Abstract:
A connecting device for power semiconductor modules with compensation for mechanical stresses includes a sleeve connected to a substrate and having a region with a given very small diameter. A wire pin is provided for insertion into the region of the sleeve during operation to form an electrical connection for a board. The wire pin has a diameter greater than the given diameter for clamping the wire pin upon insertion in the region. Axial freedom of movement of the wire pin in the sleeve makes it possible to avoid mechanical stresses resulting from different material characteristics when a temperature change takes place.

Description:
BACKGROUND OF THE INVENTION 
     Field Of The Invention 
     The invention relates to a connecting device for power semiconductor modules having a substrate and a housing with compensation for mechanical stresses which are caused by the effects of different material characteristics of components when a temperature change occurs. 
     Power semiconductors have been increasingly used in recent years in automobile electronics, energy management and, increasingly, for industrial drive and automation technology as well. As a rule, those power semiconductors are combined to form modules, which are matched to customer-specific requirements. 
     In such power semiconductor modules, individual electronic components are generally soldered to a ceramic substrate. In some cases, the ceramic substrate is in turn soldered to a baseplate (heat sink). That ensures adequate dissipation of heat from the electronic components during operation. An example of a bipolar transistor module with an integrated gate (IGBT) is described in detail below with reference to FIG.  5  and is disclosed in an article entitled “Zuverlässigkeit von Al-Dickdraht-Bondverbindungen”, [Reliability of Aluminum Thick-Wire Bonded Connections] ISHM Conference Munich 1996, Auerbach, Schwarzbauer, Lammers, Lenninger, Sommer. 
     In order to make it possible to ensure reliable bonded connections in such a power semiconductor module, the connections have to be fixed in position by complex workpiece supports during the soldering process. That means that the production process is complex, and is subject to the influences of faults. Furthermore, when using such aluminum bonded connections, a maximum permissible current intensity is limited by a bonding wire length and a bonding wire diameter. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a connecting device for power semiconductor modules with compensation for mechanical stresses, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which makes direct contact with connections of a power semiconductor on a substrate, in order to compensate for thermally dependent mechanical stresses using simple measures. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a connecting device for power semiconductor modules, comprising a sleeve connected to a substrate of a power semiconductor module. The sleeve has a region with a given very small diameter. A wire pin is provided for insertion into the region of the sleeve during operation to form an electrical connection for a board. The wire pin has a diameter greater than the given diameter for clamping the wire pin upon insertion in the region. 
     In accordance with another feature of the invention, in order to improve insulation, the housing is formed of plastic. 
     In accordance with a further feature of the invention, the sleeve is soldered to the substrate. In this context, it is feasible for the sleeve to be soldered to the substrate together with further electronic components during one furnace run. This advantageously allows the power semiconductor module to be produced efficiently. 
     In accordance with an added feature of the invention, in order to prepare for soldering, the substrate is provided with surfaces which can be wetted and surround an etched trench disposed in the surface of the substrate. Furthermore, the substrate is printed with a paste solder. Before the furnace run or the soldering process, the sleeve is placed on the paste solder. The subsequent flowing of the paste solder during the furnace run results in surface-tension forces being produced between the surfaces which can be wetted and the sleeve. Those forces center the sleeve with respect to the etched trench. This allows the sleeve to be aligned on the substrate in a very simple manner and with very accurate position and orientation tolerances, thus further simplifying the production of the power semiconductor module. 
     As mentioned above, the wire pin is introduced into the sleeve which is connected to the substrate. The sleeve and the wire pin thus form a two-part plug system, which replaces the previously used bonded connections, that were described initially, between the substrate and the board. The wire pin is clamped firmly in the sleeve by the advantageous shaping of the sleeve. 
     In accordance with an additional feature of the invention, the wire pin can still be moved axially in the sleeve under the influence of external force. This refinement advantageously makes it possible, in a state in which an end of the wire pin that is opposite the sleeve is connected to a board and the power semiconductor module is fitted to this board, for disadvantageous mechanical stresses caused by temperature changes to be dissipated directly once again. A further advantage of this exemplary embodiment results from the fact that the wire pin can still be soldered until it is fitted on the board, since the wire pin is not introduced into the sleeve until after the furnace run. 
     A further advantage of the two-part plug system including the sleeve and the wire pin for electrical connection of the substrate to the board is furthermore that the maximum permissible current intensity between the substrate and the board is greater than with the previous bonded connections. The wire diameter and bonding wire length, which in the past have been critical parameters in terms of current capacity, are of only secondary importance with the plug system according to the invention. 
     In accordance with yet another feature of the invention, both the sleeve and the wire pin are formed of tinned copper or copper alloys, thus ensuring that these components can be soldered well. In this context, appropriate material selection is generally required for the sleeve and the wire pin in order to avoid damaging contact corrosion, such as that which occurs between iron and copper. 
     In accordance with yet a further feature of the invention, the sleeve includes a bottom section and a casing section, with the casing section in the simplest case having an essentially cylindrical shape, for example. This ensures that the wire pin is clamped adequately even without any need to comply with exact tolerances on the cylinder diameter. In order to improve the capability of inserting the wire pin, it is conceivable for the wire pin to be provided on its end section with a chamfer, which is used as an insertion incline. The two-part plug system mentioned above can thus advantageously be ensured even by this simple exemplary embodiment. 
     In accordance with yet an added feature of the invention, the casing section of the sleeve has a funnel shape, thus further simplifying the insertion of the wire pin. As an alternative thereto, in this exemplary embodiment, insertion inclines can also be provided on the wire pin. When configured in the shape of a funnel, the casing section has arms between which slots are formed for the situation where the substrate is subjected to a washing process after soldering. In this case the slots advantageously ensure that flux residue can escape from the interior of the sleeve, and can thus be removed without leaving any residue. 
     In accordance with a concomitant feature of the invention, the housing of the power semiconductor module is filled with silica gel to insulate the components. The slots mentioned above in this case allow air to escape from the silica gel in the region of the sleeve, thus ensuring reliable insulation. 
     The connecting device according to the invention is assembled by placing at least one sleeve on a paste solder which has been applied to the substrate, by heating the substrate together with the sleeve in order to solder the sleeve to the substrate, and by introducing a wire pin into the respective sleeve. An electrical connection is produced between the power semiconductor module and a board through the wire pin. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a connecting device for power semiconductor modules with compensation for mechanical stresses, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention as well as within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, partly broken-away, side-elevational view of a power semiconductor module according to the invention, which is connected to a board; 
     FIG. 2 is an enlarged, fragmentary, partly-sectional view of a sleeve connected to a substrate and of a wire pin of the power semiconductor module shown in FIG. 1; 
     FIG. 3 is an elevational view of a star-shaped development of the sleeve used in FIG. 2; 
     FIG. 4 a  is a sectional view of another exemplary embodiment of a sleeve connected to the substrate; 
     FIG. 4 b  is an elevational view of a development of the sleeve shown in FIG. 4 a ; and 
     FIG. 5 is a sectional view of a power semiconductor module according to the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawings in detail and first, particularly, to FIG. 5 thereof, there is seen a lateral sectional view of a prior art bipolar transistor module with an integrated gate (IGBT). As can be seen from this view, such a power semiconductor module  15  has silicone chips  19 , which are soldered onto a ceramic substrate  17 . The ceramic substrate  17  is in turn soldered to a baseplate  18 . The silicone chips  19  are electrically connected to one another through the use of aluminum wires  20 . Furthermore, pins  16  are passed outward on the sides of a housing  22 , and are soldered to a non-illustrated board during a subsequent assembly process. Additionally, the components of the module are potted by using a silica gel  24  for electrical insulation. The housing  22  of the power semiconductor module  15  is closed by a cover  23 . 
     Furthermore, aluminum bonded connections  21  are provided in the module  15  in order to ensure an electrical connection between the silicone chips  19  through the pins  16  and the board. In this case, ends of these bonded connections are each soldered firstly to the silicone chips  19  and secondly to balcony-like sections of the pins  16 . As can be seen in FIG. 5, the aluminum bonded connections  21  assume a curved loop shape. Such so-called expansion loops or compensation sections are required in order to avoid disadvantageous mechanical stress influences, which occur due to different material characteristics of the components, when a temperature change occurs. 
     FIG. 1 is a partially broken-away illustration of power semiconductor module  1  according to the invention, having a substrate  4  and a housing  6 . In the broken-away region, it can be seen that at least one sleeve  2   a,    2   b  is connected to the substrate  4 . In this exemplary embodiment, the sleeve  2   a,    2   b  is preferably soldered to the substrate  4 . Furthermore, at least one wire pin  3  is introduced into each of the sleeves  2   a,    2   b  in such a manner that it extends approximately at right angles to a surface of the substrate  4 . In this case, another free end of the wire pin  3 , which has not been introduced into the sleeve  2   a,    2   b,  is used for electrical connection of the substrate  4  to a board  5 . 
     In the exemplary embodiment of the invention illustrated in FIG. 1, the power semiconductor module  1  is fitted on the board  5  by connecting devices  7  on the housing  6 . In the exemplary embodiment shown herein, the free end of the wire pin  3  is preferably soldered to the board  5 . The housing  6  furthermore has the connecting devices  7  in order to ensure that the power semiconductor module  1  is fitted securely to the board  5 . In this exemplary embodiment of the invention, the connecting devices  7  preferably are plastic snap-action hooks, which are clipped into the board  5 . The housing  6  also has an upper housing surface  6 a in which at least one opening  8  is provided. The upper housing surface  6 a is preferably formed integrally with the housing  6  in this exemplary embodiment. As an alternative to this, the upper housing surface  6   a  may also include an individual element which is connected to the housing  6 . 
     As can be seen from FIG. 1, the wire pin  3  extends through the opening  8  in the upper housing surface  6   a . The wire pin  3  is passed out of the housing  6  in this way. Since the wire pin  3  extends through the opening  8  in the upper housing surface  6   a,  this furthermore ensures additional guidance for the wire pin  3 . Since the upper housing surface  6   a  in this exemplary embodiment is preferably composed of plastic in the same way as the housing  6 , this ensures reliable insulation of the above-mentioned components. Furthermore, the space between the substrate  4  and the upper housing surface  6   a  is filled with a silicone gel  14 , which ensures further improved insulation of a possibly large number of sleeves  2   a,    2   b  and wire pins  3  from one another. In this exemplary embodiment, the housing  6  is filled with the silica gel  14  in such a manner that a cavity remains between the silica gel  14  and the upper housing surface  6   a.    
     In order to assist understanding of the invention, the fundamental layout of the substrate  4  together with a sleeve  2   a  and the wire pin  3  are shown on an enlarged scale in FIG. 2, which is a lateral sectional view. The substrate  4  includes a ceramic layer  4   a,  which is held layer-wise between two copper layers  4   b.  As can clearly be seen, the copper layer  4   b  facing the sleeve  2   a  has an etched trench  9   b  which is surrounded by non-illustrated surfaces that can be wetted metallically. A bottom section  11  of the sleeve  2   a  has a central hole  9   a  corresponding to the etched trench  9   b.    
     When a paste solder  10  printed on the substrate  4  starts to flow during heating, surface-tension forces acting between the surfaces (which can be wetted) of the substrate  4  and the lower surface of the bottom section  11  of the sleeve  2  result in the central hole  9 a being automatically centered with the etched trench  9   b  provided in the copper layer  4   b.  FIG. 2 shows such centering of the central hole  9   a  with respect to the etched trench  9   b.  Furthermore, the sleeve  2   a  has individual arms  12  which extend approximately at right angles to the surface of the substrate  4 . The arms  12  of this sleeve  2   a  are bent relative to those of the bottom section  11  in such a manner that the sleeve  2   a  forms a funnel shape. Furthermore, the arms  12  are curved in the direction of a center axis  9   c  of the sleeve  2   a,    2   b  in such a manner that the sleeve  2   a  has a region  13  in the funnel shape with a very small diameter, which is slightly smaller than the diameter of the wire pin  3 . The wire pin  3  can be inserted axially into the sleeve  2   a,  with a free end of the wire pin  3  facing the sleeve  2   a  being pushed past the region  13  where the sleeve  2   a  has a very small diameter, but without coming into contact with the bottom section  11 . The diameter difference mentioned above in this case leads to the wire pin  3  being clamped in the region  13  where the diameter of the sleeve  2   a  is very small. 
     FIG. 3 shows a development of the sleeve  2   a  of FIG.  2 . It can be seen that a casing section of the sleeve  2   a  includes the arms  12  which are disposed in a star shape and extend radially outward from the edge of the bottom section  11  that surrounds the central hole  9   a.  For example, such a development could be stamped from a metal sheet or a strip without any major effort. The star-shaped arms  12  which extend radially outward from the edge of the bottom section  11  are then formed in such a manner that the sleeve  2   a  assumes the funnel shape shown in FIG.  2 . As is evident from this star-shaped development, the sleeve  2   a  has slots between the arms  12  in the funnel shape. The slots ensure that any flux residues flow out during a washing process following the soldering. The slots furthermore ensure that, when introducing the silica gel  14  for potting, air can emerge in between, so that there are no undesirable air inclusions in the solidified silica gel  14 . 
     FIGS. 4 a  and  4   b  show an exemplary embodiment of a modified sleeve  2   b.  The fundamental layout of this sleeve  2   b  corresponds to that of the sleeve  2   a.  Identical features are annotated by the same reference symbols in this case, and will not be explained once again at this point. As can be seen in the lateral sectional view of FIG. 4 a,  this sleeve  2   b  has wedge sections  13   a  in the region  13  with a very small diameter. The wedge sections  13   a  each project radially inward with respect to the center axis  9   c  of the sleeve  2   b  at an acute angle. It is thus possible for the wire pin  3  to be clamped even more reliably than with the sleeve  2   a,  but without completely eliminating the axial freedom of movement of the wire pin in this case. 
     FIG. 4 b  shows a development of the casing section of the sleeve  2   b.  This figure shows that the casing section of the sleeve  2   b  is essentially formed from four arms  12  disposed in a cruciform shape. In the same way, the sleeve  2   b  can be stamped from a metal sheet or strip, with the production process for this sleeve  2   b  being simplified overall since there are fewer arms  12 . 
     The method according to the invention is carried out by placing at least one sleeve  2   a,    2   b  on a paste solder  10  applied to the substrate  4 . The substrate  4  is heated together with the sleeve  2   a,    2   b,  for example during a furnace run, in order to solder the sleeve  2   a,    2   b  to the substrate  4 . A wire pin  3  is then introduced into the respective sleeve  2   a,    2   b,  and the power semiconductor module  1  is electrically connected to the board  5  through the wire pin  3 . 
     The method according to the invention is carried out by centering a central hole  9   a  in the bottom section  11  of the sleeve  2   a,    2   b  with respect to an etched trench  9   b,  which is disposed in the substrate  4 , during the soldering process. This effect is due to surface-tension forces which are formed during the heating of the substrate  4  together with the sleeve  2   a,    2   b.  The surface-tension forces act between the lower surface of the bottom section  13  of the sleeve  2   a,    2   b  and the non-illustrated surfaces which can be wetted and are disposed on the side of the substrate  4  facing the sleeve  2   a,    2   b.  In order to insulate what may be a large number of sleeves  2   a,    2   b  and a corresponding number of wire pins  3  from one another in a suitable manner, the space between the sleeve  2   a,    2   b  and the wire pin  3  is potted with silica gel  14  in accordance with the method according to the invention. The above-mentioned slots between the arms  12  of the sleeve  2   a,    2   b  make it possible for air to be displaced through these slots, and to emerge from the silica gel  14  during the process of potting with the silica gel  14 . 
     The present invention provides a power semiconductor module which is characterized by a simple construction and can be produced without major effort and with few method steps. The axial freedom of movement of the wire pin  3  in the sleeve  2   a,    2   b  in this case ensures that no disadvantageous mechanical stresses can build up in the module in the event of temperature fluctuations to which the power semiconductor module is generally subjected during operation.