WO2002053304A1

WO2002053304A1 - Method for producing a heat sink unit

Info

Publication number: WO2002053304A1
Application number: PCT/SE2001/002872
Authority: WO
Inventors: Stellan Ekstrand
Original assignee: Sapa Profiler Ab
Priority date: 2000-12-29
Filing date: 2001-12-21
Publication date: 2002-07-11
Also published as: SE0004915D0

Abstract

When extruding profiles having fins with thin cross-section or narrow spacing, e g heat sinks, it has not been possible, without using special tool supports, to achieve a ratio between the fin height and the spacing between two adjacent fins of more than 5:1 due to loss of strength of the extrusion tool. The disclosed invention solves this problem by the use of an extrusion die comprising an aperture having a bearing portion (10) and a tapered portion (14) and where the bearing portion of the aperture is directly transferred into the tapered portion by a change of the taper angle of the aperture walls from α to β. The extreme pressures on the thin sections and sharp corners of the extrusion tool is thereby avoided and no breakdown of the extrusion tool occurs. A heat sink profile with portions of thin cross-section or narrow spacing that is produced to its final shape may thereby be produced.

Description

METHOD FOR PRODUCING A HEAT SINK UNIT

The present invention is related to the manufacturing of a heat sink unit comprising a base and a number of fins extending from the base.

The invention is particularly related to the cooling of electronic components.

According to another aspect of the invention it is also related to a die for producing extrusions with thin walled structures or of narrow spacing.

Printed circuits (ICs) are typically housed within a plastic or ceramic package. The packages have leads or surface pads that are soldered to a printed circuit board. The circuit board and package are often located within a computer chassis which contains a fan that removes the heat generated by the IC.

It is desirable to have a high rate of heat transfer from the IC package in order to maintain the junction temperatures of the integrated circuit within safe operating limits. Excessive IC junction temperatures may affect the performance of the circuit and cause a permanent degradation of the IC. Heat sinks are sometimes mounted to the top surface of the package housing to increase the thermal performance of the package. Conventional heat sinks typically have a plurality of fins that extend from a bottom base portion. The fins increase the surface area of the heat sink and the heat transfer rate of the package.

When manufacturing heat sinks it is desirable to use a method that is suitable for mass production and economical, and which produces structures that are light-weight and have a high heat conductivity. Extrusion of aluminium is a method that satisfies these criteria.

The miniaturisation of computers puts an increasing demand on computer housings to be kept as small as possible. As a consequence heat sinks that are of small size but with a high capacity for heat dissipation are required. Structures with fins of large height and with short mutual distances have hitherto been difficult to extrude.

Dies for extruding aluminium are conventionally formed by a two-stage process. This process involves cutting a series of recesses in the surface of a metal plate using a rotating cutter so as to form an elongate tapered recess along the surface of the plate, and then forming a parallel-sided elongate aperture along the bottom of the tapered recess by means of a wire spark erosion process. The thickness of the parallel-sided portion of the exit aperture controls the speed at which the aluminium passes through the aperture during extrusion. The side walls of the metal plate defining the parallel-sided portion is therefore often referred to as the bearing portion of the die. To ensure that the aluminium "beaks clean" after passing through the parallel-sided portion of the aperture during extrusion a portion of the aperture which lies adjacent to the parallel-sided portion of the aperture is thereafter removed. Typically the removed portion is formed with a right angle edge to the parallel-sided portion. These cut-outs ensure that the metal does not come into contact with the side walls of the aperture after exiting the parallel-sided portion.

When extruding profiles having thin fins extending from a base it has not been possible to achieve a profile of final shape with a high ratio of fin height to spacing between the fins due to break-down of the extrusion tool. It has been especially difficult to produce wide profiles with thin fins extending from a flat base, due to break down of the extrusion tool.

Tubular heat exchangers having thin fins extending from a tubular body are known from US3866286 (Peerless of America, Inc) and US4794985 (Peerless of America, Inc). The fins are here formed by cutting slices of the outer tube material and bending the slices to form outwardly projecting fins or ribs.

In EP1028461 (Sumitomo Metal Industries, Ltd) a heat sink fin assembly is formed by press forming a thin metal sheet to form a corrugated structure.

Extruded fin structures are known from GB2151769 and GB2079052. In GB2151769 the distance between the fins are cut in half by inserting one finned structure into another. In GB2079052 a fin height to gap ratio of 4:1 is achieved by using alternately short and long fins.

In US6138489 (Webra AB) a heat transfer unit is formed by straightening a curved structure having fins projecting rectilinearly from the base by pulling hooks arranged integrally with the arched structure. The distances between the fins are smaller after the straightening operation than in the curved structure. This method of manufacturing a heat sink requires a machining operation to remove the hooks used for pulling after the straightening has taken place and is therefore not suitable for mass production.

US6134936 discloses a heat sink with a profile ratio higher than 10:1. The heat sink is produced by using a cross-beam to support the fins when extruding. The use of these special die supports are expensive and unpractical. The resulting structure requires post straitening of the fins to be acceptable. This method is also not suitable for producing heat sinks of a greater width, due to instability of the tool. Neither of these documents solves the problem of producing a heat sink of high cooling capacity suitable for mass production. There is also no disclosure of a method for extruding a heat sink with a large fin height to spacing ratio and a large width, which requires no secondary deformation.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a method for producing a heat sink unit capable of effectively cooling heat-dissipating objects, such as electronic devices. Another object is to provide a die used for the extrusion of said heat sink unit. This is achieved by using an extrusion die comprising a metal plate formed with an elongate aperture having a tapered portion and a parallel-sided portion, wherein the parallel-sided portion of the aperture acts as a bearing during the extrusion of the metal through the aperture and where the parallel-sided portion aperture is directly transferred into a tapered portion by a change of the taper angle. A heat sink unit with projecting parallel fins extending perpendicularly from the profile base plane is hereby produced. By extruding through the die according to the invention a heat sink with fins of large height to distance ratio can be produced.

The heat sink profile is extruded to its final shape and there is no need for deformation or machining of the structure. No special arrangements to support the die, such as crossbeams, need to be used.

According to a first aspect of the invention, a heat sink unit of high ratio between the fin height and the fin spacing is produced by extruding a billet through an extrusion die and cutting the profile into the desired lengths.

According to a second aspect of the invention the extrusion takes place through a die having an aperture with a bearing portion and a tapered portion, said bearing portion being integrally formed with said tapered portion, so that no intermediate steps are formed in the tapered portion.

According to a third aspect of the invention one or more cavities are formed in the extruded profile in the side opposite the finned side, and a second heat sink, e g a copper plate, is attached in the cavity, e g by gluing or soldering or shrink fitting. Electronic components may then be attached to the copper plate. The copper plate may be omitted and the electronic components attached directly onto the heat sink unit. The cavities may be cut out for example by milling or may be formed during the extrusion.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a cross-section of a part of a conventional die for extruding aluminium.

Figure 2 is a plan view of a part of the die for extruding a heat sink according to the invention.

Figure 3 is a section taken along line A-A in figure 2.

Figure 4 shows a cross section of the extruded heat sink profile (the part of the profile corresponding to figure 2 is marked).

Figure 1 shows a cross-sectional shape of an aperture of a conventional die, which comprises a tapered portion 6 and a parallel-sided portion12 of the aperture. A cut-out with right corner edges 4 is formed in the aperture just above the parallel-sided portion. During extrusion aluminium is forced against the front surface 8 of the plate 2 and passes through the aperture, in the direction indicated by the arrow labelled D. After passing through the parallel-sided portion 12 the aluminium continues to travel in a straight line and does not come into contact with the side walls of the tapered portion of the aperture.

In figure 2 the width, d, of the die-fins corresponding to the spacing between the extruded fins is marked. If this width becomes to small in relation to the height of the die-fin h the tool will break.

In figure 3 a principle drawing of the die according to the invention shows that no release cut-out in the exit aperture is used. The only thing that separates the bearing portion 10 from the tapered portion 14 is a change of the taper angle from β to α, i. e. the angle between the extrusion direction (D) and the side walls of the aperture. In this way the extruded material is supported by the aperture walls and released in a smooth way. The angle of the bearing surface in relation to the extrusion direction β may be zero or greater. The angle of the walls of the tapered portion in relation to the extrusion direction should be greater than zero but preferably smaller than 10 degrees, most preferably smaller than 5 degrees. In figure 4 one half of a heat sink is shown. A ratio between the fin height hi and the spacing between two adjacent fins d1 of more than 10:1 can be achieved, but the method is also useful for producing heat sinks of higher or smaller ratios.

The fins may also have projecting ribs to increase the area of the fins, so that a corrugated structure is formed. The ribs may for example be formed during the extrusion.

The use of aluminium is preferred, but other materials may also be used.

It is well known to use wire sparking to remove material from extrusion dies. Wire sparking is here used as an example of how the material may be removed.

Example:

A steel tool plate is cut by wire spark erosion until the die aperture with a direct transfer of the bearing portion into a tapered portion is formed along the shape of the profile (figure 3) by a slight change of the taper angle. An aluminium billet is heated to a temperature suitable for extrusion and the metal is extruded through the die in the direction indicated by the arrow "D". The material solidifies into the finned profile according to figure 4, which is cut into desired lengths. Heat sink units with a fin height hi of 24 mm and a spacing d., between the adjacent fins of 2 mm, and the width W of 325 mm is hereby formed. Cavities are cut in the back of the heat sinks and copper plates are mounted in the cavities by heating the aluminium heat sink, placing the copper plates in the cavity and cooling to increase the heat sink capacity. Electronic components are then soldered onto the copper plates.

This example is by no means limiting, but other forms of thin walled structures or with a narrow spacing may be produced by the use of the die of this invention.

Claims

Claims:

1. A die for producing extruded products with portions of thin cross-section or narrow spacing, characterized in that said die comprises an aperture having a bearing portion (10) and a tapered portion (14) and where the bearing portion (10) of the aperture is directly transferred into the tapered portion (14) by a change of the taper angle of the aperture walls from β to α.

2. A die for producing extruded products according to claim 1, characterized in that the taper angle of the bearing portion (β) is 0 degrees.

3. A die for producing extruded products according to any of claims 1-2, characterized in that the angle of the side walls of the tapered portion in relation to the extrusion direction (α) is smaller than 10 degrees.

4. A die for producing extruded products according to any of claims 1-2, characterized in that the angle of the side walls of the tapered portion in relation to the extrusion direction (α) is smaller than 5 degrees.

5. A method of producing a heat sink unit, characterized in that an aluminium billet having a raised temperature is extruded through a die according to any of claims 1-4 to form an aluminium heat sink profile of final shape, comprising a flat base and a number of outwardly projecting fins and that the profile is cut into desired lengths to form the individual heat sink units.

6. A method of producing a heat sink unit according to claim 5, characterized in that at least one cavity is formed in the profile in the side opposite the fins during the extrusion.

7. A method of producing a heat sink unit according to any of claims 5 or 6 , characterized in that after the extrusion the at least one cavity is cut in the plate in the side opposite the fins.

8. A method of producing a heat sink unit according to any of claims 5-7, characterized in that a second heat sink is mounted in the cavity.

9. A method of producing a heat sink unit according to any of claims 5-8, characterized in that the second heat sink is mounted by shrink fitting.