Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional application of U.S. patent application Ser. No. 11/934,287, which is filed on Nov. 2, 2007 and claims the priority benefit of Taiwan application serial no. 096113893, filed on Apr. 20, 2007. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
    
    
     BACKGROUND OF THE INVENTION 
     (A) Field of the Invention 
     The present invention relates to a leadframe for a leadless package, structure and manufacturing method using the same, and more particularly, to the structure of a quad flat non-leaded (QFN) package, a leadframe thereof and a manufacturing method using the same. 
     (B) Description of the Related Art 
     To meet electronic devices&#39; requirements for lightness, thinness and small size, QFN packaging has overtaken the traditional leadframe packaging method to replace the more expensive wafer level chip size packaging (CSP). While CSP scales down the package size so that it is the same as the die size, it needs a fine-pitch ball grid array to act as pins, which makes the manufacturing process more complicated. In contrast, QFN packaging is not only with a small size, a low cost and a high manufacturing yield, it is also a high speed and power managing circuit device with a better coplanarity and high heat dissipation capacity. In addition, the QFN packaging does not need to extend the pins from its sides, and therefore its electrical performance is better than the traditional leaded packaging which needs to extend several pins from its sides, and sometime these pins serving as an inductance or a capacitor produce a lot of noise when applied to high frequency systems. 
     Furthermore, the exposed lead frame pad of QFN packaging can serve as a direct heat dissipation path, which enables the package to exhibit a better heat dissipation capacity. Normally, such a thermal pad is directly soldered on the circuit board, and the thermal via in the circuit board brings the surplus heat to the copper foil connected to the ground, so additional cooling fins can be saved. 
       FIG. 1  shows the cross-sectional schematic view of a prior art example of a QFN package structure. A QFN packaging element  80  includes a lead-frame  81 , a die  82 , an adhesive  83 , a plurality of leads  84  and a molding material  85 , wherein the die  82  is mounted on a die pad  811  of the lead frame  81  by means of the adhesive  83 , and the plurality of leads  84  electrically connect the die  82  to a plurality of pins  812  of the lead frame  81 . The die  82 , the leads  84  and the leadframe  81  are covered with the molding material  85 , while the molding material  85  does not cover the lower surfaces of the die pad  811  and the pins  812 . The uncovered parts of the surface of the pins  812  act as the contacts of the surface mounting. The uncovered part of the surface of the die pad  811  can dissipate heat directly to outside and therefore replace the heat sink of the prior art serving the same function. However, the die pad  811  is disposed on the center of the pins  812 , and needs to be kept at a proper distance from the surrounding pins  812 , so the package size is limited. In view of above, because the heat dissipation efficiency is closely related to the package size, if the uncovered area of the surface of the die pad  811  can be increased, it will help solve the more and more serious problem of heat dissipation that a multiple function die especially faces. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is to provide a leadframe for a leadless package and a package structure and a manufacturing method using the same, which improve the heat dissipation efficiency of the package by modifying the layout of the die pad and the pins in the leadframe. 
     Another aspect of the present invention is to provide a leadless package structure with steady pins, wherein each pin&#39;s four end faces are fixed and protected by a molding material, so that the package is not easily damaged by external forces. 
     Yet another aspect of the present invention is to provide a leadless package structure with steady pins, wherein each pin has a concave part and a convex part, which increases the area in contact with the molding material and thus reduces the probability of the molding material getting stripped from the pins, improving the manufacturing yield. 
     A leadframe for leadless package of one embodiment of the present invention comprises a plurality of package units and one adhesive tape. Each package unit has a die pad with a plurality of openings, and a plurality of pins disposed in the plurality of openings. The adhesive tape is adhered to the surface of the plurality of package units to fix the die pad and the plurality of pins. 
     A package structure for leadless package of another embodiment of the present invention comprises a leadframe, a die and a plurality of lead wires. The leadframe has a die pad with a plurality of openings and a plurality of pins disposed in the plurality of openings. The die is fixed on the die pad and is electrically connected to the pins by the plurality of lead wires. 
     A manufacturing method for leadless package of yet another embodiment of the present invention comprises the steps of providing a metal sheet covered by an adhesive tape; patterning the metal sheet to form a plurality of package units, wherein each of the package unit has a die pad with a plurality of openings and a plurality of pins disposed in the plurality of openings; mounting one die on each of the die pads; and covering each of the dies, each of the package units and the plurality of lead wires with a molding material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objectives and advantages of the present invention will become apparent upon reference to the following description and the accompanying drawings in which: 
         FIG. 1  shows the cross-sectional schematic view of a prior art example of a QFN package structure; 
         FIGS. 2(   a )- 2 ( e ) are schematic drawings showing each of the manufacturing steps of a leadless package structure in accordance with an embodiment of the present invention; 
         FIG. 3  shows a top view of a leadless package device in accordance with the present invention; 
         FIG. 4  shows a top view of a leadframe employed by a leadless package in accordance with the present invention; and 
         FIGS. 5(   a )- 5 ( c ) are schematic drawings showing each of manufacturing steps of a leadless package in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 2(   a )- 2 ( e ) are schematic drawings showing each of the manufacturing steps of a leadless package structure in accordance with an embodiment of the present invention. First, a metal sheet  11  covered by an adhesive tape  12  is provided, wherein the metal sheet  11  can be made of copper, aluminum, Cu—Al alloy, aluminum alloy or a combination of the above materials. As shown in  FIG. 2(   b ), by a stamping process, a plurality of concaves  132  are formed on the upper surface of the metal sheet  11 , and therefore a plurality of convexes  131  are formed on the lower surface between the metal sheet  11  and the adhesive tape  12 . By a photolithography process, the metal sheet  11  is turned into a plurality of connected die pads  14  and a plurality of separated pins  13 , wherein the pins  13  are formed in the plurality of openings  141  of the die pad  14 , as shown in  FIG. 2(   c ). And the combination of one of the die pads  14  and its surrounding pins  13  is considered as a package unit  181  of the leadframe  18 . 
     As shown in  FIG. 2(   d ), each die  15  is further mounted on the die fixing area  142  (as shown in  FIG. 3)  at the center of each die pad  14 . A plurality of lead wires  17  connect each die  15  to the pins  13  surrounding it by wire bonding. Preferably, the lead wires  17  are connected to the portions of the s pins  13  excluding the concave parts  132 . To protect the die  15  and the lead wires  17  from external forces and the environmental influence, each die  15 , each package unit  181  and the plurality of lead wires  17  are covered with a molding material  16 , as shown in  FIG. 2(   d ). After the molding material  16  hardens, the adhesive tape  12  can be removed, and the leadless package unit  20  is cut along the edges of the package units  181  by a dicing process so that the package units  181  are separated from each other, as shown in  FIG. 2(   e ). 
       FIG. 3  shows the top view of a leadless package unit in accordance with the present invention. The die  15  is mounted on the die fixing area  142  in the center of the die pad  14 , and the plurality of leads  17  are placed on the plurality of openings  141  surrounding the die pad  14 . Unlike the QFN package  80  in  FIG. 1 , the die pad  14  of the present invention extends to the four edges of the leadless package unit  20 . Except the openings  141 , the whole area of the die pad  14  can dissipate heat. Besides, since the area of the die pad  811  of the conventional QFN package  80  is approximately the same as that of the die fixing area  142  in  FIG. 3 , the heat dissipation efficiency of these two packages are remarkably different due to the different sizes of the areas of the die pads  14 . Furthermore, the four sides of each pin  13  of the present invention are fixed and protected by the molding material  16 , so the package is not easily damaged by external forces. 
       FIG. 4  shows the top view of a lead frame employed by a leadless package in accordance with the present invention. To meet the requirements of mass production, the plurality of package units  181  of the lead frame  18  can be arranged in M rows by N columns matrix. For example, the package units  181  in  FIG. 4  are arranged in two rows by N columns matrix. Instead of such arrangement, there can also be more rows to increase the unit per hour (UPH) of the molding process. 
     The convex parts  131  of pins  13  on the leadless package unit  20  increase the area in contact with the solders, and therefore enhance the solderability of the leadless package unit  20  in the surface mounting process. The concave parts  132  of pins  13  increase the area in contact with the molding material  16 , and therefore reduce the probability of the molding material getting delaminated from the pins  13 , which in turn improves the manufacturing yield. As shown in  FIG. 5(   a ), a plurality of concave parts  132 ′ are first formed on one surface of the metal sheet  11 , and accordingly a plurality of convex parts  131 ′ are formed on the other surface of the metal sheet  11 . Afterward, the plurality of concave parts  132 ′ and the adhesive tape are adhered together, as shown in  FIG. 5(   b ). Then, the same steps in  FIGS. 2(   c ) to  2 ( e ) are implemented to complete most of the manufacturing processes. Preferably, the lead wires  17  are connected to the parts of the pins  13  excluding the concave parts  132 . Finally, the leadless package unit  20 ′ is cut along edges of the package units  181  by a dicing process so that the package units  181  are separated from each other, as shown in  FIG. 5(   c ). The concave parts  132 ′ of pins  13  of the leadless package unit  20  increase the area in contact with the solders, and therefore enhance the solderability of the leadless package unit  20  in the surface mounting process. The convex parts  131 ′ of pins  13  increase the area in contact with the molding material  16 , and therefore reduce the probability of the molding material getting delaminated from the pins  13 . 
     The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.

Technology Category: 5