Thermistor Chips and Methods of Making Same

This invention relates to thermistor chips with reduced fluctuations in the normal-temperature resistance values and also to methods of making such thermistor chips.

As shown in FIGS. 11 and 12, a conventional thermistor chip 1 of this kind usually has terminal electrodes 3 provided at both end parts of a thermistor block 2 having an oxide of a transition metal such as Mn, Co and Ni as its principal component (herein referred to as "Thermistor element"). The terminal electrodes 3 each comprise an end electrode 3a formed by applying Ag/Pd or the like in a paste form and then firing and a plating layer 3b formed on its surface by using Ni or Sn. The normal-temperature resistance value (hereinafter simply referred to as "the resistance value") of such a thermistor chip is generally determined by the resistor value of the thermistor element 2 and the position of the terminal electrodes 3.

Fluctuations in the position of the end electrodes 3a, or more particularly in their width d and their separations a, are generally large because they are produced by applying a paste and firing. The so-called "3cv" value (an index of fluctuations defined as 100.times.3.sigma./(average value) where .sigma. indicates the standard deviation of fluctuations in a lot) for the resistance values is conventionally as large as 5-20%. In order to reduce it to less than 1%, as currently becoming required of the resistant values, a sorting process is necessary, and this not only affects the production cost adversely but also makes it difficult to supply a large quantity of products.

Although it has been known to make use of a laser to remove a portion of the terminal electrode 3 so as to adjust the resistance value of a thermistor chip, the use of a laser involves several problems of its own such as damage to the thermistor element due to the laser heat. In the case of a thermistor of which the resistance changes nonlinearly with respect to temperature, the adjustment of the temperature value is difficult as the temperature of the thermistor element rises due to the laser heat.

It is therefore a general object of this invention to eliminate the problems of prior art as described above.

It is a specific object of this invention to provide chip type thermistors of which the standard deviation in the fluctuations in the resistance value is reduced.

It is another object of this invention to provide a method of producing such chip type thermistors without using a laser.

A thermistor chip embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as comprising terminal electrodes which are formed on both end parts of a thermistor element but also wherein each of these terminal electrodes comprises a first metal layer having a three-layer structure and a second metal layer which is also of a three-layer structure and is formed on the surface of the first metal layer, having its edge part formed in contact with a surface area of the thermistor element. The lower layer of the three-layer structures of the first and second metal layers comprises a metal with resistance against soldering heat, the middle layer comprises a metal having both wettability to solder and resistance against soldering heat and the upper layer comprises a metal having wettability to solder. It is preferable that the lower layers comprise a material selected from Cr, Ni, Al, W and their alloys, that the middle layers comprise Ni or a Ni alloy and that the upper layers comprise Sn, a Sn--Pb alloy or Ag. It is further preferred that the first and second metal layers be formed by a dry soldering method.

A method of production embodying this invention may be characterized as comprising the steps of forming first metal layers each having a three-layer structure on both end parts of a thermistor element, forming second metal layers each having a three-layer structure above the first metal layers such that their edge parts are in contact with surface areas of the thermistor element, and adjusting the resistance value of the thermistor element. According to an alternative method, first metal layers are formed as described above, the resistance value of the thermistor element is measured, second metal layers are formed as described above on the basis of the measured resistance value and the resistance value is adjusted to a specified resistance value. The lower layer of the three-layer structures of the first and second metal layers comprises a metal with resistance against soldering heat, the middle layer comprises a metal having both wettability to solder and resistance against soldering heat and the upper layer comprises a metal having wettability to solder. It is preferable that the lower layers comprise a material selected from Cr, Ni, Al, W and their alloys, that the middle layers comprise Ni or a Ni alloy and that the upper layers comprise Sn, a Sn--Pb alloy or Ag. It is further preferred that the first and second metal layers be formed by a dry soldering method. By a method as described above, it is possible to make thermistor chips having only small fluctuations in the resistance values.

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1 is a partially cut diagonal view of an intermediate product obtained by forming first metal layers on a thermistor element for the production of a thermistor chip according to a first embodiment of this invention;

FIG. 2 is a partially sectional view of a thermistor chip according to the first embodiment of this invention;

FIG. 3 is a partially sectional view of a thermistor chip according to a second embodiment of the invention;

FIG. 4 is a sectional view of a thermistor chip according to a third embodiment of the invention;

FIG. 5 is a sectional view of a thermistor chip according to a fourth embodiment of the invention;

FIG. 6 is a diagonal view of a thermistor chip according to a fifth embodiment of the invention;

FIG. 7 is a diagonal view of an intermediate product obtained by forming first metal layers on a thermistor element according to a sixth embodiment of the invention;

FIG. 8 is a sectional view of a thermistor element according to the seventh embodiment of the invention;

FIG. 9 is a sectional view of a thermistor element according to the eighth embodiment of the invention;

FIG. 10 is a sectional view of a thermistor element according to the ninth embodiment of the invention;

FIG. 11 is diagonal view of a prior art thermistor chip; and

FIG. 12 is a sectional view of the prior art thermistor chip of FIG. 11 taken along line 12--12.

Throughout herein, like components may be indicated by the same numeral even if belonging to different thermistor chips and repetitive explanations may be omitted for simplifying the disclosure. It is also to be reminded that these figures are intended to be schematic and not true to scale. The metal layers, in particular, are generally much thinner than the thickness of the thermistor element, and hence the indications of distances in the figures are provided by ignoring the thickness of the metal layers.

The invention will be described next with reference to a thermistor chip actually produced as a first embodiment of this invention. Thermistor elements 2 with length 2.0 mm, width 1.2 mm and height 0.8 mm were prepared and first metal layers 6 of a three-layer structure were formed on both end parts as shown in FIGS. 1 and 2 by a dry soldering method such as by sputtering such that the separation A between their mutually opposite edge parts was 1.3 mm. The three-layer structure was formed by using Ni--Cr with resistance against soldering heat as lower thin-film layer 6a with thickness 0.4 .mu.m, Ni--Cu having both wettability to solder and resistance against soldering heat as middle thin-film layer 6b with thickness 0.8 .mu.m and Ag having wettability to solder as upper thin-film layer 6c with thickness 0.8 .mu.m.

Although Ni--Cu was used in the particular example, described above for the lower layers 6a, Cr, Ni, Al, W and their alloys may be used alternatively. Similarly, Ni and Ni alloys may be used for the middle layers 6b, and Sn and Sn alloys may be used for the upper layers 6c. The thickness of each layer may be appropriately varied.

The resistance value of the thermistor element 2 shown in FIG. 1 was measured by using the first metal layers 6 as electrodes. The average value of twenty samples was 10K.OMEGA. and the "3cv" of the resistance values was 15%. The lot of these samples was then divided into eleven ranks, as shown in Table 1, each corresponding to a range of 0.3K.OMEGA. in resistance. The average resistance values each corresponding to associated one of the ranks are also shown in Table 1.

Next, second metal layers 7 were formed by a dry soldering method such as sputtering on each of the ranked thermistor elements 2 such that their resistance values will fall within a specified range R=8.+-.0.2K.OMEGA.. As shown in FIG. 2, the second metal layers 7 are also of a three-layer structure (with a lower layer 7a of Ni--Cr with thickness 0.4 .mu.m, a middle layer 7b of Ni--Cu with thickness 0.8 .mu.m and an upper layer 7c of Ag with thickness 0.8 .mu.m) both on the surface of the first metal layers 6 and, extending therefrom, on a surface area of the chip type thermistor element 2. The distance B (such that B<A) between the mutually opposite edge parts of the second metal layers 7 was selected, depending on the resistance value of each rank, as shown in Table 1. The resistance values of such adjusted thermistor chips were measured and are also listed in Table 1.

As can be understood from Table 1, the difference between the maximum and minimum resistance values of the thermistor chips in this lot right after the first metal layers were formed was about 3K.OMEGA., but this was reduced to about 0.38K.OMEGA. after the second metal layers were formed to reduce the separation distance between the edges of the electrodes from A to B for each rank. Thus, the present invention makes it possible to provide thermistor chips having a desired resistance value with reduced fluctuations.

As explained above with reference to the first metal layers, the lower layer 7a of the second metal layers 7 may alternatively comprise Cr, Ni, Al, W or their alloy, the middle layer 7b may comprise Ni or a Ni alloy and the upper layer 7c may comprise Sn or a Sn--Pb alloy.

A second embodiment of this invention is explained next with reference to FIG. 3. As can be understood by comparing it with FIG. 2, this embodiment is characterized wherein the middle and upper layers 26b, 26c, 27b and 27c of the first and second metal layers 26 and 27 have smaller areas than the respective lower layers 6a and 7a such that their mutually opposite edge parts are not covered by the overlapping layers. Such a thermistor chip is produced, after the lower layers 6a are formed at both end parts of the thermistor element 2, by forming the middle and upper layers 26b and 26c with a smaller area than the lower layers 6a such that the mutually opposite edge parts of the lower layers 6a will be exposed.

As in the first embodiment of the invention, the resistance values of thermistor elements 2 thus having first metal layers 26 formed thereon are measured, and second metal layers 27 are formed on ranked thermistor elements 2 according individually to the measured resistance value such that a specified resistance value will result. The second metal layers 27 are formed such that their mutually opposite edge parts are separated by a distance B, smaller than A, determined for each rank. Middle and upper layers 27b and 27c are formed so as to have a smaller area than the lower layer 7a. This embodiment is advantageous in that the area of the middle and upper layers 26b, 26c, 27b and 27c can be independent of the areas of the lower layers 6a and 7a dictated by the desired separation distance B such that soldering onto a circuit board or the like can be carried out uniformly. The middle and upper layers 26b, 26c, 27b and 27c of the first and second metal layers 6 and 7 according to this embodiment may made of the same materials respectively as the middle and upper layers 6b, 7b, 6c and 7c of the first embodiment.

A third embodiment of this invention is explained next with reference to FIG. 4, which is similar to FIG. 2 but the second metal layer 7 is formed only at one of the end parts of its thermistor element 2.

As explained above, the thermistor elements 2 are ranked according to the measured resistance values, and the second metal layer 7 is formed on the surface of one of the first metal layers 6 and extending therefrom from its edge part onto a surface area of the thermistor element 2 so as to obtain a desired resistance value. The distance B between the edge part of the second metal layer 7 and the opposite one of the first metal layers 6 is determined for each rank. With the second metal layer 7 thus formed on each thermistor element to adjust the resistance value, thermistor chips with reduced fluctuations in the resistance values can be obtained.

A fourth embodiment of this invention is explained next with reference to FIG. 5, which is similar to FIG. 4 but its second metal layer 10 is formed so as to cover only the edge part of one of the first metal layers 6. In other respects, this embodiment is the same as the third embodiment.

Thus, as explained above, thermistor elements 2 are ranked according to the measured resistance values, and the second metal layer 10 of a three-layer structure as explained above is formed over the edge part of one of the first metal layers 6 and extending from this edge part onto a surface area of the thermistor element 2 so as to obtain a desired resistance value. The distance B between the edge part of the second metal layer 10 and the opposite one of the first metal layers 6 is determined for each rank. With the second metal layer 10 thus formed on each thermistor element to adjust the resistance value such that thermistor chips with reduced fluctuations in the resistance values can be obtained.

A fifth embodiment of the invention is explained next with reference to FIG. 6 which is similar to FIG. 1 except a second metal layer 11 is formed to cover only a portion of the edge parts with a limited length E along the edge of one of the first metal layers 6.

As explained above, thermistor elements 2 are ranked according to the measured resistance values, and the second metal layer 11 of a similar three-layer structure as explained above is formed over the edge part of one of the first metal layers 6, covering the limited distance E along the edge, and extending from this edge part onto a surface area of the thermistor element 2 so as to obtain a desired resistance value. The distance C between the edge part of the second metal layer 11 and the opposite one of the first metal layers 6 is determined for each rank. With the second metal layer 11 thus formed on each thermistor element to adjust the resistance value such that thermistor chips with reduced fluctuations in the resistance values can be obtained.

Although FIG. 6 shows a particular example of the fifth embodiment wherein the second metal layer 11 is formed on only one of the side surfaces of the thermistor element 2, a similar second metal layer may be formed on two or three of the side surfaces of the thermistor element 2 to adjust its resistance value. Moreover, two second metal layers may be formed at two places, each connecting to a different one of the first metal layers 6.

A sixth embodiment of the invention is shown in FIG. 7, which is similar to the first embodiment shown in FIG. 1 but is different wherein its second metal layers 12 are formed by leaving the side surfaces of the thermistor element 2 exposed. As shown in FIG. 7, the second metal layers 12 are formed on both end surfaces of the thermistor element 2 and portions of its upper and lower surfaces adjacent the end surfaces but not on the side surfaces which remain exposed.

As explained above with reference to the fifth embodiments, thermistor elements 2 were provided and first metal layers 12 having a three-layer structure were formed as shown in FIG. 7. After the resistance values of these thermistor elements were measured, second metal layers of various shapes as shown in FIGS. 2-6 were formed on the basis of the measured resistance values. Their resistance values were adjusted, and thermistor chips with small fluctuations could thus be obtained.

The invention has been described above with reference to thermistor elements of the kind not having any internal electrodes. Since this invention is applicable to thermistor elements with internal electrodes, such examples will be described next with reference to FIGS. 8-10.

FIG. 8 shows a seventh embodiment of the invention where use is made of a thermistor element 14 having a pair of internal electrodes 13 disposed mutually separated but on a same plane inside the element 14. FIG. 9 shows an eighth embodiment of the invention where use is made of a thermistor element 17 having internal electrodes 15 and 16 which are not in coplanar relationship but overlap mutually. FIG. 10 shows an ninth embodiment of the invention where use is made of a thermistor element 20 having two pairs of mutually coplanar, mutually separated internal electrodes 18 as well as a separated internal electrode 19 which is not coplanar with any of the other internal electrodes 18 and is not connected. The number of internal electrodes 13, 15, 16, 18 and 19 is not intended to limit the scope of the invention.

Advantages of the present invention include the following:

(1) Since the first metal layers extend farther than the second metal layers towards the center of the thermistor element, the resistance value of the thermistor chip is determined by the first metal layers and hence thermistor chips with smaller resistance values can be obtained;

(2) Since the fourth metal layers are formed over the first metal layers to adjust the resistance values, thermistor chips with smaller standard variations in the fluctuation of their resistance values can be obtained easily;

(3) Since the second and third metal layers for soldering are formed with the same size although the separating distances between the mutually opposite edge parts of the first or fourth metal layers are varied according to a specified resistance value, the areas for applying solder for attaching the thermistor chip to a circuit board can remain the same, occurrence of tombstones and solder bridges between electrodes being thereby prevented;

(4) Since the second metal layers have resistance against soldering heat and are covered by the third metal layers, wettability can be maintained and the thermistor chip can be soldered easily; and

(5) Since the first, second and fourth metal layers can be formed by a dry soldering method, electrical properties and mechanical strength of the thermistor chips are not adversely affected by wet soldering although the ceramic element is exposed unprotected.

Description of the invention is intended to be interpreted broadly. Many of the features of different embodiments of the invention described above may be combined, whenever appropriate. The invention also applies not only to thermistor elements with a negative temperature characteristic but also to thermistor elements with a positive temperature characteristic. Expressions like "resistance against soldering heat" and "wettability" are intended to be interpreted as commonly understood by ordinary persons skilled in the art. For example, if a sample is soaked in eutectic SnPb solder at 240.degree. C. for 5 seconds and over 70% of the film surface remains, it may be defined to have resistance against soldering heat. Similarly, if a sample is soaked in such solder for 2 seconds and over 70% of the film area is covered by the new solder material, it may be defined to have wettability.