Thermistor with comb-shaped electrodes

Details Thermistor with comb-shaped electrodes Thermistor with comb-shaped electrodes

1998-10-13

2001-01-09

Easthom, Karl D. (Department: 2832)

Electrical resistors

Resistance value responsive to a condition

Current and/or voltage

C338S313000, C338S332000

Reexamination Certificate

active

06172592

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to thermistors inclusive of chip-type thermistors (or “chip thermistors”) of the kind which are commonly used in a temperature-compensating circuit or a temperature detecting element. More particularly, this invention relates to such thermistors having a pair of surface electrodes formed on one surface of a thermistor block so as to face each other.
Chip-type thermistors using semiconductor ceramics having a positive or negative temperature coefficient have been widely in use, and those with many different structures have been considered so as to be easily surface-mountable to a printed circuit board. The structure of some of prior art chip-type thermistors will be described first with reference to FIGS.
16
-
19
.
FIG. 16
shows a prior art chip-type thermistor
71
with a pair of external electrodes
73
a
and
73
b
formed so as to cover both end surfaces
72
a
and
72
b
of a thermistor-forming base body (hereinafter referred to as the “thermistor block”)
72
of a semiconductor ceramic material. The resistance of the thermistor
71
is determined not only by the resistance of the thermistor block
72
but also the contact areas of the external electrodes
73
a
and
73
b
with the thermistor block
72
. The external electrodes
73
a
and
73
b
are usually formed by a dipping method, but this method tends to result in large variations in the lengths of the parts of the electrodes
73
a
and
73
b
covering the upper, lower and side surfaces of the thermistor block
72
. Since there are variations also in the specific resistance of the thermistor blocks, the overall variations in the resistance of thermistors
71
thus produced are large, and it has been difficult to produce thermistors with a desired resistance value.
In view of the above, thermistors as shown in
FIG. 17
at
75
came to be proposed, having glass layers
74
formed over the top, bottom and both side surfaces of a thermistor block
72
. Since the external electrodes
73
a
and
73
b
contact the thermistor block
72
only through the end surfaces
72
a
and
72
b
of the latter, the resistance of the thermistor
75
is determined only by the resistance of the thermistor block
72
itself and the area of its end surfaces
72
a
and
72
b
. Thus, the variations in the resistance values of the thermistors can be reduced.
If the external electrodes
73
a
and
73
b
are formed by coating a conductive paste and subjecting it to a firing process, however, the materials of the glass layers
74
and the external electrodes
73
a
and
73
b
tend to diffuse into each other where they contact each other. If a portion of the glass layer
74
falls off as a result of such diffusion, as shown in the enlarged portion of
FIG. 17
enclosed in a circle A, the outer electrode
73
b
may come to contact the thermistor block
72
directly. Since it is difficult to prevent such a diffusion phenomenon, there still remained the problem of obtaining thermistors with a desired resistance value.
Moreover, the variations in the resistance value of the thermistor blocks
72
have remained large, making it very difficult to obtain thermistors with highly accurate resistance values.
When thermistors
75
with different specified resistance values are to be produced, furthermore, thermistor blocks with different specific resistance values are required. It was thus even more difficult to accurately produce thermistors with different resistance values.
There have also been proposals to produce thermistors as shown at
77
in
FIGS. 18A and 18B
with inner electrodes
76
a
and
76
b
formed inside the thermistor block
72
. In the example shown in
FIGS. 18A and 18B
, the two inner electrodes
76
a
and
76
b
are positioned in a face-to-face relationship with each other within a same plane at a specified height inside the thermistor block
72
, one of the inner electrodes (
76
a
) being connected to one of the external electrodes (
73
a
) and the other inner electrode
76
b
being connected to the other external electrode
73
b.
Such thermistors
77
are produced by a known kind of ceramic layering technology, the inner electrodes
76
a
and
76
b
being formed by applying a conductive paste on a ceramic green sheet by a screen printing method. Thus, the gap between the two inner electrodes
76
a
and
76
b
can be varied easily, even when thermistor blocks
72
of a same size are used, by adjusting the interval between the printed areas at the time of screen printing. In other words, thermistors
77
with different resistance values can be obtained fairly easily.
When the conductive paste is applied in a printing process as described above, however, the inwardly facing edges of the inner electrodes
76
a
and
76
b
are sometimes blurred and deformed, as shown enlarged in the elliptically marked portion indicated by arrow B of FIG.
18
B. Since several ceramic green sheets are subjected to a firing process together after a conductive paste is printed thereon and they are piled up one top of another, furthermore, variations in the shrinkage accompanying the firing process also contribute to variations in the shapes of the inner electrodes
76
a
and
76
b
. As a result, it was also difficult to produce thermistors of this kind with inner electrodes accurately having a desired resistance value.
FIGS. 19A and 19B
show a chip-type thermistor
78
disclosed in Japanese Patent Publication Tokkai 6-61011, intended to reduce variations in the resistance value, characterized as having a pair of rectangular surface electrodes
79
a
and
79
b
formed on the upper surface of a thermistor block
72
in a face-to-face relationship with respect to each other with a gap of a specified width in between. Outer electrodes
73
a
and
73
b
, as described above, are formed so as to cover portions of these surface electrodes
79
a
and
79
b
on the upper surface of the thermistor block
72
. An insulating layer is formed additionally on the top surface of the thermistor block
72
so as to cover mutually facing edge portions of the surface electrodes
79
a
and
79
b
, as well as the portion of the top surface of the thermistor block
72
not covered by the surface electrodes
79
a
and
79
b
. These surface electrodes
79
a
and
79
b
can be formed accurately because the thin film technology can be used for this purpose. The requirement that the electrodes
79
a
and
79
b
must be rectangular is a drawback, however, when it is desired to produce chip-type thermistors with a very small resistance value because the separation between these surface electrodes
79
a
and
79
b
must be increased and this makes it necessary to use a larger thermistor block
72
.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide chip-type thermistors with small variations in their resistance values.
It is another object of this invention to provide such thermistors with small resistance values.
A chip-type thermistor embodying this invention, with which the above and other objects can be accomplished, may be characterized as having a pair of electrically conductive planar surface electrodes, at least one of which is comb-shaped, facing each other on one of principal surfaces of a thermistor block, an insulating layer being formed on the same surface of the thermistor block so as to cover these surface electrodes, and a pair of outer electrodes formed on end surfaces of the thermistor block so as to be each electrically connected to an associated one of the surface electrodes. Both of the pair of these surface electrodes may be comb-shaped, and the thickness of these surface electrodes is preferably 0.1-2 &mgr;m. A similar pair of surface electrodes may also be present on the other of the principal surfaces of the thermistor block, and the insulating layer preferably extends to edges between the principal surface and the end surfaces of the thermistor block.
The resistance value of such a chip-type thermistor is adjusted according to this invention by removing at least a portion of its surfa

Affiliated with

Also associated with

Rating

Thermistor with comb-shaped electrodes does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Thermistor with comb-shaped electrodes, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thermistor with comb-shaped electrodes will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2434845