Abstract:
During the manufacture of printed circuit boards, metal stencils are used as a mask over which a blade is used to squeegee solder through stencil holes onto conducting pads of the circuit board. A typical stencil is both expensive and delicate. Even minor dents, creases, or impressions (commonly referred to as “coins”) in the stencil can result in circuit board defects. Manufacturers frequently suffer significant losses scrapping their coined stencils and idling manufacturing lines while waiting for replacement stencils to be fabricated, shipped, and installed. An apparatus and method is provided for repairing metal stencil coins.

Description:
RELATED APPLICATIONS 
     This application claims priority to and incorporates by reference my U.S. provisional application No. 61/147,120, filed Jan. 25, 2009, entitled “Metal Stencil Coin Repair Method.” 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to paintless dent repair, and more particularly to several modifications of the tools and techniques of the trade to repair surface defects such as dents, creases, and impressions on solder stencils, especially framed metal stencils, used for solder paste printing of circuit boards. 
     BACKGROUND OF THE INVENTION 
     During the manufacture of printed circuit boards, metal foil stencils are used to deposit solder. The metal stencil serves as a mask over which a blade is used to squeegee solder through stencil holes onto conducting pads of the circuit board. A variety of background information about stencils can be found in U.S. Pat. Nos. 4,486,466; 4,893,403; 4,789,096; 4,976,813; 5,359,928; 5,493,075; 5,746,127; 5,800,856; 5,825,639; 5,984,166; 6,267,818; 6,273,327; 7,190,157; 7,220,975; 7,412,923; all of which are hereby incorporated by reference. 
     A typical stencil is very delicate. Stencils often range from between about 1.5 and 8 mils of thickness—and sometimes more and sometimes less. Many are made of stainless steel or a nickel alloy. Because stencils are so thin, they are very delicate and very easily damaged. Many stencils are framed to facilitate handling and mounting of the stencil without damaging the stencil. Other stencils are frameless and flexible, and can be rolled up like a street map. 
     A typical stencil—being custom made for a particular circuit board design—is also very expensive. Many cost thousands of dollars. To create the stencil, a laser is programmed to cut thousands of small, precisely-placed apertures to serve as the stencil holes. Each circuit board may require hundreds or thousands of solder points; and each stencil may be applied as a mask to multiple circuit boards simultaneously. 
     Even a minor dent, crease, or impression (commonly referred to as a “coin”) in the stencil can result in circuit board defects. Manufacturers frequently suffer significant losses scrapping their coined stencils and idling manufacturing lines while waiting for replacement stencils to be fabricated, shipped, and installed. 
     Before this invention there was no process, known to the applicant, for repairing coined metal stencils. Rather, the normal practice in the industry is to simply dispose of the damaged stencils. Accordingly, there is a need for a method of repairing coins in stencils, in order to minimize the need to replace these expensive stencils and the losses associated with lengthy production line shutdowns. 
     SUMMARY OF THE INVENTION 
     Many years ago, the applicant, a paintless dent repair technician, was challenged to attempt to repair the coins in a metal stencil. The applicant&#39;s repair attempts at that time, using the normal tools and techniques of the paintless dent repair trade, failed. Because the metal stencils were extremely thin, attempts to repair a coin frequently resulted in greater damage. Moreover, applicant was unable to repair coins to satisfaction, that is, within the tolerance ranges required for solder application. Because applicant&#39;s efforts proved unsuccessful, the applicant temporarily discontinued those efforts. 
     Much more recently, the applicant approached the metal stencil manufacturer seeking an opportunity to renew his attempts. Over a period of several months and significant trial and error, applicant developed new tools and techniques to repair metal stencil coins. These new tools and techniques succeeded where the old tools and techniques failed, and applicant has now developed a successful and profitable method of repairing metal stencil coins. 
     Applicant&#39;s renewed efforts to repair metal stencil coins, which began in October 2008, led to a number of new tools and techniques, including the following: 
     (1) New tool tips for repairing coins. The tips first used in repair attempts were formed of metal and too sharply tipped. This would tend to scar the surface of the repair. Custom tips were developed made out of soft plastic or rubber and having various tip shapes, from sharp to very blunt, for different types of coins. These new tips give a technician much more control of how much surface area of the stencil is manipulated at one time, in order to avoid the scarring the stencil further. 
     (2) New hand tool for carrying tool tips. Many of the tools of the auto paintless dent repair business are very long, in order to provide the technician the necessary leverage to repair coins in the sheet metal. These tools provide far more leverage than is appropriate for stencil coins and are also difficult to manipulate with the delicacy and control needed for stencil coins. Repairing a damaged stencil takes a very steady hand. Applicant developed a new handle—a modification of a blank screwdriver handle. The new handle facilitates direct application (rather than a leveraged application) of pressure. It also facilitates far lighter and more delicate control of pressure. 
     (3) New and coin-type-specific pressure-application techniques. It is not necessary, in the paintless dent repair trade, to repair every cosmetic coin. But a far greater degree of perfection is required to repair a solder stencil. It is necessary to repair every critical defect on a solder stencil, or the repair effort is a wasted one. For a single critical defect left unrepaired or inadequately repaired will result in the scrapping of the stencil. Accordingly, the applicant developed procedures for repairing all areas of the metal stencils, including coins in the stencil holes or apertures, coins in the steps (clearance areas in the stencils), and restoring a smooth flat surface to all other surface areas on the stencil. Furthermore, many of the developed repair techniques involve repetitive motions not generally used in the paintless dent repair trade. 
     (4) Orientation of a straight-line light source to illuminate defects. Previously, applicant had attempted to illuminate the surface of the metal stencil with washed white lighting. The washed lighting, however, failed to reveal many defects that potentially interfered with the squeegee solder application. It also failed to provide adequate visual feedback when attempting to repair a coin. Repairing a damaged stencil requires not only a very steady hand, but also very good visual feedback and hand-eye coordination. After experimentation, applicant began using an adjustable stand-mounted straight-line light source to identify defects. Distortions in the reflected light pattern proved useful in both identifying defects, locating the proper place to apply tool tip pressure, and calibrating the amount of pressure to apply. For some stencil repairs, lines of different color tapes were applied to the light fixture and bulb to give a “straight edge” in the reflection. The lines on the light improved the ability to see more exact detail as the repair proceeded. 
     (5) Improved mounting of the stencil. Previously, applicant rested the frame on a table. In this position, it was difficult to manipulate the frame to detect a coin or make a needed repair. The frame was also susceptible to being bumped or displaced during a repair, which resulted in stencil damage. To better secure the stencil, and orient it into positions more convenient for inspection and repair, a collapsible frame was acquired and modified to secure the framed metal stencil. An apparatus was built to accommodate the frames on the stencils so they could be secured to the table using ratchet clamps. 
     (6) Use of a magnifying instrument. Again, in the paintless dent repair business, the goal is to remove cosmetic defects. Accordingly, there is no need to use a magnifying instrument to detect such defects. But the solder stencil business is primarily concerned with function, and even normally imperceptible coins can result in a failed or inadequate solder joint. Accordingly, applicant incorporated a magnifying instrument to detect and repair extremely small imperfections in the metal stencil. 
     Only a small percentage of initial attempts to repair stencil coins were successful. Also, the areas of the stencil that could be successfully repaired were initially limited. This proved unsatisfactory, for the stencil still had to be scrapped due to the unrepaired coins. Applicant&#39;s development of better tool tips, a modified handle, and significant methodological improvements resulted in a much higher, and commercially acceptable level, of repairs. With Applicant&#39;s invention, repairs can now be accomplished on any part of the surface area of the stencil. Applicant&#39;s invention even—and surprisingly—succeeds in repairing coins on the stencil holes or apertures. It also succeeds in repairing step coins and long, crease-like coins that extend the entire length of the stencil surface. 
     Applicant&#39;s modification and application of paintless dent repair techniques to the metal stencils is non-obvious for many reasons. First, the auto paintless dent repair business is a field and industry far removed from the circuit board fabrication business. Paintless dent repair is directed to fixing cosmetic defects on preformed coated sheet metals. Defects in solder stencils, by contrast, are functional, and often much smaller and visibly undetectable under normal lighting conditions. It is important to repair these defects to a degree and tolerance level to eliminate defects in the solder deposition process. 
     Second, paintless dent repair is applied to preformed coated sheet metals, whose composition, thickness and other properties are very different from the properties of a tensioned solder stencil. For example, metallic auto panels have a thickness ranging from 22-28 gauge (i.e., greater than about 12 mils, thick), plus a base coat of paint 5-8 mils thick and another clear coat of paint at least 5 mils thick. Products made of sheet metal can be bent and straightened under their own tension and thickness. By contrast, solder stencils often range from between 2 and 8 mils thickness—basically, they are as thin or thinner than, and often as delicate as, a piece of foil. They cannot be straightened under their own support. 
     Third, the tools and techniques typically used in the auto panel paintless dent repair business are not nearly delicate enough for repair of solder stencils. Indeed, the inventor&#39;s initial efforts to repair solder stencils with the typical tools and techniques of the auto paintless dent repair trade caused more damage than repair. 
     Fourth, far more perfection and consistency is required to salvage a solder stencil than to make satisfactory cosmetic repairs to an automobile body panel. While it may not matter if a few dings or coins on a car panel are left unrepaired, in the solder stencil business just a single remaining critical coin along the path of the squeegee renders the stencil worthless. It is not sufficient to repair all but one critical defect on a solder stencil. Rather, every critical defect must be repaired. Therefore, methods are needed to consistently repair a variety of different types of coins commonly encountered on solder stencils. 
     The invention has been successfully tested on creases as long as 14-18 inches across, coins less than one-eighth of an inch long or diameter, and coining damages as small as a barely visible speck of dirt surrounded by a small depression. It has been successfully applied to metal stencils with thicknesses less than 3 mils, and as little as 1 to 2 mils. Surface protrusions can be corrected to within a tolerance of 1 mil or less. Coins have been repaired to a tolerance level where they are no longer detectable by sight or touch. In short, the invention has been shown to work with the very smallest functional defects. 
     There is a long-felt need and significant advantages to the invention. Repairing coining damage on metal stencils saves time, expense and materials for manufacturers when any stencils are damaged because they will be able to do in house repairs instead of ordering a replacement stencil. With the inventive process repairs can be done on site in less than an hour using the existing damaged stencil in its frame mounting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is perspective view of a framed metal stencil illustrating a squeegee application of solder. 
         FIG. 2  is a perspective view of a framed metal stencil with a small sharp coin, revealed by light reflected from a straight-line light source, in the path of the squeegee. 
         FIG. 3  is a perspective view of an apparatus for detecting and repairing metal stencil coins. 
         FIG. 4  is a side view of a handle tool with interchangeable tips for repairing metal stencil coins. 
         FIG. 5  is a perspective view of the handle tool of  FIG. 4 . 
         FIG. 6  is a perspective view of a customized tool tip, with a sharp tip, a blunt tip, and a rounded smooth side for repairing metal stencil coins. 
         FIG. 7  is a top view of the customized tool tip of  FIG. 6 . 
         FIG. 8  is a perspective view of another tool tip for the handle tool of  FIGS. 3-4 . 
         FIG. 9  is a perspective view of yet another customized tool tip for the handle tool of  FIGS. 3-4 . 
         FIG. 10  is a top view of the customized tool tip of  FIG. 9 . 
         FIG. 11  is a perspective view illustrating a metal stencil, light source, and visual apparatus relatively positioned so that light from the light source reflects off of the metal stencil in the immediate vicinity of the defect toward the visual apparatus. 
         FIG. 12  is a perspective view illustrating the step of positioning a person&#39;s chin adjacent a top surface of the metal stencil while positioning at least one of the person&#39;s hands underneath the mounted metal stencil to hold and manipulate a hand-held tool carrying the tool tip to repair the defect. 
         FIG. 13  illustrates a distortion, caused by upwardly-applied pressure against a point on the bottom surface of a framed metal stencil, in a light pattern reflected off of the stencil&#39;s top surface from a linear light source. 
         FIG. 14  illustrates the positioning of a sharp tip to repair a small-diameter metal stencil coin. 
         FIG. 15  is a side view illustrating the gentle upward application of pressure to repair a small-diameter metal stencil coin. 
         FIG. 16  illustrates the positioning of a blunt tip to repair a large-diameter metal stencil coin. 
         FIG. 17  is a side view illustrating the gentle upward and lateral, massage-like application of tool-tip pressure to repair a large-diameter metal stencil coin. 
         FIG. 18  illustrates a multiplicity of points along a larger metal stencil coin where tool-tip pressure is applied to repair a large-diameter metal stencil coin. 
         FIG. 19  illustrates a crease or line coin in the surface of a framed metal stencil. 
         FIG. 20  illustrates the rubbing motion, in the direction of the crease, used to repair a crease or line coin in the surface of a framed metal stencil. 
         FIG. 21  illustrates a step coin in a framed metal stencil. 
         FIG. 22  illustrates the downward application of pressure to repair a step coin. 
         FIG. 23  illustrates a maze technique for training a technician in the delicate art of repairing metal stencil coins. 
         FIG. 24  illustrates a dot technique for training a technician in the delicate art of repairing metal stencil coins. 
         FIG. 25  is a flow chart illustrating several steps, some preferred and others optional or situation-specific, for identifying and repairing metal stencil coins. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one embodiment of a framed metal stencil  10 . The metal stencil  10  is held in tension, at about 40-50 pounds of pressure, by metal stencil frame  12 . The metal stencil  10  is glued to, and held taut, by screen  14 . In some embodiments, the metal stencil  10  is made of stainless steel. In yet others, it is made of a brass alloy. Generally, the metal stencil  10  is moderately reflective, providing a moderately mirror-like reflection. 
     In operation, the metal stencil  10  is precisely positioned, as a mask, over a plurality of printed circuit boards.  FIG. 1  illustrates a series of rectangles  15 . Each rectangle  15  is positioned over a corresponding circuit board. Each rectangle  15  comprises hundreds of small circular holes or apertures (not shown because of their small relative size) through which solder is squeezed onto the conducting pads of the circuit board.  FIG. 1  also illustrates the direction of travel of a squeegee  16  used to apply the solder. 
     Even small, relatively imperceptible coins or surface defects in the surface of the metal stencil  10 , whether located in the rectangles  15  themselves or in the path of the squeegee  16 , result in soldering defects. Frequently, several batches of circuit boards are soldered with resulting defects before a corresponding coin is identified in the metal stencil  10 . 
     The present invention is directed to methods and apparatus for detecting and repairing metal stencil coins.  FIG. 25  is a flowchart of one embodiment  200  of such a method. The illustrated methodological embodiment  200  includes several optional steps, and the invention is intended to cover a broad array of methods that apply some, but fewer than all, of the illustrated steps.  FIGS. 2-22  illustrate various embodiments of the apparatus, relative orientations, and motions used to carry out the invention. 
     In step  205 , the metal stencil  10  is mounted in a substantially horizontal position on a wheeled frame, stand, or table  28 . The wheeled stand  28  facilitates rapid repositioning of the metal stencil  10  for purposes of detecting and repairing coins on the metal stencil surface. 
       FIG. 3  illustrates a collapsible stand  28  that can be adjusted, via telescoping rods  11  and telescoping legs  13 , to adjust to a preferred length, width, and height. In operation, the stand  28  should be adjusted to the length and width of the metal stencil frame  12 , and ratchet clamps  32  used to hold the stencil frame  12  to the stand  28  securely so that leverage and pressure can be precisely applied to the stencil without it moving. The stand  28  should also be adjusted to a height that allows a technician—preferably while sitting, or alternatively while standing or kneeling—to place his or her chin adjacent the top surface of a mounted stencil. 
     The collapsible stand  28  optionally includes wheels  29  to facilitate fast repositioning of the table relative to a fixed light source. In this manner, the lighting can be adjusted by changing the position of the stencil mounting table. The table can also be rotated a few degrees at a time to view multiple angles under the reflection of the light to assess all areas of the stencil surface for smoothness and severity of any coined or damaged areas. 
       FIG. 3  also illustrates a straight-line light source  20 —here, two elongated florescent tubes—that is mounted, via an adjustable light stand  22 , to the collapsible stand  28 . The light stand  22  comprises several articulating and pivotally-interconnected members, allowing the light source  20  to be positioned in a variety of orientations to facilitate coin detection and repair. The adjustable tube lighting can be moved to a position to provide an optimum viewing angle to perform repair process.  FIG. 3  also illustrates a magnifying instrument  26  mounted via an articulating arm  17  to the stand  28 . 
     In step  210 —which is best illustrated in FIG.  11 —the metal stencil  10 , a straight-line light source  20 , and a visual apparatus  80  are relatively positioned so that a linear pattern of light  25  is reflected from the light source  20  off of the metal stencil  10  toward the visual apparatus  80 . 
     The straight-line light source  20  preferably comprises a fluorescent tube or other elongated light fixture. Alternatively, the light source  20  may comprise a plurality of linearly-arrayed non-linear light sources or a single non-linear light source whose light is filtered, channeled, and/or reflected to provide a straight-line source of illumination. Sometimes, it is helpful to also apply strips of translucent color tapes or masks to the light source to provide a “straight edge” in the reflected light. The visual apparatus  80  preferably consists of a pair of human eyes, but alternatively comprises a camera, light detector array, or other imaging device. 
     The relative positioning of step  210  causes light from the light source to reflect in a generally linear pattern  25 , extending to the right and left of center, off of the metal stencil  10 . Observed distortions in the linearity of the reflected light pattern  25  reveal otherwise generally imperceptible defects. Looking for nonlinear distortions or irregularities in the reflected pattern of light  25  is also useful for pinpointing where the technician is applying corrective pressure. The degree of observed nonlinear distortion is useful for calibrating the amount of pressure needed or actually applied to the coin or in the area immediately surrounding the coin. 
     In step  215 , the metal stencil  10  is progressively inspected or scanned, using this defect-detection-enhancing technique, for coins. The metal stencil  10 , the straight-line light source  20 , and the visual apparatus  80  are repeatedly re-positioned to progressively reposition the observed linear pattern of light  25  over the entire critical area (the area of the stencil  10  over which the squeegee  16  travels) of the metal stencil  10 . This may be accomplished by moving the visual apparatus  80  and changing the selected position of the light source through multiple positions to inspect the stencil for coins. This may be alternatively or additionally accomplished by moving a stand  28  on which the framed metal stencil  10  is mounted through multiple positions in order to inspect the metal stencil  10  for coins. When a defect is observed, it is optionally marked with a non-permanent marker. 
     The repair process for a detected coin begins with step  220 . In step  220 , the technician identifies the direction in which a surface protrusion of a detected coin is facing. Also, the metal stencil is remounted, if necessary, so that the defective surface protrusion is facing downward. 
     Step  225  is a more particular repeat of step  210 , with the metal stencil  10 , straight-line light source  20 , and a visual apparatus  80  relatively positioned to position the reflected light pattern  25  on and to the right and left of a previously identified coin. In one embodiment, this accomplished by repositioning the straight-line light source  20  to project its light at an angle toward the metal stencil coin so that it reflects off of the coin in the direction of the visual apparatus. In another embodiment, this is accomplished by rolling and re-orienting the stand  28  to a different location relative to the light source  20 . In yet another embodiment, this is accomplished by repositioning the visual apparatus  80 . In any event, the angle of incidence  36  at the location of the metal stencil coin between the incoming light and the plane of the metal stencil  10  is approximately equal to the angle of reflection  37 , relative to the same plane, toward the visual apparatus  80 .  FIGS. 2 ,  3 , and  11  all illustrate coins  30  or  31  illuminated by a straight-line light reflection pattern  25 . 
     In step  230 , a frame-mounted magnifying instrument  26  ( FIG. 3 ), preferably a magnifying glass, is positioned between the defect and the visual apparatus to further enhance the detectibility of the defective surface protrusion. 
     In step  235 , the technician determines or categorizes the depth and diameter of the detected coin and selects a tool tip and pressure technique appropriate for that category or degree of coining damage. The extent to which the reflected light pattern is distorted provides a good indication of the length, size and depth of the coin. Preferably, a hand tool with a polymer or polymer-coated tip is selected that is adapted to press and rub away the surface protrusion to below a tolerance specification needed for squeegee solder applications.  FIGS. 4-10 , discussed further below, illustrate a handheld stencil repair tool  40  with a variety of customized tips useful in repairing different types of metal stencil coins. 
     In step  240 , the technician gently presses a polymerized or polymer-coated tool tip up and against the surface protrusion on the metal stencil, while observing the location and degree of distortions in the reflected light caused by the pressure application. The technician conditions both the location and the amount of applied pressure upon the observed location and observed degree of distortions in the reflected light. Depending on the type of coin encountered, the technician may also gently nip the tool tip up and against the surface protrusion of the coin, and in the immediate surroundings of the surface protrusion, to repair the coin. 
     Because the metal stencil  10  is so thin, considerable control and finesse is needed to repair a coin without further damaging the metal stencil  10 .  FIG. 11  illustrates a technician  81  positioning his or her chin  82  adjacent a top surface of the metal stencil frame  12  to steady his or her gaze while massaging a coin  30 .  FIG. 12  illustrates the technician simultaneously holding and carefully manipulating a handheld stencil repair tool  40  with both hands beneath the metal stencil frame  10  in order to apply gentle massaging pressure to the coin  30 . 
     In step  245 , the technician inspects and feels, with his or her finger(s), the stencil to evaluate whether the repair is adequate and whether the surface in the area of the previously-detected coin is substantially flat and free of defects. After this inspection process is done on one side of the stencil, the stencil is flipped and remounted and the same inspection is done on the opposite side. The technician may, if necessary, apply pressure from the opposite side of the stencil using the same techniques described in steps  220 - 240 . 
     After the repairs are complete and initially determined to be acceptable, then in step  250 , the technician wipes and cleans both sides of the stencil with a micro fiber cloth or paper product wetted with a 90% rubbing alcohol solution. 
     In step  255 , the repaired stencil goes through quality control where it is inspected in a controlled environment. A laser may be used to detect whether any surface displacements are within tolerance specifications. 
     The steadiness, gentleness, and control needed to repair a solder stencil also calls for a better tool than the tools typically used in the paintless dent repair trade.  FIGS. 4-10  illustrate a stencil repair tool  40  custom designed to facilitate the techniques of the present invention. The stencil repair tool  40  includes a generally cylindrical cushioned handle  41  that bears some resemblance to a blank screwdriver handle. Four oval depressions or finger holds  42  are lined up on one side of this custom-made handle  41 . Two more oval depressions or thumb holds  43  are lined up on the opposite side of the handle  41 . The handle  41  is about four to five inches in length, with a 90-degree end  46  and a 45-degree end  47 . An internally threaded bore  44  is provided in each end  46  and  47  for mounting a correspondingly externally threaded tool tip. The 90-degree end  46  holds a tool tip parallel to the longitudinal direction of the handle  41 . The 45-degree end  47  holds a tool tip at a 45-angle to the longitudinal direction of the handle  41 . 
       FIGS. 6 and 7  illustrate a customized crescent tool tip  50  especially developed for stencil repair. The crescent tool tip  50  has a carved or molded plastic body  55  mounted on an externally threaded attachment shaft  51 . The body  55  is preferably made of acetal plastic, a thermoplastic produced by the addition polymerization of an aldehyde through the carbonyl function, having a hardness, measured on the Rockwell M scale, of between 89 and 94. 
     The body  55  tapers from a generally small-diameter cylindrical cross section to a much broader-diameter crescent-shaped top that provides a blunt tip  52 , a sharp tip  53 , and a rounded smooth side  54 . The span between the blunt and sharp tips  52  and  53  is approximately 2.3 cm. The width of the body  55  is about 1 cm and the length of the body is about 2.8 cm. The variety of pressure application surfaces  52 - 54  make the crescent tool tip  50  versatile enough to repair most solder stencil coins. 
     The sharp tip  53  is best suited for applying firm, straight upward pressure against small, barely perceptible coins, located in the non-aperture section of a stencil, with a tight center in the lowest part of the coin.  FIGS. 14 and 15  illustrate the sharp tip  53  of the crescent-shaped tool  50  being used to apply straight upward pressure against a small, tight-centered coin  58 .  FIG. 14 , in particular, is not to scale, but exaggerates the size of the coin  58 . When a small coin  58  is located on or adjacent a solder aperture (e.g., in one of the rectangles  15 ), it is better to use the blunt tip  52 , because care must be taken not to create a jagged edge on the apertures. 
     When addressing a small, sharp-centered coin  58 , the pressure should applied as close to the exact center of the coin  58  as possible. Because of the coin&#39;s small size, this is a difficult practice to master. To assist in that process, the technician should carefully observe the distortions in the reflected light pattern  25  caused by application of the tool tip pressure.  FIG. 13  illustrates converging distortions  27  in a reflected light pattern  25  off of the top surface of a solder stencil  10  caused by application of tool tip pressure up and against the bottom surface of the solder stencil  10 . 
     The blunt tip  52  is best suited for repairing larger-diameter coins.  FIGS. 16-18  illustrate the blunt tip  52  of the crescent-shaped tool  50  being used to apply gentle upward and lateral, massage-like tool-tip pressure to repair a large-diameter metal stencil coin  68 . To repair a large-diameter coin  68 , the blunt tip  52  should be pressed against several different places  67  in the deepest part of the coin  68 , to gradually and incrementally smooth out the surface protrusion as evenly and smoothly as possible. Note that in  FIG. 17 , this process also involves pressing the blunt tip  52  at different angles for different points, relative to the plane of the stencil  10 , to substantially eliminate the detected coin. 
     The rounded smooth side  54  is best suited for creases, which may have many different shapes and lengths.  FIGS. 3 ,  11 ,  19  and  20  illustrate creases  30  and  59 . Creases are best addressed by using a gentle direct rubbing motion in the direction of the crease, working from one end to the other (that is, not starting in the middle). 
       FIG. 8  illustrates a rounded knob-shaped tool tip  60  having a soft plastic body  65  mounted on an externally threaded attachment shaft  61 . The body  65  is preferably made of acetal plastic having a hardness, measured on the Rockwell M scale, of between 89 and 94. Tool tip  60  is particularly suited for repairing shallow solder stencil coins that have no sharp center or crease. 
       FIGS. 9-10  illustrate a substantially-flat-but-slightly-concave-sided, flat-head tool tip  70  having a body  75  mounted on an externally threaded attachment shaft  71 . The body  75  is preferably made of a plastic having a hardness, measured on the Rockwell M scale, of between 66 and 69. Tool tip  70  is particularly suited for repairing coins located in a stencil step. A step is an area in the solder stencil that allows clearance for electrical components on the surface of the printed circuit board.  FIGS. 21 and 22  illustrate a step  79  with a coin defect  78  along an edge or corner. The flat tips of the tool tip  78  are well-suited to repairing bent-edge step coins. 
     Developing the steadiness, gentleness, and control needed to repair a solder stencil also requires practice. The amount of pressure needed to repair a defect depends on the thickness of the stencil and the overall depth of the coining damage. Pressing just enough to correct the coining defect, and not so hard as to cause more damage, is key to a successful repair. 
       FIGS. 23 and 24  illustrate two teaching techniques for training a technician in this delicate art. In  FIG. 23 , a maze  230  is superimposed (e.g., using a marker) over a stencil  10 . The technician applies gentle tool tip pressure against the underside of the stencil  10  between the maze lines. The technician moves the tool tip along the direction of the maze arrows, the entire time locating the position of the tool tip through observation of the converging distortion lines  27  in the reflected light pattern  25 . 
     In  FIG. 24 , a plurality of very small dots are drawn (again using a marker) on a stencil  10 . In  FIG. 24 , the size of the dots are exaggerated for effect. The technician presses a tool tip against the underside of the stencil  10 , attempting to consistently press the stencil at the center of each dot. Again, the technician observes the converging distortion lines  27  in the reflected light pattern  25  to identify the location of the tool tip. If the tool tip is positioned at the center of a dot, then the observed distortion lines  27  should converge at the center of the dot. 
     Having thus described exemplary embodiments of the present invention, it should be noted that the disclosures contained in  FIGS. 1-25  are exemplary only, and that various other alternatives, adaptations, and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the specific embodiments illustrated herein, but is limited only by the following claims.