Electronic component

Details Electronic component Electronic component

1999-12-14

2001-07-10

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S306100, C361S321200

Reexamination Certificate

active

06259593

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electronic components, and in particular to an improvement in structure in relation to an external electrode formed on an electronic component body.
2. Description of the Related Art
FIG. 1
is a cross-sectional view of a monolithic ceramic capacitor
1
as an exemplary conventional electronic component which is of interest in the present invention.
The monolithic ceramic capacitor
1
is provided with a chip capacitor body
2
as an electronic component body, and thick external electrodes
3
are formed at two opposing ends of the capacitor body
2
. In the formation of the external electrodes
3
, a paste containing a metal such as copper or silver as a conductive component is applied to predetermined positions of the capacitor body
2
and then is baked. In the interior of the capacitor body
2
, a plurality of internal electrodes
4
arc layered and each of the internal electrodes
4
is electrically connected to one of the external electrodes
3
.
A barrier layer
5
composed of nickel is formed on each external electrode
3
, and then an outer layer
6
composed of tin or solder is formed on the barrier layer
5
.
If the barrier layer
5
is not formed, undesirable solid-phase diffusion will occur between the external electrode
3
and the solder used for soldering the monolithic ceramic capacitor
1
onto a circuit board (not shown in the drawing) or between the external electrode
3
and the external layer
6
when the monolithic ceramic capacitor
1
is used at high temperatures. In addition, liquid-phase diffusion also may occur when the solder is in a melted state. The barrier layer
5
prevents the solid-phase diffusion. The outer layer
6
is to improve solderability when the external electrode
3
is soldered to a predetermined conductive land on the circuit board.
In such a monolithic ceramic capacitor
1
, however, the formation of the barrier layer
5
will cause other problems in some cases, as described below.
The barrier layer
5
is generally formed by wet plating. Thus, the capacitor body
2
provided with the external electrodes
3
must be dipped into a plating solution for the formation of the barrier layer
5
. This will result in reduction of the ceramic component of the capacitor body
2
, decrease in strength, deterioration of electrical characteristics or, in the worst case, interlayer separation due to penetration of the plating solution into the internal electrodes
4
.
Also, the outer layer
6
is generally formed by wet plating and the plating solution in this case will also result in similar problems. Regarding this, it is known that a plating solution used in plating of tin noticeably results in the above problems.
A means is desired to avoid solid-phase diffusion in the external electrodes
3
without the formation of the barrier layer
5
, which requires plating.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an electronic component satisfying the above requirements.
An electronic component in accordance with the present invention includes an electronic component body and external electrodes formed on the electronic component body. In order to solve the above technical problems, the invention is characterized in that a coating layer including a solder primarily composed of tin and containing the same metal contained in the external electrodes in an amount greater than the eutectic concentration is formed on each of the external electrodes. The solder is preferably lead free.
In the present invention, when the external electrodes contain copper, the solder constituting the coating layer is controlled to contain more than 0.7 percent by weight of copper, which is the eutectic concentration of the copper. More preferably, the solder contains more than about 1 percent by weight of copper. The upper limit of the copper content in the solder is preferably about 5 percent by weight.
In another aspect of the present invention, when the external electrodes contain silver, the solder constituting the coating layer is controlled to contain more than 3.5 percent by weight of silver, which is the eutectic concentration of the silver. More preferably, the solder contains more than about 5 percent by weight of silver. The upper limit of the silver content in the solder is preferably about 15 percent by weight.
In the present invention, the solder preferably contains a trace amount, that is, about 1 percent by weight or less of zinc.
In the present invention, it is preferable that the thickness of the external electrodes be greater than the thickness of the solid-phase diffused layer of the solder formable under temperature conditions to which the electronic component will be subjected, such as temperature conditions applied when the coating layer is formed.
In the present invention, the coating layer is preferably formed by a melted solder dipping process or a solder reflow process.
The present invention is advantageous when the electronic component body comprises ceramic, and is more advantageous when internal electrodes electrically connected to the external electrodes are formed in the interior of the electronic component body, such as a monolithic ceramic capacitor.
The electronic component in accordance with the present invention may further include terminal members coupled with the external electrodes via the coating layers.


REFERENCES:
patent: 5410184 (1995-04-01), Melton et al.
patent: 5489803 (1996-02-01), Kanbe et al.
patent: 0831683 A1 (1998-03-01), None
patent: 0858859 A1 (1998-08-01), None

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