Abstract:
A process for forming semiconductor packages comprises partially etching a leadframe matrix, encapsulating it with mold compound, placing a semiconductor die in a leadframe unit and singulating the leadframe matrix. A system for forming semiconductor packages comprises means for partially etching a leadframe matrix, means for encapsulating it with mold compound, means for placing a semiconductor die in a leadframe unit and means for singulating the leadframe matrix.

Description:
RELATED APPLICATIONS 
     This application claims benefit of priority under 35 U.S.C. section 119(e) of U.S. Provisional Patent Application 61/002,138 filed Nov. 6, 2007, entitled FRAME DESIGN AND MOLD DESIGN CONCEPT FOR MOLD DESIGN THICKNESS which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the field of semiconductor packaging and is more specifically directed methods of achieving optimum thicknesses for semiconductor packages. 
     BACKGROUND 
     In Leadframe Based Packaging, a leadframe matrix  100  in  FIG. 1  is processed by placing singulated semiconductor die (not shown) into each individual leadframe unit  110 . Then, bond pads on the die are wire bonded to an output pin  115  in the leadframe unit to effectuate a desired landing pattern onto a printed circuit board (not shown). Next, a mold compound is formed over the leadframe matrix  100  and allowed to set. Individual devices are marked, and the leadframe matrix is singulated by saw or other cutting apparatus into individual semiconductor devices. Although this process is well known, it does not allow for control of the thickness of the final package. The thickness of semiconductor devices is typically is dictated by the thickness of the die added to the thickness of the leadframe plus the mold compound. In certain applications, thinner semiconductor devices are desired. What is needed is a process and system for forming semiconductor leadframe packages with different desired thicknesses. 
     SUMMARY OF THE DISCLOSURE 
     One aspect of the invention is for a process of forming individual packaged semiconductor devices. To that end, a leadframe matrix has a thickness, a top surface and a bottom surface. At least one of the top surface and bottom surface is etched to form a selectively half etched leadframe having an etched pattern. The selectively half etched leadframe is mounted onto a tape carrier and then the selectively etched leadframe is encapsulated in a first mold compound such that the etched pattern is filled with mold compound. As a result, a selectively half etched molded leadframe is formed. At least one semiconductor device is mounted in at least one individual leadframe. The tape carrier is removed and the selectively half etched molded leadframe is singulated thereby forming individual packaged semiconductor devices. In some embodiments, the selectively half etched leadframe is encapsulated in a first mold compound by placing the selectively half etched leadframe between a top mold, having a film gate, and a bottom mold. A mold compound is injected in a liquid state through the film gate such that it encapsulates the selectively half etched leadframe thereby forming a selectively half etched molded leadframe. Alternatively, The selectively half etched leadframe can be encapsulated in a first mold compound by placing the selectively half etched leadframe between a top mold, having a pincer gate, and a bottom mold. Again, a mold compound in a liquid state is injected through the pincer gate such that it encapsulates the selectively half etched leadframe thereby forming a selectively half etched molded leadframe. In some embodiments, the top mold further comprises a vacuum pipe output. The process further comprises forming a near vacuum at the vacuum pipe to effectuate distribution of the first mold compound. In some embodiments, the process of further comprises encapsulating the selectively half etched molded leadframe in a second mold compound. Also, mounting at least one semiconductor device comprises mounting the at least one semiconductor device on the selectively half etched molded leadframe and wirebonding such that the at least one semiconductor device is encapsulated by the second mold compound. Alternatively, mounting at least one semiconductor device comprises mounting the at least one semiconductor device in a cavity within the individual leadframe and wirebonding such that the first mold compound encapsulates the at least one semiconductor device. 
     In another aspect of the disclosure, a system of assembling a semiconductor package comprises means for selectively etching at least one of a top surface and a bottom surface of a leadframe matrix. The system also has a means for mounting the selectively half etched leadframe onto a tape carrier and means for encapsulating the selectively half etched leadframe in a first mold compound such that the etched pattern is filled with mold compound, thereby forming a selectively half etched molded leadframe. Also, the system has a means for mounting at least one semiconductor device in at least one individual leadframe, means for removing the tape carrier, and means for singulating the selectively half etched molded leadframe thereby forming individual packaged semiconductor devices. In some embodiments, the means for encapsulating the selectively half etched leadframe in a first mold compound has a means for placing the selectively half etched leadframe between a top mold, having a film gate, and a bottom mold. A means for injecting a mold compound in a liquid state through the film gate provides the mold compound such that it encapsulates the selectively half etched leadframe thereby forming a selectively half etched molded leadframe. Alternatively, the means for encapsulating the selectively half etched leadframe in a first mold compound comprises placing the selectively half etched leadframe between a top mold, having a pincer gate, and a bottom mold. A means for injecting a mold compound in a liquid state through the pincer gate provides the mold compound such that it encapsulates the selectively half etched leadframe thereby forming a selectively half etched molded leadframe. In some embodiments, the top mold further comprises a vacuum pump output and the system further comprises means for forming a near vacuum for effectuating distribution of mold compound. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth in the appended claims. However, for purpose of explanation, several embodiments of the invention are set forth in the following figures. 
         FIG. 1  shows a standard leadframe matrix. 
         FIG. 2  shows two portions of half etched leadframe matrices from top and cross sectional views. 
         FIG. 3A  is a process of forming a molded half etched leadframe per an embodiment of the current invention. 
         FIG. 3B  shows another view of the process of forming a molded half etched leadframe of  FIG. 3A . 
         FIG. 3C  shows a process of forming a molded half etched leadframe per an alternative embodiment of the current invention. 
         FIG. 3D  is another view of the process of forming a molded half etched leadframe of  FIG. 3C . 
         FIG. 4  shows alternative half etched leadframe matrices from top and cross sectional views. 
         FIG. 5A  shows an alternative process of forming a molded half etched leadframe. 
         FIG. 5B  shows an alternative process of forming a molded half etched leadframe. 
         FIG. 5C  shows an alternative process of forming a molded half etched leadframe. 
         FIG. 5D  shows an alternative process of forming a molded half etched leadframe. 
         FIG. 6  is a process of applying the half etched leadframe to assemble semiconductor packages. 
         FIG. 7  is an alternative process of applying the half etched leadframe to assemble semiconductor packages. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details and alternatives are set forth for purpose of explanation. However, one of ordinary skill in the art will realize that the invention can be practiced without the use of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. For example, it is commonly known in the art of semiconductor device assembly that assembly is generally done on a matrix array of leadframes, often referred to as leadframe strips, each strip having a plurality of individual positions that will be processed in various ways to form individual packaged semiconductor devices. A position can have one or more semiconductor die within. The following description details exemplary embodiments of processes. It will be appreciated by the person of ordinary skill having the benefit of this disclosure that the steps taught or claimed need not be performed in the order shown. The process steps are able to be performed in an order that is not inconsistent with the teachings herein to achieve a desired end result. 
       FIG. 2  shows two portions of leadframe matrices  200 . The first is a portion of a leadframe matrix  210  that has been selectively etched from both the top and bottom surfaces. The perimeter of the matrix  211  is not etched and remains at full metal thickness with a nickel/Palladium/Gold coating to prevent corrosion. As will be shown later in a process of assembling individual packaged semiconductor devices, the perimeter  210  can be discarded in some applications. A cross section of the matrix  210  taken across the line A-A the portion of the matrix  210  having a full thickness and the portions that are etched from the bottom surface  215  and portions that are etched from the top surface  225 . The various voids and cavities are able to be formed on the top or bottom surface of the matrix  210  to provide differing specific applications. Alternatively, the leadframe matrix  220  is etched from the top surface only. The etched portions  225  are visible in the cross section taken across the line B-B as forming cavities and voids  230 . Such partially etched leadframes are referred to herein as half etched leadframes. In some embodiments, the half etched leadframe  200  is mounted to a carrier tape  230  to secure it during processing as shown in  FIGS. 4A-4D  and  5 A- 5 D. 
       FIG. 3A  shows a process of forming a molded half etched leadframe  350 . The half etched leadframe  300  in this example has been etched on both a top and a bottom surface. The half etched leadframe  300  is mounted to a tape carrier  305 . The half etched leadframe  300  and carrier tape  305  are placed between a top mold  310  having a tape loader on the mold surface  313  and a bottom mold  335 . The tape loader effectuates the movement of the half etched leadframe  300  through the mold. In some embodiments, the top mold  310  comprises a film gate  355 . The film gate  355  encircles a perimeter of the leadframe  300  to allow the injection therein of a mold compound  325 . The mold compound  325  is fed into the film gate  355  through a cavity  320 . The mold compound  325  is allowed to cool and set and the molded half etched leadframe  350  is removed from the top mold  310  and bottom mold  335 . In some embodiments, the tape carrier  305  is left attached to the molded half etched leadframe  350  to carry it through later assembly steps. The process of  FIG. 3A  is seen from top and isometric views in  FIG. 3B . The leadframe matrix  305  is shown without the top mold  310 . A mold pot and cull liquefies a mold compound and distributes the mold compound through mold runners  365  to the film gate  355 . The mold pot and cull  360  and runners  365  are removed to leave the molded half etched leadframe  350 . 
       FIG. 3C  shows an alternate embodiment of the process shown in  FIG. 3A . In this alternative exemplary embodiment, the half etched leadframe  300  is mounted to a tape carrier  305 . The half etched leadframe  300  and carrier tape  305  are placed between a top mold  310  having a tape loader on the mold surface  313  and a bottom mold  335 . The tape loader effectuates the movement of the half etched leadframe  300  through the mold. In some embodiments, the top mold  310  comprises a pincer gate  375 . The pincer gate  375  is coupled to the leadframe  300  to effectuate the injection therein of a mold compound  325 . The mold compound  325  is fed into the pincer gate  375  through a cavity  320 . The mold compound  325  is allowed to cool and set. Then, the molded half etched leadframe  350  is removed from the top mold  310  and bottom mold  335 . In some embodiments, the tape carrier  305  is left attached to the molded half etched leadframe  350  to carry it through later assembly steps. The process of  FIG. 3C  is seen from top and isometric views in  FIG. 3D . The leadframe matrix  305  is shown without the top mold  310 . A mold pot and cull liquefies a mold compound and distributes the mold compound through mold runners  365  to the pincer gate  375 . The mold pot and cull  360  and runners  365  are removed to leave the molded half etched leadframe  350 . Advantageously, no film gate  355  remains on the molded half etched leadframe  350  when the pincer gate is used to deliver the mold compound. 
       FIG. 4  shows an alternative embodiment for the leadframe matrices of  FIG. 2 . In this embodiment, the leadframe matrix  610 , comprises a support gate  620  and is etched from both a top and a bottom surface. The support gate  620  is configured to allow easy flow therein of a liquified mold compound (not shown). Alternatively, a leadframe matrix  630  comprises a vacuum pipe  640 . Applying a partial vacuum to the vacuum pipe  640  further enhances the even distribution of liquified mold compound. A leadframe matrix can comprise both the support gate  620  and the vacuum pipe  640  for still further enhanced liquid mold compound encapsulation. 
       FIG. 5A  shows an process of molding a leadframe matrix  300  having a support gate  620 . The half etched leadframe  300  is mounted to a carrier tape. The leadframe matrix  300  is placed between a top mold  310  having a film gate  315  and a bottom mold  335 . Mold compound  365  is injected in a liquid state into the film gate  315 . Advantageously, the support gate  620  is aligned with the film gate  315  to effectuate the even flow therein of liquified mold compound  365 . The top mold  310  and bottom mold  335  are removed. A film gate remainder  355  is able to be removed in a later process step or discarded when individual semiconductor devices are singulated. 
       FIG. 5B  shows an process of molding a leadframe matrix  300  having a support gate  620 . The half etched leadframe  300  is mounted to a carrier tape. The leadframe matrix  300  is placed between a top mold  310  having a pincer gate  375  and a bottom mold  335 . Mold compound  355  is injected in a liquid state into the pincer gate  375 . Advantageously, the support gate  620  is aligned with the pincer gate  375  to effectuate the even flow therein of liquified mold compound  355 . The mold compound  355  fills the cavities  325  in the half etched leadframe  300  to form a half etched molded leadframe  350 . The top mold  310  and bottom mold  335  are removed. The carrier tape  305  is able to be removed in a later process step if desired. 
       FIG. 5C  shows another alternative to the process described in  FIG. 5A . The half etched leadframe matrix  300  having a vacuum pipe  640  is mounted to a carrier tape  305 . The leadframe matrix  300  is placed between a top mold  310  and a bottom mold  335 . In this embodiment, the top mold  310  comprises a vacuum exhaust  646 . When mold compound  365  is pumped in through the film gate  315 , a partial vacuum is formed at the vacuum exhaust  646  to effectuate the more even distribution of the mold compound  365  into the cavities  325  of the half etched leadframe  300 . 
       FIG. 5D  shows another alternative to the process described in  FIG. 5B . The half etched leadframe matrix  300  having a vacuum pipe  640  is mounted to a carrier tape  305 . The leadframe matrix  300  is placed between a top mold  310  and a bottom mold  335 . In this embodiment, the top mold  310  comprises a vacuum exhaust  646 . When mold compound  365  is pumped in through the pincer gate  375 , a partial vacuum is formed at the vacuum exhaust  646  to more evenly distribution of the mold compound  365  into the cavities  325  of the half etched leadframe  300 . A more ideal vacuum is able to be formed with equipment configured to create larger negative pressures as required by specific applications that do not deviate from the teachings of this disclosure. 
       FIG. 6  shows a process  400  of applying the half etched leadframe  405 . The half etched leadframe  405  is mounted to a carrier tape  406  for transport through the process. It will be appreciated that many commercially available machines are able to carry out the steps described herein. In the next step  410 , semiconductor die  415  are mounted to individual leadframe units within the leadframe matrix. Semiconductor die  415  are mounted to the leadframe units by tape or adhesive. The semiconductor die  415  are preferably placed by pick and place automated machines. Generally, a pick and place machine comprises a robotic arm that picks up semiconductor die that have been singulated from a wafer and places them in a desired location and orientation within an individual leadframe unit. In some applications, multiple die are placed in a leadframe unit. These multiple die are able to be side by side or stacked on top of each other as applications require. Also, wire bonds  417  are attached from bond pads on the semiconductor die  415  to the leadframe units to effectuate electrical communication between the semiconductor die and the board to which the finished product will be mounted to. Next, in a step  420 , the half etched leadframe  405  is molded in a process such as the ones described in  FIGS. 3A-3D . The carrier tape  406  is removed in a step  430 . The molded leadframe is moved to a sawing step  440  where saws  445  singulate the leadframe matrix thereby forming individual semiconductor devices  450 . 
     An alternative application is shown in  FIG. 7 . The process  500  starts with a half etched leadframe  505  mounted to a carrier tape  508 . In the next step  510 , the half etched leadframe is filled with a first mold compound  515  in one of the processes described in  FIGS. 3A-3D . In the next step  520 , the carrier tape  508  is removed. In the step  530 , semiconductor die  535  are mounted to individual leadframe units. In some embodiments, the die  535  are electrically coupled to the leadframe  505  by wirebonds  538 . In a step  540 , a second mold compound  545  is used to encapsulate the leadframe having the semiconductor devices  535 . The leadframe matrix  505  is singulated by saw blades  555  in a step  540 , thereby forming individual semiconductor devices  565  in a final step  550 . The half etched leadframe technique of  FIGS. 3A-3D  is able to be used to make a thin profile semiconductor package  450  as described in  FIG. 4  or a thicker profile package  550  of  FIG. 5 .