable for producing intricate metal structures of the kind used in the heat sinks described herein. The metal structures generated are essentially homogeneous, which can help ensure that the thermal properties of the metal being used are maintained. The structures produced also tend to be strong and durable.

The process can essentially form any lattice or matrix shape. Solid shapes can also be formed within the lattice structure. Many different type of metals and ceramic composites can be used in this process. For the heat sink, materials with good thermal and radiation shielding properties are used, for example copper, bismuth, stainless steel, zinc, bronze, titanium, chromium-cobalt, or aluminium. These are provided as powders of nano-particle size. The process allows for the mixing of different materials. Ceramic materials may also be sintered onto the structure.

A mixture of metals, for example copper and bismuth, may be selected to obtain the required heat extraction and shielding properties. Typically, the majority of the material used will be one having high thermal conductivity to ensure adequate operation as a heat sink. For example, the heat sink may mainly consist of copper, which is cheap and has good thermal properties. A small amount of another material, such as steel, may then be included to improve the heat sink's wave guide properties.

A further type of heat sink will now be described having a body of a three-dimensional mesh or lattice structure as described above and a channel for a cooling medium which passes through the lattice, but which is closed off from the chambers of the lattice structure, thereby allowing cooling media to flow independently through the chambers on the one hand, and the channel on the other hand. The cooling medium used in the channel is typically a different cooling medium to that which is passed through the chambers of the body.

The main body is typically air-cooled, for example by way of a fan drawing ambient air through the chambers as described above. Channels may then be provided in the heat sink for a separate cooling medium, such as liquid (for example water) or gas (for example helium or argon).

By way of example, Figure 5 shows schematically a cross section of a liquid-cooled heat sink 44. The heat sink has a body of a metal lattice or mesh structure as described above. Additionally, a channel 46 is provided for a liquid coolant (such as water). The serpentine configuration of the channel increases the heat exchange between the heat sink and the liquid coolant. Although a channel with a two-dimensional path is shown, a more complicated, three-dimensional path may also be provided. Multiple independent or branching channels may also be provided. The channel or channels may be arranged to provide liquid cooling in the areas where the microprocessor generates the most heat. The heat sink further comprises connecting ports 50 and 52 for connection to a liquid cooling system or other suitable coolant supply.

The channel may be formed by closing off selected chambers of the lattice, though this may produce a somewhat angularly shaped channel.

Alternatively, a smooth channel surface may be provided. Additionally, for improved flow, the interior of the channels is preferably hollow. The walls of the channel are formed integrally with the metal lattice structure.

Structures of this kind may be manufactured using the selective laser remelting process described above. A fan may be provided as described above to produce an air flow through the lattice portion of the heat sink.

The channel may be larger (e.g. in diameter) than the (typically fairly small) chambers. Alternatively, fine channels could be provided of a size similar to or smaller than the chambers. For example, one or more channels may be provided inside the metal strands forming the lattice structure, i.e. by making some or all of the strands hollow.

In this way, a heat sink can be provided which combines two separate cooling systems using separate, typically different, cooling media. Features of other heat sinks described above may also be incorporated into this dual- coolant heat sink.

It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention.

For example, rather than providing the heat sink separately for attachment to the chip, the chip package and heat sink may be integrally formed.

The invention may also be applied to heat exchangers or heat extracting devices other than heat sinks for microprocessors.

Claims

1. A heat sink for a semiconductor device, comprising a body of a heat- conducting material, the body comprising: a three-dimensional array of interconnected chambers arranged so that fluid may pass through the body via the chambers; and a channel for a cooling medium, the channel passing through, but being closed off from, the array of interconnected chambers.

2. A heat sink according to Claim 1 , arranged to allow different cooling media to flow separately through the chambers and the channel.

3. A heat sink according to Claim 1 or 2, wherein the array of interconnected chambers is arranged, in use, to be open to ambient air to allow ambient air to pass through the body via the chambers.

4. A heat sink according to any of the preceding claims, comprising one or more ports for connecting the channel to an external coolant supply.

5. A heat sink according to any of the preceding claims, wherein the body is formed of a three-dimensional lattice of interconnected elongate members, the members being formed of the heat-conducting material and defining the chambers.

6. A heat sink according to any of the preceding claims, wherein the channel is defined by one or more channel walls, the walls being formed integrally with the body.

7. A heat sink according to any of the preceding claims, wherein the heat- conducting material of the body is substantially homogenous.

8. A heat sink according to any of the preceding claims, wherein the chambers are arranged to provide straight-line paths through the body in one or more directions.

9. A heat sink according to any of the preceding claims, wherein the body is formed by a process involving selective fusing of particles.

10. A heat sink according to any of the preceding claims, wherein the body is made by a selective laser remelting process.

11. A heat sink according to any of the preceding claims, wherein the chambers are in the shape of regular polyhedra.

12. A heat sink according to any of the preceding claims, wherein the heat sink comprises a recess for receiving the semiconductor device.

13. A heat sink according to Claim 12, wherein the recess is shaped so that, when the semiconductor device and the heat sink are mounted on a circuit board, the heat sink encloses the top and sides of the device.

14. A heat sink according to any of the preceding claims, further comprising a solid base.

15. A heat sink according to any of Claims 1 to 13, further comprising one or more solid pads at the base of the body corresponding to one or more areas of the semiconductor device which tend to produce more heat than other areas.

16. A heat sink according to any of the preceding claims, wherein the channel is arranged to pass adjacent to one or more areas of the semiconductor device which tend to produce more heat than other areas.

17. A heat extracting device comprising a body of a heat-conducting material, the body comprising: a three-dimensional array of interconnected chambers arranged so that fluid may pass through the body via the chambers; and a channel for a cooling medium, the channel passing through, but being closed off from, the array of interconnected chambers so as to allow different cooling media to flow separately through the chambers and the channel.

18. A method of producing a heat sink for a semiconductor device, comprising: forming a heat sink body of a heat-conducting material, the body comprising a three-dimensional array of interconnected chambers arranged to allow fluid to pass through the body via the chambers; and forming within the body a channel for a cooling medium, the channel passing through, but being closed off from, the array of interconnected chambers.

19. A method according to Claim 18, further comprising: identifying one or more areas of the semiconductor device which tend to produce more heat than other areas; and forming the channel to pass adjacent to said areas when in use.

20. A method of producing a heat sink for a semiconductor device, comprising: forming a heat sink body of a heat-conducting material, the body comprising a three-dimensional array of interconnected chambers arranged to allow fluid to pass through the body via the chambers; and wherein the body is formed using a process including selectively fusing particles of the heat-conducting material.

21. A method according to any of Claims 18 to 20, wherein the body is formed using a layerwise process including selectively fusing particles of the heat-conducting material to a previous layer to form a subsequent layer.

22. A method according to any of Claims 18 to 21 , wherein the body is formed using a selective laser remelting process.