THERMAL DRAIN FOR AN ELECTRONIC BOARD

Abstract

The present invention relates essentially to a heat sink (25) for an electronic board (11) comprising: - a central interface pad (26) intended to be pressed against a face of an electronic component (14), and - a corrugated membrane (27), referred to as the corrugated membrane, disposed around the central interface pad (26), said corrugated membrane (27) having an outer periphery (28) intended to come into direct or indirect contact with a heat sink (15), so that heat dissipated by the electronic component (14) is able to be removed by conduction through the heat sink (25) from the central interface pad (26) to the heat sink (15) via the corrugated membrane (27). Figure 4

Description

Title of the invention: HEAT DRAIN FOR AN ELECTRONIC CARD

[0001] The present invention relates to a heat sink for an electronic card. The invention finds a particularly advantageous application in the field of avionics and electronic boxes on board an aircraft.

[0002] Such electronic housings are subjected to harsh environments in which the electronic components dissipate heat in a reduced space due to space constraints. The very high ambient temperatures can range from 80°C (typical value) up to 180°C.

[0003] Electronic chips are becoming increasingly complex and therefore dissipate more and more power. For example, chips in highly integrated electronic computers dissipate more than 6 W of power. It is therefore important to be able to efficiently evacuate the heat dissipated by such electronic components.

[0004] To this end, it is known to use thermal radiators in association or not with fans which generate an air flow so as to cool electronic components. However, fans have the disadvantage of consuming energy and of being active elements likely to cause dangerous heating in the event of a breakdown.

[0005] Passive heat sinks are therefore preferably used, such as heat pipes. These heat pipes allow heat transfer to be carried out by phase transition of a fluid. For this purpose, a heat pipe has an evaporation zone for a cooling fluid intended to extract calories from an electronic component and a condensation zone for the cooling fluid intended to release the stored calories to the external environment. The liquid phase and the vapor phase of the fluid in a heat pipe are located in the same conduit. However, such devices are expensive for small series and are sensitive to gravity, which requires special precautions to be taken regarding their mounting orientation inside the aircraft. Heat pipes also impose specific operating temperature ranges.

[0006] It is also known to use thermally conductive “elastomer” mattresses which can absorb variable thicknesses and wide tolerances of electronic cards. However, the thermal efficiency of elastomer mattresses is relatively poor. In addition, an elastomer mattress generates significant compressive forces on the electronic chip against which it rests.

[0007] The invention aims to effectively remedy the aforementioned drawbacks by proposing a heat sink for an electronic card comprising: - a central interface pad intended to be pressed against one face of an electronic component, and - a wavy membrane, called a wavy membrane, arranged around the central interface pad, said wavy membrane comprising an external periphery intended to come into direct or indirect contact with a heat sink, so that heat dissipated by the electronic component is capable of being evacuated by conduction by the heat drain from the central interface pad to the heat sink via the wavy membrane.

[0008] The invention thus makes it possible to adapt to variations in the height dimensions of electronic cards, without losing conduction efficiency, and without generating high pressure constraints on the electronic component to be cooled. The invention also has the advantage of being a simple, economical and robust solution.

[0009] Compared to the use of heat pipes, the invention is insensitive to gravity. It is therefore possible to orient the heat sink in any mounting orientation within the electronics housing, and to mount said electronics housing in any mounting orientation within the aircraft.

[0010] The invention is also more efficient, at significant thickness, than conventional thermal mattresses without inducing excessive pressure on the heating component. Indeed, unlike thermal mattresses, the thermal drain according to the invention retains its thermal conductance regardless of the clearance to be absorbed.

[0011] According to one embodiment of the invention, a phase change thermal interface film is arranged between the central interface pad and the electronic component.

[0012] According to one embodiment of the invention, the central interface pad is configured to adapt to a shape of an exterior surface of the electronic component.

[0013] According to one embodiment of the invention, the corrugated membrane comprises corrugations formed in a truncated cone-shaped wall.

[0014] According to one embodiment of the invention, the undulations comprise an alternation of rounded peaks located towards the inside of the truncated cone-shaped wall and rounded troughs located towards the outside of the truncated cone-shaped wall.

[0015] According to one embodiment of the invention, a radius of curvature of the peaks is less than a radius of curvature of the troughs.

[0016] According to one embodiment of the invention, a radius of curvature of the hollows increases when moving from an inner periphery to the outer periphery of the corrugated membrane.

[0017] According to one embodiment of the invention, said heat drain further comprises a sole intended to come into contact with the heat sink, the external periphery of the corrugated membrane being fixed to the sole.

[0018] According to one embodiment of the invention, the corrugated membrane comprises a vent hole.

[0019] According to one embodiment of the invention, the corrugated membrane is produced in a copper-based material with high thermal conductivity, in particular of the order of 100 Wm-lK-1.

[0020] According to one embodiment of the invention, the corrugated membrane is made of a metallic material having good elastic properties.

[0021] According to one embodiment of the invention, the corrugated membrane is made of a malleable metallic material having an elongation at break greater than 8%.

[0022] According to one embodiment of the invention, the corrugated membrane is made of a shape memory material.

[0023] The invention also relates to an electronic housing comprising an electronic card carrying at least one electronic component, a heat sink, and a heat sink as previously defined mounted between the electronic component and the heat sink.

[0024] According to one embodiment of the invention, a damping element is arranged between the central interface pad and the heat sink.

[0025] The present invention will be better understood and other characteristics and advantages will become apparent upon reading the detailed description which follows, comprising embodiments given by way of illustration with reference to the appended figures, presented as non-limiting examples, which may serve to complete the understanding of the present invention and the description of its embodiment and, where appropriate, contribute to its definition, in which:

[0026] [Fig-1] [Fig. 1] is a perspective view of an electronic housing provided with a heat sink according to the invention;

[0027] [Fig.2] [Fig.2] is a cross-sectional view of the electronic housing of the [Fig.l];

[0028] [Fig.3] [Fig.3] is a perspective view of the electronic housing of [Fig.l] without the top cover showing an electronic board mounted on a matching frame;

[0029] [Fig.4] [Fig.4] is a cross-sectional view of a heat drain according to the invention mounted between an electronic chip and the cover of the electronic housing forming a heat sink;

[0030] [Fig.5] [Fig.5] is a cross-sectional view of the heat drain according to the invention of [Fig.4] without the electronic box cover;

[0031] [Fig.6] [Fig.6] is a perspective view of a central interface pad of the drain thermal according to the present invention;

[0032] [Fig.7a] [Fig.7a] is a perspective view of a corrugated membrane of the drain thermal according to the present invention;

[0033] [Fig.7b] [Fig.7b] is a side view illustrating a profile of a corrugated membrane according to the invention;

[0034] [Fig.8] [Fig.8] is a perspective view of a sole of the heat drain according to the present invention establishing contact between an outer periphery of the corrugated membrane and the cover of the electronic housing;

[0035] [Fig.9] [Fig.9] is a perspective view illustrating receiving cavities of a portion of the sole produced in an internal face of a cover of the electronic housing according to the invention.

[0036] It should be noted that the structural and / or functional elements common to the different embodiments have the same references. Thus, unless otherwise stated, such elements have identical structural, dimensional and material properties.

[0037] Figures 1 and 2 show an electronic box 10 intended to be installed on board an aircraft. This electronic box 10 comprises a plurality of electronic cards 11 carrying electronic components 12. The electronic components 12 may consist in particular of resistors, capacitors, coils, transistors, logic gates, or electronic chips. The electronic box 10 comprises a connector 17 for power signals and / or data signals.

[0038] The electronic cards 11 are mounted on frames 13, as shown in Figures 2 and 3. The frames 13 are stacked on top of each other, so that the electronic cards 11 are located at different levels inside the electronic housing 10. Each frame 13 delimits a portion of the side walls of the electronic housing 10, such that the stack of frames 13 forms all of the side walls of the electronic housing 10.

[0039] The electronic housing 10 is closed at each end by a cover 15 constituting a heat sink. A heat sink 15 is made of a thermally conductive material, in particular a metallic material. A heat sink 15 may comprise a plurality of fins 16 arranged on its external face. The fins 16 extend projecting towards the outside of the electronic housing 10.

[0040] As can be seen in Figures 1 and 3, in order to ensure the assembly of the different elements, the heat sinks 15 (or covers) and the frames 13 comprise fixing means 18 constituted for example by projecting ears provided with an opening for the passage of a fixing member, such as a screw or a rivet. The openings of the projecting ears 18 are arranged opposite each other to allow the passage of the fixing members (not shown).

[0041] Of course, the invention may also be implemented with any other type of electronic box 10, in particular electronic boxes 10 in which the electronic cards 11 are mounted on a slide system (system of the type "rack" in English) and more generally with any electronic box 10 comprising at least one electronic card 11 and at least one heat sink 15.

[0042] As illustrated by figures 4 and 5, one of the electronic components 12 may take the form of an electronic chip 14 of the BGA (for "Bail Grid Array" in English) type arranged on an intermediate substrate 20, which is then mounted on the electronic card 11 by means of a matrix of solderable balls 21. Preferably, this electronic chip 14 is not an encapsulated chip, that is to say that the electronic chip 14 is without a protective cover.

[0043] A heat drain 25 visible in particular in FIGS. 3, 4, and 5, is intended to evacuate by conduction the calories dissipated by the electronic chip 14 towards a heat sink 15.

[0044] For this purpose, the heat sink 25 comprises a central interface pad 26 intended to be pressed against a face of the electronic chip 14 and a membrane 27 of corrugated shape, called a corrugated membrane, arranged around the central interface pad 26. The corrugated membrane 27 comprises an external periphery 28 in direct or indirect contact with the heat sink 15, so that heat dissipated by the electronic chip 14 is capable of being evacuated by conduction by the heat sink 25 from the central interface pad 26 to the heat sink 15 via the corrugated membrane 27.

[0045] More precisely, as can be seen in Figures 4, 5, and 6, the central interface pad 26 of axis XI comprises a first portion 26.1 fitted inside a central opening 30 made in the corrugated membrane 27 and a second portion 26.2 in contact with the electronic chip 14. The central interface pad 26 is configured to adapt to a shape of an external surface of the electronic chip 14. The external surface of the electronic chip 14 is generally square but could alternatively have another shape.

[0046] The fixing of the first portion 26.1 with the corrugated membrane 27 may be carried out by brazing with the addition of material, for example tin or any other material suitable for the application. The first portion 26.1 and the second portion 26.2 may each have a cylindrical shape. The second portion 26.2 has a diameter greater than a diameter of the first portion 26.1, such that the electronic chip 14 is circumscribed inside the second portion 26.2 of the central interface pad 26. Alternatively, the second portion 26.2 may have another shape, such as for example a trapezoidal or conical shape, or any other shape allowing the central interface pad 26 to adapt to the shape of the external surface of the electronic chip 14.

[0047] The central interface pad 26 is a single-piece part made of a material that is a good heat conductor, in particular a metallic material, such as copper or any other material suitable for the application.

[0048] In the embodiment shown, the central interface pad 26 is attached relative to the corrugated membrane 27. Such an embodiment has the advantage of being able to retain the corrugated membrane 27 and of only having to replace the central interface pad 26 during a possible change of the electronic chip 14. As a variant, the central interface pad 26 may be made of the same material as the corrugated membrane 27.

[0049] A phase-change thermal interface film 32 may be arranged between the central interface pad 26 of the heat sink 25 and the electronic chip 14, as shown in FIGS. 4 and 5. This thin phase-change thermal interface film 32 ensures minimal heat loss in the contact zone between the heat sink 25 and the electronic chip 14. Preheating the heat sink 25 may be envisaged in order to melt the phase-change thermal interface film 32 during assembly, allowing direct interface pad / chip contact, with respect to imperfections in flatness and roughness. Alternatively, it will be at the time of the first power-up of the electronic chip 14, under the effect of its heating, that the melting of the phase-change thermal interface film 32 will take place.

[0050] In the example shown in Figures 3 and 4, the outer periphery 28 of the corrugated membrane 27 is fixed to a sole 34 intended to come into contact with the heat sink 15. For this purpose, the sole 34 of axis X2 shown alone in [Fig. 8] comprises an internal annular portion 34.1 and an external annular portion 34.2. The external annular portion 34.2 has a diameter greater than the diameter of the internal annular portion 34.1. The internal annular portion 34.1 is fitted inside a cavity 35 of corresponding shape made in an internal face of the heat sink 15, as shown in [Fig. 9].

[0051] The difference in diameter between the internal annular portion 34.1 and the external annular portion 34.2 defines a shoulder 36 having an external face bearing against the internal face of the cover 15. The external periphery 28 of the corrugated membrane 27 is preferably fixed to the internal face of the shoulder 36 by brazing with the addition of material, for example tin or any other material suitable for the application.

[0052] The sole 34 is a single-piece part made of a material that is a good heat conductor, in particular a metallic material, such as copper or any other material suitable for the application.

[0053] Such a configuration corresponds to an indirect fixing of the corrugated membrane 27 on the heat sink 15 by means of the sole 34. As a variant, the external periphery 28 of the corrugated membrane 27 is fixed directly on the face internal of the heat sink 15, that is to say without an intermediate element between the corrugated membrane 27 and the heat sink 15. In this case, the external periphery 28 of the corrugated membrane 27 is for example brazed onto the heat sink 15.

[0054] In order to absorb a large air gap between the electronic chip 14 and the heat sink 15, the corrugated membrane 27 of axis X3 visible in particular in FIGS. 4, 5, and 7a comprises corrugations 38 formed in a wall 39 of truncated cone shape. The corrugations 38 extend between the central opening 30 and the external periphery 28.

[0055] The corrugations 38 comprise an alternation of rounded peaks 41 and rounded troughs 42. Thus, a peak 41 is followed by a trough 42 then a peak 41 and so on. The number of peaks 41 and troughs 42 of the corrugated membrane 27 depends on the application.

[0056] In the example shown, the vertices 41 are located towards the inside of the frustoconical wall 39, that is to say on the side of the face of the wall 39 facing the axis X3, and the hollows 42 are located towards the outside of the frustoconical wall 39, that is to say on the side of the face of the wall 39 facing away from the axis X3.

[0057] Advantageously, as illustrated by [Fig.7b], a radius of curvature Rs of the peaks 41 is less than a radius of curvature Rc of the hollows 42. This makes it possible to optimize shear and bending work of the corrugated membrane 27.

[0058] Furthermore, preferably, the radius of curvature Rc of the hollows 42 increases when moving from an inner periphery to the outer periphery 28 of the corrugated membrane 27.

[0059] Alternatively, the undulations 38 may have a sawtooth shape.

[0060] Alternatively, the corrugations 38 are formed in an annular wall 39 extending in a radial plane relative to the axis X3 of the heat drain 25. The heat drain 25 then has a flattened “pancake” type shape.

[0061] The corrugated membrane 27 preferably comprises a vent hole 45 visible in [Fig.7a]. This is a through hole of small diameter, in particular less than 5 mm in diameter, made in the wall 39 of the corrugated membrane 27. This makes it possible to avoid the creation of a volume of stagnant hot air likely to deform the corrugated membrane 27 and / or to damage the electronic chip 14.

[0062] Preferably, the corrugated membrane 27 is made of a copper-based material with high thermal conductivity, in particular of the order of 100 Wm-1K-1. The corrugated membrane 27 has a certain range of elasticity to be able to compensate for the dimensional dispersions of the electronic cards 11, in particular depending on their thickness. The corrugated membrane 27 is made for example of beryllium bronze (CuBe2) or any other material suitable for the application. The corrugated membrane 27 may be made of a metallic material having good elastic properties.

[0063] Alternatively, the membrane may be made of a metallic material. malleable having an elongation at break greater than 8%, or even greater than 15%, but a reduced elastic limit with respect to the desired deformation. The advantage of such a material is that the low elastic limit makes it possible to easily deform the wall in which the corrugations 38 are made. It is thus possible to adapt on a case-by-case basis a corrugated membrane 27 made flat by giving it sufficient conical deformation to fill an axial clearance between the electronic chip 14 and the heat sink 15. The reversible elasticity at low deformation around the mounting position nevertheless makes it possible to ensure slight pressure from the central interface pad 26 on the electronic chip 14.

[0064] Advantageously, the material is chosen so that the plasticity range makes it possible to fill a gap of between 0.7 mm and 0.8 mm while the elasticity range makes it possible to adapt to a dimensional variation of the order of 0.1 mm.

[0065] The choice between using the material with a high elastic limit or the malleable material depends on the range of axial uncertainty, more or less significant, that the assembly will present and on the tolerance to axial forces that can be supported by the electronic chip 14 to be cooled.

[0066] In order to modify its mechanical characteristics to change from an elastic material to a malleable material, the material of the corrugated membrane 27, in particular beryllium bronze, may be subjected to one or more cooking phases making it possible to modify its elongation at break.

[0067] If the electronic chip 14 is intended to operate in a relatively stable thermal environment, an advantageous variant may consist of replacing the high elastic limit material (or the malleable material) of the corrugated membrane 27 with a shape memory material consisting of a copper-based alloy. Its austenitic / martensitic transformation temperature range must be compatible with the operating range of the electronic chip 14 so that the heat sink 25 works in its super-elasticity range. This makes it possible to ensure that a constant pressure is maintained over the entire super-elasticity range that can be predicted as a function of the necessary deformation rate.

[0068] Solutions based on elastic or super-elastic materials have the disadvantage of a specific conical shaping for a given nominal thickness to be absorbed. On the other hand, the assemblies and disassemblies of the electronic box 10 can be carried out without any particular precautions. This is not the case for a corrugated membrane 27 made of a malleable material which will have to be slightly re-deformed at each disassembly / reassembly in order to ensure the correct plating of the corrugated membrane 27 on the electronic chip 14.

[0069] In the case of operation in a severe vibration environment, a damping element 46 visible in [Fig.4] is arranged between the central interface pad 26 of the heat sink 25 and the heat sink 15. The damping element 46 ensures contact between the central interface pad 26 and the electronic chip 14. The damping element 46 may be made of foam or an elastomeric material.

[0070] Alternatively, the heat sink 25 may be used to cool an electronic component 12 other than an electronic chip, such as for example a resistor, a capacitor, a transistor, or any other electronic component dissipating heat.

[0071] Of course, the various features, variants and / or embodiments of the present invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive.

[0072] Furthermore, the invention is not limited to the embodiments described above and provided solely by way of example. It encompasses various modifications, alternative forms and other variants that may be envisaged by those skilled in the art within the scope of the present invention and in particular all combinations of the different operating modes described above, which may be taken separately or in association.

Claims

Claims

1. Heat sink (25) for an electronic card (11) comprising: - a central interface pad (26) intended to be pressed against a face of an electronic component (14), and - a membrane (27) of corrugated shape, called a corrugated membrane, arranged around the central interface pad (26), said corrugated membrane (27) comprising an external periphery (28) intended to come into direct or indirect contact with a heat sink (15), so that heat dissipated by the electronic component (14) is capable of being evacuated by conduction by the heat sink (25) from the central interface pad (26) to the heat sink (15) via the corrugated membrane (27), characterized in that the corrugated membrane (27) comprises corrugations (38) formed in a wall (39) of frustoconical shape.

2. Heat sink according to claim 1, characterized in that a phase change thermal interface film (32) is arranged between the central interface pad (26) and the electronic component (14).

3. A heat sink according to claim 1 or 2, characterized in that the central interface pad (26) is configured to adapt to a shape of an exterior surface of the electronic component (14).

4. Heat drain according to any one of claims 1 to 3, characterized in that the undulations (38) comprise an alternation of rounded peaks (41) located towards the inside of the frustoconical wall and rounded hollows (42) located towards the outside of the frustoconical wall (39).

5. Heat drain according to claim 4, characterized in that a radius of curvature (Rs) of the peaks (41) is less than a radius of curvature (Rc) of the troughs (42).

6. A heat drain according to claim 4 or 5, characterized in that a radius of curvature (Rc) of the hollows (42) increases when moving from an inner periphery to the outer periphery (28) of the corrugated membrane (27).

7. Heat drain according to any one of claims 1 to 6, characterized in that it further comprises a sole (34) intended to come into contact with the heat sink (15), the external periphery (28) of the corrugated membrane (27) being fixed on the sole (34).

8. A heat drain according to any one of claims 1 to 7, ca- characterized in that the corrugated membrane (27) has a vent hole (45).

9. Thermal drain according to any one of claims 1 to 8, characterized in that the corrugated membrane (27) is made of a copper-based material with high thermal conductivity, in particular of the order of 100 Wm-lK-1.

10. Heat drain according to any one of claims 1 to 9, characterized in that the corrugated membrane (27) is made of a malleable metallic material having an elongation at break greater than 8%.

11. Heat drain according to any one of claims 1 to 9, characterized in that the corrugated membrane (27) is made of a shape memory material.

12. Electronic housing (10) comprising an electronic card (11) carrying at least one electronic component (12), a heat sink (15), and a heat sink (25) defined according to any one of the preceding claims mounted between the electronic component (14) and the heat sink (15).

13. Electronic housing according to claim 12, characterized in that a damping element (46) is arranged between the central interface pad (26) and the heat sink (15).

Images