Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2000-07-18
2002-04-30
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S309000, C361S322000, C361S321400
Reexamination Certificate
active
06381118
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic electronic component having noble metal internal electrodes such as a multi-layer ceramic capacitor, a multi-layer varistor, a multi-layer dielectric resonator, and a multi-layer piezoelectric element and to a process for the production of the same.
2. Description of the Related Art
The increasingly compactness of electronic apparatuses in recent years has led to surface mounted types of components becoming the mainstream of discrete components. Among these discrete components, there is a strong demand for miniaturization of the multi-layer ceramic capacitors used for high/low voltage integrated circuits. To meet with this, there is a demand for external electrodes which are excellent in bondability with electrical circuits, superior in electrical characteristics, reliability, and mechanical characteristics, and high in bonding strength with sintered devices such as multi-layer ceramic capacitor bodies.
In general, a multi-layer ceramic capacitor has a capacitor element body obtained by printing an internal electrode conductive paste made using Pd, Pt, Ag—Pd, or another noble metal on a ceramic green sheet prepared by the doctor blade method, stacking a plurality of these ceramic green sheets, heat compressing these, then firing them. A pair of external electrodes are formed at the two ends of the capacitor element body to connect with the alternately stacked internal electrodes inside the capacitor element body.
As the external electrodes, to improve the affinity with the internal electrodes and prevent poor connection, a conductive paste comprised of a powder of Ag, Pd, Ag—Pd, or other noble metal plus glass frit is coated on the two ends of the capacitor element body, dried, then fired so as to provide first electrode layers having a good bonding state with the internal electrodes. To improve the wettability and solder resistance of the first electrode layers for connection with the conductive pattern of the circuit board on which the device is to be mounted as a multi-layer ceramic capacitor, second electrode layers comprised of a plating film of nickel and third electrode layers comprised of a plating film of tin or tin-lead are provided.
In such conventional external electrodes, however, along with the firing of the first electrode layers, the densification of the organic residue of decomposition of the vehicle included in the conductive paste was inhibited and the tensile strength lowered, so there was a problem in bonding strength with the capacitor element body. If using a noble metal to form the internal electrodes and external electrodes to electrically connect the two strongly in the hopes of obtain a high capacity by the multiple layers in the multi-layer ceramic electronic component and achieve miniaturization of the component, while the characteristics become stable, there are the defects that the bonding of the stacked ceramic body and the external electrodes is weak and therefore the mechanical characteristics become inferior.
On the other hand, Japanese Examined Patent Publication (Kokoku) No. 8-17139 and Japanese Unexamined Patent Publication (Kokai) No. 10-154633 propose to form the internal electrodes by a base metal material such as Ni in order to reduce the costs and to form the first electrode layers of the external electrodes by a conductive material comprised of Ag, Cu, and glass frit to secure the bonding strength with the internal electrodes.
With the external electrodes described in these publications, however, migration of silver occurred between electrodes along with a long period of use in a high temperature, high humidity atmosphere and therefore short-circuits easily occurred in a short period, so miniaturization of the components proved difficult. For example, in the case of a multi-layer ceramic capacitor of a miniature size of a length of 0.6 mm and a width of 0.3 mm, the distance between the pair of external electrodes formed on the two end surfaces of the capacitor element body became 0.3 mm or so at a maximum. This was less than half of the distance between external electrodes formed in a multi-layer ceramic capacitor of an ordinary size of a length of 10 mm and a width of 0.5 mm (0.6 to 0.8 mm), so migration of silver occurred particularly easily.
As opposed to this, Japanese Examined Patent Publication (Kokoku) No. 63-17217 and Japanese Examined Patent Publication (Kokoku) No. 63-17218 propose to form external electrodes by a first layer mainly comprised of silver and including palladium and a second layer of just a silver phase. Further, Japanese Unexamined Patent Publication (Kokai) No. 10-16306 proposes forming a first layer using a mixed metal resinate of silver, silicon, calcium, bismuth, etc. and an organic vehicle and forming a second layer using silver powder, glass frit, and a vehicle.
However, since the first layer was formed by a silver phase alone in each of these, while there was the effect of preventing migration of silver in the external electrodes, it was difficult to improve the bonding strength with the capacitor body.
As the conductive material forming the electrodes, it is originally desirable to use palladium powder, which is resistant to a firing temperature of over 1300° C., excellent in conductivity, and reducible, but palladium is expensive, so normally the internal electrodes are formed by a base metal and the external electrodes are formed by a conductive material mainly comprised of the good conductivity silver powder. With external electrodes formed by a conductive material mainly comprised of silver powder, however, since silver is easily oxidized in a firing furnace of a high temperature and high humidity atmosphere at a temperature near 900° C., it is necessary to add as much as 5 wt % of palladium if considering prevention of oxidation of silver and the manufacturing costs.
Even if about 5 wt % of palladium is added, however, the silver particles continue to be rich, so the silver is oxidized at a high temperature, the ionized silver (Ag+) shifts to a negative potential over a long time, and migration of silver occurs between the external electrodes with the resultant susceptibility to electrical short-circuits.
Therefore, it is necessary to realize external electrodes which can prevent the above migration of silver and ensure the bonding strength with the capacitor body when trying to miniaturize the component while securing the minimum distance between the external electrodes for preventing surface discharge.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a ceramic electronic component having a strong electrical connection between internal electrodes and an external electrode, having a high bonding strength between the external electrode and the multi-layer ceramic body, able to effectively prevent migration of silver, and able to be constructed as an extremely small component and a process for production of the same.
To achieve the above object, the ceramic electronic component of the present invention is characterized by comprising internal electrodes having a noble metal and an external electrode having a first electrode layer containing at least
a noble metal,
cuprous oxide (Cu
2
O), and
a glass ingredient
electrically connected to the internal electrodes.
Preferably, the ceramic electronic component according to the present invention has a percent composition of cuprous oxide contained in the first electrode layer of 1.0 to 10.0 wt % with respect to the noble metal.
Preferably, the ceramic electronic component according to the present invention has a percent composition of the glass ingredient contained in the first electrode layer of 4.0 to 12.0 wt % with respect to the noble metal and the cuprous oxide.
Preferably, the ceramic electronic component according to the present invention has the external electrode comprising:
a first electrode layer electrically connected to the internal electrodes,
a second electrode layer stacked on the first electrode layer and comprise
Ishigaki Takaya
Kon Shintarou
Maruno Tetuji
Sasaki Akira
Yokoyama Hideki
Dinkins Anthony
Oliff & Berridg,e PLC
TDK Corporation
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