PTC THERMISTOR

Abstract: The invention presents a PTC thermistor which is high in adhesive strengths of inner- and outer-layer electrodes composed of metallic foil respectively stuck to conductive sheets, and has a larger current breaking characteristic. It contains a laminated body (13) which is formed by alternately laminating a plurality of conductive sheets (14) and an inner-layer electrode (11) composed of metallic foil having first plated layers (12) so that the conductive sheets (14) can become the outermost layers, an outer-layer electrode (18) positioned at the outermost layer of the laminated body (13), and having a second plated layer (16) on a side facing the inner-layer electrode (11), and side-face electrode layers 20) disposed at facing sides of the laminated body 913) for connecting electrically the inner-layer electrode (11) and outer-layer electrode (18).

Description: TECHNICAL FIELD The present invention relates to a positive temperature coefficient (PTC) thermistor using a conductive polymer having a PTC characteristic. BACKGROUND ART A conventional PTC thermistor is described below. A conventional PTC thermistor is disclosed, for example, in Japanese Laid-open Patent No. 61-10203, in which a plurality of conductive sheets composed of polymer having PTC characteristic, and an inner-layer electrode and an outer-layer electrode composed of metallic foil are alternately laminated, and a side-face electrode layer is disposed at a facing side as a lead-out part. Fig. 7 is a sectional view of a conventional PTC thermistor. In Fig. 7, reference numeral (1) is a conductive sheet having carbon black or other conductive particles mixed in a crosslinked polyethylene or other polymer material. Reference numeral 2 is a metallic foil of copper, nickel or the like, having openings 3 disposed at the start end and terminal end of the conductive sheet 1 and crimped alternately, and disposed at upper and lower sides of the conductive sheet 1, and an inner-layer electrode 2a and outer-layer electrode 2b composed of this metallic foil 2 and the conductive sheet 1 are laminated alternately to form a laminated body. Reference numeral 5 is a side-face electrode layer disposed to be connected electrically with an end of the inner-layer electrode 2a and outer-layer electrode 2b at the side facing the laminated body 4. In thus constituted conventional PTC thermistor, its manufacturing method is described below. First, carbon black or other conductive particles are mixed in polyethylene, and a rectangular conductive sheet 1 is formed, and an inner-layer electrode 2a and an outer-layer electrode 2b composed of a metallic foil made of copper or nickel, of which side is shorter than at least one side of the sides of the conductive sheet 1 by 0.5 to 3.0 mm, are laminated, so that one end is alternately aligned with one end of the conductive sheet 1 and that an opening 3 may be formed at other end, so that a laminated body 4 is formed. At this time, the uppermost side and lowermost side of the laminated body 4 are formed so that the outer-layer electrode 2b composed of metallic foil may be laminated. Next, while heating the laminated body 4 to a temperature of 100 to 200 deg. C, it is compressed from above and beneath, the conductive sheet 1 is softened, and the conductive sheet 1 of the laminated body 4 and the inner-layer electrode 2a and outer-layer electrode 2b made of metallic foil are fixed. Finally, at the facing side of the laminated body 4 fixed in the preceding step, a conductive paste is applied to connect electrically with an end of the inner-layer electrode 2a and outer-layer electrode 2b composed of metallic foil 2, and a side-face electrode 5 is formed, and then by crosslinking, a PTC thermistor is manufactured. In such conventional PTC thermistor constitution, however, in order to lower the initial resistance value, the conductive sheet 1 and the inner-layer electrode 2a and outer-layer electrode 2b composed of metallic foil are laminated alternately and compressed thermally, but since they are made of different materials, when exposed to thermal impulse, peeling may occur between the conductive sheet 1 and the inner electrode layer 2a and outer electrode layer 2b made of metallic foil due to large difference in coefficient of thermal expansion, thereby increasing the resistance value. It is hence an object of the invention to present a PTC thermistor excellent in contact between the conductive sheet and inner-layer electrode and outer-layer electrode composed of metallic foil, and not increasing in the resistance value due to thermal impulse. DISCLOSURE OF THE INVENTION To achieve the object, the PTC thermistor of the invention is characterized by composing an inner-layer electrode of a metallic foil with a rough surface by forming a first plated layer on both sides, and composing an outer-layer electrode of a metallic foil with a rough surface by forming a second plated layer on a surface facing a conductive sheet. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 (a) is a perspective view of a PTC thermistor in a first embodiment of the invention, Fig. 1 (b) is a sectional view of A-A of the PTC thermistor, Fig. 2 and Fig. 3 are process charts showing a manufacturing method of the PTC thermistor, Fig. 4 is a characteristic curve showing the breakdown characteristic of the metallic foil used in the PTC thermistor, Fig. 5 is a sectional view of a PTC thermistor in other embodiment of the invention, Fig. 6 is a sectional view of a PTC thermistor in a different embodiment of the invention, Fig. 7 is a sectional view of a PTC thermistor in a prior art. BEST MODE OF CARRYING OUT THE INVENTION The invention provides a PTC thermistor as set forth in claim 1. In a preferred embodiment, the conductive sheet contains three layers or more, and the inner-layer electrode contains two layers or more, and both have a free space so as to be aligned alternately at side end potions. Preferably, the inner-layer electrode and outer-layer electrode are nickel-plated copper foils. In a further preferred embodiment, the side-face electrode layers are composed of a same metallic material as the inner-layer electrode and outer-layer electrode. The invention further provides a PTC thermistor as set forth in claim 5. In a further aspect, the invention provides a manufacturing method of a PTC thermistor, as set out in claim 12. Embodiments Referring now to the drawings, embodiments of PTC thermistor of the invention are described below. Fig. 1 (a) is a perspective view of a PTC thermistor in a first embodiment of the invention, and Fig. 1 (b) is its sectional view of A-A. In Fig. 1, reference numeral 11 is an inner-layer electrode composed of a metallic foil such as electrolytic copper foil having first plated layers 12 made of nickel or the like on upper and lower surfaces. Reference numeral 13 is a laminated body which is formed by alternately laminating the inner-layer electrode 11, and a conductive sheet 14 formed by mixing crystalline polymer composed of high density polyethylene or the like and conductive particles composed of carbon black or the like, so that the outermost layer may be the conductive sheet 14, and there is a free space 15 at the side end portion of the inner-layer electrode 11 composed of metallic foil. Reference numeral 18 is an outer-layer electrode composed of a metallic foil such as electrolytic copper foil forming a second plated layer 16 having a free space 17 in part, disposed at a side facing the inner-layer electrode 11 composed of metallic foil of the conductive sheet 14 positioned in the outermost layer of the laminated body 13, and it is laminated so that the second plated layer 16 may face the conductive sheet 14. Reference numeral 19 is a recess provided at the side facing the laminated body 13. Reference numeral 20 shows side-face electrode layers composed of a same material as the inner-layer electrode 11, disposed in facing side recesses 19 of the laminated body 13, for connecting electrically the inner-layer electrode 11 and outer-layer electrode 18. In thus constituted PTC thermistor of the first embodiment of the invention, its manufacturing method is described below while referring to the drawings. Fig. 2 and Fig. 3 are process charts showing the manufacturing method of the PTC thermistor in the first embodiment of the invention. First, as shown in Fig. 2 (a), first plated layers 22 of nickel or other metal are formed on the entire area of both upper and lower surfaces of an inner-layer electrode 21 composed of metallic foil such as electrolytic copper foil, by electroless plating method or the like, and the upper and lower surfaces are roughened by 2 microns or more. At this time, in order to cut into pieces in a later process, splitting grooves 23 may be formed in the inner-layer electrode 21 composed of metallic foil by using die press, etching method or the like, or the inner-layer electrode 21 composed of metallic foil preliminarily forming splitting grooves 23 may be used. Consequently, as shown in Fig. 2 (b), a conductive sheet 24 composed of a mixture of about 56 wt.% of crystalline polymer composed of high density polyethylene or the like with the degree of crystallization of about 70 to 90%, and about 44 wt.% of conductive particles composed of carbon black or the like with mean particle size of about 58 nm and specific surface area of about 38 m 2 /g is laminated in the upper and lower surfaces of the inner-layer electrode 21 composed of a metallic foil having the upper and lower surfaces roughened by the first plated layers 22 by 2 microns or more, thereby forming a laminated body 25. Then, as shown in Fig. 2 (c), an outer-layer electrode 27 having one surface roughed by forming a second plated layer 26 of nickel or other metal on one side of a metal of electrolytic copper foil or the like is laminated on the outermost layer of the obtained laminated body 25, so that the roughened surface may contact with the conductive sheet 24. Next, as shown in Fig. 2 (d), the laminated body 25 laminating the outer-layer electrode 27 obtained in the preceding step is pressed and formed while heating for about 1 minute at a pressure (degree of vacuum) of about 2670 Pa (20 Torr) and at a surface pressure of about 50 kg/cm 2 , by using a hot plate of about 175 deg. C higher than the melting point of the polymer by about 40 deg. C, and a laminated sheet 28 is formed. At this time, in order to cut into pieces in a later process, splitting grooves 29 may be formed in the outer-layer electrode 27 by using die press, etching method or the like, or the outer-layer electrode 27 composed of metallic foil preliminarily forming splitting grooves 29 may be used. Successively, as shown in Fig. 3 (a), through-holes 30 are formed by drilling machine, die press or the like on the upper surface of the splitting grooves 29 of the laminated sheet 28. As shown in Fig. 3 (b), at least the inner wall of the through-hole 30 is plated with copper in a thickness of 25 to 30 microns by electrolytic copper plating or electroless copper plating, and a side-face electrode layer 31 is formed. At this time, the plating applied in the inner wall of the through-hole 30 may be formed to cover around the through-hole 30, or the upper surface and lower surface of the laminated sheet 28. Then, as shown in Fig. 3 (c), a resist is formed on the upper surface of the outer-layer electrode 27 which coincides with the outermost layer of the laminated sheet 28 by screen printing or photographic method, and the resist is removed by chemical etching, using iron chloride, and a free space 32 is formed. Finally, as shown in Fig. 3 (d), by dicing the laminated sheet 28 along the splitting grooves 29, or by cutting into individual pieces 33 by die press, a PTC thermistor is manufactured. Herein, the relation between the contact of the conductive sheet 24 with the inner-layer electrode 21 and outer-layer electrode 27, and the surface pressure when pressurizing is described below. To enhance the contact of the conductive sheet 24 with the inner-layer electrode 21 and outer-layer electrode 27, when pressurizing while heating, it is required to apply a pressure of surface pressure of about 50 kg/cm 2 or more. Considering the relation with the thickness of the inner-layer electrode 21 and outer-layer electrode 27, when pressurized, the conductive sheet 24 is melted and tends to expand in the surface direction, and also by the frictional force of the of the conductive sheet 24 against the inner-layer electrode 21 and outer-layer electrode 27, a tensile stress is generated in the surface direction in the inner-layer electrode 21 and outer-layer electrode 27, and the inner-layer electrode 21 and outer-layer electrode 27 may be broken if their metallic foil is thin. Fig. 4 shows the data comparing presence and absence of breakage of the metallic foil in relation to the force applied in this surface direction (surface pressure) and the thickness of the metallic foil. In Fig. 4, the PTC thermistor in the first embodiment of the invention was crimped by hot plates heated to about 175 deg. C from above and beneath the outer-layer electrode 27, and a pressure was applied by a press machine, then releasing from the press machine, X-ray was emitted from above the outer-layer electrode 27 to inspect for presence or absence of breakage of metallic foil as the inner-layer electrode 21 of the inside. Herein, since only one side of the outer-layer electrode 27 contacts with the conductive sheet, its chance of breakage due to surface pressure is lower as compared with the inner-layer electrode 21. In Fig. 4, if the thickness of the metallic foil is less than 35 microns, it is already broken at surface pressure of less than 50 kg/cm 2 , and a pressure of 50 kg/cm 2 necessary for obtaining contact cannot be applied. Therefore, to achieve contact without breakage of metallic foil if a pressure of 50 kg/cm 2 is applied, it is known that a thickness of 35 microns or more is needed. Moreover, to enhance the contact between the conductive sheet and metallic foil, as shown in Fig. 5, by forming a junction 37 of about 30 microns by electrolytic copper plating or the like, near the connecting area of the metallic foil as the inner-layer electrode 35 having first plated layers 35 on the upper and lower surfaces, and side-face electrode layers 36, the mechanical strength is increased at the junction 37 with the side-face electrode layers 36. Therefore, to withstand the thermal impulse, both the contact with the conductive sheet 38 and the contact with the side-face electrode layers 36 can be enhanced simultaneously. In this first embodiment, by forming recesses 19 at sides, the thermal stress caused due to difference in the coefficient of thermal expansion between the conductive sheet 14 and the inner-layer electrode 11 composed of metallic foil is dispersed without being concentrated in the recesses 19, and therefore the degree of effects on the breakage in the junction between the inner-layer electrode 11 composed of metallic foil and the side-face electrode layers 20, and between the outer-layer electrode 18 and side-face electrode layers 20 can be lessened, but the side-face electrode layers 20 may be partially formed without forming recesses 19. When roughening the surface of the metallic foil as the inner-layer electrode 11 and outer-layer electrode 18, by nickel plating or plate with copper or other metal containing nickel, the surface roughness of the plated layer is greater as compare with the case of other metal. To enhance the contact between the conductive sheet 14 and the inner-layer electrode 11 composed of metallic foil, the surface roughness of 2 microns or more is needed, and to assure such surface roughness, nickel plating capable of obtaining roughness of 2 microns is effective. In the PTC thermistor in the first embodiment, the conductive sheet 14 is composed of two layers and the inner-layer electrode 11 is composed of one layer of metallic foil, but as shown in Fig. 6, three layers of conductive sheet 39, and two layers of inner-layer electrode 40 composed of metallic foil may be alternately laminated, and layers of larger numbers may be similarly manufactured, and by increasing the number of layers, a PTC thermistor capable of passing a larger current may be manufactured. In such a case, it is necessary to array the inner-layer electrodes 40 so that the free spaces 41 may be aligned alternately at the side ends. Next, a manufacturing method of PTC thermistor is described. The manufacturing method of this PTC thermistor is nearly same as in the first embodiment. This embodiment refers to a PTC the rmistor forming three layers of conductive sheets. (a) A first conductive sheet, a second conductive sheet, and a third conductive sheet containing organic polymer and having PTC characteristic are supplied. (b) Pieces of electrode material each containing a metallic foil are supplied. (c) The surfaces of the electrode material are roughened by forming a plated layer thereon to prepare a first inner-layer electrode a second inner-layer electrode, a first outer-layer electrode and a second outer-layer electrode having a roughened surface. (d) The first outer-layer electrode, the first conductive sheet, the first inner-layer electrode, the second conductive sheet, the second inner-layer electrode, the third conductive sheet and the second outer-layer electrode are laminated in this sequence. (e) The laminated body is pressurized from both sides of the first outer-layer electrode and the second outer-layer electrode, while heating to a temperature over the melting point of the organic polymer. (f) A first free space is formed in the first outer-layer electrode, an electrically separated third outer-layer electrode is formed by the first free space, a second free space is formed in the second outer-layer electrode, and an electrically separated fourth outer-layer electrode is formed by the second free space. (g) A first side-face electrode is disposed on a first side face of the laminated body by connecting to the first outer-layer electrode, the second inner-layer electrode and the fourth outer-layer electrode, and a second side-face electrode is disposed on a second side face of the laminated body by connecting to the second outer-layer electrode, the third outer-layer electrode and the first inner-layer electrode. In this procedure, the PTC thermis tor was prepared. In this embodiment, meanwhile, the third outer-layer electrode was formed by forming the first free space in the first outer-layer electrode, but it is also possible to dispose preliminarily the first outer-layer electrode and the third outer-layer electrode on the surface of the first conductive sheet. Similarly, instead of forming the fourth outer-layer electrode by forming the second free space in the second outer-layer electrode, the second outer-layer electrode and the fourth outer-layer electrode can be preliminarily disposed on the surface of the third conductive sheet. INDUSTRIAL APPLICABILITY As described herein, according to the invention, since the inner-layer electrode and outer-layer electrode are composed of metallic foils having the surface roughened by plating, it provides a PTC thermistor excellent in the contact of the conductive sheet with the inner-layer electrode and outer-electrode composed of metallic foil if exposed to thermal impulse, and having a larger current breaking characteristic. Reference Numerals 11, 35, 40 Inner-layer electrode 12, 34 First plated layer 13 Laminated body 14, 38, 39 Conductive sheet 15, 41 Free space 16 Second plated layer 17 Free space 18 Outer-layer electrode 19 Recess 20 Side-face electrode layer

Claims: A PTC thermistor comprising: a laminated body (13) containing at least two layers of conductive sheet (14, 38, 39) composed of a polymer having a PTC characteristic and at least one layer of inner-layer electrode (11, 35, 40) composed of a metallic foil having rough surfaces formed by first plated layers (12, 34) on both surfaces, the laminated body formed by alternately laminating a plurality of layers so that said inner-layer electrode (11, 35, 40) has a free space (15, 41) at the side end portion and that the outermost layer becomes said conductive sheet (14, 38, 39), an outer-layer electrode (18) disposed at a side facing said inner-layer electrode of said conductive sheet positioned at the outermost layer of said laminated body (13), having a free space (17) in part, and having a rough surface formed by a second plated layer (16) on a side facing said conductive sheet, and side-face electrode layers (20, 36) disposed at facing sides of said laminated body (13) for connecting electrically said inner-layer electrode (11, 35, 40) and outer-layer electrode (18), wherein the laminated body (13) has recesses (19) at its facing sides, and the side-face electrode layers are provided in the recesses only. A PTC thermistor of claim 1, wherein the conductive sheet (39) contains three layers or more, and the inner-layer electrode (40) contains two layers or more, and both have a free space (41) so as to be aligned alternately at side end portions. A PTC thermistor of claim 1, wherein the inner-layer electrode (11, 35, 40) and outer-layer electrode (18) are nickel-plated copper foils. A PTC thermistor of claim 1, wherein the side-face electrode layers (20, 36) are composed of a same metallic material as the inner-layer electrode (11, 35, 40) and outer-layer electrode (18). A PTC thermistor comprising: (a) a conductive sheet (14, 38, 39) having a PTC characteristic, (b) a first outer-layer electrode (18) disposed on a first surface of said conductive sheet, and a third outer-layer electrode disposed on said first surface, said third outer-layer electrode being electrically separated from said first outer-layer electrode, (c) a second outer-layer electrode (18) disposed on a second surface of said conductive sheet, and a fourth outer-layer electrode disposed on said second surface, said fourth outer-layer electrode being electrically separated from said second outer-layer electrode, each of said first, second, third and fourth outer-layer electrodes having a rough surface formed by a respective plated layer, (d) a first side-face electrode layer (20, 36) connected to said first outer-layer electrode and said second outer-layer electrode, disposed at a first side face of said conductive sheet, (e) a second side-face electrode layer (20, 36) connected to said third outer-layer electrode and said fourth outer-layer electrode, disposed at a second side face of said conductive sheet, and (f) an inner-layer electrode (15, 35, 40), having one end connected to one side-face electrode layer of said first side-face electrode layer and said second side-face electrode layer, disposed on the inside of said conductive sheet facing both said first outer-layer electrode and said second outer-layer electrode, and having a metallic foil having surfaces roughened by a fifth plated layer thereon. A PTC thermistor of claim 5, wherein said inner-layer electrode has a plurality of inner-layer electrodes (40), and each inner-layer electrode of said plurality of inner-layer electrodes has a first free space (41) alternately aligned at said first side face side and said second side face side. A PTC thermistor of claim 5, wherein said metallic foil is a nickel-plated copper foil. A PTC thermistor of claim 5, wherein said first and second side-face electrode layers (20, 36), said first outer-layer electrode (18), said second outer-layer electrode (18), and said inner-layer electrode (15, 35, 40) are made of the same material as said metallic foil. A PTC thermistor of claim 5, wherein said first side face forms a first recess (19), said second side face forms a second recess (19), said first side-face electrode layer (20, 36) is disposed in said first recess, and said second side-face electrode layer (20, 36) is disposed in said second recess. A PTC thermistor of claim 5, wherein said inner-layer electrode (15, 35, 40) has a roughened surface with a surface roughness of 2 microns or more, and said roughened surface and said conductive sheet (14, 38, 39) adhere with each other. A PTC thermistor of claim 5, further comprising a junction layer (37) disposed in part of the surface of said inner-layer electrode (35) at the end side connected to said second side-face electrode layer (36). A manufacturing method of a PTC thermistor according to one of claims 1 to 11, the method comprising the steps of: (a) supplying pieces of electrode material each containing a metallic foil, (b) roughening the surfaces of said electrode material to form an inner-layer electrode (21), a first outer-layer electrode (27) and a second outer-layer electrode (27), each having a roughened surface, by forming a plated layer thereon, (c) laminating a first conductive sheet (24) having a PTC characteristic and containing an organic polymer on a first surface of said inner-layer electrode (21), and laminating a second conductive sheet (24) having said PTC characteristic and containing said organic polymer on a second surface of said inner-layer electrode (21), (d) laminating the first outer-layer electrode (27) on the surface of said first conductive sheet (24), and laminating the second outer-layer electrode (27) on the surface of said second conductive sheet (24), (e) pressurizing a laminated body (25) containing said first outer-layer electrode (27), said first conductive sheet (24), said inner-layer electrode (21), said second conductive sheet (24) and said second outer-layer electrode (27), from both sides of said first outer-layer electrode and said second outer-layer electrode, while heating to a temperature over the melting point of said organic polymer, (f) forming a first free space (32) in said first outer-layer electrode (27) to form a third outer-layer electrode electrically separated by said first free space, and forming a second free space in said second outer-layer electrode (27) to form a fourth outer-layer electrode electrically separated by said second free space, and (g) disposing a first side-face electrode (31) on a first side face of said laminated body by connecting to said first outer-layer electrode (27) and said fourth outer-layer electrode, and disposing a second side-face electrode(31) on a second side face of said laminated body by connecting to said second outer-layer electrode (27), said third outer-layer electrode and said inner-layer electrode (21). A manufacturing method of a PTC thermistor of claim 12, wherein a surface of said electrode material is roughed to a surface roughness of 2 microns or more. A manufacturing method of a PTC thermistor of claim 12, wherein said laminated body (25) is pressurized with a pressure of about 50kg/cm 2 or more, in a state heated to a temperature over the melting point of said organic polymer. A manufacturing method of a PTC thermistor of claim 14, wherein said inner-layer electrode (21) having a thickness of 35 microns or more is formed. A manufacturing method of a PTC thermistor of claim 12, further comprising a step of disposing a junction layer (37) in part of the surface of said inner-layer electrode (21) at the end side connected to said second side-face electrode. A manufacturing method of a PTC thermistor of claim 12, further comprising a step of forming a recess (19) in at least one side face of said first side face and said second side face of said laminated body (25). A manufacturing method of a PTC thermistor of claim 12, wherein a resist is disposed on the surface of at least one outer-layer electrode of said first outer-layer electrode (27) and said second outer-layer electrode (27), and at least one free space of said first free space (32) and said second free space (32) is formed by chemical etching. A manufacturing method of a PTC thermistor of claim 12, wherein said inner-layer electrode (21) is one of a plurality of inner-layer electrodes, and said laminated body (25) is formed at said step (c) so that said inner-layer electrodes (21) and a plurality of conductive sheets containing said first conductive sheet (24) and said second conductive sheet (24) are laminated alternately, and each of said inner-layer electrodes (21) has a third free space (41) aligned alternately between the side of said first side face and the side of said second side face.

Inventors: IKEDA TAKASHI, KOJIMA JUNJI, MIKAMOTO NAOHIRO, MORIMOTO KOHICHI, IKEDA, TAKASHI, KOJIMA, JUNJI, MIKAMOTO, NAOHIRO, MORIMOTO, KOHICHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: H01C7
IPC codes: H01C7