HEAT DISSIPATOR FIXED TO THE SUBSTRATE OF AN INTEGRATED CIRCUIT HOUSING

Abstract

Integrated circuit package (1) comprising a substrate (2) and a heat sink (4) having a side panel (41) fixed to a mounting face (21) of the substrate (2) by fastening means (60, 61), wherein the fastening means (60, 61) are housed in recesses (51) of the side panel (41) and pass through the substrate (2) via first holes (50), the fastening means (60, 61) and the first holes (50) being configured to allow fastening of the side panel (41) to the mounting face (21) and relative displacement of the fastening means (60, 61) with respect to the first holes (50). Figure for the abstract: Fig 1

Description

Title of the invention: FIXED HEAT SINK ON THE SUBSTRATE OF AN INTEGRATED CIRCUIT PACKAGE

[0001] Embodiments and implementations relate to the field of electronics, in particular the field of packaging of integrated circuits and more particularly the fixing of heat sinks on the carrier substrate of integrated circuit packages.

[0002] One type of integrated circuit package, for example a "CLGA" (Ceramic Land Grid Array) package, includes a heat sink for dissipating heat produced by an integrated circuit chip during operation to the outside of the package.

[0003] The heat sink is typically fixed to a carrier substrate of the package by means of an adhesive forming chemical bonds between the sink and the carrier substrate at high temperatures, for example during thermal annealing of the package.

[0004] However, the material of the carrier substrate sometimes has a relatively low coefficient of thermal expansion (also known by the acronym "CTE" for "Coefficient of Thermal Expansion") compared to that of the material of the heat sink, such that the latter exerts mechanical stresses on the carrier substrate. These stresses occur in particular at temperatures for which the heat sink expands, for example during thermal annealing of the package, and are typically proportional to the difference between the respective coefficients of thermal expansion of the sink and the carrier substrate. In particular, these stresses can cause deformation of the carrier substrate, which can then have a curvature at its ends and not return to its initial flat shape after the temperature decreases.

[0005] The difference between the thermal expansion coefficients is particularly high for the case of a metal heat sink, for example made of copper, and a ceramic carrier substrate. The deformation of the carrier substrate has been observed in particular following the fixing of copper heat sinks whose thermal expansion coefficient is 17x106 K 1 on CLGA type package substrates generally made of ceramic such as Alumina (A12O3) whose thermal expansion coefficient is 12xlO6K *.

[0006] Therefore, a deformed carrier substrate is not suitable for mounting the package on a printed circuit board since empty spaces may appear between the curved ends of the housing and the printed circuit board once the housing is mounted on the board.

[0007] There is therefore a need to find a solution to prevent deformation of the carrier substrate when fixing the heat sink to the carrier substrate.

[0008] According to one aspect, there is provided an integrated circuit package comprising a carrier substrate and a heat sink having a side wall fixed to a mounting face of the carrier substrate by fixing means.

[0009] The fastening means are housed in housings of the side wall and pass through the carrier substrate through first orifices. The fastening means and the first orifices are configured to allow fastening of the side wall to the mounting face and relative movement of the fastening means with respect to the first orifices.

[0010] In other words, the heat sink is held fixed to the carrier substrate by fixing means whose movement is not completely restricted by the first orifices.

[0011] In particular, the fixing means move freely in the direction of expansion of the heat sink to prevent the heat sink from exerting mechanical stresses on the carrier substrate during its expansion. And, when the heat sink returns to its initial shape, i.e. when it shrinks, the fixing means can return to their initial position to prevent further mechanical stresses from being applied to the carrier substrate when the heat sink shrinks.

[0012] Thus, the housing according to this aspect comprises a heat sink fixed on a carrier substrate which does not deform during expansion of the heat sink even with significant differences between the thermal expansion coefficients of the carrier substrate and the heat sink. Such a housing according to this aspect therefore does not have any curvature at its ends.

[0013] In particular, the carrier substrate may have, for example for a certain type of ceramic, a coefficient of thermal expansion of between 10x106 K1 and 12x106 K 1 and the heat sink may have a coefficient of thermal expansion of between 15x106K' and 17x106K 1

[0014] According to one embodiment, the carrier substrate comprises a lower face opposite the mounting face and the fixing means comprise rivets each comprising a first end housed in a corresponding housing and a second end under the lower face of the carrier substrate.

[0015] The distance between the first end and the second end is equal to the thickness of the carrier substrate increased by a clearance allowing both maintenance of the side wall fixed to the mounting face of the carrier substrate and a displacement of each rivet into the first holes.

[0016] The rivets allow for quick and reliable attachment of the heat sink to the carrier substrate and are long enough to provide clearance and facilitate movement of the rivets into the first holes.

[0017] According to one embodiment, the carrier substrate has a lower face opposite the mounting face and the housings have a thread and the fixing means comprise screws adapted to be screwed into the threaded housings leaving a clearance between the screw heads and the lower face of the carrier substrate.

[0018] The screws are advantageously used as an alternative to rivets to fix the heat sink to the carrier substrate and can be tightened or loosened, or even removed completely to be replaced, for example, by new screws.

[0019] According to one embodiment, the value of the clearance (J) is between 10 and 20 pm

[0020] This range of play allows the fixing means to move freely while avoiding creating excessively large spaces between the side wall of the heat sink and the mounting face of the carrier substrate.

[0021] According to one embodiment, the first orifices have an oblong shape.

[0022] According to one embodiment, the side wall of the heat sink is shaped annular with square or rectangular section, and the carrier substrate has four first orifices respectively located opposite the four corner zones of the side wall.

[0023] This number of first orifices and therefore of fixing means is sufficient to fix a side wall having such a shape reliably on the mounting face of the carrier substrate without risk of generating mechanical stresses on the carrier substrate during expansion of the heat sink.

[0024] According to one embodiment, the axis of each first orifice in the plane of the mounting face makes an angle of 45° relative to the direction of the two edges of the side wall adjacent to this first orifice.

[0025] The oblong shape in combination with the 45° angle, allows the movement of the fixing means to be oriented so that they move in the direction of expansion or contraction of the heat sink and provides greater freedom of movement to the sink, thus reducing stresses on the carrier substrate.

[0026] The fixing means can thus move along the axis of the first orifices.

[0027] According to one embodiment, the fixing means are thermally conductive.

[0028] The fixing means thus make it possible to form a path through the carrier substrate to dissipate the heat generated by the electronic chips operating in the package.

[0029] According to another aspect, there is provided a method of manufacturing an integrated circuit package, comprising: - a provision of a carrier substrate comprising a mounting face, - a provision of a heat sink comprising a side wall, - a formation of housings in the side wall of the heat sink, - a formation of first orifices in the carrier substrate, - fixing the side wall to the mounting face of the carrier substrate by fixing means, the fixing means being housed in the housings of the side wall of the heat sink and passing through the carrier substrate through the first orifices so as to allow relative movement of the fixing means with respect to the first orifices.

[0030] According to one embodiment, the carrier substrate comprises a lower face opposite the mounting face and the fixing of the side wall on the mounting face comprises riveting by rivets each comprising a first end housed in a corresponding housing and a second end under the lower face of the carrier substrate, the distance between the first end and the second end being equal to the thickness of the carrier substrate increased by a clearance allowing both maintenance of the side wall fixed to the mounting face of the carrier substrate and movement of each rivet in the first orifices.

[0031] According to one embodiment, the carrier substrate comprises a lower face opposite the mounting face, the formation of the housings in the side wall of the heat sink comprises a threading of the housings and the fixing of the side wall on the mounting face comprises screwing by screws into the threaded housings leaving a clearance between the screw heads and the lower face of the carrier substrate.

[0032] According to one embodiment, the value of the clearance is between 10 and 20 pm.

[0033] According to one embodiment, the first orifices are made so that they have an oblong shape.

[0034] According to one embodiment, the heat sink is provided whose side wall is annular in shape with a square or rectangular section, and four first orifices are formed in the carrier substrate respectively located opposite the four corner zones of the side wall.

[0035] According to one embodiment, the axis of each first orifice in the plane of the mounting face makes an angle of 45° relative to the direction of the two edges of the side wall adjacent to this first orifice.

[0036] According to one embodiment, the fixing means are made so that they are thermally conductive.

[0037] According to one embodiment, the carrier substrate has a thermal expansion coefficient of between 10x106 K 1 and 12x106 K 1 and the dissipator thermal has a coefficient of thermal expansion between 15x106 K 1 and 17 xlO6K*

[0038] Other advantages and characteristics of the invention will appear on examining the detailed description of the embodiment and implementation, which is in no way limiting, and the appended drawings in which:

[0039] [Fig.l] ;

[0040] [Fig.2] ;

[0041] [Fig.3];

[0042] [Fig.4] and

[0043] [Fig.5] schematically illustrate modes of implementation and embodiment of the invention.

[0044] [Fig.l] schematically illustrates a sectional view of an integrated circuit package 1 according to one embodiment.

[0045] The housing 1 comprises a carrier substrate 2 comprising a mounting face 21 and a lower face 20 opposite the mounting face.

[0046] The carrier substrate 2 is of conventional structure and known per se, and may have a different composition depending on the type of package. The carrier substrate 2 comprises conductive tracks, typically made of copper, integrated in one or more layers of dielectric material. In the case of a “CLGA” type package, the dielectric material is typically ceramic such as Alumina (A12O3) or aluminum nitride (AIN).

[0047] In particular, the carrier substrate 2 may comprise low-temperature fired ceramic designated by the acronym “LTCC” (for “Low Temperature Co-Fired Ceramic” in English) of type GL773 marketed by the Japanese company Kyocera, the thermal expansion coefficient of which is between 10x106 K 1 and 12x106 KA, for example 12x10 6K '

[0048] The carrier substrate 2 comprises first orifices 50 extending in the thickness Esub of the carrier substrate 2 between the mounting face 21 and the lower face 20 of the carrier substrate 2. The thickness Esub of the carrier substrate 2 is for example between 3 and 4 mm.

[0049] The integrated circuit package 1 also comprises at least one electronic chip 3 and a heat sink 4.

[0050] The electronic chip(s) 3 may be mounted and connected to the mounting face 21 of the carrier substrate 2 in a conventional manner known per se, for example by soldering connection balls.

[0051] The heat sink 4 comprises a side wall 41 and an upper wall 40. The side wall 41 is fixed to the mounting face 21 of the carrier substrate 2 by fixing means 60 and the upper wall 40 is here fixed to the electronic chip 3 via a preferentially thermally conductive interface material 7. The side wall 41 comprises housings 51, for example orifices extending partially into the side wall 41 and aligned with the first orifices 50 of the carrier substrate 2.

[0052] Furthermore, the heat sink 4 is made of metal, for example copper (Cu), and then has a thermal coefficient of expansion equal to 17x10 6K *. Consequently, the expansion of the heat sink 4 is greater than that of the carrier substrate at high temperatures, in particular during thermal annealing of the package and would have exerted mechanical stresses on the carrier substrate if the side wall had been fixed to the mounting face by glue for example as in the prior art.

[0053] According to the invention, the fixing means and the first orifices 50 are configured to allow fixing of the side wall 41 on the mounting face 21 and relative movement of the fixing means with respect to the first orifices 50, in the plane of the mounting face 21. In particular, the fixing means pass through the carrier substrate 2 through the first orifices 50 and are housed in the housings 51 of the side wall 41.

[0054] In other words, the heat sink 4 is kept fixed to the carrier substrate 2 by fixing means whose movement is not completely restricted by the first orifices 50.

[0055] The fixing means, moving freely in the direction of expansion of the heat sink 4, prevent the sink 4 from exerting mechanical stresses on the carrier substrate 2 during its expansion, at high temperatures for example. And, when the heat sink 4 returns to its initial shape, that is to say when it shrinks in particular during a decrease in temperature, the fixing means can return to their initial position to prevent other mechanical stresses from being applied to the carrier substrate 2.

[0056] Thus, the carrier substrate 2 of the package 1 according to this aspect does not deform during expansion or contraction of the heat sink 4 even in the presence of the above-mentioned differences between the thermal expansion coefficients of the carrier substrate 2 and the heat sink 4.

[0057] Furthermore, the carrier substrate 2 of this housing 1 does not have any curvature at its ends and remains flat so that, when the housing 1 is fixed on a printed circuit board, the space between the lower face 20 of the carrier substrate and the printed circuit board is minimized.

[0058] The fastening means, as shown in [Fig.l], comprise rivets 60 each comprising a first end 601 and a second end 602 as well as a body 600 joining the first end 601 and the second end 602. The first end 601 of each rivet 60 is housed in a corresponding housing 51 of the side wall 41 and the second end 602 of each rivet 60 is located under the lower face 20 of the carrier substrate 2.

[0059] Advantageously, the distance Lfix between the first end 601 and the second end 602 is equal to the thickness Esub of the carrier substrate 2 increased by a clearance J. The clearance J allows both maintenance of the side wall 41 fixed to the mounting face 21 of the carrier substrate 2 and movement of each rivet 60 in the first orifices 50.

[0060] The value of the clearance J is advantageously between 10 and 20 μm. This range of clearance J allows the rivets 60 to move freely while avoiding creating excessively large spaces between the side wall 41 of the heat sink 4 and the mounting face 21 of the carrier substrate 2.

[0061] A person skilled in the art will know how to adapt the dimensions of the rivets 60 and the first orifices 50 to obtain such clearance values ​​J and to allow the relative movement of the rivets with respect to the first orifices 50.

[0062] Furthermore, the first end 601 and the second end 602 of the rivets 60 are configured to keep the side wall 41 fixed to the mounting face 21 and only move very slightly depending on the height of the carrier substrate 2.

[0063] Advantageously, the rivets 60 are thermally conductive and thus make it possible to form a path through the carrier substrate 2 to dissipate the heat generated by the electronic chips 3 in operation in the housing 1.

[0064] [Fig. 2] schematically illustrates a sectional view of an integrated circuit package 1 according to a variant. The heat sink and the carrier substrate, similar to those described previously, have the same references.

[0065] According to this variant, only the housings 51 of the side wall 41 and the fixing means have been changed so that the housings 51 have a thread and the fixing means comprise screws 61. The screws 61 are adapted to be screwed into the threaded housings 51 while leaving a clearance J between the screw heads 602 and the lower face 20 of the carrier substrate 2.

[0066] Everything that has been described previously for the housing 1 of [Fig.l], in particular the advantages linked to the fixing of the side wall on the mounting face and to the relative displacement of the fixing means with respect to the first orifices, applies to the housing 1 of [Fig.2].

[0067] More particularly, the screws 61 pass through the carrier substrate 2 through the first orifices 50 and have a threaded end 611 screwed into the corresponding threaded housing 51.

[0068] Furthermore, the clearance J between the screw heads 602 and the lower face 20 can be included in the same range of values, between 10 and 20 pm, as the clearance described previously in relation to [Fig.l].

[0069] Those skilled in the art will also know how to adapt the dimensions of the screws 61 relative to the dimensions of the first orifices 50 to obtain such clearance values ​​J and to allow the relative movement of the screws 61 relative to the first orifices 50.

[0070] The screws 61 can be tightened or loosened, for example to adjust the value of the clearance J, or even removed completely to be replaced by new screws.

[0071] Advantageously, the screws 61 are also thermally conductive and thus make it possible to form a path through the carrier substrate 2 to dissipate the heat generated by the electronic chips 3 in operation in the housing 1.

[0072] [Fig.3] schematically illustrates a sectional view of the housing 1 described previously in relation to [Fig.l] along the axis III-III.

[0073] The side wall 41 of the heat sink 4 is annular in shape with a square or rectangular section and comprises four housings 51 located in the four corner areas of the side wall 4L

[0074] Each housing 51 makes it possible to block the movement of the fixing means relative to the side wall 4L. The housing has, for example, an opening in the side wall 41 opening onto a cavity in which the second end 602 of the rivet 60 is housed and being sufficiently narrow so as to retain the second end 602 in the cavity.

[0075] In the alternative illustrated in [Fig.2], the threading of the housings 51 makes it possible to retain the threaded end 611 of the screw 61 and thus to block the movement of the screw 61 relative to the side wall 4L.

[0076] [Fig.4] illustrates a view from below of the housing 1 described previously in relation to [Fig.1].

[0077] The carrier substrate 2 comprises four first orifices 50 respectively located opposite the four corner zones of the side wall 4L.

[0078] The first orifices 50 preferably have an oblong shape of length Lft, greater than the diameter of the body 600 of the rivets 60 so that the rivets 60 can move freely, in the plane of the mounting face 21, during expansion or contraction of the heat sink 4.

[0079] The axis of each first orifice 50 in the plane of the mounting face 21 makes an angle of 45° relative to the direction of the two edges of the side wall 41 adjacent to this first orifice 50. The rivets 60 can thus move along the axis of the first orifices 50.

[0080] The oblong shape of the first orifices 50, in combination with the 45° angle, makes it possible to orient the movement of the rivets 60 in the direction of expansion or contraction of the heat sink 4 and provides more freedom of movement to the sink 4.

[0081] The number of housings 51 and first orifices 50 as well as their shape and posi respective operation in the side wall 41 and in the carrier substrate 2 may vary and depend for example on the shape of the side wall 41 of the heat sink to be fixed on the mounting face 21 of the carrier substrate 2.

[0082] [Fig.5] illustrates an embodiment of a method for manufacturing an integrated circuit package 1. Such a manufacturing method is for example used to manufacture a package 1 as shown in [Fig.1] or [Fig.2].

[0083] The method comprises a supply 100 of a carrier substrate 2 comprising a mounting face 21 and a lower face 20 opposite the mounting face 21. The carrier substrate 2 has a thickness Esub and comprises conductive tracks, typically made of copper, integrated in one or more layers of dielectric material which may be ceramic such as Alumina (A12O3) or aluminum nitride (AIN). The thermal expansion coefficient of such a carrier substrate 2 of such a package is approximately 12x10 6K*.

[0084] The method also comprises a provision 101 of a heat sink 4 comprising a side wall 41, for example of annular shape with a square or rectangular section. Furthermore, the heat sink 4 is made of metal, for example copper (Cu), and has a thermal expansion coefficient approximately equal to 17x106 K '.

[0085] The method further comprises a formation 102 of housings 51 in the side wall 41 of the heat sink 4 and a formation 103 of first orifices 50 in the carrier substrate 2. More particularly, the formation 102 of housings 51 in the side wall 41 comprises a threading of the housings 51 in the case of the manufacture of the housing 1 described in relation to [Fig.2].

[0086] Advantageously, four first orifices 50 are formed in the carrier substrate 2 and four housings 51 in the side wall 41 so that the first orifices 50 are respectively located opposite the four corner zones of the side wall 41 and aligned with the respective housings 51 when the side wall 4L is fixed

[0087] Furthermore, the first orifices 50 have an oblong shape. The formation 103 of the first orifices 50 can be carried out in parallel with the steps 101 of providing the heat sink and 102 of forming housings 51 or possibly before or after step 101 and / or step 102.

[0088] The method then comprises a fixing 104 of the side wall 41 on the mounting face 21 of the carrier substrate 2 by fixing means such as rivets 60 or screws 61, which may be thermally conductive. The fixing means are housed in the housings 51 of the side wall 41 of the heat sink 4 and pass through the carrier substrate 2 through the first orifices 50 so as to allow relative movement of the fixing means with respect to the first orifices 50.

[0089] In the case of manufacturing the housing 1 described previously in relation to [Fig.l], the fixing 104 comprises riveting by rivets 60 each comprising a first end 601 housed in a corresponding housing 51 and a second end 602 under the lower face 20 of the carrier substrate 2. The distance Lfix between the first end 601 and the second end 602 is equal to the thickness Esub of the carrier substrate increased by a clearance J allowing both maintenance of the side wall 41 fixed to the mounting face 21 of the carrier substrate 2 and movement of each rivet 60 in the first orifices 50.

[0090] In the case of manufacturing the housing 1 described previously in relation to [Fig.2], the fixing 104 comprises screwing by screws 61 into the threaded housings 51, leaving a clearance J between the screw heads 602 and the lower face 20 of the carrier substrate 2. The value of the clearance J is preferably between 10 and 20 μm.

[0091] The axis of each first orifice 50 in the plane of the mounting face 21 makes an angle of 45° relative to the direction of the two edges of the side wall 41 adjacent to this first orifice 50.

Claims

Claims

1. An integrated circuit package (1) comprising a carrier substrate (2) and a heat sink (4) having a side wall (41) fixed to a mounting face (21) of the carrier substrate (2) by fixing means (60, 61), wherein the fixing means (60, 61) are housed in housings (51) of the side wall (41) and pass through the carrier substrate (2) through first orifices (50), the fixing means (60, 61) and the first orifices (50) being configured to allow fixing of the side wall (41) to the mounting face (21) and relative movement of the fixing means (60, 61) with respect to the first orifices (50).

2. Housing according to claim 1, in which the carrier substrate (2) has a lower face (20) opposite the mounting face (21) and the fixing means comprise rivets (60) each comprising a first end (601) housed in a corresponding housing (51) and a second end (602) under the lower face (20) of the carrier substrate (2), the distance (Lfix) between the first end (601) and the second end (602) being equal to the thickness (Esub) of the carrier substrate increased by a clearance (J) allowing both maintenance of the side wall (41) fixed to the mounting face (21) of the carrier substrate (2) and movement of each rivet (60) in the first orifices (50).

3. Housing according to claim 1, in which the carrier substrate (2) has a lower face (20) opposite the mounting face (21) and the housings (51) have a thread and the fixing means comprise screws (61) adapted to be screwed into the threaded housings (51) leaving a clearance (J) between the screw heads (602) and the lower face (20) of the carrier substrate (2).

4. Housing according to one of claims 2 or 3, in which the value of the clearance (J) is between 10 and 20 pm.

5. Housing according to one of the preceding claims, in which the first orifices (50) have an oblong shape.

6. Housing according to one of the preceding claims, in which the side wall (41) of the heat sink (4) is annular in shape with a square or rectangular section, and the carrier substrate (2) comprises four first orifices (50) respectively located opposite the four corner zones of the side wall (41).

7. A housing according to claims 5 and 6, wherein the axis of each first orifice (50) in the plane of the mounting face (21) makes an angle of 45° relative to the direction of the two edges of the side wall (41) adjacent to this first orifice (50).

8. Housing according to one of the preceding claims, in which the fixing means (60, 61) are thermally conductive.

9. A package according to any preceding claim, wherein the carrier substrate (2) has a thermal expansion coefficient of between 10x106 K 1 and 12x106 K 1 and the heat sink (4) has a thermal expansion coefficient of between 15x106 K 1 and 17x106 K*

10. A method of manufacturing an integrated circuit package (1), comprising: - providing (100) a carrier substrate (2) comprising a mounting face (21), - providing (101) a heat sink (4) comprising a side wall (41), - forming (102) housings (51) in the side wall (41) of the heat sink (4), - forming (103) first orifices (50) in the carrier substrate (2), - fixing (104) the side wall (41) to the mounting face (21) of the carrier substrate (2) by fixing means (60, 61), the fixing means (60, 61) being housed in the housings (51) of the side wall (41) of the heat sink (4) and passing through the carrier substrate (2) through the first orifices (50) so as to allow relative movement of the means fixing (60, 61) relative to the first orifices (50).

11. Method according to claim 10, the carrier substrate (2) comprising a lower face (20) opposite the mounting face (21), in which the fixing (104) of the side wall (41) on the mounting face (21) comprises riveting by rivets (60) each comprising a first end (601) housed in a corresponding housing (51) and a second end (602) under the lower face (20) of the carrier substrate (2), the distance (Lfix) between the first end (601) and the second end (602) being equal to the thickness (Esub) of the carrier substrate increased by a clearance (J) allowing both maintenance of the side wall (41) fixed to the mounting face (21) of the carrier substrate (2) and movement of each rivet (60) in the first orifices (50).

12. Method according to claim 10, the carrier substrate (2) comprising a lower face (20) opposite the mounting face (21), in which the formation (102) of the housings (51) in the side wall (41) of the heat sink (4) comprises a threading of the housings (51) and the fixing (104) of the side wall (41) on the mounting face (21) comprises screwing by screws (61) into the threaded housings (51) leaving a clearance (J) between the screw heads (602) and the lower face (20) of the carrier substrate (2).

13. Method according to one of claims 11 or 12, in which the value of the clearance (J) is between 10 and 20 pm

14. Method according to one of claims 10 to 13, in which the first orifices (50) are made so that they have an oblong shape.

15. Method according to one of claims 10 to 14, in which the heat sink (4) is provided, the side wall (41) of which is annular in shape with a square or rectangular cross-section, and four first orifices (50) are formed in the carrier substrate (2) respectively located opposite the four corner zones of the side wall (41).

16. A method according to either of claims 14 or 15, wherein the axis of each first orifice (50) in the plane of the mounting face (21) makes an angle of 45° relative to the direction of the two edges of the side wall (41) adjacent to that first orifice (50).

17. Method according to one of claims 10 to 16, in which the fixing means (60, 61) are made so that they are thermally conductive.

18. Method according to one of claims 10 to 17, in which the carrier substrate (2) has a coefficient of thermal expansion between 10x106 K 1 and 12x106 K 1 and the heat sink (4) has a coefficient of thermal expansion between 15x106 K1 and 17x106 K1

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