Abstract:
A quad flat non-leaded (QFN) package structure with an electromagnetic interference (EMI) shielding function is proposed, including: a lead frame having a die pad, a plurality of supporting portions connecting to the die pad and a plurality of leads disposed around the periphery of the die pad without connecting to the die pad; a chip mounted on the die pad; bonding wires electrically connecting the chip and the leads; an encapsulant for encapsulating the chip, the bonding wires and the lead frame and exposing the side and bottom surfaces of the leads and the bottom surface of the die pad; and a shielding film disposed on the top and side surfaces of the encapsulant and electrically connecting to the supporting portions for shielding from EMI. A method of fabricating the package structure as described above is further proposed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to package structures and methods for fabricating the same, and more particularly, to a quad flat non-leaded (QFN) package structure with an electromagnetic interference (EMI) shielding function and a method for fabricating the same. 
         [0003]    2. Description of Related Art 
         [0004]    Generally, a lead frame can be used as a chip carrier, which has a die pad and a plurality of leads formed around the periphery of the base. A chip can be mounted to the die pad and electrically connected to the leads through bonding wires. Further, an encapsulant can be formed to encapsulate the chip, the die pad, the bonding wires and the inner portions of the leads so as to form a semiconductor package with a lead frame. 
         [0005]    In order to reduce package size, QFN package structures that dispense with outer leads protruding from the encapsulant are developed. However, such a package size can easily be influenced by external EMI noises during operation, thus adversely affecting electrical performance of the overall package structure. 
         [0006]    Accordingly, U.S. Pat. No. 5,166,772 discloses a structure with a metal shield embedded in the encapsulant thereof. 
         [0007]      FIG. 1  is a cutaway perspective view of the structure disclosed in U.S. Pat. No. 5,166,772. Referring to  FIG. 1 , a chip  11  is mounted on a substrate  10  and electrically connected to the substrate  10  through a plurality of bonding wires  12 , wherein the substrate  10  has at least a ground terminal  14 , and a perforated metal shield  13  is disposed to cover the chip  11  and electrically connected to the ground terminal  14 . An encapsulant  15  is formed to cover the metal shield  13 , the chip  11 , the bonding wires  12  and a portion of the substrate  10 , thereby embedding the metal shield  13  in the encapsulant  15 . The metal shield  13  shields the chip  11  from external EMI so as to improve electrical performance of the overall structure. Similar structures are also disclosed in U.S. Pat. No. 4,218,578, No. 4,838,475, No. 4,953,002 and No. 5,030,935. 
         [0008]    However, since an additionally fabricated metal shield  13  is required in the above-described structure, the fabrication process of the structure is complicated. Further, the metal shield  13  is required to cover the chip  11  and fixed to the substrate  10 , thus increasing the assembly difficulty. Furthermore, after the metal shield  13  is disposed on the substrate  10  to cover the chip  11 , the encapsulant  15  must pass through the metal shield  13  in order to encapsulate the chip  11 . Since the metal shield  13  is perforated, when the encapsulant  15  passes through the metal shield  13 , turbulence can easily occur in the encapsulant  15 , thus resulting in generation of air bubbles in the encapsulant  15  and causing a popcorn effect in a subsequent thermal processing. 
         [0009]      FIG. 2  is a cutaway perspective view of a structure disclosed by U.S. Pat. No. 5,557,142. Referring to  FIG. 2 , a chip  21  is mounted on a substrate  20  and electrically connected to the substrate  20  through a plurality of bonding wires  22 . Further, an encapsulant  23  is formed to encapsulate the chip  21 , the bonding wires  22  and a portion of the substrate  20 , and a metal layer  24  is formed on the exposed surface of the encapsulant  23  through coating or sputtering so as to shield the package structure from EMI. Similar structures are also disclosed in U.S. Pat. No. 5,220,489, No. 5,311,059 and No. 7,342,303. 
         [0010]    The above structures dispense with complicated processes. However, since the metal layer  24  must be formed after a singulation process and it is difficult to perform component arrangement and pickup in a singulated package structure, the above structures are not suitable for mass production. In addition, the sputtering process cannot be applied in a package structure in which the encapsulant is flush with the substrate. 
         [0011]    In a package structure disclosed by U.S. Pat. No. 7,030,469, a groove is formed on an encapsulant to expose bonding wires, and a conductive wire layer is formed in the groove and on the encapsulant to electrically connect to the bonding wires, thereby achieving a shielding effect. However, the conductive wire layer is made of a non-ferrous metal material and can only be formed on the groove and encapsulant by depositing or sputtering. Therefore, it cannot be applied in a package structure in which the encapsulant is flush with the substrate. Further, the contact between the conductive wire layer and the bonding wires is point contact, which can easily result in poor electrical connection between the conductive wire layer and the bonding wires. 
         [0012]    Therefore, it is imperative to overcome the above drawbacks of the prior art. 
       SUMMARY OF THE INVENTION 
       [0013]    In view of the above drawbacks of the prior art, the present invention provides a quad flat non-leaded (QFN) package structure with an electromagnetic interference (EMI) shielding function, which comprises: a lead frame having a die pad, a plurality of supporting portions connecting to the die pad and a plurality of leads disposed around the periphery of the die pad without connecting to the die pad; a chip mounted on the die pad; a plurality of bonding wires electrically connecting to the chip and the leads; an encapsulant for encapsulating the chip, the bonding wires and the lead frame in a manner that the side and bottom surfaces of the leads and the bottom surface of the die pad are exposed from the encapsulant; and a shielding film disposed on the top and side surfaces of the encapsulant and electrically connecting to the supporting portions. 
         [0014]    The present invention further provides a fabrication method of a QFN package structure with an EMI shielding function, which comprises: providing a metal frame with a plurality of lead frames and a plurality of transverse and longitudinal connection strips, wherein each of the lead frames has a die pad, a plurality of supporting portions connecting to the die pad and a plurality of leads disposed around the periphery of the die pad without connecting to the die pad, the lead frames being connected to the connections trips through the supporting portions and the leads thereof; mounting a chip to the die pad of each of the lead frames and electrically connecting the chip to the corresponding leads through bonding wires; forming an encapsulant to encapsulate the connection strips, the chips, the bonding wires, the die pads, the leads and the supporting portions while exposing the bottom surfaces of the connection strips, the die pads and the leads from the encapsulant; performing a first cutting process for cutting the encapsulant along the connection strips so as to form in the encapsulant a plurality of grooves which exposes the connection strips and a portion of the supporting portions; forming a shielding film on the surface and in the grooves of the encapsulant and electrically connecting to the supporting portions; and performing a second cutting process for cutting the shielding film and the metal frame along the grooves and the connection strips such that the shielding film encloses the side surfaces of the encapsulant and is flush with the leads and the supporting portions. 
         [0015]    Therein, the shielding film can be applied on the exposed surface of the encapsulant and in the grooves by screen printing and then cured. In another embodiment, the shielding film can be formed by dropping a liquid state carbon-based material or a metal powder-containing material into the grooves so as to form a first shielding film; forming a second shielding film on the exposed surface of the encapsulant and on the first shielding film in the grooves by screen printing; and curing the first shielding film and the second shielding film. 
         [0016]    According to the present invention, the grooves are formed in the encapuslant before a singulation process so as to allow the EMI shielding film to be formed therein and in contact with the connection strips, thus achieving an EMI shielding function and facilitating mass production. 
         [0017]    Further, the present invention overcomes the conventional drawback of turbulence which occurs when the encapsulant passes through a perforated metal shield, thus avoiding generation of air bubbles in the encapsulant and the popcorn effect caused thereby. Furthermore, since the shielding film and the supporting portions are in surface contact, it ensures higher electrical connection quality compared with the point contact of the prior art. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a cutaway perspective view of a package structure disclosed by U.S. Pat. No. 5,166,772; 
           [0019]      FIG. 2  is a cutaway perspective view of a package structure disclosed by U.S. Pat. No. 5,557,142; 
           [0020]      FIG. 3A  is a top view regarding a fabrication method of a QFN package structure with an EMI shielding function according to the present invention; 
           [0021]    FIG.  3 A′ is a cross-sectional view taken along a line A-A in  FIG. 3A ; 
           [0022]      FIGS. 3B ,  3 C,  3 D, and  3 E are cross-sectional views showing the fabrication method of a QFN package structure with an EMI shielding function according to the present invention; and 
           [0023]      FIGS. 3D-1  and  3 D- 2  show another embodiment of the method as depicted in  FIG. 3D . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects can be apparent to those in the art after reading this specification. 
         [0025]    Please refer to  FIGS. 3A ,  3 A′,  3 B,  3 C,  3 D, and  3 E for the drawings of a fabrication method of a QFN package structure with an EMI shielding function according to the present invention.  FIG. 3A  is a top view. FIG.  3 A′ is a cross-sectional view taken along a line A-A in  FIG. 3A .  FIGS. 3B ,  3 C,  3 D, and  3 E are cross-sectional views. 
         [0026]    Referring to FIGS.  3 A and  3 A′, a metal frame  30  made of copper is provided, which comprises a plurality of lead frames  31  and a plurality of connection strips  301  composed of a plurality of transverse connection strips  301   a  and a plurality of longitudinal connection strips  301   b . Each of the lead frames  31  has a die pad  311 , a plurality of supporting portions  312  connecting to the die pad  311 , and a plurality of leads  313  disposed at the periphery of the die pad  311  and separated from the die pad  311  by a predetermined distance. The supporting portions  312  and the leads  313  connect to the connection strips  301 . 
         [0027]    Each of the leads  313  has a top surface  313   a  and an opposing bottom surface  313   b . The leads  313  each have an extending portion  3130  with a thickness less than the leads  313  such that the top surfaces  3131  of the extending portions  3130  and the top surfaces  313   a  of the leads  313  together form a stepped structure. Generally, the top surfaces  3131  of the extending portions  3130  or the stepped structure can be formed by etching away a portion of the extending portions  3130  such that the extending portion  3130  is of a thickness less than the leads  313 . 
         [0028]    In particular, as shown in  FIG. 3A , the supporting portions  312  of each of the lead frames  31  extend from the die pad  311  to intersection points of the transverse connection strips  301   a  and the longitudinal connection strips  301   b.  The leads  313  are connected to the connection strips  301  through the extending portions  3130  thereof without connecting to the die pad  311 . Further, the thickness of the supporting portions  312  is less than the thickness of the die pad  311 . The bottom surfaces  312   b  of the supporting portions  312 , the bottom surface  311   b  of the die pad  311 , and the bottom surfaces  301   d  of the connection strips  301  together form a stepped structure. Alternatively, the top surfaces  312   a  of the supporting portions  312  are not flush with the top surface  311   a  of the die pad  311  and the top surfaces  301   c  of the connection strips  301  to thereby form a stepped structure therefrom such that the thickness of the supporting portions  312  is less than the thickness of the die pad  311 . The stepped structure can be formed by etching. 
         [0029]    Referring to  FIG. 3B , a chip  32  is mounted on the top surface  311   a  of the die pad  311  of each of the lead frames  31  and electrically connected to the top surfaces  313   a  of the leads  313  and the top surface  311   a  of the die pad  311  through a plurality of bonding wires  33 , wherein the die pad  311  is electrically grounded. Then, an encapsulant  34  is formed to encapsulate the connection strips  301 , the chips  32 , the bonding wires  33 , the die pads  311 , the leads  313  and the supporting portions  312 , in a manner that the bottom surfaces  301   d ,  311   b,    313   b  of the connections strips  301 , the die pads  311 , and the leads  313  are exposed from the encapsulant  34 . 
         [0030]    Referring to  FIG. 3C , a first cutting process is performed for cutting the encapsulant  34  along the center line of each of the connection strips  301  so as to form a plurality of grooves  340  for exposing the connection strips  301  and a portion of the supporting portions  312 . The cutting depth d for the first cutting process is greater than or equal to the thickness t of the encapsulant  34 . To be specific, the connection strips  301  and a portion of the supporting portions  312  are exposed from the grooves  340  formed at the connection positions of the connection strips  301  and the supporting portions  312 , that is, the cutting depth d at these positions is greater than the thickness t of the encapsulant  34  so as to facilitate electrical connection between a shielding film to be formed later and the supporting portions  312 . Meanwhile, the cutting depth d at other positions is less than the thickness t of the encapsulant  34  for only exposing the connection strips  301 . Further, the width of the grooves  340  is greater than that of the connection strips  301 . 
         [0031]    Referring to  FIG. 3D , a shielding film  35  is formed on the surface of the encapsulant  34  and in the grooves  340  by screen printing and is made of a carbon-based material or a metal powder-containing material. The shielding film  35  is electrically connected to the supporting portions  312  and is cured after the screen printing process so as to shield the chips  32  from external EMI, thereby ensuring normal operation of the chips  32 . 
         [0032]      FIGS. 3D-1  and  3 D- 2  show another embodiment of the method for forming the shielding film  35 . Different from the previous embodiment, the present embodiment first drops a liquid state carbon-based material or metal power-containing material into the grooves  340  to form a first shielding film  351 , as shown in  FIG. 3D-1 , and then forms a second shielding material  352  on the exposed surface of the encapsulant  34  and on the first shielding film  351 , as shown in  FIG. 3D-2 , and finally cures the first shielding film  351  and the second shielding film  352  so as to from a shielding film  35 . 
         [0033]    Referring to  FIG. 3E , a second cutting process is performed for cutting the shielding film  35  and the metal frame  30  along the center lines of the connection strips  301  in the grooves  340 , wherein the cutting width w 2  for the second cutting process is less than the width w 1  for the first cutting process such that the shielding film  35  encloses the sides  341  of the encapsulant  34  and is flush with the side surfaces of the leads  313  and the supporting portions  312 . 
         [0034]    According to the above fabrication method, the present invention further provides a QFN package structure  3  with an EMI shielding function, which comprises: a lead frame  31  having a die pad  311 , a plurality of supporting portions  312  connecting to the die pad  311  and a plurality of leads  313  disposed at the periphery of the die pad  311  and separated from the die pad  311  by a predetermined distance; a chip  32  mounted on the die pad  311  and electrically connected to the top surfaces  313   a  of the leads  313  and the top surface  311   a  of the die pad  311  through a plurality of bonding wires  33 ; an encapsulant  34  for encapsulating the chip  32 , the bonding wires  33  and the lead frame  31  while exposing the side and bottom surfaces  313   b  of the leads  313  and the bottom surface  311   b  of the die pad  311 ; and a shielding film  35  disposed on the top and side surfaces of the encapsulant  34  and electrically connected to the supporting portions  312 . Therein, the shielding film  35  is made of a carbon-based material or a metal powder-containing material. 
         [0035]    In the above structure, the encapsulant  34  separates the shielding film  35  from the leads  313 , and the package structure  3  has even side surfaces. In particular, the leads  313  each have an extending portion  3130  extending towards the corresponding sides of the package structure  3 . The extending portion  3130  is of a lesser thickness than the leads  313  such that the top surfaces  3131  of the extending portions  3130  and the top surfaces  313   a  of the leads  313  together form a stepped structure that is embedded in the encapsulant  34 . 
         [0036]    Further, the thickness of the supporting portions  312  is less than the thickness of the die pad  311 . For example, the bottom surfaces  312   b  of the supporting portions  312  and the bottom surface  311   b  of the die pad  311  together form a stepped structure, thereby embedding the supporting portions  312  in the encapsulant  34 . Furthermore, the package structure  3  has even side surfaces, and the shielding film  35  electrically connects to the supporting portions  312 . The shielding film  35  can cover the ends of the supporting portions  312 . Alternatively, as shown in  FIG. 3E , the ends of the supporting portions  312  are partially exposed from the side surfaces of the package structure  3 . 
         [0037]    According to the present invention, a chip is mounted on the die pad of each of the lead frames of a metal frame and electrically connected to the corresponding leads and the die pad through bonding wires; then, an encapsulant is formed on the metal frame, the chips and the bonding wires; thereafter, a first cutting process is performed for cutting the encapuslant along the connection strips of the metal frame such that a plurality of grooves is formed in the encapsulant for exposing a portion of the supporting portions of the lead frames; then, a shielding film is formed on the exposed surface of the encapsulant and in the grooves and electrically connects to the supporting portions; thereafter, a second cutting process is performed for cutting the shielding film and the metal frame along the grooves such that shielding film encloses the sides of the encapsulant and the structure is singulated into a plurality of package units. As such, the present invention achieves an EMI shielding function through the shielding film and meanwhile facilitates mass production. 
         [0038]    Further, the present invention overcomes the conventional drawback of turbulence which occurs when the encapsulant passes through a perforated metal shield, thus avoiding generation of air bubbles in the encapsulant and the popcorn effect caused thereby. Furthermore, since the shielding film and the supporting portions are in surface contact, it ensures higher electrical connection quality compared with the point contact of the prior art. 
         [0039]    The above-described descriptions of the detailed embodiments are only to illustrate the preferred implementation according to the present invention, and it is not to limit the scope of the present invention, Accordingly, all modifications and variations completed by those with ordinary skill in the art should fall within the scope of present invention defined by the appended claims.