Abstract:
An improved apparatus for filling patterned mold cavities formed on a surface of a mold structure with solder includes an injector head assembly comprising a solder reservoir having a bottom surface with an elongated slot for injecting solder into the mold cavities and a carriage assembly configured to carry and scan the mold structure under the injector head assembly. The mold structure is brought into contact with the solder reservoir bottom surface and a seal is formed between an area of the solder reservoir bottom surface surrounding the elongated slot and the mold structure surface and then the mold structure is scanned under the injector head assembly in a first direction from a starting position to a finish position for filling the mold cavities and in a second direction opposite to the first direction for returning back from the finish position to the starting position.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 60/908,737 filed Mar. 29, 2007 and entitled “APPARATUS AND METHOD FOR SEMICONDUCTOR WAFER BUMPING VIA INJECTION MOLDED SOLDER”, the contents of which are expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an apparatus and a method for semiconductor wafer bumping, and more particularly to semiconductor wafer bumping via an injection molded solder process. 
       BACKGROUND OF THE INVENTION 
       [0003]    Injection Molded Solder (IMS) is a process used to produce solder bumps on a semiconductor wafer surface. Referring to  FIG. 1 , the IMS process  30  includes depositing solder into mold cavities ( 34 ), forming a pattern on the semiconductor wafer surface ( 32 ), aligning the filled mold cavities with the patterned semiconductor wafer surface and then transferring the solder from the mold cavities to the semiconductor wafer surface ( 38 ). Solder bumps are formed in a glass mold plate  82  by injecting molten solder into the etched mold cavities. The etched cavities match the pattern of solder bumps required on the semiconductor wafer surface. The process provides fine pitch placement of the solder bumps in the range of 10 to 500 micrometers separation distance between adjacent solder bumps. 
         [0004]    The IMS process has been tested and applied for laboratory scale applications. It is desirable to provide a scale-up process and a high volume manufacturing (HVM) apparatus designed to optimize the high volume manufacturing process. A critical aspect of the scale-up process involves the complete filling of the mold cavities. It is desirable to fill all mold cavities uniformly with solder without leaving some cavities either not filled or partially filled or overfilled. 
       SUMMARY OF THE INVENTION 
       [0005]    In general, in one aspect, the invention features an apparatus for filling patterned mold cavities formed on a surface of a mold structure with solder. The apparatus includes an injector head assembly comprising a solder reservoir having a bottom surface with an elongated slot for injecting solder into the mold cavities and a carriage assembly configured to carry and scan the mold structure under the injector head assembly. The mold structure is brought into contact with the solder reservoir bottom surface and a seal is formed between an area of the solder reservoir bottom surface surrounding the elongated slot and the mold structure surface and then the mold cavities are filled with solder by scanning the mold structure under the injector head in a first direction from a starting position to a finish position while solder is injected into them and then the mold structure with the filled mold cavities is returned back to the starting position by scanning it in a second direction opposite to the first direction without any further solder injection 
         [0006]    Implementations of this aspect of the invention may include one or more of the following features. The solder reservoir is heated to a temperature above the solder melting point and is slightly pressurized. The apparatus further includes a preheat station where the mold structure is preheated to a temperature slightly below the solder melting temperature prior to the scanning and filling of the mold cavities with solder. The apparatus further includes a cool down station where the mold structure is slowly cooled down to room temperature after the filling of the mold cavities with solder. The apparatus further includes an aligner for aligning the mold structure prior to the scanning and filling of the mold cavities with solder. The injector head assembly remains stationary during the scanning in the first and second directions of the mold structure. The carriage assembly includes first and second parallel rails, a plate assembly movable upon the first and second parallel rails, a start parking space and a finish parking space. The mold structure is placed upon the movable plate assembly for performing the scanning under the injector head assembly. The plate assembly includes a heated plate upon which mold structure is mounted, a non-heated back plate and an insulator plate placed between the heated plate and the back plate. The mold structure is kinematically mounted onto the heated plate via three motorized pins and wherein the three motorized pins are arranged at 120 degree angle relative to each other and are used for lowering and raising the mold structure onto the heated plate. Each of the parking spaces includes a parking plate having a V-groove on its bottom surface, a heated base supporting the parking plate, an insulator plate supporting the heated base, a back plate supporting the insulator plate and first and second leveling plates configured to level the back plate. The heated base and the heated plate upon which the mold structure is mounted are heated to a temperature slightly above the solder melting point. The injector head assembly further includes a hot gas skirt surrounding the solder reservoir bottom surface and being configured to blow hot gas onto it. The injector head assembly further comprises an O-ring surrounding the elongated slot and forming the seal between an area of the solder reservoir bottom surface surrounding the elongated slot and the mold structure surface. The O-ring is made of a material capable of withstanding the solder melt temperature. The apparatus further includes an injector head slide assembly and the injector head assembly is supported and mounted onto the injector head slide assembly via a mounting mechanism that stabilizes the injector head assembly position while allowing for small thermal sidewise expansions, along the direction of the elongated slot. The apparatus further includes a support frame supporting the injector head slide assembly and first and second parking space lifters. The first and second parking space lifters are configured to pick the start and finish parking plates, respectively and raise them up or lower them down onto the corresponding heated bases. The apparatus further includes a parking plate cleaner configured to clean the parking plates. The injector head slide assembly is made of a material with low coefficient of thermal expansion. The apparatus further includes one or more thermocouples inserted into the solder reservoir at various heights and the thermocouples produce thermocouple readings used to measure solder fill levels in the solder reservoir. The injector head assembly may further include capillaries through which air is blown toward the elongated slot for freezing any molten solder. 
         [0007]    In general, in another aspect, the invention features an apparatus for forming solder bumps onto semiconductor structures including equipment for filling patterned mold cavities formed on a surface of a mold structure with solder, equipment for positioning and aligning a patterned first surface of a semiconductor structure directly opposite to the solder filled patterned mold cavities of the mold structure, a fixture tool for holding and transferring the aligned mold and semiconductor structures together and equipment for receiving the fixture tool with the aligned mold and semiconductor structures and transferring the solder from the aligned patterned mold cavities to the aligned patterned semiconductor first surface. The mold cavity filling equipment include an injector head assembly comprising a solder reservoir having a bottom surface with an elongated slot for injecting solder into the mold cavities and wherein the mold structure is brought into contact with the solder reservoir bottom surface and the elongated slot and a seal is formed between an area of the solder reservoir bottom surface surrounding the elongated slot and the mold structure surface and then the mold cavities are filled with solder by scanning the mold structure under the injector head in a first direction from a starting position to a finish position while solder is injected into them and then the mold structure with the filled mold cavities is returned back to the starting position by scanning it in a second direction opposite to the first direction without any further solder injection. 
         [0008]    In general, in another aspect, the invention features a method for filling patterned mold cavities formed on a first surface of a mold structure with solder including the following steps. First providing an injector head assembly comprising a solder filled reservoir having a bottom surface with an elongated slot for injecting solder through the elongated slot into the mold cavities. Next, providing a carriage assembly configured to carry and scan the mold structure under the injector head assembly and loading the mold structure onto the carriage assembly. Next, bringing the mold structure in contact with the solder reservoir bottom surface and the elongated slot and forming a seal between an area of the solder reservoir bottom surface surrounding the elongated slot and the mold structure surface and then scanning the mold structure under the injector head assembly in a first direction from a starting position to a finish position while injecting molten solder into the mold cavities and then scanning the filled mold structure in a second direction opposite to the first direction, without any further solder injection, for returning the mold structure back from the finish position to the starting position. The scanning step includes the following. First, positioning the injector head assembly upon the heated start parking space and then loading and centering the mold structure onto the plate assembly, so that is in contact with a heated plate. Next, moving the heated finish parking space to a fixed stop and aligning an edge of the finish parking space with an adjacent edge of the mold structure. Next, moving the start parking space with the injector head assembly to a start position and then locking the start and finish parking space positions. Next, scanning the heated plate with the mold structure in the first direction so that the injector head assembly is moved from the start parking space onto the mold structure. Next, cleaning any solder deposits from the start parking plate and lifting the start parking plate from the start parking space with the first parking space lifter. Next, continue scanning the heated plate with the mold structure under the injector head assembly and the lifted start parking plate in the first direction while injecting solder into the mold cavities until all mold cavities are filled with solder. Next, positioning the injector head assembly onto the finish parking space and placing it in contact with the finish parking plate. Next, separating the heated plate with the filled mold structure from the finish parking space. Next, lifting the injector head assembly with the finish parking plate from the finish parking space and placing the cleaned start parking plate onto the finish parking space. Next, scanning the heated plate with the filled mold structure in the second direction under the finish parking plate and injector head assembly until reaching the start parking space. Finally, placing the finish parking plate with the injector head assembly onto the start parking space and unloading the filled mold structure. 
         [0009]    The method may further include positioning and aligning a patterned first surface of a semiconductor structure directly opposite to the solder filled patterned mold cavities of the mold structure. Next, providing a fixture tool for holding and transferring the aligned mold and semiconductor structures together. Next, receiving the fixture tool with the aligned mold and semiconductor structures and transferring the solder from the aligned patterned mold cavities to the aligned patterned semiconductor first surface. The mold cavities pattern matches the semiconductor surface pattern. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    Referring to the figures, wherein like numerals represent like parts throughout the several views: 
           [0011]      FIG. 1  is a schematic diagram of a laboratory scale Injection Molded Solder (IMS) process; 
           [0012]      FIG. 2  is a schematic diagram of a scale-up IMS process according to this invention; 
           [0013]      FIG. 3  is a block diagram of the scale-up IMS process flow; 
           [0014]      FIG. 4  is a schematic diagram of the HVM IMS equipment system according to this invention; 
           [0015]      FIG. 5  is a schematic diagram of the mold fill process; 
           [0016]      FIG. 6  is a diagram of a partially filled mold plate; 
           [0017]      FIG. 7A  depicts a mold with unfilled cavities; 
           [0018]      FIG. 7B  depicts a mold with filled cavities; 
           [0019]      FIG. 8  is a schematic diagram of the solder transfer process; 
           [0020]      FIG. 9  depicts the HVM MFT equipment system of this invention; 
           [0021]      FIG. 10  depicts the mold preheat station; 
           [0022]      FIG. 11  depicts the mold preheat station of  FIG. 10  without a cover; 
           [0023]      FIG. 12  depicts the mold cooling station; 
           [0024]      FIG. 13  depicts the HVM MFT station; 
           [0025]      FIG. 14  depicts the hot plate assembly components of the HVM MFT station of  FIG. 13 ; 
           [0026]      FIG. 15  depicts the hot plate assembly with a mold; 
           [0027]      FIG. 16  is a detailed side view of the hot plate assembly of  FIG. 15 ; 
           [0028]      FIG. 17  is a detailed side view of the mold pin alignment mechanism with the pin up; 
           [0029]      FIG. 18  is a detailed side view of the mold pin alignment mechanism with the pin down; 
           [0030]      FIG. 19  is a perspective view of the staring parking space; 
           [0031]      FIG. 20  is a side view of the staring parking space; 
           [0032]      FIG. 21  is a perspective view of the finish parking space; 
           [0033]      FIG. 22  is a side view of the finish parking space; 
           [0034]      FIG. 23  is a side perspective view of the three pin motorized mount system; 
           [0035]      FIG. 24  is a front perspective view of the solder head assembly; 
           [0036]      FIG. 25  is a front perspective view of the injector head; 
           [0037]      FIG. 26  is a front view of the injector head; 
           [0038]      FIG. 27  is a top view of the injector head; 
           [0039]      FIG. 28  is a perspective view of the injector reservoir; 
           [0040]      FIG. 29  is a front view of the injector reservoir of  FIG. 28 ; 
           [0041]      FIG. 30  is a bottom view of the injector reservoir of  FIG. 28 ; 
           [0042]      FIG. 31  is a detailed view of section A-A of the injector reservoir of  FIG. 29 ; 
           [0043]      FIG. 32  is a cross-sectional view of the O-ring of  FIG. 30 ; 
           [0044]      FIG. 33-FIG .  36  are schematic diagrams of the process steps of scanning the hot plate under the injector head; 
           [0045]      FIG. 37  illustrates the mold alignment process against the finish parking space; 
           [0046]      FIG. 38-FIG .  39  illustrate the process steps of scanning the hot plate under the injector head and the function of the start and finish parking spaces; 
           [0047]      FIG. 40  depicts a bottom view of the cleaner assembly for the parking spaces; 
           [0048]      FIG. 41  depicts the air knife slide table; and 
           [0049]      FIG. 42  is another embodiment for the injector head. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0050]    Referring to  FIG. 2 , the scale-up IMS process  50  includes filling mold cavities with solder ( 34 ), inspecting the filled mold plate ( 86 ), forming a pattern on the semiconductor wafer surface ( 32 ), inspecting the wafer surface ( 75 ) and then transferring the solder from the mold cavities to the semiconductor wafer surface ( 38 ). Referring to  FIG. 3 , the scale-up IMS process  50  further includes cleaning of the molds at a mold clean station  60 , filling of the mold cavities with solder and inspecting the filled mold plate at a mold prepare station  80 , and transferring of the solder from the mold cavities onto the patterned semiconductor wafer surface at a wafer bump station  90 . The mold prepare station  80  includes a mold fill tool (MFT)  100 , a mold inspect tool (MIT)  200 , and a mold repair tool  88 . The wafer bump station  90  includes a solder transfer tool (STT)  300  and a wafer loader tool  400 . New molds  61  and previously used molds  62  pass through the mold clean station  60  where they get cleaned with an acid solution  63  and a base solution  64 . The clean molds  82  enter a mold stocker  500  and from there they are introduced into the MFT  100 . After filling the mold cavities with solder, the molds are inspected at the MIT  200  and then transferred to a ready mold stocker  550 . Molds that do not pass inspection are either recycled at the mold clean station  60  or are repaired at the repair tool  88 . Molds that are repaired pass through the MIT  200  again and upon passing the inspection are transferred to ready mold stocker  550 . In some embodiments the mold repair tool  88  is integrated with the MIT  200 . From the ready mold stocker  550  the molds are introduced into the STT  300 . Patterned wafers  74  are introduced into the wafer loader  400  and from there into the STT  300 . After the solder transfer process the bumped wafers  76  exit the wafer bump station  90  and the dirty molds  62   b  are introduced into the mold clean station  60  again. The process repeats until all wafers  74  are bumped. A schematic diagram of the HVM IMS equipment system  52  is shown in  FIG. 4 . It includes the mold clean station  60 , the mold stocker  500 , the MFT  100 , the MIT  200 , the STT  300 , the wafer loader, i.e., front open unified pod (FOUP)  400  and a mold cart  600 . In one example, the HVM system  52  has a capacity of 300 wafer per day (1 wafer every 4 minutes) and 350 molds per day (1 mold every 3.5 minutes). It provides automation of the wafer and mold transfer. The STT can process 200 mm and 300 mm wafers without any hardware changes and each mold carrier can carry up to 25 molds. The molds are identified with a bar code mechanism and the mold stocker/sorter is integrated in the process line. There is also an integrated mold and wafer tracking and management system. The system can accommodate any solder type including no lead/eutectic PbSn (low temperature) at start up and high lead later. 
         [0051]    Referring to  FIG. 5  and  FIG. 6 , the mold fill process  34  includes melting bulk solder (wire, shots, slugs) in a reservoir  81 . Reservoir  81  is heated above the melting point of the solder and is slightly pressurized. An injector head  83  communicates with the reservoir  81  and is in contact with the mold plate  82 . Mold plate  82  is scanned under the injector head  83  in the scan direction  87  and molten solder is injected through a solder slot  89  formed at the bottom of the injector head  83  and fills the empty cavities  85   a  in the mold  82 . The filled mold plate is then cooled and inspected at the MIT  200 .  FIG. 7A  depicts a glass mold plate  82  with unfilled cavities  85   a  and  FIG. 7B  depicts a glass mold with filled cavities  85   b.  Cavities  85  are etched on the glass mold  82  according to the required bump pattern. The glass mold  82  has a thermal expansion coefficient (CTE) similar to the CTE of the semiconductor wafer  72 . 
         [0052]    Referring to  FIG. 8 , the solder transfer process  38  includes bringing together a wafer  74  patterned with under bump metallurgy (UBM) structures  73  with a mold plate  82  having solder filled cavities  85   b  ( 92 ). Next, heating the mold  82  and the wafer  74  to a temperature of 20 degrees higher than the solder melting point ( 94 ) and then bringing the mold  82  and wafer  74  in close proximity ( about 20 micrometers) or soft contact so that the solder wets the UBM structures  73  ( 96 ). The solder bumps from the cavities  85   b  are transferred to the UBM structures  73  and stay on the wafer  74  after the mold  82  separates from the wafer  74  ( 98 ). A critical aspect of this process is the alignment of the mold plate  82  relative to the semiconductor wafer  74  so that the solder bumps  85   b  are transferred to precise UBM structures  73 . The alignment needs to be maintained during the transport of the aligned mold-wafer system from station to station and during the actual solder transfer process at the required temperature, atmosphere and pressure. A fixture tool for holding and transferring the aligned mold and semiconductor structures together is described in a related co-pending application Ser. No. 12/025,644, entitled “Apparatus and Method for Semiconductor Bumping via Injection Molded Solder”, the contents of which are incorporated herein by reference. 
         [0053]    Referring to  FIG. 9 , the mold fill apparatus  700  includes a mold aligner  710 , a mold pre-heat unit  720 , a mold cool unit  730 , and the mold fill tool(MFT)  100 . Inserting a room temperature glass mold plate  82  in the high temperature MFT tool  100  causes the mold sides to bend upward (warping). Placing the room temperature mold  82  in the mold pre-heat unit  720  and preheating it to a temperature just below the solder melting point before inserting it in the MFT reduces the warping problem. Referring to  FIG. 10 , and  FIG. 11  the mold pre-heat unit  720  includes a hot plate  728  heated with cartridge heaters, a robot end effector  116 , a hot plate support plate  723 , a mount and leveling assembly for the support plate  724 , a gas supply line  727  and an exhaust  726 . In one example, the mold is preheated to 200 C and the solder melting point is 217 C. Bringing the hot mold plate from the high temperature MFT tool  100  directly to room temperature also causes warping of the glass mold plate due to the uncontrolled and quick cool down process. To prevent warping during cool down, at the end of the mold filling process, the mold is placed onto the mold cool unit  730  where it is cooled down in a controlled way. Referring to  FIG. 12  the mold cool unit  730  includes a cool plate  736  which is cooled with an air chiller  732 . The mold is supported on top of the plate  736  with three lift pins  734   a - 734   c.  Gas line  735  supplies the gas for the air chiller  732 . Cool plate is mounted on top of a support plate  733  which is mounted on top of the leveling assembly  731 . Cool plate  736  is made of a good thermal conductor material. In one example, it is made of aluminum. 
         [0054]    Referring to  FIG. 13  the MFT tool  100  includes a table  101  having a top  102  upon which a carriage assembly  140  and a solder head assembly  130  are mounted. A mold plate  82  with unfilled cavities is placed in the carriage assembly  140  and is scanned (i.e., moved) under the solder head assembly  130  along direction  602   a,  shown in  FIG. 13 . During this scanning molten solder is injected from the stationary solder head assembly into the mold cavities. Carriage assembly  140  includes a hot plate assembly  110 , a start space  150  and a finish space  160 , as shown in  FIG. 14 . The hot plate assembly  110  includes a stage  112  that moves on rails  111   a,    111   b.  Stage  112  supports the hot plate  114  upon which the mold  82  plate is mounted. A robot end effector (REF)  116  moves the mold plate  82  on and off the hot plate  114 , shown in  FIG. 15 . The positioning of the mold on and off the plate is guided with four mold guide pins  181   a,    181   b,    181   c,    181   d.  The ends of each pin  181   a - 181   d  are frusto-conically shaped and are positioned and dimensioned so that their conical sides  182  guide the rounded edges  82   a  of the mold plate  82  and move the plate  82  sidewise, up or down as the pins move up or down, as shown in  FIG. 17  and  FIG. 18 , respectively. Table  101  also includes gas supply lines  103  and pressure control valves  104 . 
         [0055]    Referring to  FIG. 16 , hot plate  114  is mounted on top of a hot plate insulator  115  which in turn is mounted on top of a backing plate  115   a . In one example insulator plate  115  is made of Calcium Silicate. Cartridge heaters  117  are used for heating the hot plate  114 . The top surface  118  of the hot plate  114  includes grooves  119  having small holes through which vacuum is drawn. The vacuum is used to hold the mold  82  onto the hot plate  114 . Mold  82  is kinematically mounted on top of the hot plate  114  via a three pin motorized mount  170 , shown in  FIG. 23 . Pins  172   a ,  172   b ,  172   c  are arranged at 120 degree angles relative to each other and are used to guide the lifting and lowering of the mold  82  onto the hot plate  114 . Mount  170  includes three arms  173   a ,  173   b ,  173   c  extending from the center  175  at 120 degrees angles relative to each other. Pins  172   a ,  172   b ,  172   c  extend upward from the top surfaces of the ends of each arm  173   a ,  173   b ,  173   c , respectively. In one example pins  172   a - 172   c  are made of polyamide. Referring to  FIG. 19  and  FIG. 20  the start space assembly  150  includes a starting parking space  152  which is mounted on top of the base parking element  157 . Starting parking space  152  includes a V-groove  152   v  on it bottom surface. Base parking element  157  is mounted on top of insulator plate  154  and insulator plate  154  is mounted on top of backing plate  154   a . A ceramic spacer  153  separates the backing plate  154   a  from the first leveling plate  156  which is mounted on top of a second leveling plate  155 . Leveling plate  155  is mounted on top of support plate  158 , which in turn is set on top of the hot plate sled  111 . 
         [0056]    A mirror image finish parking space assembly  160  is on the opposite end of the carriage assembly  140 . Referring to  FIG. 21  and  FIG. 22  the finish space assembly  160  includes the finish V-groove parking space  162  which is mounted on top of the base parking element  167 . Base parking element  167  is mounted on top of insulator plate  164  and insulator plate  164  is mounted on top of backing plate  164   a . A ceramic spacer  163  separates the backing plate  164   a  from the first leveling plate  166  which is mounted on top of a second leveling plate  165 . Leveling plate  165  is mounted on top of support plate  168 , which in turn is set on top of the hot plate sled  111 . In addition to these elements, finish space assembly  160  includes a fixed stop heat sink  161  adjacent to the finish parking space  162 . An air cylinder  251  connects the fixed stop  161  with the base parking element  167 . Both parking spaces  152 ,  162  are heated to a temperature slightly above the melting point of the solder used for filling the mold cavities with cartridge heaters  174 . 
         [0057]    Referring to  FIG. 24 , the solder head (or gantry) assembly  130  includes two support elements  131   a ,  131   b , supporting an injector head slide assembly  133  carrying an injector head assembly  132 , an injector head lift cylinder  134 , two pairs of first and second parking space lifters  135   a ,  135   b  (not shown) and  135   c ,  135   d , a parking space cleaner assembly  136  and two process gas heaters  137   a ,  137   b . The parking space lifters  135   a - 135   d  are designed to pick up the start or finish parking spaces  152 ,  162  and lift them up or down in the direction of  138 . 
         [0058]    Referring to  FIG. 25- FIG .  26  the injector head slide assembly  133  includes a cross bar  133   a  having two downward extending legs  133   b ,  133   c . The bottoms of legs  133   b ,  133   c  extend sidewise to provide mounting surfaces for the injector head assembly end plates  241   a ,  241   b , shown in  FIG. 26 . The mounting mechanism in areas A and B stabilizes the position of the reservoir  230  while allowing for small thermal expansions sidewise in the direction of  242 . The injector head slide assembly is made of a material that has very low coefficient of thermal expansion (CTE) and remains stable at high temperatures. In one example, head slide assembly  130  is made of Invar alloy. 
         [0059]    Referring to  FIG. 27-FIG .  30 , the injector head assembly  132  includes a solder reservoir block  230  and hot gas skirt elements  231   a ,  231   b  having hot gas feeds  238   a ,  238   b . Reservoir block  230  has a cap  239  and thermocouple feeds  236   a ,  236   b  leading two separate thermocouples  237   a ,  237   b , to two different prearranged heights of the solder baths. A valid thermocouple reading indicates that the solder bath level is at the prearranged height where the thermocouple is mounted. The two thermocouple readings are used to monitor the fill level of the solder bath. More than two thermocouples may be used and a finer height selection may be arranged. The reservoir block  230  is supported with two hard end plates  241   a ,  241   b  at the bottom of the injector head slide assembly  133  in areas A and B, as shown in  FIG. 26 . 
         [0060]    Referring to  FIG. 30  the bottom surface  230   a  of reservoir  230  includes a slot  234  surrounded by O-ring  233 . For the mold filling process, surface  230   a  is brought into contact with the mold plate and molten solder is injected through slot  234  into the mold cavities. O-ring  233  is designed to sustain the high melt temperatures of solder. Hot gas is blown onto and around the bottom surface  230   a  through the hot gas skirt elements  231   a ,  231   b . In one example the hot gas is a mixture of nitrogen with air and the gas temperature is 200 C. The solder reservoir block is heated to a temperature above the solder melting point with cartridge heaters  243   a ,  243   b.    
         [0061]    As was mentioned above, the hot plate carriage assembly  140  carrying the mold  82  on its top is scanned under the solder head assembly  130  in the direction of  602   a  for the mold filling process. The process of scanning the mold plate under the injector and the parking space mechanism is described with reference to  FIG. 33-37 . Initially the injector head  132  is positioned on the heated start parking space  152  and the mold  82  is loaded onto the hot plate  114  which is placed in the center of the carriage assembly  140  (step  1 ), as shown in  FIG. 38 . Edge  152   a  of the start parking space is shown in  FIG. 33  to have some solder deposit from the previous run. Next, the support pins  172   a - 172   c  are lowered and the mold  82  is brought in contact with the hot plate  114  (step  2 ). The finish parking space  162  is then moved to the fixed stop  161   b  at low pressure (step  3 ) and then the edge  82   a  of the mold  82  is aligned with the finish parking space edge  162   a , as shown in  FIG. 37  (step  4 ). The start parking space  152  is also moved to the X-axis encoder position  161   a . Next, the positions of the start parking space  152  and the finish parking space  162  are locked and the vacuum for holding the mold  82  onto the hot plate  114  is turned on to secure the aligned mold onto the hot plate  114  during scanning. The hot plate assembly  140  is scanned in the direction  602   a  so that the mold plate  82  is positioned under the stationary injector head  132 , as shown in step  5  of  FIG. 33  and  FIG. 38 . The transition of the injector head  132  from the start parking space  152  onto the mold plate  82  causes solder residue to be deposited onto edge  152   b  of the parking space  152 , shown in  FIG. 33 . Next, the solder deposits from edges  152   a ,  152   b  of the start parking space  152  are cleaned at the cleaner assembly  136  (step  6 ) and then the clean start parking space  152  is picked up by the second lifter  135   b , as shown in step  7  of  FIG. 34  and  FIG. 38 . The mold  82  is scanned under the injector head  132  for filling of the mold cavities with solder while the start parking space  152  is carried by the second lifter  135   b , as shown in  FIG. 38  (step  8 ). When all cavities are filled the head  132  is at a position to be deposited onto the finish parking space  162 , as shown in step  9  in  FIG. 39 . At this transition point, edge  162   a  of the finish parking base  162  is contaminated with solder deposit from the injector head  132 . The head  132  is positioned onto the finish parking space  162  (step  9 ) and the mold/hot plate assembly separate from the finish parking space  162  (step  10 ). Next, the head  132  lifts up the finish parking space  162  (step  11 ) and the second lifter  135   b  places the clean start parking space  152  onto the finish base  167 , as shown in  FIG. 39  and  FIG. 35  (step  12 ). Then the filled mold  82  is scanned under the injector head  132  carrying the finish parking space  162  in the opposite direction  602   b  (step  13 ) until it reaches the position where the injector head is above the start base  157  (step  14 ). At this point the injector head  132  deposits the finish parking base  162  onto the start base  157  (step  15 ), shown in  FIG. 35 . The filled mold is unloaded and an empty mold is loaded onto the hot plate and the process repeats again. Excess solder escaping from the injector head when the head is transferred from the mold onto and off the parking spaces is captured at the sides of hot plate  114  in the solder catchers  199 , shown in  FIG. 16 . 
         [0062]    Referring to  FIG. 32 , the HVM MFT is designed to fill the mold cavities when the mold  82  is scanned in the direction of  602   a  and to return the filled mold back in the starting position before repeating the filling process with a new mold. This one-way deposition ensures that edge  233   a  of the injector head O-ring  233  is always in contact with the molten solder  246  that is injected through the slot  234  and edge  233   b  is always in contact with the hot air mixture of air (or oxygen) and nitrogen  246 . An oxide layer  248  is formed at the bottom surface of the O-ring and is always in contact with the mold surface, as shown in  FIG. 32 . This particular scanning limitation is important for the repeatability of the deposition parameters and reduction of defects. 
         [0063]    As was mentioned above, both parking spaces  152 ,  162  are heated to a temperature slightly above the melting point of the solder used for filling the mold cavities and the mold  82  floats above the hot plate  114  during the scanning process and is held in place by vacuum. Glass mold  82  is square shaped and the left edge of the square is aligned against the end parking space to correct for any dimensional imperfections of the mold that may lead to solder bump defect formation. In one example mold  82  is a square having a side of 14 inches long. There is a border around the square mold surface that does not carry any patterned cavities. The edges of the start and finish parking spaces  152 ,  162  that were contaminated with solder deposit during the mold fill process are cleaned at the cleaner assembly  136 , shown in  FIG. 40 . Cleaner assembly  136  includes two rotating brushes  604   a ,  604   b . An alternative injector head design that does not cause contamination of the parking space edges and mold edges is shown in  FIG. 42 . The injector head  605  includes capillaries  608  through which air is blown to “freeze off” the molten solder before lifting the injector head from the mold or parking space surfaces. 
         [0064]    Problems with the injector head during the mold fill process include chatter, tipping and warping. Chatter is caused by small vibrations of the injector head and can lead to skipping along the glass mold and either not filling a cavity or partially filling a cavity. One way of reducing chatter is to hold down the injector head onto the glass mold by clamping the solder head assembly onto the glass mold and applying pressure. Tipping refers to small forward or backward tilting of the injector head and can also lead to partially filled cavities, non-uniform sized bumps and bridges. The above mentioned clamping and applying pressure solution addresses this problem, as well. Head warping is caused by thermal gradients along the length of the injector head and can cause the injector head side ends to bend up or down. One way of reducing head warping is by heating the injector cap separately and by adding separate heaters along the length of the injector heads. In one example, the head is furnished with evenly spaced holes carrying low wattage cartridge heaters and the head is slightly compressed onto the mold. 
         [0065]    Before the mold fill process the unfilled mold plates are cleaned at the air knife table  650 , shown in  FIG. 41 . A combination of ultrasonic vibrations and blowing of ionized air is used to release and remove contaminants from the mold surface. The contaminates are removed by drawing a vacuum on the mold surface. 
         [0066]    Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.