Abstract:
A lead frame having a die support area for supporting a semiconductor die, a plurality of leads surrounding the die support area, and a dam bar connecting adjacent leads. The dam bar has a dummy tab between adjacent ones of the leads that transversely extends towards the die support area. The presence of the dummy tab reduces the volume of mold compound between the lead frame leads and thus, when the lead frame is cut via punching, only the lead frame is cut and not the molding material. This reduces mechanical stress during singulation significantly and as a result, the occurrence of package cracking is reduced. In addition, less mold compound at the dam bar inter-lead reduces debris during cutting, which in turn reduces debris from contaminating the package.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to integrated circuit (IC) packaging, and more particularly, to lead frames for semiconductor packages. 
         [0002]    Many current semiconductor assembly processes include a molding operation to encapsulate a die attached to a die attach area of a lead frame, followed by a singulation process in which adjacent devices are separated. During the singulation process, a dam bar of the lead frame also is cut off and adjacent leads are separated. 
         [0003]      FIG. 1A  shows a conventional lead frame  100  having a die support area  102  for supporting a semiconductor die (not shown), a plurality of leads  104  surrounding the die support area  102 , a dam bar  106  connecting adjacent leads  104 , and a dam bar inter-lead area  108  between adjacent leads  104  inside the dam bar  106 .  FIG. 1B  shows a partially assembled semiconductor device  110  including the conventional lead frame  100  after a molding operation. Molding material  112  flows to the dam bar inter-lead area  108  during the molding operation.  FIG. 1C  illustrates a dam bar cutting process in which the dam bar  106  is separated from the lead frame  100  with a punch  114 . During this cutting process, the molding material  112  can cause mechanical stress, which can increase the occurrence of package cracking. In addition, the molding material  112  is composed mainly of silica, which is an abrasive element that tends to accelerate singulation tool wear rate. Therefore, it would be advantageous to be able to reduce the volume of molding material in the dam bar inter-lead area  108  during the molding operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention, together with objects and advantages thereof, may best be understood by reference to the following description of preferred embodiments together with the accompanying drawings in which: 
           [0005]      FIG. 1A  is a schematic top plan view of a conventional lead frame; 
           [0006]      FIG. 1B  is a schematic top plan view of a partially assembled semiconductor device having a conventional lead frame after a molding operation; 
           [0007]      FIG. 1C  is a sectional view of the lead frame from the line  1 - 1  of  FIG. 1B  illustrating a during dam bar cutting process; 
           [0008]      FIG. 2A  is a schematic top plan view of a lead frame in accordance with an embodiment of the present invention; 
           [0009]      FIG. 2B  is a schematic top plan view of a partially assembled semiconductor device having a lead frame in accordance with an embodiment of the present invention after a molding operation; 
           [0010]      FIG. 2C  is a sectional view of the lead frame from the line  2 - 2  of  FIG. 2B  illustrating a dam bar cutting process; 
           [0011]      FIG. 3  is a schematic top plan view of a lead frame in accordance with another embodiment of the present invention; and 
           [0012]      FIG. 4  is a flow chart illustrating a method of assembling a semiconductor device in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0013]    The detailed description set forth below in connection with the appended drawings is intended as a description of presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be practised. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout. Furthermore, terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that module, circuit, device components, structures and method steps that comprises a list of elements or steps does not include only those elements but may include other elements or steps not expressly listed or inherent to such module, circuit, device components or steps. An element or step proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements or steps that comprises the element or step. 
         [0014]    In one embodiment, the present invention provides a lead frame including a die support area for supporting a semiconductor die, a plurality of leads surrounding the die support area, and a dam bar connecting adjacent leads. The dam bar has a dummy tab located between the adjacent leads and transversely extending towards the die support area. The dummy tab reduces a volume of molding material in a dam bar inter-lead area during a molding operation. 
         [0015]    In another embodiment, the present invention provides a method of assembling a semiconductor device including the step of providing a lead frame with a dam bar having a dummy tab between adjacent leads that transversely extends towards a die support area of the lead frame. The dummy tab reduces a volume of molding material in the dam bar inter-lead area during a molding operation. 
         [0016]    Referring now to  FIG. 2A , a lead frame  200  of the present invention is shown. The lead frame  200  includes a die support area  202  (indicated in dashed lines), sometimes called a die pad or flag, for supporting a semiconductor die, a plurality of leads  204  surrounding the die support area  202 , and a dam bar  206  that surrounds and is spaced from the die support area  202 . Although the die support area  202  part of the lead frame  200  is shown as square or rectangular, the die support area  202  could be otherwise formed, such as with a solid or single piece of metal that supports all or nearly all of a bottom surface of a semiconductor die, or the die support area  202  could be X-shaped. Thus, the present invention is not to be limited by the construction of the die support area  202 . The leads  204  are integral with and extend generally orthogonally from the dam bar  206 , as is known in the art. There is a dam bar inter-lead area  208  between adjacent leads  204  inside the dam bar  206 . The dam bar  206  also has dummy tabs  210  located between adjacent ones of the leads  204  and in the inter-lead areas  208 . The dummy tabs  210  extend transversely towards the die support area  202 . 
         [0017]    The lead frame  200  is preferably formed from a conductive metal such as Copper, as is known in the art. The lead frame may be plated such as with Tin or other metals, also as is known in the art. 
         [0018]    Each lead  204  has a die connect area  212 . In a preferred embodiment of the invention, each lead  204  has an inner lead portion  214  and an outer lead portion  216 , and the dam bar  206  connects the adjacent leads  204  at a location between the inner lead portion  214  and the outer lead portion  216 . In a preferred embodiment, there is a gap between the dummy tab  210  and the adjacent leads  204 . In another preferred embodiment, a length of the dummy tab  210  is less than a length of the adjacent leads  204 . 
         [0019]    Referring to  FIG. 2B , a partially assembled semiconductor device  220  having a lead frame  200  in accordance with an embodiment of the present invention after a molding operation that encapsulates the semiconductor die and portions of the lead frame  200  with a molding material  222  is shown. The dam bar  206  has the dummy tabs  210  between adjacent leads  204  that extend transversely towards the die support area  202 . During the molding operation, the dummy tab  210  reduces the volume of the molding material  222  in the dam bar inter-lead area  208 . 
         [0020]      FIG. 2C  is a sectional view of the lead frame  200  from the line  2 - 2  of  FIG. 2B  illustrating a dam bar cutting process. In the cutting process (singulation), a punch  114  is used to cut the dam bar  206  and the dummy tab  210 . Since the volume of the molding material  222  in the dam bar inter-lead area  208  has been reduced, the molding material  222  is spaced from the cutting area. As a result, the occurrence of package cracking is reduced. In addition, since the cutting tool or punch  114  does not contact with the abrasive mold material  222 , the wear on the cutting tool is reduced. 
         [0021]    Referring to  FIG. 3 , a lead frame  300  according to another preferred embodiment of the present invention is shown. The lead frame  300  includes a die support area  302  for supporting a semiconductor die (not shown), a plurality of leads  304  surrounding the die support area  302 , each lead  304  has an outer end  306 , and a dam bar  308  connecting adjacent leads  304  at the outer ends  306 . The dam bar  308  has a dummy tab  310  between the adjacent leads  304  transversely extending towards the die support area  302 . In a preferred embodiment, there is a gap between the dummy tab  310  and the adjacent leads  304 . In another preferred embodiment, a length of the dummy tab  310  is less than a length of the adjacent leads  304 . 
         [0022]      FIG. 4  is a flow chart showing the steps of a method  400  of assembling a semiconductor device with the lead frame as described above with reference to  FIGS. 2A-2B  and  4 . The method comprises providing the lead frame  200  with the dam bar  206  having the dummy tabs  210  between adjacent leads  204  and extending transversely towards the die support area  202  of the lead frame  200 . The dummy tab  210  reduces the volume of mold material proximate to the semiconductor die during a molding operation. 
         [0023]    In more detail, the method  400  starts at  402  by providing a lead frame  200  comprising a die support area  202  for supporting a semiconductor die (not shown), a plurality of leads  204  surrounding the die support area  202 , wherein each lead  204  has a die connect area  212 , and a dam bar  206  connecting adjacent leads  204 , wherein the dam bar  206  has a dummy tab  210  between the adjacent leads  204  transversely extending towards the die support area  202 . In a preferred embodiment, the dam bar  206  connects the adjacent leads  204  at a location between the inner lead portion  214  and the outer lead portion  216 . In a preferred embodiment, there is a gap between the dummy tab  210  and the adjacent leads  204 . In another preferred embodiment, a length of the dummy tab  210  is less than a length of the adjacent leads  204 . 
         [0024]    In another preferred embodiment as the lead frame  300 , the dam bar  308  connecting adjacent leads  304  at the outer ends  306  of the leads  304 . In a preferred embodiment, there is a gap between the dummy tab  310  and the adjacent leads  304 . In another preferred embodiment, a length of the dummy tab  310  is less than a length of the adjacent leads  304 . 
         [0025]    At  404 , a semiconductor die (not shown) is attached to the die support area  202  of the lead frame  200 , and is electrically connected to the die connect area  212  of the leads  204  at  406 . The connection can be made with bond wires or solder bumps. At  408 , the semiconductor die (not shown) and the die connect area  212  of the leads  204  are encapsulated with a molding material. During the molding operation, the dummy tab  210  inhibits molding material flow, therefore reduces the volume of molding material  222  in dam bar inter-lead area  208 . 
         [0026]    At  410 , the dam bar  206  is removed together with the dummy tab  210 . In a preferred embodiment, the removing step comprises cutting the dam bar  206  and dummy tab  210  with a punch. In another preferred embodiment, the removing step comprises removing the dam bar  206  and the dummy tab  210  with a saw. In a third preferred embodiment, the removing step comprises removing the dam bar  206  and the dummy tab  210  by laser cutting. In a fourth preferred embodiment, the removing step comprises etching the dam bar  206  and the dummy tab  210 . 
         [0027]    The description of the preferred embodiments of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiment disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.