Abstract:
A method for displacing an article using first and second pallets for holding and transporting an article between two process stations and an H-bar assembly for receiving the first and second pallets, the method including the steps of: receiving the first and second pallets and articles thereon by the H-bar assembly at one of the two process stations; performing a first process on the articles while received by the H-bar assembly; moving the first and second pallets and articles thereon to a second process station; and performing a second process on the articles while at the second process station.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is related to a patent application entitled “AN APPARATUS FOR DISPLACING AN ARTICLE DURING SCREENING” (Docket No. FI9-2000-0290US1), Ser. No. ______, filed even date herewith. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to screening apparatus for screening of articles and, more particularly, relates to a screening apparatus for screening ceramic greensheets in which the ceramic greensheets may be screened multiple times without losing registration with the screening apparatus.  
           [0003]    In the current manufacture of multilayer ceramic (MLC) substrates for integrated circuit semiconductor package structures, a plurality of ceramic sheets is formed by doctor blading a slurry containing a resin binder, a particulate ceramic material, solvents, and a plasticizer, drying the doctor bladed sheet, and blanking it into appropriately sized greensheets. Via holes are then mechanically punched for forming electrical interconnections through the greensheet. Electrically conductive paste is screened in the via holes, and in appropriate patterns on the surface of the greensheets, the greensheets stacked and aminated, and the assembly subsequently fired at an appropriate sintering temperature.  
           [0004]    Critical to the manufacturing process is the screening operation since the ceramic greensheets are relatively fragile, their thickness may be on the order of 0.008 inches, and their surface area is relatively large compared to their thickness. As will be appreciated by those skilled in the art, particular problems are encountered when screening such relatively fragile, paper thin, flexible greensheets which are of no concern to the general screen printing art utilizing rigid substrates.  
           [0005]    A screening mask is precisely aligned with the greensheet so that when the electrically conductive paste is screened in the via holes and in appropriate patterns on the surface of the greensheets, the screened via holes and patterns are where they are supposed to be.  
           [0006]    The screening operations of filling via holes and screening patterns are often done simultaneously, thus the screening paste has to be tailored to satisfy both the via fill and pattern deposition requirements. This approach is not without its problems since the screening paste is a compromise between the differing requirements of via fill and pattern deposition.  
           [0007]    An alternative approach is to fill the via holes with a paste Theologically tailored for vias, dismount the greensheet from the screening stage, dry it, remount it on the screening stage, align the greensheet with the screening mask for the patterns, and then screen paste that has been tailored for patterns. This approach presents problems with sheet distortion that occur during the drying process, making greensheet realignment difficult or sometimes impossible for the second screening operation.  
           [0008]    Various screening apparatus have been proposed by those skilled in the art. One particular screening apparatus is that disclosed in Davis et al. U.S. Pat. No. 4,362,486, the disclosure of which is incorporated by reference herein. This reference discloses a screening apparatus in which there are two separate loading and screening stations but only one unloading station. Two ceramic greensheets may be loaded and screened at the same time but they must be unloaded alternately as there is only one unload station. To move from one screening station to the other, it is necessary to dismount the greensheet to the unloading station, dry the greensheet, and then remount it at the second screening station.  
           [0009]    It would be desirable to be able to move from one screening station without the necessity of dismounting the greensheet and then remounting it.  
           [0010]    Accordingly, it is an object of the present invention to have a screening method in which a ceramic greensheet may be moved from one screening station to another without dismounting the ceramic greensheet from the screening stage.  
           [0011]    It is another object of the present invention to have a screening method which allows the screening of a paste tailored for the filling of via holes and the screening of a second paste tailored for the deposition of patterns.  
           [0012]    It is yet another object of the present invention to have a screening method wherein a ceramic greensheet is displaced from a first position to a second position while remaining in place on a screening stage so that realignment of the ceramic greensheet to the screening stage is unnecessary.  
           [0013]    These and other objects of the present invention will become more apparent after referring to the following description of the invention in conjunction with the accompanying drawings.  
         BRIEF SUMMARY OF THE INVENTION  
         [0014]    The purposes of the invention have been achieved by providing, according to a first aspect of the invention, a method for displacing an article using first and second pallets for holding and transporting an article between two process stations and an H-bar assembly for receiving the first and second pallets, the method comprising the steps of:  
           [0015]    receiving the first and second pallets and articles thereon by the H-bar assembly at one of the two process stations;  
           [0016]    performing a first process on the articles while received by the H-bar assembly;  
           [0017]    moving the first and second pallets and articles thereon to a second process station; and  
           [0018]    performing a second process on the articles while at the second process station.  
           [0019]    According to a second aspect of the invention, there is provided a method for displacing an article using first and second pallets for holding and transporting an article from a first position to a second position and a rotatable H-bar assembly for receiving the first and second pallets at the second position, the method comprising the steps of:  
           [0020]    moving the first and second pallets and articles thereon from the first position to the second position;  
           [0021]    receiving the first and second pallets and articles thereon by the H-bar assembly at the second position;  
           [0022]    rotating the H-bar assembly a predetermined amount so as to exchange positions of the first and second pallets and articles thereon; and  
           [0023]    moving the first and second pallets and articles thereon from the second position at the H-bar assembly to the first position.  
           [0024]    According to a third aspect of the invention, there is provided a method for displacing an article during screening using first and second paste screening stations, first and second pallets for holding and transporting an article from a first position at the paste screening station to a second position and a rotatable H-bar assembly for receiving the first and second pallets at the second position, the method comprising the steps of:  
           [0025]    screening for the first time the article on each of the pallets with a paste at the first position;  
           [0026]    moving the first and second pallets and articles thereon from the first position at the paste screening station to the second position at the H-bar assembly;  
           [0027]    receiving the first and second pallets and the articles thereon by the H-bar assembly;  
           [0028]    rotating the H-bar assembly a predetermined amount so as to exchange positions of the first and second pallets and the articles thereon;  
           [0029]    moving the first and second pallets and the articles thereon from the second position at the H-bar assembly to the first position at the paste screening station; and  
           [0030]    screening at the paste screening station for the second time the article on each of the pallets with a paste wherein the article on each of the first and second pallets has not been moved with respect to the first and second pallets between the first and second screenings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:  
         [0032]    [0032]FIG. 1 is a perspective view of the screening apparatus shown in FIG. 9 in a first, screening position with the screening stations removed.  
         [0033]    [0033]FIG. 2 is a perspective view of the screening apparatus shown in FIG. 1 in a second position.  
         [0034]    [0034]FIG. 3 is a perspective view of the screening apparatus shown in FIG. 2 where the H-bar is rotating with the first and second pallets.  
         [0035]    [0035]FIG. 4 is an enlarged perspective top view of the Hbar assembly.  
         [0036]    [0036]FIG. 5 is an enlarged perspective bottom view of the H-bar assembly.  
         [0037]    [0037]FIG. 6 is an exploded perspective view of one of the pallets and transport assemblies of the apparatus.  
         [0038]    [0038]FIG. 7 is an exploded perspective view of one of the pallets.  
         [0039]    [0039]FIG. 8 is an exploded perspective view of a nest to one of the pallets.  
         [0040]    [0040]FIG. 9 is a perspective view of screening apparatus useful for practicing the method according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]    Referring to the Figures in more detail, and particularly referring to FIG. 9, there is shown a perspective view of the screening apparatus  10  useful for practicing the method according to the present invention. One particularly preferred use of the present invention is as screening apparatus for filling via holes in and depositing patterns on ceramic greensheets  124  with a conductive paste. The present invention contemplates at least two screening stations  12 ,  14  each of which has a paste dispensing assembly  120  and a mask  122 . Paste dispensing assembly  120  is moveable along linear actuators  126 . The mask  122  shown in the screening stations  12 ,  14  is a so-called mesh mask. It should be understood that screening stations  12 ,  14  may also accommodate so-called metal masks. The screening stations  12 ,  14  also have cutout areas  128  which may be used for optical alignment or other functions. Screening stations  12 ,  14  are not part of the inventive aspects of the present invention but are shown for completeness. It should be understood that each screening station would have a load and unload station which are not shown for the sake of clarity.  
         [0042]    The screening apparatus  10  includes H-bar assembly  40  and pallets  20  which are partially obscured by the screening stations  12 ,  14 . Not shown in FIG. 9 are various apparatus such as cabinets containing electronics, safety shields, racks for moving the mask and the like which are not pertinent to the present invention.  
         [0043]    Pallets  20  are shown in FIG. 9 at their most distal points from H-bar assembly  40 . The screening apparatus  10  is capable of operating in two modes. In a first mode, a greensheet  124  is first screened at one screening station and then transferred to a second screening station for screening a second time. Thus, after screening a greensheet  124  with mask  122  and a first paste at screening station  12 , it is necessary to transfer the greensheet  124  to screening station  14  for screening of a second paste through a second mask  122 . The greensheet  124  presently at screening station  14 , having already been screened twice, will be unloaded first. Due to the desirability of maintaining registration of the greensheet  124  with the pallet  20  when being transferred from screening station  12  to screening station  14 , pallet  20  is transferred to screening station  14  along with the greensheet  124 . After unloading the greensheet  124  presently at screening station  14 , the pallet  20  at screening station  14  is transferred to screening station  12 . The net effect is that pallets  20  at screening stations  12 ,  14  are swapped. This mutual displacement of pallets  20  is accomplished by H-bar assembly  40 .  
         [0044]    In a second mode, each side of the screening apparatus  10  operates independently. Thus, a greensheet  124  is loaded onto pallet  20  while pallet  20  is captured by H-bar assembly  40 . Pallet  20  is then linearly moved to the screening station  12  or  14 , screened and then linearly moved back to H-bar assembly  40  where the greensheet  124  is unloaded. Another greensheet  124  may then be loaded onto pallet  20  to begin the process again.  
         [0045]    Referring now to FIGS.  1  to  3 , the screening apparatus  10  will be described in more detail. The screening apparatus  10  shown in FIGS.  1  to  3  has had the screening stations  12 ,  14  shown in FIG. 9 removed. As shown in FIG. 1, pallets  20  are at their distal locations with respect to H-bar assembly  40 . Pallets  20  are mounted on transfer assemblies  30  which provide air and vacuum to pallets  20  in addition to transporting pallets  20 . Transport assemblies  30  are secured to pads  16  which are linked to linear actuators  17 . Actuation of linear actuators  17  causes transport assemblies  30  to move back and forth in the directions indicated by arrows  18 . Electricity, air and vacuum are provided to transport assemblies  30  and pallets  20  through harness  22 .  
         [0046]    Referring now to FIG. 2, transport assemblies  30  and pallets  20  have been displaced to be captured by H-bar assembly  40 . To accomplish this result, transport assemblies  30  and pallets  20  have been linearly displaced by linear actuators  17  so as to be within the “H” of H-bar assembly  40 . Pallets  20  are registered with and locked into position in H-bar assembly  40  by means to be discussed hereafter. Pneumatic blocks  24  then dock with pallets  20 .  
         [0047]    Pneumatic blocks  24  are a second source of air and vacuum as air and vacuum are also provided to pallets  20  by transport assemblies  30 . Air and vacuum are necessary to maintain registration of the greensheets  124  with respect to the pallets  20 . If air and vacuum are cut off for an instant, the greensheets  124  on the pallets  20  could lose registration with the pallets  20 . Accordingly, air and vacuum need to be continuously maintained while the greensheets  124  are on pallets  20 . When control of pallets  20  is passed from transport assemblies  30  to H-bar assembly  40 , the control of air and vacuum is also seamlessly transferred from transport assemblies  30  to pneumatic blocks  24 .  
         [0048]    Thereafter, as shown in FIG. 3, H-bar assembly  40  with pallets  20  is then rotated in the direction indicated by arrow  15  until the positions of the respective pallets  20  have been swapped. At this point, pneumatic blocks  24  undock and pallets  20  are engaged by transport assemblies  30 . Transport assemblies  30  and pallets  20  are then linearly displaced by linear actuators  17  to screening stations  12 ,  14 .  
         [0049]    It should be understood that docking of the pallets with pneumatic blocks  24  and subsequent rotation of H-bar assembly  40  are only necessary when the screening apparatus  10  is operating in the first mode of the present invention.  
         [0050]    Further details of the components of the present invention will now be described with reference to FIGS. 4 through 8. Referring first to FIG. 4, there is shown an enlarged perspective view of the H-bar assembly  40 . The H-bar assembly  40  is supported by pedestal plate  26 , legs  28  and pedestals  30 . The H-bar assembly  40  comprises side plates  32  which are secured to H-bar brackets  34  which in turn are secured to H-bar plate  36 . Linear bearings  38  are secured to side plates  32  by support rails  54 . Cylinder plate  56  is secured to H-bar plate  36 . On either side of cylinder plate  56  are pneumatic block assemblies which slideably move along linear bearings  38 .  
         [0051]    Each pneumatic block assembly comprises an air cylinder  46  linked to coupler  44  which in turn is secured to push plate  42 . Push plate is secured to pneumatic block  24  by standoffs  52 . Pneumatic block  24  further includes bearing brackets  62  which allow pneumatic block  24  to slide along linear bearing  38 . In operation, pallets  20  are located on linear bearings  38 . Air cylinders/switches  48  activate pins  64  which lock pallets  20  into place in the H-bar assembly  40 . Then, air cylinders  46  in conjunction with push plates  42  push pneumatic blocks  24  along linear bearings  38 . Pneumatic blocks  24  then dock with pallets  20 . O-rings  56  denote locations in which air and vacuum are provided to pallets  20  by pneumatic blocks  24 . When undocking, travel of pneumatic blocks  24  is reversed by causing air cylinders  46  to pull push plates  42  and pneumatic blocks  24  backwards.  
         [0052]    H-bar assembly  40  is rotatable with respect to pedestal plate  26  by virtue of rotary table  58  located beneath H-bar plate  36 . The rotary table  58  is best seen in FIG. 5.  
         [0053]    Again, the docking of pneumatic blocks  24  with pallets  20  and rotation of H-bar assembly  40  are only necessary when operating in the first mode of the present invention.  
         [0054]    Referring now to FIG. 6, a transporter assembly  30  and pallet  20  are shown in more detail. Tran plate  66  supports the transporter assembly  30  and is secured to pads  16  for reciprocal movement by linear actuators  17  (as shown in FIG. 1). Actuator  72  and coupler  70  cooperate to raise and lower stage plate  80 . Stage plate  80  is loosely connected to tran plate  66  by shafts  78 , alignment bearings  76  and bushings  74 . Stage plate  80  has locator pins  82  for registration of stage plate  80  with screening stations  12 ,  14 . Harness  22  and x, y, theta table  84  are secured to the top of stage plate  80 . Pallet  20  sits on pallet base  86  which is connected to x, y, theta table  84 . Pallet base  86  has locator pins  68  for registering with pallet  20 . Air and vacuum from harness  22  are provided to pallet  20  through pallet base  86 . When pallet  20  is engaged by H-bar assembly  40 , pallet  20  separates from transporter assembly  30  by disengaging from pallet base  86 . O-rings  88  are provided on pallet base  86  to provide tight connections between pallet  20  and pallet base  86  for air and vacuum.  
         [0055]    The pallet  20  is shown in more detail in FIG. 7. Pallet  20  comprises pillow blocks  90  (one each on opposite sides) secured to pallet  20  which guide pallet  20  along linear bearing  38  of the H-bar assembly  40 . Locator blocks  92  (one each on opposite sides) and bushings  94  (one each on opposite sides)are secured to the pallet  20  proximate to the pillow blocks  92  as can be seen assembled in FIG. 6. Underneath the pallet  20  are loca pin cylinder assemblies  96 , loca pins  118  and bushings  98 , usually four each of them, which provide the location points for the ceramic greensheet  124 . Also underneath the pallet  20  are located mask pin cylinder assemblies  100 , mask pins  116  and bushings  102 , usually two each of them, for location of the mask  122 . Cover  104  fits underneath pallet  20 .  
         [0056]    Air and vacuum are provided to pallet  20  by pneumatic block  24  or transporter assembly  30  through 3-way valves  106  on the side of pallet  20 . As will be explained in more detail hereafter, when pneumatic blocks  24  dock with pallets  20 , air and vacuum are provided to pallets  20  through valves  106  extending from the sides of pallets  20 . On the other hand, when transporter assemblies  30  engage pallets  20 , air and vacuum are provided to pallets  20  through passageways on the bottom of pallets  20  which lead to valves  106 . The 3-way valves  106  can thus switch between two air and vacuum supplies. While there are five valves  106  shown, two for air and three for vacuum, the number of valves  106  can be changed of course to fit the need of the pallet  20 .  
         [0057]    Still referring to FIG. 7, valves  106  are held in place by valve plate  108 . On top of pallet  20  are located o-ring  112  which provides a seal between the pallet  20  and nest  110 . As can be seen best in FIG. 8, nest  110  further includes a porous plate  114 , preferably sintered brass or the like. The nest  110  has a number of perforations which allow a vacuum to be pulled through nest  110  and porous plate  114  to firmly hold a greensheet  124  on pallet  20 . In operation, the loca pin cylinder assemblies  96  are activated to raise the loca pins  118  so as to accurately locate a greensheet  124  which is held in place on the nest  110  and porous plate  114  by a vacuum. A mask  122  would then be accurately located on the pallet  20  by mask pins  116  of the mask pin assemblies  100 .  
         [0058]    Detailed Description of the First Mode of Operation of the Screening Apparatus:  
         [0059]    Referring first to FIG. 9, a greensheet  124  is loaded onto pallet  20  and then moved to screening station  12 . The greensheet  124  is located by loca pins  116  (as shown in FIG. 7) and held in place by a vacuum pulled through nest plate  110  (as shown in FIG. 7). Loca pins  116  can now be retracted to provide a flat surface for mask and screening operations. A mask  122  is then placed on pallet  20  and is located by mask pins  116  (as shown in FIG. 7). Since pallet  20  at screening station  14  already has a greensheet  124  in place, it is only necessary to locate a mask  122  by a similar manner on this pallet  20 .  
         [0060]    Screening stations  12 ,  14  are activated, preferably simultaneously, to screen patterns or fill via holes as desired on the greensheets  124  at these two screening stations  12 ,  14 . The mask pins  116  are lowered and the masks  122  are removed. At this point in the operation of the screening apparatus  10 , the greensheet  124  located at screening station  14  is preferably unloaded because it has already been screened twice.  
         [0061]    Referring now to FIG. 2, the transport assemblies  30  transporting pallets  20  are translated by linear actuators  17  to the H-bar assembly  40 . Linear bearings  38  (FIG. 4) engage pillow blocks  92  (FIG. 7) of the pallets  20  so as to guide the pallets  20  within the H-bar assembly  40 . After the pallets  20  are properly located within H-bar assembly  40 , the transporter assembly  30  stops moving. Air cylinders/switches  48  activate pins  64  (FIG. 4) to lock the pallets  20  in place. As best seen in FIG. 4, air cylinders  46  on the H-bar assembly  40  are activated to push pneumatic blocks  24  into docking engagement with pallets  20 . At the precise moment that air and vacuum are provided to pallets  20  through valves  106  by pneumatic blocks  24 , air and vacuum provided by the transport assembly is shut off to pallets  20 .  
         [0062]    Referring now to FIG. 6, actuators  72  through stage  20  plates  80  cause pallet bases  86  to be lowered to disengage from pallets  20 . Pallets  20  have now been completely transferred to the control of H-bar assembly  40 . As can be appreciated, air provided to loca pins  118  to locate the greensheets  124  and vaccum provided to nest plate  110  to hold down the greensheets  124  are never shut off with respect to pallets  20  during the transfer of the pallets  20  to H-bar assembly  40 .  
         [0063]    Referring now to FIG. 3, H-bar assembly  40  rotates 180° so as to swap the position of pallets  20  so that the pallet  20  having the greensheet  124  screened at screening station  12  has now been displaced so that it can be transported to screening station  14  for a second screening. Similarly, the pallet  20  previously at screening station  14  has had its greensheet  124  unloaded and is displaced so as to receive a new greensheet  124  for screening a first time at screening station  12 .  
         [0064]    The procedure described above is now reversed. Referring to FIG. 6, actuators  72  of transport assemblies  30  are activated to raise pallet bases  86  into contact with pallets  20 . At the precise moment that air and vacuum are provided to pallets  20  by transporter assemblies  30 , valves  106  are switched so that air and vacuum from pneumatic blocks  24  are shut off from pallets  20 .  
         [0065]    Again referring to FIG. 4, air cylinders/switches  48  are disengaged so that pins  64  are withdrawn, thereby releasing pallets  20  from H-bar assembly  40 . Transport assemblies and pallets  20  may now be translated by linear actuators  17  to the distal positions shown in FIG. 1.  
         [0066]    The process according to the present invention is now ready to start over.  
         [0067]    Detailed Description of the Second Mode of Operation of the Screening Apparatus:  
         [0068]    Referring first to FIG. 2, the second mode of operation begins with the pallets  20  captured by H-bar assembly  40 . Greensheets  124  are loaded onto pallets  20 . Transporter assemblies  30  transport pallets  20  to screening stations  12 ,  14  as shown in FIGS. 1 and 9. Masks  122  are placed on pallets  20  and are located by mask pins  116  (as shown in FIG. 7). Screening stations  12 ,  14  are activated to screen patterns or fill via holes as desired on the greensheets  124 .  
         [0069]    Screening apparatus  10  has been designed by the present inventors so that each side of the screening apparatus (comprising linear actuators  17 , transporter assembly  30 , pallet  20 , screener  12  or  14 , and half of H-bar assembly  40 ) can operate independently of the other side or simultaneously with it.  
         [0070]    After screening, the mask pins  116  are lowered and the masks  122  removed. Thereafter, the transport assemblies  30  transporting pallets  20  are translated by linear actuators  17  to the H-bar assembly  40 .  
         [0071]    Referring now to FIG. 2,the transport assemblies  30  transporting pallets  20  are translated by linear actuators  17  to the H-bar assembly  40 . Linear bearings  38  (FIG. 4) engage pillow blocks  92  (FIG. 7) of the pallets  20  so as to guide the pallets  20  within the H-bar assembly  40 . After the pallets  20  are properly located within H-bar assembly  40 , the transporter assembly  30  stops moving. Optionally, air cylinders/switches  48  may activate pins  64  (FIG. 4) to lock the pallets  20  in place.  
         [0072]    Screened greensheets  124  are unloaded and new or previously screened greensheets are loaded onto pallet  20  and the process begins again.  
         [0073]    It should be understood that the preferred embodiments of the present invention have been described above with reference to a screening apparatus. It can be appreciated that the present invention may have utility for other thin, flexible articles besides ceramic greensheets. Further, while the preferred embodiment of the present invention comprises two pallets and transporter assemblies such that when swapping the two pallets the H-bar assembly rotates 180°, it can be appreciated that there may be more than two transporter assemblies and pallets. Further, the screening apparatus may be configured such that the H-bar assembly rotates more or less than 180°.  
         [0074]    It will be apparent to those skilled in the art having regard to this disclosure that other modifications of this invention beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.