Structure of electronic device

Abstract: Corrosion resistivity of a thermistor (2) is improved. Corrosion resistant material is used for lead wires (5A, B) of the thermistor, and an exposed portion of electrodes (3A, B) of the thermistor and a portion (6) surrounding a weld portion of the lead wires are coated with corrosion resistant material. Since the lead wires themselves are made of corrosion resistant material, there occurs no corrosion in the welding portion and a cut working portion. Further, since the lead wires and the exposed portion of the electrodes are coated with corrosion resistant material, it is possible to provide an electronic device having the extremely high corrosion resistivity and hence the high durability and reliability, with the result that the electronic device can be used for a long period without corrosion under the heavily corrosive environment, such as sulfur dioxide gas atmosphere.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a structure of an electronic device, and more particularly to a structure with the excellent corrosion resistivity, which is suitable for an electronic device with lead wires used under the heavily corrosive environment, such as a temperature sensitive resistor for measuring the temperature of intake air in an automobile. Description of the Prior Art Referring to FIG. 1, a temperature sensitive resistor, i.e., a thermistor, as a conventional electronic device with lead wires will be described below, by taking a glass-sealed thermistor of an axial type (a diode type) as an example. As shown in the figure, the thermistor is constructed by putting thermistor element 2 in glass tube 1, which is hermetically sealed by sealing electrodes 3A, 3B. The thermistor elements 2 is sandwiched by the electrodes 3A, 3B to maintain the electric contact therebetween. Further, reference symbols 4A, 4B indicate lead wires electrically attached to the electrodes 3A, 3B, respectively. Usually, a dumet wire (JIS H4541) has been used for the sealing electrodes 3A, 3B in such a glass-sealed type thermistor as described above. FIG. 2 shows a sectional view of an example of the dumet wire As shown in the figure, a dumet wire is constructed by coating core wire 11 made of iron-nickel alloy by copper 12 as an intermediate layer, which is further covered by surface layer 13 of cuprous oxide (Cu 2 O) or borate (Cu 2 O-Na 2 B 4 O 7 ). The core of a dumet wire is made of iron-nickel alloy in order to bring the thermal expansion coefficient closer to that of glass, whereas the surface layer thereof is made of cuprous oxide for the purpose of the good melting-adhesiveness with glass. Since the sealing electrode 3A, 3B is made by cutting such a dumet wire in an appropriate length, iron-nickel alloy as core material is exposed to the atmosphere at the end surface 3a, 3b. Further, the lead wire 4A, 4B, as an example thereof being shown in FIG. 3, is formed by coating the surface of core wire 15 made of dumet, iron or iron-nickel alloy with copper 14. Metallic portions of the glass-sealed type thermistor, i.e., the outer end surfaces 3a, 3b of the sealing electrodes 3A, 3B and the surfaces of the lead wires 4A, 4B, are plated by solder to solder the thermistor onto a substrate. Further, nickeling can also be used to attach the thermistor to the substrate by spot welding or the like. As described above, the core of the dumet wire made of iron-nickel alloy is exposed to the atmosphere at the end surfaces 3a, 3b. However, the corrosion resistivity of the end surface 3a, 3b can be improved by solder-plating or nickeling. A thermistor of this kind is often used under the corrosive environment, such as a temperature sensitive resistor for measuring the temperature of intake air in an automobile. In such a case, the thermistor is required to have the sufficiently high corrosion resistivity. However, a conventional glass-sealed type thermistor as mentioned above does not have the sufficiently high corrosion resistivity and hence has a disadvantage that corrosion occurs when it is used under the heavily corrosive environment, such as sulfur dioxide gas atmosphere. In other words, a thermistor, which has solder plating to cover its metallic portions, can not have the sufficiently high corrosion resistivity. Compared with that, a thermistor, which has nickeling for the same purpose, can be much improved in the corrosion resistivity. Even with the later thermistor, however, it is difficult to say that the corrosion resistivity is really sufficient. This is because when a lead wire is cut for the length adjustment thereof, its core of easily corrosible iron-nickel alloy or iron appears at the cut surface, which is exposed to the corrosive atmosphere and from which corrosion will begin. Further, in the case of using the spot-welding, a layer of nickel plated on a lead wire is melted by welding heat, and the core of iron-nickel alloy or iron is exposed to the corrosive atmosphere and continues to be corroded from such an exposed portion. A structure of an electronic device which comprises an element with a desired electronic characteristic, electrodes electrically connected to the element, inorganic insulator for sealing or coating at least part of the element and the electrodes, and lead wires provided for the electrical connection with the electrodes is known from EP-A-0 129 997. SUMMARY OF THE INVENTION An object of the present invention solves the problem in a conventional electronic device as described above to thereby provide a structure of an electronic device, such as a thermistor, having the extremely high corrosion resistivity. A structure of an electronic device according to the present invention is characterized in that the lead wires are made of corrosion resistant material and further at least a portion surrounding a joint of the lead wires and the electrodes is coated with corrosion resistant material, said portion including said lead wires and a metallic portion of said electrodes which is exposed to the external atmosphere. Since in the present invention, the lead wires themselves are made of corrosion resistant material, there occurs no corrosion in a welding portion and a cut portion. Further, since the lead wires and the exposed portion of the electrodes are coated with corrosion resistant material, it is possible to provide a structure of an electronic device having the extremely high corrosion resistivity and hence the high durability as well as high reliability. As a result, an electronic device with a structure according to the present invention can be used for a long period without corrosion under the heavily corrosive environment, such as sulfur dioxide gas atmosphere. In one of the embodiments of the present invention, an axial type of a glass-sealed thermistor may be constructed in the following manner; namely, first of all, cylindrical electrodes made of dumet wire are welded with lead wires made of nickel. Then, a semiconductor thermistor element and the cylindrical electrodes are put in a glass tube in such a manner that the electrodes hermetically seal both ends of the glass tube. Further, nikeling is performed on an exposed portion of the electrodes and the lead wires, as well as the welded portion of the lead wires with the electrodes. In another embodiment of the present invention, a linear type of a temperature sensitive resistor with lead wires may be constructed as follows; namely, at first, a temperature sensitive element is made by forming a metallic film on the surface of a cylindrical alumina bobbin. Cap electrodes made of iron-nickel alloy are fitted by pressure to both ends of the temperature sensitive element as formed above. Then, lead wires made of nickel are welded to the cap electrodes. Part of the temperature sensitive element and the electrodes are coated with glass, and nickeling is performed on an exposed portion of the cap electrodes and the lead wires, as well as the welded portion of the lead wires with the cap electrodes. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a conventional glass-sealed type thermistor; FIG. 2 is a cross-sectional view of an example of a dumet wire used as a sealing electrode in the glass-sealed type thermistor; FIG. 3 is a cross-sectional view of an example of a lead wire used in the glass-sealed type thermistor; FIG. 4 is a cross-sectional view of a glass-sealed type thermistor in accordance with an embodiment of the present invention; and FIG. 5 is a cross-sectional view of a glass-sealed type thermistor in accordance with another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, embodiments of the present invention will be described in detail, referring to FIGs. 4 and 5. FIG. 4 is a cross-sectional view of an axial type of a glass-sealed thermistor in accordance with an embodiment of the present invention, in which the same reference numerals or symbols as those used in FIG. 1 indicate the same parts. As shown in the drawing, the glass-sealed type thermistor is constructed as follows. Namely, the glass tube 1 has the thermistor element 2 therein. Both ends of the glass tube 1 are hermetically sealed by the sealing electrodes 3A, 3B, to which nickel lead wires 5A, 5B are attached. Nickel member 6 is plated on metallic portions of an assembly as described above, i.e., outer end surface of the electrodes 3A, 3B and surface of the lead wires 5A, 5B. In this embodiment, it is preferable to use dumet wire for the sealing electrodes 3A, 3B in the same manner as conventional. Further, there is no limitation in the length and diameter of the electrode 3A, 3B. Also the glass tube 1 can be formed by a glass tube made of SiO 2 -PbO-K 2 O or the like in the same manner as conventional. Thickness of the glass tube 1 depends on the size of the thermistor element 2, but is generally 0.3 - 1.0 mm. Preferably, an inner diameter of the glass tube 1 is 1 - 1.8 times as large as the diameter of the thermistor element 2 to be inserted therein and a length thereof is 3 - 50 times as large as the thickness of the thermistor element 2. The thermistor element 2 has a thermistor ceramic with electrodes made of Ag, Pd or the like on both side thereof. The size of the thermistor element 2 is usually 0.35 - 0.6 mm square. Preferably, the nickel lead wires 5A, 5B are a wire of 0.3 - 0.5 mm in diameter. Further, the thickness of the nickeling is preferably 2 - 10 µm, since it is difficult to obtain the sufficient effect of improving the corrosion resistivity, if it is much thinner than the thickness as above, and it is uneconomical, if it is much thicker than that. In the present invention, there is no need to nickel the lead wire, since it is made by a nickel wire. If, however, the end surface of the sealing electrodes 3A, 3B is nickeled, the lead wires 5A, 5B can also be nickeled resultantly. The inventors carried out the test that the glass-sealed type thermistor as above has been spot-welded onto a substrate and used under the sulfur dioxide gas atmosphere for a long period. No occurrence of corrosion could be found in that test, however. FIG. 5 is a cross-sectional view of a linear type of a temperature sensitive resistor in accordance with another embodiment of the present invention. A temperature sensitive element according to this embodiment is constructed as follows. Thin platinum film 22 is formed by barrel spatter on the surface of solid cylindrical bobbin 21 made of alumina having a diameter of nearly 1 mm. The bobbin 21 with the platinum film 22 is further treated by heating. Cap electrodes 23A, 23B made of iron-nickel alloy are fitted by pressure on both ends of the temperature sensitive element, to which electrodes nickel lead wires 24A, 24B having a diameter of 0.3 - 0.5 mm are welded Then, adjustment of resistance is performed by laser trimming of the thin platinum film 22. Further, nickel plating 26 having the thickness of 2 - 10µm is performed on an exposed portion of the electrodes 23A, 23B and the surface of the lead wires 24A, 24B, after coating the thin platinum film portion and part of the cap electrodes 23A, 23B with glass 25. The inventors carried out the same test on this thermistor as they did on the first embodiment, too. As the result, no occurrence of corrosion could be found in this thermistor, either. As described above, with the structure of the electronic device in accordance with the present invention, it is possible to provide a temperature sensitive resistor with lead wires having the extremely high corrosion resistivity and hence the high durability and reliability. As a result, the electronic device according to the present invention can be used for a long period without corrosion under the heavily corrosive environment, such as sulfur dioxide gas atmosphere.

Claims: A structure of an electronic device comprising an element (2) with a desired electronic characteristic; electrodes (3A, B) for making the electrical connection with said element; inorganic insulator (1) for coating at least part of said element (2) and said electrodes; and lead wires (5A, B) provided for the electrical connection with said electrodes; characterized in that said lead wires (5A, B) are made of a corrosion resistant material and at least a portion (6) surrounding a joint of said lead wires and said electrodes is coated with corrosion resistant material, said portion (6) including said lead wires and a metallic portion of said electrodes which is exposed to the external atmosphere. A structure of an electronic device according to claim 1, wherein said lead wire is made of nickel, and nickel is plated at least on the portion (6) surrounding the joint of said lead wires and said electrodes. A structure of an electronic device according to claim 1, wherein said element (2) is a thermistor made of a semiconductor material having a temperature dependent characteristic, said electrodes for electrically connecting to said element are formed by dumet wire, and said inorganic insulator for coating at least part of said element and said electrodes is glass material.

Inventors: HIRAMA MASAHIRO, MIYAZAKI ATSUSHI, OCHI NAOYUKI, SAITO TAKAYUKI, TOMOTO HIROSHI, UCHIYAMA KAORU, HIRAMA, MASAHIRO, MIYAZAKI, ATSUSHI, OCHI, NAOYUKI, SAITO, TAKAYUKI, TOMOTO, HIROSHI, UCHIYAMA, KAORU, Hirama, Masahiro, Keimei-ryo, Ochi, Naoyuki, Mitsubishi-shataku 521
Applicants: HITACHI CAR ENG CO LTD, HITACHI LTD, HITACHI CAR ENGINEERING CO., LTD., HITACHI, LTD.
Application year: 1997
Main classification: H01C1
IPC codes: H01C1, H01C17, H01C7