Abstract:
An improved vertical integrated circuit device package that eliminates or reduces the need for wire bond connections and/or solder connections is provided. The package includes a vertical device having electrodes on opposite surfaces, and a compressed spring member that is used to establish compression electrical connections between the electrodes and corresponding electrically conductive elements.

Description:
TECHNICAL FIELD  
       [0001]    The invention relates to integrated circuits (IC) packages, and more particularly to electrical connections for high current vertical device IC packages. 
       BACKGROUND OF THE INVENTION  
       [0002]    Various types of circuit board components have been specifically developed for high current and high power applications, such as hybrid and electric vehicles. Such components often comprise a semiconductor device, such as a diode, thyristor, metal oxide semiconductor field effect transistor (MOSFET), isolated gate bipolar transistor (IGBT), resistors, etc., depending on the particular circuit and use desired. Vertical devices are typically formed in a semiconductor (e.g., silicon) die having metalized electrodes on its opposite surfaces, e.g., a MOSFET or IGBT with a drain/collector electrode on one surface and gate and source/emitter electrodes on its opposite surface. The die is mounted on a conductive pad for electrical contact with the drain/collector electrode, with connections to the remaining electrodes on the opposite surface often being made by wire bonding. The pad and wires are electrically connected to a lead frame having leads projecting outside a protective housing that is often formed by overmolding the lead frame and die. 
         [0003]    Components of the type described above include well-known industry standard packages, such as the TO 220 and TO 247 packages, which are pre-packaged integrated circuit components having leads adapted for attachment (e.g., by soldering) to a printed circuit board (PCB). The overmolded housings of these packages protect the die, wire bonds, etc., while typically leaving the lower surfaces of the conductive pad exposed to provide a thermal and/or electrical path out of the package. Such a path allows the package to be connected to an electrical bus for electric connection to the PCB, or a heat-sinking mass for dissipation of heat from the package. 
         [0004]    The conventional high power and high current vertical device IC packaging concepts have electrical connection methods that utilize either a wire bond approach or a complicated package with dual solder joints. These solder joints have a reduced life due to high stresses created from the mismatch between the coefficient of thermal expansion of the substrate and the coefficient of thermal expansion of the rather large integrated circuit. This mismatch becomes an even greater problem with the use of a copper slug to mount the IC device. Accordingly, improved connection technology that does not use a wire bond approach or solder joints is desired. 
       SUMMARY OF THE INVENTION  
       [0005]    The invention involves the use of a compression electrical connection for vertical IC devices. 
         [0006]    In accordance with a first aspect of the invention, springs (e.g., elastomers or metal leaf or coil springs) under compression provide a clamping force to ensure electrical contact of an integrated circuit device to a printed circuit board. 
         [0007]    In accordance with another aspect of the invention, a thermoset overmold process is used to provide a force to form the compression electrical connections. 
         [0008]    These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0010]      FIG. 1  is a cross-sectional view of an integrated circuit package utilizing elastomeric springs under compression for effecting electrical connections. 
           [0011]      FIG. 2  is a cross-sectional view of an integrated circuit package utilizing a thermoset overmold process to provide a force to establish compression electrical connections. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    Shown in  FIG. 1  is a vertical device integrated circuit package  10  utilizing springs  12   a,    12   b  and  12   c  that are under compression to provide a claming force to ensure electrical contact of an integrated circuit device  14  (e.g., an IGBT) with a printed circuit board  16 . Integrated circuit device  14  is a vertical device, more particularly, an IGBT, having metalized electrodes (electrical contacts)  20 ,  21  on opposite surfaces. An electrically conductive element  24  (e.g., copper strap) electrically connects electrical contact  20  of IGBT with an electrical contact  26  on printed circuit board  16 , while the electrical contact  21  on the opposite side of IGBT  14  is electrically connected to a separate electrical contact  28  on printed circuit board  16 . In the illustrated embodiment, a diode  32  is included in package  10 . Diode  32  includes an electrical contact  30  electrically connected to conductive element  24  and a second electrical contact  34  on a surface opposite the surface on which electrical contact  30  is located, which is electrically connected to a contact pad  36  on printed circuit board  16 . 
         [0013]    The entire assembly, including elastomeric springs  12   a,    12   b  and  12   c  is sandwiched between a heat sink  40  and a cover member  42 . 
         [0014]    Fasteners  44  (e.g., screws) are used for holding the assembly together under compression. In particular, sufficient compression is applied to compress resiliently deformable spring members  12   a,    12   b,  and  12   c  so that they urge electrical contacts together to establish compression electrical connections between corresponding electrical contacts. 
         [0015]    As shown in  FIG. 1 , the springs are preferably located directly under the interfacing electrical contacts. That is, the compressive forces from springs  12   a,    12   b  and  12   c  have a vector orthogonal to and passing through the interfacing contacts of the compression electrical connections. 
         [0016]    In order to minimize oxidation, corrosion and/or electrical resistivity at the compression electrical connections, it is desirable that the contacts  20 ,  21 ,  30 ,  34 ,  28 ,  36  and  46  are gold-plated. Similarly, it is desirable that gold-plating be provided on electrically conductive element  24  as indicated by reference numerals  26 ,  47  and  48 . 
         [0017]    A thermal interface material  50  may be disposed between electrically conductive element  24  and heat sink  40  to enhance heat transfer through conductive element  24  and into heat sink  40 , where heat can be efficiently transferred to the surrounding atmosphere. The thermal interface material may also provide dielectric isolation. 
         [0018]    Springs  12   a,    12   b  and  12   c  may comprise an elastomeric material, or a metal spring (e.g., a coil spring or leaf spring). 
         [0019]    In accordance with another aspect of the invention, as shown in  FIG. 2 , compression spring members can be formed in situ during an overmolding process. This can be achieved using spring pressure pins  60  (which are part of molding tool  62 ) to temporarily hold the assembly together during an overmolding process. Springs  60  provide a specific force so that the molding composition (e.g., a thermoset epoxy resin) does not flow into gaps between the integrated circuit and the printed circuit board or between the integrated circuit board and the electrically conductive element. During the molding process, pins  60  are retracted and the molding composition fills the volume vacated by retraction of pins  60 . In an alternative secondary process the pins may provide forces during the entire molding process, and be retracted when the mold cavity is opened. The holes formed by the pins can then be filled with epoxy after the molding process. The volume vacated by retraction of pins  60 . The molding composition adhesively bonds the printed circuit board, heat sink, and electrically conductive strip together. The maximum use temperature is always below the zero-stress state (165° C. molding temperature). This coupled with use of a material having a low coefficient of thermal expansion keeps the structure in compression. Mold locking features can also be employed in the heat sink, printed circuit board and other components of the package to mechanically lock the structure together. 
         [0020]    Advantages of the invention include simplified manufacturing, high current connection formed by gold-to-gold contact surfaces, no solder cracking to silicon integrated circuit connections, repairability (for  FIG. 1  design only), and the absence of leads (i.e., solder-free package). 
         [0021]    It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.