Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to U.S. patent application Ser. No. 15/090,176, which is a continuation of and claims priority to U.S. patent application Ser. No. 14/800,903 filed on Jul. 16, 2015, now U.S. Pat. No. 9,305,872, which is a divisional of and claims priority to U.S. patent application Ser. No. 14/481,204 filed on Sep. 9, 2014, now U.S. Pat. No. 9,171,828, which is a continuation-in-part of U.S. patent application Ser. No. 14/173,147 filed Feb. 5, 2014, now U.S. Pat. No. 9,184,121, all of which are incorporated by herein by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the invention are related in general to the field of semiconductor devices and processes, and more specifically to the structure and fabrication method of thin packaged synchronous buck converters, which are free of clips and have chips embedded outside the package in a pre-coined recess of the leadframe. 
         [0003]    Among the popular families of power supply circuits are the power switching devices for converting on DC voltage to another DC voltage. Particularly suitable for the emerging power delivery requirements are power blocks with two power MOS field effect transistors (FETs) connected in series and coupled together by a common switch node; such assembly is also called a half bridge. When a regulating driver and controller is added, the assembly is referred to as a power stage or, more commonly, as a synchronous buck converter. In the synchronous buck converter, a control FET chip, also called the high-side switch, is connected between the supply voltage V IN  and the LC output filter, and a synchronous (sync) FET chip, also called the low side switch, is connected between the LC output filter and ground potential. The gates of the control FET chip and the sync FET chip are connected to a semiconductor chip including the circuitry for the driver of the converter and the controller; the chip is also connected to ground potential. 
         [0004]    For many of today&#39;s power switching devices, the chips of the power MOSFETs and the chip of the driver and controller IC are assembled horizontally side-by-side as individual components. Each chip is typically attached to a rectangular or square-shaped pad of a metallic leadframe; the pad is surrounded by leads as input/output terminals. In other power switching devices, the power MOSFET chips and the driver-and-controller IC are assembled horizontally side-by-side on a single leadframe pad, which in turn is surrounded on all four sides by leads serving as device output terminals. The leads are commonly shaped without cantilever extensions, and arranged in the manner of Quad Flat No-Lead (QFN) or Small Outline No-Lead (SON) devices. The electrical connections from the chips to the leads may be provided by bonding wires, which introduce, due to their lengths and resistances, significant parasitic inductance into the power circuit. 
         [0005]    In some recently introduced advanced assemblies, clips substitute for many connecting wires. These clips are wide and made of thick metal and thus introduce minimum parasitic inductance. Each assembly is typically packaged in a plastic encapsulation, and the packaged components are employed as discrete building blocks for board assembly of power supply systems. 
         [0006]    In other recently introduced schemes, the control FET chip and the sync FET chip are assembled vertically on top of each other as a stack over the leadframe pad, with the physically larger-area chip of the two attached to the leadframe pad, and with clips providing the connections to the switch node and the stack top. Independent of the physical size, the sync FET chip needs a larger active area than the active area of the control FET chip, due to considerations of duty cycle and conduction loss. When both the sync chip and the control chip are assembled source-down, the larger (both physically and active area) sync chip is assembled onto the leadframe pad and the smaller (both physically and active area) control chip has its source tied to the drain of the sync chip, forming the switch node, and its drain to the input supply V IN . A first clip is connected to the switch node between the two chips; an elongated second clip of the stack top is tied to input supply V IN . The pad is at ground potential and serves as a spreader of operationally generated heat. The driver-and-control IC chip is assembled horizontally side-by-side near the stack of chips and clips and connected by bonding wires with the FET gates and the leadframe leads. Due to their forms and materials, the clips and wire bonds have resistances and inductances, which contribute to the parasitics of the system. 
         [0007]    A typical converter described in the last paragraph is depicted in  FIG. 1A , generally designated  100 . The control MOS field effect transistor (FET)  110  is stacked upon a synchronous (sync) MOSFET  120 . The control FET chip  110  of this exemplary module has a smaller area relative to sync FET chip  120 . A QFN metal leadframe has a rectangular flat pad  101 , which serves as output terminal and is destined to become the heat spreader of the package; the leads  102   a  and  102   b  are positioned in line along two opposite sides of the pad. The stacking of the FET chips is accomplished by the so-called source-down configuration: The source of sync FET  120  is soldered to the leadframe pad  101  by solder layer  121 . The low side clip  140 , soldered by solder layer  122  onto the drain of sync FET  120 , has the source of control FET  110  attached by solder layer  111 . Consequently, low side clip  140  thus serves as the switch node terminal of the converter. The high side clip  160  is connected by solder layer  112  to the drain of control FET  110 . High side clip  160  is attached to lead  102   b  of the leadframe and thus connected to the input supply V IN . The low side clip  140  and the high side clip  160  are gang placed. The driver and controller chip  130  is attached by solder layer  132  to pad  101 . Wires  133  provide the connections of the chip terminals and FET gate terminals ( 110   b,    120   b,  and  120   d ). The converter of  FIG. 1  has a height  191  of 1.5 mm and a rectangular footprint with a length  192  of 6 mm and a width  193  of 5 mm. In other known converters with smaller chips, the driver chip may be placed in top of the second clip to save board area; for these converters, however, the bonding wires have to be excessively long with significant risk of wire sweep and electrical short during the encapsulation process.  FIG. 1B  shows a cutaway along phantom line marked  1 B- 1 B. 
         [0008]    In yet another recently introduced power system, the driver-and-control chip is included in the vertical stack on top of the second clip. This assembly structure saves real estate of the leadframe pad and thus the printed circuit board, but accepts the risk of very long downhill bonding wires and thus the risk of wire sweep and subsequent electrical shorts during the encapsulation process. 
       SUMMARY 
       [0009]    A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
         [0010]    In recent years, there has been a growing trend of employing DC-DC converters in new applications, such as automotive products, which has in turn accelerates the long-standing drive towards miniaturization, lower power, higher frequency, and reduced cost. Symptoms of this trend are the pushes for reducing the height of the converters and reducing the electrical parasitics. 
         [0011]    The present disclosure realizes that a step function improvement in reducing the height of DC-DC converters could be achieved when the metallic clips used in conventional converters could be eliminated without eliminating the function of the clips. Embodiments of the disclosure solve the problem of reducing the height of the product while concurrently reducing the electrical parasitic resistances and inductances by elimination of such clips while retaining their function by assembling, for example, sync and control FET chips side-by-side in a pre-coined recess of the leadframe pad. As unexpected side benefits, it is observed that direct attachment of the FET terminals directly to a circuit board not only reduces the parasitics of the converter, but also substantially increases the thermal dissipation from active converter operation into heat sinks of the circuit board(s). As a consequence, the power handling and the operational frequency of the converter are improved. 
         [0012]    One embodiment of the disclosure relates to a DC-DC converter, which uses a QFN leadframe with leads and a pad. The pad surface facing a circuit board has a portion recessed with a depth and an outline suitable for attaching side-by-side the sync FET chip and the control FET chip. The input terminal of the control FET and the grounded output terminal of the sync FET are coplanar with the un-recessed portion of the pad, which is tied to the switch node terminal. Due to the co-planarity, all terminals can be directly and simultaneously attached to contacts of a circuit board. The direct attachment reduces the thermal resistance significantly and improves the heat dissipation to a heat sink of the circuit board. Thus, the operating frequency of the converter is enhanced (beyond 1 MHz). The driver-and-control chip is vertically stacked to the opposite pad surface and encapsulated in a packaging compound. 
         [0013]    Another embodiment of the invention is a method fabricating a power supply system. The pad of a QFN leadframe has a first and a second surface; the first pad surface has been pre-coined to have a portion recessed with a depth and an outline suitable for attaching semiconductor chips. A driver-and-control chip is attached to the second pad surface, wire bonded to respective leads, and encapsulated in a packaging compound, which leaves the first pad surface un-encapsulated. A first FET chip (the sync FET chip) is attached with its drain terminal to the recessed portion of the first pad surface so that the source and gate terminals of the first FET chip are co-planar with the un-recessed portion of the first pad surface. In addition, a second FET chip (the control FET chip) is attached with its source terminal to the recessed portion of the first pad surface so that the drain and gate terminals of the second FET chip are co-planar with the un-recessed portion of the first pad surface. 
         [0014]    Compared to conventional structure and fabrication methods of power supply systems, the invention eliminates both clips without abandoning their functions, thereby saving height of the assembled system. The invention further eliminates the corresponding clip assembly steps; and saving time and cost in the assembly process flow. The height of the finished device is further reduced by embedding both FET chips side-by-side into the partially thinned leadframe pad. Since the terminals of the assembly FET chips are coplanar with the leadframe pad terminal, all terminals can be attached to a circuit board simultaneously and directly. By avoiding thermal resistances, heat dissipation to heat sinks in the circuit boards is drastically improved, enhancing the frequency of converter operation beyond 1 MHz. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1A  shows a perspective top view of a packaged DC-DC synchronous buck converter with the driver-and-controller chip assembled adjacent to the vertically stacked FET chips and two clips on a leadframe pad according to prior art. 
           [0016]      FIG. 1B  depicts a cross section of the packaged stacked FET chips and clips of  FIG. 1A  according to prior art. 
           [0017]      FIG. 2A  illustrates a perspective top view of a packaged DC-DC synchronous buck converter according to the invention, with the driver-and-controller chip attached to the top side of a leadframe pad and the package compound encapsulating chip and wire bonds. 
           [0018]      FIG. 2B  shows a perspective bottom view of the DC-DC converter of  FIG. 2A , with both adjacent FET chips attached to the bottom side of the leadframe pad and coplanar FET terminals un-encapsulated to beg attachable to a circuit board. 
           [0019]      FIG. 3  depicts a cross section of the packaged converter of  FIGS. 2A and 2B , with the terminals of the adjacent FET chips attached to respective contacts of a circuit board. 
           [0020]      FIG. 4  displays a circuit diagram of the synchronous buck converter of  FIGS. 2A and 2B , identifying the elimination of electrical parasitics due to the avoidance of clips. 
           [0021]      FIG. 5  shows a perspective bottom view of the stamped and coined leadframe, illustrating the pad portion recessed relative to the leads and the remaining pad portion. 
           [0022]      FIG. 6  shows a perspective top view of wire bonding the terminals of the driver-and-control chip to respective leads after the chip has been attached to the top side of the leadframe pad. 
           [0023]      FIG. 7  depicts a perspective bottom view of the leadframe after encapsulating the driver-and-control chip, displaying the pre-coined recess portion of the leadframe pad. 
           [0024]      FIG. 8  shows the perspective bottom view of  FIG. 7  after depositing the adhesive polymeric layers for attaching the sync and control FET chips. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIGS. 2A and 2B  illustrate perspective views of a power supply module generally designated  200  as an exemplary embodiment of the invention,  FIG. 2A  as a top view,  FIG. 2B  as a bottom view. For explanatory reasons, module  200  is shown with a transparent encapsulation  290 . Preferred actual encapsulation uses a black-colored epoxy formulation suitable for a transfer molding technology. The exemplary module of  FIGS. 2A and 2B  has a thickness  291  in the range from 0.7 to 0.8 mm and a rectangular footprint with a module length  292  of 4.8 mm and a width  293  of 3.0 mm. Other pads may be square-shaped. 
         [0026]    Visible through the transparent encapsulation is a metal leadframe generally suitable for Quad Flat No-Lead (QFN) and Small Outline No-Lead (SON) type modules. The leadframe includes a rectangular pad  201  and a plurality of leads  202  and  203 . The pad surface visible in  FIG. 2B  is the first surface  201   a,  the pad surface visible in  FIG. 2A  is the second surface  201   b.  The leadframe is preferably made of copper or a copper alloy; alternative metal selections include aluminum, iron-nickel alloys, and Kovar™. Both surfaces of the leadframe may be prepared to facilitate epoxy adhesive attachment, for instance by a roughened surface; when other embodiments may use a solder as attachment, the leadframe surface preparation may include a layer of tin, or a sequence of plated layers of nickel, palladium, and gold. In addition, at least one surface may have a metal layer deposited to enhance thermal conductivity, for instance by a plated layer of silver. Preferred thickness of the starting metal for the exemplary embodiment shown in  FIGS. 2A and 2B  is in the range from 0.2 mm to 0.4 mm; other embodiments may use thicker or thinner leadframe metal. From the standpoint of low cost and batch processing, it is preferred to start with sheet metal and fabricate the leadframe as a strip by stamping or etching, and to singulate the leadframe for the module by trimming the strip after the encapsulation process. Electrically, pad  201  is tied to the switch node terminal V SW  of the power supply system. 
         [0027]    As illustrated in  FIG. 2B , first pad surface  201   a  has a portion  201   c  offset by a step  270  relative to portion  201   d.  Furthermore, portion  201   c  has an outline (length  271  and width  272 ) suitable for attaching semiconductor chips. The process of offsetting the pad is preferably accomplished by coining during the fabrication process of the leadframe. Alternatively, an etching process may be used; as an example, a chemical etching process may be performed so that only those surfaces (for instance copper or aluminum) are attacked which are not protected by an oxidized metal or a very thin gold layer. 
         [0028]    In the example of  FIG. 2B , the chips of two semiconductor field effect transistors (FETs) are attached to the recessed portion  201   c  of the leadframe pad. The first chip  210  is a drain-down FET, which represents the sync FET (low side FET) of a synchronous buck converter. The second chip  220  is a source-down FET, which represents the control FET (high side FET) of a synchronous buck converter. 
         [0029]      FIG. 2B  shows the sync FET (low side FET) chip  210  with a drain terminal of the FET chip attached to the recessed portion  201   c  of the first pad surface  201   a.  Herein, sync chip  210  is referred to as first FET chip. For the embodiment shown in  FIG. 2B , first chip  210  has a size of about 3.5×2.84 mm, and a thickness of about 0.1 mm. For other embodiments, the chip size and the chip thickness may have significantly greater or smaller values. The attachment is preferably achieved by a layer  211  of conductive adhesive (epoxy), which can be polymerized (cured); an alternative is a z-axis conductive polymer. The preferred thickness of the adhesive layer is at least 25 μm. The conductive adhesive provides high thermal conductivity for spreading heat, since it is filled with metallic (preferably silver) particles. Preferably, the conductive adhesive is the same for all attachment processes of device  200  so that the polymerization process can be performed by a single process simultaneously for all attachments. After attachment, the source terminal  210   a  and the gate terminal  210   b  are co-planar with the surface  201   a  of the un-recessed portion of the first pad surface. The source terminal  210   a  is available, after flipping the finished device, to be attached (by solder or conductive adhesive) to the grounded output terminal V OUT  (P GND ) on the circuit board. This attachment action also ties the sync FET gate terminal  210   b  to the respective terminal on the board. 
         [0030]    Adjacent to first FET chip  210 ,  FIG. 2B  shows the control FET (high side FET) chip  220  with a source terminal of the FET chip attached to the recessed portion  201   c  of the first pad surface  201   a.  Herein, control FET chip  220  is referred to as second FET chip. For the embodiment shown in  FIG. 2B , second chip  220  has a size of about 2.5×1.8 mm, and a thickness of 0.1 mm. For other embodiments, the chip size and the chip thickness may have significantly greater or smaller values. The attachment is preferably achieved by a layer  221  of conductive adhesive (epoxy), which can be polymerized (cured); an alternative is a z-axis conductive polymer. The preferred thickness of the adhesive layer is at least 25 μm. The conductive adhesive provides high thermal conductivity for spreading heat, since it is filled with metallic (preferably silver) particles. After attachment, the drain terminal  220   a  and the gate terminal  220   b  are co-planar with the surface  201   a  of the un-recessed portion of the first pad surface. The drain terminal  220   a  is available, after flipping the finished device, to be attached (by solder or conductive adhesive) to the input terminal V IN  on the circuit board. This attachment action also ties the control FET gate terminal  210   b  to the respective terminal on the board. 
         [0031]    As illustrated in  FIG. 2A , attached to the second surface  201   b  of the leadframe pad  201  is integrated circuit (IC) chip  230 , providing driver and controller functions for the power supply system. Chip  230  is attached to the second surface  201   b  of pad  201  preferably by a layer  231  of conductive adhesive (epoxy) of about 25 μm thickness, which can be polymerized (cured); an alternative is a z-axis conductive polymer. The conductive adhesive provides high thermal conductivity for spreading heat from chip  230  to pad  201 , since it is filled with metallic (preferably solver) particles. Chip  230  may be rectangular and 0.2 mm thick, or it may be square shaped. Other embodiments may have chips, which are smaller or greater, and thicker or thinner. As illustrated in  FIG. 2A , the terminals of chip  230  are wire bonded to respective leads  203 . The preferred diameter of bonding wires  233  is about 25 μm, but may be smaller or greater. While this bonding configuration implies so-called downhill bonding operation, which requires care during the molding operation in order to for avoid wire sweep and the correlated touching of a wire and chip  230 , the bonding in  FIG. 2A  has actually only low risk due to elongated wires and small height difference. 
         [0032]      FIG. 3  illustrates the technical advantages of the invention for applications where the thinness of the converter is at a premium, or where the cooling of the converter has to be maximized for reaching high frequencies of operation. In this embodiment, the driver-and-controller chip is assembled on top of the leadframe as in  FIG. 2A , and the height of the package is in the range between 0.7 mm and 0.8 mm. Both the low side FET  210  and the high side FET  220  are attached to the surface  201   a  of the recessed portion of the leadframe pad. The FET terminals  210   a  and  220   a  opposite the pad are co-planar with the surface  201   a  of the un-recessed portion of the pad surface. Terminals  210   a  and  220   a  are exposed so that they can be readily attached to pads  310  and  320 , respectively, of a circuit board (PC board)  300 . Concurrently, the un-recessed portions of the pad and the leads are attached to pads  301  of board  300 . The attachment can be performed by conductive polymers and by solder. As  FIG. 3  indicates, at least several of these board pads are extended as heat spreaders, or connected to heat sinks in the PC board. The direct attachment of the FET terminals to the circuit (PC) board and the effective cooling of heat spreaders and heat sinks in the PC board allows good cooling and thus low junction temperature of the FETs, and high efficiency and high frequency operation (1 MHz and above) of the converter. 
         [0033]    Assembling a synchronous buck converter according to  FIGS. 2A and 2B  reduces parasitic inductances prevalent in conventional assembly.  FIG. 4  specifies the improvements relative to the conventional assembly shown in  FIG. 1 . The electrical improvements originate from omitting both clips needed in the vertical stacking of the conventional assembly. By eliminating the high side clip (designated  160  in  FIG. 1A ), the drain terminal  220   a  of the high side FET  220  is directly mounted onto the V IN  terminal  320  of the board. The high side clip resistance is eliminated and the high side source resistance is almost negligible. Thus, a parasitic resistance of about 0.5 mΩ and a parasitic inductance of about 0.6 nH from the omitted clip are avoided; the parasitic resistance and inductance the input terminal V IN  have practically vanished. 
         [0034]    By eliminating the low side clip (designated  140  in  FIG. 1A ), the source terminal  210   a  of the low side FET  210  is directly mounted onto the grounded V OUT  terminal  310  of the board. The low side clip resistance is eliminated and the low side source resistance is almost negligible. Thus, a parasitic resistance of about 0.5 mΩ and a parasitic inductance of about 0.6 nH from the omitted clip are avoided; the parasitic resistance and inductance the output terminal V OUT  have practically vanished. 
         [0035]    Concurrently, the un-recessed portion  201   a  of the pad is attached to pad  301  of board  300 . Thereby, the pad of the leadframe is tied to the switch node terminal V SW , designated  301  in  FIG. 4 . Resistance and inductance of the connection are small, about 0.2 mΩ and 0.45 nHy respectively. For the attachment, preferably the same attachment material (conductive adhesive or solder) is used, which is employed for the attachment of the terminals. In the same fashion, leads  203  are attached to board pads  303  by low resistance connections. 
         [0036]    Another embodiment of the invention is a method for fabricating a power supply DC-DC converter system with both semiconductor chips assembled so that terminals of both chips are directly attachable to a circuit board. Compared to prior art, the chips are embedded in an outside recess of a leadframe pad, which also serves as the switch node terminal; in this fashion, the conventional two clips are eliminated, and the number of process steps are reduced so that the method is low-cost compared to prior art and produces devices of small height and small area.  FIGS. 5 to 7  depict certain steps of the assembly process flow. 
         [0037]    The process flow starts in  FIG. 5  by providing a leadframe, which is generally suitable for Quad Flat No-Lead (QFN) and Small Outline No-Lead (SON) devices. The view of  FIG. 5  depicts the first surface  201   a  of the leadframe; the second surface  201   b  is depicted in  FIG. 6 . The exemplary leadframe of  FIG. 5  has a rectangular pad  201 ; for other devices, the leadframe may have a square-shaped pad. Pad  201  will be tied to the switch terminal V SW . The leadframe is preferably made of copper or a copper alloy; alternative metal selections include aluminum, iron-nickel alloys, and Kovar™. Both surfaces of the leadframe may be prepared to facilitate solder attachment, for instance by a sequence of plated layers of nickel, palladium, and gold. The starting thickness of the leadframe metal is in the range from 0.2 mm to 0.4 mm. It is preferred to start with sheet metal and fabricate the leadframe as a strip by stamping or etching, and to singulate the leadframe for the module by trimming the strip after the encapsulation process. The top view of  FIG. 4  illustrates second surface  201   b;  the first surface  201   a  is intended to remain exposed outside the device package. 
         [0038]    First pad surface  201   a  has a portion  201   c  offset by a step  270  relative to portion  201   d.  If the area of portion  201   d  is used as a reference plane, the area of portion  201   c  appears recessed relative to area of portion  201   d.  Furthermore, portion  201   c  has an outline (length  271  and width  272 ) suitable for attaching at least two semiconductor chips. The offset of the pad is preferably accomplished by a coining technique during the fabrication process of the leadframe. The step  270  may be smaller than, equal to, or greater than the starting metal thickness. The height of step  270  is selected so that it is equal to the sum of the height of a semiconductor chip-to-be-attached and the height of the adhesive attachment layer. 
         [0039]    Alternatively, an etching process may be used; as an example, a chemical etching process may be performed so that only those surfaces (for instance copper or aluminum) are attacked which are not protected by an oxidized metal or a very thin gold layer. For some applications, the etched step may be about half of the pad thickness; consequently, a leadframe with portions of such recess is sometimes referred to as half-etched or partially etched leadframe. 
         [0040]      FIG. 6  is a top view of the second surface  201   b  of the leadframe.  FIG. 6  depicts the processes of attaching chip  230  with the driver and controller IC to the second surface  201   b  of the leadframe pad, and of connecting the chip terminals to respective leadframe leads by bonding wires. For the process of attaching, preferably a layer  231  of conductive adhesive (epoxy) of about 25 μm thickness is employed, which can be polymerized (cured); an alternative is a z-axis conductive polymer. 
         [0041]    The next process, depicted in  FIG. 7  (bottom view), includes the encapsulation of the driver-and-control chip  230  in a packaging material, preferably a molding compound  290 . The bottom view of  FIG. 7  shows that the first pad surface  201   a  remains un-encapsulated. This un-encapsulated first surface  201   a  includes the offset portion  201   c,  which has a depth  270  from portion  201   d,  and lateral dimensions suitable for attaching semiconductor chips. 
         [0042]      FIG. 8  depicts the next process, the dispensing or screen printing of layers  211  and  221  of conductive adhesive (epoxy), which can be polymerized (cured). An alternative is a z-axis conductive polymer. Preferred layer thickness is about 25 μm. The adhesive is selected so that the material is suitable for all attachment joints of the product; all adhesive layers can thus undergo the process of polymerization at an elevated temperature simultaneously during a common curing step. 
         [0043]    The result of the next processes, the attachment of the FET chips, is shown in  FIG. 2B . The first FET chip  210 , also called the sync or low side FET, is attached to adhesive layer  211  and thus onto the offset portion  201   c  of first pad surface  201   a.  The low side FET has a drain-down design and is attached with its drain terminal on the adhesive layer. Source and gate terminals are facing away from the pad surface  201   a;  after attachment, source terminal  210   a  and gate terminal  210   b  of FET chip  210  are coplanar with the pad surface of portion  201   d  and thus also co-planar with the leads  202  and  203 . Due to the co-planarity, source terminal  210   a  can be attached (for instance by solder or by conductive adhesive) to a PC board terminal functioning as input V OUT  to the system. This direct attachment of the first chip to the board has the advantage of eliminating parasitic resistance and inductance, and enhancing the heat dissipation during system operation from the system directly into a heat sink of the board. 
         [0044]    Next, the second FET chip  220 , also called the control or high side FET, is attached to adhesive layer  221  and thus onto the offset portion  201   c  of first pad surface  201   a.  The high side FET has a source-down design and is attached with its source terminal on the adhesive layer. Drain and gate terminals are facing away from the pad surface  201   a;  after attachment, drain terminal  220   a  and gate terminal  220   b  of FET chip  220  are coplanar with the pad surface of portion  201   d  and thus also co-planar with the leads  202  and  203 . Due to the co-planarity, drain terminal  220   a  can be attached (for instance by solder or by conductive adhesive) to a PC board terminal functioning as input V OUT  to the system. This direct attachment of the second chip to the board has the advantage of eliminating parasitic resistance and inductance, and enhancing the heat dissipation during system operation from the system directly into a heat sink of the board. 
         [0045]    As mentioned, the construction of device  200  and the fabrication process flow offer the opportunity to employ only conductive (metal-filled) polymeric compounds for assembly and to polymerize all compound layers simultaneously. In addition, when the direct attachment of the terminals of the FETs to a circuit board is also performed using a conductive polymer, the use of lead (Pb) for solders is completely omitted. 
         [0046]    In accordance with a further embodiment, the high current capability of the power supply module can be further extended, and the efficiency further enhanced, by adding a heat spreader to the top surface of the package. In this configuration, the module is dual cooled and can dissipate its heat from both surface sides to heat sinks. 
         [0047]    While the specific embodiments described above have been shown by way of example, it will be appreciated that many modifications and other embodiments will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing description and the associated drawings. As an example, the invention applies not only to field effect transistors, but also to other suitable power transistors. Accordingly, it is understood that various modifications and embodiments are intended to be included within the scope of the appended claims.

Technology Category: 5