DE29905056U1 - Electrical unit with power semiconductor - Google Patents

Description

18109aNA.DOC

Electrical unit with power semiconductor

The invention relates to an electrical component with at least one power semiconductor, in particular for use in automotive engineering, with the features of the preamble of claim 1.

In automotive engineering, more and more power semiconductor components have recently been used, for example as fast electronic switches for controlling brushless DC motors. Such motors are used as pump motors in power steering systems or similar.

According to the known state of the art, it is necessary, on the one hand, to electrically connect the electrical connection contacts to the corresponding connection areas of conductor tracks and, on the other hand, to connect the heat sink of the power semiconductor to another heat sink to dissipate the waste heat. This is usually done using a metallic body which, in addition to the required thermally conductive connection, is also electrically connected to the heat sink, since the heat sink integrated with the power semiconductor component is usually connected to a connection potential or an electrical connection contact of the semiconductor component, usually ground.

The heat sinks of known power semiconductor components usually have a hole for assembly, so that the semiconductor component can be connected mechanically, for example by means of screws, to the other heat sink in a thermally conductive and electrical manner. In addition, there is also the option of using an electrical and/or

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thermally conductive adhesive connection between the heat sink of the power semiconductor component and the other heat sink.

The disadvantage here is, on the one hand, the high cost involved in industrial mass production and, in the case of adhesive joints, the additional time that must be waited for until the adhesive joint has hardened before further processing of the electrical component.

With purely mechanical connections using screws, clamps or similar, there is a risk that the shocks and vibrations that occur in a vehicle will temporarily increase the electrical contact resistance between the heat sink of the power semiconductor component and the other heat sink, and the high current will lead to additional thermal stress or oxidation of the contact surfaces. This would ultimately lead to the destruction of the semiconductor.

Based on this prior art, the invention is based on the object of creating an electrical assembly with at least one power semiconductor component, which can be manufactured with little effort and in which the heat sink of the power semiconductor component has sufficient electrical contact reliability even in the event of the vibrations occurring in a motor vehicle.

The invention solves this problem with the features of claim 1.

The invention is based on the surprising finding that even commercially available power semiconductor components, such as power FETs or the like, which have an integrated heat sink, can be quickly and safely mechanically and

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can be electrically connected without the risk that the thermal stress occurring during laser welding could destroy the actual semiconductor chip.

Up to now, attempts have been made to solder the heat sink at correspondingly low temperatures. However, since a solder had to be used that can be processed at such low temperatures, there is a risk that the heat sink will be heated above the softening point of the solder due to the high power loss that often occurs in power semiconductors. As a result, simultaneous vibrations can lead to at least a temporary loosening of the mechanical and electrical connection.

The invention overcomes the prejudice that has previously existed among manufacturers of power semiconductor components that welding, including laser welding, of the heat sinks of commercially available power semiconductor components is not possible due to the excessive temperature load on the semiconductor.

Surprisingly, however, it has been shown that even commercially available power semiconductor components, which are designed for the usual types of assembly such as screwing, clamping, gluing or soldering, can be securely connected mechanically and electrically to their heat sink by laser welding to a connection contact area of a conductor track.

According to the preferred embodiment of the invention, the at least one further electrical connection contact of the semiconductor component is connected to a further connection contact element by resistance welding. Resistance welding of the connection contacts is possible because they have a relatively

have a low mass and can therefore be heated to the temperature required for welding in a short time. The energy required for this is far less than the energy that would be required to weld the heat sink using resistance welding. There is therefore no risk of thermal damage to the semiconductor chip.

The combination of the two welding processes results in the advantage of an extremely efficient and therefore cost-effective assembly of a power semiconductor component, while at the same time achieving extremely high contact reliability, even with the vibrations that occur in a vehicle.

In the case of an electrical assembly according to the invention, the conductor tracks can be designed as punched grids. Such punched grids can be produced in a simple manner, with the conductor tracks having a sufficient mass and thickness to enable laser welding of the connection areas to the heat sink of the semiconductor component. In addition, the relatively high mass of a punched grid means that it simultaneously serves to dissipate heat from the heat sink of the power semiconductor component. If necessary, this means that the use of an additional heat sink can be dispensed with or a corresponding heat sink can be dimensioned much smaller in terms of its heat dissipation performance.

In the preferred embodiment of the invention, the heat sink of the power semiconductor component has an opening that is originally intended for mechanical fastening. The connection of the heat sink to the connection area of a conductor track is then preferably carried out in such a way that the connection area of the conductor track is provided with a hole that is sufficient for heat dissipation.

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corresponding surface rests on the heat sink, with the welding taking place on the side of the breakthrough facing away from the semiconductor chip.

This has the advantage that the welding point or weld seam is relatively far from the mounting location of the chip on the heat sink, which also serves as a carrier for the chip, and the temperatures occurring at the location of the chip are relatively low during the welding process. In addition, a significant part of the locally introduced heat for the laser welding process is immediately dissipated from the connection area of the conductor tracks.

Since the welding takes place on the side of the opening facing away from the semiconductor chip or on the relatively narrow webs of the heat sink remaining on both sides of the opening, a relatively high thermal resistance results between the welding point or the weld seam and the mounting location of the chip, so that the risk of unacceptably high temperatures at the location of the chip is also reduced.

Although the welding point or weld seam is located at the outer end of the heat sink of the semiconductor component, the fact that the connection area of the relevant conductor rests on the heat sink also results in a sufficiently low heat transfer resistance in the non-welded areas to dissipate the heat loss to the connection area. The electrical contact is ensured anyway by welding at the outer end of the heat sink, although this is also where the lowest heat transfer resistance naturally exists.

Further embodiments of the invention emerge from the subclaims.

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The invention is explained in more detail below using an embodiment shown in the drawing. In the drawing:

Fig. 1 shows a partial section of an electrical unit according to the invention

in perspective view and

Fig. 2 is a perspective view of the section in Fig. 1 from below.

Fig. 1 shows a perspective section of an electrical assembly 1, which in the exemplary embodiment shown consists of a stamped grid 3 with several conductor tracks 5, 7, 9, 11, which are partially overmolded with plastic 13. The conductor tracks are connected in this way to form a one-piece assembly.

In addition to other electronic, electrical or electromechanical components or units not shown in detail, the electrical unit 1 comprises several power semiconductor components, of which two power field effect transistors 15, 17 are shown in the section shown in Fig. 1.

Each of the power field effect transistors 15, 17 has a heat sink 15a, 17a, which also serves as a carrier for the actual semiconductor chip. This is molded with plastic in the usual way. The heat sink 15a, 17a is also connected to an electrical connection contact of the semiconductor chip.

To mount the power field effect transistors 15, 17, the heat sink 15a, 17a is brought into mechanical contact with a connection area 5a or 7a of the

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Conductor tracks 5 and 7, respectively. The connection areas 5a, 7a are preferably designed so that they cover a significant part of the area of the heat sink 15a, 17a that is not covered by the plastic with which the semiconductor chip is injected. By designing the connection areas 5a, 7a with a corresponding thickness, it can be ensured that, on the one hand, sufficient mechanical strength and, on the other hand, good heat dissipation are achieved. In many cases, it will be sufficient to use only the connection areas 5a, 7a of the conductor tracks 5, 7 as heat sinks and to dispense with additional heat sinks. In any case, this ensures that an additional heat sink, if necessary, can be made much smaller.

After the heat sink has been mechanically brought into contact with the relevant connection area of the conductor tracks, the two parts are joined together using a laser welding process.

As shown in Fig. 1, the welding preferably takes place in the rear area of the heat sinks 15a, 17a or the connection areas 5a, 7a. This results in the greatest possible distance between the welding location and the position of the chip under the plastic, so that unacceptably high temperatures at the location of the chip can be safely avoided.

In addition, openings 19 can be provided in the heat sink in the usual way. If the welding points are behind the openings 19 or at most in the area of the webs 21 remaining to the side of the openings 19, the webs 21 form a higher thermal transfer resistance in the direction of the location of the chip than in a fully developed heat sink. This makes heat conduction in the direction of the chip more difficult or a significant part of the

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The heat generated by the welding process is dissipated via the connection area of the corresponding conductor track.

Although the connection areas 5a, 7a are only welded to the heat sink 15a, 17a at the rear end, there is also sufficient heat-conducting contact with the surfaces of the connection areas 5a, 7a for the rest of the underside of the connection areas 5a, 7a. Even if the electrical contact cannot be maintained in these areas or cannot be maintained constantly (particularly in the case of vibrations), the welded connection at the rear end of the connection area results in an ideal electrical (and of course also ideal heat-conducting) contact.

Due to the electrical contact between the heat sink 15a, 17a of the power field effect transistors 15, 17 and the connection areas 5a, 7a of the conductor tracks 5, 7, which is ensured even in the event of extreme vibrations and shock loads, it is possible to dispense with the additional contacting of the one of the other connection contacts 23 of the semiconductor component that is connected to the same connection of the chip as the heat sink. In the usual assembly processes for power semiconductor components, however, it is necessary to also connect this connection contact to the relevant conductor track, since a vibration-proof and shock-proof electrical contact between the heat sink and the relevant connection area could not be ensured.

The contacting of the additional connection contacts 23 (in the illustrated embodiment two of the three existing connection contacts of the power field effect transistors) is preferably carried out by resistance welding with the corresponding connection areas of the conductor tracks 9, 11.

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Resistance welding of the additional connection contacts 23 offers the advantage of less effort compared to laser welding. Laser welding, which is already more complex, would only have an advantage if the pressing of the connection contacts 23 onto the conductor tracks, which is required for resistance welding, could be dispensed with. However, such pressing would be additionally required for the already more complex laser welding process, since the thin, flexible connection contacts 23 are often not in contact with the conductor track 9 or 11 over the required length and therefore a laser welding process could fail or not be carried out with the required reliability. In contrast, laser welding of the heat sink to the connection areas of the conductor tracks is possible without major problems, since the heat sinks are so thick and rigid, as well as sufficiently flat, to enable a flat contact with the connection area in question.

In addition, the pressure required for resistance welding the relevant connection contacts 23 also ensures that the heat sink is pressed against the connection area. It is therefore advisable to carry out resistance welding and laser welding simultaneously, taking advantage of the pressure required for resistance welding anyway, or to carry out laser welding during the pressure for resistance welding.

Just for the sake of clarity, it should be noted at this point that of the three connections 23 of the power field effect transistors 15, 17 in Fig. 1 and 2, only one connection contact 23 is shown welded. The short cut-off connection contact 23 does not need to be contacted, as already mentioned, since

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this is connected to the same connection of the chip as the respective heat sink. The connection contact 23 shown bent in each case is connected in the embodiment shown to conductor tracks of another circuit board (not shown in more detail) that is mounted below the lead frame 3.

Claims (7)

18109aNA.DOC "· &idigr;#<>; Protection claims 1. Electrical unit, particularly for use in automotive engineering, a) with conductor tracks (5, 7, 9, .11) and at least one power semiconductor component (15, 17) connected to the conductor tracks, b) which has a metallic heat sink (15a, 17a) which serves as a carrier for the semiconductor chip and at the same time for heat dissipation and as a connection contact for the semiconductor chip, and c) which has at least one further electrical connection contact (23) for contacting the semiconductor chip, characterized, d) that the heat sink (15a; 17a) is connected mechanically and is electrically connected. 2. Assembly according to claim 1, characterized in that the conductor tracks (5, 7, 9, 11) are designed as a punched grid (3). 3. Construction unit according to claim 2, characterized in that the punched grid (3) is overmolded with plastic (13) in partial areas for mechanical stabilization and connection. 18109aNA.DOC ; · · * &Idigr;»' Iu! 4. Assembly according to one of the preceding claims, characterized in that at least the connection contact area (5a, 7a) of the conductor tracks (5, 7) is so strong that it simultaneously contributes to heat dissipation to such an extent that an additional heat sink for the power semiconductor element (15, 17) can be completely dispensed with or an additional heat sink can be dimensioned significantly smaller in terms of its heat dissipation performance. 5. Assembly according to one of the preceding claims, characterized in that the heat sink (15a, 17a) has an opening (19) originally intended for mechanical fastening and that the heat sink is welded to the connection region (5a, 7a) of the conductor tracks (5, 7) in at least one partial region which lies on the side of the opening (19) facing away from the semiconductor chip. 6. Assembly according to one of the preceding claims, characterized in that the power semiconductor component (15, 17) is a commercially available component with a heat sink (15a, 17a) which is designed for the usual types of assembly of screwing, clamping, gluing or soldering. 7. Assembly according to one of the preceding claims, characterized in that the at least one further electrical connection contact (23) is connected to a further connection contact element (9, 11) by resistance welding.