Abstract:
An encapsulated buck converter module includes a low side transistor and a control integrated circuit bonded to a first section on a first side of a leadframe, a first clip between a source of the low side transistor and a second section, a source contact of a high side transistor attached to the first section on a second side of the leadframe with a gate contact of the high side transistor attached to a third section, a conductive member attached to the first and second sections on the second side of the leadframe wherein the first side of the conductive member attached to the second conductive member forms a conductive path with a portion of a second side of the conductive member while any portion of the first side of the conductive member attached to the first component attachment section is insulated from the first side of the conductive member, a first plate of a capacitor attached to a drain contact of the high side transistor and a second plate of the capacitor attached to the second side of the conductive member, and means for forming an external connection to the drain contact of the high side transistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional patent application No. 61/159,562 filed Mar. 12, 2009. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to molded multiple die packages, and more particularly to a buck converter in a flip chip in a leaded and molded package (FLMP) with a capacitor inside the molded package. 
     BACKGROUND OF THE INVENTION 
     Buck converters are used as step down voltage regulators. The converters provide an output current through a series inductor that is usually driven by two switching transistors, one of which is connected to supply current to the inductor, and the other connected to draw current from the inductor. Because the buck converter is a switching type regulator the output voltage regulation is influenced by fluctuations in the input voltage to the regulator. Therefore, it is desirable to attenuate any fluctuations in the input voltage at the connections to the switching transistors. 
     The active devices of a buck converter, because of its relative simplicity compared to normal voltage regulators, can be integrated in a single molded package and therefore are often used in small electronic devices. However, there are other passive components needed for the converter, and integration of any one of these components into the molded package would be advantageous. 
     SUMMARY OF THE INVENTION 
     The invention comprises, in one form thereof, an encapsulated buck converter module with an integral capacitor comprising a low side transistor and a control integrated circuit bonded to a first component attachment section on a first side of a leadframe, a first clip between a source of the low side transistor and a second component attachment section. The encapsulated buck converter module further includes a source contact of a high side transistor attached to the first component attachment section on a second side of the leadframe with a gate contact of the high side transistor attached to a third component attachment section a conductive member attached to the first component attachment section and the second component attachment section on the second side of the leadframe wherein the first side of the conductive member attached to the second conductive member forms a highly conductive path with a portion of a second side of the conductive member while any portion of the first side of the conductive member attached to the first component attachment section is insulated from the first side of the conductive member a first plate of a capacitor attached to a drain contact of the high side transistor and a second plate of the capacitor attached to the second side of the conductive member, and means for forming an external connection to the drain contact of the high side transistor. 
     In still another form, the invention includes a method for forming an encapsulated buck converter module with an integral capacitor. The method comprises the steps of die bonding a low side transistor and a control integrated circuit to first component attachment section on a first side of a leadframe, attaching a first clip between a source of the low side transistor and a second component attachment section attaching wirebonds to the control integrated circuit. The method further comprises the steps of attaching on a second side of the leadframe a source contact of a high side transistor to the first component attachment section and a gate contact of the high side transistor to a third component attachment section, attaching to the second side of the leadframe a first side of a conductive member to the first component attachment section and the second component attachment section wherein the first side of the conductive member attached to the second conductive member forms a highly conductive path with a portion of a second side of the conductive member while any portion of the first side of the conductive member attached to the first component attachment section is insulated from the first side of the conductive member, attaching a first plate of a capacitor to a drain contact of the high side transistor and a second plate of the capacitor to the second side of the conductive member, attaching a first end of a second clip to the drain contact of the high side transistor, and encapsulating the leadframe is such a manner that part of the leadframe and a second end of the second clip is not encapsulated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other features, characteristics, advantages, and the invention in general will be better understood from the following more detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a buck converter with a two capacitors across the supply voltage; 
         FIGS. 2A and 2B  are respective top and bottom isometric views of a flip chip leaded and molded (FLMP) buck converter with an integral molded capacitor according to one embodiment of the present invention; 
         FIGS. 3A and 3B  are  FIGS. 2A and 2C , respectively, with the molding material shown in the translucent outline; 
         FIGS. 4A ,  4 B,  4 C, and  4 D are respective front, side, top, and bottom views of the FLMP buck converter shown in  FIGS. 2A and 2B ; 
         FIGS. 5A ,  5 B,  5 C, and  5 D are  FIGS. 4A ,  4 B,  4 C, and  4 D, respectively, with the molding material shown in the translucent outline; 
         FIGS. 6A ,  6 B,  6 C,  6 D,  6 E, and  6 F are isometric views of selected manufacturing stages of one method of making the FLMP buck converter shown in  FIG. 2A ; 
         FIGS. 7A ,  7 B,  7 C,  7 D,  7 E, and  7 F are isometric views of additional selected manufacturing stages of one method of making the FLMP buck converter shown in  FIG. 2A ; 
         FIG. 8  is a top isometric view of a FLMP buck converter with an integral molded capacitor with the molding material shown in outline according to another embodiment of the present invention; 
         FIGS. 9A and 9B  are isometric views of the BT or FR4 board shown in  FIG. 8 ; and 
         FIGS. 10A ,  10 B,  10 C,  10 D,  10 E, and  10 F are isometric views of selected manufacturing stages of one method of making the FLMP buck converter shown in  FIG. 8 . 
     
    
    
     It will be appreciated that for purposes of clarity and where deemed appropriate, reference numerals have been repeated in the figures to indicate corresponding features. Also, the relative size of various objects in the drawings may in some cases have been distorted to more clearly show the invention. 
     DETAILED DESCRIPTION 
     Turning now to the drawings,  FIG. 1  is a simplified schematic diagram of a buck converter circuit  20  which includes a supply voltage source  22 , a supply voltage capacitor  24 , two MOSFET driver circuits  26  and  28 , an integrated modular capacitor  30 , a high side MOSFET  32  and integral diode  34 , a low side MOSFET  36  and integral diode  38 , the gates of which are connected to the MOSFET drivers  26  and  28 , respectively. An input signal on line  40  controls the MOSFET drivers  26  and  28 . The output of a buck converter module  42 , indicated by the dashed box, on line  44  is coupled to a load  46  through an inductor  48 . The load  46  is in parallel with a load capacitor  50 . The buck converter module  42  includes the components of one embodiment of an FLMP buck converter with a molded capacitor according to the present invention. Also shown in  FIG. 1  are parasitic inductors  52 ,  54 ,  56 ,  58 ,  60 , and  62 . Parasitic inductors  52  and  54  are part of the connection between the supply voltage source  20  with its capacitor  24  and the buck converter module  42 . The parasitic inductors  56 - 62  are in the high current paths in the buck converter module  42 . The operation of a buck converter of the type shown in  FIG. 1  is well known in the art. 
     The parasitic inductors  52  and  54  between the supply voltage source  22  and the buck converter module  42  are relatively large compared to the parasitic inductors  56 - 62  in side the buck converter module  42 . The an integrated modular capacitor  30  stabilizes the supply voltage inside the buck converter module  42  by attenuating the supply voltage fluctuations resulting from the parasitic inductors  52  and  54 . 
       FIGS. 2A and 2B  are respective top and bottom isometric views of a flip chip leaded and molded (FLMP) buck converter  70  with an integral molded capacitor  130  according to one embodiment of the present invention. The FLMP shown in the drawings is a SO-8 FLMP package, although those skilled in the art will understand that the present invention is applicable to many other types of molded semiconductor packages. The integral molded capacitor  130  corresponds to the an integrated modular capacitor  30  shown in  FIG. 1 . The FLMP buck converter  70  has eight external leads,  72 ,  74 ,  76 ,  78 ,  80 ,  82 ,  84 , and  86  and is molded with an encapsulating material  88 . The top of the integral molded capacitor  130  is visible in  FIG. 2A  along with an end  90  of one tie bar. The integral molded capacitor  130  contains a dielectric material  92  separated by two capacitor plates  94  and  96 . An end  98  of a first clip  100  is visible in  FIG. 2B  together with the end  90  of the tie bar. 
       FIGS. 3A and 3B  are  FIGS. 2A and 2C , respectively, with the molding material shown in translucent outline. Each of the external leads  72 - 86  extend into the encapsulation material  88  to a component attachment section inside the encapsulation material  88  that is wider than the external leads  72 - 86  for connection to wire bonds and components inside the FLMP buck converter  70 . External leads  72 ,  74 ,  76 ,  78 , and  80  have corresponding component attachment sections  102 ,  104 ,  106 ,  108 , and  110 , respectively. External leads  82  and  84  share a large component attachment section  112 , and external lead  86  has a component attachment section  114 . 
     A high side transistor  132  is flip chip bonded to the component attachment sections  110  and  112 , with the gate solder bump attached to the top of the component attachment section  110  and the source solder bumps attached to the top of the component attachment section  112 . Transistor  132  corresponds to transistor  32  in  FIG. 1 , and thus external lead  80  corresponds to the gate of the high side transistor  32 , and the external leads  82  and  84  correspond to the output line  44  in  FIG. 1 . The first clip  100  is bonded to the drain of transistor  132 , and bends at a 90° angle such that end  98  is exposed on the bottom of the FLMP buck converter  70 , and corresponds to the connection of the buck converter module  42  to the positive supply terminal of voltage source  22  in  FIG. 1 . The end  98  of the first clip  100  is thus a leadless contact for the FLMP buck converter  70 . 
     Also attached to the drain of the high side transistor  132  is the plate  96  of the integral molded capacitor  130 . The other plate  94  of the integral molded capacitor  130  is attached to the top of a copper block  134 . The bottom of the copper block is attached to, but electrically isolated from, the top of the component attachment section  112 . The bottom of the copper block  134  is attached to, and electrically connected to, the top of the component attachment section  114  which is integral with the external lead  86 . The external lead  86  corresponds to the connection of the buck converter module  42  to the negative terminal of the voltage supply source  22  in  FIG. 1 . 
     Die bonded to the bottom of the component attachment section  114  is a low side transistor  136 , corresponding to the low side transistor  36  shown in  FIG. 1 , and MOSFET driver integrated circuit  138  which controls the gate voltages on the high side transistor  132  and the low side transistor  136 . A second clip  140  is attached to, and forms an electrical connection between, the source of the low side transistor  136  and the bottom of the component attachment section  114 . Since the bottom of the component attachment section  114  is higher than the source contact of the low side transistor  136 , the second clip  140  also has a 90° with the end of the second clip  140  attached to the bottom of the component attachment section  114 . 
     In the embodiment shown in  FIGS. 3A and 3B  the MOSFET driver integrated circuit  138  has 9 wire bond wires  142 ,  144 ,  146 ,  148 ,  150 ,  152 ,  154 ,  156 , and  158  bonded to it with an additional bond wire  160  connecting the gate of the low side transistor  136  to the component attachment section  102 . Thus external lead  72  is connected to the gate of the low side transistor  136 . 
       FIGS. 4A ,  4 B,  4 C, and  4 D are respective front, side, top, and bottom views of the FLMP buck converter  70  shown in  FIGS. 2A and 2B . 
       FIGS. 5A ,  5 B,  5 C, and  5 D are  FIGS. 4A ,  4 B,  4 C, and  4 D, respectively, with the molding material shown in translucent outline.  FIG. 5A  shows the materials used to attach the various components of the FLMP buck converter  70  together. As stated above, the high side transistor  132  is flip chip bonded to the component attachment sections  110  and  112 . Solder bumps  162  formed on the high side transistor  132  when the transistor is fabricated are shown in  FIG. 5A . The first clip  100  may be attached to the top of the high side transistor  132  using low temperature solder  164 . Alternatively, conductive epoxy may be used for the material  164 . Similarly the integral molded capacitor may be attached to the top of the high side transistor  132  and the top of the copper block  134  using low temperature solder  166  with conductive epoxy as an alternative. The bottom of the copper block  134  may be attached to the component attachment section  112  using non-conductive epoxy  168  to insulate the copper block  134  from the component attachment section  112 . The copper block may be soldered to the component attachment section  114  using a solder layer  170  to provide an electrical connection from the plate  94  of the integral molded capacitor  130  to the external lead  86  and to the source of the low side transistor  136 . The MOSFET driver integrated circuit  138  is die bonded to the component attachment section  112  using bonding material  172  which may be non-conductive adhesives such as non-conductive epoxy. Similarly, the low side transistor  136  is die bonded to the component attachment section  112  using bonding material  174  which may be solder or conductive epoxy. The second clip  140  is soldered to the source of the low side transistor  136  using low temperature melting solder  176 , and to the component attachment section  114  using solder  178 . 
       FIGS. 6A-6F  are isometric views of selected manufacturing stages of one method of making the FLMP buck converter  70 . The bottom of a leadframe blank  180  is shown in  FIG. 6A . The low side transistor  136  is die bonded to the component attachment section  112  in  FIG. 6B  using the die bond material  174 . After solder paste  176  and  178  has been applied to a section of the source contact of the low side transistor  136  and to the component attachment section  114  respectively, the second clip  140  is positioned over the solder paste  176  and  178 , and then beat is applied to reflow the solder paste as shown in  FIG. 6C . In  FIG. 6D  the MOSFET driver integrated circuit  138  has been die bonded to the component attachment section  112  using the die bond material  172 . The wire bonds  142 - 160  are then attached as shown in  FIG. 6E , and the leadframe  180  is subsequently flipped over as shown in  FIG. 6F  in preparation for attaching the remaining components of the FLMP buck converter  70  according to the process steps shown in  FIGS. 7A-7F . 
       FIGS. 7A-7F  are isometric views of additional selected manufacturing stages of one method of making the FLMP buck converter  70 . In  FIG. 7A  the high side transistor  132  has been flip chip bonded to the component attachment sections  112  and  110  with the source of the high side transistor  132  attached to the component attachment section  112 , and the gate of the high side transistor  132  attached to the component attachment section  110 . The low temperature melting solder (or conductive adhesives)  170  is printed on the component attachment section  114  and the non-conductive epoxy  168  is attached to the component attachment section  112  as shown in  FIG. 7B . In  FIG. 7C  the copper block  134  has been attached to the component attachment sections  114  and  112 . The low temperature melting solder (or conductive adhesives)  166  is attached to portions of the top of the high side transistor  132  and the copper block  134 , and the integral molded capacitor  130  is attached to the top sides of the high side transistor  132  and the copper block  134  as shown in  FIG. 7D .  FIG. 7E  shows the assembly after the low temperature melting solder (or conductive adhesives)  164  has been applied to an exposed portion of the top of the high side transistor  132 , and the first clip  100  has been attached to the high side transistor  132 . Then a second solder reflow for the low temperature melting solder or cure of the conductive adhesives is performed. The assembly is then molded as shown in  FIG. 7F . The top of the integral molded capacitor  130  is exposed on the top on assembly and the end  98  of the first clip  100  is exposed at the bottom of the assembly. The leadframe  180  is then trimmed and the external leads  72 - 86  are formed to complete the assembly of the FLMP buck converter  70 . 
       FIG. 8  is a top isometric view of a FLMP buck converter  190  with an integral molded capacitor with the molding material shown in outline according to another embodiment of the present invention. In the FLMP buck converter  190 , the copper block of the FLMP buck converter  70  has been replaced with a bismaleimide-triazine (BT) or flame retardant 4 (FR4) board  192 . The BT board  192  has a top metal plate  194 , such as copper, and two bottom metal plates, a larger bottom metal plate  196  and a smaller bottom metal plate  198 , which may also be copper, with bismaleimide-triazine or flame retardant 4  200  separating the metal plates  194 ,  196 , and  198 . Two vias  202  form a connection between the top metal plate  194  and the bottom smaller metal plate  198 , and the large bottom metal plate  196  is electrically isolated from the metal plates  194  and  198 . See  FIGS. 9A and 9B . The BT or FR4 board is attached to the component attachment sections  114  and  112  with the larger bottom metal plate  196  soldered (or bonded) to the component attachment section  112  with low temperature melting solder (or conductive adhesives)  204 , and the smaller bottom metal plate  198  soldered (or bonded) to the component attachment section  114  using the low temperature melting solder (or conductive adhesive)  170 . The top metal plate  194  is attached to the integral molded capacitor  130  using the low temperature melting solder (or conductive adhesive)  166 . 
     The assembly of the FLMP buck converter  190  begins with the attachment of the components to the bottom of the leadframe  180  as shown in  FIGS. 6A-6F .  FIGS. 10A-10F  are isometric views of selected manufacturing stages of one method of making the FLMP buck converter  190 .  FIG. 10A  is the same as  FIG. 7A . In  FIG. 10B , the non-conductive epoxy  168  of  FIG. 7B  has been replaced by solder (or conductive adhesives)  204  in  FIG. 10B . In  FIGS. 10C-10E , the copper block  134  shown in  FIGS. 7C-7E  has been replaced with the BT or FR4 board  192 . 
     Those skilled in the art will understand that the MOSFET driver integrated circuit  138  can be of many different designs, and therefore there has been no attempt to correlate the connections to the MOSFET driver circuits  26  and  28  shown in  FIG. 1  to the FLMP buck converters  70  and  190 . The integral molded capacitor  130  can be a custom capacitor, or can be a chip capacitor that is widely used in the electronics industry. In addition, the integral molded capacitor may not be exposed through the encapsulation material in some embodiments of the present invention. 
     While the invention has been described with reference to particular embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope of the invention. 
     Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope and spirit of the appended claims.