Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2000-07-21
2001-10-30
Dinkins, Anthony (Department: 2835)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S309000, C361S321200, C029S025410
Reexamination Certificate
active
06310757
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electronic component having external electrodes and method for the manufacture thereof.
BACKGROUND OF THE INVENTION
Chip-shaped electronic components such as multilayer ceramic capacitors or chip resistors are widely used as electronic elements for printed circuit boards or multilayer substrates.
Multilayer ceramic capacitors are typically manufactured by employing following steps: first, conductive paste is applied to green sheets of dielectric ceramic material to form an internal electrodes pattern, the green sheets having internal electrodes pattern are stacked and compressed to form a laminated body and this laminated body is then diced into separate capacitor elements in such a manner that successive internal electrodes are exposed at opposite end portions of the capacitor elements, after which external electrodes are formed on the opposite end portions of the capacitor elements.
When manufacturing chip resistors, first, resistor films such as metal films are disposed on ceramic substrate to form resistor bodies, the surfaces thereof are coated with protecting films and external electrodes are then formed on opposite end portions of the resistor bodies.
These external electrodes are soldered onto solder lands of the printed circuit board in order to connect the electronic components to outer circuit system. And, it is preferred that the external electrodes have heat resistance in order to prevent solder leaching and also solder wettability. These requirements can be fulfilled by using a plurality of layers composed of different metals as external electrodes. Therefore, when forming external electrodes on chip-shaped electronic components such as multilayer ceramic capacitor, chip resistor, or the like, a plurality of electrode layers composed of different materials are disposed on opposite end portions of the electronic components.
As to the multilayer ceramic capacitors, a conductive layer may be formed by applying conductive paste on the opposite end portions of sintered capacitor elements and baking the conductive paste. Or, a conductive layer can be formed by applying conductive paste on the opposite portions of unsintered capacitor elements and co-sintering them. The conductive paste is prepared by mixing an oxidizable metal such as Ni powder with a resin. The surfaces of the metal powder included in the conductive paste are usually oxidized and especially during the baking, they become more seriously oxidized due to the high temperature such that they cannot be electrolytically plated. Therefore, electroless Cu plating or electroless Ni plating has been used in order to form the heat resisting layer and then electrolytic plating is performed on the heat resisting layer.
The electroless Cu plating is performed by immersing an object into a solution containing a reducing agent and a complexing agent. An aqueous solution of sodium hydroxide containing formalin as a reducing agent and ethylenediaminetetraacetic acid (EDTA) as a complexing agent is usually used as the solution for the electroless Cu plating, which causes the production of sodium formate as well as Cu metal during the electroless plating process. Therefore, the treatment of wastewater containing formalin and sodium formate is needed and costs for pollution control systems are incurred.
Further, as nonconductive materials such as plastics can also be plated by electroless plating, electroless plated layers may be formed on the ceramic surfaces as well as on the conductive layers, which is called over-grow, and, when electronic components are soldered onto the printed circuit board having compact wiring patterns, the printed circuit board can be short-circuited by the electroless plated layers on the ceramic surfaces of the electronic elements.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an electronic component having external electrodes and a method for manufacturing thereof for which treatment of the wastewater is not needed.
It is another object of the present invention to provide an electronic component having external electrodes which have electrolytically plated layers plated only on the conductive layer but not on the ceramic surfaces and a method for manufacturing thereof.
It is still another object of the present invention to provide an electronic component having external electrodes and a method for manufacturing thereof which can be soldered onto a printed circuit board having compact wiring.
In accordance with a preferred embodiment of the present invention, there is provided an electronic component having external electrodes, which comprises:
a first layer including an oxidizable metal as a major component;
a second layer disposed on the first layer and including a metal which has smaller ionization tendency than the oxidizable metal; and
at least one electrolytically plated layer including a heat resisting layer and a solder wettable layer.
The first layer may be a film formed by baking and including the oxidizable metal. Preferably, the oxidizable metal is selected from the group consisting of copper and base metal, and, more preferably, nickel, and the like. The metal included in the second layer may be selected from the group consisting of gold, silver, platinum, copper and palladium, and the like, and, more preferably, palladium. Preferably, the heat resisting layer is an electrolytic Ni plated layer and the solder wettable layer is a solder plated layer.
The external electrode may further comprise a fourth layer, preferably Cu layer, which is electrolytically plated on the second layer and softer than the heat resisting layer.
In accordance with another preferred embodiment of the present invention, there is provided a method for manufacturing an electronic component having external electrodes, which comprises the steps of:
forming a first layer on a body of the electronic component, the first layer including an oxidizable metal as a major component;
forming a second layer on the first layer by utilizing cations of a metal having a smaller ionization tendency than the oxidizable metal; and
forming at least one electrolytically plated layer having a heat resisting layer and a solder wettable layer.
Preferably, the method further comprises a step of pretreating the first layer in order to remove oxidants from the surface thereof before the second layer forming step.
REFERENCES:
patent: 5426560 (1995-06-01), Amaya et al.
patent: 6002577 (1999-12-01), Wada et al.
patent: 6071800 (2000-06-01), Shigemoto et al.
Kawamura Sachiko
Tuzuki Kiyoshi
Dinkins Anthony
Rosenman & Colin, LLP.
Taiyo Chemical Industry Co., Ltd.
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