Abstract:
A power module (10) having a leadframe (20), a power semiconductor (30) arranged on the leadframe (20), a base plate (40) for dispersing heat generated by the power semiconductor (30) and a potting compound (50) surrounding the leadframe (20) and the power semiconductor (30), that physically connects the power semiconductor (30) and/or the leadframe (20) to the base plate (40).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a National Stage application of International Patent Application No. PCT/EP2016/051875, filed on Jan. 29, 2016, which claims priority to German Patent Application No. DE 10 2015 102 041.6, filed on Feb. 12, 2015, each of which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a power module having a leadframe, a power semiconductor arranged on the leadframe and a potting compound surrounding the leadframe and the power semiconductor. 
       BACKGROUND 
       [0003]    Power semiconductors are conventionally surrounded by plastics, that is to say organic potting compounds, to protect them against environmental influences on the one hand and to insulate them electrically on the other hand. To this end, the plastics are partly enriched with fillers that are to fulfill certain functions—for example potting compounds are enriched with fillers for adjusting a specific thermal-expansion behaviour. 
         [0004]    Despite the enrichment of the organic potting compounds with fillers having a good thermal conductivity, the increase in thermal conductivity that is achieved in the potting compound is disappointing. 
         [0005]    For this reason, heat dissipation of the semiconductor is predominantly attempted via the so-called thermal stack, that is to say via a good thermally-conducting connection of the power conductor to a substrate and via its good thermal connection to a base plate or a heatsink. 
         [0006]    The electric insulation capability of the thermal stack is ensured in some case via ceramic support layers in the substrate or in other cases via special electrically insulating organic heat-conducting films. 
         [0007]    A disadvantage of this construction, and the procedure for manufacturing power modules constructed in this way, is now that on account of the great variety of the materials needed for this purpose, attention has to be paid to their different interactions among each other, e.g. chemical or also thermo-mechanical interactions. Since on account of these interactions it is often also necessary to carry out certain process steps separate from each other, or one after the other, this produces a high outlay in terms of work and also time. 
       SUMMARY 
       [0008]    The object of the invention is therefore to provide a power module having a potting compound surrounding a power semiconductor, the module being of a simple construction and ensuring good heat dissipation of the semiconductors. A further object of the invention is to provide a simple and time-saving method for producing such a power module. 
         [0009]    These objects are achieved according to the invention by the power module having a leadframe, a power semiconductor arranged on the leadframe, a floor plate for dispersing heat generated by the power semiconductor, and a potting compound which surrounds the leadframe and the power semiconductor, and which connects the power semiconductor and/or the leadframe to the floor plate, and a method for manufacturing a power module having the following steps: connecting the power semiconductor to the leadframe followed by potting the power semiconductor connected to the leadframe with a potting compound while at the same time physically connecting the base plate to the potting compound. 
         [0010]    The potting compound acts not only as a means of encapsulation; sealing the semiconductor from the outside environment, but also as an adhesive; fixing in a permanent manner the structure comprising the semiconductor and leadframe to the baseplate. 
         [0011]    The basic idea of the invention is to accomplish the potting of power semiconductors and the connection to a heatsink by one and the same material that can be processed in a single procedure. 
         [0012]    The potting compound used may be an inorganic compound. The inorganic potting compound may exhibit in particular an aqueous binding mechanism as is, for example, known for cement. As potting compounds, cements, in particular derivatives of dental cements used in dental technology, can be considered. 
         [0013]    By cement is meant an inorganic solid which results from the mixing of one or more powders with one or more liquids. Such compounds are well known from the construction industry and from dentistry where one example is a cement which uses a basic metal oxide powder mixed with and acidic liquid. An acid/base reaction occurs when mixed, forming a metal salt which acts as a binding matrix. Other formulations are also known. The resulting solid is often stable under a range of environmental changes (such as pressure, temperature, humidity or chemical) and adheres to a variety of other materials. It is therefore suitable for physically connecting those materials in a permanent manner. 
         [0014]    The potting compound, e.g. cement, can be enriched with corresponding electrically insulating, heat-conducting fillers, so that the potting compound fulfils the function of potting, heat dissipation and physical connection of a power semiconductor applied to a leadframe and the base plate. 
         [0015]    The physical-chemical properties of cements in particular make possible the purposes mentioned above. 
         [0016]    The thermal coefficient of expansion can thus be adjusted in the order of magnitude of 5 to 10 ppm/K. 
         [0017]    The electric insulation capacity of cement may be an important parameter in particular applications, such as that of high voltage semiconductors, for example those operating at or above 600 V. It may be an advantage if the electric insulation capacity of the cement is of the same order of magnitude as that of ceramic insulation plates, such as the plates used in the commonly used Direct Bonded Copper (DBC) construction comprising ceramic plates of, for example, aluminium oxide (Al 2 O 3 ), Aluminium nitride (AlN) or silicon nitride (Si 3 N 4 ). In practice, current cements may have an electric insulation capacity of an order of magnitude lower that such ceramic plates,            
         [0018]    The thermal conductivity of cement is absolutely sufficient in the case of layer thicknesses of 20 to 200 μm to bring a leadframe and a base plate into sufficient thermal contact. 
         [0019]    The base plate for dissipating heat that has been given off by the power semiconductor can be arranged to have a high thermal capacity and/or can contribute to heat spreading. Here the base plate can be made of a metal, e.g. copper (Cu) or aluminium (Al), or a so-called metal-matrix material, in particular aluminium silicon carbide (AlSiC). 
         [0020]    Over and above this, the manufacture of the potting and of the insulation of power modules having a leadframe, a power semiconductor arranged on the leadframe and a base plate for dispersing heat generated by the power semiconductor can take place in one step, so that an inorganic potting compound fulfils equally both a protective and also a heat-connecting function. 
         [0021]    According to the invention, a power module having a leadframe, a power semiconductor arranged on the leadframe, a base plate for dispersing heat generated by the power semiconductor and a potting compound surrounding the leadframe and the power semiconductor, that connects physically the power semiconductor and/or the leadframe to the base plate, can thus be provided. 
         [0022]    The potting compound is preferably an inorganic compound, in particular cement. 
         [0023]    In general, the potting compound preferably exhibits a thermal coefficient of expansion of 5 to 10 ppm/K. 
         [0024]    The construction of the power module is in particular such that the potting compound exhibits a layer arranged between the leadframe and the base plate. In this case, this layer arranged between the leadframe and the base plate preferably exhibits a thickness of 20 to 200 μm. 
         [0025]    The base plate, however, is preferably designed as a heatsink, for example as a component which interfaces with an air or water cooler. Heat generated in the power semiconductor is transferred via the leadframe to the potting compound and then to the base plate and thus to the cooler. 
         [0026]    A method designed according to the invention, for manufacturing a power module having a leadframe, a power semiconductor, and a base plate, exhibits the following steps: connecting the power semiconductor to the leadframe, and thereafter potting the power semiconductor connected to the leadframe with a potting compound while at the same time physically connecting the base plate to the potting compound. The connection of the power semiconductor to the leadframe may be achieved by a number of known techniques. These may include soldering, brazing or sintering. 
         [0027]    Here the leadframe and the base plate are arranged relative to each other such that a layer of potting compound having a layer thickness of preferably 20 to 200 μm is designed between the leadframe and the base plate, the potting compound being an inorganic compound, in particular cement. 
         [0028]    A particularly preferable design of the invention consists in that the potting compound generally exhibits a thermal coefficient of expansion of 5 to 10 ppm/K. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Using exemplary embodiments of the invention that are of particularly preferable design, that are illustrated in the drawings, the invention is explained in more detail. In the drawings: 
           [0030]      FIG. 1  shows a schematic sectional view of a power module according to the invention; and 
           [0031]      FIG. 2  shows a flow chart for illustrating the sequence of the inventive method. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  shows a schematic cross section through a power module according to the invention of particularly preferable design. The power module  10  exhibits a leadframe  20  and a power semiconductor  30  arranged on the leadframe  20 . It is obvious that also a plurality of power semiconductors and other electronic components can be arranged on the leadframe  20 . 
         [0033]    The leadframe  20  and the power semiconductor  30  are surrounded by a preferable inorganic potting compound  50 . The potting compound  50  exhibits a function of mechanically protecting and electrically insulating the power semiconductor  30  arranged on the leadframe  20 . Over and above this, the potting compound  50  effects a physical connection between the leadframe  20  and the base plate  40 , so that the potting of the leadframe  20  and the power semiconductor  30  and physically connecting the base plate  40  to the potting compound  50  can be accomplished in one process step. 
         [0034]    The connection of the power semiconductor  30  to the leadframe  20  may be achieved by a number of known techniques. These may include soldering, brazing or sintering. The potting compound  50  acts not only as a means of encapsulation (thus sealing the semiconductor from the outside environment), but also as an adhesive (fixing in a permanent manner the structure comprising the power semiconductor  30  and leadframe  20  to the base plate  40 ). 
         [0035]    For this purpose—as  FIG. 2  shows in a flow chart of the process steps required for this purpose—in a first step  100  at first the leadframe  20  is connected to the power semiconductor  30 . In a second step  110 , the power semiconductor  30  connected to the leadframe  20  is surrounded with the potting compound  50  while simultaneously physically connecting a base plate  40  to the potting compound  50 . 
         [0036]    It can, for example, be envisaged here that the base plate  40  is inserted into a mould and the mould is filled with the potting compound  50 . The leadframe  20  supporting the power semiconductor  30  is then lowered into the liquid potting compound  50  while maintaining a predetermined distance from the base plate. After (completely) curing the potting compound  50 , the finished power module  10  can be removed from the mould. 
         [0037]    As an alternative, the base plate  40  can be inserted into a mould, the leadframe  20  supporting the power semiconductor  30  then being arranged above the base plate  40  while maintaining a predetermined distance and being immobilised in this position. A flowable potting compound  50  is only then transferred into the mould and flows around the leadframe  20  supporting the power semiconductor  30 . After (complete) curing of the potting compound  50 , the leadframe  20  supporting the power semiconductor  30  is then completely surrounded by the potting compound  50 , the base plate  40  is also physically connected to the potting compound  50 . 
         [0038]    Finally it can also be envisioned to envelope the leadframe  20  supporting the power semiconductor  30  completely with the potting compound  50  for example by immersion and to bring it into contact with a base plate  40  prior to (complete) curing. To this end, for example a leadframe  20  immersed into the potting compound  50  could be lowered on a base plate  40  for the purpose of curing the potting compound  50 . During curing of the potting compound  50 , the potting compound  50  will connect itself—like an adhesive—to the base plate  40  so that the potting compound  50  physically connects the power semiconductor  30  and/or the leadframe  20  to the base plate  40 . 
         [0039]    While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.