Abstract:
A wire clamp includes a clamp body; a pair of arms coupled to the clamp body; and a piezoelectric actuator having a longitudinal axis extending between a first end and a second end, the actuator being coupled to the pair of arms at the first end and to the clamp body at the second end. The second end of the actuator is coupled to the clamp body by a preload mechanism for applying a preload force along the longitudinal axis, and the preload mechanism comprises at least one wedge having an inclined surface which is slidable over a mating inclined surface.

Description:
TECHNICAL FIELD 
     This invention relates to a wire clamp with a piezoelectric actuator, and to a method and an apparatus for applying a preload force to the piezoelectric actuator. 
     BACKGROUND OF THE INVENTION 
     In semiconductor processing, an integrated circuit is connected to its packaging by wires in a process known as wire bonding. During wire bonding it is necessary to clamp the wire to be bonded in order to position it precisely at the desired location. One known device for clamping wires is shown in  FIG. 1 , and includes a pair of jaws  102 ,  104  which are driven by a piezoelectric actuator  110 . Applying an electric field across the actuator  110  causes the jaws  102 ,  104  to open. When the electric field is switched off, the arms close such that the wire is clamped between clamping plates  106 ,  108 . 
     When using piezoelectric actuators it is necessary to apply a preload force to the piezoelectric element of the actuator before an electric field is applied. The magnitude of the preload force is important. If insufficient preload is applied, the piezoelectric element may break due to dynamic pull forces. If too much preload is applied, the actuator may not be able to successfully actuate. 
     In the wire clamp of  FIG. 1 , a preload force is applied by using a set screw  130  to apply a force to the piezo actuator  110 , along its longitudinal axis (i.e, the axis running along the centre of the piezo actuator  110  and perpendicular to its ends  112 ,  114 ) via a preload block  120 . The set screw  130  and preload block  120  push the piezo actuator  110  against the wire clamp main body  105 . 
     A disadvantage of the prior art arrangement shown in  FIG. 1  is that the screw  130  may become loose due to vibration or insufficient friction in the mating surfaces (i.e. between the screw  130  and preload block  120 , and between the preload block  120  and piezo  110 ), resulting in a decrease of the preload force on the piezo actuator  110 , and adversely affecting the performance of the wire clamp  100 . In typical operation of a wire bonding apparatus employing the clamp  100 , an external force will repeatedly be applied to the preloading screw  130  in the axial direction, which may cause the screw to loosen. 
     A further disadvantage of the preload mechanism of  FIG. 1  is that a torque will be transmitted to the piezo stack  110  as the screw is tightened. This may damage the piezo stack  110  if not done carefully. 
     It would be desirable to provide a wire clamp which overcomes one or more of the above disadvantages, or which at least provides a useful alternative. 
     SUMMARY OF THE INVENTION 
     Embodiments relate to a wire clamp, comprising:
         a clamp body;   a pair of arms coupled to the clamp body; and   a piezoelectric actuator having a longitudinal axis extending between a first end and a second end, the actuator being coupled to the pair of arms at the first end and to the clamp body at the second end;   wherein the second end of the actuator is coupled to the clamp body by a preload mechanism for applying a preload force along the longitudinal axis, and   wherein the preload mechanism comprises at least one wedge having an inclined surface which is slidable over a mating inclined surface.       

     Other embodiments relate to a method of applying a preload force to a piezoelectric actuator of a wire clamp, the wire clamp comprising a clamp body and a pair of arms coupled to the clamp body, the piezoelectric actuator having a longitudinal axis extending between a first end and a second end and being coupled to the pair of arms at the first end and to the clamp body at the second end; the method comprising the steps of:
         providing a wedge having an inclined surface;   providing a mating inclined surface such that a gap is defined between the mating inclined surface and the piezoelectric actuator, or between the clamp body and the mating inclined surface, the wedge being slidable over the mating inclined surface;   inserting the wedge into the gap such that the inclined surface contacts the mating inclined surface; and   applying a force to the wedge in a direction transverse to the longitudinal axis of the actuator to thereby apply a preload force along the longitudinal axis.       

     Further embodiments relate to a jig for applying a preload force to a piezoelectric actuator of a wire clamp, the wire clamp comprising: a clamp body and a pair of arms coupled to the clamp body, the piezoelectric actuator having a longitudinal axis extending between a first end and a second end and being coupled to the pair of arms at the first end and to the clamp body at the second end, the second end of the piezoelectric actuator being coupled to the clamp body by a preload mechanism for applying a preload force along the longitudinal axis, the preload mechanism comprising at least one wedge having an inclined surface which is slidable over a mating inclined surface; the jig comprising:
         a clamp support having a recess to receive the clamp body;   a tensioning platform extending from the clamp support and being elevated relative to the recess, the tensioning platform having a threaded bore which, when the clamp body is received in the clamp support, is aligned with the at least one wedge; and   a tensioning screw receivable in the threaded bore such that a tip of the tensioning screw contacts the at least one wedge, and such that tightening of the tensioning screw applies a force to the at least one wedge in a direction transverse to the longitudinal axis of the piezoelectric element.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a projection view from above of a prior art wire clamp; 
         FIG. 2  is a projection view from above of a wire clamp according to some embodiments of the invention; 
         FIG. 3  is a top perspective view of the wire clamp of  FIG. 2 ; 
         FIG. 4  is a side view, in close-up, of a preload mechanism according to certain embodiments; 
         FIG. 5  is a top perspective view of the wire clamp of  FIGS. 2 to 4 , positioned in a jig for adjusting the preload force; 
         FIG. 6  is a cross-section through the line A-A of  FIG. 5 ; 
         FIG. 7  is a projection view from above and in close-up of jaws of a wire clamp; 
         FIG. 8A  and  FIG. 8B  schematically depict calibration markings used in a method according to embodiments of the invention; and 
         FIG. 9  shows the preload mechanism of  FIG. 4  comprising wedges, each wedge having respective lugs for engagement with the jig as shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring initially to  FIG. 2  and  FIG. 3 , there is shown a wire clamp  200  having a body  210 , within which is disposed a piezoelectric actuator  240 . The piezoelectric actuator  240  is preferably a multilayer stack actuator. The body  210  is substantially U-shaped and has an end portion  211  extending between two side portions  202  and  204 . The end portion  211  and side portions  202 ,  204  together define a void  232  within which the actuator  240  resides. 
     The actuator  240  contacts, at a distal end  242 , a plate  220 . The plate  220  is coupled to jaws  212 ,  214  via respective linkages  234 ,  236 . Jaw  212  has a clamp plate  216  and jaw  214  has a clamp plate  218 . Application of an electric field to the actuator  240  in a direction along the longitudinal axis results in an expansion of the actuator, thus exerting a force on plate  220 , and in turn causing the jaws  212 ,  214  to be opened (via linkages  234 ,  236 ). 
     The actuator  240  may be fastened at its distal end  242  to the plate  220 , or may be separate from the plate  220  such that it is simply in abutment with it. 
     At a proximal end  244 , the actuator  240  contacts a preload mechanism comprising wedge elements  252 ,  254 , which are interposed in a gap between the proximal end  244  and an inner surface  230  of the body  210 . Wedge element  254  may be fastened to proximal end  244  of actuator  240 . Alternatively, wedge element  252  may be fastened to the body  210  or may be integral with it such that it extends from inner surface  230 . In presently preferred embodiments, both wedge elements  252 ,  254  are separate from the actuator  240  and body  210 . Body  210  may have a support surface  208  ( FIG. 6 ) extending between its sides  202 ,  204  (for example), on which the wedge element  254  is supported. 
     The terms “distal” and “proximal” in relation to the ends of actuator  240  are used to indicate their relationship to the end portion  211  of body  210 . That is, the “proximal” end  244  is the end which is closer to end portion  211  while the “distal” end  242  is the end which is further away. 
     As shown in  FIG. 4 , wedge  252  has an inclined surface  262  and wedge  254  has a mating inclined surface  264 . Surfaces  262 ,  264  are able to slide over each other when a transverse force  400   a  or  400   b  is applied to wedge  252  or wedge  254 . Wedge  254  is supported on the support surface of the body  210  during application of the transverse force. Due to the angle of inclination of the surfaces, as a force  400   a  or  400   b  is applied, the flat-to-flat distance D between the respective flat surfaces of the wedges  252 ,  254  increases. As the distance D increases, the preload force (reaction force) applied to actuator  240  increases. 
     Transverse force  400   a  or  400   b  may continue to be applied until a desired preload force is achieved. When the desired preload force is achieved, the wedges  252 ,  254  may be secured together, for example by gluing or soldering, in order to maintain the desired preload force. 
     In presently preferred embodiments, the wedges  252 ,  254  are self-locking. Self-locking is achieved when:
 
μ&gt;tan θ,  (1)
 
where μ is the coefficient of static friction between the wedges  252 ,  254 , and θ is the angle of inclination of inclined surface  264  to flat surface  266  of wedge  254  (or equivalently, the angle of inclination of inclined surface  262  of wedge  252 ).
 
     The self-locking behaviour of the wedges  252 ,  254  is particularly advantageous, because the preload force applied to the piezo actuator  240  is more stable than that achievable by the screw-tensioning methods of the prior art. The performance of the piezo wire clamp  200  is therefore more consistent than prior art clamps. 
     The angle θ is preferably in the range 8-12 degrees although it will be appreciated that for self-locking, any angle which satisfies the self-locking condition (1) will suffice. For wedges which are not self-locking, other angles of inclination may be possible. 
     The embodiment shown in  FIGS. 2 to 4  employs two wedges  252 ,  254  in the preload mechanism. It will be appreciated that a single wedge  252  or  254  may be employed provided a suitable mating inclined surface is provided at the proximal end  244  or at the inner surface  230  as appropriate. Alternatively, in some embodiments three or more wedges may be used, provided that the surface of the wedge contacting proximal end  244  of the actuator  240  is flat so that the preload force is directed along the longitudinal axis of the actuator  240 . 
     Referring now to  FIGS. 5 and 6 , there is shown an apparatus for applying a preload force to the actuator  240 . The apparatus is a jig  300  having a base  310  from which extends a clamp support  312 . Extending in a horizontal plane from clamp support  312  is a tensioning platform  318  which is elevated with respect to the base  310 . The tensioning platform  318  has a threaded through-bore (not shown) to receive a threaded shaft  330  of a tension adjustment screw  320 . Positioned over tensioning platform  318  is a lifting platform  314 , a bore  324  of which receives the shaft  330  of screw  320 . Also received in lifting platform  314  are two smaller adjustment screws  322 . A pair of legs  350  (only one of which is shown in the drawings) extends downwardly from opposite sides of the lifting platform  314 . The legs extend from a position along the platform  314  which is approximately aligned with the bore  324 . Each leg  350  has a foot  352  which defines, together with the remainder of the leg  350 , a notch  353  sized and shaped to receive a lug  253  of wedge  252  (not shown in  FIGS. 2 to 4 , but visible in  FIG. 9 ). Between the legs  350  is a gap which is approximately the width of tensioning platform  318 . 
     To prepare the clamp  200  for preload force adjustment, the end portion  211  of the clamp body  210  is placed on recessed ledge  313  which is sized and shaped to accommodate the body end portion  211 . Lifting platform  314  is then positioned by straddling the legs  350  over tensioning platform  318  and sliding it over the tensioning platform until bore  324  is aligned with the threaded bore of platform  318 , and until the notches  353  of legs  350  align with and engage ears  253  of wedge element  252 . Large tensioning screw  320  is then passed through the aligned bores and tightened until a rounded tip  332  at the lower end of shaft  330  contacts wedge  252 . Small adjustment screws  322  are threaded into their respective bores until their lower ends contact the lifting platform  314  as shown in  FIG. 6 . 
     Once the jig  300  is assembled with the clamp  200  in place as shown in  FIGS. 5 and 6 , large tensioning screw  320  is tightened, in the process forcing wedge  252  downwards over wedge  254 . Tightening can continue until a predetermined preload force is achieved in the actuator  240 . If the preload force exceeds the desired force, it can be reduced by the use of small adjustment screws  322 . Because the lower ends of small screws  322  are contacting and abutting against the platform  318 , which is fixed, tightening them will tend to force the lifting platform  314  upwards. As lifting platform  314  is forced upwards, the feet  352  engage with lugs  253  and draw wedge  252  in an upward direction, thereby decreasing the preload force applied to actuator  240 . 
     The magnitude of the preload force can be determined by measuring the separation between clamp plates  216  and  218 , since the separation distance is directly proportional to the preload force. A calibration can be performed by measuring separation distance as a function of known preload force. Once the calibration curve is obtained, the preload force applied by the wedges can be determined by measuring the separation distance. 
     One way in which separation distance can be measured is depicted schematically in  FIGS. 7, 8A and 8B . In  FIG. 7 , fiducial marks  702 ,  704  are applied near the tips of jaws  212 ,  214  adjacent the clamp plates  216 ,  218 . The separation between clamp plates  216 ,  218  can then be measured by tracking the movement of one or both of the fiducial marks  702 ,  704 . For example, a camera (not shown) can be mounted over fiducial mark  702 . Image data from the camera may be received at a computer system which displays the captured image data on a display. An image processing module of the computer system may detect the fiducial mark  702  within the captured image, and determine its position as indicated by cross-hairs  802 ,  804  centred on the detected fiducial mark  702 . The cross-hairs may be superimposed on the displayed image as shown in  FIG. 8A . As the screw  320  is tightened and the clamp plates  216 ,  218  move further apart, fiducial mark  702  moves to the right, to a new position indicated by cross-hairs  802 ′,  804 . The measured distance d between the new x-coordinate (as indicated by vertical line  802 ′) and the old x-coordinate (as indicated by dashed vertical line  802 ) can then be compared to the previously derived calibration curve to determine the preload force. The comparison can be done manually, or automatically, by the image processing module, for example. 
     Although particular embodiments of the invention have been described in detail, many variations are possible within the scope of the invention, as will be clear to a skilled reader.