Abstract:
A direct die feeder picks known good die from a wafer and places them on a conveyor belt, which conveys the die to a pickup location. Frame ( 111 ) supports a wafer ( 105 ) which is a wafer that is sawed while adhered onto a flexible film, which is then stretched and mounted in a ring ( 110 ). A fork member ( 125 ), on which a pick head ( 150 ) is mounted slides along a horizontal axis and frame ( 111 ) slides along a vertical axis to allow the pick head ( 150 ) to access any die from the wafer. A camera ( 160 ) is directed downward at a 45° mirror ( 162 ) adjacent to the pick head to capture images of the wafer and determine the precise locations of die and qualify them. The pick head ( 150 ) can pick die from the wafer and place them directly on the conveyor belt ( 170 ) in the conventional orientation, or pass the die to a flip head ( 140 ) which then shifts to the left and lowers the die down past the pick head, placing the die on the conveyor belt in the flipped orientation. The conveyor belt operates until a die is detected at a pickup location ( 173 ) at one end of the conveyor except during placing operations of the pick head or flip head or during burst mode operations, during which the conveyor belt is loaded with closely spaced die, then fed in rapid succession to a multi-head host pick and place machine.

Description:
This application is a continuation-in-part of both U.S. patent application Ser. No. 09/025,564 filed Feb. 18, 1998 now U.S. Pat. No. 5,976,306 and U.S. patent application Ser. No. 09/033,269 filed Mar. 2, 1998 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to the manufacture of circuit boards, more particularly, the surface mounting of integrated circuits (ICs) on substrates including printed circuit boards and other platforms where components are mounted and connected. More particularly, the invention relates to a die feeder device that performs the function of removing each known good die from a sawed wafer disposed on a flexible adhesive film, placing the die on a conveyer, and conveying the die to a pick-up location. 
     In the mass production of printed circuit boards having a plurality of ICs surface-mounted thereon, automated machinery accept die from feeders and then mount them at a predetermined location on the circuit board. The die feeders have previously taken the form of tape-and-reel-type feeders (tape feeders) which remove the die from a plastic tape and present them to the host automated assembly machine. An example of a reel type tape feeder is shown in U.S. Pat. No. 4,437,232, issued to Araki et al. on Mar. 20, 1984. One advantage of the tape feeders is that they take up a small amount of space during the assembly of the circuit board, thus a large number of tape feeders can be placed adjacent to one another and utilized by a single assembly machine. Additionally, tape feeders are relatively simple in operation and are capable of feeding die reliably at a very rapid pace. The present invention takes the place of these tape feeders and obviates the steps of placing die onto the plastic tape then removing them from the plastic tape. The present invention is capable of performing this function of presenting die serially directly from a wafer without taking up significantly more space than a tape feeder. For this reason, it is called a direct die feeder, or DDF. By reducing the handling of bare die on its journey from wafer to assembly, significant expense reduction is realized, and the opportunity for damage to individual die is reduced, which improves the reliability of the end product. 
     Because the present invention is intended to replace current tape feeders, it is designed to maintain the narrow form-factor width standard of 80 mm which is standard in the industry. The narrow width enables many direct die feeders of the present invention to be positioned side by side (or along with tape feeders) and thus accessible by a single pick and place machine. By maintaining the narrow form-factor, fewer pick and place machines are needed as well as fewer pick and place stations in the assembly line. This significantly affects the overall cost and time consumed in the production of each unit. Maintaining this narrow profile required many design innovations herein disclosed. 
     The present invention represents a significant departure from the established practice of packaging dies in carrier tapes, shipping the die in the carrier tapes to a circuit board manufacturer, and then feeding the tapes to pick and place machines. When using the present invention, each die is picked from the wafer and presented directly to the host automated pick and place machine via a conveyor. 
     The feeder can also replace matrix trays to present parts to the host pick and place machine. The tray feeders represent a considerable expense as they take up a large section of the host machine interface as well as the cost of buying and stuffing trays. 
     Wafers usable by this invention are provided in a manner which is standard in the industry. A wafer which has been etched to form the micro-electric circuitry thereon is placed on a flexible adhesive film which holds the wafer in place during a sawing (or “dicing”) operation. The adhesive film is mounted in a ring which holds the flexible film during removal of the die. Ordinarily, the wafer is “expanded”, which means that the flexible adhesive film is stretched after dicing so that individual die spread out from one another. The mounting ring is used to maintain the flexible film in the stretched state. An expanded ring is characterized by an enlarged gap between adjacent dies, which improve the reliability of die extraction and prevents “chip-out” (damage to the picked die or adjacent die caused by scraping the die edges during pick-up). 
     A machine vision system is employed by the present invention to perform die recognition using the die edges, which requires gaps between the die, permitting them to be perceived by a visual imaging apparatus. The gaps caused by the dicing operation, without further expansion, is often sufficient to enable the chips to be reliably recognized and removed by the present invention, although it is recommended that the wafer be expanded prior to removing die. 
     Prior attempts at delivering chips directly from wafer to circuit board have either not been successfully implemented or industry has not accepted them. Presumably, lack of interest in the industry resulted from the failure of previous attempts to provide the specific features required by the industry, such as the high speed and small footprint currently made possible by tape feeders. 
     For example, U.S. Pat. No. 4,876,791 to Michaud et al. discloses an apparatus for removing die from an expanded wafer and placing the die on a substrate. This patent teaches holding the expanded wafer horizontally with the wafer side facing down. A pick and place head is positioned beneath the wafer and above the horizontally-disposed substrate. This configuration permits the pick and place head to remove the die from the wafer and place it on the substrate with minimal movement. However, it suffers from the disadvantage that it is not compatible with existing assembly machinery and requires a significant amount of horizontal real estate. In using the apparatus described in the Michaud et al. patent, the substrate must make a separate stop beneath each wafer and a separate mounting apparatus must be utilized for each chip to be placed on the substrate. The time and expense of using a separate machine for each die installed on the substrate and the space requirements of such an operation make this patent impractical for most assembly operations. Additionally, the horizontal orientation of the Michaud et al. patent is impractical for wafers having a diameter greater than 300 mm due to sagging of the wafer in the center which makes large wafers difficult to handle horizontally. 
     U.S. Pat. No. 5,671,530 to Combs et al. discloses an apparatus which removes die from an expanded wafer oriented vertically for use in the environment intended for the present invention. The patent to Combs et al., however, suffers from the disadvantages of only being capable of handling one die at a time using pick and place head assembly  104  shown in FIG. 6 of the Combs et al. patent. Pick and place head assembly  104  travels from a pickup location  96  to a transfer location  106 . Head assembly  104  cannot return to the pickup location  96  until the circuit board assembly machinery takes the die at transfer location  106 . This pick-move-transfer-move-pick cycle is extremely time-inefficient, making the system impractical for many customers who mass produce circuit boards. It is important to understand that the conveyor shown in the Combs et al. patent operates to transport printed circuit boards (referred to as “substrates”) during assembly, not to transport bare die to a pickup location as does the conveyor of the current invention. 
     The Combs et al. patent also suffers from the disadvantage of only being able to present chips in a “flipped” (circuit-down) orientation. There are currently two primary methods of surface-mounting chips onto a substrate or circuit board. The conventional wire-bond or tape-bond methods includes placing the chip on the substrate in the conventional, circuit-up orientation, and electrical connections being made from electrical contacts on the top of the chip to contacts on the substrate using metal wires or tape. The flip-chip method includes forming solder bumps on the art side of the die, then placing the die, circuit-down on the substrate, aligning the solder bumps on the die with metal contacts on the substrate and holding the die in position while electrical and mechanical connection is made by reflowing the solder bumps. The Combs et al. invention is limited by only being capable of presenting chips in the flipped, or circuit-down, orientation. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the prior art noted above. The direct die feeder of the present invention takes the place of conventional tape and tray feeders, and is therefore compatible with existing assembly machinery. As with tape feeders, the use of the direct die feeder is not limited to feeding die; it can feed any type of component positioned on a flexible film. Thus, for the purpose of this invention, use of the word, “die” generally refers to any type of component mounted to a flexible film. 
     Through the use of a novel conveyor belt, die may be transported to the pickup location while the pick-head assembly is busy removing more die from the wafer. Additionally, a novel flip head is selectively operable to accept a die from the pick head and then place the die in the flipped position on the conveyor. Alternatively, the pick head can place the die directly on the conveyor in the conventional orientation if required. Thus, the present invention is capable of presenting the die in the proper orientation for either the conventional or the flipped methods of mounting. 
     The die conveyor extends substantially the full length of the feeder traversing the entire wafer. This feature minimizes pick head movement as it removes die from the wafer and places them at any convenient location on the conveyor. The pick head is thus limited to two short 10 mm strokes in order to remove and register a die to the transport system where the transportation of the die is done in parallel to the picking. Prior attempts have incorporated the transport system in the pick and place mechanism resulted in requiring the die to by moved 300 mm or more to their final destination which requires a very substantial mechanism to perform this movement quickly. Such a device would be too large and bulky to fit into the narrow form factor required by the industry. The reduced motion of the pick head of the present invention translates into rapid die removal from the wafer that operates in the narrow form factor and is energy efficient. 
     The conveyor also facilitates buffering of die to the extraction point, which permits an assembly machine to take a cluster of die from the conveyor in rapid succession if it has that capability. By accepting a cluster of die in rapid succession using, for example, a rotary pick and place head assembly, the assembly machine can operate at peak efficiency. While the assembly machine is busy installing the chips on the circuit boards, the chip feeder will pick die off the wafer and place them on the conveyer, loading the conveyer for the mounting heads&#39; return to take the next cluster of die from the feeder. Buffering of die also provides the practical advantage of being able to exchange wafers without shutting down the line. 
     These and other advantages of the present invention will become clear in the following Detailed Description of the Preferred Embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the invention, fully assembled, in a false-perspective, isometric view. 
     FIG. 2 is a schematic representation of the invention showing the control system and major moving parts. 
     FIG. 3 shows certain novel components of the preferred embodiment of the present invention in perspective view, with obstructing covers and components removed. 
     FIG. 4 shows the fork assembly of the invention shown in FIG. 1 from a reverse angle. 
     FIG. 5 shows the conveyor and pick head arm of the fork assembly of FIG. 4 with the strip arm removed. 
     FIG. 6 shows a reverse angle of the conveyor and pick head arm shown in FIG. 5 with camera and flip head assembly removed to show outrigging support structure. 
     FIG. 7 is a closeup detail of the pick head in perspective. 
     FIG. 8 is an elevation view of the pick head with the pick plate in the retracted position. 
     FIG. 9 is an elevation view of the pick head with the pick plate in the extended position. 
     FIG. 10 is profile view of the pick head shown in FIG. 9 
     FIG. 11 is a cross-section of the pick head taken at section A—A as seen in FIG.  10 . 
     FIGS.  12   a,    12   b,  and  12   c  show a detail view of the pick head, which rotates to face the wafer in  12   a,  face down towards the conveyor in FIG.  12   b,  and face up towards the flip head in FIG.  12   c.    
     FIG.  12   d  shows a detail of the flip head shifted to the left and fully extended to the belt for placing a die on the belt in the flipped orientation. 
     FIG.  12   e  shows a detail of the pick head rotated to face the belt and extended for placing a die on the belt in the conventional orientation. 
     FIG. 13 shows the strip arm as viewed from the side of the wafer. 
     FIGS.  13   a  and  13   b  show a detail of the strip arm of FIG. 4 showing the movement of the strip head. 
     FIG. 14 shows a bottom view of the strip head shown in FIG. 13 with the strip head in the rotated position. 
     FIG. 15 is a cross section view of the strip arm shown in FIG. 14 taken along section line B—B. 
     FIG.  15   a  is a detail of the strip head as shown in cross section in FIG.  15 . 
     FIG. 16 shows a bottom view of the strip head shown in FIG. 13 with the strip head in its non-rotated position. 
     FIG. 17 is a cross section view of the strip arm as shown in FIG. 16 taken on section line C—C. 
     FIG.  17   a  is a detail of the strip head as shown in cross section in FIG.  17 . 
     FIG. 18 shows in a perspective view the flip head with motor and drive assembly for raising and lowering the flip head. 
     FIG.  18   a  is a reverse angle of the motor and drive assembly of FIG.  18 . 
     FIG. 19 shows the motor and drive assembly in elevation. 
     FIG. 20 shows the flip head in perspective view. 
     FIG.  20   a  shows a center cross section detail of the bottom portion of the flip head shown in FIG.  20 . 
     FIG. 21 shows the camera and flip head apparatus shifting mechanism from an angle consistent with the apparatus as shown in FIG.  3 . 
     FIG. 22 shows the camera and flip head apparatus shifting mechanism from a reverse angle from the one shown in FIG.  22 . 
     FIG. 23 shows the conveyor belt in perspective view. 
     FIG.  23   a  shows a detail of the conveyor shown in FIG. 23 wherein the structure under the belt, including exemplary guide bumps can be clearly seen. 
     FIG. 24 shows a cross-section view of the belt with a sample die placed on top. 
     FIG. 25 shows a portion of the belt with various layers removed along its length. 
     FIG. 26 shows a cross section view of a portion of the belt, the cross section taken through one of the guide bumps. 
     FIG. 27 shows a portion of the rear drive pulley of the conveyor belt with portion of the support and a sample of the conveyor belt itself, showing the interaction between these elements. 
     FIG. 28 shows the rear portion of the belt support. 
     FIG. 29 shows a conveyor clamp or pinch point along the length of the conveyor support which precisely locates the conveyor belt on the z-axis. 
     FIG. 30 shows the front portion of the support with guide wheel. 
     FIG. 31 shows, in a cross-section, the front portion of the support seen in FIG. 30 engaged with the front free pulley. 
     FIG. 32 shows a front view of the front pulley engaged with the front portion of the support. 
     FIG. 33 shows the belt cleaner apparatus in perspective view. 
     FIG. 34 shows a profile view of the belt cleaner apparatus of FIG.  33 . 
     FIG. 35 shows a cross section of the belt cleaner apparatus, the section taken along lines B—B of FIG.  34 . 
     FIG. 36 shows the die stop and sensor in perspective view. 
     FIG. 37 is a perspective view of the back panel, showing the wafer doors and control panel. 
     FIG. 38 is an example of a wafer in a mounting ring. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows the fully assembled direct die feeder  100  in a false-perspective, isometric view. Direct die feeder  100  includes a service access  94  which is transparent to allow viewing of the operation of the invention. Service access  94  pivots upward at hinge  95  to permit periodic maintenance and servicing of the moving parts inside. Cabinet  92  houses the control unit  101  and associated control systems, such as pneumatic control assembly  106 . A control panel  96 , shown in FIGS. 1 and 37, is provided which controls the function of the apparatus, which can also be controlled remotely through a network connection. The direct die feeder  100  mounts to the host assembly machine using rails  90 , which allow the direct die feeder to slide out from between other feeders making it accessible for servicing. 
     FIG. 2 provides a schematic representation of the main working parts and control system of the invention. The major components of the invention include wafer frame  111 , conveyor  170 , die strip head  130 , pick head  150 , flip head  140 , machine vision camera  160 , and control unit  101 . The basic operation of this system is best understood with reference to the schematic representation of the invention shown in FIG. 2, however reference should also be made to FIG. 3 which shows the working parts after certain covers and components are removed. 
     FIG. 2 incorporates the following conventions to clearly show the important moving parts of the invention: Each actuator is represented as a cylindrical body having rod extending therefrom which either translates along the axis of the cylindrical body or rotates on the axis of the cylindrical body. The motion of each actuator or the part it moves is indicated with an arrow near the actuator. Occasionally, the arrow is shown adjacent to the part actuated. Structural supports are represented with thick black lines. By no means is this drawing to scale, and of course actuators shown as translating actuators could be replaced with rotating actuators with rack-and-pinion gearing or other means to create a translating motion, and vice-versa. The parts in this diagram are spread out to provide room to clearly show each part and its movements. Wafer  105  is shown in phantom so that parts positioned behind the wafer, including die strip head  130 , die strip head actuator  132 , and associated control lines and support structures could be clearly shown. It should also be understood, however, that the wafer itself in not part of this invention. 
     Wafer  105  is an expanded 8 inch wafer, however the frame  111  can also position other sized wafers, and the invention can be modified to accept larger wafers. Wafers are expanded by stretching the flexible film to which the sawed wafer is attached, causing each chip on the film to move away from its neighbors. Once the film is stretched, it is placed in a wafer ring  110  (see FIG. 38) which clamps the film between two component rings. Although recommended, wafers are often not required to be expanded prior to being loaded into direct die feeder  100  in order for the feeder to function properly. Regardless of whether the wafer is expanded, it must be supported in a ring  110  that is compatible with frame  111  of the device. 
     Referring now to FIGS. 3 and 37, wafer  105  is inserted through wafer access  113  in the direction of arrow  114 , in a manner more particularly described below. Wafer  105  is positioned vertically in frame  111 , but this is not a mandatory orientation. While the vertical orientation minimizes the horizontal reach of the profile, the frame can operate at any angle as it rotates on an axis that is parallel to conveyor  170 . After wafer  105  is positioned in frame  111 , clamp  112  actuates to lock wafer  105  in place. 
     The operation of the apparatus is controlled by control unit  101 , shown in FIG. 2, which may consist of a general purpose computer system equipped with multiple i/o ports for sending and receiving signals from the various sensors and actuators. Although the signal pathways are each represented in FIG. 2 with a single fine line, it should be understood that each may consist of more than one electrical wire to and from each actuator or sensor, and in the case of actuators, the single line may also represent feedback lines from verification sensors. In the preferred embodiment, many of the actuators are electromechanical, but some are preferred to be pneumatically actuated. As is well known in the art, pneumatic control is effectuated through the use of control lines  135  which operate solenoid-controlled valves (not shown) in pneumatic control assembly  106 , which connects each of pneumatic control lines  157  and  137  with either a vacuum source  107 , ambient atmospheric pressure source  108 , or compressed air source  109  as required. 
     Control unit  101  receives input from a variety of sources, including an external advance signal generated by the host automated assembly machine  172 , die placement sensor  176  which detects die when present at the extraction point, user input from keypad  96 , door and service sensors  86 , and video input signal  166  from video camera  160 . 
     To position a selected die at the correct y-coordinate for extraction, frame  111  raises or lowers wafer  105  in response to commands from a control unit  101  to rotary actuator  112  via control line  116 . Fork-shaped support  125 , seen in FIGS. 2,  3 , and  4 , includes die strip arm  127  shown in FIGS. 2,  3 ,  4 , and  13 - 17 , which is positioned on the back side of wafer frame  111  as it is viewed in FIGS. 2 and 3. Fork-shaped support  125  also includes a pick head assembly arm  126  seen in FIGS.  2 - 6  which is positioned on the front side of wafer frame  111  as it is viewed in FIGS. 2 and 3. FIG. 6, which shows pick arm  126  from a reverse angle, shows outrigging support  120 , which provides extra support to the pick arm for increased stiffness, thereby preventing unwanted vibrations during pick operations. Outrigging support  120  rides on rail  121  mounted on the back side of the conveyor assembly. 
     In FIG. 2, fork-shaped support  125 , including die strip arm  127  and pick head assembly arm  126  are represented using thick lines. Die strip arm  127  and pick head assembly arm  126  are mechanically connected and move as a unit in the x-direction to maintain alignment between the pick head  150  and strip head  130 . 
     As discussed above, the principal purpose of this invention is to remove die from a wafer, and place them on a conveyor belt whereupon they are conveyed to a pickup location. The process of locating known good die and removing them from a wafer is well known in the art, but is not normally performed with the wafer oriented vertically and it was heretofore unknown to do so in the tight confines of a 80 mm form factor. To overcome obstacles in performing this feat, a number of innovations in design and engineering were employed relating to the operation of the pick head, strip head, flip head, and machine vision system. 
     Of course, the die removal process can be used for purposes other than placing die on a conveyor belt. For example, die removal could be carried out for stuffing carrier tape or matrix trays with die for packaging, or for delivering directly to a host pick and place machine. Use of the narrow form factor machine herein disclosed for picking and stuffing die in either carrier tape or matrix trays provides for efficient use of production space. If the tape or trays are to be fed directly to a host machine, almost unlimited buffering of the components can be realized. Likewise, the conveyor belt represents a significantly improved means of conveying dies and other small objects to a specific location with a high degree of precision and accuracy. Thus, the knowledgeable reader will understand that the die removal apparatus and the conveyor belt herein disclosed have separate and distinct utility. 
     Before removing a selected die it is desirable to know its exact location as well as its status as good or bad so that only good die or die of the correct grade are removed and used leaving disqualified die behind to be dispensed later or discarded with the spent wafer. Two basic methods are commonly employed during the testing process to indicate die quality for down stream processes. The first and most common uses ink dots to mark bad die. Typically the ink dots are black, but they can be of other colors as well. The second method is array mapping of the wafer, in which the physical location of each die, and its status or grade is provided along with the wafer. Using the on board machine vision system the invention can detect ink dots to disqualify die. Additionally, the invention may incorporate off the shelf color and luster comparison to detect ink dots by scanning die for net average variances from a known reference. Technology supports processing of pre-mapped wafers, i.e., wafers provided with maps generated during testing of the wafer that disclose array locations of known good die. By pre-map removal of a reference fiducial die off-line as part of the wafer expansion step, a suitably durable scheme could be employed using edge detection technology to reference an array starting point. 
     The machine vision system can also be employed to check for the presence of solder bumps for flip chips and to ensure that die corners are present. 
     To maintain the thin profile of the invention, camera  160  is directed generally parallel to wafer  105  and perpendicular to the direction of travel of conveyor  170  at a half-silvered mirror  162  (see FIG. 2) which is angled at 45°, for obtaining a normal view of a section of the wafer. A light source  164  having a reflector to direct light horizontally toward the wafer and not up into the camera is disposed behind the half-silvered mirror to illuminated the wafer while minimizing shadows. 
     Once the precise location of a selected known good die is known, fork-shaped support  125  and/or frame  111  is moved to align that die with pick head device  150 , which is shown in FIGS. 2 and 3, and shown in detail in FIGS.  7 - 11 . Referring now to FIGS.  7 - 11 , pick head  150  is facing up, revealing interchangeable rubber tip  51  which has a suction port  152  which operates to hold a die against tip  51  when a suction is present inside tip  51 . Tip  51  is interchangeable to permit pick head  150  to operate with different sized die, thus the size of port  152  varies with the size and shape of the die to be handled. For reasons that will become apparent, pick head tip  51  is mounted on pick plate  53  which extends as seen in FIGS.  9 - 11  and retracts as seen in FIGS. 7 and 8. Extension and retraction of pick plate  53  is effectuated via two pneumatic control lines  37  (see FIG. 2) which feed to two pneumatic ports  59  and which feed to cylinder  58  on either side of piston  59 . As air pressure is applied to either side of piston  59 , plate  53 , which rides on guide rods  56 , extends and retracts as required. Suction at port  152  is supplied via a third control line  37 , and is supplied axially through cylinder  58  and piston  59 . Hole  55  (FIGS.  11  and  12   d ) is normally covered during operation, but provides an alignment utility to align pick head  140  with pins of strip head  130 . 
     Turning to FIGS. 13,  13   a,  and  13   b,  the die strip assembly  128  is shown. Strip head  130  rotates on an axis parallel with the plane of wafer  105  and perpendicular with the direction of travel of conveyor  170 . Strip head  130  is shown in a non-rotated position in FIG.  13   a  and a rotated position in FIG.  13   b.  The head may be pneumatically actuated or electro-mechanically actuated for rotation so that wafer ring  110 , shown in FIG. 38, which is significantly thicker than wafer  105 , can pass by the non-rotated strip head  130  during insertion and ejection of the wafer and during vision imaging of wafer  105  at the extreme right side of wafer  105  as seen in FIGS. 2 and 3. By rotating the strip head as shown in FIG.  13   a,  the wafer can be removed while still maintaining the narrow profile of the invention. Guyway  129  provides a means for maintaining control lines within the narrow profile of the invention by passing the control lines over the strip head assembly  128 . 
     Operation of the strip head  130  in extracting die will now be generally described, but die removal is more particularly described below with reference to FIGS.  12   a - 12   e.  Dies are removed from wafer  105  in a manner typical in the industry. To extract a die from wafer  105 , a vacuum is applied by strip head  130  to the flexible film to stabilize it. Then, a pin or pins  134  push through the flexible film from the rear side, i.e., from within strip head  130 , while at the same time, pick head  150  applies a vacuum to the die itself on the front of the wafer. The number and configuration of pins  134  varies with the size and shape of the die. Strip head  130  is removable, and can be replaced with a different sized strip head, such as the smaller one shown in FIGS.  14 - 17   a,  and/or having a different pin configuration depending upon the size of the die to be extracted. 
     Two pneumatic control lines  157  are provided to strip head  130 . One of pneumatic control lines  137  provides a vacuum which is applied to the flexible film to stabilize it. The second of pneumatic control lines  137  operates a piston within strip head  130  which in turn operates pin or pins  134 . The rotation of strip head  130  may be operated by electromechanical rotating actuator  132  (see FIG.  2 ), which operates in response to a signal  135  from control unit  101 , but other motivating means can be used instead. 
     In the preferred embodiment, pneumatic control is implemented to rotate strip head  130 . Pins  134  are actuated pneumatically in response to air vacuum or pressure transmitted via one of control lines  137  from the pneumatic control assembly  106 , which is in turn controlled via lines  135  from control unit  101 . Although pins  134  are actuated pneumatically in the preferred embodiment, any other type of actuator suitable for the purpose can also be used. FIGS. 14 and 16 show a bottom view of strip arm  25  with strip head in rotated and non-rotated positions respectively. FIGS. 15 and 17 are sectional views of FIGS. 14 and 16 and attempt to show the altered position of rack assemblies  32 , shown in detail views  15   a  and  17   a,  which respond to pistons which are provided with pressurized air or vacuum, depending on the desired movement. Rack assemblies  32  extend in response to increased air pressure present in respective cylinders, causing pinion gears (not shown) connected to strip head  130  to rotate. To prevent strip head  130  from returning to the non-rotated position, spud  35  extends, engaging a flat surface on strip head  130 , thereby preventing unwanted rotation, which is particularly required when auto-loading or auto-ejecting a wafer, which operation is discussed in further detail in the next paragraph. 
     In addition to functioning to eject die from the flexible film on which the wafer is mounted, strip head  130  serves another important function, which is to partially automate the insertion and ejection of the wafer. Looking now to FIGS. 1 and 37, wafer entry and control doors  82  are shown in the open position. The control doors  82  have guides in the form of grooves  84  which ensure that the wafer is properly aligned to the opening during insertion and removal, minimizing the potential for damage to the wafer. By making the upper door  82  shorter than the lower door  82  it is easy to align wafer ring  110  with grooves  84 . Once the wafer is inserted, doors  82  are closed. Control unit  101  senses the position of control doors  82  via door sensor  86 . In response to doors  82  being closed, seating of ring  110  inside frame  111  is completed automatically. Fork-shaped support  125 , shown in FIGS.  2 - 4 , slides horizontally until strip head  130  is inside ring  110 . Strip head  130  then rotates to the rotated position shown in FIG.  6   b  and slides (with fork  125 ) in the positive x direction, towards pickup location  173 , until the side of strip head  130  engages the inside edge of ring  110 . Strip head  130  then continues to the positive x direction, dragging ring  110  with it until it reaches the fully mounted position shown in FIG.  2 . During the wafer ejection process, the strip head  130  performs the same operation in reverse, pushing ring  110  out from outside ring  110 , until it can be manually pulled out the rest of the way through doors  82 . The invention allows a single wafer to be inserted into the feeder multiple times for short run production, or simply exhausted in a volume environment. 
     Pick head  150  includes a suction port  152 , seen in FIGS.  7 - 11  and  12   c,  and die strip head  130  includes a pin structure  134 . During the die extraction process, the pick head  150  rotates to face the die as shown in FIGS.  5  and  12   a  and pick head plate  53  extends toward the selected die to place suction port  152  in contact with the selected die. Then the suction port  152  of pick head  150  activates to hold the die against suction port  152  once it is released from the adhesive material. Meanwhile, the strip head  130 , in the rotated position shown in FIG.  13   b,  applies suction to the flexible adhesive film that the wafer is mounted on to stabilize the film using suction ports  132 . Then, pins  134  extend from one or more of suction ports  132 , through the flexible film to which the selected die is mounted, forcing the selected die from the material and against suction port  152  where it is held by the applied suction. The number of pins utilized and their configuration depends in part on the size of the die to be extracted as is known in the art. Pins  134  then retract and pick head plate  53  retracts, separating the die from the wafer. Due to perforations caused by pins  134  in the film, a strong vacuum is necessary to maintain the film against strip head  130  when removing a die which is adjacent to a recently removed die. 
     In response to signals  156  to actuator  158 , pick head  150  rotates on an axis that is parallel with the direction of travel of conveyor belt  170 , allowing pick head to face down to belt  170  or up to flip head  140  after picking a die from wafer  105 . Flag  54 , shown in FIGS.  7 - 11 , is a simple extension to provide an indication to a feedback sensor (not shown) for confirmation of rotation actuation of pick head  59 . 
     Disposition of the picked die by the pick head depends on whether the die is required to be placed in a circuit up orientation or a “flipped” or circuit down orientation. If the die is required to be placed circuit-up then the pick head  150  rotates 90° down as seen in FIG.  12   b.  Pick head plate  53  then extends to place the die on conveyor  170  as seen in FIG.  12   e,  then the die is released by removing suction from port  152  and supplying a puff, or a series of puffs, of air to blow the die off port  152 , and finally the pick head plate  53  retracts once more and rotates to face the wafer in preparation to pick the next die. Because frame  111  raises and lowers the wafer in the y-direction, the selected die is always adjacent to conveyor  170  and pick head  150  requires only minimal movement to extract the die from the wafer and place it on the conveyor. Furthermore, since the pick head moves in the x or negative x direction, it can place multiple die on the conveyor without any requirement that the conveyor advance. This permits asynchronous operation between the pick head and the conveyor, and allows the conveyor to operate as a buffer between the pick head operation and the assembly machine retrieving die from the conveyor at pickup location  173 . 
     If the die is required to be in a flipped orientation (circuit-down) then the pick head  150  rotates 90° up as shown in FIG.  12   c  and pick head plate  153  extends upward to place the die in contact with flip head  140  which also has a suction port that is activated when pneumatic control assembly  106  applies a suction to pneumatic control line  187 . When the suction port of flip head  140  is activated, suction from port  152  of pick head  150  is removed, thus transferring the die from pick head  150  to flip head  140 . Pick head plate  153  then retracts and pick head  150  rotates to face the wafer, ready for the next selected die. 
     Meanwhile, the entire assembly  145 , indicated in FIGS.  2 - 5  and comprising pick head  150 , flip head  140 , and machine vision camera  160 , shifts to the left under the influence of linear actuator  159  (FIG. 2) so that flip head  140  can drop down past pick head  150  and place the die on conveyor  170 . This operation is most easily envisioned by referring to FIG.  2 . Linear actuator  159 , which may be an electromechanical actuator or a pneumatic actuator, operates to shift support  155  to the left in response to signal  157  from control unit  101 . Camera  160 , its associated mirror  162  and light source  164 , as well as flip head  140  are mounted to support  155  and so they all shift to the left. Flip head  140  then drops to place the die in contact with conveyor  170  as shown in FIG.  12   d.  After this is done, the suction is removed from flip head  140  to release the die and the flip head retracts to its upper position and assembly  145  and shifts back to the right, ready to receive another die from pick head  150 . Parallel processing of the flip and pick operations means the cycle times overlap, maximizing throughput. All control lines, including wires and pneumatic lines, leading to and from components in assembly  145  are provided through flexible guide  90 , shown in FIG.  5 . 
     In the preferred embodiment, shown in FIGS.  18 - 20 , flip head  140  is mounted to an elongated tube  141  formed of light-weight titanium having a flat, hardened wear plate  147  along its mid-section. The head and tube assembly shown in FIG. 20 is ultra-light weight to reduce inertial forces. Tube  141  is positioned between four roller-bearings  183  and friction drive roller  182  (see FIG.  18   a ) attached to pulley  186 . In response to signals from control unit  101 , stepper motor  188 , which is mounted to shift support  155 , activates pulley  186  via belt  187 . Pulley  186  is connected to friction drive roller  182  which is spring-biased against wear plate  147 . Spring biasing is effectuated through mounting drive arm  181  on pivot  184  and biasing mounting drive arm  181  against bear plate  147 . Tensioning arm  189  is similarly biased against belt  187  to provide tension to belt  187  through tensioning pulley  185 . 
     When pulley  186  is rotated, the friction drive roller rolls against wear plate  147 , causing entire tube  141  to raise or lower. At the top of tube  14  is suction port  144  for supplying suction to flip head  140 . Flip head  140  also has a rubber tip  142  at its bottom end which includes a suction port for holding onto a die when its transferred from pick head  150  until it is placed onto conveyor belt  170  and released. Tube  141  also includes a sensor flag  146  for providing feedback to control unit  101  in case of malfunction. 
     Because dies are released from flip head  140  by applying a puff of air (positive relative pressure), occasionally, adjacent die will move due to the rush of air. To prevent this, a mechanical push-off pin structure, shown in the cross-section view FIG.  18   b,  is implemented. Flip head  140  includes a cylindrical chamber in which piston  143  having several holes drilled through it around the periphery thereof. Piston  143  also includes a push-off pin  149  extending from its bottom and out through tip  142 . During periods of negative pressure, i.e., when holding a die, piston  143  lifts up allowing air to rush in through tip  142  and through holes in piston  143 . When a die is being released, a puff of air blows down, causing piston  143  to drop and thereby pushing a die off tip  142  with push-off pin  149 . 
     The shifting mechanism to shift assembly  140  according to the preferred embodiment will now be described with reference to FIGS. 21 and 22. FIG. 21 shows the shifting mechanism from substantially the same perspective as generally seen in FIG. 3, while FIG. 22 shows the same apparatus from a reverse perspective. Shift support  155  moves axially with respect to pick arm support  126 . Stepper motor  259  is mounted to shift support  155  while rack gear  254  is mounted to and remains stationary with pick arm support  126 . As can be seen, pinion gear  253  extending from motor  259  is engaged with rack gear  254  such that when motor  259  receives signals via line  157  from control unit  101 , pinion gear  253  rotates, causing shift support  155  to move relative to pick arm support  126 . Sensor  257  detects presence of sensor flag  256  in known manner providing feedback to control unit  101 . 
     Once the die is placed on conveyor  170  and released by either pick head  150  or flip head  160 , conveyor  170  may be activated to transport the die to pickup location  176  where stop  174  prevents the die from traveling too far and helps to precisely position the die for pickup. 
     After the die has been placed on conveyor  170 , wafer frame  111  and/or fork-shaped support  125  move into position for the next selected die to be removed. Generally, die are removed from the wafer in a right to left, then bottom to top pattern. Since pick head  150  trails the view point of camera  160  while processing a row of die, the control unit  101  must determine during the processing of a row of die whether the next pick process or vision process should be done first or if both can be done from the same location. Initially, this means that only the vision process will be running for several die, until pick head  150  is also over the wafer. Near the end of the row camera  160  will run off the left end of the wafer but there still may be die in that row for pick head  150  to pick. This further increases the efficiency of the invention by minimizing movement of the pick head  150 . When all the known good die in a row has been found and picked, pick head  150  moves back to the right most die in the current row and frame  111  lowers one die interval to the start location of the next row. 
     Conveyor belt  170  will now be described with reference to FIGS.  7 - 10 . Motor  177  drives conveyor belt  170  in response to commands from control unit  101 . Looking at FIG. 6, Motor  177  drives pinion gear  74  which drives intermediate gear  73 , which in turn drives drive gear  72 , which is attached to drive pulley  281  (see FIG.  27 ). Conveyor belt  170  includes steel belt  270 , preferably comprising a stainless steel and is advantageously a high yield 300 series stainless steel approximately 4 thousandths of an inch thick (1.0 μm). Belt  170  is tensioned via tensioning device  179  (FIG.  3 ). To dampen or prevent vibrations, the belt rides on and is magnetically coupled to a “hard deck” surface discussed in further detail below. A low friction coating  271 , such as an ultra high molecular weight polyethylene material is coated or applied with a pressure-sensitive adhesive to steel belt  270  to reduce friction between belt  170  and decking  283  (to be discussed in further detail below with reference to FIGS.  27 - 32 ). The combined thickness of polyethylene and adhesive is approximately 0.0065 inches (1.7 μm). Low friction coatings  272  and  273 , on either side on top of steel belt  270  are identical to low friction coating  271  and have the purpose of reducing wear and friction as belt passes reversing structures  172  (see FIG. 3) and pinch points  171  (see FIGS.  3  and  29 ). Finally, steel belt  270  is provided with an elastomer coating  276  on its top surface to further reduce any migration of die. Elastomer coating  276  is a polyurethane with an elastomer compound to increase the friction between the die and the belt, and preferably has a hardness of Cast-70 Shore A. As with low friction coating  271 , elastomer coating  276  is applied to steel belt  270  using a pressure-sensitive adhesive. 
     It has been found that, in certain circumstances, and particularly with dies placed in the circuit-up orientation, elastomer coating  276  can create too much adhesion between die and conveyor, causing a die to “stick” to the belt rather than being properly picked up by the host machine. To prevent or mitigate this effect of the coating, grooves  277  (shown in FIG. 24) are ground into the coating to reduce adhesion. FIG. 24 shows a die example  278  which is on the order of 1 mm wide. Die example  278  is a flip chip and includes solder bumps  279  which interface with grooves  277 , which further reduces die migration. Grooves  277  will inhibit die migration of flip chips in this manner whether or not solder bumps mesh perfectly with them. 
     Guide bumps  274  are attached in two continuous rows along the bottom of steel belt  270 . Guide bumps  274  are formed by turning, i.e., machining, or stamping them from a low-friction plastic. Preferably, guide bumps  274  are formed from acetal resin, such as that sold under the trade name Delrin by DuPont. White (virgin) Delrin is most preferably employed. Guide bumps  274  are generally doughnut-shaped and have a inner circumference that is somewhat smaller at its center than at either the top or bottom. This shape allows the guide bumps  274  to form a mechanical connection with glue  275  which holds guide bumps to steel belt  270  (see FIG.  26 ). Glue  275  is a dual-mode UV-activated acrylic based adhesive having good elastic properties and is thicksotropic, i.e., thick when applied. Glue No. 911, sold by Dymax Corp. of Torrington, CT, has been found satisfactory for this purpose. Longitudinal spacing of guide bumps  274  is not critical, though precise transverse positioning of guide bumps  274  is crucial. As is discussed in more detail below, guide bumps  274  play a key role in ensuring consistent positioning of die for ready retrieval by the host machine. 
     Referring to FIGS.  27 - 32 , guide bumps  274  are guided along channels  285  formed in deck  270  and grooves  282  formed in rear drive pulley  281  and front free pulley  289 . Deck  280  includes guide wheel  284  at the rear end of deck  280  and guide wheel  288  at its front end. Guide wheels  284  and  288  maintain a very small clearance, on the order of 0.001″ (0.025 mm), within a central circumferential groove  290  in both front and rear pulleys  79  and  71 , respectively. These guide wheels serve to position deck  280  thereby steering the belt into an accurate position on the pulleys, preventing any drift of the belt resulting from its natural camber. 
     At several key points along the length of the deck, the width of channels  285  is narrowed, forming a choke  288  such as that shown in FIG.  29 . Advantageously, these choke points are positioned at either end of drop zone  173  where die may be placed on conveyor  170 , and at the pick point  176 . At each choke point, channels  75  narrow, precisely locating guide bumps  184  along the z-axis thereby precisely locating belt  170 . Clamp  287 , shown in FIGS. 3 and 29, hold down conveyor belt  170  at one of chokes  288  to prevent guide bumps  274  from popping out channels  285 . Low friction coatings  272  and  273  formed on steel belt  270  reduce friction between clamp  287  and conveyor belt  170 . 
     Between channels  285 , decking includes a rubber magnet  286  covered with a low-friction decking material  283  which is preferably acetal resin (DELRIN) and which is laminated, e.g., using pressure-sensitive adhesive onto rubber magnet  286 . Rubber magnet  286  is best viewed in FIGS. 28 and 31 and serves the function of magnetically coupling belt  170  to deck  280 , thereby preventing and dampening vibrations of belt  170  which may occur perpendicularly to the plane of the belt. 
     Referring now to FIGS. 1,  3 , and  5 , ramp section  172  of belt  170  serves the purpose of elevating die from a first height at the drop region  173  to a second height at pickup location  176 . The ramp  172  is a necessary and important innovation in the development of a direct die feeder because it permits fork-shaped support  125  to slide back and forth in the direction of the x-axis without extending into a region of space above the conveyor to which the host machine must have access. In other words, without the ramp, drop region  173  would be at the same elevation as pickup location  176 , which would necessitate raising fork-shaped support  125  into a fly-zone of the host machine, and the pick and place head of the host machine would inevitably crash into fork-shaped support  125 . Belt reversing structures  171  hold conveyor belt  170  down against deck  70  and provide a gentle turn as conveyor belt  170  changes direction from a flat area including drop zone  173  to ramp section  172 . Low friction coatings  272  and  273  seen in FIGS. 8 and 9 reduce friction of conveyor belt  170  as it passes against belt reversing structures  171 . 
     Control unit  101  keeps track of the position of each die on conveyor belt  170  and, depending on the mode of operation, it may be programmed to advance as each die is removed from the pickup location except during the placing operation by pick head  150  or flip head  140 . To determine when a die reaches and/or is removed from the pickup location  173 , a sensor  176  (see FIG. 2) comprising two photo-emitter optical fiber bundles  198 . Sensor  176  includes a light source and a photo sensor (not show) each having one of optical fiber bundles  198  extending to stop  174  shown in FIG. 3 but shown in detail in FIG.  36 . Optical fiber bundles  198  extend through openings in stop and fiber bundle support  196  and extend to positions on either side of stop surface  197 . The fiber bundles are ground down to form a refracting surface as is known in the art to bend light so that it passes across stop surface  197 . When a die is present at pickup location  176  against stop surface  197 , light from one fiber bundle to the other is blocked. Ordinarily, upon detection of a die at pickup location  176  control unit  101  over-travels the conveyor belt  170  slightly so as to square the die against stop surface  197  and to knock the die loose from the friction-enhancing polymer surface  276  in case it has become stuck to said surface. 
     Belt cleaning device  190  will now be described with reference to FIGS. 1,  2 , and  17 - 19 . There will inevitably be some silicon particles or other material particles which come in contact with the belt  171  and perhaps stick in place. Additionally, dust is a concern, particularly when supplying chips in a flipped, or circuit-down, orientation since dust and other particles can affect the mechanical and electrical connections between die and substrate after the die is delivered to the host machine. To mitigate dust and other particles from being transferred from belt  170  to the die, a belt cleaner  190  is employed. Belt cleaner  190  includes four nylon brushes  191  mounted adjacent and below the returning portion of conveyor belt  170 . Brushes  191  are made from 0.25 inch (6.4 mm) nylon bottle brushes mounted to a twisted wire. Belt cleaner  190  is mounted to posts  195  which have a detent in them which is engaged by a spring-biased ball bearing  194  so that the cleaner snaps onto posts  195 . It is easily removed for servicing (e.g., cleaning) by grasping handle  193  and sliding it off posts  195 . Additionally, belt cleaner  190  includes a high-strength neodymium magnet  193  to pull the belt into contact with brushes  191  and remove magnetic contamination from belt  170 , such as magnetic particles. 
     The invention having now been described with particularity to a single embodiment, it is understood that the scope of protection sought is limited not by the detailed description above, but by the claims appended hereto.