Abstract:
Disclosed is a wedge bonder, comprising a wedge for bonding a wire to surfaces to form an electrical interconnection therebetween, a cleaning device for cleaning the wedge, and a positioning device to which the wedge is mounted. In particular, the positioning device is operative to move the wedge to the cleaning device for cleaning. A method of cleaning a wedge of a wedge bonder is also disclosed.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to a wedge bonder having a wedge for bonding a wire to surfaces to form an electrical interconnection therebetween. In particular, the wedge bonder further comprises a cleaning device for cleaning the wedge. The invention also relates to a method of cleaning a wedge of a wedge bonder. 
       BACKGROUND OF THE INVENTION  
       [0002]    A wedge bonder utilizes ultrasonic energy to bond a wire to a semiconductor die and to a substrate to which is mounted, so as to form an electrical interconnection between the semiconductor die and the substrate. Specifically, as the wedge presses the wire against a die pad of the semiconductor die or against the substrate, an ultrasonic transducer generates ultrasonic oscillations that are transmitted through a wedge of a bond tip of the wedge bonder and onward to the wire positioned beneath the wedge. 
         [0003]      FIG. 1  shows a bond tip  100  of a conventional wedge bonder, comprising: i) a wire guide  102  for guiding a wire  104  from a wire spool (not shown) through the bond tip  100 ; ii) a wedge  106  having a bonding portion  108  for pressing a portion of the wire  104  against a bonding pad of semiconductor die or against a substrate during bonding; and iii) a wire cutter  110  for cutting the wire  104  to detach the wire  104  from the wire spool after an electrical interconnection is formed between the semiconductor die and the substrate. 
         [0004]    Repeated bonding operations of the wedge bonder will cause residue to build up at the bonding portion  108  of the wedge  106 . For instance, Aluminum (Al) residue will build up at the bonding portion  108  of the wedge  106  if Al wire is used for wedge bonding. Hence, it will be necessary to clean the wedge  106 . Conventionally, wedge cleaning involves removing the wedge  106  from the bond tip  100 , and immersing the removed wedge  106  into a cleaning solution (e.g. NaOH solution) for about 30 minutes to remove the residue from the wedge  106 . The wedge  106  is then reinstalled onto the bond tip  100  after cleaning, before the wedge bonder resumes its bonding operations. 
         [0005]    However, reinstallation of the wedge  106  onto the bond tip  100  after cleaning requires set-up tasks such as recalibration of the ultrasonic transducer and fine component adjustment of the bond tip  100 . These tasks are not just time-consuming but also require skilled technicians to undertake. 
         [0006]    Thus, it is an object of the present invention to at least ameliorate the problems associated with cleaning the wedge of a wedge bonder. 
       SUMMARY OF THE INVENTION  
       [0007]    A first aspect of the invention is a wedge bonder comprising a wedge for bonding a wire to surfaces to form an electrical interconnection therebetween, a cleaning device operative to clean the wedge, and a positioning device to which the wedge is mounted. In particular, the positioning device is operative to move the wedge to the cleaning device for cleaning. 
         [0008]    Some preferred but optional features of the wedge bonder according to the first aspect of the invention are defined in the dependent claims. 
         [0009]    A second aspect of the invention is a method of cleaning a wedge of a wedge bonder, the wedge being for bonding a wire to surfaces to form an electrical interconnection therebetween, and the wedge bonder further comprising a cleaning device for cleaning the wedge. Specifically, the method comprises the steps of moving the wedge to the cleaning device, cleaning the wedge by the cleaning device. 
         [0010]    Some preferred but optional steps of the method according to the second aspect of the invention are also defined in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings of which: 
           [0012]      FIG. 1  shows a conventional bond tip of a wedge bonder; 
           [0013]      FIGS. 2   a - 2   c  show a wedge bonder of a preferred embodiment of the invention, comprising a bonding zone, an imaging zone, and a cleaning zone; 
           [0014]      FIG. 3  shows a configuration of the cleaning device of the wedge bonder of  FIG. 2 , comprising a rotatable cleaning wheel for cleaning the wedge; 
           [0015]      FIG. 4   a  and  FIG. 4   b  show different geometries of the rotatable cleaning wheel of  FIG. 3  for cleaning the wedge; 
           [0016]      FIG. 5  shows another configuration of the cleaning device of the wedge bonder of  FIG. 2 , comprising a plurality of chambers used for wedge cleaning; 
           [0017]      FIG. 6   a  and  FIG. 6   b  show the imaging device of the wedge bonder of  FIG. 2 , comprising a camera and a beam-splitter; 
           [0018]      FIGS. 7   a - 7   b  show respective side and bottom views of the wedge of the wedge bonder of  FIG. 2  as captured by the imaging device of  FIGS. 6   a - 6   b  before cleaning, while  FIGS. 7   c - 7   d  show respective side and bottom views of the wedge of the wedge bonder of  FIG. 2  as captured by the imaging device of  FIGS. 6   a - 6   b  after cleaning; 
           [0019]      FIGS. 8   a - 8   d  show the bond tip of the wedge bonder of  FIG. 2 , further comprising a wire-feeding mechanism for removing a wire from the bond tip before wedge cleaning and for introducing a new wire into the bond tip after wedge cleaning; and 
           [0020]      FIG. 9  is a flow chart showing a wedge-cleaning process performed by the wedge bonder of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]      FIG. 2   a - 2   c  are plan views of a wedge bonder  200 , showing a positioning device (illustrated by an XY-table  202 ) and a bond tip  204  mounted to the XY-table  202 . Similar to the bond tip shown in  FIG. 1 , the bond tip  204  may comprise: i) a wire guide for guiding a wire from a wire spool through the bond tip  204 ; ii) a wedge having a bonding portion for pressing a portion of the wire against a bonding pad of semiconductor die and a bonding pad of a substrate during bonding; and iii) a wire cutter for cutting the wire to detach the wire from the wire spool after an electrical interconnection is formed between the semiconductor die and the substrate. 
         [0022]    In addition, the wedge bonder  200  comprises: i) a bonding zone  206  for supporting a substrate (e.g. a lead frame) when performing bonding operations; ii) an imaging zone  208  for inspecting the wedge; and iii) a cleaning zone  210  for cleaning the wedge. 
         [0023]    Specifically, actuation of the XY-table  202  is controlled by a motion controller and a motor system comprising linear motors.  FIG. 2   a  illustrates the bond tip  204  positioned by the XY-table  202  relative to and above the bonding zone  206 . In particular, the XY-table  202  actuates the bond tip  204  to move from an original start position at the left of the bonding zone  206  rightward along an X-direction to bond a row of semiconductor dies mounted to the substrate. Thereafter, the substrate is then indexed upward along the Y-direction while the XY-table  202  actuates the bond tip  204  leftward along the X-direction to reposition the bond tip  204  back to its original start position before bonding another row of semiconductor dies mounted to the substrate.  FIG. 2   b  illustrates the bond tip  204  being actuated to move rightward along the X-direction by the XY-table  202  to a position relative to and above the imaging zone  208 , while  FIG. 2   c  illustrates the bond tip  204  being actuated to move downward along the Y-direction by the XY-table  202  relative to and above the cleaning zone  210 . 
         [0024]      FIG. 3  illustrates the bond tip  204  having the wire guide  302 , the wedge  304 , and the wire cutter  306 , together with a cleaning device  307  of the cleaning zone  210  of the wedge bonder  200  that cleans the wedge  304  using a dry approach. This cleaning device  307  of the cleaning zone  210  comprises: i) a motor  308 ; and ii) a rotary cleaning wheel (shown as a rotatable grinding wheel  310 ) coupled to the motor  308 . The motor  308  is controlled by a motion controller to drive the grinding wheel  310  at a controlled speed against a bonding portion  312  of the wedge  304 , to remove residue (e.g. Al residue) mechanically therefrom. 
         [0025]    Optionally, the geometry of the grinding wheel  310  may vary based on the corresponding geometry of the bonding portion  312  of the wedge  304 . 
         [0026]    For instance,  FIG. 4   a  illustrates a configuration of a wedge  400  having a flat-end bonding portion  402  for pushing a ribbon wire against a semiconductor die or a substrate during bonding. For such a configuration of the wedge  400 , the corresponding geometry of the grinding wheel  310  may comprise corresponding flat ends when viewed in a direction perpendicular to its rotational axis or to its major surface. On the other hand,  FIG. 4   b  illustrates a configuration of a wedge  404  having a bonding tip that includes a V-shaped groove  406  for at least partially housing and pushing a round wire against a semiconductor die or a substrate during bonding. In this embodiment of the wedge  404 , the corresponding geometry of the grinding wheel  310  may comprise tapered ends when viewed in a direction perpendicular to its rotational axis or to its major surface. 
         [0027]    Of course, it should be appreciated that other geometries of the grinding wheel  310  may also be possible to match corresponding geometries of the wedge  404  for the purpose of cleaning. For instance, the grinding wheel  310  may comprises trapezoidal ends when viewed in a direction perpendicular to its rotational axis or to its major surface for cleaning a corresponding U-shaped groove of a wedge-bonding tip. 
         [0028]    Besides the dry approach for cleaning the wedge  304  as described above, the wedge  304  may instead be cleaned at the cleaning zone  210  of the wedge bonder  200  using a wet approach. 
         [0029]      FIG. 5  illustrates a configuration  500  of the cleaning zone  210  based on the wet approach. Such a configuration  500  comprises three chambers  502 ,  504 ,  506 —i) a first chamber  502  for storing a cleaning solution  503  (e.g. NaOH solution) for removing residue (e.g. Al residue); ii) a second chamber  504  for storing a rinsing solution  505  (e.g. H 2 O); and iii) a third chamber  506  for housing a dryer  508  for drying. Specifically, the XY-table  202  of the wedge bonder  200  first positions the bond tip  204  relative to and above the first chamber  502  of the cleaning zone  210 , before immersing the bonding portion  312  of the wedge  304  into the cleaning solution  503  to remove the residuals from the wedge  304 . After the bonding portion  312  of the wedge  304  has been immersed in the cleaning solution  503  for a sufficient period of time, the XY-table  202  of the wedge bonder  200  withdraws the wedge  304  from the cleaning solution  503 , before immersing the same into the rinsing solution  505  to remove the cleaning solution  503  from the wedge  304  for another period of time as required. The XY-table  202  of the wedge bonder  200  then withdraws the wedge  304  from the rinsing solution  505 , before moving the wedge  304  into the third chamber  506  for drying the same. This completes the wedge-cleaning process based on the wet approach. 
         [0030]    Preferably, the first and second chambers  502 ,  504  are deep enough to store the respective cleaning and rinsing solutions  503 ,  505  to avoid spillage within the wedge bonder  200  during operation. 
         [0031]      FIG. 6   a  and  FIG. 6   b  illustrate the imaging zone  208  of the wedge bonder  200 , comprising an imaging device for capturing an image of the bonding portion  312  of the wedge  304 . Specifically, the imaging device comprises a beam-splitter  602  and a camera  600  positioned with respect to the beam-splitter  602  for capturing different images of the wedge  304 . When the wedge  304  is arranged perpendicularly to a line-of-view  604  of the camera  600  above the beam-splitter  602  as shown in  FIG. 6   a , the beam-splitter  602  allows the camera  600  to capture an image of a base of the wedge-bonding portion  312 . When the wedge  304  is aligned along the line-of-view  604  of the camera  600  such that the beam-splitter  602  is positioned between the camera  600  and the wedge-bonding portion  312  as shown in  FIG. 6   b , the beam-splitter  602  allows the camera  600  to capture an image of a side of the wedge-bonding portion  312 . 
         [0032]      FIG. 7   a  and  FIG. 7   b  illustrate sample images of a side image and a bottom image of the bonding portion  312  of the wedge  304  as captured by the imaging device before cleaning. It can be seen that the residue  701  (e.g. Al residue) accumulate at the inner walls of the wedge-bonding portion  312  after repeated bonding operations (e.g. 3000 bonding cycles).  FIG. 7   c  and  FIG. 7   d  illustrate sample images of a side image and a bottom image of the bonding portion  312  of the wedge  304  after cleaning. It can be seen that the residue (e.g. Al residue) that was previously accumulated at the inner walls of the wedge-bonding portion  312  have been removed. 
         [0033]      FIGS. 8   a - 8   d  show that the bond tip  204  of the wedge bonder  200  additionally comprises a wire-feeding mechanism  800  for removing a wire  802  from the bond tip  204  before the wedge-cleaning process begins, and for introducing a new wire  804  through the bond tip  204  after the wedge-cleaning process ends. The wire-feeding mechanism  800  may include a set of friction wheels or wire clamps for the wire-removal and wire-rethreading processes. 
         [0034]    Specifically,  FIGS. 8   a  and  8   b  show a kinked portion  801  of the wire  802  being pushed away from the wedge-bonding portion  312  by the wire-feeding mechanism  800 , in order to dislodge the wire  802  from a groove of the wedge-bonding portion  312 . Thereafter, the wire-feeding mechanism  800  retracts the wire  802  from the bond tip  204  to remove the wire  802  therefrom. 
         [0035]      FIGS. 8   c  and  8   d  show the new wire  804  being re-threaded through the bond tip  204  until the wire  804  feeds out from the wire guide  302 . Thereafter, the bond tip  204  moves downwards and backwards (as shown by arrow  806 ) during a dummy-bonding process, so that the wedge  304  pushes the new wire  804  against a dummy bond-off plate  802 . Following the dummy-bonding process, the new wire  804  is then firmly captured by the wedge-bonding portion  312  and thus, the wedge bonder  200  is able to continue its bonding operations. 
         [0036]      FIG. 9  is a flow chart  900  showing a wedge-cleaning process on the wedge bonder  200 . 
         [0037]    The flow chart  900  first begins with the step  902  of wedge bonding followed by the step  904  of incrementing a bond counter by one. Iterations of the step  902  of wedge bonding and the step  904  of incrementing the bond counter continue until the bond counter registers a count number (e.g. 3000) to trigger the cleaning of the wedge  304 . The wedge bonder  200  then switches to a “pre-cleaning” mode. 
         [0038]    During this pre-cleaning mode, the wedge bonder  200  performs the step  906  of removing the wire from the bond tip  204 . The wire-feeding mechanism  800  of the bond tip  204  actuates to remove the wire from the bond tip  204 . In particular, the kinked portion of the wire is pushed away from the wedge-bonding portion  312  of the wedge  304 , in order to dislodge the wire  802  from the groove of the wedge-bonding portion  312 . The wire-feeding mechanism  800  then retracts the wire from the bond tip  204  to remove the wire therefrom. 
         [0039]    After the step  906  of removing the wire from the bond tip  204 , the wedge bonder  200  then performs the step  908  of inspecting the wedge-bonding portion  312  during which the wedge  304  is moved to the imaging zone  208 . Thereafter, the wedge bonder  200  performs the step  910  of cleaning the wedge-bonding portion  312  in which the wedge  304  is moved to the cleaning zone  210 . Subsequently, the wedge bonder  200  switches into a “post-cleaning” mode. 
         [0040]    During this post-cleaning mode, the wedge bonder  200  again performs the step  912  of inspecting the wedge-bonding portion  312  in which the wedge is moved to the imaging zone  208 . Thereafter, the wedge bonder  200  performs the step  914  of re-threading a new wire through the bond tip  204 , before the wedge bonder  200  resumes its bonding operations. 
         [0041]    This completes an iteration of the wedge-cleaning process on the wedge bonder  200 . Such an iteration of the wedge-cleaning process may be performed either automatically or manually. Indeed, any or all of the steps of the wedge-cleaning process may be performed either automatically or manually. 
         [0042]    Optionally, the imaging device may provide feedback for the wedge-cleaning process. For instance, if no residue is detected from the sample images of the wedge-bonding portion  312  as captured by the imaging device during the pre-cleaning mode, the wedge bonder may bypass the wedge-cleaning process to resume its bonding operations. Likewise, if some residue is detected from the sample images of the wedge-bonding portion  312  as captured by the imaging device after the wedge-cleaning process, the wedge bonder may operate to reposition the wedge  304  relative to the cleaning zone  210  for another round of cleaning. If the sample images of the wedge-bonding portion  312  otherwise show that the wedge  304  has been sufficiently cleaned, the wedge bonder  200  then performs the step  914  of wire re-threading before resuming its bonding operations. 
         [0043]    Since there is no need to dismantle the wedge  304  from the bond tip  204  for cleaning and to reinstall the dismantled wedge  304  back to the bond tip  204  after cleaning, the wedge-cleaning process advantageously reduces maintenance time and reliance on skilled technicians for set-up tasks such as recalibration of the ultrasonic transducer and fine component adjustment of the bond tip  204 . Furthermore, by automating the wedge-cleaning process, a reasonable wedge-cleaning quality could be expected without human invention. 
         [0044]    It should be appreciated that other embodiments of the present invention can be envisaged without departing from the scope of the invention. For instance, although it has been described that the imaging and cleaning zones  208 ,  210  comprises the imaging and cleaning devices respectively, it can be envisaged that these devices do not have to be permanently positioned at their respective imaging and cleaning zones  208 ,  210 . Instead, each of these devices could be moved by an actuator into its respective zone  208 ,  210  to perform the necessary operations. Furthermore, the imaging zone  208  and the corresponding imaging device are optional and may thus be omitted.