Abstract:
A method of applying a bonding paste efficiently and uniformly by discharging the bonding paste from an application nozzle is presented. For applying the paste along a drawing pattern including a cross shape pattern composed of crossing plural application lines, the nozzle moves from the center of the cross shape to an end point along one application line, turns to a reverse direction at the end point, and moves to an end point at opposite side of the application line. Then, the nozzle returns from the end point to the center of the cross shape. The nozzle repeats this unit application line drawing operation for each application line. As a result, the nozzle stops less frequently near the center, has a shortened cycle time, and applies the paste uniformly.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of applying a bonding paste to a substrate. 
     BACKGROUND OF THE INVENTION 
     In a die bonding for bonding chips to a substrate such as a lead frame, a paste for bonding is applied on the substrate. A dispenser moves an application nozzle from which a paste is discharged, and applies the paste in an application area on the substrate. In this method, a moving route of the application nozzle from the application start point to the application terminating point in the application area is set. The area is determined according to the shape of the chip. The moving route is defined in various shapes depending on a pattern of the route. For example, the paste is applied in a rectangular area, for example, along a cross-shape where the application nozzle moves in an X-figure, or along an asterisk-shape where a plus (+)-figure is overlapped on the X-figure. 
     A conventional application method will be explained below with reference to FIG.  4 . 
     FIG. 4 shows a conventional application track of the paste. Within an application area A, a drawing pattern has an asterisk-shape. The application nozzle starts applying the paste from the drawing center C toward the outside, and then moves radially while commuting between turning points T 1 , T 2 , T 3 , . . . set near the center C, and end points P 1 , P 2 , P 3 , . . . set respectively at ends of application lines. The nozzle moves along all application lines, and applies the paste along the specified asterisk shape. 
     However, moving the application nozzle along the complicated application pattern such as the asterisk shape by a single stroke has the following problems. Whenever moving back and forth between the drawing center and each end, the nozzle must once stop at the turning points T 1 , T 2 , T 3 , . . . set near the center in order to change the direction. Accordingly, the application nozzle must decelerate and accelerate very frequently, such that the cycle time of the entire application operation is extended and the application efficiency is lowered. Furthermore, an average speed near the drawing center is lower than that in other areas, and the nozzle accordingly stays near the center for a longer time. As a result, the paste is applied around the center more and thus is not applied uniformly. 
     SUMMARY OF THE INVENTION 
     An application method for applying a bonding paste according to the present invention has an enhanced application efficiency and applies the paste uniformly. 
     The method of applying a bonding paste on a substrate uses an application nozzle moving and discharging the paste along a drawing pattern having a cross shape where plural application lines cross. The method includes: 
     (a) a step of moving the application nozzle from the application start point set near the center of the cross shape to a first end point on a first application line of the plural application lines, 
     (b) a step of moving the application nozzle from the first end point to a second end point of the first application line along the first application line, 
     (c) a step of moving the application nozzle from the second end point to a first application terminating point set near the central position of the first application line along the first application line, 
     (d) a step of moving the application nozzle from the first application terminating point to a third end point on a second application line of the plural application lines along the second application line, 
     (e) a step of moving the application nozzle from the third end point to a fourth end point of the second application line along the second application line, and 
     (f) a step of moving the application nozzle from the fourth end point to a second application terminating point set near the central position of the second application line along the second application line. 
     In this method, the application nozzle stops near the center less frequently, has a shortened cycle time, and applies the paste uniformly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a die bonding apparatus in an exemplary embodiment of the present invention. 
     FIG. 2 is a block diagram of a control system of the die bonding apparatus in the embodiment. 
     FIG. 3 shows a drawing pattern according to the embodiment. 
     FIG. 4 shows a conventional application track of a paste. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a perspective view of a die bonding apparatus in an exemplary embodiment of the present invention. FIG. 2 is a block diagram of a control system of the die bonding apparatus. FIG. 3 shows a drawing pattern of the die bonding apparatus. 
     In FIG. 1, in a chip feeder  1 , a wafer sheet  2  is held by a table (not shown). Multiple chips  3 , i.e., semiconductor devices, are adhered on the wafer sheet  2 . At the side of the chip feeder  1 , a conveying route  5  is provided for conveying a lead frame  6 . In the conveying route  5 , the lead frame  6  is positioned at a paste application position and a bonding position. Above the chip feeder  1 , a bonding head  4  is disposed, and the head  4  is moved in horizontal and vertical directions by a moving mechanism (not shown). 
     At the other side of the conveying route  5 , a paste application unit  9  is disposed. The application unit  9  includes a moving table  10  and a paste dispenser. The dispenser including a syringe  15  having an application nozzle  15   a  is mounted to the table  10 . The moving table  10  includes a Y-axis table  11 , an X-axis table  12  stacked up on the table  11 , and a Z-axis table  14  coupled on the X-axis table through an L-shaped bracket  13  to be disposed in the vertical direction. The Y-axis table  11 , X-axis table  12 , and Z-axis table  14  have a Y-axis motor  11   a , an X-axis motor  12   a , and a Z-axis motor  14   a , respectively. 
     By driving the X-axis motor  12   a , Y-axis motor  11   a , and Z-axis motor  14   a , the syringe  15  moves in the horizontal and vertical directions over the lead frame  6 . The syringe  15  stores a paste  7  for adhering the chips  3  on the lead frame  6 . The syringe  15 , into which air pressure is introduced, has a valve  15   b  (see FIG.  2 ), which opens and closes the application nozzle  15   a , and when opened, discharges the paste from an outlet of the application nozzle  15   a.    
     The application nozzle  15   a  has its outlet positioned in the application area  6   a , and moves while discharging the paste. As a result, the paste  7  is applied in an X-shaped application pattern in the application area  6   a  set on the lead frame  6  constituting an application object. The syringe  15 , application nozzle  15   a , and an air pressure applying unit for applying an air pressure to the syringe  15  compose a paste discharge section. The moving table  10  constitutes a moving section for moving the outlet of the application nozzle  15   a.    
     Then, the lead frame  6  is sent to a bonding position  8  on the conveying route  5 , and is positioned. On the paste  7  applied in the application area  6   a , the chip  3  picked up from the chip feeder  1  is bonded by a nozzle  4   a  of the bonding head  4 . 
     Referring to FIG. 2, the control system of the die bonding apparatus will be explained. In FIG. 2, air supplied from an air source  20  is fed into the syringe  15  through a regulator  21 . The regulator  21 , which is controlled by a controller  36 , adjusts a pressure of the air fed into the syringe  15 , and controls the discharge amount of the paste discharged from the application nozzle  15   a . A discharge control valve driver  31  drives the valve  15   b  for opening and closing the application nozzle  15   a . The discharge control valve driver  31  controlled by the controller  36  can have the application nozzle  15   a  start and stop discharging the paste. Instead of the controller  36  controlling the pressure in the regulator  21 , the regulator  21  may be controlled by a manual operation to discharge a desired amount of paste. 
     An X-axis motor driver  34 , a Y-axis motor driver  33 , and a Z-axis motor driver  32  drive the X-axis motor  12   a , Y-axis motor  11   a , and Z-axis motor  14   a  of the moving table  10 , respectively. The controller  36 , as a control section, controls the X-axis motor driver  34 , Y-axis motor driver  33 , and Z-axis motor driver  32 , and thus the moving table  10 . 
     A memory  37  stores data about the application operation of the application nozzle  15   a . The data includes the drawing pattern which includes the application starting point and application terminating point set in the application area, the nozzle passing point during the application operation, the moving speed of the application nozzle  15   a , and discharge amount of paste. On the basis of the data stored in the memory  37 , the controller  36  controls the movement of the application nozzle  15   a  driven by the moving table  10 , and the discharge operation of the paste from the application nozzle  15   a  of the syringe  15  in order to have the paste applied in the application area  6   a  in a desired drawing pattern. A bonding head driver  35 , which is controlled by the controller  36 , drives the bonding head  4 . An operation input unit  38  having an input device including a keyboard and a mouse is used to enter operation commands and numerical data. A display unit  39  is a monitor device for displaying an input guide screen and others. 
     An operation of the bonding apparatus having such configuration will be explained below. In FIG. 1, the lead frame  6  is conveyed on the conveying route  5 , and is positioned beneath the paste application unit  9 . Then, the moving table  10  is driven to position the application nozzle  15   a  of the syringe  15  above the application area  6   a  of the lead frame  6 , and the paste is applied. 
     Referring to FIG. 3, an application pattern will be explained. In FIG. 3, the application area  6   a  indicated by a square frame illustrates the range to be coated with the paste prior to a mounting of the chip  3 . Herein, cross-shaped application lines L 1  and L 2  are set as a drawing pattern. In the application area  6   a , the paste is applied along the drawing pattern. 
     A point C, the center C of the application area  6   a , is the application starting point and application terminating point in the drawing pattern. In the application area  6   a , aside from the application starting point and application terminating point, passing points of the application nozzle  15   a  are also set, and data about the points is stored in the memory  37 . The application nozzle  15   a  moves sequentially through the passing points, and applies the paste in the specified drawing pattern. 
     The application nozzle  15   a  moves to the central point C shown in FIG. 3, and is positioned at a proper height for applying the paste from a lower end of the application nozzle  15   a . Maintaining this specified height, the application nozzle  15   a  starts discharging the paste  7 , and moves along a specified route. Herein, the application nozzle  15   a  moves along application lines L 1  and L 2 . 
     The paste begins to be applied along the application line L 1 . The application nozzle  15   a  moves from a first application start point PS 1  set on the center C of application line L 1  toward an end point P 1  of the application line L 1 . At the end point P 1 , the application nozzle  15   a  turns to move in a reverse direction and moves to an end point P 2  at the opposite side along application line L 1 . At the point P 2 , the application nozzle  15   a  turns to move in a reverse direction again, and reaches a first application terminating point PE 1  set near the center of the application line L 1 . As set forth, the application operation for the application line L 1 , a unit application line drawing operation, terminates. 
     As shown in FIG. 3, a drawing track is usually set in two ways, going and returning, along a common application line. In this case, in order to avoid the same track being drawn plural times, the application nozzle  15   a  moves above a line having a line offset in a direction perpendicular to the moving direction. 
     Then, an application line L 2  is drawn. The application nozzle  15   a  is positioned at the first application terminating point PE 1  after drawing application line L 1 . The point PE 1  coincides with a second application starting point PS 2  of the application line L 2 . That is, after applying the paste along the application line L 1 , the application nozzle  15   a  immediately starts to apply the paste along the application line L 2 , and moves from the second application starting point PS 2  toward an end point P 3  of the application line L 2 . At the end point P 3 , the application nozzle  15   a  turns to move in a reverse direction and moves along the application line L 2  to an end point P 4  at the opposite side. There, the application nozzle  15   a  turns to move in a reverse direction again, and reaches a second application terminating point PE 2  set near the center of the application line L 2 . Then, the application for application line L 2  terminates, and thus a drawing of the drawing pattern terminates. 
     According to the embodiment, for applying the paste along a drawing pattern including a cross-shape composed of plural application lines crossing each other, the application nozzle moves from the application start point set near the center of the cross-shape outward to the first end point on the application line, and then moves from the first end point to the second end point at the opposite side. Further, the nozzle moves from the second end point to the application terminating point of the application line set near the center of the cross-shape, and thus finishes a unit application line drawing operation. The nozzle repeats the unit application line drawing operation for each application line. 
     According to the invention, since the application nozzle accelerates and decelerates fewer times than in the conventional method for each application line, the average moving speed of the nozzle is faster than the speed in the conventional method. Therefore, the application efficiency is enhanced. Further, as compared with the conventional method, since points of acceleration and deceleration of the application nozzle  15   a  are not concentrated around the center of the drawing pattern, the paste is not applied excessively in a limited area around the center. Hence, the paste is applied uniformly in the application area  6   a.