Abstract:
The invention provides a method and apparatus for aligning a substrate. The apparatus comprises a ball pick head for picking up a plurality of solder balls in a ball pick-up process and depositing them onto the substrate, and a vision system adapted to view and obtain positional information of the substrate. Furthermore, a carrier is provided to which the vision system is mountable, such that operation of the vision system is decoupled from movement of the ball pick head. Drivers responsive to said positional information viewed by the vision system are operative to align at least the substrate and the ball pick head for depositing solder balls onto the substrate.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for aligning a semiconductor substrate, in particular, for aligning the substrate when undergoing a semiconductor assembly process, such as during placement of solder balls onto the substrate&#39;s contact pads. 
     BACKGROUND AND PRIOR ART 
     Ball Grid Array (“BGA”) techniques are commonly used for producing high-density integrated circuit (“IC”) components. A regular array of solder balls is deposited onto the IC component at contact pads where the electrical contacts of the IC component are to be formed. Such balls forming the electrical contacts of the IC component may then be mated with corresponding connections on a printed circuit board in use. 
     During production using BGA techniques, droplets of flux and solder balls must be transferred to a substrate where they are deposited in a predetermined array. A common technique is to use a flux transfer head or pin head to transfer flux to the substrate and a ball pick head to carry solder balls in the same array configuration as is required on the substrate, and then subsequently to deposit balls onto the substrate containing flux. It is usually essential that all the electrical contact points of the IC component are covered by solder balls to ensure that the component is not defective. Conventionally, the ball pick head is formed with a plurality of locations for receiving solder balls, these locations being disposed in the same array configuration as the desired configuration of solder balls on the circuit board. The corresponding pin head must also deposit flux droplets in the same array configuration on the substrate. 
     A number of challenges are presented to the design of fast and efficient apparatus for the placement of flux droplets and solder balls. The apparatus must be designed so that the pin head and the ball pick head are brought in turn to a precise position over the substrate and since the dimensions of the array and in particular the spacing between solder ball locations on the array are small, accurate alignment techniques must be employed. Generally, a vision or pattern recognition system such as a camera is used to locate and capture images of at least two fiducial markers on the substrate to determine whether any degree of movement is required to achieve alignment with the substrate. 
     The need in a production process to accurately align pin heads and ball pick heads over the substrate in a high-speed and efficient manner has given rise to the use of two cameras to reduce the movement that might be required by a single camera having to travel between the fiducial markers. An example is U.S. Pat. No. 6,070,783 in respect of a “Conductive Ball Attaching Apparatus and Method”. An apparatus is described wherein two alignment cameras are diagonally positioned from each other and integrated to a main transfer means. The problem is that the main transfer means is involved in many process work elements, such as ball pick-up, flux transfer, alignment and ball placement on the substrate. Moreover, the apparatus has a large transfer means design, such that the individual transfer means need to travel a relatively long distance to complete one cycle, resulting in a longer alignment and ball placement process. Another feature of the apparatus is that the cameras are rigidly integrated with the transfer means, such that movement of the cameras is dependent on movement of the transfer means. This makes the system more cumbersome and complex. 
     Another example of an apparatus using two cameras to align a substrate is U.S. Pat. No. 6,355,298 for a “Placement System Apparatus and Method”. One alignment camera is mounted on a pin head and another camera is mounted on a ball pick head. The ball pick head is involved in a time-critical process and has a higher number of process work elements as compared with the pin head. The result is an unequal distribution of work-loading since the ball pick head takes a significantly longer time to complete its processes as compared to the pin head. The ball pick head is not able to perform simultaneously a ball pick-up process (which includes ball preparation into a predetermined array for pick-up) and substrate alignment. This is because the ball pick head needs to wait for the processed substrate to exit the ball mounting station and a new substrate to enter for alignment, before it can perform ball preparation for another substrate. The waiting time contributes to increased process cycle time. 
     SUMMARY OF THE INVENTION 
     It is an objective of the invention to avoid some of the disadvantages of the prior art in order to develop a relative more efficient method and apparatus for substrate alignment. 
     According to a first aspect of the invention, there is provided an apparatus for aligning a substrate comprising: a ball pick head for picking up a plurality of solder balls in a ball pick-up process and depositing them onto the substrate; a vision system adapted to view and obtain positional information of the substrate; a carrier to which the vision system is mountable, such that operation of the vision system is decoupled from movement of the ball pick head; and drivers responsive to said positional information viewed by the vision system to align at least the substrate and the ball pick head for depositing solder balls onto the substrate. 
     According to a second aspect of the invention, there is provided a method for aligning a substrate comprising the steps of: providing a ball pick head for picking up a plurality of solder balls in a ball pick-up process and depositing them onto the substrate; viewing and obtaining positional information of the substrate with a vision system mounted to a carrier, thereby decoupling operation of the vision system from movement of the ball pick head; and aligning at least the substrate and the ball pick head in response to said positional information to deposit solder balls onto the substrate. 
     It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of a method and apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a plan view of a typical BGA substrate with rectangular arrays of conductive solder pads; 
         FIG. 2  is a plan view of the BGA substrate with its fiducial marks more clearly illustrated; 
         FIG. 3  is a plan view of a substrate alignment and ball placement device according to the preferred embodiment of the invention; 
         FIG. 4  is an elevational view of dual alignment cameras mounted on a pin head of the ball placing device according to one preferred embodiment looking from direction A of  FIG. 3 ; 
         FIG. 5  is an elevational view of dual alignment cameras mounted on a non-process head according to another preferred embodiment; and 
         FIGS. 6A to 6F  illustrates an operation sequence illustrating a distribution of process work elements between the pin head and ball pick head according to the preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a plan view of a typical BGA substrate  3  with rectangular arrays of conductive solder pads  2 . Flux and solder balls must be accurately placed at the locations of the solder pads  2 . 
       FIG. 2  is a plan view of the BGA substrate with its fiducial marks more clearly illustrated. Illustrations of the solder pads  2  have been removed for clarity. 
     The substrate  3  has a number of lines defining grids  4  corresponding to outlines of the arrays of conductive solder pads  2 . These grids  4  include a number of fiducial marks  5 . An enlarged view of a fiducial mark  5  of a predetermined design is shown. In  FIG. 2 , the design of the fiducial mark  5  comprises regular L-shaped blocks, but it would be appreciated that other designs are also possible. 
       FIG. 3  is a plan view of a substrate alignment and ball placement device  22  according to the preferred embodiment of the invention. The ball placement device  22  comprises generally of a support frame  1  supporting a flux transfer head or pin head  18 , and a ball pick head  21 . The ball placement device  22  includes a number of drivers or motors. A pin head y-motor  23  attached to the support frame  1  drives the pin head  18  along a y-axis, whereas a ball pick head y-motor  24  attached to the support frame  1  drives the ball pick head  21  along the y-axis. Further, a pin head z-motor  27  and a ball pick head z-motor  28  drive the pin head  18  and ball pick head  21  respectively in the z-axis (i.e. perpendicular to the x- and y-axes). The pin head motors  23 ,  27  and ball pick head motors  24 ,  28  are mounted onto the support frame  1 . There is also a separate module that has a horizontal rail with a ball/flux mounting platform  34  and is used for flux and ball placement, and substrate transport in the x-axis. 
     With respect to the pin head  18 , it includes a pin head theta motor  25  that drives angular rotation of the pin head  18 . The pin head  18  also includes a vision system, which may be in the form of a dual camera alignment module  29 . Thus, the pin head  18  acts as a carrier to which the vision system is mounted. The vision system or dual camera alignment module  29  is adapted to view and obtain positional information of the substrate  3 . In the preferred embodiment, the vision system comprises a first camera  32  and a second camera  33 . Camera motors  30 ,  31  drive the first and second cameras  32 ,  33  respectively along linear guides  42  (see  FIG. 4 ) in the x-axis in the dual camera alignment module  29 . 
     With respect to the ball pick head  21 , it further comprises a ball pick head theta motor  26  to drive angular rotation of the ball pick head  21 . The ball pick head  21  picks up a plurality of solder balls in a ball pick-up process and deposits them onto a substrate  3  located on the ball/flux mounting platform  34  of the horizontal rail. The substrate  3  typically contains positional indicia, such as fiducial marks  11 ,  12 ,  35  and  36 . Typically, fiducial marks are read horizontally or diagonally as pairs to determine alignment of a substrate, such that fiducial mark  11  may be read simultaneously with fiducial mark  12  and fiducial mark  35  may be read simultaneously with fiducial mark  36 . Other than fiducial marks  12 , the device  22  can be programmed to recognize other positional indicia, such as conductive pads, solder pads or any other unique recognition marks on the surface of the substrate  3 . 
     At each end of the shaft, there is a flux reservoir  37  and a ball template holder  38  respectively. It should be appreciated that although one camera is sufficient for implementing the invention, two cameras are generally preferred as they may stay in relatively fixed positions in the x-axis for viewing successive substrates, provided that they do not physically obstruct each other during pattern recognition. By doing so, alignment time can be shortened. It should also be appreciated that the various motors  23 – 28 ,  30 – 31  act as drivers to control relative positions of the various components of the ball placement device  22  to align at least the substrate  3  and the ball pick head  21 , as well as the substrate  3  and pin head  18  in the described embodiments, for depositing solder balls  48  onto the substrate  3 . 
       FIG. 4  is an elevational view of the dual camera alignment module  29  mounted on the pin head  18  of the ball placement device  22  according to one preferred embodiment looking from direction A of  FIG. 3 . The camera module  29  is a modular assembly on which the first and second cameras  32 ,  33  are mounted. The pin head  18  is selected for mounting the camera module  29  as it is a relatively non time-critical process head, i.e. it executes fewer process work elements, as compared to the ball pick head  21 . 
     The first camera  32  and second camera  33  are capable of independent movement in the x-axis. There is an LED module  44 ,  45  for each camera  32 ,  33 . A linear guide  42  serves as a conduit to allow horizontal movement of the cameras  32 ,  33  and to position them with respect to the pin head  18  to view fiducial marks  11 ,  12 ,  35 ,  36  on the surface of the substrate  3 . There may be a linear guide  42  for each camera  32 ,  33  or a single linear guide  42  may be shared. The cameras  32 ,  33  are preferably placed adjacent to each other, although it is also possible to position them on different sides of the carrier or pin head  18 . A first feed screw  40  driven by the first camera motor  30  controls the motion of the first camera  32  whereas a second feed screw  41  driven by the second camera motor  31  controls the motion of the second camera  33 . 
     Also illustrated are flux transfer pins  43  on the underside of the pin head  18  that are adapted to collect flux, then contact a substrate  3  to apply flux to it. 
       FIG. 5  is an elevational view of the dual camera alignment module  29  mounted on a non-process head  47  according to another preferred embodiment. The non-process head  47  acts as a carrier for the vision system in this embodiment. The configuration is the same as that of  FIG. 4 , except that the non-process head  47  is not involved in any time-critical process or any function other than supporting and positioning the cameras  32 ,  33 . The essence of the first and second embodiments is that the vision system or dual camera alignment module  29  is adapted to obtain positional information of the substrate  3  substantially simultaneously with the ball pick head  21  undergoing the ball pick-up process (the ball pick-up process includes ball preparation into a predetermined array for pick-up). This can be done by decoupling operation of the vision system from movement of the ball pick head  21 . In this case, the non-process head  47  may be positioned such that fiducial marks  11 ,  12 ,  35 ,  36  of the substrate  3  can be viewed without extensive movement by the non-process head  47 . The ability of the non-process head  47  to move in the y-axis would be desirable. 
       FIGS. 6A to 6F  illustrates an operation sequence illustrating a distribution of process work elements between the pin head  18  and ball pick head  21  according to the preferred embodiment of the invention. The configuration is shown generally looking from direction B of  FIG. 3 . 
       FIG. 6A  shows the ball placement device  22  in a standby position. The substrate  3  is placed and accommodated on the ball/flux mounting platform  34  of the horizontal rail. The pin head  18 , that has flux transfer pins  43  and cameras  32 ,  33  attached to it, is positioned over the flux reservoir  37 . The ball pick head  21 , that has a pick head template  46 , is positioned between the ball template holder  38  and the substrate  3 . 
     In  FIG. 6B , the pin head  18  with fluxed pins is lowered so that the transfer pins  43  are dipped to a predetermined depth into the flux reservoir  37  to collect flux. 
     In  FIG. 6C , the pin head  18  is raised and is moved towards the substrate  3  with a layer of flux  49  collected on the flux transfer pins  43 . Concurrently, solder balls  48  are introduced onto the ball template holder  38  during a ball preparation stage of the ball pick-up process. The ball template holder  38  has recesses that are arranged in the same configuration as solder pads on the substrate  3 . Therefore, the solder balls  48  that are arranged on the recesses are ready to be picked up and placed onto corresponding positions on the substrate  3 . The ball pick head  21  is now positioned over the ball template holder  38  to pick up the solder balls. 
     As the pin head  18  is moved towards the substrate  3  with the cameras  32 ,  33  mounted on it, the cameras  32 ,  33  will search for and then be positioned over fiducial marks  11 ,  12 ,  35 ,  36  of the substrate  3 . As mentioned above, the fiducial marks are read in pairs. Thus, taking diagonally-located fiducial marks  11  and  12  as an example (see  FIG. 3 ), the second camera  33  is positioned over fiducial mark  12  to obtain an image of the fiducial mark  12  and the first camera  32  is positioned over fiducial mark  11  to obtain an image of the fiducial mark  11 . A combination of the relative positions of the two fiducial marks  11 ,  12  allows the ball placement device  22  to determine the extent to which the flux transfer pins  43  of the pin head  18  and the pick head template  46  of the ball pick head  21  are out of alignment with the orientation of the substrate  3 . The pin head motors  23 ,  25  and ball pick head motors  24 ,  26  of the pin head  18  and ball pick head  21  respectively are then capable of adjusting the orientations of the components accordingly in the y and theta axes to correspond with the orientation of the substrate  3  when being positioned over the substrate  3 . Compensation in the x-axis may be provided by movement of the ball/flux mounting platform  34  on the horizontal rail, or in another embodiment (not shown), movement in the x-axis of the pin head  18  and ball pick head  21 , if the pin head  18  and ball pick head  21  are so designed to travel along the x-axis. 
     If the fiducial marks  11 ,  12 ,  35 ,  36  are wide enough such that the two cameras do not physically obstruct each other during pattern recognition, a relatively shorter alignment time is necessary. The cameras  32 ,  33  may even stay in relatively fixed positions in the x-axis for viewing successive substrates. However, if the distance between reference fiducial marks  11 ,  12 ,  35 ,  36  is small, it may be necessary for the cameras  32 ,  33  to give way to each other during pattern recognition. 
     In  FIG. 6D , the flux transfer pins  43  have been aligned with the solder pads of the substrate  3  and are lowered to transfer flux onto the substrate  3 . At the same time, the pick head template  46  is lowered to pick up solder balls  48  in the next stage of the ball pick-up process, usually by vacuum suction means. 
     In  FIG. 6E , the pin head  18  has deposited a layer of flux  49  onto the substrate  3  and is moved back to its standby position. The ball pick head  21  has been raised, and its y and theta motors  24 ,  26  bring it into alignment with the orientation of the substrate  3 . 
     In  FIG. 6F , the solder balls  48  are placed onto the substrate  3  on which has been deposited a layer of flux  49 . The flux  49  helps the solder balls  48  to adhere onto the substrate  3 . At this time, the pin head  18  is lowered into the flux reservoir  37  again to collect another layer of flux  49 . Thereafter, the ball pick head  21  is raised after releasing the solder balls  48  and the substrate  3  is removed from the ball placement device  22 . Another placement cycle is started. 
     By shortening alignment time, system cycle time for solder ball placement may be shortened. As the cameras do not need to travel frequently or need to travel relatively shorter distances, potential wear problems on mechanical parts can be minimized with the reduced movement. There is further a possibility of the dual alignment cameras remaining in relatively fixed positions in the x-axis if the distance between fiducial marks is sufficiently large. With the layout according to the described embodiments, a time critical process head such as a ball pick head can focus on its task and share alignment information collected by a camera mounted to a less critical process head such as a pin head or a non-process head. As a result, the more balanced load distribution of the respective heads lead to reduced alignment time. 
     The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.