Electronic power module with potting compound covered by cover

Abstract

The invention relates to an electronic power module (12) having a circuit carrier (13) which has an upper side (14) and a lower side (15). A structured metal layer (16) is arranged on the upper side (14) of the circuit carrier (13), power semiconductors (17) being arranged on said metal layer such that the structured metal layer (16) and the power semiconductors (17) form components of an electronic power circuit. The power module (12) has a wall structure (18) having an inner side (18a) which, together with the upper side (14) of the circuit carrier (13), forms a trough (19). The circuit carrier (13) is potted with a soft potting compound (20) inside the trough (19) on the upper side (14) of the circuit carrier. A cover (21) is arranged on the potting compound (20) such that the cover (21) lies over entire surface area of the soft potting compound (20). An outer edge (22) of the cover (21) has the same contour as an inner edge of the wall structure (18).

Description

[0001] Description

[0002] Electronic power module with potting compound covered by lid

[0003] The present invention relates to an electronic power module,

[0004] - wherein the power module has a circuit carrier which has a top and a bottom,

[0005] - wherein a structured metal layer is arranged on the top side of the circuit carrier and power semiconductors are arranged on the structured metal layer, such that the structured metal layer and the power semiconductors form components of an electronic power circuit,

[0006] - wherein the power module has a wall structure with an inside which, together with the top of the circuit carrier, forms a trough,

[0007] - wherein the circuit carrier inside the trough is encased on its upper side with a soft potting compound.

[0008] The present invention further assumes a converter unit comprising at least one such power module.

[0009] Power modules of the aforementioned type and the associated converter units are generally known.

[0010] During operation of such electronic power modules, heat is generated in the power semiconductors, which must be dissipated. For this reason, a heat sink or other heat sink is usually arranged on the underside of the circuit carrier and thermally coupled to it. To optimize thermal coupling, the circuit carrier typically has an (unstructured) metal layer on its underside. Optionally, thermal paste may also be applied between the underside of the circuit carrier and the heat sink.

[0011] To minimize the contact resistance between the circuit carrier and the heat sink, the circuit carrier should be pressed against the heat sink over as large a surface area and as evenly as possible. This pressure is particularly important in the areas above which the power switches are located. Prior art methods employ spring elements, load terminals, or pressure pieces in the immediate vicinity of the power semiconductors to apply pressure to the top of the circuit carrier, thus pressing the underside of the circuit carrier against the heat sink. This approach has the disadvantage that the corresponding areas of the top of the circuit carrier must be kept clear for pressure application or at least be suitable for applying pressure. This restricts the design (layout) freedom of the electronic power circuit.

[0012] More recently, it has become possible to apply pressure directly to the power semiconductors. However, this approach requires a so-called planar interconnect technique on the power semiconductors. Therefore, it is not always applicable.

[0013] The circuit carrier within the housing is typically covered or encased on its upper surface with a potting compound. This potting compound is generally necessary to achieve sufficiently high dielectric strength. It is often a soft compound that is initially liquid and then cures to form an elastic, rubbery mass. Even in its cured state, such a soft casting compound retains considerable flexibility and cannot directly transmit forces.

[0014] It has already been implemented to encase the circuit carrier within the housing on its upper surface not with a soft, but with a hard potting compound, or so-called mold. Such a hard potting allows pressure to be applied to the surface of the potting compound and transferred over a large area to the circuit carrier. Alternatively, it is possible to apply the pressure not directly to the potting compound itself, but to the housing, which then transfers it to the potting compound. The potting compound, in turn, transfers the pressure to the circuit carrier, pressing it against the heat sink. However, the materials used to create a hard potting compound—usually epoxy resins—are carcinogenic in their raw state. Therefore, handling them is problematic.

[0015] The object of the present invention is to provide a means by which pressure can be applied to the top surface of the circuit carrier over a large area, and particularly in the area of ​​the power semiconductors, in a simple manner, despite the use of a soft potting compound. This object is achieved by an electronic power module with the features of claim 1. Advantageous embodiments of the power module are the subject of dependent claims 2 to 12.

[0016] The problem is further solved by a method for manufacturing an electronic power module with the features of claim 13. Advantageous embodiments of the method are the subject of dependent claims 14 to 15.

[0017] According to the invention, an electronic power module of the type mentioned at the outset is designed by arranging a lid on the potting compound so that the lid rests fully against the soft potting compound, and by forming an outer edge of the lid that is contour-matched to an inner edge of the wall structure.

[0018] The circuit carrier is typically designed as a substrate. The circuit carrier, including the metal layer on it, can be designed, in particular, as a DCB substrate, an AMB substrate, or an IMS substrate. As is generally known to those skilled in the art, the abbreviations DCB, AMB, and IMS stand for "direct copper bonding," "active metal bracing," and "insulated metal substrate," respectively. The metal of which the metal layer is made can, in particular, be copper.

[0019] The structured metal layer and the power semiconductors, as components of an electronic power circuit, typically implement a number of half-bridges in a converter circuit, usually either a single half-bridge or three half-bridges. A half-bridge consists of (at least) two semiconductor switches connected in series, each with a diode connected in parallel—either intrinsically or as a separate component. The two ends of the half-bridge can each be connected to a high and a low DC potential, and a junction between the two semiconductor switches can be connected to an AC potential. When multiple half-bridges are formed, the two DC potentials are generally the same for all half-bridges, while the AC potentials are specific to each half-bridge.

[0020] The wall structure typically contains electrical contact elements, which, individually or in groups, each form one of the connections for the high and low DC voltage potentials and the AC voltage potential(s), as well as connections for a control signal(s). The contact elements usually protrude beyond the wall structure on the upper side facing away from the circuit carrier. However, these details are of minor importance within the scope of the present invention. The present invention focuses solely on the formation of the trough.

[0021] Soft potting compounds are well known to professionals. They are usually silicone-based and possess permanently elastic properties.

[0022] The term "contour-identical" means that the contour of the lid is similar to the course of the inner edge of the wall structure. While the lid may have a slight distance from the inner edge, its edge always follows the inner edge of the wall structure. Therefore, the contour of the lid is not only identical to the contour of the inner edge, but also includes a contour that is formed, so to speak, by a central scaling of the inner edge's contour by a factor slightly less than 1 – in other words, it is slightly shrunken from the inner edge's contour.

[0023] In many cases, the lid forms an area smaller than the cross-section enclosed by the wall structure. In this case, when placed on the potting compound, the lid creates a gap with the wall structure. This design makes it possible, in particular, to subsequently place the lid onto the potting compound without exerting significant force.

[0024] If the lid has a surface area smaller than the cross-section enclosed by the wall structure, it is necessary to reliably prevent the soft potting compound from creeping through the gap. Two main designs are possible for this purpose, which can also be combined if necessary.

[0025] Firstly, the gap may have a maximum width of 0.3 mm, usually 0.2 mm, and particularly 0.1 mm. With such a small gap, it may be sufficient to leave it completely open. Secondly, the gap may be closed by an additional structure. This structure acts as a seal, sealing the gap regardless of its exact width. As mentioned earlier, these two designs can be combined.

[0026] The design of the additional structure can be customized as needed, provided it achieves the desired purpose: sealing the gap and thus preventing the soft potting compound from creeping through it. For example, the additional structure could be designed as a circumferential seal located on the outer edge of the lid. In this case, the lid would have a circumferential seal along its outer edge. The seal could be a separate element, injection-molded onto the lid, or foamed onto it. Alternatively, it could be a tapered section in the lid material, forming a sealing lip.

[0027] Alternatively, the additional structure can be designed as a sealing mat located on the underside of the lid facing the potting compound and projecting beyond the outer edge of the lid. In this case, the lid has a sealing mat on its underside facing the potting compound.

[0028] Alternatively, the additional structure could be designed as a connecting structure running around the outer edge of the lid, through which the lid is connected to the wall structure. In this case, the lid is connected to the wall structure via the connecting structure. This connecting structure could, in particular, be a weld or consist of adhesive.

[0029] Alternatively, the additional structure could be designed as a connecting structure running around the outer edge of the lid, through which the lid is connected to the potting compound. In this case, the lid is connected to the potting compound, not to the wall structure, via this connecting structure. This connecting structure could, in particular, consist of an adhesive.

[0030] Alternatively, the wall structure may have a circumferential groove on its inner surface, and the cover may snap into this groove. In this case, the cover forms a surface that is not smaller, but in fact slightly larger, than the cross-section enclosed by the wall structure. Snapping the cover into the groove ensures that it is held securely in place. Furthermore, the labyrinthine effect of the groove prevents the potting compound from creeping through it.

[0031] The problem is further solved by a converter unit which has at least one power module according to the invention.

[0032] The properties, features, and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more readily understandable in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawings. These drawings show, in schematic representation:

[0033] FIG 1 a block diagram,

[0034] FIG 2 shows a converter topology,

[0035] FIG 3 shows a power module in cross-section,

[0036] FIG 4 shows a power module and a cover from above,

[0037] FIGS. 5 to 9 show ways of arranging a lid on a potting compound.

[0038] FIG 10 shows a sectional view of a power module with springs,

[0039] FIG 11 shows a sectional view of a power module with brackets and

[0040] FIG 12 shows a sectional view of a power module with a screw.

[0041] According to FIG. 1, a load 1 is to be supplied with electrical energy via a converter unit 2 from an energy source 3 – in FIG. 1, energy source 3 is represented as a multiphase supply network. The converter unit 2 comprises a rectifier 4, an inverter 5, and a DC link 6 between them, typically with a DC link capacitor 7. However, the converter unit 2 could also be configured differently, for example, without a rectifier 4 if the energy source is a DC source, or as a direct converter.

[0042] FIG. 2 shows a typical configuration of the inverter 5. According to FIG. 2, the inverter 5 comprises several half-bridges 8, each having at least two semiconductor switches 9 connected in series. The semiconductor switches 9 can be, for example, IGBTs or field-effect transistors, in particular MOSFETs. A diode 10 is connected in parallel to each semiconductor switch 9. The diodes 10 can be separate components or integral parts of the semiconductor switches 9, as required. A junction 11 is located between the two semiconductor switches 9 of each half-bridge 8. An AC voltage potential can be tapped at each junction 11 and supplied to the load 1. A high and a low DC voltage potential – indicated by a plus sign and a minus sign, respectively – can be applied to the ends of each half-bridge 8.

[0043] The rectifier 4 can be constructed in the same way or differently. In particular, the rectifier 4 can alternatively include diodes instead of some or all of the semiconductor switches, and in the simplest case, consist entirely of diodes. An electronic power module 12 – shown in FIG. 3 – has a circuit carrier 13, which in turn has a top surface 14 and a bottom surface 15. A structured metal layer 16 is arranged on the top surface 14 of the circuit carrier 13. Power semiconductors 17 are arranged on the structured metal layer 16, so that the structured metal layer 16 and the power semiconductors 17 form components of an electronic power circuit. The remaining part of the electrical power circuit is implemented via electrical connecting elements (bond wires, not shown) arranged above the power semiconductors 17.The power circuit formed generally corresponds to one of the half-bridges 8 of FIG. 2 or to all three half-bridges 8 of the inverter 5 of FIG. 2. It can also correspond to half-bridges of the rectifier 4. In this case, the power semiconductors 17 correspond to one of the semiconductor switches 9 or diodes 10 of the inverter 5 or to semiconductor switches or diodes of the rectifier 4. Furthermore, the inverter unit 2 thus comprises at least one such electronic power module 12.

[0044] The power module 12 further comprises a wall structure 18. The wall structure 18 surrounds the circuit carrier 13 at its upper surface 14. The wall structure 18 can be placed on top of the circuit carrier 13 or surround the upper surface 14 radially, as required. In either case, the wall structure 18 and the upper surface of the circuit carrier 13 together form a trough 19, which is bounded by an inner surface 18a of the wall structure 18 and the upper surface 14 of the circuit carrier 13.

[0045] Within the trough 19, the circuit carrier 13 is encased on its upper surface 14 with a potting compound 20. The potting compound 20 is a soft potting compound, usually silicone-based. It exhibits permanently elastic, rubber-like properties.

[0046] A cover 21 is arranged on the potting compound 20. The cover 21 can, in particular, be placed on top of the potting compound 20. The cover 21 rests fully against the soft potting compound 20. An outer edge 22 of the cover 21 is formed according to FIG. 4 with the same contour as an inner edge of the wall structure 18. Thus, if the trough 19 is formed, as shown in FIG. 4, essentially as a rectangle with a length 11, a width b1, and rounded corners, then the cover 21 is likewise formed essentially as a rectangle with a length 12, a width b2, and rounded corners, wherein the lengths 11 and 12, the widths b1 and b2, and also the rounding of the corners are at least substantially the same. In many cases, the cover 21 will form an area that is smaller than the cross-section enclosed by the wall structure 18.In this case, when the lid 21 is placed on the potting compound 20, it forms a gap 23 with a gap width s with the wall structure 18, as shown in FIG. 3. Various configurations are also possible in this case.

[0047] As shown in FIG. 3, the gap width s can be a maximum of 0.3 mm. With such a small gap 23, even when a large pressure force F is applied to the cover 21, the pressure can be transferred via the potting compound 20 to the circuit carrier 13, the structured metal layer 16, and the power semiconductors 17 without the potting compound 20 escaping through the gap 23 over time. In this case, the cover 21 can be placed on the potting compound 20 only after the potting compound 20 has cured. Preferably, the gap width s is chosen to be as small as possible, even below 0.3 mm, for example, a maximum of 0.2 mm and particularly a maximum of 0.1 mm.

[0048] Alternatively, the gap 23 may be closed by an additional structure 24. In this case, the gap 23 may—but does not necessarily have to—have a gap width s of more than 0.3 mm. Possible embodiments of the additional structure 24 are shown in FIGS. 5 to 8.

[0049] For example, as shown in FIG. 5, the additional structure 24 can be designed as a seal 25, which is arranged at the outer edge 22 of the cover 21 and extends along the outer edge 22. In this case, too, it is possible to place the cover 21 onto the potting compound 20 only after the potting compound 20 has cured.

[0050] Alternatively, as shown in FIG. 6, the additional structure 24 can be designed as a sealing mat 26, which is arranged on the underside of the lid 21 and projects beyond the edge 22 of the lid 21. In this case, the sealing mat 26 should project at least to the inner side 18a of the wall structure 18, and possibly even slightly beyond. This projection is possible because the sealing mat 26 is flexible enough to be bent upwards when the lid 21 is placed on top. The underside of the lid 21 is the side of the lid 21 that faces the potting compound 20. In this case as well, it is possible to place the lid 21 onto the potting compound 20 only after the potting compound 20 has cured. The sealing mat 26 can be a full-surface layer, as required, or, as shown in FIG. 6, it can be designed as a relatively wide strip that runs along the edge 22 of the lid 21.Another possibility is to design the additional structure 24 as a connecting structure 27 that runs around the outer edge 22 of the cover 21. In this case, the cover 21 can be connected via the connecting structure 27 either to the wall structure 18 as shown in FIG. 7 or to the potting compound 20 as shown in FIG. 8. In the embodiment shown in FIG. 7, the cover 21 is placed onto the potting compound 20 directly after the potting compound 20 has been poured or somewhat later, but generally before the potting compound 20 has completely cured. The cover 21 can be welded or bonded to the wall structure 18 via the connecting structure 27, for example. In the case of welding, laser welding is particularly suitable. In the embodiment shown in FIG. 8, the cover 21 is generally placed onto the potting compound 20 only after the potting compound 20 has completely cured.For example, as a connecting structure 27, a so-called bead of adhesive can first be applied to the (already cross-linked) potting compound 20 and then the lid 21 can be placed on top.

[0051] FIG. 9 shows another possibility for sealing the cover 21 against the wall structure 18. According to FIG. 9, the wall structure 18 has a groove 28 on its inner side 8a. The groove 28 is circumferential. In the embodiment of FIG. 9, the cover 21 is snapped into the groove 28. The edge 22 of the cover 21 is therefore located in the groove 28.

[0052] As an alternative to subsequently placing the lid 21 onto the potting compound 20, it may even be possible to place the lid 21 onto the trough 19 before pouring the potting compound 20. In this case, the lid 21 must be secured, for example by gluing. Furthermore, an opening must remain through which the potting compound 20 can be poured into the area below the lid 21. The potting compound 20 must be sufficiently fluid in this case. Finally, the opening must be sealed afterwards.

[0053] The present invention offers many advantages. In particular, despite the use of a conventional, "problem-free" soft potting compound 20, it is possible to exert pressure on the circuit carrier 13 and the elements arranged on the circuit carrier 13 (structured metal layer 16, power semiconductors 17) via the potting compound 20, starting from the cover 21. This also applies when the cover 21 is fixed with respect to the wall structure 18 – for example, in the embodiments shown in FIGS. 7 and 9. Even then, the pressure force F can still be exerted on the central area of ​​the cover 21, which is transmitted as a quasi-hydrostatic pressure to the entire potting compound 20. The pressure force F acts not only alongside the power semiconductors 17 but also directly on them. This results in good contact and thus a low thermal resistance. Furthermore, a conventional soft potting compound 20 can still be used.The electrical connection of the power semiconductors 17 is still possible via conventional bond wires. The freedom for the circuit layout is not restricted in the power module 12 according to the invention. The exact location where the pressure force F is exerted on the cover 21 is irrelevant because the pressure is distributed evenly over the potting compound 20. The possibility of improving the contact with the heat sink by pre-bending the circuit carrier 13 remains.

[0054] FIG. 10 shows a sectional view of a power module 12 with springs 30. The springs 30 are, for example, designed as coil springs, leaf springs, or flat springs and are arranged between a surface 32, in particular a flat surface, of a pressure structure 32. The pressure structure 32 can, for example, be part of a housing. The pressure force F is transmitted to the cover 21 via the springs 30. In particular, the springs 30 are clamped in at least a partially compressed state between the cover 21 and the pressure structure 32 to generate a permanent contact pressure. The further embodiment of the power module 12 in FIG. 11 corresponds to that in FIG. 2.

[0055] FIG. 11 shows a sectional view of a power module 12 with brackets 36. The brackets 36 are detachably attached to the wall structure 18 and are at least partially elastic. The brackets 36 are attached to the wall structure 18 in such a way that a permanent compressive force F is generated on the cover 21, the cover 21 transmitting the compressive force F via the potting compound 20 to the circuit carrier 13, the structured metal layer 16, and the power semiconductors 17. In particular, the brackets are clamped between the wall structure 18 and the cover 21 to generate a permanent contact pressure. Alternatively, the brackets 36 can be attached outside the wall structure 18, for example, to a housing structure arranged laterally outside the wall structure 18, to generate the compressive force F on the cover 21. The further embodiment of the power module 12 in FIG. 11 corresponds to that in FIG. 2.

[0056] FIG. 12 shows a sectional view of a power module with a screw 38 which is arranged through a thread 40 in the pressure structure 32. The screw 38 presses against an elastic pressure element 42, which is connected to the cover 21. The elastic pressure element 42 is, for example, made of an elastomer and has a contact area 44 that is larger than the cross-sectional area of ​​the screw. The pressure force F is transmitted from the screw 38 to the cover 21 via the elastic pressure element 42. The elastic pressure element 42 is compressed by screwing in the screw 38 to generate a permanent contact pressure.

[0057] Although the invention has been further illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived from them by the person skilled in the art without leaving the scope of protection of the invention.

Claims

Patent claims 1. Electronic power module, - wherein the power module has a circuit carrier (13) having a top (14) and a bottom (15), - wherein a structured metal layer (16) is arranged on the top side (14) of the circuit carrier (13) and power semiconductors (17) are arranged on the structured metal layer (16), such that the structured metal layer (16) and the power semiconductors (17) form components of an electronic power circuit, - wherein the power module has a wall structure (18) with an inside (18a) which together with the top (14) of the circuit carrier (13) forms a trough (19), - wherein the circuit carrier (13) is encapsulated on its upper surface (14) within the trough (19) with a soft potting compound (20), characterized in that a cover (21) is arranged on the potting compound (20) so that the cover (21) rests fully against the soft potting compound (20), and that an outer edge (22) of the cover (21) is formed to be contour-matched to an inner edge of the wall structure (18), wherein the power module is designed such that a pressure force (F) acts on the circuit carrier (13), the structured metal layer (16) and the power semiconductors (17) starting from the cover (21) via the potting compound (20).

2. Power module according to claim 1, characterized in that the cover (21) forms an area which is smaller than the cross-section enclosed by the wall structure (18), so that when the cover (21) is arranged on the potting compound (20) it forms a gap (23) with the wall structure (18).

3. Power module according to claim 2, characterized in that the gap (23) has a width (s) of a maximum of 0.3 mm, preferably of a maximum of 0.2 mm, in particular of a maximum of 0.1 mm.

4. Power module according to claim 2 or 3, characterized in that the gap (23) is closed by an additional structure (24).

5. Power module according to claim 4, characterized in that the additional structure (24) is designed as a circumferential seal (25) arranged on the outer edge (22) of the cover (21).

6. Power module according to claim 4, characterized in that the additional structure (24) is designed as a sealing mat (26) arranged on an underside of the lid (21) facing the potting compound (20) and projecting beyond the outer edge (22) of the lid (21).

7. Power module according to claim 4, characterized in that the additional structure (24) is designed as a connecting structure (27) circumferentially around the outer edge (22) of the cover (21), via which the cover (21) is connected to the wall structure (18).

8. Power module according to claim 4, characterized in that the additional structure (24) is designed as a connecting structure (27) circumferentially around the outer edge (22) of the cover (21), via which the cover (21) is connected to the potting compound (20).

9. Power module according to claim 1, characterized in that the wall structure (18) has a circumferential groove (28) on its inside (18a) and that the cover (21) is snapped into the groove (28).

10. Power module according to one of the preceding claims, comprising at least one, in particular elastic, pressure element which is connected to the cover (21) and configured to permanently transmit the pressure force (F) to the cover (21).

11. Power module according to claim 1, wherein the at least one, in particular elastic, pressure element comprises a spring (30) and / or a bracket (36).

12. Converter unit, wherein the converter unit comprises at least one power module (12) according to any one of the above claims.

13. Method for manufacturing an electronic power module, - wherein the power module has a circuit carrier (13) having a top (14) and a bottom (15), - wherein a structured metal layer (16) is arranged on the top side (14) of the circuit carrier (13) and power semiconductors (17) are arranged on the structured metal layer (16), such that the structured metal layer (16) and the power semiconductors (17) form components of an electronic power circuit, - wherein the power module has a wall structure (18) with an inside (18a) which together with the top (14) of the circuit carrier (13) forms a trough (19), - wherein the circuit carrier (13) is encased on its upper surface (14) within the trough (19) with a soft potting compound (20), wherein a cover (21) is arranged on the potting compound (20) so that the cover (21) rests fully against the soft potting compound (20), and wherein an outer edge (22) of the cover (21) is formed to be contour-matched to an inner edge of the wall structure (18), wherein the power module is designed such that a pressure force (F) acts on the circuit carrier (13), the structured metal layer (16) and the power semiconductors (17) starting from the cover (21) via the potting compound (20).

14. Method according to claim 13, wherein the pressure force (F) is permanently transferred to the lid (21) via at least one, in particular elastic, pressure element.

15. Power module according to claim 14, wherein the at least one, in particular elastic, pressure element comprises a spring (30) and / or a bracket (36).

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