Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2001-07-11
2003-07-22
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S209000, C174S258000
Reexamination Certificate
active
06596382
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multilayered board and to a method for fabricating the same. More particularly, the invention relates to a multilayered board using a glass-containing material which can be fired at low temperatures, and to a method for fabricating the same.
2. Description of the Related Art
As the level of functionality of electronic components is increased and the size of electronic components is reduced, more and more semiconductor ICs and other surface-mounting devices are required be mounted on circuit boards. In order to meet such requirements, various types of multilayered ceramic boards, which are fabricated by laminating ceramic green sheets with applied electrode paste for forming electrodes, followed by firing, have been developed and widely used.
Recently, multilayered boards (low-temperature-fired multilayered boards) have been developed using glass-containing materials, such as glass-ceramic materials, which can be fired at low temperatures.
When a multilayered board is fabricated, an Ag paste containing Ag powder as a conductive component, for example, is widely used as the electrode paste for forming electrodes.
However, when an Ag paste comprising Ag powder, or Ag powder and an oxide serving as a chemical bonding agent, is applied to a board, in which a large amount of ceramic is included in order to increase the strength, and firing is performed simultaneously, if the glass component content is low in the ceramic layer and in the electrodes, it is difficult to obtain satisfactory electrode bonding strength.
The electrode bonding strength may be increased by adding a glass component to the Ag paste so as to improve its wettability on the board, or by including a large amount of a glass component in the board. However, in such cases, the glass component floats on the surfaces of the electrodes, resulting in a decrease in the solderability of the electrodes or in the flexural strength of the board.
SUMMARY OF THE INVENTION
The present invention provides a multilayered board which has a high bonding strength between the electrodes and the glass-containing insulating layers, satisfactory solderability, and a high flexural strength, and provides a method for fabricating the same.
In one aspect of the present invention, a multilayered board includes a laminate including a plurality of glass-containing insulating layers, each glass-containing insulating layer being provided with an electrode on the surface thereof. The glass-containing insulating layer is formed by firing a layer containing 60% by volume or less of a glass component before firing, a portion of the glass component is segregated in the surface region of the glass-containing insulating layer by firing, and the electrode is bonded to the surface of the glass-containing insulating layer by means of the segregated glass component.
In the multilayered board of the present invention, since the glass-containing insulating layer before firing has a glass component content of 60% by volume or less, and the electrode is bonded to the surface of the glass-containing insulating layer by means of the glass component which has been segregated in the surface region by firing, the multilayered board does not have an excessively high glass component content in the glass-containing insulating layer so as to decrease its flexural strength, and it does not have an excessively high glass component content in the electrode so as to decrease its solderability, whereby a high electrode bonding strength can be secured.
“The glass component content of 60% by volume or less before firing” conceptually means that the glass component content in the inorganic component constituting a material for forming the glass-containing insulating layer before firing (green sheet) is 60% by volume or less.
Preferably, in the multilayered board of the present invention, the glass component content is in the range of 60% to 90% by volume within a depth of 1 &mgr;m from the surface of the glass-containing insulating layer provided with the electrode.
By segregating the glass component by firing and by increasing the glass component content within a depth of 1 &mgr;m from the surface of the glass-containing insulating layer to 60% to 90% by volume, it is possible to improve the electrode bonding strength even if the glass component content in the entire glass-containing insulating layer is not increased. Consequently, it is possible to prevent the glass component content in the entire glass-containing insulating layer from increasing excessively, and a high flexural strength can be maintained, thus ensuring the effectiveness of the present invention.
Preferably, the glass-containing insulating layer is composed of a glass-ceramic material.
In the present invention, although it is possible to use materials in which glass and various other insulating components are mixed as the glass-containing insulating layer, in order to attain various electrical characteristics and mechanical characteristics, a glass-ceramic material containing a glass and a ceramic is preferably used. In such a case, the advantage of the present invention is reliably obtained.
Preferably, the electrode contains Ag as a principal conductive component.
In the present invention, although it is possible to use various materials for the electrode, when a material containing Ag as a principal conductive component is used for the electrode, the electrode can have a low resistance and superior bonding strength, thus ensuring the effectiveness of the present invention.
Preferably, the electrode and the glass-containing insulating layer are simultaneously fired.
Although the electrode and the glass-containing insulating layer may be fired separately, by simultaneously firing, it is possible to improve the bonding strength between the electrode and the glass-containing insulating layer and also the fabrication process can be simplified.
In accordance with the present invention, the initial bonding strength of the electrode to the glass-containing insulating layer can be improved, and the initial bonding strength to the glass-containing insulating layer in a tensile test can be 5 N/mm
2
or more on average.
In accordance with the present invention, since the glass component content in the glass-containing insulating layer is not increased to such an extent that the mechanical strength of the glass-containing insulating layer is greatly decreased, the flexural strength which does not present a difficulty in practical use, namely, a flexural strength of 200 MPa or more, can be achieved.
In the multilayered board of the present invention, preferably, the glass component in the glass-containing insulating layer is a crystallized glass.
By using the crystallized glass as the glass component in the glass-containing insulating layer, the mechanical strength can be further improved, thus further ensuring the effectiveness of the present invention.
In another aspect of the present invention, a method for fabricating a multilayered board, in which a plurality of glass-containing insulating layers are laminated, each glass-containing insulating layer being provided with an electrode on the surface thereof, includes the steps of applying an electrode paste for forming electrodes to the surfaces of green sheets containing 60% by volume or less of a glass component; laminating and pressure-bonding the green sheets applied with the electrode paste to form a pressure-bonded laminate; and heat-treating the pressure-bonded laminate so that the green sheets and the electrode paste are simultaneously fired, a portion of the glass component is segregated in the surface regions of the glass-containing insulating layers, and the electrodes are bonded to the surfaces of the glass-containing insulating layers by means of the segregated glass component.
Since the method for fabricating the multilayered board of the present invention includes the steps described above, it is possible to reliably fabricate the multilayered board which has high e
Dickstein , Shapiro, Morin & Oshinsky, LLP
Lam Cathy
Murata Manufacturing Co. Ltd.
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