Patent Publication Number: US-2023133764-A1

Title: Chip resistor and method of manufacturing chip resistor

Description:
TECHNICAL FIELD 
     The present invention relates to a chip resistor and a method of manufacturing the same. 
     BACKGROUND ART 
     A chip resistor typically includes a rectangular parallelepiped insulating substrate, a pair of front electrodes facing each other with a predetermined interval therebetween and provided on a surface of the insulating substrate, a pair of back electrodes facing each other with a predetermined interval therebetween and provided on the back surface of the insulating substrate, a pair of end face electrodes for electrically connecting the front electrodes and the back electrodes, a pair of external plating layers for covering each of the electrodes above, a resistor for bridging the pair of front electrodes, and an insulating protective film for covering the resistor. 
     In this type of chip resistor, usually, the front electrodes are made of an Ag (silver) based metal material having low resistivity, and the external plating layers are formed so as to cover the front electrodes. Since strongly corrosive sulfide gas or the like easily enters a gap which is a boundary portion between the external plating layer and the protective film, a front electrode portion at a boundary position between the front electrode and the protective film may be corroded by the sulfide gas. Such corrosion may cause problems, for example, change in resistance and disconnection. 
     In view of these problems, as illustrated in  FIG.  9 A , there has been conventionally proposed a chip resistor in which an auxiliary film  100  made of resin containing metal particles and carbon particles is formed on a top surface of a front electrode  101 , and the auxiliary film  100  is disposed at a boundary position between an external plating layer  102  and a protective film  103 , whereby the auxiliary film  100  blocks sulfide gas entering from a boundary portion between the external plating layer  102  and the protective film  103  to avoid exposure of the front electrode  101  to the sulfide gas (for example, see Patent Literature 1). 
     In this type of chip resistor, electroplating, which is advantageously lower in cost and has shorter plating time than electroless plating, is widely employed for formation of the external plating layers. In such electroplating, a plating layer is formed on a surface of an object to be plated having conductivity, and thus, as illustrated in  FIG.  9 A , the external plating layer  102  is formed so as to cover a surface of the end face electrode  104  and that of the auxiliary film  100 . Here, in  FIGS.  9 A and  9 B , reference signs  105 ,  106 ,  107  indicate an insulating substrate, a resistor, and a back electrode, respectively. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP-B-5957693 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the case of a chip resistor having a low resistance (for example, 100 mΩ or less), a resistance of a front electrode affects a resistance of the entire chip resistor, and thus the lower the resistance of the chip resistor is, the more a TCR increases. The chip resistor described above and illustrated in  FIG.  9 A  has such a structure that a portion of the protective film  103  is inserted between the auxiliary film  100  and the resistor  106 . In this structure, in a mounted state in which the external plating layer  102  is soldered to a land of a circuit board, as illustrated by an arrow in  FIG.  9 B , the electric current supplied from the external plating layer  102  flows from the auxiliary film  100  to the resistor  106  through the front electrode  101  immediately below the protective film  103 . This increases the length of a portion of the front electrode  101  which is between the auxiliary film  100  and the resistor  106  in the electric current path from the electrode region (entire region including the front electrode  101 , the external plating layer  102 , and the auxiliary film  100 ) to the resistor  106 , resulting in increase of a resistance component of the front electrode  101  located in this portion and thus deterioration of the TCR by the increased amount. 
     Furthermore, in the chip resistor illustrated in  FIG.  9 A  described above, the auxiliary film  100  blocks the sulfide gas entering from the boundary portion between the external plating layer  102  and the protective film  103 , however, in the case where the protective film  103  is formed of a resin material such as epoxy, especially in a moisture-resistant atmosphere, the sulfide gas may permeate through the protective film  103  and thus the portion of the front electrode  101  which is located immediately below the protective film  103  is easily sulfurized. 
     The present invention has been made in view of the circumstances of the prior art described above, and a first object thereof is to provide a chip resistor capable of ensuring a low TCR even in the case where it has a low resistance while maintaining sulfurization resistance, and a second object thereof is to provide a method of manufacturing such a chip resistor. 
     Solution to Problem 
     In order to achieve the first object, the present invention provides a chip resistor comprising: a rectangular parallelepiped insulating substrate; a pair of electrodes provided at both ends of a main surface of the insulating substrate; a resistor that connects between the pair of electrodes; a first insulating protective film provided on the resistor; a second insulating protective film provided on the first protective film; a conductive auxiliary film provided so as to be over a connecting portion between one of the electrodes and the resistor at a position away from an end face of the insulating substrate; a pair of end face electrodes extending at both end faces of the insulating substrate so as to be connected to the electrodes; and a pair of external plating layers covering the end face electrodes, the electrodes, and the auxiliary film, wherein the auxiliary film is formed of a material with sulfide-resistant properties more than that of the electrodes, and a portion of the auxiliary film is sandwiched between the first protective film and the second protective film. 
     The chip resistor having the structure as described above includes the auxiliary film which is provided inside the boundary portion where the external plating layer and the second protective film are in contact with each other. The auxiliary film is formed of a material with sulfide-resistant properties more than that of the electrodes, and a portion of the auxiliary film is sandwiched between the first protective film and the second protective film. The auxiliary film blocks the sulfide gas that has entered from the boundary portion between the external plating layer and the second protective layer, and thus the sulfide gas cannot reach the electrodes. This enables prevention of sulfidation of the electrodes. Furthermore, forming the auxiliary film at a position to be over a connecting portion between one of the electrodes and the resistor shortens the length of a portion of the electrode located between the auxiliary film and the resistor in the electric current path from the electrode region (entire region including the front electrode, the external plating layer, and the auxiliary film) to the resistor. Still further, the thickness of the auxiliary film allows the electric current to easily flow. Therefore, reduction in the resistance of the electrode portion can be realized and thus it is possible to ensure a low TCR even in the case of a chip resistor having a low resistance. 
     In the chip resistor having the structure as described above, a material of the auxiliary film is not particularly limited as long as it has conductivity. Meanwhile, preferably, forming the auxiliary film with a resin material containing conductive particles such as carbon particles and metal particles enables easy formation of the auxiliary film. 
     In the chip resistor having the structure as described above, in the case where a connecting portion of the resistor with one of the electrodes is an exposed portion that is not covered with the first protective film and the exposed portion is entirely covered with the auxiliary film, the area of the electrode connected to the external plating layer at a position not covered by the auxiliary film can be increased. In other words, the length of a portion of the electrode which is located in the electric current path from the electrode region through the auxiliary film to the resistor can be shortened. This enables reduction in the resistance of the electrode portion and thus more effective suppression of the deterioration in the TCR. 
     In the chip resistor having the structure as described above, in the case where both the first protective film and the second protective film are set to be short with respect to a length of the resistor in a direction between the electrodes, the auxiliary film is located above the connecting portion between the resistor and the electrode and the outer plating layer is formed further above the auxiliary film, and thus the entire surface of the electrode is covered with the outer plating layer. As a result, the resistivity of the electrode portion can be further lowered, thereby improving the TCR. 
     In the chip resistor having the structure as described above, in each of the electrodes, forming a portion thereof other than a connecting portion with the resistor is thicker than the connecting portion with the resistor causes a concave step at the boundary position between the electrode and the resistor. This concave step enables accurate formation of the auxiliary film at a predetermined position. 
     A method of manufacturing a chip resistor, comprising: forming, on an insulating substrate, a resistor and electrodes connected to both ends of the resistor; forming a first protective film made of a glass material so as to cover at least a portion of the resistor; forming an auxiliary film made of a resin material containing conductive particles at a position over a connecting portion between one of the electrodes and the resistor; forming a second protective film made of a resin material so as to cover a portion of the auxiliary film and the first protective film; forming an end face electrode connected to corresponding one of the electrodes by performing sputtering of metal particles onto an end face of the insulating substrate; and forming an external plating layer covering the end face electrode, the corresponding one of the electrodes, and the auxiliary film by performing electroplating. 
     In the case where the method of manufacturing the chip resistor having the structure as described above further comprises, in each of the electrodes, forming a second electrode on a portion other than a connecting portion with the resistor as a preliminary step of forming the auxiliary film, such formation of the second electrode causes a concave step in the boundary position of the electrode with the resistor. This concave step enables suppression of flow of the resin onto the second electrode when the auxiliary film made of a resin material is formed in this concave step, and therefore, it is possible to accurately form the auxiliary film at a predetermined position. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a chip resistor capable of ensuring a low TCR even in the case where it has a low resistance while maintaining sulfurization resistance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a plan view of a chip resistor according to a first embodiment. 
         FIG.  2    is a cross-sectional view taken along the line II-II of  FIG.  1   . 
         FIGS.  3 A and  3 B  are a cross-sectional view of an enlarged left half of  FIG.  2   . 
         FIG.  4 A to  4 H  are a plan view illustrating manufacturing processes of the chip resistor. 
         FIG.  5 A to  5 H  are a cross-sectional view illustrating the manufacturing processes of the chip resistor. 
         FIG.  6    is a cross-sectional view illustrating a main portion of a chip resistor according to a second embodiment. 
         FIG.  7    is a cross-sectional view illustrating a main portion of a chip resistor according to a third embodiment. 
         FIG.  8    is a cross-sectional view illustrating a main portion of a chip resistor according to a fourth embodiment. 
         FIGS.  9 A and  9 B  are a cross-sectional view of a chip resistor according to the prior art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
       FIG.  1    is a plan view of a chip resistor according to a first embodiment of the present invention,  FIG.  2    is a cross-sectional view taken along the line II-II of  FIG.  1   ,  FIGS.  3 A and  3 B  are a cross-sectional view of the enlarged left half of  FIG.  2   ,  FIG.  4 A to  4 H  are a plan view illustrating manufacturing processes of the chip resistor, and  FIG.  5 A to  5 H  are a cross-sectional view illustrating the manufacturing processes of the chip resistor. 
     As illustrated in  FIG.  1    to  FIGS.  3 A and  3 B , the chip resistor  1  according to the first embodiment includes a rectangular parallelepiped insulating substrate  2 , a pair of front electrodes  3  provided at both ends in the longitudinal direction of the insulating substrate  2  on the upper surface thereof, a pair of back electrodes  4  provided at both ends in the longitudinal direction of the insulating substrate  2  on the lower surface thereof, a rectangular resistor  5  provided so as to connect between the pair of front electrodes  3 , an undercoat layer (first protective film)  6  provided so as to cover the entire resistor  5  including connecting portions with the front electrodes  3 , an overcoat layer (second protective film)  7  provided so as to cover the undercoat layer  6 , a pair of auxiliary films  8  provided at both ends of the undercoat layer  6  and overcoat layer  7 , a pair of end face electrodes  9  extending at both end faces of the insulating substrate  2  so as to electrically connect between the front electrodes  3  and the back electrodes  4 , and a pair of external plating layers  10  provided so as to cover the entire end face electrodes  9  and also cover portions of the back electrodes  4  which are exposed from the end face electrodes  9 . 
     The insulating substrate  2  is made of ceramics or the like, which is obtained by dividing a large-sized substrate, which will be described later, along a primary division groove and a secondary division groove extending vertically and the horizontally, respectively. 
     The front electrodes  3  are obtained by screen-printing an Ag (silver) based paste containing 1 wt % to 5 wt % of Pd (palladium) and drying and firing the printed paste. The back electrodes  4  are obtained by screen-printing the Ag paste and drying and firing the printed paste. 
     The resistor  5  is obtained by screen-printing a resistive paste such as ruthenium oxide and drying and firing the printed paste. Both ends in the longitudinal direction of the resistor  5  overlap the front electrodes  3 . The resistor  5  is provided with a trimming groove  5   a  for adjusting a resistance, and the trimming groove  5   a  is formed by irradiating a laser beam from above the undercoat layer  6 . 
     The undercoat layer  6  is obtained by screen-printing a glass paste and drying and firing the printed paste. The undercoat layer  6  is formed so as to cover the entire resistor  5  prior to formation of the trimming groove  5   a.    
     The overcoat layer  7  is obtained by screen-printing a resin paste such as epoxy or phenol and heating and curing (burning) the printed paste. The overcoat layer  7  is formed so as to cover the undercoat layer  6  after the trimming groove  5   a  is provided in the undercoat layer  6 . An insulating protective film having a double layer structure is formed with these undercoat layer  6  and overcoat layer  7 . 
     The auxiliary films  8  are made of a material with sulfide-resistant properties more than that of the front electrodes  3 , and specifically, they are obtained by screen-printing a resin paste containing carbon particles and heating and curing the printed paste, or obtained by screen-printing a resin paste containing metal particles such as Ag, Cu, or Ni and heating and curing the printed paste. The auxiliary films  8  are formed on portions of the front electrodes  3  which are away from the end faces of the insulating substrate  2 , and provided at positions over the connecting portions between the front electrodes  3  and the resistor  5 . Furthermore, a portion of each of the auxiliary films  8  is sandwiched between the undercoat layer  6  and the overcoat layer  7 , and each end portion of the overcoat layer  7  overlaps a portion (inner end portion) of each of the auxiliary film  8 . 
     The end face electrodes  9  are obtained by sputtering of nickel (Ni)/chromium (Cr) or the like so as to electrically connect between the front electrodes  3  and the back electrodes  4  which are separated via the end faces of the insulating substrate  2 . The end face electrodes  9  cover not only the end faces of the insulating substrate  2 , but also cover portions of the lower surfaces of the back electrodes  4  which are located near the end faces of the insulating substrate  2 , the upper surfaces of the front electrodes  3 , and the surfaces of the auxiliary films  8 . 
     Each of the external plating layers  10  has a double layer structure composed of a barrier layer  11  provided on the inner layer side and an external connection layer  12  provided on the outer layer side to cover the barrier layer  11 . The barrier layer  11  is a Ni plating layer formed by electroplating, and is formed so as to cover the entire end face electrode  9  and a portion of the back electrode  4  which is exposed from the end face electrode  9 . The external connection layer  12  is an Sn plating layer formed by electroplating, and is formed so as to cover the entire surface of the barrier layer  11 . 
     Next, a method of manufacturing the chip resistor  1  having the structure as described above will be explained with reference to  FIG.  4 A to  4 H  and  FIG.  5 A to  5 H . 
     Firstly, a large-sized substrate  2 A provided with the primary division groove and the secondary division groove, which extend to form a grid, is prepared. Providing the primary division groove and the secondary division groove allows both front and back surfaces of the large-sized substrate  2 A to be divided into a number of chip forming regions, and each of the chip forming regions serves as a single unit of the insulating substrate  2 .  FIG.  4 A to  4 H  and  FIG.  5 A to  5 H  illustrate a single chip forming region as a representative example, however, in practice, a large number of such chip forming regions are arranged in a grid pattern. 
     Then, after the Ag paste is screen-printed on the back surface of the large-sized substrate  2 A, the printed paste is dried and fired at 850° C., thereby forming the pair of back electrodes  4  facing each other with a predetermined interval therebetween at both ends in the longitudinal direction of each of the chip-forming regions. 
     Next, after the Ag—Pd paste is screen-printed on the surface of the large-sized substrate  2 A, the printed paste is dried and then fired at 850° C., thereby, as illustrated in  FIG.  4 A  and  FIG.  5 A , forming a pair of front electrodes  3  facing each other with a predetermined interval therebetween at both ends in the longitudinal direction of each of the chip forming regions. Note that the order of forming the front electrodes  3  and the back electrodes  4  may be reversed, or the front electrodes  3  and the back electrodes  4  may be formed simultaneously. 
     Next, after the resistive paste containing ruthenium oxide or the like is screen-printed on the surface of the large-sized substrate  2 A, the printed paste is dried and then fired at 850° C., thereby, as illustrated in  FIG.  4 B  and  FIG.  5 B , forming the rectangular resistor  5  whose both end portions overlap the front electrodes  3 . 
     Next, after the glass paste is screen-printed in a region covering the resistor  5 , the printed paste is dried and then fired at 600° C., thereby, as illustrated in  FIG.  4 C  and  FIG.  5 C , forming the undercoat layer  6  which covers the entire resistor  5 , including the connecting end portions with the front electrodes  3 . 
     Next, after the resin paste containing metal particles such as Ag (or Cu, Ni) is screen-printed on the connecting portions between the front electrodes  3  and the resistor  5 , the printed paste is dried and then heated and cured (burned) at 200° C., thereby, as illustrated in  FIG.  4 D  and  FIG.  5 D , forming the pair of auxiliary films  8 , each of which extends in a band shape over the connecting portion between the front electrode  3  and the resistor  5 . Since each of the auxiliary films  8  is formed so as to be over the connecting portion where the front electrode  3  and the resistor  5  overlap each other, it has an upwardly convex cross-sectional shape (bowl shape). 
     Next, a laser beam is irradiated from above the undercoat layer  6 , thereby, as illustrated in  FIG.  4 E  and  FIG.  5 E , forming a trimming groove  5   a  passing through the undercoat layer  6  and resistor  5  for the purpose of adjustment of a resistance of the resistor  5 . 
     Next, after the resin paste of epoxy or phenol is screen-printed from above the undercoat layer  6 , the printed paste is dried and then heated and cured at 200° C., thereby, as illustrated in  FIG.  4 F  and  FIG.  5 F , forming the overcoat layer  7  that covers the entire surface of the undercoat layer  6  and also covers the portions of the auxiliary films  8  which overlap the undercoat layer  6  and are close to the ends thereof. At this time, since each of the auxiliary films  8  has the bowl-shaped cross section as described above, forming the end portions of the overcoat layer  7  on the inner side inclined surfaces of the auxiliary films  8  enables suppression of sagging of the printed paste of the overcoat layer  7  on the auxiliary films  8 . Note that an insulating protective film having a double layer structure is formed with these undercoat layer  6  and overcoat layer  7 . 
     The processes described so far are collectively performed with respect to the large-sized substrate  2 A. On the other hand, in the next process, the large-sized substrate  2 A is primarily divided along the primary division groove to obtain a strip-shaped substrate  2 B whose width-dimension corresponds to the longitudinal direction of the chip forming region. 
     Next, sputtering of Ni/Cr is applied toward the divided faces (end faces) of the strip-shaped substrate  2 B, thereby, as illustrated in  FIG.  4 G  and  FIG.  5 G , forming the pair of end face electrodes  9  that electrically connects between the front electrodes  3  and the back electrodes  4 . The end face electrodes  9  cover the entire end faces of the strip-shaped substrate  2 B, portions of the lower surfaces of the back electrodes  4  which are located closer to the end faces of the strip-shaped substrate  2 B, the upper surfaces of the front electrodes  3 , and the surfaces of the auxiliary films  8 . 
     Next, after the strip-shaped substrate  2 B is secondarily divided along the secondary division groove to obtain a plurality of chip-shaped substrates  2 C, an Ni electroplating layer is formed on each of the chip-shaped substrates  2 C, thereby forming the barrier layers  11  covering the entire end face electrodes  9  and also covers the portions of the back electrode  4  which are exposed from the end face electrodes  9  Thereafter, an Sn electroplating layer is formed on each of the chip-shaped substrates  2 C, thereby, as illustrated in  FIG.  4 H  and  FIG.  5 H , forming the external connection layers  12  covering the entire surfaces of the barrier layers  11 . The external plating layers  10  having a double layer structure is formed with these barrier layer  11  and external connection layer  12 . In this way, the chip resistor  1  as illustrated in  FIG.  1    to  FIG.  3 A  can be obtained. 
     As described above, the chip resistor  1  according to the first embodiment includes the auxiliary films  8  which are provided inside the boundary portions where the external plating layers  10  are in contact with the overcoat layer  7 . The auxiliary films  8  are formed of a resin material containing conductive particles, and portions of the auxiliary films  8  are inserted between the undercoat layer  6  and the overcoat layer  7 . Therefore, even if the sulfide gas enters from the boundary portions between the external plating layers  10  and the overcoat layer  7 , the auxiliary films  8  block the sulfide gas and thus the sulfide gas cannot reach the front electrodes  3 . This enables prevention of sulfidation of the front electrodes  3 . 
     In the case of a chip resistor having a low resistance (for example, 100 mΩ or less), a resistance of the front electrodes  3  affects a resistance of the entire chip resistor, and thus the lower the resistance of the chip resistor is, the more a TCR increases. However, the chip resistor  1  according to the first embodiment includes the auxiliary films  8  formed at positions over the connecting portions between the front electrodes  3  and the resistor  5 , and accordingly, as illustrated by the arrow in  FIG.  3 B , the length of a portion of the front electrode  3  which is located between the auxiliary film  8  and the resistor  5  in the electric current path from the electrode region (entire region including the front electrode  3 , the external plating layer  10 , and the auxiliary film  8 ) to the resistor  5  can be shortened. Furthermore, the thickness of the auxiliary film  8  allows the electric current to easily flow, resulting in reduction in the resistance of the electrode portion and thus ensuring of a low TCR even in the case of the chip resistor having a low resistance. 
     Furthermore, in the chip resistor  1  according to the first embodiment, since the auxiliary films  8  are formed of a resin material containing conductive particles, it is possible to easily form the auxiliary films  8  using a thick film technique of printing, heating, and curing the resin paste. In particular, in the present embodiment, the auxiliary films  8  are formed of a resin material including metal particles such as Ag and Cu. Reaction of the metal particles with the sulfide gas causes fixation of the sulfide gas to the inside of the auxiliary films  8 , thereby reliably preventing the sulfide gas from entering the inside. 
       FIG.  6    is a cross-sectional view illustrating a main portion of a chip resistor  20  according to a second embodiment. In  FIG.  6   , features corresponding to those of  FIG.  1    to  FIGS.  3 A and  3 B  are provided with the same reference signs. 
     The chip resistor  20  illustrated in  FIG.  6    is different from the chip resistor  1  according to the first embodiment in that the connecting portion of the resistor  5  with the front electrode  3  is an exposed portion  5   b  which is not covered with the undercoat layer  6 , and the auxiliary film  8  is formed so as to cover the entire exposed portion  5   b . The other features of the chip resistor  20  are basically the same as those of the resistor  10 . 
     In the chip resistor  20  according to the second embodiment having the structure as described above, the area of the front electrode  3  connected to the external plating layer  10  at a position not covered with the auxiliary film  8  can be increased, in other words, the length of a portion of the front electrode  3  which is located between the auxiliary film  8  and the resistor  5  in the electric current path from the electrode region (the entire region including the front electrode  3 , the external plating layer  10 , and the auxiliary film  8 ) to the resistor  5  can be shortened. This enables reduction in the resistance of the electrode portion and thus more effective suppression of the deterioration in the TCR. 
       FIG.  7    is a cross-sectional view illustrating a main portion of a chip resistor  30  according to a third embodiment. In  FIG.  7   , features corresponding to those of  FIG.  1    to  FIGS.  3 A and  3 B  are provided with the same reference signs. 
     The chip resistor  30  illustrated in  FIG.  7    is different from the chip resistor  1  according to the first embodiment in that a second front electrode  3   a  is formed on the top surface of the front electrode  3  other than the connecting portion with the resistor  5 , and the auxiliary film  8  is formed at the inner side of the second front electrode  3   a . The other features of the chip resistor  20  are basically the same as those of the chip resistor  10 . Here, the front electrode  3  and the second front electrode  3   a  are made of the same material, and the second front electrode  3   a  is formed on the front electrode  3  before the auxiliary film  8  is formed. 
     In the chip resistor  30  according to the third embodiment having the structure as described above, forming the second front electrode  3   a  on the front electrode  3  other than the connecting portion with the resistor  5  causes a concave step in the connecting portion of the front electrode  3  with the resistor  5 . This concave step enables suppression of flow of the resin onto the second front electrode  3   a  during formation of the auxiliary film  8  made of a resin material in the next step, and therefore, it is possible to accurately form the auxiliary film  8  at a predetermined position. 
     Furthermore, in the chip resistor  30  according to the third embodiment, both the undercoat layer  6  and the overcoat layer  7  are set to be short with respect to the length of the resistor  5  in the direction between the electrodes. In addition, the auxiliary film  8  is located on the upper portion of the connecting portion between the front electrode  3  and the resistor  5  and the outer plating layer  10  is formed further on the upper portion of the auxiliary film  8 , and thus the entire surface of the front electrode  3  is covered with the outer plating layer  10 . As a result, the resistivity of the electrode portion can be further lowered, thereby improving the TCR. 
       FIG.  8    is a cross-sectional view illustrating a main portion of a chip resistor  40  according to a fourth embodiment. In  FIG.  8   , features corresponding to those of  FIG.  1    to  FIGS.  3 A and  3 B  are provided with the same reference signs. 
     The chip resistor  40  illustrated in  FIG.  8    is different from the chip resistor  1  according to the first embodiment in that a Cu layer  13  is provided inside the barrier layer  11  that is a part of the external plating layer  10 . The other features of the chip resistor  40  according to the fourth embodiment are basically the same as those of the first embodiment. 
     In the chip resistor  40  according to the fourth embodiment having the structure as described above, providing the Cu layer  13  inside the barrier layer  11  lowers the resistance of the electrode portion, thereby reducing the TCR. The thickness of the Cu layer  13  is preferably 15 μm to 35 μm, and further providing a Ni layer inside the Cu layer  13  enables stable formation of the Cu layer  13 . 
     The present invention is not limited to the embodiments described above, and various modifications can be made without departing from the technical concept thereof. For example, in the embodiments described above, a chip resistor provided with back electrodes to be electrically connected to the front electrodes on the back surface of the insulating substrate has been described, however, the present invention is also applicable to a chip resistor of the type not provided with such back electrodes. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  20 ,  30 ,  40  chip resistor 
           2  insulating substrate 
           2 A large-sized substrate 
           2 B strip-shaped substrate 
           2 C chip-shaped substrate 
           3  front electrode (electrode) 
           3   a  second front electrode (second electrode) 
           4  back electrode 
           5  resistor 
           5   a  trimming groove 
           5   b  exposed portion 
           6  undercoat layer (first protective film) 
           7  overcoat layer (second protective film) 
           8  auxiliary film 
           9  end face electrode 
           10  external plating layer 
           11  barrier layer 
           12  external connection layer 
           13  Cu layer