Patent Publication Number: US-2010127368-A1

Title: Lead frame

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a lead frame, and more specifically to a lead frame that can prevent itself from thermal deformation during the wire bonding process. 
     2. Description of the Related Art 
       FIG. 1  shows a unit  10  of a lead frame according to a prior art. The unit  10  comprises an external frame  11 , a plurality of leads  12  connected with the external frame  11 , a die mount pad  13  for supporting a chip, and four support bars  14  connected with the external frame  11  and the die mount pad  13 . In order to serve special needs of products, a short bar  15  is provided to connect the die mount pad  13  to one of the leads  12  for eliminating static electricity. 
     During the wire bonding process, the lead frame is attached to a heat block and then heated at about 200° C. Since the die mount pad  13  is restricted by the short bar  15  and the support bars  14  that are bridged between the external frame  11  and the die mount pad  13  have a straight shape without a thermal deformation-absorbing design, the thermal expansion stress concentrated in the unit  10  can&#39;t be released, causing displacement or deformation of the die mount pad  13 . Under this circumstance, if the wire bonding process continues, some units may be unable to have wire bond connections correctly or a leakage of resin encapsulant may occur during the molding process that follows the wire bonding process due to the displacement or deformation of the die mount pad  13 , resulting in decrease of the product yield. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in view of the above-noted circumstances. It is one objective of the present invention to provide a lead frame, which provides a stretchable design to release the thermal expansion stress during the wire bonding process so as to increase the product yield. 
     To achieve this objective of the present invention, the lead frame comprises a plurality of units arranged in a matrix manner. The units each include an external frame defining an accommodation area, a die mount pad disposed in the accommodation area of the external frame, a plurality of leads connected with the external frame and arranged around the die mount pad, a short bar having two ends respectively electrically connected with the die mount pad and one of the leads, and a plurality of support bars each having a straight section connected with the external frame, and a continuous curved section connected with the die mount pad. 
     Accordingly, the lead frame of the present invention is capable of releasing the thermal expansion stress caused by the wire bonding process by means of the continuous curved sections of the support bars to prevent each of the units of the lead frame from thermal deformation and displacement, thereby raising the yield of the package. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic drawing of a unit of a lead frame according to a prior art; 
         FIG. 2  is a schematic drawing of a lead frame according to a first preferred embodiment of the present invention; 
         FIG. 3  is a schematic drawing of a unit of the lead frame according to the first preferred embodiment of the present invention, and 
         FIG. 4  is a schematic drawing of a unit of the lead frame according to a second preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 2 , a lead frame  20  in accordance with a first preferred embodiment of the present invention comprises a plurality of units  22  arranged in a matrix manner. As shown in  FIG. 3 , each of the units  22  includes an external frame  30 , a die mount pad  40 , a plurality of leads  50 , a short bar  60  and four support bars  70 . 
     The external frame  30  defines a square accommodation area  32 . 
     The die mount pad  40  is disposed in the center of the accommodation area  32  for supporting a chip (not shown). 
     Each of the leads  50  is connected with the external frame  30  and arranged around the die mount pad  40 . 
     The short bar  60  is a straight line, having two ends respectively electrically connected with a lateral side of the die mount pad  40  and one of the leads  50  to provide a grounding function. 
     The support bars  70  each have a straight section  72  and an S-shaped continuous curved section  74 . The straight section  72  has a distal end connected with a corner of the external frame  30 , and the continuous curved section  74  has a distal end connected with a corner of the die mount pad  40 . 
     When the lead frame  20  of the present invention is attached to a heat block and heated during the wire bonding process, the thermal expansion stress induced in each of the units  22  can be absorbed and released through the continuous curved sections  74  of the support bars  70  that have a continuous change of curvature respectively. As a result, each of the units  22  of the lead frame  10  of the present invention will not deform or displace despite the existence of the short bar  60  during the wire bonding process, thereby enhancing the yield of the package. 
     It is to be mentioned that the short bar is not limited to a straight line. As shown in  FIG. 4 , a unit  80  of a lead frame in accordance with a second preferred embodiment of the present invention comprises a short bar  82  that has two turns  822  and  824  to form a curved shape, such as an N-shaped curve, for assisting the continuous curved sections  842  of the support bars  84  to release the thermal expansion stress generated in the lead frame. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.