Abstract:
An aspect of the present invention provides a power module for automotive switching applications including a plurality of semiconductor chips and a unitary silicon nitride substrate. The plurality of semiconductor chips are attached to the silicon nitride substrate and the substrate is configured to have a thermal coefficient of expansion substantially the same as the plurality of semiconductor chips.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to an electronic power module. More specifically, the invention relates to electronic power modules having a silicon nitride substrate for automotive applications.  
         [0003]     2. Description of Related Art  
         [0004]     Power modules including IGBT (insulated gate bipolar transistor), FREDs (fast recovery epitaxial diode), MOSFETs, and other semiconductor chips have been used in automotive applications for many years. The power modules must be able to operate at a high ambient temperature. In addition, the high power dissipation required by the power modules further increases the temperature variations affecting the components of the power module. With such large temperature swings, the difference in thermal expansion of the components in the power module often causes reliability issues. A major cause of reliability issues is due to warping of the substrate. Substrate warpage of even 250 microns across a die can present a problem. Typically, the printed solder paste thickness is only 125 microns thick. Therefore, a warpage of 250 microns would not allow the die to be placed flat on the solder print.  
         [0005]     Many manufacturers have used smaller powered dies on multiple substrates to address the warpage problem. For example, four 50 Amp chips have been used on four separate substrates rather than a single 200 Amp chip on a single substrate as desired. However, using multiple substrates requires the use of a metal base plate which adds cost and complexity to the module. In addition, a thermal expansion mismatch between the die and substrate can also cause mechanical stress in the electrical connections thereby compromising reliability. Repetitive high power switching applications tend to increase temperature and shorten the life expectancy of solid-state power devices.  
         [0006]     One application that subjects the power module to repetitive high power switching is the electric automobile application, especially when the electric vehicle is driven in a city where stopping at a red light every few minutes is typically required. The power required to accelerate a vehicle from a standing start is substantially greater than the power required to maintain a constant speed. Another example is an electric assisted automotive power steering system while in a parking maneuver. Quite often in this situation the tires come in contact with the curb. Tires contacting the curb drives the power assist system to provide maximum output until the controlling processor reacts to the “stall load” demand by reducing the amount of assist. Automotive applications typically require very high peak power demands followed by a short cool down in a repetitive cycle. To address the power dissipation problem caused by the frequent high power switching, power modules were designed with multiple chips and multiple substrates, as shown in  FIG. 1 .  
         [0007]      FIG. 1  shows a IGBT module  10  with a first semiconductor chip  12  and a second semiconductor chip  26 . The first semiconductor chip  12  is attached to a ceramic substrate  14 . The ceramic substrate  14  includes a copper layer  16  that includes a circuit pattern (not shown). The first semiconductor chip  12  is attached to the copper layer  16  by wire bonds  20 . In addition, substrate  14  has a copper layer  22  which can be used for grounding or thermal dissipation. The copper layer  22  is attached to a base plate  24 . The base plate  24  acts as a foundation or support for all of the semiconductor components.  
         [0008]     The second semiconductor chip  26  is attached to a separate second ceramic substrate  28 . The second ceramic substrate  28  has a copper layer  30  which includes a circuit pattern (not shown). Second semiconductor chip  26  is electrically connected to the circuit pattern of copper layer  30  by wire bonds  32 . The second substrate  28  also has a copper layer  34  for grounding, mechanical balancing, and dissipating heat. Copper layer  34  is also attached to base plate  24 . Attached to the circuit pattern on copper layers  16 ,  30  are leads  42 ,  44 ,  46 ,  48  for connecting the IGBT module with other devices or circuits external of module  10 . To improve the thermal dissipation and protect the components of module  10  a silicone gel  40  is disposed over the semiconductor chips  12 ,  26  to cover and protect them. In addition, an epoxy resin  38  is disposed on top of silicone gel  40  to further protect and seal module  10 . The epoxy resin case  36  is attached to the base plate  24  and provides additional structural protection for the components of IGBT module  10 .  
         [0009]     The multiple ceramic substrates  14  and  28  help to mitigate thermal expansion problems. However, the multiple ceramic substrates also complicate the manufacture of module  10 , add additional weight, and add significant cost to the module.  
         [0010]     In view of the above, it is apparent that there exists a need for an improved power module for automotive applications that is easier to manufacture, weighs less, and has lower component costs.  
       SUMMARY  
       [0011]     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides a power module for automotive switching applications including a plurality of semiconductor chips and a unitary silicon nitride substrate. The plurality of semiconductor chips are attached to the silicon nitride substrate and the substrate is configured to have a thermal coefficient of expansion substantially the same as the plurality of semiconductor chips.  
         [0012]     In another aspect of the invention, at least one of the plurality of semiconductor chips is an IGBT semiconductor chip.  
         [0013]     In another aspect of the invention, at least one of the plurality of semiconductor chips is a FRED semiconductor chip.  
         [0014]     In yet another aspect of the invention the substrate includes a copper layer. The copper layer is between 0.1 and 0.5 mm thick and is actively brazed to the substrate. The plurality of semiconductor chips are attached to the copper layer by solder and connected to the circuit pattern of the copper layer by wire bonds.  
         [0015]     In yet another aspect of the invention the substrate includes holes for fastening the substrate. The substrate may be fastened to a heat sink. Further, the housing of the power module is fastened to the substrate.  
         [0016]     Further aspects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a side cutaway view of a prior art module using a conventional design; and  
         [0018]      FIG. 2  is a side cutaway view of an IGBT module according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]     Referring now to the drawings, a power module embodying the principles of the present invention is illustrated therein and designated at  10 . As its primary components, power module  10  includes a first semiconductor chip  52 , a second semiconductor chip  60 , and a substrate  54 .  
         [0020]     The first and second semiconductor chip  52 ,  60  are both attached to a single substrate  54 . The substrate  54  is made of silicon nitride (Si 3 N 4 ) and is between 0.5 and 1.5 mm thick. One method of attaching semiconductor chips  52 ,  60  to the substrate  54  is by soldering. The thermal coefficient of expansion of the silicon nitride substrate  54  is configured to match the coefficient of expansion of the first and second semiconductor chips  52 ,  60  thereby eliminating the need for multiple ceramic substrates while reducing the stress on the components and connections. Both Si 3 N 4  substrate  54  and semiconductor chips  52  and  60  have thermal coefficients of expansion of 3 ppm/C. By matching the coefficients of expansion dies of 200 Amps or higher can be accommodated which was previously not possible. A conductive layer shown as copper layer  56  is attached to substrate  54 . The copper layer  56  is between 0.1 and 0.5 mm thick and is preferably actively brazed onto the substrate  54 . The copper layer  56  also includes a circuit pattern. The first semiconductor chip  52  is connected to the circuit pattern of copper layer  56  by wire bonds  58 . Similarly, the second semiconductor chip  60  is connected to copper layer  56  by wire bonds  64 . The substrate  54  also has a second copper layer  66  for grounding, mechanical balancing, and heat dissipation purposes.  
         [0021]     The substrate  54  also serves as the foundation or support for power module  50 . The substrate  54  also includes holes  82  for fastening power module  50  to a heat sink  84 , other device or a circuit board. Leads  72 ,  74 ,  76 , and  78  are attached or soldered to solder pads on the circuit pattern of copper layer  56  for electrically connecting power module  50  to external circuits or devices. To protect the components of power module  50 , a silicone gel  68  is disposed over and surrounds the electrical components of power module  50 . In addition, an epoxy resin  70  is placed on top of silicone gel  68  to further seal and protect the components. An epoxy resin case  80  is attached to the substrate  54  and further protects the components of power module  50  by providing structural stability to the module.  
         [0022]     Therefore, silicon nitride has a thermal conductivity of 70 watts/M-C almost twice the value of alternative substrates. The single silicon nitride substrate  54  provides better thermomechanical continuity allowing better heat dissipation while also providing better resistance to warpage thereby increasing reliability. Further, the manufacture of the device is simplified using a single substrate and the substrate can be used to replace metal base plate (shown in  FIG. 1 ). Therefore the cost and weight of the module is reduced by eliminating the multiple ceramic substrates and the base plate of the prior art designs.  
         [0023]     Another aspect of the invention provides a method of manufacture for a power semiconductor module. The method begins with a unitary silicon nitride substrate. A copper layer including a circuit pattern is actively brazed onto the silicon nitride substrate. Semiconductor chips are attached to the silicon nitride substrate. In another aspect of the invention, at least one IGBT and at least one FRED semiconductor chips are attached to the silicon nitride substrate. The semiconductor chips are attached to the silicon nitride substrate using a high temperature soft solder, such as 95 Pb/2.5Sn/2.5Ag. Each of the semiconductor chips are wire bonded to the printed circuit pattern using aluminum wire. Power terminals are attached to the copper layer using a low temperature soft solder, such as 60 Pb/40Sn. Additional terminals are attached to the copper layer by manual soldering. Sides of the plastic housing are attached to the silicon nitride substrate using an epoxy adhesive. A layer of silicone gel is applied into the cavity formed by the plastic housing to cover and protect the semiconductor chips and wire bonds. An epoxy resin is applied into the cavity formed by the substrate and the plastic housing on top of the silicone gel. A plastic lid of the housing is attached by an epoxy adhesive.  
         [0024]     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.