GB2330690A

GB2330690A - A clip-on heat sink

Info

Publication number: GB2330690A
Application number: GB9722655A
Authority: GB
Inventors: Lau Kwong Thin; Ng Seng Huat; Zee Kum Yoong
Original assignee: Thomson Multimedia SA
Current assignee: Technicolor SA
Priority date: 1997-10-27
Filing date: 1997-10-27
Publication date: 1999-04-28
Also published as: GB9722655D0; WO1999022405A1

Abstract

A heat sink is provided with a chimney by stamping out a part 10 of a plate 2. An active electronic component 6, eg a transistor, is attached to the stamped out part 10 by a clip 5. Heat sink legs 3 are provided with elastic members 21 to clip the sink into holes in the printed circuit board 30. The active electronic component 6 may be clipped to the inside or the outside of the chimney. The side of the chimney to which the component is attached may have mounting pads 14 stamped out to accurately locate the component by abutment with the pads. The heat sink may be mainly planar or mainly dihedral (as shown). The chimney cross section may be a trapezium or a rectangle. The leg 3c may be omitted.

Description

Heat sink.

Field of the invention.

This invention relates generally to heat sinks for use with transistors and the like. More particularly, but not by way of limitation, this invention relates to a heat sink wherein the heat sink is attached to a transistor or the like by the force exerted of a resilient clip that straps the transistor to the heat sink.

Background.

Most commonly heat sinks are made in two parts, a plate generally in aluminium or aluminium alloy and a clip in resilient material such as steel for springs.

Aluminium or aluminium alloy are generally considered to be malleable and have little resilience but do have excellent heat transfer characteristics. The aluminium materials can be easily formed so that the eat sinks can be manufactured by mass production techniques and are, generally speaking relatively inexpensive.

The aluminium plate and clips may have various forms. For prior art in the field one can read for instance US patent N" 3,572,428 about a clamping heat sink or 4,609,040 about a self securing heat sink or 4,899,255 about a self securing heat sink.

An exempie of a heat sink of known prior art which seems the nearest of the present invention is shown in figure 1. This example is used in Thomson multirnédia power supply for television or video recorder set, to cool a MOS FET transistor used as switching transistor for a switched mode power supply. Good cooling is necessary because MOS FET have positive temperature coefficient and so the resistor of the transistor is increasing with temperature and then the transistor resistor increases and so on until the transistor is damaged.

Referring to figure 1 the heat sink comprises an aluminium plate 2 having two holes 1. Holes 1 are for insertion of a clip 5 which hold a transistor 6 strapped on plate 2. Plate 2 is folded at right angle. Plate 2 have on one side, legs 4 and a solder pin 3. Such an heat sink and transistor are assembled on a printed circuit board 30.

Said board 30 has been shematically represented in perspective view. Legs 4 and pin 3 are inserted in through holes 31 of board 30. Assembly is made as follows: -The legs 4 and solder pin 3 are inserted in through holes 31 of the board.

Then the legs 4 are twisted so that the heat sink is mechanically held on the board.

Transistor 6 is clipped on the heat sink by clip 5. When the element of the board are wave welded, solder pin 3 is welded to a mass of the board and then the heat sink is well enough fixed to the board.

Advantages of the invention.

Although such a heat sink has given good result, the purpose of the invention is to simplify the work on an assembly line of a printed circuit board including the heat sink. It is also to reduce the risk of short circuit that may occurs between the other element of the board and the twisted legs of the sink. It is also to reduce waste of aluminium in order to increase the efficiency of cooling by increasing the dissipating surface and to reduce the amount of scrapped matter to get a greenest production line. It is also to improve the air convection around the sink to make cooling more efficient.

Summary of the invention.

The invention is for a heat sink for an electronic component said heat sink comprising a plate of malleable material having a high heat transfer coefficient, wherein the plate has a main flat part and a boss stamped out of the plate said boss having a perimeter line delimiting the main flat part from the boss, said line having part which are common to the main flat part and to the boss, and part which are not, one at least of said part which is not common to the main part and the boss delimiting with a line of the main part an opening of the sink, said boss having a flat part at a distance from the main part.

In a preferred embodiment, the plate is folded so that it has a dihedral form, the boss or protruding part is parallel to the part of the sheet from which it is protruding. In an other preferred embodiment the protruding part includes three small mounting pad that are in contact each with a different side of the transistor to be cooled, so as to ease the positioning of the transistor. In an other preferred embodiment the protruding part has an axis of symmetry and is of a rectangular shape with two parallel sides perpendicular to the axis of symmetry and two jointing part with a cylindrical form made by a straight line which is always parallel to the axis of symmetry.

Brief description of the drawings.

The foregoing and additional objects and advantages of the invention will become more apparent as the following detailed description is read in conjunction with the accompanying drawing wherein like reference characters denote like parts in all views and wherein: Fig. 1 already commented is a perspective view of a prior art heat sink.

Fig 2 is a schematic perspective view of a heat sink according to the invention with a transistor attached to the sink.

Fig 3 is a top view of a heat sink according to the invention without the clip.

Fig 4 is a cross section of a leg of the heat sink of fig. 2.

In fig. 2 is represented in perspective view, a heat sink according to the invention. Said heat sink is made of a formed aluminium sheet 2, folded in this example at right angle, around a line of fold 13, so that the sheet 2 is split in two parts 7 and 8 forming with fold 13 a dihedral. A boss 9 is stamped out of side 7 of the dihedral, so that with said boss, side 7 comprises two parts, a first one referred to as the main part 26, which is a flat part from which boss 9 has been stamped out, and boss 9. Boss 9 is delimited from the main part by a line 25 which is referred to as the perimeter of boss 9. According to a feature of boss 9 the perimeter 25 of boss 9 has parts 25c, which are common to boss 9 and to main part and part 25d, which are detached from main part 26. At the places where there are part 25c of the perimeter 25 which are common to main part 26 and boss 9, there is continuity between main part 26 and the beginning of boss 9. That means that from those part 25c of the perimeter 25 there is a continuous material slope going from the level of main part 26 to the top part of boss 9, the top part being the one which is at the greatest distance from the level of main part 26. On the reverse for the parts 25d which are detached from main part 26, there is no material continuum and consequently there is an opening 24. In the preferred embodiment of the invention boss 9 comprises three parts, two slope parts 11 and 12, that start from parts 25c of the perimeter line 25 which are common to main part 26 and boss 9 and a flat part 10 in between.

Preferably and as represented on figure 2 slope part 11 and 12 are two symmetrical parts 11, 12 which form an angle with the plane of side 7. Those two parts are symmetrical relatively to a plane A perpendicular to the plane of side 7. Said plane A is represented in doted line in figure 3. Third part 10 is in the form of a plane parallel to the plane of side 7, and at a distance of it which is a function of the width of first 11 and second 12 symmetrical parts of boss 9. Boss 9 being made through stamping out, the maximum width of symmetrical sides 11 and 12 is limited by the need to avoid any breaking stress while stamping. It is to be noted that said width does not need to be great. Transistor 6 is on the outer surface 15 of the third part 10 of boss 9. The meaning of "outer surface" will be defined here below. Plate 2 has two surfaces separated by the thickness of the plate. When it is spoken of the outer surface of third part 10, it is meant the surface which is the remotest from the two surfaces of the non stamped part 26 of plate 2. Transistor 6 is fixed on the outer surface 15 of boss 9 by mean of a clip 5, having two side, a first 16 and a second 17.

One of the side 16, is applied on the transistor 6 and the second side 17 is applied on the inner face 18 of third part 10. Said arrangement has been made for easiness of assembly. In some cases where bulkiness is important transistor 6 could be fixed on the inner face 18 of third part 10, one side of the clip 5 being applied on the outer face 15 of third part 10. In the embodiment represented on figure 2 boss 9 has one edge 19, which before stamping was a part of an edge 20 of plate 2. In such a way, only one cut of plate 2 is necessary to make boss 9. This allows for a simplification of the stamping tool. Edge 19 is a part 25d of the perimeter line 25 which is not common to main part 26 and boss 9.

To ease positioning of transistor 6 on boss 9, three mounting pads 14 are stamped out in boss 9. They are placed so that three faces of transistor 6 abut on said pads 14 and among those three faces of transistor 6, a face 32 from which connections 22 of transistor 6 are coming out. Preferably the two other faces of abutting are faces which are adjacent to face 32.

The foldable legs 3 of known prior art have been replaced in the preferred embodiment of the invention by self locked solder pin 3a, 3b, and 3c. The term self locked will be explained later on in relation with figure 4. The number of pins 3 and their location may be varied according to the stability of the assembly which is required before soldering for handling the printed board on which the sink will be assembled, and the mechanical stresses to be applied to an apparatus in which the board is incorporated. In the preferred embodiment, there are only two pins 3, one 3a at an end of one side 7, of the dihedral said end being the remotest from fold 13, and a second 3b on the other side 8 of the dihedral, said end being the remotest from fold 13. Pins 3 are fixed to plate 2 by any known means for instance by soldering.

The term self locked will now be explained in relation with figure 4 which is a magnified side view of a pin 3. Pin 3 comprise an elastic member 21, which spring out of the surface of pin 3. When pin 3 is pushed in a slot of the printed circuit board on which the heat sink is to be assembled, elastic member 21 is first pressed against the main surface of pin 3, and then spring out when the thickness of the board has been crossed. Preferably elastic member 21, is stamped out of the plate that constitute pin 3.

It is to be noted that in the example shown, a part 25d of said perimeter line 25 of the boss 9 which is not common to the boss 9 and to the main part 26 is in a plane which also contains the line of the main part which delimit the opening 24 of the sink. That is because the stamp out tool is moving with a translation movement.

Generally, this movement being perpendicular to the plane of the main part 26, said part 25d, is in a plane perpendicular to the plane of main part 26.

It has been found that compared to the known prior art of figure 1, the heat sink according to the invention has a better cooling efficiency of about 10%. The inventors think that this improvement is due for part to a greater surface of aluminium for the same outside dimensions of plate 2, coming from the fact that no holes 1 are punched out for mounting clip 5, for part from a chimney or venturi effect coming from the form of boss 9, and for part from the fact that the new form of the clip 5, allowed by the new form of the heat sink, allows for a greater applied pressure to press transistor 6 on the sink.. It has also been found that there is a greater ease for positioning and fixing transistor 6 on the sink, due to the mounting pads 14, and to the form of the clip 5. There is also a greater ease of assembly of the sink on a printed board due to the self locket pins 3. Last, there is a reduction of the hazard of short circuit because of the replacement of legs 4 by soldering pins 3a, 3b, and 3c, coming from the fact that there is no more manual twisting movements.

Claims

CLAIMS 1. A heat sink for an electronic component, said sink comprising a plate of malleable material having a high heat transfer coefficient, said plate having a main flat part and a boss part, said boss having a perimeter line delimiting the boss, said line having parts which are common to the main flat part and to the boss, and parts which are not, one at least of said parts which is not common to the main part and the boss delimiting with a line of the main part an opening of the sink, said boss having a flat part at a spacing from the main part.
2. A heat sink according to claim 1 wherein said plate has a dihedral form with one fold and two sides, one of the side of the dihedral having the main flat part and said boss part.
3. A heat sink according to claim 1 or 2 wherein said flat part of the boss is parallel to the plane of the main part.
4. A heat sink according to claim 2 wherein said flat part at a spacing from the main part, of said boss, has a general rectangular shape with four sides, two parallel sides being at a distance from the plane of the main part of the plate and wherein two parallel other sides are adjacent to slopes joining the flat part of said boss to the main part.
5. A heat sink according to any one of claims 1 to 4 wherein a part of said perimeter line of the boss which is not common to the boss and the main part is in a plane which also contains the line of the main part which delimit the opening of the sink.
6. A heat sink according to any one of claims 1 to 5 wherein a part of said perimeter line of the boss is in a plane, said plane containing an edge of the main part.
7. A heat sink according to any one of claims 1 to 6 wherein a part of said perimeter line of the boss is in a plane perpendicular to the plane of the main part.
8. A heat sink according to any one of claims 1 to 7 wherein flat part of said boss is fitted with three mounting pads.
9. A heat sink according to any one of claims 1 to 8 wherein the plate comprises pins, to assemble said plate on a printed circuit board, said pins having an elastic member to clip said plate over the printed circuit board.
1 0. A heat sink substantially as herein described with reference to Figs. 2 to 4 of the accompanying drawings.
11. An assembly of an electronic component and of a heat sink according to any one of claims 1 to 10, the electronic component being pressed on the sink by a clip.

1 2. An assembly of an electronic component and of a heat sink according to claim 8 or 9, the electronic component being pressed on the sink by a clip and three sides of the electronic component abutting each, on one mounting pad that spring out from flat part.

1 3. An electronic appliance containing an assembly according to claim 11 or
12.