JPH0538802U - Chip thermistor - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a stable chip thermistor having excellent heat resistance and a low resistance change rate after mounting on a substrate. In a chip thermistor 7 having a substantially rectangular ceramic body 8 and end electrodes 9a and 9b formed on both ends of the body, the tip thermistor 7 is made of lead by electrolytic plating. First metal coating 10
a and 10b are formed, and then a second metal coating 1 made of tin is formed on the surface of the first metal coating by electrolytic plating.
1a and 11b are formed. As a result, the solder wettability at the time of mounting is improved, the solder heat resistance is improved, and the solder erosion of the end electrodes is reduced. It is possible to provide a highly stable chip thermistor capable of significantly reducing

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a chip thermistor having a plate-shaped or substantially rectangular, columnar, or cylindrical ceramic body.

[0002]

[Prior Art]

 A chip thermistor is a plate-shaped or substantially rectangular, columnar, or cylindrical ceramic body that is a sintered body of a ceramic material, and has end electrodes made of silver, palladium alloy (Ag / Pd), copper (Cu), or the like at both ends. Are insulated from each other.

FIG. 4 shows a conventional example of a chip thermistor.

The chip thermistor 1 has a first end electrode 3a and a second end electrode 3b formed at both ends of a flat ceramic body 2, and as shown in FIG. The first end electrode 3a and the second end electrode 3b are soldered with the solder 5 to be mounted on a desired printed circuit board 6.

However, at the time of this soldering, the end electrodes 3a and 3b are heated by the melted solder 5, and the end electrode material partially migrates into the solder due to diffusion of metallurgical properties, and the solder is generally eaten away. The phenomenon of migration of electrodes, called

It is possible to suppress the influence of solder erosion by selecting an electrode material. For example, when an Ag / Pd alloy is used as the electrode material, the Pd content should be increased. However, the solder erosion can be reduced, but the cost is increased because Pd is expensive.

In addition to the solder erosion, the resistance value of the chip thermistor 1 after mounting on the printed circuit board 6 changes due to the melting and heating of the solder.

As a means for solving the above-mentioned drawback, it can be considered that a solder paste made of lead, tin or the like is printed and applied on the surface of the end electrode, and then heat treatment is performed to form a metal film.

However, according to this method, since the number of heat treatments increases, the thermal stress applied to the flat plate-shaped ceramic body becomes larger, and the resistance value of the chip thermistor after mounting on the printed circuit board 6 decreases, and Causes a problem that the resistance change with time also increases

[0010]

[Problems to be solved by the device]

 As described above, conventionally, the chip thermistor is mounted on a printed circuit board by soldering, so that the heat transfer from the chip causes solder erosion, which changes the resistance value of the chip thermistor after board mounting. there were.

Therefore, the present invention has been made in view of the above circumstances, and has good solder wettability at the time of mounting, little change in resistance value of the chip thermistor after mounting and little change with time, and excellent solder heat resistance. It is intended to provide a chip thermistor.

[0012]

[Means for Solving the Problems]

 In order to achieve the above object, the present invention provides a chip thermistor having a plate-shaped or substantially rectangular-shaped, column-shaped, cylindrical ceramic element body and end electrodes that are formed insulated from each other at both ends of the element body. A metal coating is formed on the surface of the end electrode by electrolytic plating. The metal coating is made of tin, or the metal coating has a two-layer structure in which lead is the first layer and tin is the second layer. Further, the end electrode is formed by baking a metal containing 50% or more of palladium in an oxygen atmosphere, and then performing heat treatment in a reducing or neutral atmosphere.

[0013]

[Action]

 According to the chip thermistor configured as described above, the metal coating is formed on the surface of the end electrode by the electrolytic plating method before the board is mounted, so that the solder wettability during board mounting is improved and the solder heat resistance is improved. This reduces solder erosion of the end electrodes due to heat transfer during board mounting, and reduces the resistance change rate of the chip thermistor after board mounting. Further, the change in the resistance value can be suppressed also by forming the end electrodes by heat treatment in a reducing or neutral atmosphere.

[0014]

【Example】

 An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a sectional view of a chip thermistor 7 according to an embodiment of the present invention.

At both ends in the longitudinal direction of the substantially rectangular ceramic body 8 made of a sintered body containing manganese oxide MnO, cobalt oxide CoO, nickel oxide NiO, etc. as main components, first end portions insulated from each other are formed. An electrode 9a and a second end electrode 9b are provided. The end electrodes 9a and 9b are formed of, for example, silver or palladium alloy (Ag / Pd) containing 50% or more of palladium Pd. Further, first metal coatings 10a, 10b made of lead are formed on the surfaces of the end electrodes 9a, 9b by electrolytic plating, and second metal coatings 11a, 11b made of tin are formed on the surfaces by electrolytic plating. It is formed . That is, a metal coating having a two-layer structure is formed on the surfaces of the end electrodes 9a and 9b.

After that, the chip thermistor 7 is soldered together with the metal films 10a, 10b made of lead and the metal films 11a, 11b made of tin on the desired printed circuit board by forming the end electrodes 9a, 9b by the electrolytic plating method. It is implemented by attaching.

Next, a method of manufacturing the chip thermistor 7 in the above embodiment will be described.

First, for example, a paste of silver and a palladium alloy (Ag / Pd) containing 50% or more of palladium Pd is applied to both ends of the substantially rectangular ceramic body 8, and the paste is baked in an oxygen atmosphere. The baked electrode is subjected to heat treatment in a reducing or neutral atmosphere. Then, the end electrodes 9a and 9b are formed by heat treatment in a neutral atmosphere such as N _{2 at the} same temperature as the substrate mounting temperature, that is, at 180 to 200 ° C. for 5 minutes. After that, as described above, the surfaces of the end electrodes 9a, 9b of the chip thermistor element are subjected to lead plating by electrolytic plating at room temperature to form the first metal coatings 10a, 10b. Next, tin plating is similarly applied to the surface to form second metal coatings 11a and 11b.

Finally, the chip thermistor 7 is completed through cleaning and the like.

As described above, since the substantially rectangular ceramic body 7 and the end electrodes 9a and 9b which are heat-treated in the neutral atmosphere are initially aged, the heat transfer for soldering after mounting on the board is performed. The change in the resistance value of the flat plate-shaped ceramic body 11 itself hardly occurs. Further, since the metal coatings 10a, 11a, 10b, 11b are formed by subjecting the surfaces of the end electrodes 9a, 9b to electrolytic plating, the solder wettability is improved and the electrolytic plating is performed at room temperature. The heat resistance is improved, and the resistance change rate due to the solder erosion of the end electrodes 9a 1 and 9b is also reduced.

Therefore, the resistance change rate of the chip thermistor due to the mounting on the printed circuit board is significantly reduced, so that the variation in the resistance value is reduced.

FIG. 3 shows a result obtained by an experiment conducted by the present inventors on the chip thermistor 7 manufactured as described above. This figure is a diagram showing the rate of resistance change after mounting on the substrate with respect to the melting temperature of the solder, and is a comparison between the chip thermistors 1 and 7 in the conventional example and the present example. As can be seen in FIG. 3, the rate of change in resistance after mounting the substrate also rises as the solder temperature rises, but at the same solder temperature, this example is considerably improved compared to the conventional example. I understand.

The chip thermistor 7 is mounted by mounting the chip thermistor 7 on a printed circuit board and immersing it in a solder bath. FIG. 2 shows the heat resistance of the chip thermistor of this embodiment. The relationship between the immersion time and the resistance change rate when the composition ratio of Ag / Pd, which is the material of the partial electrodes 9a and 9b, is changed is shown. In this case, the greater the amount of Pd with respect to Ag, the lower the resistance change rate of the chip thermistor 7, and it is best if only Pd is used as the material (Pd = 100%). Because of the high cost, in this embodiment, Pd is included at 50% or more.

In the present embodiment, the metal coating is formed by two layers of lead plating and tin plating, but the same effect can be obtained by forming only the tin plating.

Further, although the ceramic body has a substantially rectangular shape in the present embodiment, the same effect can be obtained by using a plate shape, a columnar shape, or a cylindrical shape.

The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention.

[0028]

[Effect of the device]

 According to the present invention described in detail above, after the chip thermistor having the end electrodes formed thereon is heat-treated in a neutral atmosphere, a metal film is formed on the end electrode surfaces by electrolytic plating at room temperature. The solder wettability at the time is improved, and the solder heat resistance is also improved, so that the solder erosion of the end electrodes is reduced, and the rate of resistance change of the chip thermistor and the change with time of the resistance after mounting on the board are significantly reduced. It is possible to provide a highly stable chip thermistor that can be reduced.

[Brief description of drawings]

FIG. 1 is a sectional view of a chip thermistor according to an embodiment of the present invention.

FIG. 2 is a diagram showing heat resistance of a chip thermistor according to an embodiment of the present invention.

FIG. 3 is a diagram showing an effect of one embodiment of the present invention.

FIG. 4 is a sectional view of a conventional chip thermistor.

[FIG. 5] Mounting process diagram of a conventional chip thermistor

[Explanation of symbols]

 7 Chip Thermistor 8 Substantially Rectangular Ceramic Element 9a, 9b End Electrode 10a, 10b First Metal Film 11a, 11b Lead Metal Second Metal Film 11a, 11b

Claims (6)

[Scope of utility model registration request] 1. A chip thermistor having a plate-shaped or substantially rectangular, columnar, or cylindrical ceramic body and end electrodes insulated from each other at both ends of the body, wherein a surface of the end electrode is provided. A chip thermistor having a metal coating formed by an electrolytic plating method. 2. The chip thermistor according to claim 1, wherein the metal coating is made of tin. 3. The thermistor according to claim 1, wherein the metal coating has a two-layer structure in which lead is the first layer and tin is the second layer. 4. The chip thermistor according to claim 1, wherein the end electrode has an electrode layer formed of a metal containing 50% or more of palladium. 5. The chip thermistor according to claim 1, wherein the end electrode is a baked electrode baked in an oxygen atmosphere. 6. The chip thermistor according to claim 5, wherein the baked electrode is formed by heat treatment in a reducing or neutral atmosphere.