Abstract:
A stencil for a printer used to print a substance onto a substrate using a blade that passes over the stencil during printing. In embodiments of the invention, the stencil includes a first section having a first printing pattern formed thereon, a second section having a second printing pattern formed thereon, a seam between the first section and the second section, and a strip that covers at least a portion of the seam to allow the blade to pass smoothly over the seam.

Description:
RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application Serial No. 60/064,938, filed Nov. 7, 1997, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a stencil apparatus used in a stencil or screen printer and more specifically to a multiple image stencil apparatus used in a stencil printer for printing solder paste onto a circuit board or some other similar substrate. 
     BACKGROUND OF THE INVENTION 
     In typical surface-mount circuit board manufacturing operations, a stencil printer is used to print solder paste onto a circuit board. Typically, a circuit board having a pattern of pads or some other, usually conductive, surface onto which solder paste will be deposited is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to properly align the circuit board with the stencil or screen of the stencil printer. In some prior art systems, an optical alignment system is used to align the circuit board with the stencil. Examples of optical alignment systems for stencil printers are described in U.S. Pat. No. 5,060,063, issued Oct. 21, 1991 to Freeman, and in U.S. Pat. No. Re. 34,615, issued Jan. 31, 1992, also to Freeman, each of which is incorporated herein by reference. 
     Once the circuit board has been properly aligned with the stencil in the printer, the circuit board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures in the stencil and onto the board. The stencil typically consists of a thin but relatively stiff sheet of stainless steel or brass in which fine lines or apertures for the passage of solder paste have been formed, for example, by etching or laser cutting. 
     More recently, the development of dual image and dual lane solder paste stencil printers has created the need for two aperture patterns on one stencil. In particular, U.S. patent application Ser. No. 08/802,934, entitled Dual Tracking Stencil/Screen Printer, incorporated herein by reference, discloses a stencil printer having dual lanes, each of which is designed to receive a circuit board to be printed upon with solder paste. In embodiments of the invention disclosed in U.S. patent application Ser. No. 08/802,934, one stencil having two aperture images or patterns (one for each lane of the printer) is used to print solder paste on circuit boards. The two images on the stencil may be the same, for example, when each of the lanes is used for circuit boards having the same configuration, or may be different to accommodate different circuit boards (i.e., different products) in each of the lanes. In some applications, one lane of the printer is used to print solder paste on the top of a circuit board and the other lane of the printer is used to print solder paste on the bottom of the circuit board. In this application, each of the patterns on the stencil is typically unique. 
     Stencils used in solder paste stencil printers are relatively expensive and therefore, it is desirable to replace the stencils as infrequently as possible. However, it is not uncommon for a schematic change to be made to a printed circuit board requiring replacement of the stencil used to print solder paste on the circuit board. For dual-image stencils, used, for example, to print on the top and bottom of a circuit board as discussed above, the frequency of schematic changes requiring replacement of the stencil will be significantly greater than in a single image stencil. 
     For applications in which a dual lane screen printer is simultaneously used in the production of two different products, so that the two images on the stencil are different, it is often desirable to replace one of the products with a different product, to meet production demands. In these applications, a unique stencil is typically provided for each of the possible product combinations, resulting in the need to purchase and store a large number of stencils. 
     Another drawback associated with prior art dual image stencils is that when one of the images on a stencil is damaged, the entire stencil must typically be replaced, even though the other image on the stencil is still fine for printing. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention overcome drawbacks of the dual image stencils described above, by providing multiple image stencils in which one or more of the images on the stencil may be replaced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to the drawings which are incorporated herein by reference and in which: 
     FIG. 1 shows a multiple image stencil in accordance with a first embodiment of the present invention; 
     FIG. 2 shows an alternate version of the multiple image stencil shown in FIG. 1; 
     FIG. 3 shows a multiple image stencil in accordance with a second embodiment of the present invention; 
     FIG. 4 shows a multiple image stencil in accordance with a third embodiment of the present invention; 
     FIG. 5 shows a side view of the multiple image stencil shown in FIG. 4; 
     FIG. 6 shows a first alternate version of the multiple image stencil shown in FIG. 4; 
     FIG. 7 shows a second alternate version of the multiple image stencil shown in FIG. 4; 
     FIG. 8 shows a multiple image stencil in accordance with a fourth embodiment of the present invention; 
     FIG. 9 shows a side view of the multiple image stencil shown in FIG. 8; 
     FIGS. 10 and 11 show a cap used in the embodiment of FIG. 8 in greater detail; 
     FIG. 12 shows a retaining bar used in the embodiment of FIG. 8 in greater detail; 
     FIG. 13 shows a multiple image stencil in accordance with a fifth embodiment of the present invention; 
     FIG. 14 shows placement of a cap in the embodiment shown in FIG. 13; and 
     FIGS. 15 and 16 show a multiple image stencil in accordance with a sixth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     For purposes of illustration, embodiments of the present invention will now be described with reference to a stencil printer used to print solder paste onto a circuit board. One skilled in the art will appreciate, however, that embodiments of the present invention are not limited to stencil printers that print solder paste onto circuit boards, but rather, may be used in other applications that utilize a stencil or a screen. 
     FIG. 1 shows a multiple image stencil  10  in accordance with one embodiment of the present invention for use in a dual image or dual lane printer that prints solder paste onto a circuit board. The dual image stencil comprises a first stencil section  12  and a second stencil section  14 , each having a respective aperture image  16  and  18 . Each of the aperture images consists of a pattern of holes or lines formed, for example, by etching or laser cutting the stencil section. In one embodiment, each of the stencil sections is made from a thin sheet of stainless steel. 
     The stencil  10  is loaded into a frame for use in a stencil printer, so that one edge  17  of the first stencil section  12  is in contact with one edge  19  of the second stencil section. The frame may be an integral part of the stencil printer or may be separable from the stencil printer. Either the frame or the stencil printer is configured to hold the stencil sections  10  and  12  of the stencil in position during printing and to provide tension to the stencil  10  by applying pressure to the stencil in the direction of arrows  20  and  22 . It is desirable to provide tension to the stencil to maintain the stencil in a rigid manner during printing. The stencil  10  may have one or more holes or slots (not shown) at the edges of the stencil to facilitate the application of pressure to the stencil by the frame or by the stencil printer. The first and second stencil sections could also be fastened in the stencil frame using bonded polyester mesh as is known in the art. 
     In one embodiment, the stencil sections  10  and  12  are loaded into a frame, and tension is applied to the stencil using one of several techniques known in the art, such as spring-loaded fasteners, or pneumatic, hydraulic or electrically driven fasteners. In other embodiments of the present invention, the stencil  10  is fastened using the stencil support apparatus disclosed in Published PCT Application No. PCT/GB93/01021, entitled Improved Stencil or Mask For Applying Solder to Circuit Boards and Support Frame Therefor, incorporated herein by reference. 
     As shown in FIG. 2, a thin strip  24  may be placed over the seam  26  between the first stencil section  12  and the second stencil section  14  of the stencil  10 . The thin strip  24  is used to cover the seam to prevent solder paste from penetrating through the seam and to provide a smooth transition for the squeegee of the stencil printer as it passes over the seam. The thin strip  24  may be implemented using adhesive backed tape or a thin strip of metal, plastic or some other material held in place using an adhesive, a clip or some other fastening device. 
     FIG. 3 shows another embodiment of a multiple image stencil  30  for use in a dual image or dual lane printer. The multiple image stencil  30  is comprised of the same first and second stencil sections  12  and  14  as the dual image stencil  10  discussed above with reference to FIG.  1 . Multiple image stencil  30  differs from multiple image stencil  10  in that the first stencil section  12  overlaps the second stencil section  14  to prevent a continuous vertical seam through which solder paste can penetrate. In one embodiment, the width of the overlapped area W is approximately 0.50 inches. 
     The multiple image stencil  30  is mounted in a stencil printer using one of the same techniques discussed above for the stencil  10 . As with stencil  10  discussed above, when the stencil  30  is mounted in a screen printer, tension is provided to the stencil in the directions shown by arrows  20  and  22 . 
     FIG. 4 shows another embodiment of a multiple image stencil  40  for use in a dual image or dual lane printer. The multiple image stencil  40  is comprised of a first stencil section  42  and a second stencil section  44  that are similar to stencil sections  12  and  14 , except that stencil sections  42  and  44  have curled edges  43  and  45  respectively. The multiple image stencil  40  also includes a “C” clip  46  that is used to hold the stencil sections  42  and  44  together as shown in FIG. 5 without a continuous seam through which solder paste can penetrate. 
     In one embodiment, the “C” clip is constructed from a resilient material, and the clip is installed by bending the clip and placing it over the curled edges of each of the stencil sections. In another embodiment, the clip is installed by sliding it over the curled edges of the stencil sections. 
     The multiple image stencil  40  is mounted in a screen printer using one of the same techniques discussed above for the stencil  10 . The mounting of stencil  40  differs from the mounting of stencil  10  in that, when the stencil  40  is mounted in a screen printer, tension is provided to the stencil by applying pressure to the stencil in the directions shown by arrows  20  and  22  and in the directions shown by arrows  47  and  48 . 
     As shown in FIG. 6, the “C” clip may be covered with a piece of self-adhesive tape  59  to provide a smooth transition for the squeegee of the stencil printer as it passes over the “C” clip. In other embodiments, in place of the tape  59 , a metal or plastic strip may be attached to the “C” clip to provide the smooth transition. 
     In another version of the embodiment shown in FIGS. 4-6, the “C” clip is replaced by a spring-loaded clip  62 , as shown in FIG.  7 . Similar to the “C” clip, the spring-loaded clip  62  may be covered by a piece of tape  59  or a metal or plastic strip. 
     FIGS. 8-12 show another embodiment of a multiple image stencil  70  for use in a dual image or dual lane printer. The multiple image stencil  70  is comprised of a first stencil section  72  and a second stencil section  74  that are similar to stencil sections  12  and  14 , except that stencil sections  72  and  74  each have a number of square perforations  76  arranged in a line along one edge of the stencil section. A retaining bar  78 , having a triangular cross section and a number of holding pins  80 , is used to hold the stencil sections  72  and  74  together. The holding pins are arranged in a first row  82  and a second row  84  respectively on a first face  86  and a second face  87  of the retaining bar. As shown in greater detail in FIG. 12, the holding pins  80  have a square cross-section to mate with the perforations  76 . 
     A cap  88  is used to cover the retaining bar  78  to provide a smooth transition for the squeegee as it passes from one stencil section to the other. The cap  88  has a first groove  90  and a second groove  92  to mate respectively with the first row  82  and the second row  84  of holding pins. The cap  88  is made from a resilient material such as stainless steel which allows the cap to be deflected when placed on and removed from the retaining bar  78 . To place the cap  88  onto the bar, or to remove it from the bar, force is provided to the cap at the positions shown by arrows  91 ,  92  and  94  in FIG.  11 . When the cap is mounted to the retaining bar  78 , it is in a deflected state, and the spring tension of the cap holds it securely in place on the retaining bar. Although the perforations and the holding pins are shown as square, they could also be circular or of some other shape. The dual image stencil  70  is mounted in a stencil printer using one of the same techniques discussed above for stencils  10  and  40 . 
     FIGS. 13 and 14 show another embodiment of a multiple image stencil  100  for use in a dual image or dual lane printer. The multiple image stencil  100  includes the first stencil section  72  and the second stencil section  74  of the stencil  70  described above, and also includes a cap  102 . The cap  102  is similar to the cap  88  except that the grooves in the cap  88  are replaced by a first row  104  and a second row  106  of pins  108 . The stencil  100  differs from stencil  70  in that the retaining bar is not used in stencil  100 , but rather, the pins in the cap  102  are used to hold the first and second stencil sections together. As with the cap  88 , spring tension is used to hold cap  102  in place and to lock the first and second stencil sections onto the pins  108 . 
     FIGS. 15 and 16 show another embodiment of a multiple image stencil  110  for use in a dual image or dual lane printer. The multiple image stencil  110  includes a first stencil section  112  and a second stencil section  114 . In contrast with the previously described embodiments of the present invention, the first and second stencil sections are not arranged side-by-side, but rather, are placed one on top of the other. Specifically, in the embodiment shown in FIGS. 15 and 16, the first stencil section  112  is placed on top of the second stencil section  114 . Each of the stencil sections  112  and  114  have a respective aperture image  113  and  115 , similar to the aperture images of the embodiments discussed above. In addition, each of the stencil sections has a rectangular cut-out  116  and  118  having an area that is larger than the areas of the aperture images 
     As shown in FIG. 16, the aperture images and the cut-outs are arranged such that the cut-out of each stencil section is aligned with the aperture image of the other stencil section when the stencil sections are arranged with one on top of the other. The stencil  110  may be secured in a stencil printer with or without a frame in the same manner as the stencils described above. The stencil  110  is designed to receive tension in the directions shown by arrows  120 ,  122 ,  124  and  126 . 
     Using the stencil  110 , two different images may be printed on one circuit board in a dual image printer, or one image can be printed on two different circuit boards. Tape, or a similar material, could be placed at the interface between the cut-out of the first stencil section and the second stencil section to provide a smooth transition for the squeegee as it passes from the first stencil section to the second stencil section. 
     Several embodiments of dual image stencils for use in dual image or dual lane stencil printers have been described above. As understood by those skilled in the art, the present invention is not limited to stencils having two images, but rather, the embodiments described above, can be readily adapted to accommodate a number of images greater than two by, for example, utilizing a number of stencil sections greater than two. 
     Embodiments of the present invention overcome the drawbacks discussed above with prior art dual image stencils by providing multiple image stencils in which aperture images may readily be replaced. Thus, embodiments of the present invention allow only one image to be replaced when it has been damaged, when the corresponding circuit board has had a schematic design change, when manufacturing demand changes, or for any other reasons. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the scope and spirit of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting.