JPH03250604A - Thermistor - Google Patents

Abstract

PURPOSE:To obtain a thermistor element excellent in solderability and solder heat resistance and little in the variation of a resistance value by coating the surface of a thermistor element assembly except the area which comes into contact with electrodes, with the inorganic composite material of glass and inorganic crystal, and by forming the electrodes through plating. CONSTITUTION:A thermistor is equipped with a thermistor element assembly 1, which lowers its electric resistance as the temperature rises in the operating temperature range, and with two electrodes provided on the surface of this thermistor element assembly 1. In this case, the surface of the thermistor element assembly 1 is covered with an inorganic composite material layer 4 including a glass and inorganic crystal excepting the part of two electrodes, and the two electrodes respectively consist of a baked electrodes 2 and plated layer 3. Therefore, it is possible to plate only the electrode surface and to improve both solderability and solder heat resistance. Also, the inorganic composite material becomes a mask at the time of forming the electrode so that the area of electrode adhesion becomes substantially constant in the case of elements of the same manufacture and the variation of a resistance value is reduced after manufacture.

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 forming this by plating, it is possible to provide 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, the oscillation frequency is temperature-compensated.5 Conventional thermistors that are surface-mounted on printed circuit boards or alumina substrates have silver-palladium as a main component on both ends of the thermistor body. It has a structure in which electrodes are formed. 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 recommended 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 caused 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.

FIGS. 4 and 5 show microscopic photographs of cross sections of conventional thermistors subjected to plating treatment. In the example shown in FIG. 4, the thermistor element body has been eroded. Furthermore, in the example shown in FIG. 5, plating is attached to the surface of the thermistor body.

Further, even if the erosion is not as pronounced as shown in FIG. 4, the interface between the electrode and the thermistor element or a part of the element surface may be 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 and between manufacturing lofts.

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 except for the electrode portion is coated with an inorganic composite material containing glass and an inorganic crystal, and the electrode includes a plating layer.

Examples of thermistors in which the surface of the thermistor body is coated with glass or an inorganic substance are disclosed in Japanese Utility Model Application Publication No. 1983-177201 and Japanese Patent Application Publication No. 63-177402, respectively. These known techniques use a glass layer or an inorganic material in order to prevent characteristic deterioration during use due to the exposed thermistor body, and do 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 an inorganic composite material to solve the problems associated with this. It is.

Further, although alumina is exemplified as an inorganic material in the above-mentioned Japanese Patent Application Laid-Open No. 63-177402, it is not known to use an inorganic composite material containing glass and an inorganic crystalline material.

To manufacture the thermistor of the present invention, a coating of an inorganic composite material is formed on the surface of the thermistor body except for the portion where electrodes are to be formed. After this, an electrode containing silver as a generating component is formed on the part not covered with the inorganic composite material, and metal plating such as nickel tin is applied thereon.

The volume ratio of glass and inorganic crystalline material contained in the inorganic composite material is preferably in the range of 50 to 98% for glass and 2 to 50% for inorganic crystalline material. When the volume ratio of glass is less than 50%, it becomes difficult to make the coating dense, and problems such as a change in the resistance value of the thermistor element arise when plating is performed. If the volume ratio of glass exceeds 98%, problems such as adhesion of elements to each other or sticking to a firing jig will occur, and the effect of using the inorganic composite material will be lost.

The glass included in the inorganic composite material has a softening point of 40
Materials with a temperature range of 0 to 1000°C are desirable. Furthermore, it is desirable to use more materials with a lower softening point and less materials with a higher softening point. The range of the softening point is actually determined in consideration of the electrode firing temperature. The firing temperature when using baked silver for electrodes is 600 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 damage to the elements or to the firing process. The yield may decrease due to sticking to the ingredients.

Furthermore, if the softening point is higher than 1000°C, the temperature during coating formation will be substantially around 1000°C, causing problems such as deterioration of the thermistor element.

Specific glass materials include SiO□, B2O3, Ba
O-based ones are preferable, but in addition to these, Na5
The material may contain ions such as Mg, Sr, Zn5CdSPb, AR, etc., and the present invention is not limited to these materials.

The inorganic crystalline substance contained in the inorganic composite material may be any insulating material, but in particular, alumina, zirconia, zircon, titania, mullite, magnesia, forsterite, cordierite, silica, and barium titanate. , zinc oxide and other inexpensive materials can be used.

The linear thermal expansion coefficient of the inorganic composite material is preferably 40% or more and 100% or less of the linear thermal expansion coefficient of the thermistor element. In this range, the flexural strength of the thermistor increases compared to the case without the inorganic composite material coating. Especially 5
When it is 0 to 90%, the bending strength increases by 20 to 70%. On the other hand, outside the range of 40 to 100%, the flexural strength will be lower than when no coating is provided.

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 thought to be that compressive stress remains in the inorganic composite material coating on the surface of the element.

That is, when the thermistor element and the coating, which were thermally expanded during manufacture, cool down, the thermistor element with a larger coefficient of thermal expansion will shrink more, and the coating 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 coating of the thermistor body 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 coating, the bending strength increases compared to the case where there is no coating.

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.

As the material for the base of the plating layer, a baked electrode made of 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 or nickel can be used, and the base layer can also be formed by thermal spraying.

[For production]

By covering the parts other than the electrodes with an inorganic composite material,
It is possible to prevent erosion of the thermistor element during plating, adhesion of plating to the element, and erosion of a different color between the electrode and the element, and it is possible to plate only the electrode surface. Therefore, both solder adhesion and solder heat resistance can be improved.

In addition, since the inorganic composite material serves as a mask during electrode formation,
If the elements have the same structure, the electrode adhesion area force 1 will be substantially constant, and the variation in resistance value after manufacturing will be reduced.

Even when a film made of glass alone is used, the above effects can be obtained. However, since the glass is melted during film formation or electrode formation, problems such as adhesion of elements to each other and sticking to a firing jig may occur, resulting in a decrease in yield.

Further, in the case of a coating made of an inorganic crystalline substance alone, the temperature required for coating formation exceeds 1000° C. due to the properties of its constituents. For this reason, problems such as excessive reaction with the thermistor element occur, and problems arise in which a dense coating cannot be obtained and the plating solution penetrates.

On the other hand, in the case of a composite film, it can be manufactured at a high yield by preventing elements from adhering to each other or sticking to the firing jig, and the film can be formed at a relatively low temperature, allowing the film and thermistor element to A dense coating can be obtained without excessive reaction with

Furthermore, the thermal expansion coefficient can be freely selected by combining glass and inorganic crystalline material, and a thermistor with high bending strength can be obtained.

〔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 element l is coated with an inorganic composite material layer 4 containing glass and an inorganic crystal material except for the two electrodes, and the two electrodes are It includes a baked electrode 2 and a plating layer 3.

Specific examples will be described below.

(Example 1) First, commercially available manganese carbonate, nickel carbonate, and cobalt carbonate were used as starting materials, weighed at a metal atomic ratio of 3:1:2 in terms of MnQ2:NiO:CoO, and mixed in a ball mill for 16 hours. After that, it was dehydrated and dried. Next, this mixture was calcined at 900° C. for 2 hours, mixed again in a ball mill, and dehydrated and dried. To the calcined raw material, 6% by weight of polyvinyl butyral, 30% by weight of ethanol, and 30% by weight of butanol were added to prepare a mixed slurry. Using this slurry, a thickness of 0 was obtained using the doctor blade method.
, an 80 mm sheet was produced. 2゜3 from this sheet
A chip with a size of 4 mm x 1.48 mm was punched out and baked at 1200°C for 4 hours to form a chip with a length of 1.9 mm and a width of 1.
A fired body having a thickness of 2 mm and a thickness of 0.65 tnm was obtained.

This fired body was used as a thermistor body 1, and an inorganic composite material H4 was formed on the surface thereof. As a raw material for the inorganic composite material layer 4, a mixed powder of 70% glass powder with a softening point of 720° C. whose main components are Sl 02 , B2O3 and Ba0 and 30% alumina powder was used. An organic binder and a solvent were added to this mixed powder to form a paste, and this paste was printed on the surface of the thermistor body 1 except for the end face (1.2 mm Xo, 65 mm face), and fired at 850°C. As a result, an inorganic composite material layer 4 having a thickness of 15±5 μm was obtained.

Next, silver paste was applied to the exposed surface of the thermistor body 1 and a part of the surface of the inorganic composite material layer 4, and baked at 800° C. to form a baked electrode 2. The dimensions of the element at this stage are approximately 2.5 mm in length. Omm, width approx. 1.25mm, thickness approx. 0.7
It was mm.

Following this, a plating layer 3 was formed on the surface of the baked electrode 2. As the plating method, an electrolytic plating method is used to form a nickel layer with a thickness of 2 to 3 μm on the surface of the baked electrode 2,
Subsequently, a tin layer having a thickness of 4 to 5 mm was similarly formed thereon.

The thermistor thus obtained was tested 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, 4
It was immersed for a second and the solder adhesion area was observed. The results are shown in Table 1. Regarding 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.

Table 1 Results of solder adhesion test Table 2 Results of solder heat resistance test As described above, the thermistor obtained in Example 1 had excellent solder adhesion and solder heat resistance.

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 lot numbers 5 to 8 are comparative examples.

Table 3: Average resistance value and variation among manufacturing lots As described above, the elements of the example had small variation in resistance value within a loft, and variation in average value between lots was also small.

(Example 2) A thermistor was produced in the same manner as in Example 1, except that the starting materials in Example 1 were replaced with manganese carbonate and nickel carbonate, and the metal atomic ratio was 80:20 in terms of MnO2:Ni○. This thermistor was soldered to a printed circuit board, and the change in resistance value at 25° C. over time was measured when the thermistor was left in a mixed state for 1000 hours at 85° C. and 85 RH%. 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, loft number 3 indicates a comparative example without plating, and loft number 4 indicates a comparative example with plating. The number of elements in each loft is 300.

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

(Example 3) As raw materials for the inorganic composite material layer 4, the glass powder with a softening point of 720°C used in Example 1 and various inorganic crystal powders were used, and the volume ratio was changed to produce the same material as in Example 1. I made a thermistor. Tables 5 and 6 show the material of the inorganic crystal powder, the volume ratio to glass, the yield, and the rate of change in the loft average value of resistance before and after the plating process. The yield is expressed as the percentage of non-defective products excluding defective products caused by adhesion of elements to each other or sticking to the firing jig during the formation of the inorganic composite material layer 4 and the electrode formation. The number of samples in each loft is 2000. In addition, lot number 1.9.10.
In sample No. 11.12, the volume ratio of glass to inorganic crystalline material is outside the range of 50 to 98% glass and 2 to 50% inorganic crystalline material.

(Hereinafter, this page margin) As shown in Tables 5 and 6, the volume ratio of glass to inorganic crystals is 50 to 98% glass and 2 to 5% inorganic crystals.
In the range of 0%, there are few problems such as adhesion of elements to each other or sticking to a baking jig, and virtually no change in resistance value occurs during plating. 55-80% glass, 2 inorganic crystals
It is more desirable if it is 0 to 45%.

(Example 4) As raw materials for the inorganic composite material layer 4, 70% by volume of various glass powders with different softening points and 30% by volume of alumina powder were used.
A thermistor was produced in the same manner as in Example 1 using a mixed powder of Table 7 shows the softening point of the glass used, the yield, the average resistance value within the lot, and the rate of change in the average value before and after the plating process for the obtained thermistors. The yield is expressed as the percentage of non-defective products excluding defective products caused by adhesion of elements to each other or sticking to the firing jig during the formation of the inorganic composite material layer 4 and the electrode formation. The number of samples in each loft is 2000. In addition, loft number 1
．． 2. Samples 14 and 15 have glass softening points of 4
It is outside the range of 00°C to 1000°C.

(Hereinafter, this page margin) As shown in Table 7, if the softening point of the glass constituting the inorganic composite material layer 4 is in the range of 400 to 1000°C, the elements will not stick to each other or to the firing jig. There are few problems such as these, and there is virtually no change in resistance value due to deterioration of the thermistor element during film formation.

The range of the softening point of the glass is more preferably 540 to 860°C.

(Example 5) In order to measure the bending strength of the thermistor based on the thermal expansion coefficient of the inorganic composite material layer, Mn5CoS was used as the thermistor element.
Using materials with various linear thermal expansion coefficients obtained by mixing and firing oxides such as Ni and Cu5Af, as an inorganic composite material layer, ■ Glass only, linear thermal expansion coefficient α nee h = 65 Xl0-7/t ■
■Glass with zirconia (linear thermal expansion coefficient 105Xl)
0-'/l) added to 20% by volume, linear thermal expansion coefficient α3-t = 73X10-'/l ■■ Glass with 30% zirconia added, linear thermal expansion coefficient α: +-h =77 Xl0-7/t■
■ Glass with alumina (linear thermal expansion coefficient 83XIO-
7/l) added at 20% by volume, linear thermal expansion coefficient α:
106 A thermistor having a length β of about 20 mm and a width W of about 1.25 mm was formed before electrodes were formed by using a temperature of 1.25 mm. We also created a thermistor without an inorganic composite material layer. Both ends of these thermistors in the length direction were placed on two tables arranged at an interval of 1.2++ on, and a downward force was applied at a downward speed of 20 mm/m1n to the middle position between the two tables. The weight was measured.

The results are shown in Table 8 and FIG. The values shown above are obtained by measuring the average value of 20 pieces of each combination of the same thermistor body material and inorganic composite material, and the bending strength f when there is no inorganic composite material layer is:, -□2 ．． The ratio of the bending strength f=+-t when there is an inorganic composite material layer is expressed as a percentage.

(Hereinafter, this page margin) In FIG. 3, the horizontal axis shows the ratio of linear thermal expansion coefficients, and the vertical axis shows the ratio of bending strength. The linear thermal expansion coefficient α iodine in the inorganic composite material layer is 40, which is the linear thermal expansion coefficient α −58 of the thermistor element.
When ~100% of the material was used, the refraction strength increased compared to the absence of that layer.

In particular, in the case of 50-90%, the strength increased by 20-70%. On the other hand, when the coefficient of linear thermal expansion α was outside the above-mentioned range, the bending strength decreased compared to the case where there was no inorganic composite material 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 element that determines the resistance value is set in advance, 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.

Thirdly, there are fewer defective products due to elements adhering to each other or sticking to firing jigs during the manufacturing process, there is less change in thermistor resistance value during coating formation, and the average resistance within a lot is lower before and after the plating process. There is also little change in value. Therefore, there is an effect that a high quality and highly reliable thermistor can be manufactured.

Fourthly, it is possible to use silver, copper, or the like, which is cheaper than silver-palladium, as an electrode material, and there is an effect that a thermistor can be manufactured at a low cost.

Fifth, by selecting the coefficient of thermal expansion of the coating for the thermistor body, there is an effect that the bending 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 changes in the transverse strength ratio with respect to the ratio of linear thermal expansion coefficients. FIG. 4 is a micrograph showing the cross-sectional crystal structure of the electrode portion of a conventional thermistor after plating. FIG. 5 is a micrograph showing the cross-sectional crystal structure of the bone of the electrode part of the conventional thermistor after plating. 1... Thermistor element body, 2... Baked electrode, 3...
Plating layer, 4... inorganic composite material layer.

Claims (4)

[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. 2. A thermistor, wherein the surface of the thermistor body except for a portion where the thermistor body is covered with an inorganic composite material containing glass and an inorganic crystal, and wherein the two electrodes include a plating layer. 2. The volume ratio of glass and inorganic crystalline material contained in the inorganic composite material is 50 to 98% for glass and 2 to 50% for inorganic crystalline material.
%. 3. The glass included in the inorganic composite material has a softening point of 40
The thermistor according to claim 1, wherein the temperature is in the range of 0 to 1000C. 4. 2. The thermistor according to claim 1, wherein the inorganic composite material has a linear thermal expansion coefficient of 40% or more and 100% or less of the linear thermal expansion coefficient of the thermistor element.