Abstract:
A ball-implantation method and a system applying the method are provided. To begin with, solder balls are implanted onto a flux applied to each of the ball pads on a substrate plate. Then, a vibration force of preset magnitude is exerted on the substrate plate, inducing vibration and causing any solder balls that have deviated from positions corresponding to the ball pads exposed from the openings of a solder mask provided on the substrate plate to return to the correct orientation and be kept therein by the vibration force and gravity. Subsequently, the ball implantation process is completed using a reflow process to solder the implanted solder balls. Using this method and the system thereof, the problem of missing or misaligned solder balls that occurs after the reflow process is solved, thereby dispensing with rework and improving the production yield and product reliability.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to ball implantation methods and systems applying the method, and more specifically, to a method of implanting solder balls onto ball pads on a substrate plate that prevents missing solder balls and a ball implantation system applying the method. 
       BACKGROUND OF THE INVENTION 
       [0002]    In order to reduce packaging costs and increase production yield, the packaging industry employs a batch substrate plate comprising multiple substrate units in array configuration, on which a molding process is performed after semiconductor chips are attached and electrically connected to respective substrate units on the substrate plate, thus forming encapsulated or molded bodies on the top surface with a semiconductor chips being disposed therein. Then, a ball implantation process is applied using a reflow process to implant solder balls on the bottom surface of the substrate plate. Lastly, a singulation process is performed to form a plurality of discrete semiconductor packages corresponding to the substrate plates. 
         [0003]    The batch-type method of forming one or more encapsulants or mold bodies is advantageous in that it allows multiple packages to be fabricated in batches at one time, and thus the molding process need not be repetitively performed on the substrate units, thereby reducing the cost of fabrication. However, due to differences in the coefficient of thermal expansion (CTE) of the materials of the encapsulant, the substrate plate and the semiconductor chip, thermal stresses generated from the temperature cycle in the packaging process easily causes the substrate plate to become warped at places along the longitudinal direction of its two sides, as indicated in  FIGS. 1A and 1B , respectively. 
         [0004]    Moreover, the size of the substrate plate is preferred to be as large as possible to efficiently increase the production yield by producing as many substrate units as possible on a single substrate plate. However, the bigger the size of a substrate plate, the more serious the warpage problem mentioned above. Since the positions of the flux applicator and ball implantation are fixed, when the substrate plate  12  warps at or along its two longitudinal sides  10   a ,  10   b  during the temperature cycle (to become a substrate plate  13 ), the ball pads  11  situated at the two warped sides  15   a ,  15   b  deviate from their intended positions, as depicted by the predetermined ball positions  16   a ,  16   b  of  FIG. 1B . However, application of flux and implantation of solder balls do not allow changing of positions so as to adjust to the position deviation of the ball pads  11 , resulting in positional deviation of a flux  21  applied to the respective ball pads  204  and the solder balls  22  implanted onto the flux  21 . That is, the flux  21  and solder balls  22  deviate from the center of the ball pads  204 , which in turn prevents the solder balls  22  from being aligning with ball pads  204  and thus securely trapped by the flux  21 , eventually causing the problem of missing balls and affecting the yield of process as a result. 
         [0005]    Therefore, it is desirable to provide a ball implantation method that prevents the problem of missing balls after the reflow process and yet does not compromise the size of the substrate plate used for this kind of batch-type molding process. 
       SUMMARY OF THE INVENTION 
       [0006]    In view of the foregoing drawbacks associated with the conventional technology, a primary objective of the present invention is to provide a ball implantation method and a system applying the method that can prevent the problem of missing balls to increase the production yield as a result. 
         [0007]    In order to achieve the foregoing and other objectives, the ball implantation method proposed by the present invention comprises the steps of: providing a substrate plate comprised of a plurality of substrate units; applying a flux to a plurality of ball pads exposed from the substrate plate; implanting a plurality of solder balls onto the flux; exerting a vibration force of preset magnitude on the substrate plate to enable any solder balls on any warped portions that have deviated from positions corresponding to ball pads to return to the positions corresponding to the ball pads by the vibration force and gravity; and performing a reflow process to implant the solder balls onto the substrate plate. 
         [0008]    In the method of ball implantation, a solder mask is formed on the substrate plate and has a plurality of openings formed therein to expose corresponding solder pads underneath the solder mask therefrom. In the process, the flux is glutinous and does not harden until being processed during the reflow operation. For any implanted solder balls not aligned with corresponding ball pads, a vibration force exerted on the substrate plate in whole enables the solder balls to move within the desired range of the applied flux until the solder balls return to and are kept in the openings thereof, thereby securely trapping the solder balls on their respective ball pads to solve the problem of missing balls after the reflow process. 
         [0009]    The vibration force can be produced by any conventional vibration equipment, such as ultrasonic oscillators or mechanical vibrators, provided that the vibration equipment exerts a controllable vibration force on the substrate plate to effectuate the purposes. The vibration force is applied sideward, vertically, or both, but is not limited thereto 
         [0010]    The present invention further proposes a ball implantation system applying the method described above, comprising a carrier for carrying the substrate plate comprised by a plurality of substrate units, wherein a solder mask is provided on the substrate plate, the solder mask having a plurality of openings formed therein to expose corresponding ball pads of the substrate therefrom; a flux applicator for applying a flux to each of the solder pads, a solder ball implanter for implanting solder balls onto each respective flux; a vibration force generating unit for exerting a vibration force of preset magnitude on the substrate plate; and a reflow unit for soldering solder balls onto the substrate plate. 
         [0011]    Accordingly, the ball implantation method and system proposed by the present invention enable solder balls not coupled to corresponding solder pads to move and return to and be kept in the openings of the solder mask, thereby preventing the problem of missing balls as encountered in the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
           [0013]      FIG. 1A  and  FIG. 1B  are cross-sectional views illustrating warpage on the two longitudinal sides of a substrate plate; 
           [0014]      FIGS. 2A through 2E  are cross-sectional views illustrating the steps of implementing the method of ball implantation according to the present invention; and 
           [0015]      FIGS. 3A through 3E  are cross-sectional views illustrating the system of implementing the method of ball implantation according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    The present invention is hereunder described with specific embodiments, such that one skilled in the pertinent art can easily understand other advantages and effects of the present invention from the disclosure of the invention. The present invention may also be implemented and applied according to other embodiments, and the details may be modified based on different views and applications without departing from the spirit of the present invention. 
         [0017]    The following embodiments describe the ball implantation method and the system applying the method. The drawings are simplified to show the essential features of the present invention in an understandable manner, and only components directly related to the present invention are shown, but details of the remaining components are omitted for brevity. 
         [0018]      FIGS. 2A through 2E  are cross-sectional views illustrating the steps of implementing the method of ball implantation according to the present invention. As shown in  FIG. 2A , a substrate plate  20  comprising a plurality of substrate units  200  is provided, the substrate plate  20  having first and second surfaces  201 ,  202 , wherein a chip and an encapsuant enclosing the chip are formed on the exposed surface of each of the substrate units  200 . In view of the well-known nature of this process technology, the related processes of mounting chips and forming encapsulants on the substrate units  200  are not specifically depicted herein for brevity. In an embodiment of the ball implantation method, a solder mask  203  is formed on the second surface  202  of the substrate plate  20 , and the solder mask  203  has a plurality of openings  203   a  formed therein to expose each of the ball pads  204  underneath the solder mask  203  therefrom. The substrate plate  20  includes, but is not limited to, common flip-chip substrates, Ball Grid Array (BGA) substrates and Window BGA substrates. 
         [0019]    As illustrated in  FIG. 2B , a flux  21  is applied to each of the ball pads  204  exposed from the substrate plate  20  by means of a conventional flux applicator (not shown). However, with warpage at the two longitudinal sides of the substrate plate  20  as indicated by the arrows during the temperature cycle, the flux  21  applied by the flux applicator to two longitudinal sides  20   a ,  20   b  of the substrate plate  20  deviates from the center of the ball pads  204 . 
         [0020]      FIG. 2C  illustrates implanting a plurality of solder balls onto the flux  21  by a conventional ball implanter (not shown). In that the positions of the flux applicator and ball implantation are predetermined, the flux  21  being applied to the two longitudinal sides  20   a ,  20   b  deviates from the intended preset positions and thus causes deviation of ball implantation on both sides where the warpage occurred, thus affecting the positioning of the solder balls  22  on the ball pads  204  exposed from the openings  203   a.  That is, some of the solder balls  22  are misaligned with respect to the ball pads  204 . 
         [0021]    Subsequent to the process of ball implantation, as illustrated in  FIG. 2D , a vibration force F of preset magnitude is exerted on the substrate plate  20  by, for example, an ultrasonic vibrator to induce vibration of the substrate plate  20  before the flux  21  hardens, thereby enabling solder balls  22  that initially deviated from their respective ball pads  204  to move within the range of the applied flux by the vibration force and then return to the openings  203   a  of solder mask  203  by gravity, thus limiting and grabbing solder balls  22  therein. The vibration force can be produced by conventional vibration equipment, such as ultrasonic oscillators or mechanical vibrators, provided that the vibration equipment exerts a controllable vibration force on the substrate plate  20  to effectuate the purposes. The vibration force is applied sideward, vertically, or both, but is not limited thereto. It should be noted that the solder balls  22  positioned on the ball pads  204  and arranged along the two longitudinal sides  20   a ,  20   b  of the substrate plate  20  are confined to the openings  203   a  of solder mask  203  and therefore do not roll despite a vibration force exerted on the substrate plate  20 . Hence, the vibration force exerted on the substrate plate in whole enables the solder balls to move within the desired range of the applied flux until the solder balls return to and are kept in the openings thereof, thereby securely trapping the solder balls on their respective ball pads. 
         [0022]    Lastly, as shown in  FIG. 2E , a reflow process is performed on the substrate plate  20  in order to securely solder the implanted solder balls  22  thereon, thereby overcoming the problem of missing balls and improving the production yield and product reliability. 
         [0023]      FIGS. 3A through 3E  are cross-sectional views illustrating the system of implementing the method of ball implantation according to the present invention. 
         [0024]    As depicted in  FIG. 3A , the ball implantation system applying the method described above comprises: a carrier  30 , a flux applicator  31 , a solder ball implanter  32 , a vibration force generating unit  33 , a reflow unit  34 , and a substrate plate  20  comprised of a plurality of substrate units  200  and carried by the carrier  30 , wherein a solder mask  203  is formed on the substrate plate  20 . The solder mask  203  has a plurality of openings  203   a  formed therein to expose the ball pads  204  of the substrate plate  20  therefrom. The substrate plate  20  includes, but is not limited to, common flip-chip substrates, Ball Grid Array (BGA) substrates and Window BGA substrates. 
         [0025]    As illustrated in  FIG. 3B , the flux applicator  31  applies the flux  21 , via an output portion  31   a,  to ball pads  204  exposed from the substrate plate  20 . However, with warpage at the two longitudinal sides of the substrate plate  20  during the temperature cycle, the flux  21  applied by the flux applicator  31  to the two sides  20   a ,  20   b  of the substrate plate  20  deviates from the center of the ball pads  204 . Subsequently, as indicated in  FIG. 3C , the solder ball implanter  32  is provided to implant solder balls  22  onto the flux  21  applied to the substrate plate  20 . In that the positions of the flux applicator  31  and ball implantation are predetermined, the flux  21  being applied to the two longitudinal sides  20   a ,  20   b  of the substrate plate  20  deviates from the intended positions, which in turn causes the positions of ball implantation on both sides where warpage occurred to deviate, thus affecting the positioning of the solder balls  22  on the ball pads  204  exposed from the openings  203   a.    
         [0026]    Subsequent to the process of ball implantation, as illustrated in  FIG. 3D , a vibration force F of preset magnitude is exerted on the substrate plate  20  by means of an ultrasonic vibrator to induce vibration to the substrate plate  20  before the flux  21  hardens, such that the implanted solder balls  22  that deviated from the ball pads  204  can move within the range of the applied flux  21  by the vibration force and then return to the openings  203   a  of the solder mask  203  by gravity to be limited and secured therein. The vibration force can be produced by conventional vibration equipment, such as ultrasonic oscillators or mechanical vibrators, provided that the vibration equipment exerts a controllable vibration force on the substrate plate  20  to effectuate the purposes. The vibration force is applied sideward, vertically, or both, but is not limited thereto. It should be noted that the solder balls  22  positioned on the ball pads  204  and arranged along the two longitudinal sides  20   a ,  20   b  of the substrate plate  20  are confined to the openings  203   a  of solder mask  203  and therefore do not roll despite a vibration force exerted on the substrate plate  20 . Hence, the vibration force exerted on the substrate plate in whole enables the solder balls to move within the desired range of the applied flux until the solder balls return to and are kept in the openings thereof, thereby securely trapping the solder balls on their respective ball pads. 
         [0027]    Lastly, as shown in  FIG. 3E , a reflow process is performed on the substrate plate  20  in order to securely solder the implanted solder balls  22  thereon to the openings  203   a  of solder mask  203 , thereby overcoming the problem of missing balls and improving production yield and product reliability. 
         [0028]    In another embodiment, the vibration force generating unit  33  of the present invention can be concurrently applied together with the ball implanter  32 . In yet another embodiment, the vibration force generating unit  33  of the present invention works in conjunction with the reflow unit  34  concurrently. 
         [0029]    In summary, the ball implantation method and system proposed by the present invention is characterized by enabling solder balls not aligned with ball pads to move and return to openings of a solder mask so as for the solder balls to be secured in position thereto, thereby preventing the problem of missing balls as encountered in the prior art. 
         [0030]    The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.