GB2236899A - "Clamping electrical components to heat sinks" - Google Patents

Abstract

A method of clamping an electrical power device to a heat sink comprising the steps of: positioning a power device (42) proximate to said heat sink (44) whereby one face of said power device is to be in contact with said heat sink; positioning shrinkable securing means such as a sleeve (48) made from shrinkable thermoplastics material round the power device and heat sink arrangement; and shrinking said shrinkable securing means onto said power device and heat sink arrangement whereby said power device is clamped to said heat sink. A semiconductor assembly produced by such a method is disclosed. <IMAGE>

Description

Mounting Power Devices This invention relates to clamping a power device such as a power transistor to a heat sink.

Heat sinks are generally required to be used in power applications in order to protect power devices from temperature rises which raise the temperature above the maximum value for a particular power device. For such applications, it is important that good thermal conduction is obtained at the interface between the power devices and the heat sinks so as to avoid the generation of any temperature gradients which would compromise the protection afforded by the heat sink.

The most common method of ensuring good thermal conduction is to use a screw at one end of the power device to clamp the power device to the heat sink. Such an arrangement is shown in Figure 1 which shows a power device 2 coupled to a PCB 4, only part of which is shown, and clamped to a heat sink 6 by a screw 8 with a washer 9 at one end 10 of the power device. However, this method has a number of disadvantages. For instance, the heat is generated by the main body 12 of the power device and since the power device is clamped to the heat sink 6 at one end 10 thereof, the power device is not clamped where the heat is generated and thus the thermal conduction between the power device and the heat sink in the main body area is poor. In addition, it is difficult to automate the assembly process of clamping the power device to the heat sink using a screw.Therefore, the assembly process is normally performed by an operator which involves time-consuming effort and hence a reduction in the production capabilities of the production line. Also, screws can easily become lost or dropped during assembly.

In order to circumvent these problems, an alternative method is known which uses a spring element to clamp the power device to the heat sink. Such a spring element arrangement is shown in Figure 2.

The spring element 28 comprises a U-shaped piece of flexible material which can be resiliently extended during assembly so as to clip over and onto the power device 22 and heat sink 26. Depending on the resilience of the material a large clamping force can be applied by the ends 32 and 34 of the spring element which are respectively in contact with the power device and the heat sink. This method, however, has a number of disadvantages in that the spring elements are costly, they take up a significant amount of space and may be lost during assembly. In addition, since the point of contact 30 between the power device and one end 32 of the spring element is small, a large force is applied to a very small area and hence the average thermal conduction is poor over the surface interface between the power device and the heat sink.

A further disadvantage of both the above described methods is that during assembly of electronic modules incorporating the power device and heat sink arrangement of Figure 1 or 2, springs and screws are respectively used which are both conductive and which can easily be lost in the electronic modules. The loss of such conductive components can seriously affect the insulation provided for in the electronic modules.

In accordance with a first aspect of the present invention there is provided a method of clamping a power device to a heat sink comprising the steps of: providing a heat sink; positioning a power device proximate to said heat sink whereby one face of said power device is to be in contact with said heat sink; the method characterised by the steps of, positioning shrinkable securing means round the power device and heat sink arrangement; and shrinking said shrinkable securing means onto said power device and heat sink arrangement whereby said power device is clamped to said heat sink.

In accordance with the second aspect of the present invention there is provided a semiconductor assembly comprising: a power device; a heat sink in thermal contact with said power device; and means for clamping said heat sink and power device together whereby thermal contact is maintained, characterised in that said clamping means comprises shrinkable securing means configured to substantially surround said power device and heat sink.

In a preferred embodiment, the securing means comprises a sleeve made from shrinkable thermoplastic material which can be easily slid over the power device and heat sink arrangement prior to the shrinking step.

Such a shrinkable sleeve costs far less than springs and provides for easier assembly.

Thus, it will be appreciated that since the securing means is arranged so that on shrinking it surrounds the power device and heat sink arrangement applying a clamping force thereto, the clamping force is applied over the main body of the power device thereby allowing for good uniform thermal conduction over the interface between the power device and the heat sink.

In addition, it will also be appreciated that unlike the method requiring the use of a screw, the present invention does not require accurate alignment for assembly, which simplifies the assembly process and makes it easier to automate.

It will also be appreciated that due to the simple design of the invention the power device can be clamped to the heat sink during manufacture of electronic modules incorporating power devices and heat sinks as an in-line operation involving no extra cost and that this, together with the elimination of screws and springs, results in considerable potential for cost savings.

A method of clamping power devices to heat sinks and a semiconductor assembly produced by such a method will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a power device clamped to a heat sink by a first prior art method; Figure 2 shows a power device clamped to a heat sink by a second prior art method; Figure 3 shows a power device clamped to a heat sink in accordance with the present invention; and Figure 4 shows a power device in contact with a heat sink before being clamped thereto using a method in accordance with the present invention.

Referring to Figure 3, in a preferred embodiment of the present invention an electronic module, only the power device and heat sink arrangement 40 of which is shown, comprises a power device 42 coupled to a PCB 46 of the electronic module and clamped to a heat sink 44 by a heat shrink sleeve 48. The heat sink is coupled to a case (not shown) which houses the electronic module so as to provide a heat conduction path. The electronic module may be for example one which is used in car engines to control the operation of the engine and the power device may be for example a power transistor.

The heat shrink sleeve comprises a tube made from material which shrinks on exposure to heat and which is long enough to substantially cover the main body 50 of the power device.

The manner in which the power device 42 is clamped to the heat sink 44 will now be described with reference also to Figure 4.

The power device 42 mounted on a PCB 46 is positioned so that one of its faces 43 is in contact with the heat sink 44. The heat shrink sleeve, whose cross-sectional dimensions are arranged so that the sleeve can easily surround the power device and heat sink coupled together, is slid over the power device and heat sink arrangement so that it encompasses the power device and heat sink arrangement.

Heat is then applied in the vicinity of the heat shrink sleeve whereby the material shrinks bringing the heat shrink sleeve into contact with the outer surface 45 of the power device 42 and the outer surface 52 of the heat sink 44. Once the heat shrink sleeve has shrunk the force applied by the sleeve in contact with the power device and the heat sink, clamps these two components together. Since the shrinkage is not reversible, a permanent clamping force is applied to the power device and the heat sink.

It is important that the heat shrink sleeve is made from material which is stable over the operating temperature range of the electronic module. For the electronic module used in car engines the temperature range can be between -40 degrees centigrade to +140 degrees centigrade and a suitable material for this temperature range is Kynar* manufactured by Raychem Ltd.

Kynar is suitable for use between -55 degrees centigrade and +175 degrees centigrade.

The molecular structure of Kynar is such that as the Kynar material is exposed to infrared radiation adjacent molecules are permanently coupled together or crosslinked whereby the material shrinks and can be formed into any desired shape. Kynar has further advantages in that it has high flame retardance and also retains a high clamping force.

Thus, it will be appreciated that since the heat shrink sleeve substantially covers the outer face 45 of the main body 50 of the power device, the clamping force is applied over the whole of the main body of the device. High force concentrations are therefore avoided which provides for improved thermal conduction.

It will also be appreciated that since the heat shrink sleeve is made from a thermoplastic material no electrical coupling problems arise from losing the heat shrink sleeve in the electronic module.

* Kynar is a trademark of Pennwalt Chemicals Ltd.

It will be appreciated that the step of shrinking the heat shrink sleeve by infrared or other forms of radiant energy can be performed during solder reflow of the PCB which makes the process of clamping the power device to a heat sink according to the present invention simple and easy to automate.

Claims (11)

Claims

1. A method of clamping a power device to a heat sink comprising the steps of: providing a heat sink; positioning a power device proximate to said heat sink whereby one face of said power device is to be in contact with said heat sink; the method characterised by the steps of, positioning shrinkable securing means round the power device and heat sink arrangement; and shrinking said shrinkable securing means onto said power device and heat sink arrangement whereby said power device is clamped to said heat sink.

2. A method of clamping a power device to a heat sink according to claim 1 wherein said shrinking step comprises exposing said shrinkable securing means to high energy radiation.

3. A method of clamping a power device to a heat sink according to claim 1 wherein said shrinking step comprises exposing said shrinkable securing means to heat.

4. A method of clamping a power device to a heat sink according to claim 1, 2 or 3 wherein said shrinkable securing means comprises a sleeve made from shrinkable material, said sleeve comprising a tube and being configurated to substantially cover a face of said power device which opposes said one face of said power device.

5. A method of clamping a power device to a heat sink according to claim 1, 2, 3 or 4 wherein said power device is coupled to a PCB such that the shrinking step is performed during solder reflow of the PCB.

6. A semiconductor assembly comprising: a power device; a heat sink in thermal contact with said power device; and means for clamping said heat sink and power device together whereby thermal contact is maintained, characterised in that said clamping means comprises shrinkable securing means configured to substantially surround said power device and heat sink.

7. A semiconductor assembly according to claim 6 wherein said shrinkable securing means comprises a sleeve made from shrinkable material.

8. A semiconductor assembly according to claim 7 wherein said shrinkable material shrinks on exposure to heat.

9. A semiconductor assembly according to claim .7 wherein said shrinkable material shrinks on exposure to high energy radiation.

10. A method of clamping a power device to a heat sink substantially as hereinbefore described with reference to Figures 3 and 4.

11. A semiconductor assembly substantially as hereinbefore described with reference to Figures 3 and 4.