Abstract:
The present invention relates to a segmented squeegee for depositing a medium onto a surface, such as depositing solder paste onto a printed wiring board. The segmented squeegee may include a plurality of independent squeegee segments or elements, a support structure and a plurality of independent connections or linkages connecting the squeegee segments to the support structure. The segmented squeegee may be used in connection with a conventional stencil such that the independent linkages and the squeegee segments may be structured and arranged to maintain substantial contact between the stencil and the printed wiring board.

Description:
This application claims priority from U.S. provisional patent application Ser. No. 60/552,963, filed Mar. 12, 2004, entitled “SEGMENTED SQUEEGEE FOR STENCILING” and, the disclosure of which is incorporated herein, in its entirety, by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the use of squeegees in the production of printed wiring boards (“PWBs”), and more particularly to an apparatus and method for utilizing a segmented squeegee on PWBs. 
     BACKGROUND OF THE RELATED ART 
     The production of PWBs includes a variety of techniques to deposit solder paste on a substrate. One method of depositing solder paste includes stenciling. This method includes the use of a stencil with cutouts in the stencil corresponding to the desired solder pattern for a PWB. The stencil, typically constructed of metal, is applied to the surface of a PWB and solder paste is applied to the stencil. A straight, rigid edge element, commonly referred to as a squeegee, is pressed down on the stencil and is wiped or moved across the stencil to deposit an even, smooth portion of solder paste into the cutouts of the stencil. Once the squeegee and stencil are removed, a solder pattern is left behind on the PWB. 
     The print quality of the PWB depends on the consistency of the dimensions and thickness of the solder paste after deposition. The dimensions and the patterns of the stencil in the stenciling process typically control the amount and thickness of the deposited solder paste. However, accurate deposition of the solder paste requires the stencil to be flush or in contact with the surface of the PWB as solder paste is deposited. Typically, the stencil is forced into contact with the PWB by the squeegee during the stenciling process. 
     Unfortunately, the non-coplanarity of PWBs significantly affects the print quality of the stenciling process because contact between the stencil and the PWB cannot be maintained during the stenciling process. As shown in  FIG. 1 , despite downward pressure from the squeegee, the stencil may not remain in contact with the PWB during stenciling if the warpage or non-coplanarity creates valleys or low lying depressions. As a result, printing quality may be insufficient to meet minimum standards, which results in additional production costs, repetition of work, and increased use and wear on production equipment. 
     The effect on print quality is particularly problematic for large PWBs. As the size of the PWB grows, the warpage or non-coplanarity typically worsens. This makes stenciling on large sized PWBs difficult with conventional equipment. Unfortunately, most large sized boards contain warpage and non-coplanarity characteristics that are incompatible with the use of conventional equipment, even if the boards meet standard specifications (0.75% max warpage per inch), such as IPC-2221 for surface mount technology. 
     The conventional equipment available for stenciling solder paste or adhesives on large size PWBs, such as boards greater than 18×24 inches, includes a metal stencil and a long, straight, rigid squeegee. The conventional squeegee is typically greater than 18 inches long and constructed from metal, generally stainless steel. 
     Stenciling large sized PWBs using a conventional long, straight squeegee results in unacceptable print quality because the squeegee is incapable of conforming to the non-coplanarity of the large sized boards. For example, as solder paste is spread over the stencil, a long, straight, rigid squeegee rides on the peaks of a warped large sized board without adequately pressing into the low lying areas of the board. As a result, conventional squeegees inadequately maintain contact between the stencil and the surface of the PWB. Consequently, deposition of solder paste onto low lying areas of a warped large sized board is inconsistent and insufficient to meet minimum print quality. 
     In previous attempts to overcome non-coplanarity have included increasing the downward pressure from the squeegee, using a flexible squeegee, and even trying to improve the coplanarity requirements on PWBs. Unfortunately, increased pressure from the squeegee results in damage to the stencil and the underlying PWB during the stenciling process. Further, increased pressure and friction between the stencil and squeegee results in significantly increased wear of and increased replacement of stenciling equipment. 
     Another attempt includes the use of flexible squeegees, which provide some ability to conform to the contours of the PWB. However, flexible squeegees are significantly less durable and more difficult to clean. While rigid squeegees provide a durable and consistent edge, which is necessary for uniform and accurate solder deposition, flexible squeegees have edges that degrade quickly under repeated use and cleaning. 
     Finally, attempts to require more consistent and coplanar PWBs are not practical for large sized PWBs. The increased cost of producing PWB with greater copalanarity is prohibitive, especially due to the fact that much of the warpage of the PWB is due to local heating and cooling during subsequent processing of the PWB. 
     Therefore, there exists a need for a squeegee capable of improving PWB print quality and compensating for the non-coplanar characteristics of large sized boards. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention generally relates to a squeegee assembly for applying a medium to a surface. The squeegee assembly includes a plurality of squeegee segments and a support structure. The plurality of squeegee segments and the support structure are joined by a plurality of independent linkages. 
     Another embodiment of the present invention relates to a configuration of the squeegee segments. The squeegee segments are positioned in a staggered and overlapped configuration. The squeegee segments allow excess solder paste to be transferred across the stencil in the squeegee direction and in a substantially perpendicular direction to the squeegee direction. 
     Another embodiment of the present invention relates to a method of stenciling a medium onto a top surface of a substrate. The method steps include positioning a stencil on the substrate, such that the bottom surface of the stencil is in substantial contact with the top surface of the substrate and applying solder paste to the top surface of the stencil. The method also includes squeegeeing the top surface of the stencil with a plurality of independent squeegee segments in a predetermined direction and maintaining substantial contact, beneath each of the plurality of independent squeegee segments, between the bottom surface of the stencil and the top surface of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it may be believed the same will be better understood from the following description taken in conjunction with the accompanying drawings, which illustrate, in a non-limiting fashion, the best mode presently contemplated for carrying out the present invention, and in which like reference numerals designate like parts throughout the figures, wherein: 
         FIG. 1  is a front view of a prior art stencil and squeegee assembly; 
         FIG. 2  is a front view of a segmented squeegee and stencil according to one embodiment of the present invention; 
         FIG. 3  is a top view of a segmented squeegee and stencil according to one embodiment of the present invention; 
         FIG. 4  is a detailed front view of a squeegee segment according to one embodiment of the present invention; and 
         FIG. 5  is a detailed side view of a squeegee segment according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. However, one of ordinary skill in the art would readily recognize that the same principles are equally applicable to, and can be implemented in, many types of printing or stenciling processes and that any such variations do not depart from the true spirit and scope of the present invention. Moreover, in the following detailed description, references are made to the accompanying figures, which illustrate specific embodiments. Electrical, mechanical, logical and structural changes may be made to the embodiments without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense and the scope of the present invention is defined by the appended claims and their equivalents. 
     In  FIG. 1 , a prior art embodiment of a conventional stenciling assembly is shown. The conventional stenciling assembly includes a straight, long squeegee  10  held by a support structure  15  and a support link  30 . Due to the structural demands and the need to wash and clean the equipment, the support structure  15 , support link  30 , and long squeegee  10  typically are constructed from a strong rigid material such as steel. 
     The conventional stenciling assembly shown in  FIG. 1  illustrates an example of the disadvantages of the long squeegee  10 . As shown, PWBs  50  is warped such that the two sides of the PWB  50  are raised with a low lying valley in the center. Accordingly, a gap  52  between the PWB  50  and the squeegee  10  may be present during deposition of the solder paste  70 . The conventional squeegee  10  is incapable of bending or flexing with the PWB  50 . As a result, the stencil  60  does not remain in contact with the surface of the PWB  50  during deposition of solder paste  70 . 
     The stencil  60  typically controls the thickness and pattern of solder paste  70  deposited on the PWB  50 . As shown in  FIG. 1 , the center of the PWB  50  and the stencil  60  are not in contact due to the warpage and the gap  52 . Accordingly, the amount of solder paste  70  deposited on the center of the PWB  50  may be sporadic or inconsistent with the outer sides of the PWB  50 . The inconsistency may range from only slight deposition of solder paste  70  to complete lack of deposition of solder paste  70  in the low lying sections of a warped PWB  50 . 
     Due to the concentration of stresses on the high sections of a warped PWB  50  (shown as the outer sides in  FIG. 1 ), deposition of solder paste on high sections of the PWB  50  may also be too thin or otherwise inconsistent. Even slight inconsistencies in the application of solder paste  70  on a PWB  50  may be unacceptable in certain applications such as high reliability systems for space applications and/or military applications. As a result, the use of conventional stenciling equipment on large PWBs increases production costs through unnecessary repeated use and wear of equipment and low quality control. 
     It should be understood that the non-coplanarity of the PWB  50  shown in  FIG. 1  is only an example. As would be obvious to one of ordinary skill, large sized PWBs may be warped in many different configurations to which the application of the present invention is intended to apply. For example, the PWB may be warped in the opposite direction of that shown in  FIG. 1  with two sides that are lower than a high section in the center. Further, a single PWB may include multiple high sections and/or multiple low lying sections. 
     Referring now to  FIG. 2 , a segmented squeegee stenciling system according to one embodiment of the present invention is shown in a front view. As with the conventional system of  FIG. 1 , a support structure  20  provides the structural foundation for the squeegee during the stenciling process. However, instead of having a single long rigid squeegee  10 , the embodiment of the present invention shown in  FIG. 2  includes squeegee elements or segments  100 A,  100 B,  100 C, and  100 D. 
     Each of the squeegee segments  100 A,  100 B,  100 C, and  100 D are connected to the support structure  20  with a mechanical linkage or connection  105 . The linkage  105  includes a lower beam or lower squeegee holder  110 , a hinge or flexible joint  120 , a upper beam or upper squeegee holder  130 , and a biasing member  140 . The segmented squeegee assembly connects via a bracket or connector  80  to a transmission or motor for moving the segmented squeegee assembly during the process of stenciling. It should be noted that the linkages  105  are independent and permit independent movement of each of the squeegee segments  100 A,  100 B,  100 C, and  100 D. 
     The squeegee segments  100 A,  100 B,  100 C, and  100 D and their independent movement allow for each segment to exert independent forces and individually apply the soldering paste  70 . Because each segment is responsible for a smaller section of the stencil  60  and the PWB  50 , less force may be required to maintain contact between the stencil  60  and the PWB  50 . This may reduce the total amount of force required and reduce the amount of stress applied to the stencil  60  and the PWB  50  during the stenciling process. Further, the reduced total force may decrease the wear on the squeegee and stencil, prolonging the usable life of the components. 
     The biasing member  140  biases each of the squeegee segments  100 A,  100 B,  100 C, and  100 D such that the squeegee segments  100 A,  100 B,  100 C, and  100 D conform to the surface of the PWB  50  as shown in  FIG. 2 . The hinges  120  allow the squeegee segments  100 A,  100 B,  100 C, and  100 D to rotate and align the flat surface of the squeegee segments  100 A,  100 B,  100 C, and  100 D with the localized surface of the PWB  50  under each squeegee segment. The squeegee segments  100 A,  100 B,  100 C, and  100 D, as shown in  FIG. 2 , are configured to independently maintain contact between the stencil  60  and the PWB  50  despite the curvature of the PWB  50 . 
     The individual squeegee segments  100 A,  100 B,  100 C, and  100 D are shorter in length than the conventional squeegee  10 , allowing the smaller squeegee segments to press down, between the high sections of the PWB  50 , into the low-lying areas. By comparison, the long squeegee  10 , as shown in  FIG. 1 , rests on the outer sides of the PWB  50  without extending down to the gap  52 . 
       FIG. 2  also illustrates how the squeegee segments  100 A,  100 B,  100 C, and  100 D impart consistent downward forces on the stencil  60  and the PWB  50  despite whether or not the squeegee segments fall on high sections or low sections of the PWB  50 . Because of the independent action of the squeegee segments of the present invention, the stencil  60  maintains contact with the curvature of the PWB  50  during the process of solder paste deposition without forcing the PWB  50  to flex or change shape during stenciling. The biasing member  140  and the hinge  120  may apply a downward force on each of the squeegee segments  100 A,  100 B,  100 C, and  100 D to locally press the stencil  60  onto the PWB  50 . By maintaining contact between the stencil  60  and any low-lying areas of the PWB  50 , the consistency of the deposition of the solder paste  70  may be improved without additional localized stress on the high sections of the PWB. 
     The support structure  20  is illustrated as a solid plate approximately the same width as the squeegee segments  100 A,  100 B,  100 C, and  100 D. However, the support structure  20  may include hollow structures, beams, tubes, or other structures known to one skilled in the art so long as the structure is capable of withstanding the forces exerted through connector  80  and supporting the squeegee segments  100 A,  100 B,  100 C, and  100 D. The support structure  20  must also be sufficiently stiff such that the support structure  20  may react against the biasing members  140  during the stenciling process in order to press the stencil  60  into contact with the surface of the PWB  50  and to maintain consistent contact as the support structure  20  moves. 
     It should be noted that the hinge or joint  120  and the biasing member  140  are configured to provide each of the squeegee segments  100 A,  100 B,  100 C, and  100 D with two degrees of freedom of motion. The first degree of freedom includes vertical movement up and down with the biasing member  140  forcing the squeegee segments in a downward direction. The second degree of freedom includes angular movement about the joint or pivot point  120 . 
     The two degrees of freedom of motion enables each of the squeegee segments  100 A,  100 B,  100 C, and  100 D to engage to PWB  50  with as little gap between the squeegee segments  100 A,  100 B,  100 C, and  100 D and the PWB  50  as possible. For example, in  FIG. 2 , the squeegee segment  100 A engages the PWB  50  at an angle with the right hand side of the squeegee segment  100 A engaging the PWB  50  at a higher point that the left hand. The squeegee segment  100 A rotates about the joint  120  to achieve the orientation shown. 
     The biasing member  140  provides the ability to for the squeegee segments  100 A,  100 B,  100 C, and  100 D to apply consistent downward forces and to engage the PWB  50  at different elevations as seen in  FIG. 2 . For example, the squeegee segments  100 B and  100 C extend further into the low-lying areas in the center of the PWB  50 . The squeegee segments  100 A,  100 B,  100 C, and  100 D may be capable of about 0.010 inches of vertical travel and about 2 degrees of rotation about the flexible joint  120 . 
     Referring now to  FIG. 3 , the configuration of the squeegee segments  100 A,  100 B,  100 C, and  100 D according to an embodiment of the present invention is shown in a top view. The squeegee segments  100 A,  100 B,  100 C, and  100 D may be configured into a staggered and overlapped configuration. The squeegee segments may be moved across the stencil  60  and the PWB  50  in the direction shown as arrow A in  FIG. 3 . This staggered and overlapped configuration allows excess solder paste to be transferred across the stencil  60  in the direction of arrow A and in a direction perpendicular to arrow A. The staggered and overlapped configuration also allows the squeegee segments  100 A,  100 B,  100 C, and  100 D to individually interact with the PWB surface without interference with other segments to remove excess solder paste without leaving streaks or solder clumps. 
     The staggered and overlapped configuration also improves consistency by continuously removing excess solder paste forward and to the side of the PWB  50  during the stenciling process. While the prior art allows solder paste to clump and build up in front of the single squeegee  10 , the staggered and overlapped configuration directs clumps and excess solder paste to the side. This may reduce solder paste clumps and excess accumulation from creating divots or other inconsistencies in the solder paste as the squeegee segments pass over the deposition patterns in the stencil  60 . 
     The staggered and overlapped configuration of the squeegee segments  100 A,  100 B,  100 C, and  100 D includes each of the squeegee segments  100 A,  100 B,  100 C, and  100 D being angularly disposed from the direction of arrow A. The angle of each squeegee segment  100 A,  100 B,  100 C, and  100 D, as shown in  FIG. 3 , directs (as a function of squeegee speed) excess solder paste  70  toward the top of  FIG. 3  as the squeegee assembly travels in the direction of arrow A. It should be noted that, although the squeegee segments are allowed to rotate about hinge  120  as shown in  FIG. 2 , the angular position of the squeegee segments  100 A,  100 B,  100 C, and  100 D as shown in  FIG. 3  may be substantially fixed. These angular positions allow the staggered and overlapped configuration to drive excess solder paste to the side of the PWB  50  without allowing clumps to be left behind by the squeegee or to excessively build up on the squeegee during stenciling. 
     As solder paste  70  is pushed forward and to the side by the angular position of the squeegee segments  100 A,  100 B,  100 C, and  100 D, some amount of solder paste  70  falls to the side of each squeegee segment and is left behind. To avoid any solder paste  70  being left behind on the stencil  60 , the squeegee segments  100 A,  100 B,  100 C, and  100 D are staggered and overlapped. The squeegee segments  100 A,  100 B,  100 C, and  100 D are staggered in the direction of arrow A with squeegee segment  100 A being position behind squeegee segment  100 B. Likewise, squeegee segment  100 B is behind squeegee segment  100 C, which is behind squeegee segment  100 D. Further, the squeegee segments are overlapped as shown by the overlap  150  between squeegee segments  100 A and  100 B. 
     The overlap  150  and staggered positioning allows for the staggered and overlapped configuration where each squeegee segment picks up the solder paste  70  left behind by the squeegee segment in front. This staggered and overlapped effect provides a stenciling process that avoids leaving solder paste clumps behind and avoids having excessive build up of solder paste on the squeegee segments  100 A,  100 B,  100 C, and  100 D during stenciling. The squeegee segment  100 A pushes excess solder paste  70  to the side of the stencil  60  and away from the PWB  50 . Then, the excess solder paste  70  may be easily cleaned or removed without affecting the PWB  50  under the stencil  60 . 
     It should be noted that the squeegee segments  100 A,  100 B,  100 C, and  100 D provide independent action between the segments such that each segment can maintain contact, through the stencil, with the PWB as the squeegee segments are wiped across the stencil in the direction of arrow A. As opposed to the conventional squeegee  10  and the gap  52 , the squeegee segments  100 A,  100 B,  100 C, and  100 D may eliminate a substantial amount of gap  52 , as shown in  FIG. 1 , and minimize any gap under the individual segments. By reducing any gaps between the PWB  50  and the stencil  60 , the squeegee segments  100 A,  100 B,  100 C, and  100 D may reduce the uneven or inconsistent deposition of solder paste  70 . 
       FIG. 3  illustrates one embodiment of the staggered and overlapped configuration according to the present invention. However, the staggered and overlapped configuration of the squeegee segments may be changed or reconfigured without deviating from the true spirit and scope of the present invention. The angular position of the squeegee segments  100 A,  100 B,  100 C, and  100 D may be varied or even reversed such that the squeegee segments  100 A,  100 B,  100 C, and  100 D work in concert to drive the solder paste  70  forward and to either side or even both sides of the PWB  50 . 
       FIG. 3  also shows the squeegee segment  100 D as the most forward squeegee segment in the configuration. However, squeegee segment  100 A may be positioned to in the most forward position with the squeegee segments  100 B,  100 C, and  100 D staggered in the opposite direction of that shown in  FIG. 3 . It is also contemplated that the configuration of the squeegee segments could represent an arrowhead shape where the squeegee segments push solder paste to both sides of the stencil  60 . 
     It is important to note that the configuration of the squeegee segments  100 A,  100 B,  100 C, and  100 D as shown in  FIGS. 2 and 3  is only representative in nature. The number of the squeegee segments may be more or less depending on the individual size of the squeegee segments, the type of staggered and overlapped configuration, and the overall size of the stencil. Although the squeegee segments are shown having identical shape and size, the squeegee segments may be shaped and sized differently, especially to avoid squeegee segment edges from passing over sensitive components of the PWB  50  and/or cutouts in the stencil  60 . 
     Referring now to  FIG. 4 , a detail front view of the mechanical linkage  105  is shown. The link  105  connects the squeegee segments  100 A,  100 B,  100 C, and  100 D with the support structure  20 . The biasing member  140  is fastened to the support structure  20  and to the upper squeegee holder  130 . The biasing member  140  biases the upper squeegee holder  130  in a downward direction such that the squeegee segments  100 A,  100 B,  100 C, and  100 D may be biased against the stencil  60  and the PWB  50 . 
       FIG. 5  illustrates a cross sectional view of the upper squeegee holder  130 , the hinge  120 , the lower squeegee holder  110  and the squeegee segment  100 A. The pin  122  is shown passing through the upper and lower squeegee holders and creating the hinge  120 . 
     The lower squeegee holder  110  and the upper squeegee holder  130  are joined by a hinge or joint  120 . As shown in  FIG. 4 , the hinge  120  includes an end of the upper squeegee holder  130  and an end of the lower squeegee holder  110  with a hole in each. A pin  122  is placed through the holes in the upper and low squeegee holders  110  and  130  such that the lower squeegee holder  110  and the squeegee segment  100 A can rotate about the pin  122 . 
     Although, the hinge  120  is shown in  FIGS. 4 and 5  as a standard hinge, the hinge  120  may include many different types of rotating members or joints and be constructed from many different methods known to those skilled in the art. It is also contemplated that in other embodiments the connection between the upper and lower squeegee holders may not have any moving parts but may include flexible material that is designed or constructed to flex in the same direction as the hinge shown in  FIG. 4  but to be stiff in other directions. The upper and lower squeegee holders may also be a single element with a flexible or weakened section representing the hinge  120 . 
     In  FIGS. 4 and 5 , the hinge  120  is shown positioned equally between the biasing member  140  and the squeegee segment  100 A. However, the hinge  120  may be positioned at different heights. The hinge  120  may also be mounted directly to the biasing member  140  or the squeegee segment  100 A such that one of the upper or lower squeegee holders may no longer be needed. 
     The biasing member  140  is shown as a common commercially available coil spring and is made up of spring steel. The biasing member  140  accommodates an up-down movement of the holder  130  as the squeegee segments  100 A,  100 B,  100 C, and  100 D follow the height of the PWB surface. This up-down movement is typically less than 0.010 inch. An alignment pin (not shown) may be used to prevent the squeegee segments  100 A,  100 B,  100 C, and  100 D from rotating about the axis of the upper squeegee holder  130  due to the flexibility of the biasing member  140 . As would be obvious to one of ordinary skill, the alignment pin may engage the support structure  20  and the upper squeegee holder  130  as necessary to control any squeegee segment rotation about the axis of the upper squeegee holder  130  and to maintain the squeegee segment orientation as shown in  FIG. 3 . Although the biasing member is shown as a common coil spring, the biasing means may also be accomplished using other springs and elements as would be obvious to one of ordinary skill. For example, the biasing member  140  may be commercially available hard (shore D 40 to 80) rubber bumpers, or beryllium-copper springs. 
     Although the embodiment shown in  FIGS. 2-5  is directed toward the deposition of solder paste on a PWB, the segmented squeegee assembly described may be used to deposit controlled amounts of other mediums. For example, the present invention may be used to deposit adhesives dots on PWBs or to deposit controlled amounts of fill material in vias, i.e. holes in PWBs. 
     While the invention has been described with reference to the exemplary embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments without departing from the true spirit and scope. The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. In particular, although the method has been described by examples, the steps of the method may be performed in a different order than illustrated or simultaneously. Those skilled in the art will recognize that these and other variations are possible within the spirit and scope as defined in the following claims and their equivalents.