Abstract:
A conductive ball mounting apparatus for mounting conductive balls by providing an array mask having through holes, into which conductive balls are to enter, above a mounting target placed on a stage, by arranging a ball reservoir having an opening for reserving a plurality of conductive balls, in the bottom, by moving the ball reservoir along the array mask, by dropping the conductive balls into the individual through holes of the array mask, adopts the following means. Firstly, the conductive ball mounting apparatus comprises moving means for moving the array mask and the stage relative to each other in horizontal directions. Secondly, the positions of the conductive balls in the through holes are arranged by finely moving at least one of the array mask and the stage relative to each other in the horizontal directions after the balls were dropped.

Description:
[0001]     This application is based on Japanese Patent Application No. 2005-132062, which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an improvement in an apparatus for mounting a number of conductive balls by moving a ball reservoir for reserving the conductive balls along the upper face of an array mask and, more particularly, to a conductive ball mounting apparatus which is improved with a main view to improving the positional precision of the conductive balls in the through holes of the array mask.  
         [0004]     2. Description of the Related Art  
         [0005]     As the conductive ball mounting apparatus for mounting conductive balls after the adhesive material was applied to individual electrodes formed in a predetermined array pattern on the mounting target, there exists in the related art an apparatus for mounting the conductive balls, after sucked, arrayed and adsorbed by the ball mounting head having an array plate, on the individual electrodes on the mounting target, as disclosed in JP-A-2001-358451. However, as the mounting target product such as a wafer becomes larger, the number of solder balls to be mounted at one time exceeds one million. This makes it difficult at present to reduce the defects in the array of solder balls and the defects at the mounting time.  
         [0006]     As disclosed in JP-A-2002-538970 and Japanese Patent No. 3177370, therefore, there has been provided an apparatus, in which an electronic substrate or a mounting target printed with flux is provided with an array mask and in which solder balls are directly dropped onto electrodes of the electronic substrate. As compared with the diameter of the conductive balls, however, the diameter of the through holes of the array mask is larger. This raises a problem that the positions of the conductive balls in the through holes are not arranged merely by dropping the conductive balls into the through holes of the array mask.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention has an object to solve the aforementioned problem and to provide a conductive ball mounting apparatus which is enabled by arranging the positions of conductive balls in through holes to improve the precision even in positions for mounting a large number of conductive balls.  
         [0008]     In order to solve the aforementioned problem, a first aspect of the invention adopts the following means in the conductive ball mounting apparatus for mounting a conductive ball comprising an array mask having a through hole, into which a conductive ball is to enter, the array mask being provided above a mounting target placed on a stage, a ball reservoir having an opening for reserving a plurality of conductive balls in the bottom thereof, the ball reservoir moving along the array mask and dropping the conductive balls into the through hole of the array mask, adopts the following means.  
         [0009]     Firstly, the conductive ball mounting apparatus comprises moving means for moving at least one of the array mask and the stage relative to each other in a horizontal direction.  
         [0010]     Secondly, a position of the conductive ball in the through hole is arranged by moving at least one of the array mask and the stage relative to each other in a horizontal direction after the balls was dropped.  
         [0011]     According to a second aspect of the invention, the moving at least one of the array mask and the stage comprises a straight movement in a horizontal direction. According to a third aspect of the invention, the moving at least on of the array mask and the stage further comprises a sequential movement in a direction perpendicular to the horizontal directions. According to a fifth aspect of the invention, the moving at least one of the array mask and the stage comprises a movement to turn in a horizontal plane.  
         [0012]     According to a fourth aspect of the invention, a distance of the moving at least one of the array mask and the stage is at least a difference between a diameter of the conductive ball and a diameter of the through hole. According to a sixth aspect of the invention, after the moving at least one of the array mask and the stage, the at least one of the array mask and the stage is so moved that the conductive ball leaves the inner wall of the through hole.  
         [0013]     In the first aspect of the invention, the positions of the conductive balls in the through holes are arranged by finely moving the array mask and the stage relative to each other in the horizontal directions after the balls were dropped. As a result, the conductive ball mounting apparatus can improve the positioning precision for mounting the conductive balls. Moreover, the apparatus mounts the conductive balls in position by arranging the array mask having the through holes for dropping the conductive balls, above the mounting target, and by moving the ball reservoir to reserve the numerous conductive balls, by the ball reservoir moving means, along the array mask. As a result, the apparatus can precisely mount the numerous conductive balls, the number of which increases as the mounting target products such as wafers are large-sized.  
         [0014]     In the second, third or fifth aspect of the invention, the fine movements of the array mask and the stage are straight movements in one direction of horizontal directions, sequential movements in two directions perpendicular in the horizontal directions, or movements to turn in a horizontal plane. Even at less steps or by smaller actions, it is possible to improve the mounting position precision of the conductive balls better. In the fourth aspect of the invention, moreover, the fine moving distances of the array mask and the stage in one direction are at least the difference between the conductive ball diameter and the through hole diameter. In the sixth aspect of the invention, the array mask and the stage are finely moved relative to each other and are then so moved that the conductive balls may leave the inner walls of the through holes. As a result, it is possible to improve the mounting position precision of the conductive balls better. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a schematic top plan view showing the entirety of a solder ball mounting apparatus according to the embodiment;  
         [0016]      FIG. 2  is a schematic top plan view of the case, in which a wafer feeding unit and a wafer housing unit are disposed in the same direction;  
         [0017]      FIG. 3  is a partially sectional, explanatory side elevation showing a ball mounting unit;  
         [0018]      FIG. 4  is a top plan view of the ball mounting unit;  
         [0019]      FIGS. 5A-5C  are explanatory views showing a fine moving procedure of the ball array mask and the wafer transfer stage; and  
         [0020]      FIG. 6  is a front elevation showing the movement of a wafer transfer stage. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     An embodiment of the invention is described in the following with reference to the accompanying drawings. In the invention, a semiconductor wafer (as will be simplified into the wafer), an electronic circuit substrate or a ceramic substrate is exemplified as a target for mounting conductive balls, but a wafer  14  is used in the embodiment. Moreover, electrodes  38  are formed in a predetermined pattern on the surface of the mounting target. On the other hand, flux, solder paste or a conductive adhesive is used as an adhesive material.  
         [0022]      FIG. 1  is a schematic top plan view showing the entirety of a solder ball mounting apparatus  1 . This solder ball mounting apparatus  1  includes a carry-in wafer transfer unit  2 , a flux printing unit  3 , a ball mounting unit  4 , and a carry-out wafer transfer unit  5 . A wafer feeding unit  6 , a primary alignment unit  7  and a carry-in robot  8  exist at the pre-step of the solder ball mounting apparatus  1 , and an inversion unit  9 , a wafer housing unit  10  and a carry-out robot  11  exist at the post-step of the solder ball mounting apparatus  1 .  
         [0023]     The primary alignment unit  7  for the pre-step turns the wafer  14  in a horizontal plane to detect the position of an orientation flat or notch of the wafer  14  thereby to correct the position of the wafer  14  approximately and to direct the wafer  14  to be mounted on the wafer transfer unit  2 , in a predetermined direction. On the other hand, the inversion unit  9  for the post-step turns the wafer  14  in the horizontal direction so that the wafer  14  is turned to bring its orientation flat or notch to a predetermined position and is housed in a magazine  32 .  
         [0024]     The solder ball mounting apparatus  1  is equipped with a wafer transfer stage  12  and a transfer passage  13  for transferring the wafer  14  from the wafer transfer unit to the flux printing unit  3 , the ball mounting unit  4  and the wafer transfer unit  5 . The transfer passage  13  is equipped with an X-axis (longitudinal, as shown) drive mechanism  40  of the transfer stage  12 .  
         [0025]     The flux printing unit  3  is equipped with a flux feeding device  16 , a printing mask  15  for printing flux, or the adhesive material on the wafer  14 , and vertical observation cameras  31  for observing the alignment marks of the wafer  14  and the printing mask  15  thereby to register the wafer  14  and the printing mask  15 . The printing mask  15  has through holes formed along with the pattern of the electrodes  38  on the wafer  14 . Two (not-shown) alignment marks are formed at two portions on the lower face of the printing mask  15  in a through hole forming area  36 . The printing mask  15  is applied to a molding box  17  and is held by a fixing unit such as a frame. The flux feeding device  16  moves the (not-shown) squeezee along the upper face of the printing mask  15  so that the flux is printed in the through holes of the printing mask  15  and fed onto the electrodes  38  of the wafer  14 . Here, numeral  33  in the drawing designates a cleaning unit for cleaning off the flux adhered to the printing mask  15 .  
         [0026]     The ball mounting unit  4  is equipped with a solder ball feeding device  20 , a ball array mask  19  having through holes  18  formed along with the pattern of the electrodes  38  on the wafer  14 , and vertical observation cameras  34  for observing the alignment marks of the wafer  14  and the ball array mask  19  thereby to register them.  
         [0027]     The ball array mask  19  has a thickness substantially equal to the diameter of solder balls  21  to be fed, and the through holes  18  have a diameter slightly larger than that of the solder balls. Like the printing mask  15 , the ball array mask  19  has the (not-shown) alignment marks formed at two portions on the lower face of the through hole forming area  36 . The ball array mask  19  is adhered to a molding box  37  and is held by a fixing unit such as the frame.  
         [0028]     The solder ball feeding device  20  is equipped with a ball hopper  22  for reserving a number of solder balls  21 , a ball cup  23  for dropping the solder balls  21  into the ball array mask  19 , a mask height detecting sensor  27 , and a carriage unit  24  not only for moving the ball cups  23  along an X-axis guide  25  and a Y-axis guide  26  but also for displacing the same in a Z-axis direction. Here, the ball hopper  22  is exchanged according to the size and material of the solder balls  21 . Inside of and in the lower portion of the inner wall face of the ball cup  23 , there is formed a recess  35  for causing the solder balls  21  housed therein to circulate, as indicated by an arrow in the ball cup  23  in  FIG. 3 .  
         [0029]     The mask height detecting sensor  27  may be of either the contact type or the non-contact type. Specifically, a laser sensor or an electrostatic capacity sensor is used as the mask height detecting sensor  27 . The mask height detection is made by bringing the molding box  37  of the ball array mask  19 , when exchanged at an initial setting time or at a mold exchanging time, into abutment against a stopper or the like, and by positioning and fixing the molding box  37  by means of a clamp. Specifically, after the ball array mask  19  was fixed, the ball cup  23  empty of the solder balls  21  is moved sequentially on a plurality of height detection points preset outside of the through hole forming area  36 , and the height of the upper face of the ball array mask  19  is measured.  
         [0030]     On the other hand, the height of the upper face of the ball array mask  19  in the through hole forming area  36  is determined by calculations. Moreover, the heights at the individual positions are calculated by considering the weight which is applied when the solder balls  21  are housed in the ball cup  23 . At the ball mounting time, the ball cup  23  is so moved on the basis of the determined height, while being controlled by the moving unit  24 , that the clearance between the upper face of the ball array mask  19  and the lower face of the ball cup  23  may not exceed a predetermined distance.  
         [0031]     The wafer transfer stage  12  is a stage for placing the wafer  14  thereon and is mounted on the X-axis drive mechanism  40  through a Y-axis drive mechanism  28  acting as moving means in the direction (i.e., the Y-axis direction) perpendicular to the transfer passage  13  of the wafer  14 , a θ-axis drive mechanism  29  acting as turning means, and a Z-axis drive mechanism  30  acting as vertically moving means.  
         [0032]     The actions of the solder ball mounting apparatus  1  of the embodiment are described. At first, the wafer  14  to have the solder balls  21  mounted thereon is housed in the magazine  32  of the wafer feeding unit  6 . Then, one wafer  14  is extracted from the magazine  32  of the wafer feeding unit  6  and carried in the primary alignment unit  7 . In this primary alignment unit  7 , the wafer  14  is turned to detect the position of the orientation flat or notch thereby to correct the position of the wafer  14  approximately and to set the orientation flat or notch at a predetermined position. Subsequently, the wafer  14  is carried by the carry-in robot  8  from the primary alignment unit  7  to the wafer transfer stage  12  on standby at the wafer transfer unit  2 .  
         [0033]     The wafer transfer stage  12  having the wafer  14  mounted thereon is moved by the X-axis drive mechanism  40  along the transfer passage  13  to the flux printing unit  3  and stops at a predetermined position. Here, the alignment marks of the wafer  14  and the printing mask  15  are individually observed by the vertical observation cameras  31 , and the wafer transfer stage  12  is so positioned in the X-axis direction by the X-axis drive mechanism  40  of the transfer passage  13 , in the Y-axis direction by the Y-axis drive mechanism  28  and in the θ-axis direction by the θ-axis drive mechanism  29  that the electrodes  38  of the wafer  14  and the through holes of the printing mask  15  may be centrally aligned. After positioned, the wafer transfer stage  12  is raised by the Z-axis drive mechanism  30  so that it is stopped at a predetermined height position with respect to the printing mask  15  having been prepared with the flux. In this state, the printing mask  15  is fed with the flux at its one end portion in the Y-axis direction, and the squeezee is moved toward the other end portion to print the flux on the electrodes  38  of the wafer  14  from the through holes of the printing mask  15 .  
         [0034]     After the flux-printing, the wafer transfer stage  12  is moved downward by the Z-axis drive mechanism  30  and is moved to the ball mounting unit  4  by the X-axis drive mechanism  40  so that it is stopped at a predetermined position. Here, the alignment marks of the wafer  14  and the ball array mask  19  are also individually observed by the vertical observation cameras  34 , and the wafer transfer stage  12  is positioned in the X-axis direction by the X-axis drive mechanism  40  of the transfer passage  13 , and in the Y-axis direction and in the θ-axis direction by the Y-axis drive mechanism  28  and the θ-axis drive mechanism  29 , respectively. After this, the wafer transfer stage  12  is moved upward by the Z-axis drive mechanism  30  so that it is stopped while leaving the predetermined clearance from the ball array mask  19 .  
         [0035]     As shown in  FIG. 3 , the ball cup  23  moves over the ball array mask  19  to drop the solder balls  21  into the through holes  18  of the ball array mask  19  so that the solder balls  21  are mounted on the wafer  14 . After this ball dropping operation, the wafer transfer stage  12  is finely moved straight (in the X-axis direction) with respect to the ball array mask  19  to correct the positions of the solder balls  21  in the through holes  18  thereby to arrange the positions of the solder balls  21 . Here, it is preferred that the fine movement distance of the wafer transfer stage  12  is at least the distance of the difference between the diameter of the solder ball  21  and the diameter of the through holes  18 .  
         [0036]      FIGS. 5A  to  5 C are explanatory views showing one example of a fine moving procedure of the ball array mask  19  and the wafer transfer stage  12 . The centers of the electrodes  38  of the wafer  14  are so positioned that they are deviated in the X-axis direction from the centers of the through holes  18  of the ball array mask  19  by a half distance of the difference between the diameter of the solder balls  21  and the diameter of the through holes  18 . At first,  FIG. 5A  shows the stage immediately after the solder balls  21  were dropped into the through holes  18  of the ball array mask  19 . In this state, the solder balls  21  are not always mounted in the centers of the through holes  18 . As shown in  FIG. 5B , therefore, the wafer transfer stage  12  is finely moved in the X-axis direction (leftward of the drawing) by the distance of the difference between the diameter of the solder balls  21  and the diameter of the through holes  18 . As a result of this fine movement, the solder balls  21  finely move in the X-axis direction together with the wafer transfer stage  12  thereby to come into abutment against the lefthand inner walls, as shown, of the through holes  18  so that their positions in the X-axis direction are corrected. Simultaneously with this, the solder balls  21  moved along the lefthand inner walls, as shown in  FIG. 5B , of the through holes  18  so that their positions in the Y-axis direction are corrected. In short, the solder balls  21  come into the state, in which they are precisely mounted at the central portions of the electrodes  38  of the wafer  14 . As shown in  FIG. 5C , the wafer transfer stage  12  is moved backward of the fine movement from  FIG. 5A  to  FIG. 5B  by the distance of one half of the difference between the diameter of the solder balls  21  and the diameter of the through holes  18 . Then, all the solder balls  21  in the through holes  18  leave the inner walls of the through holes  18  and are mounted in the positions homogeneous in the X-axis direction and in the Y-axis direction. As a result, the solder balls  21  can leave without any deviation from the ball array mask  19  when the wafer transfer stage  12  moves downward. Here, the fine movement distance may be longer than the difference between the diameter of the solder balls  21  and the diameter of the through holes  18 , but the initial position of the wafer transfer stage  12  at the ball mounting unit is set according to that distance.  
         [0037]     In the embodiment thus far described, the fine movements of the ball array mask  19  and the wafer transfer stage  12  are straight in one direction, i.e., in the X-axis direction of the horizontal directions. It is, however, natural that the fine movements may be one in another direction such as in the Y-axis direction, sequential ones in two perpendicular directions in the horizontal directions, or a turning one in a horizontal plane. Here in the sequential movements in the two directions perpendicular in the horizontal directions, the wafer transfer stage  12  is finely moved in the X-axis direction by a distance equal to at least the difference between the diameter of the solder balls  21  and the diameter of the through holes  18 . After this, the wafer transfer stage  12  is finely moved in the Y-axis direction perpendicular to the X-axis direction by the half distance of the difference between the diameter of the solder balls  21  and the diameter of the through holes  18 , and is then finely moved so that the solder balls  21  may return to the centers of the through holes  18 . In the turning motions in the horizontal plane, on the other hand, the wafer transfer stage  12  is turned with a turning radius of one half of the difference between the diameter of the solder balls  21  and the diameter of the through holes  18  and is so turned or moved straight that the solder balls  21  may finally return to the centers of the through holes  18 .  
         [0038]     In this embodiment, as the moving means for that fine movement, the fine movement in the X-axis direction is caused by the X-axis drive mechanism  40  of the transfer passage  13 , and the fine movement in the Y-axis direction is caused by the Y-axis drive mechanism  28  of the wafer transfer stage  12 , so that the wafer transfer stage  12  is finely moved by that moving means. Since the fine movements may be relative ones, however, the ball array mask  19  may also be provided with a moving device for moving the ball array mask  19  finely in the X-axis direction and in the Y-axis direction.  
         [0039]     After the solder balls mounting operation, the wafer transfer stage  12  is moved downward by the Z-axis drive mechanism  30  so that it is moved to stop at the carry-out wafer transfer unit. In the wafer housing unit  10 , the wafer  14  is transferred from the wafer transfer stage  12  to the inversion unit  9  by the carry-out robot  11 , and the wafer  14  is turned to bring the orientation flat or notch to the predetermined position. The wafer  14  is further transferred by the carry-out robot  11  from the inversion unit  9  to the magazine  32  of the wafer housing unit  10 . When the carry-out robot  11  takes out the wafer  14  from the wafer transfer stage  12 , the wafer transfer stage  12  returns to the original position or the wafer transfer unit  2 , thus completing one step. The present apparatus repeats the actions thus far described.  
         [0040]     In the embodiment shown in  FIG. 1 , the wafer feeding unit  6  is disposed in front of the solder ball mounting apparatus  1 , and the wafer housing unit  10  is disposed at the back. Since the wafer transfer stage  12  returns to the original position, as described above, the wafer feeding unit  6  and the wafer housing unit  10  may also be disposed on one side, as shown in  FIG. 2 .  
         [0041]     With the structure thus made, the carry-out robot  11  can be replaced by the carry-in robot  8 , and the wafer  14  is held and housed in the same direction as that of the wafer  14  being carried in, so that the inversion unit  9  can be omitted. Moreover, one of the wafer transfer units  2  and  5  can also be omitted so that the number of structural components can be reduced.  
         [0042]     Moreover, this embodiment employs the vertical observation cameras  31  and  34  for photographing the alignment marks of the wafer  14  and the printing mask  15  or the ball array mask  19  simultaneously at the stop time of the wafer transfer stage  12 , as the means for positioning the printing mask  15  and the ball array mask  19 , and the wafer  14 . However, the invention should not be limited thereto but can be conceived to have various structures.