Technical field

The present invention relates to a heat sink and a method for production of a heat sink. The heat sink is primarily intended for cooling of component casings intended for outdoor placement and is primarily arranged for natural cooling.

Background of the invention

Heat sinks are used for cooling purposes in different applications such as, e.g., in cooling of electronic components arranged in component casings on printed circuit boards (PCB) to form an electronic device. The heat sink is often arranged on the component casing with a thermally conductive gap filler between the component casing and the heat sink. It is also common to place the PCB with its components directly on the heat sink. In many applications the device with the heat sink is to be arranged outdoors on masts. Examples of such devices are transmitters, receivers and base stations for mobile communication. In such applications when the device is arranged on a mast it is important to keep the overall weight of the device as low as possible to minimize the dimensions and thereby also the cost of the mast equipment.

Heat sinks are usually manufactured with a base which is arranged to collect heat from the component to be cooled and a number of fins which are arranged to lead heat out from the base. The heat is then to be dissipated into the air surrounding the fins. In order to increase the heat dissipation from the fins the air may be forced to pass the fins using a fan. One type of heat sink that has been used in the prior art is constituted by an aluminium alloy that has been manufactured by extrusion or molding. The heat dissipation from the heat sink base depends on the heat transfer from the heat sink base to the heat sink fins and the heat dissipation from the surface of the heat sink fins. The heat transfer into the fins depends on the thermal conductivity of the material and the cross sectional area of the fins. In order to maximise the heat transfer into the fins the width of the fins should be maximised. On the other hand the heat dissipation from the fins is maximised by maximising the area of the fins. For a given size of the heat sink base and a given height of the heat sink fins the area of the heat sink fins is maximised by maximising the number of fins. This may be done by minimising the width of the fins and the separation of the fins. However, when using extrusion or molding to manufacture the heat sink there is a limit on how thin the fins may be made and also a limit on how small the separation of the fins may be.

Another method of constructing the heat sink fins is to use a thin sheet of metal which is then folded in a wave pattern and fastened to the heat sink base for example by soldering or gluing.

Summary of the invention

The object of the present invention is to provide a heat sink which casing is an alternative to the casings of the prior art.

Another object of the present invention is to provide a casing with a heat sink device which at least partly solves one of the problems with the prior art.

A further object of the present invention is to provide a heat sink which is lighter in weight than the heat sink of the prior art while still providing the same or better heat dissipation.

According to a first aspect of the present invention a heat sink is provided for cooling of an electronic device. The heat sink comprises a heat sink base with a contact surface to be attached to an electronic device and a number of heat sink fins arranged on the heat sink base. The heat sink is characterized in that the heat sink fins comprise a laminate comprising at least one carbon layer with a layer of sheet metal on each side of the carbon layer.

Such a heat sink is favourable in that the heat sink fins may be made considerably thinner than the heat sink fins of the prior art as carbon has a superior heat conductivity compared with aluminium and copper which are the common materials used in heat sinks of the prior art.

The carbon is preferably in the form of graphite, but may also be in the form of graphene or diamond.

By arranging the carbon in a laminate between two layers if sheet metal the strength of the heat sink fins is greatly improved. Thus, the mechanical strength of carbon is not an issue as the layers of sheet metal contributes to the mechanical strength of the laminate.

Each one of the heat sink fins may be constituted by a separate essentially flat laminate comprising a single carbon layer between two layers of sheet metal, which laminates are attached to the heat sink base. This is a construction of the heat sink that is known from the prior art. However, only metal fins have been described in the prior art.

The heat sink fins may be arranged in grooves in the heat sink base essentially perpendicular to the contact surface of the heat sink base, i.e. so that the normal to the heat sink fins is essentially perpendicular to the normal to the contact surface of the heat sink base. It is possible to arrange the fins also on other surfaces of the heat sink base such as on a side surface of the heat sink base.

As an alternative a number of heat sink fins may be part of a common continuous laminate with a length axis, which laminate is folded at different positions along its length axis and which comprises at least one carbon layer with a layer of sheet metal on each side of the carbon layer. Such a heat sink is somewhat easier to manufacture as no grooves have to be formed in the heat sink base. Furthermore, the a suitable length of the laminate may be taken from a long roll of laminate which is then folded to form some or all of the heat sink fins.

Along its length axis the folded laminate may comprise essentially flat portions which are arranged essentially perpendicular to the contact surface of the heat sink base, bottom portions which are attached to the heat sink base and which connect two adjacent flat portions and top portions which connect two adjacent flat portions in the opposite side of the flat portions. The bottom portions preferably comprise a flat portion to provide a surface for contact with the heat sink base. The folded laminate is attached to the heat sink base by some appropriate method such as, e.g., gluing, welding or soldering.

Even though the layers of sheet metal may be easily bent the carbon layers in the laminate is brittle and may be cracked if it is bent too narrowly. To avoid this problem the laminate, periodically along the length axis, may comprise a number of first areas with a carbon layer and a layer of sheet metal on each side of the carbon layer, which first areas are divided by second areas comprising a single layer of sheet metal. By the arrangement of such second areas the bending of the carbon layer may be avoided by arranging all the bends in the second areas. The second areas could alternatively be constituted by two or more layers of sheet metal with an adhesive in between. Thus, preferably the first areas are arranged in the flat portions of the folded laminate and the top portions and the bottom portions are constituted by the second areas. The second areas could be constituted by the two layers of sheet metal that are arranged on the sides of the carbon layers. The metal may be chosen from the group consisting of copper, aluminium and alloys thereof. It is possible within the scope of the invention to utilise other metals with a high thermal conductivity such as gold or silver. However, for economical reasons the choice of metals is preferably limited to aluminium and copper.

The height of the heat sink fins from the base may be 5-100 mm. This is the practical limit for the height of the heat sink fins. It is possible within the scope of the invention to have lower fins, but the heat dissipation gain will then be limited compared to the case with no heat sink. It is also possible to have higher fins, but for most applications that will be impractical while adding only marginally to the heat dissipation.

The thickness of the laminate may be 0.1-2 mm. It is difficult to make the overall thickness smaller than 0.1 mm. If the thickness of the fins is made larger than 2 mm the gain over conventional heat sinks will be marginal.

According to a second aspect of the present invention a method for production of a heat sink for cooling of an electronic device is provided. The heat sink comprises a heat sink base with a contact surface to be attached to an electronic device and a number of heat sink fins arranged on the heat sink base. The method is characterized in that the method comprises the steps of providing a metal heat sink base, forming a laminate comprising at least one carbon layer with a layer of sheet metal on each side of the carbon layer, and arranging the laminate as heat sink fins on the heat sink base.

With the method according to the invention a more light weight heat sink is provided while still providing equally good or better heat dissipation.

In the method of the invention the laminate may be formed by the steps of providing a first layer of sheet metal and a second layer of sheet metal and a layer of carbon, preferably in the form of a graphite sheet, applying an adhesive on the first layer of sheet metal, attaching the carbon layer on the first layer of sheet metal, applying an adhesive on the carbon layer, and attaching the second layer of sheet metal on the carbon layer.

It is possible to attach the carbon layer to the layers of sheet metal by other means than by the use of an adhesive. It is for example possible to utilize chemical vapour deposition to apply the carbon layer on the sheet metal.

Further advantages of the invention will be apparent from the following detailed description.

Brief description of the drawings

The appended drawings are intended to clarify and explain different embodiments of the present invention in which: Fig.. 1 shows a schematic view of a casing with a heat sink device according to the invention in an exploded view.

Fig. 2. shows in more detail a part of the heat sink in Fig. 1. Fig. 3. shows a heat sink according to an alternative embodiment

Fig. 4. is a flow diagram over a method according to an embodiment of the present invention. Fig. 5. is a flow diagram over a method according to an embodiment of the present invention. Detailed description of embodiments of the invention

In the following description of preferred embodiments of the invention similar features will be denoted with the same reference numeral. It is to be noted that the drawings are not drawn to scale. Fig. 1 shows a heat sink 1 for cooling of an electronic device (not shown), which heat sink 1 comprises a heat sink base 2 with a contact surface 3 to be fastened on an electronic device and a number of heat sink fins 4 arranged on the heat sink base2. Fig. 2 shows in more detail a part of the heat sink with the heat sink fins 4. The heat sink fins 4 comprise a laminate 21 comprising at least one layer of carbon 5 with a layer of sheet metal 6, 7, on each side of the layer of carbon. The carbon in the carbon layer may be in the form of graphite. The sheet metal is preferably aluminium or an aluminium alloy. Alternatively the sheet metal may be copper or a copper alloy. Both aluminium and alloy have a high thermal conductivity and suitable mechanical properties for making thin sheets. However, aluminium and aluminium alloys have better corrosion resistance, lower cost and better mechanical properties than copper making it a better choice as the metal in the metal layers. In Fig. 1 and Fig. 2 the heat sink fins 4 are part of a common continuous laminate 21 with a length axis 8. The laminate 21 is folded at different positions along its length axis 8. Along the length axis 8 the folded laminate 21 comprises essentially flat portions 9 which are arranged essentially perpendicular to the contact surface 3 of the heat sink base 2,. The folded laminate 21 also comprises bottom portions 10 which comprise flat portions which are fastened to the heat sink base 2 and which connect two adjacent flat portions 9 and top portions 11 which connect two adjacent flat portions 9 in the opposite side of the flat portions 9. The laminate 21 comprises at least one carbon layer 5 with a layer of sheet metal 6, 7, on each side of the carbon layer. As carbon has a considerably higher specific thermal conductivity the laminate 21 may be made considerably thinner than an all aluminium heat sink fin having the same thermal conductivity.

As can be seen in Fig. 2 the laminate 21 has a constant cross section along its entire length. With such a constant cross section the carbon layer 5 has to be bent at the bends 12 between the flat portions 9 and the bottom portions and also in the bends 13 between the flat portions and the top portions 11. As graphite is rather brittle the bending of the laminate 21 in said bends 12, 13, may result in breakage of the carbon layer 5 resulting in cracks in the carbon layer 5. The thermal conductivity over such cracks will be low. In order to avoid this problem the laminate 21, periodically along the length axis, may comprise a number of first areas 14 with a carbon layer and a layer of sheet metal on both sides of the carbon layer, which first areas are divided by second areas 15 comprising a single layer of sheet metal. In Fig. 2 the second areas are delimited by the dashed lines 16. Thus, the first areas 14 are arranged in the flat portions 9 of the folded laminate 21 and the top portions and the bottom portions are constituted by the second areas 10, 11. The heat sink according to claim 1, wherein the height of the heat sink 4 fins from the base preferably is 5-100 mm. Furthermore, the thickness of the laminate 21 is 0.1-2 mm. It would, however, be possible to have a height of the heat sink fins and a thickness of the laminate 21 outside the described intervals. In Fig. 1 and Fig. 2 the heat sink fins 4 are arranged on the side of the heat sink base 2 being opposite to the contact surface 3 of the heat sink base 2. However, it is possible to arrange the heat sink fins on the side surface of the heat sink base. This may be done altematively or additionally. Fig. 3 shows a heat sink 1 according to an alternative embodiment. In the heat sink 1 shown in Fig. 3 each one of the heat sink fins 4 is constituted by a separate essentially flat laminate 21 comprising a single carbon layer 5 between two layers 6, 7, of sheet metal, which laminates 21 are attached to the heat sink base 2. The heat sink fins 4 are arranged in grooves 20 in the heat sink base essentially perpendicular to the contact surface 3 of the heat sink base 2, i.e. so that the normal 17 to the heat sink fins is essentially perpendicular to the normal 18 to the contact surface 3 of the heat sink base 2.

A method for production of a heat sink 1 according to the invention, for cooling of an electronic device, is schematically shown in Fig. 4. The heat sink comprises a heat sink base 2 with a contact surface 3 to be attached to an electronic device and a number of heat sink fins 4 arranged on the heat sink base 2. The method comprises the steps of providing 101 a metal heat sink base 2, forming 102 a laminate 21 comprising at least one carbon layer 5 with a layer of sheet metal 6, 7, on each side of the carbon layer 5, and arranging 103 the laminate 21 as heat sink fins 4 on the base.

As shown in Fig. 5 the step of forming 102 the laminate 21 comprises the steps of providing 104 a first layer of sheet metal 6 and a second layer of sheet metal 7 and providing a carbon layer 5 in the form of a graphite sheet. In step 105 an adhesive is applied on the first layer of sheet metal and in step 106 the graphite sheet 5 is arranged on the first layer of sheet metal 6. Then an adhesive is applied 107 on the graphite sheet and the second layer of sheet metal is attached 108 on the graphite sheet 5. The present invention is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claim. Thus, it is possible to combine features from the embodiments described above.

In the described embodiments all heat sink fins are equally high. However, it is possible to have varying heights of the heat sink fins. In the described embodiments the heat sink fins are arranged equidistant. However, it is possible to arrange the heat sink fins with the distance between adjacent fins varying across the heat sink 1.

It is possible to have the fins at an arbitrary angle to the contact surface 3 of the heat sink base 2.

It is of course possible to have the carbon in other forms than graphite. It is possible to have the carbon in the form of grapheme or diamond.

It is possible to have an arbitrary shape on the heat sink fins.