Abstract:
An apparatus for supporting double sided printed circuit boards during printing a solder past pattern or the like on the second side of the printed circuit board after the first side is already loaded. The apparatus includes components for holding the printed circuit boards in position and support posts with different footprints for supporting the printed circuit board at locations on the first side not occupied by components. The apparatus uses a vacuum chamber and/or vacuum cups for holding the printed circuit board in place. It further includes components for proper positioning the printed circuit boards prior to holding the same in position.

Description:
BACKGROUND OF THE INVENTION 
     During production of electronic circuit boards and especially of double sided circuit boards a pattern of solder paste is printed on the circuit board prior to placing and soldering of electronic components to the circuit board using reflow solder methods. For proper application of solder paste the circuit board has to be held in a desired position with high precision, and it has to be supported in various locations to prevent any bending of the circuit board during the printing process. 
     PRIOR ART 
     The prior art includes vacuum tables with sidewalls which can be adjusted to the size of the particular printed circuit board to be handled. None of the known vacuum tables includes a plurality of openings, arranged in rows and columns, which can be closed off, inside and outside of the vacuum chamber, and which can be used for positioning support posts for the printed circuit board or left open inside the vacuum chamber to apply a vacuum. None of the prior art systems has a number of different types of support posts, such as support posts with centric or off-center supports, and support posts with suction cups to improve holding power on the printed circuit board during the printing operation. 
     SHORT DESCRIPTION OF THE INVENTION 
     The universal suction plate of the present invention prevents damage of electronic components during an application of e.g. a pattern of solder paste, on the second side of a printed circuit board. While printing on the second side of a printed circuit board the first, lower side contains components, such as capacitors, resistors, integrated circuit of different sizes and heights, which can not be used for supporting the printed circuit board. The universal suction plate is used in combination with a printing apparatus, and a conveyor system for supplying printed circuit boards and removing the same after the print operation. The suction plate for holding and supporting electronic circuit boards is placed underneath and aligned with the printing apparatus. The conveyor system supplies the circuit boards to the suction board, which includes sensors to determine the position of a circuit board. A separate apparatus lifts the suction plate to the lower plane of the circuit board. A strong vacuum is generated in the suction chamber or one or more suction cups to hold the circuit board in place. To avoid any buckling or bending of the circuit board by the vacuum and during the print operation, support posts of different forms and shapes are provided at various locations underneath the circuit. After completion of the print operation the vacuum is released, the suction plate is lowered, and the circuit board is forwarded by the conveyor system to the next production step. 
     The suction plate includes a base plate and a support plate with a large number of openings for receiving support posts and sensors. The base plate includes a connector to a vacuum pump. A space between base plate and support plate connects the vacuum pump with the selected openings in the support plate and at least a vacuum chambwe or a vacuum suction cup. 
     The vacuum chamber is defined by walls which are movable in accordance with the size of the circuit board to be handled in the printing operation. Attached to the sidewalls of the vacuum chamber or the support platw there are sensors which detect when a conveyor system supplies a new circuit board. The sensors control the conveyor system to advance the circuit board to the proper print position. A Z-axis drive lifts the suction plate until the circuit board rests in the side walls of the vacuum chamber or the support posts on the support plate. The vacuum is applied and the print operation can be performed. Additional positioning means can be used to move a circuit board to a desired position with high accuracy independent of the conveyor system. 
     The vacuum generated underneath the circuit board is of sufficient strength to overcome leakage of air into the chamber through openings in the circuit boards, such as vias, and minor openings in the movable side walls. 
     The support means are placed in suitable ones of a large number of openings in the support plate. These support means include plain support posts, posts with offset extensions, posts with suction cups. These different types of posts are required to maintain circuit boards in a plane position. The supports are positioned directly under the circuit board and do not interfere with any of the components on the lower side of the circuit board, where these components reach into the vacuum chamber. The posts with offset extensions are used to support the circuit board at locations which do not align with an opening in the support plate. 
     Support posts with suction cups are used to further inhibit bulging against the vacuum of the circuit board, and to further reduce horizontal movement of the printed circuit board during the print process. These suction cups are connected through the support posts to the vacuum generator. 
     Short cavity plugs are provided to seal unused openings in the suction plate, and especially all of the openings outside a vacuum chamber. 
    
    
     SHORT DESCRIPTION OF THE FIGURES 
     FIG. 1 is an illustration of the placement of the arrangement suction plate in a manufacturing place. 
     FIG. 2 is a perspective view of the suction plate. 
     FIGS. 3A-3F are illustrations of various types of a support posts. 
     FIGS. 3G-3I are top views of support posts illustrated in FIGS. 3C-3E, respectively. 
     FIG. 3J is a top view of FIG.  3 F. 
     FIG. 4 is an illustration of a cavity plug. 
     FIGS. 5A and 5B are side view illustrations of a dual foot post. 
     FIGS. 6A and 6B are illustrations of pneumatic board positioner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is an illustration of the arrangement of a suction plate in a manufacturing place. FIG. 1 depicts an instance when three printed circuit boards  1 A,  1 B, and  1 C, which are of the same type and size, pass the printing station on a conveyor belt in the direction as indicated by arrow  6 . Board  1 B is in the position for printing a pattern of solder paste on top side  7 . Conveyor belt and printing mechanism are not shown in FIG.  1 . Underneath the printing position there is a suction plate  3  of the present invention with a vacuum chamber  2 , which is set up for the size of boards  1 A- 1 C. Suction plate  3  is supported by worktable  4 . A Z-drive  5  is attached underneath worktable  4 . Z-drive  5  lifts workbench  4 , suction plate  3  with vacuum chamber  2  as indicated by arrow  7  to a height that board  2  rests on top rim  8  of vacuum chamber  2 . Inside vacuum chamber  2  there are additional posts for supporting board  2  during the print process. Upon completion of the print process Z-drive  5  lowers the equipment to its original position and the boards  1 A- 1 C are advanced by one process step position to the right. 
     FIG. 2 is a perspective view of the suction plate. Suction plate  200  has a base plate  202  and a support plate  204 . Suction plate  200  has a large number of cavities, such as cavities  268  shown in the lower left corner of suction plate  200 . These cavities are provided for mounting support posts, and are arranged in rows and columns. A side wall  206 , consisting of lower section  206 A and upper section  206 B, is mounted near edge  207  of base plate  200 . Near the two edges  209  and  211 , which are normal to edge  207  there are rail  254  and guiding edge  213  for positioning of movable sidewall  208 , consisting of lower section  208 A and upper section  208 B. Two sidewall components  210 A and  212 A are slidably mounted in rails  222 A and  222 B of sidewall  206 . Two sidewall components  208 B and  210 B are slidably mounted in rails  220 A and  220 B of sidewall  208 . Sidewall components  210 A,  210 B,  212 A,  212 B, and sidewalls  206  and  208  define a vacuum chamber, and their expose to edges provide support for a circuit board during the printing operation. Preferably, the sidewalls  206  and  208 , and sidewall components  210 A,  210 B,  212 A,  212 B, support the printed circuit board at the edges of the board where there are no components mounted on the underside of the printed circuit board. Positioning of the sidewalls  206  and  208 , and sidewall components  210 A,  210 B,  212 A,  212 B is done using a matrix of rows and columns, marked by scales  250 ,  252 , marking rows, and scale  260  marking columns. Scales  250  and  252  carry markings ‘A’ through ‘Z’ and ‘AA’ through ‘HH’, scale  260  is marked ‘-21’ through ‘0’ to ‘21’, whereby scale marking ‘0’ is preferably referring to the center column of the cavities. However, other row and column designations are possible or may be more suitale in special applications. For each column positioned in each of the rows suction plate  3  has a cavity. A cavity van be occupied by a plug closing the cavity, or a post for supporting printed circuit board during printing operation. An additional opening  270  is provided near the center of sidewall  206  for connection to a vacuum suction pump, and to other means which are no part of the suction plate of the present invention. Opening  270  can be closed by a cover plate (not shown) to maintain the vacuum by plugs in openings  268  in support plate  204  exclusively. A recess  264  is provided for attaching tools and sensors to rails  222 A and  222 B inside vacuum chamber between sidewall components  210 A and  212 A. 
     Elements  240 ,  242 ,  244 , and  246  as shown in FIG. 2 are illustrative of the forms of the support post as will be discussed in more detail below. 
     Sidewall  206  includes a second scale  251  for setting sidewall components  210 A and  212 A at a desired position and spacing. Scale  251  is also used to position a fixed stop  230  for the printed circuit board to be processed, and to position a variable stop  232  used for pushing a printed circuit board towards a fixed stop  230 . FIG. 6A is a more detailed illustration of fixed stop  230 . FIG. 6B is a more detailed illustration of variable stop  232 . 
     During a processing the printed circuit board is moved by a conveyor system in parallel with sidewalls  206  and  208  across the vacuum chamber towards fixed stop  230 . After suction plate  200  is sufficiently lifted so that the printed circuit board can rest on the upper edges of sidewalls  206  and  208 , and sidewall components  210 A,  210 B,  212 A,  212 B, variable stop  232  is activated and pushes the printed circuit board against fixed stop  230 . At that time the printed circuit board is properly positioned and the printing process can begin. 
     FIGS. 3A-3F are illustrations of various types of a support posts. As illustrated in FIG. 3A a support post  48  has a foot  50  and a stem  52 . Face  54  at the upper end of stem  52  is the interface for supporting a printed circuit board during a printing operation. At the lower end stem  52  has a cross-section  56  which is slightly larger than the cross-section  58  of cavity  60  into which foot  50  of support post  48  has been placed. Support post  48  rests with ring surface on surface  57  of suction plate  62 . Thus the length of stem  52  determines the height of face  54  above the top surface of suction plate  62 . Foot  50  does not reach bottom  64  of cavity  60 . Co-axial position of foot  50  and cavity  60  is established by two elastic rings  66  and  68 . Elastic ring  66  aligns foot  50  with cavity  60  in base plate  72 . Elastic ring  68  aligns foot  50  with hole  74  in support plate  76  of suction plate  62 . A vacuum chamber is provided between base plate  72  and support plate  76 . 
     FIG. 3B is an illustration of a support post  78  with a vertical air suction channel  80  which is connected to a short horizontal channel  82 , which in turn connects with the vacuum chamber  70  of suction plate  62  (see FIG.  3 A). Support post  78  rests with ring surface  79  on the suction plate. The upper end  82  will hold in place the printed circuit board by the applied vacuum as soon as the surface of the printed circuit board rests on face  84  of support post  78 . 
     FIG. 3C is an illustration of a support post  90  with a suction cup  92  for folding a printed circuit board. A vertical air suction channel  91  which is connected to a short horizontal channel  93 , which connects with the vacuum chamber  70  of suction plate  62  (see FIG.  3 A). Top end  94  of post  90  is suitably formed for receiving suction cup  92 . Face  96  of top end  94  is at the same height above suction plate  76  as face  54  of post  48 . To maintain a vacuum inside suction cup  92  while a circuit board rests on face  96  there is a groove  98  extending radially from suction channel  91 . In non-loaded condition upper rim  98  of suction cup  94  is slightly higher than face  96 . The difference in height of rim  99  and face  96  depends on the elasticity of suction cup  92  and the surface characteristic of the printed circuit board, i.e. what differences in planarity of the printed circuit board have to be dealt with. 
     FIG. 3D is an illustration of a support post  100  with thinner central extension  102  of stem  104 . A support post of the type shown in FIG. 3D is used if the space between components on the circuit board is too small for using a post as shown in FIGS. 3A,  3 B or  3 C, and if the used support space on the circuit board is just above the center of a cavity in the suction plate. 
     FIG. 3E is an illustration of a support post  106  with an extension  108  off center to stem  110 . Post  106  can be turned in the cavity it is placed to have extension  108  support the printed circuit board at a place not just above the center of the particular cavity. 
     FIG. 3G-3I are top view illustrations of support posts illustrated in FIGS. 3C-3E, respectively. In FIG. 3G groove  98  is shown to cross face  96  of top end  94 . FIG. 3H is the top view illustration of support post  100  with a round central extension  102  of stem  104 . FIG. 3I is the top view illustration of support post  106  with an offset extension  108  of stem  110 . Extension  108  is shown to have a square cross-section. However, the cross-section may have other forms, depending on the manufacturing methods or the demands of the free space of the printed circuit board it is to support. The free space of the printed circuit board available for supporting with a support post during a printing process may also determine the offset of the extension from the center of the support post. 
     FIG. 3J is an illustration of a support post  106  with three feet  124 ,  128 , and  130  to be placed into three adjacent cavities of the same row or column of a suction plate of the present invention. A support post as shown in FIG. 3J can be used to hold special measuring tools or support means. Stem  120  can be of a type discussed above with reference to FIGS. 3 a  through  3 G. A Block  126  is attached to post  120  to hold two of the three feet  124 ,  128  and  130 . This block can be used to hold additional tools. Stem  120  may have a suction hole  122 . 
     FIG. 4 is an illustration of a cavity plug in a cavity of the suction plate. The suction plate includes base plate  142  and support plate  140 . A space  144  provides the connection between cavities in the suction plate and the vacuum pumps, in FIG. 4 cavity plug  152  is inserted in cavity  158 . The lower end  154  of cavity plug  152  contacts the bottom of cavity  158 . This ensures that the upper end of cavity plug  152  does not protrude the top surface of support plate  140 . An elastic ring  156  closes cavity  158 . Al cavities of the suction plate outside the vacuum chamber have to be closed off with such cavity plugs. 
     FIGS. 5A and 5B are side view illustrations of a dual cavity plug  160 , as an example for a row or column oriented post. It includes two feet  161 A and  161 B to mount is in a preferred direction in the suction plate, and a recess  163  into which optical or mechanical sensing means can be mounted e.g. for generating an edge signal when the edge of the next printed wiring board to be handled in a printing process is passing. An edge signal can be used to stop the conveyor means which supply the printed wiring board A proximity signal can be used to lift the suction plate towards the printed wiring board until the printed wiring board is within operating proximity of the printed wiring board positioning means. The shape of the body  162  can be modified to need and requirement. The top end can be used for supporting the printed circuit board if the board has a surface sufficiently large enough to match the top surface of boy  162 . 
     FIGS. 6A and 6B are illustrations of pneumatic board positioning means, which include an adjustable printed circuit stop  170  and a adjustable controlled pneumatic print wiring board pusher  190 . When the suction plate is lifted in operating proximity of the printed wiring board positioning means the Printed wiring board is located between printed circuit stop  170  and print wiring board pusher  190 . Print wiring board pusher  190  receives a signal from a operational cycle controller and pushes the printed wiring board towards printed circuit stop  170 , which places the printed wiring board in a position in which it can rest on the walls of the vacuum chamber. 
     Adjustable printed circuit stop  170  is attached to rail  180 , one of the rails  222 A and  222 B in FIG. 2 at one side of the vacuum chamber. It is adjusted to a desired position using scale  182  (scale  251  in FIG. 2) and held in that position by a clamping means under control of knob  174 . Main body  172  carries board stop  178 , which is settable at a desired distance from sidewall  171  by screw  182 . 
     Mounting block  192  of print wiring board pusher  190  (see FIG. 6B) is attached to rail  180  using set screw  195  at the other side of the vacuum chamber. Rail  180  is one of the rails  222 A and  222 B in FIG.  2 . Upon receiving a positioning signal via pneumatic control line  196  slider  197  and push rod  188  are moved to the left, towards the printed wiring board and pushes the board towards board stop  178 , as indicated by arrow  189 . Print wiring board pusher  190  is adjusted to adesired position using scale  191  (scale  251  in FIG. 2) and held in that position by a clamping means under control of set screw  195 . Push rod  188  is attached to slider  197  by an adjustment block  199  so that pushrod  188  can be set to a desired distance from sidewall  193  by screw  194 . It is within the scope of the invention to replace the pneumatic control of slider  197  by other means of activation, such as electrical, or mechanical. 
     The above description discloses the use of the suction plate with fixed and movable walls, which establish a vacuum chamber. However, as already indicated, support posts with suction cups can be used when there is either no continuous space avaible at the lower side of the printed circuit board to establish a vacuum chamber or when one or more support posts with suction cups hold the printed circuit board in position while other support posts support the printed circuit board as needed during the print operation. In the latter case, fixed and movable walls may be used for providing lokal support for the printed circuit board without establishing a vacuum chamber.