Abstract:
An apparatus for subdividing a sheet of brittle insulating material with a plurality of semiconductor chips disposed thereon. The chips are separated by row and column kerfs each of which contains a respective scribed line. The subdivision of the sheet is accomplished by placing the sheet in a flexible conformable carrier having an open grid, formed of ribs, with each rib positioned over a respective scribe line on the surface of the sheet and forcing the sheet against an arched anvil, thereby fracturing the sheet along the scribe lines.

Description:
FIELD OF THE INVENTION 
   This invention generally relates to a method and an apparatus for subdividing or dicing a sheet of brittle insulating material into a plurality of discrete or individual regular sections or arrays and more particularly relates to an apparatus and method for the automatic separation or singularization of a sheet of brittle insulating material, having a plurality of semiconductor chips mounted thereon, so that each discrete chip is provided with its own respective insulating substrate such that it can easily be included in an electronic package. 
   BACKGROUND OF THE INVENTION 
   As is well known, a plurality of semiconductor circuits, are formed in large semiconductor wafers. The circuits are typically arranged on the wafer in rows and columns separated one from the other by regions known as kerfs. Once the circuits are formed in the wafer; metallic connections are provided on the surface of each circuit; the circuits are then separated one from the other; each circuit is then mounted on a nonconductive base or substrate such that it may be further handled or provided with a protective cover. 
   Such nonconductive bases or substrates are typically divided out of a larger, thin sheet of a rigid insulating material, such as ceramic, that has arranged on one surface thereof a plurality of individual wiring blocks. Each wiring block on the sheet comprises a wiring path and connections designed for the semiconductor chip to be placed thereon. These wiring blocks are formed on the insulating sheet in rows and columns that are separated one from the other by isolating regions known as kerfs. 
   A respective semiconductor chip is then electrically bonded to each respective block and the sheet is then divided in each kerf to singularize the blocks one from the other. In this way, each chip is provided with a respective substrate or base that is suitable for receiving a protective cover over the chip. 
   An early method of individualizing or singularizing such substrates from such a larger sheet consisted of sawing the insulating sheet in the kerf areas between the blocks. This method was found to be very slow and caused fine debris to be deposited on the surface of the blocks requiring follow-up cleaning steps. Moreover such cleaning steps were often ineffective in removing debris lodged beneath chips that were mounted on each block prior to the sawing action. 
   A later method consisted of scribing, on the sheet, a first set of scribed lines in the row kerfs and a second set of scribed lines in the column kerfs, which are perpendicular to the first set of scribed lines to create a plurality of cross hatched lines on the sheet surface. Using this process, the scribe lines enclose a plurality of defined enclosed regions on the substrate surface. A printed circuit wiring block is then formed in each scribe defined region and a respective chip is mounted on each printed circuit block. Next the chip carrying scribed sheet is placed on an elastic base and the sheet is fractured along the scribed lines by passing a roller over the sheet in a first direction parallel to the first set of the scribed lines and then passing the roller over the sheet in a second direction that is parallel to the second set of scribed lines, and at right angles to the first direction. This method, however, proved unsatisfactory because of variations in the force exerted on the roller and in the elasticity of the base, causing fractures to occur in the sheet in regions other than along the scribed lines. Still further, because chips had been mounted on the sheet prior to passing the roller over the surface of the unit, the rolling action was found to cause deformations and/or breaking of the conductive bonds between the chips and the underlying wiring blocks on the ceramic substrate. All of these difficulties resulted in excessive failure rates. 
   Another method employed a machine in which the chip carrying scribed sheet was placed against a convex die and a steel band was drawn against the chip mounted sheet, forcing the sheet against the die and causing the sheet to fracture along the first set of scribed lines, and then turning the partially broken substrate ninety degrees to fracture the substrate on the first set of lines. This machine also had difficulties associated with it for when used on a production line the tension on the band was found to be difficult to control and if the tension was even just slightly excessive, the blocks themselves were broken or cracked in undesired regions. Also the movement of the band across the surface of the chips mounted on the blocks often caused deformations and/or breaking of the conductive bonds between the chips and the underlying wiring blocks again resulting in undesirable losses. 
   A further method of subdividing such a scribed sheet carrying chips thereon used two sets of mating convex and concave arched dies positioned inline. The direction of the curves of the dies of the second set being positioned at a right angle with respect to the direction of the curves in the first set. The scribed sheet, to be subdivided, is mounted on an adhesive tape passing between both sets of dies. The sheet, is then positioned between the dies of the first set so that the convex die could be moved to force the scribed sheet against its mating concave die to break the sheet along a first set of the crosshatched scribed lines, following which the convex die was retracted, the tape moved between the second set of dies and the process repeated to fracture the sheet along the second set of scribed lines. In this arrangement each die in a set must be exactly positioned with respect to its mating die, for any misalignment of the dies or improper spacing between the dies when closed can result in either un-separated portions of the sheet or breakage of the sheet in undesired areas or breaking or distortion of the chip to block bonds. Further it was found that the first set of mating dies could cause a distortion in the tape resulting in an inappropriate shifting and misalignment of the sheet under the second set of dies. These problems also resulted in undesirable failure rates. Because of these difficulties a better mechanism and process for subdividing or singularizing a chip carrying sheet has long been sought. 
   SUMMARY OF THE INVENTION 
   The present invention has been designed to overcome the difficulties found in the prior art and does so through the use of an apparatus using a single die or anvil having a fixed convex arch and a conformable carrier mechanism carrying a previously scribed sheet of brittle material that is to be divided into regular individual pieces. 
   The surface of the sheet is cross hatched by forming first and second sets of scribe lines in the kerf areas between the chips. Each scribe line in a respective set is positioned in its own kerf and traverses the entire width of the sheet and is parallel to every other scribe line in the same set. The lines in one set are arranged perpendicular to the lines in the other set thus providing the cross hatched lines across the surface of the sheet. 
   Once scribed and populated with chips, the sheet is loaded into and sandwiched in a carrier having been formed of flexible and conformable, upper and lower carrier elements. The loaded carrier is then placed beneath an arched anvil of a suitable drive mechanism capable of applying fluidic pressure, such that a first set of scribed lines is parallel to the arch of the anvil. The carrier, carrying the sheet, is then forced by the fluid of the drive mechanism to conform to the arch of the anvil, while the anvil is rigidly held in a first known position at a specified distance above the carrier. Because the force used to conform the carrier to the anvil is a fluid it is applied with equal force over the entire surface of the carrier and it need only be sufficient to force the carrier, containing the scribed sheet to conform to the arch of the anvil, thereby fracturing or breaking the sheet along the set of scribe lines parallel to the arch of the anvil. Once the sheet is so fractured, the force is removed from the drive mechanism allowing the carrier to return to its initial position, the anvil is then raised above the carrier and rotated ninety degrees to a second known and fixed position such that the arch of the anvil, in this second position, is now ninety degrees to its previous direction. Once the anvil is in this second position, the anvil is lowered once again towards the top surface of the carrier and held there as described above. The drive mechanism is again used to force the carrier and the scribed, chip populated sheet contained therein to again conform to the arch of the rotated anvil thereby fracturing the sheet along the second set of scribe lines. 
   Further by designing the upper element of the carrier with a grid, formed by a plurality of ribs forming an open cross hatched framework with each rib overlying the scribed cross hatched lines in the sheet, breakage of the sheet only along the crossed hatched scribe lines is assured. Furthermore by having the openings defined by the ribs overlying the chips on the surface of the pressure applied by the drive mechanism is not applied to the surface of the chips thus assuring that the chips and their wiring block connections are not stressed, deformed or broken as could occur with the prior art techniques. 
   Still further, by providing such a grid in the conformable carrier of the invention and using fluidic pressure to force the conformable carrier containing the chip loaded sheet against the same anvil to fracture the sheet along both sets of scribe lines, the damage to the separated chips, the substrates, or the chip to substrate connections that were encountered by when using the prior art mechanisms is eliminated. 
   Accordingly, it is an object of the present invention to provide an apparatus for subdividing or singularizing a sheet of brittle material into a plurality of discrete or individual regular sections 
   It is a another object of the present invention to provide a method for the accurate, automatic separation or singularization of a sheet of brittle insulating material into a plurality of discrete chip carrying substrates ready for installation in an electronic package. 
   It is still a further object of the present embodiments of the invention to provide a method and an apparatus for singularizing a ceramic sheet carrying thereon a plurality of semiconductor circuits arranged and connected to circuitry on the sheet in a predetermined array without damaging the semiconductor circuits or the connections between the sheet and the circuits affixed thereto. 
   These objects, features and advantages of the present embodiments of the invention will become further apparent to those skilled in the art from the following detailed description taken in conjunction with the accompanying drawings wherein: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an apparatus provided in accordance with a preferred embodiment of the present invention for automatically singularizing a sheet of a brittle material such as ceramic provided with a regular arrangements of circuit blocks each of which has a respective semiconductor device electrically bonded thereto; 
       FIG. 2  is an exploded view of the lower portion of  FIG. 1  containing the fluid drive mechanism of the apparatus; 
       FIG. 3  is a partial exploded view of the anvil positioning mechanism of the apparatus illustrated in  FIG. 1 ; 
       FIG. 4  is an exploded view of the conformable carrier used in the apparatus of  FIG. 1 ; 
       FIG. 5  depicts a scribed ceramic sheet provided with wiring blocks and chips thereon; 
       FIG. 6  is a cross sectional view of the conformable carrier containing a chip mounted and scribed and positioned beneath the arched anvil before the apparatus is pressurized; and 
       FIG. 7  is a cross sectional view of the conformable carrier containing a chip mounted and scribed when the apparatus is pressurized and the carrier is forced to conform to the arch of the anvil. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The apparatus  10 , as shown in  FIGS. 1 ,  2  and  3 , is comprised of a lower portion  11  containing an enclosed carrier support on a fluidic pressure applying unit, an anvil and an upper portion  12  that contains an anvil positioning and drive mechanism. 
   The lower portion  11  is contained in an enclosure  16 . This enclosure  16  is supported on a base plate  15 . The enclosure  16  has two sections  18  and  32 . The lower section  18  contains a pressure containment apparatus and its upper section  32  provides an anvil chamber. On the base plate  15  there is provided a pair of parallel rails  17   a  and  17   b  which carry the pressure containment apparatus which is shown in detail in  FIG. 2 . 
   As shown in  FIG. 2 , the pressure containment apparatus is a 28,900 mm square box  19  having side walls  20   a  and a unitary bottom  20   b.  The top of the side walls  20   a  are sealed with a 3.2 mm thick, metallic cover plate  21 . Over this cover plate  21  there is disposed a thin flexible, elastic, resilient and conformable membrane  23 . The membrane  23  may be comprised, for example, of a sheet of high grade Buna-N Rubber (Electrostatic Discharge Protection) that is 1.6 mm thick. The cover plate  21 , has a central circular aperture  22  that is of a diameter smaller than the longest diagonal of the carrier to be placed therein, such that the weight of the carrier is effectively supported by the cover plate  21 , and not by the membrane  23 . For example, when the sheet to be singularized is 50 mm by 50 mm the carrier size is 81 mm by 81 mm. The diagonal of the carrier is 114.5 mm, the central aperture  22  of plate  21  will be at a maximum of 100 mm in diameter. Over the membrane is positioned a rigid metallic carrier locator plate  24  that is 12 mm in thickness which has a central aperture  25 , for example, in the form of a quadrate cross whose central square section must accommodate the carrier to be placed therein, as will be described below. It should be understood that the details given above as to dimensions, shapes and the materials used can and will vary depending on the size and thickness of the sheet being fractured and can be readily established by any competent engineer. 
   The cover plate  21 , the membrane  23  and the locator plate  24  are all secured to the top of the side walls  20   a,  of chamber or box  19 , by a plurality of screws  29  that pass through holes  30   c  around the edges of the plate  21 , holes  30   b  around the edges of membrane  23 , and holes  30   a  around the edges of the carrier locator plate  24  into tapped holes  31  in the top of the side walls  20   a  to form a leak proof pressure seal with the upper rim of the side walls  20   a  of the chamber  19 . A flexible pressure line  27 , couples the interior of the pressure chamber  19  to a suitable fluid supply such as an air compressor  26 . Above the chamber  19  and behind door  30  there is positioned the anvil chamber  32  which contains an anvil  50  as shown in  FIG. 3 . 
   Above the anvil chamber  32  there is provided on the top surface  33  of enclosure  16 , a first spaced pair of L shaped supports  34  and  35  that are spanned by a support cross piece  38  that carries on its underside a reciprocating piston  40  such as a CDQ2KB63F-15DM-A73HL piston sold by the SMC company that has an extended central shaft  41  attached to it. The upper end of shaft  41  passes up through a suitable aperture in the cross piece  38  where it is connected by a coupling  43  to the shaft  44  of a suitable mechanism  45  for rotating the anvil  50 . This rotating mechanism  45  may be, for example, any suitable device such as a rotary pneumatic cylinder or an electrically driven stepping motor and is held a fixed distance above the cross piece  38 , by a pair of support arms  46  and  47  and a mount plate  48 . 
   As shown in  FIG. 3 , the lower end of the central shaft  41  of the piston  40  is coupled via support  49  to an anvil  50 , having an arched lower surface  51 , such that the anvil  50  can be suspended directly in the anvil chamber  32  over the central square section of the quadrate cross opening  25  in the center of plate  24  in which a flexible carrier apparatus  60  can be placed. As noted previously, the central square opening in the quadrate cross opening  25  is designed to accommodate the carrier  60  that is to be placed therein and the anvil  50  is designed such that its length will be approximately equal to the length of the carrier sited in the central square section of the quadrate cross  25 . 
   The carrier  60 , shown in  FIGS. 4 ,  6  and  7  is comprised of a base or lower element  61  which is flat and formed from a sheet of black polyurethane having a thickness of about 9.53 mm. The central portion  63  of the lower carrier element  61  is enclosed by a wall  62  in which a scribed and printed ceramic sheet  55 , populated with semiconductors chips or devices, as illustrated in  FIG. 5  can be placed. If needed, the bottom surface of the central portion  63  can be configured, in order to accommodate any connections (pins, BGA, etc.) that might be provided on the bottom of the sheet. The height of the wall  62  is set equal to or slightly thinner than the thickness of the ceramic sheet  55  to be placed therein. On each corner  59  of the portion of the lower carrier element  61  outside of the wall  62  are secured positioning devices  64 , each of which is provided with a set of upright studs  67 . Each respective device  64  interfaces with a respective set of locating holes  66  in each corner of an upper carrier element  65 . The center of the upper carrier element has an open grid pattern  70  formed by a plurality of cross hatched ribs  68  therein. The ribs  68  are designed to match, and to overlie the crossed hatched scribe lines  54  on the underlying ceramic sheet  55  placed in the carrier as depicted in  FIG. 6 . The openings  70 , in the upper carrier element, defined by the ribs  68 , are designed to be larger than the chips and underlying wiring blocks placed on the ceramic sheet and to overlie the scribe lines in the underlying sheet  55 . By making the height of these ribs  68  greater than the height of the chips  56  bonded to the ceramic sheet  55 , as shown in  FIG. 6 , the arched surface  51  of the anvil  50 , used to fracture the sheet along the scribed lines, is prevented from coming in to contact with the chips or applying any force to the chips or their connections to their underlying printed circuit blocks. By so protecting the chips, these ribs, assure that each chip and its underlying connections to its wiring block cannot be stressed by the anvil and thus will remain undisturbed during the fracturing and separation of the sheet contained in the carrier. 
   The process of accomplishing this protective separation of a scribed chip mounted ceramic sheet will be now be particularly described especially in conjunction with  FIGS. 4 ,  5 ,  6  and  7 . 
   Initially, as shown in  FIG. 5 , a previously scribed ceramic sheet  55 , typically 50 mm by 50 mm and ranging from 0.7 to 1.5 mm in thickness has a plurality of wiring blocks (not shown) formed thereon by a state of the art process. The wiring blocks are laid out on the surface of the ceramic sheet within defined kerf areas which contain the notched cross hatched scribe lines  54   a  and  54   b  as is well known to the art. Subsequently, a respective semiconductor chip  56  is bonded to each wiring block again using processes well known to the art. The wiring blocks are, of course, designed to accommodate the chips to be placed thereon. It should be noted that the area within the cross hatched scribe lines  54   a  and  54   b  can be either square or rectangular in form. It is also to be noted that the sheet is broken along these scribe lies  54   a  and  54   b  to form singularized substrates. 
   At present, square singularized substrates typically range from 8 to 15 mm square and rectangular singularized substrates are typically, 4 mm by 6 mm or larger. It is of course understood that both the chips and the ceramic sheets can be either smaller or larger than these described sizes and that the carrier will be designed to match the sheet and/or chips affixed thereto. Usually when square substrates are to be singularized from the ceramic sheet the grid ribs  68  will form square openings and when oblong substrates are to be singularized from the ceramic sheet, grid ribs  68  will form oblong openings. 
   Once the scribed and cleaned ceramic sheet  55  to be singularized has been provided with chips  56  as shown in  FIG. 5 , it is loaded into the walled enclosure  63  provided in the carrier base  61  and a carrier cover  65  having an appropriate grid formation  68  is placed thereon. The cylinder  40  is activated to raise and hold the lowest surface  51  of anvil  50  about 6 mm above the top surface of plate  24 . Pressure drawer  19  is now opened by moving it out of enclosure  11  on rails  17   a  and  17   b.  Once drawer  19  is outside enclosure  11 , a carrier containing a scribed, ceramic sheet  55  is placed in the square center of quadrate cross opening  25  in the plate  24  so that it rests over of flexible membrane  23  affixed to the top of drawer  19 . The bulk of the carrier is thus positioned over opening  22  in plate  21 . It is of course to be recalled that the corners of the carrier extend beyond the edges of opening  22  and thus the corners are positioned over plate  21 . Drawer  19  is then moved back into enclosure  11  such that the carrier is positioned directly beneath raised anvil  51 . 
   Cylinder  40  is now activated to push anvil  50  down, about 6 mm, such that, as shown in  FIG. 6 , the lowest point of its arch  51  is held just above or just touching the center of the upper surface of carrier  60  containing scribed ceramic sheet  55  that is to be separated. Thus, as illustrated in  FIG. 6 , arch  51  of anvil  50  is parallel to a first set of scribe lines in the sheet. 
   The interior of drawer  19  is now pressurized to between about 20 and 40 PSI. As shown in  FIG. 7  this causes membrane  23  to expand upwards through quadrate cross opening  25  such that as carrier  60  and the scribed, chip loaded, ceramic sheet  55 , contained therein, is forced to conform to arch  51  of anvil  50 , positioned above the carrier, sheet  55  is caused to fracture along scribed lines  54   a.  Because carrier  60  is now being forced upwards against anvil  50 , it is to be understood that anvil  50  must be held fixed in its position with a force equal to or greater than the force being applied to carrier  60  by the expanded membrane. By so holding the anvil in affixed position carrier  60  and its contained ceramic sheet  55  is forced to conform to the arch of anvil  50  and the sheet breaks along first set of scribe lines  54   a  parallel to the direction of arch  51  of anvil  50  as shown in  FIG. 7 . Once the sheet is broken the pressure in drawer  19  is reduced permitting the membrane  23  and carrier  60  to return to their initial flat positions. 
   At this time, anvil  50  is again raised above the carrier surface about 6 mm by activating cylinder  40 . Once raised the cylinder is rotated 90 degrees by rotating apparatus  45  to place arch  51  of anvil  50  perpendicular with respect to its original position and parallel to the second set of scribe lines  54   b  in the sheet. Cylinder  40  is again activated to lower anvil  50 , as described above, such that the center of arch  51  of anvil  50  is again just touching the upper surface of the carrier. Again, the interior of drawer  19  is pressurized and membrane  23  expands upwards through quadrate cross opening  25  causing carrier  60  and the ceramic sheet contained therein to be forced upwards against arch  51  of anvil  50  positioned above the carrier. Because anvil  50  is now positioned at 90 degrees to its first position the sheet in the carrier is broken along second set of scribe lines  54   b.  Again it is to be noted that the pressure in cylinder  40  must be sufficient to hold anvil  50  in a fixed position as carrier  60  is forced against anvil  50  with a force sufficient to break carrier enclosed sheet  55  along the second set of scribe lines  54   b.  When ceramic sheet  55  in the carrier fractures along this second set of scribe lines the chips are singularized. 
   Once sheet  55  has been so singularized the pressure in drawer  19  is reduced, anvil  50  is again retracted from the surface of the carrier, drawer  19  is opened, the carrier removed, and the singularized substrates are removed therefrom. At this time, a new scribed, chip carrying ceramic sheet may be inserted in the carrier and the entire cycle described above is repeated. 
   Because the grid in the upper segment is open, force is applied by anvil  50  to the sheet contained in the carrier only by ribs  68  that lie along the scribe lines. Thus in the embodiment of the present invention, force is not applied to the chips, mounted on the sheet being fractured, as occurred in the prior art techniques. By eliminating such direct pressure on the chips mounted on the sheet, as occurred in the prior art, the present invention assures that the electrical connections or bonds between the chips and their underlying circuits on the sheet are preserved and remain unaffected during the sheet breaking action. This embodiment of the present invention thus avoids the problems of distortion or breakage of the chip to circuit connections encountered by the prior art equipment or processes. 
   The present invention thus teaches a simple, inexpensive and automatic machine and process for overcoming the difficulties found in the prior art and does so through the use of an apparatus using but a single convex arched anvil against which a conformable carrier mechanism carrying a previously scribed sheet of brittle material that is to be divided into regular individual pieces can be forced by an equalized fluid pressure. 
   This completes the description of the preferred embodiment of the invention. Since changes may be made in the above construction without departing from the scope of the invention described herein, it is intended that all the matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
   Other alternatives and modifications will now become apparent to those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.