Ceramic electronic part

Details Ceramic electronic part Ceramic electronic part

1999-11-29

2002-03-12

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S321200, C361S303000

Reexamination Certificate

active

06356430

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic electronic part such as, for example, a monolithic capacitor, a ceramic inductor, or the like, and particularly to a ceramic electronic part in which the structure of terminal electrodes formed on the outer surface of a ceramic element assembly is improved.
2. Description of the Related Art
Conventional methods of forming terminal electrodes of ceramic electronic parts include coating and baking of conductive paste, wet plating, thin film deposition such as vapor deposition, sputtering, and the like. In order to enable surface mounting of ceramic electronic parts on a printed circuit board, a metal layer having excellent solderability is frequently formed on the outermost surface of a terminal electrode.
FIG. 7
shows a monolithic capacitor
51
as an example of a conventional ceramic electronic part. Monolithic capacitor
51
comprises a ceramic sintered compact
52
made of a dielectric ceramic material. The ceramic sintered compact
52
contains internal electrodes
53
a
to
53
f
which are arranged to overlap each other with ceramic layers held therebetween. The internal electrodes
53
a
,
53
c
and
53
e
extend to an end surface
52
a
of the ceramic sintered compact
52
and the internal electrodes
53
b
,
53
d
and
53
f
extend to an end surface
52
b
. Terminal electrodes
54
and
55
are formed to cover the end surfaces
52
a
and
52
b
, respectively.
Each of the terminal electrodes
54
and
55
includes a first electrode layer
54
a
or
55
a
, a second electrode layer
54
b
or
55
b
, and a third electrode layer
54
c
or
55
c
which are laminated together. The first electrode layers
54
a
and
55
a
are formed by coating and baking a conductive paste which is mainly composed of a metal powder of Ag, Pd, Ag—Pd, Cu, or the like, and are electrically connected to the internal electrodes
53
a
,
53
c
, and
53
e
or the internal electrodes
53
b
,
53
d
and
53
f
with excellent reliability. The third electrode layers
54
c
and
55
c
are formed by plating Sn or a Sn alloy, for improving solderability. The intermediate second electrode layers
54
b
and
55
b
are respectively provided for preventing diffusion of Ag from the first electrode layers
54
a
and
55
a
into solder during soldering, and preventing diffusion of Sn, contained in the third electrode layers
54
c
and
55
c
and the solder used in soldering, into the first electrode layers
54
a
and
55
a
and the ceramic sintered compact
52
, or at least decreasing the diffusion rate. Diffusion of Sn to the interfaces between the terminal electrodes
54
and
55
and the ceramic sintered compact
52
deteriorates the adhesion of the terminal electrodes
54
and
55
to the sintered compact
52
, thereby failing to obtain desired electric characteristics.
This is due to the fact that the adhesion between the terminal electrodes
54
and
55
and the ceramic sintered compact
52
depends upon oxygen bonding between the metal which forms the terminal electrodes
54
and
55
, and the ceramic material, and the oxygen bonding between the electrode forming metal and the ceramic material is broken in the presence of Sn, resulting in a deterioration of the adhesion.
Therefore, the monolithic capacitor
51
includes the second electrode layers
54
b
and
55
b
to prevent the diffusion of Sn or to at least decrease the diffusion rate. Each of the second electrode layers
54
b
and
55
b
comprises a deposited film of a metal such as Ni, Ti, Cr or Mo, or an alloy thereof.
The above-described terminal electrodes having the three-layer structure are used in not only monolithic capacitors, but also various other ceramic electronic parts such as a monolithic varistor, a monolithic piezoelectric part, and the like; a single-chip ceramic capacitor, a ceramic thermistor, and the like, which are not monolithic.
While, the electrode layers
54
b
and
55
b
(comprising a deposited film of a metal such as Ti, Ni, Cr or Mo, or an alloy mainly composed of any of these metals) prevent (or slow down) the diffusion of Sn to the first electrode layers
54
a
and
55
a
, respectively, in some cases, the plating solution used to deposit the metal film enters the ceramic sintered compact
52
. Therefore, the interfaces between the ceramic material, which forms the ceramic sintered compact
52
and the internal electrodes
53
a
-
53
f
are reduced resulting in defects such as layer separation, cracks, etc., and failure to obtain desired electric characteristics such as electrostatic capacity. Particularly, in the formation of a single film of a metal having high affinity for oxygen, such as Ti, Cr, Mo, or the like, in a ceramic electronic part used at a relatively high voltage, i.e., in a medium-to-high-voltage region, there is oxygen depletion in the ceramic material located at the end surface
52
a
of the ceramic sintered compact, significantly deteriorating electric characteristics and conductivity with reduction of ceramic. The effectiveness of Ni in preventing diffusion of Sn to the first electrode layers
54
a
and
55
a
insufficient, compared to Ti, Cr, and Mo.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to solve the defects of the above-described conventional technology, and provide a ceramic electronic part comprising terminal electrodes capable of suppressing diffusion of Sn contained in solder into a ceramic element assembly, and causing less reduction of ceramic, thereby securely exhibiting desired electric characteristics.
The present invention provides a ceramic electronic part comprising a ceramic element assembly, and terminal electrodes formed on the outer surface of the ceramic element assembly, wherein the portions of the terminal electrodes, which adhere to the surface of the ceramic element assembly contain both a main component element which forms the primary component of said portions of the terminal electrodes, and a metal having higher affinity for oxygen than Sn.
In the terminal electrodes of the ceramic electronic part of the present invention, the metal having higher affinity for oxygen than Sn may be mixed in the terminal electrodes, or may be alloyed with the main component metal. More generally, the metal having higher affinity for oxygen than Sn can be present in various forms in the terminal electrodes.
The terminal electrodes may comprise a single electrode layer, or a lamination of a plurality of electrode layers. In the terminal electrodes comprising a lamination of a plurality of electrode layers, a layer which adheres to the ceramic element assembly contains the metal having higher affinity for oxygen than Sn.
As the metal having higher affinity for oxygen than Sn, it is preferable to select at least one metal from the group consisting of Ti, Mo, W, and V.
The metal having higher affinity for oxygen than Sn is mixed so that it is present in the faces of the terminal electrodes which adhere to the ceramic element assembly at a ratio of 0.1 to 20% by weight of the main component metal.


REFERENCES:
patent: 4604676 (1986-08-01), Senda et al.
patent: 6043973 (2000-03-01), Nagashima et al.
patent: 2106714 (1983-04-01), None
Patent Abstracts of Japan, vol. 016, No. 367 (E-1245), Aug. 7, 1992, & JP 04 116910 A (Dai Ichi Kogyo Seiyaku Co Ltd. Others: 02), Apr. 17, 1992; Abstract.

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