Abstract:
A bondhead is provided that comprises a bondhead body for holding a bonding tool, such as an ultrasonic transducer, and a bondhead actuator coupled to the bondhead body for moving the bonding tool with respect to a bonding surface. A wire clamping device, which may comprise a wire clamp holder and a wire clamp, is movable relative to the bondhead body for feeding a bonding wire to the bonding tool. A wire clamping device actuator is operative to move the wire clamping device relative to the bondhead body for improved control of the feeding of bonding wire to the bonding tool.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a wire bonding apparatus for making electrical wire connections on semiconductor devices, and in particular to a bondhead of such a wire bonding apparatus.  
       BACKGROUND AND PRIOR ART  
       [0002]     In a wire bonding process, electrically conductive wires are bonded between electrical bonding pads found on semiconductor devices, such as between a semiconductor die and a substrate onto which the die is attached. The substrate is usually a semiconductor leadframe. The electrical connection could also be made between bonding pads found on separate semiconductor dice. The bond is formed by a bonding tool which may be in the form of a capillary attached to an ultrasonic transducer for generating ultrasonic energy to the capillary tip.  
         [0003]     In modern day wire bonders for making so-called “ball-bonds”, a bondhead which carries the bonding tool is designed to execute a rocking motion about a suitably located pivot. For ultrasonic bonding, the bonding tool is an ultrasonic transducer mounted onto the bondhead, the ultrasonic transducer comprising a piezoelectric driver stack coupled to a horn, and a capillary at an end of the horn. Bonding wire, which is typically made of gold, aluminum or copper, is fed from a spool of bonding wire through a hole in the capillary to the tip of the capillary. Bonding is done by welding the wire at the tip of the capillary to the bonding pad through the application of ultrasonic energy to the capillary tip.  
         [0004]     It is common to utilize a wire clamp to control feeding of bonding wire to the capillary tip. For example, the clamp may be closed to hold onto and fix a length of wire relative to the capillary, or opened to allow wire to slide through the capillary. The wire clamp is also closed to hold the wire in position during the making of wire bonds on the bonding pads. The clamp is further commonly used to facilitate looping of a length of bonding wire between electrical bonding points on the die and/or substrate, and/or to pull and break wires from bonds after the bonds have been made. The wire needs to be held firmly, fed to the bonding site and stripped off at appropriate junctures in the process. Over the years, the operational speed of wire bonding machines has increased considerably, with the result that the wire clamp and bondhead need to be actuated at high speeds while exerting controlled force on the wire being clamped without damaging the wire.  
         [0005]      FIG. 1  is an example of a prior art bondhead  100 . The bondhead  100  generally comprises a bondhead body  102 , a wire clamp  104  fixed to the body  102  and a transducer  106  mounted to the bondhead  100 . The transducer  106  has a capillary  108  attached to one end of the transducer, and is generally movable in tandem with the movement of the bondhead  100 .  
         [0006]     Bonding wire  110  is fed from a spool of wire (not shown), and is relayed past the jaws of the wire clamp  104  and threaded through a hole in the capillary  108 . The wire clamp  104  is arranged along the path of the bonding wire so as to control feeding of the wire to the capillary  108 , in particular, to the capillary tip.  
         [0007]     The bondhead body  102  is pivoted at a pivot point  112  for turning motion, and turning movement of the bondhead body  102  about the pivot point  112  is actuated by a bondhead actuator  114 . The bondhead actuator  114  may comprise a voice coil motor including a coil that is movable relative to a magnet by way of electromagnetic interaction when current flows through the coil. When actuated by the bondhead actuator  114 , the body  102  and wire clamp  104  are driven to turn along a turning arc  116 . Bonding wire  110  is drawn from the spool of wire towards a bonding location when the wire clamp  104  is closed, and the bondhead  100  is turned away from the spool of bonding wire. The wire clamp  104  may further be opened and the bondhead  100  turned towards the spool in order to position the wire clamp  104  to clamp and draw more bonding wire  110 .  
         [0008]     During a bonding cycle and before starting to weld the first bond, a molten ball has to be formed at a tail end of the bonding wire  110  protruding out of the capillary tip. The molten ball is later lowered onto a bonding pad to form a first ball bond. The molten ball is formed at the end of this protruding bonding wire  110  by melting the wire through electro-sparking, so a sufficient length of wire must be available at the tail end of the bonding wire  110  to do so. An electronic flame-off (“EFO”) device creates an electrical spark and melts the wire to form the molten ball.  
         [0009]     To leave a tail of bonding wire  110  protruding from the capillary tip after completion of a bond, the bondhead  100  has to follow a variety of programmed motions. More specifically, during ball-bonding processes, the bondhead  100  needs to move up a short distance with the wire clamp  104  open after the bonding wire  110  has been welded at a second bond location to complete a wire connection. Then, the bondhead  100  stops and the wire clamp  104  is closed to clamp the bonding wire  110 . After that, the bondhead  100  moves up further to a higher position. During this further upward motion, the bonding wire  110  is pulled up and broken at the second bond location, and gets ready for the start of the first bond of the next wire connection. This is called tail formation, to ensure that a predetermined length of bonding wire protrudes from the capillary tip after each wire connection is established. The consistency of the length and linearity of the protruding wire determines the repeatability of ball formation and the ball size formed.  
         [0010]     Another feature of the prior art bondhead  100  is that it uses an air tensioner to ensure the centering of the bonding wire  110  and the molten ball with respect to the capillary tip. This is to ensure accuracy of placement of the bonded ball at the first bond. After EFO sparking, the formed ball is pulled up by the air tensioner to sit in a central position under the capillary  108 . The consistency of ball centering relies on the stability of the pulling force exerted by the air tensioner. Therefore, periodic checking and cleaning of the air tensioner is required to ensure consistency of ball centering.  
         [0011]     The existing tail-formation process has a number of drawbacks. One drawback is that it requires precise synchronization between operation of the wire clamp  104  and motion of the bondhead  100 . This becomes much more difficult when the bondhead  100  moves at very high speeds and acceleration. The process is also very demanding on the stability of the bondhead structure and motion. It is difficult to produce consistently straight tails with uniform lengths when bonding wires of smaller and smaller diameters are used. Any variation in the process causes corresponding variation of the wire shape of the next bonded wire, resulting in inconsistency. Furthermore, operational stoppages can result when the bonding wire  110 , especially thin bonding wire, is broken prematurely at the second bond location when the bondhead  100  moves up while the wire clamp  104  is still open. Additionally, more process time is required to form the protruding bonding wire  110  by manipulating the bondhead  100 , so that bond cycle time is increased for each bonded connection.  
       SUMMARY OF THE INVENTION  
       [0012]     It is an object of the invention to seek to provide an improved bondhead for a wire bonding apparatus that helps to reduce cycle time for making each bonded connection and also avoids some of the disadvantages associated with prior art bondheads.  
         [0013]     According to a first aspect of the invention, there is provided a bondhead comprising: a bondhead body for holding a bonding tool; a bondhead actuator coupled to the bondhead body for moving the bonding tool with respect to a bonding surface; a wire clamping device which is movable relative to the bondhead body for feeding a bonding wire to the bonding tool; and a wire clamping device actuator for moving the wire clamping device relative to the bondhead body.  
         [0014]     According to a second aspect of the invention, there is provided a method of bonding a wire comprising the steps of: forming a wire bond with a bonding tool; moving the bonding tool away from the wire bond while clamping the wire with a wire clamping device and releasing a length of wire between the wire bond and the bonding tool; then moving the clamping device and the bonding tool away from the wire bond while clamping the wire to separate the bonding wire from the wire bond.  
         [0015]     It would be convenient hereinafter to describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Examples of various embodiments of bondheads in accordance with the invention will now be described with reference to the accompanying drawings, in which:  
         [0017]      FIG. 1  is a side view of a prior art bondhead;  
         [0018]      FIG. 2  is a side view of a bondhead according to a first preferred embodiment of the invention;  
         [0019]      FIG. 3  is a side view of a bondhead according to a second preferred embodiment of the invention;  
         [0020]      FIG. 4  is a side view of a bondhead according to a third preferred embodiment of the invention;  
         [0021]      FIG. 5  is a side view of a bondhead according to a fourth preferred embodiment of the invention;  
         [0022]      FIG. 6  is a side view of a typical wire loop formed when electrically connecting two bond pads at different bonding locations;  
         [0023]     FIGS.  7 ( a ) and  7 ( b ) are cross-sectional side view representations of the pulling and centering of a molten ball after EFO using an air tensioner, and performing the same operation using a bondhead according to a preferred embodiment of the invention respectively;  
         [0024]      FIG. 8  shows a bonding sequence using the prior art bondhead; and  
         [0025]      FIG. 9  shows a bonding sequence using a bondhead according to a preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]      FIG. 2  is a side view of a bondhead  10  according to a first preferred embodiment of the invention. The bondhead  10  comprises a bondhead body  11 , to which is mounted a bonding tool, preferably an ultrasonic transducer  13 , with a capillary  15  attached to one end of the transducer  13 . The bondhead body  11  is mounted on a bondhead pivot  16  approximately near a center of gravity of the bondhead assembly. This allows the capillary  15  to be movable towards and away from a bonding position in a generally up and down motion about the bondhead pivot  16  in a pivotal motion. The bondhead body  11  is coupled to a bondhead actuator, such as a voice coil motor comprising a coil bracket  18  disposed between and movable relative to permanent magnets  19  by way of electromagnetic interaction, for controlling movement of the transducer  13  and capillary  15 . The movement of the transducer  13  and capillary  15  is along a first turning arc  22 .  
         [0027]     Also mounted on the bondhead pivot  16  is a wire clamping device, such as a wire clamp holder  14  comprising a wire clamp  12  at one end located along a path of a length of bonding wire  26  to clamp it. The wire clamp holder  14  is mounted such that its mounting axis passes through the bondhead body  11  and is movable about the bondhead pivot  16  relative to the bondhead body  11 . In this embodiment, the mounting axis thus corresponds to a pivoting axis at the bondhead pivot  16 . Its pivotal movement is controlled by a wire clamping device actuator, which may also be in the form of a voice coil motor comprising a coil bracket  20  disposed between permanent magnets  21  and movable with respect thereto. It should be appreciated that other forms of actuators, including without limitation other types of linear motors, piezoelectric motors or pneumatic motors, are applicable for implementing a bondhead according to the invention. Moreover, other driving mechanism such as DC servomotors and lead screw mechanisms may be used.  
         [0028]     The movement of the wire clamp  12  is along a second turning arc  24 . Although the wire clamp holder  14  and the bondhead body  11  share the same turning axis about the bondhead pivot  16 , they are preferably mounted on a pivot axis of the pivot  16  at different points so that their motions are decoupled.  
         [0029]     As the bondhead body  11  and wire clamp holder  14  are driven by separate actuators, they are capable of independent movement and are driven independently. Thus, the position of the wire clamp  12  is not constrained by the position of the bondhead  10  as in the prior art. Vertical motion of the wire clamp  12  with respect to the transducer  13  can feed bonding wire  26  upwards or downwards through the capillary  15  on the transducer  13  independently of the bondhead body  11 . An advantage of this first preferred embodiment of the invention is that the bondhead body  11  and wire clamp holder  14  pivot about a common axis, namely the bondhead pivot  16  or pivot axis to which both said components are mounted. This allows easier coordination of the relative movements of the bondhead body  11  including the transducer  13  and capillary  15  on the one hand, and the wire clamping device including the wire clamp holder  14  and the wire clamp  12  on the other.  
         [0030]     The motion control of the wire clamp  12  can be either designed on an open loop system without feedback sensing, or on a closed-loop feedback system. Although the bondhead bodies and wire clamping devices are configured to move in pivotal motions in the described embodiment, it should be appreciated that the bondhead body and wire clamping devices respectively may be configured and actuated to move in linear motions instead.  
         [0031]      FIG. 3  is a side view of a bondhead  10  according to a second preferred embodiment of the invention. In this second embodiment, the wire clamp body  14  has a different shape as compared to the first embodiment. Essentially, the wire clamping device actuator is arranged so that the wire clamping device actuator, comprising the coil bracket  20  and permanent magnets  21 , is positioned higher than the bondhead body actuator, whereas the bondhead body actuator is positioned higher than the wire clamping device actuator in the first embodiment. As the second embodiment functions in a substantially similar manner to the first embodiment, it will not be further elaborated upon. The choice of embodiment to adopt would depend on the configuration of the wire bonder itself, and the manner in which it would be more convenient to locate the wire clamping device actuator relative to the bondhead body actuator.  
         [0032]      FIG. 4  is a side view of a bondhead  10  according to a third preferred embodiment of the invention. In this embodiment, the bondhead body  11  is mounted on a bondhead pivot  16 , but the wire clamp holder  14  is mounted on a separate wire clamp pivot  17  located elsewhere on the bondhead body  11 . Again, the bondhead body  11  is driven by a bondhead actuator comprising a coil bracket  18  disposed between permanent magnets  19  and movable with respect thereto by way of electromagnetic interaction. The wire clamp holder  14  is driven by a wire clamping device actuator comprising a coil bracket  20  disposed between permanent magnets  21  and movable with respect thereto. Both actuators are capable of independent movement. However, the turning axes of the bondhead body  11  and wire clamp holder  14  are different because they are fixed at separate locations on the bondhead body  11 . The operational parameters should be modified accordingly to take into account the modified turning arcs  22 ,  24 .  
         [0033]      FIG. 5  is a side view of a bondhead  10  according to a fourth preferred embodiment of the invention. In this embodiment, the wire clamp holder  14  is mounted such that its mounting axis (which is the same as its pivoting axis) does not pass through the bondhead body  11 . The mounting axis passes through a wire clamp pivot  17  at a location that comprises a pivoting structure that is separate from the bondhead body  11 . The wire clamp holder  14  is configured for pivotal movement relative to the bondhead body  11 . The turning arcs  22 ,  24  are again different from the other embodiments. In other respects, the design and working principles are similar to the previous embodiment and will not be further elaborated upon.  
         [0034]      FIG. 6  is a side view of a typical wire loop  27  formed when electrically connecting a first bond pad  31  to a second bond pad  33  at different bonding locations. A first bond is made at the first bond pad  31  using a ball bond  30 . The capillary  15  is then moved to the second bond pad  33 , at which a wedge bond  32  is formed to complete the electrical connection between the first bond pad  31  and the second bond pad  33 . A variety of programmed motions are used by the capillary  15  when moving from the first bond pad  31  to the second bond pad  33  to shape the wire loop  27 . A ball bond would next be formed by the capillary  15  at another bond pad (not shown) at the next bonding location.  
         [0035]     FIGS.  7 ( a ) and  7 ( b ) are cross-sectional side view representations of the pulling and centering of a molten ball after EFO using an air tensioner, and performing the same operation using a bondhead according to a preferred embodiment of the invention respectively. Referring first to  FIG. 7 ( a ), which is a prior art design, an EFO device  36  produces an electrical spark to form a molten ball  38  at the tail end of a length of bonding wire  110  controlled by a wire clamp  104  and threaded through a transducer  106  and capillary  108 . After the molten ball  38  is formed, the molten ball  38  has to be located at the mouth of the capillary  108 , and this can be done by using air tensioning to pull the wire up. The wire clamp  104  is open during said air tensioning.  
         [0036]     Referring now to  FIG. 7 ( b ), the EFO device  36  produces an electrical spark to form a molten ball  38  at the tail end of a length of bonding wire  26 . The bonding wire  26  is controlled by a wire clamp  12  and threaded through a transducer  13  and capillary  15 . To position the molten ball  38  at the mouth of the capillary  15 , the wire clamp  12  is closed and the bonding wire  26  is pulled up by the wire clamp  12 . Pulling the bonding wire  26  using the wire clamp  12  is more precise than using an air tensioner, and avoids causing damage to the bonding wire  26  or molten ball  38 .  
         [0037]      FIG. 8  shows a bonding sequence using the prior art bondhead  100 . After formation of the molten ball  38 , the bonding wire  110  is pulled upwards by air tensioning so that the molten ball  38  is positioned at the mouth of the capillary  108 , as described with respect to  FIG. 7 ( a ). The molten ball  38  is lowered onto a first bond pad to form a ball bond  30 . The capillary  108  is then moved to a second bond pad to form a wedge bond  32 . Thereafter, the capillary  108  is lifted by a certain distance with the wire clamp open in order to leave a tail end of the bonding wire  110  hanging from the mouth of the capillary  108 , then the wire clamp  104  closes and the bonding wire  110  is pulled to break it from the wedge bond  32 . The lifting of the transducer  106  and capillary  108  is done by lifting the whole bondhead  100 . Tail formation of the bonding wire  110  is necessary for the EFO to form a molten ball  38  to create a ball bond at the next bonding location. The additional step of raising the capillary  108  with the wire clamp  12  open for tail formation results in increased cycle time.  
         [0038]      FIG. 9  shows a bonding sequence using a bondhead  10  according to a preferred embodiment of the invention. After formation of the molten ball  38 , the bonding wire  26  is pulled upwards by the wire clamp  12  in a direction away from the transducer  13  so that the molten ball  38  is positioned at the mouth of the capillary  15 , as described with respect to  FIG. 7 ( b ). The molten ball  38  is lowered onto a first bond pad to form a ball bond  30 . The capillary  15  is then moved to a second bond pad to form a wedge bond  32 . Thereafter, the transducer  13  and capillary  15  are lifted by the bondhead body  11  away from the wedge bond  32  while the wire clamp  12  is closed and clamping the bonding wire  26 , thereby releasing a length of bonding wire  26  between the wedge bond  32  and the capillary  15  to form the tail wire. At the same time, the closed wire clamp  12  preferably moves down in the direction of the transducer  13  whereby to increase the length of bonding wire  26  released between the wedge bond  32  and the capillary  15  that is hanging out at the mouth of the capillary  15 . The total traveling distance of the wire clamp  12  and capillary  15  can be made equal to the tail length desired. Thereafter, the wire clamp  12  and transducer  13  move away from the wedge bond  32  together while clamping the bonding wire so as to break and separate the bonding wire  26  from the wedge bond  32 . With the tail wire produced, the EFO device then produces an electrical spark to form another molten ball  38  on the bonding wire  26 , and the next ball bond  30  can be formed with the molten ball  38  as described above.  
         [0039]     Instead of having to raise the whole bondhead  10  with the wire clamp  12  open to create a tail of protruding wire, and then closing the wire clamp  12  to break the wire  26 , tail formation is performed with the wire clamp  12  closed. Tail wire formation is assisted by the wire clamp  12  pulling bonding wire  26  through the capillary  15 , so that the bondhead body  11  may travel for a shorter distance. As a result, a tail wire is formed without start and stop motions of the bondhead  10  and opening and closing of the wire clamp  12 , as in the prior art. Bonding cycle time can be reduced.  
         [0040]     It should be appreciated that the wire clamp  12  according to the preferred embodiments of the invention is movable relative to the transducer  13  mounted on the bondhead  10 , so that less bondhead manipulation is necessary to control a length of bonding wire  26  fed to the capillary tip. The bonding wire  26  is assisted directly by the wire clamp  12  to be fed through the capillary  15  for better control of the tail end of bonding wire  26  protruding from the capillary tip.  
         [0041]     Further, the preferred embodiments reduce vulnerability of the molten ball  38  to damage during ball centering caused by air tensioning forces. Centering between the bonding wire  26  and the molten ball  38  with respect to the capillary tip, and thus the position of the ball bond  30 , can be more accurate. Stoppages of the bonding system due to premature breaking of the bonding wire  26  or a missing tail wire induced by motion of the tail wire at high bonding speeds can also be reduced, so that bonding system stability is increased. Moreover, overall bonding cycle time is reduced for faster operation.  
         [0042]     The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.