Abstract:
A system for manufacturing a print board assembly, which is implemented in a system having a computer for storing parts insertion instruction diagrams and displaying parts insertion instructions, and a soldering unit for soldering a printed circuit board, includes: assembling different types of the printed circuit boards from pieces of parts and inspecting them in a multitude of cells; installing different types of the printed circuit boards in carriers to assemble them separately according to the parts insertion instructions displayed by the computer of each cell, and moving them to the soldering unit through a transfer conveyor line; and returning the carriers having the printed circuit boards completely soldered to each cell identified for the carriers through a return conveyor line.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of Applicant&#39;s Ser. No. 08/978,469, filed in the U.S. Patent &amp; Trademark Office on Nov. 25, 1997 now U.S. Pat. No. 6,145,190. 
    
    
     CLAIM OF PRIORITY 
     This application makes reference to and claims all benefits accruing under 35 U.S.C. §§119 and 120 from applications for PRINT BOARD ASSEMBLY PRODUCING SYSTEM AND METHOD earlier filed in the Korean Industrial Property Office: on the 25th day of Nov. 1996 and there to duly assigned Ser. No. 1996-57795; on the 29th day of Nov. 1996 and there duly assigned Ser. No. 1996-60056; on the 29th day of Nov. 1996 and there duly assigned Ser. No. 1996-61342; and on the 29th day of Nov. 1996 and there duly assigned Ser. No. 1996-61340. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printed circuit board assembly manufacturing system and, more particularly, to a such manufacturing system that can simultaneously produce various kinds of printed circuit boards in one production line. 
     2. Description of the Related Art 
     The extraordinary growth in demand for consumer electronics products in recent years has resulted in a concomitant growth in demand for printed circuit board (PCB) assemblies used in such products. Rapid technological advances have also shortened the life cycles of electronics products, as consumer preferences adjust to the availability of new product features and improved quality. To compete in markets where consumers demand ever-increasing quality along with low prices, electronics manufacturers must reduce the production costs associated with PCB assembly production. 
     PCB assembly (PCBA) manufacture has tended to carry particularly high costs arising from two sources: set-up operations and production defects. Set-up of a production line for a different PCBA product entails refitting line equipment, retraining line operators, adjusting product transport equipment such as conveyors, and distributing parts for the new product to the various work stations. A change of the line set-up from one product to another therefore requires extensive expertise and substantial amounts of time. These operations have necessarily idled the entire production line for the duration of the retooling process. The shortened product life cycles that have become characteristic of the electronics industry only exacerbate the impact of set-up costs, because shorter production runs necessarily increase the per unit costs of production. 
     Line parallelism has been proposed as a way to mitigate the effect of set-up operations on overall productivity. For example, U.S. Pat. No. 4,719,694, issued Jan. 19, 1988 to Herberisch et al., discloses a system that divides a large number of PCB process steps into several groups of related operations performed in succession at several parallel cells of work stations. Each cell has a buffer area to store incoming units for which the cell is not ready or outgoing units for which a succeeding cell is not ready. Changing the production line over to a new product model can therefore proceed from cell to cell sequentially, rather than stopping the entire production line until every cell has completed its set-up for the new model. 
     This system substantially reduces down time costs, but its effectiveness is limited because it requires extensive coordination as the set-up process progresses through successive cells of the production line. Once a set-up process begins, it must propagate through the entire production line before the line can resume an optimal production rate. Moreover, the set-up process cannot easily be suspended or interrupted. The disclosed system also only rearranges a multiplicity of individual process steps, and it therefore does not address the issue of quality control. 
     A related approach is proposed in U.S. Pat. No. 5,170,554, issued Dec. 15, 1992 to Davis et al., which discloses a method of reducing set-up overhead by arranging a known production bottleneck into temporal cells. A temporal cell is defined in terms of a group of products, each of which is capable of being processed through the bottleneck using a single machine set-up. The method provides for careful planning of the duration and scope of these cells to allow a reduction in the number of set-up cycles and to overlap set-up cycles with production cycles. 
     This method shares some of the advantages of the &#39;694 system discussed above. However, the approach of the &#39;554 patent also demands careful coordination between production and set-up. Also like the system of the &#39;694 patent, this system does not enhance quality control. It also does not consider potential gains from reconfiguring the physical arrangement of the production line. 
     An alternative approach uses multiple production lines to manufacture several different product models simultaneously. For example, U.S. Pat. No. 5,355,579, issued Oct. 18, 1994 to Miyasaka et al., discloses a production line system that comprises several individual lines arranged in parallel. Redundancy in the system is reduced by directing the output of the several parallel lines to a common packaging station. Each individual line in the system operates independently of the other lines to produce a particular product model, with the only interaction between lines occurring indirectly through the packaging station. 
     The substantial parallelism of the &#39;579 patent&#39;s system reduces idle time for the overall system by decoupling the system into semi-independent sub-lines. This result comes, however, at the price of equipment duplication to implement the parallel production lines. More importantly, set-up for a production line generates similar overhead costs whether the line stands alone or in parallel with several other lines. Each of the disclosed system&#39;s sub-lines must be stopped to be changed over to a new product set-up, just as if that sub-line stood as an independent production line. The sharing a common packaging station at the end reduces these costs somewhat. However, the relative benefit of this measure declines as each of the individual lines grows in complexity and length. 
     None of the references mentioned above addresses the second major source of lost productivity in manufacturing systems for PCB assemblies: the incidence of production defects. These defects arise because the assemblies typically are structurally and functionally complex. The production system must therefore reliably assemble numerous different components and provide for a multitude of testing procedures to ensure proper operation of completed units. Faced with these system requirements, a traditional view of mass production has taught the division of the process into numerous specialized tasks, whereby the performance of each task (whether by a human operator or by automated equipment) can be designed to maximize throughput. 
     Extensive task specialization can cause its own problems, however, because it generally multiples the number of discrete operations in a fabrication process. This discretization can quickly erode the efficiency gains of task specialization because it increases the costs of adjusting the system to changed circumstances. Task discretization therefore tends to increase both the occurrence of production problems and the difficulty of quickly detecting those problems. 
     This problem arises whether the production line consists of fully automated process station or also employs human operators. For example, suppose that a process involves three or four operators working in serial relation. If a recurrent defect begins to arise in work units output from the first operator station, the problem can be quickly corrected even if it is not detected until the affected work units begin to arrive at the last station. If the system distributes the same process steps across 15 or 20 workers, in contrast, it may incorporate the defect into many more units before the defect is noticed. The effect can be even more pronounced with automated stations, where defect detection may be delayed by test and measurement limitations. 
     Two other mechanisms particularly affect production systems that employ human operators. First, the presence of a latent defect may be more difficult to detect in a highly discretized process. For example, a human operator carrying out a highly specialized task is more prone to boredom or inattentiveness, which can cause errors or prevent their prompt detection. Moreover, in a line with more process stations (whether operator controlled or fully automated), the nature and cause of a defect and its likely effects must be assessed for more stations. 
     In the case of human-operated stations, problem assessments and corresponding instructions must be quickly communicated to the station operators. This communication facilitates a coordinated response and is particularly important when the defect results from the combined work operations of several stations. In addition, similar problems may occur across several parallel sub-lines, and an adequate response to these situations requires an additional level of communication and coordination. 
     It follows that when a manufacturing system includes a large number of discrete work stations, problems are more difficult to detect rapidly and coordinated responses are more difficult to implement. Parallelism, as disclosed in the references cited above, reduces the overhead costs of line set-up, but it does not mitigate and may exacerbate the difficulty of responding to problems that arise during production. 
     We have therefore found that the electronics manufacturing arts have lacked a production arrangement that reduces overhead costs from both line set-up and production defects. Such an arrangement should minimize the need to reconfigure line equipment for production of a different PCBA product. It should also allow line elements not involved in a set-up operation to continue production, as well as allowing rapid detection and response to problems that arise in production. Preferably, such an arrangement would address these goals at all levels of the system design. Ideally, such a system would amplify its efficiency gains both by incorporating modem computerized process control technologies and by realizing the system responsiveness that human operators can provide. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a cell line for a PBA manufacturing system which can enhance productivity by assembling different types of PCBs in several work frames simultaneously while providing a common soldering facility and return of each soldered unit to its respective work frame for inspection. 
     Another object of the present invention is to provide a PBA manufacturing system by which a small number of skilled workers assigned to each work frame do the overall procedures in a production line covering from assembly to inspection for different kinds of parts. 
     According to one aspect of the present invention, a system for manufacturing a print board assembly, which has a spray flux for flux-coating a printed circuit board installed in a carrier, and a soldering unit for soldering the printed circuit board coated with flux, comprising: a multitude of work frames having a multitude of work frame upper conveyors for placing each of the carriers thereon by a number of workers, installing different types of the printed circuit boards in the carriers and inserting parts into the printed circuit boards according to parts insertion instructions, the carriers with the printed circuit boards soldered being returned to the work frame upper conveyors by types of the printed circuit board; an upper conveyor for transferring the carriers from the work frames to the soldering unit, with a multitude of transfer switch turned on; a carrier transfer distance controller for adjusting a transfer distance of the carriers on the upper conveyor so as to prevent the carriers from colliding with each other; a multitude of up/down and rotation units for moving the carriers up to the height of the spray flux and rotating them in a 90-degree arc; a slope conveyor for moving one side of the carriers down so as to have a declination during soldering; an elevator for moving the carriers down when the carriers are transferred from the slope conveyor and enter the elevator; a rotation unit for rotating the carriers reversely in a 90-degree arc so as to change their orientations; a lower conveyor for transferring the carriers rotated reversely by the rotation units to return, the carriers being moved down to the lower conveyor by the elevator when they are transferred from the slope conveyor; a multitude of lifters for moving the carriers returned by the lower conveyors up to the work frame upper conveyors; a multitude of carrier type sensing units installed in each designated position of the lower conveyor to detect positions of cell-recognizing indicators of the carriers; a controller for controlling the carriers to return to each work frame designated in response to a determining signal received from a carrier type discrimination unit to determine a transfer status of the carriers, generating a driving control signal for the work frame upper and lower conveyors and a carrier control signal to move up or down the carriers, and generating a motor control signal to drive base boards for supplying the parts in the clockwise or counterclockwise direction; and a multitude of sensors installed on the upper and lower conveyors to sense the transfer status of the carriers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawing figures, in which like reference symbols indicate the same or similar components, but which are attached only by way of example, wherein: 
     FIG. 1 schematically illustrates a PCB assembly system with several discrete process stations in serial order; 
     FIG. 2 is a perspective view of a system configured as a line of parallel work frames, according to the present invention for manufacturing various types of PCBA products in small quantities; 
     FIG. 3 is a perspective view of a carrier as shown in FIG. 2; 
     FIG. 4 is a cross sectional view taken along line A-A′ of the carrier shown in FIG. 3; 
     FIG. 5 is a cross sectional view of a part B shown in FIG. 3; 
     FIG. 6 is a diagram illustrating the carrier of FIG. 3 on which a PCB is mounted; 
     FIG. 7 is a perspective view of a work frame shown in FIG. 2; 
     FIG. 8 is a front view of the work frame of FIG. 7; 
     FIG. 9 is a front view illustrating a state in which a base board is mounted in the work frame of FIG. 7; 
     FIG. 10 is a perspective view of a base board which can put a variety of parts according to the present invention; 
     FIG. 11 is a plan view of a parts information label showing the list of parts that are assembled in a PCB according to the present invention; 
     FIG. 12 is a schematic configuration of sensors for sensing the movement status of carriers and motors, according to the present invention; 
     FIG. 13 is a circuit diagram of a motor driver installed on a conveyor according to the present invention; 
     FIGS. 14 to  17  are circuit diagrams of the motor drivers installed on each work frame according to the present invention; 
     FIG. 18 is a block diagram of a device for driving an elevator or a plurality of lifters according to the present invention; and 
     FIGS. 19 to  98  are diagrams illustrating the operation of a control program of a PLC according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 schematically illustrates a serial PCB assembly manufacturing system having several discrete work stations. A number of bare PCBs (not shown) are loaded in a PCB loading box  10 . A worker takes out a PCB from PCB loading box  10 , places it on a conveyor belt  14 , and inserts corresponding parts, indicated on a work instruction sheet  18  and provided in parts boxes  16 , into the PCB as the PCB slowly moves. The next worker takes out the parts indicated in the work instruction sheet  18  from parts boxes  16  and inserts them into the PCB as it travels past the second worker on conveyor belt  14 . 
     In the serial arrangement of FIG. 1, each worker only inserts his assigned parts into the PCB and passes the PCB on to the next worker through the conveyor belt  14 . When all the required parts are inserted into the PCB by the workers, the PCB is conveyed to a spray flux station  20 . At spray flux station  20  the PCB is coated with soldering flux, and then it is delivered to a soldering device  22 . The soldering machine  22  solders the fluxed PCB, after which the soldered PCB is cooled at a cooling station  24 . A worker then shifts the soldered and cooled PCB to a cutting device  26 . After cutting the PCB, anther worker checks whether the parts are correctly inserted by using a parts insertion detector  28 . This operation is repeated with each PCB assembled. 
     FIG. 2 shows a production line embodying the present invention. The disclosed system  100  illustratively includes first through fourth work frames  101  to  104 , at which a number of workers place carriers  710  on a plurality of work frame upper conveyors  111  to  114 . The workers then install a PCB (not shown) in each carrier  710  in preparation for inserting parts into the PCB. After parts insertion at work frames  101 - 104 , the parts-installed but unsoldered PCBs are transferred to an automated spray flux station  150  and an automated soldering unit  170 . Carriers  710  containing soldered PCBs are returned for collection at the right side end of the work frame upper conveyors  111  to  114 . 
     Work frames  101 - 104  each have the following features that facilitate their operation. Each one of work frames  101 - 104  includes a respective one of a plurality of computers  240 - 243 . Computers  240 - 243  store a program for displaying parts insertion instruction diagrams according to the type of PCB to be assembled at their respective work frames. Each carrier  710  can be rotated both clockwise and counterclockwise about a vertical axis, which facilitates the insertion process by allowing the PCB to be in either a clockwise or counterclockwise orientation. 
     Each of the computers  240 - 243  displays, on a monitor, work instructions for inserting the parts in the clockwise or counterclockwise orientation. Clockwise and counterclockwise rotation switches  252  and  253  display the work instructions for insertion of parts in the clockwise and counterclockwise orientations, respectively. Alternatively, a foot switch  256  (see FIG. 7) controls the same display function for inserting the parts in the clockwise orientation. 
     Work frames  101 - 104  also collectively include a plurality of baseboards  231 - 238  with, for example, two such base boards included in each work frame. As shown in FIG. 10, each of base boards  231 - 238  has a generally round cross-section and is divided by partitions into a plurality of parts containers  54  to be loaded with different kinds of parts. Each of base boards  231 - 238  supplies required parts to a worker at a respective one of work frames  101 - 104  by rotating under the control of a PLC (Programmable Logic Controller)  220 . In the illustrative embodiment of FIG. 2, two of base boards  231 - 238  are installed in each work frame. 
     The operation of base boards  231 - 238  will be described with reference to base board  238  as a concrete example. Pressing one of rotation switches  252  or  253  generates a key signal that causes PLC  220  to control base board  238  to rotate in the clockwise or counterclockwise direction, respectively. Actuation of foot switch  256 , which is connected to clockwise rotation switch  252 , similarly causes base board  238  to rotate in the clockwise direction. Pressing a reset switch  254  causes a key signal to rotate base board  238  to its original position. 
     The process and equipment for transfer of PCBs in system  100  from work frames  101 - 104  to flux and soldering stations  150  and  170  will now be described, with exemplary reference to work frame  104 . When the parts insertion process has been completed for one or more carrier-mounted PCBs at work frame  104 , a worker presses a transfer switch  251 , which causes a key signal to be generated. The key signal controls work frame upper conveyor  114  to move carriers  710  from the right side end of conveyor  114  to a first conveyor  120 . When transfer switch  251  is pressed, an up-down unit  131  moves up to work frame upper conveyor  114  so as to receive a loaded carrier  710 . When carrier  710  has been completely transferred to up-down unit  131 , then up-down unit  131  moves down to load first conveyor  120  with carrier  710 . 
     First conveyor  120  then conveys carrier  710  thus received from work frame  104  toward soldering unit  170  for soldering of the PCB installed in carrier  710 . An up-down and rotation unit  140  moves carrier  710  up to a designated height and rotates it through a 90-degree angle. This rotation ensures proper formation of solder junctions during soldering. Spray flux station  150  flux-coats the PCB installed in carrier  710  in a manner well known in the art. Spray flux station  150  discharges carrier  710  horizontally, but then an auxiliary conveyor  160  transfers carrier  710  at an incline of  6  degrees to soldering unit  170 . 
     Soldering unit  170  applies solder to the PCB in a manner well known in the art. Carrier  710  is then discharged from soldering unit  170  at a designated position. A slope conveyor  180  transfers carrier  710  from soldering unit  170  to a first elevator  190 . Slope conveyor  180  may be configured, for example, to perform this transfer horizontally from soldering unit  170  to first elevator  190 , but with one side of carrier  710  lowered to maintain the PCB at an inclined angle after carrier  710  exits soldering unit  170 . 
     First elevator  190  receives carrier  710  from slope conveyor  180  at a designated position and transfers carrier  710  down to a second conveyor  210 . A rotation unit  200  then receives carrier  710  from first elevator  190  and rotates it through a 90-degree angle. This second rotation operation returns carrier  710  to its original orientation, thus preparing carrier  710  for return to work frame  104  from whence it came. A second conveyor  210  transfers carrier  710  back to work frame  104 . 
     Fifth through eighth up-down units  135 - 138  are installed at the bottom of second conveyor  210  and aligned vertically with first through fourth up-down units  131 - 134 . Carrier  710  transferred by second conveyor  210  is lifted up to a work frame lower conveyor  118  of work frame  104  by up-down unit  138 . Lower conveyor  118  transfers carrier  710  from up-down unit  138  to the right hand end of work frame  104 . A lifter  224  raises carrier  710  from work frame lower conveyor  118  up to work frame upper conveyor  114 . 
     Work frames  101 - 103  of course include respective lower up-down units  131 - 133 , frame unit lower conveyors  115 - 117 , and lifters  221 - 223 . This arrangement of conveyors, up-down units, and lifters thus enables a specific carrier  710  originating from any of work frames  101 - 104  to return to that same work frame for post-soldering inspection. In contrast with the systems of the cited references, therefore, the present invention enables the same workers who performed the initial component insertion process to inspect the boards for flaws after the boards have been soldered. 
     The operation of computers  240 - 243  in conjunction with base boards  231 - 238 , to be described in more detail below, substantially reduces the rate of defects in PCBAs produced in accordance with the present invention. However, system  100  enables further improvements in productivity by sending each soldered PCB back to its frame of origin for testing. The expertise and observations of the workers who inserted components into a PCB are thus used to advantage a second time by returning each of the soldered PCBs to its frame of origin for inspection by the workers who inserted the components. 
     In effect, the serial line of FIG. 1, wherein parts insertion, trimming, and inspection are each performed at discrete stations, is “folded” after the soldering step. With this folded configuration, the defect detection and communication problems associated with discretized process steps are largely eliminated. This is possible because the same workers who perform the parts insertion steps, where most production defects may originate, also perform the inspection steps to detect defects and report them for correction. 
     The operation of system  100  depends extensively upon computerized control systems. PLC  220  stores a program to control the overall operation of system  100 . PLC  220  receives output signals from a plurality of sensors that monitor the movement status of carrier  710 . In accordance with these sensor output signals and its stored program, PLC  220  initiates and suspends movements of carrier  710  at each transfer stage. For example, pressing transfer switch  251  signals PLC  220  to drive work frame upper conveyor  114  (or one of work frame upper conveyors  111 - 113  corresponding to work frames  101 - 1 - 3 , respectively). PLC  220  also controls each of base boards  231 - 238  for rotation in response to signals from their respective rotation switches  252  and  253 , as well as for clockwise rotation in response to a respective foot switch  256 . 
     The basic configuration of system  100 , described above, differs substantially from and provides definite advantages over the systems disclosed in the cited references. However, particular structural features of system  100  warrant further detailed description because these features, while not necessary to the basic system, are included in a preferred version of the system and enhance its effectiveness. The discussion will therefore now turn to a description of preferred embodiments for carriers  710 , work frames  101 - 104 , and base boards  231 - 238 . 
     FIG. 3 is a perspective view of carrier  710  shown in FIG.  2 . Carrier  710  includes front and rear bars  711   a  and  711   b,  which together with right and left bars  713   a  and  713   b  define a generally rectangular opening. U-shaped guide grooves  714   a  and  714   b  are formed along the top surfaces of the front and rear bars  711   a  and  711   b,  respectively. Guides  716   a  and  716   b  are respectively inserted into guide grooves  714   a  and  714   b  and move freely in a back direction  760  and a forth direction  750  along the common length of guide grooves  714   a  and  714   b.    
     A positioning bar  712  having front and rear ends  715   a  and  715   b,  respectively, is disposed between front bar  711   a  and rear bar  711   b  and generally parallel to right and left bars  713   a  and  713   b.  Guides  716   a  and  716   b  are also inserted into a pair of C-shaped grooves formed at front end  715   a  and rear end  715   b,  respectively, and are fastened to positioning bar  712  with suitable fastening means, such as screws. Guides  716   a  and  716   b  are clamped into fixed positions in guide grooves  714   a  and  714   b  with clamping screws  718   a  and  718   b,  installed at front and rear ends  715   a  and  715   b,  respectively. 
     The heads of clamping screws  718   a  and  718   b,  for fixing guides  716   a  and  716   b,  project above positioning bar  712 . Screws  718   a  and  718   b  also have shanks that pass through holes formed in front and rear ends  715   a  and  715   b  of positioning bar  712  and through guides  716   a  and  716   b  and press against the bottoms of the guide grooves  714   a  and  714   b.  Coupling grooves  732   a  and  732   b  are formed on inside edges of front and rear bars  711   a  and  711   b,  respectively. The lower parts of the C-shaped grooves formed at the ends of positioning bar  712  are inserted through coupling grooves  732   a  and  732   b.    
     A solder preventive sill  728  extends along the bottom surface of front bar  711   a  and generally parallel thereto. Sill  728  has a side rail that extends downward from front bar  711   a  and a bottom rail that projects horizontally from the bottom edge of the side rail and into the rectangular opening defined by carrier  710 . Sill  728  is fastened to front bar  711   a  by suitable means, such as a screw. 
     Two holes, which are used for attaching a cell detection plate  730 , are drilled in the left end of each of front and rear bars  711   a  and  711   b.  Cell detection plate  730  carries an identifying indicium that, when detected by a sensor to be described later, enables PLC  220  to determine the correct work frame to which carrier  710  corresponds. Plate  730  may be attached to either front bar  711   a  or rear bar  711   b  by screws threaded into the holes described above. The position and orientation of cell detection plate  730  will depend on the location of the work frame where the operation for the PCB is performed. 
     Right and left bars  713   a  and  713   b  have defined therein stop grooves  734   a  and  734   b,  respectively, each of which is formed at the center of an outside edge of its respective bar. L-shaped brackets  722   a,    722   b,    722   c,  and  722   d  are fastened to and project from the bottom of an inside edge of right bar  713   a.  Brackets  722   a-d  are separated by predetermined intervals that increase in length from front bar  711   a  to rear bar  711   b . Brackets  720   a ,  720   b ,  720   c , and  720   d  are fastened to and project from the bottom right side of positioning bar  712  and respectively face brackets  722   a-d.  Each one of brackets  720  has a lower end that defines a groove therein. 
     Attached to the right side of positioning bar  712  are a plurality of leaf springs  724 , with each leaf spring  724  corresponding to one of brackets  720   a -d.  upper ends of leaf springs  724   a-d  attach to the top of the positioning bar  712  with coupling screws  726   a,    726   b,    726   c,  and  726   d,  respectively. The lower ends of leaf springs  724   a-d  are curved at the edge of positioning bar  712  at a first predetermined angle (not exceeding 90°) and offset from the right side of positioning bar  712 . Therefore, the bottom ends of leaf springs  724  project into the grooves in their respective brackets  720  but do not touch the left and right sides of the grooves. 
     FIG. 4 shows a cross-section of carrier  710  along the line A-A″ and depicts details of bracket  722   c  and its facing bracket  720   c . As shown in FIG. 4, the right face of bracket  720   c  must extend downward collinearly from the right (i.e., inner) side of positioning bar  712 . Bracket  720   c  has a lower protrusion that extends horizontally at a vertical level substantially equal to the level of the bottom rail of solder preventive sill  728 . 
     Leaf spring  724   c  has a free lower end and a fixed upper end that is attached to the top of positioning bar  712  with coupling screw  726   c.  From its fixed upper end, leaf spring  724   c  curves away from the right side of positioning bar  712  at the first predetermined angle (not exceeding 90°). This arrangement defines a space between the side of positioning bar  712  and leaf spring  724   c.  Closer to its free lower end leaf spring  724   c  curves again, but this time toward the side of the positioning bar  712 , at a second predetermined angle. The free lower end of leaf spring  724   c  is formed so that it does not touch the left and right sides of the groove defined in bracket  720   c . 
     The double curvature of leaf spring  724   c  thus defines the space between the right side of positioning bar  712  and leaf spring  724   c  to be generally triangular and enables leaf spring  724   c  to deform elastically under tension. 
     Bracket  722   c  is fastened to and projects from the bottom left side of right bar  713   a.  As FIG. 4 indicates, the right face of bracket  722   c  must extend downward from and collinearly with the left (i.e., inner) side of right bar  713   a.  Bracket  722   c  has a lower protrusion that extends horizontally at substantially the same level vertically as the bottom rail of solder preventive sill  728  and the lower end of bracket  720   c . 
     FIG. 5 shows a cross-section detail of the front area designated as B in FIG.  3 . Guide  716   a  slidably resides in guide groove  714   a  formed in the top of front bar  711   a . A top protrusion of the front end  715   a  of the positioning bar  712 , which along with A top protrusion thereof defines the C-shaped groove of front end  715   a,  extends over guide  716   a  and the top of front bar  712 . The lower protrusion extends under the bottom of front bar  712 . Guide  716   a  fastens to the upper protrusion of positioning bar  712  by suitable means, such as with two screws as shown. The arrangement of the upper and lower protrusions retain front end  715   a  in proper vertical alignment with respect to front bar  712 . 
     The head of clamping screw  718   a  projects above the top of the positioning bar  712 . Its shank extends through threaded holes through positioning bar  712  and guide  716   a,  and butts against the bottom of guide groove  714   a.  Clamping screw  718   a,  when screwed into contact with the bottom of guide groove  714   a,  therefore fixes guide  716   a  and positioning bar  712  at a selected position along the length of guide groove  714   a.  That is, the end of the shank of clamping screw  718   a  presses against the bottom of guide groove  714   a  and thereby generates an upward tension against the upper protrusion of front end  715   a  of positioning bar  712 . Solder preventive sill  728 , which may be attached to the bottom of front bar  711  a with screws, projects inward from the bottom of front bar  711   a  at the same vertical level as brackets  720   a-d  and  722   a-d.    
     Carrier  710  as described above effectively holds PCBs of various dimensions and allows a PCB to be mounted easily therein. FIG. 6 illustrates, by way of example, a PCB  740  mounted on carrier  710  of FIG. 3. A worker may loosen the clamping screws  718   a  and  718   b  by turning them counterclockwise, which relaxes any tension between clamping screws  718  and the lower protrusions of front and rear ends  715  of positioning bar  712 . This allows guides  716  to slide freely left  750  and right  760  (which directions are represented in FIG. 6 with corresponding arrows) along their respective guide grooves  714   a  and  714   b.    
     The worker then can relocate positioning bar  712  to the left side of carrier  710 , which facilitates insert of PCB  740  between positioning bar  712  and right bar  713   a.  Right bar  713   a,  it will be recalled, carries brackets  722   a-d,  some or all of which will support the right side of PCB  740 . When positioning bar  712  is moved to the left, guides  716  and their respective ends  715  of positioning bar  712  move in the left direction  750  along guide grooves  714 . 
     With positioning bar  712  positioned sufficiently far from right bar  713   a  for PCB  740  to be easily inserted therebetween, the worker places the right edge of PCB  740  upon one or more of brackets  722   a-d  and the front edge of PCB  740  upon the bottom rail of solder preventive sill  728 . The worker then returns guides  716   a  and  716   b  in the right direction  760 , along their respective guide grooves  714   a  and  714   b,  until the left side of PCB  740 , which is not yet supported by carrier  710 , is contacted by one or more of leaf springs  724   a-d.  The left side of PCB  740  slides onto the lower protrusions of one or more of brackets  722  and depresses the contacted leaf springs  724 . Positioning bar  712  can then be adjusted left  750  or right  760  to generate an amount of tension in leaf springs  724  to snugly retain PCB  740  within carrier  710 . 
     With the desired tension in leaf springs  724 , the worker tightens down clamping screws  718   a  and  718   b  to fix positioning bar  712  and guides  716   a  and  716   b  in the selected position. Winding clamping screws  718   a  and  718   b  clockwise forces the ends of the clamping screws  718   a  and  718   b  against the bottoms of guide grooves  714   a  and  714   b,  which in turn generates an upward force against ends  715  of positioning bar  712 . The lower protrusions of ends  715  are thereby forced against the bottoms of front and rear bars  711   a  and  711   b,  and guides  716   a  and  716   b  and positioning bar  712  are thus clamped in the selected position. 
     If PCB  740  is not correctly mounted on brackets  720 , although its left edge presses against leaf springs  724  with the desired tension, the worker can press PCB  740  down to correctly position it on brackets  720  using the elasticity of leaf springs  724 . When correctly positioned on brackets  720 , PCB  740  is clamped to carrier  740  such that the left side of PCB  740  is held by the gaps formed by the lower curved ends of the leaf springs  724  and thus is prevented from warping during the soldering process. 
     The PCB mounting operation described above need be performed only the first time a particular carrier  710  for a run of a given PCBA product through system  100 . Once carrier  710  has been adjusted for the specific product, the worker need only has remove the soldered PCBA from carrier  710  by pressing it leftward against the tension of leaf springs  724  and lifting. A new PCB can then be mounted without repeating the adjustment operation described above. It is noted that positioning bar  712  can be installed in or removed from carrier  710  by positioning the lower protrusions of ends  715  of positioning bar  712  at the respective coupling grooves  732   a  and  732   b.    
     FIGS. 7 &amp; 8 illustrate the work frame of the present invention and will be used to describe the advantages thereof. FIG. 7 is a perspective view of work frame  101  shown in FIG.  2 . FIG. 8 is a front view of the same. FIGS. 7 and 8 do not show carrier  710 . Although system  100  includes a plurality of work frames  101 - 104 , only first work frame  101  is illustrated in FIGS. 7 &amp; 8 by way of example. 
     Referring to FIGS. 7 and 8, a plurality of parts are placed in circular base boards  231  and  232  in groups divided according to part type. A gear  46  is fixed to the underside of each of base boards  231  and  232  to enable rotation thereof (both clockwise and counterclockwise). A drive motor  44  has a motor gear  45  that meshes with gear  46  so as to rotate the base boards  231  and  232 . Motor  44  is fixed to a support plate  42 . Work frame upper conveyor  111 , which conveys carriers  710  to first conveyor  120  (see FIG.  2 ), is attached to a working plate  40 , which in turn is positioned above base boards  231  and  232 . Work frame lower conveyor  115  is positioned below base boards  231  and  232  and conveys carriers  710  with soldered PCBs from second conveyor  210 . 
     Rotation switches  252  (for clockwise rotation) and  253  (for counterclockwise rotation) are located adjacent to a frame rail  34   a  of work frame upper conveyor  111  and are used to signal for rotation of base boards  231  and  232 . Reset switch  254  is located adjacent to rotation switches  252  and  253  and is used to signal for base boards  231  and  232  to be returned to an initial position. Work frame  101  according to the present invention may be configured to allow screws of the basic frame structure to be tightened, whereby work plate  40 , base plates  41 , and support plate  42  are fixed at specified vertical positions. 
     Base boards  231  and  232  are used to organize, according to type, various parts to be inserted into PCBs mounted in carriers  710 . A label  52  (see FIG. 10) is attached to the upper portion of each of base boards  231  and  232  in order to prevent the parts from being incorrectly inserted. A plurality of holes  50  are defined through label  52 . The motion of base boards  231  and  232  is controlled by PLC  220  through motor  44  in accordance with signals from an optical sensor  51 . Base boards  231  and  232  then rotate by stages, and the worker removes appropriate parts therefrom in accordance with assembly instructions displayed by computer  240  (see FIG.  2 ). A plurality of guides  231   a  on the underside of each of base boards  231  and  232 , and corresponding guide recesses  30  are formed in each base plate  41 . If base plate  41  is rotated by motor  44 , the corresponding base board  231  also rotates. 
     A worker may pull base boards  231  and  232  in order to load them with parts. To load them, base boards  231  and  232  are pulled out by a sliding operation between guides  231   a  and the guide recesses  30 . Gear  46  is fixed to base plate  41  and meshes with motor gear  45  of drive motor  44 . Therefore, if drive motor  44  is operated, base plate  41  is rotated together with the corresponding base board  231 . The angle of rotation through which drive motor  44  turns base board  231  is controlled by PLC  220  in accordance with key signals from rotation switches  252  and  253  and reset switch  254 . 
     Work frame upper and lower conveyors  111  and  115 , installed at the upper and lower portions of work frame  101 , respectively, are of a rail type, as shown in FIG.  7 . Conveyor frame rails  34   a,    34   b,    54   a  and  54   b  provide frames for work frame upper and lower conveyors  111  and  115 . Chains  33  and  53  mesh with opposed pairs of wheels  82  and  92 , which are rotatably attached to frame rail  34  and  54 , respectively. Each pair of wheels  82  and  92  is connected together by a respective one of shafts  31  and  61 . Therefore, if a drive motor of conveyor  111  or  115  is actuated, the corresponding wheels and chains are sequentially rotated and a carrier  710  is conveyed to or from first conveyor  120 . 
     At a worker position of work frame  101 , cut-away portions  40   a  are formed in work plate  40  to enable the worker to easily take out the parts contained in base boards  231  and  232 . A sensor  535  is located at the inner side of each cut-away portion  40   a  in order to detect an obstruction, such as a worker″s hand, in cut-away portion  40   a  and thus prevent injuries that might otherwise occur when base boards  231  and  232  are rotated. If the worker puts his hand in the cut-away portion  40   a,  sensor  535  generates a signal that inhibits PLC  220  from controlling base boards  231  and  232  to rotate. 
     Rotation switches  252  and  253  and reset switch  254  are installed adjacent to frame rail  34   a  of work frame upper conveyor  111  at the worker position of work frame  101 . For the convenience of the worker, foot switch  256  is connected in parallel with clockwise rotation switch  252  and is installed on the floor. Foot switch  256  is used to operate base boards  231  and  232  by foot. Rotation switches  252  and  253  are also used to allow access to desired parts by rotating base boards  231  and  232  in the desired direction. 
     A shelf  80 , for supporting computer  240  including a keyboard thereof, is attached to frame  101   a  of work frame  101  above the rear of work frame upper conveyor  111 . A parts box  246  also may be fixed to frame  101   a  for storage of parts that do not easily fit into base boards  231  and  232 . Optical sensor and stopper pairs  258  are installed at the terminal end of work frame lower conveyor  115  and between conveyor  115  and second conveyor  210  to detect when a carrier  710  is conveyed from second conveyor  210 . A speed controller  86 , for sensing the convey speed of carriers  710 , is installed at frame rail  54   a  of work frame lower conveyor  115  and generates speed signals enabling conveyor  115  to convey the carrier at a constant speed. This operation is controlled by PLC  220 . 
     In operation, the worker puts a carrier  710  on work frame upper conveyor  111 , confirms from computer  240  a work instruction set including parts insertion positions, parts names, and the quantity of each part, and accordingly inserts appropriate parts into a PCB fixed to carrier  710 . After the parts are inserted, the worker transfers carrier  710  toward first conveyor  120  by signaling for transfer with transfer switch  251 . Carrier  710  is then conveyed to first conveyor  120  by work frame upper conveyor  111  in the direction denoted by reference character A in FIG.  7 . 
     Carrier  710  thereafter passes through spray flux station  150  and soldering unit  170 , after which it is conveyed to second conveyor  210  by elevator  190 . Carrier then returns to the work frame from which it originated through detection of the indicium on cell detection plate  730 . That is, the indicium is sensed by the optical sensor of optical sensor and stopper pair  258  located between second conveyor  210  and work frame lower conveyor  115 . When PLC  220  determines that the indicium corresponds to the work frame from which the indicium has been read, carrier  710  is transferred to work frame lower conveyor  115  by lower up-down unit  135 . (This occurs at work frame  101  because, in this example, carrier  710  originated from work frame  101 ). 
     PLC  220  stops the progress of carrier  710  in accordance with a signal from optical sensor and the stopper pair  258  at the terminal end of work frame lower conveyor  115 . The signal indicates the presence there of carrier  710 . Lifter  221  then conveys carrier  710  to work frame upper conveyor  111 , as indicated in FIGS. 7 &amp; 8 by direction reference character D. Conveyed carrier  710  is then situated at work frame upper conveyor  111  by a timing belt in direction E. The worker inspects to soldered PCB mounted in carrier  710  for proper parts insertion and soldering and confirms the same, the assembly of the PCB is completed. 
     FIG. 9 is a front view of a work position of work frame  101  detailing the arrangement of base board  231  and associated features. FIG. 10 is a perspective view of base board  231  and label  52  and illustrates how base board  231  can organize a variety of parts. FIG. 11 is a plan view of parts information label  52  that carries a list of parts to be inserted into the PCB. 
     As shown in FIG. 9, opening  68 , in which base board  231  is mounted, is defined between work plate  40  and support plate  42 . Cut-away portions  40   a  (see FIG. 7) are defined in work plate  40  adjacent to the upper side of opening  68 . Drive motor  44  is installed below opening  68 . Motor gear  45 , mounted on drive motor  44 , transmits torque from drive motor  44  to a driving gear  32 . An L-shaped bracket  74  is located on the underside of driving gear  32  and provides a reference position allowing base board  231  to return to its starting position. A sensor  36  detects bracket  74  and generates a signal indicating that base board  231  is in the reference position. Base plate  41 , which is fixed to the top side of driving gear  32 , rotates with driving gear in response to the rotation of drive motor  44  and thereby positions base board  231  at any of a plurality of angular positions. 
     Base board  231  has a preferred construction, as shown in FIGS. 10 and 11, as a squat circular cylinder. The cylinder is divided into a plurality of parts containers  54  disposed at equal intervals around in the axis of the cylinder. Containers  54  store parts in accordance with a particular assembly procedure or in accordance with part types. Distribution of the parts among parts containers  54  helps to prevent mis-insertion of the parts in PCBs under assembly. In the embodiment of FIG. 10, base board  231  is divided into 16 equal segments wherein parts containers  54  are formed. Preferably, cut-away portion  40   a  has dimensions corresponding to the dimensions of parts containers  54 . Thus, at each angular position, cut-away portion  40   a  exposes a specified parts container  54 . The width of each parts container  54  tapers from the perimeter to the center of the cylinder, and cut-away portion  40   a  defined in work plate  40  conforms to this shape. 
     Label  52  is located on a parts information label mounting face  60  at a central region of base board  231 . Label  52  is printed for each PCB assembly job to identify the various parts contained in parts containers  54 . FIG. 11 shows that, in one embodiment, label  52  is divided into 16 equal segments, thereby providing one segment for each parts container  54 . In a preferred embodiment, label  52  is divided into 18 equal segments. 
     A first rotation sensing aperture  56  is formed in the parts information label mounting face  60  adjacent to each parts container  54 . A second rotation sensing aperture  50  is formed in each segment label  52 . When label  52  is mounted on parts information label mounting face  60 , second rotation sensing apertures  50  align with first rotation sensing apertures  56 . The aligned pairs of rotation apertures  50  and  56  allow the angular position of base board  231  corresponding to a specified part to be selected. Optical sensor  51 , installed in work plate  40 , detects when a pair of rotation apertures  50  and  56  is aligned below sensor  51 . When a given parts container  54  is not used, second rotation sensing aperture  50  corresponding to the empty part container  54  is covered with a reflective material, such as silvered paper, to prevent passage of light to photo sensor  51 . 
     A first original point setting aperture  44  is formed in parts information label mounting face  60 . A corresponding second original point setting aperture  62 , formed in label  52  at a position radially inward from second rotation sensing apertures  50 , aligns with first original point setting aperture  44  when label  52  is attached to mounting face  60 . The pair of original point apertures  44  and  62  enable return of base board  231  to its original reference position. As with the pairs of rotation apertures  50  and  56 , A second sensor (not shown) detects the aligned pair of original point apertures  44  and  62 . This second sensor indicates when base board  231  has returned to its original position after completing an assembly cycle. 
     Label  52  is correctly positioned on label mounting face  60  through alignment of first and second fixing apertures  46  and  64 . First fixing apertures  46  are formed in information label mounting face  60  and align with second fixing apertures  64 , respectively. When label  52  is correctly mounted on parts information label mounting face  60 , each second original point setting aperture  62  aligns with a corresponding first fixing aperture  46 . Second fixing apertures  64  are formed in the upper and lower portions of the label  52  for correctly positioning label  52  on parts information label mounting face  60 . 
     A model indicator  66  is marked in the center of label  52  and identifies the product model for which the parts organized in base board  231  are intended. Each segment of label  52  further provides four separate information fields disposed at successive radial positions in the segment. A work procedure indicator  70  identifies a work procedure for the respective parts contained in the parts container  54  corresponding to the label segment. A quantity indicator  72  specifies the number of the given parts type needed for each unit under assembly. A parts code indicator  74  provides a part number of the parts in the particular parts container  54 . Finally, a parts name indicator  76  shows the name of the part. 
     Label  52 , listing information on parts to be loaded into base board  231 , is mounted on parts information label mounting face  60  before the parts are loaded. The respective apertures  44  and  62 ,  46  and  64 , and  56  and  50  are aligned to ensure that the parts container  54  for each group of parts will be correctly identified. A worker then loads the various parts to be inserted into the PCBs, into parts containers  54  in accordance with the respective indicators  66 ,  70 ,  72 ,  74  and  76 . When a given parts container  54  is not to be used, its second rotation sensing aperture  50  is covered with silvered paper to block light from reaching optical sensor  51 . Base board  231  is then inserted into opening  68  and smoothly mounted to base plate  41  by sliding guides  23  la into the corresponding guide recesses  30  of the base plate  41 . 
     To begin the parts insertion process for a PCB, the worker activates reset switch  254  or a corresponding foot switch (not shown). This generates a key signal, in accordance with which PLC  220  controls drive motor  44  to rotate base board. The second sensor detects when the aligned pair of original point apertures  44  and  62  pass therebelow. Base board  231  continues to advance. A light source included in optical sensor  51  continuously emits light. The light thus emitted cannot reach an optical detector of optical sensor  51 , however, until it passes through first and second rotation sensing apertures  56  and  50  corresponding to an initial parts container  54 . Optical sensor  51  detects the presence of light and generates a signal, in accordance with which PLC  220  stops drive motor  44 , thereby stopping the base board  231 . It is noted that when a second rotation sensing aperture  50  is covered, such as with silvered (i.e., reflective) paper, the light blocked from photo sensor  51  and base board  231  therefore continues to rotate beyond to angular position for access to the unused parts container  54 . 
     After this, the worker retrieves parts from the initial parts container  54 , through cut-away portion  40   a,  and inserts them into a PCB. The worker repeats this operation until all parts to be inserted at his work station are correctly positioned on the PCB. Upon completion of the insertion process, base board  231  rotates until aligned first and second original point setting apertures  44  and  58  are positioned below the light of the second sensor (not shown), which generates a signal for PLC  220  to return base board  231  to its original position. Alternatively, when one of rotation switches  252  and  253  operates, driving gear  32  rotates until bracket  74  of the driving gear  32  is sensed by a optical sensor  36 , whereby base board  231  returns to its original position. 
     FIGS. 12A and 12B schematically show a configuration of sensors for sensing the movement status of a carrier  710  and the motors included in the present invention. Referring to FIGS. 12A and 12B, when specified moving conditions are not satisfied under the control of PLC  220 , a plurality of stoppers Y 25 D, Y 243 , Y 250  to Y 252 , Y 255  to Y 257 , Y 260  to Y 262 , Y 265  to Y 267 , Y 270  to Y 272 , Y 275  to Y 277 , Y 280  to Y 287 , and Y 285  to Y 287  intermittently stop carriers  710  in order to space them at regular intervals on first and second conveyors  120  and  210 . 
     Photosensors X 051  to X 053 , X 05 F, X 058  and X 061 , X 064 , X 072  to X 079 , X 07 A to X 07 C, X 07 F, X 082 , X 091  to X 099 , X 09 A to X 09 F, X 102 , X 111  to X 119 , X 11 A to X 11 F, X 122 , X 131  to X 139 , and X 13 A to X 13 F detect the movement carriers  710  and transmit the results to PLC  220 . These photosensors employ transparent and reflective fiber sensors. Fiber sensors X 078 , X 098 , X 118  and X 138  detect the type of each carrier  710  and generate signals used by PLC  220  to transfer each carrier  710  back to its original work frame. The fiber sensors may either scan the indicium located on cell detection plate  730  of each carrier  710  or, alternatively, detecting the presence of a bracket (not shown) attached to each carrier  710  in a position indicative of the origin of the carrier. 
     A plurality of reflective fiber sensors X 085 -X 086 , X 10 A,B, X 105 -X 106 , X 12 A,B, X 125 -X 126 , X 14 A,B, and X 145 - 146  detect the rotation status of base boards  231 - 238  and generate signals indicating the same for use by PLC  220 . Limit switches X 050 , X 081 , X 101 , X 121  and X 141  sense that a carrier  710  has been fully lowered by levator  190  or one of lifters  221 - 224 , respectively, and send signals indicating that status to PLC  220 . 
     FIG. 13 shows a block circuit diagram of a conveyor motor driver for use with system  100  of the present invention. Referring to FIG. 13, a power supply unit  513  supplies power to the circuit. A relay  514  is switched on and off by a motor control signal transferred from PLC  220 . A switching unit  515  receives the motor control signal from PLC  220  and, in accordance therewith, drives a motor M 11  in the clockwise or counterclockwise direction. Specifically, motor M 11  drives elevator  190 . 
     Magnet contact switches C 1  to C 10  supply power to motor drivers  501  to  510  connected thereto. Motor driver  501  drives motors M 1  to M 10 . Motors M 1  and M 2  drive first conveyor  120 , and motors M 3  to M 6  transfer carriers  710  to elevator  190 . Motor M 7  transfers carriers  710  on slope conveyor  180 , and motor M 8  drives a conveyor between up-down and rotation unit  140  and spray flux station  150 . Motors M 9  and M 10  are spare motors. Motors M 12  and M 13  drive conveyor belts in rotation unit  200  and up-down and rotation unit  140 , respectively. Switches SW 1  and SW 2  are turned on by a control signal from PLC  220 , thus applying a power from power supply unit  513  to motor drivers  511  and  512 . 
     FIGS. 14 to  17  are block circuit diagrams of motor drivers installed at first to fourth work frames  101  to  104 , respectively, in accordance with the present invention. These motor drivers are all constructed as described next with respect to FIG. 14 corresponding to work frame  101 . With rotation switches  252  and  253  on, respective rotation signals are generated. Pressing foot switch  256  similarly generates a clockwise rotation signal. Sensing unit  535  comprises two reflective fiber sensors. When PLC  220  receives a key signal from one of rotation switches  252  and  253  or foot switch  256 , it generates a motor control signal to rotate base boards  231  and  232  unless inhibited by a sensing signal detected by sensing unit  535 . 
     Upon receipt of a control signal from a base board control unit (not shown), PLC  220  turns on switches  520 ,  521  and  522  to apply power from power supply unit  513  to motor units DM 1 , DM 2 , and DM 3 , respectively. A control signal from the base board control unit also turns on switches  523 ,  524  and  525 , thereby running motor units DM 1 , DM 2 , and DM 3  in either the clockwise or the counterclockwise direction. Motor units DM 1  and DM 2  each rotates one of base boards  231  and  232 ; work frames  101 - 104  comprise eight such motor units in total. 
     Motor DM 3  transfers carriers  710  into lifters  221 . Switches  526  to  529  are turned on by the motor control signal received from PLC  220 , thereby applying power from power supply unit  513  to motor drivers  530  to  533 , respectively. Motor drivers  530  to  533  drive motors SM 1  to SM 4 , respectively, with the power from power supply unit  513 . Motor SM 1  is installed in first up-down unit  131 . Operation of first up-down unit  131 , by supplying power to motor SM 1 , transfers carriers  710  from work frame upper conveyor  111  to first conveyor  120  in accordance with the control signal of PLC  220 . Motor SM 2  is installed in eighth up-down unit  138  and functions in a manner substantially similar to motor SM 1 , in accordance with an appropriate control signal from PLC  220 . Motor SM 3  drives work frame upper conveyor  111  and motor SM 4  drives work frame lower conveyor  115 , both under the control of PLC  220 . 
     FIG. 18 is a block diagram of a device for driving each of elevator  190  and lifters  221 - 224  according to a preferred embodiment of the present invention. Each such device has the same construction as described below with respect to elevator  190 . Referring to FIG. 18, PLC  220  generates a solenoid control signal when the presence of a carrier  710  is sensed by sensor X 051  (see FIG.  12 A). A solenoid valve  601  drives a cylinder  602  in accordance with the solenoid control signal of PLC  220  and thereby moves carrier  710  from slope conveyor  180  down to rotation unit  200 . A device as illustrated in FIG. 18 is provided as a driving unit for each of elevator  190  and lifters  221 - 224 . 
     FIGS. 19-98 illustrate the operation of a control program of PLC  220  according to the present invention. The operation of a preferred embodiment of the present invention will now be described in detail with reference to FIGS. 2 to  98 . To realize the embodiment, computers  240  to  243  must be programmed to display part insertion instruction diagrams according to different kinds of PCB corresponding thereto in the following procedures. 
     PCB CAD data is changed from *.CDI file into *.DXF file, so that an edit operation is available in EASY CAD (TM) to highlight certain parts or practice character works. This may be done separately with computers  240  to  243  if each has a Fab-Master (TM) software program installed thereon. The program is operated with two clicks the Fab icon in the initial screen of Window 95 (TM). Operation proceeds by clicking TEST twice and then INCPROC twice. Choose REDAC.EXE and click INPUT in the next screen. 
     In the Select REDAC Source File window, choose either drive A or C, and Folder. Check if REDAC[*.CDI] is selected in the List File of Type window. Select Files to Work from the File name menu and click OK. Then a *.CDI file is displayed in a poly window. At this time, the Fab program runs and the *.CDI file is opened. Alphanumeric characters are shown in a new window. When a message “press any key” is displayed, press any key. If the file has no error, circuit diagrams and their code numbers are displayed in a square box. Select a diagram to be saved as a DXF file with two clicks, in response to which three circuit diagrams (specifically, Parts, Drill and Signals) are shown. 
     For the first circuit diagram, choose Parts to display only the Parts circuit diagram. Press F6 to designate the name of Parts as Plot. Rename the file name as the code name of the circuit diagram edited and click OK to store directories and files that have been edited in C:\TEST.Fab. Accordingly, a desired circuit diagram exists as a file having the name of “code name of circuit diagram.dxf” under the directory “code name.job”. When a mistake is made after or during the work, discard the icon of the directory or file under C:\TEST direction (i.e., “throw it into the wastebasket”) to delete it. 
     After registering the circuit diagram as described above, quickly click the Easy CAD (TM) icon two times to display the Easy CAD (TM) Tool. If opening the file to edit, the File List menu is displayed. Choose a desired dxf file from the File List menu and click Yes to open .DXF file. Open Edit and click the Erase menu, and the Menu Option will be displayed. Click the Window icon. Select the Coordination Line with the mouse (or pointer), and press the right button of the mouse (or pointer) to show the Menu Option. Click Do it to remove the selected coordination line. Click Select icon from the icons arranged in right side of the Easy CAD (TM) window to display the circuit diagram of .dxf file without coordination lines. 
     If.DXF file taken from the Fab Master (TM) is not displayed in standard white color, click EXIT and choose Explode to show the Menu Option. Click the Window button and drag the left button from the+mouse pointer to select overall PCB. Click the left button of the mouse once more to convert the color and show the Menu Option. Click Do it, and the color of the entire circuit diagram is converted into white. Select EXIT and choose Color from the Change menu to show the Menu Option. Click the Window icon and select the desired portion of the circuit diagram by drawing a box with the left button of the mouse. Then the color is changed to white and the Menu Option is displayed. When a message is displayed indicating to select NEW Color in the Command menu, choose White from the Color icons at the left to show the entire circuit diagram. 
     Draw a note column to insert text such as work items and notes. To draw a box, choose Draw and Click Paths and Polys and then select BOX. To draw a line, choose Draw and click LINE from the subordinate menu of Lines. Then insert the desired text. Designate Text Size by choosing the T icon from the icons arranged on right side. When a Menu Prompt is shown, the user designates Height of the Metrics group according to the size of the Box, usually about 0.5. Character Style and Font are designated by Default. Click Default Text to insert into the BOX. Click Draw and Text as a subordinate menu from the Text menu to display the Edit Text menu. Check Multi-line in order to use several lines for entering the name of the note therein. 
     Copy this basic format for each of the different types of parts. Select the COPY command displayed and click the subordinate menu COPY to display the Window Comer selection menu on Command Line. Select the entire display with the left button of the mouse and Click Do it to show the Copy origin command display. Select the left bottom end of the entire of the display as Origin. Choose Origin Point at the left bottom end and set the Origin Point. With a message, place the mouse pointer to the position to be copied and click the left button of the mouse. 
     To designate the position and color of the parts to insert, the parts are recognized through a basic indicating method of each parts. Select Change from the Edit menu. Then click Color to display the menu options. Click Window icon from the menu options to select the parts to change by using a pointer. If parts are selected, their color is changed when the Menu Option appears. Click Do it to display a message of NEW Color on the Command Line. 
     To change to colors of the part into yellow, select a yellow out of the color icons arranged in the left side. To paint a BOX, click the checkerboard button from the icons arranged in the right side to display the Menu Option. Choose the style Default, then click DRAW from the Easy CAD (TM) command menu and select Paths and Polys. Select BOX to paint the position of parts to insert on the screen. All positions of the parts to insert are indicated by assigning only one SEC-CODES to one PBA. All information about the parts such as name, quantity, position and notes are recorded for a product. The file is then saved. 
     The saved file can be changed to a .gif file by using the Capture Professional as follows. On the initial screen of Window 95 (TM), click the Easy CAD (TM) and Capture icons twice each to execute the Easy CAD (TM window and the Capture Professional v2.0 window. Prior to a work for the program, click Capture from the menu of the Capture window to adjust and set up the size of the Easy CAD (TM) window into 800×600. Select File from the menu of the Easy CAD window to display the File menu. If Open is chosen from the File menu, the Open Drawing command menu will be displayed. When a dialog box is displayed, search for a directory Dcsecw and click twice. 
     Enlarge the first circuit diagram out of several circuit diagrams displayed as large as possible by clicking a square button once and dragging the desired circuit diagram from the left top to the right bottom. Click Capture from the menu of the Capture Professional v2.0 window once to remove if the Capture Professional v2.0 window and change the shape of a cursor from an arrow to a camera form. Capture the enlarged circuit diagram with the camera-shaped cursor from the left top to the right bottom, (780-790)×(580-590). Enlarge the circuit diagram again as described above. When all the circuit diagram desired are captured, click File from the Capture Professional v2.0 window menu once and click Save to display a dialog box of Capture Professional-File Save. 
     For example, if the code name of the PBA is 9200004A and the captured circuit diagram is the eighth diagram, search for 9200004A from the directory box in the right side to click it twice and type aa08 for a name of the file in the left side and press Enter. Then the file is saved as aa08 .gif and the dialog box disappears. Close the circuit diagram saved. Click File from the Capture window menu and click Save once. With a dialog box displayed again, type aa07 for a new name of the file and press Enter. In the same manner, the other circuit diagrams are saved as files by typing aa06, aa05, aa04, aa03, aa02 and aa01. For 20 circuit diagrams, type aa20, aa19, aa18, . . . , aa02 and aa01 to save the diagrams as aa20.gif, aa19.gif, aa18.gif, . . . , aa02.gif and aa01.gif. 
     After the files are saved, the computerized work instructions are displayed on each monitor of the computers  240  to  243  in the following procedures. 
     Turn on the power of, for example, computer  240 . To ensure the process, for example, if a test process is performed in the first and second work frames  101  and  102 , type CD test on the C drive and press Enter. Type S in the C drive Test path and press Enter. If there is no test process, type CD test-no on the C drive and press Enter. Then type S in the C drive Test-no path and press Enter. Choose a PBA code to work on with the arrow key and press Enter. Press T key to Tag the file *.gif displayed. Position the cursor on aa01.gif out of the files displayed and press T key to convert the color of the Text and Tag process. Ensure that all gif files are tagged. Type F9.Operation&gt;Side show and press Enter and space bar. 
     To display the desired parts insertion instructions at, for example, work frame  101 , follow air the precedures as follows. When the worker turns the power switch of PLC  220  on, all magnet contact switches C 1  to C 10  are turned on to drive all of the conveyors. Motors M 1  to M 10  drive first and second conveyors  120  and  210 . With the parts insertion instructions displayed on the monitor of computer  240 , the worker ensures that the materials of base board  232  is identical to the material indicated on the display. He takes a carrier  710  out of a carrier case and places it on work frame upper conveyor  111  to install a PCB in it. The parts loaded on base board  232  are inserted into the PCB installed in carrier  710 . 
     This operation may be done by a group of two persons, or one person. In case of a group of two persons, the worker on the left side inserts from compartments  246  large-sized parts that are hard to load on the base board  232 , or parts loaded on the base board  231 , into the PCB and takes them over the other worker on the right side. When the parts on base board  232  are all inserted, the worker presses clockwise rotation switch  252  to rotate base board  232 , divided into  16  (or, in the preferred embodiment,  18 ) sections, and move to the next set of parts to insert. At this time, the monitor of computer  240  displays the parts insertion instructions for the next parts. 
     Pressing clockwise rotation switch  252  makes PLC  220  generate a control signal to turn on switches  521  and  524 . Thus motor DM 2  is driven to rotate base board  232 , which is provided with label having therein  16  (or  18 ) rotation sensing apertures  50  in order for photosensor X 08 B to detect the position of the next set of parts on rotating base board  232 . Photosensor X 08 B senses the position of the next rotation sensing aperture  50  on the rotating base board  232  while apertures  50  are detected. Then PLC  220  turns off switches  521  and  524  to interrupt motor DM 2 . Apertures  50  corresponding to a section without parts loaded thereon must be sealed with a reflective material (such as silvered paper or aluminum) to block detection by photosensor X 08 B. Thus the base board control unit (not shown) rotates base board  232  until  51  detects the next aperture  50 . 
     The worker then takes parts loaded succeeding parts compartment  54  of base board  232  as indicated by the parts insertion instructions displayed on the monitor of computer  240  and inserts them into the PCB. Clockwise rotation switch  252  is pressed again to rotate base board  232  to the position of the next parts to insert. 
     When the PCB is completed through repetition of these operations, the worker places carrier  710  at the right side end of work frame upper conveyor  111  and presses transfer switch  251 . Then PLC  220  controls magnet sensor X 076  to check that up-down unit  134  is moved up. If the up-down unit  134  is moved up, motor driver  532  is powered up by moving stopper Y 250  down and turning on switch  528  as shown in FIG. 14, thereby driving motor SM 3  and, thereby, work frame upper conveyor  111 . PLC  220  turns on switch  526  as shown in FIG. 14 to supply power to motor driver  530  for driving motor SM 1 . Carrier  710  is transferred to fourth up-down unit  134 , so that photosensor X 072  senses that carrier  710  is completely transferred. 
     PLC  220  then turns off switches  526  and  528  to stop motors SM 1  and SM 3  and restore stopper Y 250 . When driving solenoid valve  601  shown in FIG. 18, cylinder  602  is operated to move fourth up-down unit  134  down to its original position. Then carrier  710  is transferred onto first conveyor  120 . Similarly, in second, third and fourth work frames  102 ,  103  and  104 , carriers  710  with different types of PCBs installed therein are transferred to first conveyor  120  through the same operation described above with respect to first work frame  101 . 
     When photosensor X 073  senses carriers  710  transferred, PLC  220  considers that one cycle of the work is completed and returns up-down unit  134  to its original state. If photosensor X 073  installed on the transfer path of first conveyor  120  senses the motion of carrier  710  but no other carriers  710  are sensed by photosensors X 061  and X 064 , then PLC  220  moves down stopper Y 25 D and drives motor M 13  for driving up-down and rotation unit  140 . When photosensor X 64  senses that carrier  710  is moving, PLC  220  moves up stopper Y 25 D to prevent a next carrier  710  from being transferred, then suspending driving motor Ml 3  for a required time. PLC  220  moves up carrier  710  by a cylinder Y 244 . 
     Photosensor X 066  detects the completion of the upward motion of cylinder Y 244 , and similarly for cylinder Y 246  moving up the up-down and rotation unit  140 . Photosensor X 068  senses that cylinder Y 246  is completely moved up, a cylinder Y 248  rotates carrier  710  in the clockwise direction through a 90-degree angle. When the rotation is sensed by photosensor X 06 A, PLC  220  drives a cylinder  24 A to adjust the width of the conveyor. With this adjustment sensed by photosensor X 06 C, a cylinder Y 245  is moved down so as to place carrier  710  on a transfer conveyor (not shown). Carrier  710  is transferred through spray flux station  150  via the transfer conveyor. 
     If photosensor X 61  senses that carrier  710  transferred, then PLC  220  returns up-down and rotation unit  140  to its initial state. PLC  220  repeatedly operates as described above for each carrier  710  sensed by photosensor X 073 . Each carrier  710  is transferred to spray flux station  150  to coat it with flux. While carrier  710  is in on auxiliary conveyor  160 , it is transferred at an incline of  6  degrees to soldering unit  170 . Carrier  710  is soldered in soldering unit  170  and transferred to slope conveyor  180 . 
     PLC  220  recognizes carrier  710  sensed by photosensor XO 5 F and controls motor driver  507  to stop motor M 7 . A slope cylinder Y 23 D is moved down. At slope conveyor  180 , PLC  220  drives cylinder  602  to lower one side of carrier  710 . When magnet switch X 057  recognizes the downward motion of slope conveyor  180 , PLC  220  controls motor drivers  506  and  507  to drive motors M 6  and M 7 , transferring carrier  710  to elevator  190 . If carrier  710  is sensed by photosensor X 51 , then PLC  220  recognizes that carrier  710  is out of slope conveyor  180  and controls motor driver  507  to stop motor M 7  and return slope conveyor  180  to its original position. With photosensor X 051  sensing, PLC  220  receives a signal from photosensor X 054  sensing that elevator  190  is moved up and drives motor M 11  of elevator  190 . Photosensor X 050  senses that carrier  710  is transferred to elevator  190  by motor M 11 . PLC  220  then stops motors M 6  and M 11 , driving a cylinder Y 233  to move elevator  190  down. 
     When photosensor X 054  detects carrier  710  without photosensors X 053  and X 058  sensing, then PLC  220  drives motors M 11  and M 12  to transfer carrier  710  to rotation unit  200 . With carrier  710  detected by photosensor X 053 , PLC  220  determines that carrier  710  is completely transferred and stops motor M 11 . Elevator  190  is moved up to its original position by driving a cylinder Y 232 . With photosensor X 059  sensing, PLC  220  recognizes that rotation unit  220  is completely moved up and drives motor Y 23 A to rotate carrier  710  through a 90-degree angle. 
     Driving cylinders Y 238  and Y 237  adjusts the width of the conveyor and moves down rotation unit  200 , respectively. When magnet sensor X 05 A senses the upward motion of rotation unit  200 , the cylinder rod is moved down by driving stopper Y 235  to drive motor M 12  and thus transfer carrier  710  to second conveyor  210 . With photosensor X 058  sensing the transfer of carrier  710 , PLC  220  drives a cylinder Y 236  to move up the turn table of rotation unit  200  and rotates the turn table reversely by a cylinder Y 23 B. PLC  220  then drives cylinder Y 237  to move down the turn table and returns the width of the conveyor to its original state by a cylinder Y 239 , recognizing that one cycle of the operation is completed. 
     If photosensor X 07 B is sensed, then PLC moves down stopper Y 257 . Thus carrier  710  is transferred to second conveyor  210 . With photosensor X 07 A sensing, PLC  220  moves up stopper Y 257  but moves down the cylinder rod of stopper Y 256 . With photosensor X 078  sensing, PLC  220  controls the cylinder rod of stopper  255  to move up, thereby interrupting the transfer of carrier  710 . After an interval of t seconds, the cylinder rod is moved up by using cylinder Y 258  in return (i.e., lower) up-down unit  135 . With photosensor X 07 D sensing, PLC  220  drives motor SM 2  to transfer carrier  710  to work frame lower conveyor  115 . When photosensor X 07 C senses, motor SM 2  is suspended and PLC  220  drives cylinder Y 259  to move down return up-down unit  135 . With photosensor X 07 E sensing, PLC  220  controls the operation according to the sensing of the photosensor concerned. 
     Motor SM 4  is always operative. With photosensor X 07 F sensing, PLC  220  moves up stopper Y 25 A to prevent the transfer of carrier  710 . If a lifer moving-down signal is detected by photosensor X 084 , then PLC  220  moves down stopper Y 25 A and drives motor DM 3  to transfer carrier  710  to lifter  221 . With limit switch X 081  sensing, PLC  220  moves up the stopper, stopping motor DM 3  and driving cylinder Y 25 B to move lifter  221  up to work frame upper conveyor  115 . When magnet sensor X 083  senses, PLC  220  detects that lifter  221  is completely moved up. If photosensor X 082  senses that carrier  710  has been transferred, then PLC  220  stops motor DM 3  and controls cylinder Y 25 C to move down lifter  221 . With photosensor X 084  sensing, PLC  220  determines that lifter  221  is completely moved down. Photosensor X 079  releases the stopper in order for reset switch  254  to transfer carrier  710  to return up-down unit  135 . 
     When photosensor X 078  does not sense the type of carrier  710  but photosensor X 079  senses the transfer of carrier  710  to second conveyor  210 , then PLC  220  continues to transfer carrier  710  to second conveyor  210 . With the photosensor sensing, PLC  220  moves down stopper Y 267  to transfer the carrier  710  to the second conveyor  210 , which is sensed by the photosensor X 09 A. The PLC  220  moves up the stopper Y 267  but moves down the cylinder rod of the stopper Y 265 . With the photosensor X 098  sensing, the PLC  220  moves up the cylinder rod of the stopper Y 265  to stop the transfer of the carrier  710 . In t seconds, the cylinder Y 268  in the return up-down unit  136  is moved up. With the photosensor X 09 D sensing, the PLC  220  drives the motor SM 2  to transfer the carrier  710  to the work frame lower conveyor  116 D. Through the photosensor X 09 C sensing, the motor SM 2  is stopped to drive the PLC  220 , thus moving return up-down unit  136  down. 
     If magnet sensor X 09 E senses, then PLC  220  resets return up-down unit  136  for the next operation for a subsequent carrier  710 . Motor SM 4  is always operative. With photosensor X 09 F sensing, PLC  220  moves stopper Y 26 A up to prevent the transfer of carrier  710 . When photosensor X 104  senses that lifter  222  is completely moved down, then PLC  220  moves stopper Y 26 A down and drives motor DM 3  to transfer carrier  710  in lifter  222 . Pressing it switch X 101  makes PLC  220  move stopper Y 26 A up. PLC  220  stops motor DM 3  to move lifter  222  up to work frame upper conveyor  115 . With photosensor X 103  sensing, PLC  220  determines that lifter  222  is completely moved up and maintains the state of the lifter. Turning photosensor X 102  off drives motor DM 3  to transfer carrier  710 . If carrier  710  is sensed by photosensor X 102 , then PLC  220  stops motor DM 3  and moves lifter  222  down to cylinder Y 26 C. With photosensor X 104  sensing, PLC  220  determines that lifter  222  is completely moved down. 
     In third work frame  103 , carrier  710  is recognized and returned to the work frame with the same precedures. If fourth work frame  104  is the last one, then any carrier  710  that is not returned before reaching fourth work frame  104  is considered as originating from fourth work frame  104  and thus returned thereto. 
     When carriers  710  with soldered PCBs are transferred to work frame upper conveyors  111  to  114 , the workers separate the PCBs from their respective carriers  710  and trim them, followed by inspecting them for incorrect insertion of parts. 
     To sense the type of carrier  710  transferred to second conveyor  210 , transparent fiber sensors X 078 , X 098  and X 118  are installed in different positions on second conveyor  210  as photosensors. Transparent fiber sensors X 078 , X 098  and XI  18  may detect a bracket (not shown) attached to carrier  710  as a cell-recognizing indicator and recognize the carrier  710  as that of the work frame. In the preferred embodiment, the indicium of cell detection plate  730  is detected for cell recognition. For a the last work frame, no separate sensing device is installed. 
     To return base boards  231 - 238  to their original positions for the purpose of insertion of parts, photosensors X 08 B, X 086 , X 10 B, X 106 , X 12 B, X 126 , X 14 B and X 146 DMF may be installed under the gear for rotating base boards  231 - 238 . Base boards  231  to  238  are rotated by pressing rotation switches  252  and  253  to drive a motor. Alternatively, base boards  231 - 238  may be rotated manually after the insertion of the parts. 
     According to the present invention, carriers with different types of PCB installed therein are transferred to a soldering unit through an upper conveyor. After soldering, each of the carriers is moved down to a lower conveyor to return to its work frame designated according to detecting the position of a cell-recognizing indicator installed on the carrier. The PCBAs on the carriers are then inspected by the same workers who inserted the parts into the PCBs. With these procedures, the present invention can enhance productivity and thus decrease production cost. 
     One benefit of the present invention is that the failure rate may be reduced, because the worker is in charge of an entire production process from assembly through inspection. The workers can have greater job satisfaction in a production system for different processes. The present invention also prevents the waste of manpower that may be caused while a new production set-up is established. 
     It should be understood that the present invention is not limited to the particular embodiment disclosed herein as the best mode contemplated for carrying out the present invention, and neither is the present invention limited to the specific embodiments described in this specification except as defined in the appended claims.