Method for manufacturing a power module for an electric vehicle

Abstract

Method for producing a power module (10) for an electric vehicle, comprising the following steps: a) encapsulating a half-bridge inverter in a first plastic encapsulation (16) such that at least one first busbar (18), which has a rectangular cross-section and is electrically coupled to the half-bridge inverter, protrudes from the first plastic encapsulation (16); b) encapsulating an intermediate circuit capacitor in a second plastic encapsulation (12) such that at least one second busbar (14), which has a rectangular cross-section and is electrically coupled to the intermediate circuit capacitor, protrudes from the second plastic encapsulation (12); c) aligning at least the first and second busbars (14, 18) such that they are arranged one above the other; d) welding the upper (18) to the lower busbar (14) using a laser beam, characterized by the following steps to be carried out before step d): e) determining a predetermined bending point (20) of the upper busbar (18); f) Partial melting of the upper busbar (18) in the area of the predetermined bending point (20); g) While the upper busbar (18) is partially melted: Applying a joining force (F1) to the upper busbar (18) in such a way that the upper busbar (18) buckles in the region of the predetermined bending point (20) so that it rests flat on the lower busbar (14), the applied joining force (F1) being dimensioned in such a way that the force component (F2) which is transmitted via the upper busbar (18) to the lower busbar (14) lies below a first predeterminable threshold value.

Description

[0001] The present invention relates to a method for producing a power module for an electric vehicle, comprising the following steps: First, a half-bridge inverter is encapsulated in a first plastic encapsulation such that at least one first busbar, which has a rectangular cross-section and is electrically coupled to the half-bridge inverter, protrudes from the first plastic encapsulation. Furthermore, an intermediate circuit capacitor is encapsulated in a second plastic encapsulation such that at least one second busbar, which has a rectangular cross-section and is electrically coupled to the intermediate circuit capacitor, protrudes from the second plastic encapsulation. Subsequently, at least the first and second busbars are aligned such that they are arranged one above the other. Finally, the upper busbar is welded to the lower busbar using a laser beam.

[0002] The combination of half-bridge inverter and intermediate circuit capacitor represents the so-called commutation cell, which in a hybrid or electric vehicle serves to supply the electric motor with power.

[0003] Three-phase half-bridge modules are typically used for half-bridge inverters. These are connected to a corresponding number of busbars of the DC link capacitor via a corresponding number of busbars. For simplicity, a single-phase implementation is described below. In the prior art, the busbar of the half-bridge inverter is arranged above the busbar of the DC link capacitor, and a uniformly distributed joining force is then applied to the stacked busbars. An increased pressing force is then applied to the part of the half-bridge inverter's busbar that extends beyond the busbar of the DC link capacitor in order to create a tight connection between the stacked busbars. This pressing is always necessary due to existing design and manufacturing tolerances in order to create a flat contact surface.It is a prerequisite for welding the joint in the next step.

[0004] With this well-known procedure, unwanted failures are repeatedly detected during subsequent operation.

[0005] WO 2021 / 013 915 A1 discloses a method for electrically contacting a terminal lug of an electrical component with a contacting partner. In order to establish a gap-free welded contact between the terminal lug and the contacting partner, the terminal lug is bent open at a bending point such that it is rigidly bent towards the contacting partner. When the contacting partner is placed against the terminal lug in the intended mounting position, it presses the terminal lug against its bending direction. Although this creates a gap-free contact surface, an increased force is applied to the terminal lug in the area of the bending point. The increased stress in the area of the bending point can lead to a breakage of the terminal lug in this area, which in turn can lead to undesirable failures.

[0006] DE 42 28 528 A1 discloses a method for sheet metal processing, wherein a laser beam is used to weld and modify a metal sheet.

[0007] The object of the present invention is therefore to improve a generic method for producing a power module for an electric vehicle in such a way that greater reliability can be ensured.

[0008] This problem is solved by a method having the features of patent claim 1.

[0009] The present invention is based on the finding that the strong bending of the busbars during the joining process can cause stresses in the encapsulation of the intermediate circuit capacitor and the encapsulation of the half-bridge inverter. These stresses in the material can lead to cracking, which can lead to material spalling and thus to direct component failure. Likewise, the cracking can allow moisture penetration over the lifetime of the component, which can also lead to damage and failure.

[0010] Particularly due to the rectangular shape of the busbars, the bending process used in the prior art, particularly the subsequent pressing under considerable force, can lead to excessive material stress at the transition between the plastic encapsulation and the corner areas of the associated busbar, which can lead to the undesirable effects mentioned above. If the busbars were round, this problem would be reduced; however, in the power electronics sector, rectangular busbars are desirable to maximize the contact area between the two busbars, which would not be possible with busbars with a round cross-section.

[0011] According to the invention, before the welding step, a predetermined bending point of the upper busbar is first defined, this is then partially melted in the area of the predetermined bending point and finally, while the upper busbar is partially melted, a joining force is applied to the upper busbar in such a way that it comes to rest flat on the lower busbar, wherein the applied joining force is dimensioned in such a way that the force component which is transferred via the upper busbar to the lower busbar is below a predeterminable threshold value.

[0012] Due to the melting process, only a very small mechanical stress on the plastic encapsulation of the DC link capacitor and the plastic encapsulation of the half-bridge inverter occurs for a short time, reliably preventing the risk of stresses in these plastic encapsulations that could lead to cracking. Even a small joining force is sufficient to cause the upper busbar to buckle, so that it rests flatly on the busbar below.

[0013] In a reference measurement, the force component acting on the lower busbar can be determined, for example, by a pressure measuring device placed between the two busbars and compared to this by measuring the joining force applied to the upper busbar. If the determined joining force is then applied to the upper busbar, as in the actual application, the force component acting on the lower busbar corresponds to the force component previously determined in the reference measurement.

[0014] Advantageously, the inventive procedure completely avoids critical mechanical stresses in the plastic encapsulations. The busbars are bent at the positions specified by the predetermined bending point, whereby the force required for bending can be set to almost zero. This completely eliminates the risk of cracking and delamination (adhesive failure) between the respective busbar and the respective plastic encapsulation, which is typically feared in the prior art.

[0015] Preferably, the laser used to melt the upper busbar is used, which is used in step d) to weld the upper and lower busbars. This also allows the process to be implemented particularly cost-effectively.

[0016] Preferably, a force component is aimed for that lies between the threshold value and 20% below this threshold value, since excessive melting of the upper busbar must be avoided. An excessively large melted area leads to material weakening and thus to an undesirable negative influence on conductivity. Therefore, a range is determined for the at least one laser parameter that, on the one hand, enables bending of the upper busbar with the least possible force, but, on the other hand, prevents excessive melting of the upper busbar.

[0017] Preferably, the melting in step f) is carried out by means of a laser, wherein at least one laser parameter, in particular the laser power and / or the laser beam width and / or the laser wavelength and / or the laser feed rate, is adjusted such that a region of the upper busbar is melted such that the joining force to be applied in step g), which is necessary to bring the upper busbar into contact with the lower busbar over its entire surface, leads to the transfer of a force component to the lower busbar that is below the first predeterminable threshold value. If too small an area were melted, a relatively high joining force would still have to be applied to bring the two busbars into contact with one another. Accordingly, the force component transferred from the upper to the lower busbar could potentially be undesirably high.However, by appropriately dimensioning the melted area, it is possible for the force component transferred from the upper to the lower busbar to be below the first predeterminable, non-critical threshold value.

[0018] While the force acting on the lower busbar has been considered so far, it is still important to consider the force acting on the upper busbar. In this context, in step g), while the upper busbar is partially melted, the joining force applied to the upper busbar to bring the upper busbar into flat contact with the lower busbar is dimensioned such that the force acting on the upper busbar is below a second predefined threshold value. It is also important to ensure that the load acting via the upper busbar on the associated plastic encapsulation is low enough to avoid the problems mentioned in connection with the prior art at the transition between the upper busbar and its associated plastic encapsulation.

[0019] In this context, the melting in step f) is preferably carried out by means of a laser, wherein at least one laser parameter, in particular the laser power and / or the laser beam width and / or the laser wavelength and / or the laser feed rate, is adjusted such that a dimensioned area of the upper busbar is melted such that the joining force to be applied in step g), which is necessary to bring the upper busbar into flat contact with the lower busbar, results in a force acting on the upper busbar that is below the second predeterminable threshold value. Furthermore, insufficient melting would lead to an undesirably high load at the transition between the upper busbar and the plastic encapsulation, so care must be taken to ensure that a suitably large area is melted in order to avoid the problems mentioned in connection with the prior art.

[0020] In this context, the at least one laser parameter is preferably also set such that a region of the upper busbar is melted such that the joining force to be applied in step g), which is necessary to bring the upper busbar into flat contact with the lower busbar, results in a force acting on the upper busbar that is above a third predeterminable threshold value. If, as already mentioned, too large an area is melted, the resulting material outflow could lead to an undesirable change in the cross-section of the upper busbar, in particular to a reduction in the cross-sectional area, which would lead to increased line resistance and thus to partial overheating, which would, on the one hand, lead to an undesirable voltage drop and, on the other hand, would have a negative impact on the service life.

[0021] According to a preferred embodiment, the first and / or second and / or third predeterminable threshold value is determined as a function of the moment acting as a result of the joining force introduced into the respective busbar at the point where the busbar emerges from the associated plastic encapsulation. This moment depends on the lever arm, i.e. the length of the busbar and the one point at which the force is theoretically introduced into the busbar, as well as the introduced force. The greater this length, the greater the lever arm via which the introduced joining force acts on the connection between the respective busbar and the point where it emerges from the associated plastic encapsulation.

[0022] Furthermore, the first and / or second and / or third predeterminable threshold value is preferably determined as a function of the ratio of the density of the plastic encapsulation assigned to the respective busbar and the density of the material of the respective busbar. The greater the difference in density between the two materials, the less force may be transferred to the respective busbar in order to reliably prevent cracking or delamination. Polished copper is preferably used as the material for the busbar in order to achieve a particularly low line resistance. Epoxy resin, silicone, or thermoplastics, in particular polypropylene, are preferably used for the plastic encapsulation.

[0023] It has also proven advantageous if the first and / or second and / or third predeterminable threshold values are determined depending on the length of the respective busbar that runs within the associated plastic encapsulation. The longer this portion of the busbar, the better the busbar is anchored in the respective plastic encapsulation and the better it can absorb forces without cracking or delamination.

[0024] It is further preferred if the first and / or the second and / or the third predefinable threshold value is determined as a function of the width of the respective busbar. The wider a busbar is, the less effective the torsional forces are, which may occur under certain circumstances when the joining force is applied, with exit points on the plastic encapsulation where the edges of the busbars are located being particularly critical. In this context, it must be taken into account that, due to tolerances, the joining force may be introduced into the upper busbar in a non-central, i.e. non-symmetrical manner, or correspondingly, the force transmitted to the lower busbar may be introduced in a non-central or non-symmetrical manner, so that a torsional moment may occur at the exit point of the respective busbar from the associated plastic encapsulation.

[0025] The second busbar preferably runs in an arc shape and encloses an angle of more than 90°, preferably between 91° and 100°. In this context, the second busbar preferably emerges vertically from the associated plastic encapsulation. In this way, the free end of the lower busbar in the manufacturing arrangement is the highest point of the busbar. The projection of this highest point onto the upper busbar above it therefore preferably determines the location of the predetermined bending point. Due to this angle, the upper busbar must be bent downwards at the location of the predetermined bending point, which is possible if the upper side of the upper busbar is melted by the action of the laser. The area of the upper busbar facing the laser experiences the greatest expansion due to the bending, in that the area of the upper busbar that lies beyond the predetermined bending point is bent downwards to form a flat contact with the lower busbar.

[0026] The joining force is preferably applied via a spring-mounted punch. Due to the spring-mounted design, force peaks can be reliably avoided by appropriately dimensioning the spring used.

[0027] According to a preferred development, in step d), the welding depth is adjusted by appropriate operation of the laser so that only the upper busbar is welded. As a result, the upper busbar is softened by heating and the weld seam so that it is pressed onto the lower busbar by the joining force, preferably applied by the aforementioned spring-loaded punch. This ensures particularly small gaps, in particular gaps with a gap width of almost zero, between the upper and lower busbars.

[0028] Further preferred embodiments emerge from the subclaims.

[0029] The invention also encompasses combinations of the features of the described embodiments. The invention therefore also encompasses implementations that each have a combination of the features of several of the described embodiments, unless the embodiments are described as mutually exclusive.

[0030] Exemplary embodiments of the invention are described below. Shown are: Fig. 1 is a schematic diagram to explain the method according to the invention for producing a power module for an electric vehicle; Fig. 2 a schematic representation of a busbar encapsulated in a plastic potting compound, wherein an intermediate circuit capacitor is located in the plastic potting compound, to which the busbar is electrically coupled; and Fig. 3 a signal flow diagram for an embodiment of the method according to the invention.

[0031] The exemplary embodiments explained below are preferred embodiments of the invention. In the exemplary embodiments, the described components of the embodiments each represent individual features of the invention that can be considered independently of one another, each of which also further develops the invention independently of one another.

[0032] Therefore, the disclosure is intended to encompass combinations of the features of the embodiments other than those shown. Furthermore, the described embodiments can also be supplemented by further features of the invention already described.

[0033] In the figures, the same reference symbols denote elements with the same function.

[0034] Fig. 1 shows a schematic representation to explain a method for manufacturing a power module 10 for an electric vehicle. The power module 10 comprises a half-bridge inverter (not shown) that is encapsulated in a first plastic encapsulation 16. Electrically coupled to the half-bridge inverter is a first busbar 18, which, in the illustrated assembly arrangement, emerges horizontally from the plastic encapsulation 16. Arranged in a second plastic encapsulation 12 is an intermediate circuit capacitor (not shown), which is electrically coupled to a second busbar 14, which protrudes vertically from the second plastic encapsulation 12 and extends in an arc shape, wherein the arc encloses an angle α of more than 90°. The two busbars 14, 18 are made of polished copper and have a rectangular cross-section.The total length of busbar 14 is between 4 and 10 cm, and that of busbar 18 in the initial state is between 6 and 12 cm. The width (B) of the busbars is between 0.5 and 1 cm, and the height is between 0.2 and 0.5 cm.

[0035] Epoxy resin, silicone, or thermoplastics, particularly polypropylene, are preferably used as the material for the plastic encapsulation 12, 16. In general, busbars 14, 18 are preferably made of copper or a copper-containing alloy.

[0036] The plastic encapsulation 16 is arranged on a cooling device 22, wherein the half-bridge inverter is thermally conductively coupled to the cooling device 22.

[0037] The procedure is described below in summary by Fig. 1 and Fig. 3 explains: First, in a first step 100, the half-bridge inverter is encapsulated in the plastic encapsulation 16 such that the busbar 18 protrudes horizontally from the plastic encapsulation 16. Subsequently, in a second step 120, the intermediate circuit capacitor is encapsulated in a plastic encapsulation 12 such that the second busbar 14 protrudes vertically from the plastic encapsulation 12 and describes an arc that encloses an angle α of more than 90 degrees. The angle α is preferably between 91° and 100°. Steps 100 and 120 can, of course, also be performed in reverse order.

[0038] Subsequently, in a step 140, the busbar 18 is aligned with the busbar 14 such that both busbars 18, 14 are arranged one above the other. Due to design or manufacturing tolerances, the distance between the two busbars 18, 14 is either too large or too small. If the distance is too large, the two busbars 18, 14 cannot be welded with high quality; if the distance is too small, a bending moment occurs at the respective exit point of the respective busbar 18, 14 from the associated plastic encapsulation 16, 12 due to the applied joining force F1.

[0039] Therefore, according to the invention, in a step 160, a predetermined bending point 20 of the upper busbar, in the representation of the Fig. 1 of the busbar 18. Preferably, the positioning of the predetermined bending point 20 takes place at the position of the upper busbar 18 that corresponds to the projection of the uncast end of the lower busbar 14 onto the upper busbar 18. This position is melted in a subsequent step 180 using a laser.

[0040] In a subsequent step 200, while the upper busbar 18 is partially melted, a joining force F1 is applied to the upper busbar 18 such that it rests flat on the lower busbar 14, wherein the applied joining force F1 is dimensioned such that the force component F2, which is transferred via the upper busbar 18 to the lower busbar 14, lies below a first predeterminable threshold value. The force F2 is the theoretical force F2 acting at a point on the busbar 18 as a result of the joining force F1. The projection of the upper busbar 18 beyond the lower busbar 14, see in Fig. 1 the part of the upper busbar 18 designated L1) is necessary because a force cannot be introduced at a specific location on the busbar 18 and this location cannot be subjected to a laser for welding at the same time.

[0041] Subsequently, in a step 220, the upper 18 is welded to the lower busbar 14 by a corresponding operation of a laser, wherein the welding depth is adjusted such that only the upper busbar 14 is welded.

[0042] In step 240, the excess length L1, which was only required for assembly purposes to apply the joining force F1, is optionally cut off. For clarity, the length L1 is shown in the figure larger than actually required; in other words, the length L1 can also be selected to be significantly smaller than indicated in the figure.

[0043] In the presentation of Fig. 1, the dashed line shows how the upper busbar 18 bends at the predetermined bending point 20 as a result of the action of the joining force F1, so that the upper busbar 18 and the lower busbar 14 lie flat against one another. During melting in step 180, at least one laser parameter, in particular the laser power and / or the laser beam width, is adjusted such that a region of the upper busbar 14 dimensioned such that the joining force F1 to be applied, which is necessary to bring the upper busbar 18 into flat contact with the lower busbar 14, leads to the transmission of a force component F2 to the lower busbar 14 that lies below the first predefinable threshold value.Furthermore, in step 200, while the upper busbar 18 is partially melted, the joining force F1 applied to the upper busbar 18 in order to bring the upper busbar 18 into flat contact with the lower busbar 14 is dimensioned such that the force acting on the upper busbar 18 is below a second predeterminable threshold value.

[0044] In this case, at least one laser parameter, in particular the laser power and / or the laser beam width and / or the laser wavelength and / or the laser feed rate, is set such that a region of the upper busbar 18 dimensioned such that the joining force F1 to be applied in step 200, which is necessary to bring the upper busbar 18 into contact with the lower busbar 14, leads to a force acting on the upper busbar 18 which is below the second predeterminable threshold value.

[0045] It should be noted that a region of the upper busbar 18 is melted such that the joining force F1 to be applied in step 200, which is necessary to bring the upper busbar 18 into flat contact with the lower busbar 14, results in a force acting on the upper busbar 18 that is above a third predeterminable threshold value. This third predeterminable threshold value is preferably between 60 and 90% of the second threshold value.

[0046] Preferably, the first and / or the second and / or the third predeterminable threshold value are determined as a function of the moment acting on the respective busbar 18, 14 as a result of the joining force, the ratio of the density of the plastic encapsulation 16, 12 assigned to the respective busbar 18, 14 and the density of the material of the respective busbar 18, 14 as well as the length of the respective busbar 18, 14 that runs within the assigned plastic encapsulation 16, 12, and finally as a function of the width (B) of the respective busbar 18, 14.

[0047] In step 180, by adjusting the energy input, in particular the selection of laser parameters such as laser feed rate, laser power, laser beam width, laser wavelength, the temperature shortly before buckling is heated very locally in order to achieve a phase transformation in the predetermined bending point 20.

[0048] Fig. Figure 2 shows, by way of example, the busbar 14 emerging from the plastic encapsulation 12. In particular, the stresses that arise at the exit point of the busbar 14 from the plastic encapsulation 12 in the method known from the prior art are shown, which can arise with the high joining force F1 used in the prior art when the introduction is not absolutely symmetrical, as can easily occur due to manufacturing tolerances. As shown by the arrow P1, the representation of Fig. 2 at the exit point, at the right front edge of the busbar 14, a high stress in the plastic encapsulation 12, which can lead to delamination or even cracking in the plastic encapsulation 12. Such stresses are reliably avoided in the method according to the invention by the significant reduction of the joining force F1 to be used.

[0049] In this context, it should be taken into account that the resistance to detachment of the polished copper preferably used for the busbars 18, 14 is very low, so that in the prior art, detachment of the plastic encapsulation 16, 12 from the copper of the busbars 18, 14 can very easily occur, in particular at the exit point of the busbars 18, 14 from the respective plastic encapsulation 16, 12.

Claims

[1] A method for manufacturing a power module (10) for an electric vehicle, comprising the following steps: a) encapsulating a half-bridge inverter in a first plastic encapsulation (16) such that at least one first busbar (18), which has a rectangular cross-section and is electrically coupled to the half-bridge inverter, protrudes from the first plastic encapsulation (16); b) encapsulating an intermediate circuit capacitor in a second plastic encapsulation (12) such that at least one second busbar (14), which has a rectangular cross-section and is electrically coupled to the intermediate circuit capacitor, protrudes from the second plastic encapsulation (12); c) aligning at least the first and second busbars (14, 18) such that they are arranged one above the other; d) welding the upper (18) to the lower busbar (14) using a laser beam, characterized bythe following steps to be carried out before step d): e) determining a predetermined bending point (20) of the upper busbar (18); f) Partial melting of the upper busbar (18) in the area of the predetermined bending point (20); g) While the upper busbar (18) is partially melted: Applying a joining force (F1) to the upper busbar (18) in such a way that the upper busbar (18) buckles in the region of the predetermined bending point (20) so that it rests flat on the lower busbar (14), the applied joining force (F1) being dimensioned in such a way that the force component (F2) which is transmitted via the upper busbar (18) to the lower busbar (14) lies below a first predeterminable threshold value. [2] Method according to claim 1, characterized bythat the melting in step f) is carried out by means of a laser, wherein at least one laser parameter, in particular the laser power and / or the laser beam width and / or laser wavelength and / or laser feed speed, is set such that a region of the upper busbar (18) dimensioned such that the joining force (F1) to be applied in step g), which is necessary in order to bring the upper busbar (18) into contact with the lower busbar (14) in a flat manner, leads to the transmission of a force component (F2) to the lower busbar (14) which lies below the first predeterminable threshold value. [3] Method according to one of claims 1 or 2, characterized byin that in step g), while the upper busbar (18) is partially melted, the joining force (F1) applied to the upper busbar (18) in order to bring the upper busbar (18) into flat contact with the lower busbar (14) is dimensioned such that the force acting on the upper busbar (18) is below a second predeterminable threshold value. [4] Method according to claim 3, characterized bythat the melting in step f) is carried out by means of a laser, wherein at least one laser parameter, in particular the laser power and / or the laser beam width and / or laser wavelength and / or laser feed speed, is set such that a region of the upper busbar (18) dimensioned such that the joining force (F1) to be applied in step g), which is necessary in order to bring the upper busbar (18) into contact with the lower busbar (14) in a flat manner, leads to a force acting on the upper busbar (18) which is below the second predeterminable threshold value. [5] Method according to claim 4, characterized bythat the at least one laser parameter is set such that a region of the upper busbar (18) is melted such that the joining force (F1) to be applied in step g), which is necessary to bring the upper busbar (18) into flat contact with the lower busbar (14), leads to a force acting on the upper busbar (18) which is above a third predeterminable threshold value. [6] Method according to claim 5, characterized by that the first and / or the second and / or the third predeterminable threshold value is determined as a function of the moment acting as a result of the joining force introduced into the respective busbar at the exit point of the busbar (14, 18) from the associated plastic encapsulation (12, 16). [7] Method according to one of the preceding claims 5 or 6, characterized bythat the first and / or the second and / or the third predeterminable threshold value is determined as a function of the ratio of the density of the plastic encapsulation (12, 16) assigned to the respective busbar (14, 18) and the density of the material of the respective busbar (14, 18). [8] Method according to one of the preceding claims 5 to 7, characterized by that the first and / or the second and / or the third predeterminable threshold value is determined as a function of the length of the respective busbar (14, 18) which runs within the associated plastic encapsulation (12, 16). [9] Method according to one of the preceding claims 5 to 8, characterized by that the first and / or the second and / or the third predeterminable threshold value is determined as a function of the width (B) of the respective busbar (14, 18) at the point of exit from the associated plastic encapsulation (12, 16). [10] Method according to one of the preceding claims, characterized by that the second busbar (14) is curved and encloses an angle (α) of more than 90 degrees, preferably between 91 and 100 degrees. [11] Method according to one of the preceding claims, characterized by that in step d) the welding depth is adjusted by appropriate operation of the laser so that welding is carried out only into the upper busbar (18).

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