Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
1999-05-19
2001-03-06
Kincaid, Kristine (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S308100, C361S301400, C501S019000
Reexamination Certificate
active
06198618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to conductive paste used for forming a thick film electrode and the like, and a ceramic electronic part comprising an electrode formed by using the conductive paste.
2. Description of the Related Art
A typical example of an electronic part having a structure in which an external electrode is provided on a ceramic element assembly is a monolithic ceramic capacitor as shown in FIG.
1
.
The monolithic ceramic capacitor comprises a ceramic element assembly (capacitor body)
13
formed by alternately laminating a ceramic layer
11
as a dielectric layer and an internal electrode
12
, and then sintering the resultant laminate; and external electrodes
14
arranged on both sides of the ceramic element assembly
13
so as to cause conduction with the internal electrodes
12
.
The internal electrodes
12
are formed by using a conductive paste which can be burned with ceramic and which contains a metal powder of Pd, Ag-Pd or the like as a conductive component.
The external electrodes
14
are formed by coating a conductive paste on the burned ceramic element assembly
13
and then burning the paste. As the conductive paste for forming the external electrodes (referred to as “the external electrode bodies” or “the thick film electrodes” hereinafter)
14
used has been a metal powder of Ag, Ag-Pd or the like, and zinc borosilicate glass frit dispersed in an organic vehicle.
In order to improve the heat resistance and solderability of the external electrodes, the surfaces of the external electrode bodies (the thick film electrodes) formed by baking the conductive paste are plated with nickel, tin or solder to form plating films of nickel, tin or solder on the thick film electrodes.
In order to prevent the occurrence of a tombstone phenomenon during mounting, a ceramic element assembly having small dimensions is often plated with solder to form solder plating films on the outermost layers of the external electrode bodies (the thick film electrodes).
The above-described monolithic ceramic capacitor is generally surface-mounted by soldering the external electrodes to a land of a wiring board. However, the solder plating film on the outermost layer is re-melted by the heat generated by soldering during mounting, and at the same time, the melted solder plating film is scattered due to bumping (violent escape) of the moisture trapped in the external electrode bodies during the plating step in the formation of the external electrodes, thereby causing a short-circuit with adjacent mounted electronic parts in some circumstances. The above-described problem is true for not only the case in which the outermost layer is plated with solder, but also the case of a ceramic electronic part in which a plating film of any of various low melting point materials such as tin and the like is formed.
SUMMARY OF THE INVENTION
The present invention has been achieved for solving the above problem, and an object of the present invention is to provide conductive paste which can prevent scattering of a plating film, such as a solder plating film, which is formed on a thick film electrode, during surface mounting of a ceramic electronic part comprising the thick film electrode (an external electrode or the like) formed by using the conductive paste, and a ceramic electronic part manufactured by using the conductive paste.
One preferred embodiment of the present invention provides a conductive paste, comprising: a metal powder, a glass frit and SiO
2
dispersed in an organic vehicle where the compounding ratio of SiO
2
is in the range of about 0.3 to 1.5% by weight.
The about 0.3 to 1.5% by weight of SiO
2
is mixed with the conductive paste containing the metal powder and the glass frit, which are dispersed in the organic vehicle, so that the vacancies (in which moisture is trapped during plating) produced in the thick film electrode formed by baking the conductive paste are filled with a single SiO
2
material or a complex of SiO
2
and glass frit, thereby suppressing the entrance of moisture in the thick film electrode. As a result, during surface mounting of a ceramic electronic part comprising the thick film electrode (the external electrode or the like) formed by using the conductive paste, moisture bumping in the thick film electrode due to the heat generated during mounting can be suppressed, and thus scattering of the plating film such as the solder plating film formed on the thick film electrode can be suppressed and prevented.
In the above described conductive paste, the reason why the compounding ratio of SiO
2
is about 0.3 to 1.5% by weight is that with a SiO
2
compounding ratio of less than about 0.3% by weight, the effect of filling the vacancies produced in the thick film electrode, which is formed by baking the conductive paste, becomes insufficient, while with a SiO
2
compounding ratio of over about 1.5% by weight, it is undesirably difficult to increase the thickness of the plating film, for example, when a Ni plating film is formed on the thick film electrode formed by baking the conductive paste.
The compounding ratio of SiO
2
is more preferably about 0.5 to 1.2% by weight.
In the above described conductive paste, the average particle diameter of SiO
2
is preferably about 0.1 &mgr;m or less.
By using SiO
2
having an average particle diameter of about 0.1 &mgr;m or less, it is possible to securely fill the vacancies of the thick film electrode, which is formed on a predetermined object such as an electronic part or the like by coating and baking, and suppress trapping of moisture in the thick film electrode. The reason why the average particle diameter of SiO
2
is about 0.1 &mgr;m or less is that with an average particle diameter of over about 0.1 &mgr;m, little SiO
2
undesirably enters the vacancies.
SiO
2
having an average particle diameter of about 0.02 to 0.05 &mgr;m is more preferably used.
In the above described conductive paste, a glass frit having a glass transition point of about 300 to 600° C. is employed as the glass frit.
The reason why glass frit having a glass transition point of about 300 to 600° C. is used as the glass frit is that the use of a glass frit having a glass transition point of less than about 300° C. causes over-sintering of the electrode and deterioration in mechanical strength, while the use of glass frit having a glass transition point of over about 600° C. possibly causes insufficient sintering of the electrode and deterioration in reliability. The use of a glass frit having the glass transition point of about 300 to 600° C. permits more secure filling of the vacancies produced in the thick film electrode with a complex of SiO
2
and the glass frit, making the present invention more effective.
In the above described conductive paste, zinc borosilicate glass frit may be used as the glass frit. The use of zinc borosilicate glass frit as the glass frit permits efficient filling of the vacancies produced in the thick film electrode, which is formed on a predetermined object such as an electronic part or the like by coating and baking, without deteriorating sinterability, thereby suppressing the entrance of moisture in the thick film electrode, and making the present invention more effective.
Another preferred embodiment of the present invention provides a ceramic electronic comprising: a ceramic element assembly; and an external electrode comprising a thick film electrode provided on the ceramic element assembly; wherein the external electrode is formed by plating the thick film electrode provided by coating and baking the above described conductive paste.
In the ceramic electronic part comprising the external electrode formed by plating the thick film electrode, which is formed by coating and baking the conductive paste of the present invention, the vacancies produced in the thick film electrode are filled with a SiO
2
single material or a complex of SiO
2
and glass frit, suppressing the entrance of moisture in the thick film electrode. As a result, it is possible to suppr
Fujiyama Masaki
Ohtani Akira
Kincaid Kristine
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Thomas Eric W.
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