Abstract:
The present disclosure relates to a stacked package of a voltage regulator and a method for fabricating the same. The method comprises: providing a first chip and a second chip which are integrated with each other, the first chip and the second chip each having a front surface provided with a plurality of bumps; forming a non-conductive layer and a conductive layer side by side on the back surface of the second chip; providing a first leadframe and a second leadframe each having at least a group of leads, the plurality of bumps on the first chip being electrically coupled to the first leadframe, the plurality of bumps on the second chip being electrically coupled to the second leadframe, and the back surface of the second chip being electrically coupled to a back surface of the first leadframe; the first leadframe being electrically coupled to the second leadframe; the first chip, the second chip, the conductive layer, the non-conductive layer, the bumps, the first leadframe, and the second leadframe forms a stacked package to reduce the footprint of a chip and reducing manufacture cost.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Chinese Patent Application No. 201310378482.X, filed on Aug. 27, 2013, which is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure generally relates to the field of semiconductor chip package, and more specifically, to a stacked package of a voltage regulator and a method for fabricating the same. 
       BACKGROUND 
       [0003]    As the demand for miniaturization, low manufacture cost, high density and multi-functionality of electronic products, chip packages continue to advance towards the package technology that a plurality of dies are encapsulated in one encapsulant. 
         [0004]    An example of a switching voltage regulator will now be described to show some features of the prior package technology, with reference to  FIG. 1A to 1C . As will be described below, the switching voltage regulator has a multi-chip package structure. 
         [0005]      FIG. 1A  illustrates a schematic diagram of an example structure of a switching voltage regulator. The switching voltage regulator includes a control chip u 1 , a top power switch Q 1  and a bottom power switch Q 2 , all of which are independent components. For example, the top power switch and the bottom power switch are both MOSFETs. The control chip has 5 I/O terminals I/O 1 -I/O 5 , 2 driving terminals C 1  and C 2 . The control chip provides signals from the driving terminals C 1  and C 2  to gates G 1  and G 2  of the two power switches Q 1  and Q 2  to control their on-off state. A drain of the power switch Q 1  is electrically coupled to an input power supply VIN, a source of the power switch Q 1  is electrically coupled to a drain of the power switch Q 2 , and a source of the power switch Q 2  is grounded. The drain of the power switch Q 2  provides an output signal LX. A chip package may be formed by the prior package technology, which incorporates the switching voltage regulator as shown in  FIG. 1A . Top views of the resultant multi-chip package structure before and after encapsulation are shown in  FIGS. 1B and 1C , respectively. In the multi-chip package structure shown in  FIGS. 1B and 1C , the control chip u 1  and the power switches Q 1  and Q 2  are substantially coplanar with each other, and arranged on a carrier of a leadframe. Pads of each chip are electrically coupled to the corresponding leads of the leadframe by bonding wires, so that the chip may be further electrically coupled to an external printed circuit board. 
         [0006]    An important concern in the chip package technology is a ratio of a chip footprint to a package area. The package technology is advantageous if the ratio has a value of approximately 1. However, the above chip package structure means that the chip package has an area at least larger than a sum of the three independent chips. Consequently, it has a large package area and results in a high manufacture cost. 
       SUMMARY 
       [0007]    One object of the present disclosure is to provide a stacked package of a voltage regulator and a method for fabricating the same. 
         [0008]    To solve the above problem, there is provided a method for fabricating a stacked package of a voltage regulator, comprising: 
         [0009]    forming a first chip by integrating a gate driving circuit with a first power switch, forming a second chip having a second power switch, arranging a plurality of terminals on a front surface of the first chip and on a front surface of the second chip, and arranging a plurality of terminals on a back surface of the second chip; 
         [0010]    forming a group of bumps corresponding to the plurality of terminals on the front surface of the first chip and on the front surface of the second chip; 
         [0011]    forming a non-conductive layer and conductive layer side by side on the back surface of the second chip, for electrically insulating the first chip from the second chip and electrically coupling the first chip to the second chip respectively; 
         [0012]    providing a first leadframe and a second leadframe each having at least a group of leads, all of the leads of the first leadframe being electrically coupled to the terminals on the front surface of the first chip by means of the bumps of the first chip, and some of the leads of the second leadframe being electrically coupled to the terminals on the front surface of the second chip by means of the bumps of the second chip; 
         [0013]    electrically coupling the back surface of the first leadframe to the back surface of the second chip; 
         [0014]    electrically coupling the leads of the first leadframe to the corresponding ones of the second leadframe by using a group of electrical wires so that the leads of the second leadframe may be coupled to an external circuit; and 
         [0015]    encapsulating the first chip, the second chip, the conductive layer, the non-conductive layer, the plurality of bumps, the first leadframe, the second leadframe and the group of electrical wires in a stacked package. 
         [0016]    Preferably, the method further comprises: attaching a printed circuit to the back surface of the second leadframe. 
         [0017]    Preferably, the leads are finger-like or block-like. 
         [0018]    Preferably, the conductive layer has a thickness the same as that of the non-conductive layer. 
         [0019]    Preferably, the conductive layer has an area larger than that of the non-conductive layer. 
         [0020]    Preferably, the electrical wires are copper clips. 
         [0021]    Preferably, the bumps are one selected from the group consisting of copper posts, solder bumps, and composite bumps formed by electroless nickel immersion gold. 
         [0022]    Preferably, the voltage regulator is a switching voltage regulator. 
         [0023]    Another object is to provide a stacked package comprising: 
         [0024]    a first chip having a gate driving circuit and a first power switch which are integrated with each other, the first chip having a front surface with a plurality of terminals thereon; 
         [0025]    a second chip having a second power switch, the second chip having a front surface and a back surface with a plurality of terminals thereon; 
         [0026]    a non-conductive layer and conductive layer side by side on the back surface of the second chip, for electrically insulating the first chip from the second chip and electrically coupling the first chip to the second chip respectively; 
         [0027]    a first leadframe and a second leadframe each having at least a group of leads, the back surface of the first leadframe being electrically coupled to the back surface of the second chip; 
         [0028]    a group of bumps corresponding to the terminals on the front surface of the first chip and on the front surface of the second chip respectively, for electrically coupling the terminals on the front surface of the first chip to the leads of the first leadframe and electrically coupling the terminals on the front surface of the second chip to some of the leads of the second leadframe respectively; 
         [0029]    a group of electrical wires for electrically coupling the leads of the first leadframe to the leads of the second leadframe; and 
         [0030]    an encapsulant for encapsulating the first chip, the second chip, the conductive layer, the non-conductive layer, the plurality of bumps, the first leadframe, the second leadframe and the group of electrical wires. 
         [0031]    Preferably, the voltage regulator further comprises: a printed circuit attaching to the back surface of the second leadframe. 
         [0032]    Preferably, the leads are finger-like or block-like. 
         [0033]    Preferably, the conductive layer has a thickness the same as that of the non-conductive layer. 
         [0034]    Preferably, the conductive layer has an area larger than that of the non-conductive layer. 
         [0035]    Preferably, the electrical wires are copper clips. 
         [0036]    Preferably, the bumps are one selected from the group consisting of copper posts, solder bumps, and composite bumps formed by electroless nickel immersion gold. 
         [0037]    In view of that above technical solutions, the present disclosure relates to a method for fabricating a stacked package of a voltage regulator, comprises: forming a first chip by integrating a gate driving circuit with a first power switch, forming a second chip having a second power switch, arranging a plurality of terminals on a front surface of the first chip and on a front surface of the second chip, and arranging a plurality of terminals on a back surface of the second chip; forming a group of bumps corresponding to the plurality of terminals on the front surface of the first chip and on the front surface of the second chip; forming a non-conductive layer and conductive layer side by side on the back surface of the second chip, for electrically insulating the first chip from the second chip and electrically coupling the first chip to the second chip respectively; providing a first leadframe and a second leadframe each having at least a group of leads, all of the leads of the first leadframe being electrically coupled to the terminals on the front surface of the first chip by means of the bumps of the first chip, and some of the leads of the second leadframe being electrically coupled to the terminals on the front surface of the second chip by means of the bumps of the second chip; electrically coupling the back surface of the first leadframe to the back surface of the second chip; electrically coupling the leads of the first leadframe to the corresponding ones of the second leadframe by using a group of electrical wires; and encapsulating the first chip, the second chip, the conductive layer, the non-conductive layer, the plurality of bumps, the first leadframe, the second leadframe and the group of electrical wires in a stacked package. In the present method for fabricating the stacked package, the conductive layer and the non-conductive layer are used for electrically insulating and coupling different parts of the integrated circuit. The stacked package of the voltage regulator is obtained by stacking the first chip on the second chip by using the first leadframe and the second leadframe. Thus, the stacked package formed by the present method has a small size, a high integration density and improved performance. Moreover, no bonding wires are used in the chip package where the first chip is stacked on the second chip by using a first leadframe and a second leadframe, which results in less power loss and higher efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1A  illustrates a schematic diagram of an example structure of a conventional switching voltage regulator; 
           [0039]      FIG. 1B  is a top view showing an example multi-chip package structure of a conventional switching voltage regulator; 
           [0040]      FIG. 1C  is a top view showing an example multi-chip package structure after encapsulation of a conventional switching voltage regulator; 
           [0041]      FIG. 2  illustrates a flow chart of a method for fabricating a stacked package of a voltage regulator according to an embodiment of the present disclosure; 
           [0042]      FIGS. 3A to 3J  are cross-sectional views in various stages of a method for fabricating a stacked package of a voltage regulator according to an embodiment of the present disclosure; 
           [0043]      FIG. 4  illustrates a schematic diagram of an example structure of a switching voltage regulator which incorporates a first chip and a second chip according to an embodiment of the present disclosure; and 
           [0044]      FIG. 5  is a top view showing an example stacked package structure after encapsulation of a voltage regulator according to an embodiment of the present disclosure. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0045]    Objects, features and advantages of the present disclosure will become apparent from the following detailed description of particular embodiments of the present disclosure, with reference to the accompanying drawings. 
         [0046]    Some particular details will be described for thorough understanding of the present disclosure. However, the present disclosure may be practiced with or without these particular details by one skilled person, without departing the principles of the present disclosure. Thus, the present disclosure is not limited to the particular embodiments below. 
         [0047]    As an example, a method for fabricating a stacked package of a voltage regulator according to the present disclosure will be described with reference to the flow chart shown in  FIG. 2 , in connection with  FIGS. 3A to 3J . The method may be used for a switching voltage regulator circuit. 
         [0048]    Firstly, a first chip U 1  is formed by integrating a gate driving circuit u 1  with a power switch Q 1 , as shown in  FIG. 3A . The first chip U 1  has a front surface and a back surface. A source terminal, a drain terminal, a gate terminal, I/O terminals and driving terminals are arranged on the front surface of the first chip U 1 . Moreover, a group of conductive bumps are formed for the respective terminals on the front surface of the first chip U 1 . A second chip U 2  is formed by incorporating a power switch Q 2 , as shown in  FIG. 3B . The second chip U 2  also has a front surface and a back surface. A gate terminal and a source terminal are arranged on the front surface of the second chip, and a drain terminal is arranged on the back surface of the second chip. Moreover, a group of conductive bumps are formed for the respective terminals on the front surface of the second chip U 2 . 
         [0049]    Alternatively, a first chip U 1  is formed by integrating a gate driving circuit u 1  with a power switch Q 2 , as shown in  FIG. 3A . The first chip U 1  has a front surface and a back surface. A source terminal, a drain terminal, a gate terminal, I/O terminals and driving terminals are arranged on the front surface of the first chip U 1 . Moreover, a group of conductive bumps are formed for the respective terminals on the front surface of the first chip U 1 . A second chip U 2  is formed by incorporating a power switch Q 1 , as shown in  FIG. 3B . The second chip U 2  also has a front surface and a back surface. A gate terminal and a drain terminal are arranged on the front surface of the second chip, and a source terminal is arranged on the back surface of the second chip. Moreover, a group of conductive bumps are formed for the respective terminals on the front surface of the second chip U 2 . 
         [0050]    Only the previous arrangement will be described in the following steps, as an example of the method for fabricating the stacked package of the voltage regulator according to the present disclosure. However, it will be understood by one skilled person that the other arrangement may also be used in the method for fabricating the stacked package of the voltage regulator according to the present disclosure. 
         [0051]    In the present embodiment, the two power switches Q 1  and Q 2  are metal-oxide-semiconductor field effect transistors (MOSFETs). It should be understood that the two power switches may be any semiconductor switches in the art. Specifically, the gate driving circuit u 1  has input/output terminal I/Os and driving output terminals C 1  and C 2 , as shown in  FIG. 4 . The driving output terminal C 1  is electrically coupled to a gate G 1  of the power switch Q 1 . Only the driving output terminal C 2  will be described below, because the gate driving circuit u 1  is integrated with the power switch Q 1  in the first chip U 1  and the driving output terminal C 1  and the gate G 1  are combined into one terminal. The driving output terminal C 2  is electrically coupled to a gate G 2  of the power switch Q 2 . The power switch Q 1  has a source for receiving an input power supply VIN and a drain which is electrically coupled to a drain of the power switch Q 2 . The power switch Q 2  has a source which is electrically coupled to ground GND and a drain which provides an output signal LX. The gate and the drain of the power switch Q 2  are formed on the front surface of the power switch Q 2 , and the source of the power switch Q 2  is formed on the back surface of the power switch Q 2 . 
         [0052]    Preferably, the bumps are one selected from the group consisting of copper posts, solder bumps, and composite bumps formed by electroless nickel immersion gold. 
         [0053]    Next, a non-conductive layer A and a conductive layer B are formed in two different areas on the back surface of the second chip, for electrically insulating the first chip U 1  from the second chip U 2  and electrically coupling the first chip U 1  to the second chip U 2  respectively. Preferably, the conductive layer has a thickness the same as that of the non-conducting layer so that a first leadframe may then be firmly attached to the back surface of the second chip. The conductive layer has an area larger than that of the non-conductive layer to ensure electrical insulation from and coupling to the first chip, as shown in  FIGS. 3C and 3D .  FIG. 3D  is a top view showing the structure of  FIG. 3C . 
         [0054]    Next, a first leadframe L/F 1  is provided, as shown in  FIG. 3E . The first leadframe L/F 1  has a plurality of leads which are electrically coupled to the respective terminals of the first chip by means of the bumps corresponding to the terminals on the front surface of the first chip. The bumps are used instead of bonding wires for electrical coupling, to avoid performance deterioration due to electrical resistance and electrical inductance introduced by the bonding wires. The leads may be finger-like or block-like. For example, the leads for the drain terminal, the I/O terminals, the gate terminal G 1 , the driving terminal C 2  of the first chip U 1  may be finger-like, the lead for the source terminal of the first chip U 1  may be block-like. Alternatively, the lead for the source terminal of the first chip U 1  may also be finger-like, which is block-like in this embodiment. 
         [0055]    The first leadframe L/F 1  has a group of finger-like leads at one side, including leads VIN 1  being electrically coupled to the drain terminal of the first chip U 1 , leads I/Os 1  being electrically coupled to the terminals I/Os 1  of the first chip U 1 , a lead C 2  being electrically coupled to the driving terminal C 2  of the first chip U 1 . As mentioned above, the driving output terminal C 1  and the gate G 1  are combined into one terminal in the first chip U 1 . Thus, there may be no lead G 1  for being electrically coupled to the gate G 1  of the first chip U 1 . The first leadframe L/F 1  has a block-like lead LX 1  being electrically coupled to the source of the first chip U 1  at the other side. The leads of the first leadframe L/F 1  are insulated from each other. That is, the leads VIN 1 , I/Os 1 , G 1 , C 2  and LX 1  are insulated from each other. Reference may be further made to  FIG. 3F , which is a cross-sectional view taken along line AA′ in  FIG. 3E . 
         [0056]    Next, a second leadframe L/F 2  is provided, as shown in  FIG. 3G . The second leadframe L/F 2  has a plurality of leads which may be finger-like or block-like. For example, the second leadframe L/F 2  includes finger-like leads for being electrically coupled to a gate terminal G 2  on the front surface of the second chip U 2 , and the lead VIN 1 , the leads I/Os 1  and the lead LX 1  of the first leadframe L/F 1 , and a bock-like lead (alternatively, a finger-like lead) for being electrically coupled to the drain terminal of the second chip U 2 . Some leads of the second leadframe L/F 2  are electrically coupled to the gate terminal G 2  and the source terminal on the front surface of the second chip by means of the bumps corresponding to the terminals on the front surface of the second chip to avoid adverse effects of the bonding wires. 
         [0057]    The second leadframe UF 2  has a group of finger-like leads at one side, including a lead G 2  for electrically coupling to the gate terminal G 2  of the second chip U 2 , leads VIN 2  for being electrically coupled to the leads VIN 1 , and leads I/Os 2  for being electrically coupled to the leads I/Os 1 . The second leadframe has a block-like lead in a middle area for being electrically coupled to the source terminal on the front surface of the second chip. The second leadframe L/F 2  has a group of finger-like leads or a block-like lead LX 2  at the other side, for being electrically coupled to the lead LX 1 . The leads of the second leadframe L/F 2  are electrically insulated from each other. That is, the leads VIN 2 , the leads I/Os 2 , the lead LX 2  and the lead G 2  are electrically insulated from each other. Reference may be further made to  FIG. 3H , which is a cross-sectional view taken along line BB′ in  FIG. 3G . 
         [0058]    Next, the first leadframe is attached to the second chip in a back to back manner. One side of the first leadframe L/F 1  where the finger-like leads are arranged is attached to the non-conducting layer of the back surface of the second chip, and the other side of the first leadframe where the block-like leads is attached to the conductive layer of the back surface of the second chip, respectively, as shown in  FIG. 3I . Accordingly, the terminals I/Os, the drain terminal, the gate terminal G 1  and the driving terminal C 2  of the first chip U 1  are electrically insulated from the drain terminal on the back surface of the second chip U 2  by means the non-conductive layer, and the source terminal of the first chip U 1  is electrically coupled to the drain terminal on the back surface of the second chip U 2  by means of the conductive layer. 
         [0059]    Next, the leads of the first leadframe are electrically coupled to the corresponding leads of the second leadframe by means of a group of electrical wires, as shown in  FIG. 3J . In this embodiment, 
         [0060]    1. the lead C 2  of the first leadframe, which is in turn electrically coupled to the driving terminal C 2  of the first chip U 1 , is electrically coupled to the lead G 2  of the second leadframe, which is in turn electrically coupled to the gate terminal G 2  of the power switch Q 2  of the second chip U 2 ; the leads I/Os 1  of the first leadframe, which is in turn electrically coupled to the input/output terminal I/Os of the first chip U 1 , is electrically coupled to the leads I/Os 2  of the second leadframe, for providing the lead I/Os for an external circuit; 
         [0061]    2. the leads LX 1  of the first leadframe, which are in turn electrically coupled to the source terminal of the power switch Q 1  of the first chip U 1 , are electrically coupled to the leads LX 2  of the second leadframe, which are in turn electrically coupled to the drain terminal of the power switch Q 2  of the second chip U 2 , for providing the lead LX for an external circuit; and 
         [0062]    3. the leads VIN 1  of the first leadframe, which are in turn electrically coupled to the drain terminal of the power switch Q 1  of the first chip U 1 , are electrically coupled to the leads VIN 2  of the second leadframe, for providing the lead VIN for an external circuit. 
         [0063]    Moreover, the block-like lead of the second leadframe, which is in turn electrically coupled to the source terminal of the power switch Q 2  of the second chip U 2 , provides the lead GND for the external circuit. 
         [0064]    Furthermore, the lead GND of the second leadframe is a block-like lead having a relatively large area. Thus, the block-like GND lead may be directly used as a pad for soldering, when the second leadframe is attached to the printed circuit board (PCB). Thus, the resultant stacked package may have less number of leads, small package size, and convenience for routing on the PCB. 
         [0065]    Furthermore, the electrical wires may be implemented with various interconnect technologies. In this embodiment, the electrical wires may be copper clips. 
         [0066]    Finally, the first chip, the second chip, the conductive layer, the non-conductive layer, the plurality of bumps, the first leadframe, the second leadframe and the group of electrical wires, as shown in  FIG. 3J , are encapsulated into a stacked package of a voltage regulator, as shown in  FIG. 5 .  FIG. 5  is a top view of the stacked package after encapsulation. The stacked package according to the present disclosure has a reduced package area. Moreover, the stacked package according to the present disclosure incorporates a flip chip package using the bumps, instead of the bonding wires, which further reduces the package size and increases the integration density of the chip package. 
         [0067]    The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical applications, to thereby enable others skilled in the art to best utilize the disclosure. The disclosure is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the disclosure as defined by the appended claims.