Electrical device

Abstract

Electrical device with electronics (20) enclosed in an aluminium housing (10), wherein components (22) of the electronics (20) are arranged on a first main surface (31) of a circuit carrier (30), wherein the circuit carrier (30) consists of a metal base plate (33) and at least one layer (35) of conductor tracks (36) on a dielectric layer (34), and wherein the circuit carrier (30) is inserted into an opening of the aluminium housing (10) and rests there with edge sections on a circumferential shoulder (50) and the metal base plate (33) is connected to the aluminium housing (10) by a welded connection (40), characterized by that the welded joint (40) is produced by friction stir welding, and that at least one metallic contact pad (38) is arranged between the circumferential shoulder (50) and the circuit carrier (30), which is electrically connected to the conductor tracks on the circuit carrier (30) and which is pressed against the shoulder (50).

Description

[0001] The invention relates to an electrical device with electronics that are housed in an aluminum casing, wherein components of the electronics are arranged on a first main surface of a circuit carrier, wherein the circuit carrier consists of a metal base plate and at least one layer of conductor tracks on a dielectric layer, and wherein the circuit carrier is inserted into an opening of the aluminum casing and rests there with edge sections on a circumferential shoulder and the metal base plate is connected to the aluminum casing by a welded connection.

[0002] Such an electrical device and a method for its manufacture are described in the unpublished German patent application DE 10 2017 001 351 A1. This electrical device has a fluid-tight aluminum housing in which a circuit carrier is used as a partition to a cooling channel, thus bringing it into direct contact with a fluid cooling medium. The circuit carrier has a metal base plate that is fluid-tightly connected to the aluminum housing by a weld. Friction stir welding is mentioned in this patent application as a preferred method for producing the weld. A perspective view of this device is shown in the Fig. This is illustrated in section 3 of this document and will be explained in more detail below. Such circuit carriers are also known as IMS printed circuit boards (IMS stands for insulated metal substrate) and are circuit carriers in which conductive traces and insulating layers are laminated directly onto an insulated metal plate. A particular advantage of such circuit boards lies in their good heat dissipation, or more generally, their efficient heat transfer. Therefore, these circuit boards are often used in power electronics, where significant power losses must be dissipated.

[0003] In many electronic devices in motor vehicles, the ground connection, i.e., the "battery negative" potential, is routed through the housing and subsequently through the vehicle body. Therefore, it is advantageous to implement the ground connection of a circuit arrangement on the circuit board via the conductive housing itself, in order to save on wires and connection points.

[0004] However, attaching conductive surfaces of a circuit board to the surfaces of an aluminum housing often leads to unsatisfactory results. This is because aluminum surfaces quickly form a thin oxide layer when exposed to air, which thickens over time. Since this layer is quite stable, it protects the underlying aluminum from further oxidation, or rather, after an initial increase, the oxide layer grows only very slowly as long as it is not attacked. However, this oxide layer becomes electrically very poor as its thickness increases.

[0005] These oxide layers are particularly thick in the form of a casting skin. Therefore, if an electrically conductive connection to the aluminum casting housing is to be created, the relevant contact area must be freed from the casting skin by a mechanical process, such as milling. In the case of an IMS plate to be welded in, it is necessary to mill a pocket to maintain manufacturing tolerances, thus removing the casting skin of the housing in the contact area only once. However, a new oxide layer forms within fractions of a second after the milling process, although this layer is initially very thin. It is therefore essential to ensure rapid further processing or to take measures to limit or prevent the growth of the oxide layer.This can be achieved in the form of a conductive coating; it is also useful to keep the time window between mechanical processing and further processing short in order to keep the thickness of the oxide layer as low as possible.

[0006] On such a surface prepared for the connection, a sufficiently high contact force for a good conductive, i.e. low-resistance connection can then be achieved, for example, by means of a screw connection.

[0007] The production of such a high-quality, i.e., very low-resistance, electrical ground connection from the electrical conductors and components arranged on a circuit carrier to the housing ground of an aluminum housing is therefore generally a relatively complex process step, which, in addition to housing preparation and a free area in the housing, also requires a connecting line and at least two contact ends.

[0008] The ground connection of an IMS board to an aluminum housing is generally made via terminals on the component side of the circuit board. These terminals can be soldered bolts or nuts, or soldered terminal blocks. Such connection elements share the disadvantages of increased costs, additional assembly effort, and space requirements on the circuit board. For electrical connection, an electrical cable is sometimes used, which is laboriously connected manually to electrical connectors located on the circuit board's conductor tracks and on the metal body of the device housing.

[0009] US patent 2016 / 0291272A1 discloses an optical module with a metal housing in which an optical component and a multilayer flexible printed circuit board structure are integrated.

[0010] DE 10 2017 001 351 A1 discloses an electrical device with power electronics in which a circuit carrier as part of a housing wall is in direct contact with a fluid cooling medium (e.g. water) to enable particularly effective heat dissipation. The task arose to design a generic electrical device in such a way that a high-quality ground connection could be produced in a particularly simple, space-saving and cost-effective manner and without manual assembly.

[0011] This problem is solved according to the invention by producing the welded joint by friction stir welding, and by arranging at least one metallic contact pad between the circumferential shoulder and the circuit carrier, which is electrically connected to the conductor tracks on the circuit carrier and pressed against the shoulder.

[0012] A metallic contact pad is defined here as a planar structure consisting of at least one metal layer of sufficient thickness. The contact pad therefore comprises a planar extension of a copper conductor track, preferably reinforced by additional metallization. This additional metallization can consist of a soft metal, preferably tin or gold. The additional metallization can be applied, in particular, by an electroplating process during the manufacturing of the printed circuit board and may include intermediate layers between the copper and the respective surface metal, such as a nickel barrier layer, which are beneficial for the layer structure.

[0013] A metal contact pad on the circuit board alone does not create a good conductive connection to an aluminum surface. The decisive factor for the connection's conductivity is the actual contact area at the microscopic level. This requires high contact pressure, which smooths the surface microstructures, or alternatively, melting a component, which then conforms very precisely to the corresponding contact area, thereby creating a very large contact area. Furthermore, high contact pressure can also cause the thin and relatively brittle aluminum oxide layer to break up through microscopic deformation of the surface, thus also improving the electrical contact.

[0014] According to the invention, it is additionally provided to form a welded connection between the circuit carrier and the aluminum housing using the friction stir welding method. As welding tests have shown, the friction stir welding process, in which a fluid-tight seam is created by high contact pressure, mechanical friction and stirring of the base material, succeeds in producing an electrically conductive connection from a contact pad to an aluminum housing.

[0015] The exact mechanism at work is not fully understood. What is certain is that, unlike other welding processes such as arc or laser welding, friction stir welding generates a high contact pressure from the base plate of the circuit carrier onto the contact pad, which firmly presses the contact pad against the shoulder of the aluminum housing.

[0016] The metal of the contact pad likely bonds mechanically with the irregularities in the microstructure of the aluminum oxide layer, creating numerous microscopic contacts with the underlying aluminum and resulting in an overall low contact resistance. This effect can be enhanced by high contact pressure and a large surface area. It is assumed that the thin oxide layer of aluminum newly formed after milling remains intact, or at most, only cracks locally due to the high contact pressure.

[0017] To achieve a particularly good mechanical and electrical connection, the contact pad can be made of a particularly soft material, such as gold, which adheres particularly well to the microstructures of the aluminum oxide layer under the contact pressure during friction stir welding.

[0018] To achieve the same goal for the additional metallization of the contact pad, a metal such as tin can be advantageously chosen, which softens or even melts under the heat of friction stir welding. The heat input from the friction stir welding process can therefore be appropriately greater than would be required for simple friction stir welding of the housing to the circuit carrier.

[0019] It is assumed that a molten tin layer does not create a metallurgical bond (as in a soldered joint) to the aluminum or aluminum oxide of the housing, but that instead the softened tin or the molten tin fills in irregularities in the microstructure of the aluminum oxide layer, thus creating a very large contact area which in turn enables a low contact resistance.

[0020] Of course, this type of connection can also be used not only for grounding, but also, for example, for mounting so-called Y-capacitors. These capacitors are used, for example, in mains filters.

[0021] The electrical device according to the invention will now be explained in more detail with reference to an embodiment shown in the drawing. The drawing shows... Fig. 1 a section of the partition wall and the circuit carrier from the welding side, Fig. 2 a section of the partition and circuit carrier from the component side, and Fig. 3. A perspective view of the electrical device.

[0022] The Fig. Figure 3 shows a perspective view of an electrical device.

[0023] The Fig. Figure 3 shows an aluminum housing 10, preferably made of die-cast aluminum. The housing 10 is horizontally divided by a partition 11. The upper half of the partition 11, as shown in the illustration, can be brought into contact with a fluid cooling medium. Water is typically used as the cooling medium, flowing along the inner surfaces of the housing 10 in a cooling channel. The cooling channel is formed by a water guide element (not shown) located between the partition 11 of the housing 10 and a cover (not shown in the drawing). The cover is attached to the upper edge of the housing 10 to close or enclose the cooling channel. The cooling water is supplied through an inlet nozzle 60 and exits the cooling channel through an outlet nozzle 70 after flowing past the partition 11 of the housing 10. The water guide element can be, for example, made of a flexible material.be designed as a meandering plastic part that is clamped between the partition 11 of the housing 10 and the lid. The partition 11 of the aluminum housing 10 is partially formed by a circuit carrier 30, which is inserted into an opening initially present in the aluminum housing 10 and, after completion, seals this opening by welding. A first main surface 31 of the circuit carrier 30, the so-called component side, forms a section of one side of the partition 11 of the aluminum housing 10. This component side 31 is provided with electrical conductor structures on which electronic components 22 of a power electronics system are arranged. These electronic components 22 are primarily power semiconductors, which can generate considerable heat during operation of the electrical control unit.

[0024] The second main surface 32 of the circuit carrier 30, opposite component side 31, forms a section of the other side of the partition 11 of the housing 10. Since this side 32 of the partition 11 is in direct contact with the water used for cooling, the circuit carrier 30 must be fluid-tightly connected to the rest of the aluminum housing 10. This fluid-tight connection is made by a weld 40, shown here only partially.

[0025] Before welding, the circuit carrier 30 is inserted from the water side into an opening in the partition 11 and rests there on a circumferential shoulder 50. This has the advantage that the water pressure in the cooling channel does not stress the weld joint 40. Instead, the circuit carrier 30 is even pressed against the circumferential shoulder 50, i.e., against the housing 10, as soon as it is pressurized from the water side.

[0026] The arrangement of the circuit carrier 30 is shown based on the sketchy representations of the Fig. 1 and Fig. 2 further clarifies this. The circuit carrier 30 is designed as an IMS printed circuit board, which has a layered structure, as shown in the Fig. 2 is more clearly recognizable. The IMS printed circuit board 30, which can also be referred to as a metal core printed circuit board, has a base plate 33 made of aluminum as its metal core, onto which one or more conductor track layers 35 are laminated, which are insulated from the base plate 33, and in the case of several conductor track layers also from each other, by a dielectric layer 34. The one shown here in the Fig. The circuit carrier 30 shown here is purely an example and has only a single conductor track layer 35.

[0027] The Fig. Figure 1 shows a section of the partition 11 to which the circuit carrier 30 is already attached. A section of a weld 40 running around the edge of the circuit carrier 30 is indicated by hatching, through which the metal side of the circuit carrier 30 is connected to the partition 11.

[0028] To create this weld joint 40, the friction stir welding method is specifically used. In friction stir welding, a rapidly rotating stirring pin (not shown) generates frictional heat that is below the actual melting point of aluminum, so that the aluminum in the vicinity of the stirring pin does not actually molten, but merely has a pasty consistency. By moving the stirring pin along the joint line of the objects to be welded, this process produces a very fine-grained microstructure and a gas-tight weld seam.

[0029] The Fig. 2 shows the rear view of the item in the Fig. 1. Arrangement shown. As the Fig. As illustrated in Figure 2, the aluminum housing 10 forms an opening with a surrounding ledge 50. The surface area of ​​the opening and that of the circuit carrier 30 are coordinated such that the edge sections of the circuit carrier 30 rest on the surrounding ledge 50 on all sides. The height of the ledge 50 is designed such that the second main surface 32 of the circuit carrier 30 is flush with a surface of the partition 11.

[0030] During the manufacturing process of the printed circuit board, the contact pad 38 is formed as an extension of one or more conductor tracks 36 and provided with a corresponding metallization. The circuit carrier 30 is placed in the opening of the partition 11 such that the contact pad 38 is located between the uppermost layer of the circuit carrier 30 and the edge of the shoulder 50 that overlaps the circuit carrier 30. The contact pad 38 extends more or less extensively between the shoulder 50 and the circuit carrier 30. In particular, it is also possible to provide several contact pads 38 or even a continuous contact pad 38 arranged circumferentially along the shoulder 50 beneath the shoulder 50.

[0031] During the friction stir welding process of welding the circuit carrier 30 to the aluminum housing 10, the contact pad 38 is pressed against the shoulder 50 with a high contact force. Depending on the metal of the contact surface of the contact pad 38 facing the shoulder 50, this material can soften or even melt due to the heat input of the friction stir welding process. In any case, the surface material of the contact pad 38 bonds tightly to the surface of the shoulder 50, creating a highly conductive electrical connection to the aluminum housing 10. The contact pressure applied by the friction stir welding pin during the friction stir welding process is partially retained even after the welding process is complete, pressing the circuit carrier 30, now attached to the aluminum housing 10, against the contact pad 38, and the pad in turn against the shoulder 50.

[0032] The contact pad 38 can be connected to conductor tracks 36 or directly to terminals of electronic components 22 on the circuit carrier 30 during the manufacturing process at virtually no additional cost. In this way, a large-area and electrically conductive connection to the ground of the aluminum housing 10 is created in a cost-effective and space-saving manner, and can reliably transmit even larger currents.

[0033] In the case of a multi-layered construction of the circuit carrier 30, care should be taken to ensure that the conductor tracks 36 to be connected to the housing ground belong to the outer layer and that no layers of other potentials run in the contact area, i.e. between the housing shoulder 50 and the circuit carrier 30. Reference sign 10 aluminum housings 11 Partition wall 20 Electronics 22 electronic components 30 circuit carriers 31 (first) main surface (component-side) 32 (second) main area (waterside) 33 metal base plate 34 Dielectric layer 35 conductor track position 36 conductor tracks 38 contact pads 40 Friction stir welding Paragraph 50 60 inlet nozzles 70 outlet nozzles

Claims

[1] Electrical device with electronics (20) enclosed in an aluminium housing (10), wherein components (22) of the electronics (20) are arranged on a first main surface (31) of a circuit carrier (30), wherein the circuit carrier (30) consists of a metal base plate (33) and at least one layer (35) of conductor tracks (36) on a dielectric layer (34), and wherein the circuit carrier (30) is inserted into an opening of the aluminium housing (10) and rests there with edge sections on a circumferential shoulder (50) and the metal base plate (33) is connected to the aluminium housing (10) by a welded connection (40), characterized by , that the welded joint (40) is produced by friction stir welding, and that at least one metallic contact pad (38) is arranged between the circumferential shoulder (50) and the circuit carrier (30), which is electrically connected to the conductor tracks on the circuit carrier (30) and which is pressed against the shoulder (50). [2] Electrical device according to claim 1, characterized by , that the contact pad (38) is formed from at least one metal layer. [3] Electrical device according to claim 1 or 2, characterized by , that the contact pad (38) is formed by the end section of a conductor track (36). [4] Electrical device according to claims 2 and 3, characterized by , that the contact pad (38) is formed by the end section of a conductor track (36) onto which an additional metal layer is applied. [5] Electrical device according to claim 4, characterized by that the additional metal layer consists of gold. [6] Electrical device according to claim 4, characterized bythat the additional metal layer consists of tin. [7] Electrical device according to claim 4, characterized by that the additional metal layer is applied by an electroplating process. [8] Electrical device according to claim 6, characterized by , that the tin layer is melted during the production of the friction stir welding joint (40). [9] Electrical device according to claim 1, characterized by , that an electrical connection of the electronics (20) to the housing mass of the aluminum housing (10) is established via the contact pad (38) which is pressed onto the shoulder (50) of the aluminum housing (10). [10] Electrical device according to claim 1, characterized by that several contact pads (38) are provided or that a contact pad (38) is provided that runs around the shoulder (50).

Images