Abstract:
A stencil for applying a viscous material to a substrate includes a substantially planar plate affixed to the substrate, the plate having at least one aperture of a first dimension and at least one relief hole of a second dimension, the at least one aperture configured to align with a printing surface of the substrate and the at least one relief hole configured to align with a feature having a height greater than the printing surface; and a mask attached to a top surface of the plate and positioned over the at least one relief hole to protect the feature having the height greater than the printing surface from

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to stencil printing, and more particularly to squeegee printing on descended surfaces. 
         [0002]    One of the most important parts of a surface mount assembly process is the application of solder paste to a printed circuit board (PCB). One aim of this process is to accurately deposit the correct amount of solder paste onto each of the pads to be soldered. This is typically achieved by screen printing the solder paste through a stencil. It is widely believed that this part of the process, if not controlled correctly, accounts for the majority of assembly defects. 
         [0003]    In general, stencil printing, also referred to a squeegee printing, is a process by which a viscous material is deposited through aperture openings of a stencil onto a substrate. The configuration of the stencil apertures determines the basic layout of the deposits. 
         [0004]    In every squeegee printing application, a surface to be printed on must be the tallest surface because the process requires a bottom of the stencil to contact a top of the printing surface. If the printing surface is not the tallest surface, the bottom of the stencil will contact the tallest surface prior to reaching the printing surface, resulting in damage to the stencil. 
       SUMMARY OF THE INVENTION 
       [0005]    The following presents a simplified summary of the innovation described herein. This summary is not an extensive overview of the invention. It is intended to neither identify key nor critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
         [0006]    The present invention generally relates to stencil printing, and more particularly to squeegee printing on descended surfaces. 
         [0007]    In one aspect, the invention features a stencil for applying a viscous material to a substrate, the stencil including a substantially planar plate affixed to the substrate, the plate having at least one aperture of a first dimension and at least one relief hole of a second dimension, the at least one aperture configured to align with a printing surface of the substrate and the at least one relief hole configured to align with a feature having a height greater than the printing surface; and a mask attached to a top surface of the plate and positioned over the at least one relief hole to protect the feature having the height greater than the printing surface from a squeegee applying the viscous material over the plate. 
         [0008]    In another aspect, the invention features a method including providing a substrate having at least one printing area and at least one feature with a height greater than the printing area, providing a stencil comprising an aperture configured to align with the printing area and a relief hole configured to align with the feature with a height greater than the printing area, securing the substrate to a positioning fixture, securing the stencil to the substrate, aligning the aperture with the printing area and the relief hole with the feature, and securing a mask over the feature in the relief hole. 
         [0009]    These and other features and advantages will be apparent from a reading of the following detailed description and a review of the associated drawings. It is to be understood that both the foregoing general description and the following detailed description are explanatory only and are not restrictive of aspects as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The features and advantages of the invention are apparent from the following description taken in conjunction with the accompanying drawings in which: 
           [0011]      FIG. 1  is a cross-section of a first exemplary stencil printing application. 
           [0012]      FIG. 2  is a cross-section of a second exemplary stencil printing application. 
           [0013]      FIG. 3  is a cross-section of a third exemplary stencil printing application. 
           [0014]      FIG. 4  is an illustration of an exemplary notched squeegee. 
           [0015]      FIG. 5  is a flow diagram. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The following detailed description of the invention references the accompanying drawings that illustrate specific embodiments in which the invention can be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
         [0017]    In this description, references to “one embodiment,” “an embodiment,” or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment,” “an embodiment,” or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, and so forth, described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein. 
         [0018]    As shown in  FIG. 1 , a cross-section of an exemplary stencil printing application  100  includes a PCB  105 . The PCB  105  includes one or more printing surfaces  110  (e.g., bond pads). A stencil  115  is secured to a top surface of the PCB  105 . In general, stencils, such as stencil  115 , can be manufactured from, for example, stainless steel or nickel. The stencil  115  includes apertures  120  designed to align with respective printing surfaces  110  on the PCB  105 . In general, apertures, such as apertures  120 , are formed in stencil  115  using, for example, laser cutting technology. A bottom of the stencil  115  contacts a top of the printing surfaces  110 . An angled blade  125 , referred to as a squeegee, is used to drive a viscous material  130  across a surface of the stencil  115  at a controlled speed and force. 
         [0019]    In general, squeegees, such as squeegee  125 , are manufactured from, for example, metal or plastic such as polyurethane. Example viscous materials include, but are not limited to, solder paste, glass paste, epoxy, conductive adhesives, such as silver paste, and so forth. As a result of the squeegee&#39;s  125  movement across the stencil  115 , the apertures  120  on the stencil  115  are filled with the viscous material  130 . When the stencil  115  is released from the printing surfaces  110  the resulting contents (i.e., the viscous material  130 ) of the filled apertures are transferred to the printing surfaces  110 , thereby forming deposits. 
         [0020]    In every squeegee printing application, such as application  100  described above, the printing surfaces  110  must always be the tallest surface on the PCB  105  because any squeegee printing application requires a bottom of the stencil  115  to contact the top of the printing surface(s)  110 . 
         [0021]    As shown in  FIG. 2 , a cross-section of an exemplary stencil printing application  200  includes a PCB  205 . The PCB  205  includes one or more printing surfaces  210  (e.g., bond pads) and a tall component  212 . A stencil  215  is secured to a top surface of the PCB  205 . However, while the stencil  215  includes apertures  220  designed to align with respective printing surfaces  210  on the PCB  205 , the tall component  212  prevents a bottom of the stencil  215  from contacting a top of the printing surfaces  210 . Attempts to secure the stencil  210  to the PCB  205  can result in stencil  210  deformation. In addition, when an angled blade  225 , referred to as a squeegee, is used to drive a viscous material  230  (e.g., solder paste, glass paste, epoxy, conductive adhesive, and so forth) across a surface of the stencil  215  at a controlled speed and force, the stencil  215  may also become deformed and/or the viscous material  230  entering the apertures  220  may flow uncontrollably across a top surface of the PCB  205 . 
         [0022]    As shown in  FIG. 3 , a cross-section of an exemplary stencil printing application  300  in accordance with the principles of the present invention includes a PCB  305 . The PCB  305  includes one or more printing surfaces  310  (e.g., bond pads) and a tall component  312  (also referred to as a tall feature). A stencil  315  is secured to a top surface of the PCB  305 . The stencil  315  includes apertures  320  designed to align with respective printing surfaces  310  on the PCB  305 . The stencil  315  also includes a relief hole  322 . Although in this representative cross-section only one relief hole  322  is shown, it should be appreciated that other examples may include multiple relief holes to match multiple tall features present on a PCB. The inclusion of a relief hole  322  enables a bottom of the stencil  315  to contact a top surface of the printing surfaces  310  by providing a pass through the stencil  315  for the tall component  312 . More specifically, the relief hole  322  is cut in the stencil  315  in the same location and pitch as the tall component  312 . An angled blade  325 , referred to as a squeegee, is used to drive a viscous material  330  (e.g., solder paste, glass paste, epoxy, conductive adhesive, and so forth) across a surface of the stencil  315  at a controlled speed and force. Prior to initiating squeegee  325  movement, a cover or mask  314  is adhered to the top surface of stencil  315  to protect the tall component  312  from contact with the viscous material  330  or any other contamination. The mask  314  is configured with the same pitch as the tall feature  312 . As a result of the squeegee&#39;s  325  movement across the stencil  315 , the apertures  320  on the stencil  315  are filled with the viscous material  330 . When the stencil  315  is released from the printing surfaces  310  the resulting contents (i.e., the viscous material  330 ) of the filled apertures are transferred to the printing surfaces  310 , thereby forming deposits. Removing the stencil  315  also exposes the tall component  312 . Thus, the configuration shown in  FIG. 3  enables the printing to be performed on variable height surfaces. 
         [0023]    In one embodiment, a blade of the squeegee  325  is relief cut to pass over and/or around the cover  314  on the top surface of the stencil  315 . 
         [0024]    In one example, the viscous material  330  is glass paste applied on an automotive sensor with a surface  310  that requires printing below one or more tall features  312 . 
         [0025]    As shown in  FIG. 4 , during one exemplary manufacturing, a stencil  410  covers a number of automotive sensors (not shown) that are secured by a respective number of pairs of port flanges  415  to a tooling fixture (not shown). Printing patterns  420  (i.e., apertures) within the stencil  410  are aligned with respective printing surfaces on the PCBs. The stencil  410  includes relief holes (not shown) for each of the pairs of port flanges  415 . Covers  425 ,  430  are adhered to a top surface of the stencil  410  and create a channel  435  for the viscous material to print. A blade  445  of the squeegee  450  is notched to print in the channel  435  without contacting the covers  425 ,  430 . The blade  445  of the squeegee  450  travels inside the channel  435  created by the covers  425 ,  430 , printing the pattern  420  on the ports below. 
         [0026]    As shown in  FIG. 5 , a process  500  includes providing ( 510 ) a substrate having at least one printing area and at least one feature with a height greater than the at least one printing area. 
         [0027]    Process  500  provides ( 520 ) a stencil comprising an aperture configured to align with the at least one printing area and a relief hole configured to align with the at least one feature. 
         [0028]    Process  500  secures ( 530 ) the substrate to a positioning fixture. 
         [0029]    Process  500  secures ( 540 ) the stencil to the substrate, aligning the aperture with the at least one printing area and the relief hole with the at least one feature. 
         [0030]    Process  500  secures ( 550 ) a mask over the feature in the relief hole. 
         [0031]    Process  500  applies ( 560 ) a viscous material to a first end of the stencil. 
         [0032]    Process  500  drives ( 570 ) the viscous material across the stencil at a controlled speed and force with a squeegee. 
         [0033]    Process  500  releases ( 580 ) the stencil from the substrate. 
         [0034]    In summary, the present invention enables a printing to be performed on surfaces that are not the tallest surface. In a squeegee printing process where components or features prevent the bottom of the stencil from contacting the printing surface, the present invention uses relief holes cut in the stencil in the same location and pitch as the components or features. This allows the tallest components to pass through the stencil when the stencil is lowered to the printing surface. 
         [0035]    Once the components or features pass through the stencil, they need to be protected during printing cycles from printing media contamination. This is accomplished by adhering a cover mask to the top side of the stencil around the perimeters of the relief hole cuts. 
         [0036]    In some implementations, the squeegee blade is also relief cut to pass over or around the protective mask on the top surface of the stencil. 
         [0037]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. Accordingly, any apparently limiting statements are made only with regard to a particular embodiment, and are not limiting of the invention. In addition, many modifications will be appreciated by those skilled in the art to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.