FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention relate to a heat sink mounting apparatus and method, for example a heat sink mounting apparatus and method for a use with a subsea electronics module for a subsea control module as part of a well control system, such as a control system for a subsea well. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is well-known to use heat sinks as cooling devices to transfer away heat generated in electronic components. As illustrated in  FIG. 1 , removal of heat from a high power electronic component housed within a device housing  1 , is typically achieved by mounting the electronic component on a metal plate heat sink  2  (shown as hatched for clarity), which is generally formed from an aluminium alloy. The electronic component is omitted for clarity, but it can be attached to the heat sink  2  by a variety of well-known methods. The heat sink  2  is in turn attached to the wall of the device housing  1 , thus providing a good thermal conduction path from the electronic component to the device housing  1 , which then dissipates the heat to the surrounding environment by conduction and/or radiation. 
         [0003]    Typically such heat sinks  2  need to be removable from the device housing  1  to facilitate replacement and repair. As shown in  FIG. 1 , the heat sink  2  is typically secured in the device housing  1  by clamping the heat sink  2  within a slot  3  in the container wall by a screw and wedge device, known commercially as a wedgelock  8 . A further empty slot  3  is shown adjacent to the slot  3  to aid understanding, but in practice more than one heat sink  2  may be attached to the device housing  1 . 
         [0004]    The wedgelock  8  consists of an assembly of a pair of wedges  4 ,  5  at the top of the slot  3  in the wall of the device housing  3  and a further pair of wedges at the bottom of the slot (not shown, only the top section of the housing being shown for clarity). The top and bottom pairs of wedges are connected by a screwed rod  6 , capped with a slotted head  7 . Typically the edge of the heat sink  2  is machined to accommodate the wedgelock  8 . Rotation of the screwed rod  6  causes wedge  5 , and its corresponding wedge at the bottom, to move relative to the other wedge  4  of the pair. Movement of wedge  5 , and its corresponding wedge at the bottom, causes the wedges  4 ,  5  to exert a force against the heat sink  2  and clamp the heat sink  2  to the side of the slot  3  in the wall of the device housing  1 . The wedgelock  8  is glued to the heat sink  2  edge. Although wedgelocks  8  generally function satisfactorily in securing heat sinks  2  to device housings, they are expensive to manufacture and the gluing process is time consuming. 
       SUMMARY OF THE INVENTION 
       [0005]    According to a first aspect of the present invention there is provided a heat sink mounting apparatus for mounting a heat sink to a wall of a device housing, the apparatus comprising: a heat sink; and a device housing, the housing having a slot in a wall of the housing for receiving a heat sink, wherein the heat sink and the housing are adapted such that the heat sink can be secured within the slot with an interference fit. 
         [0006]    The use of an interference fit to secure the heat sink to the device housing eliminates the need to use wedgelocks and glue, thus significantly reducing the costs and time associated with securing the heat sink in the device housing. 
         [0007]    In an embodiment of the invention the heat sink comprises an elongate protrusion which is arranged to be received in a corresponding groove in the slot in the wall of the housing. The interaction of the protrusion and the groove help to achieve the interference fit. The protrusion will have sufficient flexibility to permit it to be press fitted into the groove. 
         [0008]    In an embodiment of the invention a hollow channel is provided within the protrusion. The hollow channel serves to facilitate temporary distortion of the protrusion as the heat sink is fitted into the device housing. 
         [0009]    In an embodiment of the invention the protrusion has a curved upper surface. In an embodiment of the invention the protrusion is dome-shaped in cross-section. 
         [0010]    In an embodiment of the invention the hollow channel has generally the same shape as the protrusion. 
         [0011]    In an embodiment of the invention the heat sink is machined from a solid block. In an embodiment of the invention the heat sink is formed from aluminium alloy. 
         [0012]    In an embodiment of the invention a plurality of protrusions are provided in the heat sink for engaging with a plurality of corresponding grooves in the slot in the wall of the housing. 
         [0013]    In an embodiment of the invention a plurality of heat sinks are provided for engaging with a plurality of slots in a wall of the housing. 
         [0014]    In an embodiment of the invention the housing is a housing of a subsea electronics module for a subsea control module of a control system for a subsea well. 
         [0015]    According to a second aspect of the present invention there is provided a method of mounting a heat sink to a wall of a device housing, the housing having a slot in a wall of the housing for receiving the heat sink, the heat sink and the housing being adapted such that the heat sink can be secured within the slot with an interference fit, the method comprising the steps of: aligning the heat sink with the slot in the wall of the housing; and applying a force to heat sink in the direction of the slot such that at least a portion of the heat sink is received within the slot. 
         [0016]    In an embodiment of the invention the heat sink comprises an elongate protrusion which is arranged to be received in a corresponding groove in the slot in the wall of the housing, and the force applied is sufficient to cause a temporary distortion of the protrusion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  shows a partial perspective view of a prior art heat sink mounting apparatus; and 
           [0019]      FIG. 2  shows a partial perspective view of a heat sink mounting apparatus according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 1  shows a prior art heat sink mounting apparatus as described above. 
         [0021]      FIG. 2  shows a heat sink mounting apparatus  10  according to an embodiment of the present invention, for example for the securing of heat sinks in a subsea electronics module (SEM) housed within a subsea control module (SCM) mounted, for example, on a subsea well tree, as part of the well control system for a subsea well, such as a hydrocarbon production well. 
         [0022]    The heat sink mounting apparatus  10  comprises a heat sink  12  and a device housing  14 , such as a housing of a SEM. The device housing  14  has a slot  16  in a wall  18  of the housing  14  for receiving the heat sink  12 . The heat sink  12  (shown as hatched for clarity) is provided to remove the heat from an electronic component, and is typically machined from a solid block, as it is often not a simple plate and box construction with edges machined to suit a wedgelock, as in the prior art. The electronic component is omitted from  FIG. 2  for clarity, but it can be attached to the heat sink  12  by a variety of well-known methods. The heat sink  12  is typically machined from a solid block of aluminium alloy using a CNC machine. The heat sink  12  has an elongate protrusion  20  machined along a side edge  22  of the heat sink  12 . The elongate protrusion  20  has a curved upper surface which is dome-shaped in cross-section. To facilitate the temporary distortion of the protrusion  20 , which will be described in more detail below, a hollow channel  24  is machined in the protrusion  20 . The shape of the hollow channel  24  matches the shape of the protrusion, but in practice any shape of channel, even a simple hole, will often suffice. 
         [0023]    The slot  16 , in the wall  18  of the device housing  14 , is also machined with a “female” groove  26 , to match the “male” half of the protrusion  20 , on the heat sink  12 . An adjacent unoccupied slot  16  is shown to further illustrate the groove  26 . Insertion of the heat sink  12  (with attached electronic component) into the housing is achieved by sliding the heat sink  12  into the slot  16 , until the protrusion  20  engages with the groove  26  in the slot  16 . Once the protrusion  20  comes into contact with the groove  26  in the slot  16  substantial pressure must be applied to temporarily distort the protrusion  20  to complete the insertion of the heat sink  12  into the groove  26 . The hollow channel  24  serves to facilitate temporary distortion of the protrusion  20  as the heat sink  12  is fitted into the device housing  14 . 
         [0024]    The tolerances of the protrusion  20  and the groove  26  are arranged such that on completion of the insertion of the heat sink  12 , the protrusion  20  of the heat sink  12  is still slightly in compression, thus ensuring a firm pressure and good thermal conduction of the heat sink  12  to the slot  26 . To ensure consistent pressure across the slot  16 , protrusions  20  are, typically, machined at the top and bottom of the heat sink  12 , and the slots  16 . In order to extract the heat sink  12 , and the attached electronic component, from the device housing  14  a simple extraction tool may be necessary, or the end of the heat sink  12  may be machined to allow a screwdriver to be inserted to lever the heat sink  12  out of its slot  16 . 
         [0025]    A key advantage of the present invention is the elimination of expensive wedge locking devices and the gluing process to attach them, in exchange for the low additional cost of machining the protrusion in the heat sink  12  and the corresponding groove  26  in the wall  18  of the device housing  14 .
A heat sink mounting apparatus including: a heat sink; and a device housing, the housing having a slot in the wall of the housing configured to receive a heat sink, wherein the heat sink and the housing are configured such that the heat sink can be secured within the slot with an interference fit.