Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
2000-10-27
2003-04-08
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C501S032000, C501S065000, C428S689000
Reexamination Certificate
active
06544654
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thick-film resistor containing no lead and a ceramic circuit board.
2. Description of the Prior Art
In forming a resistor on a surface of a ceramic substrate by a thick-film method, a thick-film resistor pattern is conventionally printed using a thick-film resistor paste. The thick-film resistor pattern is then fired to be formed into a thick-film resistor. At present, a mixture of ruthenium oxide and glass is generally used to form a thick-film resistor in order to adjust the firing temperature and resistance value. However, the glass used for the formation of the thick-film resistor contains lead (Pb) for the following reasons:
(1) Electrical resistance of the thick-film resistor is obtained by resistance due to contact of fine powder of an electrically conductive material (ruthenium oxide) and resistance due to a thin film of glass between the conductive materials. However, a quantity of conductive material is reduced when a thick-film resistor to be fabricated has a high resistance at or above 100 k&OHgr;/square. As a result, the resistance due to a thin film of glass between the conductive materials is dominant and accordingly, the resistance value tends to be changed even by a slight variation in a firing step. As a countermeasure, a ruthenium composite oxide such as Pb
2
RU
2
O
6
, Bi
2
Ru
2
O
7
, etc., each of which has a higher resistivity than RuO
2
, is used as the conductive material such that a blending ratio of the conductive material is increased, whereupon a rate of electrical conduction by the contact of the conductive materials is increased.
However, the ruthenium composite oxide partially decomposes in the firing step, thereby rendering the characteristic of the thick-film resistor unstable. For example, Bi
2
Ru
2
O
7
decomposes as follows:
Bi
2
Ru
2
O
7
→2RuO
2
+Bi
2
O
3
The thick-film resistor contains both RuO
2
and Bi
2
Ru
2
O
7
as the result of the decomposition. Glass used for the formation of the thick-film resistor needs to contain PbO in order that the aforesaid decomposition may be prevented.
(2) When the glass of the thick-film resistor contains PbO, characteristics of the glass such as a melting point, thermal expansion coefficient, etc. can readily be adjusted and accordingly, the characteristics of the thick-film resistor can readily be adjusted. However, the use of lead is undesirable from the point of view of environmental protection. A thick-film resistor using no lead needs to be developed early.
A compressive force needs to be applied from the ceramic substrate to the thick-film resistor to prevent progress of microcrack in order that the stability of thick-film resistor may be ensured for a long period of time. For this purpose, the thermal expansion coefficient of the thick-film resistor needs to be rendered smaller than that of the ceramic substrate. The ruthenium composite oxide has a thermal expansion coefficient of 8.0 to 10.0×10
−6
/° C., which value is rather larger than the thermal expansion coefficient, 4 to 6×10
−6
/° C., of the ceramic substrate. Accordingly, RuO
2
, (5 to 6×10
−6
/° C.) is desirable as the conductive material for the ceramic substrate. However, since RuO
2
has a lower resistivity than the ruthenium composite oxide as described above, the blending ratio of RuO
2
, needs to be reduced and that of glass needs to be increased when a resistor having the resistance at or above 100 k&OHgr;/square is fabricated from an RuO
2
thick-film resistor. As a result, the resistance value tends to be changed even by a slight variation in the firing step.
Furthermore, a glass paste is conventionally printed and fired on the surface of a thick-film resistor or thick-film conductor fabricated on the ceramic substrate so that a film of overcoat glass is fabricated. The surface of the thick-film resistor or thick-film conductor is covered with the overcoat glass for insulation of the thick-film resistor or conductor, whereby the electrical characteristic of the resistor or conductor is stabilized.
Although the conventional overcoat glass contains PbO for adjustment of the characteristics such as a firing temperature, thermal expansion coefficient, etc., the use of lead is undesirable from the point of view of environmental protection. A thick-film resistor using no lead needs to be developed early. In view of this point, the prior art has proposed an unleaded overcoat glass containing no Pb component. However, since the proposed unleaded overcoat glass has a large thermal expansion coefficient, the thermal expansion coefficient of the overcoat glass becomes larger than that of the ceramic substrate when the proposed overcoat glass is used for a ceramic substrate having a low thermal expansion coefficient, whereupon the ceramic substrate applies a tensile force to the overcoat glass.
One of important purposes of the overcoat glass is to limit progress of microcrack caused in the thick-film resistor during laser trimming to thereby reduce the variation of the resistance value with age. The overcoat glass needs to apply a compressive force to the thick-film resistor to fully accomplish the purpose. As described above, however, the ceramic substrate applies the tensile force to the overcoat glass. The tensile force reduces the compressive force applied to the thick-film resistor, whereupon the effect of limiting the progress of microcrack is reduced after the laser trimming and the variations in the resistance value with age are increased.
SUMMARY OF THE INVENTION
Therefore, a primary object of the present invention is to provide a thick-film resistor which is unleaded or contains no lead, which is hard to be influenced by the variations in the firing step, which has a stable resistance value, which can be fabricated efficiently, and which can improve the productivity and quality.
Another object of the invention is to provide a ceramic circuit board which uses an overcoat glass which is unleaded and in which characteristics of the overcoat glass such as the firing temperature, thermal expansion coefficient, etc. can properly be adjusted without use of lead component.
To achieve the primary object, the present invention provides a thick-film resistor comprising RuO
2
and an SiO
2
—B
2
O
3
—K
2
O glass having a composition of 60 wt %≦SiO
2
≦85 wt %, 15 wt %≦B
2
O
3
≦40 wt %, 0.1 wt %≦K
2
O≦10 wt %, and impurity ≦3 wt %.
The thick-film resistor is fired at or below 900° C. in most cases and more specifically at about 850° C. The reason for this is that a metal with a low melting point, for example, Ag or Au, is used as a surface conductor of the ceramic substrate. Another reason is for prevention of evaporation of RuO
2
. In order that the thick-film resistor may be fired at 850° C., it is desired that glass contained in it have a transition point at or below 650° C. In the present invention, the SiO
2
—B
2
O
3
—K
2
O glass contained in the thick-film resistor has the above-described composition such that a transition point thereof is at or below 650° C. As a result, the thick-film resistor can be fired at 850° C. In this case, K
2
O contained in the glass serves to lower the glass transition point. Accordingly, when an amount of K
2
O is smaller than 0.1 wt %, the glass transition point becomes higher than 650° C., whereupon it is difficult to fire the thick-film resistor at 850° C. Na
2
O or Li
2
O can lower the glass transition point, instead of K
2
O. However, when Na
2
O or Li
2
O is used, a temperature coefficient of resistance (TCR) changes to a large extent to thereby take a negative value. This deteriorates the temperature characteristic of the thick-film resistor. Since K
2
O is used in the present invention, the glass transition point can be lowered without deterioration of the temperature characteristic of the thick-film resistor. The thermal expansion coefficient (TEC) of the glass is increased when a quantity of K
2
O contained in the glass is excessively increased. Accord
Fukaya Masashi
Higuchi Chiaki
Shibata Koji
Watanabe Yoshinobu
Blackwell-Rudasill G.
Foley & Lardner
Jones Deborah
Sumitomo Metal (SMI) Electronics Devices Inc.
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