Abstract:
A method for removing tiebars of a lead frame after one or more semiconductor components have been encapsulated in a resin to form a package body attached to the lead frame, includes preparing a support member and cutters, the support member including holes engageable with the cutters, positioning the lead frame in place on the support member, moving the cutters relative to the support member so that the cutters are fitted into corresponding holes of the support member, punching out tiebars of the lead frame, and thereafter, removing any scraps left between leads of the lead frame.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to a process for manufacturing a semiconductor chip package and, in particular, to a semiconductor chip package using a lead frame having tiebars.  
       BACKGROUND OF THE INVENTION  
       [0002]      FIG. 10A  illustrates a conventional packaged semiconductor device having a resin package body in which one or more semiconductor components are encapsulated and a plurality of leads each connected electrically with the components and projected outwardly from the package body. In the molding, semiconductor components on a lead frame L of the semiconductor device are encapsulated with resin to form a package body  2 . As shown in  FIG. 10B , the leads are connected with the lead frame L and also connected with the neighboring lead or leads  3  through a bridge or tiebar  4  leaving a small gap from a periphery of the package body to be molded. During the encapsulation process, the tiebars  4  are supposed to function as a barrier to minimize an amount of resin leaking out of a small opening defined between upper and lower mold halves. Disadvantageously, the resin leaked out of the opening and then stopped by the tiebars  4  form unnecessary residuals or flashing  5  which should be removed afterwards together with the tiebars  4 .  
         [0003]     For this purpose, a press cutting mechanism is provided on a downstream side of the molding in the manufacturing process. In general, the cutting mechanism has a die on which the lead frame L is placed. The die has a plurality of holes defined in a surface adjacent to the packaged device at respective portions opposing the tiebars  4  and residuals  5  when the lead frame L is positioned in place on the die. The cutting mechanism also has a plurality of press cutters provided above the die. The cutters are designed to move in and out of the corresponding holes so that the tiebars  4  and residuals  5  are cut or punched out into the holes and removed from the package body  2 , as shown in  FIG. 10C .  
         [0004]     With the arrangement, the tiebars  4  and residuals  5  are well removed from the package body  2 . However, some of the cut portions or scraps can adhere to the cutters after the cutoff operation and, as a result, be brought back with the cutters moving out of the holes and then clog between the neighboring leads  3 , which results in an unwanted short-circuit of the leads  3  and lowers the productivity of the device.  
       SUMMARY OF THE INVENTION  
       [0005]     Hence, it is an object of the present invention to provide an apparatus and method for removing tiebars without leaving any scraps in the lead frame.  
         [0006]     It is another object of the present invention to provide a method for manufacturing a semiconductor device capable of removing tiebars with high reliability.  
         [0007]     To achieve the above object, an apparatus of the present invention for removing tiebars of a lead frame after one or more semiconductor components have been molded to form a package body attached to the lead frame includes first and second removing stations. The first removing station includes first and second support members opposed to each other capable of putting the lead frame therebetween; and a tiebar-cutting punch held by the first support member for reciprocation relative to the first support member. The tiebar-cutting punch and second support member have projections or cutters and holes, respectively, engageable with each other. In the process, the lead frame is positioned in place, the lead frame is put between the first and second support members, and then the cutters are fitted into the corresponding holes in the second support, thereby punching out the tiebars. The second removing station includes third and fourth support members opposed to each other capable of putting the lead frame therebetween; and a scrap-removing punch held by the third support member for reciprocation relative to the third support member for removing the possible scraps left in between the leads of the lead frame after the tiebars have been removed in the first removing station. The scrap-removing punch and fourth support member have projections and holes, respectively, engageable with each other. In the process, the lead frame is positioned in place, the lead frame is put between the third and fourth support members, and then the scrap-removing punch is moved relative to the fourth support member so that the projections are fitted into the corresponding holes in the fourth support member, thereby pushing the possible scraps out of the lead frame to the holes.  
         [0008]     According to the removing apparatus, after the tiebars (and the residuals) are removed in the first removing station, the possible scraps left in between the leads are removed in the second station. Therefore, the tiebars (and the residuals) can be positively removed.  
         [0009]     Note that, although according to the present invention the residuals as well as the tiebars can be removed as described above, reference is not made hereinafter to the removal of the residuals and the cut portions thereof for the sake of descriptive simplicity.  
         [0010]     A method according to the present invention for removing tiebars of a lead frame after one or more semiconductor components have been molded to form a package body attached to the lead frame includes the steps of preparing a support member and cutters, the member being formed with holes engageable with the cutters; positioning the lead frame in place on the support member; moving the cutters relative to the support member so that the cutters are fitted into the holes of the support member, thereby punching out the tiebars; and removing the possible scraps left in between the leads of the lead frame after the moving step.  
         [0011]     According to the removing method, after the tiebars are removed, the scraps that may be left in between the leads are removed. Therefore, the tiebars can be positively removed.  
         [0012]     A method according to the present invention for manufacturing a semiconductor device includes the steps of molding one or more semiconductor components positioned on a lead frame to form a package body; removing remaining gates after the semiconductor components have been molded; and removing tiebars after the remaining gates have been removed. The step of removing the tiebars includes the steps of positioning the lead frame in place on a support member formed with holes; moving cutters relative to the support member so that the cutters are fitted into the corresponding holes of the support member, thereby punching out the tiebars; and removing the possible scraps left in between the leads of the lead frame after the moving step.  
         [0013]     According to the manufacturing method in which the “remaining gates” are removed before the removing process of the tiebars, components in the first and/or second removing station are prevented from being damaged because of the remaining gates during the removing process of the tiebars and/or scraps. Therefore, a high reliable removing operation of the tiebars can be performed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic view of the tiebar-removing device according to the present invention;  
         [0015]      FIG. 2  is a cross sectional view of a first removing station, taken along line the II-II in  FIG. 1 , which is perpendicular to a guiding direction of a lead frame;  
         [0016]      FIG. 3  is a cross sectional view of a second removing station, taken along the line III-III in  FIG. 1 , which is perpendicular to a guiding direction of a lead frame;  
         [0017]      FIG. 4A  is an enlarged cross sectional view of punching projections of the second removing station;  
         [0018]      FIG. 4B  is an enlarged cross sectional view of a variant of punching projections of the second removing station;  
         [0019]      FIG. 5  is a schematic diagram showing projections of a tiebar-cutting punch, fitted into corresponding through holes of a die in the first removing station;  
         [0020]      FIG. 6  is a cross sectional view of another die in the first removing station;  
         [0021]      FIG. 7  is a cross sectional view of a mechanism for detecting the detachment of the lead frame;  
         [0022]      FIG. 8  is a cross sectional view of a mechanism for removing resins remaining at the gate;  
         [0023]      FIG. 9A  is a partial cross sectional view of the mechanism in  FIG. 8  in the process of cutting the remaining resins;  
         [0024]      FIG. 9B  is a partial cross sectional view of the mechanism in  FIG. 8 , which has just cut the remaining resins;  
         [0025]      FIG. 10A  is a side view of a semiconductor device having a resin molded chip;  
         [0026]      FIG. 10B  is a top view of a portion of a typical lead frame having a tiebar structure; and  
         [0027]      FIG. 10C  is a top view of a tiebarless lead frame. 
     
    
       [0028]     Corresponding reference numbers indicate corresponding parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]      FIG. 1  shows schematically an embodiment of a tiebar-removing device according to the present invention. The device has a first removing station  6  for removing tiebars after a molding process and a second removing station  7  downstream of the station  6  with respect to the guiding direction of the lead frame for removing scraps that may be left in between leads after the first removing station  6  has removed the tiebars.  
         [0030]     Referring to  FIG. 2 , the first removing station  6  includes a lower die set  8 , to which guide posts  9  extending in the vertical direction are fixed. An upper die set  10  is supported on the guide posts  9  for vertical movement relative to and along the guide posts  9 .  
         [0031]     A die plate  12  is fixed on the lower die set  8 . A tiebar-cutting die  16  is removably mounted through a spacer  14  on the die plate  12 . The die  16  constitutes a lower supporting member (second supporting member) which, in combination with an upper supporting member described below, serves to engage a lead frame L therebetween during the cutting process of the tiebars. In order that, where the die  16  is abraded to some extent, it is detached and polished to reuse, the spacer  14  is replaced with a new one which is thicker by the thickness of the polished portion of the die  16  to adjust the height of the top surface of the die  16 . The die  16  has a plurality of through holes  16   a  in the upper portion which, in combination with a plurality of projections of the tiebar-cutting punch described below, serve to punch out or cut the tiebars. The hole pitch is equal to the lead pitch of the lead frame L. The through holes  16   a  are connected with a scrap-exhausting hole  16   b  formed in the lower portion of the die  16 . In order to discard the cut portions or scraps of the tiebars from the device, scrap-exhausting holes  8   a,    12   a  and  14   a  are formed in the lower die set  8 , the die plate  12  and the spacer  14  connected with the scrap-exhausting hole  16   b.    
         [0032]     A backing plate  18  is fixed on the lower surface of the upper die set  10 . On the lower surface of the backing plate  18  is fixed a punch plate  20 , on which guide posts  24  extending in the vertical direction are fixed for supporting a stripper plate  22  for vertical movement. On the stripper plate  22  is fixed a stripper piece  28 , on the lower surface of which pilot pins  26  are mounted in place. The stripper piece  28  constitutes an upper supporting member (first supporting member) which, in combination with the lower supporting member, serves to engage the lead frame L therebetween during the cutting process of the tiebars.  
         [0033]     On the upper surface of the stripper plate  22  are fixed one ends of push rods  30 , which are extended in the vertical direction through the backing plate  18  and punch plate  20 . The other ends of the push rods  30  are connected with respective coil springs  32  provided within the upper die set  10  for biasing the push rods  30  downwardly. In the drawing, reference number  34  indicates a bolt for incorporating the respective coil spring  32  in the upper die set  10 .  
         [0034]     The first removing station  6  includes a tiebar-cutting punch  36 . The punch  36  has a plurality of projections or cutters  36   a  which, in combination with the through holes  16   a  of the die  16 , serve to, punch out the tiebars. For this purpose, the size and pitch of the projections  36   a  are designed so that they are capable of being fitted into the corresponding through holes  16   a  of the die  16 .  
         [0035]     The punch plate  20  is formed with an opening for incorporating therein and holding the upper part of the tiebar-cutting punch  36 . A spacer  38  is provided between the upper end of the punch  36  and the backing plate  18 . In order that, where the punch  36  is abraded to some extent, it is detached and polished to reuse, the spacer  38  is replaced with a new one which is thicker by the thickness of the polished portion of the punch  36  to adjust the height of the lowermost surface of the punch  36 . The punch  36  is removably attached to the punch plate  20  by means of punch holders  42 , each of which is fastened by a bolt  42 .  
         [0036]     The stripper piece  28  is provided with a plurality of guides  28   a  that are fitted in the respective recesses between the adjacent projections  36   a  of the punch  36 . The guides  28   a  are designed with a high degree of geometrical accuracy so that they can guide the projections  36   a  of the punch  36  without rocking in the transverse direction when the punch  36  is moved in the vertical direction relative to the stripper piece  28  as will be described hereinafter.  
         [0037]     A pair of opposing guide rails  44  is provided between the die  16  and stripper piece  28  for guiding the lead frame L from a molding device (not shown) to the first removing station  6  and then to the second removing station  7 . The guide rails  44  have a plurality of through holes  44   a  for receiving the pilot pins  28  of the stripper piece  28  to position the lead frame L relative to the stripper piece  28 . The guide rails  44  are designed so that it can be moved in the vertical direction between a home position shown where they are located between and spaced from the die  16  and stripper piece  28  and a tiebar-removing position where they are sandwiched between the die  16  and stripper piece  28 . As described below, the downward movement of the stripper piece  28  allows the guide rails  44  to be pressed by the stripper piece and move downwardly to the tiebar-removing position.  
         [0038]     Referring to  FIG. 3 , the second removing station  7  is similar to the first removing station  6  except for the details described hereinafter. Specifically, the second removing station  7  includes a scrap-removing punch  136  with projections  136   a  that are capable of fitting into corresponding through holes  116   a  of a die  116  to push out the scraps that may clog between the leads and drop them to a scrap-exhausting hole  116   b  formed in the die  116 . The stripper piece  128  and die  116  constitute an upper or third supporting member and a lower or fourth supporting member, respectively, to put the lead frame L therebetween during the removing process of the scraps.  
         [0039]     It should be noted that the projections  136   a  of the second removing station  7  need not function as cutters, unlike the first removing station  6 . Each of the projections  136   a  may have a cross section identical to that of the projection  36   a  in the first removing station  7  (and thus the tiebar portion to be cut by each projection  36   a ). However, each of the projections  136   a  may be designed so that it has a cross section smaller than that of the projection  36   a  in the first removing station  7  (and thus the tiebar portion to be cut by each projection  36   a ) as shown in  FIG. 4A , as long as the cross section of the projection  136   a  is large enough to push out the scraps T caught in between the leads. Preferably, as shown in  FIG. 4B , the projections  136   a  may be tapered in the downward direction to improve the strength thereof. In addition, the punch  136  (or at least the projections  136   a  thereof) may be formed with resin, so that the lead frame L is prevented from being damaged in case where the lead frame L and projections  136   a  may interfere with each other during the removing process of the scraps T.  
         [0040]     The punch  36  of the first removing station  6  is aligned and guided with high accuracy by the guides  28   a  of the stripper piece  28  that are fitted in between the corresponding adjacent projections  36   a  of the punch  36 . On the other hand, the punch  136  of the second removing station  7  can remove the scraps that may be left in between the leads even if the alignment accuracy of the projections  136   a  is lower than that of the projections  36   a . Therefore, guiding portions such as the guides  28   a  may be omitted in the stripper piece  128  and only the periphery of the punch  136  may be guided by the stripper piece  128 , as shown in  FIG. 3 . This result in the constructional simplicity of the stripper piece  128 , providing a tiebar-removing device at lower cost.  
         [0041]     The operation of the tiebar-removing device will now be described. Referring to  FIG. 2 , the lead frame L is guided along the guide rails  44  and transported to the region above the die  16  of the first removing station  6 . Next, the upper die set  10  is moved downwardly in the vertical direction along the guide posts  9 . Accordingly, the stripper plate  22  and stripper piece  28  fixed thereto are moved downwardly in the vertical direction along the guide posts  24 , so that the pilot pins  26  on the lower surface of the stripper piece  28  are inserted through the holes  44   a  of the guide rails  44  into corresponding locating holes not shown in the lead frame L. As the lead frame is positioned relative to the stripper piece  28 , the stripper piece  28  presses the guide rails  44  downwardly.  
         [0042]     The further downward movement of the upper die set  10  brings the stripper piece  28  in touch with the die  16  to press the lead frame L toward the die  16 . With the additional downward movement of the upper die set  10 , the stripper piece  28  presses the compressed coil springs  32  via the respective push rods  30 , which in turn increases the reaction force to compress the lead frame L toward the die  16 . The punch  36  is also moved downwardly along the guides  28   a  of the stripper piece  28  so that the through holes  16   a  of the die  16  and the corresponding projections  36   a  of the tiebar-cutting punch  36  engage with each other to punch out the tiebars.  FIG. 5  shows the fit of the projections  36   a  of the punch  36  into the corresponding through holes  16   a  of the die  16 . The tiebars that have been cut are discarded through the exhausting holes  16   b,    14   a,    12   a  and  6   a  in this order out of the removing device.  
         [0043]     Then, the upper die set  10  is moved upwardly to permit components of the first removing station  6  to operate reversely so that the guide rails  44  are returned to their home position. Note that when the punch  36  is moved upwardly to return to its original position, the stripper piece  28  prevents the lead frame L from moving upwardly together with the punch  36 .  
         [0044]     As described above, when the projections or cutters  36   a  of the punch  36  in the first removing station  6  are pulled out of the holes  16   a  of the die  16 , the portions that have been cut may be lifted and caught in between the neighboring leads of the lead frame L. According to the present invention, for removal of such cut portions or scraps, the lead frame L is transported along the guide rails  44  to the second removing station  7 .  
         [0045]     The transported lead frame L is held in the second removing station  7  so that the regions in the lead frame L where the tiebars have been cut are right above the through holes  116   a  of the die  116  (see  FIG. 4A ). Then, components in the second removing station  7  are operated in the substantially same way as the corresponding components in the first removing station  6 . Specifically, the projections  136   a  of the punch  136  are fitted into the through holes  116   a  of the die  116  to push out the possible scraps caught in between the leads so that they are discarded through the exhausting holes  116   b,    14   a,    12   a  and  4   a  in this order out of the removing device.  
         [0046]     According to this embodiment of the tiebar-removing device, the scraps caught in between the leads in the tiebar removing process in the first removing station  6  are completely removed in the second removing station  7 , which allows the yield of a semiconductor device to be improved.  
         [0047]     Since the present invention has been described by way of the examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be effected without departing from the spirit and scope of the present invention.  
         [0048]     For example, as shown in  FIG. 6 , the exhausting holes  216   b  in the die  216  of the first removing station  6  may be connected with one or more vent holes  217  for supplying gas (e.g. air) to the projections or cutters of the punch when they are fitted in the holes of the die  216 . This restrains the scraps attached on the tips of the projections from being lifted when the projections of the punch are pulled out of the holes of the die  216 .  
         [0049]     Also, a mechanism for detecting the detachment of the lead frame L from the guide rails may be provided in the removing device in order to prevent the lead frame and/or components of the removing stations from being damaged in the tiebar- and/or scrap-removing operation. An exemplary detecting mechanism is shown in  FIG. 7  in which a pair of opposing guide rails  344 , which is formed with grooves  345  along which the lead frame L is guided, has openings  347  positioned above and below the corresponding grooves  345 . Sensors  343  (e.g. photo sensors such as light emitters and receivers) are provided externally of the openings  345  so-that they can detect the detachment of the lead frame L from the grooves  345 .  
         [0050]     As known in the art, resins may remain at the gate (which is herein referred to as “remaining gates”) after the lead frame has been molded to form a package body. Therefore, when the tiebars are removed in the first removing station  6  with the resins remaining at the gate, they may damage components such as die. Accordingly, it is preferable to trim off the remaining gates before the cutting process of the tiebars in the first removing station  6 .  
         [0051]      FIG. 8  shows a partial cross sectional view of a gate cutting station. The station  50  has a configuration similar to that of the first removing station  6 . In  FIG. 8 , the similar elements are indicated by reference numbers with a hundreds digit of 4. The gate cutting station  50  is configured so that it cooperates with the first and second removing stations  6  and  7 . Specifically, the gate cutting, first removing and second removing stations  50 ,  6  and  7  are positioned in this order with respect to the guiding direction of the lead frame L, which is in the form of a ribbon and guided along the guide rails  44 . The downward movement of the upper die set  10 , which is a common element to the gate cutting, first removing and second removing stations  50 ,  6  and  7 , allows the gate-, tiebar- and scrap-removing process to be simultaneously performed for respective lead frame portions positioned in place in the stations  50 ,  6  and  7 , respectively.  
         [0052]     The station  50  has a lifter  52  with a projection  52   a  opposed to the remaining gates G of the lead frame L. The lifter  52  is connected at its one end with a coil spring  54 , which is provided within a hole formed in the lower die set  408 , die plate  412  and die  416  for biasing the lifter  52  upwardly. In the drawing, reference number  56  indicates a bolt for incorporating the coil spring  54  in the hole.  
         [0053]     The stripper piece  428  has a projection  428   b  opposed to the projections  52   a  of the lifter  52  so that the lead frame L transported along the guide rails  44  is interposed therebetween. The projection  52   a  serves to support the surface opposite the surface with the remaining gates G thereon when the stripper piece  428  is moved downwardly. By the further downward movement of the stripper piece  428 , the projection  428   b  cooperates with the projection  52   a  of the lifter  52  to trim the remaining gates.  
         [0054]     A press pin  58  is fixed to the plate  420 . The press pin  58  is fitted for vertical movement in the through hole formed in the stripper plate  422  and stripper piece  428 . The upper surface  52   b  of the lifter  52  is opposed to the lower surface  58   a  of the press pin  58  to make contact with the press pin  58  when the press pin  58  moves downwardly. In order to simplify an understanding of the construction, the press pin  58  is shown as if it is provided for downward movement across the lead frame L toward the lifter  52  in the drawing. However, the press pin  58  is designed so that it does not interfere with the lead frame L during its downward movement.  
         [0055]     The lowermost surface  422   a  of the stripper plate  422  is set to have the substantially same height as that of the lower surface  428   a  of the stripper piece  428 . The uppermost surface  412   a  of the die plate  412  is set to have the substantially same height as that of the tip of the projection  52   a  of the lifter  52  in its top position biased by the spring  54 .  
         [0056]     With the gate cutting station  50  so constructed, the lead frame L is transported along the guide rails  44  to the region above the die  416  in the gate cutting station  50 . Next, the upper die set (not shown) is moved downwardly in the vertical direction along the guide posts (not shown). Accordingly, the stripper plate  426  and stripper piece  428  fixed thereto are moved downwardly in the vertical direction along the guide posts (not shown), so that the pilot pins  426  on the lower surface of the stripper piece  428  are inserted through the holes (not shown) of the guide rails  44  into corresponding locating holes (not shown) in the lead frame L. Thus, the lead frame L is brought in alignment with the stripper piece  428 . Then, as shown in  FIG. 9A , the die plate  412  blocks the movement of the stripper plate  422  while the projections  428   b  and  52   a  of the stripper piece  428  and lifter  52 , respectively, trim off the remaining gates. At this moment, the tiebar/scrap-removing process is not performed in the removing stations  6  and  7 .  
         [0057]     The further downward movement of the upper die set and the plate  420  fixed thereto allows the press pin  58  to be guided downwardly in the through hole formed in the stripper plate  422  and stripper piece  420 . Then, as shown in  FIG. 9B , the press pin  58  is brought in touch with the lifter  52 , so that the lifter  52  is pushed downwardly against a biasing force provided by the spring  54  to space the projection  52   a  of the lifter  52  from the lead frame L.  
         [0058]     With the projection  52   a  of the lifter  52  spaced away from the lead frame L to the extent that the projection  52   a  does not make contact with the lead frame L, the tiebar/scrap-removing process is performed for the lead frame portions in the first and second removing stations  6  and  7 . Use of such gate cutting mechanism for vertical movement allows the remaining gate, tiebar and scrap removing processes to be performed with high reliability and efficiency.  
         [0059]     According to the present invention, it is ensured in the manufacturing of a semiconductor that the tiebars are completely removed after the molding process, which result in an improved yield of the semiconductor.