Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
1999-10-25
2002-01-15
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S901000, C174S251000, C252S514000, C252S515000
Reexamination Certificate
active
06338893
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a conductive paste used for a ceramic printed circuit substrate formed of a glass ceramic and to a ceramic printed circuit substrate that uses a conductive paste as material for a printed circuit.
2. Description of the Related Art
Given recent trends toward increasingly compact and higher performance electronic apparatus, it is apparent that demands for further compactness and further improvement in electric characteristics of electronic components and printed circuit substrates will become more severe. Conventionally, a sequential lamination process and a simultaneous lamination process have been employed in order to attain compactness of electronic components and printed circuit substrates. The sequential lamination process repeats a step of printing circuits on an insulation layer and a subsequent firing step. According to the simultaneous lamination process, ceramic green sheets on which circuits are printed by use of a conductive paste are laminated and are then fired simultaneously.
The simultaneous lamination process, which involves simultaneous firing, uses a ceramic material, such as alumina or lead titanate, as an insulator. Since a firing temperature for such a ceramic material is 1000° C. or higher, a wiring material must be a metal having a melting point not lower than 1000° C., such as tungsten or palladium. These high melting point metals have a high electrical resistivity and thus involve a drawback in that an electric signal flowing through a conductor of such a metal suffers a large loss (so-called conductor loss). As a result, electronic components and printed circuit substrates using these materials encounter difficulty in satisfying market demands for electric characteristics.
In order to meet market demands that are becoming increasingly severe, there have been developed various kinds of electronic components and printed circuit substrates that use a low firing temperature material, which can be fired at a temperature not higher than 1000° C., and that allow use of a conductor material having low electrical resistivity, such as silver, gold, or copper. Particularly, since silver has low electrical resistivity and can be fired in an oxidizing atmosphere, in distinction from copper, the technology of simultaneous firing of silver and a low firing temperature material has been studied and developed, thereby improving electric characteristics of ceramic printed circuit substrates.
However, silver involves problems in relation to reliability. Specifically, circuit lines of silver are apt to suffer solder erosion, and silver ions migrate and cause a short circuit between circuit lines. Accordingly, a printed circuit substrate has not been able to employ a surface circuit pattern formed of silver. Conventionally, a surface circuit pattern of silver is plated with, for example, nickel, in order to prevent solder erosion during, for example, solder mounting of a printed circuit substrate on another board or mounting of an integrated circuit chip on the printed circuit substrate. However, this plating practice involves a problem in that pretreatment by means of a chemical solution, such as acid or alkali, impairs adhesion of conductors to the substrate. Also, employment of the plating step involves a corresponding increase in cost.
Use of a silver-palladium conductor or silver-platinum conductor is effective for solving problems involved in using a silver conductor, such as solder erosion and migration of silver ions. The silver-palladium and silver-platinum conductors are obtained through addition to silver of palladium or platinum having high heat resistance. The palladium concentration of the silver-palladium conductive material is 5 to 30%, and the platinum concentration of the silver-platinum conductive material is 0.1 to 5%. These conductive materials are often used in the form of a conductive paste that undergoes postfiring metallization in a thick-film process. Postfiring in the thick-film process is performed after a conductive paste is applied to a fired substrate in a defined circuit pattern through printing and is adapted to fire the applied conductor at a temperature not higher than the substrate firing temperature.
The technology of forming a surface circuit pattern through postfiring of a thick-film process is disclosed in, for example, the following publications. Japanese Patent Application Laid-Open (kokai) No. 4-88067 discloses a conductive paste formulated through addition of an oxide of manganese, chromic oxide, and glass frit to a silver-palladium material. This disclosed conductive paste yields the effect of improving bonding strength and solder wettability of a surface circuit pattern that has undergone high-temperature aging. Japanese Patent Publication (kokoku) No. 6-50705 discloses a conductive paste formulated through addition of silicon dioxide and glass frit to silver. This disclosed conductive paste yields the effect of improving bonding strength and solder erosion resistance of a surface circuit pattern. Japanese Patent Publication (kokoku) No. 5-14363 discloses a conductive paste formulated through addition to silver of bismuth oxide, copper oxide, manganese dioxide and glass frit. This disclosed conductive paste yields the effect of improving bonding strength and solder erosion resistance of a surface circuit pattern. Since the disclosed techniques employ postfiring of a thick-film process for formation of a surface circuit pattern, a corresponding increase in labor is involved, causing an increase in fabrication cost. Since glass frit is softened during firing and stagnates between conductor particles, when the surface of a conductor layer is eroded by solder, a conductor surface having glass portions appears, with the result that the solder is repelled. Since a silver-palladium material has a relatively high electrical resistivity, a surface circuit pattern formed of the material involves a problem in that an electric signal suffers a relatively large conductor loss.
In order to reduce fabrication cost, the technique for simultaneously firing a surface circuit pattern formed of a silver-based material and a substrate has been studied. In contrast to the above-mentioned thick-film process, not many disclosed inventions employ a simultaneous firing process for forming a surface circuit pattern on a substrate. The simultaneous firing process in this context is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) Nos. 9-198919 and 9-74256 and Japanese Patent Publication (kokoku) No. 5-74166. Japanese Patent Application Laid-Open Nos. 9-198919 and 9-74256 disclose a printed circuit substrate that uses a conductive paste formulated through addition of vanadium pentoxide to silver, thereby improving bonding strength and solder wettability of a surface circuit pattern, as well as warp resistance of the substrate. Japanese Patent Publication No. 5-74166 discloses a conductive paste formulated through addition of molybdenum and/or tungsten to a noble metal, such as silver, palladium, or platinum. This disclosed conductive paste yields the effect of improving solder wettability of a surface circuit pattern.
Even when a surface circuit pattern is formed of a silver-platinum conductor in an attempt to reduce the electrical resistivity of the surface circuit pattern and to improve resistance of the surface circuit pattern to soldering heat and solder erosion, the following problems arise. (1) Initial bonding strength of a circuit conductor is low; (2) the surface circuit pattern exhibits a significant deterioration of strength in an aging test conducted at a temperature as high as 150° C.; (3) the surface circuit pattern suffers solder erosion at a soldering heat resistance test conducted by use of a solder having a temperature as high as 260° C.; and (4) a mismatch in firing timing between the surface circuit pattern and the corresponding substrate causes warpage of the resultant printed circuit substrate.
A conceivable cause for the abo
Kodera Eiji
Nagura Hitoshi
Sato Hironori
Taga Shigeru
Brinks Hofer Gilson & Lione
Lam Cathy
NGK Spark Plug Co. Ltd.
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