The present invention relates to aluminium heatsinks for semiconductor devices, of the type in which said semiconductor devices include solderable means arranged to be soldered to a circuit board with tin/lead solder.

The use of heatsinks in the electronics art is very well known and essentially consists of increasing the surface area of a heat-dissapating device, using a material with good thermal conduction properties. The material most commonly used for fabricating heatsinks is aluminium and a suitable shape may be obtained by either pressing aluminium plate or extruding the heated material through a die.

When fabricating low volume circuits, often using expensive electronic components, the cost of making heatsinks and applying them to the circuit is relatively small compared to the total cost of designing and making the circuit, however, over recent years the cost of electronic components has reduced significantly and many circuits are now mass produced in their thousands. The field is also highly competitive, therefore, manufacturers are constantly looking for ways in which the cost of passive components and board assembly may be reduced.

Large power transistors require large heat sinks and these are often secured to a metal back-plane or cabinet by means of a bolt. However, in many applications, a large number of smaller transistors are required and it would be impossible to arrange all of these along the edges of the circuit board. An arrangement for providing a heat sink for many transistors of this type is shown in European Patent Application No. 0 124 715 of International Business Machines Corporation, US in which a plurality of transistors are supported in a row along a common heatsink, which maybe bolted or glued to the circuit board. However, a problem with this approach is that the

transistors must be regularly spaced over the circuit board, placing an additional restriction on the circuit designer.

It is also known to provide a separate heatsink for each transistor on a board and many transistors are provided with a metal support plate, having a hole therein, allowing said plate to be bolted to a suitable heatsink. However, if the heatsink is not secured to a back-plane etc. the weight of the heatsink must be supported by the electrical contacts (legs) of the electronic device, which may in turn lead to failure, due to excessive force being applied to said contacts. Bolting a heatsink to an active device is also an expensive operation and, for this reason, heatsinks are also available which include interference clipping means, for holding the transistor while ensuring good thermal conductivity at the interface of the two components. However, in addition to the risk of applying excessive force to the legs of the active device, it is also possible for this type of heatsink to become dislodged, again leading to failure. It is therefore an object of the present invention to provide an improved heatsink for semiconductor devices, such as transistors, voltage regulators and amplifiers etc. SUMMARY OF THE INVENΗON

According to a first aspect of the invention, there is provided an aluminium heatsink, comprising means for receiving a semiconductor device, said semiconductor device including solderable means arranged to be soldered to a circuit board with tin/lead solder; characterised by securing means arranged to secure said heatsink to said circuit board, wherein said securing means is a portion of said aluminium heatsink arranged to securably engage with said circuit board.

An advantage of the invention is that it allows heatsinks to be included in the design of a circuit, after the main circuit design has been completed. Heatsinks may be applied to active components at any position on the circuit board and the designer is not restrained by having to place all active, ie. heat-

dissipating, components at positions determined by their respective heatsinks. Furthermore, the heatsink is secured to the circuit board by a portion of the heatsink which extends downwards in the same direction as the electrical contacts of the electronic device. Thus, the circuit designer need only provide additional holes for securing the heatsink, in a similar way to which holes are provided for the electrical contacts, which are used to restrain the heatsink, thereby taking the weight of the heatsink away from the electrical contacts by providing its own support means.

In a preferred embodiment, two portions of aluminium form the securing means and each of said portions are coated with solder, allowing them to be soldered into the circuit board. In this way, the heatsink may be soldered to the circuit board during the operation for soldering the electrical contacts to said circuit board. Preferably, said portions are coated with aluminium solder, onto which conventional tin/lead solder may be applied, providing the advantage of allowing electrical contacts and heatsink portions to be soldered to the circuit board during a common operation, as performed during the mass production of circuit boards.

In an alternative embodiment, said portions of said heatsink include means for engaging into respective holes in a circuit board to provide an interference fit, thereby securing said heatsink to the circuit board. Preferably, said securing means is an angled tine, facilitating the entry of said portion into said hole, whereafter said tine prevents removal of said portion from said hole. BRIEF DESCRIPTION )F THE DRAWINGS Figure 1 shows a heatsink with means for connecting said heatsink to a circuit board, and a transistor suitable for co-operating with said heatsink; and,

Figure 2 shows an alternative means for connecting a heatsink to a circuit board.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aluminium heatsink 15 is shown in Figure 1, fabricated by cold- pressing a piece of aluminium sheet, typically having a thickness of 0.5-2.5 mm. The heatsink is arranged to receive a semiconductor device 16 (such as a Texas Instruments 8827, TIP 31 A, transistor or similar package) having a metal plate connected thereto to facilitate heat transfer. The transistor is housed within an enclosure 17, formed by the heatsink, with the metal plate forced into contact with the back of the inner surface of the heatsink by means of leaf springs 18, formed at the edges of the pressed plate. The terminal legs of the transistor extend from the transistor-body/heatsink assembly, for connection to a circuit board using conventional tin/lead solder.

Extending from the side walls 19 of the heatsink, in the direction of contacts 20, are securing portions 21, arranged to engage with holes in the circuit board. The securing portions ensure that the transistor/heatsink assembly is held firmly to the circuit board, thereby substantially reducing the possibility of the heatsink being accidentally moved from the transistor and also substantially reducing the risk of transistor-contact (20) breakage.

The transistor contacts 20 are secured to the circuit board by means of tin/lead solder which, in addition to providing mechanical retention, also facilitates electrical conduction. In a first embodiment, the securing portions 21 are also arranged to engage with holes in the circuit board and are secured to said holes by a soldering process, similar to that used for securing the contacts. In most applications, a small area of copper is provided on the circuit board, which does not make electrical contact with any other copper tracks, however, in some applications, a designer may wish to place said portions 21 in circuit, usually by connecting them to ground.

To facilitate the manual assembly of components, solder is applied to the ends of the portion during manufacture which, during the application of a soldering iron, melts to form a mechanical bond. However, in an alternative

embodiment, particularly suited for mass production, the ends of portion 21 are coated, to ensure that they attract the type of solder used when applying solder to the circuit board as a whole.

In a preferred embodiment, the securing portions 21 are first coated with aluminium solder 22, such as that manufactured in the United KingdorJl by Multicore Solders Limited and sold under the Trade Mark ALUSOL 45. ^' However, once applied, the aluminium solder is not actually used as solder, but provides a coating for the conventional tin/lead solder, used to connect the heatsink to the circuit board. The aluminium solder melts at a significantly higher temperature than that of the tin/lead solder and therefore said aluminium solder does not melt during the soldering process.

The heatsink is fabricated by pressing and forming the required shape from a sheet of aluminium. After fabrication, the aluminium is anodised, using an electrolyte of sulphuric acid, to produce a very thin porous layer of oxide. The anodised aluminium is then introduced to a black dye, which is fixed to give a smooth, homogeneous black finish to the device which, at this stage, extends to the ends of the securing portions 21. The black dye and oxide layer is removed from the ends of portions 21 by dipping said portions into a solution of sodium hydroxide, after which, ALUSOL 45 flux is applied before dipping them into a bath of aluminium solder, maintained at a temperature of 250-350 °C.

In some situations, a designer may wish to apply heatsinks after the soldering process has been completed, in which case it is not appropriate to solder the heatsinks to the circuit board as this may affect components soldered during the earlier operation. An alternative embodiment is therefore shown in Figure 2, in which, like the device shown in rigure 1, securing means are arranged to secure the heatsink to the circuit board, wherein said securing means are portions of the aluminium heatsink, arranged to securably engage with the circuit board. The heatsink 25 of Figure 2 shown

in outline, however, its shape and manner of manufacture is styled substantially the same to that used in the manufacture of the device shown in Figure 1. Securing portions 26 extend through holes 27 in a circuit board 28 but the size of the portions and the size of the holes are such that force must be exerted to place portions in said holes 27, which are then retained by an interference fit. In addition, the securing portions 26 are stamped so as to create a barbed tine 29, which facilitates the entry of the portions 26 into the holes 27, while providing substantial resistance to the removal of said portions from said holes.