Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/576,703, filed on Jun. 3, 2004, the entirety of the contents of which is hereby incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a module comprising flip chip devices forming at least the semiconductor portion of a circuit. 
   2. Description of the Related Art 
   Numerous types of electrical circuits, such as DC to DC converters, synchronous converters, and the like, require a number of semiconductor components such as MOSFETs and ICs. Such circuit components can be found in portable electronics apparatus and the support components are commonly separately housed and mounted individually on a support board. The separately housed parts take up board space and each part generates heat. If the part is near other components, such as microprocessors, the part can interfere with the operation of the microprocessor. 
   To address the twin problems of heat generation and the occupation of board space, at least two different approaches have been applied previously. 
   One approach has been to arrange various semiconductor components in a planar fashion in a single housing. An exemplary circuit diagram, including semiconductor components, which are arranged in a planar fashion on a single substrate, is shown in  FIG. 1 , which was originally shown as  FIG. 2  of U.S. Pat. No. 6,388,319. 
     FIG. 1  shows a synchronous buck converter circuit having an N-channel MOSFET  4  as a switching device, and an N-channel synchronous MOSFET  5  and a Schottky diode  6  in parallel for synchronous rectification. The N-channel MOSFET  4 , N-channel MOSFET  5 , and Schottky diode  6  are arranged in a planar fashion within a common housing  7 . However, the control circuit  8  connected to the gates of MOSFETs  4  and  5  is not contained within the previously mentioned common housing  7 . Since the source (top) of die  4  is connected to the drain (bottom) of die  5 , it was required to insulate die  5  from the substrate, and wire bond. In addition, control chip  8  was separate because it had to be insulated from the board. 
   Other patents taking the approach of arranging semiconductor components in a planar fashion within a common housing include U.S. Pat. Nos. 5,977,630, 6,144,093, 6,404,050, 6,448,643, 6,465,875, 6,593,622, and 6,696,321. Significantly, however, in all of the references previously cited, wire bonding was used extensively to make the necessary connections between the semiconductor components. Such wire bonding leads to higher resistance and inductance. Additionally, although packaging the components in a common housing reduces the thermal effects of the components when compared to the components being housed separately, further improvement in thermal management is desirable. 
   A second approach toward confronting the problems of occupation of board space and thermal management involves the use of stacked or superimposed die contained within a common housing. Such an approach is exemplified by U.S. Pat. Nos. 5,770,480, 6,798,044 and 6,858,922. Such an approach should conserve board space even more than the planar arrangement approach. In addition, wire bonding between the die can be eliminated. However, such an approach may involve increases in the thermal effects the operation of one die would have on the other die stacked or superimposed on it over the planar arrangement approach. 
   SUMMARY OF THE INVENTION 
   In light of the above problems and considerations discovered through an examination of the related art, the present invention has several objects. 
   First, the present invention seeks to provide a planar arrangement of semiconductor components in a common housing so as to conserve circuit board space. 
   Second, it is an object of the present invention to provide such a planar arrangement, while eliminating the need to provide wire bonding to effect connections between the various die in the common housing. 
   Third, it is an object of the present invention to provide a planar arrangement of die in a common housing that exhibits superior thermal management over conventional planar arrangements of die and over stacked or superimposed arrangements of die. 
   Therefore, the invention comprises a plurality of flip chip devices arranged on a common lead frame or other circuit substrate, the arrangement being in substantially planar fashion without any stacking or superimposing of one flip chip device over any other flip chip device. The connections between the flip chip devices are made without any wire bonding. The connection between at least some of the flip chip devices provides improved thermal management over that available with conventional planar arrangements of die or stacked or superimposed arrangements of die in a common housing. 
   Thus, for a buck converter circuit, the control FET can be a top drain MOSFET, of the type disclosed in U.S. Provisional Application No. 60/539,549, filed Jan. 26, 2004, with its drain and gate on the top of the die, the drain and gate contacting the lead frame and the source on top when the die is flipped. The synchronous FET can be a Direct FET™ type, which was disclosed in U.S. Pat. No. 6,624,522, having its drain on the die top, and its source and gate on the die bottom, so that it can be connected to the source of the top drain FET through a T-PAC type packaging structure, which was disclosed in U.S. Patent Publication No. 2004/0061221 A1, the entirety of the contents of which are hereby incorporated by reference herein. Other conventional vertical conduction device packages can also be used to connect the synchronous FET and the top drain FET. The IC is also a flip chip so its bottom can be connected by traces patterned in the lead frame to the gates of the control FET and the synchronous FET. Thus, the need for wire bonding is avoided. 
   Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing a conventional arrangement of a semiconductor portion of a circuit in a common housing. 
       FIG. 2  is a circuit diagram of the invention, showing a semiconductor portion of a circuit arranged in a common housing. 
       FIG. 3  is a plan view of an embodiment of the invention corresponding to the components of the circuit shown in  FIG. 2  as being included in a common housing. 
       FIG. 4  is a cross-sectional view of  FIG. 3  taken through section lines  4 - 4  in  FIG. 3 . 
       FIG. 5  is a cross-sectional view of  FIG. 3  taken through section lines  5 - 5  in  FIG. 3 .  FIG. 5  also schematically shows the flow of the current therein, corresponding to the circuit shown in  FIG. 2 . 
       FIG. 6  is a partial elevation view of a T-PAC packaging structure showing ridges in the top surface thereof. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows a circuit diagram of a buck converter circuit including a MOSFET of top drain construction  15 , a synchronous MOSFET  25 , the synchronous MOSFET  25  being of a DirectFET™ type, a flip chip type IC  94 , controlling MOSFETs  15  and  25  in a PWM mode to obtain a constant output DC voltage, an inductor  10  and a capacitor  11 . The buck converter circuit, sometimes known as a step down converter, is commonly used to reduce voltages. Therefore, the input voltage V IN  is greater than the output voltage V OUT . The MOSFET die  15 , the MOSFET die  25 , and the IC die  94  are arranged in a common housing  21 . The die  15 ,  25 ,  94  are arranged in a planar fashion on a lead frame  20  or other substrate which is both thermally and electrically conductive. 
   The thermal conductivity of the lead frame or other substrate is needed to assure effective transmission of heat away from die  15 ,  25 ,  94  and toward one or more heatsinks (not shown) below the lead frame or other substrate. The electrical conductivity of the lead frame or the substrate is needed to permit electrical connections between the IC  94  and the MOSFETs  15 ,  25 , as will be described in more detail, and to allow transmission of the input voltage V IN  and an output voltage V 1 , (see  FIG. 2 ), to and from the common housing  21 , respectively. Substrates other than lead frames which are thermally and electrically conductive include direct-bond copper (DBC), printed circuit boards (PCB), printed wiring boards (PWB), and flexible circuits. 
   Referring to  FIG. 3 , IC  94  is directly bonded to the lead frame  20  or other substrate by solder or conductive epoxy (not shown). (As used herein, the term “flip chip” denotes a chip which is attached with a surface down directly to the lead frame or other substrate without any wire bonding, the flip chip having appropriately prepared bond pads.) Lead frame  20  has contact areas  30  and  31  which receive a gate electrode  32  and a drain electrode  33 , respectively, of the MOSFET  15 . The lead frame  20  also has source contacts  40  and  41  to contact source electrodes  43  and  44 , respectively, of MOSFET  25 . In addition, the lead frame  20  has a gate contact area  42  to receive the gate electrode  45  of MOSFET  25 . The lead frame  20  or other substrate also has traces  52 , shown schematically, patterned in the lead frame or other substrate, connecting the IC  94  to gate contact areas  30 ,  42 . 
   Gate contact areas  30 ,  42  are in turn connected to gate electrodes  32 ,  45 , respectively, through solder or conductive epoxy  58  and through solder or conductive epoxy  54 , respectively. Likewise, referring to  FIG. 5 , source contact areas  40  and  41  are connected to source electrodes  43  and  44 , respectively, through solder or conductive epoxy  62  and solder or conductive epoxy  60 , respectively. The drain contact area  31  is connected to the drain electrode  33  through solder or conductive epoxy  66 . 
   MOSFET  25  is of DirectFET™ construction manufactured by International Rectifier Corporation. Thus, referring to  FIG. 5 , MOSFET  25  is passivated on the surface  70  of the die, on which the source and gate electrodes  43 ,  44 ,  45  are located, in order to prevent shorting between source and gate electrodes and to protect them from moisture and other contamination. The drain contact  74  of MOSFET  25  is connected to the source electrode  78  of MOSFET  15  through the conductive T-PAC type packaging structure  50 , which also provides a pathway to conduct the output voltage V 1 , shown in  FIG. 2 , to lead frame  20  or other substrate, as shown schematically in  FIG. 5 . 
   In addition, the use of the T-PAC type packaging  50  provides improved thermal management. The T-PAC type packaging structure  50  is comprised of a connective portion  80  and a web portion  82 . The web portion  82  is connected to the lead frame  20  or other substrate by solder or conductive epoxy  84 . The connective portion  80  is connected to drain contact  74  of MOSFET  25  by conductive epoxy or solder  86 ,  88  (see  FIG. 4 ), and is also connected to source contact  78  by solder or conductive epoxy (not shown). The connective portion  80  and the web portion  82  are integrally formed into a unitary body. 
   In order to implement the circuit of  FIG. 2 , both source contacts  40  and  41  are grounded, as shown schematically in  FIG. 5 , and V IN  is supplied, through the lead frame  20  or other substrate, to drain electrode  33 , as also shown schematically in  FIG. 5 . A conventional molded housing  90  of resin or other conventionally nonconductive material encapsulates the T-PAC packaging structure  50  and all the other components of the circuit package above the lead frame  20  or other substrate. 
   It should be noted that the connective portion  80  of the T-PAC packaging structure covers the entire area of the lead frame  20  or other substrate, while the web portion  82  is of sufficient dimension to only make contact with a portion of the upper surface of lead frame  20  or other substrate, the contact portion being shown as a hatched area  92  in  FIG. 3 . (The plan view of  FIG. 3  is taken with the T-PAC type packaging structure  50  removed in order to simplify the view.) It should be understood, furthermore, that the lower surface of the connector portion  80  is above the upper surface of flip chip IC  94 , and that the nonconductive material  90  composed of resin or other non-conductive material electrically isolates the flip chip IC  94  from the T-PAC packaging structure  50 . The extension of the connector portion  80  of the T-PAC packaging structure  50  over the entire area of the lead frame  20  or other substrate may provide improved thermal management of the heat generated by the die over other conventional planar, stacked, or superimposed arrangements of die in common housings. Additional improvements in thermal management may be obtained by including ridges  96  in the top surface of connector portion  80  of the T-PAC packaging structure  50 , as disclosed in  FIG. 8A  of U.S. Publication No. 2004/0061221 A1, and also shown in  FIG. 6  herein. Such ridges may not only help dissipate more heat, they may also help connector portion  80  adhere better to nonconductive material  90 . 
   Although only the MOSFET die  15 , the MOSFET die  25 , and the IC die  94  have been described previously as being arranged in a planar fashion on lead frame  20  or other substrate, it can easily be conceived that the inductor  10  of the buck converter circuit of  FIG. 2  can also be located on lead frame  20  or other substrate. In such a case, the inductor  10  can be connected to drain contact  74  of MOSFET  25 , by traces patterned in the lead frame  20  or another substrate and by the T-PAC type packaging structure  50  or other packaging structure, thus implementing yet another portion of the circuit shown in  FIG. 2 , without the need for wire bonding. Finally, the capacitor  11  could also be placed on the lead frame  20 , and appropriate traces patterned in the lead frame  20  to connect the capacitor  11  to the inductor  10  and to the source contacts  40  and  41 , thus contacting the source electrodes  43  and  44 , respectively, of MOSFET  25 , again without wire bonding. The addition of inductor  10  and capacitor  11  to lead frame  20  would complete the implementation of the entire buck converter circuit shown in  FIG. 2  on a single lead frame or other substrate. 
   Although the foregoing disclosure has focused on the planar packaging of the semiconductor portion of a conventional buck converter circuit in a common housing, it should be appreciated that the invention is not limited to this particular arrangement. On the contrary, the invention can generally be applied to arrange flip chip devices, which are the semiconductor portions of various circuits, in a planar fashion in a common housing without the necessity of wire bonding. In addition, the use of a T-PAC packaging structure for the package may allow for improved heat management of the package. 
   It should be understood, of course, that insulating barriers or layers will be present, as needed, in the lead frame  20  or other substrate to prevent shorting between and among contacts of any semiconductor components of the circuit, any T-PAC or other packaging structure, and any passive devices on the lead frame  20  or other substrate. 
   Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Technology Category: 5