Abstract:
A singulation saw for sawing either substrate or wafers includes a pair of counter-rotating saw blades mounted for independent movement in a vertical direction for alternatively engaging with a substrate to be singulated. The singulation saw further includes a transport system including a pair of substrate carriers reciprocates the substrates. While the first substrate is being cut, the second substrate or other substrate carrier sequentially unloads a cut substrate, loads a new uncut substrate and then moves the uncut substrate to a vision system for determining the position of the substrate relative to the second carrier and then positions the second carrier and its substrate in a standby position ready to be cut by the pair of saw blades that are cutting the first substrate. As the first cut substrate is moved to an unload position, the new uncut substrate is moved into a cutting position.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an improved singulation saw that is adaptable to be used as a dicing or wafer saw. More particularly, the present invention relates to a novel substrate saw having a plurality of rotating saw blades adapted to cut or saw substrates in two opposite directions of relative movement. 
   2. Description of the Prior Art 
   Dicing saws and singulation saws are known. Dicing saws fitted with special saw blades have been used to singulate or separate semiconductor die from a semiconductor wafer mounted on a layer of adhesive on a stretchable plastic film. The dicing saw is preferably programmed to cut streets between rows and columns of die to a depth that penetrates through the wafer into the adhesive layer, thus, completely saw-cutting one die from others. Most prior art saws are designed to saw a single street or cut across a wafer then raise the saw blade and return to the same side and start the next cut. 
   Advance Packaging (AP) devices include a die and a carrier. The sawn die are mounted onto a carrier such as a substrate or printed circuit (PC board) that is provided with conductive leads and/or conductive balls and/or pins. AP technology includes Ball Grid Array (BGA); Micro BGA (μBGA); Flip-Chip devices; Chip Scale Packaging, etc. All such devices are preferably mounted on the substrate with other AP die devices and need to be singulated one from another. 
   Quite often the AP devices are separated from each other by wide streets which cannot be cut and removed by wide saw blades. This requires two saw cuts to remove material forming the street that separate the devices. This dual cut street also causes wafer parts and street scrap to fall away or to be projected by the moving saw blade into fragile parts of the dicing saw such as resilient bellows found in dicing saws. A further problem is that the dicing saw must be shut down to remove the scrap that accumulates. Thus, prior art slurry drain systems are not designed to pass substrate scraps and the conversion of dicing saw systems for use in singulating substrates require a complete shut-down and clean-up periodically. Large pieces of stripped substrate cannot be flushed by liquid means as possible when only wafer slurry is present. 
   Some AP devices are separately encapsulated on a substrate apart from other devices, however, some manufacturers gang encapsulate all AP devices on the substrate with a uniform layer of encapsulating resin such as epoxy which shrinks when passing into the cured state. The gang encapsulated process causes bowing or distortion of the substrate strip and presents problems for the singulation saws when separating one device from another. 
   Prior art dicing saws have been provided with wafer handlers. When the wafer handler is coupled to one side of the saw it inputs a wafer to be cut and subsequently removes a wafer after it is cut. Such handlers are sometimes called in-and-out handlers. The disadvantage of the in-and-out handler is that no productive sawing can be accomplished during the unloading and reloading of the wafers. When dicing saws are adapted to singulate encapsulated AP devices the same non-productive time occurs for unloading and loading strips or substrates with plural AP devices embedded thereon. 
   It has been suggested that prior art dicing saws be modified so that the input handler could supply a wafer to be cut at one side of the saw and the output handler could receive a wafer after being cut. No such saw is known to exist because this would require two handlers that take up twice the handler floor space as well as a major redesign of the transport system in the saw itself. 
   Prior art dicing saws have been provided with two or more saw blades mounted on a single spindle for simultaneously cutting plural paths in a wafer or substrate. This results in a decreased time for cutting a complete wafer. It is not possible to use such systems where very high precision is needed or the street between the devices requires two cuts to remove street material. Further, only when the die being cut is perfectly square and the saw blade is the same width as the street is a multi-blade on one spindle system of practical use. 
   It would be desirable to provide a new design singulation saw that recognizes and solves all of the above-mentioned problems found in modification or conversion of dicing saws to perform singulation of AP devices. It would be desirable to provide a singulation saw that may be modified or converted for use as a dicing saw while retaining all of the improvements that result in increasing through-put. 
   SUMMARY OF THE INVENTION 
   It is a principal object of the present invention to provide a new singulation saw/dicing saw that increases throughput of devices without increasing saw blade speed. 
   It is another principal object of the present invention to provide a new transport system in a singulation saw/dicing saw that performs loading and unloading during sawing time. 
   It is a principal object of the present invention to provide a highly efficient singulation saw with a plurality of saw blades, at least two of which are counter-rotational to permit sawing of substrates in two opposite directions of relative movement. 
   It is a principal object of the present invention to provide a method of partially cutting through thick distorted substrates with the first saw blade in a first direction of cut and then completing the singulation with a different saw blade cutting in a second direction. 
   It is a principal object of the present invention to provide a substrate scrap or parts removal system which virtually eliminates down time for clean-up and removal of substrate scrap. 
   It is a principal object of the present invention to provide a novel transport support and positioning system for a bi-directional sawing system which presents and positions substrates to be cut to a rotating saw blade with a minimum loss of transport time. 
   It is a principal object of the present invention to provide a novel bi-directional sawing system which permits sawing substrates in two directions of relative movement and virtually eliminates the time lost in relocating the saw blade before a second cut. 
   It is a principal object of the present invention to provide a singulation saw for sawing or singulating substrates into individual devices. 
   It is a principal object of the present invention to provide a singulation saw having a higher through-put of devices than prior art saws. 
   It is a principal object of the present invention to provide a novel singulation saw with two separate and distinct saw blades for sawing substrates in different directions of relative movement. 
   It is a principal object of the present invention to provide a new transport system with separate and distinct substrate carriers whose movement is independently controlled for movement in two directions under a relatively fixed saw blades. 
   It is a principal object of the present invention to provide a novel singulation saw having two separate and distinct substrate carriers that alternately reciprocate under a pair of alternately cutting saw blades. 
   According to these and other objects of the present invention there is a provided a pair of counter-rotating saw blades mounted for movement in a vertical direction for alternately engaging with a first substrate to be singulated. A transport system comprising a pair of substrate carriers reciprocates the first substrate under the pair of saw blades and alternate ones of the saw blades are engaged to cut the substrate. While the first substrate is being cut, the second or other substrate carrier sequentially unloads a cut substrate, loads a new uncut substrate and then moves the uncut substrate to a vision system for determining the position of the substrate relative to the second carrier and then positions the second carrier and its substrate in the position ready to be cut by the pair of saw blades that are cutting the first substrate. As the first cut substrate is moved to an unload position, the new uncut substrate is moved into a cutting position with a minimum loss of cutting time. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a prior art reciprocating wafer saw and an I/O wafer handler that has been converted for a singulation saw; 
       FIG. 2  is a block diagram of the present invention bi-directional singulation saw and an I/O substrate handler that may be converted to a wafer saw; 
       FIG. 3  is a block diagram of another embodiment of the present invention bi-directional singulation saw coupled to an input substrate handler and an output substrate handler for continuous throughput performance; 
       FIG. 4  is a schematic plan view of a substrate used to illustrate a plurality of different sizes of semiconductor devices to be singulated; 
       FIG. 5  is an enlarged section in elevation of a singulated Ball Grid Array (GBA) device separated from a substrate; 
       FIG. 6  is an isometric drawing of the present invention singulation saw showing the support cabinet with covers thereon; 
       FIG. 7  is an isometric drawing showing a pair of preferred embodiment substrate transport linear actuators operable below a pair of counter-rotating cutting saw blades; 
       FIG. 8  is an isometric drawing of the singulation saw showing the counter-rotating cutting saw blades and the Y-axis positioning drive for the saw blades, and a support gantry for the rocking frame or head that supports the saw blades; 
       FIG. 9  is front elevation of the singulation saw of  FIG. 8  showing the vision system and the rocking drive motor for positioning the saw blades in a Z-axis; 
       FIG. 10  is an isometric drawing of the rocking head drive motor and its linkage system for positioning the saw blades in a Z-axis; 
       FIG. 11  is an isometric drawing of the counter-rotating saw blades and their spindle drive motors mounted on a rocking head support that is positioned by the locking drive motor shown in  FIG. 10 ; 
       FIG. 12  is an enlarged isometric drawing of one of the substrate carrier support systems showing a preferred embodiment support system for gross and fine positioning of a substrate in two orthogonal cutting positions; 
       FIG. 13  is a schematic plan drawing of the front carrier in a short axis loading position and the rear carrier in a long axis cutting position; 
       FIG. 14  is a schematic plan drawing of the front carrier rotated to permit passage by the rear carrier in the long axis cutting position; 
       FIG. 15  is a schematic plan drawing of the front carrier positioned below the vision system while the rear carrier is still in the long axis cutting position; 
       FIG. 16  is a schematic plan drawing of the front carrier rotated 90 degrees and still positioned below the vision system and the rear carrier has rotated so that the singulation saw is cutting in the short axis cutting position; 
       FIG. 17  is a schematic plan drawing of the front carrier after being positioned in the vision system and the saw is now cutting in the long axis cutting position while the rear carrier is positioned in the right-most position for unloading a singulated substrate and for subsequently loading a new substrate to be singulated; 
       FIG. 18  is a schematic elevation drawing of the front carrier moving in the plus X direction while being cut by the right-most saw blade moving in a counter-clockwise direction; 
       FIG. 19  is a schematic elevation drawing of the singulation saw of  FIG. 18  after completion of the plus X direction cut and the two saw blades are raised to a neutral or non-cutting position; and 
       FIG. 20  is a schematic elevation drawing of the singulation saw of  FIGS. 18 and 19  after the beginning the minus X direction cut with the carrier moving in the minus X direction to engage the left-most saw blade rotating in the clockwise direction. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Refer now to  FIG. 1  showing a block diagram of a prior art wafer saw  1  coupled to an in-and-out handler  2  having a load/unload station  3  for loading and unloading wafers cut at the cutting station  5  by the single spindle saw  4 . 
   Refer now to  FIG. 2  showing a block diagram of a preferred embodiment of the present invention bi-directional saw showing singulation saw  6  coupled to a prior art type handler  2  adapted for substrates and having a load and unload station  3 . The saw  6  comprises a cutting station  5 , a vision station  7 , and a dual spindle saw  8 . 
   Refer now to  FIG. 3  showing a block diagram of another embodiment of the present invention bi-directional singulation saw  6  coupled to an output handler  11  having an unload station  10 . There is shown an input handler  9  coupled to the singulation saw  6  comprising a vision station  7  and a cutting station  5 . 
   Refer now to  FIG. 4  showing a schematic plan view of a substrate illustrating a plurality of different type semiconductor devices to be singulated. There are shown square devices  13  separated by wide streets S of H and large rectangular devices  14  separated by wide streets in two directions and two other types of rectangular devices  15  and  16  also separated by vertical streets S of V. It will be understood that this substrate/strip  12  is not typical but is used to illustrate the different types and sizes of devices that present the problems that can be solved by the novel singulation saw making two cuts along each side of the streets S of H and S of V in order to singulate the devices  13  to  16 . 
   Refer now to  FIG. 5  showing an enlarged section in elevation of a singulated Ball Grid Array (BGA) device  13  comprising a die or semiconductor  17  mounted on a substrate base  18  and showing conductive balls  19  which are connected to the circuitry on the substrate base  18  by vias not shown. Conductive wires  21  are shown connected between lead-out pads  22  and pads  23  on die  17  which connect to circuitry  24  on the substrate base  18 . Encapsulation material  25  completely covers the die  17  and its electrical connections to complete a BGA device. 
   Refer now to  FIG. 6  showing an isometric drawing the preferred embodiment of the present invention singulation saw base  26 , also showing the support cabinet for the base comprising side cover  29 , a computer or controller door D 1 , power supply and amplifier access door D 2 , a pull-out drawer  31  provided with apertures or drain holes  32  which may be provided in a pull-out basket or sieve. The base  26  is preferably a high-density casting of very high precision and provided with recesses  27  for receiving and accurately positioning transport means and having a sink or basin  28  which empties both slurry and substrate parts of particles or scrap into pull-out drawer  31  so that the singulation saw does not have to be shut down for the purpose of cleaning out passageways below the saw but may be prepared for use by simply emptying the basket or drawer  31 . 
   Refer now to  FIG. 7  showing an isometric drawing of a pair of substrate transport means  33  and  34  each comprising a pair of carrier supports  37  and  38  and a pair of linear actuators  35  and  36  that are driven by motors  39  and  40 . It will be understood that the linear actuators shown employ ball screws and drive motors but may be linear motors for positioning the carrier supports  37  and  38 . There are shown four mounting and alignment blocks  41  that slip into the recesses  27  shown in  FIG. 6  and are provided with means for fine adjustment and alignment so that the linear actuators  35  and  36  are positioned accurately relative to the rotating saw blades and parallel to each other. 
   Refer now to  FIG. 8  showing an isometric drawing of the singulation saw  8  showing the counter-rotating saw blades and a Y-axis positioning drive for the saw and the support gantry for the working head that supports the dual spindle saws. The linear actuators  35  and  36  are shown supporting their carrier supports  37  and  38  mounted on mounting and alignment blocks  41  which fit into the recesses  27  of the base or support  26  for the saw  8 . It will be noted that the substrate carriers  42  and  43  at the top of the carrier supports  37  and  38  are adapted to receive rectangular substrate strips and are provided with a vacuum source V which extends below the substrate (not shown) and that the substrate carriers  42  and  43  are accurately positionable from one position shown to an orthogonal position 90 degrees from that shown so that the substrate carriers may pass each other during operation of the transport system. The novel system includes a vision system comprising a vision system camera  44  mounted on a Y-axis linear actuator  45  comprising a Y-axis motor  46 . The Y-axis actuator is moveably mounted on a mounting bracket  48  which mounts on the base or support  26 . It will be understood that the camera  44  may be accurately positioned in X and Y over a substrate carried by one of the substrate carriers  42  or  43  in either of their orthogonal positions when at the left-most vision station end under the vision camera system position as shown at substrate carrier  42 . 
   Refer now to both  FIGS. 8 and 9  showing the singulation saw  8  which is supported by the base or support  26  through a mounting plate or block  49  which supports a Y-gantry  50 . The Y-gantry  50  is coupled to the upper fixed frame  53  through a spacer  52  for movement in the Y-axis. The upper frame  53  supports the lower rocking frame  56  and its rocking shaft  55  which is fixed to the lower rocking frame  56  for movement by the rocker drive motor  54 . The rocking frame  56  supports spindles  59  and  60  and their drive motors which are mounted on the rocking frame  56  and carry the counter-rotating saw blades  57  and  58 . 
   Refer now to  FIG. 10  showing an isometric drawing of the rocking frame drive motor and the linkage system for positioning the saw blades in a Z-axis. Also refer to  FIG. 11  showing an isometric drawing of the counter-rotating saw blades  57  and  58  and their spindle drives  59  and  60  mounted on the lower rocking frame  56  that is positioned by the rocking drive motor  54 . The upper fixed frame  53  is shown attached to the spacer adapter  52  for movement in the Y-axis by the Y-axis drive motor  51 . A support bracket  61 S is mounted on the side of the fixed frame  53  for supporting the flexible E-chain  61  shown in  FIG. 8 . The rocking drive motor  54  is shown mounted on a bearing block bracket  62  and coupled to lever drive block  63  for linear movement. The drive block  63  is mounted through a pivot to a pivot block  64  for driving a lever assembly  65 . The lever  65  is fixed to the rocker drive shaft  55  which extends through the upper fixed frame  53  and is fixedly attached to the rocker frame  56 . Thus, it will be understood that when the rocker drive motor  54  is actuated it can pivot the lever  65 , rotate the shaft  55  which is attached to the lower rocker frame and rocks the rocking frame and moves the singulation saw blades  57  and  58  in the Z-axis. Further, there is provided a brake  66  which couples to a disc on the shaft  55  and may be actuated to fix the saw blades  57  in a Z-direction once they are properly positioned for sawing or singulating a substrate.  FIG. 11  shows the spindle drive motors  67  for the spindle systems  59  and  60  and also show the clamp bracket  68  for holding the spindle drive systems  59  and  60  in the lower rocking frame  56 . 
   Having explained  FIGS. 9 ,  10  and  11  showing the rocking frame  56  and the system for rocking the rocker drive shaft  55 , it will be understood that the rocker drive motor is preferably a stepping motor which can be rotationally positioned to a high degree of accuracy for imparting a high degree of rotational accuracy to the rocker shaft  55  which in turn imparts a high degree of Z-movement to the singulation cutting blades  57  and  58 . 
   Refer now to  FIG. 12  showing an enlarged isometric drawing of the one the substrate carrier supports  37  showing a preferred embodiment system for both gross and fine positioning a substrate in two orthogonal cutting positions 90 degrees from each other. The carrier support  37  is carried by a bearing block  69  which has an aperture  71  for receiving a ball screw (not shown). The bearing block is mounted on a mounting bracket  72  which supports the system identified as a carrier support  37 . The carrier support  37  comprises a rotary actuator  73  for gross positioning the substrate carrier  42  in one of two preferred orthogonal positions. A coupling  74  is mounted between the actuator  73  and a driven disc  75  which has a lever arm  76  mounted thereto. The lever arm  76  is driven in an X-axis direction by a fine positioning theta motor  77  mounted on a bracket  78 . It will be understood that the coupling  74  does not directly couple to the disc  75  but is coupled through a mechanism which allows the rotary actuator  73  to position the pins  83  in one of two rotary positions. Mounted above the disc  75  is bearing housing  79  which is adapted to provide a Z-motion to the substrate carrier when the substrate carrier needs to be positioned in a Z-axis to avoid collision of the substrate carriers  42  and  43  when they are adapted to carry large wafers. In the preferred embodiment of the present invention the substrate carrier support  37  employs an adapter plate  81  and gasket  82  which is designed to mate with the devices on a particular substrate to be sawn so that the vacuum system is applied to each of the substrate apertures  84  which apply the vacuum to the gasket apertures  85  which coincide in position with the devices on the substrate so that when the devices are singulated, the vacuum continues to hold the device even though it is separated from other devices. However, the vacuum system does not hold the edges of the substrate which are sawn loose from the outer perimeter or the streets between the devices which fall down into the sink  28  in the base  26 . 
   Refer now to  FIG. 13  showing a schematic plan drawing of the front carrier  43  in the loading position and the rear carrier  42  in the long axis cutting position. It will be observed that the block  7  defines the vision station and the block  5  defines the cutting station and the block  3  defines the loading and unloading station in this figure. While the substrate  12  in the cutting position is being cut, the substrate  12  in the loading and unloading position  3  is being unloaded and loaded with a new substrate  12  for subsequent cutting. 
   Refer now to  FIG. 14  showing a schematic plan drawing of the front carrier  43  rotated to a long axis position to permit passing the rear carrier  42  in the long axis cutting position. 
   Refer now to  FIG. 15  showing a schematic plan drawing of a front carrier  43  positioned below the vision system  44  while rear carrier  42  is still in the long axis cutting position in the cutting station  5 . 
   Refer now to  FIG. 16  showing a schematic plan drawing of the front carrier  43  positioned below the vision system  44  while the rear carrier  42  is positioned in the short axis cutting position in the cutting station  5 . 
   Refer now to  FIG. 17  showing a schematic plan drawing of the front carrier  43  after being positioned by the vision system  44  and moved into the cutting station in the long axis cutting position while the rear carrier  42  is positioned in the right-most position for unloading a singulated substrate  12  and for loading a new substrate  12  to be singulated. 
   Refer now to  FIG. 18  showing a schematic elevation drawing of the front carrier  43  moving in the plus-X direction while being cut by the saw blade  57  moving in a counter-clockwise direction. 
   Refer now to  FIG. 19  showing a schematic elevation drawing of the singulation saw  8  of  FIG. 18  after completion of the plus-X direction cut and the two saw blades  57  and  58  are raised to a neutral or non-cutting position. 
   Refer now to  FIG. 20  showing a schematic elevation drawing of the singulation saw  8  of  FIGS. 18 and 19  after beginning the minus-X direction cut moving the substrate carrier  43  in the minus-X direction for engagement with the left-most saw blade  58  rotating in a clockwise direction. 
   Refer now to Table 1 which is designed to show the difference between the prior art and the present invention which saves considerable time without having to use both an unload and a load station. 
   
     
       
             
             
           
         
             
               TABLE 1 
             
             
                 
             
           
           
             
               1. 
               Load 
             
             
               2. 
               Position the substrate on the carrier 
             
             
               3. 
               Move the substrate relative to the vision system 
             
             
               4. 
               Vision align the substrate 
             
             
               5. 
               Reposition the substrate for a first cut 
             
             
               6. 
               Lower the blade outside of the substrate 
             
             
               7. 
               Move the substrate through the saw blade at cutting speed 
             
             
               8. 
               Raise the saw blade 
             
             
               9. 
               Retract the substrate at high speed 
             
             
               10. 
               Reposition the Y-axis of the saw blade 
             
             
               11. 
               Rotate the substrate 90 degrees and repeat the cutting sequence 
             
             
               12. 
               Move the substrate to a load/unload station 
             
             
               13. 
               Unload the singulated strip 
             
             
                 
             
           
        
       
     
   
   It will be noted that the cutting sequence numbered  6  through  10  happens multiple times until the substrate is completed sawn in one direction. After rotating 90 degrees then the identical sawing sequence is repeated using the steps shown  6  through  10  a different number of times. Once the substrate is completely cut or singulated it is then moved to the unload station where the singulated strip is removed and an uncut strip is replaced on the carrier. 
   The steps  1  through  5  in the present invention are being performed simultaneously with the operation which occurs in the steps  6  through  11 . Further the preferred embodiment invention cuts in two directions; thus, the step shown at step  9  where the saw blade is retracted is completely eliminated and the cutting operation is performed at least 15 percent faster using the same linear movement of the substrate and the same rotational blade speed. 
   Having explained a preferred embodiment dual spindle saw using counter-rotating saw blades to enable a reciprocating substrate to be cut in two directions of horizontal movement under saw blades that remain fixed in the Z-direction, it will now be understood that the rocking frame which supports the dual spindles may be replaced by two independent Z-actuators each of which support one spindle and is independently programmable in the Z-direction at greater expense. 
   Similarly, the saw blades in the saw system could be replaced by a laser saw system incorporating the novel transport system to unload and load substrates to be singulated while the cutting operation is being performed on a different substrate. Presently, the dual spindle saw blades cut faster and cooler than any known commercially available laser. It is known that lasers cut hot and slower than saw blades and are considered to be a less desirable modification than the preferred embodiment described herein. 
   An example of the benefit of the present invention is that two identical substrates were cut, one using the best prior art system known and the other using the present invention. The best prior art invention saw required 183seconds to singulate two substrates while the present invention did the identical or superior cutting operation in 133 seconds. This results in an approximately 37-½ percent faster cutting time. 
   Prior art dicing saws use several liters of expensive deionized water to cool flush and clean a wafer during cutting. Since cutting time is reduced by the present invention, so is the amount of deionized water needed. 
   In a preferred embodiment singulation saw, it is possible to further reduce actual cutting time by starting each cut in each direction by moving the saw blade vertically to engage the substrate and simultaneously starting horizontal movement. For lack of a better descriptive name this start cut is called a plunge cut. In similar manner at the end of a cut when the saw blade breaks through the bottom edge, the saw blade may be immediately moved upward to a neutral position in a motion which is the reverse of a plunge cut. As soon as the blade is clear of the substrate being cut the Y-axis gantry positions the rocking frame (or head mount) ready for the next plunge cut and start of singulation of a substrate.