Patent Publication Number: US-2011061849-A1

Title: Device for cooling semi-conductors

Description:
The invention relates to a device for cooling electronic structural components which includes a cooling body and a metal base plate constructed as a part of a support of the structural element, wherein the cooling body is arranged adjacent the base plate and wherein a connecting element is arranged at least over certain areas between the base plate and the cooling body. 
     Connecting elements of this type according to the prior art are typically constructed as so-called heat conducting pastes. As a rule, such heat conducting pastes contain silicon and/or graphite. It is also already known to use as connecting elements coated metal foils or non-metal heat conducting elements. Generally, such connecting elements serve for an improvement of the heat transfer between the electronic structural component, typically a power semi-conductor element, and the cooling body. 
     From the field of constructing computers, it is also already known to use heat conducting foils. Such heat conducting foils have a melting point in the range of above 58° C., typically in the range of 60° C., and they are therefore not suitable for use in the field of power electronics because in that field frequently cooling body temperatures above 100° C. are encountered. Such cooling body temperatures may lead to the discharge of molten heat conducting foils and, consequently, there is the danger of short circuits. 
     It is the object of the present invention to construct a device of the above-described type in such a way that an effective cooling of electronic structural elements is made possible even at higher cooling body temperatures. 
     In accordance with the invention, this object is met by constructing the connecting element at least over areas thereof of a metal which has a melting temperature of at least 60° C., and the connecting element is provided with a frame-like seal. 
     Because of the selection of the above-mentioned melting temperature for the metal and the use of a frame-like seal, it is possible even at higher cooling temperatures to utilize the very good heat transfer between the structural element to be cooled, the metal connecting element and the cooling body, and to still prevent the discharge of liquefied metal when the melting point of the metal connecting element is exceeded. The frame-like seal surrounds the metal connecting element and rests against a support side, on the one hand, and a cooling body, on the other hand. In this connection, it is possible to provide either a direct contact between the connecting element and the electronic structural element; in accordance with other embodiments, it is also possible to position the electronic structural element on a metal support. It is essential for effective cooling that, starting from the ambient temperature and prior to reaching a maximum operating temperature, the metal changes its state of aggregation. 
     To prevent molten metal from being discharged, it is proposed that the seal is constructed of a metal having a melting temperature above 200° C. 
     In particular, it is considered to construct the connecting element so as to be foil-like. 
     For compensating for increased volumes due to melting of the metal, it is possible to arrange at least one recess in the area of the connecting element. 
     Moreover, it is also being considered to arrange at least one material thickness reduction in the area of the connecting element. 
     It is also possible that the connecting element extends at least over areas thereof at a distance from the seal. 
     In accordance with another embodiment, it is provided that at least one recess facing the connecting element is arranged in the area of the seal. 
     Moreover, it is also possible that the seal has in a direction extending transversely of the cooling body a greater height than a thickness of the metal foil. 
    
    
     
       In the drawings, embodiments of the invention are schematically illustrated. In the drawing: 
         FIG. 1  is a partial representation of a cross-section through a support for an electronic structural element which is in contact to a cooling body through a metal connecting element; 
         FIG. 2  is a top view of a metal connecting element which is surrounded by a frame-like seal; 
         FIG. 3  is an embodiment modified in comparison to  FIG. 2 , with expansion spaces; 
         FIG. 4  is a further modified embodiment with a differently constructed expansion space. 
     
    
    
     In accordance with the embodiment of  FIG. 1 , an electronic structural element  1 , typically a chip, is positioned in the area of a support  2 , typically a module. The support  2  has a metal base plate  3  which is coupled through a connecting element  4  to a cooling body  5 . The cooling body  5  has for supporting its heat transfer to the ambient air cooling ribs  6  which extend preferably in a direction which is modified as compared to the structural element  1 . Alternatively or as a supplement to the use of cooling ribs  6 , it is also possible to use liquid cooling. In particular, it is intended to conduct an appropriate cooling medium through assigned cooling ducts. 
     In accordance with the embodiment of  FIG. 2 , the connecting element  4  is composed of a metal foil  7  which is surrounded by a frame-like seal  8 . In the mounted state illustrated in  FIG. 1 , the seal  8  is clamped between the base plate  3  and the cooling body  5  in order to ensure a necessary tightness. 
     In accordance with the embodiment of  FIG. 3 , one or more recesses  9  are arranged in the area of the metal foil  7 . The recesses  9  ensure a sufficient tightness of the frame  8  even when the metal foil  7  melts as a result of temperature influence and, as a result, a volume increase of the material of the metal foil  7  is caused. Thus, the recesses  9  constitute expansion spaces in the event of a corresponding volume increase. 
     In accordance with the embodiment of  FIG. 4 , the metal foil  7  has a smaller thickness than a height of the seal  8 . In this case, the height of the seal  8  corresponds to a distance between the base plate  3  and the cooling body  5 . The appropriate dimensioning of the seal  8  ensures that above the metal foil  7  an expansion space  10  is arranged which can compensate for the corresponding volume increases of the material of the metal foil  7  in the case of liquefication. In particular, the metal foil  7  can in a liquefied state compensate even for different distances between the base plate  3  and the cooling body  5  which are caused by the fact that the base plate  3  assigned to the module typically has an arched shape. 
     A preferred use of the metal connecting element  4  according to the invention takes place in connection with electronic structural elements  1  which are constructed as power semi-conductors. They are used in stationary as well as in mobile fields. Such mobile uses refer in particular to power electric devices or components. The improved cooling effect can be utilized for achieving different advantages. For example, it is possible to increase the power density as a result of the improved cooling effect. Also, it is possible to facilitate an operation with increased cooling agent temperatures and/or at increased ambient temperatures. Generally, it is also possible to increase the service life of the structural elements  1  by reducing the operating temperatures. 
     When the structural elements are constructed as power semi-conductors, especially considered are realizations as IGBT or MOSFET. These may be present in discrete construction or in modular construction. 
     The metal connecting elements  4  according to the invention reduce the DIE temperature by about 4 to 6° K. Also, compared to a use of heat conducting pastes, a markedly reduced assembly work is required. The reproducibility of the achieved cooling effect is improved and, as a result, quality requirements of the production process can be safely adhered to. 
     Used as material for the metal foil  7  are typically suitable alloys. The melting point can be influenced through the composition of the alloy. A typical alloy consists of indium, tin and bismuth. As necessary, indium can be replaced fully or partially by gallium. 
     Used as the material for the seal  8  can be tin, aluminum or copper or an alloy which contains one or more of the aforementioned elements. 
     A typical thickness of the metal foil  7  is about 30 micrometers. However, also usable are metal foils  7  with a thickness in the range of 20 micrometers up to 40 micrometers. A typical thickness of the seal  8  is about 60 micrometers. Basically, also usable are material thicknesses in the range of 40 micrometers up to 80 micrometers. 
     The metal foil  7  used has during assembly a solid state of aggregation. In a conventional operation, the metal foil  7  has a liquid state of aggregation and a discharge of the molten metal is prevented by the seal  8 . During each longer interruption of operation, the metal foil  7  returns into the solid state of aggregation. The changes from the liquid state into the solid state as well as from the solid state into the liquid state take place completely reversibly. 
       FIG. 5  once again illustrates the arched configuration of the base plate  3  as already described above and the limitation of the expansion space  10  resulting therefrom. After a liquefication of the metal foil  7 , the latter fills out the expansion space  10  and leads to a large area contact between the base plate  3  and the cooling body  5  which is not illustrated.