Abstract:
A surface mount resistor is formed from an elongated resistive body having first and second terminal ends and a raised center portion formed therebetween. The raised center portion includes slots in its edges which form a serpentine current path through the raised center portion of the resistor. A dielectric surrounds and encapsulates the raised center portion and an electrically conductive material coats the first and second terminal ends. The method for manufacturing involves utilizing an elongated ribbon which is of unitary construction and which is formed to create a carrier strip and a raised center portion for the resistors ultimately to be formed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a formed 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 resistive material. The resistive material is coated with epoxy 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 formed surface mount resistor and method for making same. 
     A further object of the present invention is the provision of a method for making a formed surface mount resistor which utilizes a single ribbon of material for the resistor body and the carrier strip. 
     A further object of the present invention is the provision of an improved formed 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 formed 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 a formed 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 resistive body formed from a single piece of electrically resistive material. The resistive body includes first and second terminal ends and a raised center portion positioned above first and second terminal ends. The raised center portion includes first and second opposite edges and has a plurality of slots extending into the lateral edges so as to create a serpentine current path through the raised center portion from first terminal end to the second terminal end. A dielectric material surrounds and encapsulates the raised center portion. An electrically conductive material coats the first and second terminal ends. 
     The method for making the surface mount resistor of the present invention comprises taking an elongated ribbon of electrically resistive material having upper and lower ribbon edges. The ribbon is partially separated into a plurality of individual body members, each having opposite side edges and first and second terminal ends with a central portion therebetween. The ribbon includes a carrier portion interconnecting the plurality of body members. A plurality of slots are formed in the opposite side edges of the body members so as to create a serpentine current path from the first terminal end through the central portion to the second terminal end in each of the body members. The cross sectional shapes of the body members are then formed so that the central portion is raised above the first and second terminal ends. The raised central portion is then encapsulated within a dielectric material and the terminal ends of the body members are coated with an electrically conductive material. 
     In one embodiment of the method the step of forming the cross sectional shape of the body members is performed by forming the ribbon before the separating step is accomplished. 
     In another embodiment of the method of the present invention the step of forming the cross sectional shape of the body members is performed on the body members after the separating step has been performed. 
     Various types of forming methods may be used, including roll forming to create the forming of the raised portion or stamping may also be used. Preferably the roll forming method is used when the forming is accomplished before separating the strip into the various body members. Stamping is the preferred method if the forming is accomplished after the body members have been separated. 
    
    
     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. 3 is an enlarged sectional view taken along line  3 — 3  of FIG.  2 . 
     FIG. 3A is an elevational view taken from the left 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 the drawings the numeral  10  generally designates the surface mount resistor of the present invention. Resistor  10  includes a raised center  12  and first and second end terminals  14 ,  16 . The bottom surfaces of terminals  14 ,  16  form first and second stand offs  18 ,  20  which permit the resistor to be mounted on a surface with the raised center  12  spaced above the surface on which the resistor is mounted. 
     FIG. 2 shows a schematic representation of the method for manufacturing the resistor of the present invention. A reel  22  includes a unitary ribbon  24  wound around it. The ribbon  24  is shown in enlarged detail in FIGS. 3 and 3A. It includes a carrier portion  26 , an upper terminal portion  28 , and a lower terminal portion  30 . Between portions  28 ,  30  is a raised center portion  34 . A cut line  32  is represented by a dashed line  32 , and later in the process a longitudinal cut will be made along this line to produce the individual resistors. The ribbon  24  is of unitary construction, and is formed of an electrically resistive material. The preferred material for the resistive material is copper nickel, but other well known resistive materials such as nickel iron, nickel chromium or a copper based alloy may be used. 
     In one form of the present invention the forming of the raised portion  34  is done by roll forming the resistive strip either before it is wound upon the reel  22 , or after it has been unwound from the reel  22 , but before it has been punched or formed into individual resistors. 
     In another form of the invention, the resistive material  24  is in a flat unformed state on the reel  22 , and is unwound and formed into individual resistors before the raised portion  34  is formed. In this modified form of the invention the forming of the raised portion  34  may be accomplished by stamping and is preferably accomplished before the individual resistors are separated from the strip. 
     The numeral  36  in FIG. 2 represents the step of roll forming the resistive strip either before it is placed on reel  22  or immediately thereafter. 
     The carrier portion  26  of strip  24  is used as an indexing device for carrying the resistors through the entire manufacturing operation. 
     The next step which is performed on the strip  24  is the punching of transfer holes, represented by block  38  in FIG.  2 . Holes  40  are punched into the carrier strip  26  and are used for indexing the strip through the manufacturing process. 
     The next step performed on the strip  24  is the step of separating the individual resistor bodies from one another and is represented by the block  42  in FIG.  2 . FIG. 4 illustrates the manner in which this separation process is accomplished. The upper edge of strip  24  is trimmed to provide an upper edge  44  for each of the resistor elements. At the same time a separating slot  46  is formed between each of the resistor bodies. The slots  46  protrude downwardly slightly below the cut line  32 . While various methods may be used for cutting or forming the edges  44  and the slots  46 , the preferred method is to do so by stamping the strip  24 . 
     FIG. 5 illustrates the result of the adjusting and calibrating step performed on the resistors and represented by the block  60  in FIG.  2 . Side slots  48 ,  50  are formed in the edges of the resistor body so as to create a serpentine path represented by arrow  52  for the current to pass from terminal  28  to  30 . During this adjusting process, the slots  48 ,  50  are cut preferably by laser and the resistance of the resistance body is monitored and measured until the precise resistance value is achieved. 
     The next step to be performed on the resistor is the encapsulation of the central portion within a dielectric material, and is represented by block  62  in FIG.  2 . As can be seen in FIGS. 6 and 6A the dielectric material  54  is applied so that it surrounds the entire central portion  34  of the resistor blank. 
     The purposes of the encapsulating operation include providing protection from various environments to which the resistor may be exposed; adding rigidity to the resistance element which has been weakened by the value adjustment operation; and providing a dielectric insulation to insulate the resistor from other components or metallic surfaces it may contact during its actual operation. The encapsulating material  54  is applied in a manner which only covers the central portion  34 . A liquid high temperature coating material roll coated to both sides of the central portion  34  is the preferred method. The terminal ends  28 ,  30  of the resistor blank are left exposed. 
     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  65  where the individual resistors are cut away from the carrier strip  24 . The individual resistors are plated with solder to create a solder coating  58  as shown in FIG.  7 A. The individual resistors  10  are then complete and they are attached to a plastic tape  68  at a packaging station represented by the numeral  66 . 
     The forming of the raised portion  34  can be accomplished at various stages of the manufacturing process as desired. For example, the raised portion can be roll formed before the strip  24  is placed on reel  22 , or it can be roll formed immediately after it is unwound from reel  22 . A further modified form of the method may involve waiting until after the separation step  42  and the adjust and calibrate step  60  before stamping the individual resistor blanks to create the raised portion  34 . The advantage of this later method is that the raised portion is not deformed or bent during the performance of the punching step  38 , the separating step  42 , or the adjust and calibration step  60 . 
     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 stamping or punching. However, other methods such as cutting with lasers, drilling, etching, and 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 solder that is applied may be a plating which is preferable or could also be a conventional solder paste or hot solder dip material. 
     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. 
     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.