NTC thermistor to be embedded in a substrate, and method for producing the same

Abstract

An NTC thermistor to be embedded in a substrate includes a thermistor body that is a ceramic sintered body and includes two opposed main surfaces, two opposed side surfaces, and two opposed end surfaces, a plurality of internal electrodes provided inside the thermistor body, and two external electrodes provided on outer surfaces of the thermistor body, and electrically connected to the plurality of internal electrodes. Each of the external electrodes includes a first electrode layer covering one of the end surfaces of the thermistor body, a second electrode layer provided on each of the main surfaces of the thermistor body, the second electrode layer including at least one layer, one end of the second electrode layer being in contact with the first electrode layer, and another end thereof extending in a direction of another end surface, and a third electrode layer including at least one layer and covering the first electrode and the second electrode layers.

Claims

1. An NTC thermistor comprising: a thermistor body that is a ceramic sintered body and includes two opposed main surfaces, two opposed side surfaces, and two opposed end surfaces; a plurality of internal electrodes provided inside the thermistor body; and two external electrodes provided on outer surfaces of the thermistor body, and electrically connected to the plurality of internal electrodes; wherein each of the external electrodes includes: a first electrode layer covering only one of the end surfaces of the thermistor body; a second electrode layer provided on each of the two opposed main surfaces of the thermistor body, the second electrode layer including at least one layer, one end of the second electrode layer being in contact with the first electrode layer, and another end thereof extending in a direction of another end surface; and a third electrode layer including at least one layer and covering the first electrode and the second electrode layers.

2. The NTC thermistor according to claim 1, wherein each of the second electrode layers includes a sputtering film including at least one layer.

3. The NTC thermistor according to claim 1, wherein the third electrode layer is a plated film.

4. The NTC thermistor according to claim 1, wherein the first electrode layer is a fired conductive paste layer.

5. The NTC thermistor according to claim 1, wherein the second electrode layer is flatter than the first electrode layer.

6. The NTC thermistor according to claim 1, wherein the thermistor body includes a non-covered region that is not covered with the external electrodes on outer surfaces at a center portion thereof, and the non-covered region includes a step lower than a covered region covered with the external electrodes.

7. The NTC thermistor according to claim 1, wherein the thermistor body is a rectangular or substantially rectangular parallelepiped ceramic sintered body.

8. The NTC thermistor according to claim 1, wherein the thermistor body includes ceramics containing an oxide of at least one of Mn, Ni, Fe, Ti, Co, Al, and Zn.

9. The NTC thermistor according to claim 1, wherein the thermistor body includes ceramics containing manganese oxide as a principal component, and containing one or more of nickel oxide, cobalt oxide, alumina, iron oxide, and titanium oxide.

10. The NTC thermistor according to claim 1, wherein the first electrode layer includes one of Ag, Pd, Pt, and Au.

11. The NTC thermistor according to claim 1, wherein a thickness of the first electrode layer is about 10 m or more and about 60 m or less.

12. The NTC thermistor according to claim 1, wherein a thickness of the first electrode layer is about 10 m or more and about 50 m or less.

13. The NTC thermistor according to claim 1, wherein a thickness of the first electrode layer is about 15 m or more and about 40 m or less.

14. The NTC thermistor according to claim 1, wherein a thickness of the second electrode layers is about 0.5 m or more and about 10 m or less.

15. The NTC thermistor according to claim 1, wherein a thickness of the second electrode layers is about 1 m or more and about 10 m or less.

16. The NTC thermistor according to claim 1, wherein a thickness of the second electrode layers is about 1.5 m or more and about 5 m or less.

17. The NTC thermistor according to claim 1, wherein a thickness of the third electrode layers is about 5 m or more and about 20 m or less.

18. A method for producing the NTC thermistor of claim 1, the method comprising: a step of forming each of the external electrodes; wherein the step of forming each of the external electrodes further comprises: a step of forming a first electrode layer covering only one end surface of a thermistor body; a step of forming a second electrode layer formed on each main surface of the thermistor body and including at least one layer, one end of the second electrode layer being in contact with the first electrode layer, and another end thereof extending in a direction of another end surface; and a step of forming a third electrode layer including at least one layer and covering the first electrode layer and the second electrode layers.

19. The method according to claim 18, wherein the step of forming the third electrode layer includes a step of forming the third electrode layer by a plating method, and after forming the second electrode layers, for a non-covered region of the outer surfaces at the central portion of the thermistor body that is not covered with the second electrode layer, removal of the body surface portion is performed.

20. The method according to claim 18, wherein in the step of forming the second electrode layer, the second electrode layer is formed so as to be flatter than the first electrode layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic longitudinal cross-sectional view showing a structure of an NTC thermistor to be embedded in a substrate according to a preferred embodiment of the present invention.

(2) FIG. 2 is a partially-enlarged longitudinal cross-sectional view in FIG. 1.

(3) FIGS. 3A to 3D are schematic longitudinal cross-sectional views showing a method for producing the NTC thermistor to be embedded in a substrate according to a preferred embodiment of the present invention.

(4) FIGS. 4A to 4D are schematic longitudinal cross-sectional views showing a method for mounting the NTC thermistor to be embedded in a substrate according to a preferred embodiment of the present invention on the component-embedded substrate.

(5) FIG. 5 is a schematic longitudinal cross-sectional view showing one example of a structure of a conventional NTC thermistor to be embedded in a substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) Hereinafter, referring to the drawings, preferred embodiments of the present invention will be described in detail.

(7) An NTC thermistor to be embedded in a substrate according to a preferred embodiment of the present invention includes a thermistor body that is a ceramic sintered body, and including two opposed main surfaces, two opposed side surfaces, and two opposed end surfaces, a plurality of internal electrodes provided inside the thermistor body, and two external electrodes provided on outer surfaces of the thermistor body and electrically connected to the plurality of internal electrodes, wherein each of the external electrodes includes a first electrode layer covering one of the end surfaces of the thermistor body, a second electrode layer provided on each of the main surfaces of the thermistor body, the second electrode layer including at least one layer, one end of the second electrode layer being in contact with the first electrode layer, and another end thereof extending in a direction of another end surface, and a third electrode layer including at least one layer and covering the first electrode and the second electrode layers.

(8) FIG. 1 is a schematic cross-sectional view showing a structure of an NTC thermistor according to a preferred embodiment of the present invention. An NTC thermistor 1 includes a thermistor body 2 preferably made of a rectangular or substantially rectangular parallelepiped ceramic sintered body. The thermistor body 2 includes two opposed main surfaces 13, 14, two opposed side surfaces (not shown), and two opposed end surfaces 11, 12. Sizes of the two main surfaces may be the same as one another, or may be different from one another. Moreover, sizes of the two side surfaces may be the same as one another, or may be different from one another. Further, sizes of the two end surfaces may be the same as one another, or may be different from one another.

(9) Inside the thermistor body 2, a plurality of internal electrodes 3a, 3b, 3c, 3d, 3e are disposed so as to overlap one another with thermistor body layers interposed therebetween. On outer surfaces of the thermistor body 2, two external electrodes 8, 9 are provided and are electrically connected to the plurality of internal electrodes 3a, 3b, 3c, 3d, 3e. The internal electrodes 3a, 3b, 3c, 3d, 3e are one preferred example, and the number of the internal electrodes is not particularly limited, as long as a plurality of internal electrodes are provided.

(10) The ceramic sintered body of the thermistor body 2 is preferably made of ceramics having a negative resistance temperature characteristic. As the ceramics having the negative resistance temperature characteristics, appropriate ceramics conventionally used to make the NTC thermistor, for example, ceramics containing oxide of transition metal such as Mn, Ni, Fe, Ti, Co, Al, and Zn may preferably be used. Preferably, for example, ceramics containing manganese oxide as a principal component, and containing one or more of nickel oxide, cobalt oxide, alumina, iron oxide, and titanium oxide are used. Moreover, a shape of the thermistor body 2 is not particularly limited, as long as the thermistor body 2 includes the two corresponding end surfaces 11, 12, the two corresponding main surfaces 13, 14, and the two opposed side surfaces. While in FIG. 1, the example of the rectangular or substantially rectangular parallelepiped shape is described, a cube shape may also be provided, for example.

(11) The internal electrodes 3a, 3b extend to the first end surface 11 of the thermistor body 2. Moreover, the internal electrodes 3d, 3e extend to the second end surface 12 opposed to the first end surface 11. The internal electrodes may preferably be formed by application and baking of a conductive paste. For the internal electrodes, a simple substance or an alloy of noble metal such as Ag, Pd, Pt, and Au or a base metal such as Cu, Ni, Al, W, and Ti may preferably be used.

(12) On the outer surfaces of the thermistor body 2, the two external electrodes 8, 9 are provided. The first external electrode 8 includes a first electrode layer 4a covering the one end surface 11 of the thermistor body 2, two second electrode layers 5a, 5b provided on the respective main surfaces of the thermistor body 2, in each of which one end is in contact with the first electrode layer 4a and another end extends in a direction of the other end surface, and a third electrode layer 6 covering the first electrode layer 4a and the second electrode layers 5a, 5b. Moreover, the second external electrode 9 includes a first electrode layer 4b covering the one end surface 12 of the thermistor body 2, two second electrode layers 5c, 5d provided on the respective main surfaces of the thermistor body 2, in each of which one end is in contact with the first electrode layer 4b and another end extends in a direction of the other end surface, and a third electrode layer 7 covering the first electrode layer 4b and the pair of second electrode layers 5c, 5d.

(13) The first electrode layers 4a, 4b include noble metal, such as Ag, Pd, Pt, and Au, and for example, are formed by application and baking of a conductive paste. Immersing the end surfaces of the thermistor body in the conductive paste allows the electrode layers covering the end surfaces to be easily formed. A thickness of each of the first electrode layers 4a, 4b is preferably about 10 m or more and about 60 m or less, more preferably about 10 m or more and about 50 m or less, and even more preferably about 15 m or more and about 40 m or less, for example. In this case, the thickness of each of the first electrode layers is a length between a joining position between the end surface and the main surface of the thermistor body and an outer surface of the first electrode layer (E1 in FIG. 1), and is the same or almost the same as the thickness of the first electrode layer.

(14) The NTC thermistor is left standing at about 125 C. for about 1000 hours, and a reliability test is conducted to measure a change in resistance value after being left standing with respect to a resistance value before being left standing. The following results are obtained.

(15) TABLE-US-00001 TABLE 1 Thickness of first electrode layer (m) R (%) 35 <1% 10 <1% 5 >1%

(16) Here, if a change in resistance value at about 25 C. after being left at about 125 C. with respect to a resistance value R.sub.25 at about 25 C. before being left at about 125 C. is R.sub.25, R (%) is a value defined by the following formula.
(R25/R.sub.25)100

(17) It is indicated that when the value R (%) is small, the resistance value change of the NTC thermistor is small. As is evident from the above-described results, it is confirmed that when the thickness of the first electrode layer is smaller than about 10 m, the resistance value change increases, and that the reliability deteriorates.

(18) For each of the second electrode layers, a single or a plurality of metal layers containing metal, preferably, Au, Ag, Cu, or Ti may be used. For the single layer or an outermost layer, Au or Ag, which is hard to oxidize, is preferably used. A thickness of each of the second electrode layers is preferably about 0.5 m or more and about 10 m or less, more preferably about 1 m or more and about 10 m or less, and even more preferably about 1.5 m or more and about 5 m or less, for example. It is preferable that the second electrode layers are flat. For example, an R value preferably is less than about 1 m, and preferably less than about 0.5 m. Here, the R value is an index indicating a degree of roughness of a surface. Referring to FIG. 2, a description will be provided. FIG. 2 is a partially-enlarged longitudinal cross-sectional view of an A portion in FIG. 1. The NTC thermistor is subjected to cross section polishing to measure the thickness of the second electrode layer 5a at five positions. A maximum value and a minimum value of the thicknesses at the five positions were determined, and a difference therebetween is defined as the R value. The smaller R value indicates that the surface is flatter. In FIG. 2, the thicknesses on an inner side spaced from the end surface of the thermistor body by about 10 m or more are measured. While the second electrode layers may be formed using a sputtering method or a printing method, it is preferable to use the sputtering method, which makes simplifies film thickness control, and enables a flatter film to be formed.

(19) Each of the third electrode layers is a single layer or a plurality of metal or alloy layers, and as the metal, Ni, Cu or Au, and as the alloy, alloys of these metals may preferably be used. For a single layer or an outermost layer, Cu or Au is preferable. This is because burning during laser light emission is effectively reduced or prevented. Moreover, a thickness of each of the third electrode layers is preferably about 5 m or more and about 20 m or less, and more preferably about 6 m or more and about 15 m or less, for example. The third electrode layer may preferably be formed using a plating method, for example.

(20) Moreover, the thermistor body 2 includes non-covered regions that are not covered with the external electrodes on outer surfaces of a central portion thereof. For example, the one main surface 13 includes a covered region 13a covered with the external electrode 8, a covered region 13b covered with the external electrode 9, and a non-covered region 13c not covered with either of the external electrodes 8, 9. Moreover, the other main surface includes a covered region 13d covered with the external electrode 8, a covered region 13e covered with the external electrode 9, and a non-covered region 13f not covered with either of the external electrodes 8, 9. Here, sizes of the covered regions and the non-covered regions of the main surfaces may be selected in view of prevention of ruptures, such as a crack of an NTC thermistor element. For example, the sizes may be set so that the first external electrodes and the second external electrodes satisfy conditions described below. That is, if in a length direction of the thermistor 1 (a direction from the one end surface to the other end surface 12), an overall length of the thermistor 1 is L, a length of the first external electrode 8 (a length of the covered region) is E2, and a length of the second external electrode 9 (a length of the covered region) is E3, ()LE2+E3(0.95L), and preferably ()LE2+E3(0.90L), for example, is satisfied. While it is preferable that the length E2 of the first external electrode 8 and the length E3 of the second external electrode 9 are the same or substantially the same to facilitate production, the lengths E2 and E3 may be different.

(21) Here, the overall length L of the thermistor 1 indicates a length between both the ends in the length direction of the thermistor 1. Moreover, the length E2 of the first external electrode 8 indicates a length between one end and another end of the first external electrode 8 in the length direction of the thermistor 1. Further, the length E3 of the second external electrode 9 indicates a length between one end and another end of the second external electrode 9 in the length direction of the thermistor 1. For example, if a size of the thermistor 1 is JIS standard 0603 size [(0.60.03) mm (the length direction)(0.30.03) mm (a width direction)], L is preferably about 0.6 mm, and E2 and E3 are preferably each about 0.2 mm or more, for example. This satisfies E2+E3()L. A thickness of the thermistor 1 is preferably more than about 0.1 mm, and less than about 0.3 mm, for example. Moreover, the thickness of the thermistor 1 may preferably be about 0.3 mm or more, for example. The size of the thermistor 1 may preferably be a size in a range of JIS standard 0402 to 2012, for example.

(22) Moreover, the non-covered regions 13c, 13f may preferably include step portions 15, 16, respectively, which are lower than the covered regions covered with the two external electrodes 8, 9. Step differences of the step portion 15 and the step portion 16 may be the same as or different from one another. The step difference is preferably about 1 m or more and about 30 m or less, and more preferably about 1 m or more and about 15 m or less, for example. In FIG. 1, a total of the step differences of the step portion 15 and the step portion 16 is defined by a difference between a thickness T2 of the thermistor body 2 (a thickness between the covered regions 13a and 13d) and a thickness T1 (a thickness between the non-covered regions 13c and 13f), and for example, each of the step differences may preferably be a value of almost about of the above-described total of differences. The step portions may preferably be formed, for example, by polishing treatment, acid treatment, plating treatment, or grinding treatment. While in the figure, an example of one step is shown, a plurality of step portions may be provided.

(23) As described above, according to a preferred embodiment of the present invention, the second electrode layer, which is thinner and flatter than the first electrode layer, and in which the one end is in contact with the first electrode layer, and the other end extends in the direction of the other end surface, is provided in each of the main surfaces, so that flat portions of the external electrodes are large. Therefore, since a strict accuracy of position is not required with respect to the laser light during the laser light emission, the component built-in type substrate is able to be produced with easier simplified processes.

(24) As required, insulating layers may be provided so as to cover the non-covered regions 13c, 13f. A material for the insulating layers is not particularly limited, but proper synthetic resin, for example, may preferably be used.

(25) The NTC thermistor element according to preferred embodiments of the present invention may be produced, for example, using a production method according to a preferred embodiment of the present invention described below, and the production method includes at least a step of producing a thermistor body including internal electrodes, and a step of forming external electrodes in the thermistor body.

(26) In the step of producing the thermistor body, an organic binder is preferably added to row material powder calcined as required, and is mixed to be turned into a slurry state, and is then molded, using a doctor blading method or other suitable method to produce a ceramic green sheet. Subsequently, using the conductive paste for internal electrode, screen printing is performed on the ceramic green sheet to form an electronic pattern. Next, the plurality of ceramic green sheets on each of which the electrode pattern has been printed are layered, and then sandwiched by the ceramic green sheets on which no electrode pattern is printed from above and below to be press-bonded and produce a layered body. Subsequently, after the obtained layered body is subjected to debinding treatment, the resultant is fired to produce the thermistor body in which the internal electrodes and the thermistor body layers are alternately layered.

(27) FIGS. 3A to 3D are schematic longitudinal cross-sectional views showing the step of forming the external electrodes of the thermistor element. In FIGS. 3A to 3D, the portions similar to those in FIG. 1 are denoted by the same reference characters, and descriptions thereof will be omitted. FIG. 3A is a schematic longitudinal cross-sectional view of the thermistor body 2 in which the internal electrodes 3a, 3b, 3c, 3d, 3e and the thermistor body layers are alternately layered. FIG. 3B shows a step of forming the first electrode layers of the external electrodes. The end surfaces 11, 12 of the thermistor body 2 in which the internal electrodes and the thermistor body layers are alternately laminated are immersed in the conductive paste, and are baked to form the first electrode layers 4a, 4b. FIG. 3C shows a step of forming the second electrode layers. By a sputtering method, the pair of second electrode layers 5a, 5b and the pair of second electrode layers 5c, 5d are formed on the respective main surfaces, wherein the one end of each of the second electrode layers 5a, 5b is in contact with the first electrode layer 4a and the other end thereof extends in the direction of the other end surface, and the one end of each of the second electrode layers 5c, 5d is in contact with the first electrode layer 4b and the other end thereof extends in the direction of the other end surface. FIG. 3D shows a step of forming the third electrode layers, and shows an example in which each of the third electrode layers includes two layers. A first layer 7a and a second layer 7b of the third electrode layer 7 are preferably formed, for example, by a plating method. This enables the NTC thermistor element to be produced.

(28) Moreover, in the case where the step of forming the third electrode layers is a step of forming the third electrode layer by the plating method, after the second electrode layers are formed, body surface treatment, such as oxide treatment, polishing treatment, plating treatment, and grinding treatment may be applied to the non-covered regions, each of which is the outer surface at the central portion of the thermistor body, and is not covered with the second electrode layer. At this time, the step portions 15, 16 are formed. While in the case where the second electrode layers are formed using the sputtering method, there is a possibility that a portion of the electrode material adheres to the non-covered regions of the thermistor body, performing the above-described body surface treatment removes the portion of the electrode material when the step portions 15, 16 are formed. Thus, when the third electrode layers are formed using the plating method, an island-shaped plating is prevented from occurring in the non-covered regions of the thermistor body. For example, for the NTC thermistor element in the JIS standard 0603 size, between a case where the above-described body surface treatment is performed after the second electrode layers are formed by the sputtering method, and a case where the body surface treatment is not performed thereafter, the occurrence of the island-shaped plating is compared. When SEM observation is conducted on the non-covered regions of the thermistor body, in 1000 elements subjected to the above-described body surface treatment, the occurrence of the island-shaped plating having a diameter of about 0.5 m or more is not observed. In contrast, in the case where the above-described surface treatment is not performed, in all of the 1000 elements, the occurrence of the island-shaped plating having a diameter of about 0.5 m or more is observed.

(29) Moreover, in the above-described example, the example in which the second electrode layers are used, using the sputtering method has been described. However, a screen printing method may be used to form the second electrode layers.

(30) FIGS. 4A to 4D are schematic longitudinal cross-sectional views showing one example of a step of mounting the NTC thermistor element on the component-embedded substrate. First, as shown in FIG. 4A, a thermistor element 20 is placed on a substrate 30. At this time, a first external electrode 21 and a second external electrode 22 are bonded to the substrate 30 through an adhesive agent.

(31) Next, as shown in FIG. 4B, a sealing body 23 is applied onto a surface of the substrate 30 so as to seal the thermistor element 20. This enables the thermistor element 20 to be embedded in the sealing body 23.

(32) Next, as shown in FIG. 4C, the laser light is emitted to portions of the sealing body 23 immediately under which the first external electrode 21 and the second external electrode 22 are located, and a hole portion 24a through which the first external electrode 21 is exposed, and a hole portion 24b through which the second external electrode 22 is exposed are formed. At this time, since each of the second electrode layers, which is thinner and flatter than the first electrode layer, and in which the one end is in contact with the first electrode layer, and the other end extends in the direction of the other end surface, is provided on each of the main surfaces, a flat portion of each of the external electrodes becomes larger. Therefore, the strict accuracy of position is not required with respect to the laser light during the laser light emission, so that the component built-in type substrate can be produced with easier and simpler processes.

(33) Next, as shown in FIG. 4D, the hole portions 24a, 24b of the sealing body 23 and the upper surface of the sealing body 23, and a back surface of the substrate 30 are plated with metal materials 25, 32 such as Cu, for example, as wiring. This enables insides of the hole portions 24a, 24b to be filled with the metal material. Further, an insulating layer and a conductive layer (not shown) are preferably laminated to produce the component built-in type substrate.

(34) While NTC thermistors have been described in accordance with the above preferred embodiments of the present invention, additional preferred embodiments of the present invention may also be applied to a PTC thermistor having internal electrodes and external electrodes, which has been described, for example, in WO 2014/017365 A. That is, when preferred embodiments of the present invention are applied to a PTC thermistor, a substrate-embedded PTC thermistor is able to be provided that includes a thermistor body made of a ceramic sintered body, and including two opposed main surfaces, two opposed side surfaces, and two opposed end surfaces, a plurality of internal electrodes provided inside the thermistor body, and two external electrodes provided on outer surfaces of the thermistor body and electrically connected to the plurality of internal electrodes, wherein each of the external electrodes includes a first electrode layer covering one of the end surfaces of the thermistor body, a second electrode layer provided on each of the main surfaces of the thermistor body and including at least one layer, one end of the second electrode layer being in contact with the first electrode layer, and another end thereof extending in a direction of another end surface, and a third electrode layer including at least one layer and covering the first electrode layer and the second electrode layers. In this case, as in the NTC thermistor, an extremely high accuracy of position is not required when a via hole is formed by laser light. In the case of the PTC thermistor, ceramics having a positive resistance temperature characteristic may preferably be used for the thermistor body. Moreover, for the internal electrodes and the first electrode layers, the second electrode layers, and the third electrode layers of the external electrodes, the materials described above may preferably be used.

(35) According to preferred embodiments of the present invention, NTC thermistors capable of being easily embedded in a component-embedded substrate are provided.

(36) While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.