Substrate alignment method and apparatus

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.

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'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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a plan view of a typical BGA substrate3with rectangular arrays of conductive solder pads2. Flux and solder balls must be accurately placed at the locations of the solder pads2.

FIG. 2is a plan view of the BGA substrate with its fiducial marks more clearly illustrated. Illustrations of the solder pads2have been removed for clarity.

The substrate3has a number of lines defining grids4corresponding to outlines of the arrays of conductive solder pads2. These grids4include a number of fiducial marks5. An enlarged view of a fiducial mark5of a predetermined design is shown. InFIG. 2, the design of the fiducial mark5comprises regular L-shaped blocks, but it would be appreciated that other designs are also possible.

FIG. 3is a plan view of a substrate alignment and ball placement device22according to the preferred embodiment of the invention. The ball placement device22comprises generally of a support frame1supporting a flux transfer head or pin head18, and a ball pick head21. The ball placement device22includes a number of drivers or motors. A pin head y-motor23attached to the support frame1drives the pin head18along a y-axis, whereas a ball pick head y-motor24attached to the support frame1drives the ball pick head21along the y-axis. Further, a pin head z-motor27and a ball pick head z-motor28drive the pin head18and ball pick head21respectively in the z-axis (i.e. perpendicular to the x- and y-axes). The pin head motors23,27and ball pick head motors24,28are mounted onto the support frame1. There is also a separate module that has a horizontal rail with a ball/flux mounting platform34and is used for flux and ball placement, and substrate transport in the x-axis.

With respect to the pin head18, it includes a pin head theta motor25that drives angular rotation of the pin head18. The pin head18also includes a vision system, which may be in the form of a dual camera alignment module29. Thus, the pin head18acts as a carrier to which the vision system is mounted. The vision system or dual camera alignment module29is adapted to view and obtain positional information of the substrate3. In the preferred embodiment, the vision system comprises a first camera32and a second camera33. Camera motors30,31drive the first and second cameras32,33respectively along linear guides42(seeFIG. 4) in the x-axis in the dual camera alignment module29.

With respect to the ball pick head21, it further comprises a ball pick head theta motor26to drive angular rotation of the ball pick head21. The ball pick head21picks up a plurality of solder balls in a ball pick-up process and deposits them onto a substrate3located on the ball/flux mounting platform34of the horizontal rail. The substrate3typically contains positional indicia, such as fiducial marks11,12,35and36. Typically, fiducial marks are read horizontally or diagonally as pairs to determine alignment of a substrate, such that fiducial mark11may be read simultaneously with fiducial mark12and fiducial mark35may be read simultaneously with fiducial mark36. Other than fiducial marks12, the device22can be programmed to recognize other positional indicia, such as conductive pads, solder pads or any other unique recognition marks on the surface of the substrate3.

At each end of the shaft, there is a flux reservoir37and a ball template holder38respectively. 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 motors23–28,30–31act as drivers to control relative positions of the various components of the ball placement device22to align at least the substrate3and the ball pick head21, as well as the substrate3and pin head18in the described embodiments, for depositing solder balls48onto the substrate3.

FIG. 4is an elevational view of the dual camera alignment module29mounted on the pin head18of the ball placement device22according to one preferred embodiment looking from direction A ofFIG. 3. The camera module29is a modular assembly on which the first and second cameras32,33are mounted. The pin head18is selected for mounting the camera module29as it is a relatively non time-critical process head, i.e. it executes fewer process work elements, as compared to the ball pick head21.

The first camera32and second camera33are capable of independent movement in the x-axis. There is an LED module44,45for each camera32,33. A linear guide42serves as a conduit to allow horizontal movement of the cameras32,33and to position them with respect to the pin head18to view fiducial marks11,12,35,36on the surface of the substrate3. There may be a linear guide42for each camera32,33or a single linear guide42may be shared. The cameras32,33are preferably placed adjacent to each other, although it is also possible to position them on different sides of the carrier or pin head18. A first feed screw40driven by the first camera motor30controls the motion of the first camera32whereas a second feed screw41driven by the second camera motor31controls the motion of the second camera33.

Also illustrated are flux transfer pins43on the underside of the pin head18that are adapted to collect flux, then contact a substrate3to apply flux to it.

FIG. 5is an elevational view of the dual camera alignment module29mounted on a non-process head47according to another preferred embodiment. The non-process head47acts as a carrier for the vision system in this embodiment. The configuration is the same as that ofFIG. 4, except that the non-process head47is not involved in any time-critical process or any function other than supporting and positioning the cameras32,33. The essence of the first and second embodiments is that the vision system or dual camera alignment module29is adapted to obtain positional information of the substrate3substantially simultaneously with the ball pick head21undergoing 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 head21. In this case, the non-process head47may be positioned such that fiducial marks11,12,35,36of the substrate3can be viewed without extensive movement by the non-process head47. The ability of the non-process head47to move in the y-axis would be desirable.

FIGS. 6A to 6Fillustrates an operation sequence illustrating a distribution of process work elements between the pin head18and ball pick head21according to the preferred embodiment of the invention. The configuration is shown generally looking from direction B ofFIG. 3.

FIG. 6Ashows the ball placement device22in a standby position. The substrate3is placed and accommodated on the ball/flux mounting platform34of the horizontal rail. The pin head18, that has flux transfer pins43and cameras32,33attached to it, is positioned over the flux reservoir37. The ball pick head21, that has a pick head template46, is positioned between the ball template holder38and the substrate3.

InFIG. 6B, the pin head18with fluxed pins is lowered so that the transfer pins43are dipped to a predetermined depth into the flux reservoir37to collect flux.

InFIG. 6C, the pin head18is raised and is moved towards the substrate3with a layer of flux49collected on the flux transfer pins43. Concurrently, solder balls48are introduced onto the ball template holder38during a ball preparation stage of the ball pick-up process. The ball template holder38has recesses that are arranged in the same configuration as solder pads on the substrate3. Therefore, the solder balls48that are arranged on the recesses are ready to be picked up and placed onto corresponding positions on the substrate3. The ball pick head21is now positioned over the ball template holder38to pick up the solder balls.

As the pin head18is moved towards the substrate3with the cameras32,33mounted on it, the cameras32,33will search for and then be positioned over fiducial marks11,12,35,36of the substrate3. As mentioned above, the fiducial marks are read in pairs. Thus, taking diagonally-located fiducial marks11and12as an example (seeFIG. 3), the second camera33is positioned over fiducial mark12to obtain an image of the fiducial mark12and the first camera32is positioned over fiducial mark11to obtain an image of the fiducial mark11. A combination of the relative positions of the two fiducial marks11,12allows the ball placement device22to determine the extent to which the flux transfer pins43of the pin head18and the pick head template46of the ball pick head21are out of alignment with the orientation of the substrate3. The pin head motors23,25and ball pick head motors24,26of the pin head18and ball pick head21respectively are then capable of adjusting the orientations of the components accordingly in the y and theta axes to correspond with the orientation of the substrate3when being positioned over the substrate3. Compensation in the x-axis may be provided by movement of the ball/flux mounting platform34on the horizontal rail, or in another embodiment (not shown), movement in the x-axis of the pin head18and ball pick head21, if the pin head18and ball pick head21are so designed to travel along the x-axis.

If the fiducial marks11,12,35,36are wide enough such that the two cameras do not physically obstruct each other during pattern recognition, a relatively shorter alignment time is necessary. The cameras32,33may even stay in relatively fixed positions in the x-axis for viewing successive substrates. However, if the distance between reference fiducial marks11,12,35,36is small, it may be necessary for the cameras32,33to give way to each other during pattern recognition.

InFIG. 6D, the flux transfer pins43have been aligned with the solder pads of the substrate3and are lowered to transfer flux onto the substrate3. At the same time, the pick head template46is lowered to pick up solder balls48in the next stage of the ball pick-up process, usually by vacuum suction means.

InFIG. 6E, the pin head18has deposited a layer of flux49onto the substrate3and is moved back to its standby position. The ball pick head21has been raised, and its y and theta motors24,26bring it into alignment with the orientation of the substrate3.

InFIG. 6F, the solder balls48are placed onto the substrate3on which has been deposited a layer of flux49. The flux49helps the solder balls48to adhere onto the substrate3. At this time, the pin head18is lowered into the flux reservoir37again to collect another layer of flux49. Thereafter, the ball pick head21is raised after releasing the solder balls48and the substrate3is removed from the ball placement device22. 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.