Abstract:
A power conversion device including a low-side MOSFET, a high-side MOSFET and an integrated control IC chip is disclosed. The power conversion device further includes a substrate comprising a first mounting area having a first group of welding discs and a second mounting area having a second group of welding discs; a first chip flipped and attached to the first mounting area; a second chip flipped and attached to the second mounting area; a metal clip; and a molding body covering a front surface of the substrate, the first chip, the second chip and the metal clip. Metal pads on a front side of the first chip is attached to the first group of welding discs. Metal pads on a front side of the second chip is attached to the second group of welding discs. The metal clip connects a connection pad to a back metal layer of the first chip.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to a power conversion device. More particularly, the present invention relates to a DC-DC voltage converter. The DC-DC voltage converter includes a low-side (LS) metal oxide semiconductor field-effect transistor (MOSFET), a high-side (HS) MOSFET and a control integrated circuit (IC) chip. 
     BACKGROUND OF THE INVENTION 
     In a DC-DC voltage converter, the power consumption of a MOSFET is high. The source terminal or the drain terminal of the MOSFET is required to dissipate heat. Conventionally, part of a lead frame is exposed from a molding body to improve heat dissipation of the MOSFET. For example, as shown in  FIG. 6  (same as FIG. 1 of the U.S. Pat. No. 8,981,539), a DC-DC converter  10  includes a HS MOSFET  11 , a LS MOSFET  13 , and a control chip  12 . The control chip  12  outputs a pulse width modulation (PWM) or pulse frequency modulation (PFM) signal to the HS MOSFET  11  and the LS MOSFET  13  and then receives feedback signals. Electrode pads arranged on the HS MOSFET  11  and the LS MOSFET  13  are electrically connected to input/output (I/O) metal pads of the control chip  12  through a plurality of leads. The HS MOSFET  11  and the LS MOSFET  13  are respectively attached to a substrate  21  and a substrate  23 . The substrate  21  and the substrate  23  are separated from each other. The source of the HS MOSFET  11  is connected to the substrate  23  through a metal sheet  15 . The source of the LS MOSFET  13  is connected to the pin  24  through a metal sheet  16 . The control chip  12  is attached to another separated metal substrate  22 . The respective bottom surfaces of the metal substrate  21  and the metal substrate  23  are exposed from a bottom surface of a molding body of the DC-DC converter  10  (not shown). The exposed bottom surfaces are used as terminals electrically connected to external circuits for heat dissipation. The disadvantage of the DC-DC converter  10  is that dimensions of metal substrate  21  and the metal substrate  23  are large. Therefore, a device package size of the DC-DC converter  10  is large and the corresponding fabrication cost is high. 
     US patent application publication US20120061813A1 discloses a DC-DC voltage converter for power conversion. The DC-DC voltage converter includes a HS MOSFET and a LS MOSFET. The HS MOSFET and the LS MOSFET are arranged side by side on a substrate. A control chip is stacked on the HS MOSFET and the LS MOSFET. It requires that a metal contact lead of the HS MOSFET and the LS MOSFET must have a lower loop height to avoid the control chip to contact the metal contact lead. The heat radiating route of the HS MOSFET and the LS MOSFET is blocked by the control chip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  to  FIG. 1D  are schematic diagrams illustrating a circuit board in examples of the present disclosure. 
         FIG. 2A  to  FIG. 2I  are schematic diagrams illustrating a process of mounting a chip on a front surface the circuit board. 
         FIG. 3A  to  FIG. 3C  are schematic diagrams of metal welding pads with larger heat dissipation areas on a back surface of the circuit board. 
         FIG. 4A  to  FIG. 4C  are schematic diagrams of the metal welding pads for heat dissipation on the back surface of the circuit board. 
         FIG. 5A  to  FIG. 5F  are schematic diagrams illustrating a process of mounting three separated chips on a front surface of the circuit board in examples of the present disclosure. 
         FIG. 6  is a schematic diagram of a conventional DC-DC converter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1A  is a schematic diagram of an insulating substrate  100  in examples of the present disclosure. In one example, the substrate  100  is a printed circuit board for electrical connection of electronic parts and components. The substrate  100  includes a first mounting area  110  and a second mounting area  120  for mounting the semiconductor chips. The first mounting area  110  and the second mounting area  120  are close to each other and are arranged side by side on a front surface of the substrate  100 . 
       FIG. 1B  is another schematic diagram of the substrate  100 . The substrate  100  further includes a group of pin pads  151   a - 151   d  and another group of pin pads  152   a - 152   d  located on a back surface of the substrate  100 . Pin pads  151   a - 151   d  are located near a top edge of the substrate  100 . The other group of pin pads  152   a - 152   d  are located near a bottom edge opposite the top edge. In examples of the present disclosure, the pin pads  151   a - 151   d  are arranged in a row and the pin pads  152   a - 152   d  are arranged in another row. 
       FIG. 1C  is a zoomed-in view of the first mounting area  110 .  FIG. 1D  is a zoomed-in view of the second mounting area  120 . In  FIG. 1C , the first mounting area  110  includes a first set of welding discs  110   a  and a second set of welding discs  110   b . In examples of the present disclosure, the first mounting area  110  includes a plurality of first set of welding discs  110   a  and only one second set of welding disc  110   b . In  FIG. 1D , the second mounting area  120  includes a third set of welding discs  121   a  and  121   b , a fourth set of welding discs  124   a - 124   e , a fifth set of welding discs  122   a  and a sixth set of welding discs  123   a . The third set of welding discs  121   a - 121   b  are not interconnected. The fourth set of welding discs  124   a - 124   e  are not interconnected. The fifth set of welding discs  122   a  are interconnected through a metal wiring  101  on the front surface of the substrate  100 . The sixth set of welding discs  123   a  is interconnected through another metal wiring  101  on the front surface of the substrate  100 . 
     Referring to  FIG. 1A ,  FIG. 1C  and  FIG. 1D , the second set of welding disc  110   b  in the first mounting area  110  is directly interconnected with the third set of welding discs  121   b  in the second mounting area  120  through the metal wiring  101 . One of the first set of welding discs  110   a  in the first mounting area  110  is directly interconnected with of the third set of welding disc  121   a  in the second mounting area  120  through another metal wiring  101 . 
     Referring to  FIG. 1A ,  FIG. 1B  and  FIG. 1D , each welding disc in the fourth set of welding discs  124   a - 124   e  is connected to a respective pin pad located on the front surface of the substrate  100  through the wiring  101 . For example, a fourth set of welding disc  124   a  ( FIG. 1D ) is connected to one welding disc  142   a  located on the front surface of the substrate  100 . The welding disc  142   a  is aligned with and overlapped with the pin pad  152   a  ( FIG. 1B ) on the back surface of the substrate  100 . A through hole penetrating through the thickness of the substrate  100  is arranged between the welding disc  142   a  on the front surface and the pin pad  152   a  on the back surface of the substrate  100 . The through hole is lined and filled with a metal material forming the interconnecting via  102 . Therefore, the welding disc  142   a  on the front surface of the substrate  100  is electrically connected with the pin pad  152   a  on the back surface. Similarly, one fourth set of welding disc  124   b  ( FIG. 1D ) is connected to one welding disc  142   b  ( FIG. 1A ) located on the front surface of the substrate  100 . The welding disc  142   b  is aligned with the pin pad  152   b  ( FIG. 1B ). Therefore, the fourth set of welding disc  124   b  and the pin pad  152   b  are electrically connected by another interconnecting via  102 . The fourth set of welding disc  124   c  ( FIG. 1D ) is connected to one welding disc  142   c  ( FIG. 1A ) located on the front surface of the substrate  100 . The welding disc  142   c  is aligned with a pin pad  152   c  on the back surface of the substrate  100 . Therefore, the welding disc  142   c  is electrically connected to the pin pad  152   c  by yet another interconnecting via  102 . The fourth set of welding disc  124   d  ( FIG. 1D ) is connected to one welding disc  142   d  ( FIG. 1A ) located on the front surface of the substrate  100 . The welding disc  142   d  is aligned with the pin pad  152   d  ( FIG. 1B ) on the back surface of the substrate  100 . Therefore, the fourth set of welding disc  124   d  is electrically connected with the pin pad  152   d  by still another interconnecting via  102 . One fourth set of welding disc  124   e  ( FIG. 1D ) is connected to one welding disc  141   b  ( FIG. 1A ) located on the front surface of the substrate  100  through the wiring  101 . The welding disc  141   b  and the pin pad  151   d  ( FIG. 1B ) are aligned and are electrically connected by yet still another interconnecting via  102 . Therefore, the fourth set of welding disc  124   e  is electrically connected to the pin pad  151   d.    
     Referring to  FIG. 1A  to  FIG. 1D , the fifth set of welding discs  122   a  are electrically connected together. The fifth set of welding discs  122   a  are also electrically connected to a metal connection pad  130  on the front surface of the substrate  100  through the wiring  101 . The metal connection pad  130  is near the first mounting area  110 . The connection pad  130  is preferably in a shape of long strip. The welding disc  141   a  and the welding disc  141   b , on the front surface of the substrate  100 , are arranged along a direction of an extended line of the connection pad  130 . The connection pad  130 , the welding disc  141   a  and the welding disc  141   b  are located near the top edge of the substrate  100  and are overlapped with the first row of pin pads  151   a - 151   d  ( FIG. 1B ). The welding discs  142   a - 142   d  are located near the bottom edge of the substrate  100  and are overlapped with the second row of pin pads  152   a - 152   d  ( FIG. 1B ). As mentioned above, each of the group of welding discs  142   a - 142   d  on the front surface of the substrate  100  are aligned with and are overlapped with a respective pin pad of the row of pin pads  152   a - 152   d  on the back surface of the substrate  100 . The connection pad  130  ( FIG. 1A ) on the front surface of the substrate  100  is aligned with and is overlapped with two pin pads  151   a  and  151   b  ( FIG. 1B ) on the back surface of the substrate  100 . The connection pad  130  is electrically connected to the two pin pads  151   a  and  151   b  by interconnecting vias  102  formed in the through holes penetrating through the connection pad  130  and the two pin pads  151   a  and  151   b . Therefore, the fifth set of welding discs  122   a  and the connection pad  130  are electrically connected to the two pin pads  151   a  and  151   b  ( FIG. 1B ). 
     Referring to  FIG. 1A  to  FIG. 1D , all the sixth set of welding discs  123   a  are interconnected with each other through the metal wiring  101  on the front surface of the substrate  100  and are further connected to one welding disc  141   a  through the wiring  101 . The welding disc  141   a  on the front surface of the substrate  100  ( FIG. 1A ) is aligned with and is overlapped with one pin pad  151   c  on the back surface of the substrate  100  ( FIG. 1B ). The welding disc  141   a  and the pin pad  151   c  are electrically connected by an interconnecting via  102  formed in the through hole penetrating through the welding disc  141   a  and the pin pad  151   c . All the sixth set of welding discs  123   a  are electrically connected with the pin pad  151   c . In examples of the present disclosure, the metal pin pads on the back surface of the substrate  100  are respectively electrically connected to all metal welding discs on the front surface of the substrate  100  or the metal connection pad  130 . 
     Another example is shown in  FIG. 2A , which is slightly different from  FIG. 1B . The first row of pin pads  151   a - 151   d  and the second row of pin pads  152   a - 152   d  are arranged in the edge areas of the back surface of the substrate  100 . A first metal welding pad  161  and a second metal welding pad  162  are arranged in a center area of the back surface of the substrate  100 . The first metal welding pad  161  and the second metal welding pad  162  are side by side and are separated from each other. Referring to  FIG. 1A  and  FIG. 2A , the first set of welding discs  110   a  formed in the first mounting area  110  on the front surface of the substrate  100  are aligned with and are overlapped with the first metal welding pad  161  on the back surface of the substrate  100 . The through holes are penetrating through the first set of welding discs and the first metal welding pad  161  with the interconnecting via  102  formed in the through hole. Therefore, the first set of welding discs  110   a  and the first metal welding pad  161  are electrically connected. However, the second set of welding discs  110   b  arranged in the first mounting area  110  is not electrically connected to the first metal welding pad  161 . Referring to  FIG. 1D  and  FIG. 2A , the fifth set of welding discs  122   a  arranged in the second mounting area  120  are aligned with and are overlapped with the second metal welding pad  162  with the through holes penetrating through the welding discs and the second metal welding pad  162 . The through holes are filled with interconnecting vias  102 . Therefore, the fifth set of welding discs  122   a  are electrically connected to the second metal welding pad  162 . 
     In  FIG. 2B , one first semiconductor chip  201  is mounted on the first mounting area  110  and a second semiconductor chip  202  is mounted on the second mounting area  120 . The first semiconductor chip  201  and the second semiconductor chip  202  are flipped and attached to the front surface of the substrate  100 . 
     In  FIG. 2C , for clarity, the first semiconductor chip  201  and the second semiconductor chip  202  are drawn as transparent to show an alignment mode of the first semiconductor chip  201  and the second semiconductor chip  202  overlapping on the first mounting area  110  and the second mounting area  120 . In a conventional DC-DC voltage converter, a LS power MOSFET and a HS power MOSFET are usually connected between a power voltage VIN and a ground terminal GND in serial. A control circuit (IC) controls the on-off of the LS power MOSFET and the HS power MOSFET. In examples of the present disclosure, the first semiconductor chip  201  is the LS power MOSFET and the second semiconductor chip  202  with the IC chip integrated thereon is the HS power MOSFET of the DC-DC converter. 
     In  FIG. 2D , a metal clip  211  is attached to the connection pad  130  and the back metal layer on the back side of the first flipped chip  201 . The metal clip  211  electrically connects the connection pad  130  and the back metal of the first flipped chip  201 . In examples of the present disclosure, the metal clip  211  can be replaced by the bonding wires  212  as shown in  FIG. 2E . In examples of the present disclosure, the metal clip  211  can be replaced by a conductive band or other similar structure. 
       FIG. 2F  is a cross-sectional view of the structure along A-A plane in  FIG. 2D . The metal clip  211  includes an upper end and a lower end. The upper end is attached to the back metal layer of the first flipped chip  201  through a conductive bonding material. The lower end is attached to the connection pad  130  through the conductive bonding material. A height between the upper end and the lower end of the metal sheet is reduced. The power MOSFET usually includes a gate, a source and a drain. The electrical connection between the first chip  201  and the second chip  202  and the substrate  100  is illustrated in  FIG. 1D  and  FIG. 2C . 
     In examples of the present disclosure, a first metal pad located on the front side of the first chip  201  is configured as the source S 1  of the LS MOSFET. A second metal pad on the front side of the first chip  201  is configured as the gate G 1 . A third metal layer on the back side of the first chip  201  is configured as the drain. In  FIG. 2C , the source S 1  of the first chip  201  is overlapped and mounted on the first set of welding discs  110   a  formed in the first mounting area  110 . In one example, the source S 1  and the first set of welding discs are connected by solder balls or the other similar metal bumps placed on the source S 1  of the first chip  201 . The gate G 1  of the first chip  201  is overlapped and mounted on the second set of welding disc  110   b  formed in the first mounting area  110 . In one example, the gate G 1  and the second set of welding discs are connected by solder balls placed on the gate G 1  of the first chip  201 . The second chip  202  includes a HS MOSFET and an integrated control circuit module. In  FIG. 2C , a plurality of source metal pads S 2  of the HS MOSFET located on the front side of the second chip  202  are overlapped and mounted on the fifth sets of welding discs  122   a  formed in the second mounting area  120 . Each source metal pad S 2  is attached to a respective welding disc of the fifth set of welding discs  122   a . In one example, the attachment is by solder balls or other similar metal bumps. A plurality of drain metal pads D 2  of the HS MOSFET located on the front side of the second chip  202  are attached on the sixth set of welding discs  123   a  formed in the second mounting area  120 . Each drain pad D 2  is attached to a respective welding disc of the sixth set of welding disc  123   a . In one example, the attachment is by solder balls. Because the HS MOSFET and the control circuit in the second chip  202  are integrated on the same chip, the gate of the HS MOSFET in the second chip  202  can be directly coupled to the control circuit in the same chip. Therefore, it is unnecessary to form the gate pad on the HS MOSFET. 
     As shown in  FIG. 2C , the third set of welding disc  121   a  is electrically connected to one of the first set of welding disc  110   a . The third set of welding disc  121   b  is electrically connected to one of the second set of welding discs  110   b . The fourth set of welding discs  124   a - 124   b  are connected to a respective pin pad of the pin pads  152   a - 152   d . The fourth set of welding disc  124   e  is connected to the pin pad  151   d . Therefore, metal pads IO 1 -IO 7  of various input or output terminals of the control circuit integrated on the second chip  202  can be arranged on the front side of the second chip  202 . The metal pads IO 1 -IO 7  are attached to a respective welding disc of the third set of welding discs  121   a - 121   b  and the fourth set of welding discs  124   a - 124   e  by a respective solder ball or a respective metal bump. 
     In  FIG. 2C , the source S 1  of the LS MOSFET of the first chip  201  is attached to the first set of welding discs  110   a . The gate G 1  of the LS MOSFET is attached to the second set of welding disc  110   b . The first set of welding discs  110   a  are connected to the first metal welding pad  161  ( FIG. 2A ) on the back surface of the substrate  100 . Therefore, the source S 1  of the LS MOSFET is also electrically connected to the first metal welding pad  161  on the back surface of the substrate  100 . The drain of the LS MOSFET, i.e., the back metal of the first chip  201  is connected to the connection pad  130  through a metal clip  211 . The source S 2  of the HS MOSFET integrated in the second chip  202  is electrically connected to the connection pad  130  through the fourth set of welding discs  122   a . Therefore, the source S 2  of the HS MOSFET and the drain of the LS MOSFET are electrically connected. To electrically connecting the pin pads  151   a  and  151   b  on the back surface of the substrate  100  with the connection pad  130 , nodes (LX) interconnected by the source S 2  of the HS MOSFET and the drain of the LS MOSFET are configured in the pin pads  151   a  and  151   b . The drain D 2  of the HS MOSFET integrated in the second chip  202  is connected to the welding disc  141   a  on the top edge of the substrate  100  through the fifth set of welding discs  123   a . The welding disc  141   a  is connected to one pin pad  151   c  on the back surface of the substrate  100  so that the power voltage VIN is transmitted to the drain D 2  of the HS MOSFET through the pin pad  151 C as the drain D 2  of the HS MOSFET is electrically connected to one pin pad  151   c  on the back surface of the substrate  100 . 
     When the control circuit integrated in the second chip  202  controls the on or off of the LS MOSFET, a control signal is sent from the metal pad IO 1  to drive the LS MOSFET. As the metal pad IO 1  of the control circuit is attached to the third set of welding discs  121   b , which is connected to the second set of welding discs  110   b  through the wiring  101 , and the gate G 1  of the LS MOSFET is attached to the welding disc  110   b , the metal pad IO 1  of the control circuit is coupled to the gate G 1  of the LS MOSFET through the route. The metal pad IO 2  of the control circuit integrated in the second chip  202  is attached to the third set of welding discs  121   a , which is further connected to one or a plurality of the first set of welding discs  110   a  through the wiring  101 , and the source S 1  of the LS MOSFET is attached to the welding disc  110   a  and the first set of welding discs  110   a  is further electrically connected to the first metal welding pad  161  ( FIG. 2A ) on the back surface of the substrate  100 , so the metal pad IO 2  of the control circuit is electrically coupled to the first metal welding pad  161  on the back surface of the substrate  100 . Each of the metal pads  103 - 106  of the control circuit are electrically connected to a respective welding disc of the welding discs  142   a - 142   d  near the bottom edge of the substrate  100  through the wiring  101 . The metal pad IO 7  of the control circuit is electrically connected to the welding disc  141   b  near the top edge of the substrate  100  through the wiring  101 . The welding discs  142   a - 142   d  on the front surface of the substrate  100  are correspondingly connected to the pin pads  152   a - 152   d  on the back surface of the substrate  100 . The welding disc  141   b  on the front surface of the substrate  100  is correspondingly connected to the pin pad  151   d  on the back surface of the substrate  100 . Therefore, the metal pads IO 3 -IO 6  of the control circuit are electrically connected to the pin pads  152   a - 152   d  on the back surface of the substrate  100  correspondingly. The metal pad IO 7  of the control circuit is electrically connected to the pin pad  151   d  on the back surface of the substrate  100  correspondingly. The first welding pad  161  ( FIG. 2A ) with a larger size is arranged on the back surface of the substrate  100  improving the heat dissipation generated in the HS MOSFET and the LS MOSFET. In examples of the present disclosure, the first metal welding pad  161  serves as the GND pin (the source S 1  of the LS MOSFET) and also improves the heat dissipation of the device. 
       FIG. 2F  and  FIG. 2G  are cross sectional view of the structure in  FIG. 2D  along the dotted lines AA and BB respectively. The first chip  201  is flipped and mounted on the welding discs  110   a  and  110   b  through solder balls  220  placed on the source S 1  and the gate G 1  of the LS MOSFET. The HS MOSFET integrated in the second chip  202  is flipped and attached on the fifth and sixth sets of welding discs through the solder balls  220  placed on the source S 2  and the drain D 2  of the HS MOSFET. The metal pads IO 1 -IO 7 , as the terminals of the control circuit, integrated in the second chip  202  are mounted on the third and fourth sets of welding discs through the solder balls  220 . After the first and second chips  201  and  202  are attached to the substrate  100 , a molding layer  230  is formed to cover the first chip  201 , the second chip  202  and the metal clip  211  as shown in  FIG. 2H  and  FIG. 2I . 
     In  FIG. 2A  and  FIG. 2C , a second metal welding pad  162  is arranged in the center area on the back surface of the substrate  100 , and the plurality of fifth set of welding discs  122   a  are aligned with and overlapped with the second metal welding pad  162  with a through hole filled with an interconnecting via  102  penetrating through the welding discs  122   a  and the second metal welding pad  162 . Therefore, the second metal welding pad  162  is connected to the fifth set of welding discs  122   a  and is further connected to the source S 2  of the HS MOSFET and the drain D 1  of the LS MOSFET. As a result, the second metal welding pad  162  serves as the LX pin (the drain D 1  of the LS MOSFET and the source S 2  of the HS MOSFET) and can also improve the heat dissipation of the device. 
       FIG. 3A  to  FIG. 3C  is similar to  FIG. 2A  to  FIG. 2I  except that only one metal welding pad  161   a  is formed in the center area of the back surface of the substrate  100 . In  FIG. 3A  to  FIG. 3C , the first metal welding pad  161   a  is larger and is arranged in the entire center area of the back surface of the substrate  100 . The first metal welding pad  161   a  occupies all the area of the first and second metal welding pads  161  and  162  of  FIG. 2A . 
       FIG. 4A  to  FIG. 4C  is similar to  FIG. 2A  to  FIG. 2I . In  FIG. 2A , the fifth set of welding discs  122   a  are connected to the second metal welding pad  162  through the interconnecting via  102 . In  FIG. 4B , the sixth set of welding discs  123   a  are aligned with and overlapped with the second metal welding pad  162  instead of the fifth set of welding discs  122   a . therefore, the sixth set of welding discs  123   a  and the second metal welding pad  162  are electrically connected together by the interconnecting via  102  formed in the through hole penetrating through the substrate  100 . In  FIG. 4B , the drain D 2  of the HS MOSFET integrated in the second chip  202  is electrically connected to the sixth set of welding discs  123   a  through the wiring  101 . Therefore, the second metal welding pad  162  is also connected to the drain D 2  of the HS MOSFET. Furthermore, the power voltage VIN is an input from the pin pad  151   c  electrically connected to the drain D 2  of the HS MOSFET. Therefore, the second metal welding pad  162  can be an input terminal for inputting the power voltage VIN and with larger area. It also improves heat dissipation of the device. 
     In  FIG. 5A  and  FIG. 5B , the front surface of the substrate  300  includes a first mounting area  310 , a second mounting area  340 , a third mounting area  320  and a connection pad  330 . The back surface of the substrate  300  includes a first metal welding pad  461 . In  FIG. 5B , a plurality of pin pads are arranged on the back surface of the substrate  100  with a row of the first group of pin pads  351   a - 351   e  arranged near the first edge of the square substrate  100 . A row of the second group of pin pads  352   a - 352   e  are arranged near the second edge opposite the first edge of the square substrate  100 . In  FIG. 5A , the second mounting area  340  and the third mounting area  320  are arranged side by side in one half of the area of the substrate  100 . The first mounting area  310  is located on the other half of the substrate. Therefore, the first mounting area  310 , the second mounting area  340  and the third mounting area  320  are in a compact T shape to reduce a required area of the circuit board. 
       FIG. 5A  is a schematic diagram of the insulating substrate  300  or the printed circuit board, the first mounting area  310 , the second mounting area  340  and the third mounting area  320 . The front surface of the substrate  300  are mainly used for mounting three different semiconductor chips. The first mounting area  310  includes the first set of welding discs  310   a  and the second set of welding discs  310   b . The first set of welding discs includes a plurality of welding discs  310   a . In examples of the present disclosure, the second set only includes one welding disc  310   b . The number of welding discs in the first set or second set may vary. The second mounting area  340  includes the third set of welding discs  341   a - 341   b , the fourth set of welding discs  343   a , the fifth set of welding discs  342   a  and the sixth set of welding discs  342   b.    
     In  FIG. 5A , the second set of welding disc  310   b  in the first mounting area  310  are directly interconnected with a third set of welding disc  341   b  in the second mounting area  340  through the metal wiring  101 . A first set of welding disc  310   a  in the first mounting area  310  is directly interconnected with a third set of welding disc  341   a  in the second mounting area  340  through the metal wiring  101 . 
     In  FIG. 5A , each of the fourth set of welding disc  343   a  is connected to a respective pin pad through the wiring  101 . Specifically, one fourth set of welding disc  343   a  is connected to one welding disc  442   a  located on the front surface of the substrate  300 , which is essentially aligned with and is overlapped with the pin pad  352   a  ( FIG. 5B ) on the back surface of the substrate  300 . A through hole penetrating through the thickness of the substrate  300  between the welding disc  442   a  on the front surface and the pin pad  352   a  on the back surface is filled with a metal material so as to form an interconnecting via  102  electrically connecting the welding disc  442   a  on the front surface and the pin pad  352   a  on the back surface of the substrate  300 . Therefore, the fourth set of welding disc  343   a  is electrically connected to the pin pad  352   a . Similarly, another fourth set of welding disc  343   a  is connected to a welding disc  442   b  located on the front surface of the substrate  300 , which is aligned with and is electrically connected to the pin pad  352   b  ( FIG. 1B ) by the interconnecting via  102 . The other fourth set of welding disc  343   a  is connected to one welding disc  442   c  located on the front surface of the substrate  300 , which is aligned with and is electrically connected to the pin pad  352   c  ( FIG. 1B ) by the interconnecting via  102 . Similarly, one fourth set of welding disc  343   a  is connected to one welding disc  442   d  on the front surface of the substrate  300  through the wiring  101 , which is aligned with and is electrically connected to the pin pad  352   c . Therefore, the welding disc  343   a  is electrically connected to the pin pad  352   d . A fourth set of welding disc  343   a  is connected to one welding disc  442   e  on the front surface of the substrate  300  through the wiring  101 , which is aligned with and is electrically connected with the pin pad  352   e . Therefore, the welding disc  343   a  is electrically connected with the pin pad  352   e.    
     In  FIG. 5A , the fifth set of welding disc  342   a  is electrically connected with a bonding pad  361  through the wiring  101  on the front surface of the substrate  300 . The sixth set of welding disc  342   b  is electrically connected with the other bonding pad  362  through the wiring  101  on the front surface of the substrate  300 . The bonding pad  361  and the bonding pad  362  are preferably located near the first edge of the substrate  300 . One metal die pad  321  with a long and thin metal portion  321   a  extending to the first edge of the substrate  300  is formed in the third mounting area  320 . The connection pad  330  is arranged between the third mounting area  320  and the first edge of the substrate  300 . The connection pad  330  is overlapped with the first row of pin pad  351   c - 351   d  ( FIG. 5B ) and is connected to the pin pads  351   c  and  351   d  by interconnecting vias  102  formed in the through holes penetrating the substrate  300  between the pin pads  351   c - 351   d  and the connection pad  330 . The wirings  101  interconnected with the first set of welding discs  310   a  on the front surface of the substrate  300  are extending near the first edge of the substrate  300  to overlap with the pin pads  351   a - 351   b  ( FIG. 5B ). Therefore, the wirings  101  are electrically connected to the pin pads  351   a - 351   b  by the interconnecting vias  102  formed in the through holes penetrating through the substrate  300  between these wirings  101  and the pin pads  351   a - 351   b . The extended metal portion  321   a  of the metal die pad  321  is overlapped with the pin pad  351   e  in the first row ( FIG. 5B ). Therefore, the extended metal portion  321   a  is electrically connected to the pin pad  351   e  by interconnecting via  102  formed in the through hole penetrating the substrate  300  between the metal portion  321   a  and the pin pad  351   e . The metal die pad  321  is also connected to the pin pad  351   e.    
     In  FIG. 5B , the first row of pin pads  351   a - 351   e  and the second row of pin pads  352   a - 352   e  are located at the back surface near the first and second edges of the substrate  300 . A first metal welding pad  461  is attached in the center area at the back surface of the substrate  300 . Optionally, a second metal welding pad  462  is attached in the center area at the back surface of the substrate  300 . The second metal welding pad  462  is arranged side by side with the first metal welding pad  461 . The first set of welding discs  310   a  formed in the first mounting area  310  on the front surface of the substrate  300  are aligned with and are overlapped with the first metal welding pad  461  on the back surface of the substrate  300 . Therefore, the welding discs  310   a  and the first metal welding pad  461  are electrically connected by the interconnecting vias  102  formed in the through holes penetrating through the substrate  300  between the welding discs  310   a  and the first metal welding pad  461 . However, the second set of welding disc  310   b  formed in the first mounting area  310  is not electrically connected to the first metal welding pad  461 . In examples of the present disclosure, when the second metal welding pad  462  is formed on the back surface of the substrate  300 , it is separated from the first metal welding pad  461 . 
     In  FIG. 5A  to  FIG. 5B , the metal die pad  321  located in the third mounting area  320  is overlapped with at least a portion of the second metal welding pad  462  on the back surface of the substrate  300 . Therefore, the metal die pad  321  is electrically interconnected with the second metal welding pad  462  by an interconnecting via  102  formed in the through hole penetrating through the substrate  300  between the metal die pad  321  and the second metal welding pad  462 . 
     In  FIG. 5C , the first semiconductor chip  301  is flipped and mounted on the first mounting area  310 . A second semiconductor chip  302  is flipped and mounted on the second mounting area  340 . A third semiconductor chip  303  is mounted on the third mounting area  320  without flipping. For clarity, the first chip  301 , the second chip  302  and the third chip  303  are drawn as transparent in  FIG. 5D  to show the position alignment mode of three chips in the first mounting area  310 , the second mounting area  340  and the third mounting area  320  respectively. As mentioned above, in a conventional DC-DC voltage converter, a LS power MOSFET and a HS power MOSFET are usually connected between a power voltage VIN and a ground terminal GND in serial. A control circuit controls the on-off of the HS power MOSFET and the HS power MOSFET. In one example, the first chip  301  is the LS power MOSFET, the second chip  302  is the control circuit, and the third chip  303  is the HS power MOSFET. 
     In  FIG. 5E , a metal clip  450  is mounted on the first chip  301  and the third chip  303 . The metal clip  450  is essentially an L-shape metal sheet. The metal clip  450  includes a first portion  450   a  and a second portion  450   b  perpendicular to the first portion  450   a . The first portion  450   a  is attached to the back metal layer of the flipped first chip  301  through a conductive material. The second portion  450   b  is attached to a source metal pad  303   b  on the front side of the third chip  303 . An extending portion  450   c  extends away from the second portion  450   b . The free end of the extending portion  450   c  is directly attached to the connection pad  330  through the conductive material. The extending portion  450   c  extends in an opposite direction to the first portion  450   a . The first portion  450   a  and the second portion  450   b  are coplanar. Therefore, a height from the free end of the extending portion  450   c  attached on the connection pad  330  to the first portion  450   a  and the second portion  450   b  is small. The connection pad  330  is electrically connected to the source metal pad  303   b  on the front side of the third chip  303  and the drain metal layer on the back side of the first chip  301  through the metal clip  450 . The power MOSFET usually includes a gate, source and drain. The electrical connection between the first chip  301  and the third chip  303  as well as the substrate  300  is illustrated in  FIG. 5D  and  FIG. 5E . 
     In examples of the present disclosure, a metal pad located on the front side of the first chip  301  is configured as the source S 1  of the LS MOSFET. Another pad is configured as the gate G 1 . The metal layer on the back side of the first chip  301  is configured as the drain. In  FIG. 5D , when the first chip  301  is flipped and attached to the first mounting area  310 , the source S 1  of the first chip  301  is attached to the first set of welding discs  310   a  through a solder ball or other metal bump. The gate G 1  of the first chip  301  is attached to the second set of welding discs  310   b  through the solder ball or the other metal bump. 
     As mentioned above in  FIG. 5D , the third set of welding disc  341   a  is electrically connected to one first set of welding disc  310   a . The other third set of welding disc  341   b  is electrically connected to the second set of welding disc  310   b . In  FIG. 5A  and  FIG. 5D , the metal pads IO 1 -IO 9  of various input or output (I/O) terminals of the control circuit integrated in the second chip  302  can also be arranged on the front side of the second chip  302 . The metal pads IO 1 -IO 9  need to be connected to the third to sixth sets of welding discs. Specifically, each of the fourth set of welding discs  343   a  ( FIG. 5A ) is connected to a respective pin pad of the pin pads  352   a - 352   e  ( FIG. 5B ). In one example in  FIG. 5D , one welding disc  343   a  (attached to the IO 9 ) is connected to one welding disc  442   a  overlapped with the pin pad  352   a  on the front surface of the substrate  300 . The welding disc  442   a  is connected to the pin pad  352   a . Therefore, the welding disc  343   a  attached to the IO 9  is connected with the pin pad  352   a . Similarly, one welding disc  343   a  (attached to the IO 8 ) is connected to one welding disc  442   b  overlapped with the pin pad  352   b  on the front surface of the substrate  300  through the wiring  101 . The welding disc  442   b  is connected with the pin pad  352   b . Therefore, the welding disc  343   a  attached to the IO 8  is connected with the pin pad  352   b . One welding disc  343   a  attached to the IO 7  is connected with pin pad  352   c . The welding disc  343   a  attached to the IO 6  is connected with the pin pad  352   d . The welding disc  343   a  welded with the  106  is connected with the pin pad  352   e.    
     In  FIG. 5D , one metal pad IO 1  of the control circuit integrated in the second chip  302  is attached to the third set of welding disc  341   b  through the solder ball or other metal bump. The welding disc  341   b  is further connected to the second set of welding disc  310   b  through the wiring  101 . Therefore, a driving signal sent by the control circuit integrated in the second chip  302  can be transmitted to the second set of welding disc  310   b  through the welding disc  341   b  and then to the gate G 1  of the LS MOSFET. A metal pad IO 2  of the control circuit integrated in the second chip  302  is attached to the third set of welding disc  341   a  through the solder ball or the other metal bump. The welding disc  341   a  is further connected to the first set of welding discs  310   a . The terminal IO 2  of the control circuit integrated in the second chip  302  and the first set of welding discs  310   a  are connected to the first metal welding pad  461  ( FIG. 5B ). The fifth set of welding disc  342   a  (attached to the IO 3 ) in  FIG. 5D  is connected to one bonding pad  361  on the front surface of the substrate  300  through the wiring  101 . One sixth set of welding disc  342   b  (attached to the IO 4 ) in  FIG. 5D  is connected to one bonding pad  362  on the front surface of the substrate  300  through the wiring  101 . 
     In  FIG. 5D  and  FIG. 5E , the source S 1  of the LS MOSFET of the first chip  201  is attached to the first set of welding discs  310   a . The gate G 1  of the LS MOSFET is attached to the second set of welding disc  310   b . The first set of welding discs  310   a  is connected to the first metal welding pad  461  ( FIG. 5B ) on the back surface of the substrate  300 . The source S 1  of the LS MOSFET is also electrically connected to the first metal welding pad  461  on the back surface of the substrate  300 . The drain of the LS MOSFET, i.e., the back metal layer of the first chip  301  is connected to the source metal pad  303   b  of the third chip  303  through the metal clip  450 . The extending portion  450   c  of the metal clip  450  is attached to the connection pad  330   b . Therefore, the node (LX) interconnecting the drain D 1  of the LS MOSFET with the source S 2  of the HS MOSFET is reflected in the connection pad  330  as well as the pin pad  351   c  and the pinpad  351   d . The source S 1  of the LS MOSFET is reflected in the pin pad  351   a  and the pin pad  351   b  that are connected to the source S 1 . The back metal layer  303   c  (shown in dashed lines) of the HS MOSFET in the third chip  303 , i.e., the drain D 2 , is attached to the metal die pad  321  in the third mounting area  320  through the conductive bonding material. The metal die pad  321  and the metal portion  321   a  are connected to the pin pad  351   e  through the interconnecting via  102 . A power voltage VIN can be applied to the drain D 2  of the HS MOSFET through the pin pad  351   e  as the drain D 2  of the HS MOSFET is electrically connected to one pin pad  351   e  on the back surface of the substrate  300 . 
     In  FIG. 5B , the metal die pad  321  is further connected with the second metal welding pad  462  on the back surface of the substrate  300  through the interconnecting vias  102 . Therefore, the second metal welding pad  462  is served as the grounding VIN pin. It improves the heat dissipation. Furthermore, the first metal welding pad  461  is served as the grounding GND pin (connected to the source S 1  of the LS MOSFET) and also improves the heat dissipation of the device. 
     In  FIG. 5D  and  FIG. 5E , the fifth set of welding disc  342   a  are electrically connected to one bonding pad  361  through the wiring  101  on the front surface of the substrate  300 . The sixth set of welding disc  342   b  are electrically connected to the other bonding pad  362  through the wiring  101  on the front surface of the substrate  300 . Before or after mounting the metal clip  450 , a bond wire  451  is used to connect one bonding pad  362  to one source metal pad  303   b  on the front side of the third chip  303 . A bond wire  451  is also used to connect between one bonding pad  362  and one gate metal pad  303   a  on the front side of the third chip  303 . The fifth set of welding disc  342   a  (attached to the IO 3 ) in  FIG. 5D  is connected to the source metal pad  303   b  of the HS MOSFET. The sixth set of welding disc  342   a  (attached to the IO 4 ) in  FIG. 5D  is connected to the gate metal pad  303   a  of the HS MOSFET. Therefore, the control circuit integrated in the second chip  302  can drive the HS MOSFET on-off through the route of the sixth set of welding disc  342   b  and the bonding pad  362 , and sense the potential change of the source metal pad  303   b  through the fifth set of welding disc  342   a  and the bonding pad  361 . 
       FIG. 5F  is a cross sectional view of the device in  FIG. 5E  along the dotted line C-C. After the installation of the chips, a molding layer  230  is formed to cover the first chip  301 , the second chip  302 , the third chip  303 , the metal clip  450  and the bond wires  451 . 
     Those of ordinary skill in the art may recognize that modifications of the embodiments disclosed herein are possible. For example, the number of welding discs in the first and second mounting areas may vary. Other modifications may occur to those of ordinary skill in this art, and all such modifications are deemed to fall within the purview of the present invention, as defined by the claims.