Abstract:
An apparatus and method for depositing material on a surface of an electronic substrate includes a frame, a substrate support coupled to the frame to support the electronic substrate, a stencil coupled to the frame having at least one aperture to receive a material to be deposited through the aperture onto the surface of the electronic substrate, a controller that controls dispensing of material on the substrates, and a vibration system, coupled to one of the stencil, the frame and the substrate support, and coupled to the controller, that introduces a vibration to the stencil, wherein the vibration has a frequency controlled by the controller based on characteristics of at least one of the material, the stencil and the electronic substrate. Release of material from the apertures is accomplished while the vibration is applied to the stencil.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to apparatus and methods for screen printing, and more specifically to apparatus and methods for the screen printing of electronic substrates such as circuit board assemblies.  
         BACKGROUND OF THE INVENTION  
         [0002]    The manufacturing of circuit boards involves many processes, one of which is the screen printing of solder paste and adhesives onto printed circuit boards so that electronic components can thereafter be deposited onto the boards. To apply solder paste and surface mount components to a surface of a circuit board, a stencil with an aperture or a plurality of apertures defines a pattern that corresponds to the pattern of pads already disposed on the circuit board. The stencil is then placed substantially parallel to the surface of the board to be printed and the apertures are aligned with the desired pattern on the substrate surface. The solder paste or other material to be deposited is then placed on top of the stencil for deposition into the aperture or apertures and onto the board. Once the apertures are filled with material, excess material may be removed from the top of the stencil, using for example a squeegee, so that substantially all of the material that remains is in the aperture or apertures. The stencil is then separated from the board and the surface tension between the board and the material causes most of the material to stay on the board.  
           [0003]    The pads used on circuit boards are continuing to become smaller and smaller, and accordingly, the size of apertures on stencils is also becoming smaller. For small apertures, the surface tension between the material and the board is not always sufficient to cause most of the material in the aperture to transfer from the stencil to the board. Known means of improving the release of materials from stencils are described in Hertz, U.S. Pat. No. 6,138,562; and Ray, U.S. Pat. No. 5,407,488. However, it is desirable to provide a screen printer with improved material release than those known in the prior art.  
         SUMMARY OF THE INVENTION  
         [0004]    One aspect of the present invention is directed to an apparatus for performing operations on a surface of an electronic substrate. The apparatus includes a frame, a substrate support, coupled to the frame, to support the electronic substrate, a stencil coupled to the frame having at least one aperture to receive a material to be deposited through the aperture onto the surface of the electronic substrate, a controller that controls dispensing of materials on substrates, and a vibration system, coupled to one of the stencil, the frame and the substrate support and coupled to the controller, that introduces a vibration to the stencil. The vibration has a frequency controlled by the controller based on characteristics of at least one of the material, the stencil and the electronic substrate.  
           [0005]    The vibration system can include an actuator coupled to the stencil to cause vibration of the stencil. The vibration system can include an actuator coupled to the substrate support to cause vibration of the substrate. The vibration system can include an actuator coupled to the frame to cause vibration of the frame. At least one aperture can have an aperture size, and the controller can control the frequency based on the aperture size. The controller can control the frequency based on the material deposited through the aperture onto the electronic substrate. The controller can control the frequency over a range from 3000 Hz to 30,000 Hz. The electronic substrate can have a marking, whereby the controller controls the frequency based on detection of the marking on the electronic substrate. The stencil can have a marking, whereby the controller controls the frequency based on detection of the marking on the stencil.  
           [0006]    Another aspect of the present invention is directed to a method of dispensing material on a surface of an electronic substrate. The method includes loading an electronic substrate into a printing apparatus, supporting the substrate with a substrate support during dispensing, aligning a stencil above a surface of the electronic substrate, the stencil having at least one aperture having an aperture size through which material is deposited onto the surface of the electronic substrate, determining a vibration frequency based on the characteristics of at least one of the material, the stencil and the electronic substrate, depositing the material onto the surface of the electronic substrate, and vibrating one of the stencil, the frame and the substrate support at the vibration frequency.  
           [0007]    The method can further include coupling an actuator to the stencil to vibrate the stencil. The method can further include coupling an actuator to the substrate support to vibrate the substrate support. The method can further include coupling an actuator to the frame to vibrate the frame. The method can still further include controlling the vibration frequency based on the aperture size, characteristics of the material, or characteristics of the substrate. The method can further include loading a second electronic substrate into the printing apparatus and changing the vibration frequency according to characteristics of the second electronic substrate. The method can include adjusting the frequency of the vibration over a range from 3000 Hz to 30,000 Hz., and vibrating the stencil while separating the stencil from the electronic substrate following deposition of material on the electronic substrate.  
           [0008]    Yet another aspect of the invention is directed to an apparatus for dispensing material on at least one surface of an electronic substrate. The apparatus includes a frame, a substrate support, coupled to the frame, to support the electronic substrate, a stencil coupled to the frame having at least one aperture to receive a material to be deposited through the aperture onto the surface of the electronic substrate, a controller that controls dispensing of materials on substrates, and means for vibrating the stencil with an adjustable frequency based on the characteristics of at least one of the material, the stencil and the electronic substrate. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0009]    For a better understanding of the present invention, reference is made to the drawings which are incorporated herein by reference and in which:  
         [0010]    [0010]FIG. 1 is a perspective view of a printing apparatus in accordance with one embodiment of the invention;  
         [0011]    [0011]FIG. 2 is a schematic diagram of a solder paste release system in one embodiment of the present invention;  
         [0012]    [0012]FIG. 3 is a schematic diagram of a solder paste release system in another embodiment of the present invention;  
         [0013]    [0013]FIG. 4 is a schematic diagram of a solder paste release system in another embodiment of the present invention;  
         [0014]    [0014]FIG. 5 is a schematic diagram of a solder paste release system in another embodiment of the present invention; and  
         [0015]    [0015]FIG. 6 is a flow chart diagramming the solder paste release method in accordance with one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]    Embodiments of the present invention are described below with reference to screen printers or stencil printers that print solder paste onto circuit boards. As understood by those skilled in the art, embodiments of the present invention can be used with electronic substrates other than circuit boards and with machines other than screen printers.  
         [0017]    Referring to FIG. 1, a printer  10  in accordance with one embodiment of the invention that applies solder paste or other materials to substrates, such as circuit boards, is shown. The printer is an improvement over the screen printer described in U.S. Pat. No. 5,794,329 and U.S. Pat. No. 6,324,973, each of which is hereby incorporated by reference.  
         [0018]    As shown in FIG. 1, the printer  10  includes a tractor feed mechanism  12 , cameras  14  carried on a carriage  16 , a controller  18  and a monitor  19 , a squeegee/solder dispenser  28 , a stencil  50 , and an actuator  56 . The cameras  14  carried on the carriage  16  are moveable along tracks  24  so that they may move in both a linear X-axis of motion and a linear Y-axis of motion. The solder stencil  50  is attached to the printer frame above the circuit board  22 . The circuit board enters the printer on the tractor feed mechanism  12 . The squeegee/solder dispenser  28  is attached to the printer  10  in a position above the level of the solder stencil  50 . The solder stencil  50  has apertures  52  (shown in FIG. 2) through which solder is deposited on the surface of a circuit board.  
         [0019]    Boards  22  fed into the printer  10  typically have a pattern of pads or other, usually conductive surface areas onto which solder paste will be deposited. When directed by the controller  18  of the printer, the tractor feed mechanism  12  supplies boards  22  to a location where the camera  14  records an image of the circuit board  22 . The image is sent to the controller, which signals to shuttle the board  22  to a second location over a board support  20  and under the stencil  50 . Once arriving at a position under the stencil  50 , the circuit board  22  is in place for a manufacturing operation. To successfully deposit solder paste on the board  22 , the board  22  and the stencil  50  are aligned, via the controller. When the solder stencil  50  and the circuit board  22  are aligned correctly, the stencil is lowered toward the board  22  for application of the solder paste through the apertures  52 , or the board can be raised toward the stencil by a support mechanism  20 .  
         [0020]    The pattern of the apertures  52  corresponds to the pattern of conductive surfaces or pads already on the board  10 . The squeegee/solder dispenser  28 , positioned above the circuit board  22 , can vary the amount of solder paste delivered on the stencil  50  and applied by the squeegee. The squeegee  28  wipes across the stencil, thereby pushing solder paste into the stencil apertures, and from here, the solder paste will be released onto the board  22 . After solder paste has been deposited on the circuit board  22  in the proper pattern, the support mechanism  20  moves downward away from the position of the board, or the stencil moves upward away from the board, under control of the controller. The controller then controls movement of the board  22  to the next location using the tractor mechanism, where electrical components will be placed on the board  22  in position where the solder paste was printed.  
         [0021]    As discussed, during the deposition of solder paste onto the surface of the boards  22 , the stencil  50  is aligned so that apertures  52  correspond in shape and size to areas on the circuit board onto which solder paste is to be deposited. When aligned, the solder paste dispenser  28  releases solder paste or other adhesive onto the surface of the stencil whereby it fills the apertures  52 . The squeegee  28  aids in the process of filling the apertures  52  by wiping over the top surface of the stencil  50  to push the solder paste through each of the apertures  52  and onto the desired locations on the surface of the circuit board. Solder paste is released onto the surface of the board  22  when the stencil  50  is moved away from the board, or when the board  22  is moved down and away from the stencil. During this phase, it is desirable to apply a substantially consistent amount of solder paste over each surface of the board where electronic substrates will be placed.  
         [0022]    Referring to FIG. 2, because the apertures are generally small in size, in prior art printers, solder paste often gets caught in the apertures  52  as the solder paste is deposited onto the circuit board  22 . In embodiments of the present invention, to separate the solder paste from the edges of the apertures  52 , an actuator  56  is used to introduce a vibration to the stencil that aids in the removal of solder paste from the stencil  50 . The actuator may be a magnetostrictive device, manufactured by Etrema of Ames, Iowa, or it may be any device that introduces vibrations in a mechanical system to enhance the release of solder paste from the aperture of a stencil, such as piezoelectrics and voice coils.  
         [0023]    The actuator  56  is placed on the stencil  50  so that the stencil vibrates as the stencil and circuit board are separated, causing the solder paste to loosen from the sides of the apertures  52  and thereby improving the release of the solder paste onto the circuit boards. In other embodiments, as shown in FIGS. 3 and 4, the actuator  56  may be used to vibrate the frame of the stencil  54 , which thereafter creates vibration in the stencil itself. The actuator  56  can be attached to the stencil frame to cause vibration in the horizontal direction, as shown in FIG. 4, or it can be attached to cause vibration in the vertical direction, as shown in FIG. 3.  
         [0024]    An additional embodiment, shown in FIG. 5, depicts the actuator  56  in contact with the substrate support system  20 , which is in contact with the circuit board  22 . When the circuit board  22  is vibrated, the stencil is vibrated, causing solder paste in contact with the stencil to release to the surface of the substrate. By vibrating the stencil  50  or the substrate  22 , solder paste will be transferred from the stencil to the circuit board to provide substantially consistent volumes of paste during printing operations. The vibration originating with the actuator  56  can be applied during filling of the apertures  52  with solder paste and during release of the solder paste onto the circuit board  22 .  
         [0025]    Typically, the size and shape of the apertures, the number of apertures, the particular solder paste used, and the thickness of the apertures, for example, are characteristics that affect the release of solder paste from the apertures for deposit onto the circuit boards. In embodiments of the present invention, the frequency and/or amplitude of vibration applied to the stencil or board is optimized based on the characteristics of the stencil, board and material. In one embodiment, the controller controls the frequency and/or amplitude of the vibration to be selected in accordance with the size of the apertures and the type of solder paste. The frequency can be selected by either the user or by the system itself. For example, the system may be programmed to respond to particular dimensions or materials so that it can respond with a proper vibration frequency, or a user may input a desired frequency of vibration. Additionally, the frequency may be adjusted by using the controller, where the controller is programmed to detect distinct markings that appear on the stencil or on the circuit board and adjust the frequency according to the markings. In one embodiment, the frequency is controllable between 3000 Hz and 30,000 Hz. Using frequencies in this range has been found to cause a stencil displacement of 1-50 microns when the vibration is applied perpendicular to the plane of the stencil  50 .  
         [0026]    A flow chart depicting a method  69  by which the vibration system operates is found in FIG. 6. The first stage  70  is to load the circuit board  22  into the printer  10 , where it is then transported to a next location and aligned under the stencil  50  in step  72 . According to the next stage  74 , solder paste is deposited onto the surface of the circuit board  22  in a position determined by the pads already on the surface of the board. After deposition of the solder paste, the frequency or amplitude of vibration of the stencil  50  is adjusted in stage  76 , either by the controller  18  or by the user, so that the stencil  50  can be vibrated to achieve the release of the solder paste onto the circuit board  22 . The stencil  50  is then vibrated at the selected frequency in the next stage  78 , and thereafter the circuit board  22  is transported to the next printing phase with the solder paste on its surface.  
         [0027]    In the embodiments of the present invention described above, the actuator contacts the stencil, stencil frame or the substrate support system. As understood by those skilled in the art, other configurations may include an actuator that is not in contact with the stencil, stencil frame, or substrate support system, but that creates a vibration that vibrates the stencil or substrate at the desired frequencies without direct contact.  
         [0028]    In the embodiments of the present invention described above, the stencil deposits solder paste onto the surface of a circuit board. As understood by those skilled in the art, other configurations may include materials other than solder paste deposited onto other electronic substrates or some other substrate.  
         [0029]    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. The invention&#39;s limit is defined only in the following claims and the equivalents thereto.