U.S. patent application number 11/483834 was filed with the patent office on 2007-02-01 for solid electrolytic capacitor element, manufacturing method therefor, and solid electrolytic capacitor.
This patent application is currently assigned to SANYO Electric Co., Ltd.. Invention is credited to Takahisa Iida, Mamoru Kimoto, Mutsumi Yano.
Application Number | 20070025064 11/483834 |
Document ID | / |
Family ID | 37674331 |
Filed Date | 2007-02-01 |
United States Patent
Application |
20070025064 |
Kind Code |
A1 |
Iida; Takahisa ; et
al. |
February 1, 2007 |
SOLID ELECTROLYTIC CAPACITOR ELEMENT, MANUFACTURING METHOD
THEREFOR, AND SOLID ELECTROLYTIC CAPACITOR
Abstract
The present invention provides a solid electrolytic capacitor
with low leakage current. In the solid electrolytic capacitor, an
anode has an anode lead made of tantalum, a surface layer made of
niobium formed on the anode lead, and a rectangular block shaped
base body having a porous sintered body made from niobium
particles, and the anode lead is partially embedded in the base
body. Onto the anode, an oxide layer made of niobium oxide, an
electrically conductive polymer layer, and a cathode laminated with
a first electrically conductive layer and a second electrically
conductive layer are sequentially laminated. Onto the top surface
of an area surrounding the cathode, a cathode terminal is formed
through an electrically conductive adhesive layer. Also, an anode
terminal is connected onto an edge of the anode lead.
Inventors: |
Iida; Takahisa; (Osaka,
JP) ; Yano; Mutsumi; (Osaka, JP) ; Kimoto;
Mamoru; (Osaka, JP) |
Correspondence
Address: |
MASUVALLEY & PARTNERS
8765 AERO DRIVE
SUITE 312
SAN DIEGO
CA
92123
US
|
Assignee: |
SANYO Electric Co., Ltd.
|
Family ID: |
37674331 |
Appl. No.: |
11/483834 |
Filed: |
July 10, 2006 |
Current U.S.
Class: |
361/528 |
Current CPC
Class: |
H01G 9/052 20130101;
H01G 9/012 20130101; H01G 9/15 20130101; H01G 9/042 20130101 |
Class at
Publication: |
361/528 |
International
Class: |
H01G 9/04 20060101
H01G009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 28, 2005 |
JP |
2005-218817 |
Claims
1. A solid electrolytic capacitor element comprising: an oxide
layer containing niobium oxide, the oxide layer being formed on an
anode; a cathode formed on the oxide layer, wherein the anode has:
an anode lead; a surface layer containing niobium, the surface
layer being formed on the anode lead; and a base body comprised of
a porous sintered body containing niobium, the base body being
formed on the surface layer.
2. The solid electrolytic capacitor element according to claim 1,
wherein the anode lead contains tantalum and the anode lead not
formed there around with the base body has thereon a region not
formed with the surface layer.
3. The solid electrolytic capacitor element according to claim 1,
wherein the anode lead contains tantalum, and a first region formed
with the surface layer is formed on the anode lead, and a terminal
is formed outside of the first region.
4. The solid electrolytic capacitor element according to claim 1,
wherein a film thickness of the surface layer falls within a range
of 0.01 .mu.m to 2 .mu.m.
5. The solid electrolytic capacitor element according to claim 4,
wherein a measurement of the film thickness of the surface layer is
performed by an energy dispersive X-ray analysis (EDX) method.
6. The solid electrolytic capacitor element according to claim 4,
wherein the anode lead contains tantalum and the anode lead not
formed there around with the base body has thereon a region not
formed with the surface layer.
7. The solid electrolytic capacitor element according to claim 4,
wherein the anode lead contains tantalum, a first region formed
with the surface layer is formed on the anode lead, and a terminal
is formed outside of the first region.
8. The solid electrolytic capacitor element according to claim 1,
wherein the surface layer suppresses a leakage current between the
cathode and the anode to be less than 600 .mu.A.
9. The solid electrolytic capacitor element according to claim 1,
wherein the surface layer suppresses a leakage current between the
cathode and the anode to be less than 300 .mu.A.
10. The solid electrolytic capacitor element according to claim 4,
wherein the film thickness of the surface layer preferably falls
within a range of 0.1 .mu.m to 0.5 .mu.m.
11. The solid electrolytic capacitor element according to claim 10,
wherein a measurement of the film thickness of the surface layer is
performed by an energy dispersive X-ray analysis (EDX) method.
12. A method for manufacturing a solid electrolytic capacitor
element, the method comprising the steps of: forming on an anode
lead a surface layer containing niobium; forming on the surface
layer an anode having a base body comprised of a porous sintered
body containing niobium; forming on the anode by anodization an
oxide layer containing niobium oxide; and forming a cathode on the
oxide layer.
13. The method for manufacturing a solid electrolytic capacitor
element according to claim 12, wherein the step of forming the
surface layer containing niobium comprises a cathodic reduction in
an element selected from a group consisting of: molten lithium
fluoride; sodium fluoride; potassium fluoride; and potassium
hexafluoroniobate (LiF-NaF-KF-K.sub.2NbF.sub.6).
14. The method for manufacturing a solid electrolytic capacitor
element according to claim 12, wherein the step of forming the
surface layer containing niobium comprises a cathodic reduction in
an element selected from a group consisting of: molten lithium
fluoride; potassium fluoride; and potassium hexafluoroniobate
(LiF-KF-K.sub.2NbF.sub.6).
15. The method for manufacturing a solid electrolytic capacitor
element according to claim 12, wherein the step of forming the
surface layer containing niobium comprises a cathodic reduction in
an element selected from a group consisting of: molten lithium
fluoride and potassium hexafluoroniobate
(LiF-K.sub.2NbF.sub.6).
16. A solid electrolytic capacitor comprising: an oxide layer
containing niobium oxide, the oxide layer being formed on an anode;
a cathode formed on the oxide layer, wherein the anode has: an
anode lead; a surface layer containing niobium, the surface layer
being formed on the anode lead; and a base body comprised of a
porous sintered body containing niobium, the base body being formed
on the surface layer, and an anode terminal is formed on the anode,
a cathode terminal is formed on the cathode, and a mold-packaging
resin is formed such that respective edges of the anode terminal
and the cathode terminal are led outside.
Description
TECHNICAL FIELD
[0001] The present invention relates to a solid electrolytic
capacitor element, a manufacturing method therefor, and a solid
electrolytic capacitor.
BACKGROUND OF INVENTION
[0002] In recent years, a solid electrolytic capacitor has been
requiring the decrease in size and the increase in capacitance, and
a solid electrolytic capacitor using niobium oxide having a large
dielectric constant have been proposed, instead of using aluminum
oxide or tantalum oxide as a dielectric material. An example of
such proposal includes Japanese published unexamined patent
application No. 2000-68157.
[0003] FIG. 8 is a cross-sectional view for illustrating a
structure of a conventional solid electrolytic capacitor. The
structure of the conventional solid electrolytic capacitor will be
described in reference to FIG. 8.
[0004] As shown in FIG. 8, in the conventional solid electrolytic
capacitor, an anode 101 comprises an anode lead 101a made of
tantalum and a rectangular block shaped base body 101c comprised of
a porous sintered body made from niobium particles, and the anode
lead 101a is partially embedded in the base body 101c.
[0005] Onto the anode 101, an oxide layer 102 is formed so as to
cover an area surrounding the base body 101c. The oxide layer 102
functions as a dielectric layer.
[0006] Onto the oxide layer 102, an electrically conductive polymer
layer 103 made of polypyrrole or the like is formed so as to cover
an area surrounding the oxide layer 102. The electrically
conductive polymer layer 103 functions as an electrolyte layer.
Onto the electrically conductive polymer layer 103, a cathode 104
is formed, which is laminated with a first electrically conductive
layer 104a containing carbon particles formed so as to cover an
area surrounding the electrically conductive polymer layer 103 and
a second electrically conductive layer 104b containing silver
particles formed so as to cover an area surrounding the first
electrically conductive layer 104a.
[0007] Onto the top surface of an area surrounding the cathode 104,
an electrically conductive adhesive layer 105 is formed, onto which
a cathode terminal 106 is further formed. An anode terminal 107 is
connected onto the anode lead 101a, which is exposed from the base
body 101c and the oxide layer 102. Further, a mold-packaging resin
108 is formed around the cathode 104, the cathode terminal 106 and
the anode terminal 107 such that respective edges of the cathode
terminal 106 and the anode terminal 107 can be led outside. The
conventional solid electrolytic capacitor is thus configured.
[0008] FIGS. 9 to 13 are cross-sectional views for illustrating a
fabrication process of the conventional solid electrolytic
capacitor. In reference to FIGS. 9 to 13, the fabrication process
of the conventional solid electrolytic capacitor having the
structure configured as above is now described.
[0009] First, as shown in FIG. 9, the anode 101 comprising the
anode lead 101a made of tantalum and the rectangular block shaped
base body 101c comprised of the porous sintered body made from
niobium particles is formed. The base body 101c is formed by heat
treatment process in vacuum a molding made from niobium particles,
in which the anode lead 101a is partially embedded.
[0010] Then, the anode 101 is anodized in an aqueous solution such
as a phosphoric acid solution to thereby form the oxide layer 102
on the base body 101c so as to cover the area surrounding the base
body 101c as shown in FIG. 10.
[0011] Subsequently, as shown in FIG. 11, the electrically
conductive polymer layer 103 made of polypyrrole or the like is
formed onto the oxide layer 102 by polymerization or the like so as
to cover the area surrounding the oxide layer 102.
[0012] Subsequently, as shown in FIG. 12, a carbon paste is coated
onto the electrically conductive polymer layer 103 so as to cover
the area surrounding the electrically conductive polymer layer 103,
and then dried to thereby form the first electrically conductive
layer 104a containing carbon particles. After that, a silver paste
is coated on the first electrically conductive layer 104a so as to
cover the area surrounding the first electrically conductive layer
104a, and then dried to thereby form the second electrically
conductive layer 104b containing silver particles. This allows the
cathode 104, which is laminated with the first electrically
conductive layer 104a and the second electrically conductive layer
104b, to be formed so as to cover the area surrounding the
electrically conductive polymer layer 103.
[0013] Subsequently, as shown in FIG. 13, after an electrically
conductive adhesive has been coated onto the cathode terminal 106,
the cathode terminal 106 is bonded to the top surface of the area
surrounding the cathode 104 through the electrically conductive
adhesive. Further, drying the electrically conductive adhesive
allows the electrically conductive adhesive layer 105 to be formed,
through which the cathode 104 and the cathode terminal 106 are
interconnected. Also, the anode terminal 107 is welded onto the
anode lead 101a exposed from the base body 101c and the oxide layer
102.
[0014] Finally, as shown in FIG. 8, the mold-packaging resin 108 is
formed around the cathode 104, the cathode terminal 106 and the
anode terminal 107 such that the respective edges of the cathode
terminal 106 and the anode terminal 107 can be led outside.
[0015] FIG. 14 is a schematic cross-sectional view for illustrating
a condition of the oxide layer near the anode lead in the
conventional solid electrolytic capacitor. Referring to FIG. 14, in
the conventional solid electrolytic capacitor, the oxide layer 102
is formed on the surface of the anode lead 101a at positions of
spaces that exist between the anode lead 101a and the base body
101c due to the base body 101c comprised of the porous sintered
body, as well as on the surface of the base body 101c. Note that
because the anode lead 101a is made of tantalum, an oxide layer
102a made of tantalum oxide is formed on the anode lead. On the
other hand, because the base body 101c is made of niobium, an oxide
layer 102b made of niobium oxide is formed on the base body 101c.
For this reason, in the spaces existing between the anode lead 101a
and the base body 101c, the oxide layer 102 is in a condition of a
mixture of the oxide layer 102a made of tantalum oxide and the
oxide layer 102b made of niobium oxide.
[0016] The above conventional solid electrolytic capacitor can
achieve large electrostatic capacitance because the oxide layer 102
is comprised of niobium oxide except for an area in the vicinity of
the interface between the anode lead 101a and the base body 101c as
described.
[0017] However, in the above conventional solid electrolytic
capacitor, the oxide layer 102 in the vicinity of the interface
between the anode lead 101a and the base body 101c is in the
condition of the mixture of the oxide layer 102a made of tantalum
oxide and the oxide layer 102b made of niobium oxide, and therefore
stress due to the difference in thermal expansion coefficient
between the oxide layer 102a made of tantalum oxide and the oxide
layer 102b made of niobium oxide arises during heat treatment
process such as reflow soldering process. For this reason, there
have arisen problems that the oxide layer 102 is likely to be
peeled off from the anode lead 101a and the base body 101c and that
cracks are likely to be generated inside of the oxide layer 102. As
a result, there has arisen another problem that leakage current
between the cathode 104 formed on the oxide layer 102 and the anode
101 is likely to be increased.
[0018] The present invention has been made in order to solve the
problems described above, and it is therefore one object of the
present invention to provide a solid electrolytic capacitor element
or a solid electrolytic capacitor having low leakage current.
[0019] Another object of the present invention is to provide a
method for manufacturing the solid electrolytic capacitor element
having low leakage current.
BRIEF SUMMARY OF THE INVENTION
[0020] In order to accomplish the above objects, the solid
electrolytic capacitor element comprises: an oxide layer that is
formed on an anode and contains niobium oxide; and a cathode formed
on the oxide layer, wherein the anode has: an anode lead; a surface
layer that is formed on the anode lead and contains niobium; and a
base body that is formed on the surface layer and comprised of a
porous sintered body containing niobium.
[0021] In the solid electrolytic capacitor element according to the
present invention, the anode has the anode lead and the base body
containing niobium formed on the anode lead as described above, and
therefore the oxide layer containing niobium oxide formed on the
anode is also formed on a surface of the base body as well as on a
surface of the anode lead at positions of spaces existing between
the anode lead and the base body.
[0022] Note that because the anode lead is formed thereon with the
surface layer containing niobium, the oxide layer is formed on the
surface layer containing niobium. That is, at the positions of the
spaces, the oxide layer containing niobium oxide is formed on the
surface layer and the base body both containing niobium.
[0023] For this reason, the oxide layer containing niobium oxide is
unlikely to be peeled off from the surface layer and the base body
both containing niobium even during heat treatment process such as
reflow soldering process, and cracks inside of the oxide layer are
also unlikely to be generated. As a result, an increase in leakage
current between the cathode formed on the oxide layer and the anode
can be suppressed, so that a solid electrolytic capacitor element
with low leakage current can easily be obtained.
[0024] Preferably, in the above-described solid electrolytic
capacitor element, the anode lead contains tantalum and the anode
lead around which the base body is not formed has thereon a region
not formed with the surface layer. In other words, the anode lead
contains tantalum, a first region formed with the surface layer is
formed on the anode lead, and a terminal is formed outside of the
first region.
[0025] Configuring as described allows a tantalum base material,
which is more chemically stable than niobium and is unlikely to be
oxidized, to be exposed in a region on the anode lead where the
base body and the surface layer are not formed, and therefore an
oxide coating is unlikely to be formed on the region during heat
treatment process.
[0026] This enables a satisfactory electrical connection between
the anode lead and the anode terminal to be made when the anode
terminal is connected onto the region, so that a solid electrolytic
capacitor element with low equivalent series resistance (ESR) can
be obtained.
[0027] Preferably, in the solid electrolytic capacitor element, a
film thickness of the surface layer falls within the range of 0.01
.mu.m to 2 .mu.m. A measurement of the film thickness of the
surface layer is performed by an energy dispersive X-ray analysis
(EDX) method.
[0028] Configuring as described enables a stress arising between
the anode lead and the oxide layer to be sufficiently suppressed
and adhesiveness of the surface layer on the anode lead to be
improved, and therefore peeling off of the oxide layer and cracks
in it are unlikely to be generated. This enables leakage current to
be further reduced.
[0029] Also, the above-described surface layer suppresses a leakage
current between the cathode and the anode to be less than 600
.mu.A, and more preferably less than 300 .mu.A.
[0030] Further, the method for manufacturing the solid electrolytic
capacitor element according to the present invention comprises the
steps of forming on an anode lead a surface layer containing
niobium; forming on the surface layer an anode having a base body
comprised of a porous sintered body containing niobium; forming on
the anode by anodization an oxide layer containing niobium oxide;
and forming a cathode on the oxide layer.
[0031] The step of forming the surface layer containing niobium may
comprise cathodic reduction in molten lithium fluoride-sodium
fluoride-potassium fluoride-potassium hexafluoroniobate
(LiF-NaF-KF-K2NbF6) systems. Alternatively, the step of forming the
surface layer containing niobium may comprise cathodic reduction in
any of molten lithium fluoride-potassium fluoride-potassium
hexafluoroniobate (LiF-KF-K2NbF6) systems or molten lithium
fluoride-potassium hexafluoroniobate (LiF-K2NbF6) systems.
[0032] In the method for manufacturing the solid electrolytic
capacitor element, as described above, surfaces of the anode lead
having the surface layer containing niobium and the base body that
is formed on the anode lead and comprised of a porous sintered body
containing niobium are formed thereon with the oxide layer
containing niobium oxide, so that the oxide layer containing
niobium oxide can be readily formed also at positions of spaces
existing between the anode lead and the base body.
[0033] For this reason, the oxide layer containing niobium oxide is
unlikely to be peeled off from the surface layer and the base body
both containing niobium even during heat treatment process such as
reflow soldering process, and cracks inside of the oxide layer are
also unlikely to be generated. As a result, an increase in leakage
current between the cathode formed on the oxide layer and the anode
can be suppressed, so that a solid electrolytic capacitor with low
leakage current can easily be manufactured.
[0034] Also, the solid electrolytic capacitor according to the
present invention comprises: an oxide layer that is formed on an
anode and contains niobium oxide; and a cathode formed on the oxide
layer, wherein the anode has: an anode lead; a surface layer that
is formed on the anode lead and contains niobium; and a base body
that is formed on the surface layer and comprised of a porous
sintered body containing niobium, the base body being, an anode
terminal is formed on the anode, a cathode terminal is formed on
the cathode, and a mold-packaging resin is formed such that
respective edges of the anode terminal and the cathode terminal are
led outside.
[0035] For this reason, the oxide layer containing niobium oxide is
unlikely to be peeled off from the surface layer and the base body
both containing niobium even during heat treatment process such as
reflow soldering process, and cracks inside of the oxide layer are
also unlikely to be generated. As a result, an increase in leakage
current between the cathode formed on the oxide layer and the anode
can be suppressed, so that a solid electrolytic capacitor with low
leakage current can easily be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a cross-sectional view for illustrating a
structure of a solid electrolytic capacitor according to Example 1
of the present invention.
[0037] FIG. 2 is a cross-sectional view for illustrating a first
step in a fabrication process of the solid electrolytic capacitor
according to Example 1 of the present invention.
[0038] FIG. 3 is a cross-sectional view for illustrating a second
step in the fabrication process of the solid electrolytic capacitor
according to Example 1 of the present invention.
[0039] FIG. 4 is a cross-sectional view for illustrating a third
step in the fabrication process of the solid electrolytic capacitor
according to Example 1 of the present invention.
[0040] FIG. 5 is a cross-sectional view for illustrating a fourth
step in the fabrication process of the solid electrolytic capacitor
according to Example 1 of the present invention.
[0041] FIG. 6 is a cross-sectional view for illustrating a fifth
step in the fabrication process of the solid electrolytic capacitor
according to Example 1 of the present invention.
[0042] FIG. 7 is a schematic cross-sectional view for illustrating
a condition of an oxide layer near an anode lead in the solid
electrolytic capacitor according to Example 1 of the present
invention.
[0043] FIG. 8 is a cross-sectional view for illustrating a
structure of a conventional solid electrolytic capacitor.
[0044] FIG. 9 is a cross-sectional view for illustrating a first
step in a fabrication process of the conventional solid
electrolytic capacitor.
[0045] FIG. 10 is a cross-sectional view for illustrating a second
step in the fabrication process of the conventional solid
electrolytic capacitor.
[0046] FIG. 11 is a cross-sectional view for illustrating a third
step in the fabrication process of the conventional solid
electrolytic capacitor.
[0047] FIG. 12 is a cross-sectional view for illustrating a fourth
step in the fabrication process of the conventional solid
electrolytic capacitor.
[0048] FIG. 13 is a cross-sectional view for illustrating a fifth
step in the fabrication process of the conventional solid
electrolytic capacitor.
[0049] FIG. 14 is a schematic cross-sectional view for illustrating
a condition of an oxide layer near an anode lead in the
conventional solid electrolytic capacitor.
DETAILED DESCRIPTION OF INVENTION
[0050] Examples of the present invention will hereinafter be
described in reference to the drawings.
EXAMPLE 1
[0051] FIG. 1 is a cross-sectional view for illustrating a
structure of a solid electrolytic capacitor according to Example 1
of the present invention.
[0052] As shown in FIG. 1, in the rectangular block shaped solid
electrolytic capacitor according to Example 1 of the present
invention, an anode 1 comprises an anode lead 1a made of tantalum,
a surface layer 1b made of niobium having a film thickness of
approximately 0.1 .mu.m formed on the anode lead 1a and a
rectangular block shaped base body 1c with a size of approximately
3.3 mm.times.approx. 2.7 mm.times.approx. 1.7 mm comprised of a
porous sintered body made from niobium particles having an average
particle size of approximately 2 .mu.m, and the anode lead 1a is
partially embedded in the base body 1c. Also, the surface layer 1b
is not formed on an edge 1d of the anode lead 1a where the anode
lead 1a is not embedded in the base body 1c, and therefore a
tantalum base material is exposed there. In addition, the edge 1d
is one example of a "region where a surface layer is not formed"
according to the present invention.
[0053] Onto the anode 1, an oxide layer 2 made of niobium oxide is
formed so as to cover an area surrounding the anode 1. The oxide
layer 2 functions as a so-called dielectric layer.
[0054] Onto the oxide layer 2, an electrically conductive polymer
layer 3 made of polypyrrole is formed so as to cover an area
surrounding the oxide layer 2. The electrically conductive polymer
layer 3 functions as a so-called electrolyte layer. Onto the
electrically conductive polymer layer 3, a cathode 4 is formed,
which is laminated with a first electrically conductive layer 4a
containing graphite particles formed so as to cover an area
surrounding the electrically conductive polymer layer 3 and a
second electrically conductive layer 4b containing silver particles
formed so as to cover an area surrounding the first electrically
conductive layer 4a.
[0055] Onto the top surface of an area surrounding the cathode 4,
an electrically conductive adhesive layer 5 is formed, onto which a
cathode terminal 6 is further formed. Also, the edge 1d of the
anode lead 1a where the anode lead 1a is not embedded in the base
body 1c is exposed from the surface layer 1b, the base body 1c and
the oxide layer 2, and an anode terminal 7 is connected onto the
edge 1d. Further, a mold-packaging resin 8 is formed around the
cathode 4, the cathode terminal 6 and the anode terminal 7 such
that respective edges of the cathode terminal 6 and the anode
terminal 7 can be led outside. The solid electrolytic capacitor
according to Example 1 of the present invention is thus
configured.
[0056] FIGS. 2 to 6 are cross-sectional views for illustrating a
fabrication process of the solid electrolytic capacitor according
Example 1 of the present invention. In reference to FIGS. 2 to 6,
the fabrication process of the solid electrolytic capacitor having
the structure illustrated above according to Example 1 of the
present invention is now described.
[0057] First, as shown in FIG. 2, the surface layer 1b made of
niobium having a film thickness of approximately 0.1 .mu.m was
formed on the anode lead 1a by cathodically reducing the anode lead
1a for approximately 3 minutes in lithium fluoride-sodium
fluoride-potassium fluoride-potassium hexafluoroniobate
(LiF-NaF-KF-K.sub.2NbF.sub.6) systems that had been heated at
approximately 550.degree. C. to be thereby molten. The film
thickness of the surface layer 1b may be identified by an energy
dispersive X-ray analysis (EDX) method or the like. The cathodic
reduction was performed without immersing one edge 1d of the anode
lead 1a, whereby the tantalum base material was exposed at the edge
1d.
[0058] Then, the anode 1 comprising the anode lead 1a and the
rectangular block shaped base body 1c with a size of approximately
3.3 mm.times.approx. 2.7 mm.times.approx. 1.7 mm comprised of a
porous sintered body made from niobium particles having an average
particle size of approximately 2 .mu.m was formed. The base body 1c
was formed by heat treatment process in vacuum a molding comprised
of niobium particles in which the other edge of the anode lead 1a
was embedded.
[0059] Subsequently, the anode 1 was anodized in an approximately
0.5 wt. % phosphoric acid solution, which was held at approximately
60.degree. C., by applying a constant voltage of approximately 10 V
for approximately 8 hours, whereby the oxide layer 2 made of
niobium oxide was formed so as to cover the area surrounding the
base body 1 as shown in FIG. 3.
[0060] Subsequently, as shown in FIG. 4, the electrically
conductive polymer layer 3 made of polypyrrole was formed on the
oxide layer 2 by polymerization or the like so as to cover the area
surrounding the oxide layer 2.
[0061] Subsequently, as shown in FIG. 5, a graphite paste was
coated onto the electrically conductive polymer layer 3 so as to
cover the area surrounding the electrically conductive polymer
layer 3, and then dried at approximately 80.degree. C. for
approximately 30 minutes to thereby form the first electrically
conductive layer 4a containing graphite particles. After that, a
silver paste was coated on the first electrically conductive layer
4a so as to cover the area surrounding the first electrically
conductive layer 4a, and then dried at approximately 170.degree. C.
for approximately 30 minutes to thereby form the second
electrically conductive layer 4b containing silver particles. This
allowed the cathode 4, which was laminated with the first
electrically conductive layer 4a and the second electrically
conductive layer 4b, to be formed so as to cover the area
surrounding the electrically conductive polymer layer 3.
[0062] Subsequently, as shown in FIG. 6, after an electrically
conductive adhesive had been coated in an amount of approximately 2
mg on the cathode terminal 6 made from iron foil of which a
thickness was approximately 0.1 mm and a surface was nickel-plated,
the cathode terminal 6 was bonded to the top surface of the area
surrounding the cathode 4 through the electrically conductive
adhesive. Further, drying the electrically conductive adhesive at
approximately 60.degree. C. for approximately 30 minutes allowed
the electrically conductive adhesive layer 5 to be formed, through
which the cathode 4 and the cathode terminal 6 were
interconnected.
[0063] Also, the anode terminal 7 made from iron foil of which a
thickness was approximately 0.1 mm and a surface was nickel-plated
was welded onto the edge 1d of the anode lead 1a where the anode
lead 1a was exposed from the surface layer 1b, the base body 1c,
and the oxide layer 2.
[0064] Finally, as shown in FIG. 1, the mold-packaging resin 8 was
formed around the cathode 4, the cathode terminal 6 and the anode
terminal 7 such that the respective edges of the cathode terminal 6
and the anode terminal 7 were able to be led outside. The solid
electrolytic capacitor according to Example 1 was thus fabricated
as shown in FIG. 1.
[0065] In addition, in the present invention, a condition in which
the cathode terminal 6 and the anode terminal 7 are formed and the
mold-packaging resin 8 is formed around the cathode 4, the cathode
terminal 6 and the anode terminal 7 is referred to as a solid
electrolytic capacitor, and also, a prior condition as an element
in which the terminals have not been connected is referred to as a
solid electrolytic capacitor element.
[0066] FIG. 7 is a schematic cross-sectional view for illustrating
a condition of the oxide layer near the anode lead in the solid
electrolytic capacitor according to Example 1 of the present
invention. Referring to FIG. 7, in the solid electrolytic capacitor
according to Example 1 of the present invention, the oxide layer 2
formed on the anode 1 is also formed on the surfaces of the surface
layer 1b and the base body 1c at positions of spaces that exist
between the anode lead 1a and the base body 1c due to the base body
1c comprised of the porous sintered body. Because the surface layer
1b and the base body 1c contain niobium, the oxide layer 2
containing niobium oxide is unlikely to be peeled off from the
surface layer 1b and the base body 1c even during heat treatment
process such as reflow soldering process, and cracks inside of the
oxide layer 2 are also unlikely to be generated. As a result, an
increase in leakage current between the cathode 4 formed on the
oxide layer 2 and the anode 1 can be suppressed, so that the
leakage current can be decreased.
[0067] Also, in the above-described Example 1, the anode lead 1a is
made of tantalum and a tantalum base material is exposed at the
edge 1d of the anode lead 1a where the anode lead 1a is exposed
from the base body 1c and the oxide layer 2, so that an oxide
coating is unlikely to be formed on the edge 1d. This enables a
satisfactory electrical connection between the anode lead 1a and
the anode terminal 8 to be made when the anode terminal 8 is
connected onto the edge 1d, so that ESR can be reduced.
[0068] Further, in the above-described Example 1, because the oxide
layer 2 containing niobium oxide is formed by anodizing the anode 1
having the above configuration, the oxide layer 2 containing
niobium oxide can readily be formed also on the surface layer 1b
and the base body 1c both containing niobium at the positions of
the spaces existing between the anode lead 1a and the base body 1c.
For this reason, the oxide layer 2 containing niobium oxide is
unlikely to be peeled off from the surface layer 1b and the base
body 1c even during heat treatment process such as reflow soldering
process, and cracks inside of the oxide layer 2 are also unlikely
to be generated. As a result, an increase in leakage current
between the cathode 4 formed on the oxide layer 2 and the anode 1
can be suppressed, so that a solid electrolytic capacitor with low
leakage current can easily be manufactured.
[0069] As the next step, in order to examine an effect of Example 1
of the present invention, the following comparative experiments
were performed.
COMPARATIVE EXAMPLE 1
[0070] In Comparative Example 1, the conventional electrolytic
capacitor shown in FIG. 8 was fabricated.
[0071] In other word, in Comparative Example 1, the solid
electrolytic capacitor similar to that in Example 1 was fabricated,
except that the capacitor in Comparative Example 1 employed the
anode lead made of tantalum onto which the surface layer in Example
1 was not formed.
COMPARATIVE EXAMPLE 2
[0072] In Comparative Example 2, a solid electrolytic capacitor
similar to that in Comparative Example 1 was fabricated, except
that the capacitor in Comparative Example 2 employed an anode lead
made of niobium, instead of the anode lead made of tantalum in
Comparative Example 1.
[0073] Also, leakage current measurements were performed on the
solid electrolytic capacitors in the above Example 1, and
Comparative Examples 1 and 2.
[0074] In the leakage current measurements, after heat treatment
process of each of the fabricated solid electrolytic capacitors had
been performed at approximately 240.degree. C. for approximately 5
minutes, a constant voltage of approximately 5 V was applied
between the cathode terminal and the anode terminal and a current
value observed approximately 20 seconds after the application was
measured on each capacitor. Measurement results are listed in Table
1. TABLE-US-00001 TABLE 1 Leakage current (.mu.A) Example 1 200
Comparative Example 1 600 Comparative Example 2 800
[0075] As listed in Table 1, it has turned out that the solid
electrolytic capacitor in Example 1 exhibits a large reduction in
leakage current in comparison with those in Comparative Examples 1
and 2.
EXAMPLE 2
[0076] As the next step, an effect of a film thickness of the
surface layer 1b in the solid electrolytic capacitor in Example 1
of the present invention was examined.
[0077] In other words, solid electrolytic capacitors similar to
that in Example 1 were fabricated, except that a film thickness of
the surface layer 1b was varied in Example 2 by varying a time
period for cathodically reducing the anode lead 1a in molten
LiF-NaF-KF-K2NbF6 systems in Example 1 in the range of
approximately 10 seconds to approximately 2 hours 30 minutes as
shown in Table 2.
[0078] Similarly to the case of Example 1, a film thickness of the
surface layer 1b was measured by EDX and a leakage current was also
measured. Measurement results are listed in Table 2. TABLE-US-00002
TABLE 4 Cathodic reduction Surface layer Leakage time thickness
(.mu.m) current (.mu.A) Example 2 10 seconds 0.005 560 Example 2 15
seconds 0.0075 480 Example 2 20 seconds 0.01 270 Example 2 2
minutes 0.05 230 Example 1 3 minutes 0.1 200 Example 2 15 minutes
0.5 200 Example 2 30 minutes 1.0 210 Example 2 1 hour 2.0 210
Example 2 1 hour 30 minutes 3.0 460 Example 2 2 hours 30 minutes
5.0 540
[0079] As listed in Table 2, the film thickness of the surface
layer 1b is varied in the range of approximately 0.005 .mu.m to
approximately 5.0 .mu.m, depending on the variation in the time
period for the cathodic reduction, and in this range, any of the
capacitors exhibits a lower leakage current than those in
Comparative Examples 1 and 2. Also, it has turned out that in the
case where the film thickness of the surface layer 1b falls within
the range of approximately 0.01 .mu.m to approximately 2.0 .mu.m,
the leakage current is particularly low.
[0080] In addition, the examples disclosed herein should be
considered exemplary in all aspects, and not as a limitation on the
present invention. The scope of the present invention is defined
not by the above description of the examples but by the appended
claims, and further includes meanings equivalent to the claims and
any modifications within the scope of the claims.
[0081] Also, in the above-described examples, the surface layer 1b
made of niobium was employed; however, the present invention is not
limited to this, but the surface layer 1b may contain another
element such as tantalum, titanium, or nitrogen, besides
niobium.
[0082] For example, in the examples, the surface layer 1b was
formed by the cathodic reduction performed in molten
LiF-NaF-KF-K2NbF6 systems; however, the present invention is not
limited to this, but another molten salt containing niobium such as
lithium fluoride-potassium fluoride-potassium hexafluoroniobate
(LiF-KF-K.sub.2NbF.sub.6) systems or lithium fluoride-potassium
hexafluoroniobate (LiF-K.sub.2NbF.sub.6) systems may be employed,
or alternatively the layer may be formed by another thin film
fabrication method. Also, after the fabrication of the surface
layer 1b by the use of one of above methods, a heat treatment may
be performed in a nitrogen atmosphere.
[0083] Further, in the above-described examples, the anode lead 1a
made of tantalum was employed; however, the present invention is
not limited to this, but the anode lead 1a may be comprised of, for
example, a tantalum alloy containing another metal, such as niobium
or titanium, or another element such as nitrogen, in addition to
tantalum.
[0084] Still further, in the above-described examples, the porous
sintered body made from niobium particles was employed as the base
body 1c; however, the present invention is not limited to this, but
the base body 1c may be comprised of a porous sintered body made
from, for example, niobium alloy particles containing another metal
such as tantalum or titanium, or another element such as nitrogen,
in addition to niobium.
[0085] Still further, in the above-described examples, the oxide
layer 2 was formed by anodizing the anode 1 in an aqueous solution;
however, the present invention is not limited to this, but the
oxide layer 2 may be formed by another method.
[0086] Still further, in the above-described examples, the
electrically conductive polymer layer 3 made of polypyrrole was
formed; however, the present invention is not limited to this, but
another electrically conductive polymer such as polythiophene or
polyaniline may be employed, or alternatively another electrically
conductive material such as manganese oxide may be employed.
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