Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2000-09-07
2002-04-30
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S301400, C361S320000, C361S321300
Reexamination Certificate
active
06381117
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramic electronic components. In particular, the present invention relates to the structure of a terminal portion and an improvement in material of a ceramic electronic component, such as a monolithic ceramic capacitor, which includes a ceramic electronic component body (hereinafter referred to as a component body) and is surface-mounted.
2. Description of the Related Art
FIG. 1
is a cross-sectional view of a surface-mounted ceramic electronic component
1
which is one of interest in the present invention. The ceramic electronic component
1
includes a rectangular parallelepiped component body
2
having two end faces
3
and
4
opposing each other and four side faces
5
which connect the end faces
3
and
4
.
Terminal electrodes
6
and
7
are formed on the end faces
3
and
4
, respectively. The terminal electrodes
6
and
7
are formed by, for example, coating and baking a conductive paste, and extend over edge portions of the side faces
5
.
The component body
2
is a main constituent of a monolithic ceramic capacitor and includes a plurality of internal electrodes
8
and
9
which are alternately arranged in the interior thereof. The internal electrodes
8
are electrically connected to the terminal electrode
6
, whereas the internal electrodes
9
are electrically connected to the terminal electrode
7
.
A mounting board
10
for mounting the ceramic electronic component
1
has conductive lands
11
and
12
corresponding to the terminal electrodes
6
and
7
, respectively. In surface mounting of the ceramic electronic component
1
onto the mounting board
10
, the terminal electrodes
6
and
7
are aligned onto the conductive lands
11
and
12
, respectively, and the terminal electrodes
6
and
7
are soldered to the conductive lands
11
and
12
, respectively, for example, by a solder reflow process. In the drawing, reference numerals
13
and
14
represent solder provided by the solder reflow process. The solder
13
and
14
is provided on the end faces
3
and
4
, respectively, including the extensions which extend over the edge portions of the side faces
5
.
In the above surface mounting of the ceramic electronic component
1
, distortion of the mounting board
10
and thermal shock cycles involving repeated rising and falling of temperature cause relatively large stresses in the terminal electrodes
6
and
7
and the component body
2
. As a result, cracks
15
form in the component body
2
, as shown in FIG.
1
.
Further, as shown in
FIG. 2
, there is a case in which solders
13
and
14
are provided not only on the end surfaces
3
and
4
of the terminal electrodes
6
and
17
, but also on a side face
5
a
. In such a case a crack
15
may be provided on the side face
5
a
side.
Since the stresses causing such cracks
15
particularly affect the component body
2
at the extensions of the terminal electrodes
6
and
7
over the side faces
5
,
5
a
, the cracks
15
readily form in the vicinity of edges
16
and
17
of the terminal electrodes
6
and
7
, respectively. Moreover, the solder
13
and
14
provided on the extensions of the terminal electrodes
6
and
7
over the side faces
5
,
5
a
enhances the stress.
When the mounting board
10
is a metal-core mounting board, such as an aluminum board composed of an aluminum base covered with an insulating coating, a large difference in thermal expansion coefficients between the mounting board
10
and the component body
2
causes a large stress during the thermal shock cycles, and thus the cracks
15
readily form.
When the component body
2
is a high-capacitance monolithic ceramic capacitor composed of a Pb-based ceramic dielectric material, the component body
2
has a relatively low bending strength. Thus, the cracks
15
more readily form.
The cracks
15
cause a decrease in humidity resistance and a decrease in thermal shock resistance of the ceramic electronic component
1
. Moreover, the cracks
15
cause decreased electrical characteristics such as insulation resistance. As a result, the ceramic electronic component
1
is less reliable.
In order solve the above problems, a conductive resin composed of a resin and metal powder is used for the formation of the terminal electrodes
6
and
7
so that the deformable conductive resin can relax the stress. However, the adhesive strength of the conductive resin of the terminal electrodes
6
and
7
to the component body
2
decreases after the ceramic electronic component
1
is placed in a high-temperature atmosphere, e.g., of approximately 150° C., for a long period or in a high-temperature, high-humidity environment, of e.g., at 85° C. and an 86% humidity. As a result, the terminal electrodes
6
and
7
are easily detached from the component body
2
.
FIG. 3
is a cross-sectional view of another ceramic electronic component
1
a
which is one of interest in the present invention. In
FIG. 3
, elements corresponding to the elements shown in
FIG. 1
are referred to with the same reference numerals and a repeated description is omitted. The ceramic electronic component
1
a
is provided in order to solve the above problems regarding the cracks
15
. In the ceramic electronic component
1
a
, a resin coat
18
is applied over the extensions of the terminal electrodes
6
and
7
and the side faces
5
. Thus, the solder
13
and
14
is provided only on the end faces
3
and
4
of the terminal electrodes
6
and
7
, respectively.
When the ceramic electronic component
1
a
is mounted onto the mounting board
10
, the solder
13
and
14
is not provided on the extensions of the terminal electrodes
6
and
7
on the side faces
5
. Thus, the resin coat
18
contributes to a decreased stress and prevents the formation of the cracks
15
.
The surface mounting of the ceramic electronic component
1
a
shown in
FIG. 3
, however, inhibits direct contact of the terminal electrodes
6
and
7
to the conductive lands
11
and
12
, respectively, and causes a decreased contact area of the terminal electrodes
6
and
7
with the solder
13
and
14
, respectively. Thus, the bonding strength, particularly the shear strength, of the ceramic electronic component
1
a
to the mounting board
10
is not so high. As a result, the ceramic electronic component
1
a
may become detached from the mounting board
10
.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a ceramic electronic component which does not cause the formation of cracks and decreased shear strength.
The present invention is directed to a ceramic electronic component comprising at least one component body having two end faces opposing each other and side faces connecting the two end faces, and terminal electrodes formed on the component body, each extending from each end face to edge portions of each side face of the component body. The terminal electrode is characterized as follows in order to solve the above problems.
Each of the terminal electrodes comprises a metal layer formed on at least each end face of the component body, and a conductive resin layer for covering at least portions of the side faces of the component body, the conductive resin layer extending from the metal layer including the edge of the metal layer to the portions of the side faces, and comprising a conductive resin containing metal powder and resin. The thickness of the conductive resin layer above the side faces is at least about 10 &mgr;m. A metal plating film covers the outer surface of the terminal electrode.
The metal layer ensures sufficiently high bonding strength to the component body and the plating film facilitates soldering to the terminal electrodes of the ceramic electronic component.
The conductive resin layer relaxes the stress due to distortion of a mounting board and thermal shocks so that cracks do not form in the component body. Thus, the ceramic electronic component has highly reliable electrical characteristics.
Preferably, the thickness of the conductive res
Nakagawa Takuji
Takagi Yoshikazu
Watanabe Toru
Yoneda Yasunobu
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Reichard Dean A.
Thomas Eric W.
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