Abstract:
A metal strip resistor is provided. The metal strip resistor includes a metal strip forming a resistive element and providing support for the metal strip resistor without use of a separate substrate. There are first and second opposite terminations overlaying the metal strip. There is plating on each of the first and second opposite terminations. There is also an insulating material overlaying the metal strip between the first and second opposite terminations. A method for forming a metal strip resistor wherein a metal strip provides support for the metal strip resistor without use of a separate substrate is provided. The method includes coating an insulative material to the metal strip, applying a lithographic process to form a conductive pattern overlaying the resistive material wherein the conductive pattern includes first and second opposite terminations, electroplating the conductive pattern, and adjusting resistance of the metal strip.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to low resistance value metal strip resistors and a method of making the same. 
     Metal strip resistors have previously been constructed in various ways. For example, U.S. Pat. No. 5,287,083 to Zandman and Person discloses plating nickel to the resistive material. However, such a process places limitations on the size of the resulting metal strip resistor. The nickel plating method is limited to large sizes because of the method for determining plating geometry. In addition, the nickel plating method has limitations on resistance measurement at laser trimming. 
     Another approach has been to weld copper strips to the resistive material to form terminations. Such a method is disclosed in U.S. Pat. No. 5,604,477 to Rainer. The welding method is limited to larger size resistors because the weld dimensions take up space. 
     Yet another approach has been to clad copper to the resistive material to form terminations such as disclosed in U.S. Pat. No. 6,401,329 to Smjekal. The cladding method is limited to larger size resistors because of tolerances in the skiving process used to remove copper material thus defining the width and position of the active resistor element. 
     Still further approaches are described in U.S. Pat. No. 7,327,214 to Tsukada, U.S. Pat. No. 7,330,099 to Tsukada, and U.S. Pat. No. 7,326,999 to Tsukada. Such approaches also have limitations. 
     Thus, all of the methods described have one or more limitations. What is needed is a small sized low resistance value metal strip resistor and a method for making it. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore, it is a primary object, feature, or advantage of the present invention to improve over the state of the art and to provide a small sized low resistance value metal strip resistor and a method for making it. 
     According to one aspect of the present invention, a metal strip resistor is provided. The metal strip resistor includes a metal strip forming a resistive element and providing support for the metal strip resistor without use of a separate substrate. There are first and second opposite terminations overlaying the metal strip. There is plating on each of the first and second opposite terminations. There is also an insulating material overlaying the metal strip between the first and second opposite terminations. 
     According to another aspect of the present invention, a metal strip resistor is provided. The metal strip resistor includes a metal strip forming a resistive element and providing support for the metal strip resistor without use of a separate substrate. There are first and second opposite terminations sputtered directly to the metal strip. There is plating on each of the first and second opposite terminations. There is also an insulating material overlaying the metal strip between the first and second opposite terminations. 
     According to yet another aspect of the present invention, a metal strip resistor is provided. The resistor includes a metal strip forming a resistive element and providing support for the metal strip resistor without use of a separate substrate. There is an adhesion layer sputtered to the metal strip. There are first and second opposite terminations sputtered to the adhesion layer. There is plating on each of the first and second opposite terminations and an insulating material overlaying the metal strip between the first and second opposite terminations. 
     According to another aspect of the present invention, a method for forming a metal strip resistor wherein a metal strip provides support for the metal strip resistor without use of a separate substrate is provided. The method includes coating an insulative material to the metal strip, applying a photolithographic process to form a conductive pattern overlaying the resistive material wherein the conductive pattern includes first and second opposite terminations, electroplating the conductive pattern, and adjusting resistance of the metal strip. 
     According to another aspect of the present invention, a method for forming a metal strip resistor wherein a metal strip provides support for the metal strip resistor without use of a separate substrate, is provided. The method includes mating a mask to the metal strip to cover portions of the metal strip, sputtering an adhesion layer to the metal strip, the mask preventing the adhesion layer from depositing on the portions of the metal strip covered by the mask, the portions of the metal strip covered by the mask forming a pattern including first and second opposite terminations. The method further includes coating an insulative material to the metal strip and adjusting resistance of the metal strip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of one embodiment of a resistor. 
         FIG. 2  is a cross-sectional view of a resistance material with an adhesion layer and a mask during the manufacturing process. 
         FIG. 3  is a cross-sectional view after applying a conductive pattern and electroplating during the manufacturing process. 
         FIG. 4  is a cross-sectional view after stripping material away during the manufacturing process. 
         FIG. 5  is a top view of a resistive sheet during the manufacturing process. 
         FIG. 6  is a top view of the resistive sheet during the manufacturing process after resistance has been adjusted. 
         FIG. 7  is a top view of the resistive sheet during the manufacturing process where insulating material covers exposed resistor material between terminators. 
         FIG. 8  is a cross-sectional view of a resistor after the plating process. 
         FIG. 9  is a top view of the resistive sheet showing four-terminal resistors. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to metal strip resistor and a method of making metal strip resistors. The method is suitable for making an 0402 size or smaller, low ohmic value, metal strip surface mount resistor. An 0402 size is a standard electronics package size for certain passive components with 0.04 inch by 0.02 inch (1.0 mm by 0.5 mm) dimensions. One example of a smaller size of packaging which also may be used is an 0201 size. In the context of the present invention, a low ohmic value is generally a value suitable for applications in power-related applications. A low ohmic value is generally one that is less than or equal to 3 Ohms, but often times in the range of 1 to 1000 milliohms. 
     The method of manufacturing the metal strip resistor uses a process wherein the terminations of a resistor are formed by adding copper to the resistive material through sputtering and plating. This method utilizes photolithographic masking techniques that allow much smaller and better defined termination features. This method also allows the use of the much thinner resistance materials that are needed for the highest values in very small resistors yet, the resistor does not use a support substrate. 
       FIG. 1  is a cross-sectional view of one embodiment of a metal strip resistor of the present invention. A metal strip resistor  10  is formed from a thin sheet of resistance material  18  such as, but not limited to EVANOHM (nickel-chromium-aluminum-copper alloy), MANGANIN (a copper-manganese-nickel alloy), or other type of resistive material. The thickness of the resistance material  18  may vary based on desired resistance. However, the resistance material may be relatively thin if desired. Note that the resistance material  18  is central to the resistor  10  and provides support for the resistor  10  and there is no separate substrate present. 
     The resistor  10  shown in  FIG. 1  also includes an optional adhesion layer  16  which may be formed of CuTiW (copper, titanium, tungsten). The adhesion layer  16 , where used, is sputtered over the surface of the resistive material  18  for the copper plating  14  to bond to. Some resistance materials may require the use of the adhesion layer  16  and others do not. Whether the adhesion layer  16  is used, depends on the resistance material&#39;s alloy and if it allows direct bonding of copper plating with adequate adhesion. If an adhesion layer  16  is desirable and both sides of the resistance material  18  are to receive pads then both sides of the resistance material  18  should be sputtered with an adhesion layer  16 . 
     Prior to the sputtering process a metal mask (not shown in  FIG. 1 ) may be mated with the sheet of resistance material  18  to prevent the CuTiW material from depositing onto areas of the sheet that will later become the active resistor areas. This mechanical masking step allows one to eliminate a gold plating and etch back step later in the process thus reducing cost. Where gold plating is used or other highly conductive plating, the gold plating  24  overlays the copper plating  14 . A plating  28  is provided which may be a nickel plating. A tin plating  12  overlays the nickel plating  28  to provide for solderability. 
     Also shown in  FIG. 1  is an insulative coating material  20  which is applied to the resistance material  18 . The insulative coating material  20  is preferably a silicone polyester with high operating temperature resistance. Other types of insulating materials may be used which are chemical resistant and capable of handling high temperature. 
       FIG. 2  illustrates a relatively thin sheet of resistance material such as EVANOHM, MANGANIN or other type of resistance material  18 . The resistance material  18  serves as the substrate and support structure for the resistor. There is no separate substrate present. The thickness of this sheet of resistance material  18  may be selected to achieve higher or lower resistance value ranges. A field layer of CuTiW (copper, titanium, tungsten) or other suitable material is sputtered over the surface of the resistive material  18  as an adhesion layer  16  for the copper plating to bond to. Prior to the sputtering process, a metal mask may be mated with the sheet of resistance material  18  to prevent the CuTiW material or other material for the adhesion layer  16  from depositing onto areas of the sheet that will later become the active resistor areas. This mechanical masking step eliminates a gold plating and etch back step later in the process thus reducing cost. 
     Next a photolithographic process is performed. The photolithographic process may include laminating a dry photoresist film  22  to both sides of the resistance material  18  to protect the resistance material  18  from copper plating. A photo mask may then be used to expose the photoresist with a pattern corresponding to the copper areas to be deposited onto the resistance material. The photoresist  22  is then developed, exposing the resistive material in only the areas where copper or other conductive material is to be deposited as shown in  FIG. 2 . 
       FIG. 3  illustrates the copper pattern  14 . The copper pattern may include individual terminal pads, stripes, or near complete coverage except in areas that will be the active resistor area. The pad size may be defined at the punching operation in cases where stripes and near-full coverage patterns are used. The terminal pad geometry and number can vary depending on the PCB mounting requirements and electrical connections required such as 2-wire or 4-wire circuit schemes, or multi-resistor arrays. Copper  14  is plated in an electrolytic process. A thin layer of Au (gold)  24  is electroplated over the copper. The photoresist material is then stripped as shown in  FIG. 4  and subsequently the CuTiW material  16  not covered by copper plating  14  is stripped from the active resistor areas in a chemical etch process. In another embodiment the gold layer  24  is not added and the CuTiW layer  16  is not stripped back after removing the photoresist layer to save manufacturing cost but at the expense of electrical characteristics. In a further embodiment the gold is not added and stripping is not necessary because the CuTiW material was mechanically masked at the sputtering step. 
     The resulting terminated plate may be processed as a sheet, sections of a sheet, or in strips of one or two rows of resistors. The sheet process will be described from this point on but these subsequent processes also apply to sections and strips. As shown in  FIG. 5 , the sheet  19  is a continuous solid (although alignment holes may be present) and areas of the sheet  19  may then be removed to define the resistor&#39;s design dimensions of length and width. Preferably this is done with a punch tool but may also be done by a chemical etching process or by laser machining or mechanical cutting away of the unwanted material. 
     The resistance values of the unadjusted resistors are determined by the copper pad spacing, defined by the photo mask, length, width, and the thickness of the sheet of resistive material. As shown in  FIG. 6 , adjustment of the resistance value may be accomplished by a laser or other means of removing material  26  to increase the resistance while at the same time measuring the resistance value. Adjustment of the resistance value may also be accomplished by adding more termination material, or other conductive material, in areas where the resistive material is still exposed to reduce the value. The resistors work equally as well with no material removed or added but the resistance value tolerance is much broader. 
     As shown in  FIG. 7  and  FIG. 8 , exposed resistor material between the terminations is covered by a coating material  20  which is an insulating material to prevent electroplating onto the resistive element and changing its resistance value. The coating material  20  is preferably a silicone polyester with high operating temperature resistance but may be other insulating materials that are chemical resistant and capable of handling high temperatures. The coating material  20  is preferably applied by a transfer blade. A controlled amount of coating material  20  is deposited on the edge of the blade and then transferred to the resistor by contact between the blade and resistor. Other methods of applying the coating material  20  may be used such as screen printing, roller contact transfer, ink jetting, and others. The coating material  20  is then cured by baking the resistors in an oven. Any markings that are put on the coating material  20  would be applied by ink transfer and baking or by laser methods at this point in the process. A die cutter may be used to remove each single resistor from the carrier plate. Other methods to singulate the resistors from the carrier may be used such as a laser cutter or photoresist mask and chemical etching. 
     Individual resistors are then put into a plating process where nickel  28  and tin  12  are added to make the part solderable to a PCB as shown in  FIG. 1 . Other plating materials may be used for other mounting methods such as gold for bonding applications. DC resistance may be checked on each piece and those in tolerance are placed into product packaging, usually tape and reel, for shipment. 
     Therefore a low resistor value material strip resistor has been disclosed. The resistor may achieve a small size, including an 0402 size or smaller package. The present invention contemplates numerous variations including variations in the materials used, whether an adhesion layer is used, whether the resistor is 2 terminal or 4 terminal, the specific resistance of the resistor, and other variations. In addition a process for forming a low resistance value metal strip resistor has also been disclosed. The present invention contemplates numerous variations, options and alternatives, including the manner in which a coating material is used, whether or not a mechanical masking step is used, and other variations.