Electricity: electrical systems and devices – Electrolytic systems or devices – Solid electrolytic capacitor
Reexamination Certificate
2001-11-27
2003-12-09
Nguyen, Ha Tran (Department: 2812)
Electricity: electrical systems and devices
Electrolytic systems or devices
Solid electrolytic capacitor
C361S531000, C361S533000, C361S537000, C361S538000, C361S540000, C029S025030
Reexamination Certificate
active
06661645
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a solid electrolytic capacitor and to a manufacturing method thereof. More particularly, the present invention relates to a solid electrolytic capacitor comprising a capacitor element having a substrate comprising a valve-acting metal having a dielectric film on the surface thereof and a solid electrolyte layer on the substrate, the capacitor element having lead wires (lead frames). The solid electrolytic capacitor has excellent strength and heat resistance at the bonding portion between the capacitor element and the lead frames and is highly reliable.
BACKGROUND ART
To keep up with recent advancement of digitization or high frequency driving of electric equipment for attaining downsizing or electric power savings, demands for solid electrolytic capacitors having low impedance at high frequency range, high reliability and high capacitance are increasing.
Generally, a solid electrolytic capacitor has a basic structure that includes a plurality of single plate capacitor elements stacked one on another. Each single plate capacitor element has an etched valve-acting metal such as aluminum, tantalum or titanium, having a dielectric oxide film on the surface thereof. It also has a solid electrolyte layer comprising an organic substance layer such as a layer of an electroconductive polymer or an inorganic substance layer such as a layer of a metal oxide on the dielectric oxide film. Furthermore, it has an anode lead wire connected to an anode terminal of valve-acting metal (surface portion of end part where no solid electrolyte is provided) and on the other hand a cathode wire connected to an electroconducting part composed of a solid electrolyte (cathode part). The entire structure is sealed with an insulating resin such as an epoxy resin.
To manufacture a solid electrolytic capacitor having such a structure as described above and also having high reliability, the capacitor must have high strength and excellent heat resistance at the bonded portions between the capacitor element and the lead frames. In particular, chip-type capacitors which are surface mounted on an electronic circuit substrate are designed to have durability against heat at the reflow soldering by using a highly heat resistant material or by constructing the capacitor to enable relaxation of the thermal stress. These solid electrolytes have low resistance but are poor in the recovering activity of dielectric film. Accordingly, the dielectric film may occasionally undergo microscopic destructions due to the thermal stress or the like to increase the leakage current.
Some known structures for bonding between a capacitor element and a lead frame do not have always-sufficient heat resistance. For example, according to the method of JP-A-6-69084, a projecting metal plate is provided on the anode part of a stacked layer element, so that the element damages at the time of connecting to a lead frame can be reduced. According to the method of JP-A-9-320895, a lead frame is formed into a special shape so as to protect the element and then a stacked layer element is integrated therein. The examples shown in the figures of these patent applications have similar arrangement of an element to that in the present invention. However, the relationship in proper positioning between the lead frame and the element is not referred to and the effect thereof is not described. Furthermore, JP-A-10-144573 discloses a structure in which a projection is provided on the anode side of a lead frame and the element anode part is provided with a positioning part so as to be positioned to engage with the projection. The structure is essentially different from the present invention in that the anode part of capacitor element has a positioning part.
In the case of conventional solid electrolytic capacitors, when a lead frame composed of copper, a copper alloy or the like is bonded to the anode end part of a capacitor element, they are bonded with an electroconductive adhesive or mechanically connected by bending and caulking the terminals. Alternatively, they are bonded by welding with a lead based solder material, laser welding or the like. However, the bonding method using an electroconductive adhesive takes a long time for applying the adhesive. In particular, when a number of single plate capacitor elements are stacked and bonded, the working is very cumbersome. The mechanical bonding method by caulking the connected parts of lead frame is not suitable for those having small bonding parts and results in unstable bonding. Furthermore, in the case of welding with a lead based solder material, there is a fear that excessive lead removed from the welded part would cause problems such as environmental pollution. The bonding method by laser welding has the problem of increased costs and so forth.
In addition to these bonding methods, resistance welding of a terminal of capacitor element to a lead frame is known (JP-A-3-188614). This is to perform resistance welding using exclusively an iron nickel alloy (42 alloy) as the lead frame material. In addition, in the case where aluminum foil is used as the valve-acting metal of the capacitor element, the lead frame composed of copper, copper alloy or the like cannot be bonded by simple resistance welding. This is because resistance welding is a bonding method in which the metal at the part to be welded is molten for welding by heat generation (Joule heat) due to electric resistance and aluminum, copper, copper alloy and the like materials having high electroconductivity have low resistance so that they generate less heat. In addition because of good heat conductivity, the part to be bonded can be molten only insufficiently so that it is difficult to weld these materials.
Furthermore, among conventional solid electrolytic capacitors, those having a capacitor element bonded to a lead frame that has plating over the entire surface thereof are also known. However, when the lead frame is plated on its entire surface, superposed on the capacitor element and heat treated, it may be resulted that the plating metal is molten not only in the portion to be bonded with the capacitor element but also in the portion to be contact with the mold resin and a defect called solder ball will occur. The known structure to avoid such an inconvenience is obtained by a method of plating a copper substrate of a lead frame on the entire surface thereof with solder, removing the plating where the mold resin contacts when sealing therewith to expose the copper substrate, roughening the exposed surface, and then mounting a capacitor element on the roughened surface and bonding it to the surface (JP-A-5-21290). However, the method has problems that the amount of plating on the bonding part of the capacitor element is insufficient and that the bonding strength is low.
OBJECT OF THE INVENTION
The present invention is intended to provide a solid electrolyte capacitor and manufacturing method thereof free of the above-mentioned problems encountered in the conventional technology.
In order to reduce impedance, the contact area between a capacitor element and the cathode part of a lead frame (cathode-side lead frame) may be made as large as possible. However, this causes an increase in the leakage current after the reflow soldering or the like. The contact area is made large for the purpose of reducing the resistance on the cathode part of a capacitor element as much as possible and in addition, for protecting the element from mechanical or thermal stress at the time of lamination of elements, anodic welding or armoring with resin.
Despite these effects, if the cathode-side end corner part of a lead frame is present in the vicinity of the boundary between the cathode part and the insulating part of an element, there is a risk that stress concentration may occur in the vicinity of the boundary due to bending stress to rupture the dielectric film. Furthermore, silver paste used for bonding the lead frame to the element may enter from the boundary between the insulating part and the cathode electr
Ichimura Takashi
Kawamura Naoki
Nitoh Hiroshi
Sakai Atsushi
Nguyen Ha Tran
Showa Denko K.K.
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