Patent Application
20080198565
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.
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°.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Where like parts appear in more than one drawing, it has been attempted to use like reference numerals for clarity.
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.
Turning to
The SMT connector 10, including the foot 20, is cut and formed from pre-plated sheet stock 70 (
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.
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
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.
As also illustrated in
The coined edge surface 22 may be a beveled edge surface 22a or it may be a smooth curve resulting in a radial edge surface 22b. As shown in
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.
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.
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.
