Abstract:
A method of holding a curved circuit board in a flat fashion and a wire bonding apparatus thereof, including suction cavities provided in a suction stage to which a curved circuit board is suctioned. While evacuating air from the vacuum suction cavities, a capillary attached to the tip end of a bonding arm is lowered to press a part of the curved circuit board down thus having at least one of the vacuum suction cavities sealed by the circuit board, allowing the remaining vacuum suction cavities to be sealed successively so that the curved circuit board is straightened to be flat and suction-held on the suction stage. The vacuum suction cavities are sealed with by straightened circuit board, and the circuit board is held on the suction stage in place.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method of suctioning a circuit board in place on a suction stage of a wire bonding apparatus and to a wire bonding apparatus executing the method. 
         [0002]    A typical wire bonder that connects a circuit board and a semiconductor die mounted on the circuit board by a wire performs wire-bonding while suctioning a lead frame being transferred by vacuum and fixing the lead frame to the upper face of a suction stage. On the other hand, with recent demands for thinned semiconductor packages with improved functionalities and manufacturing efficiency, circuit boards are increasingly thinned and large-sized and multi-layer mounting of dies or so-called stacking is more and more commonly employed. However, a thinned circuit board often involves a curvature or warpage, resulting in a case in which the wire bonding cannot be performed because it is not possible to suction and fix such a circuit board to a suction stage by vacuum. 
         [0003]    In order to ensure suctioning and fixing of the curved circuit board provided with a semiconductor die to the suction stage, there is a suction fixation apparatus that moves a suction tube upward and downward. The suction tube has a vacuum suction hole therein and penetrates through the suction stage. This apparatus is such that, when the curved circuit board is mounted with the semiconductor die and transferred to the suction stage, the suction tube is raised so as to be in contact with the circuit board and to suction the circuit board with the vacuum suction hole, then the suction tube is lowered to suction and fix the circuit board with the vacuum suction hole of the substrate suction surface (for example, see Japanese Patent Application Unexamined Publication No. 2000-138253). 
         [0004]    Further, there is a method with which a bellow-type vacuum gripper is attached movably up and down through a penetrating hole in the suction stage so as to draw the circuit board to the substrate suction surface (for example, see Japanese Patent Application Unexamined Publication No. 2001-203222, which corresponds to European Patent Publication Nos. 1109205A1 and 1109207A1). 
         [0005]    In wire bonding, there is a case in which either the circuit board or the semiconductor die on the circuit board is required to be heated when connecting a wire to the circuit board. However, an arrangement for suctioning the circuit board of the conventional art shown in Japanese Patent Application Unexamined Publication No. 2000-138253 or 2001-203222 is provided with an up-down moving arrangement for moving up and down the suction tube and the vacuum gripper at a lower portion of the suction stage. Therefore, such an arrangement cannot be provided with a heating arrangement at the lower portion of the suction stage, and consequently cannot be used for a bonding apparatus that performs bonding and heating at the same time. 
         [0006]    For this reason, a method is proposed with which the circuit board is pressed down to the suction stage from an upper face of the circuit board instead of drawing the circuit board up on the suction stage from the bottom of the suction stage. 
         [0007]    For example, as shown in  FIG. 6 , there is a method with which the circuit board  15  is suctioned and fixed through a vacuum suction hole  27  by blowing air from an air nozzle  71  provided above the circuit board  15  to a curved circuit board  15  that is transferred on a suction stage  23  along a transfer guides  22  so that the circuit board  15  is pressed down to a substrate suction surface  23   a  of the suction stage  23  (for example, see Japanese Patent Application Unexamined Publication No. 2000-138253). 
         [0008]    Further, in the method shown in  FIGS. 7(   a ) through  7 ( c ) both ends of the circuit board  15  mounted with a semiconductor die  14  are gripped by gripper elements  73   a  and  73   b  of the grippers  73 , respectively, so that the circuit board  15  is pulled to both sides to be flattened, and then the grippers  73  are moved down to press the circuit board  15  to the suction stage  23 . Then, the grippers  73  are moved further down so that the circuit board  15  is pressed to the substrate suction surface  23   a  of the suction stage  23  to be suctioned and fixed with the vacuum suction hole  27  (for example, see Japanese Patent Application Unexamined Publication No. 2001-176915). 
         [0009]    However, in the conventional art shown in  FIG. 6 , there is a problem that a wire  12  connects the semiconductor die  14  and the circuit board  15  is bend by a wind pressure when air is blown from the upper face, and consequently that the wire  12  and the wire  12  are brought into contact with each other. Further, in the conventional art shown in  FIGS. 7(   a ) through  7 ( c ), there is a problem that the apparatus becomes large and complex because it is necessary to provide a large driving arrangement besides the suction stage  23  even though a gripper driving arrangement is not necessary at a lower portion of the suction stage  23 . 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    In view of the above noted problems, an object of the present invention is to provide a simple method and bonding apparatus capable of straightening a curved circuit board and suction-hold the circuit board in a flat fashion on a suction stage in an efficient manner. 
         [0011]    According to the present invention, the method of fixing a curved circuit board in a wire bonding apparatus comprises the steps of: providing a suction stage having at least one vacuum suction cavity on a substrate suction surface thereof that suctions a circuit board thereon; providing a wire bonding tool attached to a tip end of a bonding arm; and pressing down the curved circuit board using the wire bonding tool to the suction stage so that the circuit board seals the at least one vacuum suction cavity. 
         [0012]    Further, it is preferable that the method further includes a step of pressing down the curved circuit board using the wire bonding tool toward the at least one vacuum suction cavity of the suction stage. 
         [0013]    According to the present invention, the method of fixing a curved circuit board in a wire bonding apparatus comprises the steps of: providing a suction stage having at least one vacuum suction cavity on a substrate suction surface thereof that suctions a circuit board thereon; providing a wire bonding tool attached to a tip end of a bonding arm; a circuit board setting step for activating a vacuum apparatus after confirming the position of the curved circuit board that is transferred onto the suction stage; a ball formation step for forming a tip end of a wire that is inserted through the wire bonding tool into a ball after the circuit board setting step; a circuit board pressing position setting step for setting a position in an X-Y direction after the ball formation step, at which position the curved circuit board is to be pressed down; a bonding tool moving step for moving the wire bonding tool to the position in the X-Y direction that has been set in the circuit board pressing position setting step; a circuit board pressing step for pressing down the curved circuit board to the suction stage by moving the wire bonding tool downwardly after the bonding tool moving step; a vacuum suction state confirming step for confirming the state of vacuum suction after the circuit board pressing step; and a preliminary bonding step for performing preliminary bonding for the ball after the vacuum suction state confirming step. 
         [0014]    Further, it is preferable that the method further includes a circuit board pressing position setting changing step for changing the position in the X-Y direction at which the curved circuit board is pressed down when the state of the vacuum suction is not confirmed in the vacuum suction state confirming step. 
         [0015]    It is also preferable that the ball is for pressing down the curved circuit board, and the diameter of the ball is larger than a ball for normal wire bonding. 
         [0016]    According to the present invention, the wire bonding apparatus comprises a suction stage having at least one vacuum suction cavity on a substrate suction surface thereof that suctions a circuit board thereon; a wire bonding tool attached to a tip end of a bonding arm; and a computer for controlling the wire bonding apparatus, the computer including, a circuit board setting means for activating a vacuum apparatus after confirming a position of the curved circuit board that is transferred onto the suction stage; a ball formation means for forming the tip end of a wire that is inserted through the wire bonding tool into a ball; a circuit board pressing position setting means for setting a position in an X-Y direction, at which position the curved circuit board is pressed down; a bonding tool moving means for moving the wire bonding tool to the position in the X-Y direction that has been set by the circuit board pressing position setting means; a circuit board pressing means for pressing down the curved circuit board to the suction stage by moving the wire bonding tool downwardly; a vacuum suction state confirming means for confirming the state of vacuum suction; and a preliminary bonding means for performing preliminary bonding for the ball. 
         [0017]    Further, it is preferable that the computer further includes a circuit board pressing position setting changing means for changing the position in the X-Y direction at which the curved circuit board is pressed down when the state of the vacuum suction is not confirmed by the vacuum suction state confirming means. 
         [0018]    It is also preferable that the ball is for pressing down the curved circuit board, and the diameter of the ball is larger than a ball used for normal wire bonding. 
         [0019]    The present invention provides an advantageous effect that a curved circuit board can be efficiently fixed to a suction stage using a simple method and bonding apparatus. 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0020]      FIG. 1  is a diagram showing a system structure of a wire bonder to which a method of fixing a curved circuit board and a program therefor according to the present invention are applied; 
           [0021]      FIG. 2  is a top plan view showing the upper face of a suction stage of the wire bonder shown in  FIG. 1 ; 
           [0022]      FIG. 3  is a cross-sectional view of the suction stage of the wire bonder shown in  FIG. 1 ; 
           [0023]      FIG. 4  is a flowchart of an embodiment according to the present invention; 
           [0024]      FIG. 5   a  is an illustrative view showing a state in which the curved circuit board is set; 
           [0025]      FIG. 5   b  is an illustrative view showing a state in which the capillary is moved to a position at which the curved circuit board is pressed downward; 
           [0026]      FIG. 5   c  is an illustrative view showing a state in which the capillary starts pressing the curved circuit board downward; 
           [0027]      FIG. 5   d  is an illustrative view showing a state of the curved circuit board when the capillary reaches a position at which the downward movement stops; 
           [0028]      FIG. 5   e  is an illustrative view showing a state in which the curved circuit board is suctioned and fixed; 
           [0029]      FIG. 6  is a diagram illustrating a method of fixing a circuit board in conventional art; and 
           [0030]      FIGS. 7(   a ) through  7 ( c ) are diagrams illustrating another method of fixing a circuit board in conventional art. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Hereinafter, preferred embodiments of the present invention will be concretely described with reference to the accompanying drawings.  FIG. 1  is a diagram showing a system structure of a wire bonder to which a method of fixing a curved circuit board and a program therefor according to the embodiment of the present invention are applied.  FIG. 2  is a top plan view showing the upper face of a suction stage of the wire bonder.  FIG. 3  is a cross-sectional view of the suction stage of the wire bonder taken along the centerline  35  in an X-direction shown in  FIG. 2 . 
         [0032]    As shown in  FIG. 1 , a wire bonder  10  is structured such that a bonding head  19  is disposed on an X-Y table  20 , and the bonding head  19  is provided with a bonding arm  13  whose tip end is driven by a motor in a Z direction that is an up-down direction. A capillary  16  which is a bonding tool is attached to the bonding arm  13 . The X-Y table  20  and the bonding head  19  structures a transfer device  18 , which is able to move the bonding head  19  to any given positions within a horizontal plane (X-Y plane) using the X-Y table  20 . The transfer device  18  is further able to freely move the capillary  16  attached to the tip end of the bonding arm  13  in the X, Y, and Z directions when the bonding arm  13  that is attached to the transfer device  18  is activated. 
         [0033]    The X-Y table  20  is provided with an X-Y position detecting unit  31  that detects a position of the tip end of the capillary  16  in the X-Y direction. This X-Y position detecting unit  31  detects an X-Y coordinate position of a predetermined portion of the bonding head  19  and corrects the distance between the predetermined portion and the tip end of the capillary  16  in the X-Y direction, thereby detecting an X-Y position of the tip end of the capillary  16 . The X-Y position detecting unit  31  can be of a noncontact type such as electrical or optical, or of a contact type such as mechanical. Further, the X-Y position detecting unit  31  can be an X-Y position sensor that can directory measure the position of the tip end of the capillary  16  without correcting the measured value of the X-Y position of the predetermined position of the bonding head  19 , if it is possible to measure the X-Y position of the tip end of the capillary  16 . Moreover, the X-Y position detecting unit  31  can be a linear scale. 
         [0034]    The bonding head  19  is provided with a load sensor  28  that detects a load on the tip end of the capillary  16 . 
         [0035]    A wire  12  is inserted through the capillary  16  at the tip of the bonding arm  13 . The wire  12  is wound around a spool  11 . The wire  12  that is wound around the spool  11  is connected to a conducting state obtaining unit  38  that obtains a conducting state either between the wire  12  and a semiconductor die  14 , or between the wire  12  and a circuit board  15 . 
         [0036]    In the vicinity of the tip end of the capillary  16 , a ball formation unit  17  is provided for forming a tip end of the wire  12  into a ball. The ball formation unit  17  can be an electric flame off probe that causes a discharge between ball formation unit  17  and the wire  12  to form a ball, or can be a gas torch. 
         [0037]    Furthermore, the bonding head  19  is attached with an imaging unit  21  that images the capillary  16 , the semiconductor die  14 , and the circuit board  15 . 
         [0038]    Below the capillary  16 , a suction stage  23  for suctioning and fixing the circuit board  15  mounted with the semiconductor die  14  is attached to a wire bonder frame that is not shown in the drawing. 
         [0039]    Transfer guides  22  that guide the circuit board  15  are fixed to both sides of the suction stage  23 , respectively, and the circuit board positioning clamping device  30  that holds so as to sandwich the circuit board  15  and move to a predetermined position is attached in the vicinity of the transfer guides  22 . The suction stage  23  is provided with a plurality of vacuum suction cavities  24 , which are evacuated when the upper face of the suction stage  23  is sealed by the circuit board  15 , and a vacuum suction hole  27  is opened at a lower portion of each vacuum suction cavity  24 . The vacuum suction hole  27  is connected to a vacuum apparatus  26  via a vacuum plumbing  33 . 
         [0040]    A pressure sensor  29  that detects the pressure in the vacuum suction cavities  24  is provided in the vicinity of the suction stage  23 . The pressure sensor  29  can consecutively output a signal of a measured pressure. Alternatively, the pressure sensor  29  can be a pressure switch that outputs a signal when the pressure reaches a predetermined level. A plurality of pressure sensors  29  can be provided so that each pressure sensor  29  detects the pressure of each vacuum suction cavity  24 , or a single pressure sensor  29  can be provided at the vacuum suction hole  27  under the vacuum suction cavities  24 . Further, the pressure sensor  29  can be provided at one or more portions of the vacuum plumbing  33  that connects the vacuum apparatus  26  and the vacuum suction hole  27 . 
         [0041]    In addition, a heat block  25  that heats the circuit board  15  mounted with the semiconductor die  14  that is suctioned by vacuum is attached to the lower portion the suction stage  23 . 
         [0042]    The transfer device  18  is connected to a transfer device interface  59 , the ball formation unit  17  is connected to a ball formation unit interface  61 , the conducting state obtaining unit  38  is connected to a conducting state obtaining unit interface  56 , and the X-Y position detecting unit  31  is connected to an X-Y position detecting unit interface  60 . In addition, the load sensor  28  is connected to a load sensor interface  57 , the imaging unit  21  is connected to an imaging unit interface  55 , the circuit board positioning clamping device  30  is connected to a circuit board positioning clamping device interface  53 , and the pressure sensor  29  is connected to a pressure sensor interface  51 . Furthermore, the vacuum apparatus  26  is connected to a vacuum apparatus interface  47 , and the heat block  25  is connected to a heat block interface  49 . 
         [0043]    Each one of the above-described interfaces is connected through a data bus  43  to a control unit  41  of a computer  40  for controlling bonding actions. The above-described interfaces are respectively component parts of the computer  40 . The control unit  41  of the computer  40  includes a CPU for controlling the bonding action. Further, to the data bus  43  is connected a memory unit  45  for storing control data and programs including a circuit board setting program, a ball formation program, a circuit board pressing position setting program, a bonding tool moving program, a circuit board pressing program, a vacuum suction state confirming program, preliminary bonding program, and a circuit board pressing position setting changing program. 
         [0044]    Referring to  FIG. 2  and  FIG. 3 , the suction stage  23  and related portions of the wire bonder  10  will be described below in detail. In the following description, as shown in  FIG. 2 , a direction across the horizontal plane along which the circuit board is carried is referred to as a Y direction, and a direction perpendicular to the direction along which the circuit board is carried is referred to as an X direction. 
         [0045]    As shown in  FIG. 2 , the suction stage  23  is a flat rectangular plate that is disposed between the transfer guides  22 . The suction stage  23  has a plurality of vacuum suction cavities  24   a - 24   e  lined up along the X-direction centerline  35  in the middle of a substrate suction surface  23   a  of the suction stage  23 . Each of the vacuum suction cavities  24   a - 24   e  is structured by a pair of grooves with a rectangular vertical cross-section that cross with each other such that the grooves form substantially 45 degrees with the X-direction center line  35  and the Y-direction center line  36 , respectively, so that the cross-section of each cavity taken along the horizontal plane is in an X shape. Vacuum suction holes  27   a - 27   e  respectively open up at center portions of the X-shaped vacuum suction cavities  24   a - 24   e.    
         [0046]    Each groove in each of the X-shaped cavities  24   a - 24   e  is slightly shorter than a diagonal line of the semiconductor die  14  mounted to the circuit board  15 . The shape of the vertical section of the grooves of the vacuum suction cavities  24   a - 24   e  is not limited to the rectangular shape, and can be any other shape such as a semicircle. Moreover, the horizontal cross-section is not limited to the X shape as mentioned above, and can be any other shape such as either a cross shape with the grooves forming an angle other than 45 degrees to the X-direction center line  35  and the Y-direction center line  36 , respectively, or a star shape having more than two grooves, as long as the grooves extend radially from the center portion of each of the vacuum suction holes  27   a - 27   e  so that the circuit board  15  can be efficiently suctioned. 
         [0047]    In bonding, the position of the circuit board  15  is set so that the center of the semiconductor die  14  mounted to the circuit board  15  is on the X-direction center line  35 . This position of the circuit board  15  is referred to as a bonding position. 
         [0048]      FIG. 3  is a cross-sectional view of the suction stage  23  taken along the X-direction centerline  35  as shown in  FIG. 2 , and each of the vacuum suction cavities  24   a - 24   e  is shown in a cross-section in the lengthwise direction of the groove. 
         [0049]    As shown in  FIG. 3 , the suction stage  23  is overlaid on and fixed to the heat block  25  that is fixed to a base  34 . Each of the vacuum suction holes  27   a - 27   e  at the center portion of each of the vacuum suction cavities  24   a - 24   e  is connected, within the suction stage  23 , to a header, and guided from the suction stage  23  to the lower portion of the base  34 . The vacuum suction holes  27   a - 27   e  are connected to the vacuum plumbing  33  at the lower portion of the base  34 . Instead of structuring a header within the suction stage  23 , each of the vacuum suction holes  27   a - 27   e  can be structured so as to penetrate to the lower portion of the suction stage  23  and the vacuum plumbing  33  is connected to each of the penetrating portions and bound up into a single header connected to the vacuum apparatus  26 . 
         [0050]    Next, the operation in the method of fixing a curved circuit board and the program therefor will be described with reference to  FIG. 4  and  FIGS. 5   a  to  5   e .  FIG. 4  is a flowchart showing the operation in this embodiment, and  FIGS. 5   a  to  5   e  are illustrative views showing the operational states. 
         [0051]    As shown in  FIG. 5   a , when the wire bonder  10  is activated, the curved circuit board  15  on which the semiconductor dies  14  are mounted is guided by the transfer guides  22  at both sides of the suction stage  23 , and transferred to the suction stage  23 . Because the circuit board  15  curves to a large degree as being a thin substrate, there is a gap between the circuit board  15  and the substrate suction surface  23   a  of the suction stage  23 . Further, the bonding arm  13  provided with the capillary  16  at the tip end stands by at a standby position that is not on a transfer path of the circuit board  15 , with the capillary  16  at a rising position. In this embodiment, the standby position of the bonding arm  13  is a position at which the capillary  16  at the tip end comes outside the area between the transfer guides  22 . However, this standby position can be within the area between the two transfer guides  22 , as long as the capillary  16  at the tip end at the standby position does not interfere the curved circuit board  15  that is being transferred. Moreover, when the wire bonder  10  is activated, the control unit  41  of the computer  40  outputs an instruction for starting up the heat block to the heat block interface  49 , which then activates a heat source  25   a  to start heating the heat block  25  according to this instruction. 
         [0052]    In Step S 101  in  FIG. 4 , when the circuit board  15  is being transferred by the circuit board positioning clamping device  30 , whether or not the circuit board  15  is carried to a predetermined position by the circuit board positioning clamping device  30  is detected, and a detection signal of the detection is converted by the circuit board positioning clamping device interface  53  into a signal that can be inputted to the control unit  41  of the computer  40 , and then inputted to the control unit  41 . The control unit  41  detects whether or not the circuit board  15  has been reached the bonding position as shown in  FIG. 5   b , based on this signal using the imaging unit  21 . 
         [0053]    When the circuit board  15  reaches the bonding position, the circuit board positioning clamping device  30  transmits a bonding position reaching signal, which is inputted to the control unit  41  from the circuit board positioning clamping device interface  53  of the computer  40 . In the next Step S 102  in  FIG. 4 , the control unit  41  determines based on the input of the signal that the circuit board  15  has reached the predetermined bonding position, and outputs an instruction to stop the transfer operation to a transfer apparatus that is not depicted in the drawing. Then, the circuit board  15  stops at the bonding position. 
         [0054]    Next, the control unit  41  of the computer  40  activates the vacuum apparatus  26  in Step S 103 . Because there is a gap between the circuit board  15  transferred to the bonding position and the substrate suction surface  23   a  as shown in  FIG. 5   a , the pressure inside each one of the vacuum suction cavities  24   a - 24   e  is not brought to vacuum even after the air in each of the vacuum suction holes  27   a - 27   e  is evacuated by the vacuum apparatus  26 , and it is not possible to suction and fix the circuit board  15 . Consequently, the gap between the circuit board  15  and the substrate suction surface  23   a  remains the same. 
         [0055]    In the next Step S 104  in  FIG. 4 , the control unit  41  of the computer  40  outputs a signal for ball formation to the ball formation unit interface  61 . In response to the ball formation signal, the ball formation unit interface  61  causes a discharge between the wire  12  lead out at the tip end of the capillary  16  and the ball formation unit  17  to form a ball  37  as shown in  FIG. 5   a . The formation of the ball  37  is for a smooth shift to a bonding step after the circuit board  15  is suctioned and fixed in place. Further, in forming the ball  37 , it is also preferable to cause the discharge with greater power than in forming a common ball for bonding to form the ball  37  for pressing down that is larger than the common ball for bonding. The formation of a larger ball increases an area that presses down the curved circuit board  15 , allowing the ball to press the circuit board  15  down more efficiently. 
         [0056]    In the next Step S 105  in  FIG. 4 , the control unit  41  of the computer  40  sets a most suitable pressing position of the circuit board  15  in an X-Y direction based on data of the circuit board  15  that has been inputted into the memory unit  45  in advance. In this embodiment, the pressing position is set to an end portion of the semiconductor die  14  disposed at the center of the circuit board  15 . This is because the position in the vicinity of the center of the circuit board  15  is appropriate to press the entirety of the circuit board down to the suction stage  23  evenly, and because no semiconductor circuit element is provided at the end portion of the semiconductor die  14  so that the pressing down can not cause any damage to the semiconductor circuit element of the die. Further, the pressing position can be any position on the circuit board  15  at which the semiconductor die  14  is not disposed as long as one of the vacuum suction cavities  24   a - 24   e  can be sealed. In addition, this pressing position can be immediately above one of the vacuum suction holes  27   a - 27   c , depending on the type of the semiconductor die  14  and the type of the circuit board  15 . This immediately-above position is a position at which the circuit board  15  can be pressed most directly down to a the center of each of the vacuum suction cavities  24   a - 24   e  than at other positions, and the circuit board  15  can be more effectively suctioned. 
         [0057]    Once the pressing position of the circuit board  15  is set, in Step S 106 , the control unit  41  of the computer  40  outputs, to the transfer device interface  59 , an instruction for moving the position of the tip end of the capillary  16  toward the set position in the X-Y direction. As shown in  FIG. 5   b , the transfer device interface  59  (see  FIG. 1 ) drives the X-Y table  20  based on this instruction, and starts moving the bonding head  19  so that the capillary  16  at the tip end of the bonding arm  13  comes to the pressing position that has been set in the X-Y direction. The control unit  41  obtains the detection signal from the X-Y position detecting unit  31  through the X-Y position detecting unit interface  60  in Step S 107 , and monitors the difference between the position of the tip end of the capillary  16  and the pressing position. Further, the difference between the pressing position and an instructed value computed by the computer can be monitored by taking images of the capillary  16 , the semiconductor die  14 , and the circuit board  15  using the imaging unit  21 , inputting the image data to the control unit  41  through the imaging unit interface  55 , and obtaining the tip end position of the capillary  16  based on image processing using the control unit  41 . Then, when the control unit  41  determines that the difference has crossed over a threshold value, the control unit  41  inputs an instruction to stop the movement of the tip end position of the capillary  16  to the transfer device interface  59  in Step S 108 . In response to the instruction, the transfer device interface  59  stops the movement of the bonding head  19 , and consequently stops the movement of the tip end position of the capillary  16  in the X-Y direction. When the movement of the bonding head  19  stops, the tip end position of the capillary  16  is at the pressing position which is at the end portion of the semiconductor die  14  disposed at the center of the circuit board  15  as shown in  FIG. 5   b , and the tip end of the capillary  16  is at a raised position away from the circuit board  15  or the semiconductor die  14 . 
         [0058]    In the next Step S 109  in  FIG. 4 , the control unit  41  of the computer  40  outputs, to the transfer device interface  59 , an instruction for moving the tip end of the capillary  16  downward. Based on this instruction, the transfer device interface  59  drives a motor, which is for driving the bonding arm  13  and is provided in the bonding head  19 , to output a signal for moving the tip end of the capillary  16  downward. Then, the motor of the bonding head  19  is driven and the bonding arm  13  starts to rotate downwardly. A signal for detecting the load at the tip end of the capillary by the load sensor  28  is inputted from the load sensor interface  57  to the control unit  41 . The control unit  41  monitors the difference between the signal and a predetermined grounding load of the capillary in Step S 110 . The grounding load is a load detected when the tip end of the capillary  16  grounds (or touches the pressing position), which is smaller than a pressure-bonding load of a wire in normal bonding so that the ball  37  at the tip end of the capillary is prevented from being pressure-bonded to the semiconductor die  14  or the circuit board  15  by pressing down the circuit board  15 . 
         [0059]    As shown in  FIG. 5   c , the tip end of the capillary  16  starts to move down to the curved circuit board  15  by the downward movement of the bonding arm  13 . Then, the ball  37  formed at the wire at the tip end of the capillary is brought into contact with the semiconductor die  14 . In this state, there is a gap between the circuit board  15  and the substrate suction surface  23   a , and air flows through the gap to the vacuum apparatus  26  (see  FIG. 1 ), and accordingly, any of the vacuum suction cavities  24   a - 24   e  is not sealed, and the circuit board  15  cannot be suctioned by vacuum. 
         [0060]    During the downward movement of the bonding arm  13 , the control unit  41  of the computer  40  (see  FIG. 1 ) continues monitoring whether or not the load at the tip end of the capillary is equal to or more than the predetermined grounding load based on the signal inputted from the load sensor  28 . Then, when the difference between the input signal from the load sensor  28  and the grounding load crosses over the predetermined threshold value, the control unit  41  determines that the capillary  16  is grounded and outputs an instruction for stopping the downward movement of the capillary  16  to the transfer device interface  59  in Step S 111  in  FIG. 4 . According to this instruction, the transfer device  18  stops the downward movement of the bonding arm  13  of the bonding head  19 , to stop the downward movement of the capillary  16 . Alternatively, the detection as to whether or not the capillary  16  is grounded can be made by detecting conducting current using the conducting state obtaining unit  38 . In this case, when the capillary  16  is grounded, a current for detecting the conducting state flows through the wire  12 . The conducting current from the wire  12  to the semiconductor die  14  or the circuit board  15  is detected by the conducting state obtaining unit  38 , and the detected signal is inputted to the control unit  41  from the conducting state obtaining unit interface  56 . Based on the signal input, the control unit  41  detects the grounding of the capillary  16  to stop the downward movement of the capillary  16 . The conducting state obtaining unit  38  can be a direct type that detects changes in a direct current between the wire  12  and the semiconductor die  14  or the circuit board  15 , or can be an alternating type that detects changes in an alternating current. 
         [0061]    After the above-described process, as shown in  FIG. 5   d , by the downward movement of the capillary  16 , the central portion of the circuit board  15  is pressed down to the substrate suction surface  23   a  of the suction stage  23 , the portion of the circuit board  15  below the semiconductor die  14  in the center of the circuit board  15  is brought into contact with the substrate suction surface  23   a , and the circuit board  15  covers the upper face of the vacuum suction cavity  24   c . In this situation, a portion of the air flow path from the vacuum suction cavities  24   a - 24   e  to the vacuum apparatus  26  is blocked, and an amount of air flow to the vacuum apparatus  26  decreases. Consequently, the pressure in all the vacuum suction cavities  24   a - 24   e  is reduced. Then, when the pressure in all the vacuum suction cavities  24   a - 24   e  becomes lower than the atmosphere pressure, the circuit board  15  starts to be pressed to the suction stage  23  by the atmosphere pressure. This causes the lower face of the circuit board  15  to closely attach in the vicinity of the vacuum suction cavity  24   c  where the gap between the lower face of the circuit board  15  and the substrate suction surface  23   a  is the smallest, thereby sealing the upper face of the vacuum suction cavity  24   c . By this, the central portion of the circuit board  15  is suctioned to the substrate suction surface  23   a . As shown in  FIG. 5   d , the circuit board  15  above the vacuum suction cavities  24   a ,  24   b ,  24   d , and  24   e  is not closely attached to the substrate suction surface  23   a . However, because the vacuum suction cavity  24   c  is closed (substantially) entirely, the amount of air flow evacuated by the vacuum apparatus  26  is further decreases, and the pressure in the entire vacuum suction cavities  24   a - 24   e  is further reduced. By this, the circuit board  15  over the vacuum suction cavities  24   a ,  24   b ,  24   d , and  24   e  is pressed even more strongly to the suction stage  23  by the atmosphere pressure. 
         [0062]    Then, when the circuit board  15  seals the upper face of any one of the vacuum suction cavities  24   a ,  24   b ,  24   d , and  24   e , the air does not flow from the vacuum suction cavities to the vacuum apparatus  26 , resulting in further reduction in the pressure in the entire vacuum suction cavities  24   a - 24   e  to increase the difference from the atmosphere pressure. As the pressure difference increases, the strength to press the circuit board  15  down also increases, and the vacuum suction cavities  24   a - 24   e  are sealed by the circuit board  15  one by one. When all of the vacuum suction cavities  24   a - 24   e  are sealed by the circuit board  15 , the circuit board  15  is completely suctioned to the suction stage by vacuum. As seen from the above, when one of the vacuum suction cavities is sealed, the pressure in the entire vacuum suction cavities is reduced in a chain reaction, and together with the pressure difference from the atmosphere pressure, the circuit board  15  is rapidly suctioned to the substrate suction surface. When all of the vacuum suction cavities  24   a - 24   e  are sealed by the circuit board  15 , the circuit board  15  is suctioned and fixed to the substrate suction surface  23   a  as shown in  FIG. 5   e , and the pressures in each of the vacuum suction cavities  24   a - 24   e , in each of the vacuum suction holes  27   a - 27   e , and in the vacuum plumbing  33  are brought into substantially vacuum. 
         [0063]    In Step S 112 , the control unit  41  of the computer  40  (see  FIG. 1 ) obtains a pressure measured by the pressure sensor  29  as a pressure signal from the pressure sensor interface  51 , and determines whether or not the difference between the pressure and a predetermined vacuum pressure has crossed over a predetermined threshold value. Then, when the difference crosses over the predetermined threshold value, the control unit  41  determines that all of the vacuum suction cavities  24   a - 24   e  are brought to vacuum and the circuit board  15  is suctioned and fixed on the suction stage  23 . 
         [0064]    After confirming the vacuum suction state of the circuit board  15 , the control unit  41  of the computer  40  outputs an instruction to the transfer device interface  59  for raising and moving the capillary  16  to a preliminary bonding position, such as at the circuit board  15  or an end portion of the circuit board, at which wire bonding cannot be interfered. The transfer device  18  drives the motor for driving the bonding arm  13  to raise the bonding arm  13 , and then moves the tip end of the capillary by the X-Y table to the preliminary bonding position of the circuit board  15  according to the instruction. When the tip end of the capillary reaches the preliminary bonding position, the control unit  41  moves the capillary  16  downwardly to perform bonding at the preliminary bonding position. In this case, as in the common bonding, the ball  37  formed at the tip end of the capillary  16  is pressure-bonded to the preliminary bonding position. Once the ball  37  is pressure-bonded by the capillary  16 , the current for detecting the conducting state flows through the wire  12 . The conducting current is detected by the conducting state obtaining unit  38 , and a signal of the current is inputted to the control unit  41  through the conducting state obtaining unit interface  56 . The control unit  41  determines that the ball  37  is pressure-bonded based on the input of the signal. Then, the control unit  41  raises the capillary  16  to cut the tail wire. After cutting the tail wire, the current for detecting the conducting state that has been flowed through the wire  12  stops flowing. The state in which the conducting current stops is detected by the conducting state obtaining unit  38 , and a signal for the state is inputted to the control unit  41  through the conducting state obtaining unit interface  56 . The control unit  41  determines that the wire  12  is normally cut based on the signal. Then, the control unit  41  determines that the preliminary bonding has been performed normally when the signal for the conductive state based on the pressure-bonding of the ball  37  and the signal based on the absence of the conducting current by the cutting of the wire are both inputted. When the preliminary bonding is normally completed in the preliminary bonding step as described above, the control unit  41  finishes the fixation program of the curved circuit board. When there is an error in the preliminary bonding in the preliminary bonding step, the control unit  41  performs an error stop process and deactivates the wire bonding apparatus in Step S 116  in  FIG. 4 . 
         [0065]    On the other hand, when the pressure difference as described above crosses over the predetermined threshold value, the control unit  41  of the computer  40  determines that any of the vacuum suction cavities remains unsealed and is drawing the air, and the circuit board  15  is not fully suctioned and fixed. Then, when the vacuum suction state of the circuit board  15  cannot be confirmed, the control unit  41  repeats the press down movement of the circuit board  15  in Step S 112 . 
         [0066]    In Step S 115  in  FIG. 4 , the control unit  41  of the computer  40  outputs an instruction to the transfer device interface  59  for resetting the height of the capillary to an ascending position. According to this instruction, the transfer device interface  59  drives the motor for the bonding arm  13  to raise the bonding arm  13 , and resets the position of the tip end of the capillary  16  to the ascending position as an initial state. Then, the control unit  41  changes the setting of the pressing position of the circuit board in the X-Y direction in Step S 105 . The position can be set to an end surface of the semiconductor die  14  adjacent to the previous pressing position. Further, more than one pressure sensor  29  can be attached to the vacuum suction cavities  24   a - 24   e , respectively, so that a position in the vicinity of a vacuum suction cavity with the largest pressure is pressed down. 
         [0067]    Next, upon completion of the setting of the pressing position, the control unit  41  of the computer  40  again moves the tip end of the capillary  16  to the pressing position by the transfer device  18  in Step S 106 , and repeats the press down movement of Steps S 109  to S 111 . Then, in Step S 112 , when the vacuum state of the vacuum suction cavities is confirmed, the control unit  41  determines that the circuit board  15  is suctioned to the suction stage  23 . After this, the preliminary bonding step starts in Step S 113 , and when the preliminary bonding is completed normally, the control unit  41  ends the fixation program of the curved circuit board in Step S 114 . 
         [0068]    When the circuit board  15  is suctioned and fixed to the suction stage  23 , the circuit board  15  and the semiconductor die  14  are heated by the heat block  25  that is provided below the suction stage  23  and has already been heated up to a temperature at which the heating is possible, so that wire-bonding can be performed. The control unit  41  of the computer  40  runs a bonding program and performs the wire bonding between the semiconductor die  14  and the circuit board  15 . When the bonding of the semiconductor dies  14  that are lined up along the X-direction center line  35  is completed, the vacuum is released to release the vacuum suction fixation of the circuit board  15 . Then, the circuit board  15  is transferred till the next row of the semiconductor dies  14  comes to the X-direction center line  35 , and the suction operation of the circuit board  15  is repeated in the same manner as described above. 
         [0069]    In the bonding apparatus of the present invention describe above, the pair of transfer guides  22  are operated so as to move toward each other and away from each other so that the both side edges of the curved circuit board and straightened circuit board can be snugly held and carried smoothly thereby. In the process shown from  FIG. 5   a  to  FIG. 5   e , the transfer guides  22  are gradually moved away from each other as the process proceeds. 
         [0070]    As described above, according to this embodiment, the curved circuit board  15  can be advantageously suctioned and fixed to the suction stage  23  by vacuum in a simple and efficient manner only by changing the control program without adding any special arrangement to the wire bonder  10  of a common structure. Further, because there is no need to provide a movable arrangement below the suction stage  23 , it is possible to perform suction and heating of the circuit board at the same time, thereby advantageously improving the bonding efficiency.