Patent Document

RELATED APPLICATION  
       [0001]     This application claims benefit of U.S. Provisional Application No. 60/486,688, filed Jul. 9, 2003. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to microelectronics device assembly and, in particular, to a flip chip (FC) device assembly machine and related processes.  
       BACKGROUND OF THE INVENTION  
       [0003]     Product functionality for devices such as handheld telephones, laptop computers, and other personal electronic items has driven a trend towards compactness of design and improved packaging processes. Flip chip technology offers design and processing advantages. Design advantages include smaller device footprint, improved electrical performance, better thermal dissipation properties, and lower cost resulting from better use of silicon real estate. Processing advantages include shorter assembly cycle times, fewer operations, and higher yields.  
         [0004]     A range of packages is available for flip chip packaging including FC-chip scale packaging (FC-CSP), FC-ball grid arrays (FC-BGA), high-performance FC-BGAs (HFC-BGA), and FC pin grid arrays (FC-PGA) among others. These packages can be compared with reference to I/O count and package size. The methodology of flip chip die bonding is rooted in die bonding with certain modifications. There are critical requirements for high volume flip chip die bonding. Key components of the flip chip process are substrate handling, die flipping, and flux dipping and are described from the initial point of picking the die through fluxing and to the actual placement of the die, including material handling. Work holder planarity and flux control represent aspects of the flip chip die bonding process that materially affect high yield, high volume production. Process control and high throughputs represent aspects of underfill dispensing that materially affect cost effective production.  
         [0005]     There is a rapid increase in the number of electronic packages implemented with flip chip technology. The ongoing expansion of the Internet, mobile phones, personal data assistants, desktop and laptop computers, digital camcorders, digital cameras, and other electronic based consumer products has spurred a revolution of innovation in flip chip technology. Product functionality has never been more demanding, and time to market and volume production is more critical than ever. Flip chip packages exist for a range of products from few-lead radio-frequency identification devices to greater than 2000 lead BGAs. Substrate technology has transitioned from traditional ceramics to a wide range of organic materials, thereby enabling a multitude of different package applications built around flip chip technology.  
         [0006]     There are a number of inherent advantages of flip chip technology. A key advantage of flip chip technology is size. Flip chip packages do not require peripheral space for the wire bonds and, therefore, can be made smaller than wire bond packages with a similar input/output (I/O) count. For die with a high I/O count, flip chip technology offers large space savings because the I/O can be arranged in an array on the die and the substrate. This eliminates the need for traces to the chip edge from internal interconnect points. At the substrate level, routings can be directed through multiple internal layers. This array architecture can be used to achieve space savings, similar to the savings between BGA and quad flat pack (QFP) packaging. Ultimately, when taking into account die shrinkage enabled by flip chip, overall material cost (package and die) is less. Flip chip technology also offers the potential for lower total package height because no extra clearance is required for wire bonds or encapsulation/mold compound above the die. The space savings of flip chip technology translate into a geometry that delivers the solution for today&#39;s high I/O consumer end products, such as digital video cameras.  
         [0007]     Another advantage is improved performance. A short signal path provides for low inductance, resistance, and capacitance, resulting in faster signal and better high frequency characteristics. Flip chip technology provides improved functionality in terms of an increased number of I/Os and the concentration of more signal, ground, and power connections in a smaller area. The technology offers better thermal capabilities, since an external heat sink can be directly added above the chip to remove heat.  
         [0008]     A further advantage is that a solder reflow flip chip has fewer process steps compared to traditional epoxy die attach and wire bonding. Operations such as wire bonding and encapsulation or molding are eliminated. Flip chip technology integrates all package assembly steps in one operation. The assembly time, total number of process steps, overall capital equipment costs, the number of pieces of equipment, as well as other factors, result in a reduced cost of ownership.  
         [0009]     As stated above, there are multiple types of flip chip packages, including FC-BGA, HFC-BGA, ceramic FC-BGA/PGA, and FC-CSP. FC-BGA and HFC-BGA packages support I/Os of 100 to over 1500 with bismaleimide triazine (BT) laminate or sophisticated multi-layer substrates. HFC-BGA packages are thermally enhanced by the attachment of a metal heat sink that can effectively remove the heat and improve thermal characteristics. Ceramic FC-BGA/PGA is a ceramic package that provides better heat dissipation for high thermal conductivity and a coefficient of thermal expansion more closely matched to that of silicon. The FC-CSP package offers chip scale geometry for packages with fewer than 200 I/Os and provides better protection for the die than chip on board (COB) technology. FC-CSP prevails over known good die in low-cost test and burn-in. It is intended to provide thin, small profile, and lightweight packaging. Applications include RF and memory integrated circuits (ICs).  
         [0010]     Table 1 below summarizes the characteristics of these types of flip chip packages.  
                                     TABLE 1                           Common Flip Chip Packages            Package           Substate   Ball       Type   Nr. I/O   Package size   Type   Pitch               FC-CSP   36˜200    7 × 7˜15 × 15   Laminate   0.8/1.0        Ceramic FC   &lt;1421   27 × 27˜50 × 50   Ceramic   0.8˜       BGA/PGA               1.27       FC-BGA   100˜1521   11 × 11˜40 × 40   Laminate   1.0/1.27       HFC-BGA   256˜1521   27 × 27˜40 × 40   Laminate   1.0/1.27                  
 
         [0011]     What is still needed is next generation flip chip production equipment, including flip chip bonders. Future flip chip assembly machines require many advanced features to satisfy the new manufacturing requirements and to minimize the cost of ownership of integrated device manufacturers and subcontract manufacturers.  
       SUMMARY OF THE INVENTION  
       [0012]     An object of this invention is, therefore, to provide an apparatus and a method for high-throughput flip chip assembly of electronic components.  
         [0013]     Another object of this invention is to provide an apparatus and a method for applying flux to the electronic components prior to their assembly.  
         [0014]     A further object of this invention is to provide a flip-to-flux pick and place subassembly for further improving electronic component assembly throughput.  
         [0015]     Still another object of this invention is to provide a substrate carrier conveyor assembly for rapidly conveying the movement of carriers into and out of the flip chip assembly machine.  
         [0016]     A flip chip assembly machine (“FCAM”) is a piece of equipment responsible for picking a die from a wafer, flipping the die, dipping it into flux, and placing it in proper alignment on the substrate. A next generation FCAM offers 300 mm (12 in) wafer capability. The first step in a die bond process is to load substrates to the FCAM. Substrates are unloaded from magazines and indexed into the FCAM. High system speeds are possible when the substrate loading operation can be done in parallel with pick and place operations. The loader is configured to handle substrates in strip form (e.g., BGA strip) as well as singulated substrates in carrier boats. Carrier boats or substrates are loaded into magazines, and the magazines are placed in the loader. The carrier boats are then indexed, one at a time, into the flip chip die bonder.  
         [0017]     The substrate strip or carrier is indexed into the work area, and the substrates are locked in place with vacuum pressure using a vacuum chuck. Alternatively, mechanical clamping is sometimes used. The vacuum chuck is manufactured to have very good planarity relative to the die placement head, which places a die on a substrate. A vacuum chuck that is easy to exchange and set up ensures rapid changeover capability. Vacuum sensing ensures that the substrates are secured at all times to enable accurate placement. Use of a “down facing” camera to align the substrates affords an accurate die placement capability. The FCAM determines the substrate coordinates, using substrate fiducials or alignment marks. Most die bonder systems currently use pattern recognition in addition to geometric feature recognition. Pixel size and vision repeatability are factors that affect accuracy. Quality optics and programmable-intensity lighting, together with various light types and colors, are used to obtain better definition.  
         [0018]     Die are presented in wafer format with the bumps up. At this stage, wafers have been fully tested and diced. “Good die” on the wafer are either determined by an ink dot scheme or based on a wafer result map. Electronic wafer mapping is usually preferred over ink dot when processing flip chip die. The handling of 300 mm (12 in) wafers includes an ability to dock an industry standard wafer cassette Personal Guided Vehicle. The wafer is loaded from a wafer cassette (which can hold up to 25 wafers), onto a wafer table. During the loading process, a bar code located on the wafer frame is read to cause a download of the correct wafer map file from the server. The wafer is stretched to prevent die edge chipping, and the first good die is located using a wafer camera. The wafer table is indexed to the correct location for a die flipping mechanism to pick and flip the die. The wafer map file (cyber wafer) is aligned to the wafer, and the machine begins to pick good die.  
         [0019]      FIGS. 1A, 1B ,  1 C, and  1 D show a die flipping process employed by this invention.  FIG. 1A  shows a die flipper  10  picking a good die  12  from a wafer  14  having multiple die with solder bumps  16  facing up. A die ejector  18  is positioned under good die  12  and projects ejector pins  20  to separate good die  12  from wafer  14 .  FIG. 1B  shows die flipper  10  attached to good die  12  by vacuum pressure and flipping good die  12  over so that solder bumps  16  are facing down. Die flipper  10  then releases its vacuum pressure.  FIGS. 1C and 1D  show a flux head  22  attaching to good die  12  by vacuum pressure and lifting good die  12  off die flipper  10  with solder bumps  16  facing down in a flux-ready orientation.  
         [0020]     After picking good die  12  from wafer  14 , die flipper  10  moves straight up before translating to a rotational movement. This prevents good die  12  from colliding with other die on wafer  14 . The vacuum actuated pickup tool on die flipper  10  must not damage solder bumps  16  while having sufficient vacuum pressure to securely hold good die  12  during flipping. Die flipper  10  movement, speed, and acceleration are programmed and synchronized with die ejector  18  and ejector pin  20  movements to prevent die damage and maximize throughput.  
         [0021]      FIGS. 2A, 2B , and  2 C show a flux dipping process employed by this invention. As shown in  FIG. 1D , good die  12  is lifted from die flipper  10  by flux head  22  (or by a bond head if flip-to-flux pick and place not used).  FIG. 2A  shows flux head  22  positioning good die  12  over a flux well  24 .  FIG. 2B  shows flux head  22  dipping solder bumps  16  of good die  12  into flux well  24 . Flux head  22  then moves back into position to retrieve the next good die from die flipper  10  as shown in  FIG. 1D .  FIG. 2C  shows a bond head  26  withdrawing good die  12  from flux well  24 . Flux well  24  is a precision-machined depression in a plate that is part of a fluxer that is described with reference to  FIG. 9 . The flux thickness is determined by the depth of flux well  24  and the surface tension of the fluxing fluid. A range of plates can be exchanged to achieve different thicknesses. The flux delivery system improves yield and throughput. Fine control over the depth of flux is achievable with attention to the properties of the flux and to the mechanics of the delivery system. By programming the speeds of die dipping and depositing flux in flux well  24 , throughput can be optimized while still attaining precise control over volume. Fluxer indexing speed is programmed to account for different flux viscosities. Heating the flux can help to reduce flux viscosity and thereby achieve optimum wetting of solder bumps  16 .  
         [0022]     Fluxer planarity contributes to good process control and prevents open joints because the amount of flux on solder bumps  16  directly influences solder bump reflow. The flux plate and flux well  24  are designed for easy exchange and cleaning, without the need for special tools.  
         [0023]     Flux dwell time is programmed in accordance with the type of flux used and its particular wetting capabilities. The amount of time spent applying flux to the chip directly influences system throughput. However, by performing the flux operation in parallel with other operations, such as picking die from wafers and placing fluxed die on substrates, the die fluxing step is removed from the critical processing path. Such parallel operations can increase the unit per hour rate (UPH) of the system by as much as 50%. The FCAM of this invention performs fluxing in parallel with the pick and place cycle to achieve improved throughput rates.  
         [0024]     Die pick and place to a substrate is performed following flux dipping. Bond head  26  picks good die  12  from flux well  24  for presentation to an upward looking camera to perform vision alignment. The upward looking camera is described with reference to  FIG. 4 . The vision system determines the X, Y, and θ offsets from good die  12  to bond head  26 . Based on this offset determination, an adjustment of the position of bond head  26  ensures that fluxed good die  12  is placed accurately on the substrate. Lighting is an important part of the vision process to ensure accurate location of the fluxed bumps, which can be challenging to basic vision systems.  
         [0025]     Bond head planarity to the substrate affects accurate die placement. Small deviations can cause the die to shift during placement. Bond force control and bond force repeatability are factors in achieving accurate and repeatable placements. Closed-loop controlled bond force ensures highly accurate placements and repeatability, thus achieving a stable process and a high Process Capability index (Cpk).  
         [0026]     After the substrates are populated with die, the carrier is either loaded back into a magazine or transported to a solder reflow oven. The offloading of carriers offers another opportunity for throughput gains. Improvement is realized if carriers can be exchanged sufficiently quickly to be done in parallel with the pick and place cycle. Although it exhibits fast action performance, the indexing of carriers operates smoothly to prevent die shifting. A preferred way of performing fast carrier exchange entails combining the carrier conveyor with a mechanical device. With this approach, the conveyor can be used to bring carriers to and from the die bonder system, but the faster mechanism can be used for rapid delivery of the carrier to the assembly area. By controlling acceleration and deceleration of the carrier mechanism motion, the fastest movements are possible without disturbing the placed die. Performing carrier exchange in parallel with the die pick and place cycle is especially important when there is a low number of die for each carrier. This is so because carriers with low numbers of die are exchanged frequently.  
         [0027]     The final step of the flip chip die bonding assembly process is solder reflow. Solder bumps are reflowed in an oven with an inert atmosphere, creating a solder joint that also acts as the electrical interconnect. A typical reflow oven used in flip chip applications has multiple heat zones and can reach temperatures of up to 400° C. The actual reflow profile is a function of oven indexer belt speed and heat zone temperature settings. Carriers with reflowed chips are either loaded back into a magazine or transported to a next process step.  
         [0028]     The dispense of underfill follows solder reflow. Underfill material is dispensed alongside the die, and the material is drawn between the die and the substrate via capillary action. Underfill material is used to protect the interconnect area from moisture. It also reinforces the mechanical connection between the substrate and the die. Underfill compensates for any difference in the thermal coefficient of expansion (TCE) between the chip and the substrate.  
         [0029]     After underfill dispense is finished, the carrier is indexed into the post heat area. Post-heating allows the underfill material to finish flowing, and allows any air bubbles (voids) to escape, while keeping moisture content low. Having a separate post-heat station increases package reliability at no cost to system UPH.  
         [0030]     After the underfill dispensing process is finished, the processed carriers are loaded into magazines or transported into a cure oven. Temperatures and dwell times depend on the type of underfill material used and the package size. Once the underfill is cured, the part is a complete, bonded, interconnected, packaged system.  
         [0031]     The flip chip assembly machine offers several advantages. First, the flip chip assembly machine is designed with a main gantry and a substrate camera gantry that are configured to operate independently of each other and, respectively, support a die and a substrate camera for alignment purposes. Second, the flip chip assembly machine imparts motion to the flux reservoir by variable, uniform speed motor operation to allow for different motion speeds, depending on flux viscosity. Third, a flip-to-flux pick and place subassembly is configured to pick up a die and place it in flux independently of the operation of the main gantry. The main gantry is, therefore, made available to perform another task during the flux dwell time. Fourth, the substrate carrier conveyor operates in association with a walking beam to synchronize the movement of the substrates to that of the conveyor belts. The synchronism achieved allows rapid and controlled acceleration and deceleration of the substrate carrier to speed the movement of carriers into and out of the flip chip assembly machine. The synchronized movement also eliminates rubbing of the substrate carrier against the belt caused by a speed difference between them and thereby minimizes wear and particle generation.  
         [0032]     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the embedded and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIGS. 1A, 1B ,  1 C, and  1 D are simplified pictorial elevation views of a die picking and flipping process employed by operation of the FCAM.  
         [0034]      FIGS. 2A, 2B , and  2 C are simplified pictorial elevation views of a die flux dipping process employed by the FCAM.  
         [0035]      FIG. 3  is an isometric view of an embodiment of an FCAM of the present invention shown enclosed within its system cabinet.  
         [0036]      FIG. 4  is a diagram of the FCAM of  FIG. 3  with its system cabinet removed to reveal its main subassemblies.  
         [0037]      FIG. 5  is an isometric view of a wafer handling subassembly of the FCAM of  FIG. 4 .  
         [0038]      FIG. 6  shows an enlarged view of a needle array die ejector.  
         [0039]      FIGS. 7A, 7B , and  7 C are cross sectional schematic views showing the operational sequence of using the die ejector of  FIG. 6  to release a die adhered to a sticky film.  
         [0040]      FIG. 8  is an isometric view of a die flipper mechanism and a flip-to-flux mechanism that cooperate to pick a die from a wafer and manipulate the die into position for dipping in a flux well at a flux station.  
         [0041]      FIG. 9  is an isometric view of a flux station that includes a flux well into which a die is dipped to apply flux to the solder bumps on the die.  
         [0042]      FIG. 10  is an isometric view of a main gantry that spans the width and moves along the length of the FCAM to position the die pickup tool and its associated camera.  
         [0043]      FIG. 11  is an enlarged isometric view of a θ-axis die pickup head and associated die pickup tools carried by the main gantry of  FIG. 10 .  
         [0044]      FIG. 12  is an isometric view of the underside of a substrate gantry positioned below the main gantry and carrying the downward-looking substrate camera shown in  FIG. 4 .  
         [0045]      FIG. 13  is an isometric view from one end of the substrate conveyor.  
         [0046]      FIGS. 14 and 15  are isometric views of, respectively, the side and top of the conveyor of  FIG. 13  showing the components of the walking beam mechanism.  
         [0047]      FIG. 16  is an isometric view of the top of a tooling lift assembly that is embedded in the conveyor of  FIG. 13  to lift the substrates off the conveyor belts.  
         [0048]      FIGS. 17 and 18  are isometric views of, respectively, substrate magazine unloader/elevator and substrate magazine reloader-elevator subassemblies of the flip chip assembly machine of  FIG. 4 .  
         [0049]      FIG. 19  is a block diagram of the control system governing the overall operation of the flip chip assembly machine of  FIG. 4 .  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0050]     As described in the background of this invention, flip chips are small die that carry arrays of tiny balls of solder (solder “bumps”) that are precisely aligned with and placed on corresponding pads on the circuit substrate. The flip chip assembly machine places flip chips precisely onto the substrates. The flip chip-substrate assemblies are then delivered to an owner-provided downstream oven to reflow the solder bumps and thereby complete the attachment process.  
         [0051]      FIG. 3  shows the overall external appearance of an FCAM  30  in which the preferred embodiments of this invention are implemented.  
         [0052]      FIG. 4  shows an internal view of FCAM  30 , the subassemblies and operation of which are described below. FCAM  30  accepts cassettes  32  of frame-mounted silicon wafers  34  that are diced into discrete flip chips. FCAM  30  further accepts magazines of substrates on which the flip chips are to be assembled. Substrates, or pallets containing a number of substrates, are placed in a loader  36  and are conveyed through FCAM  30  and positively locked into position for die placement. A substrate camera  38  mounted on a camera gantry  40  examines the substrates and precisely determines the position coordinates of the substrate pads. The solder bumps are applied to wafers  34  before dicing and are positioned on the “top” surface of the die. A vision system including a wafer camera  42  locates the positions of the solder bumps. A flipper mechanism  44  acquires each die and inverts it 180°, so that the solder bumps face down, in the correct orientation for placement onto the target substrate. The die is then placed briefly in a shallow flux well  46  to apply flux to each solder bump, facilitating the downstream solder-reflow process. A pick-and-place mechanism  48  mounted to a main gantry  50 , and moving independently above substrate camera gantry  40 , acquires the die from flux well  46 .  FIG. 4  shows wafer camera  42  and flipper mechanism  44  swung aside in a maintenance position for purposes of illustration to reveal flux well  46 . Motions are described with reference to the X-, Y-, Z-, and θ-(theta) axes shown in  FIG. 4 .  
         [0053]      FIG. 4  further shows a wafer cassette elevator  51  that is designed to accept all standard cassettes for 300 mm (12 in) or, optionally, 200 mm (8 in) wafer frames. The vertical motion of wafer cassette elevator  51  is constrained by a pair of anti-friction slides  52  (one shown), and an elevator platform  54  is positioned by a ball-bearing lead screw  56  that is rotated by a closed-loop stepper or servomotor (not shown). The top end of the range of motion of wafer cassette elevator  51  positions cassette  32  at a convenient loading elevation. The bottom end of the range of wafer cassette elevator  51  motion positions the top of cassette  32  below other moving parts of the machine to provide clearance. Between these extremes, cassette  32  is indexed from one wafer frame  34  to the next, positioning frames  34  in a programmed sequence so they can be withdrawn from cassette  32  for delivery to a wafer handling system  60  for processing. A fluidic shock-absorbing system (not shown) positioned at the bottom of the stroke of the cassette elevator cushions the descent, if the drive system should fail.  
         [0054]      FIG. 5  shows further details of wafer handling system  60 , which includes a wafer-frame holder  62  that moves along the ±X and ±Y axes, a wafer-fetch gripper device  64  that acquires wafer frames  66  from cassette  32  ( FIG. 4 ), and a stretch ring  68  that applies tension to a sticky wafer frame film (not shown) on which the wafer rests. Wafer-fetch gripper device  64  later acts as a pusher to return wafer frames  66  to cassette  32 . Stretch ring  68  provides a known elevation (Z-axis direction) for the wafers. The 300 mm (12 in) wafer-handling system can be modified to handle 200 mm (8 in) wafers by exchanging in the field a subassembly and a few change parts.  
         [0055]     Wafer frame holder  62  is set on two pairs of rails  70  allowing motion in the X- and Y-axis directions (only Y-axis rails are shown). An X-axis servomotor  72  and Y-axis servomotor (not shown) drive ball screws (not shown) that move wafer frame holder  62  to locate each die on the wafer precisely over an ejector assembly  74  so that individual die can be acquired for processing. After being ejected by ejector assembly  74 , each die is acquired by a vacuum tool on flipper mechanism  44  ( FIG. 4 ), after which wafer frame holder  62  moves in preprogrammed X- and Y-axis amounts to bring the next die into position over ejector assembly  74 .  
         [0056]     Wafer-fetch gripper device  64  is sandwiched within wafer handling system  60  and is powered by a closed-loop stepping motor (not shown) to extend outwardly (in the “−X” direction) and pneumatically actuated by air cylinders  76  to grip the one of wafer frames  66  that is elevated to the correct position. The stepping motor then retracts wafer-fetch gripper device  64  (in the +X direction), to pull wafer frame  66  into position.  FIG. 5  shows wafer frame  66  mostly pulled into position within wafer frame holder  62 . During its motion, wafer frame  66  is supported and guided by rows of grooved rollers  78  (only two shown) on each side of wafer frame holder  62 . When wafer frame  66  arrives at the correct position, a pair of pneumatically actuated clamps (not shown) lock it in position. Wafer-fetch gripper device  64  then releases wafer frame  66  and retracts to a “home” position that does not interfere with subsequent operations.  
         [0057]     When wafer frame  66  is locked into position, a pressure plate  80  positioned above wafer frame  66  moves downward a short distance to bring the wafer frame sticky film into contact with stretch ring  68 , which has a diameter that is halfway between the outer diameter of the wafer and the inner diameter of wafer frame  66 . Pressure plate  80  continues pushing wafer frame  66  downward a pre-programmed distance to slightly tension and stretch the sticky film under the diced wafer. Pressure plate  80  is lowered by a closed-loop stepping motor  82  that rotates four jack-screws  84  (only two shown) that are synchronously linked by a drive chain  86 . In some cases it may not be necessary to stretch the sticky film, so the stretching process is a machine option.  
         [0058]     After each die is removed from wafer frame  66  and the next die is indexed into position, down-looking wafer camera  42  ( FIG. 8 ) checks features on the die to be sure it is accurately placed. If there is an error, the X- and Y-axis servomotors of wafer handling system  60  make corrections as needed to place the die in the correct pickup location. Wafer camera  42  also examines each die for inspection marks that indicate a faulty component. If a die is so marked, wafer handling system  60  automatically indexes to the next die on the wafer. Wafer camera  42  has a programmable-intensity, light-emitting diode (LED) light source that provides uniform on-axis illumination of the wafer surface.  
         [0059]      FIG. 6  shows further details of ejector assembly  74 , which includes a servomotor driven vertical slide that accepts interchangeable ejector pins  20 . Ejector pins  20  protrude from a dome  92  that includes annular vacuum channels  94  for holding the sticky film down while ejector pins  20  push upward. Preferably, the center one of ejector pins  20  moves farther upward than the surrounding ejector pins  20  move.  
         [0060]      FIG. 7A  shows die  12 A,  12 B, and  12 C that are separated by fine saw cuts  96  and adhered to a sticky film  98 , which is urged against dome  92  by vacuum pressure in vacuum channels  94 . Die  12 B represents one of a group of known good die on the diced wafer. Ejector pins  20 A,  20 B, and  20 C are shown in their retracted positions. Ejector pin  20 B represents the center-most of ejector pins  20 .  
         [0061]      FIG. 7B  shows ejector pins  20 A,  20 B, and  20 C protruding upward from dome  92  to form one or more “hills”  100  in sticky film  98  under die  12 B, thereby reducing the effective area of sticky film  98  contacting die  12 B so it can be readily picked from above by a vacuum tool  102  ( FIG. 7C ) on flipper mechanism  44  ( FIG. 4 ). For ejecting very small die, only ejector pin  20 B may be used; for larger die, ejector pins including  20 A,  20 B, and  20 C may be used; and for very large die, independently actuated concentric arrays of ejector pins  20  may be used.  
         [0062]      FIG. 7C  shows ejector pin  20 B protruding upward a distance farther than the protrusion distance of ejector pins  20 A and  20 C to break die  12 B mostly free from hills  100  in sticky film  98  and causing die  12 B to protrude upward from die  12 A and die  12 C, thereby facilitating the picking of die  12 B by vacuum tool  102 .  
         [0063]      FIG. 8  shows further details of flipper mechanism  44  that acquires with vacuum tool  102  each die with bumps up, such as die  12 B in a diced wafer  104 , of which only a portion is shown. Flipper mechanism  44  moves vertically along a Z-axis and 180° rotationally about a φ-axis. An arm  106  is coupled to a φ-axis flipper motor  108 , and an outer end of arm  106  includes a holder for interchangeable vacuum tools, such as vacuum tool  102 .  
         [0064]     With arm  106  rotated to the die picking position shown in  FIG. 8  and vacuum pressure applied to vacuum tool  102 , a flipper elevator motor  110  moves flipper mechanism  44  downward along the Z-axis to contact die  12 B. Die  12 B is contacted from above by vacuum tool  102  and from below by ejector assembly  74  essentially simultaneously. When a predetermined amount of vacuum pressure is sensed at vacuum tool  102  (signaling that die  12 B is firmly gripped), flipper elevator motor  110  and ejector pins  20  ( FIGS. 7A  to  7 C) move upward along the Z-axis simultaneously and at the same velocity. At a predetermined Z-axis elevation, vacuum pressure is released from vacuum channels  94  ( FIG. 7C ) and ejector assembly  74  retracts downward while vacuum tool  102  continues moving upward along the Z-axis. When flipper mechanism  44  has elevated sufficiently high for clearance, flipper motor  108  is actuated to impart “underhanded” (i.e., clockwise rotation to arm  106  such that die  12 B rotates downward, swinging closely past wafer  104 , and upward to a 180° inverted position (shown in dashed line) with solder bumps down and adjacent to a vacuum tool  112 . Skilled persons will appreciate that Z-axis motion could be reduced and the rotation of arm  106  reversed (clockwise). In this particular design, however, clockwise rotation could cause interference with a light source  114  associated with wafer camera  42 .  
         [0065]     After flipper mechanism  44  has inverted die  12 B, it is transferred to flux well  46  by either pick and place mechanism  48  associated with main gantry  50  ( FIG. 4 ), or to minimize cycle time, preferably by a flip-to-flux mechanism  116 . Flip to flux mechanism  116  elevates vacuum tool  112  along the Z-axis and 90° rotationally about a swing θ-axis in a horizontal plane. After flipper mechanism  44  positions die  12 B just below vacuum tool  112 , a pneumatic actuator  118  moves a swing arm  120  downward along the Z-axis a short distance such that vacuum tool  112  acquires die  12 B. The vacuum pressure in flipper mechanism  44  vacuum tool  102  is then released and pneumatic actuator  118  moves vacuum tool  112  upward slightly. A θ-axis motor  122  swings die  12 B 90-degrees horizontally into position above flux well  46 . Pneumatic actuator  118  then lowers and presses die  12 B into flux in flux well  46 , releases the vacuum pressure on vacuum tool  112 , and returns to dwell position above the die transfer point shown in dashed lines in  FIG. 8 .  
         [0066]     A locking assembly  124  allows flipper mechanism  44 , flip to flux mechanism  116 , and their associated assemblies to be swung aside from the operational position shown to a position that allows access to other mechanisms of FCAM  30  that would, otherwise, be obscured.  
         [0067]      FIG. 9  shows a flux station  130  that facilitates consistent, uniform application of flux to solder bumps  16  of each die  12  processed by FCAM  30 . Flux application is accomplished by reciprocating an open-bottom flux reservoir  132  over and back across flux well  46  to deposit a consistent layer of flux in the shallow depression forming flux well  46 . Then, as described above, die  12 B is placed in flux well  46  so that every solder bump touches the bottom of flux well  46  and, therefore, acquires the same amount of flux.  
         [0068]     Flux station  130  includes a base  134  on which an interchangeable flux plate  136  is accurately mounted. Base  134  includes an upper surface that is manufactured to ensure accurate and permanent alignment that is parallel to the horizontal X- and Y-axes of FCAM  30 . Each interchangeable flux plate  136  contains a shallow flux well depression sized to fit the largest die to be processed and having a depth suitable to match the solder bump sizes and flux properties employed. Flux well  46  depths preferably range from about 25 μm to about 250 μm (0.001 in to 0.01 in). Flux plates  136  are easily removed without tools.  
         [0069]     The shallow depressions forming flux wells  46  in flux plates  136  can be formed by several techniques. One technique entails masking an area of an extremely flat metal plate and then plating (e.g., by electroless nickel plate process) all around the masked area to raise the surface. For example, after removal of the masking material, a 0.002-inch plating thickness creates a flux well of 50 μm in depth. A second technique entails masking all areas except the well area and etching to the desired depth by electrochemical milling processes. A third technique entails using an electrode of the same profile as that of the desired well shape and creating a depth by employing an electrical discharge machining (EDM) process. A fourth technique entails forming a rectangular through hole in a rectangular plate member of about 6 mm (0.24 in) in thickness. A rectangular piston having cross-sectional dimensions equal to the dimensions of the hole and having a length less than the 6 mm (0.24 in) thickness of the plate member is fit into the hole to plug it. Because its length is shorter than the thickness of the plate member, the piston plugging the hole forms a shallow recess in the member and thereby a flux well of a desired depth. The length of the plug can be set by a grinding operation to remove material, and the plugged hole can be sealed by a seal ring placed between the plug and the member from the bottom (exterior) side of the member.  
         [0070]     For greater and less-critical depths, precision milling or grinding processes can create flux wells, which necessarily have rounded corners that require the wells to be considerably larger than the die, an undesirable result. All parts that can come in contact with flux are fabricated from or plated with corrosion-resistant materials.  
         [0071]     A precision low-friction linear slide (not shown) is attached to base  134 . A carriage  138 , mounted on the slide, holds open-bottomed flux reservoir  132 . A screw-actuated device  140  coupled to carriage  138  provides an adjustable spring force for pressing the bottom of flux reservoir  132  lightly against flux plate  136 . The component  140 A that retains flux reservoir  132  under spring force may be opened, either manually or automatically, to allow easy removal of flux reservoir  132  and flux plate  136 . Preferably they can be removed individually or as a pair. A quick release latch  141  facilitates removal.  
         [0072]     A closed-loop stepping motor  142  and timing-belt drive  144  move carriage  138  back and forth, causing flux reservoir  132  to reciprocate across flux well  46 . The bottom perimeter surfaces or edges of flux reservoir  132  adjacent to flux plate  136  are polished to minimize friction and provide a good “doctoring” action, thereby depositing a smooth flux surface in flux well  46 . A film of flux between the bottom perimeter surfaces of flux reservoir  132  and flux plate  136  functions as a lubricant between them. Stepping motor  142  provides control of flux reservoir  132  velocity over flux well  46 . For example, if the flux rheology requires a low-shear doctoring effect, flux reservoir  132  can be advanced quickly then retracted slowly.  
         [0073]     To further facilitate removal of interchangeable flux plate  135 , a cammed lever  146  is coupled to flux reservoir  132 . When stepping motor  142  moves flux reservoir  132  to a maximum +X-axis location, cammed lever  146  engages a wheel  150  that presses down on cammed lever  146 , thereby raising flux reservoir  132  off flux plate  136  and facilitating its removal.  
         [0074]     In use, flux reservoir  132  is partly filled with flux, and stepping motor  142  is cycled once to fill and smoothly doctor the flux in flux well  46  prior to placing die  12 B in the flux. Flip-to-flux mechanism  116  ( FIG. 8 ) places die  12 B in the flux and then pick and place mechanism  48  associated with main gantry  50  ( FIG. 4 ) positions die  12 B on the substrate. As soon as the flux well area is clear, flux reservoir  132  is automatically cycled again in preparation for receiving the next die.  
         [0075]     The following are some alternatives to the above-described preferred flux station embodiment. The motor could be one of a closed-loop stepper motor, a servomotor, a conventional direct-current (DC) motor, or an alternating-current (AC) motor. In any event, the motor facilitates maintaining constant velocity over the flux well and allows different velocities on the “fill” (advance) and “doctor” (retract) portions of the flux depositing cycle. A cable/capstan, a fast-pitch lead screw, a rack and pinion, a linkage, or any of other numerous devices for obtaining straight-line motion could replace the belt drive. A linear motor could be used, eliminating the need to convert from rotary to linear motion. Base  134  could incorporate a controlled heating device to raise the temperature of the flux, if necessary, to reduce its viscosity or improve its chemical activity. A practical range of temperatures is from 20° C. (68° F.) to 50° C. (122° F.). The base could also incorporate a cooling device to reduce the temperature of the flux. The adjustable spring force that presses the reservoir against the flux plate could be “fixed” to reduce cost.  
         [0076]     An optional electrically heated block, lightly pressed against the bottom of the flux plate, heats the area of the flux well to facilitate dispensing and doctoring very viscous or waxy fluxes. Many fluxes require a finite amount of time to react with the solder bumps. If this reaction time is significant, the overall cycle time is reduced by having flip to flux mechanism  116  place a die in flux well  46  and then return to the dwell position. After the appropriate flux reaction time has elapsed, pick and place mechanism  48  associated with main gantry  50  ( FIG. 4 ) then acquires the die and places it on the target substrate. If fast fluxes are used, however, flip-to-flux mechanism  116  may not be needed. Instead, pick and place mechanism  48  on main gantry  50  can acquire the die directly from flipper mechanism  44 , touch the die briefly in the flux, and then place the die on the target substrate.  
         [0077]     Somewhat similar fluxing systems employ air cylinders to provide motion, but velocities are not well controlled and results are inconsistent. For small die, a rotary system has been employed in which flux is applied to a slowly rotating disk that passes under a fixed doctor blade to control film thickness. This is impractical for large die (diameter becomes too large for practical application). It is very difficult to maintain an even film thickness at the tolerances required (approximately ±5 μm). It is also impractical to use low-viscosity fluxes and virtually impossible to match shear rates along the radius of the circular disk. Another prior design uses a fixed flux reservoir and an oscillating flux plate. Because the plate is moving, not the reservoir, it is difficult to keep the flux-well bottom accurately parallel to the X-Y machine axes, which is necessary to ensure that all solder bumps are coated equally.  
         [0078]     Referring again to  FIG. 4 , after it has acquired a fluxed die from flux well  46 , pick and place mechanism  48  on main gantry  50  cannot move the die directly to the substrate until the exact position of the die is ascertained to ensure correct placement. This is accomplished by moving the die from flux well  46  to a position over an up-looking camera  152 , which locates printed fiducial marks on the die that relate to the solder bump positions. Up-looking camera  152  is attached to the front of a substrate conveyor assembly  154  that is positioned centrally within FCAM  30 .  
         [0079]     For a very small die (e.g., less than 2 mm×3 mm (0.08 in×0.12 in)), up-looking camera  152  can view the entire die. For a larger die, pick and place mechanism  48  positions the die so up-looking camera  152  first views one corner, then an opposite corner of the die. The camera-acquired data are then processed by an industrial PC  156  to direct pick and place mechanism  48  and main gantry  50  to align the die with the target substrate in X-, Y-, and E-axis directions. Up-looking camera  152  includes a programmable-intensity LED ring light source containing two rows of alternating red, blue, and green LEDs. The LEDs are controlled independently to change the illumination angle of the die. The intensities of the differently colored LED are also variable, in accordance with product-specific programming, to provide a wide range of light colors for maximizing image contrast.  
         [0080]      FIG. 10  shows further details of main gantry  50 , which moves a carriage  160  that carries pick and place mechanism  48  for picking and placing die and a down-looking camera  162  for viewing the working area of FCAM  30 . The X- and Y-axis motions of main gantry  50  are powered by linear motors. Y-axis motion employs two linear motors  164  running in synchronism, one on each side of main gantry  50 . A lightweight, but stiff gantry beam  166  spans the width of main gantry  50  and moves on precise linear Y-axis bearing rails  168  that are positioned adjacent to Y-axis linear motors  164 . Gantry beam  166  further includes an X-axis linear motor  170  for driving carriage  160  on X-axis bearings along X-axis rails  174 . X-axis motion is limited by X-axis shock absorbers  176 .  
         [0081]     Pick and place mechanism  48  includes a Z-axis motor  178  and a theta-axis motor  180  for moving a vacuum pickup tool  182  in respective Z-axis and theta-axis directions. Down-looking camera  162  further includes a lens  184  for viewing the working area under vacuum pickup tool  182 . The working area is illuminated selectively by an on-axis light source  186  and a ring light  188  having illumination characteristics similar to those of the ring light associated with up-looking camera  152  ( FIG. 4 ). Ring light  188  is a shallow angle illuminator that provides off-axis illumination.  
         [0082]      FIG. 11  shows further details of pick and place mechanism  48 . The precise angular orientation of vacuum pickup tool  182  is measured by a precision glass-scale encoder  190  in a closed-loop relationship with theta-axis motor  180 . Z-axis positioning of vacuum pickup tool  182  is augmented by a short, precision Z-axis slide  192 . Vacuum pickup tool  182  is one of a set of interchangeable pickup tools, such as pickup tool  182 ′, that are held by vacuum pressure in a tool collet  194  that includes a conical seat. Vacuum pickup tools  182  and  182 ′ are hollow and employ controlled vacuum pressure supplied at a vacuum port  196  for picking up die. Vacuum pickup tools  182  and  182 ′ each further include a conical surface that mates with the conical seat in tool collet  194  for securing pickup tools  182  and  182 ′ by controlled vacuum pressure delivered to a vacuum port  200 . Vacuum pickup tool  182  has a relatively large working end  202  that is preferably round, includes an inserted O-ring  204 , and is suitable for picking and placing relatively large die. Conversely, vacuum pickup tool  182 ′ has a relatively small working end  206  that is preferably pointed, includes a rubber or elastomeric tip  208 , and is suitable for picking and placing relatively small die. Interchangeable vacuum pickup tools, such as tools  182  and  182 ′ are stored in a tool holder described with reference to  FIG. 14 .  
         [0083]      FIGS. 4 and 12  show substrate camera gantry  40 , which moves along X-axis and Y-axis directions beneath main gantry  50 . Down-looking substrate camera  38  is carried on a carriage  210  including X-axis bearings  212  that glide along X-axis rails  214 . Substrate camera gantry  40  includes Y-axis bearings  216  that glide along Y-axis rails  218  ( FIG. 4 ). X-axis motion of carriage  210  is accomplished by an X-axis linear motor  220 , with the precise positioning of carriage  210  measured by an X-axis encoder  222  that senses an X-axis encoder scale  224 . Y-axis motion of substrate camera gantry  40  is accomplished by a Y-axis linear motor  226 .  
         [0084]     The purpose of substrate camera gantry  40  is to save cycle time by positioning down-looking substrate camera  38  while main gantry  50  is busy elsewhere. Just as up-looking camera  152  determines the locations of the die fiducials with respect to die vacuum pickup tool  182  coupled to main gantry  50 , down-looking substrate camera  38  determines the positions of corresponding fiducials on the substrates. Because different substrates may have different thicknesses, the focal plane of down-looking substrate camera  38  is varied by a motorized focus actuator 228 employing a DC motor and encoder. Initial focus may be set using [+] and [−] controls at an operator interface terminal  230  ( FIG. 4 ), with which the initial focal plane position is captured in a part-specific program. As with other cameras in FCAM  30 , a ring light  232  provides on-axis illumination of the substrates. While main gantry  50  is moving pick and place mechanism  48  between flux well  46  and two locations typically needed to view die fiducials, down-looking substrate camera  38  locates the correct substrate position for placing the die. As soon as substrate images are acquired, down-looking substrate camera  38  quickly moves in the −X direction to be clear of subsequent pick and place mechanism  48  operations. The substrate image locations are quickly processed, and main gantry  50  and pick and place mechanism  48  move as necessary in X-, Y-, Z-, and θ-axis directions to place the die in the correct location on the substrate.  
         [0085]      FIGS. 13, 14 , and  15  show further details of substrate conveyor assembly  154 , which carries substrates through FCAM  30  on a parallel pair of conveyor belts  240  and  242  that move at the same rate in the +X direction. Conveyor belt  242  is fixed in its X-axis position, while conveyor belt  240  can be adjusted in the Y-axis direction by turning a conveyor width knob  243 . Regardless of the width adjustment (narrowest width is shown), conveyor belt  240  remains parallel to conveyor belt  242 , to carry substrates having widths ranging from about 35 mm (1.38 in) to about 180 mm (7.09 in). The substrates can be carried on conventional stainless steel boats or carriers, of either flat or “J”-type, or they can be separate thin printed-circuit strips or boards. Conveyor belts  240  and  242  are narrow and support the substrates or substrate carriers by their edges. Optional pinch-rolls can be added to allow conveyor belts  240  and  242  to transport very thin or warped substrates. A single motor  244  simultaneously drives both belts, which are suspended between pairs of idler pulleys  246 . The tension of conveyor belts  240  and  242  is adjustable by idler pulley tension adjustments  248 . The functioning of conveyor belts  240  and  242  is augmented by conveyor belt support rails  250  (one shown) and conveyor belt guards  252  (one shown).  
         [0086]     With particular reference to  FIGS. 14 and 15 , conveyor belts  240  and  242  are augmented by a reciprocating walking beam mechanism  260  that is positioned alongside conveyor belt  240  and moves along the X-axis parallel to the lengths of conveyor belts  240  and  242 . Mounted to walking beam mechanism  260  is an interchangeable tool having fingers  262  and  264  that when deployed straddle the respective leading and trailing edges of the substrate or carrier. The spacing between fingers  262  and  264  is slightly greater than the length of the substrate or carrier. Skilled persons will appreciate that the fingers could also engage holes in carriers, if holes are made available for the purpose. In use, finger  264  of walking beam mechanism  260  quickly moves the substrate or carrier into an operating station while finger  262  simultaneously pushes the just-completed substrate or carrier out of the operating station. Conveyor belts  240  and  242  then carry the completed pallet or device farther downstream.  
         [0087]     Walking beam mechanism  260  is reciprocated by a drive motor/encoder  266  that drives a drive belt  268 , which is suspended around an idler puller  270  and tensioned by a guide belt tensioner  272 . Drive belt  268  moves a walking beam support bracket  274  along a walking beam guide rail  276 . Fingers  262  and  264  are coupled to a walking beam pivot bar  278  that is actuated by an air cylinder  280  to engage and disengage fingers  262  and  264  from the substrate or carrier.  
         [0088]     The motion of walking beam mechanism  260  is more positive than could be achieved by conveyor belts  240  and  242  alone because they depend on friction to accelerate the substrate or carrier. Conveyor belts  240  and  242  and walking beam mechanism  260  are driven by servomotors or closed-loop stepping motors so acceleration and deceleration may be accurately controlled. Controlled acceleration minimizes the chance of dislodging assembled parts that are lightly adhered (e.g., prior to curing an adhesive or re-flowing solder). Moreover, controlled deceleration minimizes potentially harmful impact at the operating station “stop” position. Walking beam mechanism  260  can also move a carrier through multiple small steps, thus acting as an indexer. This is particularly useful when an operation is performed at several locations along a carrier and the operating equipment has limited mobility. Synchronously accelerating and decelerating conveyor belts  240  and  242  and walking beam mechanism  260  eliminates the wear and resultant particle generation that would occur if the carrier and conveyor belts moved at different speeds.  
         [0089]     Substrate conveyor assembly  154  further includes stops with presence sensors  282  for properly positioning the substrates or carriers for processing at the operating station. Fiducial marks  284  provide operating station reference locations for down-looking camera  38  ( FIG. 12 ). A tool change station  286  adjacent to the operating station includes a small grooved tool holder for holding vacuum pickup tools of various sizes, such as tools  182  and  182 ′ ( FIG. 11 ) to cover die sizes from less than 1 mm (0.04 in) to about 53 mm (2.09 in) square. Defective parts or assemblies can be temporarily stored in a reject bin  288 .  
         [0090]     Referring to  FIG. 15 , conveyor belts  240  and  242  are secured to idler pulleys  246  by pinch roller assemblies  290 . Turning conveyor width knob  243  turns a pair of width-adjusting, anti-backlash lead screws (not shown), one at each end of substrate conveyor assembly  154 , that together move a conveyor width adjusting frame  292  along guide rails  294 . Motor  244  rotates a splined drive shaft  296  that engages splined nuts  298  in idler pulleys  246  driving conveyor belts  240  and  242 .  
         [0091]     Substrate conveyor assembly  154  further includes a reloader mechanism  300  for reloading processed substrates or carriers back into magazines ( FIG. 17 ) for further processing. An air cylinder slide  302  first moves a cam actuator  304 , which swings a reloader finger  306  down 90-degrees in back of a completed substrate or carrier. As air cylinder slide  302  continues its motion, reloader finger  306  pushes the completed pallet or device several inches into the magazine.  FIG. 13  more clearly shows a swing and push pathway  308  followed by reloader finger  306 .  
         [0092]      FIG. 16  shows a tooling lift mechanism  310  that is located beneath conveyor belts  240  and  242  for elevating and locking a tooling plate  312  at a predetermined Z-axis elevation slightly above conveyor belts  240  and  242 . Tooling lift mechanism  310  supports and holds by vacuum pressure substrates, carriers, or devices at the operating station location for processing. Tooling plate  312  shown in  FIG. 16  is a mid-sized example that is designed to hold twelve individual substrates  313  that are carried on a conventional Auer Precision “J” boat. Tooling plate  312  is supported by a spacer plate  314  that is coupled by linear guide shafts  316  to a lift table  318 . Tooling plate  312  is raised by an eccentric cam (under lift table  318 ) that rotates  180 ° from bottom to top positions. Rotation of the cam is accomplished by an air cylinder  320  that pulls a cogged belt  322  that rotates a cogged pulley  324  that is coupled to the cam. Cogged belt  322  is supported by an idler pulley  326 , tensioned by a tensioning clevis  328 , and supported by a guide rail  330 . (Portions of lift table  318  and air cylinder  320  are revealed in  FIG. 15 .) Alternative embodiments of tooling lift mechanism  310  may include a wedge system, screws, or any number of linkages driven by motors or pneumatic actuators. Substrates are preferably locked by vacuum pressure to tooling plate  312 , although mechanical clamps or grippers could also be employed.  
         [0093]     This embodiment of tooling lift mechanism  310  is advantageous because has a very low profile and can accurately position tooling plate  312  within 0.005 mm (0.0002 in) in a horizontal reference plane. An adjustable lift stop  332  and a fixed lift stop  334  ensure planarity and a travel limit. Tooling plate  312  is sufficiently wide to support the largest width tooling substrate conveyor assembly  310  can handle. Narrower tooling plates are preferably coupled to lift table  318  near the fixed (front) conveyor belt  242 .  
         [0094]     The operating sequence of tooling lift mechanism  310  starts at the completion of processing a carrier of substrates at the operating station:  
         [0095]     1. With finger  264  of walking beam mechanism  260  engaging a next carrier, both walking beam mechanism  260  and conveyor belts  240  and  242  synchronously accelerate and then decelerate. At the completion of motion, the next carrier to be processed is in the operating station, and the previous carrier is at rest on the conveyor downstream of the operating station.  
         [0096]     2. Tooling lift mechanism  310  rises to lift tooling plate  312  slightly off conveyor belts  240  and  242 , and vacuum pressure is applied to secure the substrate carriers to tooling plate  312 . At this time, stops  282  are actuated, and walking beam fingers  262  and  264  lift and retract to await the next carrier.  
         [0097]     3. Conveyor belts  240  and  242  now advance at a low velocity to convey a new carrier into position against upstream stop  282  and to convey the processed carrier out of FCAM  30  to subsequent processes.  
         [0098]     4. When the new carrier arrives at upstream stop  282 , conveyor belts  240  and  242  stop and walking beam fingers  262  and  264  engage the front and rear ends of the new carrier awaiting a signal to advance.  
         [0099]     5. When carrier processing is completed, the operating sequence returns to step 1.  
         [0100]      FIG. 17  shows a rear view of substrate magazine elevator/loader  36 . Substrates or carriers  342  are brought to FCAM  30  in metal or plastic magazines  344  that are supported by magazine carriers  346 . Magazine carriers  346  are adjustable to accommodate different width magazines. Magazines  344  contain a series of shelves on each side for supporting multiple carriers  342 . To improve throughput, FCAM  30  magazine elevator/loader  36  automatically transfers magazine carriers  346  sequentially onto substrate conveyor assembly  154  ( FIGS. 13, 14 , and  15 ) on demand. An operator of FCAM  30  can place a fresh magazine in one position and remove an empty magazine from a second position, while carriers in a third magazine are being loaded onto substrate conveyor assembly  154 . Sensors  348  detect the presence of magazines  344  and are adjustable up and down. Forward stops  350  limit the forward travel of magazines  344 , and retaining bars  352  prevent carriers  342  from drifting forward in magazines  344 .  
         [0101]     A stepper motor  354  (inside enclosure) drives a lead screw  356  that elevates magazines  344  to the load position shown in  FIG. 17 . Dual precision slides  358  and a load position photocell  360  ensure alignment of carriers  346  with substrate conveyor assembly  154 . When aligned, a pusher  362  pushes carriers  342  one at a time out of magazines  344 . Pusher  362  includes an adjustment  364  for magazine length and carrier height.  
         [0102]      FIG. 18  shows a substrate magazine elevator/unloader  370 , which may not be required in applications where processed carriers/substrates  346  are conveyed directly into a downstream oven. However, when needed, substrate magazine elevator/unloader  370  is very similar to substrate magazine elevator/loader  36 , but lacks pusher  362 . Instead, processed magazine carriers  346  are pushed into substrate magazine elevator/unloader  370  by reloader mechanism  300  ( FIG. 15 ) on substrate conveyor assembly  154 .  
         [0103]      FIG. 19  shows a control system governing the overall operation of FCAM  30 . Industrial PC  156  and ancillary process control boards respond to operational commands implemented in software and operational information provided by machine sensors to actuate the motors that position main gantry  50 , camera gantry  40 , and cameras  38 ,  42 ,  152 , and  162 . An Ethernet link connects industrial PC  156  to an Ethernet Hub for controlling machine subassembly operations such as those of substrate conveyor assembly  154 , wafer handling system  60 , die flipper motors  108 ,  110 , and  122 , and their associated sensors and actuators. Images acquired by the four cameras that contribute to die and substrate alignment and pick and place operations are processed by a four-channel image frame grabber  380  under control of industrial PC  156 . A keyboard and a liquid crystal display (LCD) monitor constitute operator interface terminal  230  functions for industrial PC  156 .  
         [0104]     Referring back to  FIG. 3 , FCAM  30  is covered to prevent accidental contact with moving parts and ensure process cleanliness. A sheet-metal top “cap”  390  holds a multi-color indicator-lamp tower  392  to show machine status. A panel in cap  390  can be exchanged with a set of blowers and high efficiency particulate air (HEPA) filters, if desired by the user. From near the floor to “waist” level, the front and back of FCAM  30  have sheet-metal panels that are readily removable for access. The side panels are solid sheet metal from top to bottom, except for locations where the conveyor protrudes. Sliding or upward-swinging doors  394  with clear high-impact, static-dissipative-plastic windows  396  cover the front and rear of FCAM  30  from waist-level up to cap  390 . At the front right of the machine, an articulated support  398  holds operator interface terminal  230 , and a box  400  that contains Start, Stop and Emergency-Stop buttons.  
         [0105]     Skilled workers will recognize that portions of this invention may be implemented differently from the implementations described above for preferred embodiments. For example, depending on specific product requirements, some components could be eliminated to reduce cost, though at the expense of throughput. As noted in previous sections, the substrate camera and its gantry and related controls could be eliminated, saving cost but increasing cycle time (reduce throughput). However, this may also increase accuracy somewhat by eliminating sources of error (e.g., substrate camera system resolution and substrate gantry position encoders). The flip-to-flux pick and place could be eliminated if the flux used was very fast acting. In this case, the main gantry would acquire the die directly from the flipper, move to and quickly place the die in the flux well, then immediately take the die to the up-looking camera. The conveyor walking-beam mechanism could be eliminated if carriers held a large number of substrates (carrier load/unload time would be a small proportion of the total time). With simple change tooling, the machine can be quickly reconfigured in the field to handle 200 mm (8 in) wafers, as well as the 300 mm (12 in) wafers for which it was designed. Some optional additions may be desired, such as heated substrate tooling.  
         [0106]     The basic equipment can be used, with additions and/or subtractions of components, as a more conventional pick-and-place machine. In this case, the die (chips) are not “flipped” over, but simply picked from the wafer and placed on a substrate. For such applications, some of the changes might include optimizing the “Z”-stroke of the main gantry to pick directly from the wafer and adding a glue-application station either upstream or internal to the machine.  
         [0107]     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of this invention should, therefore, be determined only by the following claims.

Technology Category: 5