Abstract:
The power semiconductor component has a semiconductor body which is electrically supplied through a contact clip. A solder ball connects the semiconductor body to the contact clip. The contact clip has a meandering electrical supply to a solder land, into which the solder ball is inserted.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending International Application PCT/DE97/01754, filed Aug. 14, 1997, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an electrical connection for a power semiconductor component, which is formed inside a baseplate region. The assembly comprises a semiconductor body, a contact clip and a soldered joint which connects the semiconductor body to the contact clip. 
     U.S. Pat. No. 3,519,896, for example, discloses an electrical connection without any solder, by means of metal clips, for a power semiconductor component. European patent disclosure EP 0 155 473 describes the use of firmly soldered contact clips to make contact with power semiconductor modules. 
     In power semiconductor components, an electrical supply to a semiconductor body is often formed by a copper layer which is applied on a ceramic substrate. The heat that is produced in the semiconductor body during the operation of the power semiconductor component is also dissipated via the copper layer. Owing to this one-sided heat dissipation and cooling and the temperature drop across the ceramic substrate, which is only a poor heat conductor, power semiconductor components reach their operating temperature in just a few seconds. This is particularly true when they are used in modular form with, for example, a plurality of power semiconductor components being arranged alongside one another. 
     The fact that the operating temperature is reached quickly means that the power semiconductor components are subject to large numbers of alternating load cycles in many applications, that is to say to a large number of heating and cooling processes. For example, more than 105 alternating load cycles may be reached per annum. Each heating and cooling process, that is to say each alternating load cycle, causes thermomechanical stresses in the power semiconductor component and its electrical supply leads. Large numbers of alternating load cycles can thus cause severe damage in the electrical supply leads on the semiconductor body. 
     Since the fatigue in the material caused by large numbers of alternating load cycles becomes less the better the coefficient of thermal expansion of the material of the electrical supplies is matched to the coefficient of thermal expansion of the semiconductor body, that is to say the coefficient of thermal expansion of silicon, and of the ceramic substrate, molybdenum or copper-plated Invar is often used as the material for the electrical supplies, and is respectively soldered to the semiconductor body or the ceramic substrate. The use of such materials is disclosed, for example, in the European patent disclosure EP 0 432 867. However, it has been found that there is always a residual mismatch, although this may be small, between the coefficients of thermal expansion of the various materials and, furthermore, mechanical stresses are caused by non-uniform heating of solder connections on the one hand and, particularly with modular construction, of the module base on the other hand. Together with the mechanical stresses, this mismatch leads to slow fatigue in the respective soldered joints between the semiconductor body or the ceramic substrate and electrical supplies. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide an electrical connection for a power semiconductor component which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which suffers virtually no material fatigue, particularly at the soldered joints, even after large numbers of alternating load cycles. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, an electrical connection for a power semiconductor component, comprising: 
     a baseplate, a semiconductor body disposed on the baseplate, and a contact clip with a soldered joint connected to the semiconductor body; 
     the soldered joint being formed as a small-area solder point between the semiconductor body and the contact clip; and 
     the contact clip including at least one strain-relieving solder land carrying the small-area solder point, whereby the contact clip and the semiconductor body are thermally and mechanically decoupled from one another. 
     In accordance with an added feature of the invention, a meandering electrical supply connects the strain-relieving solder land to the contact clip. 
     In accordance with an additional feature of the invention, the electrical supply, the solder land, and the contact clip are formed from one and the same material. 
     In accordance with another feature of the invention, the electrical supply and the solder land are formed in a plane that is closer to the semiconductor body than the plane defined by the contact clip. 
     In accordance with a further feature of the invention, the contact clip is formed from a tri-layer bimetallic sheet or molybdenum. 
     In accordance with again an added feature of the invention, the contact clip is formed of a tri-layer bimetallic sheet with a layer sequence composed of materials A-B-A. In a preferred embodiment, the material A is copper and the material B is Invar. 
     In accordance with again an additional feature of the invention, a filling is provided between the contact clip and the semiconductor body. In accordance with a preferred embodiment, the filling is an adhesive having a coefficient of thermal expansion matched to the coefficient of thermal expansion of the contact clip and the semi-conductor body. 
     In accordance with again another feature of the invention, the baseplate comprises a copper layer supporting the semiconductor body and a ceramic substrate supporting the copper layer. 
     In accordance with again a further feature of the invention, the contact clip forms a large-area connection with the copper layer and/or the ceramic substrate at an end of the contact clip facing away from the semiconductor body so that the contact clip is heated only slightly above a temperature of the copper layer. 
     In accordance with a concomitant feature of the invention, during an operation of the power semiconductor component, the ceramic substrate assumes a temperature T 1 , the contact clip assumes a temperature T 3 , and the semiconductor body assumes a temperature T 2 , and the temperatures are related by the expression: 
     
       
         T 2 &gt;&gt;T 3 &gt;T 1 .  
       
     
     In other words, in the electrical connection according to the invention, the soldered-on contact clip and the semiconductor body of the actual power semiconductor component are thermally and mechanically decoupled. The electrical supply is no longer soldered over the entire area on the surface of the semiconductor body, but with the aid of the at least one solder land, possibly at a few small-area solder points, by means of the solder balls incorporated in the respective solder land. The connection between the solder point and the actual contact clip, namely the electrical supply to the solder point, is designed, for example, in a meandering shape and comprises a thin, flexible leg or strip, which is stamped together with the solder land out of the sheet metal of the contact clip and is bent downward into a plane that is located closer to the surface of the semiconductor body. The length and cross section of this electrical supply from the leg or strip are chosen such that the power loss remains low and such that there is only a small amount of heat dissipation between the semiconductor body and the contact clip through the electrical supply. The electrical conductivity and, above all, the thermal conductivity of the actual contact clip, which forms a connecting clip for the electrical supply via the solder land, are in contrast set to be sufficiently high that the connecting clip is heated only slightly above the temperature of a base plate, which is composed of a ceramic substrate and a copper layer, on which the semiconductor body is fitted. In this case, the temperature T 1  of the ceramic substrate, the temperature T 3  of the contact clip and the temperature T 2  of the semiconductor body are intended to be related by the above expression T 2 &gt;&gt;T 3 &gt;T 1 . 
     In consequence, even if major temperature fluctuations occur, only extremely small thermomechanical stresses are produced between the electrical supply or the contact strip and the semiconductor body or the actual power semiconductor component. By additionally filling the area or the gap between the semiconductor body and the contact clip with a material, preferably an adhesive, whose coefficient of thermal expansion is matched with the coefficient of thermal expansion of the contact clip and of the semiconductor body, any relative movement can be absorbed elastically without deformation and fatigue at the solder points. The contact clip and the meandering electrical supply are produced from a tri-layer bimetallic sheet with a “symmetrical” layer sequence. Suitable layers are, for example, copper-Invar-copper. However, a molybdenum sheet may also be used instead of these layers. The solder lands, into which the soldered balls will later be incorporated, and the electrical supply to them can be formed from such a metal sheet by etching or laser cutting. 
     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 an electrical connection for a power semiconductor component, 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 and 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 plan view of a contact clip; 
     FIG. 2 is a partial section taken through the contact clip along the line II—II in FIG. 1, with the contact clip fitted on a power semiconductor component; 
     FIG. 3 is a partial section taken through the filling for the contact clip; and 
     FIG. 4 is a partial sectional view showing a layer sequence for the contact clip. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a contact clip  1 , from which, in regions  2 , electrical supplies  3  to solder lands  4  are formed by etching or laser cutting. 
     With reference to FIG. 2, the electrical supply  3  and the solder land  4  are located “underneath” the plane of the actual contact clip  1 . This can be attained by an appropriate bending process after etching or laser cutting. 
     The contact clip  1 , which represents a connecting clip for the electrical supply  3 , the electrical supply  3 , and the solder land  4  are produced from the same material and are composed, for example, of molybdenum or they comprise a tri-layer bimetallic sheet with a symmetrical layer sequence. This avoids any bimetallic effect with bending to one side. One such exemplary layer sequence is shown in FIG.  4 . The sequence comprises a copper layer  5 , an Invar layer  6 , and a further copper layer  7 . Invar is chosen since it has a particularly low coefficient of thermal expansion. 
     Referring again to FIG. 2, the contact clip  1  is passed between a copper layer  8  and a semiconductor body  9 . The copper layer  8  is applied, for example using the DCB technique (DCB=direct copper bonding), onto a ceramic substrate  10 , preferably composed of aluminum oxide, and is interrupted by a gap  11 . The “left-hand” part of the copper layer  8  thus forms an electrode  12  for the semiconductor body  9 . The gap  11  may be produced, for example, by etching, once the copper layer  8  has been applied onto the ceramic substrate  10 . 
     In the exemplary embodiment in FIGS. 1 and 2, two regions  2  are each provided with a solder land  4 . A solder ball  14  is introduced into the opening  13  in the solder land  4  in order to make contact with the semiconductor body  9 , so that there are two small-area solder points here. 
     The electrical supply  3  is designed in a meandering shape for strain relief. It forms a thin, flexible leg or strip, whose length and cross section are designed such that the power loss and heat dissipation from the semiconductor body  9  to the contact clip  1  remain low. 
     At its end  15  facing away from the semiconductor body  9 , the contact clip  1  is connected to the copper layer  8 , which is separated by the gap  11  from the electrode  12 . The contact clip  1  can thereby be bent in a U-shape to provide further strain relief. 
     The ceramic substrate  10  and the copper layer  8  form a baseplate  16  for the semiconductor body  9  and the contact clip  1 , that is to say for the actual power semiconductor component. The electrical conductivity and, above all, the thermal conductivity of the contact clip  1  to the copper layer  8  and to the baseplate  16  are chosen to be sufficiently high that the contact clip  1  is heated only slightly above the baseplate temperature and that a temperature T 1  of the baseplate  16  or of the copper layer  8  (in the region outside the electrode  12 ), a temperature T 3  of the contact clip  1  and a temperature T 2  of the semiconductor body are related by the following expression: 
     
       
         T 2 &gt;&gt;T 3 &gt;T 1 .  
       
     
     As has already been mentioned above, two solder points are provided in the present exemplary embodiment. However, more than two solder points and thus regions  2  with electrical supplies  3  and solder lands  4  can also be used. In special cases, even only one solder point is sufficient, provided the power loss in the single electrical supply remains sufficiently low. 
     As FIG. 3 shows, a filling  17  can be introduced into the area  18  between the contact clip  1  and the semiconductor body  9 . This filling  17  is composed of an adhesive whose coefficient of thermal expansion is matched to the coefficient of thermal expansion of the semiconductor body  9  and of the contact clip  1 . One suitable adhesive, for example, is a conventional flipchip filler. This allows any relative movement to be absorbed elastically, without deformation and fatigue of the solder points. 
     The invention thus provides an electrical connection for a power semiconductor component, which exhibits virtually no material fatigue, particularly at the solder points or soldered joints, even after large numbers of alternating load cycles.