WO2016005088A1 - Semiconductor module with a spring-loaded base plate - Google Patents

Abstract

A semiconductor module (10) comprises at least one substrate (36), which supports at least one semiconductor chip (38), a base plate (12), on which the at least one substrate (36) is fastened in such a way that heat from the substrate (36) can be dissipated into the base plate (12), at least one fastening element (14) for fastening the base plate (12) to a heat sink (16), wherein the fastening element (14) is routed through the base plate (12), a spring element (22), which is pressed against the base plate (12) by the fastening element (14) when the fastening element (14) is fastened to the heat sink (16), and a plastically deformable spacer element (28), which is accommodated between the spring element (22) and the base plate (12) and which is designed to absorb some of the force which is produced between the spring element (22) and the base plate (12) when the fastening element (14) is fastened to the heat sink (16). The spring element (22) presses onto the base plate (12) at a first contact point (30) above the spacer element (28) and the spring element (22) presses against the base plate (12) at a second contact point (34) when the fastening element (14) is fastened to the heat sink (16).

Description

 Semiconductor module with spring-loaded base plate

FIELD OF THE INVENTION

 The invention relates to a semiconductor module, in particular a power semiconductor module with one or more power semiconductor chips. BACKGROUND OF THE INVENTION

 Power semiconductor modules generate heat that must be dissipated efficiently to maintain the semiconductor chips within their operating temperature. Therefore, these modules typically have a large base plate that can be mounted to a heat sink.

 Normally, thermal compound is distributed on the base plate and the module is screwed to the heat sink by means of several screws. Even relatively small torques of 5 Nm, with which the screws are screwed, cause each screw presses the base plate with up to 5000 N to the heat sink.

 Due to these high forces, high frictional forces occur between the base plate and the heat sink, and by repeated thermal expansion and contraction, the base plate can be friction welded to the heat sink. Thereafter, high thermo-mechanical stresses between the heat sink and the base plate may arise in which the base plate plastically deform and a gap between the base plate and the heat sink may arise. The cooling of the module deteriorates, which can shorten its life.

DE 10 2012 215 052 A1 proposes various solutions for reducing stresses in the base plate, such as the use of a spring washer or an elastic object between the screw head and the base plate. It is also proposed to fasten a spring piece to the edge of the base plate, which presses the base plate against the heat sink via a screw arranged outside the base plate. US Pat. Nos. 7,450,389 B2 and 7,477,518 B2 also show solutions in which spring elements press the base plate downwards.

 In JP 2005 045 105 A, the edge of the base plate is pressed by a pressing element onto the cooling element.

 All of these solutions require relatively many components and / or a relatively large amount of space.

JP 2003 243 584 A discloses a semiconductor module which has a substrate which carries a semiconductor chip. Furthermore, a base plate is provided, on which the substrate is fixed in such a way that heat can be dissipated from the substrate into the base plate. In addition, a fastening element in the form of a screw for fastening the base plate is provided on a heat sink, wherein the screw is guided by the base plate.

 The US 6,477,050 B1 and US 2006/098408 A1 each show a fastening mechanism for a heat sink, wherein a plastically non-deformable spacer is provided which is received between a leaf spring and a base plate, wherein the spacer element serves as a stop so that the leaf spring does not span and thus not overloaded.

 US 2014/124915 A1 discloses a semiconductor module in which a heat dissipation plate is deformed so that it is convex in one direction.

 DE 102006008807 A1, EP 2003691 A2, JP 20041 1 1645 A, JP 2013214566 A and JP 201 1096758 A each indicate generic semiconductor modules and associated attachments.

SUMMARY OF THE INVENTION

 It is an object of the invention to provide a low-cost and compact semiconductor module with a long service life.

 This object is solved by the subject matter of the independent claims. Further embodiments of the invention will become apparent from the dependent claims and from the following description.

 The invention relates to a semiconductor module which can carry, for example, the semiconductor switches of a half-bridge circuit. The semiconductor chips that provide these semiconductor switches may be power semiconductor chips configured to process currents greater than 10 A and / or voltages greater than 1000 V. The semiconductor module may, for example, be an IGBT module in which semiconductor chips carry one or more IGBTs.

According to one embodiment of the invention, the semiconductor module comprises at least one substrate carrying at least one semiconductor chip, a base plate on which the Substrate is mounted such that heat from the substrate into the base plate is dissipated, at least one fastener for securing the base plate to a heat sink, said fastener is passed through the base plate (for example, through an opening such as a slot or a bore), and a spring member which is pressed by the fastener against the base plate when the fastener is attached to the heat sink. In this way, the heat sink can be brought into thermal contact with the semiconductor module or the base plate in a planar manner.

 Further, the semiconductor module comprises a plastically deformable spacer element which is received between the spring element and the base plate and which is designed to receive a part of the force which arises when fastening the fastening element on the heat sink between the spring element and the base plate, wherein the spring element in a presses first contact point on the spacer on the base plate and presses the spring element (for example by means of a spring arm) at a second contact point against the base plate when the fastener is attached to the heat sink.

 The spacer may, for example, surround the fastener annular and / or be made of plastic. For example, the spacer may be a plastic washer. Above all, the spacer element can have a shape such that it is pressed against the base plate only at the first contact point. At the second contact points, the spacer element is not present and / or the spring element touches the base plate directly. By the relaxation (plastic deformation) of the spacer, the force of the base plate can be adjusted to the heat sink, so that although a good thermal contact is present, but the above problems (such as friction welding between the base plate and the heat sink) or not only diminished occur.

 In addition, the number of fasteners and the number of holes in the heat sink (which may be threaded in the case of screws) can be reduced, which can reduce the complexity of the assembly and hence the cost of the semiconductor module.

According to one embodiment of the invention, a thickness of the spacer element and a spring action of the spring element are coordinated such that a force on the second contact point is less than 1 .000 N (for example less than 500 N). In this way it can be achieved that even when the fastener (initially) exerts a high force on the base plate, the force is predefined at the second contact point. The base plate remains in good thermal contact with the heat sink, reducing the risk of friction welding. According to one embodiment of the invention, the spacer element is plastically deformable such that a force on the first contact point after fastening of the fastening element drops below 50% (for example below 25% or below 10%) of the original force. In addition to the lower force on the second contact point can be reduced to this, the force on the first contact point, even if, for example, a screw as a fastener firmly (or too tight) is tightened.

 The plastic deformation usually takes some time. After fixing, a plastic spacer will still transfer the original force for some time. Only with time, the tension in the spacer element decreases, because the plastic material softens by plastic deformation and / or flows away. For example, the force may have dropped below 50% after one week at less than or equal to 100 ° C.

 In this way, the fastener can first press the base plate arbitrarily strong against the heat sink. Due to the thickness of the spacer element, the force is reduced to the second contact point and the plastic deformation reduces the force at the first contact point. This leads to defined spring forces on all contact points, without the contact force of the fastener would have to be adjusted or adjusted exactly. In particular, the torque does not need to be precisely controlled when fastening a screw.

 According to one embodiment of the invention, the spring element comprises or is a leaf spring. The spring element may be a curved and / or rectangular metal strip, which develops a spring action, for example, when deformed into a planar shape.

 According to one embodiment of the invention, the spring element is pressed against the base plate at a first, middle contact point and presses at two ends against two further, second contact points against the base plate, so that the base plate against the central contact point and the two further contact points (34) against the heat sink is pressed. By the fastener (for example, a screw), the spring element is pressed in the middle and at its ends against the base plate. Thus, with only one fastener, the base plate and the heat sink disposed thereunder can be acted upon by three contact points with force.

 According to an embodiment of the invention, the semiconductor module further comprises a housing which is fixed on the base plate and covers the substrate. Such a housing may also carry the electrical contacts of the semiconductor module.

According to one embodiment of the invention, a spacer element between the spring element and the base plate is provided by the housing. The housing can for example, be cast from plastic and / or the spacer be made in one piece with the housing. Such a spacer can be provided without additional effort and additional costs.

 According to one embodiment of the invention, the housing has a (rectangular) receptacle in which the spring element is accommodated. For example, the housing may have a box-shaped basic shape, which covers the substrate in the middle and has on one side a (for example rectangular) receptacle, in which the spring element is inserted.

 In addition, the spring element is secured by the spacer element of the housing from falling out of the receptacle. In this way, the housing can be equipped with the spring element and transported together with it, before it is fastened together with the substrate and the base plate on the heat sink.

 According to one embodiment of the invention, the spring element has an opening (approximately a hole or a slot) through which the fastener leads. When the spring element is secured in the receptacle in the housing, the spring element can secure the fastener before it is screwed to the heat sink.

 According to one embodiment of the invention, the first contact point is arranged between two corners of the base plate and / or the second contact point is arranged at one of these corners. The base plate may for example be rectangular and be pressed at its two corner pairs of one spring element down. In this way, the base plate with only two fasteners and, for example, two leaf springs can be pressed flat against the heat sink.

 According to one embodiment of the invention, the semiconductor module comprises two arrangements of a fastening element and a leaf spring, which press the base plate on opposite sides against the heat sink. Each of these assemblies may be received on opposite sides of the housing in the housing.

According to one embodiment of the invention, the fastening element comprises or is a screw which has a screw head and a thread fastened therein.

According to one embodiment of the invention, the screw on a screw head on a corrugation, which is designed to prevent the screw solves independently. This corrugation on the underside of the screw head can produce a higher friction between the screw head and spring element, which prevents the screw can turn independently despite lower pressure force. According to one embodiment of the invention, the screw and / or the spring element have a ratchet mechanism which is designed to prevent the screw from coming loose on its own. The corrugation on the screw head can be provided with asymmetrical ramps in the direction of rotation, which can engage in corresponding ramps on the spring element. The spring element can thus serve as a locking mechanism for the screw.

BRIEF DESCRIPTION OF THE FIGURES

 Embodiments of the invention will now be described in detail with reference to the accompanying drawings.

 Fig. 1 shows a cross section through a semiconductor module according to an embodiment of the invention prior to assembly.

 Fig. 2 shows a cross section through the semiconductor module of FIG. 1 directly after tightening.

 Fig. 3 shows a cross section through the semiconductor module of FIG. 1 after some time, when the voltage in the spacer has largely degraded.

 4 shows a perspective cross section through a semiconductor module according to an embodiment of the invention.

 FIG. 5 shows a perspective view of the semiconductor module from FIG. 4.

 6 shows a perspective view of the housing of the semiconductor module from FIG. 4.

 7 shows a perspective view of a blocking mechanism for a semiconductor module according to an embodiment of the invention.

 The reference numerals used in the figures and their meaning are listed in summary form in the list of reference numerals. Basically, identical or similar parts are provided with the same reference numerals.

DETAILED DESCRIPTION OF EMBODIMENTS

 FIGS. 1 to 3 show a semiconductor module 10 having a base plate 12 secured to a heat sink 16 by a fastener 14 in the form of a screw. The base plate 12 is a flat flat (for example, cuboid) body which rests flat on the heat sink 16. The screw 14 leads through an opening 13 in the base plate 12th

The screw 14 is screwed with a thread into a corresponding internal thread in an opening 18 in the heat sink 16. Between a head 20 of the screw 14 and the base plate 12 is a spring element 22 having an opening 24 through which guides the body 26 of the screw 14 with the thread. The head 20 of the screw 14 presses the spring element 22 against the base plate 12th

 The spring element 22 may be a leaf spring, i. an elongated, rectangular strip of elastic material, such as metal. In its relaxed 5 ground state, the spring element 22 is bent.

 Between the spring element 22 and the base plate 12 is a spacer element 28, for example a plastic washer, which surrounds the screw body 26.

 Fig. 1 shows the spring element 22 in the unloaded state and between the o spacer 28 and the spring element 22 is a gap visible.

 In Fig. 2, the state of the semiconductor module after the complete screwing of the screw 14 (with, for example, 5 Nm torque) is shown. The screw head 20 presses the spring element 22 in its center against the spacer 28 and the spacer presses the base plate 12 at a first, 5 middle contact point 30 against the heat sink 16. This creates, for example, a force of about 5,000 N between the spring element 22 and the base plate 12 and the heat sink 16.

 Due to the spring action of the spring element 22, the two ends 32, which are located remote from the spacer element 28 and contact the base plate 12 directly, are pressed against the base plate 12 at two further, second contact points 34.

 The spring action of the spring element 22 and the thickness of the spacer 28 are, for example, coordinated so that the ends 32 press at about 500 N at the contact points 34 on the base plate 12. The remaining approximately 4,000 N act on the central contact point 30 via the spacer 28th

Since the spacer 28 is plastically deformable, it begins to deform by the force effect, whereby the force effect at the central contact point 30 between the spring element 22 and the base plate 12 and the heat sink 16 decreases.

This is shown in Fig. 3, where, for example, after about 15 minutes, the force at the central contact point 30 has dropped to about 200 N (for a spacer element 28 made of plastic to about 20 N / mm ² ). The force at the middle contact point 30 has dropped to a low value, while the force at the contact points 34 has assumed a value defined by the spacer element and the spring element 22. In this way, the base plate 22 is pressed against three contact points 30, 34 with a defined force against the base plate 12.

For the sake of completeness, FIG. 3 shows that a substrate 36 which carries semiconductor chips 38 is fastened on the base plate 12. The semiconductor chips 38 may carry, for example, IGBTs. 4 shows a cross section through a semiconductor module 10, in which a housing 40 covers the substrate 36 with the chips 38 in the manner of a box. On the housing electrical contacts 42 are provided.

 The semiconductor module 10 of FIG. 4 has the same pressing mechanism as that of FIGS. 1 to 3, wherein the spacer 28 is provided by the housing 40.

 In the semiconductor module 10 are two leaf springs 22 with two screws 14 on opposite sides of the base plate 12. The screws 14 and the central contact points 30 are located in the middle of the respective side, the ends 32 of the spring element 22 and the outer contact points are located at the corners of each page.

 The housing 40 has receptacles 44 which serve to receive the leaf springs 22. Each receptacle 44 has the same cross section as the outer shape of the spring element 22 (for example, rectangular) to guide the spring element 22. The receptacles 44 are attached to opposite sides of the housing 40.

Next, the housing 40 has a guide or opening 46 for the screw 14, through which the screw 14 can be screwed for example.

 As is apparent from Fig. 5 and from Fig. 6, which shows the housing 40 from below, it can be seen, the spacer 28 is formed as a tab with openings, which projects from a wall of the housing 40 in the receptacle 44. The tab also prevents the spring element 22 from falling out of the housing 40 during transportation of the housing 40.

 FIG. 7 shows a ratchet mechanism which prevents the screw 14 from automatically loosening again. The screw head 20 has on the spring element 22 side facing asymmetrical ramps 48 which can slide on corresponding ramps 48 on the spring element 22 when the screw 14 is tightened, but lock against each other when the screw 14 is rotated in the opposite direction.

The structure with the ramps in the spring element 22 can be embossed, for example.

 Alternatively, it is also possible that only one (symmetrical) corrugation 48 is provided on the screw head 20. Here, the spring element 22 may also have the ramps 48, against which the corrugation can lock when the screw is rotated in the release direction.

In addition, it should be understood that "comprising" does not exclude other elements or steps and "a" or "an" does not exclude a plurality. "Further, it should be noted that features or steps described with reference to one of the above embodiments also in combination with others Features or steps of other embodiments described above can be used. Reference signs in the claims are not to be considered as limiting.

LIST OF REFERENCE SIGNS

10 semiconductor module

 12 base plate

 13 opening in base plate

 14 fastening element (screw)

16 heat sinks

 18 opening in the heat sink

 20 screw head

 22 leaf spring

 24 opening in the leaf spring

26 screw body

 28 spacer element

 30 first, middle contact point

32 end of the leaf spring

 34 second, outer contact point

36 substrate

 38 semiconductor chip

 40 housing

 42 electrical contact

 44 recording

 46 leadership

 48 ribbing / ramps

Claims

A semiconductor module (10) comprising:  at least one substrate (36) carrying at least one semiconductor chip (38);  a base plate (12) on which the at least one substrate (36) is mounted so that heat from the substrate (36) into the base plate (12) can be dissipated;  at least one fastener (14) for securing the base plate (12) to a heat sink (16), the fastener (14) being passed through the base plate (12);  a spring member (22) urged by the fastener (14) against the base plate (12) when the fastener (14) is attached to the heat sink (16); and  a plastically deformable spacer member (28) received between the spring member (22) and the base plate (12) and adapted to carry a portion of the force applied when securing the fastener (14) to the heat sink (16) between the spring member (16); 22) and the base plate (12) is formed to receive;  wherein the spring element (22) presses on the base plate (12) at a first contact point (30) via the spacer element (28) and pushes the spring element (22) against the base plate (12) at a second contact point (34) when the fastening element (14) is attached to the heat sink (16). A semiconductor module (10) according to claim 1,  wherein a thickness of the spacer element (28) and a spring action of the spring element (22) are coordinated with one another such that a force on the second contact point (34) is less than 1 000 N; and or,  wherein the spacer element (28) is plastically deformable such that a force on the first contact point (30) after fastening of the fastening element (28) falls below 50% of the original force. 3. The semiconductor module (10) according to claim 1 or 2,  wherein the spacer element (28) surrounds the fastening element (14) in an annular manner. 4. Semiconductor module according to one of the preceding claims, wherein the spacer element (28) is made of plastic. 5. Semiconductor module according to one of the preceding claims, wherein the spring element (22) comprises a leaf spring. Semiconductor module according to one of the preceding claims,  wherein the spring element (22) is pressed against the base plate (12) at a middle contact point (30) and presses against the base plate (12) at two ends at two further contact points (34), so that the base plate (12) is pressed against the middle plate Contact point (30) and the other two contact points (34) is pressed against the heat sink. Semiconductor module (10) according to one of the preceding claims, further comprising:  a housing (40) mounted on the base plate (12) and covering the substrate (36). A semiconductor module (10) according to claim 7,  wherein a spacer (28) is provided between the spring member (22) and the base plate (12) from the housing (40). A semiconductor module (10) according to claim 8,  wherein the housing (40) has a receptacle (44) in which the spring element (22) is received;  wherein the spring element (22) is secured by the spacer element (28) from falling out of the receptacle (44). Semiconductor module (10) according to one of the preceding claims,  wherein the spring element (22) has an opening through which the Fixing element (28) leads. Semiconductor module (10) according to one of the preceding claims, wherein the first contact point (30) is arranged between two corners of the base plate (12) and the second contact point (34) is arranged at one of these corners. 12. The semiconductor module (10) according to one of the preceding claims, wherein the semiconductor module (10) comprises two arrangements of a fastening element (14) and a spring element (22) which press the base plate (12) on opposite sides against the heat sink (16). 13. The semiconductor module (10) according to one of the preceding claims,  wherein the fastener (14) comprises a screw. 14. The semiconductor module (10) according to claim 13,  the screw (14) on a screw head (20) having a corrugation (48) designed to prevent the screw (14) from coming loose on its own; and or  wherein the screw (14) and / or the spring element (22) comprise a ratchet mechanism designed to prevent the screw (14) from coming loose on its own. 15. Semiconductor module according to one of the preceding claims,  wherein the semiconductor module is an IGBT module.