Abstract:
A stencil printing method and apparatus for printing paste material in a given pattern on a substrate from a paste dispenser through a stencil to define the stencil pattern on a substrate. The apparatus is provided with a cleaning module that can be moved against the printing face of the stencil and moved, along a given path beneath the stencil, for cleaning the printing face of the stencil. The cleaning module includes a blade assembly mounted there on transverse to the path of movement of the cleaning module. The print apparatus further including selectively operable means for actuating the blade assembly during a select cleaning cycle to engage the leading end of its blade with the underside of the stencil as the cleaning module is passed beneath said stencil whereby the leading edge of the blade scrapes debris from the underside of said stencil. The stencil printing and cleaning method of the invention uses the steps of: dispensing a paste material through a stencil onto a substrate surface to define a given pattern thereon; and passing a cleaning module, provided with a scraping blade that can be extended above the module to contact the lower surface of the stencil to clean the underside thereof when the leading edge of the blade is in contact with the underside of the stencil.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention generally relates to a screen printing apparatus for fabrication of substrates, circuit boards and other electronic circuit components, and more particularly relates to a method and apparatus for cleaning the mask, or stencil, utilized in such screen printing arrangements.  
       BACKGROUND OF THE INVENTION  
       [0002]     The complexity and compactness of present day electronic products requires increased packing density of various conductive circuit configurations on the surface of substrates, circuit boards, and other components. One method of accomplishing this is by a contact printing apparatus such as a 265 Infinity printer apparatus commercially available from DEK International of Flemington, N.J. Such machines print the desired pattern by depositing a conductive paste, such as solder paste, through a metal stencil directly onto the surface of the component. However, the precision of such printed patterns is often compromised by paste material and other debris accumulating on the stencil surface that contacts the surface of the component being printed.  
         [0003]     At present, some such commercially available screen printers include various arrangements for facilitating the cleaning of the printing surface of the metal stencil after each printing step. For example, after a printing operation, one prior art printer automatically draws a paper strip across the printing surface of the stencil to wipe its operational surface. To enhance the cleaning of the stencil, selected chemical solvents are often disposed on to the paper strip just prior to, or during its engagement with the stencil surface. A subsequent additional step applies a vacuum through the cleaning paper to draw particles from the stencil to the paper as the paper wipes the stencil surface for a second time. Although these cleaning steps do remove some or even most of the paste residue from the printing face of the stencil they leave slight amounts of residue or debris on its printing surface of the stencil. After a number of substrates are printed it has been found such residue builds up and hardens on the printing surface of the stencil such that errors result in the pattern being printed resulting in a high substrate defect rate. To reduce this defect rate it is necessary, after printing twenty or so substrates to remove the stencil from the machine and clean the stencil face of the hardened material. This procedure results in increased machine downtime resulting in reduced production and increased cost of the component.  
         [0004]     Additionally, as electronic assembles become smaller and denser, the printed patterns also become smaller, and cleaning of the stencil becomes more critical. In this instance the number of substrates that can be printed before of the stencils must be removed and cleaned of hardened material becomes reduced as even smaller amounts of such residues on the stencil face can cause undesirable increases in machine downtime and component scrap rate. Thus, a solution that would provide for a more complete cleaning of the printing stencils without extended machine downtime or reduced production has long been sought.  
       SUMMARY OF THE INVENTION  
       [0005]     A stencil printing apparatus for printing paste material in a given pattern on a substrate in the fabrication of electronic components and assemblies comprises a paste dispenser configured for dispensing paste material through a stencil onto the substrate to define the given pattern thereon, and a cleaning module for passing beneath the stencil to clean the underside thereof having a blade extending at an angle therefrom such that its leading edge can be engaged against the underside for scraping any adherent or hardened debris from the under side of the stencil as the cleaning module passes beneath the stencil.  
         [0006]     In the printing method of the invention, a substrate, to be printed with a paste material, is disposed beneath a stencil having a pattern defined therein. It should be noted that the substrate can be a module, wafer, fixture, board or other component Paste is then dispensed, through the stencil, onto the underlying substrate to create, on the substrate, the pattern defined by the stencil. The printed substrate is then removed from beneath the stencil and the bottom of the stencil is cleaned to remove any excess paste material from the bottom of the stencil. This cleaning of the bottom of the stencil can be done in several steps. The first step consists of wiping the lower surface of the stencil with a liquid impregnated paper; the second step is a vacuum cleaning of the substrate and the third requires raising the leading edge of a blade into engagement with the underside of the stencil and passing the blade across the underside of the stencil to remove any debris not removed by the prior cleaning steps.  
         [0007]     Accordingly, it is an object of the present invention to provide an improved method of cleaning a printing stencil employed in printing apparatus designed for depositing precise patterns of solder paste, solder flux paste, or other metallic alloys and pastes thereof on such substrates.  
         [0008]     Another object of the present invention is to provide a method of employing a cleaning blade for cleaning hardened materials from the printing face of a stencil employed in stencil printing apparatus.  
         [0009]     A still further object of the invention is to provide a blade assembly operable in conjunction with other cleaning devices in a stencil printing apparatus for cleaning the printing surface of the stencil.  
         [0010]     Another object of the invention is to provide a printing apparatus that deploys a blade at an appropriate time in a select cycle of the printing apparatus for scraping the printing face or surface of the stencil.  
         [0011]     These and other objects and features of the present invention will become further apparent from the following description taken in conjunction with the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a diagrammatic sectional view of the stencil printing apparatus of a preferred embodiment of the invention for printing a precise pattern of material, such as a solder paste, on a substrate in the fabrication of components for electronic assembles.  
         [0013]      FIG. 2  is a view of the printing apparatus depicted in  FIG. 1  and illustrates a first cleaning cycle of the apparatus, following the printing of the substrate;  
         [0014]      FIG. 3  is an enlarged exploded view in perspective of the elements of the cleaning blade assembly shown in  FIGS. 1 and 2 ;  
         [0015]      FIG. 4  is an enlarged view in perspective of the cleaning blade assembly, whose elements are individually illustrated in  FIG. 3 ; and  
         [0016]      FIG. 5  is a view of the printing apparatus as embodiment of the invention illustrating a cleaning cycle utilizing operation of the blade assembly for cleaning the underside of the stencil in accordance with the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     The preferred embodiment of the invention will now be described, with regard to the figures wherein:  FIG. 1  is a diagrammatic sectional view of the stencil printing apparatus of a preferred embodiment of the invention  FIG. 2  is a view of the printing apparatus depicted in  FIG. 1 ;  FIG. 3  is an enlarged exploded view in perspective of the elements of the cleaning blade assembly shown in  FIGS. 1 and 2 ;  FIG. 4  is an enlarged view in perspective of the cleaning blade assembly illustrated in  FIG. 3 ; and  FIG. 5  is a view of the preferred embodiment printing apparatus of the invention illustrating a cleaning cycle utilizing the blade assembly in accordance with the invention.  
         [0018]      FIG. 1  is a diagrammatic cross-sectional view of a printer apparatus  10  and comprises: a paste dispensing head  12 ; a stencil  14  having an upper and lower surface  16  and  18  respectively; a substrate  20  for receiving the paste in the pattern defined by the stencil; and a cleaning module  22  positioned for cleaning of the stencil&#39;s lower surface  18 .  
         [0019]     The paste dispensing head  12  has an internal chamber  24 , filled with a suitable viscous paste material, such as a solder paste  26 , that is to be passed through the stencil  14  and deposited onto the upper surface  30  of the substrate  20  in a pattern defined by the stencil pattern, (not shown). It is, of course, to be understood that the stencil is designed to print the desired pattern required for of that particular substrate. A mesh,  32  is affixed to the bottom of the chamber  24  to aid in retaining the paste  26  in the otherwise open bottom of the chamber, until the paste is forced from the chamber  12  as will later be explained in detail with regard to operation of the printing apparatus  10 .  
         [0020]     The operation of the printing apparatus  10  is controlled by a computer system (not shown). This system, in conjunction with the monitoring of a variety of sensor signals from different areas and different operations of the apparatus, coordinates all steps in the process defined by the apparatus.  
         [0021]     The process of printing a substrate consists of the steps of: selecting a stencil  14 ; filling the chamber  12  with a suitable solder paste  26 ; placing the stencil beneath and in contact with the under side of the chamber  12 ; placing the substrate  20  beneath and in contact with the underside  18  of the stencil; and pressurizing the chamber to force a select amount of the paste  26 , from the chamber  12  through the stencil  14  where it is deposited on the surface  30  of the substrate  20 .  
         [0022]     The initial arrangement of the dispensing head  12 , the stencil  14  and the substrate  20 , in readiness for the actual printing step described above is shown in  FIG. 1 . The stencil  14 , held in a fixed position by a frame, (not shown), is moved beneath and then up against the bottom  16  of the dispensing head  12  by any convenient elevating and positioning means. With the stencil properly positioned beneath the dispensing head  12 , a substrate  20  is laterally transferred from a standby position, (not shown,) by any conventional means, such as a pick and place mechanism (not shown), into position beneath the stencil  14 . Once in position it is also raised to bring its upper surface  30  in contact with the stencil&#39;s lower surface  18 .  
         [0023]     Next, pressure, via input  28 , is applied to piston  34 , in chamber  24 , to force a selected amount of the paste  26  from the chamber  24 , through the underlying mesh  32  and stencil  14  to replicate a paste deposit of the stencil&#39;s pattern on the upper surface  30  of the substrate  20 . Once the specified amount of paste is deposited onto the surface  30 , the pressure on piston  34  is relieved to stop the flow of paste onto the surface  30 .  
         [0024]     When necessary, additional paste can be added to the chamber by any conventional means, known to those skilled in the art, so that chamber will always be ready for another dispensing cycle.  
         [0025]     At this time, the dispensing head  12  is raised to its standby position, as illustrated in  FIGS. 2 and 5 ; the apparatus is then prepared for cleaning of the stencil to occur. Hence, as the head  12  is raised or just subsequent to thereto, the substrate  20  is dropped slightly to release it from engagement with the stencil&#39;s lower surface  18  and then shifted laterally there from, by a pick and place arrangement or any other conventional transfer means, not shown, to move the now printed substrate away from the printing position. This lateral movement of the substrate  20  leaves an open path for movement of the cleaning module  22  beneath the stencil  14  for cleaning the latter, and ultimately for the subsequent positioning of the next substrate in the vacated printing position.  
         [0026]     The cleaning module  22 , of the present invention, is illustrated, in section, in  FIGS. 1, 2  and  5 . As shown in these  FIGS. 1, 2  and  5 , the module is comprised of a carriage  36  carried on a support plate  56 . The carriage  36  has an open top and supports therein a pair of spaced apart rolls, i.e., a feed roll  38  and a take up roll  40 . Centrally positioned, above and between the rolls  38  and  40 , is an axially sectioned or divided tube  44  having two sections  46  and  48 , to provide means for applying either a vacuum or a liquid to the paper passing over the tube  44 . The portion  46  is perforated and coupled to a suitable vacuum source (not shown). The portion  48  is also perforated and coupled to a liquid solvent dispensing means (not shown). A sheet of paper  42 , from a roll of paper on the feed roll  38 , is fed from the roll  38  over the tube  44  to take up roll  40 . In accordance with the present invention, the cleaning module  22  further has a blade assembly  58  mounted thereon. This blade assembly  58 , specifically illustrated in an exploded view in  FIG. 3 , is mounted on the trailing end  23  of the cleaning module  22 .  
         [0027]     In  FIG. 3 , this blade assembly  58  is comprised of a support member  64  provided with a blade  60 , formed of a material as hard as the material forming the lower face  18  of the stencil  14 . The blade  60  preferably is secured within a slot by any convenient means, such as screws, (not shown), in the support member  64 , such that it can be readily removed for replacement or sharpening of its leading edge  62  as necessary. The support member  64  is bolted to a flat on a rod  66 . At each end, the rod  66  carries extending round axles  68   a  and  68   b  configured for insertion in journals  70   a  and  70   b  respectively. These axles and journals are provided to permit positioning of the blade by rotation of the rod axles in the journals such that the leading edge  62  of the blade  60  can be positioned to contact the lower stencil face  18  at any desired angle. In the present apparatus, it was found that this angle should be approximately 60 degrees. Setscrews, (not shown) are used is to secure the rod axles in the journals such that the blade is held at the desired angle.  
         [0028]     Once the journals  70   a  and  70   b  are each secured to a respective axle  68   a ,  68   b , they are attached to a support member  72  that holds the journals in a fixed, spaced relationship to each other, to confine the rod  66  there between, thus forming a blade support subassembly  73 . This sub assembly  73  is then mounted on a support bracket  76 , which carries a pair of vertical spaced apart mounting posts  78  each of which is proved with a respective spring  82 . The distance between these posts is such that each post will engage a respective mounting hole  80  in a respective one of the journals  70   a  and  70   b.    
         [0029]     As shown in these figures, the support bracket  76  is under cut, i.e. provided with a notch  84 , between the posts, to accommodate the subassembly  73  there between such that there is sufficient clearance for journals to rest on the springs  82  of the mounting posts  78  without bottoming out on the bracket  76 . Accordingly, the springs will maintain a specified force between the blade&#39;s leading edge  62  and the lower face  18  of stencil  14  when the blade is engaged with the stencil as explained below.  
         [0030]     The support bracket  76  also carries a rack arrangement  86 , as well as a pair of upright slots  88  spaced evenly on either side of and from the rack  86  the use of which will be described below. The slots  88  are arranged to hold blade assembly  58  on the trailing edge  90  of the support bracket and to maintain the alignment of the blade assembly as it is raised to force the blade edge  62  against the bottom surface  18  of the stencil  14 .  
         [0031]     A stepper motor  92 , or other conventional means, is also mounted on the support plate  56  adjacent the trailing end of the carriage  22 . This motor is positioned so that its shaft, carrying a gear  96  fixed in engagement with the rack  86  on the subassembly  73 , will raise and lower the assembly  73  at specified times during the cleaning of the lower face  18  of the stencil  14 .  
         [0032]     Referring now to  FIGS. 1, 2 , and  5  the cleaning of the lower face  18  of the stencil  18  following the printing of the substrate  20 , as taught above in conjunction with  FIG. 1 , will now be explained. Once the above-described printing step is complete, the printed substrate  20  is removed from beneath the stencil  14 ; the paste dispenser  12  is raised from the top of the stencil  14  and the control system, initiates cleaning of the lower face  18  of stencil  14  by delivering an appropriate activation signal to the cleaning module  22  to begin the first cleaning cycle. This first cleaning cycle is, a wet paper cycle and requires moving the module  22  from its start position, as shown in  FIG. 1 , to cause it to traverse the lower face  18  of stencil  14 .  
         [0033]     As noted previously the paper  42  passes over the tube  44  that is positioned between and higher than the rolls  38  and  40 . The module  22  is raised, by the support plate  56  just as the tube  44  begins to pass beneath the leading edge of the stencil  14 . This forces the paper, as it passes over the tube  44 , into engagement with the lower face  18  to wipe the face  18  as the carriage traverses the stencil  14 .  
         [0034]     Simultaneously to enhance the cleaning of the face  18 , the fluid dispensing section  46  of pipe  44  is filled with a paste dissolving solvent that wets the paper, as it passes over the tube  44 . As the carriage  22  traverses beneath the face  18 , fresh solvent wetted paper is drawn, from the roll  38 , across the tube  44  and against the face  18 , by the take-up rolls  40 , such that the entire lower face  18  of stencil  14  is cleaned by fresh, solvent wetted paper. It is of course to be understood that the solvent dispensing pipe  46  is coupled to a controlled pressurized solvent source, (not shown) that will controllably dispense and deliver the solvent to the tube  44 . Such an arrangement is within the skill of any competent engineer.  
         [0035]     When the tube  44  reaches the trailing end of the stencil  14 , the control system ceases delivery of solvent through tube  46 , and lowers the support plate  56  so that the module  22  can return unhindered to its initial position and be ready to begin the second cleaning cycle. In the second cleaning cycle, the carriage  22  is again raised and caused to wipe, once again, the lower surface or face  18  of the stencil  14 . Now however, instead of applying a solvent to the paper, a vacuum is applied to the paper as the paper passes over the tube  44 , in contact with the lower surface or face  18  of stencil  14  to remove any loose particles or fibers from the lower stencil face  18 . Again it is to be understood that that the vacuum is created by a suitable vacuum source coupled to the tube  44  that will apply and control the amount and delivery of the vacuum to the tube  44 . Again such an arrangement is within the skill of any competent engineer. In this second cleaning cycle the vacuum, drawn through the paper  42  as it passes over the tube  44 , pulls any loose or lightly held particles from the face  18  of stencil  14  on to the paper  42  being taken up on the take up roll  40 .  
         [0036]     Again, as the tube  44  reaches the far or trailing end of the stencil  14 , the support plate  56  is lowered, the vacuum in the pipe  48  terminated and, at approximately the same time, the direction of travel of the cleaning module  22  is reversed to return the module  22  back to its start position.  
         [0037]     Although the above-described procedure is adequate and removes the bulk of any paste deposited on the surface of the stencil, a residue remains and this residue can and does build up. This residue accumulates on the lower surface of the stencil especially around the openings in the stencil and hardens. In some instances, this hardened residue can clog an opening entirely or, at the very least, will reduce the size of an opening so that the amount of paste being passed on to the substrate is insufficient causing the created solder deposit, to vary and produce solder joints which are less reliable or anomalous. In other cases, such accumulations can cause the face of the stencil to fail to properly mate with the surface of the substrate being printed. This situation can cause too much solder to be deposited at selected points resulting in improper electrical characteristics or in undesired interconnections and/or short circuits in the circuit formed on the substrate surface again increasing substrate defect rate. Prior to the present invention the only way of avoiding these problems was to periodically shut down the machine on a regular basis in order to remove and replace the stencil. This increases the down time of the machine.  
         [0038]     The present inventors have found, by introducing the present invention, i.e., the assembly  58  described in  FIGS. 3 and 4  and adding a third cleaning step employing the assembly  58 , that the above-described residue accumulation problems that create this substrate defect rate and/or machine downtime can be minimized if not avoided entirely.  
         [0039]     The procedure that incorporates this third cleaning step and uses the above described blade assembly  58 , shown in  FIGS. 3 and 4 , will now be described in detail with reference to  FIGS. 3, 4 , and  5 .  
         [0040]     The cleaning procedure of the invention begins this third cleaning step following completion of the first and second cleaning cycles described above. Immediately after the second cycle is completed, this third cycle begins. The support plate  56  again raises the cleaning module  22  just as the tube  44  begins to pass beneath the leading edge of the stencil  14 . Again, this forces the paper, as it passes over the tube  44 , into engagement with the lower face  18  to wipe the face  18  as the carriage traverses stencil. Again if desired the paper may be wetted with the paste dissolving solvent however this is generally unnecessary. Carriage  22  is now caused to begin to traverse beneath the face  18  and as the leading edge  62  of the blade  60  becomes positioned just beneath the leading edge of the stencil  14  the stepper motor  92  coupled to rack  86  is activated to raise bar  76 , carrying the blade subassembly  73 , to cause the leading edge  62  of the blade  60  to engage the lower surface  18  of stencil  14  and just compress the springs  82 . Preferably, the edge  62  of blade  60  will form an angle approximate sixty degrees (60°) with respect to the stencil&#39;s lower surface  18 . Because of the springs  82  the force with which the blade edge  62  contacts the lower face  18  of the stencil is constant. The module  22  now moves beneath the stencil  14  and the leading edge  62  of blade  60  scrapes the stencil&#39;s lower surface  18  clean of residue. When the cleaning module  22  reaches a point wherein the blade assembly  58  reaches the trailing end of the stencil  14 , the stepper motor  92  is triggered and the blade assembly  58  is lowered. Simultaneously, the support plate  56  is lowered and the cleaning module  22  is returned to its start position.  
         [0041]     It is to be noted that the stencil is typically formed of invar or stainless steel having a hardness between 70 and 97 HRB and the blade  60  is preferably formed of a similar material. A blade slightly harder than that of the material from which the stencil is formed has an improved ability to scrape residue off the lower side  18  of the stencil  30 .  
         [0042]     It should be noted that when the prior art, wet paper and vacuum cleaning cycles are regularly employed after printing of a substrate, the blade scraping step of the present invention may not need to be required after the printing of each substrate but should be employed once a selected number of substrates have been printed. The frequency of the scraping step of the invention is determined by the stencil and paste used for printing. However the present inventors have found that the typical stencil need only be scraped, in accordance with the present invention, only after twenty or more stencils have been printed. Additionally, whereas the blade cleaning step is described in the preferred embodiment in a third cycle following a wet paper and a vacuum cycle, it may be utilized alone, either before or after a respective one of these cycles or in conjunction with either or both of these cycles.  
         [0043]     This completes the description of the preferred embodiment of the invention, and since changes may be made in the above construction without departing from the scope of the invention described herein, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense. Thus, other alternatives and modifications will now become apparent to those skilled in the art without departing from the spirit and scope of the invention.