Abstract:
A surface mount device having a surface mount foot and a method for making the same is disclosed. The method includes providing a piece of pre-plated conductive stock of a conductive base material and a corrosion-resistant conductive plating material different from the conductive base material at least partially overlying the conductive base material. The stock is cut and formed to create a surface mount device having a surface mount foot extending therefrom. The surface mount foot has a top surface, a plated bottom surface and an at least one unplated side surface intermediate the top and bottom surfaces. The plated bottom surface is swaged to create a new plated coined edge surface intermediate the unplated side surface and the plated bottom surface of the surface mount foot.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is generally directed to electrical connectors for use with printed circuit boards and more particularly to surface mount devices having one or more surface mount feet for soldering to electrical devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    Surface mount devices include connectors commonly used in electronic assemblies that typically utilize one or more feet, referred to as surface mount feet or SMT feet, to which the device is soldered to a printed circuit board (PCB) or other electrical device. Although the stock material from which the SMT devices are formed is conductive, it is generally not corrosion resistant. Corroded surfaces resist wetting of applied solder, resulting in unacceptable solder joints within the IPC industry standard IPC J-STD-001D: “Requirements for Soldered Electrical and Electronic Assemblies” which generally requires wetting angles less than 90°. 
         [0003]    To provide suitable solder-ability, current SMT feet are manufactured from unplated sheet stock and then plated, post-manufacturing, with a corrosion resistant material so that all cut and formed surfaces are coated prior to soldering. However, this requires the additional step of post-plating the SMT foot after cutting and forming, adding time and expense to the process. 
         [0004]    It would be less expensive to directly cut and form the SMT device containing the SMT foot from pre-plated metal stock. However, in cutting and forming the SMT foot from pre-plated metal stock, some surfaces, including the sides of the foot to which solder is applied, would still have exposed base material due to the profile cutting (or blanking) process and would not be plated. Over time, the exposed, unplated surfaces corrode. As a result, when solder is applied to those unplated, corroded surfaces, the solder does not properly wet the surface and thus does not consistently produce an acceptable solder joint. 
         [0005]    What is needed is an SMT foot that can be cut and formed from pre-plated metal stock, reducing post-processing costs, while still providing for an acceptable solder joint. 
       SUMMARY OF THE INVENTION 
       [0006]    According to an exemplary embodiment of the invention, a surface mount device is disclosed. The surface mount device has at least one surface mount foot extending therefrom, the surface mount foot comprising a conductive base material and a corrosion-resistant conductive material at least partially overlying the conductive base material. The surface mount foot has a top surface, a plated bottom surface, two unplated side surfaces, and at least one plated coined edge surface. The plated coined edge surface is intermediate the plated bottom surface and an unplated side surface. 
         [0007]    According to another exemplary embodiment of the invention, a method for making a surface mount device comprises providing a piece of pre-plated conductive sheet stock of a conductive base material and a corrosion-resistant conductive plating material different from the conductive base material overlying the conductive base material and cutting and forming the piece of pre-plated conductive sheet stock to create a surface mount device having a surface mount foot extending therefrom. The surface mount foot has a top surface, a plated bottom surface and at least one unplated side surface intermediate the top surface and the bottom surface. The method further includes swaging the plated bottom surface to create a plated coined edge surface intermediate the unplated side surface and the plated bottom surface of the surface mount foot. 
         [0008]    According to another exemplary embodiment of the invention, a method for attaching a surface mount device to an electrical device comprises soldering the surface mount foot made by the foregoing method to the electrical device. 
         [0009]    One advantage of exemplary embodiments of the invention is that surface mounted devices suitable for soldering to printed circuit boards can be manufactured using pre-plated stock, which is less expensive than employing subsequent post-processing plating techniques. 
         [0010]    Another advantage of exemplary embodiments of the invention is that the surface mount device has a surface mount foot with a plated coined edge surface to which solder will adhere. 
         [0011]    Other features and advantages of the present invention will be apparent from the following more detailed description of exemplary embodiments, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates a side view of a surface mount connector attached to an electronic device in accordance with an exemplary embodiment of the invention. 
           [0013]      FIG. 2  illustrates an isometric view of a portion of a surface mount connector in accordance with an exemplary embodiment of the invention. 
           [0014]      FIG. 3  illustrates a front elevation of a surface mount foot in accordance with an exemplary embodiment of the invention. 
           [0015]      FIG. 4  illustrates a perspective view of a surface mount foot at an intermediate step in a method according to an exemplary embodiment of the invention. 
           [0016]      FIG. 5  illustrates a sheet of pre-plated stock for use in accordance with exemplary embodiments of the invention. 
       
    
    
       [0017]    Where like parts appear in more than one drawing, it has been attempted to use like reference numerals for clarity. 
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0018]    Exemplary embodiments of the invention are directed to surface mount devices, including connectors for use in electronic assemblies, having one or more surface mount feet by which the device is soldered to a printed circuit board or other electronic device. 
         [0019]    Turning to  FIG. 1 , a surface mount (SMT) connector  10  is mounted on a printed circuit board  100  by solder connections (omitted for clarity) to one or more SMT feet  20  in contact with a surface  110  of the printed circuit board  100 . As shown, two SMT feet  20  are provided with the connector  10 , although a single SMT foot could be used. In a similar fashion, three or more SMT feet  20  may extend from a single SMT connector  10  to the surface  110  of the printed circuit board  100 . At least one of the SMT feet  20  has a plated coined edge surface  22  that increases the plated surface area of the SMT foot  20  for a solder fillet to be applied and attach the foot  20  to the printed circuit board  100 . 
         [0020]    The SMT connector  10 , including the foot  20 , is cut and formed from pre-plated sheet stock  70  ( FIG. 5 ). That is, the connector  10  is initially formed from a sheet of metallic base material pre-plated on at least one side with a layer of a corrosion-resistant conductive material. Typically, the sheet  70  is pre-plated on the top and bottom sides  72 ,  74  of the sheet  70 . The base material may be any conductive material suitable for use as a surface mount device and is typically copper or an alloy thereof. Exemplary copper alloys include, without limitation, bronze, phosphor bronze, brass, tin brass, leaded brass, beryllium copper, copper silicon nickel, and combinations thereof. Other exemplary base materials include copper-iron alloys and iron-nickel alloys. 
         [0021]    The plating layer may be any layer of corrosion-resistant, conductive material, such as tin, tin-lead alloy, zinc, nickel, gold, silver, platinum, palladium, alloys thereof, and combinations thereof, by way of example only. 
         [0022]    In order to manufacture the connector  10  to its desired shape, the connector is cut and formed from the pre-plated sheet stock  70  in any manner customarily used to create surface mount devices from sheet stock. These manufacturing operations result in creating new, unplated surfaces by exposing underlying base material in the formed connector  10 , such as the side surfaces  24  of the foot  20  (better seen in  FIG. 2 ). Conversely, the surfaces of the top and bottom sides  72 ,  74  of the sheet  70  are generally unaltered by manufacturing operations, and thus provide other surfaces of the formed connector  10  that remain plated, such as the top and bottom surfaces  26 ,  28  of the SMT foot  20 . In the embodiment shown in  FIG. 2 , the unplated side surfaces  24  are shown with cross-hatching, but it will be appreciated that this is for illustration only and is not meant to suggest that no other surfaces of the connector  10  are unplated as a result of manufacturing operations. The exposed base material, as described above, is typically a copper alloy or other corrosion susceptible material that does not support a satisfactory solder joint. 
         [0023]    It has been determined by the inventors that by using pre-plated stock to make surface mount devices, the deficiencies of the exposed surfaces of base material that result from production can be overcome by subsequently imparting a coined (also referred to as swaged) edge surface  22  to the intersection of the plated bottom surface  28  and the unplated side surface  24  of the SMT foot  20 . This causes the still-plated bottom surface  28  of the foot  20  to essentially “wrap” around the side of the foot  20 , decreasing the area of the unplated side surface  24 . In doing so, a new, plated, coined edge surface  22  is created to which a solder fillet will be wetted during the soldering operation. As a result, an acceptable solder joint that at least meets IPC J-STD-001D and has a wetting angle less than 90 degrees may be created. 
         [0024]    As also illustrated in  FIG. 2 , the SMT foot  20  may include an optional jog portion  25  that extends away from the connector receptacle portion  15  of the connector  10  downward to the foot  20 . 
         [0025]    The coined edge surface  22  may be a beveled edge surface  22   a  or it may be a smooth curve resulting in a radial edge surface  22   b . As shown in  FIG. 3 , the foot  20  may have a combination of both styles of coined edge surfaces  22 , although both coined edge surfaces  22  are ordinarily of the same style.  FIG. 3  also illustrates how a solder fillet  40  wets the plated coined edge surfaces  22 , but generally not the unplated side surfaces  24 , to yield an acceptable solder joint when the SMT foot  20  is soldered to the surface  110  of a printed circuit board  100 . It will be appreciated that while the SMT foot  20  is illustrated as being soldered to the surface  110  of a printed circuit board  100 , the SMT foot  20  may also be attached to the surface of a molded interconnect device or any other suitable surface of an electronic device. 
         [0026]    As will be appreciated by those of ordinary skill in the art, soldering generally involves applying heat to the parts to be joined in the presence of solder wire, causing the solder to melt and be drawn into the joint by capillary action and to bond to the materials to be joined by wetting action after the metal cools. Exemplary embodiments of the invention may be used with both lead-base and lead-free (e.g. tin-base) solders. 
         [0027]      FIG. 4  shows an example of an SMT foot  20  after the cutting and forming operation, but prior to the swaging operation to impart the coined edge surfaces  22 . Any suitable swaging process may be used. According to one embodiment, the coined edge surface  22  is produced by a swaging process that uses an automated die to compress the plated bottom surface  28  of the foot  20  toward the top surface  26  at corner edges  29  that define the interface of the bottom surface  28  and the unplated side surfaces  24  (again shown with cross-hatching for illustration purposes). The boundaries of the new coined edge surface  22  to be created are shown in broken line. The corner edges  29  are compressed along at least a portion of the length of the foot  20 , and the coined edge surface  22  may extend the entire length of the SMT foot  20 . 
         [0028]    In one embodiment, the foot  20  is about 0.090 inches long, about 0.050 inches wide, and about 0.0125 inches high. It will be appreciated, however, that the dimensions of the foot  20  may vary based on the overall dimensions of the connector  10 , as well as the application in which the connector  10  will be used. It will further be appreciated that the original dimensions may be altered slightly by the swaging operation, which may, for example, cause minor bulging in the top and bottom surfaces  26 ,  28  of the foot  20 . After swaging, the height of the coined edge surface  22  may be about 20% to about 70% of the height of the foot  20 , typically at least about 30%, and more typically about 50% of the height of the foot  20 . For example, with respect to the 50% embodiment, if the pre-coined height of the foot  20  is 0.010 inches, the interface of the new coined edge surface  22  and the remaining side surface  24  is at a height of about 0.005 inches above the plane in which the bottom surface  28  is situated. 
         [0029]    While the foregoing specification illustrates and describes exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.