Patent Abstract:
Apparatus and methods for applying solder paste to circuits, such as integrated circuits, are disclosed. The apparatus and methods comprise a squeegee blade having a pair of elongated face sides spaced apart by a selected thickness and a corresponding pair of elongated substantially parallel narrow sides spaced apart by a selected width. The elongated face sides and elongated narrow sides join together to form squeegee operating edges. The squeegee blade is free of mounting aperture as to provide four operating edges. The squeegee blade is mounted to a resilient clamping structure which applies a regular and controlled gripping force so as to avoid deformation of the squeegee blade edge due to excessive mounting force. The plurality of fasteners are received by the clamping structure for adjusting the gripping force to the squeegee blade.

Full Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to apparatus and methods of manufacturing and using a squeegee apparatus with machinery for applying solder paste to circuit connections, and more specifically, a method and apparatus for mounting the squeegee blade such that all four long edges are available for use instead of the typical two edges. The invention further relates to a method of setting up and aligning the squeegee blade to avoid errors due to deformation caused by clamping forces used to mount the resilient squeegee blade (typically made of rubber).  
         BACKGROUND OF THE INVENTION  
         [0002]    Electrical connections between circuit components and a PCB (printed circuit board) or integrated circuit substrate are typically made by solder ball connections or by dipping the side of a board into molten solder. The PCB in combination with a screen mask or stencil is typically placed connector or back side up such that only related connections are exposed by the stencil or screen mask. The combination PCB and screen mask is placed on a machine such as a MicroStar BGA Solder Ball Attaching Machine. The machine then provides a selected volume of solder paste at one edge of the PCB in front of the squeegee apparatus. The squeegee apparatus is then placed so as to contact the PCB before it is moved across the PCB. As the squeegee moves across the PCB, it evenly distributes solder paste through apertures in the screen mask or stencil to the exposed connection points. Thus, the apertures assure that the selected connections will be coated by solder paste before they receive solder. To assure even distribution of the solder paste, the squeegee blade must be carefully aligned so that the squeegee edge contacting the PCB is parallel to the surface of the PCB. Since a straight and undamaged squeegee edge is necessary to assure the even distribution of the solder paste, the squeegee blade is frequently replaced and/or reoriented such that a new and undamaged edge is available. Further, since the squeegee blade is typically made of a resilient rubber, mounting of the squeegee blade must be accomplished with extreme care to assure that the edge of the squeegee blade that will contact the PCB is parallel to the PCB. In addition, prior art methods of mounting the squeegee blade typically require attaching the blade to holding apparatus by a plurality of bolts. However, the bolts must be carefully torqued or tightened to avoid deforming the rubber due to unequal pressure being applied by the plurality of bolts. Finally, significant amounts of solder paste may be wasted if it is allowed to flow around the ends of the squeegee and beyond the PCB.  
         SUMMARY OF THE INVENTION  
         [0003]    One embodiment of the present invention provides method and apparatus for simplifying the mounting procedures to ensure a properly aligned edge of a squeegee blade and which also doubles the number of edges of the squeegee blade available for use. Another embodiment incorporates over flow end guards to prevent excess solder paste from running around the squeegee ends.  
           [0004]    According to the present invention, there is provided methods and apparatus for applying solder paste to circuit connections comprising a squeegee blade having a pair of elongated face sides spaced apart by a selected thickness and a pair of elongated substantially parallel narrow sides spaced apart by a selected width. The elongated face sides and the elongated narrow sides join together at a squeegee operating edge. There is also included a slightly resilient clamping structure defining an elongated rectangular cavity for receiving the squeegee blade. The cavity has a depth less than the selected width of the squeegee blade&#39;s first and second longer sides which are separated by a short dimension which is less than the selected thickness of the squeegee blade. Consequently, the clamping structure can apply a gripping force to the squeegee blade when the blade is received within the cavity. Also included is a plurality of fasteners, such as threaded bolts, received by the clamping structure for adjusting the gripping force applied to the squeegee blade to hold the blade firmly into the clamping structure. Unlike the prior art squeegee blades, the squeegee blade of this invention is free of mounting apertures and consequently, each squeegee blade may be oriented such that there are four available edges to provide the squeegee action. Another embodiment of the apparatus further includes solder paste overflow guards which are located at each end of the squeegee blade and perpendicular to the squeegee blade such that solder paste cannot flow beyond the edges of the circuits and be wasted.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a schematic or skeleton illustration of a squeegee apparatus for applying solder paste to circuit connections.  
         [0006]    [0006]FIG. 2 is a cross-sectional illustration showing a squeegee operation of applying solder paste through a screen mask or template to selected circuit connections.  
         [0007]    [0007]FIG. 3A is a side view and FIG. 3B is a front view of prior art squeegee apparatus.  
         [0008]    [0008]FIG. 4 is an oblique view of the prior art squeegee blade of FIGS. 3A and 3B.  
         [0009]    [0009]FIG. 5A is a side view and FIG. 5B is a front view of one embodiment of the squeegee apparatus of the present invention.  
         [0010]    [0010]FIG. 6 is an oblique view of the squeegee blade of the squeegee apparatus of the present invention shown in FIGS. 5A and 5B.  
         [0011]    [0011]FIG. 7 illustrates another embodiment of the present invention further including side guards to prevent the loss of excess solder. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0012]    Referring now to FIG. 1, there is shown a schematic view of an apparatus for applying paste to connections on circuit boards which are to be electrically connected by solder. As shown, the apparatus will include a machine bed  10  for supporting at least one printed circuit board or integrated circuit  12 . Squeegee apparatus  20  is supported by holding or support apparatus  22  which moves the squeegee apparatus  20  from a first end  24  of the machine bed  10  to an opposite end  26  of the machine bed. In operation, solder is introduced in front of the squeegee blade and then the squeegee blade is dragged across one or more printed circuit boards or integrated circuits such as integrated circuits  12 ,  14 ,  16  and  18  so as to evenly distribute the solder paste across the circuit boards. The solder paste has been applied to PCB  12  as indicated by the solid connections  21 . The solder paste has been applied to some of the connections on PCB  14  as indicated by solid connection  21 , but not to others as indicated by the empty circuit  21   a . Solder paste has not been applied to any of the connections on PCBs  16  and  18  as indicated by all of the connections being open as indicated by  21   a.    
         [0013]    Referring now to FIG. 2, there is shown an enlarged schematic of the squeegee blade  20  as it moves across the printed circuit board  14  as indicated by arrow  23 . As shown, there is also included a mask screen or stencil  28  which includes apertures, such as aperture  30 , which expose areas or connection points on the printed circuit board  14  where solder is to be used for making an electrical connection. As shown, the squeegee edge  32  is dragged or passes over the screen mask or stencil  28  and moves a roll of solder paste  34  across the screen mask  28 . In those areas of a circuit board where there are no apertures, such as aperture  30 , the squeegee edge  32  of squeegee  20  will typically wipe the screen mask or stencil  28  to clean off the solder paste  34 . However, where apertures exist, the solder paste will be forced down into the aperture  30  to provide a coating of the solder paste in those connection areas of the circuit exposed by the stencil  28 . Thus, it will be appreciated that the solder paste  34  will only be deposited where a connection is to be made as provided for by the screen mask or stencil. It should also be understood that although there is only a single aperture  30  that has been shown in FIG. 2, there typically will be a plurality (tens or hundreds) of such apertures on a circuit board which receives the solder paste as is better indicated by the connections  21  and  21   a  of FIG. 1.  
         [0014]    Referring now to FIGS. 3A and 3B, there is shown a side view and a front view, respectively, of a prior art squeegee apparatus. As shown, there is a clamping portion  36  typically mounted to the holding apparatus  22  shown in FIG. 1 by a bolt or mounting rod indicated at reference number  38 . The clamping portion  36  typically includes a ledge or cut-out area  40  for receiving a squeegee blade  42  as shown in FIGS. 3A and 3B. A perforated metal strip  44  shown in the left most portion of FIG. 3B or individual washers  45  receive threaded bolts  46  which are received by threaded apertures  48  in the clamping portion  36  as shown in FIG. 3A. FIG. 4 shows a prior art squeegee blade  42 , including a series of mounting apertures  50   a  through  50   h , that allows for the bolts  46  which pass through the perforated strip  44  and for washer  45  to also pass through the apertures  50   a  through  50   h  into the threaded aperture  48  of metal clamping portion  36  of the apparatus. Thus, referring to FIG. 3A, it can be seen that if the combination clamping structure  36  and squeegee blade  42  are moved in the direction as indicated by arrow  52 , the edge  54  of squeegee blade  42 , which is in the front or forward position, will bear against the screen mask as it moves across the one or more circuit boards and evenly distributes the solder paste. Referring to FIG. 4, it is seen that there are two possible useable edges  54  and  56  for distributing the solder paste. For example, if edge  54  becomes damaged or worn, edge  56  may be used as the contact or front edge by removing squeegee blade  42  from the clamping apparatus  36  and flipping the blade  42  over (180°). The edges  58  and  60  at the top of the solder blade  42  typically are not available for use as squeegee edges because of the mounting apertures  50   a  through  50   h.    
         [0015]    It will also be appreciated by those skilled in the art that the mounting bolts  46  used to clamp squeegee blade  42  to the clamping structure  36  must be carefully tightened or torqued to avoid distortion of the resilient rubber material used in the squeegee blade  42 . As will be appreciated by those skilled in the art, if a bolt  46  is tightened an excessive amount, the rubber will deform and may likely cause a protrusion or uneven area at the squeegee edge which contacts the screen mask and thereby makes it impossible to evenly distribute the solder paste. Such an uneven edge may result in such uneven distribution that a gap is formed such that solder paste passes under the blade edge and is not moved across the face of the screen mask.  
         [0016]    Finally, it will be appreciated that excessive solder paste indicated at reference numeral  62  in FIG. 3B accumulating in front of the edge  54  of squeegee blade  42 , may run around ends  64  and  66  and beyond the edges of the printed circuit boards such that it is not useable and will be wasted.  
         [0017]    Referring now to side view FIG. 5A and front view FIG. 5B, there is shown the squeegee apparatus of the present invention. As shown, there is again included a squeegee blade  42 A mounted to clamping apparatus  36 A. However, as is clear, mounting apparatus and squeegee blades  42 A include significant changes and improvements over the prior art apparatus. As shown, clamping apparatus  36 A is constructed from a partially resilient material, such as for example, hard rubber, and includes a backing portion  68  and a front or clamping portion  70 . Backing portion  68  and clamping portion  70  comprising clamping apparatus  36 A are typically two separate pieces or portions, but as shown, could be molded with a thin flexible rubber hinge  72 . The backing portion  68  and clamping portion  70  define an elongated rectangular cavity  74  for receiving the squeegee blade  42 A. As seen, the backing portion  68  also includes an extension  76  which provides backing support to the more flexible squeegee blade  42 A when the squeegee apparatus with blade  42 A is moved in the direction as indicated by arrow  78 . The front or clamping portion  70  of the clamping apparatus  36 A defines a series of apertures  80   a  through which threaded mounting bolts  82  pass and are received in matching apertures  80   b  of backing portion  68  such that a clamping force can be provided to secure the squeegee blade  42 A within the cavity  74  discussed above. Threaded inserts  84  may be included within the apertures  80   b  of the backing portion  68  for receiving the threaded bolts  82 . Front or clamping portion  70  further typically includes a lip portion  86  which bears against the full length of the squeegee blade  42 A to assure sufficient clamping force to maintain the squeegee blade in position. In addition, the partially resilient clamping apparatus allows its mounting bolts to simply be tightened down without having to carefully torque its bolts. Also as shown, the new clamping structure  36 A of the present invention further includes expansion spaces  88   a  and  88   b  to relieve distortion of the resilient squeegee blade  42 A.  
         [0018]    Referring now to FIG. 6, there is illustrated the squeegee blade  42 A of the present invention. As shown, the squeegee blade  42 A of FIG. 6 is free of mounting holes which were necessary for the prior art mounting technique discussed with respect to the squeegee blade shown in FIG. 4. Consequently, in addition to the two edges  54 A and  56 A shown in FIG. 6, which are available for use for squeegee operation or activity, the present blade may also be rotated such that the squeegee blade edges  58 A and  60 A are available for use. Thus, it will be appreciated that the present invention allows or provides twice the number of available squeegee blade edges for use in squeegee operations. Further, by using the hard rubber mounting structure  36 A, the clamping forces can be more easily controlled and thereby avoid areas of excess pressure which will tend to distort or deform the squeegee edges used to contact the screen mask. Further, the expansion spaces  88   a  and  88   b  as illustrated in FIG. 5A allow some expansion of the top edges to further relieve excess pressure and deformation.  
         [0019]    Referring now to FIG. 7, there is shown a further embodiment of the present invention. As shown, in addition to the improved squeegee blade apparatus  36 A as discussed above, the embodiment of FIG. 7 further includes a pair of over flow guard members  90  mounted at each end of the squeegee blade apparatus so as to prevent the excess overflow of solder paste around the ends of the squeegee blade, such as shown at  64 A. The over flow guards  90  may be mounted to either the squeegee clamping apparatus  36 A or to the holding apparatus  22  shown in FIG. 1 which supports the squeegee apparatus.  
         [0020]    Thus, there has been described unique apparatus and methods of this invention for applying solder paste by the use of squeegee blades which methods and apparatus doubles the number of available squeegee edges on a squeegee blade and also helps eliminate distortion or deformation of the squeegee blade to assure a regular undamaged and aligned squeegee blade edge is in contact with a circuit screen mask or stencil. Further, although the invention has been described with respect to specific methods and apparatus, it is not intended that such specific references be considered limitations upon the scope of the invention except as is set forth in the following claims.

Technology Classification (CPC): 7