Capacitor element with thick cathode layer

Details Capacitor element with thick cathode layer Capacitor element with thick cathode layer

2003-05-07

2004-10-05

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Liquid electrolytic capacitor

C361S523000, C361S525000, C361S528000, C361S529000, C361S530000, C029S025030

Reexamination Certificate

active

06801423

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a capacitor element which is less likely to break due to heat as compared with prior art capacitor elements. The present invention also relates to a method of making such a capacitor element.
2. Description of the Related Art
FIG. 9
illustrates an example of prior art solid electrolytic capacitor. The capacitor includes a resin package
90
and a capacitor element
91
sealed in the package. The capacitor element
91
includes a porous sintered body
91
A, an anode wire
91
B partially embedded in the sintered body
91
A, and a cathode layer
91
C formed on the sintered body
91
A. As shown in the figure, the cathode layer
91
C is connected to a cathode lead
02
, whereas the anode wire
91
B is connected to an anode lead
93
. The cathode lead
92
and the anode lead
93
include portions
92
a
,
93
a
sealed in the package
90
and portions
92
b
,
93
b
projecting outward from the package
90
. The outer portions
92
b
,
93
b
extend downwardly along the side surfaces of the package
90
and then extend horizontally along the bottom surface of the package
90
. With such a structure, the capacitor
9
can be surface-mounted on a circuit board by soldering, for example.
In the prior art capacitor
9
, the cathode layer
91
C has a relatively small thickness in the range of 20-30 &mgr;m, for example. However, such a small thickness causes the following problems.
As methods for mounting the capacitor
9
on a circuit board, there exist flow soldering and reflow soldering. In these methods, the capacitor
9
is heated in melting the applied solder material, so that the package
90
and the cathode layer
91
C (and other components) undergo thermal expansion. However, since cathode layer
91
C differs from the package
90
differs in thermal expansion coefficient, stress is applied to the cathode layer
91
C. As a result, the cathode layer
91
C which has a relatively small thickness may be broken. Particularly, portions
91
c
indicated in
FIG. 9
(which correspond to corner portions of the sintered body
91
A) are further thinner than other portions of the cathode layer
91
C. Therefore, there is much possibility that the thin-walled portions
91
c
break during the flow soldering (or reflow soldering). Further, due to such breakage, the cathode layer
91
C becomes likely to be removed from the sintered body
91
A.
The above-described defect of the cathode layer
91
C is not preferable, because it causes an increase in impedance and dielectric loss of the capacitor element
91
. Further, when such defect is serious, the intended capacitor characteristics cannot be obtained at all.
SUMMARY OF THE INVENTION
An object of the present invention, which is conceived under the circumstances described above, is to provide a capacitor element capable of preventing the cathode layer from being broken or removed during the soldering.
According to a first aspect of the present invention, there is provided a capacitor element comprising a porous member made of valve metal powder, an anode wire projecting from the porous member, and a cathode layer formed on the porous member. The cathode layer has a thickness of no less than 35 &mgr;m. More preferably, the thickness of the cathode layer is no less than 40 &mgr;m.
Preferably, the impedance after flow soldering is no more than twice the impedance before the flow soldering. More preferably, the impedance after flow soldering is no more than 1.5 times the impedance before the flow soldering.
Preferably, the cathode layer is formed by heating conductor paste applied on the porous member.
Preferably, the conductor paste contains conductor powder, binder and solvent.
Preferably, the cathode layer is formed by repetitively performing a process which comprises applying conductor paste on the porous member and heating the conductor paste.
According to a second aspect of the present invention, there is provided a capacitor element comprising a porous member made of valve metal powder, an anode wire projecting from the porous member, a cathode layer formed on the porous member, an anode lead connected to the anode wire, a cathode lead connected to the cathode layer, and a resin package sealing the porous member, the anode wire and the cathode layer. The cathode layer has a thickness of no less than 35 &mgr;m.
According to a third aspect of the present invention, there is provided a method of making a capacitor element comprising the steps of preparing a porous member of valve metal powder, and heating conductor paste applied on the porous member to form a cathode layer. The cathode layer is formed to have a thickness of no less than 35 &mgr;m.
Preferably, the thickness of the cathode layer is controlled by adjusting the viscosity of the conductor paste.
Other objects, features and advantages of the present invention will become clearer from the description of the preferred embodiment given below.


REFERENCES:
patent: 4935844 (1990-06-01), Burn
patent: 5179507 (1993-01-01), Iijima
patent: 5734546 (1998-03-01), Kuriyama et al.
patent: 6423104 (2002-07-01), Omori et al.

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