Abstract:
Integrated circuit packages incorporating an inductor and methods for their fabrication. The lead frame used in packaging the integrated circuit includes a first area for receiving the integrated circuit, and a second area having a plurality of connections from one side to the other side of the lead frame, thereby forming coil segments. After mounting the integrated circuit and wire bonding its connections, the lead frame is placed on a ferrite plate, the assembly is encapsulated in resin, and the leads trimmed and bent. Mounting of the packaged integrated circuit on a properly prepared printed circuit interconnects the coil segments in the package to coil segments on the printed circuit, thereby forming a single, multi-turn coil around the ferrite plate. Various embodiments are disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of integrated circuit packaging. 
     2. Prior Art 
     A number of well known prior art circuits require an inductor for proper operation. Such circuits include switching power supplies which receive an input voltage, typically unregulated, and provide a regulated voltage to the circuits powered thereby. For large power supplies, the switching transistors, as well as the inductor and output capacitor, are normally discrete devices, though for smaller power supplies, the switching transistors can be integrated into the controller integrated circuit. In an effort to make smaller and simpler power supplies, the industry has been considering various methods of incorporating the inductor into the integrated circuit package for such supplies. Most of these efforts have been around an integrated circuit process that incorporates the inductor or a substrate that can have the magnetic material and minimal winding to form the inductor inside the package. All of the prior art produces small inductance with high resistance and high cost. However the ability to incorporate the inductor into the integrated circuit, or at least the integrated circuit package, is still a highly desirable goal to reduce component count and the circuit board area required, both of which reduce costs and enhance product appeal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top view of an exemplary lead frame that may be used with the present invention. 
         FIG. 2  is a schematic top view of the exemplary lead frame of  FIG. 1  on a ferrite plate and with an integrated circuit mounted thereon. 
         FIG. 3  is a cross section of the assembly of  FIG. 2 . 
         FIG. 4  is a perspective view of the assembly of  FIG. 2 . 
         FIG. 5  is a top view of the completed package comprising the encapsulated assembly of  FIGS. 2 ,  3  and  4 . 
         FIG. 6  is a cross section, similar to  FIG. 3 , of an alternate embodiment. 
         FIG. 7  is a perspective of the packaged circuit as mounted on a printed circuit board. 
         FIG. 8  is a plan view of exemplary printed circuit board traces in accordance with the present invention. 
         FIG. 9  is a cross section, similar to  FIGS. 3 and 6 , of a still further alternate embodiment. 
         FIG. 10  is a schematic top view of an alternate exemplary lead frame that may be used with the present invention. 
         FIG. 11  is a schematic top view of the exemplary alternate lead frame of  FIG. 1  on a ferrite plate and with an integrated circuit mounted thereon using flip-chip techniques. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described herein and illustrated in the Figures. It is to be understood, however, that the Figures illustrate the invention schematically, though are illustrative of the principles of the invention. Also the phrase “lead frame” as used herein and in the claims to follow refers to a complete lead frame having lead frame leads and typically a pad for the mounting of an integrated circuit, all initially physically connected together adjacent the lead frame periphery, as used in the packaging of integrated circuits, and may also refer to component parts of a lead frame before or after physical separation of the component parts thereof. By way of example, the phrase lead frame leads may refer to the lead portion of a complete lead frame, or the leads after physical separation, depending on the context such phrase is used. 
       FIG. 1  is a plan view of a lead frame  20  in accordance with the present invention. The lead frame is comprised of two primary lead frame areas, namely, the lead frame area  22  at the left of the lead frame  20  and the lead frame area  24  at the right of the lead frame  20 . The lead frame area  24  is of substantially conventional design, being comprised of a pad  26  for receiving an integrated circuit and a plurality of leads  28 , all supported by an integral lead frame region  30  circumscribing the lead frame. The right area  22  of the lead frame is simply comprised of a plurality of connections  32 , extending from one side of the lead frame to the opposite side. 
     In the process of packaging an integrated circuit, the integrated circuit  34  will be mounted on pad  26  and wire bonded to the leads  28  and to the first coil segment  32  at the left side of the integrated circuit by wire bonding connections  36  as shown in  FIG. 2 , with a side cross section being shown in  FIG. 3 . These Figures show the entire lead frame assembly being positioned on a ferrite plate  38  which extends under the connections  32  in the left area  22  of the lead frame to beyond the edge of pad  26 , as also illustrated in the perspective view of  FIG. 4 . The ferrite plate may be first coated with a suitable insulative material, if desired, to augment the normally very high resistivity of ferrite. The integrated circuit  34  and ferrite plate  38  are then encapsulated in resin  40  as shown in  FIG. 5 , with the leads  28  extending from the molding being cut from the lead frame periphery  30  and bent for resting on contact pads on a printed circuit board for soldering to the printed circuit board, as illustrated in  FIG. 7 . 
     The local layout of the printed circuit board itself is shown in  FIG. 8 . The printed circuit board would have contact pads  40  for the integrated circuit, with the routing (not shown) being in accordance with the connection of the integrated circuit to other circuits on the printed circuit board. The printed circuit board would also have contacts  42  which connect one end of lead frame connections  32  to the other end of the next lead frame connection  32 . This connects each of connections  32  over the top of ferrite plate  38  to a connection  44  under the ferrite plate, and from there to the next connection  32  over the top of the ferrite plate, etc. In essence, connections  32  over the top of the ferrite plate  38  and connections  44  on the printed circuit under the ferrite plate form coil segments which, when the packaged integrated circuit is soldered to the printed circuit board, are all connected in series to form one continuous multi-turn coil around the ferrite plate. In the embodiment schematically shown, this provides approximately seven and one-half turns around the ferrite plate, the multi-turn coil having a first end  46  and a second end  48 . With the printed circuit board connection shown in  FIG. 8 , one the left coil end is wire bonded to the integrated circuit, and the other end is connected to respective printed circuit contact pad  40 . 
     The specific embodiment so far disclosed uses a ferrite plate that forms the core of the inductor, and further extends under the integrated circuit. This is convenient, as it is highly desirable to provide a flat surface for the lead frame to rest on while encapsulating the assembly. However this is not a necessity, as the ferrite plate need not extend under the integrated circuit for the inductor to function. Any loss in inductance would be relatively small, and easily more than made up for, if necessary, by using a top ferrite plate  50  ( FIG. 6 ) over the lower ferrite plate and the coil segments on the lead frame, closing the magnetic circuit. Even if the ferrite plate  50  did not extend under the integrated circuit, one could still provide a flat surface for the lead frame to rest on while encapsulating the assembly by placing a plastic plate  52  of the same thickness as the ferrite plate  50  under the integrated circuit. For integrated circuits having substantial power dissipation, such as switching regulators with integrated switching transistors, a metal plate might be used for a more uniform temperature within the integrated circuit and better heat conduction away from the integrated circuit. While this would slightly further reduce the inductance, again this would easily more than be made up for, if necessary or desired, by using a top ferrite plate over lower ferrite plate and the coil segments on the lead frame, closing or nearly closing the magnetic circuit. In that regard, closing the magnetic circuit, particularly by using a “U” shaped upper ferrite plate essentially eliminates any part of a ferrite plate that extends under the integrated circuit from the magnetic circuit. Still, it is preferred to use a ferrite plate that forms the core of the inductor and further extends under the integrated circuit, as it reduces the number of parts used in the packaged device. In one embodiment, adequate inductance was provided without any top ferrite plate being used. Of course, if a top plate is used, it may be a simple flat ferrite plate  54 , coated with insulation or not, as shown in  FIG. 9 , rather than the shaped top plate shown in  FIG. 6 . 
     Also it should be noted that in the embodiment disclosed, the coil segments forming part of the lead frame are aligned straight across the lead frame and the traces (coil segments) on the printed circuit board are effectively slanted to connect the coil segments into a single, multi-turn coil. However the reverse could be used. The coil segments forming part of the lead frame could be slanted across the lead frame and the traces (coil segments) on the printed circuit board effectively straight across to connect the coil segments into a single, multi-turn coil. Also, while connections between the multi-turn coil and the integrated circuit in the embodiment disclosed are made on the circuit board, such connections may be made by wire bonding prior to encapsulation. 
     By way of example,  FIGS. 10 and 11  illustrate an embodiment using Flip-chip packaging. The lead frame itself, shown schematically in  FIG. 10 , is similar to that of  FIG. 1 , though with a smaller integrated circuit pad to allow the individual lead frame leads to extend under the integrated circuit  34  for solder ball bonding thereto. The printed circuit connections for this embodiment may be the same as shown in  FIG. 8 . 
     While the coil segments in the embodiments disclosed are connected so that the coils are physically in order, this is not a limitation of the invention. The coils may be connected in any physical order, provided they are connected with the same winding sense. This may be of benefit if both the start and finish connections are to be connected to the integrated circuit, as it would allow both the start and the finish connections to be adjacent the integrated circuit for more convenient connections. 
     Thus while certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.