JP2591206B2 - Thermistor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a thermistor whose resistance value decreases with an increase in temperature within a use temperature range. In particular, it relates to the element structure of a thermistor. Thermistors are used for temperature compensation of electronic devices, surface temperature measurement sensors, and the like.

〔Overview〕

The present invention relates to a thermistor in which the resistance value decreases as the temperature rises within the operating temperature range, wherein the surface of the thermistor body other than the portion in contact with the electrode is coated with an inorganic composite material of glass and an inorganic crystal, Is formed by plating to provide a thermistor element having excellent solder adhesion and solder heat resistance, and having a small variation in resistance value.

[Conventional technology]

The thermistor has a large negative temperature coefficient of resistance, and the 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.

Conventional thermistors that are surface-mounted on a printed circuit board or alumina substrate, etc., have silver-
It has a structure in which an electrode mainly composed of palladium is formed. The reason why silver-palladium is used for the electrodes is to prevent the electrodes from eluting and disappearing in the solder when soldering the thermistor to the printed circuit board, that is, to obtain solder heat resistance.

In order to form a silver-palladium electrode, a method of immersing a part of the thermistor body in a silver-palladium paste is used.

[Problems to be solved by the invention]

However, conventional thermistors have limitations on soldering conditions, such as insufficient soldering heat resistance and inability to perform soldering at a high temperature for a long time.

In order to improve the solder heat resistance, the amount of palladium may be increased. However, as the amount of palladium increases, the solder adhesion decreases, and the amount of solder attached to the thermistor electrode decreases. For this reason, there have been problems such as a weak fixing force of the thermistor to the printed circuit board and an incomplete electrical connection between the thermistor and the wiring on the circuit.

In order to improve both solder adhesion and solder heat resistance, a method of applying metal plating to the surface of the electrode is considered.
However, during the plating process, the thermistor body erodes or plating adheres to the body surface.

FIGS. 4 and 5 show micrographs of the cross section of the thermistor when a plating treatment is applied to the conventional thermistor. In the example shown in FIG. 4, the thermistor body is eroded. Further, in the example shown in FIG. 5, plating is attached to the thermistor body surface.

In addition, even when the erosion is not so remarkable as shown in FIG. 4,
The interface between the electrode and the thermistor body or a part of the body surface may be eroded, and the reliability such as moisture resistance may be reduced.

Further, in the method of immersing the thermistor body in silver-palladium, the amount of silver-palladium adhered to each element varies, and the distance between the electrodes varies. As a result, there has been a problem that the resistance value distribution of the thermistor is widened, and the variation within a manufacturing lot and between manufacturing lots is increased.

An object of the present invention is to solve the above problems and to provide a thermistor having a structure excellent in solder adhesion and solder heat resistance and capable of reducing variation in resistance value.

[Means for solving the problem]

In the thermistor of the present invention, the surface of the thermistor body is coated with an inorganic composite material containing glass and an inorganic crystal except for the electrode portion, and the electrode is coated with the inorganic composite material on the surface of the thermistor body. A base electrode covering the non-existing part;
And a plating layer provided on the surface of the base electrode, wherein the inorganic composite material has a linear thermal expansion coefficient of 40% to 100% of the linear thermal expansion coefficient of the thermistor body.

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 Laid-Open No. 63-67201 and Japanese Patent Laid-Open No. 63-177402, respectively.
These known techniques use a glass layer or an inorganic substance in order to prevent deterioration of characteristics during use due to the exposed thermistor body, and do not consider plating.

On the other hand, the present invention aims to improve solder adhesion and solder heat resistance by plating an electrode surface, and uses an inorganic composite material to solve the accompanying problems. It is.

In the above-mentioned JP-A-63-177402, alumina is exemplified as an inorganic substance, but it is not known to use an inorganic composite material containing glass and an inorganic crystal.

In order 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 a portion where an electrode is to be formed. Thereafter, an electrode containing silver as a main component is formed in a portion not covered with the inorganic composite material,
Metal plating such as nickel and tin is applied thereon.

The volume ratio of glass to inorganic crystal contained in the inorganic composite material is desirably 50 to 98% for glass and 2 to 50% for inorganic crystal. If the volume ratio of the glass is less than 50%, it becomes difficult to densify the coating, and problems such as a change in the resistance value of the thermistor body during plating treatment occur. When the volume ratio of glass exceeds 98%, there occurs a problem that elements adhere to each other and stick to a firing jig, and the effect of using the inorganic composite material is lost.

The glass contained in the inorganic composite material has a softening point of 40
Materials in the range of 0 to 1000 ° C are desirable. Further, it is desirable to increase the number of materials having a lower softening point and to reduce the number of materials having a higher softening point. The range of the softening point is actually determined in consideration of the electrode firing temperature. When baked silver is used for the electrode, the firing temperature is 600 to 850 ° C. If the softening point of the glass is significantly lower than that temperature, the glass rises to the electrode surface during electrode firing, and the elements or elements and the element are cured. In some cases, sticking to the tool may occur, and the yield may decrease. If the softening point is higher than 1000 ° C., the temperature at the time of coating formation is substantially 1000 ° C.
The temperature is around ℃ and adverse effects such as deterioration of the thermistor body occur.

As a specific glass material, SiO ₂ , B ₂ O ₃ , and BaO-based materials are preferable, but in addition, Li, K, Na, Mg, Sr, Zn,
It may contain ions such as Cd, Pb, and Al, and the present invention is not limited to these materials.

As the inorganic crystal contained in the inorganic composite material, any material may be used as long as it is an insulator.
Zirconia, zircon, titania, mullite, magnesia, forsterite, cordierite, silica, barium titanate, zinc oxide and other inexpensive materials can be used.

It is desirable that the linear thermal expansion coefficient of the inorganic composite material is 40% or more and 100% or less of the linear thermal expansion coefficient of the thermistor body. In this range, the transverse rupture strength of the thermistor increases as compared with the case where the inorganic composite material is not coated. Especially 50
At ~ 90%, the flexural strength increases by 20-70%. On the other hand, outside the range of 40 to 100%, the transverse rupture strength is reduced as compared with the case where no coating is provided.

The transverse rupture strength refers to the breaking strength when both ends of the element are placed on two pedestals arranged at an interval and a weight is applied to the center of the element. This is a measure of how much the element can withstand the stress caused by heat due to solder or the like when the element is mounted on the surface mount substrate or a thermal cycle after the mounting.

It is considered that the transverse rupture strength is increased because compressive stress remains in the inorganic composite material film on the element surface. That is,
When the thermistor element that has been thermally expanded at the time of manufacturing and the coating cools, the thermistor element having a larger thermal expansion coefficient shrinks more, and the coating is in a compressed state. When a bending force is applied to the thermistor in this state, a compressive stress is generated inside the bend and a tensile stress is generated outside the bend. Both the ceramic material and the coating of the thermistor body are strong against compressive stress but weak against tensile stress, and when a bending force of a certain degree or more is applied, cracks occur outside the bending. At this time, since compressive stress is applied to the outer coating from the beginning,
The flexural strength increases as compared with the case without the coating.

As the material of the thermistor body, an oxide sintered body of one or more metals selected from manganese, cobalt, nickel, aluminum, and copper can be used, but the present invention is not limited to these materials.

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

As a material to be a base of the plating layer, a baked silver paste electrode containing silver as a main component, which has better electric conductivity, better heat resistance and lower cost than silver-palladium alloy, can be used. However, the present invention is not limited to this. For example, copper, nickel, or the like can be used, and a base layer can be formed by a thermal spraying method.

[Action]

By coating the parts other than the electrodes with the inorganic composite material, it is possible to prevent erosion of the thermistor element body during plating, adhesion of plating to the element body, and erosion of the interface between the electrode and the element body. Plating can be performed.
Therefore, both solder adhesion and solder heat resistance can be improved.

In addition, since the inorganic composite material serves as a mask when forming the electrodes, if the elements have the same structure, the electrode attachment area becomes substantially constant, and the variation in the resistance value after manufacture is reduced.

The above effects can be obtained even when the coating of glass alone is used. However, since the glass is melted at the time of film formation or electrode formation, problems such as adhesion of elements and sticking to a firing jig occur, and as a result, the yield may be reduced.

Further, in the case of the inorganic crystal single film, the temperature required for forming the film exceeds 1000 ° C. due to the properties of the constituents. For this reason, a problem such as an excessive reaction with the thermistor body occurs, and a problem that a plating solution invades without obtaining a dense film occurs.

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

Furthermore, the coefficient of thermal expansion can be freely selected by a combination of glass and an inorganic crystal, and a thermistor with high bending strength can be obtained.

〔Example〕

FIG. 1 is a perspective view of a thermistor according to an embodiment of the present invention,
FIG. 2 shows a cross-sectional view along the length direction. Here, an element having a prismatic structure in which electrodes are provided at both ends will be described as an example.

This thermistor comprises a thermistor element body 1 whose electric resistance decreases as the temperature rises within the operating temperature range,
And two electrodes provided on the surface of the thermistor body 1. Here, the feature of this embodiment is that the surface of the thermistor body 1 is covered with an inorganic composite material layer 4 containing glass and an inorganic crystal except for two electrodes, and the two electrodes are respectively The printing electrode 2 and the plating layer 3 are included.

 A specific embodiment 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 MnO ₂ : NiO: CoO, and then ball-milled for 16 hours. After mixing, the mixture was dehydrated and dried. Next, this mixture was calcined at 900 ° C. for 2 hours, mixed again with a ball mill, and dehydrated and dried. Based on the calcined raw material, polyvinyl butyral 6% by weight,
Add 30% by weight of ethanol and 30% by weight of butanol,
A mixed slurry was prepared. Using this slurry, a sheet having a thickness of 0.80 mm was produced by a doctor blade method.
A chip having a size of 2.34 mm × 1.48 mm was punched from the sheet and fired at 1200 ° C. for 4 hours to obtain a fired body having a length of 1.9 mm, a width of 1.2 mm and a thickness of 0.65 mm.

This fired body was used as a thermistor body 1, and an inorganic composite material layer 4 was formed on the surface thereof. As a raw material of the inorganic composite material layer 4, a mixed powder of 70% of a glass powder having a softening point of 720 ° C. containing SiO ₂ , B ₂ O ₃ and BaO as main components and 30% of an alumina powder was used. An organic binder and a solvent were added to the mixed powder to form a paste. The paste was printed on the surface of the thermistor body 1 excluding the end face (1.2 mm × 0.65 mm face) and fired at 850 ° C. Thus, an inorganic composite material layer 4 having a thickness of 15 ± 5 μm was obtained.

Next, the exposed surface of the thermistor body 1 and the inorganic composite material layer 4
A silver paste was applied to a part of the surface of the substrate and baked at 800 ° C. to form a baked electrode 2. The dimensions of the element at this stage were about 2.0 mm in length, about 1.25 mm in width, and about 0.7 mm in thickness.

Subsequently, a plating layer 3 was formed on the surface of the baked electrode 2. As a plating method, a nickel layer having a thickness of 2 to 3 μm was formed on the surface of the baked electrode 2 by using an electroplating method, and a tin layer having a thickness of 4 to 5 μm was similarly formed thereon.

The thus obtained thermistor was tested for solder adhesion and solder heat resistance. As a comparative example, a silver-palladium electrode was used for thermistor body at 850.
Tests were also performed on those baked at ° C. The solder adhesion was immersed in a 230 ° C. solder bath for 4 seconds, and the solder adhesion area was observed. Table 1 shows the results.
Regarding the solder heat resistance, the electrode was immersed in a solder bath at 350 ° C. for 30 seconds and the disappearance of the electrode was observed. Table 2 shows the results.

Thus, the thermistor obtained in Example 1 was very excellent in solder adhesion and solder heat resistance.

In addition, the same manufacturing method was used to manufacture 300 thermistors of one lot and four lots of thermistors, and the average resistance value and variation of each lot at 25 ° C. were measured. Four lots of the comparative example were manufactured, and the average resistance value and the variation were measured in the same manner.
Table 3 shows the results. Lot numbers 1 to 4 are examples, and lot numbers 5 to 8 are comparative examples.

As described above, in the device of the example, the variation in the resistance value among the lots was small, and the variation in the average value between the lots was also small.

(Example 2) The starting material in Example 1 was changed to manganese carbonate and nickel carbonate, and the metal atomic ratio was converted to MnO ₂ : NiO to 80: 2.
As 0, a thermistor was produced in the same manner as in Example 1. This thermistor was soldered to a printed circuit board, and the change over time in the resistance value at 25 ° C. when left in a wet state at 85 ° C., 85 RH% for 1000 hours was measured. As a comparative example, a thermistor body baked with a silver-palladium electrode,
The same measurement was carried out for the plating-treated product. Table 4 shows the results. In this table, lot numbers 1 and 2 are obtained by the examples, lot number 3 is a comparative example without plating, and lot number 4 is a comparative example with plating. The number of elements in each lot is 300.

As is clear from the table, in the comparative example manufactured by the conventional technique and subjected to the plating treatment, the change with time is large, and the moisture resistance is reduced by the plating treatment. On the other hand, the element of the example has a small change with time or is superior to the non-plated element manufactured by the conventional technique even after plating, and has excellent reliability.

(Example 3) As a raw material of the inorganic composite material layer 4, the glass powder having a softening point of 720 ° C used in Example 1 and various inorganic crystal powders were used. A thermistor was manufactured. Tables 5 and 6 show the material of the inorganic crystal powder, the volume ratio to glass, the yield, and the rate of change of the average value of the resistance values before and after the plating step in the lot. The yield is indicated by the ratio of non-defective products excluding defective products caused by adhesion of elements generated during the formation of the inorganic composite material layer 4 and electrode formation and sticking to a firing jig. The number of samples in each lot is 2000. Lot numbers 1, 9, 10, 11, 12
In the sample, the volume ratio between glass and inorganic crystal was 50
9898%, inorganic crystal material 25050%.

As shown in Tables 5 and 6, the volume ratio between glass and inorganic crystal was 50 to 98% for glass and 2 to 50% for inorganic crystal.
Within the range, there are few problems such as adhesion between elements and sticking to a firing jig, and there is substantially no change in resistance during plating. More preferably, the glass is 55 to 80% and the inorganic crystal is 20 to 45%.

(Example 4) As a raw material of the inorganic composite material layer 4, a mixed powder of 70% by volume of various glass powders having different softening points and 30% by volume of alumina powder was used. Produced. For the obtained thermistor, Table 7 shows the softening point of the glass used, the yield, the average value of the resistance values in the lot, and the rate of change of the average value before and after the plating step. The yield is indicated by the ratio of non-defective products excluding defective products caused by adhesion of elements generated during the formation of the inorganic composite material layer 4 and electrode formation and sticking to a firing jig. The number of samples in each lot is 2000. Lot numbers 1, 2, 14
And 15 samples have a softening point of 400-1000 ° C.
Is out of the range.

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., there are few problems such as adhesion between elements and sticking to a firing jig,
Substantially no change in resistance value due to deterioration of the thermistor body at the time of film formation. The range of softening point of glass is 54
0-860 ° C is more preferred.

Example 5 In order to measure the bending strength of the thermistor based on the coefficient of thermal expansion of the inorganic composite material layer, Mn, Co, and N were used as the thermistor bodies.
i, Cu, Al, etc. are mixed and baked with various linear thermal expansion coefficients. As the inorganic composite material layer, only glass, linear thermal expansion coefficient α _coat = 65 × 10 ^-7 / ° C Glass with zirconia (linear thermal expansion coefficient 105 × 10
⁻⁷ / ° C.), 20% by volume, linear thermal expansion coefficient α _coat = 73 × 10 ⁻⁷ / ° C., zirconia added at 30% by volume, linear thermal expansion coefficient α _coat = 77 × 10 ^{− 7} / ° C. glass alumina (coefficient of linear thermal expansion 83 × 10 ^-7 /
C) is 20% by volume, linear thermal expansion coefficient α- _coat = 101 × 10 ⁻⁷ / ° C., glass is 40% by volume alumina added, linear thermal expansion coefficient α- _coat = 106 × 10 ⁻⁷ / ° C. Glass only, using linear thermal expansion coefficient α _coat = 110 × 10 ^-7 / ° C, length l = about 2.0 mm before electrode formation,
A thermistor having a width w of about 1.25 mm was formed. A thermistor without an inorganic composite material layer was also made. Both ends of these thermistors in the length direction were placed on two tables arranged at a distance of 1.2 mm, and a force was applied to the middle position between the two tables at a pressing speed of 20 mm / min. Was measured.

The results are shown in Tables 8 and 3. The values shown in these for a combination of the same thermistor element material and an inorganic composite material, respectively to measure the 20 average values, and _no bending strength f _coat in the absence of the inorganic composite material layer, an inorganic The ratio of the transverse rupture strength f with the composite material layer to the presence of the _{coat is} expressed in percentage.

In FIG. 3, the horizontal axis shows the ratio of the linear thermal expansion coefficient, and the vertical axis shows the ratio of the transverse rupture strength. Linear thermal expansion coefficient α for inorganic composite material layer
_Coat of linear thermal expansion coefficient α _thermistor of the thermistor element
When 40 to 100% of the material was used, the flexural strength increased as compared with the case where the layer was not provided. In particular, in the case of 50 to 90%, the strength increased by 20 to 70%. Linear thermal expansion coefficient α _coat contrast when outside the range described above, as compared when no inorganic composite material layer bending strength had decreased.

〔The invention's effect〕

As described above, the thermistor of the present invention has the effect of improving both the solder adhesion and the solder heat resistance, firstly, since the plated electrode is used.

Second, since the area of the electrode in contact with the thermistor element for determining the resistance value is set in advance, the reproducibility of the target resistance value is good, and the variation is small. That is, there is an effect that the yield of products can be improved.

Third, in the manufacturing process, there is little occurrence of defective products due to adhesion of elements or sticking to a firing jig, little change in thermistor resistance during coating formation, and average resistance in the lot before and after the plating process. There is little change in the value. Therefore, there is an effect that a high quality and highly reliable thermistor can be manufactured.

Fourth, silver or copper or the like, which is cheaper than silver-palladium, can be used as an electrode material, and there is an effect that a low-cost thermistor can be manufactured.

Fifth, by selecting the coefficient of thermal expansion of the coating with respect to the thermistor body, there is an effect that the bending strength of the thermistor can be increased.

[Brief description of the drawings]

FIG. 1 is a perspective view of a thermistor according to an embodiment of the present invention. FIG. 2 is a sectional view. FIG. 3 is a diagram showing a change in a transverse rupture strength ratio with respect to a linear thermal expansion coefficient ratio. FIG. 4 is a micrograph showing a cross-sectional crystal structure of an electrode portion after plating of a conventional thermistor. FIG. 5 is a micrograph showing a cross-sectional crystal structure of an electrode portion after plating of a conventional thermistor. 1 ... Thermistor body, 2 ... Bake electrode, 3 ... Plating layer, 4 ... Inorganic composite material layer.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masahiro Hirama 2270 Yokoze, Yokoze-cho, Chichibu-gun, Saitama Mitsubishi Mining Cement Co., Ltd. Mining Cement Co., Ltd., Ceramics Research Laboratory (72) Inventor Moriyasu Asami, 2270 Yokoze, Yokoze-cho, Chichibu-gun, Saitama Mitsubishi Mining Cement Co., Ltd. Mining Cement Co., Ltd. Ceramics Research Laboratory (72) Inventor Masami Koshimura 2270 Yokoze, Yokoze-cho, Chichibu-gun, Saitama Prefecture Mitsubishi Mining Cement Co., Ltd. Ceramics Research Laboratory (56) References Showa 60-701 ( P, A)

Claims (3)

(57) [Claims] 1. A thermistor comprising: a chip-shaped thermistor element whose electric resistance decreases as the temperature rises within a use temperature range; and two electrodes provided on the surface of the thermistor element. The surface of the thermistor body is coated with an inorganic composite material containing glass and an inorganic crystal, except for a portion where the two electrodes are in electrical contact with each other. The two electrodes are each a surface of the thermistor body. A base electrode covering a portion not covered with the inorganic composite material, and a plating layer provided on the surface of the base electrode, wherein the inorganic composite material has a linear thermal expansion coefficient of a linear thermal expansion coefficient of a thermistor body. A thermistor characterized by being 40% or more and 100% or less. 2. The volume ratio of glass to inorganic crystal contained in the inorganic composite material is 50 to 98% for glass and 2 to 2 for inorganic crystal.
2. The thermistor according to claim 1 in the range of 50%. 3. The thermistor according to claim 1, wherein the glass contained in the inorganic composite material has a softening point in the range of 400 to 1000 ° C.