Abstract:
A device for assembling electronic circuits comprises a placing station ( 7 ) for placing circuit components on a circuit carrier ( 1 ) held at a placing location ( 17 ), a fixing station ( 19 ) for fixing the circuit components to the circuit carrier ( 1 ) under the effect of heat, and a belt conveyor means ( 9 ) for conveying a circuit carrier from the placing location ( 17 ) to a take-over location ( 29 ) of the fixing station ( 19 ). The fixing station ( 19 ) comprises a heatable zone ( 22 ) and a manipulator ( 21 ) for raising the circuit carrier ( 1 ) from the take-over location ( 29 ) and placing it in the heatable zone ( 22 ). A continuous belt conveyor ( 9 ) of the belt conveyor means extends from the placing location ( 17 ) up to the take-over location ( 29 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a division of U.S. patent application Ser. No. 10/498,836, filed Nov. 29, 2004, now abandoned, which is a Rule 371 application of International Patent Application No. PCT/IB02/05753, filed Dec. 13, 2002. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to an apparatus and a method for automated assembly of electronic circuits. 
   Usually, for this purpose, program controlled placing devices are used which are capable of taking a circuit component from a stack and inserting it at a given position of a circuit board. When the placing process is finished, the circuit board is heated together with the components placed on it in order to melt solder and to bring the circuit components into contact with conductors of the circuit board. 
   The process steps of placing and heating are carried out by different devices which, for a completely automated operation, must comprise internal conveyor means which receive a circuit board to be processed at an input of the device, and convey it to a processing location where the circuit board is packed with the components or the components are soldered, respectively, and further to an output of the device. These internal conveying means are often formed with conveyor belts. At a transition from one device to another, these conveyor belts are generally butt jointed, so that a circuit board which is delivered by a conveyor belt of the placing device may slide across the joint onto the conveyor belt of the soldering device and may be further transported by it. Lateral guide rails ensure that the circuit board arrives on the conveyor belt of the soldering device exactly in a correct orientation. 
   Such a device set-up provides satisfactory results when circuits having working frequencies in the MHz range are assembled. For assembling radio frequency circuits with working frequencies in the GHz range, they have proved unsatisfactory. The reason for this is that, in radio frequency circuits, a much higher positional accuracy is required for the individual circuit components. There are placing devices available which are capable of placing the individual circuit components with the required accuracy onto a circuit board; however, after soldering it is often found that the circuit components are no longer at the intended locations with the required accuracy. An undesirable scatter in the technical properties of the circuit or even circuit failures may result. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide an apparatus for assembling electronic circuits which is suitable for manufacturing electronic circuits with well reproducible technical properties, in particular for assembling circuit components on a board with a positioning accuracy in a range of ±50 mm, as required for radio frequency circuits. 
   The object is achieved by an apparatus having the features of claim  1 . 
   The inventors have found that in current assembling devices an essential reason for the deviations of the soldered circuit components from their intended position is the hand-over of the circuit board from the conveyor means of the placing device to those of the soldering device. If the conveyor means of these two devices are not exactly parallel, there is an abrupt change of direction at the hand-over of the circuit board which may cause the circuit components to slip. The same applies if the planes in which the conveyor means support the circuit boards have slightly different heights or are not exactly parallel. Also, small differences in the conveying speeds may cause abrupt movements of the circuit board during hand-over. 
   All these problems are avoided in the apparatus of claim  1  by departing from the conventional conception of automatic devices working autonomously and conveying from the input to the output, and by using, instead, a continuous conveyor belt which bridges the output of the placing device by extending directly from a placing location to take-over location within the fixing device, where a circuit board conveyed on the conveyor belt is taken over by a manipulator in order to place it in a heatable zone of the soldering device. 
   In order to provide a shock-free transition of a circuit board with components placed on it, it is desirable that each circuit board comes to rest at an exactly reproducible position of the conveyor belt. For this purpose, preferably, a first sensor for detecting a passage of a circuit board is located at the conveyor belt, and a control device connected to this sensor brakes the conveyor belt with a predefined velocity profile after detection of the passage of a circuit board, in order to bring the circuit board to rest at a predefined location. 
   A second sensor is advantageously located at the conveyor belt at the location where the circuit board should come to rest; it allows for a post-correction of the speed profile in order to compensate inaccuracies of speed control. 
   According to the first embodiment of the invention, fixing of the circuit components on the circuit board is carried out by soldering. A solder oven employed for this purpose should preferably be evacuable in order to prevent the formation of cavities in the solder layer which might impair the behaviour of radio frequency components. 
   For reproducible soldering of good quality, it is important that in each solder process the temperature distribution in the solder oven is reproduced as accurately as possible. This may be difficult if the number of circuit boards to be soldered in one solder process varies. In order to achieve a reproducible solder process under these circumstances, too, a parking area for a dummy circuit board is preferably provided in the movement range of the manipulator so that the manipulator may fill the solder oven with the dummy if not all places present in it are occupied by circuit boards. 
   Another possibility of fixing circuit components on the board is to use a heat-sealing film placed between the board and the circuit components. In order to fix the circuit components on the board using such a film, only a heatable plate is required on which the circuit board having the components placed on it may be put by the manipulator. 
   In order to protect the highly accurate and susceptible placing station from heat and, possibly, vapours from the solder oven, the placing station is preferably enclosed in a casing having an opening through which the conveyor belt emerges. 
   If a continuous conveyor belt extends from a pick-up location at the input of the placing station to its output or, as described above, to a take-over location, where a circuit board may be taken over by the manipulator of the fixing station, there is a problem in that a new circuit board without components may not straightforwardly be loaded onto the conveyor belt at the pick-up location as long as a circuit board with components placed on it is still on the conveyor belt. Any vibration during loading might cause a displacement of components on the circuit board equipped with them. Since the circuit board must be conveyed very slowly and carefully between the placing location and the take-over location or, at least, may only be accelerated and braked very slowly, this transport takes considerable time which increases the cycle time of the apparatus. In order to make this cycle time as short as possible, it is proposed to split the conveyor means into two conveyor belts, a first one extending between the pick-up location for the unequipped circuit board and a hand-over location upstream of the placing location and a second conveyor belt extending from the hand-over location via the placing location to the take-over location. Due to this division, it is possible to convey a new, unequipped circuit-board to the hand-over location—which should appropriately be in close proximity to the placing location—before the completely equipped circuit board has reached the take-over location where it is taken over by the manipulator. As soon as it is taken over, the new circuit board only has to be conveyed over a minimum distance on the second conveyor belt to the placing location. 
   In order to be able to place the circuit components with high accuracy, it is desirable that during the placing procedure the circuit board is not supported by the conveyor means, which might be compliant. For this purpose, preferably an elevating table is provided at the placing location for lifting the circuit board off the conveyor means. 
   In order to provide a reproducible up and down movement of the elevating table, it is linearly guided by rails. It is difficult to provide absolute parallelism between these rails and the movement of an actuator that drives the up and down movement of the elevating table. In order to prevent abrupt movements from being caused by deviations from parallelism when the elevating table reaches its abutments, a clearance is provided in the connection of the actuator to the elevating table. 
   The actuator for raising and lowering the elevating table is preferably driven by a pressurized fluid, i.e. hydraulically or pneumatically. The free cross-section of an exhaust line by which pressure fluid is exhausted from the actuator when lowering the elevating table is smaller than the free cross-section of a feed line by which pressurized fluid is fed into the actutator when raising the elevating table. This measure allows for a reduction of the descending speed of the actuator without requiring sophisticated controls, so that while the circuit board equipped with circuit components is being deposited on the conveyor belt, absolutely no vibrations can occur. 
   Preferably, the cross-section of the exhaust line is reduced only locally by a throttle. The free cross-section of this throttle should be adjustable, so that the speed of the downward movement can be adapted to current requirements by adjusting the cross-section. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention become apparent from the subsequent description of embodiments referring to the appended drawings. 
       FIG. 1  is a schematic top view of a device for assembling electronic circuits according to the invention; 
       FIG. 2  is a detailed view of the take-over location of the device of  FIG. 1 ; 
       FIG. 3  is a perspective view of the take-over location; 
       FIG. 4  is a schematic section of the placing location of the device of  FIG. 1 ; and 
       FIG. 5  is a schematic representation of the pneumatic layout of the placing location. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The assembling device of  FIG. 1  is transited by the circuit carrier  1  to be equipped from left to right. The first apparatus of the device is a magazine loader, which, being known as such, is not represented in detail in the drawing. Such a magazine loader comprises a rack which receives a plurality of circuit carriers and is adjustable in height, so that the circuit carriers  1  may successively be placed in front of a slider  2  which pushes them onto a first belt conveyor  3 . 
   The circuit carriers  1  are here formed of a carrier board  4  of metal having a width selected such that it fits with little clearance onto the belt conveyor  3  and which is slightly tapered at its front end in order to ensure that the carrier board can safely be pushed onto the first belt conveyor  3  without risk of tilting. A metal casing  6  in which circuit components are to placed is fixed to the carrier board  4  by several screwed pads  5 . Alternatively, the circuit carrier might be formed only of such a casing if it has appropriate dimensions for safe transport on the belt conveyor  3 . 
   The first belt conveyor  3  is part of a computer controlled placing apparatus  7 . The placing apparatus  7  comprises a table top  8  on which the first belt conveyor  3 , a subsequent second belt conveyor  9  and magazine containers  10  for the circuit components to be placed in the casing  6  of circuit carrier  1  are located. Above the table top  8 , a gripper arm  13  is mounted to a granite board (not shown) displaceably in vertical and, by means of rails  11 ,  12 , in two horizontal directions. The two belt conveyors  3 ,  9  are essentially similar in construction. Both comprise a base plate  14  around which two conveyor belts  15  are wound and are driven by a motor. The base plate is delimited at its longitudinal sides by side plates  16  that-define a transport path for the circuit carrier  1  and that are slightly divergent at their upstream end in order to provide a safe transition of the circuit carriers  1 . 
   The second belt conveyor  9  differs from the first  3  essentially in that in its upstream portion an elevating, table  17  is inserted into the base plate  14 , and that its length is considerably greater than that of the first belt conveyor  3 , so that it extends out of the casing  18  of the placing device and into a subsequent soldering device  19 . The section of the second belt conveyor extending into the soldering device will subsequently be further described with reference to  FIGS. 2 and 3 . 
   The soldering device  19  comprises, mounted to a table top  20 , a manipulator formed by a highly precise and vibration-free robot arm  21 , an evacuable solder oven  22 , a support  23  for a circuit carrier dummy and a cooling-off plate  24  formed by a metal plate having cooling water ducts arranged inside it and a slider for pushing a circuit carrier cooled-off on the cooling plate  24  out of the soldering device  19  to a further belt conveyor  26 . 
   The operation of the illustrated device is as follows. At the pickup location  27 , at the beginning of the first belt conveyor  3 , a circuit carrier  1  is pushed onto the first belt conveyor  3  using slider  2 . The first belt conveyor  3  conveys the circuit carrier  1  to a hand-over location  28  for hand-over to the second belt conveyor  9 . At this hand-over location  28 , the two belt conveyors  3 ,  9  are butt-jointed, and the occurrence of vibrations during hand-over of the circuit carrier  1  cannot be completely avoided, but is harmless since there are no circuit components placed on it yet. 
   The second belt conveyor  9  has conveyor belts  15  which, at that time, circulate with essentially the same speed as the conveyor belts  15  of the first belt conveyor  3 . They convey the circuit carrier  1  further and come to rest at a placing location  69 , at which the circuit carrier is immediately above the elevating table  17 . By raising the elevating table  17 , the circuit carrier  1  loses contact with the conveyor belts  15  and, instead, has its planar bottom side stably resting on the elevating table  17 . In this way, it is made sure that during the placing procedure, the circuit carrier  1  cannot be displaced by pressure exercised by gripper arm  13 , so that the complete placing procedure can be carried out at a high accuracy. 
   During the placing process, there is no further circuit carrier  1  on the second belt conveyor  9 ; however, a further circuit carrier from the magazine loader may have already arrived on the first belt conveyor  3 . 
   When the placing procedure is finished, the elevating table  17  is descended again, so that the circuit carrier  1  located on it is again deposited on the conveyor belts  15  of second belt conveyor  9  carefully and without vibrations. 
   From this position, the circuit carrier  1  equipped with the circuit components is further conveyed by the second belt conveyor  9 , the acceleration of the circuit carrier being selected low enough to prevent vibrations and slipping of the placed circuit components. 
   Before reaching a take-over location  29  at the end of the second belt conveyor  9 , a gentle deceleration of the circuit carrier  1  begins. For this purpose, as can be seen in the top view of  FIG. 2 , a first sensor  30 , e.g. a photo detector, is inserted into the base plate  14  of the second belt conveyor  9  in order to detect the passage of the leading edge of a circuit carrier  1  at a distance from its intended rest position at the take-over location  29 , which is sufficient to bring the circuit carrier to rest without causing vibrations. As soon as the passage of the circuit carrier is detected at the first sensor  30 , it is e.g. begun to brake the belt conveyor  9  with a constant deceleration, which is selected such that, provided that the control is free from tolerances and errors, the circuit carrier will come to rest exactly at the take-over location, as indicated by a dashed outline of the leading edge of the circuit carrier in  FIG. 2 . 
   In order to bring the circuit carrier  1  to rest exactly at this predefined position  29 , the second belt conveyor  9  is preferably further equipped with a second sensor  31  that extends over approximately 1 to 2 mm in transport direction and provides an output signal proportional to its degree of coverage by the circuit carrier. Based on this output signal, a control (not shown) of the belt conveyor  9  is capable of detecting whether the circuit carrier reaches the second sensor  31  before or after an expected instant, and whether, in consequence, the braking procedure must be shortened or prolonged in order to reach the desired target position  29 . 
     FIG. 3  shows a perspective view of the second belt conveyor  9  having a circuit carrier  1  placed at the takeover location, and, above, an end effector  32  of robot arm  21 . The end effector  32  comprises two claws  33  that are moveable transversely to the transport direction in the direction of arrow P, each having two downward projecting fingers  34 , that have inwardly directed tips  35  at the ends. In the side wings  16  of the second belt conveyor  9 , two cutouts  36  adapted to the size and distance of the fingers  34  are formed, which allow the tips  35  to engage longitudinal recesses  37  at the bottom of carrier plate  4  of the circuit carrier  1  and thus, to lift it safely. 
   The robot arm  21  afterwards places the circuit carrier in the solder oven  22 . The oven shown has two locations for circuit carriers, but might also be larger or smaller.  FIG. 1  shows the solder oven  22  with an open lid  38 , a circuit carrier  1  placed inside the oven  22  and a dummy circuit carrier  39  placed next to it. This dummy  39  is placed in the oven  22  if a solder process is to be carried out on a single circuit carrier  1 . Its thermal properties are practically the same as of an equipped circuit carrier, so that the temperature distribution in the solder oven  22  is the same in each solder process, regardless of how many locations of the solder oven are occupied by circuit carriers or by dummies. 
   After loading the solder oven  22 , the lid  38  is closed; the solder process is carried out in a conventional manner under vacuum. 
   When the solder process is finished, the lid  38  is opened, and the robot arm  21  extracts the soldered circuit carrier or carriers and places them on the water cooled cooling plate  24 . After cooling off, the slider  25  is operated, and the soldered and cooled circuit carriers  1  are pushed out onto the belt carrier  26 . 
     FIG. 4  shows a schematic longitudinal section through the placing location and the elevating table  17 . The elevating table  17  is a solid metal plate having two through bores in which cylindrical columns  41  are fixedly inserted. The columns  41  are exactly parallel to each other and extend with minimum clearance through bores  42  of table top  8 . The elevating table  17  is vertically displaceable by means of a pneumatic actuator  43  having a double acting piston and a piston rod  45 . Switching valves  44  that control ingress and egress of air into/from the two chambers of actuator  43  are located on the table top  8  at a minimum distance from actuator  43 , in order to minimize pressure vibrations in the chambers which might provoke an uneven movement of the elevating table  17 . 
   Since the columns  41  must be guided with minimum clearance in the bores  42 , a horizontal component of the acting force of actuator  43  may cause the elevating table  17  to jam during up and down movement if the piston rod  45  is not exactly parallel to the columns  41 . This must be avoided. For this purpose, the head of the piston rod  45  is received with a clearance in a recess of plate  17 . In the Figure this head is shown as a sphere engaging a similarly spherical recess; the required clearance may also simply be achieved by a threaded head of the piston rod  45  engaging a threaded bore of elevating table  17  without being fastened to it. 
   The range of movement of the elevating table  17  is limited in upward direction by projections (here formed by rings  46 ) at the lower ends of columns  41 , which, in a completely raised position of the elevating table  17  abut against the bottom side of table top  8 . This construction allows to fix the elevating table  17  in the raised position shown in the Figure without having to squeeze any parts, by merely providing pressure in one of the chambers of actuator  43 . This pressure is selected such that a pressing force exerted from above by the gripper arm  13  when placing a circuit component is not sufficient to override the supporting force of the piston rod  45 . 
     FIG. 5  schematically shows the pneumatic layout of the placing location according to a preferred embodiment. The switching valves are here combined into a single valve  44 ′ having two inputs, two outputs and three switching positions. In a first switching position, the valve  44 ′ connects a pressure fluid source, e.g. a compressor, to a lower chamber  51  of the cylinder of the double-acting actuator  43  by a feed line  49 . A check valve  53  in the feed line  49  ensures that pressurized air can only flow through the feed line  49  in a direction from the valve  44 ′ to the lower chamber  51 . 
   Air entering the lower chamber  51  urges the piston of the cylinder upward, so that the elevating table  17  mounted to the piston rod  45  is raised. Air from the upper chamber  52  is exhausted by a line  45  that goes through valve  44 ′. 
   When the elevating table  17  has reached a desired level, the valve  44 ′ is switched over into a second position in which the connections between its inputs and outputs are all interrupted. The air thus enclosed in the chambers  51 ,  52  of the actuator  43  opposes any vertical displacement of the elevating table  17 . In such a raised state, a circuit carrier on the elevating table  17  may have circuit components placed on it. 
   When after placing the circuit components the elevating table  17  is to be descended again, the valve  44 ′ is switched to a third position in which the inputs and outputs are cross-coupled with respect to the first switching position. Thus, now the upper chamber  52  is provided with pressurized air from compressor  47  by line  54 , and air from the lower chamber  51  escapes by an exhaust line  50  by-passing feed line  49 . The exhaust line  50  has a check valve  55  arranged opposite to the check valve  53 , and a throttle  48  having a controllable free cross-section. The throttle  48  considerably reduces the free cross-section of the exhaust line  50  in comparison with the feed line  49  and the line  53 . The throttle  48  thus causes the air to escape from the lower chamber  51  during the downward movement of the piston much more slowly than from the upper chamber  52  in case of an upward movement. In a simple way, this arrangement allows a fast upward movement of the circuit carrier  1  which is desirable in view of short cycle times, and a slow downward movement which is required in order to avoid vibrations when the circuit carrier settles on the belt conveyor  9 . The speed of the downward movement can be set by controlling the cross-section of throttle  48 . 
   As an alternative to the design shown in  FIG. 5 , one-way feed and output lines might also be assigned to the upper chamber  52 . In that case, a throttle for braking the downward movement would have to be placed in the feed line.