Resistor and method for making same

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.

Claims

1. A metal strip resistor, comprising: a metal strip having a generally planar top surface and forming a resistive element and providing support for the metal strip resistor without use of a separate substrate; first and second photolithographically formed termination areas overlaying the top surface of the metal strip adjacent opposite first and second side ends of the metal strip; copper plating on each of the first and second termination areas; a first metal plating layer extending from a bottom edge of the resistive element adjacent the first side end of the metal strip, along the first side of the metal strip, and along the top surface of the metal strip, the first metal plating layer covering the copper plating of the first termination area; and, a second metal plating layer extending from a bottom edge of resistive element adjacent the second side of the metal strip, along the second side of the metal strip, and along the top surface of the metal strip, the second metal plating layer covering the copper plating of the second termination area; and, an insulating material overlaying the top surface of the metal strip between the first and second termination areas; wherein the copper plating of each termination area does not overlap the first insulating material.

2. The metal strip resistor of claim 1, wherein the metal strip is a metal alloy comprising at least one of nickel, chromium, aluminum, manganese, and copper.

3. The metal strip resistor of claim 1, further comprising an adhesion layer applied to the metal strip.

4. The metal strip resistor of claim 1, further comprising a pair of photolithographically formed terminations on a bottom surface of the metal strip.

5. The metal strip resistor of claim 4, further comprising an insulating material overlaying the metal strip on a bottom surface of the metal strip.

6. The metal strip resistor of claim 1, further comprising first and second solderable layers formed along opposite outer sides of the metal strip.

7. A method for forming a metal strip resistor wherein a generally planar metal strip provides support for the metal strip resistor without use of a separate substrate, the method comprising: applying a photolithographic process to form a conductive pattern overlaying a top surface of the metal strip, wherein the conductive pattern includes first and second termination areas adjacent opposite first and second side ends of the metal strip; electroplating the first and second areas with copper; plating a first metal layer extending from a bottom edge of the resistive element adjacent the first side end of the metal strip, along the first side of the metal strip, and along the top surface of the metal strip, the first metal layer covering the electroplated copper of the first termination area; and, plating a second metal plating layer extending from a bottom edge of resistive element adjacent the second side of the metal strip, along the second side of the metal strip, and along the top surface of the metal strip, the second metal plating layer covering the electroplated copper of the second termination area; and, overlaying an insulating material along the top surface of the metal strip between the first and second termination areas; wherein the electroplated copper of each termination area does not overlap the insulating material.

8. The method of claim 7, wherein the metal strip is a metal alloy comprising at least one of nickel, chromium, aluminum, manganese, and copper.

9. The method of claim 7, further comprising applying an adhesion layer to the metal strip prior to electroplating.

10. The method of claim 7, further comprising applying a photolithographic process to form a conductive pattern overlaying a bottom surface of the metal strip, wherein the conductive pattern includes third and fourth opposite terminations.

11. The method of claim 10, further comprising electroplating the conductive pattern on the bottom surface of the metal strip.

12. The method of claim 11, further comprising applying an insulating material overlaying the metal strip between the third and fourth opposite terminations.

13. The method of claim 7, further comprising plating first and second solderable layers along opposite outer sides of the metal strip.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of one embodiment of a resistor.

(2) FIG. 2 is a cross-sectional view of a resistance material with an adhesion layer and a mask during the manufacturing process.

(3) FIG. 3 is a cross-sectional view after applying a conductive pattern and electroplating during the manufacturing process.

(4) FIG. 4 is a cross-sectional view after stripping material away during the manufacturing process.

(5) FIG. 5 is a top view of a resistive sheet during the manufacturing process.

(6) FIG. 6 is a top view of the resistive sheet during the manufacturing process after resistance has been adjusted.

(7) FIG. 7 is a top view of the resistive sheet during the manufacturing process where insulating material covers exposed resistor material between terminators.

(8) FIG. 8 is a cross-sectional view of a resistor after the plating process.

(9) FIG. 9 is a top view of the resistive sheet showing four-terminal resistors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) The present invention relates to metal strip resistors 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.

(11) 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.

(12) 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.

(13) 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'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.

(14) 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.

(15) 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.

(16) 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.

(17) 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.

(18) 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.

(19) 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'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.

(20) 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.

(21) 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.

(22) 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.

(23) 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.