Abstract:
The present invention provides a circuit board including a first conductor layer forming a plurality of conductive circuit traces for interconnecting electronic components. The circuit board includes a substrate for supporting the first conductor layer and a pedestal formed from the substrate for supporting at least one of the plurality of electronic components. The pedestal provides a heat conduction path for conducting heat away from the at least one of the plurality of electronic components and a aperture in the substrate adjacent the pedestal for allowing a fluid to pass through the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    The present invention claims priority to U.S. Provisional Application Serial No. 60/387,629, filed Jun. 10, 2002, entitled “Etched Trimetal Thermal Management”. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to systems and methods for mounting power devices in electronic assemblies and to systems and methods for removing heat from such devices.  
         BACKGROUND  
         [0003]    Conventional electronic assemblies include surface mount devices that may be transistors, amplifiers, or the like. These devices in operation create heat that must be removed from the surface of the device to avoid overheating and damage to the components. Prior art methods for removing heat from power devices include drilling thermal via holes through the bond pad and circuit board and metallizing these holes to provide a thermal path to the opposite side (or bottom side) of the board. These vias must be filled, typically with epoxy material, to prevent flux (which results in corrosion) from flowing through the holes to the opposite side of the circuit board. The heat removed to the bottom side of the circuit board must then be transported to a heat sink especially designed to dissipate the heat to the surrounding environment.  
           [0004]    All the power devices that are thermally connected to the heat sink must be electrically isolated. Typically, this is accomplished by attaching the heat sink to the circuit board metallizations on the bottom side of the circuit board via a thermally conductive, electrically insulative adhesive. The adhesive layer contributes significantly to the overall thermal resistance necessitating a larger heat sink and limiting the heat removal capacity of the thermal stack. However, this process of removing heat from the power devices is costly due to the complex procedure of drilling metallizing and filling vias.  
           [0005]    Other prior art methods for removing heat from power devices utilize, etched tri-metal (ETM) structures. In such ETM based systems, etched pedestals are created from ETM structures and serve the function of the filled vias that connect one side of a circuit board to the other side. More specifically, heat from power components passes through the entire copper backed ETM stack before being dissipated through a heat sink on the opposite side of the circuit board. For example, the heat sink may be a metal cross-car beam that supports an instrument panel in a vehicle. This construction provides a thorough path while electrically isolating the power components. Advantageously, such constructions create a thermal path through the substrate that is unimpeded by poorly conducting material. For example, a solder connection could be established between the top side circuit (onto which the power device is soldered) and the bottom side circuit. Alternatively, two solder connections to a core layer, one with the top side circuit and the other with the bottom side circuit could form a pass through with no poorly conducting materials in the thermal path. If a core pedestal is used, the pedestal may be electrically isolated from the ground by creating an island in the core, as disclosed in the above references. If a solder slug is used to connect the top and bottom layers, a hole in the ground plane may be drilled or etched to a larger diameter than the ETM layers ensuring that the solder does not wet the core.  
           [0006]    While the above-mentioned prior art systems and a method do indeed reduce the resistance of the thermal stack upstream of the adhesive layer that bonds the circuit to the heat sink, increased heat transfer rates would be desirable. All of the heat transfer within the substrate is accomplished through heat conduction. One known problem that the prior art does not address is that heated air, in some instances, is trapped against the substrate and results in a reduced heat transfer efficiency.  
           [0007]    Therefore, there is a need to improve the heat transfer rates through and around electronic circuit board and substrate that is absorbing and transferring heat from power devices. Therefore, the new and improved system and method for transferring heat from the circuit board should increase the rate of heat transfer from the thermal stack.  
         SUMMARY  
         [0008]    In an aspect of the present invention a circuit board adapted to remove heat from a power component is provided. The circuit board includes a first conductor layer forming a plurality of conductive circuit traces for interconnecting electronic components. Further, a substrate is provided for supporting the first conductor layer and a pedestal formed from the substrate for supporting at least one of the plurality of electronic components. The pedestal provides a heat conduction path for conducting heat away from the at least one of the plurality of electronic components. A aperture is provided in the substrate adjacent the pedestal for allowing a fluid to pass through the substrate.  
           [0009]    In another aspect of the present invention, the circuit board includes the first conductor layer which is an etched tri-metal circuit and a second conductor layer forming a plurality of conductive circuit traces for interconnecting electronic components.  
           [0010]    In yet another aspect of the present invention, the circuit board includes a substrate having a plurality of metal layers including a copper metal layer, and a first intermediate layer disposed between the first conductor layer and the substrate.  
           [0011]    In yet another aspect of the present invention, the circuit board includes a pedestal portion of the first intermediate layer and a portion of the substrate.  
           [0012]    In yet another aspect of the present invention, a solder layer is disposed between the first conductor layer and the substrate and between the at least one of the plurality of electronic components and the pedestal.  
           [0013]    In still another aspect of the present invention, a plurality of bridges are connected at a first end to the pedestal and at a second end to the substrate. The bridges are formed from the first conductor layer and the substrate.  
           [0014]    In still another aspect of the present invention, apertures are defined by the area between the pedestal, and the plurality of bridges and are formed from the first conductor layer.  
           [0015]    These and other aspects and advantages of the present invention will become apparent upon reading the following detailed description of the invention in combination with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0016]    [0016]FIG. 1 a  is a plan view of a portion of a circuit board illustrating the electrical and mechanical connection of a power device to the circuit board, in accordance with the present invention;  
         [0017]    [0017]FIG. 1 b  is a cross-sectional view through the power device and circuit board illustrated in FIG. 1 a , in accordance with the present invention;  
         [0018]    [0018]FIG. 2 is a perspective view of a power device to be mounted on a portion of a circuit board illustrating the thermal stack and apertures through the circuit board, in accordance with the present invention;  
         [0019]    [0019]FIG. 3 is a perspective view of a portion of a circuit board including a vented thermal stack wherein the core layer in the thermal stack is separated from the rest of the core layer in the circuit board, in accordance with the present invention; and  
         [0020]    [0020]FIG. 4 is a perspective view of a portion of a circuit board including a thermal stack for mounting a power device wherein the thermal stack includes a core layer that is mechanically attached to the circuit board core layer, in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0021]    [0021]FIG. 1 a  is a plan view of a portion of a circuit board  10  adapted for attaching a power device  12 . As conventionally known, power device  12  includes connector tabs  14  for electrically interconnecting (soldering) the power device to conductor pads  15  are contiguous with conductor traces on circuit board  10 . The power devices along with other electrical components and the conductor traces form the electrical circuit on circuit board  10 . In an embodiment of the present invention, through vents or apertures  16  disposed in circuit board  10  and surrounding power device  12  are provided. Apertures  16  as will be further described and illustrated, provides a flow path that allows fluid such as air to circulate through circuit board  10  and around power device  12 . Thus, during operation power device  12  is cooled through heat convection whereby the heat generated by power device  12  is transferred to the air flowing over the power device.  
         [0022]    Referring now to FIG. 1 b , a cross-sectional view through power device  12  and circuit board  10  is further illustrated, in accordance with the present invention. As shown in FIG. 1 b , circuit board  10  in an embodiment of present invention includes a core layer or substrate  30  having a first ETM layer  32  attached to a first core layer surface  34  and a second ETM layer  36  attached to a second core layer surface  38 . An adhesive layer  39  secures first and second ETM layers  32 ,  36  to core layer  30 . Adhesive layer  39  may be any dielectric adhesive suitable for securing and electrically isolating, layers  32 ,  36  from core layer  30 . First and second ETM layers  32 ,  36  typically are comprised of three metals, such as copper, aluminum and copper. The present invention contemplates various ETM configurations wherein for example aluminum is the middle layer and copper material forms the outer layers to form a copper aluminum copper ETM layer. Core layer or substrate  30  may be copper or any other suitable material. The construction and manufacturing of ETM is disclosed for example in the following U.S. patents having U.S. Pat. Nos. 6,391,211; 5,738,797; 4,404,059; and 3,801,388, incorporated herein by reference.  
         [0023]    A center stack or pedestal  40  is created in circuit board  10  by forming through apertures  16  through first and second ETM layers  32 ,  36  and core layer  30 . Center stack  40  may include the same number and kind of layers as circuit board  10  or the number and layer configurations of stack  40  may be different than the circuit board  10 . For example, center stack  40  includes a first and second ETM layer  42  and  44  disposed on either side of a core layer  46 . Furthermore, center stack  40  may be comprised of the same or different materials such as copper and aluminum as found in the rest of circuit board  10 . Through apertures  16  may completely surround power device  12  or be positioned on only one side of power device  12 .  
         [0024]    Power device  12  is attached to stack  40  via a solder paste or other die attachment material  48 . Connector tabs  14  are electrically interconnected, in a conventional manner, to connector pads  15  on circuit board  10  through solder or other material  50 . As will be shown in the subsequent figures, stack  40  is mechanically connected to circuit board  10  through bridges  52 . As will be described and illustrated, bridges  52  may be comprised of portions of the core layer or ETM layers. Air flow through circuit board  10  and across side surfaces  54  of stack  40  is illustrated and represented by arrows A. Thus, it is readily apparent how heat transfers from the power die to the stack and then to the surrounding air occurs through natural convection.  
         [0025]    Referring now to FIG. 2, a perspective view of circuit board  10  having apertures  16  and a center stack  40  is further illustrated, in accordance with the present invention. FIG. 2 further illustrates the attachment of stack  40  to the rest of circuit board  10  as shown. Apertures  16  divide or separate stack  40  from the rest of circuit board  10 . The plurality of bridges  52  mechanically connect stack  40  to circuit board  10 . As conventionally known, as disclosed in the above-referenced patents, the ETM layers  32  may be etched to form power die connector pads  60  and circuit traces  62  that connect power die  12  to the rest of the circuit on circuit board  10 .  
         [0026]    Referring now to FIG. 3, a center stack  70  is illustrated in a perspective view, in accordance with an embodiment of the present invention. In the present embodiment, center stack  70  is mechanically attached to the rest of circuit board  10  by bridges  74  and  76  that are formed from portions of a first ETM layer  80  and a second ETM layer  82 , respectively. More specifically, as previously described ETM layer  80  and  82  may be comprised of an aluminum layer sandwiched between two copper layers. Thus, as illustrated in FIG. 3, a portion of one of the copper layers  86  forms bridges  74  and a portion of one of the copper layers  88  forms bridges  76 . In this manner, core layer  90  of center stack  20  is isolated from the rest of the core layer of circuit board  10  via cooling apertures  16 .  
         [0027]    Referring now to FIG. 4, an alternate embodiment of a center stack is illustrated in a perspective view and identified by reference numeral  100 , in accordance with the present invention. In the present embodiment, center stack  100  is mechanically attached to the rest of circuit board  10  by bridges  102  that are formed from portions of the core layer of circuit board  10 . More specifically, as previously described circuit board  10  is comprised of a core layer sandwiched between two ETM layers  106  and  108 . Thus, as illustrated in FIG. 4, a portion of the core layers of circuit board  10  forms bridges  102 . Thus, core layer  104  of center stack  20  is contiguous with the rest of the core layer of circuit board  10 . ETM layers  106  and  108  are, as shown in FIG. 4, isolated from the rest of circuit board  10  by cooling apertures  16 .  
         [0028]    As any person skilled in the art of systems and methods for mounting power devises in electronic assemblies and systems and methods for removing heat from such devices will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.