Electrical device comprising an electrical component mounted on a heat sink

Abstract

The invention relates to an electrical device (2) comprising a heat sink (4) having an upper face, at least one electrical component (8) mounted on the upper face (6) of the heat sink, a thermal attachment means (12) positioned between the electrical component (8) and the upper face (6) of the heat sink (4) along a main axis (X), the electrical device being characterized in that the thermal attachment means comprises a metal foam.

Description

[0001] DESCRIPTION

[0002] Title of the invention: Electrical device comprising an electrical component mounted on a heat sink

[0003] The present invention relates to the field of electrical devices for electric or hybrid cars.

[0004] Electrical devices include various electrical components such as power modules, coils, and capacitors, though this list is not exhaustive. These electrical components generate heat when the device is in use.

[0005] Electrical devices are used in systems that require ever-increasing power levels. This increase in power is accompanied by a growing production of heat during the operation of electrical devices, necessitating solutions to reduce these thermal stresses, which can lead to malfunctions in the components of the electrical device.

[0006] More specifically, it is known to equip electrical devices with a heat sink through which a heat transfer fluid circulates. This heat sink has a top surface against which the electrical components to be cooled are placed, particularly the power modules, which are the components that generate a lot of heat and whose performance depends on their temperature. The electrical components are joined to the heat sink by a brazing process during which the components and the heat sink are placed in a furnace to polymerize the material between the electrical components and the heat sink. In other words, the electrical components, and especially the power modules, are held together by a thermal bonding agent designed to liquefy when it reaches a certain temperature and then geometrically solidify the interface.

[0007] The power module is an increasingly common component in electrical systems because it allows for the assembly of multiple semiconductor chips in a compact space. More specifically, the power module consists of a package containing semiconductor chips. The package includes a base, formed by power connectors, onto which the semiconductor chips are mounted, and a cover plate that encloses the chips and, together with the base plate, forms an internal volume within which the chips are housed. The chips are connected to electrical pins extending outside the power module, notably to allow the electrical connection of the chips inside the package to a control module, which typically includes a printed circuit board (PCB).

[0008] The power module has a rectangular shape, defined by the rectangular shape of the package. Several power modules are arranged on the heatsink. The heatsink is larger than the combined area of ​​the electrical components mounted on it. The cover of the power module package is made of an insulating material with different thermal characteristics than the heatsink. This is also true for the module socket and the chips, which have different thermal characteristics than the package and heatsink, as well as for the semiconductor chips.

[0009] It is known that cooling devices and / or electrical components expand and contract when placed in an oven and cooled. It follows that when the elements of this electrical device are placed in the oven and heated, they expand differently, due to both differences in material and geometric variations. The rigid assembly of these elements creates deflection resulting from these different types of expansion behavior.

[0010] Such a difference in deformation, even when the electrical components are fixed to the heatsink, can damage the power modules, potentially causing them to detach from the heatsink or creating excessive geometric distortion that renders the device non-compliant. It has also been observed that the attachment of electronic components to the heatsink can be degraded due to the inherent warping of the electrical components, particularly the power modules. During operation of the electrical system in a vehicle, the ambient temperature can vary between -40°C and +125°C, in addition to internal heating that can exceed 175°C within the electrical component. To mitigate this inherent deformation, rigid fastening using a clamping device is a known method.The bending of electrical components under the effect of temperature leads to mechanical stresses in the electrical component, particularly the power module, and rapid aging at the interface.

[0011] Increasing the thickness of the thermal attachment medium to compensate for module warping leads to additional problems such as: - deep transverse notches are created, or

[0012] - metallic projections that contaminate the heat sink, with risks to the electrical insulation.

[0013] In order to resolve all or part of the various problems mentioned and summarized here: rapid aging of the interface, internal electrical failures of the electrical component, poorly formed solder joints and metallic spatter, an electrical device according to the invention is proposed which comprises:

[0014] a heat sink with a top surface,

[0015] at least one electrical component mounted on the upper surface of the heat sink, a thermal attachment means disposed between the electrical component and the upper surface of the heat sink along a main axis,

[0016] The electrical device is remarkable in that the thermal attachment means includes a metallic foam.

[0017] This thermal attachment method secures the electrical component to the heatsink. It reduces the amount of material required while improving the component's adhesion to the heatsink due to the absorption of mechanical stresses by the metallic foam. This thermal attachment method helps prevent aging of the electrical component's attachment and internal electrical failures, particularly in power modules containing semiconductor chips. The claimed attachment method also minimizes the presence of notches in the thermal attachment and metal spatter.

[0018] According to one aspect of the invention, the electrical component may consist of a power module. The power module may include semiconductor chips.

[0019] According to one aspect of the invention, the electrical device may comprise a plurality of electronic components, in particular three, in particular six. Each electrical component may be mounted on the upper face of the heat sink.

[0020] According to one aspect of the invention, the upper face of the heat sink is normal to the main axis. The thermal attachment means is normal to the main axis. The electrical component is normal to the main axis.

[0021] According to one aspect of the invention, the metallic foam comprises pores. The metallic foam may have a pore density, in PPI (Pores Per Inch), of between 10 and 1000, in particular between 300 and 700, in particular approximately equal to 500. Such a density makes it possible to obtain a low tensile modulus and a high elongation at break, improved compared to a solid material.

[0022] According to one aspect of the invention, the pores form the cavities of a metallic mesh. This mesh can act as a set of springs. The pores provide flexibility between the electrical component and the heat sink.

[0023] According to one aspect of the invention, the metallic foam comprises copper, in particular a copper alloy. The metallic foam can be made with more than 90% copper, in particular more than 95% copper, in particular more than 99% copper. Such a percentage of copper makes it possible to obtain a highly conductive material which, in particular, helps to compensate for the presence of pores.

[0024] According to one aspect of the invention, the metallic foam may comprise nickel, in particular a nickel alloy.

[0025] According to one aspect of the invention, the thermal bonding means comprises a metallic top layer, a metallic bottom layer, and an intermediate layer. The intermediate layer may comprise metallic foam. The intermediate layer, sandwiched between the brazing preforms, is in the form of a foam sheet of varying density, which allows this interface layer to absorb thermomechanical stresses during the operation of the power modules.

[0026] According to one aspect of the invention, the lower layer is opposite the heat sink, in contact with the upper face of said heat sink.

[0027] According to one aspect of the invention, the upper layer faces the electrical component, specifically in contact with a base of the electrical component forming an underside of the electrical component. The base may include power substrates, on which semiconductor chips are mounted in the case of a power module. This base may include copper or may be made of copper.

[0028] According to one aspect of the invention, the intermediate layer is substantially parallelepiped-shaped. The intermediate layer can be defined between a lower surface and an upper surface, normal to the principal axis. The lower surface of the intermediate layer passes through the lowest point of the metal foam. The upper surface of the intermediate layer passes through the highest point of the metal foam.

[0029] According to one aspect of the invention, the upper layer and / or the lower layer does not have metallic foam.

[0030] According to one aspect of the invention, the intermediate layer has a thickness of between 30% and 90%, in particular between 60% and 80%, of the thickness of the thermal attachment means. The thickness is measured along the principal axis. The thickness of the thermal attachment means may be between 0.3 mm and 0.5 mm. The thickness of the intermediate layer may be between 0.1 mm and 0.3 mm.

[0031] According to one aspect of the invention, the lower and / or upper layer are formed of a tin alloy, in particular a tin, silver, and copper alloy known as "SAC" in English terminology, specifically a SAC-Sb or even SAC-Sb-In alloy. The composition of the lower and upper layers may be identical. The lower and / or upper layers may comprise a single material. According to one aspect of the invention, the intermediate layer also comprises a second material formed of a tin alloy, this second material filling, in particular, a portion of the pores of the metal foam. This second material may be the material of the lower and / or upper layer. This second material may fill only a portion of the pores of the metal foam. In particular, the intermediate layer may include end zones, especially in the direction of the principal axis.These end areas may contain metallic foam penetrated by the second material.

[0032] According to one aspect of the invention, the intermediate layer may include a zone, in particular a central zone, in which the pores are empty, specifically devoid of a second material. This central zone provides flexibility to the thermal attachment zone.

[0033] The invention also relates to an electrical converter, in particular an inverter, suitable for equipping an electric or hybrid motor vehicle, comprising an electrical device as defined above.

[0034] In the case of an inverter, each power module can be associated with one or two phases of a rotating electrical machine.

[0035] The converter can have the function of enabling the transformation of a direct current (DC) supplied by a battery into an alternating current (AC) usable by the electrical components of the electric vehicle, and in particular the components of an electric vehicle drive system.

[0036] The invention also relates to a method for fixing the electrical device comprising the following successive steps:

[0037] The heat sink is included.

[0038] A lower layer preform is disposed on the upper face of the heat sink.

[0039] A pre-cut metallic foam is placed on the lower layer preform.

[0040] A preformed top layer is placed on the pre-cut metallic foam,

[0041] The electrical component is placed on the top layer preform. The process is remarkable in that the heat sink, each of the preforms, the pre-cut metallic foam and the electrical component then undergo a heating step, in particular in an oven or in a drying oven, under a temperature between 200 °C and 300 °C, for example 230 °C.

[0042] According to one aspect of the invention, the preforms are brazing preforms.

[0043] According to one aspect of the invention, the furnace is a reflow furnace.

[0044] According to one aspect of the invention, each of the preforms comprises a single material, in particular the same material is used for both preforms.

[0045] During the heating stage, the preform material penetrates the metal foam. This penetration occurs primarily through capillary action. The density of the metal foam described above allows for good penetration, enabling a strong intermetallic bond. The proposed density also provides sufficient sealing of the preform material to prevent the metal foam from filling and losing its mechanical flexibility. According to one aspect of the invention, during the heating stage, a force is applied to the electrical component along its principal axis. This improves penetration between the preforms and the pre-cut metal foam.

[0046] According to one aspect of the invention, the fixing process also includes a surface treatment step, in particular with formic acid. This surface treatment can take place during an initial temperature plateau and / or for a predetermined duration, for example, between 150°C and 200°C for a duration of 20 to 180 seconds.

[0047] According to one aspect of the invention, a gap-maintaining device can be used. This allows for the precise maintenance and control of a gap between the heat sink and the electrical device, in particular a uniform gap throughout the heating operation. This maintaining device makes it possible to define the optimal gap to control penetration into the metal foam in order to maintain proper mechanical flexibility.

[0048] According to one aspect of the invention, the retaining device can be formed by specific shapes on the electrical device and / or on the heat sink. For example, bosses or shoulders. The retaining device can also be an added element, such as a filler wire or shims. The retaining device must not melt at the heating or brazing temperature. This allows for the definition of a minimum clearance height.

[0049] According to one aspect of the invention, the minimum playing height can be between 100pm and 200pm.

[0050] Other features and advantages of the invention will become apparent from the following description on the one hand, and from several illustrative and non-limiting examples of embodiments given with reference to the attached schematic drawings on the other hand, in which:

[0051] [fig 1] is a schematic representation of a converter comprising a first example of an electrical device according to the invention; [fig 2] is a cross-sectional view in a plane comprising the principal axis of a second example of an electrical device;

[0052] [fig 3] is an enlargement centered on the thermal attachment means of the electrical device in the previous figure; and

[0053] [fig 4] is a representation of steps of an example of a fastening process according to the schematic invention.

[0054] Figure 1 is a schematic representation of a converter 1 comprising an electrical device 2. The converter 1 is here an inverter suitable for equipping an electric or hybrid motor vehicle.

[0055] In the example considered, the electrical device 2 comprises a heat sink 4 having an upper face 6 and at least one electrical component 8 mounted on the upper face of the heat sink. Here, there are three components, all mounted on the upper face 6. Only one electrical component 8 is shown in the middle; on either side, the thermal attachment means 12 are visible from the top of the electrical device 2. Each electrical component 8 is fixed to the heat sink by a thermal attachment means 12 arranged between the electrical component and the upper face of the heat sink along a principal axis X. In the example considered, the electrical components 8 are power modules comprising semiconductor chips. Each power module can be associated with one or two phases of a rotating electrical machine.

[0056] In the example considered, each electrical component 8 comprises a package 10 in which semiconductor chips are arranged. The package 10 has a base 11, which is formed here of power substrates, on which the semiconductor chips are mounted, and a cover wall that encloses the semiconductor chips and, in combination with the base, forms an internal volume in which the semiconductor chips extend. The package 10 thus defines the external surface of each electrical component 8. The cover wall is made of an insulating material. The base 11 may be made of copper or may be copper-based. The base forms a lower face of the electrical component 8.

[0057] This is not shown in Figure 1, but the semiconductor chips can be connected to electrical connection pins that extend outside the electrical component 8, in particular to allow the electrical connection of the semiconductor chips to a control module including a printed circuit board (PCB in English terminology).

[0058] In the example considered, each electrical component 8, here each power module, has a rectangular shape, defined by the rectangular shape of the case 9.

[0059] Figure 2 shows a second example of an electrical device 2 in a plane including the principal axis X. This plane is perpendicular to that of Figure 1. Figure 2 shows a single electrical component 8. The top face 6, the thermal attachment means 12 and the electrical component 8 are normal to the principal axis X.

[0060] Figure 3 shows an enlargement centered on the thermal attachment means 12 of the electrical device of Figure 2. It is visible in the figure that the thermal attachment means 12 comprises a metallic foam 14.

[0061] In the example considered, the metal foam 14 includes pores 16. The metal foam 16 has a pore density, in PPI (Pores Per Inch) between 10 and 1000, in particular between 300 and 700. Here it is approximately equal to 500 PPL. The pores 16 form cavities in a metal mesh, which can act as a set of springs.

[0062] In the example considered, the metallic foam 14 comprises copper, in particular a copper alloy. The metallic foam 14 can be made of more than 90% copper, in particular more than 95% copper, in particular more than 99% copper.

[0063] In the example considered, the thermal attachment means 12 comprises a metallic upper layer 18, a metallic lower layer 19 and an intermediate layer 20 which comprises the metallic foam 14. The lower layer 19 is opposite the heat sink 4, in contact with the upper face 6. The upper layer 18 is opposite the electrical component 8 in contact with a base 11.

[0064] In the example considered, the intermediate layer 20 is essentially parallelepiped-shaped. The intermediate layer 20 is defined between a lower surface and an upper surface, normal to the principal axis X. The lower surface of the intermediate layer passes through the lowest point of the metal foam 14, and the upper surface of the intermediate layer passes through the highest point of the metal foam. These surfaces are represented by dashed lines in Figure 3.

[0065] In the example considered, the intermediate layer 20 has a thickness between 60 and 80% of the thickness of the thermal bonding medium 12 measured along the main axis. The thickness of the thermal bonding medium 12 can be between 0.3 mm and 0.5 mm. The thickness of the intermediate layer 20 can be between 0.1 mm and 0.3 mm.

[0066] In the example considered, the lower layer 19 and the upper layer 18 are formed of a tin alloy, specifically an alloy (tin, silver and copper) known as "SAC" in English terminology, specifically a SAC-Sb or SAC-Sb-In alloy. The composition of the lower and upper layers is identical and they comprise a single material.

[0067] In the example considered, the intermediate layer also includes a second material, which is the same as that found in the lower layer 19 and the upper layer 18. This second material fills part, and only part, of the pores 16 of the metal foam 14. With reference to Figure 4, it will be explained how the second material transfers from the upper layer 18 and the lower layer 19 to the intermediate layer 20.

[0068] In the example considered, the intermediate layer 20 includes end zones, in the direction of the principal axis X, penetrated by the second material. The intermediate layer 20 also includes a central zone, in which the pores 16 are devoid of the second material.

[0069] With reference to Figure 4, an example of a method for fixing the electrical device 2 is shown. This method comprises, successively, a first step in which the heat sink 4 is provided. The heat sink 4 is mounted on a base 24. In the second step, a lower-layer preform 25 is placed on the upper face 6 of the heat sink 4. These first two steps are shown in the upper image of Figure 4. In the third step, a pre-cut metallic foam 26 is placed on the lower-layer preform 25. This is visible in the third image of Figure 4 from the top. In the fourth step, an upper-layer preform 27, having the same composition as the lower-layer preform 25, is placed on the pre-cut metallic foam. This is visible in the second image of Figure 4 from the bottom.During the fifth step, three electrical components 8 are arranged on each of the top layer preforms 27. This is visible in the first image of Figure 4 from the bottom.

[0070] In the example considered, the assembly consisting of the heat sink 4, each of the preforms, the pre-cut metal foam 26, and the electrical components 8 is then placed in an oven or furnace to undergo a heating step at a temperature between 200 °C and 300 °C, for example, 230 °C. This heating step allows the material from the preforms to penetrate the metal foam 14, specifically the pores 16 of the metal foam 14. Only a portion of the pores is filled by the material from the two preforms. This heating step can be carried out by applying a force to the electrical components along the principal X-axis.

[0071] The invention, as described above, achieves its intended purpose and provides an electrical device. Variations not described here could be implemented without departing from the scope of the invention, provided that, in accordance with the invention, they include a heat sink support device capable of reducing the heat sink's deflection as described in the invention.

Claims

DEMANDS 1- Electrical device (2) comprising a heat sink (4) having an upper face, at least one electrical component (8) mounted on the upper face (6) of the heat sink, a thermal attachment means (12) disposed between the electrical component (8) and the upper face (6) of the heat sink (4) along a principal axis (X), the electrical device being characterized in that the thermal attachment means comprises a metallic foam (14). 2- Electrical device (2) according to the preceding claim, in which the metallic foam (14) comprises pores (16), the metallic foam having a pore density, in PPI (“Pores Per Inch” in English) between 10 and 1000, in particular between 300 and 700, in particular about equal to 500. 3- Electrical device (2) according to any one of the preceding claims, wherein the metallic foam (14) comprises copper, in particular a copper alloy, or a nickel alloy. 4- Electrical device (2) according to any one of the preceding claims, wherein the thermal attachment means (12) comprises a metallic top layer (18), a metallic bottom layer (19) and an intermediate layer (20), the intermediate layer (20) comprising the metallic foam (14). 5- Electrical device (2) according to the preceding claim, the intermediate layer (20) has a thickness between 30% and 90%, in particular between 60% and 80%, of a thickness of the thermal attachment means. 6- Electrical device (2) according to any one of the two preceding claims, wherein the lower layer (19) and / or the upper layer (18) are formed of a tin alloy, in particular of an alloy (tin, silver and copper) called "SAC", in particular of an alloy SAC-Sb or SAC-Sb-In. 7- Electrical device (2) according to any one of claims 4 to 6, wherein the intermediate layer (20) also comprises a second material formed of a tin alloy, this second material filling in particular a part of the pores (16) of the metallic foam (14). 8- Electrical converter (1), in particular an inverter, suitable for equipping an electric or hybrid motor vehicle, comprising an electrical device (2) according to any one of the preceding claims. 9- Method for fixing the electrical device (2) according to any one of claims 4 to 7, comprising the successive steps: The heat sink (4) is supplied, A lower layer preform (25) is disposed on the upper face (6) of the heat sink (4), A pre-cut metallic foam (26) is placed on the lower layer preform (25), A top layer preform (27) is placed on the pre-cut metallic foam (26), The electrical component (8) is placed on the top layer preform (27). The heat sink (4), each of the preforms (25, 27), the pre-cut metallic foam (26) and the electrical component (8) then undergo a heating step, in particular in an oven or in a drying oven, under a temperature between 200 °C and 300 °C, for example 230 °C. 10- Method of fixing the electrical device (2) according to the preceding claim, wherein, during the heating step, a force is applied to the electrical component (8) along the main axis (X).

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