Abstract:
A wire bonder includes a cut clamper having a pair of supporting members and a pair of electrodes formed in rectangular shape. Each electrode is rotatably connected to the supporting member. Each electrode has an uneven surface. A heater is incorporated in one or both of supporting.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Japanese Patent Application No. 11-37368, filed Feb. 16, 1999, the entire subject matter of which is incorporated herein of reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a wire bonder which connects a pad on a semiconductor chip with a pad on a circuit board so as to make a looping with a bonding wire. 
     2. Description of the Related Art 
     A wire bonder of an ultrasonic thermocompression type includes a pipe-shaped capillary through which a gold bonding wire made of gold can be passed through, a cut clamper and a torch rod. The cut clamper has a pair of disk-shaped clamping electrodes made of cermet (Cr—SiO), which are placed on parallel plates. The clamping electrodes can clamp the gold bonding wire and apply a fixed electric potential to it. 
     In a case where a tip of the gold bonding wire is to be connected to a pad on a circuit board, a gold ball should be formed at the tip as the tip exits the end of the capillary. Then, the gold ball is pressed by the tip of capillary against the pad on the circuit board, and is bonded to the pad using an ultrasonic vibration. In a case in which the gold bonding wire connected to the pad on the circuit board at its tip, is to be connected to a pad on a semiconductor chip, the capillary is moved to a position above the pad on the chip. Then, the gold bonding wire is pressed by the tip of capillary against the pad, and is bonded to the pad by applying ultrasonic vibration. After the connection has been completed, the gold bonding wire is clamped by the clamping electrodes of the cut clamper, and is pulled by the cut clamper. The gold bonding wire is cut by tension. 
     The gold ball is formed using the following method. The gold bonding wire is clamped by the clamping electrodes. Then, by applying an electrical voltage between the electrodes and the torch rod, a melted gold ball is formed by atmospheric discharge against the gold bonding wire as it exits the end of the capillary. Then, the melted gold ball is solidified by cooling. 
     The circuit board is heated when the gold bonding wire is bonded thereto. Therefore, the gold bonding wire which is pressed by the tip of the capillary is ultrasonically thermocompressed by applying ultrasonic vibration to the gold bonding wire. 
     However, because the face of the electrode for clamping the gold bonding wire is disk-shaped, if the gold bonding wire is clamped in a position tilted from the axis of disk-shaped electrode, the distance between the end of the electrode and the tip of the gold bonding wire is different from what is desired. Discharge voltage is varied with a parameter of the distance. This may cause the gold ball to be made in various sizes. Additionally, the face of the electrode is worn because of the repetition of the atmospheric discharge. This also causes the gold balls to vary in size. The strength of the gold ball when pressed on the pad depends on its size. Thus, as the size varies so does the bonding strength. Therefore, nonuniformity in gold ball size disadvantageously results in nonuniformity in bonding strength. 
     Further, the faces of the clamping electrodes should be precisely in parallel to clamp the gold bonding wire. As the result, as electrodes come out of parallel with use, the electrodes must be periodically changed. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved wire bonder that corrects the problems mentioned above. 
     To achieve this object, a wire bonder according to the invention includes a cut clamper having a first supporting member, a second supporting member which is opposite the first supporting member, the second supporting member being movable toward and away from said first supporting member, a first clamping electrode connected to the first supporting member, the first clamping electrode having a first rectangular main surface, and a second clamping electrode connected to the second supporting member, the second clamping electrode having a second rectangular main surface which faces the first rectangular main surface in parallel relation thereto. 
     Further, to achieve the object, the first clamping electrode and the second clamping electrode are rotatably connected to the first supporting member and the second supporting member, respectively. Also, the first and second clamping electrodes have even surfaces, and a heater is incorporated in one of the support members, or in both. 
     In addition, to further achieve the object, the cut clamper is divided into two independent clampers having different functions. The first clamper serves to apply an electric potential to the gold bonding wire, and the second clamper serves to clamp the gold bonding wire in order to pull the gold bonding wire. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more particularly described with reference to the accompanying drawings, in which: 
     FIG. 1A is a diagrammatic illustration of a wire bonder for carrying out a first embodiment of the invention; 
     FIG. 1B is an enlarged side view of a cut clamper; 
     FIG.  2 A(I) is a cross-sectional view taken on line I—I of FIG. 1A; 
     FIG. 2B is an enlarged detail illustration of the tip of the cut clamper; 
     FIG. 3 is an enlarged detail illustration showing the relationship between a clamping electrode and a gold bonding wire in the related art and the first embodiment of the invention; 
     FIG. 4 is an alternative clamping electrode of the first embodiment of the invention; 
     FIG. 5 is a diagrammatic illustration of a wire bonder for carrying out a second embodiment of the invention; 
     FIG.  6 (II) is cross-sectional view taken on line II—II of FIG. 5; 
     FIG.  7 (III) is cross-sectional view taken on line III—III of FIG. 5; and 
     FIG.  8 (IV) is cross-sectional view taken on line VI—VI of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1A, a wire bonder  100  preferably is made up of a spool unit  1  in which a spool  1   a  is set, a gold bonding wire guide  3 , a pipe-shaped air clamper  4  having a hole  4   a,  a transducer  5 , a capillary  6  having a center hole  6   a,  a torch rod  7  and a cut clamper  10 , which can be moved toward the spool unit  1 . A gold bonding wire  2  is spooled on the spool  1   a.  The gold bonding wire  2  coming from the spool  1   a  is led by the gold bonding wire guide  3  to pass through the hole  4   a  in the air clamper  4 , and it is also passed through the center hole  6   a  of the capillary  6  which is supported by the transducer  5 . The gold bonding wire  2  is stretched with air by the air clamper  4  toward the spool  1   a.  In this wire bonder  100 , the transducer  5  has two main functions. One is to move the capillary  6  so as to let its tip press against the pad on the circuit board or the pad on the chip. The other function is to apply ultrasonic vibration to the capillary  6 . The torch rod  7  is disposed adjacent the tip of the capillary  6 , and the cut clamper  10  is disposed between the capillary  6  and the air clamper  4 . 
     FIG.  2 A(I) shows the structural details of the cut clamper  10 . Referring to FIG.  2 A(I), the cut clamper  10  has a fixed plate  12  that is fixed to a shaft  11 , and a movable plate  14  that is fixed to a elastic shaft  13  that can expand and contract. The fixed plate  12  and the movable plate  14  are disposed in parallel. A distance between the fixed plate  12  and the movable plate  14  can be changed by a solenoid  15 . That is, the movable plate  14  can be moved toward and away from the fixed plate  12 . The cut clamper  10  also has a pair of clamping electrodes  16 ,  17  made of cermet. As shown in FIG. 2B, the electrode  16  is connected to the fixed plate  12  by a screw  19   a  and the electrode  17  is connected to the movable plate  14  by a screw  19   b.  Therefore, the electrodes can be rotated. Further, each electrode has a rectangular, preferably square main surface. The main surfaces of the electrodes  16 ,  17  oppose each other in parallel relation. The gold bonding wire  2  is clamped between the main surfaces of the electrodes  16 ,  17 , and a fixed electrical potential is applied to the gold bonding wire by these electrodes. A heater  20  is incorporated in the movable plate  14  to heat the electrode  17 . Alternatively, the heater  20  may be incorporated in the fixed plate  12  or in both of the plates  12 ,  14 . A spring  18  is connected between the movable plate  14  and the shaft  11  to adjust the movement of the movable plate  14 . 
     A method of bonding the gold bonding wire with the above-mentioned wire bonder, is explained below. 
     For connection of the pad on a semiconductor chip to the pad on a circuit board with the gold bonding wire, first, a gold ball  2   a  is formed at the tip of the gold bonding wire  2  as it exits the end of the capillary  6 . At this time, the movable plate  14  is moved toward the fixed plate  12  by the solenoid  15  so as to clamp the gold bonding wire  2  by the electrodes  16 ,  17 . The gold bonding wire  2  can be clamped anywhere on the surfaces of the electrodes  16 ,  17 . That is, it is not required to adjust the location where the gold bonding wire  2  is placed. Then, by applying a particular electrical potential between the electrodes  16 ,  17  and the torch rod  7 , a melted gold ball is formed at the tip of the gold bonding wire  2  by an atmospheric discharge against the gold bonding wire  2 . Then, the gold ball  2   a  is solidified as it cools down after the application of the voltage is halted. 
     After the gold ball  2   a  is solidified, the electrodes  16 ,  17  release the gold bonding wire  2  by moving the movable plate  14  away from the fixed plate  12 . Then, the tip of the capillary  6  presses the gold ball  2   a  against the pad on the circuit board by moving the transducer  5 . The gold ball  2   a  is compressed against the pad. Then, the capillary  6  is vibrated ultrasonically by the transducer  5  so that the ultrasonic vibration is transmitted to the gold ball  2   a.  Further, the electrode  17  is heated by the heater  20 , and the heat is transmitted to the gold ball  2   a.  Accordingly, the gold ball  2   a  at the end of the gold bonding wire is bonded to the pad on the circuit board by ultrasonic thermocompression. 
     After the gold bonding wire  2  has been connected at its tip to the pad on the circuit board, the capillary  6  is moved to a position above the pad on the semiconductor chip. Then, the gold bonding wire  2  is pressed by the tip of capillary  6  against the pad. Then, the capillary  6  is vibrated ultrasonically by the transducer  5 , and the ultrasonic vibration is transmitted to the gold bonding wire  2 . Further, the electrode  17  is heated by the heater  20 , and the heat is transmitted to the gold bonding wire  2 . Accordingly, the gold bonding wire  2  is bonded to the pad on the semiconductor chip by ultrasonic thermocompression so that the pads are connected so as to make a looping with the gold bonding wire  2 . 
     After the connection has been completed, the gold bonding wire  2  is clamped by the clamping electrodes  16 ,  17 . Referring to FIG. 1B, then, by moving the cut clamper  10  toward away from the pad on the chip, the gold bonding wire  2  is cut by tension. The electrodes  16 ,  17  are rotated 90 degrees by the screws  19   a,    19   b  if the main surfaces of the electrodes  16 ,  17  are worn because of the repetition of the above-described operations. 
     According to the first embodiment of the invention, the following advantages can be obtained. 
     As shown in FIG. 3, the gold bonding wire  2  should always be put through the center of the disk-shaped electrode in the conventional device to keep the gold bonding wire  2  contacting the electrode  16 ,  17  uniform in length. However, it is not necessary to put the gold bonding wire  2  through the center of the electrodes  16 ,  17  in this embodiment because the main surface of the electrode is rectangular. Therefore, as the length of the gold bonding wire  2  which contacts the electrodes  16 ,  17  is invariable, an electrical potential which is consistently the same atmospheric discharge can be applied. Thus, a gold ball that is consistently same size can be formed. Further, as it is not necessary to adjust the location of the gold bonding wire on the electrodes precisely, it becomes easy to control the clamping movement of the movable plate  14  and the fixed plate  12 . 
     In the conventional device, when a worn electrode is replaced to a new one, it is necessary to place the new electrodes in parallel very precisely. However, where the main surfaces of the rotatable electrodes  16 ,  17  are square, the gold bonding wire  2  can be clamped with fresh surfaces of the electrodes  16 ,  17  by rotating the electrodes by 90 degrees, and moreover, the frequency of electrode replacement can be reduced. 
     Furthermore, as the gold bonding wire  2  is heated by the heater  10  when the atmospheric discharge is performed, the gold ball can consistently be formed with the same hardness. Additionally, because the gold bonding wire  2  can be heated directly by the electrode  17 , various kinds of the connection-specification of the gold bonding wire  2 , e.g. a wire loop height, can be controlled by heat. Therefore, a stable looping connection can be performed. 
     Referring to FIG. 4, alternative clamping electrodes  29   30  can be used in the wire bonder  100 . Many microprojections such as sandpaper, are formed on the surfaces of the electrodes  29 ,  30 . The microprojections prevent the gold bonding wire  2  from slipping on the electrode surfaces. As the distance from the tip of the gold bonding wire  2  to the edge of the electrodes  29 ,  30  is always the same, the gold ball which is consistently same size can be formed. 
     Referring to FIG. 5, a wire bonder  200  according to the second embodiment of the invention has a spool unit  31  in which a spool  31   a  is set, a gold bonding wire guide  33 , a pipe-shaped air clamper  34  having a hole  34   a,  a transducer  35 , a capillary  36  having a center hole  36   a,  a torch rod  37 , and a cut clamper unit  40  that includes of a first cut clamper  38  and a second cut clamper  39 . A gold bonding wire  32  is spooled on the spool  31   a.  The cut clamper unit  40  can be moved toward the spool unit  31 . As the parts of the wire bonder  200  other than the cut clamper unit  40  are the same as in the wire bonder  100 , the description of the second embodiment is focused on the cut clamper unit  40 , which replace the cut clamper  10 . Referring to FIG.  6 (II), the cut clamper unit  40  is disposed between the pipe-shaped air clamper  34  and the transducer  35 . Referring to FIG.  7 (III), the first cut clamper  38  has the same structure as the cut clamper  10  of the first embodiment. That is, the first cut clamper  38  has a first fixed plate  38   b  which is fixed to a shaft  41 , and a first movable plate  38   a  that is fixed to an elastic shaft  43  that can expand and contract. The first fixed plate  38   b  and the first movable plate  38   a  are disposed in parallel. A distance between the first fixed plate  38   b  and the first movable plate  38   a  can be changed using a solenoid  45  to move the first movable plate  38   a  toward and away from the first fixed plate  38   b.  The first cut clamper  38  has a pair of clamping electrodes  38   c,    38   d  made of cermet. The electrode  38   d  is connected to the first fixed plate  38   b  by a screw  50   b  and the electrode  38   c  is connected to the first movable plate  38   a  by a screw  50   a.  Therefore, the electrodes  38   c,    38   d  can be rotated. Further, each electrode has a rectangular main surface, that is, preferably square. The main surfaces of the electrodes  38   c,    38   d  oppose each other in parallel relation. The gold bonding wire  32  is clamped between the main surfaces of these electrodes. A heater  49  is incorporated in the first movable plate  38   a  to heat the electrode  38   c.  Alternatively, the heater  49  may be incorporated in the first fixed plate  38   b  or heaters may be incorporated in both of the first fixed plate  38   b  and the first movable plate  38   a.  A spring  48  is connected between the first movable plate  38   a  and the shaft  41  to adjust the movement of the first movable plate  38   a.    
     Referring to FIG.  8 (IV), the second cut clamper  39  has a second fixed plate  39   b  that is fixed to a shaft  141 , and a second movable plate  39   a  that is fixed to a elastic shaft  143  that can expand and contract. The second fixed plate  39   b  and the second movable plate  39   a  oppose each other in parallel relation. The distance between the second fixed plate  39   b  and the second movable plate  39   a  can be changed by activating a solenoid  145 . As the result, the second movable plate  39   a  can be moved toward and away from the second fixed plate  39   b.  The second cut clamper  39  also has a pair of metal plate  39   c,    39   d,  preferably made of cermet. The metal plate  39   d  is connected to the second fixed plate  39   b  by a screw  150   b,  and the metal plate  39   c  is connected to the second movable plate  39   a  by a screw  150   a.  Therefore, the metal plates  39   c,    39   d  can be rotated. Further, each metal plate has a rectangular, preferably square, main surface that opposes the other in parallel relation. The gold bonding wire  32  is clamped between these main surfaces. A spring  148  is connected between the second movable plate  39   a  and the shaft  141  to adjust the movement of the movable plate  39   a.    
     A method of bonding the gold bonding wire with the above-described wire bonder  200  is explained below. 
     For connection of the pad on a semiconductor chip to the pad on a circuit board with the gold bonding wire, first, a gold ball  32   a  is formed at the tip of the gold bonding wire  32  as it exits the end of the capillary  36 . At this time, the first movable plate  38   a  is moved toward the first fixed plate  38   b  by the solenoid  45  so as to clamp the gold bonding wire  32  by the electrodes  38   c,    38   d.  The gold bonding wire  32  can be clamped anywhere on the surfaces of the electrodes  38   c,    38   d.  That is, it is not required to adjust the location where the gold bonding wire  32  is placed. Then, by applying a particular electrical potential between the electrodes  38   c,    38   d  and the torch rod  37 , a melted gold ball is formed by an atmospheric discharge against the gold bonding wire  32 . Then, the gold ball  32   a  is solidified as it cools down after the application of the voltage is halted. 
     After the gold ball  2   a  is solidified, the electrodes  38   c,    38   d  release the gold bonding wire  32  by moving the first movable plate  38   a  away from the first fixed plate  38   b.  Then, the tip of the capillary  36  presses the gold ball  32   a  against the pad on the circuit board by moving the transducer  35 . The gold ball  32   a  is compressed against the pad. Then, the capillary  36  is vibrated ultrasonically by the transducer  35  so that the ultrasonic vibration is transmitted to the gold ball  32   a.  Further, the electrode  38   c  is heated by the heater  49 , and the heat is transmitted to the gold ball  32   a.  Accordingly, the gold ball  32   a  at the end of the gold bonding wire  32  is bonded to the pad on the circuit board using ultrasonic thermocompression. 
     After the gold bonding wire  32  has been connected at its tip to the pad on the circuit board, the capillary  36  is moved to a position above the pad on the semiconductor chip. Then, the gold bonding wire  32  is pressed by the tip of capillary  36  against the pad, Then, the capillary  36  is vibrated ultrasonically by the transducer  35 , and the ultrasonic vibration is transmitted to the gold bonding wire  32 . Further, the electrode  38   c  is heated by the heater  20 , and the heat is transmitted to the gold bonding wire  32 . Accordingly, the gold bonding wire  32  is bonded to the pad on the semiconductor chip by ultrasonic thermocompression so that the pads are connected so as to make a looping with the gold bonding wire  32 . 
     After the connection has completed, the second movable plate  39   a  is moved toward the second fixed plate  39   b  by the solenoid  145  so as to clamp the gold bonding wire  32  by the metal plates  39   c,    39   d.  Then, by moving the second cut clamper  39  toward away from the pad on the chip, the gold bonding wire  32  is cut by tension. The electrodes  38   c,    38   d  or the metal plates  39   c,    39   d  are rotated 90 degrees by the screws  19   a,    19   b  if the main surfaces of the electrodes  38   c,    38   d  or the main surface of the metal plates  39   c,    39   d  are worn because of the repetition of the above-described operations. 
     According to the second embodiment of the invention, the following advantages can be obtained. In this second embodiment, the first cut clamper  38  is used for applying voltage to get the atmospheric discharge, and the second cut clamper  39  is used for cutting the gold bonding wire  32 . That is, as the first cut clamper  38  and the second cut clamper  39  can be changed by the purpose of the use, the stable looping connection can be performed and a gold ball which is consistently same size can be formed. 
     Abrasion on the main surface of the electrodes  38   b,    38   d  by the atmospheric discharge and abrasion on the main surface of the metal plates  39   b,    39   d  by slipping the gold bonding wire, have different proprieties. However, in this embodiment, as the first cut clamper  38  receives a influence of the atmospheric discharge only, and as the second cut clamper  39  receives a influence of the slipping the gold bonding wire, it is easy to control the maintenance. 
     Furthermore, the alternative clamping electrodes  29 ,  30  which are shown in FIG. 4 can be used as the metal plates in the wire bonder  200 . 
     In addition, it may be allowed that the first cut clamper  38  is disposed between the second cut clamper  39  and transducer  35 . 
     While the present invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrated embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as falls within the true scope of the invention.