Abstract:
A capillary holder for mounting a capillary onto a horn is provided that comprises a mounting hole formed in the horn that has a first width along a first axis that is smaller than a width of the capillary and a second width along a second axis perpendicular to the first axis that is larger than the width of the capillary. In particular, the mounting hole is configured such that application of a flexion force to reduce the second width simultaneously expands the first width so as to fit the capillary when the first width is larger than the width of the capillary, and removal of said flexion force contracts the first width whereby to grip the capillary using an elastic force of the horn.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to an arrangement for mounting a capillary to a body of a bonding tool, such as a horn of an ultrasonic transducer for the purpose of performing wire bonding.  
       BACKGROUND AND PRIOR ART  
       [0002]     Wire bonding is a common procedure used in semiconductor packaging whereby electrical wire connections are made between electrical contact pads of different electronic components, or of different contacts pads on an electronic component. For example, wire connections are commonly made between contact pads of an integrated circuit (“IC”) chip and a leadframe carrier on which the IC chip is attached before the IC chip is molded during assembly.  
         [0003]     A capillary is often used to apply bonding energy to weld bonding wire, typically made from gold, aluminum or copper, to the contact pads. The capillary is also used to feed wire to the contact pad and break the wire from the bond on the contact pad once a wire connection is made. Bonding energy, such as ultrasonic force, power and/or thermal energy is applied at a tip of the capillary. Therefore, it is very important for the capillary to be fastened securely so as to ensure that the wire bonds are formed properly and consistently according to set parameters.  
         [0004]     Conventionally, a horn of an ultrasonic transducer is formed with a slit at its tip. The capillary is mounted in a recess in the slit. Threaded screw holes are made in the slit to receive a screw, which is screwed into the holes and tightened in order to close the slit and provided a clamping force to grip the capillary securely. However, it has been found that there are many disadvantages associated with the use of a screw to secure the capillary. One disadvantage is that since wire bonding is performed repeatedly at high frequency under stressful conditions, over time, the screw threads on the screw and screw holes become worn and loosen the screw. The problem is that wire bonding requirements are so precise that any loosening of the capillary mount may adversely affect the performance of the wire bonding tool. Another undesirable effect is the possible bending vibration caused by the screw.  
         [0005]     One method of avoiding the aforesaid disadvantage is to utilize the body of the horn itself to grip the capillary without reliance on a foreign object to secure the capillary. This approach is disclosed in U.S. Pat. No. 6,422,448 entitled “Ultrasonic Horn for a Bonding Apparatus”. An ultrasonic horn is provided with a capillary attachment hole formed so as to be smaller than the capillary and a jig insertion hole that communicates with the capillary attachment hole. A jig is insertable into the jig insertion hole to widen the capillary attachment hole. Therefore, a cross-sectional area of the capillary attachment hole is increase, so that the capillary can be received in the capillary attachment hole. The capillary is fastened in place by the elastic force that is generated by the horn material when the jig is removed from the jig insertion hole and the cross-sectional area of the capillary attachment hole is thereby reduced.  
         [0006]     In typical ultrasonic transducers, the ultrasonic drivers are arranged such that ultrasonic oscillation is generated down the horn in a longitudinal direction along the length of the horn. A disadvantage of the gripping force introduced by the aforesaid prior art is that the elastic force is exerted on sides of the capillary that are transverse to the longitudinal oscillation axis of the horn. By gripping the capillary transversely to this longitudinal direction, the gripping force is less effective, and there is a greater risk of movement of the capillary in the longitudinal direction relative to the horn if the elastic fastening force is not sufficiently large to prevent this.  
       SUMMARY OF THE INVENTION  
       [0007]     It is therefore an object of the invention to seek to provide an improved arrangement for fastening a capillary to a horn of a bonding tool without the use of screws to provide a clamping force on the capillary. It is related object of the invention in a preferred embodiment of the invention to seek to optimize a gripping force acting on the capillary in a direction of oscillation of the capillary.  
         [0008]     Accordingly, the invention provides a capillary holder for mounting a capillary onto a horn, comprising a mounting hole formed in the horn that has a first width along a first axis that is smaller than a width of the capillary and a second width along a second axis perpendicular to the first axis that is larger than the width of the capillary, wherein the mounting hole is configured such that application of a flexion force to reduce the second width simultaneously expands the first width so as to fit the capillary when the first width is larger than the width of the capillary, and removal of said flexion force contracts the first width whereby to grip the capillary using an elastic force of the horn.  
         [0009]     It would be convenient hereinafter to describe the invention in greater detail by reference to the accompanying drawings which illustrate preferred embodiments 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  
       [0010]     Examples of preferred embodiments of capillary holders in accordance with the invention will now be described with reference to the accompanying drawings, in which:  
         [0011]      FIG. 1  is an isometric view of an ultrasonic horn including an arrangement for holding a capillary according to a first preferred embodiment of the invention;  
         [0012]      FIG. 2  is an isometric view of the ultrasonic horn of  FIG. 1  including a capillary mounted onto it;  
         [0013]      FIG. 3  is an isometric view of an ultrasonic horn according to a second preferred embodiment of the invention;  
         [0014]      FIGS. 4   a  to  4   c  are plan views of the ultrasonic horn according to the first preferred embodiment looking from direction A of  FIG. 1  and  FIG. 2 , illustrating the manner in which a tip of the ultrasonic horn can be manipulated to secure a capillary in a mounting hole;  
         [0015]      FIGS. 5   a  to  5   c  are plan views of an ultrasonic horn according to the second preferred embodiment of the invention looking from direction A of  FIG. 3  including recesses formed in the mounting hole that are shaped to receive a capillary; and  
         [0016]      FIGS. 6   a  to  6   c  are plan views of an ultrasonic horn according to a third preferred embodiment of the invention, wherein the mounting hole includes three gripping positions to grip a capillary. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]      FIG. 1  is an isometric view of an ultrasonic horn  10  including an arrangement for holding a capillary according to a first preferred embodiment of the invention. A mounting hole  14  is located at a tip  12  of the ultrasonic horn  10  for inserting and fastening a capillary. However, it should be appreciated that the mounting hole  14  can be located at other positions depending on the design and use of the horn. The horn  10  is connected to an ultrasonic driver (not shown) that is configured to generate ultrasonic oscillation along an oscillation axis  17  that is parallel to the longitudinal axis  18  of the horn  10 .  
         [0018]      FIG. 2  is an isometric view of the ultrasonic horn  10  of  FIG. 1  including a capillary  20  mounted onto it. Generally, a cross-sectional area of the mounting hole  14  is larger than a cross-sectional area of the capillary  20 . The capillary  20  has a capillary tip  22  through which bonding wire is fed and bonding force and power is applied at the capillary tip  22 . When the ultrasonic horn  10  is driven by ultrasonic drivers in use, the capillary  20  is driven to oscillate along the oscillation axis  17  that is parallel to the longitudinal axis  18  of the horn  10 .  
         [0019]      FIG. 3  is an isometric view of an ultrasonic horn according to a second preferred embodiment of the invention. In this embodiment, a slot  16  is located adjacent to the mounting hole  14 , preferably opposite a distal end of the tip  12 . It is aligned with the mounting hole  14  along a longitudinal axis  18  of the horn  10  and is divided from the mounting hole  14  by a portion of the horn  10 . The slot  16  is positioned such that an edge of the mounting hole  14  next to the slot  16  is configured to flex in the direction of the slot  16  when a compression force is exerted on the sides of the tip  12  of the horn  10 . An advantage of this design is that a portion of the periphery of the mounting hole  14  furthest from the tip of the horn  10  is allowed to flex and thus allow greater flexion to be achieved for the mounting hole  14  generally. In the first embodiment, this portion of the mounting hole  14  is relatively more rigid.  
         [0020]      FIGS. 4   a  to  4   c  are plan views of the ultrasonic horn  10  according to the first preferred embodiment looking from direction A of  FIG. 1  and  FIG. 2 , illustrating the manner in which a tip  12  of the ultrasonic horn  10  can be manipulated to secure a capillary  20  in the mounting hole  14 . In  FIG. 4   a,  there is no force acting on the tip  12  of the horn  10 , and the mounting hole  14  is generally in an elliptical shape at rest. The cross-sectional area of the mounting hole  14  is larger than the cross-sectional area of the capillary  20 . The elliptical mounting hole  14  comprises two primary widths. A first width, w 1 , of such ellipse that lies along a first axis or longitudinal axis  18  of the horn  10  is smaller than a width of the capillary  20 . A second width, w 2 , of the mounting hole  14  along a second axis  19  perpendicular to the first or longitudinal axis  18  is larger than the width of the capillary  20 . This ensures that the capillary  20  is gripped at points along the first or longitudinal axis  18  of the horn  10  when the capillary  20  is inserted and fastened. Additionally, the capillary  20  cannot be inserted into the mounting hole  14  without the application of a flexion force on the sides of the horn tip  12  (as described below) to flex the edges of the mounting hole  14  so as to increase the width of the mounting hole  14  along the longitudinal axis.  
         [0021]     In  FIG. 4   b,  flexion forces in the form of compression forces  24  that are parallel to the second axis are exerted on the sides of the horn tip  12  to flex the edges of the mounting hole  14  along the longitudinal axis  18  of the horn  10 . The second width of the mounting hole  14  is reduced while simultaneously, the first width of the mounting hole  14  along the longitudinal axis  18  is expanded. The change in the shape of the mounting hole  14  makes it possible to insert a capillary  20  once the first width is larger than the width of the capillary  20 . Unlike in the prior art, it would be noted that the cross-sectional area of the mounting hole  14  before application of the flexion or compression forces  24  is substantially the same as the cross-sectional area of the mounting hole  14  during application of the forces.  
         [0022]     The compression forces  24  may be provided in a variety of ways, such as simply by using a pair of pliers to compress the sides of the horn tip  12 . Accordingly, a special tool is not necessary for mounting the capillary  20 . However, a jig with a clamp that is controllable to compress the horn tip  12  by a predetermined distance is preferable to achieve consistency in the distance or amount of compression to be applied. When the edges of the mounting hole  14  are sufficiently flexed to fit the capillary  20 , the capillary  20  is inserted.  
         [0023]     In  FIG. 4   c,  once the capillary  20  is properly positioned, the compression forces  24  are removed. The edges of the mounting hole  14  will tend to return to their original unflexed positions, thereby contracting the first width of the mounting hole  14  along the longitudinal axis  18 . A gripping force is thereby exerted on the capillary  20  by the elastic force of the horn material tending to return the mounting hole  14  to its original shape, and the capillary  20  is mounted and secured. In particular, the two gripping positions of the mounting hole  14  on the capillary  20  are located along and substantially apply gripping forces that are parallel to the oscillation axis  17  or longitudinal axis  18  of the horn  10 . Therefore, the capillary  20  is more effectively secured along its oscillation direction parallel to the longitudinal axis  18  of the horn  10 .  
         [0024]      FIGS. 5   a  to  5   c  are plan views of an ultrasonic horn  10  according to the second preferred embodiment of the invention looking from direction A of  FIG. 3  including recesses  26  formed in the mounting hole  14  that are shaped to receive a capillary  20 . In  FIG. 5   a,  there is no force acting on the tip  12  of the horn  10 , and the mounting hole  14  is generally in an elliptical shape at rest, as in the first embodiment. Additionally, the mounting hole  14  includes two recesses  26  aligned along the longitudinal axis  18  of the horn  10  that are shaped in conformance with a portion of a circumference of the capillary  20 . The recesses  26  are formed to receive the capillary  20  and may enable the mounting hole  14  to more readily receive the capillary  20  upon the application of the compression forces  24 .  
         [0025]     In  FIG. 5   b,  compression forces  24  are exerted on the sides of the tip  12  of the horn  10  to flex the edges of the mounting hole  14  along the longitudinal axis  18  of the horn  10 . Once the edges of the mounting hole  14  along the longitudinal axis  18  are sufficiently flexed to fit the capillary  20 , the capillary  20  is inserted and positioned.  
         [0026]     In  FIG. 5   c,  the capillary  20  has been properly positioned and the compression forces  24  are removed. Since the recesses  26  are shaped according to a portion of the circumference of the capillary  20 , the capillary  20  fits more snugly in the mounting hole  14 .  
         [0027]      FIGS. 6   a  to  6   c  are plan views of an ultrasonic horn  10  according to a third preferred embodiment of the invention, wherein the mounting hole  14  includes three gripping positions  30   a,    30   b,    30   c  for gripping the capillary  20 . In  FIG. 6   a,  the mounting hole  14  is at an unflexed position. Three recesses  28  are formed in the mounting hole  14  in between the three gripping positions  30   a,    30   b,    30   c.    
         [0028]     In  FIG. 6   b,  when compression forces  24  are applied to the tip  12  of the horn  10  adjacent to the mounting hole  14 , an edge of the mounting hole  14  at a distal end of the horn  10  is allowed to flex to expand the mounting hole  14 . The compression forces  24  are applied on the horn tip  12  along the second axis  19  such that the first width of the mounting hole  14  along the longitudinal axis  18  of the horn  10  is expanded to allow a capillary  20  to be inserted. Once the width of the mounting hole  14  along the longitudinal axis is large enough to accommodate a capillary, a capillary  20  is inserted into position.  
         [0029]     In  FIG. 6   c,  the capillary  20  has been positioned and the compression forces  24  are removed. In this embodiment, the recesses  28  are preferably formed such that the mounting hole  14  grips the capillary  20  at three gripping positions  30   a,    30   b,    30   c  around the circumference of the capillary  20 . The arrangement of the gripping positions may be generally triangular. In particular, at least one gripping position  30   a  preferably lies along the longitudinal axis  18  of the horn  10 . The other gripping positions  30   b,    30   c  should preferably also act on the capillary  20  in such a way that they provide gripping forces that comprise force components that are parallel to the oscillation axis  17 . In this way, the capillary  20  is gripped securely along the oscillation axis  17  as the gripping positions  30   a,    30   b,    30   c  are still arranged to substantially apply forces that are parallel to the oscillation axis  17  or longitudinal axis  18 .  
         [0030]     At least a portion of the horn  10  at which the mounting hole  14  is formed is preferably made from titanium. Therefore, the horn  10  could be manufactured from one single piece of material that is made from titanium, or it could comprise two or more pieces that are connected together. In the latter case, it is preferred that the piece of material at which the mounting hole  14  is formed is made from titanium.  
         [0031]     It would be appreciated that since there is no need to use screws to provide clamping forces for mounting the capillary in the above preferred embodiments of the invention, a purer longitudinal oscillation or vibration of the capillary can be achieved and conventional problems relating to the screws wearing off can be avoided. A simpler design is possible such that ultrasonic properties of the bonding tool can be made more consistent. The simpler design also results in easier manufacturability and quality control.  
         [0032]     Another advantage of the capillary holders according to the preferred embodiments of the invention is that the capillary is held by a direct compressive force, not by a frictional force as in conventional bonding tools using screws. As a result, a lesser clamping force to the capillary is required and there is less wear on the mounting hole of the horn. Further, by gripping the capillary at points along the longitudinal axis of the horn, the capillary can be held more securely during longitudinal vibration of the capillary when ultrasonic bonding is carried out.  
         [0033]     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.