Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2000-12-21
2002-08-20
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C428S901000, C174S257000
Reexamination Certificate
active
06436316
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a conductive paste, and more specifically, it relates to a conductive paste used in order to form an electrode, a wiring pattern, etc., made of a thick film on a substrate, and relates to a printed wiring board formed using the same.
2. Description of the Related Art
In general, so called thick film techniques have been widely used in practice wherein conductive pastes are coated on insulating substrates made of glass, ceramics, etc., by a screen printing method or a direct drawing method, and thereafter these are fired to form desired conductive patterns such as electrodes and wiring patterns.
The conductive pastes used in the thick film techniques can be classified, in accordance with firing temperatures, into high temperature firing types which are fired at about 800° C. to 950° C. and middle temperature firing types which are fired at about 750° C. or less, for example, about 600° C.
When the high temperature firing type conductive pastes are used, it is possible to obtain electrodes and wiring patterns superior in conductive characteristics, in particular, conductivity and adhesiveness to substrates, while there are drawbacks in that printed resistors and dielectrics are thermally damaged.
On the other hand, the middle temperature firing type conductive pastes have advantages in that these can be fired at lower temperatures and therefore electrodes and wiring patterns can be formed without causing large thermal damage to common printed resistors, dielectrics, etc.
In general, the middle temperature firing type conductive pastes are composed of metallic powders and glass frits dispersed in organic vehicles, wherein the metallic powders are sintered during the firing to be thick film conductors, the glass frits enter the liquid state during the firing to improve the sinterability of metals and to function as adhesives of formed thick film conductors and substrates, and the organic vehicles function as organic liquid media for making these powders possible to print.
The function of the aforementioned glass frits of adhering thick film conductors to substrates is called the glass bonding function. The glass frits melt and move to the interface with the substrates during the firing of the conductive pastes to function as adhesives of fired films to substrates.
Therefore, there are more metallic components in the upper layer part of the thick film conductors after firing, and there are more glass components in the lower layer part. Thus the glass fills the role of a mechanical bonding of the substrates and thick film conductors, wherein the glass is in the shape as if it were reaching out its hand between metallic particles from above the surface of the substrates. In the common middle temperature firing type conductive pastes, in order to simplify the pulverizing process of the glass frits, usually, glass frits of about 3 &mgr;m or more in mean diameter are used.
The thick film conductors, however, such as electrodes and wiring patterns, formed using the middle temperature firing type conductive pastes are likely to be inferior in the conductivity characteristics compared to the high temperature firing type conductive pastes.
For example, in the case of the middle temperature firing type conductive pastes using glass frits, the glass frits melt and move to the interface with the substrates during the firing, and pores are formed at the places where the glass frits have been before the firing. Because the pores are formed in accordance with particle diameters of the used glass frits, there are problems in that the conductivity of the obtained thick film conductors may be lowered, the strength of the obtained thick film conductors themselves may be lowered and furthermore, degradation in the tolerance to thermal aging may be caused due to the penetration of solder into the pores.
As the aforementioned middle temperature firing type glass, usually glass containing lead and softening at a low temperature is used. Recent years, however, the environmental pollution by the elution of lead has caused problems, and conductive pastes containing no lead have been required.
SUMMARY OF THE INVENTION
The present invention was made in consideration of the aforementioned circumstances. It is an object of the present invention to provide a conductive paste which can form a thick film conductor containing no lead, able to be fired at a middle temperature, superior in the conductivity and having sufficient adhesive strength to a substrate, and to provide a printed wiring board in which an electrode and a wiring pattern are formed using the conductive paste.
In order to attain the aforementioned objects, a conductive paste according to the present invention is a conductive paste comprising: at least one metallic powder selected from the group consisting of Cu, an alloy containing Cu and a mixture containing Cu; a glass frit; and an organic vehicle, and is characterized in that a glass powder which has a composition represented by the formula xBi
2
O
3
—yB
2
O
3
—zSiO
2
, where x, y, and z are indicated in mole %, and the composition ratio (x, y, z) of which is within the range surrounded by point A (
15
,
35
,
50
), point B (
25
,
60
,
15
), point C (
45
,
40
,
15
) and point D (
45
,
20
,
35
) in a ternary compositional diagram shown in
FIG. 1
, is used as the glass powder constituting the aforementioned glass frit.
By using the glass frit containing the glass powder which has the composition represented by the formula xBi
2
O
3
—yB
2
O
3
—zSiO
2
, and the composition ratio (x, y, z) of which is within the range surrounded by point A, point B, point C and point D shown in
FIG. 1
as the glass frit, it becomes possible to lower the firing temperature without using the lead glass, and therefore it becomes possible to provide a conductive paste which can form a thick film conductor containing no lead, being superior in conductivity and having sufficient adhesive strength to a substrate even when it was fired at a middle temperature.
That is, by using the glass powder, the composition ratio (x, y, z) of which is within the range surrounded by the point A, point B, point C and point D shown in
FIG. 1
as the glass powder, it becomes possible to make it surely function as the liquid phase preventing the increase of the glass softening point and the crystallization of the glass during the firing of the conductive paste, and it becomes possible to improve the sinterability of a metallic powder, being a conductive material, selected from the group consisting of Cu, an alloy containing Cu and a mixture containing Cu, even when it is fired under a non-oxidative atmosphere. Therefore, it becomes possible to lower the firing temperature without using the lead glass, and it becomes possible to provide a conductive paste which can form the thick film conductor containing Cu as a primary component, superior in the conductivity and having sufficient adhesive strength to the substrate even when it was fired at the middle temperature under the non-oxidative atmosphere.
By using the powder (copper material) selected from the group consisting of Cu, the alloy containing Cu and the mixture containing Cu, it becomes possible to suppress the material cost, to reduce the wiring resistance, and to improve the migration characteristics compared to the case in which noble metals, for example, gold, silver, palladium, etc., are used.
Furthermore, it becomes possible to lower the firing temperature to about 700° C. or less, and therefore it becomes possible to reduce the thermal damage to elements such as a resistor formed beforehand on the substrate.
In the conductive paste according to the present invention, a mean particle diameter of the aforementioned glass powder is preferably within the range of about 0.1 to 1.0 &mgr;m. By using the glass powder, the mean particle diameter of which is within the range of about 0.1 to 1.0 &mgr;m as the glass powder constituting the glass frit, it becomes possible to improve the reactivity
Dickstein , Shapiro, Morin & Oshinsky, LLP
Kopec Mark
Murata Manufacturing Co. Ltd.
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