Electricity: electrical systems and devices – Safety and protection of systems and devices – Circuit interruption by thermal sensing
Reexamination Certificate
2000-07-31
2003-06-10
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Circuit interruption by thermal sensing
C361S058000, C361S103000
Reexamination Certificate
active
06577486
ABSTRACT:
TECHNICAL FIELD
This invention relates to a stacked-type electronic device such as a stacked-type ceramic capacitor, a stacked-type piezoelectric ceramic actuator, and others having a ceramic body with a plurality of internal electrodes and an external electrode formed on the ceramic body and connected, as a lead terminal, to the internal electrodes, and, in particular, to such a stacked-type ceramic device having a fuse element for breaking abnormal current.
BACKGROUND ART
A typical one of the stacked type electronic devices is a ceramic capacitor which comprises a ceramic body of a ceramic material with first and second sets of internal electrodes and first and second external electrodes formed on the ceramic body at different positions. The first set of internal electrodes and the second set of internal electrodes are alternately disposed in the ceramic body and spaced from each other by the ceramic material. The first and second sets of internal electrodes are connected to the first and second external electrodes are connected to the first and second internal electrodes. The first and second external electrodes serve as lead terminals for connecting the capacitor to an external circuit.
The first set of internal electrodes and the second set of internal electrodes will often be referred to as first internal electrodes and second internal electrodes, respectively, hereinafter.
In production of the ceramic capacitor, first and second internal electrodes are formed by screen printing method on ceramic green sheets respectively. Then, green sheets having first internal electrodes and green sheets having second internal electrodes are alternately stacked to form a ceramic body. The ceramic body is fired or baked to form a hard ceramic body having the first and second internal electrodes. Then, external electrodes are formed on the ceramic body.
In the stacked-type capacitor, when adjacent ones of the first and second internal electrodes are short-circuited by any fault, for example, pin holes existing in the ceramic body, an abnormal current flows between the first and second external electrodes through the short-circuited internal electrodes to cause destruction of the external circuit as well as the capacitor itself. This is a serious problem.
The stacked-type piezoelectric ceramic actuator has a similar structure of the capacitor as described above. The ceramic material is a piezoelectric material and is polarized between the adjacent first and second internal electrodes. The stacked-type piezoelectric ceramic actuator has also the similar problem.
Generally speaking, the electronic devices having a ceramic body with a plurality of internal electrodes connected to an external electrode also suffers from the similar problem.
In order to resolve the problem, JP-A 63-305506 (Reference I) discloses to form a narrow width portion in each of the internal electrodes adjacent the external electrode. The narrow width portion is referred to as a fuse electrode. The fuse electrode is melted and broken out by the Joule heat caused by the abnormal electrode flowing therethrough.
However, there is another problem that the Jule heat may cause the ceramic body itself cracked.
JP-A 3-1514 (Reference II) discloses to use a wire-shape fuse element for connecting the internal electrodes with the external electrode. An outer electrode is formed on an outer surface of the ceramic body and is connected to the internal electrodes. The outer electrode is connected to the external electrode by the wire-shape fuse element.
It is troublesome that the wire-shape fuse element extends on the outer surface of the ceramic body. Further, when a pair of adjacent internal electrodes is only short-circuited, the fuse element is broken out. This means the capacitor can no longer be used.
OBJECT OF THE INVENTION
Therefore, it is an object of this invention to provide a stacked-type ceramic electronic device having a plurality of internal electrodes connected to an external electrode and a fuse function for protecting against abnormal current, wherein that ones of the internal electrodes through which the abnormal current flows are only removed or separated by the fuse function from the connection between the internal electrodes and the external electrode without any crack being caused in the ceramic body, so that the device can then be operable while still maintaining its electronic properties.
It is another object of this invention to provide the stacked-type ceramic electronic device with the fuse function which is of a surface mount type.
It is another object of this invention to provide the stacked-type electronic device with the fuse function which has an improved equivalent series resistance.
DISCLOSURE OF INVENTION
According to the present invention, a stacked-type electronic device is obtained which comprises:
a ceramic body of a ceramic material having an outer surface including a side surface;
a set of internal electrodes disposed in the ceramic body and spaced from each other by the ceramic material, the internal electrodes extending in parallel with each other, the internal electrodes having their individual lead-out electrodes extending therefrom to their end surfaces exposed in the side surface;
an external electrode mounted on the ceramic body for connecting to an external electric circuit;
a film electrode connected to the external electrode, the film electrode being deposited on the side surface and overlying the end surfaces of the individual lead-out electrodes in common to be directly connected to the lead-out electrodes.
At least, the film element partially fuses by a heat caused due to an abnormal current flowing therethrough from the external electrode to a particular one of the internal electrodes to break the abnormal current from flowing to the particular internal electrode.
In an aspect, the film electrode comprises metallic material having a melting point lower than that of the internal electrodes.
In an embodiment, the film electrode has a thickness of 0.5-50 micrometers (&mgr;m). Each of the lead-out electrodes has a width of 50-800 micrometers (&mgr;m).
In an example, it can make use of metals or alloys selected from a group of Ag, Pd, Cu, Ni, Sn, Zn, Bi, Pb and Cd for the metallic material of the film element.
In a preferred embodiment, the film electrode is a baked film formed by an electrode metal paste being coated on the side surface and then baked.
Preferably, the electrode metal paste is a metallic powder paste comprising electrode metallic powder and a carrier solvent suspending the powder therein.
More preferably, the metallic powder paste further contains at least one selected from glass forming elements of SiO
2
, Al
2
O
3
and B
2
O
3
.
Alternatively, the electrode metal paste is an metallo-organic compound past which comprises resinate of metallo-organic compound/organometallic compound and organic solvent resolving the resinate therein. The reginate of the metallo-organic compound/organometallic compound is one comprises the metallo-organic compound/organometallic compound and resin mixed to the the metallo-organic compound/organometallic compound.
The metallo-organic compound preferably contains at least one of glass forming elements of Si, Al, and B.
The electrode metal paste may be a mixed paste which comprises the metallic powder paste and the metallo-organic compound paste mixed with the metallic powder paste. It is preferable that the metallic powder paste further contains at least one selected from glass forming elements of SiO
2
, Al
2
O
3
and B
2
O
3
, or the metallo-organic compound may contain at least one of glass forming elements of Si, Al, and B.
In an embodiment of the stacked-type electronic device according to this invention, the external electrode overlaps the film electrode. The external electrode and the film electrode can be made of the same material.
Alternatively, the external electrode and the film electrode can be spaced from each other by a hollow space formed therebetween, or an insulating glass layer may partially be interposed between the
Aisawa Syuji
Ito Shinji
Kurano Masayuki
Nishimiya Yukio
Frishauf Holtz Goodman & Chick P.C.
Jackson Stephen W.
NEC Tokin Corporation
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