Patent Publication Number: US-8111130-B2

Title: Chip resistor and method for manufacturing the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a chip resistor and to a method for manufacturing a chip resistor. 
     2. Description of the Related Art 
     A conventional chip resistor is described in JP-A-2007-49207, for example. The chip resistor disclosed in this document includes a resistor element and a pair of electrodes provided on a surface of the resistor element. The paired electrodes are insulated from each other and directly bonded to the surface of the resistor element. An overcoat layer for protecting the resistor element is formed on the surface of the resistor element which is opposite from the surface formed with the electrodes. By making a resistor element using a metal plate of an alloy having a low temperature coefficient of resistance (TCR), a chip resistor having an extremely low resistance of a few milliohms (mΩ) in addition to a low TCR is obtained. To achieve a higher resistance of e.g. several tens to hundreds of milliohms, the width of the resistor element is reduced partially or part of the resistor element is removed by laser trimming or etching. 
     In the conventional chip resistor, however, the resistor element itself is made of a relatively thin metal plate, and the overcoat layer does not have a sufficient thickness to support the resistor element and the electrodes. Thus, when the resistor element is made narrow or partially removed, the sufficient strength to support the electrodes is not provided by the metal plate and the overcoat layer. Further, when current is applied, the resistor element is heated. In the conventional structure, the heat is trapped in e.g. the narrow portion and cannot be dissipated efficiently. 
     SUMMARY OF THE INVENTION 
     The present invention has been proposed under the circumstances described above. It is therefore an object of the present invention to provide a chip resistor which has a sufficient strength and is capable of efficiently dissipating heat. 
     According to a first aspect of the present invention, there is provided a chip resistor comprising: a resistor element including a first surface and a second surface opposite to the first surface; a reinforcing member bonded to the first surface of the resistor element; and a pair of electrodes formed on the second surface of the resistor element, where the resistor element is formed with a slit located between the pair of electrodes. 
     Preferably, the chip resistor of the first aspect may further comprise an insulating bonding layer provided between the resistor element and the reinforcing member. 
     According to a second aspect of the present invention, there is provided a chip resistor comprising: a plurality of resistor elements each including a first surface and a second surface opposite to the first surface; and a reinforcing member bonded to the first surfaces of the plurality of resistor elements. The second surface of each of the plurality of resistor elements is formed with a pair of electrodes. The plurality of resistor elements are spaced from each other in a direction perpendicular to a facing direction of the pair of electrodes. Each of the plurality of resistor elements is formed with a slit located between the pair of electrodes. 
     Preferably, the chip resistor of the second aspect may further comprise an insulating bonding layer provided between the reinforcing member and the plurality of resistor elements. 
     According to a third aspect of the present invention, there is provided a method for manufacturing a chip resistor. In accordance with the method, first, an assembly is prepared, which is made up of at least one metal plate and a substrate bonded to each other, where the metal plate is provided for a resistor element, and the substrate is provided for a reinforcing member. The metal plate is etched, and a protective film is formed to partially cover the metal plate. A plurality of electrodes are formed by plating on exposed portions of the metal plate, in other words, portions that are not covered by the protective film. Then, the assembly, together with the protective film, is divided into a rectangular chip (or chips each) including at least one pair of electrodes stemming from the plurality of electrodes. 
     Preferably, the etching of the metal plate may include forming a slit in the metal plate between the pair of electrodes. 
     Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a chip resistor according to a first embodiment of the present invention; 
         FIG. 2  is a sectional view taken along lines II-II in  FIG. 1 ; 
         FIG. 3A  shows a step of a method for manufacturing the chip resistor shown in  FIG. 1 ; 
         FIG. 3B  shows another step of the method; 
         FIG. 4A  shows another step of the method; 
         FIG. 4B  shows another step of the method; 
         FIG. 5  shows another step of the method; 
         FIG. 6  is a perspective view showing a chip resistor according to a second embodiment of the present invention; and 
         FIG. 7  is a perspective view showing a chip resistor according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
       FIGS. 1 and 2  show a chip resistor according to a first embodiment of the present invention, and  FIGS. 3-5  illustrate an example of method for manufacturing the chip resistor. 
     As seen from  FIGS. 1 and 2 , the chip resistor A of the first embodiment includes a resistor element  1 , a pair of electrodes  2 , a bonding layer  3 , a reinforcing member  4  and an electrode insulator  5  (not shown in  FIG. 1 ). The chip resistor A is a surface-mounting type having a relatively low resistance ranging from several tens mΩ to several hundreds mΩ, for example. 
     The resistor element  1  comprises a metal plate having a relatively low temperature coefficient of resistance (TCR). Such a metal plate may be made of an Ni—Cr-based alloy, an NI—Cu-based alloy, an Fe—Cr-based alloy or a Cu—Mn-based alloy, for example. The resistor element  1  has a thickness of about 10 to 100 μm. The resistor element  1  is formed with slits  1   a  for adjusting the resistance, but continuously extends from one electrode  2  to the other. 
     The paired electrodes  2  are directly bonded to a surface of the resistor element  1  (the lower surface, in the illustrated example) and spaced from each other by the interposed electrode insulator  5  (see  FIG. 2 ). A solder layer for improving soldering reliability may be formed on the lower surface of each electrode  2 . The electrodes  2  may have a thickness of about 50 to 200 μm and can be formed by plating with the use of Cu, as will be described below. 
     The bonding layer  3  is interposed between the resistor element  1  and the reinforcing member  4  for fixing the resistor element  1  and the reinforcing member  4  to each other. The bonding layer  3  is made of an insulating material. Thus, when the reinforcing member  4  is made of an electroconductive material, the bonding layer  3  serves as insulator for preventing the reinforcing member  4  from being connected to the resistor element  1 . The bonding layer  3  has a thickness of about 60 to 100 μm, for example. As will be described below, the bonding layer  3  is made of a prepreg (insulating bonding film) which may be glass fiber impregnated with a bonding resin. 
     The reinforcing member  4  is attached, by the bonding layer  3 , to the upper surface of the resistor element  1 , which is the opposite surface to the electrode-formed lower surface. The reinforcing member  4  may be made of alumina, glass-fiber-reinforced epoxy resin or a metal (Cu, for example). The reinforcing member  4  mechanically supports the resistor element  1  (which is not stiff by itself due to its thinness), while also serving as radiator for dissipating the heat generated by the resistor element  1  upon current application. To fulfill the above functions, the reinforcing member  4  may have a thickness of about 200 to 400 μm, for example. 
     The electrode insulator  5  is arranged between the electrodes  2  to insulate them from each other. The electrode insulator  5  may be formed by applying e.g. an epoxy-resin-based electrically insulating material to the region between the electrodes  2 . 
     Referring to  FIGS. 3-5 , a method for manufacturing the chip resistor A will be described below. 
     First, as shown in  FIG. 3A , a prepreg  30  (which is for the bonding layer  3 ) is placed between a metal plate  10  (for the resistor element  1 ) and a substrate  40  (for the reinforcing member  4 ). Then, the metal plate  10 , the substrate  40  and the prepreg  30  are bonded together by vacuum press. The metal plate  10 , the substrate  40  and the prepreg  30  are large enough to produce a desired number of chip resistors A collectively. The metal plate  10 , the substrate  40  and the prepreg  30  have thicknesses corresponding to those of the resistor element  1 , the reinforcing member  4  and the bonding layer  3 , respectively. 
     Then, etching is performed with respect to the metal plate  10  to produce a desired pattern as shown in  FIG. 3B . As seen from  FIG. 3B , the pattern includes a plurality of resistor elements  1  mutually connected at this stage. In this etching step, slits  1   a  are formed at predetermined portions of the respective resistor elements  1 . With such slits, each of the resistor elements  1  is still continuous as one piece extending as a whole in the lateral direction (the right-left direction in the illustrated example). 
     Then, as shown in  FIG. 4A , a plurality of protective films or strips  50  are formed to cover the slits  1   a  of the resistor elements  1 . Each of the protective films  50  corresponds to the electrode insulator  5  described above. In this film making step, the resistor elements  1  are left exposed on the two sides of each protective film  50 , on which the electrodes  2  are to be formed later. 
     Then, as shown in  FIG. 4B , the unit made up of the resistor elements  1 , the prepreg  30 , the substrate  40  and the protective films  50  is plated with copper by rack plating. Thus, electrode layers  20 , which are to become the electrodes  2 , are formed on the exposed portions of the resistor elements  1  on the two sides of each protective film  50 . To improve soldering reliability, a solder layer of e.g. Sn may be formed on the electrode layers  20  by plating. 
     Finally, as shown in  FIG. 5 , the unit made up of the resistor elements  1 , the prepreg  30 , the substrate  40 , the protective films  50  and the electrode layers  20  is divided into a plurality of chips by cutting the unit along vertical cutting lines L 1  extending along the electrode layers  2  and horizontal cutting lines L 2  extending between adjacent two electrode layers  20 . As a result, a plurality of chip resistors A as shown in  FIGS. 1 and 2  are obtained. 
     In the chip resistor A described above, the resistor element  1  and the electrodes  2  are supported by the reinforcing member  4 . Thus, even when the resistor element  1  does not have a sufficient mechanical strength due to the provision of the slits  1   a , the chip resistor A as a whole is appropriately rigid. Further, the provision of the reinforcing member  4 , which is larger than the resistor element  1  in volume, ensures efficient heat dissipation from the chip resistor A. 
       FIG. 6  shows a chip resistor according to a second embodiment of the present invention. The illustrated chip resistor B includes a total of four electrodes  2  (two electrodes  2  at the respective ends of the resistor element  1 ). With this arrangement, the chip resistor B is provided with a relatively large contact area for soldering, so that it can be fixed to e.g. a printed circuit board more reliably. 
       FIG. 7  shows a chip resistor according to a third embodiment of the present invention. The illustrated chip resistor C as a whole is in the form of an elongated rectangle and includes a plurality of resistor elements  1  spaced from each other in the longitudinal direction (perpendicular to the direction in which a pair of electrodes  2  on the same resistor element  1  face each other. The arrangements of each resistor element  1  and the relevant pair of electrodes  2  in the third embodiment may be the same or similar to those of the counterparts of the first embodiment shown in  FIGS. 1 and 2 . In the illustrated example, the chip resistor C has three resistor elements  1 . It should be noted that the present invention is not limited to this example, and a chip resistor may be configured to have two or more than three resistor elements. 
     As illustrated in  FIG. 7 , the entire upper surfaces of the respective resistor elements  1  are bonded, via the bonding layer  3 , to the common reinforcing member  4 . To fix the chip resistor C to e.g. a wiring board by soldering, the electrodes  2  on the respective two sides of the chip resistor C are bonded to corresponding one of two electrode pads formed on the wiring board, so that the three resistor elements  1  are connected in parallel with each other. 
     The method for manufacturing the chip resistor C is substantially the same as the method shown in  FIGS. 3-5  except for some modification made in the cutting step of  FIG. 5 . Specifically, in the cutting step for the third embodiment, while cuttings along the vertical cutting lines L 1  are performed in the same manner as for the chip resistor A, the latter cuttings (i.e. the ones along horizontal cutting lines L 2 ) are not performed for all the cutting lines L 2  depicted in  FIG. 5 , but only for some of them. The illustrated example ( FIG. 5 ) depicts four cutting lines L 2  (the first or nearest cutting line L 2 , the second, the third, and the fourth or furthermost cutting line L 2 ). Among these cutting lines, it is only two lines (the first and the fourth cutting lines L 2 , which are not adjacent to each other) along which cutting are performed. 
     In the chip resistor C of the third embodiment, which is in the form of an elongated rectangle as a whole, the larger reinforcing member  4  provides higher mechanical strength. Also, since no cuttings along the second and the third cutting lines L 2  need to be performed, it is possible to make an efficient mass-production of chip resistors C.