Solid electrolytic capacitor and manufacturing method thereof

Details Solid electrolytic capacitor and manufacturing method thereof Solid electrolytic capacitor and manufacturing method thereof

1999-05-13

2001-04-10

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Solid electrolytic capacitor

C361S528000, C029S025030

Reexamination Certificate

active

06215652

ABSTRACT:

BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a solid electrolytic capacitor in which niobium is used as an anode (hereinafter referred to as “the niobium solid electrolytic capacitor”), and more specifically, it relates to a solid electrolytic capacitor in which the change of a capacitance before and after a reflow step can be restrained, and a manufacturing method of the solid electrolytic capacitor.
(ii) Description of the Prior Art
A solid electrolytic capacitor generally comprises an oxide layer as a dielectric layer on the surface of a valve action metal sinter, a cathode layer formed thereon, and a cathode outgoing lead extending from the cathode layer.
Here, the above valve action metal sinter can be obtained by compressing/molding a valve action metal powder, and then heating the thus molded article at a high temperature in vacuum. On the other hand, the oxide layer can be formed by anodization.
The valve action metal means a metal having a valve action, and examples of such a valve action metal include aluminum, titanium, zirconium, niobium, hafnium and tantalum. Among them, the practical metals are limited to aluminum and tantalum at present. Aluminum is usually used in the form of an etched aluminum foil as an anode. Therefore, it is no exaggeration that the solid electrolytic capacitor using the valve action metal sinter is limited to a solid electrolytic capacitor alone in which tantalum is used as the anode (hereinafter referred to as “the tantalum solid electrolytic capacitor”).
The reason why the valve action metal is limited to tantalum alone is that its capacitance is stable. For example, in the case of the niobium solid electrolytic capacitor, the oxide layer of the dielectric layer is much more easily affected by heat as compared with the case of the above tantalum solid electrolytic capacitor, so that the capacitance easily changes. Particularly in a reflow step of parts, heating is given at a temperature of 200 to 260° C. for a period of several seconds to about 10 seconds, but this heating causes the capacitance of the niobium solid electrolytic capacitor to change. For this reason, the niobium solid electrolytic capacitor cannot attain a practical level.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a niobium solid electrolytic capacitor in which the change of a capacitance before and after a reflow step during the mounting of parts can be restrained.
Another object of the present invention is to provide a method for manufacturing this niobium solid electrolytic capacitor.
The first aspect of the present invention is directed to a solid electrolytic capacitor in which a dielectric layer formed on the surface of an anode obtained by molding and then sintering a niobium metal powder comprises a niobium oxide layer and a niobium nitride region.
The second aspect of the present invention is directed to a method for manufacturing a solid electrolytic capacitor which comprises a step of molding and sintering a niobium metal powder to form an anode, and then subjecting the surface of this anode to a nitriding treatment, and a step of anodizing the thus nitrided anode to form a dielectric layer comprising a niobium oxide layer and a niobium nitride region.
According to the present invention, it is possible to remarkably restrain the fluctuation of a capacitance in a reflow step during the mounting of parts.
The present inventors can presume this reason as follows. That is to say, in a heating step such as the reflow step, oxygen diffusion occurs from the niobium oxide layer to the base side of the niobium anode, so that the capacitance increases. In this heating step, however, nitrogen contained in the dielectric layer does not diffuse so much as oxygen. In consequence, a relative ratio of the nitride in the dielectric layer increases. Here, a dielectric constant of niobium nitride is lower as compared with that of niobium oxide, and therefore, the increase of the relative ratio of the nitride leads to the effect of lowering the capacitance. As described above, the present inventors can suppose that the increase of the capacitance by the diffusion of oxygen is offset by the increase of the relative ratio of the nitride having a low dielectric constant.


REFERENCES:
patent: S52-039164 (1977-03-01), None
patent: H03-069108 (1991-03-01), None
patent: 4-69914 (1992-03-01), None
patent: H07-508618 (1995-09-01), None
patent: WO98/38660 (1998-09-01), None
WO94/25971, published Nov. 10, 1994.

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