Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2002-04-08
2003-09-02
Dinkins, Anthony (Department: 2835)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S306100
Reexamination Certificate
active
06614641
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramic electronic components, and, more particularly, to a ceramic electronic component including a ceramic body, two-layered terminal electrodes, lead terminals and solder.
2. Description of the Related Art
A conventional ceramic electronic component of such a type will be described with reference to
FIGS. 1A and 1B
. As shown in the drawing, a ceramic electronic component
11
includes a ceramic body
12
mainly composed of a dielectric material, insulator, semiconductor, piezoelectric material, magnetic material or the like; terminal electrodes
13
formed on both principal surfaces of the ceramic body
12
; lead terminals
15
joined to the corresponding terminal electrodes
13
with solder
14
; and an outer resin section
16
which is formed so as to cover the ceramic body
12
, the terminal electrodes
13
, the solder
14
and parts of the lead terminals
15
.
The terminal electrodes
13
include thin films or thick films formed by sputtering or vapor deposition and plating films containing, for example, a noble metal, such as Ag or Pd, or an alloy thereof, or a base metal, such as Ni or Cu, or an alloy thereof, as a conductive constituent. As the solder
14
, an alloy containing Sn/Pb as a principal constituent is commonly used. The lead terminal
15
, for example, includes a core composed of Cu, Fe or the like, which is coated by Sn-Pb or Sn plating.
However, when the ceramic electronic component in accordance with the conventional technique is exposed to a high temperature environment for a long period of time, intermetallic compounds are formed at the joint interfaces between the terminal electrode and the solder and between the terminal electrode and the ceramic body due to interdiffusion between Sn contained in the solder and the conductive constituent of the terminal electrode mainly composed of Ag, Cu, Ni or the like. Since the intermetallic compound is rigid and brittle, bonding reliability is decreased and separation occurs between the ceramic body and the terminal electrode when the entire terminal electrode is transformed into the intermetallic compound, resulting in degradation in electrical characteristics.
When Sn/Pb solder is used, a Pb-rich phase is generated at the interface between the intermetallic compound and the solder due to the diffusion of Sn. If the ceramic electronic component has a structure in which the soft Pb-rich phase and the rigid, brittle intermetallic compound are brought into contact with each other, cracking occurs in the terminal electrode, etc., when stress is applied, resulting in degradation in bonding reliability.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a ceramic electronic component in which stable electrical characteristics are ensured even if exposed to a high temperature environment for a long period of time and which has satisfactory bonding strength between the ceramic body and terminal electrodes and between the terminal electrodes and lead terminals.
In one aspect of the present invention, a ceramic electronic component includes a ceramic body, terminal electrodes formed on the ceramic body, and lead terminals joined to the terminal electrodes with solder containing Sn. Each terminal electrode includes a first electrode layer formed on the ceramic body and a second electrode layer formed on the first electrode layer, and the second electrode layer contains a conductive constituent containing at least Zn, Ag and/or Cu, and Sn. The Zn content in the second electrode layer is about 4% by weight or more in relation to 100% by weight of the conductive constituent, and is within the solubility limit so that it does not form AgZn and/or CuZn intermetallic compounds.
In the ceramic electronic component in which the lead terminal is joined to the second electrode layer with the solder, AgZn and/or CuZn intermetallic compounds must be prevented from being formed after soldering in order to achieve the object of the present invention. Incidentally, the solubility limit for not forming AgZn and/or CuZn intermetallic compounds are not determined by the Zn content directly and exclusively. It is determined by the cooling rate from the molten state to the solid state and the solid-state temperature after cooling. In particular, when soldering is performed as in the case of the present invention, since the cooling rate is fast, the nonequilibrium state is brought about, and it is not possible to numerically express the solubility limit using the Zn content. Therefore, the Zn content is defined as being within the solubility limit which does not form AgZn and/or CuZn intermetallic compounds.
In another aspect of the present invention, a ceramic electronic component includes a ceramic body, terminal electrodes formed on the ceramic body, and lead terminals joined to the terminal electrodes with solder containing Sn. Each terminal electrode includes a first electrode layer formed on the ceramic body and a second electrode layer formed on the first electrode layer, and a barrier layer is formed in the first electrode layer and/or the second electrode layer due to the flow and concentration of Zn in the second electrode layer.
Preferably, the barrier layer lies in the first electrode layer and is formed within a range of about 2 &mgr;m from the interface with the ceramic body.
Preferably, the first electrode layer is composed of a thin film or a thick film and contains a conductive constituent containing at least Ag and/or Cu.
Preferably, the lead terminals are coated with an alloy which does not contain Pb apart from incidental impurities or a metal other than Pb, and the solder does not contain Pb apart from incidental impurities.
Preferably, the second electrode layer is formed by hot dipping.
A ceramic electronic component of the present invention includes two-layered terminal electrodes, each including a first electrode layer formed on a ceramic body and a second electrode layer formed on the first electrode layer. The second electrode layer contains a conductive constituent containing at least Zn, Ag and/or Cu, and Sn.
When the ceramic electronic component is subjected to thermal aging after the second electrode layer is provided on the ceramic electronic component, Zn in the second electrode layer is concentrated in the vicinity of the interface with the first electrode layer to form a barrier layer. The barrier layer flows into the first electrode layer as thermal aging advances, and then reaches the vicinity of the interface between the first electrode layer and the ceramic body. Since the barrier layer suppresses interdiffusion between Sn contained in the solder formed on the second electrode layer and the parent metal in the first electrode layer, intermetallic compounds are inhibited from being formed at the interface between the second electrode layer and the solder and at the interface between the ceramic body and the first electrode layer, and thus separation between the terminal electrode and the solder and separation between the ceramic body and the terminal electrode are prevented. Therefore, stable electrical characteristics are ensured even if exposed to a high temperature environment for a long period of time and satisfactory bonding strength is obtained between the ceramic body and the terminal electrodes and between the terminal electrodes and the lead terminals. Additionally, the barrier layer is not necessarily required to be formed in the entire region or to the same level in the second electrode layer or the first electrode layer, and the barrier layer may be formed substantially as a film of varying thickness so that interdiffusion with the parent metal in the first electrode layer is inhibited.
The Zn content in the second electrode layer must be about 4% by weight or more in relation to 100% by weight of the conductive constituent constituting the second electrode layer, and must be within the solubility limit not forming AgZn and/or CuZn intermetallic compounds. If the Zn content is a
Hamada Kunihiko
Kayatani Takayuki
Takaoka Hidekiyo
Dickstein Shapiro Morin & Oshinsky LLP.
Dinkins Anthony
Murata Manufacturing Co. Ltd
Thomas Eric
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