IL270123B1

IL270123B1 - Thermal diffusion interface

Info

Publication number: IL270123B1
Application number: IL270123A
Authority: IL
Original assignee: Thales Sa
Priority date: 2018-10-25
Filing date: 2019-10-23
Publication date: 2023-08-01
Also published as: EP3644354A1; FR3087938A1; IL270123B2; IL270123A

Description

THERMAL DIFFUSION INTERFACE The present invention relates to a thermal diffusion interface, such as a base of a housing of an electronic component or a heat sink. Thermal conduction or diffusion is a mode of heat transfer brought about by a difference in temperatures between two regions of the one same medium, or between two mediums that are in contact, and which occurs without net movement (on a macroscopic scale) of substance, as opposed to convection which is another mode of heat transfer. It can be interpreted as being the knock-on transmission of the thermal agitation, an atom or a molecule gives up some of its kinetic energy to the adjacent atom. For example, in the field of housings for electronic components, the internal chips need to dissipate a very large amount of power over a very small surface measuring for example a few square millimeters. In this field, it is known practice to use housings made of Cu/Mo (copper/molybdenum) alloy having a thermal conductivity comprised between 200 W.m-1.K-1 and 400 W.m-1.K-1 depending on the proportions of Cu and of Mo. It is also known practice to use Cu/D (copper/diamond) with conductivity performance of the order of 500 W.m-1.K-1, but the cost of which is of the order of 60 times more than the Cu/Mo solution. Figure 1 schematically illustrates one exemplary embodiment of a known thermal diffusion interface 1, in this case a base 1 of a housing 2 of an electronic component. In this instance, the base 1 is made of Cu/Mo alloy and needs to diffuse the thermal power or heat supplied by a chip 3. The base 1 is fixed, for example by brazing, welding or bonding, to a structure 4, for example made of an aluminium Al belonging to a system such as an emitter and/or receiver. 35 One object of the invention is to alleviate the abovementioned problems and notably to provide a thermal diffusion interface that has improved diffusivity or conductivity, based on a material that is easy to produce, and for a low cost. One aspect of the invention proposes a thermal diffusion interface made from a material with isotropic thermal diffusivity, comprising an insert made from material with anisotropic thermal diffusivity containing graphite having at least one favoured plane of thermal conduction perpendicular to the mean plane of the interface, the upper surface of the insert opening onto the upper surface of the interface, and the lower surface of the insert being covered with the isotropic thermal diffusivity material of the interface. Graphite, which generally comes in the form of bars or sheets, is anisotropic, and has thermal conductivity essentially in the directions of the mean plane of the sheet, and practically zero conductivity in the direction orthogonal to this mean plane of the sheet. The presence of such an insert makes it possible to reduce the thermal resistance of the base and therefore limit the heating of the element mounted in contact with the upper part thereof, the heat supplied by which needs to be dissipated. The lower surface of the insert is covered with the isotropic thermal diffusivity material of the interface, and this avoids the flaking of the anisotropic thermal diffusivity material containing graphite which may generate pollution, and this material is protected. Furthermore, the thermal conductivity between the anisotropic thermal diffusivity material containing graphite and the isotropic thermal diffusivity material is improved because assembly can be performed by moulding. In one embodiment, the lower surface of the insert is larger than the upper surface of the insert The region from which the thermal power originates defines the size of the upper surface of the insert, and the lower surface is determined in such a way as to maximize the diffusion of this power into the isotropic matrix 35 of the thermal diffusion interface, such as copper or a Cu/Mo (copper/molybdenum) alloy. That makes it possible to make best use of the orthotropic properties of graphite, while minimizing the need for diffusion in the unfavourable direction and maximizing this thermal diffusion in the favourable plane, while at the same time encouraging exchange of heat with the isotropic matrix of the thermal diffusion interface. According to one embodiment, the isotropic thermal diffusivity material contains copper. Copper is commonly used in supports for electronic components because of its thermal diffusivity properties combined with its ease of use and its isotropy, and the properties of copper are well known and mastered. In one embodiment, the insert is in the shape of a cross with a raised central part corresponding to the upper surface of the insert opening onto the upper surface of the interface Such a shape of insert allows the heat drained from the source to be channelled via its graphite composition in order thereafter to favour thermal diffusion in the volume of the matrix. It is a matter of taking advantage of the high thermal diffusivity of graphite while seeking to overcome the effects of its orthotropy using the isotropic matrix. The thermal diffusion in the (insert+graphite) structure can thus be improved, thereby protecting the electronic components. According to one embodiment, the raised central part has edges inclined by an angle comprised between 30 and 60° with respect to an axis perpendicular to the mean plane of the interface, for example 45°. Such inclined edges improve the diffusion of heat from the insert towards the rest of the thermal diffusion interface, and do so optimally for an inclination of 45°. The inclination of the edges makes it possible to improve the diffusion of heat by taking advantage of a larger surface area of contact between the graphite and the matrix.

For example, the thermal diffusion interface is configured to be a base of a housing of an electronic component. For example, the thermal diffusion interface is configured to be a heat sink, the principle being to position the element whose heat is to be dissipated in contact with it so as to benefit from the exchange of heat by conduction like, for example, in aluminium heat-sink radiators on power transistors or chips. In one embodiment, the upper surface of the insert is smaller than that of the surface of the element delivering the heat, which is intended to be in contact with the said upper surface of the insert Thus, the diffusion of heat is improved by taking advantage of a larger surface area of contact with the copper matrix. The invention will be better understood from studying a number of embodiments described by way of entirely nonlimiting examples and illustrated by the attached drawings in which: - Figure 1 schematically illustrates one exemplary embodiment of a known thermal diffusion interface, in this instance a base 1 of a housing 2 of an electronic component according to the prior art; - Figure 2a schematically illustrates a thermal diffusion interface according to one aspect of the invention; - Figures 2b schematically illustrates a thermal diffusion interface configured to be a base of a housing of an electronic component, according to one aspect of the invention, and - Figure 3 schematically illustrates a graphite insert in the shape of a cross, according to one aspect of the invention; - Figure 4 schematically illustrates a plurality of graphite inserts in the form of bars, in this instance four of them, according to one aspect of the invention; and - Figure 5 schematically illustrates a thermal diffusion interface comprising a plurality, in this instance four, of graphite inserts in the form of bars, according to Figure 4, according to one aspect of the invention.

In all of the figures, elements that have identical references are similar. In the present description, the embodiments described are entirely non limiting, and the features and functions that are well known to those skilled in the art are not described in detail. Figure 2a schematically depicts a thermal diffusion interface 1 with a graphite insert 5, according to one aspect of the invention. The graphite insert 5 comprises at least one favoured plane of thermal conduction perpendicular to the mean plane of the interface 1, the upper surface of the insert 5 opening onto the upper surface of the interface 1, and the lower surface 5a of the insert 5 being covered by the isotropic thermal diffusivity material of the interface 1. The thermal conductivity of graphite in a favoured plane of thermal conduction perpendicular to the mean plane of the interface 1 is of key importance. It may be ensured during the manufacture of the graphite by a very high temperature annealing process that makes it possible to obtain a conductivity of the order of 1000 W.m-1.K-1. The isotropic thermal diffusivity material is preferably based on copper Cu, because it has a fairly high thermal conductivity value and its use in the field of electronics and thermal diffusion is well known. The process of incorporating the pyrolytic graphite insert 5 into the isotropic thermal diffusivity material such as copper Cu consists in coating, in a furnace, copper brought to its melting point in a mould made of graphite for example. After the copper has cooled, the graphite insert is coated in the copper and reworked for example by mechanical means in order to have an optimal contact surface on one side by surfacing the copper and on the other side by revealing the active part of the insert 5. 35 Figure 2b schematically depicts a thermal diffusion interface 1 in one exemplary embodiment of a known thermal diffusion interface 1, in this instance a base 1 of a housing 2 of an electronic component. In this particular instance, the base 1 contains copper Cu and the graphite insert 5 has to diffuse the thermal power or heat supplied by a chip 3. The base 1 is fixed, for example by brazing, welding or bonding, to a structure 4, for example made of aluminium Al, of a system such as an emitter and/or receiver. The lower surface 5a of the insert may be larger than the upper surface of the insert, in order to maximize the exchange of heat between the graphite and its copper matrix. Figure 3 schematically depicts a graphite insert 5 in the shape of a cross with a raised central part 6 corresponding to the upper surface of the insert 5 opening onto the upper surface of the interface 1. Because of the strong orthogonality of the thermal conductivity of graphite, a cross shape is determined to allow a better dissipation of the thermal flux through the copper coating. The raised central part 6 of the graphite insert 5 has edges inclined by an angle comprised between 30 and 60° with respect to an axis perpendicular to the mean plane of the interface, ideally of 45°. That makes it possible to have the greatest possible surface area of graphite for exchange of heat with the copper. As an alternative, the thermal diffusion interface may be a heat sink, such as fingers or any other means adapted to increasing the area of exchange of heat between the graphite and the copper. Figure 4 schematically depicts a plurality of graphite inserts 5, in this instance 4, in the form of parallel bars, with a raised central part 6

Claims

1.CLAIMS 1. Thermal diffusion interface (1) made from a material with isotropic thermal diffusivity, comprising an insert (5) made from material with anisotropic thermal diffusivity containing graphite having at least one favoured plane of thermal conduction perpendicular to the mean plane of the interface (1), the upper surface of the insert (5) opening onto the upper surface of the interface (1), and the lower surface (5a) of the insert (5) being covered with the isotropic thermal diffusivity material of the interface (1).

2. Thermal diffusion interface (1) according to Claim 1, wherein the lower surface (5a) of the insert (5) is larger than the upper surface of the insert (5).

3. Thermal diffusion interface (1) according to one of the preceding claims, wherein the isotropic thermal diffusivity material contains copper.

4. Thermal diffusion interface (1) according to one of the preceding claims, wherein the insert (5) is in the shape of a cross with a raised central part (6) corresponding to the upper surface of the insert (5) opening onto the upper surface of the interface (1).

5. Thermal diffusion interface (1) according to Claim 4, wherein the raised central part (6) has edges (7) inclined by an angle comprised between 30 and 60° with respect to an axis perpendicular to the mean plane of the interface (1).

6. Thermal diffusion interface (1) according to Claim 5, wherein the raised central part (6) has edges (7) inclined by an angle of 45° with respect to an axis perpendicular to the mean plane of the interface (1).

7. Thermal diffusion interface (1) according to one of Claims to 6, configured to be a base of a housing of an electronic component. 9

8. Thermal diffusion interface (1) according to one of Clams to 6, configured to be a heat sink.

9. Thermal diffusion interface (1) according to one of the preceding claims, wherein the upper surface of the insert (5) is smaller than that of the surface of the element delivering the heat, which is intended to be in contact with the said upper surface of the insert (5).