Thermistor Glass Metal Seal

A thermistor with glass-metal seal using dumet wires as lead-in-wires with an electronic device structure with excellent corrosion resistivity 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.

Corrosion resistant material is used for lead-in-wires of thermistors and an exposed portion of electrodes of the thermistor and a portion surrounding a weld portion of the lead-in-wires are coated with corrosion resistant material. Since the lead-in-wires themselves are made of corrosion resistant material hence no corrosion occurs in the welding portion and a cut working portion. Further, the lead-in-wires and the exposed portion of the electrodes are also coated with corrosion resistant material. Because of this a thermistor having extremely high corrosion resistivity and high durability and reliability is produced with the result that the thermistor can be used for a long period without corrosion under the heavily corrosive environment.

A thermistor is an electronic device, comprising:

1. an element made of a semiconductor material having a temperature dependent characteristic;
2. a pair of electrodes comprising dumet material for making electrical connection with the element;
3. a glass member covering the element and a part of the electrodes hermetically seal the element between the pair of electrodes;
4. a pair of lead-in-wires made of corrosion resistant metal, each electrically joined by welding to the corresponding pair of electrodes at a portion not covered by the glass member; and
5. a corrosion resistant metal plating separately formed subsequent to the lead-in-wire joining on the surfaces of the non-covered portion of the electrodes except for the joint portions with the lead-in-wires and on the surfaces of the lead-in-wires where the corrosion resistance of the metal plating is higher than a corrosion resistance of the dumet electrodes.
6. lead-in-wire is made of nickel, and nickel plating is provided on the portion surrounding the joint of said lead wires and said electrodes.

A conventional temperature sensitive resistor, i.e., a thermistor, is typically an electronic device with lead-in-wires made of dumet wire for glass-metal sealing. In particular, a glass-sealed thermistor of an axial type or a diode type is the best example. These thermistors are constructed by putting the thermistor element in glass tube, which is hermetically sealed by sealing electrodes. The thermistor element is sandwiched by the electrodes to maintain the electric contact inbetween. Further, the lead-in-wires are electrically attached to the electrodes. A dumet wire is used for the sealing electrodes in such a glass-sealed type thermistor.

The dumet wire is constructed by coating core wire made of iron-nickel alloy by copper as an intermediate layer, which is further covered by surface layer of cuprous oxide (Cu₂O) or borate (Cu₂O—Na₂B₄O₇). 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 is made of cuprous oxide for the purpose of the good melting adhesiveness of the material with glass. Since the sealing electrode is made by cutting such a dumet wire in an appropriate length, the iron-nickel alloy which is the core material is exposed to the atmosphere at the end surface. Further, the lead-in-wire is formed by coating the surface of core wire made of dumet, iron or iron-nickel alloy with copper.

The metallic portions of the glass-sealed type thermistor - the outer end surfaces of the sealing electrodes and the surfaces of the lead wires, are also plated by solder so as to solder the thermistor onto the required substrate. Nickeling can also be used to attach the thermistor to the substrate by spot welding. The core of the dumet wire made of iron-nickel alloy is exposed to the atmosphere at the end surfaces. However, the corrosion resistivity of the end surface can be improved by solder-plating or nickeling.

A thermistor of this kind is often used in a 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 sufficiently high corrosion resistivity. However, a conventional glass-sealed type thermistor does not have sufficiently high corrosion resistivity and hence has a disadvantage that corrosion occurs when it is used in a heavily corrosive environment such as sulfur dioxide gas atmosphere. Thus, a thermistor which has solder plating to cover its metallic portions cannot have sufficiently high corrosion resistivity. Compared with a soldered thermistor, a thermistor which has nickeling for the same purpose can be much improved in corrosion resistivity.

Even with the later corrosion resistivity is less than desirable. This is because when a lead-in-wire is cut for length adjustment, its core of easily corrodible iron-nickel alloy or iron appears at the cut surface which is exposed to the corrosive atmosphere and from which corrosion will begin. 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.

The electronic device structure for corrosion resistant thermistor comprises of an element with the desired electronic characteristic and electrodes electrically connected to the element with an inorganic insulator for sealing or coating at least part of the element and the electrodes and lead-in-wires provided for the electrical connection with the electrodes wherein the lead wires are made of corrosion resistant material and a portion surrounding the joint of the lead wires and the electrodes is coated with corrosion resistant material.

Since the lead-in-wires themselves are made of corrosion resistant material, no corrosion occurs in the welding portion and the cut portion. Since the lead-in-wires and the exposed portion of the electrodes are coated with corrosion resistant material, the structure of the thermistor has an extremely high corrosion resistivity and high durability with high reliability. Hence, the thermistor can be used for long periods of time without corrosion in a heavily corrosive environment.

An axial type glass-sealed thermistor can be constructed with cylindrical electrodes made of dumet wire which are welded with lead-in-wires of nickel. Then, a semiconductor thermistor element and the cylindrical electrodes are put in a glass tube with the electrodes hermetically sealed at both ends of the glass tube. Next, nickeling is performed on the exposed portion of the electrodes and the lead-in-wires, as well as the welded portion of the lead wires with the electrodes.

Another type is a linear type of temperature sensitive resistor with lead wires constructed with a temperature sensitive element 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. Then, lead-in-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 the exposed portion of the cap electrodes and the lead-in-wires as well as the welded portion of the lead wires with the cap electrodes.

Thermistor Construction

The glass-sealed type thermistor is constructed as follows: 

1. The glass tube has the thermistor element inside. 
2. Both ends of the glass tube are hermetically sealed by the sealing electrodes.
3. Nickel lead-in-wires are attached to the sealing electrodes.
4. A nickel member is plated on metallic portions of an assembly on the outer end surface of the electrodes and surface of the lead-in-wires.

1. Dumet wire is used for the sealing electrodes with no limitation in the length and diameter of the electrode.
2. The glass tube is formed by a glass tube made of SiO2—PbO—K2O conventionally. Thickness of the glass tube depends on the size of the thermistor element. The inner diameter of the glass tube is 1 to 1.8 times as large as the diameter of the thermistor element to be inserted inside and the length is 3 to 50 times as large as the thickness of the thermistor element.
3. The thermistor element is a thermistor ceramic with electrodes made of Ag, Pd or similar metal on both sides.
4. The nickel lead-in-wires are 0.3 to 0.5 mm in diameter. The thickness of the nickeling is preferably 2 to 10 since it is difficult to improve the corrosion resistivity if it is much thinnerand is uneconomical if it is thicker.
5. There is no need to nickel the lead wire, since it is made of a nickel wire. However, if the end surface of the sealing electrodes is nickeled then the lead-in-wires can also be nickeled as a result.

In the linear type of a temperature sensitive resistor, a temperature sensitive element is constructed as follows:

1. A thin platinum film is formed by barrel spatter on the surface of a solid cylindrical bobbin made of alumina. The bobbin with the platinum film is further treated by heating.
2. Cap electrodes made of iron-nickel alloy are fitted by pressure on both ends of the temperature sensitive element to which the electrodes of nickel lead-in-wires are welded.
3. Adjustment of resistance is done by laser trimming of the thin platinum film. 
4. Nickel plating is performed on the exposed portion of the electrodes and the surface of the lead-in-wires after coating the thin platinum film portion and part of the cap electrodes with glass.

With the structure of such an electronic device, a temperature sensitive resistor with lead wires is provided with extremely high corrosion resistivity with high durability and reliability. This thermistor can be used for a long period without corrosion in a heavily corrosive environment.