Patent Publication Number: US-RE39660-E

Title: Surface mounted four terminal resistor

Description:
This application is based upon the applicants&#39; provisional application Ser. No. 60/074,570 filed Feb. 13, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a surface mounting four terminal current sensing resistor of very low ohmic value and high stability. 
     Surface mounted current sensing resistors have been available for the electronic market for many years. Their construction comprises a flat plate made of a resistive alloy like the Cu—Mn—Ni alloy onto which are plated lands of high conductivity metal forming the four terminals. The voltage-sensing node is set in the resistive alloy. 
     When applied to ohmic values in the range of a few millohms or less, this construction introduces additional Joule losses due to the resistance between the point of connection of the current carrying wires and the a/m nodes. This leads to an additional temperature rise and results in drifts of the measurements. 
     The primary object of this invention is to provide an improved very low value surface mounted current sensor characterized by high stability when subjected to high ambient temperatures and to pulses of high power. 
     A further object of this invention is the provision of a resistor made of an alloy of high resistivity in order to increase its thermal capacity. 
     A still further object of this invention is the provision of a resistor in which the dimensions of the resistive plate are chosen in a way to minimize the length of the trimming cuts thus avoiding hot spots at points where the current makes a turn of 180 degrees. 
     A still further object of this invention is the provision of a resistor with terminals made of thick, high thermal conductivity material, which acts also as a heat sink during a power pulse. 
     A still further object of this invention is the provision of a resistor, which is constructed in a way to be capable of withstanding pulses of high power by choice of materials withstanding high temperatures and by reducing thermal resistance within the resistor. 
     A further object of this invention is the provision of a resistor which can be mass produced by stamping, laser trimming and coating by methods described in U.S. Pat. No. 5,604,477 and which can receive a high power rating when cemented to a metal base for soldering to a heat sink. 
     A still further object of this invention is the provision of a resistor that has terminals plated, for interconnection either by soldering or by welding. 
     SUMMARY OF THE INVENTION 
     A surface mounted resistor is formed by welding to each side of a resistive strip of Ni—Cr alloy two strips, one narrow and another wide, of a Ni plated high conductivity copper. The thickness and width of the resistive strip are chosen to form a resistance value below but close to the requested target, and therefore to minimize the extent of posterior laser trimming. This composite strip is punched to form individual resistors in a way described in the U.S. Pat. No. 5,604,477, but with an additional slot in the terminations in order to divide them into distinct current and sense pads, the current pad being at least twice as long as the sense pad. The depth of the slots is optimized to get the best stability of resistance readings with changing ambient temperature and under influence of the self-heating effect. The punched resistors remain attached to the wide copper strip by one current pad. This configuration permits four terminal (Kelvin) measurements of resistors on a continuous strip during subsequent trimming operation. 
     Solder coating is applied to the pads in case the application calls for interconnection by soldering. 
     When the intended interconnection is by ultrasonic bonding of aluminum wires, the Nickel coating applied before welding the strips serves this purpose. Next, the resistors are cut off the strip. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the device of this invention; 
         FIG. 2  is an end elevational view; 
         FIG. 3  is a top plan view; 
         FIG. 4  is a top plan view of a punched wide copper strip, and 
         FIG. 5  is a side elevational view of FIG.  4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The numeral  10  designates the resistor of this invention. It includes a resistor plate  12  with a pair of pads  14  secured thereto. Each pad has a current pad portion  16  and a sense pad portion  18 . Resistor  10  is adapted for mounting on substrate  20 . Specifically, the surface mounted resistor  10  is formed by welding to each side of the resistive strip  12  of Ni—Cr alloy two strips  14 , one narrow and another wide, of a Ni plated high conductivity copper. The thickness and width of the resistive strips  12  are chosen to form a resistance value below but close to the requested target, and therefore to minimize the extent of posterior laser trimming. This composite strip is punched on lines  22  ( FIG. 4 ) to form individual resistors  10  in a way described in the U.S. Pat. No. 5,604,477 (incorporated herein by reference), but with an additional slot  24  in the terminations in order to divide them into distinct current and sense pads, the current pad  16  being at least twice as long as the sense pad  18 . The depth of the slots is optimized to get the best stability of resistance readings with changing ambient temperature and under influence of the self-heating effect. One current pad  16  of the punched resistors remains attached to the wide copper strip  26 . This strip  26  configuration permits four terminal (Kelvin) measurements of resistors on a continuous strip during subsequent trimming operation. 
     As previously indicated, solder coating is applied to the pads in case the application calls for interconnection by soldering. 
     When the intended interconnection is by ultrasonic bonding of aluminum wires, the Nickel coating applied before welding the strips serves this purpose. Next, the resistors  10  are cut off of the strip  26  on lines  14 . 
     In case the application calls for mechanical assembly by soldering the device to a metal substrate  20 , the resistors  10  are bonded with electrically insulating cement of high thermal conductivity to a metal base. The bottom of the base may be plated with nickel and gold for better solderability to the substrate. 
     The layers of resistor  10  are secured together with a high thermal conductivity dielectric cement, such as ceramic powder filled high temperature cements. Use of beryllium oxide in such cements is a component that functions well.