Abstract:
The present invention employs tie bar(s) of a lead frame as contact(s) so as to increase the number of contacts in a package structure. Therefore, a die can be packaged in a package structure with a smaller dimension to lower packaging cost of an integrated circuit.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to Taiwan Application Serial Number 099140266, filed Nov. 23, 2010 which is herein incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    (1) Field of the Invention 
         [0003]    This invention relates to a package structure, and more particularly relates to a package structure with bonding wires for electrically connecting tie bars and the die. 
         [0004]    (2) Description of the Prior Art 
         [0005]    After finishing the process of fabricating an integrated circuit on a die, a packaging process is usually demanded for protecting the fragile die from outside damages. In the beginning, a wafer is sliced into a plurality of dies. Then the die is placed on a lead frame, and a conductive pad on the die is wire bonded to pins of the lead frame. Afterward, the die is covered by the packaging material. Thus, the package structure can provide a stiff shell body for protecting the die located therein from the damages of moisture, heat, or noise. On the other hand, the package structure must be able to allow the exchange of signals between the die and the outside of the package structure. For achieving the above two objects, the lead frame plays an important role. 
         [0006]      FIG. 1  is a typical wire-bonded lead frame before being cut into pieces. As shown in  FIG. 1 , a die  10  is adhered on a die pad  20 . A tie bar  25  connecting the die pad  20  and a rail  30  is utilized for supporting the die pad  20 . A pin  40  is connected to a dam bar  35 , and the dam bar  35  is connected to the rail  30  for supporting the pin  40 . The bonding wires  45  are utilized for electrically connecting bonding pads  15  on the die  10  to the pins  40 . The wire-bonded lead frame is then cut and separated into pieces after encapsulation. 
         [0007]    The trend for the development of integrated circuit (IC) technology is directed to high layout integration for reducing die size and material cost. However, the dimension of package structure is restricted by the increasing number of pins demanded for the die with various functions. Because packaging cost reaches about 50% of chip cost, cost saving by the reduction of die size is limited. Thus, it is an important issue for the development of IC technology to figure out a method for further reducing the cost under the limitations mentioned above. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the aforementioned restrictions, the present invention uses tie bars of a lead frame as contacts for connecting a die such that the number of available contacts can be increased and the die can be packaged in a package structure with a smaller dimension, thus reducing packaging cost. 
         [0009]    For achieving the aforementioned object, a package structure is provided in accordance with an embodiment of the present invention. The package structure has a die pad, a die, a set of contacts, and at least one tie bar. The die is located on the die pad. The contacts are spaced apart from the die pad. The tie bar is connected to the die pad. The die is wire bonded to the contact by using at least one first conductive wire, and is wire-bonded to the tie bar by using at least one second conductive wire. 
         [0010]    Another package structure is also provided in accordance with another embodiment of the present invention. The package structure has a plurality of die pads, a plurality of dies, a set of contacts, and at least one tie bar. The contacts are spaced apart from the die pads. The dies are located on the die pads respectively, and are wire bonded to the contacts by using a plurality of first conductive wires. The tie bar is connected to at least one of the plurality of die pads, and is wire-bonded to at least one of the plurality of dies by using at least one second conductive wire. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which: 
           [0012]      FIG. 1  is a typical wire-bonded lead frame before being cut into pieces; 
           [0013]      FIG. 2  is a schematic view of a package structure in accordance with a first preferred embodiment of the present invention; 
           [0014]      FIG. 3  is a schematic view of a package structure in accordance with a second preferred embodiment of the present invention; and 
           [0015]      FIG. 4  is a schematic view of a multi-chips package structure in accordance with a third preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]      FIG. 2  is a schematic view of a package structure in accordance with a first preferred embodiment of the present invention. As shown in  FIG. 2 , the package structure  100  has a die  110 , a die pad  120 , a set of contacts  140 , and at least one tie bar  125 . The die  110  is located on the die pad  120 . A plurality of conductive pads  115  are located on an upper surface of the die  110  as a signal input or a signal output for the circuit within the die  110 . The contacts  140  are wire bonded to the conductive pads  115  on the die  110  by using first conductive wires  145 , such that the die  110  is electrically connected to an external circuit through the contacts  140  for outputting or receiving signals. The tie bar  125  is connected to the die pad  120  for supporting the die pad  120  until the lead frame is cut into pieces. In the present embodiment, the package structure is a QFN 2×2 package structure with twelve contacts  140  (three contacts on each side of the package structure) and four tie bars  125  aligned to the four corners of the package structure. Some of the conductive pads  115  on the die  110  are wire bonded to the tie bars  125  through second conductive wires  145 A. In the present embodiment, one conductive pad  115  is electrically connected to the tie bar  125 , and thus, thirteen contacts in total are provided in the QFN 2×2 package structure of the present embodiment. In addition, by using the tie bar  125  as another contact, the idea of the present invention may be applied to a typical lead frame without the needing to change the model of the lead frame. Thus, the package structure provided in the present embodiment is quite suitable for the present package process. 
         [0017]      FIG. 3  is a schematic view of a package structure in accordance with a second preferred embodiment of the present invention. A major feature of the present embodiment different from the embodiment in  FIG. 2  is described below. As shown in  FIG. 3 , the integrated circuit layout of the die  110  includes a control circuit  105 A and a driving circuit  105 B. The control circuit  105 A is utilized for receiving the input signals from the outside and for controlling the operation of the driving circuit  105 B according to the input signals. The driving circuit  105 B is utilized for driving outside circuits, such as integrated circuit or MOSFET transistors. Because of driving requirements, the power demanded for the driving circuit  105 B is much greater than that for the control circuit  105 A. If the two circuits  105 A and  105 B are connected to the same contact for receiving driving power, the noise resulted from the driving circuit  105 B may interfere with the operation of the control circuit  105 A. Thus, two separate contacts are provided as the power inputs for the control circuit  105 A and the driving circuit  105 B respectively. In the present embodiment, the control circuit  105 A and the driving circuit  105 B are wire bonded to two different tie bars  125  by using the second conductive wires  145 A for receiving the power from a common driving power source. Preferably, the two tie bars  125  may be the tie bars  125  located at the diagonally opposite corners of the package structure. Because the two tie bars  125  for electrically connecting the control circuit  105 A and the driving circuit  105 B are not connected directly, the noise generated by the driving circuit  105 B is attenuated by the intermediate circuit elements (such as the die pad  120 ) between the two tie bars  125  before reaching the control circuit  105 A. Thus, the undesirable affects of noise to the control circuit  105 A can be significantly reduced. In the present embodiment, the two tie bars  125  are electrically connected to the conductive pads  115 , and thus, fourteen contacts are provided in the QFN 2×2 package structure of the present embodiment. In addition, the number of tie bars  125  electrically connected to the die pad  120  and the way for fixing the die  110  on the die pad  120  may be varied according to the circuit design on the die  110 . For example, the die  110  may be adhered to the die pad  120  by using conductive glue or insulation glue. 
         [0018]      FIG. 4  is a schematic view of a multi-chips package structure in accordance with a third preferred embodiment of the present invention. As shown in  FIG. 4 , a package structure  200  has a plurality of die pads  220 A,  220 B and  220 C, a plurality of dies  210 A,  210 B,  210 C and  210 D, a set of contacts  240 , and at least one tie bar  225 . In the present embodiment, the die  210 A has a control circuit formed thereon, and each of the dies  210 B,  210 C and  210 D has a N-type MOSFET structure formed thereon, and the package structure is a QFN 5×5 package structure with forty contacts  240  (ten contacts  240  on each side of the package structure). Some of the contacts  240  are directly connected to the die pads  220 B and  220 C as drain contacts for the dies. Other contacts  240  are wire bonded to the dies  210 A,  2106 ,  210 C and  210 D through first conductive wires  245 . Some of the conductive pads  215  on the die  210 A are wire bonded to the tie bar  225  by using second conductive wires  245 A. 
         [0019]    In the present embodiment, the die pad  220 A and  220 B also have bonding areas  255  which are located thereon and electrically connected to the dies  210 A and  210 B through third conductive wires  245 B. As the size of the die and the die pad is reduced, excess portion of the resin for binding the die on the die pad may bleed to the boding areas to fail the wire bonding process of the third conductive wires  245 B. To solve this problem, a resin bleed prevention structure  250  is formed between the die attach area (with respect to the location of the die) and the bonding area  255  to prevent resin bleeding to the boding area  255 , thereby guaranteeing enough space for wire bonding. The resin bleed prevention structure  250  may be a groove or a bump. 
         [0020]    In conclusion, the present invention using the tie bar of the lead frame as the contact for increasing the number of available contacts is helpful for packaging a given die into a smaller package structure. Thus, the packaging cost as well as the total fabrication cost of the IC can be reduced. 
         [0021]    While the preferred embodiments of the present invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the present invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the present invention.