Abstract:
A stencil foil assembly includes a stencil foil, having a stencil pattern thereon, of a planar configuration secured to a rigid frame with an upstanding wall and a support flange emanating inwardly from the upstanding wall. The stencil foil resides on the support flange with the bottom surface of the periphery of the stencil foil being in communication with the top surface of said support flange. The outer edge of the stencil foil is positioned adjacent to the inner surface of said upstanding wall. The periphery of the stencil foil is secured to the rigid frame. The stencil foil assembly protects a user from injury by preventing contact with the sharp outer edges of the stencil foil. The assembly protects the stencil foil portion from damage and greatly facilitates handling thereof.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to the manufacture of circuit boards and pertains, more specifically, to the placement of pads of solder paste in a prescribed pattern on a printed circuit board being prepared for the surface mounting of components onto the circuit board by subsequent soldering.  
           [0002]    Current manufacturing techniques for making printed circuit boards call for the mounting of components to the circuit board by “surface mounting”. That is, rather than passing the leads of a component through holes in a circuit board and then soldering the leads at the holes, the circuit board is provided with pads of solder paste and the components are placed on the pads for soldering directly to the surface of the board.  
           [0003]    In order to place the pads of paste in appropriate locations on the circuit board, just before affixing the components to the board, a stencil-like screen is juxtaposed with the circuit board and provides a pattern of openings registered with the locations where the pads are to be placed. The paste then is spread over the screen with a squeegee to lay down the pattern of pads. The stencil screen is in the form of a thin foil which is supported by a stretcher frame in an appropriately tensioned state. The frame fits into a machine which registers the tensioned screen with the circuit board and then applies the paste. Typically, the machine also stretches the stencil to achieve the desired tension of the stencil to ensure accuracy during printing.  
           [0004]    However, the foregoing stencils pose serious safety and operational problems. For example, traditional foil stencils are potential safety hazards to those technicians that handle them due to the presence of sharp edges around the entire periphery of the foil stencil. Such sharp edges can easily cut a person handling the foil stencil and putting them at risk of exposure to infection. This is a particular concern due to the presence of solder paste, which can be toxic, in the work environment. In light of the aforementioned safety hazards, transportation and storage of foil stencils are problematic because technicians are constantly transporting foil stencils to and from a storage location during the normal course of circuit board manufacture. During this normal handling, the foil stencil undergoes bending, for example, that threatens its integrity. As a result, foil stencils are prone to damage.  
           [0005]    There have been many attempts in the prior art to address the foregoing problems with foil stencils relating to safety to the handler and damage during handling and storage. For example, it is common in the prior art to mount foil stencils to an aluminum frame with a polyester border to ensure proper tensioning of the foil stencil. While the encapsulation of the periphery of the foil stencil with polyester keeps the foil tight and covers the sharp edges from the handler, it is time consuming and expensive to encapsulate the foil stencil and requires special molding machinery. As a result, such polyester encapsulated foil stencils require an additional preparation step which typically requires outsourcing to a special subcontractor for such operation. Moreover, these encapsulated foil stencils are difficult to store and cannot be easily retrieved when stacked.  
           [0006]    In view of the foregoing, there is a demand for a frame for a foil stencil assembly that protects that handler from the sharp edges to avoid injury. There is a further demand for a foil stencil assembly that can be easily stored and retrieved. There is a demand for a foil stencil assembly that can lay flat or hang from a vertical hook in a high density storage cabinet. There is a further demand for a foil stencil assembly that is inexpensive and easy to assemble without special tools or special equipment. There is also a demand for a foil stencil assembly that has stencil tension that does not decrease over time and is machine washable. In addition, there is a further demand for a foil stencil assembly with a large print area.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention preserves the advantages of prior art stencil foil assemblies for the surface mount solder paste stencil printing industry. In addition, it provides new advantages not found in currently available assemblies and overcomes many disadvantages of such currently available assemblies.  
           [0008]    The invention is generally directed to a stencil foil assembly includes a stencil foil, having a stencil pattern thereon, of a planar configuration. The assembly also includes a rigid frame having a generally L-shaped profile with an upstanding wall and a support flange emanating inwardly from the upstanding wall. The stencil foil resides on the support flange with the bottom surface of the periphery of the stencil foil being in communication with the top surface of said support flange. The outer edge of the stencil foil is positioned adjacent to the inner surface of said upstanding wall. The periphery of the stencil foil is secured to the rigid frame. The stencil foil assembly protects a user from injury by preventing contact with the sharp outer edges of the stencil foil. The assembly protects the stencil foil portion from damage, maintains it in a tensioned condition and greatly facilitates handling thereof.  
           [0009]    The stencil foil may be secured to the support flange of the outer rigid frame in a number of different ways. Preferably, a number of studs are provided on the support flange which engage with mounting holes on the stencil foil itself. The mounting holes are, preferably, of a star washer configuration whereby they grippingly engage with the studs on the support flange. For assembly, the outer rigid frame is positioned with the top surface of the support flange facing upwards. The shape of the outer rigid frame and the stencil foil are substantially identical. The stencil foil is positioned over the support flange so that the studs of the support flange align with their corresponding mounting holes. The periphery of the stencil is pushed down onto the support flange to snap the stencil foil into place thereby securing the stencil foil to the outer rigid frame in preparation for handling and subsequent use.  
           [0010]    It is therefore an object of the present to provide a stencil foil assembly that protects the user from injury from sharp edges during handling of the foil.  
           [0011]    Another object of the present invention is to provide a stencil foil assembly that protects the stencil foil from damage during shipping and handling.  
           [0012]    It is a further object of the present invention to provide a stencil foil assembly that is inexpensive and easy to manufacture and assemble.  
           [0013]    It is a further object of the present invention to provide a stencil foil assembly that enables a larger printing area than prior art stencil foil assemblies.  
           [0014]    It is another object of the present invention to provide a stencil foil assembly that has reduced storage requirements.  
           [0015]    A further object of the present invention is to provide a stencil foil assembly that allows shorter manufacturing turn around times.  
           [0016]    Yet a further object of the present invention is to provide a stencil foil assembly that eliminates the need to return used frame castings.  
           [0017]    Another object of the present invention is to provide a stencil foil assembly that reduces setup time and the need for tools for such setup.  
           [0018]    A further object of the present invention is to provide a stencil foil assembly where the stencil tension does not decrease with extensive use.  
           [0019]    Another object of the present invention is to provide a stencil foil assembly that is machine washable. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The novel features which are characteristic of the present invention are set forth in the appended claims. However, the invention&#39;s preferred embodiments, together with further objects and attendant advantages, will be best understood by reference to the following detailed description taken in connection with the accompanying drawings in which:  
         [0021]    [0021]FIG. 1 is perspective view of the stencil foil assembly of the present invention;  
         [0022]    [0022]FIG. 2 is an exploded perspective view of the stencil foil assembly of the present invention;  
         [0023]    [0023]FIG. 3 is a top plan view of the stencil foil of FIG. 1;  
         [0024]    [0024]FIG. 4 is a cross-sectional view through the line  4 - 4  of FIG. 3;  
         [0025]    [0025]FIG. 5 is an exploded perspective view of an alternative embodiment of the present invention which employs separate star washers;  
         [0026]    [0026]FIG. 6 is a perspective view of the stencil foil assembly of the present invention being installed on a stretcher frame;  
         [0027]    [0027]FIG. 7 is a top plan view of the stencil foil assembly of the present invention in preparation for installation onto the stretcher frame;  
         [0028]    [0028]FIG. 8 is a partial top view of the stencil foil assembly of the present invention residing on the stretcher frame prior to stretching; and  
         [0029]    [0029]FIG. 9 is a partial top view of the stencil foil assembly of the present invention residing on the stretcher frame after stretching. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]    Referring first to FIGS.  1 - 4 , the foil stencil assembly  10  of the present invention is shown. For illustration purposes only, the foil stencil assembly  10  is shown and described in an orientation where a foil stencil  12  is placed on top of the rigid frame  14 . An inverted orientation is also considered within the scope of the present invention. In FIGS.  5 - 8 , the foil stencil assembly  10  is shown in this inverted orientation for installation onto a stretcher frame  16  (as shown in FIG. 5) in preparation for printing.  
         [0031]    In FIGS.  1 - 4 , the foil stencil assembly  10  is shown to include a foil stencil  12  with an outer edge  18 , a top surface  25  and a bottom surface  24 . By way of example, the configuration of the assembly  10  is shown as being square with four sides but it may be of other configurations to suit the application at hand. The foil stencil  12  includes a central region  20  in which an etched circuit board lead pattern  22  resides therethrough. With the assembly  10  in an inverted orientation over a circuit board (not shown), solder paste is drawn across the bottom surface  24  of the foil stencil  12  in the central region  20  and lead pattern  22 . Details of the foil stencil printing for circuit boards is so well known in the art, it need not be discussed in detail herein. The foil stencil  12  further includes a number of mounting apertures  26  which are, preferably, in the form of star washers with a circumferential array of gripping fingers as well as registration apertures  28  for installing the assembly on a stretcher frame  16 , as will discussed in detail below. The foil stencil  12 , as is known in the art, is typically made of brass or stainless steel and is typically in the range of 0.003 to )0.008 inches thick. In accordance with the present invention, a standard foil stencil may be employed to create the foil stencil  12  used in the assembly of the present invention. For example, during the etching process for creating the lead pattern  22  in the central portion  20  of the foil stencil  12 , the mounting apertures  26  in the form of star washers and the registration apertures  28  are easily simultaneously created.  
         [0032]    An outer frame  30  provides a rigid support for the flexible and delicate foil stencil  12 . The outer frame  30  is preferably made of plastic and ideally has a thickness in the approximate range of 70-80 mils. However, other materials and thicknesses may be employed depending on the type of circuit board and foil stencil  12 . The outer frame  30  preferably has an L-shaped profile having an upstanding wall  32  and a support flange  34  that emanates inwardly therefrom. Preferably, the support flange  34  emanates from the bottom edge of the upstanding wall  32  and is present about the entire periphery of the upstanding wall  32 . However, the support flange  34  may be notched or discontinuous (not shown) to save materials and weight. Further, the profile of the outer frame may be modified to provide proper engagement with a stretcher frame  16  (as shown in FIG. 5) onto which the foil stencil assembly  10  is installed for printing.  
         [0033]    An array of studs  36 , each having a free end  38 , emanate upwardly from the top surface  40  of the support flange  34 . For example, of a total of  28  studs are preferably provided to respectively engage with corresponding mounting apertures  26  through a peripheral region  38  of the foil stencil  12 . An number of pass-through apertures  41  are provided through the flange  34  of the outer frame  30 . To assemble the foil stencil assembly  10 , as best seen in FIG. 2, the foil stencil  12  is placed over the outer frame  30  so that the studs  36  are aligned with the corresponding mounting apertures  26 . The foil stencil  12  is then pushed down onto the mounting apertures  26 , namely the integral star washers, so that the foil stencil  12  is secured to the outer frame  30 . As shown in FIG. 4, installation of the foil stencil  12  to the outer frame  30  causes the fingers  26   a  of the star washer of the mounting aperture  26  to grab to the circumference of the stud  36  thereby securing the foil stencil  12  to the outer frame  30 . The remaining  27  studs similarly engage their corresponding mounting apertures  26  and associated star washers. When the stencil foil  12  is installed on the outer frame  34 , registration apertures  28  of the stencil foil  12  and the pass-through apertures  41  through the outer frames are aligned with one another.  
         [0034]    Alternatively, as shown in FIG. 5, separate star washers  42  may be employed where the mounting apertures  26  are open pass-through holes to secure the foil stencil  12  to the outer frame  30 . In this alternative embodiment, the studs  36  are routed through open mounting apertures  27  to grippingly engage with separate star washers  42 . In addition, the foil stencil  12  may secured to the outer frame  30  by ultrasonic welding (not shown). While the use of integrated star washers at apertures  26  through the periphery  39  of the foil stencil  12 , the foregoing alternative connection methods are also considered within the scope of the present invention.  
         [0035]    Referring to FIG. 4, the sharp outer edge  18  of the foil stencil  12  is embraced by the junction of the upstanding wall  32  and the support flange  34 . As a result, a handler of the foil stencil  12  carrying the outer frame  30  is protected by the dangerous sharp outer edge  18 . Further, as seen in FIGS.  1 - 3 , a foil stencil  12  is provided that now has a rigid, lightweight frame  30 . Such a framed foil stencil or foil stencil assembly  10  is handled with ease by a technician and can be easily stored in a compact fashion. As can be understood, the storage of bare foil stencils  12  is very cumbersome in addition to being dangerous. Searching for the correct foil stencil  12  from a group of stencil fit with an outer frame  30  of the present invention is greatly facilitated because they can be easily labeled on the outer surface  44  of the upstanding wall  32 . As a result, the foil stencil assemblies  10  can be pushed into a storage rack requiring only a {fraction ( 1 / 2 )} inch per frame or hung vertically from hooks and still be easily and quickly searched and handled.  
         [0036]    Another advantage of the foil stencil assembly  10  of the present invention is the savings in shipping costs. Typically, bare foil stencils  12  are packaged in shipping containers that are much larger than the foil stencil  12  itself to allow for additional packaging to protect the delicate foil stencil  12 . Such larger packages incur higher shipping costs. In contrast, a foil stencil  12  configured as the foil stencil assembly  10  of the present invention with outer frame  30 , the sharp edges  18  are protected and the foil stencil  12  itself is tensioned and much less susceptible to bending and other damage. Therefore, normal packaging can be used when shipping a foil stencil assembly  10  of the present invention thus avoiding the cost of larger and heavier packaging materials.  
         [0037]    The foil stencil assembly  10  of the present invention also has the advantage of being of a standard size and configuration to fit any stencil printer. For example, the frame  30  has a 20 inch by 20 inch dimension with standard registration apertures  28  for accommodating standard screen printer fixtures. Further, the foil stencil assembly  10  of the present invention has a standard 1 inch thick frame  30 . In that connection, the foil stencil assembly  10  can be easily and safely cleaned in an ultrasonic cleaner without damage to the foil stencil  12 .  
         [0038]    Unlike prior art foil stencils, the foil stencil assembly  10  of the present invention can be quickly and easily installed on a stretcher frame for printing without fear of injury to the handler. FIGS.  6 - 9  illustrate the foil stencil assembly  10  of the present invention is installed on a stretcher frame  16 . It should be understood that stretcher frame  16  is just one example of many different types of stretcher frames or machines that can accommodate the foil stencil assembly  10  of the present invention. In FIG. 6, an exploded perspective view of the foil stencil assembly  10  in the process of being installed on a stretcher frame  16  is shown. The foil stencil assembly  10  is inverted compared to FIG. 1 so that top surface  40  of the support flange  34  faces toward the stretcher frame  16 . A top view of the foil stencil assembly  10 , in an inverted position in preparation for installation on the stretcher frame  16 , is shown in FIG. 7.  
         [0039]    The stretcher frame  16  includes a main body frame  46  with a number of pneumatically spring controlled pins  48  thereon. The registration apertures  28  (not seen in FIG. 6) through the periphery  39  of the foil stencil  12  and pass-through apertures  41  of the outer frame  30  are aligned with the pneumatically controlled pins  48  on the stretcher frame  16 . In FIG. 8, the foil stencil assembly  10  is lowered so that the pneumatic pins  48  are routed through the respective registration apertures  28  through the foil stencil assembly  10  and the corresponding pass-through apertures  41  through the outer frame  34 . The registration apertures  28 , as shown in FIG. 8, each have a preferably oblong shape with a portion with a larger diameter opening  43  and a portion with a smaller diameter opening  49 . This permits passage of the head of each of the pins  48  through each of the larger diameter portions  43  of the registration apertures  28  so that the foil stencil assembly  10  can easily rest on the top surface of the stretcher frame  16 . The corresponding pass-through apertures  41  through the outer frame  30  are slightly larger than the registration apertures  28  and do not hinder passage of the pins  48  therethrough. In FIG. 9, the pneumatic pins  48  are actuated so that they move in an outward direction as indicated by the arrows. As a result, the reduced neck portion  50  of each pin, as seen in FIG. 6, is received by the smaller diameter opening  49  of the registration apertures  28  of the foil stencil assembly  10 . With the pneumatic pins  48  engaged, the foil stencil assembly  10  is secured to the stretcher frame  16  and is stretched in preparation for printing.  
         [0040]    In view of the foregoing, the foil stencil assembly  10  of the present invention enjoys significant cost savings when stencils  12  are ordered with the outer frame  30  already installed. Alternatively, circuit board manufacturers, who manufacture their own foil stencils  12 , may install the outer frame  30  themselves. Since the outer frame  30  is very low profile, a large print area  20  is available for use and require smaller storage space than prior art foil stencil assemblies. With the foil stencil assembly  10  of the present invention, return of used frame castings is eliminated and setup can be achieved quickly without the need for tools.  
         [0041]    It would be appreciated by those skilled in the art that various changes and modifications can be made to the illustrated embodiments without departing from the spirit of the present invention. All such modifications and changes are intended to be covered by the appended claims.