Abstract:
A chip type resistor includes terminal electrodes having main upper electrodes, extensions or enclaves of a resistive film formed on left and right sides of the upper surface of main upper electrodes, and auxiliary upper electrodes formed on upper surfaces of extensions or enclaves. Therefore, step between the upper surface of terminal electrodes at opposing ends of resistive film and an upper surface of cover coat covering the resistive film can be reduced or eliminated at a low cost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a structure and manufacturing method of a chip type resistor having a resistive film and terminal electrodes positioned at opposing ends of the resistive film, formed on a chip type insulating substrate. 
     2. Description of the Background Art 
     In a conventional chip type resistor such as described in Japanese Patent Laying-Open No. 60-27104, an upper surface of a cover coat covering a resistive film formed on an upper surface of a chip type insulating substrate is protruded from a surface of terminal electrodes positioned at opposing ends of the resistive film, so that there is a considerable step between the upper surface of the cover coat and the upper surface of the terminal electrodes. Therefore, it often suffers from the problem that when the chip type resistor is soldered on a printed board with the side of the resistive film facing the printed board, one side of the chip type resistor rises or floats, preventing secure soldering of terminal electrodes at the opposing ends. 
     In view of the foregoing, Japanese Patent Laying-Open No. 4-102302 discloses a structure of a chip type resistor in which terminal electrodes 3 are formed at opposing ends of a resistive film 2 on left and right end portions of a chip type insulating substrate 1 such that the terminal electrodes 3 each includes a main upper electrode 3a formed on the surface of insulating substrate 1 and conductive to resistive film 2, an auxiliary upper electrode 3b formed heaped up on the surface of main upper electrode 3a, a side electrode 3c formed on either side of insulating substrate 1, and a metal plate layer 3d formed over the surfaces of auxiliary upper electrode 3b and side electrode 3c, as shown in FIGS. 28 and 29. This laid-open patent application proposes, by this structure, to reduce or eliminate the step between the upper surface of terminal electrode 3 and the upper surface of cover coat 4 using auxiliary upper electrode 3b. 
     Cover coat 4 has a three-layered structure including an undercoat 4a directly covering resistive film 2, a middle coat 4b covering undercoat 4a, and an overcoat covering middle coat 4b, or a two-layered structure with under coat 4a or middle coat 4b omitted. 
     The chip type resistor disclosed in Japanese Patent Laying-Open No. 4-102302 is manufactured through the following steps. 
     Step 1. On an upper surface of the insulating substrate 1, main upper electrodes 3a are formed and thereafter resistive film 2 is formed. Alternatively, resistive film 2 is formed first and thereafter main upper electrodes 3a are formed. 
     Step 2. Undercoat 4a of glass is formed on resistive film 2. Thereafter, resistive film 2 and undercoat 4a are engraved to form a trimming groove by laser beam irradiation, for example, while resistance value of resistive film 2 is measured by a conductive probe which is brought into contact with main upper electrodes 3a so that the resistance value of resistive film 2 is within a prescribed tolerable range. 
     Step 3. Middle coat 4b of glass is formed on the surface of undercoat 4a to fill the trimming groove. Thereafter, overcoat 4c of glass or a synthetic resin is formed covering resistive film 2, part of the main upper electrodes 3a, undercoat 4a and middle coat 4b. 
     Step 4. On the upper surface of main upper electrodes 3a, auxiliary upper electrodes 3b are formed. Thereafter, side electrodes 3c are formed on end surfaces of insulating substrate 1, and surfaces of auxiliary upper electrodes 3b and side electrodes 3c are subjected to metal plating, whereby a metal plate layer 3d is formed. 
     In the above described chip type resistor, auxiliary electrodes 3b are formed on the upper surfaces of the main upper electrodes 3a by directly applying the conductive paste by screen printing. Therefore, in order to reduce or eliminate the step between upper surfaces of terminal electrodes 3 and cover coat 4, auxiliary electrodes 3b must be made thick. 
     In order to increase thickness of auxiliary upper electrode 3b, the number of application of the conductive paste for the auxiliary upper electrodes 3b must be increased. Therefore, amount of conductive paste used is increased, resulting in considerable increase in manufacturing cost of the chip type resistor. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a chip type resistor having such a structure that can perfectly prevent rising or floating of one side of the resistor when the resistor is mounted by soldering to a printed board. 
     Another object of the present invention is to provide a method of manufacturing the chip type resistor attaining the above described object, which suppresses increase in cost of the chip type resistor by reducing amount of material used for the chip type resistor. 
     The above described objects of the present invention can be attained by the chip type resistor in accordance with one aspect of the present invention having terminal electrodes positioned at opposing ends of a resistive film formed at left and right end portions of an insulating substrate, the terminal electrodes each including at least a main upper electrode formed on an upper surface of the insulating substrate and conductive to the resistive film, and an auxiliary upper electrode formed on an upper surface of the main upper electrode, the chip type resistor further having a cover coat covering the resistive film, formed on the upper surface of the insulating substrate, wherein 
     said resistive film has extensions or enclaves (isolated patches) formed on left and right sides of the upper surface of the main upper electrode, and the auxiliary upper electrodes are formed on an upper surface of the extensions or enclaves. 
     Preferably, the auxiliary upper electrode is provided between the extensions or between the enclaves of the resistive film on the upper surface of the main upper electrode. 
     Preferably, the main upper electrode is formed with left and right sides superposed on the extensions or enclaves of the resistive film, and the auxiliary upper electrode is formed on that portion of the upper surface of the main upper electrode which is superposed on the extension or enclave, or which is between the overlapping portions. 
     The above described objects of the present invention is attained by the method of manufacturing a chip type resistor in accordance with the present invention including the steps of: forming a pair of left and right main upper electrodes on an upper surface of an insulating substrate; forming a resistive film having opposing ends conductive to said main upper electrodes such that extensions or enclaves of the resistive film are provided on left and right sides of the upper surface of said main upper electrodes; forming a cover coat covering the resistive film; and forming auxiliary upper electrodes on upper surfaces of the extensions or enclaves of the resistive film or on portions of the upper surfaces of the main upper electrodes which are between the extensions of enclaves. 
     As described above, extensions or enclaves of the resistive film are formed on both left and right sides on the upper surface of the main upper electrode and auxiliary upper electrodes are formed on upper surfaces of the extensions or enclaves. Therefore, the left and right terminal electrodes come to have a three-layered structure including the main upper electrode, the extension or enclave of the resistive film and the auxiliary upper electrode. Therefore, the height from the upper surface of the insulating substrate to the upper surface of the auxiliary upper electrode includes thickness of the conventional auxiliary upper electrode and of the main upper electrode plus the thickness of the resistive film. As a result, the thickness of the auxiliary upper electrode can be made thinner by the thickness of the resistive film as compared with the prior art. 
     Since extensions or enclaves of the resistive film are formed on both left and right sides on the upper surface of the main upper electrode and the auxiliary upper electrode is formed between the extensions or enclaves on the upper surface of the main upper electrode, when the auxiliary upper electrode is formed by applying a conductive paste by screen printing or the like, the extensions or enclaves positioned on the left and right sides block flow of the conductive paste applied to the portion therebetween in the widthwise direction of the insulating substrate. As a result, the conductive paste in small amount can be heaped up high. 
     Here, when the extensions or enclaves of the resistive film are formed both on the left and right sides on the upper surface of the main upper electrode, it is possible when the resistive film is adjusted by trimming while measuring resistance value thereof, to bring a conductive probe directly into contact with each of the main upper electrodes. Therefore, exact trimming adjustment of the resistive film is possible. 
     According to the present invention, the amount of conductive paste used for forming the auxiliary upper electrodes can be reduced and the number of application of the conductive paste by screen printing can be reduced, without sacrificing exactness of trimming adjustment, whereby manufacturing cost can significantly be reduced. 
     Further, such function and effects can also be attained by a structure in which extensions or enclaves of the resistive film are formed superposed on both left and right sides on the upper surface of the main upper electrode and the auxiliary upper electrode is formed between the extensions or enclaves of the resistive film on the upper surface of the main upper electrode, or a structure in which the main upper electrode is formed with left and right sides superposed on extensions or enclaves of the resistive film, and an auxiliary upper electrode is formed on that portion of the upper surface of the main upper electrode which is superposed on the extension or enclave of the resistive film. 
     Particularly, according to the manufacturing method of the present invention, the extensions or enclaves of the resistive film are formed simultaneously with the resistive film, and therefore the number of process steps is not increased to form the extensions or enclaves. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing main upper electrodes formed on an insulating substrate in accordance with a first embodiment. 
     FIG. 2 is a perspective view showing a resistive film formed on the insulating substrate in accordance with the first embodiment. 
     FIG. 3 is a plan view of FIG. 2. 
     FIG. 4 is a cross section taken along the line 4-4&#39; of FIG. 3. 
     FIG. 5 is a cross section taken along the line 5-5&#39; of FIG. 3. 
     FIG. 6 is a plan view showing a state in which an undercoat is formed on the insulating substrate in accordance with the first embodiment. 
     FIG. 7 is a cross section taken along the line 7-7&#39; of FIG. 6. 
     FIG. 8 is a plan view showing a state in which a middle coat is formed on the insulating substrate in accordance with the first embodiment. 
     FIG. 9 is a cross section taken along the line 9-9&#39; of FIG. 8. 
     FIG. 10 is a plan view showing a state in which an overcoat is formed on the insulating substrate in accordance with the first embodiment. 
     FIG. 11 is a cross section taken along the line 11-11&#39; of FIG. 10. 
     FIG. 12 is a plan view showing a state in which an auxiliary upper electrode is formed on the insulating substrate in accordance with the first embodiment. 
     FIG. 13 is a cross section taken along the line 13-13&#39; of FIG. 12. 
     FIG. 14 is a cross section taken along the line 14-14&#39; of FIG. 12. 
     FIG. 15 is a vertical cross section showing the chip type resistor in accordance with the first embodiment. 
     FIG. 16 is a cross section taken along the line 15-15&#39; of FIG. 15. 
     FIG. 17 is a plan view showing a modification of the first embodiment. 
     FIG. 18 is a plan view showing another modification of the first embodiment. 
     FIG. 19 is a cross section taken along the line 18-18&#39; of FIG. 18. 
     FIG. 20 is a plan view showing a state in which an auxiliary upper electrode is formed on the insulating substrate in a second embodiment. 
     FIG. 21 is a cross section taken along the line 21-21&#39; of FIG. 20. 
     FIG. 22 is a cross section taken along the line 22-22&#39; of FIG. 20. 
     FIG. 23 is a vertical cross section showing the chip type resistor in accordance with the second embodiment. 
     FIG. 24 is a cross section taken along the line 24-24&#39; of FIG. 23. 
     FIG. 25 is a cross section showing the same portion as that shown in FIG. 20, showing a modification of the second embodiment. 
     FIG. 26 is a cross section showing the same portion as that shown in FIG. 22 showing another modification of the second embodiment. 
     FIG. 27 is a vertical cross section showing the chip type resistor in accordance with a further embodiment. 
     FIG. 28 is a perspective view showing a conventional chip type resistor. 
     FIG. 29 is a cross section taken along the line 29-29&#39; of FIG. 28. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described with reference to the figures. 
     (First Embodiment) 
     FIGS. 1 to 19 show the first embodiment. 
     The chip type resistor in accordance with the first embodiment is manufactured through the following method. 
     Step 1. First, referring to FIG. 1, a pair of left and right main upper electrodes 13a are formed on an upper surface of a chip type insulating substrate 11, by applying a row material conductive paste by screen printing, followed by sintering. 
     Step 2. Thereafter, referring to FIGS. 2 to 5, on the upper surface of insulating substrate 11, a resistive film 12 is formed by application of a raw material paste by screen printing, followed by sintering. At this time, simultaneously with the formation of resistive film 12, extensions 12&#39; integrally extending from resistive film 12 are formed superposed on both left and right sides on the upper surface of main upper electrodes 13a. 
     Step 3. On the upper surface of insulating substrate 11, referring to FIGS. 6 and 7, an undercoat 14a of glass covering resistive film 12 is formed by application by screen printing of a raw material, followed by sintering. Thereafter, resistive film 12 and undercoat 14a are engraved to form a trimming groove 12a by laser beam irradiation, for example, while resistance value of resistive film 12 is measured by a conductive probe (not shown) brought into contact with that portion of the upper surface of main upper electrodes 13a which is between the extension 12&#39; so that the resistance value of resistive film 12 is within a prescribed tolerable range, whereby trimming adjustment is realized. 
     Step 4. Referring to FIGS. 8 and 9, a middle coat 14b of glass to fill trimming groove 12b is formed on the upper surface of insulating substrate 11, by application of a raw material by screen printing, followed by sintering. 
     Step 5. Referring to FIGS. 10 and 11, an overcoat 14c entirely covering middle coat 14b is formed on the upper surface of insulating substrate 11 by application of a raw material by screen printing, followed by sintering. In this manner, cover coat 14 having a three layered structure including undercoat 14a, middle coat 14b and overcoat 14c is formed. 
     Step 6. Referring to FIGS. 12 to 14, an auxiliary upper electrode 13b is formed on the upper surface of main upper electrode 13a such that the auxiliary upper electrode is superposed over both the extensions 12&#39; of resistive film 12 and the upper surface of main upper electrode 13a, by application of a raw material conductive paste by screen printing, followed by sintering. 
     Step 7. Referring to FIGS. 15 and 16, side electrodes 13c are formed on left and right surfaces of insulating substrate 11 by application of a raw material conductive paste and sintering, and thereafter metal plating is performed entirely, so as to form metal plate layers 13d on auxiliary upper electrodes 13b and on side electrodes 13c, and thus terminal electrodes 13 are completed. 
     In the chip type resistor manufactured in this manner, the portion of terminal electrode 13 on the upper surface of insulating substrate 11 has a three layered structure as shown in FIGS. 15 and 16. More specifically, terminal electrode 13 has a main upper electrode 13a, an extension 12&#39; of resistive film 12 on left and right sides of the upper surface of main upper electrode, and auxiliary upper electrode 13b formed on the upper surface of extension 12&#39;. The height H1 from the upper surface of insulating substrate 11 to the upper surface of auxiliary upper electrode 13b (more exactly, height between the upper surface of insulating substrate 11 to the upper surface of metal plate layer 13c) corresponds to the thickness Tb of auxiliary upper electrode 13b, thickness Ta of main upper electrode 13a plus thickness T1 of resistive film 12, when the height H1 is made exactly or approximately the same as the height H0 from the upper surface of insulating substrate 11 to the upper surface of overcoat 14c. Therefore, the thickness Tb of auxiliary upper electrode 13b can be made thinner by the thickness T1 of resistive film 12 as compared with the prior art. 
     The extension 12&#39; of resistive film 12 may, alternatively, be formed as an enclave 12&#34; separate from resistive film 12 as shown in FIG. 17. Alternatively, the extension may have a shape appropriately combining extension 12&#39; and enclave 12&#34;. For example, the extension 12&#39; may be formed on one of the main upper electrodes 13a and an enclave 12&#34; may be formed on the other one of the main upper electrodes. 
     In the first embodiment described above, main upper electrodes 13a are formed, thereafter resistive film 12 is formed with extensions 12&#39; or enclaves 12&#34; superposed on left and right sides of main upper electrodes 13a, and thereafter auxiliary upper electrodes 13b are formed superposed on extensions 12&#39; or enclaves 2&#34; of resistive film 12. Alternatively, referring to FIGS. 18 and 19, resistive film 12 together with extensions 12&#39; or enclaves 2&#34; may be formed first, thereafter main upper electrodes 13a may be formed with the left and right sides superposed on the extensions 12&#39; or enclaves 12&#34; of resistive film 12 and, thereafter, auxiliary upper electrodes 13b may be formed on portions of the upper surface of the main upper electrodes 13a which are on the extensions 12&#39; or enclaves 12&#34; of resistive film 12 (other structures are the same). 
     (Second Embodiment) 
     FIGS. 20 to 24 show a second embodiment. 
     The second embodiment is similar to the first embodiment until formation of overcoat 14c. The present embodiment differs from the first embodiment in subsequent formation of auxiliary upper electrodes 13b. 
     More specifically, when auxiliary upper electrode 13b is formed on the upper surface of main upper electrode 13a, auxiliary upper electrode 13b&#39; is formed only at a portion between extensions 12&#39; or enclaves 12&#34; on the upper surface of main upper electrode 13a as shown in FIGS. 20 to 22. 
     The auxiliary upper electrode 13b&#39; may be formed by application of the raw material conductive paste by screen printing and sintering as described above. Alternatively, it may be formed simultaneously with formation of side electrodes 13c on both end surfaces of insulating substrate 11. More specifically, when the raw material conductive paste for the side electrodes 13c is applied to left and right end surfaces of insulating substrate 11, part of the raw material conductive paste may be heaped up to the upper surface side of main upper electrode 13a for forming the auxiliary upper electrode. 
     Thereafter, referring to FIGS. 23 and 24, side electrodes 13c are formed in the similar manner on both left and right end surfaces of insulating substrate 11, metal plating is performed entirely, and a metal plate layer 13d is formed on surfaces of auxiliary upper electrodes 13b and side electrodes 13c. Thus terminal electrodes 13 are completed. 
     Chip type resistor manufactured in this manner has extensions 12&#39; or enclaves 12&#34; of resistive film 12 formed on both left and right sides of main upper electrodes 13a of terminal electrodes 13 as shown in FIGS. 23 and 24. Auxiliary upper electrode 13b is formed at a portion between extensions 12&#39; or enclaves 12&#34; on the upper surface of main upper electrode 13a. When auxiliary upper electrode 13b&#39; is formed by applying a raw material conductive paste by screen printing or the like, the extensions or enclaves position on left and right sides function as a dam for preventing flow of the conductive paste applied to the portion therebetween in the widthwise direction of the insulating substrate. Therefore, the conductive paste in small amount can be heaped up high. 
     In the second embodiment, extensions 14a&#39; (or enclaves) of undercoat 14a may be formed on the upper surface of extension 12&#39; or enclaves 12&#34; of resistive film 12 as shown in FIG. 25, whereby the height from the upper surface of main upper electrode 13a can be increased. As a result, when auxiliary upper electrodes 13b are formed, the effect that a small amount of conductive paste can be heaped up high is further enhanced. Further, extensions 12&#39; or enclaves 12&#34; of resistive film 12 can be covered by the extensions 14a&#39; (or enclaves) of undercoat 14a. Therefore, at the time of metal plating, metal plating layer 13c can be formed only on auxiliary upper electrodes 13b as shown in FIG. 26. Further, formation of metal plate layer 13c on surfaces of extensions 12&#39; or enclaves 12&#34; of resistive film 12 can be avoided. 
     Here, on the upper surface of extensions 12&#39; or enclaves 12&#34; of resistive film 12, extensions (enclaves) of middle coat 14b or extensions (or enclaves) or overcoat 14c may be formed, in place of extensions 14a&#39; (or enclaves) of undercoat 14a, as described above. Further, extensions (or enclaves) of at least two of undercoat 14a, middle coat 14b and overcoat 14c may be formed. Further, in the second embodiment, main upper electrodes 13a are formed, thereafter resistive film 12 is formed with extensions 12&#39; or enclaves 12&#34; superposed on both left and right sides of main upper electrodes 13a, and thereafter auxiliary upper electrodes 13b are formed on the upper surface of main upper electrodes 13a superposed between extension 12&#39; or enclaves 12&#34; of the resistive film 12. Alternatively, as in the example shown in FIGS. 18 and 19, resistive film 12 together with extensions 12&#39; or enclaves 12&#34; may be formed first, main upper electrodes 13a may be formed with left and right sides superposed on extensions 12&#39; or enclaves 12&#34; of resistive film 12, and thereafter auxiliary upper electrodes 13b may be formed superposed on the upper surface of the main upper electrodes 13a superposed on extensions 12&#39; or enclaves 12&#34; of the resistive film 12. 
     Further, in each of the embodiment described above, a lower electrode 13e constituting part of terminal electrode 13 may be formed on a lower surface of insulating substrate 11 as shown in FIG. 27, whereby it becomes possible to mount the chip on a printed board with the side of resistive film 12 facing upward. 
     Generally, lower electrode 13c is formed by screen printing and sintering of a raw material paste in a process step before forming main upper electrodes 13a or resistive film 12. However, it is possible to form lower electrode 13e simultaneously with formation of side electrodes 13c on both end surfaces of insulating substrate 11. More specifically, the lower electrode may be formed, when a raw material conductive paste for side electrodes 13c are applied to left and right end surfaces of insulating substrate 11, by heaping up part of the raw material conductive paste on the lower surface of insulating substrate 11. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.