Patent Publication Number: US-7907046-B2

Title: Chip resistor and method for producing the same

Description:
TECHNICAL FIELD 
     The present invention relates to a chip resistor including a chip-shaped insulating substrate and a resistor film provided on the substrate, and it also relates to a method for manufacturing such a chip resistor. 
     BACKGROUND ART 
     As disclosed in Patent Document 1, a conventional chip resistor of the above-mentioned type includes a pair of terminal electrodes provided on both ends of an insulating substrate chip. The upper surface of the insulating substrate is formed with a resistor film electrically connected to the paired terminal electrodes. The chip resistor may be mounted on a printed circuit board by soldering. 
     Patent Document 1: JP-A-2000-133507 
     When the voltage of a power source is supplied to the printed circuit board carrying the chip resistor, the voltage is to be applied between the paired terminal electrodes. Since the single resistor film is provided between the paired terminal electrodes, all the power supplied between the electrodes concentrates on the resistor film. The concentration of the power on the resistor film increases the temperature of the resistor film. Accordingly, it is unpractical to use the chip resistor for a high-power circuit. 
     One way to address the above problem may be to arrange a plurality of resistor films in parallel on the upper surface of the insulating substrate between the paired terminal electrodes. With this arrangement, the power supplied between the paired terminal electrodes is divided into each resistor film. Hence, the temperature increase in the respective resistor films is suppressed, so that the chip resistor is applicable to a circuit to which high power is to be supplied. 
     The resistor film of a chip resistor is formed with a trimming groove. By forming a trimming groove, the resistance between the paired terminal electrodes is set to be within a predetermined allowable range. 
     In a chip resistor in which a plurality of resistor films are arranged in parallel between the paired terminal electrodes, each of the resistor films is electrically connected to the terminal electrodes. Thus, it is extremely difficult to make the dimensions of the trimming grooves of all the resistor films equal or substantially equal to each other. In other words, it is difficult to make the resistances of all the resistor films equal or substantially equal to each other. As a result, excessive temperature increase may occur at some of the resistor films which have a relatively high resistance. 
     DISCLOSURE OF THE INVENTION 
     An object of the present invention, which is proposed under the above-described circumstances, is to provide a chip resistor which is capable of suppressing temperature increase at some of the resistor films and a method for manufacturing such a chip resistor. 
     According to a first aspect of the present invention, there is provided a chip resistor comprising: an insulating substrate in the form of a chip; a pair of terminal electrodes formed on both ends of the insulating substrate; a plurality of resistor films formed on an obverse surface of the insulating substrate in parallel with each other between the paired terminal electrodes; and a cover coat formed on the obverse surface of the insulating substrate to cover the resistor films. One of the paired terminal electrodes comprises individual upper electrodes each formed on the obverse surface of the insulating substrate to be independently connected to a respective one of the resistor films and a side electrode formed on a side surface of the insulating substrate to be connected to all the individual upper electrodes. 
     Preferably, the other one of the paired terminal electrodes comprises individual upper electrodes each formed on the obverse surface of the insulating substrate to be independently connected to a respective one of the resistor films and a side electrode formed on the other side surface of the insulating substrate to be connected to all the individual upper electrodes. 
     Preferably, the chip resistor further comprises an auxiliary upper electrode formed on each of the individual upper electrodes to cover the individual upper electrode. Part of the auxiliary upper electrode overlaps an end of the cover coat. 
     Preferably, the other one of the paired terminal electrodes comprises a common upper electrode formed on the obverse surface of the insulating substrate to be connected to all the resistor films and a side electrode formed on the other side surface of the insulating substrate to be connected to the common upper electrode. 
     Preferably, the chip resistor further comprises auxiliary upper electrodes formed on the individual upper electrodes and the common upper electrode to cover the individual upper electrodes and the common upper electrode. Part of the auxiliary upper electrodes overlaps ends of the cover coat. 
     According to a second aspect of the present invention, there is provided a method for manufacturing a chip resistor. The method comprises the steps of forming, on an obverse surface of an insulating substrate in the form of a chip, a plurality of resistor films in parallel with each other and individual upper electrodes independently connected to both ends of each of the resistor films, forming a trimming groove for resistance adjustment in each of the resistor films, forming a cover coat for covering the resistor films on the obverse surface of the insulating substrate, and forming side electrodes on opposite side surfaces of the insulating substrate to be connected to all the individual upper electrodes. 
     Preferably, the method further comprises the step of forming, after the cover coat formation step, an auxiliary upper electrode on each of the individual upper electrodes to cover the individual upper electrode so that part of the auxiliary upper electrode overlaps an end of the cover coat. 
     According to a third aspect of the present invention, there is provided a method for manufacturing a chip resistor. The method comprises the steps of forming, on an obverse surface of an insulating substrate in the form of a chip, a plurality of resistor films in parallel with each other, individual upper electrodes each independently connected to a first end of a respective one of the resistor films and a common upper electrode commonly connected to respective second ends of the resistor films, forming a trimming groove for resistance adjustment in each of the resistor films, forming a cover coat for covering the resistor films on the obverse surface of the insulating substrate, forming a side electrode on a side surface of the insulating substrate to be connected to all the individual upper electrodes, and forming a side electrode on the other side surface of the insulating substrate to be connected to the common upper electrode. 
     Preferably, the method further comprises the step of forming, after the cover coat formation step, auxiliary upper electrodes on the individual upper electrodes and the common upper electrode to cover the individual upper electrodes and the common upper electrode so that part of the auxiliary upper electrodes overlaps ends of the cover coat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view partially cut away, showing a chip resistor according to a first embodiment of the present invention. 
         FIG. 2  is a sectional view taken along lines A-A in  FIG. 1 . 
         FIG. 3  shows a process for manufacturing the chip resistor according to the first embodiment. 
         FIG. 4  shows a process for manufacturing the chip resistor according to the first embodiment. 
         FIG. 5  shows a process for manufacturing the chip resistor according to the first embodiment. 
         FIG. 6  shows a process for manufacturing the chip resistor according to the first embodiment. 
         FIG. 7  shows a process for manufacturing the chip resistor according to the first embodiment. 
         FIG. 8  shows a process for manufacturing the chip resistor according to the first embodiment. 
         FIG. 9  shows a process for manufacturing the chip resistor according to the first embodiment. 
         FIG. 10  shows a process for manufacturing the chip resistor according to the first embodiment. 
         FIG. 11  is a plan view showing a chip resistor according to a second embodiment of the present invention. 
         FIG. 12  is a sectional view taken along lines B-B in  FIG. 11 . 
         FIG. 13  is a plan view partially cut away, showing a chip resistor according to a third embodiment of the present invention. 
         FIG. 14  shows a process for manufacturing the chip resistor according to the third embodiment. 
         FIG. 15  shows a process for manufacturing the chip resistor according to the third embodiment of the present invention. 
         FIG. 16  is a plan view showing a chip resistor according to a fourth embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It is to be noted that the same or similar elements are designated by the same reference signs throughout the figures. 
       FIGS. 1 and 2  show a chip resistor  1  according to a first embodiment of the present invention. 
     The chip resistor  1  includes an insulating substrate  2  which is made of a heat-resistant material such as a ceramic material and generally in the form of an elongated rectangle in plan view, a pair of terminal electrodes  3 , 4  formed at widthwise ends of the insulating substrate  2 , a plurality of resistor films  5  arranged on an obverse surface of the insulating substrate  2  in parallel with each other in the longitudinal direction of the insulating substrate  2 , and a cover coat  6  formed on the obverse surface of the insulating substrate  2  to cover the resistor films  5 . 
     When the chip resistor  1  is mounted on a non-illustrated printed circuit board, the terminal electrodes  3 ,  4  are connected by soldering to a circuit pattern (not shown) of the printed circuit board. 
     The cover coat  6  is made of glass or a heat-resistant synthetic resin. An undercoat  7  made of glass and covering each of the resistor films  5  individually is provided under the cover coat  6 . In  FIG. 1 , the illustration of the undercoat  7  is omitted. 
     The terminal electrode  3  includes individual upper electrodes  8  and a side electrode  9 . Each of the individual upper electrodes  8  is formed on the upper surface of the insulating substrate  2  to be electrically connected individually to an end of a respective resistor film  5 . The individual upper electrodes  8  are made of silver-based conductive paste. The side electrode  9  is formed on a longitudinal side surface  2   a  of the insulating substrate  2  to be electrically connected to all the individual upper electrodes  8 . 
     The terminal electrode  4  includes individual upper electrodes  10  and a side electrode  11 . Each of the individual upper electrodes  10  is formed on the upper surface of the insulating substrate  2  to be electrically connected individually to an end of a respective resistor film  5 . The individual upper electrodes  10  are made of silver-based conductive paste. The side electrode  11  is formed on a longitudinal side surface  2   b  of the insulating substrate  2  to be electrically connected to all the individual upper electrodes  10 . 
     Lower electrodes  12  and  13  are provided on the lower surface of the insulating substrate  2  independently with respect to each of the resistor films  5 . Alternatively, the lower electrodes  12  and  13  may be provided to be common to all the resistor films  5 . The lower electrode  12  is connected to the side electrode  9  along the longitudinal side surface  2   a  of the insulating substrate  2 . The lower electrode  13  is electrically connected to the side electrode  11  along the longitudinal side surface  2   b  of the insulating substrate  2 . 
     Though not illustrated, a solder plating layer is formed, via a nickel plating layer as an underlayer, on the surfaces of the individual upper electrodes  8 ,  10 , side electrodes  9 ,  11  and lower electrodes  12 ,  13 . The nickel plating layer may be omitted. 
     A method for manufacturing the chip resistor  1  will be described below. 
     First, as shown in  FIG. 3 , a material board Al is prepared which integrally includes a plurality of insulating substrates  2  arranged in rows and columns. 
     The material board A 1  is later to be divided by breaking or dicing along the division lines B 1  in the longitudinal direction and the division lines B 2  in the widthwise direction into a plurality of insulating substrates  2 , which will be described later. 
     Then, as shown in  FIG. 4 , individual upper electrodes  8  and  10  are formed on the upper surface of the material board A 1  at appropriate portions in each of the insulating substrates  2  by the application of a metal-based conductive paste such as silver-based paste by screen printing and the subsequent baking. Similarly, lower electrodes  12  and  13  (not shown) are formed on the lower surface of the material board Al at appropriate portions in each of the insulating substrates  2  by the application of a metal-based conductive paste such as silver-based paste by screen printing and the subsequent baking. 
     Then, as shown in  FIG. 5 , a plurality of resistor films  5  are formed on the upper surface of the material board A 1  at appropriate portions in each of the insulating substrates  2  by the application of material paste by screen printing and the subsequent baking. 
     Unlike the above, the resistor films  5  may be formed before the individual upper electrodes  8  and  10  are formed. 
     Then, as shown in  FIG. 6 , an undercoat  7  of glass is formed on each of the resistor films  5  by the application of material paste by screen printing and the subsequent baking. Then, the total resistance between the paired terminal electrodes  3 ,  4  (see  FIGS. 1 and 2 ) is so adjusted as to lie within a predetermined allowable range. The resistance adjustment is performed by forming a trimming groove  5   a  in each of the resistor films  5 . Specifically, with probes for energization held in contact with the individual upper electrodes  8  and  10 , the trimming groove  5   a  is formed to have an appropriate dimension while measuring the resistance of the resistor film  5 . 
     In this way, in this method for manufacturing the chip resistor  1 , the trimming groove  5   a  is formed in each of the resistor films  5  before the side electrodes  9 ,  11  are formed. Since each of the resistor films  5  and the individual upper electrodes  8 ,  10  on the ends thereof are independent from others, the formation of the trimming groove  5   a  is properly performed individually with respect to each of the resistor films  5  while measuring the resistance of the resistor film  5 . 
     Thus, it is possible to make the dimensions of the trimming grooves  5   a  of all the resistor films  5  be equal or substantially equal to each other. That is, it is easy to make the resistances of the resistor films  5  be equal or substantially equal to each other. 
     Then, as shown in  FIG. 7 , cover coats  6  are formed on the upper surface of the material board A 1  at an appropriate portion in each of the insulating substrates  2 . Specifically, when glass is used as the material, the cover coat is formed by the application of glass paste by screen printing and the subsequent baking. When a synthetic resin is used as the material, the cover coat  6  is formed by the application of the synthetic resin paste by screen printing and the subsequent drying. 
     Then, as shown in  FIG. 8 , the material board A 1  is divided along the division lines B 1  in the longitudinal direction into material boards A 2  in the form of a bar. 
     Then, as shown in  FIG. 9 , side electrodes  9 ,  11  are formed on opposite side surfaces A 2   a , A 2   b  of the bar-shaped material board A 2 . Specifically, when metal-based conductive paste is used as the material, the side electrodes are formed by the application of the material paste by screen printing and the subsequent baking. When nonmetal-based conductive paste is used as the material, the side electrodes  9 ,  11  are formed by the application of the material paste by screen printing and the subsequent drying. 
     Then, as shown in  FIG. 10 , the bar-shaped material board A 2  is divided along the division line B 2  in the widthwise direction into insulating substrates  2 . Then, plating such as barrel plating is performed. Thus, the chip resistor  1  is obtained. 
     As noted above, in making the chip resistor  1 , the trimming groove  5   a  is formed in each of the resistor films  5  before the side electrodes  9 ,  11  are formed. Since each of the resistor films  5  and the individual upper electrodes  8 ,  10  on the ends thereof are independent from others, the formation of the trimming groove  5   a  is properly performed individually with respect to each of the resistor films  5  while measuring the resistance of the resistor film  5 . Thus, it is possible to make the resistances of all the resistor films  5  be equal or substantially equal to each other, so that excessive temperature rise at any of the resistor films  5  is prevented. 
       FIGS. 11 and 12  show a chip resistor  1 A according to a second embodiment of the present invention. 
     The chip resistor  1 A differs from the chip resistor  1  of the first embodiment in that auxiliary upper electrodes  14 ,  15  for covering the individual upper electrodes  8 ,  10  on the insulating substrate  2  are formed on the individual upper electrodes  8 ,  10 . The auxiliary upper electrodes  14 ,  15  partially overlap the ends of the cover coat  6 . The auxiliary upper electrodes  14  and  15  are electrically connected to the side electrodes  9  and  10 , respectively. The structure of other parts is the same as that of the first embodiment. The auxiliary upper electrodes  14 ,  15  may be provided individually for each pair of the individual upper electrodes  8 ,  10  or may be so formed as to extend continuously over all the individual upper electrodes  8 ,  10 . 
     When the individual upper electrodes  8 ,  10  are made of silver-based conductive paste having a low resistivity, the auxiliary upper electrodes  14 ,  15  reliably prevent the corrosion such as migration of the individual upper electrodes  8 ,  10  due to sulfur components in the air. The auxiliary upper electrodes  14 ,  15  eliminate or reduce the stepped portion formed between the upper surface of the terminal electrode  3 ,  4  and the upper surface of the cover coat  6 . Further, the provision of the auxiliary upper electrodes  14 ,  15  reduces the resistance between the terminal electrodes  3 ,  4 . 
     To manufacture the chip resistor  1 A of the second embodiment, after the cover coat  6  is formed (see  FIG. 7 ), auxiliary upper electrodes  14 ,  15  are formed on the upper surface of the material board Al to cover the upper surfaces of the individual upper electrodes  8 ,  10 . The auxiliary upper electrodes may be formed by the application of metal-based conductive paste by screen printing and the subsequent baking. When nonmetal-based conductive paste is used as the material, the auxiliary upper electrodes  14 ,  15  may be formed by the application of the material paste by screen printing and the subsequent drying. Thereafter, as shown in  FIG. 8 , the material board A 1  is divided along the division lines B 1  in the longitudinal direction into bar-shaped material boards A 2 . 
       FIG. 13  shows a chip resistor  1 B according to a third embodiment of the present invention. 
     The chip resistor  1 B of the third embodiment differs from that of the first embodiment in that a common upper electrode  16  electrically connected to all the resistor films  5  is provided on the insulating substrate  2  instead of the individual upper electrodes  8  of the terminal electrode  3 . The structure of other parts is the same as that of the first embodiment. With this structure again, the same advantages as those of the first embodiment are obtained. 
     To manufacture the chip resistor  1 B of the third embodiment, as shown in  FIG. 14 , the individual upper electrodes  10  and the common upper electrodes  16  are formed on the material board A 1  at appropriate portions in each of the insulating substrates  2  by the application of metal-based conductive paste such as silver-based paste by screen printing and the subsequent baking. 
     Then, as shown in  FIG. 15 , a plurality of resistor films  5  are formed at appropriate portions in each of the insulating substrates  2  by the application of material paste by screen printing and the subsequent baking so that each of the individual upper electrodes  10  and the common upper electrode  16  are connected to each other. The subsequent steps are the same as those of the manufacturing process of the first embodiment. 
       FIG. 16  shows a chip resistor  1 C according to a fourth embodiment of the present invention. 
     The chip resistor  1 C includes auxiliary upper electrodes  17  and  18  formed to cover the common upper electrode  16  and the individual upper electrodes  10  on the insulating substrate  2 . The structure of other parts is the same as that of the third embodiment. The auxiliary upper electrode  18  may be provided individually for each of the individual upper electrodes  10  or may be so formed as to extend continuously over all the individual upper electrodes  10 . With this structure again, the same advantages as those of the third embodiment are obtained. 
     The present invention is not limited to the foregoing embodiments. For instance, the present invention is also applicable to a multiple chip resistor including a single insulating substrate formed with a plurality of resistor films and a pair of terminal electrodes formed at the ends of each of the resistor films. 
     The specific structure of each part of the chip resistor according to the present invention may be varied in design in various ways without departing from the spirit of the invention.