Abstract:
A precisor plate for an automatic wire bonder having a heat block attachment portion thereon to precisely attach a heat block thereto provides a simple and quickly adjustable precisor plate and method of alignment. The precisor plate includes fixed alignment pins to align the precisor plate relative to a lead frame in an automatic wire bonding machine. In a preferred embodiment, the heat block is integral with the precisor plate and includes internal bores and or passageways for receiving a heating element, a thermocouple, and/or a vacuum source.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/333,080, filed Jun. 14, 1999, now U.S. Pat. No. 6,237,830 B1, issued May 29, 2001, which is a divisional of application Ser. No. 08/794,877, filed Feb. 5, 1997, now U.S. Pat. No. 5,971,256, issued Oct. 26, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to work stations or work holders used in indexing semiconductor lead frames through an automatic wire bonder and, more specifically, to a precisor plate employed in such a work holder or work station adapted to be quickly readjustable to accommodate differently configured lead frames. 
     2. State of the Art 
     There are a limited number of companies in the semiconductor industry that manufacture automatic wire bonders used to form wire bonds on semiconductor devices. A leading manufacturer of such wire bonding machines is Kulicke and Soffa Industries, Inc. of Willow Grove, Pa., manufacturer of a work holder designated as model number 2810. These automatic wire bonders typically employ dedicated work holders which are adapted to receive a specifically configured lead frame. The work holders are coupled to or connected to at least three mechanical drives, one for advancing the lead frame, one for positioning the lead frame on the bonding station and one or more drives for clamping the lead frame in the station. Heretofore, changing the work holder required skilled technicians to disassemble and reassemble or partially rebuild the work holder to accommodate each differently configured lead frame and ensure that the new, differently configured lead frames will be precisely aligned with respect to the wire bonder. Such a reconfiguration process typically takes an hour or more with current work holders. 
     A typical work holder for a Kulicke and Soffa wire bonding machine includes a box-like frame attached to the machine through which lead frames are indexed, the box-like frame forming part of the work station at which lead frames are wire bonded. In addition, a precisor plate is attached to a vertical slider, the slider vertically moving the precisor plate beneath the lead frame. The precisor plate, in conjunction with a heat block, typically rectangular in shape, supports the lead frame in the work holder and keeps the lead frame from moving during the wire bonding operation. 
     An attempt to provide a modular work station or work holder which is easily removed from and replaced in an automatic wire bonder is disclosed in U.S. Pat. No. 4,765,531 (&#39;531 patent) assigned to Kulicke and Soffa Industries, Inc., the manufacturer of the aforementioned model 2810 wire bonder. While the &#39;531 patent is generally described with respect to Kulicke and Soffa automatic wire bonder models 1481 and 1482, many of the components and their configurations are similar to the components of the model 2810 wire bonder. In the &#39;531 patent, the problem of precisely aligning the precisor plate and heat block relative to the rest of the work holder is addressed by immobilizing the entire work station while various pins and bolts align and secure these components together and to the wire bonding machine. While the quick change work station described in the &#39;531 patent may have been an improvement over the work holder stations of its time, the &#39;531 patent work holder requires separate alignment of both the precisor plate and the heat block. 
     The work holder that is provided by Kulicke and Soffa with the 2810 model is as difficult, if not more, to align than the work holder described in the &#39;531 patent. In addition to not providing a work station that can be quickly realigned, the precisor plate provided with the 2810 model wire bonder has moveable pins that must themselves be properly aligned to precisely engage tooling holes in the lead frame. In addition, the heat block must be separately aligned, and the precisor plate itself requires planarity adjustment both right-to-left and front-to-back. 
     When a wire bonder as herein described is out of service, production on that machine is lost, resulting in a decrease in manufacturing efficiency. Accordingly, in order to avoid down time for realignment of the work holder, such automatic wire bonders are kept on line as long as possible, wire bonding similarly configured lead frames. In order to accommodate low numbers of production items, where only a few parts must be wire bonded, intermittent batch production, where the configuration of the wire bonder is frequently changed, or long production runs, where realignment of the precisor plate must be periodically performed, it would be desirable to provide a quick change precisor plate that can be simply and quickly adjusted. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, a precisor plate and method of aligning the precisor plate to an automated wire bonding work station is herein disclosed. A first embodiment of the precisor plate in accordance with the present invention comprises a plate having fixedly attached alignment pins positioned to engage tooling holes in a lead frame or lead frame strip, whether stamped, etched or otherwise manufactured, and a heat block attachment or mounting portion on the plate for attachment of a heat block thereto. The heat block is held in precise position relative to the precisor plate by a pin which extends from the heat block mounting portion into the heat block and an abutment surface adjacent the heat block mounting portion against which the heat block must abut in order to fit onto the pin. The heat block is secured to the heat block mounting portion by threaded screws which fit through holes provided in the heat block mounting portion and into matingly threaded holes in the heat block. 
     In another preferred embodiment, at least two pins are provided on the heat block mounting portion and engage an equal number of pin holes in the heat block. The pin holes, which have an internal configuration to substantially match the outer configuration of the pins, must engage with the pins and thus properly align the heat block to the precisor plate. 
     In yet another preferred embodiment of a precisor plate in accordance with the present invention, the precisor plate and heat block comprise a one-piece component such that the heat block is either permanently attached to the precisor plate or integrally formed as part of the precisor plate. Such a device removes entirely the need to align the heat block relative to the precisor plate, or to separately align the heat block to the work station. 
     The heat block according to the present invention may be provided with various internal passages, in addition to those used for securing purposes. Such internal passages may include bores sized to receive heating elements and temperature measuring devices, such as thermocouples, and passages to provide a vacuum at the wire bonding surface of the heat block to help secure the lead frame relative to the heat block during the wire bonding process. 
     In order to align and attach a precisor plate of the present invention to a work station, the heat block is attached to the precisor plate (if separate components) and the precisor plate is attached to a vertical slider of the wire bonding machine by inserting screws through holes provided in the precisor plate and into threaded holes provided in the slider. A lead frame is then indexed into the work station. Before tightening these screws into the threaded holes in the slider, the precisor plate is moved until, upon actuation of the slider, the alignment pins of the precisor plate engage the tooling holes of the lead frame without disturbing the position of the lead frame. When the alignment pins can be engaged into the tooling holes without lifting (i.e., disturbing) the lead frame, the precisor plate is in the correct position and can be securely attached to the slider by tightening the screws into the slider. 
     To facilitate movement of the precisor plate relative to the work station, one or more holes may be formed in the precisor plate into which grasping members, such as pins or threaded screws, may be inserted, These grasping members may be grasped by a user to move the precisor plate relative to the work station. After proper alignment and upon securement of the precisor plate relative to the work station, the grasping members may be removed. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1A is an isometric exploded view of a first embodiment of a precisor plate in accordance with the present invention positioned within a work station of a wire bonding machine; 
     FIG. 1B is an isometric top view of the precisor plate of FIG. 1A; 
     FIG. 1C is an isometric bottom view of the precisor plate of FIG. 1A with an associated heat block; 
     FIG. 2 is an isometric top view of a second embodiment of a precisor plate in accordance with the present invention; 
     FIG. 3A is an isometric top view of a third embodiment of a precisor plate in accordance with the present invention; 
     FIG. 3B is an isometric bottom view of the precisor plate of FIG. 3A; and 
     FIG. 4 is a side cut-away view of the precisor plate of FIG. 1A positioned within the work holder and attached to a wire bonding machine. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A illustrates a precisor plate  10  in accordance with the present invention positioned within a modular, box-shaped work station  12  known in the art. The work station  12  is comprised of a bottom plate  14  having a pair of rail guides  16  and  18  consisting of longitudinally extending steps providing abutting edges  20  and  22  against which a pair of side rails  24  and  26  can be positioned, respectively. The tops  28  and  30  of the side rails  24  and  26  are also provided with longitudinally extending abutment surfaces  32  and  34  for holding a top plate  36  along similarly configured longitudinally extending abutment surfaces  38  and  40  of the top plate  36 . The top plate  36  is provided with a first major opening  42  defined in the top surface  44  of the top plate  36 . The opening  42  provides access to the lead frame and die (not shown) that are to be wire bonded by a wire bonding head of a wire bonding machine to which the work station  12  is attached. Other apertures or openings  46 ,  47 ,  48 , and  49  defined in the top plate  36  may also be provided for positioning and/or access purposes. 
     The side rail  26  defines an opening  50  therein large enough to receive the precisor plate  10  and allow vertical movement of the precisor plate  10  within the opening  50 . When assembled, the work station  12 , in conjunction with the precisor plate  10 , helps to provide physical, immobile support for a lead frame during the wire bonding process to ensure that the lead frame does not move. In addition, movement of the precisor plate  10  relative to the work station  12  is provided to allow lead frames to be indexed into and out of the work station  12 . 
     As better illustrated in FIGS. 1B and 1C, the precisor plate  10  is a one-piece component comprised of a plate  60  having a heat block attachment portion or ledge  62  depending therefrom, the ledge  62  having a top surface  64  on a different horizontal plane than a top surface  66  of the plate  60 . A heat block abutment surface  68  is provided between and substantially perpendicular to the plate  60  and the ledge  62 . In order to attach a heat block  69  to the precisor plate  10  and properly position it relative to the precisor plate  10 , holes  70  and  72  are provided in the ledge  62  configured to match the position of threaded holes  74  and  76  in the heat block  69 . The holes  70  and  72  are spaced from the abutment surface  68  substantially the same distance as the holes  74  and  76  are spaced from the abutment side  82  of the heat block  69 . In addition, a pin  78  attached to the top surface  64  of the ledge  62  is positioned to engage pin hole  80  in the heat block  69  with the pin  78  being spaced from the abutment surface  68  substantially the same distance the pin hole  80  is from the abutment side  82 . Accordingly, the combination of abutting the abutment side  82  of heat block  69  against the abutment surface  68  and engaging the pin  78  into the pin hole  80  results in a properly positioned and aligned heat block  69  relative to the precisor plate  10 . Threaded screws (not shown), as known in the art, can then be inserted through holes  70  and  72  and engaged with threaded holes  74  and  76 , respectively, so that the heat block  69  is immobilized in proper alignment relative to the precisor plate  10 . 
     The ledge  62  also defines a semicircular opening  110  to allow access through the ledge  62  to the opening  112  of internal bore  114  in the heat block  69  by the wire bonding machine, as will be further described with reference to FIG.  4 . 
     The plate  60  of the precisor plate  10  is also provided with other holes, both threaded and unthreaded. For example, holes  84  and  86  are defined through the plate  60  to attach the precisor plate  10  to a wire bonding machine. In addition, holes  88  and  90  are positioned to substantially match the spacing of tooling holes provided in a lead frame strip as is known in the art. Alignment pins (see FIG. 4) can then be secured into the holes  88  and  90 . Other holes  92 ,  94 , and  96  are positioned about the top surface  66  of the plate  60  into which a grasping member such as a threaded fastener  98  can be attached if holes  92 ,  94 , and  96  are internally threaded, or a pin  100  sized to match the size of the holes  92 ,  94 , or  96  if holes  92 ,  94 , and  96  are not threaded. The threaded fasteners  98  and/or pins  100  can be grasped to move the precisor plate  10  to the work station  12  while aligning the precisor plate  10  to the work station  12 , as will be subsequently described. After proper alignment of the precisor plate  10 , the threaded fasteners  98  and/or pins  100  can be removed from the precisor plate  10  and are not otherwise necessary for the wire bonding process. 
     It is also contemplated, as illustrated in FIG. 2, that multiple pins  118  and  119  or other positioning members such as raised and/or recessed portions may be employed to align a heat block  113  to a precisor plate  115  of the present invention so that, in contrast to the configuration shown in FIG. 1B, the abutment surface  68  and ledge  62  are not required. Accordingly, such a precisor plate  115  could be formed from a plate  116  having a top surface  117  in a single plane with the heat block  113  attached directly to the top surface  117  of the plate  116 . Referring now to FIGS. 3A and 3B, another preferred embodiment of a precisor plate  120  according to the present invention is illustrated. Similar to the precisor plate  10  of FIGS. 1A,  1 B and  1 C, the precisor plate  120  is comprised of a single element  122 . The heat block  124 , however, is integral with the single element plate  122  and may be formed as a single piece with the precisor plate  120 . The heat block  124  may be provided with various passages such as an internal bore  126  extending therein and sized to receive a heating element (not shown) in a male/female relationship. A hole  128  may also be provided proximate the opening  130  of the internal bore  126  to which the heating element (not shown) may be secured. As shown in FIG. 3B, the heat block  124  may also include another bore  132  therein into which a thermocouple (not shown) can be inserted to monitor the temperature of the heat block  124 . Referring again to FIG. 3A, the top surface  134  of the heat block  124  may also have a pair of vacuum outlet ports  136  and  138  through which a vacuum may be drawn to assist in immobilizing a lead frame until the heat block  124  abuts the top plate  36  (see FIG.  4 ). Referring again to FIG. 3B, in the bottom surface  140  of the heat block  124 , a vacuum inlet port  142  in communication with vacuum outlet ports  136  and  138  can be attached to a vacuum source of a wire bonding machine. 
     FIG. 4 illustrates the precisor plate  10  attached to a wire bonding machine  150  known in the art. A lead frame  152  is in a bonding position within the work station  12  with the heat block  69  holding the lead frame  152  against the bottom surface  154  of the top plate  36 . Alignment pins  153  engage the index or tooling holes  155  in the lead frame  152 . 
     After being clamped in position by the heat block  69 , the lead frame  152  is further supported by an air cylinder  157  which is located inside a center frame  166  of the wire bonding machine  150  and which engages the heat block  69 . The additional force provided by air cylinder  157  further stabilizes the heat block  69  so that the lead frame  152  is more solidly held in the wire bonding machine  150 . In addition, the air cylinder  157  may provide a vacuum source  159  to the heat block  69  through internal passages (not shown) in the air cylinder  157  connected to a vacuum outlet (not shown) in the wire bonding machine  150  so that, as previously described, the lead frame  152  is held in position relative to the heat block  69  until the heat block  69  abuts the top plate  36 . After the bonding operation, drive rollers (not shown), as are known on such wire bonding machines, actuated by the wire bonding machine  150  move the lead frame  152  to the next index position once the alignment pins  153  are lowered out of the index holes  155  of the lead frame  152 . 
     The precisor plate  10  is attached by screw  156  and lock washer  158  to vertical slider  160  that vertically moves the precisor plate  10  up and down relative to the work station  12 . The slider  160  is mounted for movement on a movable front frame member  168  which is locked in place by a split clamp  170  and screw  172 . The front frame member  168  is movable on a pair of shaft members  174  (only one of which is shown) and, when properly indexed by a set screw  176  engaging a side edge of the bottom plate  14 , positions both the front frame member  168  and the slider  160  in the proper operating position. Similarly, the rear frame member  178  is movable on the same shaft members  174 , which is a part of the base of the automatic wire bonder, and is set in place by set screws  180  engaging the bottom plate  14 . There is also provided a split clamp  182  and screw  184  to hold the rear frame member  178  in place. The heat block  69  is attached to the ledge  62  of the precisor plate  10  by screw  162  and lock washer  164  and further held in position by pin  78 . 
     In practice, the work station  12  is first mounted on the center frame  166  and the precisor plate is attached to the slider  160 . The screw  156  is tightened enough to hold the precisor plate to the slider  160  but is loose enough to allow movement of the precisor plate  10  relative to the slider  160 . In a lowered position and with the top plate  36  removed, the precisor plate  10  is moved relative to the slider  160  until alignment pins are positioned proximately below the tooling or index holes  155  of the lead frame  152 . Grasping members such as threaded fasteners  98  and/or pins  100  as illustrated in FIG. 1B may be employed to assist movement of the precisor plate  10 . The precisor plate  10  is then raised by the slider  160  to ensure that the alignment pins  153  do not substantially disturb the position of the lead frame  152 . Typically, such disturbance can be detected by observing vertical movement of the lead frame  152  by the alignment pins  153 . If the alignment pins  153  do not precisely engage the index holes  155  of the lead frame  152 , the precisor plate  10  can be adjusted until the alignment pins  153  fit (i.e., ±1 mil between the centers of alignment pins  153  and index holes  155 ) within the index holes  155  without lifting the lead frame  152 . Once the precisor plate  10  is in position, the screws  156  can be tightened to secure the precisor plate  10  relative to the rest of the work station  12 . 
     It is noted that other configurations of modular work stations may also be employed in accordance with the precisor plate  10  of the present invention. Thus, while a modular work station as herein described has been shown with respect to a Kulicke and Soffa Industries, Inc. wire bonding machine, it is contemplated that a precisor plate in accordance with the present invention may be adapted to and/or utilized with other wire bonding machines and work stations known in the art. Those skilled in the art will also appreciate that various combinations and modifications of the preferred embodiments may be made without departing from the spirit of this invention and the accompanying claims.