Patent Publication Number: US-2022221414-A1

Title: Production method for sulfidation detection sensor

Description:
TECHNICAL FIELD 
     The present invention relates to a method of manufacturing a sulfidation detection sensor capable of detecting a cumulative amount of sulfide in a corrosive environment. 
     As an internal electrode of an electronic component such as a chip resistor, generally, an Ag (silver) based electrode material having a low specific resistance is used. However, silver sulfide occurs when silver is exposed to sulfide gas and the silver sulfide is an insulator, which may result in malfunction such as disconnection of the electronic component. In recent years, measures against sulfidation, such as forming an electrode that hardly gets sulfurized by adding Pd (palladium) and Au (gold) to Ag, or forming the electrode into a structure that prevents the sulfide gas from reaching the electrode have been taken. 
     However, even when such measures against sulfidation are taken for the electronic component, in the case where the electronic component is exposed to sulfide gas for a long time or exposed to high-concentration sulfide gas, disconnection cannot be prevented completely. Accordingly, it is necessary to detect the disconnection in advance to prevent failure from occurring at an unexpected timing. 
     With this regard, as described in Patent Literature  1 , there has been proposed a sulfidation detection sensor capable of detecting a level of cumulative sulfide in an electronic component to detect a risk of failure such as disconnection which occurs in the electronic component due to sulfidation. 
     Patent Literature 1 discloses a sulfidation detection sensor configured such that a sulfidation detection body mainly made of Ag is provided on an insulation substrate, a transparent protective film having sulfide gas permeability is provided so as to cover the sulfidation detection body, and end face electrodes connected to the sulfidation detection body are provided, respectively, at both side end portions of the insulation substrate. When the sulfidation detection sensor configured as above is mounted on a circuit board together with other electronic components and then the circuit board is used in an atmosphere containing sulfide gas, the other electronic components get sulfurized over time, and the sulfide gas passes through the protective film of the sulfidation detection sensor and comes into contact with the sulfidation detection body, whereby decreasing the volume of silver forming the sulfidation detection body in accordance with the concentration of the sulfide gas and the elapsed time. Accordingly, by detecting change in the resistance values and disconnection in the sulfidation detection body, it is possible to detect the level of sulfidation. 
     Patent Literature 1 further discloses a sulfidation detection sensor in which a sulfidation detection body is exposed to the outside without being covered with a protective film so that the sulfidation detection body can detect sulfidation with high sensitivity. In a method of manufacturing the sulfidation detection sensor configured as above, as illustrated in  FIG. 14A , a large-sized substrate  100  from which multi-piece insulation substrates are obtained is prepared, and a sulfidation detection body  101  is formed on a front surface of the large-sized substrate  100 . After a pair of back electrodes  102  is formed on a back surface of the large-sized substrate  100 , a protective film  103  made of a soluble material is formed on the center portion of the sulfidation detection body  101 . Next, after the large-sized substrate  100  is primarily divided along primary slits to obtain strip-shaped substrates, end face electrodes  104  are formed at both end portions of each strip-shaped substrate by coating or vapor deposition. Then, after the strip-shaped substrate is secondarily divided along secondary slits to obtain chip substrates having the same size as that of the insulation substrate, external electrodes  105  which covers both end portions of the sulfidation detection body  101  and also covers the front surfaces of the end face electrodes  104  and those of the back electrodes  102  by sequentially performing Ni plating and Sn plating with respect to the chip substrates. Thereafter, the protective film  103  is removed by using a solvent or the like, whereby, as illustrated in  FIG. 14B , it is possible to obtain the sulfidation detection sensor configured such that the sulfidation detection body  101  whose center portion is exposed is formed on the insulation substrate  100 A. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A-2009-250611 
     SUMMARY OF INVENTION 
     Technical Problem 
     According to Patent Literature 1, since the sulfidation detection sensor is mounted on a circuit board in a state where the sulfidation detection body  101  is exposed, the sensitivity to sulfidation is improved so that the sulfidation can be detected with high sensitivity. However, both the end portions of the protective film  103  formed by printing of resin paste have inclined surfaces, and thus plating materials of the external electrodes  105  are formed in close contact with the inclined surfaces of the protective film  103 . Accordingly, as illustrated by the arrows P in  FIG. 14B , the end portions of the external electrodes  105  after the protective film  103  is removed are pointed in angular shapes, which causes a problem that the external electrodes  105  are easily detached from the substrate. 
     The present invention has been made in view of the circumstances above of the prior art, and an object of the present invention is to provide a sulfidation detection sensor capable of detecting sulfidation with high sensitivity while preventing detachment of an external electrode. 
     Solution to Problem 
     In order to achieve the object above, the present invention provides a method of manufacturing a sulfidation detection sensor, comprising: a conductor forming step of forming a sulfidation detection conductor on a main surface of a large-sized substrate; a protective film forming step of forming a pair of first protective films made of an insoluble material on the sulfidation detection conductor, and forming a second protective film made of a soluble material so as to cover the sulfidation detection conductor positioned between the pair of first protective films; a primary dividing step of primarily dividing the large-sized substrate into strip-shaped substrates after the protective film forming step; an end face electrode forming step of forming end face electrodes on divided faces of each of the strip-shaped substrates, respectively; a secondary dividing step of secondarily dividing each of the strip-shaped substrates into a plurality of chip substrates after the end face electrode forming step; and an external electrode forming step of forming external electrodes on outer sides of the pair of first protective films, respectively, by performing electrolytic plating with respect to each of the chip substrates, wherein surface height of the second protective film is set to be lower than surface height of the first protective films. 
     According to the manufacturing method of the sulfidation detection sensor including the steps above, at the time of forming the external electrodes by plating with respect to each chip substrate obtained by secondarily dividing each strip-shaped substrate, a plating material adheres to an end portion of each first protective film but does not adhere to the second protective film. Accordingly, in a state of the product in which the second protective film is removed to expose the sulfidation detection portion of the sulfidation detection conductor, a pointed portion having an angular shape does not appear at an end portion of each external electrode, which is in close contact with the corresponding first protective film. As a result, it is possible to realize the sulfidation detection sensor with high sensitivity in which the sulfidation detection portion is exposed to the outside while preventing detachment of the external electrodes. Furthermore, since the surface height of the second protective film is set to be lower than the surface height of the first protective films, even in the case of forming the end face electrodes by sputtering from the end face sides of each strip-shaped substrate, the first protective films block the sputtering film and prevent the sputtering film from being formed on the second protective film. In addition, in the case of performing sputtering with respect to a plurality of vertically stacked strip-shaped substrates, it is possible to prevent the strip-shaped substrates from sticking to each other due to the adhesive force of the second protective film made of a soluble material. 
     As a further aspect, the method of manufacturing a sulfidation detection sensor described above further comprises: an internal electrode forming step of forming a pair of internal electrodes on the main surface of the large-sized substrate so as to be connected to both end portions of the sulfidation detection conductor, respectively, wherein the pair of first protective films is formed so as to cover overlapping portions in which the sulfidation detection conductor overlaps with each of the pair of internal electrodes. According to the aspect above, the pair of first protective films is formed on the overlapping portions, respectively, in which the sulfidation detection conductor overlaps with each of the pair of internal electrodes. As a result, it is possible to easily make the surface height of the first protective films higher than the surface height of the second protective film. 
     As a still further aspect, the method of manufacturing a sulfidation detection sensor described above further comprises: a resistor forming step of forming a pair of resistors on the main surface of the large-sized substrate so as to be connected to both end portions of the sulfidation detection conductor, respectively; an internal electrode forming step of forming a pair of internal electrodes that is connected to the pair of resistors, respectively; and a trimming step of forming trimming grooves on the pair of resistors to ad 0017 just resistance values, wherein each of the first protective films has an undercoat layer and an overcoat layer which cover corresponding one of the pair of resistors, and the pair of external electrodes is formed so as to cover the pair of internal electrodes, respectively. According to the aspect above, the pair of resistors and the pair of internal electrodes are arranged, respectively, in symmetrical positions interposing the second protective film on the center portion. As a result, it is possible to overlap a plurality of strip-shaped substrates in a stable posture. 
     As a still further aspect, the method of manufacturing a sulfidation detection sensor described above further comprises: an internal electrode forming step of forming an internal electrode on the main surface of the large-sized substrate so as to face one end portion of the sulfidation detection conductor with a certain space therebetween; a resistor forming step of forming a resistor that connects between the sulfidation detection conductor and the internal electrode; and a trimming step of forming a trimming groove on the resistor to adjust a resistance value, wherein one of the first protective films has an undercoat layer and an overcoat layer which cover the resistor while the other one of the first protective films is formed on the sulfidation detection conductor. According to the aspect above, at the time of providing a trimming groove for adjustment of the resistance value of the resistor, trimming can be performed while bringing probes into contact with the sulfidation detection conductor, which is positioned on the outer side of the other one of the first protective films, and the internal electrode, which is connected to the resistor. As a result, it is possible to prevent the sulfidation detection portion of the sulfidation detection conductor from being damaged by the probes. 
     Advantageous Effects of Invention 
     According to a method of manufacturing a sulfidation detection sensor of the present invention, it is possible to detect sulfidation with high sensitivity by exposing a sulfidation detection portion of a sulfidation detection conductor while preventing detachment of an external electrode. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view of a sulfurization detection sensor 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 . 
       Each  FIG. 3A ˜ 3 H is a plan view illustrating a manufacturing process of the sulfurization detection sensor according to the first embodiment. 
       Each  FIG. 4A ˜ 4 H is a cross-sectional view illustrating the manufacturing process of the sulfurization detection sensor according to the first embodiment. 
         FIG. 5  is a plan view of a sulfurization detection sensor according to a second embodiment of the present invention. 
         FIG. 6  is a cross-sectional view taken along the line VI-VI of  FIG. 5 . 
       Each  FIG. 7A ˜ 7 H is a cross-sectional view illustrating a manufacturing process of the sulfurization detection sensor according to the second embodiment. 
         FIG. 8  is a plan view of a sulfurization detection sensor according to a third embodiment of the present invention. 
         FIG. 9  is a cross-sectional view taken along the line IX-IX of  FIG. 8 . 
       Each  FIG. 10A ˜ 10 H is a cross-sectional view illustrating a manufacturing process of the sulfurization detection sensor according to the third embodiment. 
         FIG. 11  is a plan view of a sulfurization detection sensor according to a fourth embodiment of the present invention. 
         FIG. 12  is a cross-sectional view taken along the line XII-XII of  FIG. 11 . 
       Each  FIG. 13A ˜ 13 H is a cross-sectional view illustrating a manufacturing process of the sulfurization detection sensor according to the fourth embodiment. 
       Each  FIG. 14A and 14B  is a cross-sectional view illustrating a manufacturing process of the sulfurization detection sensor 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 sulfidation detection sensor 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 . 
     As illustrated in  FIG. 1  and  FIG. 2 , a sulfidation detection sensor  10  according to the first embodiment mainly includes a rectangular parallelepiped insulation substrate  1 , a sulfidation detection conductor  2  provided on a front surface of the insulation substrate  1 , a pair of first protective films  3  provided on the sulfidation detection conductor  2  with a certain space therebetween, a pair of back electrodes  4  provided on a back surface of the insulation substrate  1  at both end portions in the longitudinal direction thereof, respectively, a pair of end face electrodes  5  provided on the insulation substrate  1  at both the end portions in the longitudinal direction thereof, respectively, and a pair of external electrodes  6  provided on front surfaces of the end face electrodes  5  and those of the back electrodes  4 , respectively. 
     The sulfidation detection conductor  2  is obtained by scree-printing, drying, and firing Ag-based paste that contains silver as a main component. The sulfidation detection conductor  2  is formed so as to cover the front surface of the insulation substrate  1 . 
     The pair of first protective films  3  is made of an insoluble material having a property that is insoluble in a solvent used in the subsequent second protective film removing step, and is obtained by screen-printing, curing, and heating epoxy-based resin paste. The first protective films  3  are formed at two positions, respectively, which are separated from each other across the center portion of the sulfidation detection conductor  2 . As will be described later, the center portion of the sulfidation detection conductor  2  interposed between the pair of first protective films  3  serves as a sulfidation detection portion  2   a.    
     The pair of back electrodes  4  is obtained by screen-printing, drying, and firing Ag-based paste that contains silver as a main component. The back electrodes  4  may be formed in a step different from the step of forming the sulfidation detection conductor  2 , whereas they may be formed at the same time. 
     The pair of end face electrodes  5  is obtained by sputtering Ni/Cr or applying Ag-based paste on both end faces of the insulation substrate  1  and then heating and curing the paste. The pair of end face electrodes  5  is formed in the shape having a U-shaped cross section. 
     The pair of external electrodes  6  is composed of a double layer structure having a barrier layer and an external connection layer. The barrier layer is a Ni plating layer formed by electrolytic plating while the external connection layer is an Sn plating layer formed by electrolytic plating. Each of the external electrodes  6  covers the front surface of corresponding one of the back electrodes  4  which is exposed from corresponding one of the end face electrodes  5 , and also covers the entire surface of the corresponding one of the end face electrodes  5 . 
     Next, a manufacturing process of the sulfidation detection sensor  10  will be described with reference to  FIG. 3  and  FIG. 4 . Each  FIG. 3A  to  FIG. 3H  is a plan view illustrating a surface of a large-sized substrate used in the manufacturing process. Each  FIG. 4A  to  FIG. 4H  is a cross-sectional view of one of the chips within the large-sized substrate, which is taken along a center portion of the large-sized substrate in the longitudinal direction thereof. 
     As illustrated in  FIG. 3A  and  FIG. 4A , the first step of the manufacturing process of the sulfidation detection sensor  10  is to prepare a large-sized substrate  10 A from which multi-piece insulation substrates  1  are obtained. In the large-sized substrate  10 A, primary division grooves and secondary division grooves are provided in advance to form a grid pattern, and each one of the grids divided by the primary division grooves and the secondary division grooves serves as a single chip region.  FIG. 3  illustrates the large-sized substrate  10 A corresponding to a single chip region as a representative, but practically, each step described below is collectively performed with respect to the large-sized substrate corresponding to multi-piece chip regions. 
     That is, as illustrated in  FIG. 3B  and  FIG. 4B , after screen-printing Ag-based paste (Ag—Pd 20%) on a back surface of the large-sized substrate  10 A, by drying and firing the screen-printed paste, the step of forming the pair of back electrodes  4  (back electrode forming step) is performed. 
     Next, after screen-printing Ag-based paste containing Ag as a main component on a front surface of the large-sized substrate  10 A, by drying and firing the screen-printed paste, as illustrated in  FIG. 3C  and  FIG. 4C , the step of forming the sulfidation detection conductor  2  extending longitudinally on the front surface of the large-sized substrate  10 A (conductor forming step) is performed. In this connection, note that the order of formation of the back electrodes  4  and that of the sulfidation detection conductor  2  may be reversed, or performed at the same time. 
     Next, after screen-printing epoxy-based resin paste on a front surface of the sulfidation detection conductor  2 , by heating and curing the screen-printed paste, as illustrated in  FIG. 3D  and  FIG. 4D , the step of forming the pair of first protective films  3  made of an insoluble material on two positions, respectively, near the center portion of the sulfidation detection conductor  2  is performed (first protective film forming step). The first protective films  3  may have a single layer structure, whereas a multi-layer structure having two or more layers realizes the first protective films  3  whose film thickness is large. 
     Next, after screen-printing soluble resin paste such as phenol resin on the sulfidation detection conductor  2  so as to cover an exposed portion of the sulfidation detection conductor  2  interposed between the pair of first protective films  3 , by heating and curing the screen-printed paste, as illustrated in  FIG. 3E  and  FIG. 4E , the step of forming the second protective film  7  which covers the sulfidation detection conductor  2  between the pair of first protective films  3  (second protective film forming step) is performed. The second protective film  7  is made of a soluble material having a property that is soluble in a solvent but is insoluble in plating solution to be used in the subsequent external electrode forming step. The second protective film  7  is formed such that the surface height thereof is lower than the surface height of the first protective films  3 . In this connection, it is preferable to form the second protective film  7  by using a material having viscosity lower than that of the first protective films  3  so as to easily make the film thickness thereof small. 
     Next, after primarily dividing the large-sized substrate  10 A along the primary division grooves to obtain strip-shaped substrates  10 B (primary dividing step), by sputtering Ni/Cr on divided faces of each strip-shaped substrate  10 B, as illustrated in  FIG. 3F  and  FIG. 4F , the step of forming the pair of end face electrodes  5  on the divided faces of each strip-shaped substrate  10 B, respectively, each of which covers one of both the end portions of the sulfidation detection conductor  2  in the longitudinal direction thereof and also covers most of corresponding one of the back electrodes  4  (end face electrode forming step) is performed. The sputtering above is performed with respect to the plurality of vertically stacked strip-shaped substrates  10 B. At the time of sputtering, since the first protective films  3  protrude upward more than the second protective film  7 , the pair of first protective films  3  provided on an arbitrary one of the strip-shaped substrates  10 B comes into contact with the lower surface of another one of the strip-shaped substrates  10 B arranged on the upper side. As a result, the first protective film  3  can block the sputtering film and prevent the sputtering film from reaching the second protective film  7 , and even in the case where the second protective film  7  is cured at a low temperature or the like so as to be easily removed in a later step and thus gets adhesive force, it is possible to prevent the vertically stacked strip-shaped substrates  10 B from sticking to each other due to the adhesive force of the second protective film  7 . In this connection, instead of sputtering Ni/Cr on the divided faces of each strip-shaped substrate  10 B, Ag-based paste may be applied thereto, heated, and cured to form the end face electrodes  5 . 
     Next, after secondarily dividing the strip-shaped substrates  10 B along the secondary division grooves to obtain a plurality of chip-shaped substrates  10 C (secondary dividing step), by performing electrolytic plating with respect to each chip-shaped substrate  10 C to form a Ni—Sn plating layer, as illustrated in  FIG. 3G  and  FIG. 4G , the step of forming the pair of external electrodes  6  which covers the entire surfaces of the end face electrodes  5  and also covers exposed portions of the back electrodes  4  (external electrode forming step) is performed. 
     Next, by immersing each chip-shaped substrate  10 C in an alkaline solution or the like which dissolves the second protective film  7  but does not dissolve the first protective films  3  to remove the second protective film  7 , as illustrated in  FIG. 3H  and  FIG. 4H , the step of exposing the sulfidation detection portion  2   a  of the sulfidation detection conductor  2  between the pair of first protective films  3  (second protective film removing step) is performed. In this way, the manufacturing process of the sulfidation detection sensor  10  illustrated in  FIG. 1  and  FIG. 2  is completed. In this connection, note that the second protective film removing step can be performed after mounting the sulfidation detection sensor  10  on a circuit board. In this case, the sulfidation detection portion  2   a  can be protected until being mounted on the circuit board. 
     As described above, in the method of manufacturing the sulfidation detection sensor  10  according to the first embodiment, at the time of forming the external electrodes  6  by plating with respect to each chip-shaped substrate  10 C which has been obtained by the secondary dividing step of dividing the strip-shaped substrates  10 B, plating materials of the external electrodes  6  do not adhere to the second protective film  7 . Accordingly, even in the case of exposing the sulfidation detection unit  2   a  to the outside by removing the second protective film  7 , angular shaped pointed portions do not appear at end portions of the external electrodes  6 , which are in close contact with the first protective films  3 , respectively. As a result, it is possible to realize the sulfidation detection sensor  10  with high sensitivity in which the sulfidation detection portion  2   a  is exposed to the outside while preventing detachment of the external electrodes  6 . Furthermore, since the surface height of the second protective film  7  is set to be lower than the surface height of the first protective films  3 , even in the case of forming the end face electrodes  5  by sputtering from the end face sides of the strip-shaped substrate  10 B, the first protective films  3  can block the sputtering film and prevent the sputtering film from being formed on the second protective film  7 . Still further, since the surface height of the second protective film  7  is set to be lower than the surface height of the first protective films  3 , at the time of performing sputtering with respect to the vertically stacked strip-shaped substrates  10 B, it is possible to prevent the strip-shaped substrates  10 B from sticking to each other due to the adhesive force of the second protective film  7  made of a soluble material. 
       FIG. 5  is a plan view of a sulfidation detection sensor  20  according to a second embodiment of the present invention.  FIG. 6  is a cross-sectional view taken along the line VI-VI of  FIG. 5 . In the following, portions corresponding to those illustrated in  FIG. 1  and  FIG. 2  are provided with the same reference signs, and repetitive explanation therefor will be omitted. 
     As illustrated in  FIG. 5  and  FIG. 6 , in the sulfidation detection sensor  20  according to the second embodiment, a pair of internal electrodes  21  is formed on the front surface of the insulation substrate  1  at both end portions in the longitudinal direction thereof, respectively, and a sulfidation detection conductor  22  is connected to the pair of internal electrodes  21  such that both end portions of the sulfidation detection conductor  22  overlap with the pair of internal electrodes  21 , respectively. A pair of first protective films  23  is formed so as to cover the overlapping portions, respectively, in which the sulfidation detection conductor  22  overlaps with each of the pair of internal electrodes  21 , and a sulfidation detection portion  22   a  of the sulfidation detection conductor  22  is exposed between the first protective films  23 . The other configurations are basically the same as those of the sulfidation detection sensor  10  according to the first embodiment. 
     In the following, a manufacturing process of the sulfidation detection sensor  20  configured as above will be described with reference to the cross-sectional view of  FIG. 7 . Firstly, as illustrated in  FIG. 7A , after screen-printing Ag-based paste (Ag—Pd  20 %) on a back surface of a large-sized substrate  20 A, by drying and firing the screen-printed paste, the step of forming the pair of back electrodes  4  facing each other with a certain space therebetween (back electrode forming step) is performed. At the same time, or before or after this step, after screen-printing Ag-based paste (Ag—Pd 20%) on a front surface of the large-sized substrate  20 A, by drying and firing the screen-printed paste, as illustrated in  FIG. 7B , the step of forming the pair of internal electrodes  21  facing each other with a certain space therebetween (internal electrode forming step) is performed. 
     Next, after screen-printing Ag-based paste on the front surface of the large-sized substrate  20 A, by drying and firing the screen-printed paste, as illustrated in  FIG. 7C , the step of forming the sulfidation detection conductor  22  connected to the pair of internal electrodes  21  (conductor forming step) is performed. Since the sulfidation detection conductor  22  and the pair of internal electrodes  21  are connected such that their end portions overlap with each other, each of the overlapped portions is composed of a double layer structure having large film thickness. 
     Next, after screen-printing epoxy-based resin paste so as to cover the overlapped portions, in which the sulfidation detection conductor  22  overlaps with each of the pair of internal electrodes  21 , by heating and curing the screen-printed paste, as illustrated in  FIG. 7D , the step of forming the pair of first protective films  23  made of an insoluble material on both the end portions of the sulfidation detection conductor  22 , respectively, (first protective film forming step) is performed. 
     Next, after screen-printing soluble resin paste such as phenol resin on the front surface of the sulfidation detection conductor  22  interposed between the pair of first protective films  23 , by heating and curing the screen-printed paste, as illustrated in  FIG. 7E , the step of forming the second protective film  24  which covers the sulfidation detection conductor  22  between the pair of first protective films  23  (second protective film forming step) is performed. The second protective film  24  is made of a soluble material having a property that is soluble in a solvent but is insoluble in plating solution to be used in the subsequent external electrode forming step. The second protective film  24  is formed such that the surface height thereof is lower than the surface height of the first protective films  23 . 
     Next, after primarily dividing the large-sized substrate  20 A along the primary division grooves to obtain strip-shaped substrates  20 B (primary dividing step), by applying Ag-based paste on divided faces of each strip-shaped substrate  20 B and then heating and curing the applied paste, as illustrated in  FIG. 7F , the step of forming the pair of end face electrodes  5  on divided faces of each strip-shaped substrate  20 B, respectively, each of which connects corresponding ones of the internal electrodes  21  and the back electrodes  4  (end face electrode forming step) is performed. In this connection, instead of applying Ag-based paste to form the end face electrodes  5 , in the same manner as the first embodiment described above, the end face electrodes  5  may be formed by sputtering Ni/Cr on the divided faces of each strip-shaped substrate  20 B. 
     Next, after secondarily dividing the strip-shaped substrates  20 B along the secondary division grooves to obtain a plurality of chip-shaped substrates  20 C (secondary dividing step), by performing electrolytic plating with respect to each chip-shaped substrate  20 C to form a Ni—Sn plating layer, as illustrated in  FIG. 7G , the step of forming the pair of external electrodes  6  which covers the entire surfaces of the end face electrodes  5  and also covers exposed portions of the internal electrodes  21  and those of the back electrodes  4  (external electrode forming step) is performed. 
     Next, by immersing each chip-shaped substrate  20 C in an alkaline solution or the like which dissolves the second protective film  24  but does not dissolve the first protective films  23  to remove the second protective film  24 , as illustrated in  FIG. 7H , the step of exposing the sulfidation detection portion  22   a  of the sulfidation detection conductor  22  between the pair of first protective films  23  (second protective film removing step) is performed. In this way, the manufacturing process of the sulfidation detection sensor  20  illustrated in  FIG. 5  and  FIG. 6  is completed. In this connection, note that the second protective film removing step can be performed after mounting the sulfidation detection sensor  20  on a circuit board. In this case, the sulfidation detection portion  22   a  can be protected until being mounted on the circuit board. 
     As described above, in the manufacturing method of the sulfidation detection sensor  20  according to the second embodiment, after forming the pair of internal electrodes  21  connected to both the end portions of the sulfidation detection conductor  22 , respectively, on the front surface of the large-sized substrate  20 A, the step of forming the first protective films  23  on the overlapped portions in which the sulfidation detection conductor  22  overlaps with each of the pair of internal electrodes  21  is performed. As a result, in addition to the effects that can be obtained by the first embodiment, the second embodiment can obtain an advantageous effect that the surface height of the first protective films  23  can be easily set to be higher than the surface height of the second protective film  24 . 
       FIG. 8  is a plan view of a sulfidation detection sensor  30  according to a third embodiment of the present invention.  FIG. 9  is a cross-sectional view taken along the line IX-IX of  FIG. 8 . In the following, portions corresponding to those illustrated in  FIG. 1  and  FIG. 2  are provided with the same reference signs, and repetitive explanation therefor will be omitted. 
     As illustrated in  FIG. 8  and  FIG. 9 , in the sulfidation detection sensor  30  according to the third embodiment, a pair of internal electrodes  31  is formed on the front surface of the insulation substrate  1  at both end portions in the longitudinal direction thereof, respectively, and a pair of resistors  32  is connected to a sulfidation detection conductor  33  in series between the internal electrodes  31 . The resistors  32  are covered with first protective films  34 , respectively, each of which is composed of a double layer structure having an undercoat layer  34   a  and an overcoat layer  34   b,  and a sulfidation detection portion  33   a  of the sulfidation detection conductor  33  is exposed between the first protective films  34 . The other configurations are basically the same as those of the sulfidation detection sensor  10  according to the first embodiment. 
     In the following, a manufacturing process of the sulfidation detection sensor  30  configured as above will be described with reference to the cross-sectional view of  FIG. 10 . Firstly, as illustrated in  FIG. 10A , after screen-printing Ag-based paste on a back surface of a large-sized substrate  30 A, by drying and firing the screen-printed paste, the step of forming the pair of back electrodes  4  facing each other with a certain space therebetween (back electrode forming step) is performed. 
     Next, after screen-printing Ag-based paste containing Ag as a main component on a front surface of the large-sized substrate  30 A, by drying and firing the screen-printed paste, as illustrated in  FIG. 10B , the step of forming the sulfidation detection conductor  33  and the pair of internal electrodes  31  with certain spaces therebetween (conductor forming step and internal electrode forming step) is performed. In this way, by forming the sulfidation detection conductor  33  and the pair of internal electrodes  31  at the same time by using the same material, spaces between the sulfidation detection conductor  33  and each of the pair of internal electrodes  31  can be formed without variation. 
     Next, after screen-printing resistor paste such as ruthenium oxide on the front surface of the large-sized substrate  30 A, by drying and firing the screen-printed paste, as illustrated in  FIG. 10C , the step of forming the pair of resistors  32  such that both ends thereof are connected to the sulfidation detection conductor  33  and each of the internal electrodes  31 , respectively (resistor forming step) is performed. 
     Next, after screen-printing glass paste so as to cover the resistors  32  and then drying and firing the screen-printed paste to form the undercoat layers  34   a,  the step of forming trimming grooves (not illustrated) for adjustment of resistance values from above the undercoat layers  34   a  (trimming step) is performed. Thereafter, by screen-printing epoxy-based resin paste so as to cover the undercoat layers  34   a  and then heating and curing the screen-printed paste, as illustrated in  FIG. 10D , the step of forming the pair of first protective films  34 , which is composed of the double layer structure having the undercoat layer  34   a  and the overcoat layer  34   b,  at both end portions of the sulfidation detection conductor  33 , respectively, (first protective film forming step) is performed. 
     Next, after screen-printing soluble resin paste such as phenol resin on the front surface of the sulfidation detection conductor  33 , by heating and curing the screen-printed paste, as illustrated in  FIG. 10E , the step of forming the second protective film  35  which covers the sulfidation detection conductor  33  between the pair of first protective films  34  (second protective film forming step) is performed. The second protective film  35  is made of a soluble material having a property that is soluble in a solvent but is insoluble in plating solution to be used in the subsequent external electrode forming step. The second protective film  35  is formed such that the surface height thereof is lower than the surface height of the first protective films  34 . 
     Next, after primarily dividing the large-sized substrate  30 A along the primary division grooves to obtain strip-shaped substrates  30 B (primary dividing step), by applying Ag-based paste on divided faces of each strip-shaped substrate  30 B and then heating and curing the applied paste, as illustrated in  FIG. 10F , the step of forming the pair of end face electrodes  5  on divided faces of each strip-shaped substrate  30 B, respectively, each of which connects corresponding ones of the internal electrodes  31  and the back electrodes  4  (end face electrode forming step) is performed. In this connection, instead of applying Ag-based paste to form the end face electrodes  5 , in the same manner as the first embodiment described above, the end face electrodes  5  may be formed by sputtering Ni/Cr on the divided faces of each strip-shaped substrate  30 B. 
     Next, after secondarily dividing the strip-shaped substrates  30 B along the secondary division grooves to obtain a plurality of chip-shaped substrates  30 C (secondary dividing step), by performing electrolytic plating with respect to each chip-shaped substrate  30 C to form a Ni-Sn plating layer, as illustrated in  FIG. 10G , the step of forming the pair of external electrodes  6  which covers the entire surfaces of the end face electrodes  5  and also covers exposed portions of the internal electrodes  31  and those of the back electrodes  4  (external electrode forming step) is performed. 
     Next, by immersing each chip-shaped substrate  30 C in an alkaline solution or the like which dissolves the second protective film  35  but does not dissolve the first protective films  34  to remove the second protective film  35 , as illustrated in  FIG. 10H , the step of exposing the sulfidation detection portion  33   a  of the sulfidation detection conductor  33  between the pair of first protective films  34  (second protective film removing step) is performed. In this way, the manufacturing process of the sulfidation detection sensor  30  illustrated in  FIG. 8  and  FIG. 9  is completed. In this connection, note that the second protective film removing step can be performed after mounting the sulfidation detection sensor  30  on a circuit board. In this case, the sulfidation detection portion  33 a can be protected until being mounted on the circuit board. 
     As described above, in the manufacturing method of the sulfidation detection sensor  30  according to the third embodiment, after forming the pair of resistors  32  and the pair of internal electrodes  31  which are connected, respectively, in series on both sides across the sulfidation detection conductor  33  positioned at the center portion, the step of forming the pair of first protective films  34 , which is composed of the double layer structure having the undercoat layer  34   a  and the overcoat layer  34   b,  at the positions covering the resistors  32 , respectively, is performed. As a result, in addition to the effects that can be obtained by the first embodiment, the third embodiment can make it possible to easily make the surface height of the first protective films  34  higher than the surface height of the second protective film  35 , use the sulfidation detection sensor  30  as a sulfidation detection sensor equipped with a chip resistor, and manufacture the sulfidation detection sensor  30  by the same process as that of a general chip resistor. 
       FIG. 11  is a plan view of a sulfidation detection sensor  40  according to a fourth embodiment of the present invention.  FIG. 12  is a cross-sectional view taken along the line XII-XII of  FIG. 11 . In the following, portions corresponding to those illustrated in  FIG. 1  and  FIG. 2  are provided with the same reference signs, and repetitive explanation therefor will be omitted 
     As illustrated in  FIG. 11  and  FIG. 12 , in the sulfidation detection sensor  40  according to the fourth embodiment, an internal electrode  41  and a sulfidation detection conductor  42  are formed with a certain space therebetween on a front surface of the insulation substrate  1  at both end portions in the longitudinal direction thereof, respectively, and a resistor  43  is formed between the internal electrode  41  and the sulfidation detection conductor  42 . The resistor  43  is covered with a first protective film  44  that is composed of a double layer structure having an undercoat layer  44   a  and an overcoat layer  44   b,  and a trimming groove (not illustrated) for adjustment of a resistance value is formed on the resistor  43  and the undercoat layer  44   a.  In addition, another first protective film  45  that is composed of a double layer structure having an undercoat layer  45   a  and an overcoat layer  45   b  is formed on the center portion of the sulfidation detection conductor  42 , and a sulfidation detection portion  42   a  is exposed between the first protective film  45  and the first protective film  44  that is positioned on the resistor  43 . The other configurations are basically the same as those of the sulfidation detection sensor  10  according to the first embodiment. 
     In the following, a manufacturing process of the sulfidation detection sensor  40  configured as above will be described with reference to the cross-sectional view of  FIG. 13 . Firstly, as illustrated in  FIG. 13A , after screen-printing Ag-based paste on a back surface of a large-sized substrate  40 A, by drying and firing the screen-printed paste, the step of forming the pair of back electrodes  4  facing each other with a certain space therebetween (back electrode forming step) is performed. 
     Next, after screen-printing Ag-based paste containing Ag as a main component on a front surface of the large-sized substrate  40 A, by drying and firing the screen-printed paste, as illustrated in  FIG. 13B , the step of forming the internal electrode  41  and the sulfidation detection conductor  42  on the front surface of the large-sized substrate  40 A with a certain space therebetween (conductor forming step and internal electrode forming step) is performed. 
     Next, after screen-printing resistor paste such as ruthenium oxide on the front surface of the large-sized substrate  40 A, by drying and firing the screen-printed paste, as illustrated in  FIG. 13C , the step of forming the resistor  43  whose both ends are connected to the internal electrode  41  and the sulfidation detection conductor  42 , respectively, (resistor forming step) is performed. 
     Next, after screen-printing glass paste on the position covering the resistors  43  and the center portion of the sulfidation detection conductor  42 , and then drying and firing the screen-printed paste to form the pair of undercoat layers  44   a,    45   a,  the step of forming a trimming groove (not illustrated) for adjustment of a resistance value from above the undercoat layer  44   a  which covers the resistor  43  (trimming step) is performed. At this time, by bringing a pair of probes into contact with the sulfidation detection conductor  42 , which is located on the outer side of the undercoat layer  45   a,  and the internal electrode  41 , trimming can be performed while measuring the resistance value of the resistor  43 , whereby making it possible to prevent the portion of the sulfidation detection conductor  42  serving as the sulfidation detection portion  42   a  from being damaged by the probes. Thereafter, by screen-printing epoxy-based resin paste so as to cover both the undercoat layers  44   a,    45   a  and then heating and curing the screen-printed paste, as illustrated in  FIG. 13D , the step of forming the first protective film  44 , which is composed of the undercoat layer  44   a  and the overcoat layer  44   b,  on the resistor  43  as well as the first protective film  45 , which is composed of the undercoat layer  45   a  and the overcoat layer  45   b,  on the center portion of the sulfidation detection conductor  42  (first protective film forming step) is performed. In this connection, the pair of first protective films  44 ,  45  has the same configuration (double layer structure with a glass material and a resin material) so as to be formed at the same time. Meanwhile, since it is not necessary to provide a trimming groove on the first protective film  45  positioned on the sulfidation detection conductor  42 , the first protective film  45  may be made of only a resin material such as epoxy resin. Furthermore, in the case of forming the first protective film  44  and the first protective film  45  at symmetrical positions of the front surface of the insulation substrate  1 , it is possible to eliminate the directionality. 
     Next, after screen-printing soluble resin paste such as water-soluble phenol resin on the front surface of the sulfidation detection conductor  42 , by heating and curing the screen-printed paste, as illustrated in  FIG. 13E , the step of forming the second protective film  46  which covers the sulfidation detection conductor  42  between the pair of first protective films  44 ,  45  (second protective film forming step) is performed. The second protective film  46  is made of a soluble material having a property that is soluble in a solvent but is insoluble in plating solution to be used in the subsequent external electrode forming step. The second protective film  46  is formed such that the surface height thereof is lower than the surface height of the first protective films  44 ,  45 . 
     Next, after primarily dividing the large-sized substrate  40 A along the primary division grooves to obtain strip-shaped substrates  40 B (primary dividing step), by applying Ag-based paste on divided faces of each strip-shaped substrate  40 B and then heating and curing the applied paste, as illustrated in  FIG. 13F , the step of forming the pair of end face electrodes  5  on divided faces of each strip-shaped substrate  40 B, respectively, each of which connects the internal electrodes  41  and the back electrodes  4  (end face electrode forming step) is performed. In this connection, instead of applying Ag-based paste to form the end face electrodes  5 , in the same manner as the first embodiment described above, the end face electrodes  5  may be formed by sputtering Ni/Cr on the divided faces of each strip-shaped substrate  40 B. 
     Next, after secondarily dividing the strip-shaped substrates  40 B along the secondary division grooves to obtain a plurality of chip-shaped substrates  40 C (secondary dividing step), by performing electrolytic plating with respect to each chip-shaped substrate  40 C to form a Ni—Sn plating layer, as illustrated in  FIG. 13G , the step of forming the pair of external electrodes  6  which covers the entire surfaces of the end face electrodes  5  and also covers exposed portions of the internal electrode  41  and those of the back electrodes  4  (external electrode forming step) is performed. 
     Next, by immersing each chip-shaped substrate  40 C in an alkaline solution or the like which dissolves the second protective film  46  but does not dissolve the first protective films  44 ,  45  to remove the second protective film  46 , as illustrated in  FIG. 13H , the step of exposing the sulfidation detection portion  42   a  of the sulfidation detection conductor  42  between the pair of first protective films  44 ,  45  (second protective film removing step) is performed. In this way, the manufacturing process of the sulfidation detection sensor  40  illustrated in  FIG. 11  and  FIG. 12  is completed. In this connection, note that the second protective film removing step can be performed after mounting the sulfidation detection sensor  40  on a circuit board. In this case, the sulfidation detection portion  42   a  can be protected until being mounted on the circuit board. 
     As described above, in the manufacturing method of the sulfidation detection sensor  40  according to the fourth embodiment, after the resistor  43  that connects between the internal electrode  41  and the sulfidation detection conductor  42  is formed, the first protective film  44  is formed on the position covering the resistor  43  and the first protective film  45  is formed on the predetermined position of the sulfidation detection conductor  42 , and then the sulfidation detection portion  42   a  is exposed between the first protective films  44 ,  45 . Accordingly, the pair of probes can be brought into contact with the sulfidation detection conductor  42 , which is positioned on the outer side of the undercoat layer  45   a,  and the internal electrode  41  at the time of providing a trimming groove to adjust the resistance value of the resistor  43 . As a result, in addition to the effects that can be obtained by the first embodiment, the fourth embodiment can advantageously prevent the sulfidation detection portion  42   a  from being damaged by the probes. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  20 ,  30 ,  40  sulfidation detection sensor 
           1  insulation substrate 
           2 ,  22 ,  33 ,  42  sulfidation detection conductor 
           2   a,    22   a,    33   a,    42   a  sulfidation detection portion 
           3 ,  23  first protective film 
         back electrode 
         end face electrode 
         external electrode 
           7 ,  24 ,  35 ,  46  second protective film 
           21 ,  31 ,  41  internal electrode 
           23 ,  32 ,  43  resistor 
           34 ,  44 ,  45  first protective film 
           34   a,    44   a,    45   a  undercoat layer 
           34   b,    44   b,    45   b  overcoat layer 
           10 A,  20 A,  30 A,  40 A large-sized substrate 
           10 B,  20 B,  30 B,  40 B strip-shaped substrate 
           10 C,  20 C,  30 C,  40 C chip-shaped substrate