Abstract:
A surface mount resistor includes an elongated piece of resistive material having strips of conductive material attached to its opposite ends. The strips of conductive material are separated to create an exposed central portion of the resistive material therebetween. According to the method the resistive strip is attached to a single co-extensive strip of conductive material and a central portion of the conductive material is removed to create the exposed central portion of the resistive strip.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is a division of application Ser. No. 09/471,622, filed Dec. 21, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an overlay surface mount resistor and method for making same. 
     Surface mount resistors have been available for the electronics market for many years. Their construction has comprised a flat rectangular or cylindrically shaped ceramic substrate with a conductive metal plated to the ends of the ceramic to form the electrical termination points. A resistive metal is deposited on the ceramic substrate between the terminations, making electrical contact with each of the terminations to form an electrically continuous path for current flow from one termination to the other. 
     An improvement in surface mount resistors is shown in U.S. Pat. No. 5,604,477. In this patent a surface mount resistor is formed by joining three strips of material together in edge to edge relation. The upper and lower strips are formed from copper and the center strip is formed from an electrically reactive material. The resistive material is coated with a high temperature coating and the upper and lower strips are coated with tin or solder. The strips may be moved in a continuous path for cutting, calibrating, and separating to form a plurality of electrical resistors. 
     A primary object of the present invention is the provision of an improved overlay surface mount resistor and method for making same. 
     A further object of the present invention is the provision of an improved overlay surface mount resistor and method for making same which reduces the number of steps and improves the speed of production from that shown in U.S. Pat. No. 5,604,477. 
     A further object of the present invention is the provision of an improved overlay surface mount resistor and method for making same wherein the resulting resistor is efficient in operation and improved in quality. 
     A further object of the present invention is the provision of an overlay surface mount resistor and method for making same which is economical to manufacture, durable in use and efficient in operation. 
     SUMMARY OF THE INVENTION 
     The foregoing objects may be achieved by a surface mount resistor comprising an elongated resistance piece of electrically resistive material having first and second end edges, opposite side edges, a front face and a rear face. The resistance piece of resistive material includes a plurality of slots formed in its side edges that create a serpentine current path for current moving between the first and second ends or the resistor. 
     First and second conductive pieces of conductive metal are each formed with a front face, a rear face, first and second opposite side edges, and first and second opposite and edges. The first and second conductive pieces each have their front faces in facing engagement and attached to the front face of the resistive material and are spaced apart from one another to create an exposed area of the front face of the resistive material therebetween. A dielectric material covers the exposed area of the front face of the resistive material. 
     The method of the present invention includes taking elongated resistive strip of electrically resistive material having first and second opposite ends, an upper edge, a lower edge, a front flat face, and a rear flat face. The method includes joining a first elongated conductive strip and a second elongated conductive strip of conductive material to the front flat face of the resistive strip in spaced relation to one another so as to create an exposed portion of the front flat face of the resistive strip between the first and second conductive strips. The joined strips are then sectioned into a plurality of separate body members. Next a plurality of slots are cut through the exposed portion of the resistive strip to create a serpentine current path in the resistive material of each of the body members. Next the resistive strips of each body member are encapsulated in a coating of electrically insulating material. 
     According to one feature of the invention, the attaching step comprises attaching an elongated wide conductive strip over substantially the entire surface of the front face of the resistive strip and then removing a central portion of the wide conductive strip to create the first and second elongated conductive strips and the exposed portion of the elongated resistive strip therebetween. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS 
     FIG. 1 is a perspective view of a resistor made according to the present invention. 
     FIG. 2 is a schematic flow diagram showing the process for making the present resistor. 
     FIG. 2A is an enlarged view taken along line  2 A— 2 A of FIG.  2 . 
     FIG. 3 is a sectional view taken along line  3 — 3  of FIG.  2 . 
     FIG. 3A is a partial elevational view of the ribbon of FIG.  3 . 
     FIG. 4 is an enlarged view taken along line  4 — 4  of FIG.  2 . 
     FIG. 5 is an enlarged view taken along line  5 — 5  of FIG.  2 . 
     FIG. 6 is an enlarged view taken along line  6 — 6  of FIG.  2 . 
     FIG. 6A is a sectional view taken along line  6 A— 6 A of FIG.  6 . 
     FIG. 7 is an enlarged view taken along line  7 — 7  of FIG.  2 . 
     FIG. 7A is a sectional view taken along line  7 A— 7 A of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 the numeral  10  generally designates the surface mount resistor of the present invention. 
     Resistor  10  includes a central portion  12 , first termination  14 , and second termination  16 . Terminations  14 ,  16  each include on their lower surfaces a first standoff  18  and a second standoff  20  respectively. Standoffs  18 , 20  permit the resistor to be mounted on a surface with the central portion  12  spaced slightly above the surface of the circuit board. 
     Referring to FIGS. 2 and 2A, a reel  22  comprising a plurality of strips joined together into one continuous ribbon designated by the numeral  21 . Ribbon  21  comprises a carrier strip  24  which is welded to an overlay strip  26  along a weld line  36 . Overlay strip  26  comprises a resistive strip  28  having first and second conductive strips  30 ,  32  attached to one surface thereof. 
     The method for manufacturing the continuous ribbon  21  is as follows: Beginning with a strip of metallic resistance material  28  of the proper width and thickness and a single strip of copper of the same width, the two metals are joined together through a metal cladding process to form overlay strip  26 . The cladding process is a process well known in the art for joining dissimilar metals through the application of extremely high pressure without braising alloys or adhesives. The resulting overlay strip  26  is of double thickness, one thickness being the copper strip and one thickness being the resistive strip. 
     The next step in the process involves removing a center portion of the conductive strip so as to create the upper conductive strip  30  and the lower conductive strip  32  with an exposed portion  34  therebetween. The removal may be accomplished by grinding, milling, skiving (shaving) or any other technique well known in the art for removing metal. Once removed, the exposed portion  34  electrically separates the upper conductive strip  30  and the lower conductive strip  32 . This can be readily seen in FIGS. 3 and 3A. In FIG. 2A the block  38  represents the attaching of the carrier strip  24  to the overlay strip  26  by welding, and the block  40  represents the removal of the center of the conductive strip to create the upper and lower conductive strips  30 ,  32 . 
     Next in the manufacturing process is the punching step represented by block  42  in FIG.  2 . In this punching step holes  44  are punched in the carrier ribbon to permit the ribbon to be indexed throughout the remainder of the manufacturing process. 
     Next the block  46  represents the separating step for separating each of the various electrical resistors into separate bodies. This step in shown in detail in FIG.  4 . The upper portion of overlay strip  26  is trimmed to create the upper edges  40  of each of the body members. Then a vertical separating slot  50  is cut or stamped between each of the bodies  51 . 
     A cut line is represented by the dotted line  37 , and represents where a cut will be performed later in the process. Slots  50  extend below cut line  37 . 
     The separated resistor bodies are next moved to an adjustment and calibration station  52 . At this station each body is adjusted to the desired resistance value. Resistance value adjustment is accomplished by cutting alternative slots  54 ,  56  (FIG. 5) through the exposed portion  34  of the resistance material of resistance strip  20 . This forms a serpentine current path designated by the arrow  58 . The serpentine path increases the resistance value of the resistor. The slots are cut through the resistance material using preferably a laser beam or any instrument used for the cutting of metallic materials. The resistance value of each resistor is continuously monitored during the adjustment cutting until the desired resistance is achieved. 
     After the resistors are adjusted to their proper resistance value the bodies are moved to an encapsulation station  60  where a dielectric encapsulating material  62  is applied to the exposed front and rear surfaces and edges of the resistive strip  28 . The purposes of the encapsulating operation are to provide protection from various environments to which the resistor may be exposed; to add rigidity to the resistance element which has been weakened by the value adjustment operation; and to provide a dielectric insulation to insulate the resistor from other components or metallic surfaces it may contact during its actual operation. The encapsulating material  62  is applied in any manner which covers only the resistive element materials  28 . A liquid high temperature coating material roil coated to both sides of the resistor body is the preferred method. The conductive elements  30 ,  32  of each body are left exposed. These conductive strips  30 ,  32  of the resistor serve as electrical contact points for the resistor when it is fastened to the pointed circuit board by the end user. Since the ends  30 ,  32  on the resistor are thicker than the resistive element  28  in the center of the resistor, the necessary clearance is provided for the encapsulation on the bottom side of the resistor as shown in FIG.  6 A. 
     Next in the manufacturing process is the application of marking information, printing, to the encapsulated front surface of the resistor. This step is represented by block  64  in FIG.  2 . This is accomplished by transfer printing the necessary information on the front surface of the resistor with marking ink. The strip is then moved to the separating station represented by block  70  where the individual resistors are cut away from the carrier strip  24 . The individual resistors are plated with solder to create a solder coating  68  as shown in FIG.  7 A. The individual resistors  10  are then complete and they are attached to a plastic tape  74  at a packaging station represented by the numeral  72 . 
     The above process can be accomplished in one continuous operation as illustrated in FIG. 2 or it is possible to do the various operations one at a time on the complete strip for example, the attachment and removing steps can be accomplished either before or after the continuous ribbon  21  is wound on a spool. The punching of the transfer holes  44 , the trimming and the separation can then be accomplished by unwinding the spool and moving the strip through stations  46 ,  52 ,  60  to accomplish these operations. Similar operations can be accomplished one at a time by unwinding the spool for each operation. 
     For the welding of weld joint  36  the preferred method of welding is by electron beam welding. However, other types of welding or attachment may be used. The preferred method for forming the transfer holes, for trimming the upper edge of the strip to length, and forming the separate resistor blanks is punching. However, other methods such as cutting with lasers, drilling, etching, or grinding may be used. 
     The preferred method for calibrating the resistor is to cut the resistor with a laser. However, punching, milling, grinding or other conventional means may be used. 
     The dielectric material used for the resistor is preferably a rolled high temperature coating, but various types of paint, silicon, and glass in the forms of liquid, powder or paste may be used. They may be applied by molding, spraying, brushing or static dispensing. 
     The marking ink used for the resistor is preferably a white liquid, but various colors and types of marking ink may be used. They may be applied by transfer pad, ink jet, transfer roller. The marking may also be accomplished by use of a marking laser beam. 
     The solder used in the present invention may be a plating which is preferable, or a conventional solder paste or hot tin dip may be used. 
     In the drawings and specification there has been set forth a preferred embodiment of the invention, and although specific terms are employed, these are used in a generic and descriptive sense only and not for purposes of limitation. Changes in the form and the proportion of parts as well as in the substitution of equivalents are contemplated as circumstances may suggest or render expedient without departing from the spirit or scope of the invention as further defined in the following claims.