JPH03250603A - Thermistor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thermistor whose resistance value decreases as the temperature rises within the operating temperature range.

In particular, it relates to the element structure of a thermistor. The thermistor is
Used for temperature compensation of electronic equipment, surface temperature measurement sensors, etc.

〔overview〕

The present invention provides a thermistor whose resistance value decreases as the temperature rises within the operating temperature range, by covering the surface of the thermistor body other than the part where the electrodes come into contact with a glass layer, and forming the electrodes by plating. The present invention provides a thermistor element that has excellent solder adhesion and solder heat resistance, and has small variations in resistance value.

[Conventional technology]

A thermistor has a large negative temperature coefficient of resistance, and its resistance value decreases as the temperature rises within the operating temperature range. Utilizing this characteristic, for example, in communication equipment, temperature compensation of the oscillation frequency is performed.

A conventional thermistor surface-mounted on a printed circuit board or an alumina substrate has a structure in which electrodes mainly composed of silver and palladium are formed at both ends of a thermistor element.

The reason why silver-palladium is used for the electrode is to prevent the electrode from being eluted into the solder and disappear when the thermistor is soldered to a printed circuit board, that is, to obtain solder heat resistance.

To form a silver-palladium electrode, silver-palladium.
A method such as immersing a part of the thermistor element in paste is used.

[Problem to be solved by the invention]

However, conventional thermistors have insufficient soldering heat resistance and cannot be soldered for long periods of time at high temperatures, so there are restrictions on soldering.

In order to improve solder heat resistance, it is preferable to increase the amount of palladium. However, as the amount of palladium increases,
Solder adhesion deteriorates, and the amount of solder adhering to the thermistor electrode decreases.

This has led to problems such as weakening of the thermistor's adhesion to the printed circuit board and incomplete electrical connection between the thermistor and the wiring on the circuit.

A possible method for improving both solder adhesion and solder heat resistance is to apply metal plating to the surface of the electrode. However, the thermistor element may be eroded during the plating process, or
Plating adheres to the surface of the element. In addition, the interface between the electrode and the thermistor element and a part of the element surface are eroded, resulting in a decrease in reliability such as moisture resistance.

Further, in the method of immersing the thermistor body in silver-palladium, variations occur in the amount of silver-palladium deposited on each element, and variations occur in the spacing between electrodes.

As a result, the resistance value distribution of the thermistor becomes wider, causing problems such as greater variation within the manufacturing loft and between manufacturing lots.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and provide a thermistor having a structure that has excellent solder adhesion and solder heat resistance, and can reduce variations in resistance value.

[Means to solve the problem]

The thermistor of the present invention is characterized in that the surface of the thermistor body is coated with a glass layer except for the portions where the electrodes make electrical contact with each other, and the electrodes include a plating layer.

As a thermistor in which the surface of the thermistor body is coated with a glass layer, a chip-type thermistor disclosed in Japanese Utility Model Application Publication No. 63-67201 is known. This known technique uses a glass layer in order to prevent characteristic deterioration during use due to the exposure of the thermistor body, and does not take plating treatment into consideration.

In contrast, the present invention aims to improve solder adhesion and solder heat resistance by plating the electrode surface, and uses a glass layer as a lever to solve the problems associated with this. It is.

To manufacture the thermistor of the present invention, a glass paste is coated or printed on the surface of the thermistor body except for the areas where electrodes are to be formed, and this is fired to form a glass layer. After this, an electrode mainly composed of silver is formed on the part not covered with the glass layer, and nickel and
Plating with metal such as tin.

The material for the glass layer has a softening point of 400 to 1000°C.
It is desirable that the coefficient of linear thermal expansion is 40 to 100%, particularly 50 to 90%, of the thermistor element.
Preferably.

The range of the softening point is actually determined in consideration of the electrode firing temperature. The firing temperature when using baked silver for the electrode is 6
00 to 850°C, and if the softening point of the glass is significantly lower than that temperature, the glass will float to the electrode surface during electrode firing, causing the elements to stick to each other or the elements and the firing jig, resulting in a decrease in yield. There are things to do. A desirable softening point range is ±50° C. of the electrode firing temperature.

If the softening point is higher than 1000°C, the temperature for forming the glass layer will substantially exceed 1000°C, causing problems such as deterioration of the thermistor element.

When the linear thermal expansion coefficient of the glass is 40 to 100% of the linear thermal expansion coefficient of the thermistor element, the bending strength increases compared to the case of the thermistor element alone. Especially 50-9
When it is 0%, the bending strength increases by 20 to 70%. On the other hand, outside the range of 40-100%, the bending strength will be lower than that without the glass layer.

The bending strength refers to the breaking strength when a load is applied to the center of the element with both ends of the element placed on two stands spaced apart. This is a measure of how well the device can withstand the heat caused by soldering or the like when attaching the device to the surface mounting board, and the stress and strain caused by subsequent thermal cycles.

The reason for the increase in bending strength is considered to be that compressive stress remains in the glass layer on the surface of the element. In other words, when the thermistor element and glass layer, which were thermally expanded during manufacturing, cool down,
A thermistor element having a larger coefficient of thermal expansion will shrink more, and the glass layer will be in a compressed state. When a bending force is applied to the thermistor in this state, compressive stress is generated on the inside of the bend, and tensile stress is generated on the outside. Both the ceramic material and the glass layer of the thermistor element are strong against compressive stress but weak against tensile stress, and if a bending force exceeding a certain level is applied, cracks will occur on the outside of the bend. At this time, since compressive stress is originally applied to the outer glass layer, the bending strength increases compared to the case where there is no glass layer.

As a specific material for the glass layer, plating-resistant S1
02, B203, and BaO-based ones are preferred, but in addition to these, Ll, K, Na, and Mg5Sr.

The material may contain ions such as Zn, Cd, Pb, and AR, and the present invention is not limited to these materials.

Materials for the thermistor body include manganese, cobalt,
A sintered body of oxide of one or more metals selected from Nikkenope aluminum and copper can be used, but the present invention is not limited to these materials.

The thermistor body preferably has a prismatic or cylindrical shape, but the present invention is not limited to these shapes.

To form the plating layer, a conductive layer with good plating adhesion is formed on the thermistor body, and a plating layer is further formed on the surface of the conductive layer. As a material for the conductive layer, a baked electrode made of a silver paste containing silver as a main component can be used, which has better electrical conductivity than a silver-palladium alloy, has excellent heat resistance, and is inexpensive. However, without being limited thereto, for example, copper, nickel, etc. can also be used, and it can also be formed by a thermal spraying method.

[For production]

By covering the thermistor body with a glass layer except for the parts where the electrodes come into contact, it prevents erosion of the thermistor body during plating, plating adhesion to the body, and erosion of the interface between the electrode and the body. This makes it possible to plate only the electrode surface. By plating the electrode surface, both solder adhesion and solder heat resistance can be improved.

Further, if the glass layer serves as a mask during electrode formation, and the elements have the same structure, the electrode adhesion area will be substantially constant, and variations in resistance value after manufacture will be reduced. In particular, if the coefficient of thermal expansion is selected appropriately, the bending strength can be increased.

〔Example〕

FIG. 1 shows a perspective view of a thermistor according to an embodiment of the present invention, and FIG. 2 shows a cross-sectional view along the length direction.

Here, an element having a prismatic structure with electrodes provided at both ends will be explained as an example.

This thermistor includes a thermistor element 1 whose electrical resistance decreases as the temperature rises within the operating temperature range, and two electrodes provided on the surface of the thermistor element 1. Here, the feature of this embodiment is that the surface of the thermistor body 1 is covered with a glass layer 4 except for the parts where the two electrodes are in electrical contact with each other, and the two electrodes are plated with the baked electrode 2. Layer 3 is included.

As a specific example, two types of elements were manufactured in which the thermistor body 1 was made of different materials. These embodiments will be described below.

(Example 1) FIG. 3 shows a method of manufacturing the example device.

First, using commercially available manganese oxide and nickel oxide as starting materials, the molar ratio of MnO□:N10 was set to 8:2,
Based on the weight of this raw material, polyvinyl butyral is 6% by weight, ethanol is 30% by weight, and butanol is 30% by weight.
In addition, a mixed slurry was prepared. Using this slurry, a sheet with a thickness of 0.39 mm was prepared by a doctor blade method, and this sheet was punched out to a size of 70 mm x 70 mm. Next, this sheet was fired at 1200° C. for 4 hours. The dimensions of the sheet 31 obtained by firing are as follows:
Length a x width x thickness C is 50mm x 5Qmm x Q
, 55 mm (Fig. 3(a)).

5102, B2O3 and B on both sides of this sheet 310.
Print glass paste containing aO as the main component and heat at 850°C.
(Fig. 3 et al.) to form a glass layer 32 having a thickness d=20±10 μm). Next, make this width e-1, 20m
Figure 3 (C)) is cut out into a rectangular shape of m, and the cut surface is coated with a thickness f = 20 ± 10μ by the above-mentioned glass layer forming method.
A glass layer 33 of s was formed (FIG. 3(d)).

Furthermore, it was cut into chips with a length g = 1.90 mm in a direction perpendicular to the cut surface obtained by the above-mentioned cutting (FIG. 3(e)).

Silver paste was applied as an electrode 34 to this cut surface and the surrounding glass layers 32, 33, and baked at 800° C. (FIG. 3(f)). The dimensions of the element at this stage are:
Length β - about 2.0 mm, width W - about 1.25 mm, thickness h
= approximately 0.75 mm.

Next, this element is plated using an electrolytic plating method, and a nickel layer with a thickness of 2 to 3 μm and a tin layer with a thickness of 4 to 5 μm are laminated on the surface of the electrode 34, and a double-structured electrode surface layer is formed. was formed.

FIG. 4 shows a cross-sectional micrograph of the electrode portion of the thermistor obtained through the above steps.

For comparison, a similar plating process was performed on an element whose thermistor body was not coated with a glass layer. Cross-sectional micrographs of the electrode portion at this time are shown in FIGS. 5 and 6.

In the example shown in FIG. 5, the thermistor element body has been eroded by the plating process. Furthermore, in the example shown in FIG. 6, plating is also attached to the surface of the element other than the portion where the electrode is intended to be formed.

On the other hand, in the thermistor shown in FIG. 4, the element body is not eroded. The electrode is also attached to the surface of the glass layer, but this part is insulated from the element body.
It does not affect the resistance value of the element. In other words, it is equivalent to providing electrodes only on the end faces of the element body.

Next, regarding the thermistor obtained by the above manufacturing method,
Tests were conducted for solder adhesion and solder heat resistance. As a comparative example, a thermistor body with a silver-palladium electrode baked at 850°C was also tested. Regarding solder adhesion, the sample was immersed in a solder bath at 230° C. for 4 seconds, and the solder adhesion area was observed. The results are shown in Table 1. Regarding solder heat resistance, 350
The electrode was immersed in a solder bath for 30 seconds, and the state of disappearance of the electrode was observed.

The results are shown in Table 2.

(Hereinafter, the margin of this page) Table 1 Results of solder adhesion test Table 2 Results of solder heat resistance test As shown in 2, the thermistor obtained in Example 1 has very excellent solder adhesion and solder heat resistance. was.

Further, 300 and 40 thermistors of 10 t were manufactured using the same manufacturing method, and the average resistance value and variation of each loft at 25° C. were measured. Forty pieces of Comparative Example were also manufactured, and the average resistance value and variation were similarly measured. The results are shown in Table 3. Loft numbers 1 to 4 are examples, and loft numbers 5 to 8 are comparative examples.

Table 3: Average resistance value and variation of manufacturing lofts As can be seen, the variation in resistance value within a lot was small, and the variation in average value between lofts was also small.

(Example 2) The starting materials in Example 1 were replaced with manganese oxide, nickel oxide and cobalt oxide, and MnO2:N10:CO
A thermistor was produced in the same manner as in Example 1 except that the molar ratio of O was 3:1:2. Solder this thermistor to the printed circuit board, 85℃, 85RH%, 1000℃.
The time-dependent change in resistance value at 25° C. was measured while the sample was left in the mixed state for some time. Furthermore, as comparative examples, the same measurements were performed on a thermistor body with a silver-palladium electrode baked on it and a thermistor body with a plating treatment applied thereto. The results are shown in Table 4. In this table, loft number 1.2 indicates the sample obtained in the example, lot number 3.4 indicates a comparative example without plating, and loft number 5.6 indicates a comparative example with plating. The number of elements in each loft is 300.

Table 4 Resistance change over time (%) after 1000 hours of moisture resistance As is clear from the table, the comparative example manufactured using the conventional technology and subjected to plating showed a large change over time, and the plating treatment caused a decrease in moisture resistance. Indicates that On the other hand, even if the element of the example is subjected to plating treatment, the change over time is as small as, or even greater than, an element manufactured by the prior art that is not subjected to plating treatment, and the element is excellent in reliability.

(Example 3) In order to measure the bending strength of the thermistor based on the thermal expansion coefficient of the glass layer, Mn, Co, N1,
Cu, AI! Using various linear thermal expansion coefficients made by mixing and firing oxides such as, etc., and using various materials as the glass layer, the length before forming the electrodes! = about 2. Qm
A thermistor with a width W of approximately 1.251T1m was formed. We also created a thermistor without a glass layer. Both ends of these thermistors in the length direction were placed on two tables placed at a distance of 1.2 mm, and a downward force was applied at a downward speed of 20 mm/rnin to a position midway between the two tables. The weight was measured.

The results of the second test are shown in Table 5 and Figure 7. The values shown are 20% for the same combination of linear thermal expansion coefficients.
The bending strength f when there is no glass layer is measured.
:I-, li:L and bending strength f when there is a glass layer
The ratio with fj9a:, -h is expressed as a percentage.

Table 5 (fkau; t+-, / f+-IL) X10
Value of 0 In FIG. 7, the horizontal axis shows the ratio of linear thermal expansion coefficients, and the vertical axis shows the ratio of bending strength. When a material having a linear thermal expansion coefficient α of 40 to 100% of that of the thermistor body was used, the refraction strength increased compared to the case without a glass layer. especially,
In the case of 50-90%, the strength increased by 20-70%. On the other hand, the coefficient of linear thermal expansion is α.

When it was out of the above-mentioned range, the bending strength was lower than when there was no glass layer.

〔Effect of the invention〕

As explained above, the thermistor of the present invention has, firstly,
Since plated electrodes are used, both solder adhesion and solder heat resistance can be improved.

Second, since the area of the electrode in contact with the thermistor body that determines the resistance value is preset, the reproducibility of the target resistance value is good and there is little variation. That is, there is an effect that the yield of products can be improved.

Third, it is possible to use silver, copper, or the like, which is cheaper than silver-palladium, as the underlying material for the plating electrode, which has the effect of making it possible to manufacture a low-cost thermistor.

Fourthly, by appropriately selecting the coefficient of thermal expansion of the glass layer, the flexural strength of the thermistor can be increased.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a thermistor according to an embodiment of the present invention. Figure 2 is a sectional view. FIG. 3 is a diagram showing the manufacturing method. FIG. 4 is a micrograph showing the cross-sectional crystal structure of the electrode portion of the example. FIG. 5 is a micrograph showing the cross-sectional crystal structure of the electrode portion after plating without a glass layer. FIG. 6 is a micrograph showing the cross-sectional crystal structure of the electrode portion after plating without a glass layer. FIG. 7 is a diagram showing changes in the transverse strength ratio with respect to the ratio of linear thermal expansion coefficients. 1... Thermistor element body, 2... Baked electrode, 3...
Plating layer, 4.32.33...Glass layer, 31...
- Sheet, 34...electrode. Tsunen Kununen (Q) (b) (C) Part A ((1) (e) 4 (f) 6 α thermistor o = 75 xlO-7/”C Δ=76 0:83 ■=85 ・=95

Claims (2)

[Claims] 1. In a thermistor comprising a thermistor element whose electrical resistance decreases as the temperature rises within the operating temperature range, and two electrodes provided on the surface of this thermistor element, the two electrodes are in electrical contact with each other. A thermistor characterized in that the surface of the thermistor body except for the parts covered with a glass layer is coated with a glass layer, and the two electrodes include a plating layer. 2. The glass layer has a softening point of 400°C or more and 1000°C or less, and a linear thermal expansion coefficient of 40% the linear thermal expansion coefficient of the thermistor element.
The thermistor according to claim 1, wherein the thermistor is made of a glass material having a value of % or more and 100% or less.