Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
1999-01-19
2001-01-16
Gupta, Yogendra (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C252S509000, C252S508000, C252S513000, C252S519140, C252S519300
Reexamination Certificate
active
06174462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an insulating paste composition and a conductive paste composition for a circuit board.
2. Related Art
A thick film circuit board is known as a circuit board, which comprises an insulating substrate formed by, for example, alumina, having thereon a conductive paste composition printed and baked. The conductive paste composition comprises conductive metal powder main components of which are silver and palladium (Pd) or a noble metal such as platinum, and an inorganic binder such as a glass frit, which are dispersed in an organic vehicle.
Recently, a circuit board is highly demanded to have a high density, a compact size and environment durability. For example, in an IC circuit board for an automobile, it is considered that high integration due to the use of hybrid ICs is demanded, and the installation in an engine room of the board, which has been installed in a vehicle cabin, is required.
Accordingly, wiring parts formed on an insulating substrate are required to have higher reliability characteristics than the past. For example, even when a voltage is applied between the wiring parts having a narrow wiring distance therebetween and the board is put under an outer environment for a long period of time, the insulating property between the wiring parts should not be deteriorated.
However, it is generally known that, when the wiring part is formed by using a conductive paste containing silver on an insulating substrate, silver contained in the wiring part is ionized on application of a voltage in the presence of water or water vapor and moves from a positive electrode part to a negative electrode part causing dendrites of silver, and the so-called migration of silver is liable to occur, which results in deterioration in insulating property between the wiring parts. Therefore, a sealing structure has been employed to prevent the migration of silver.
However, the inventors found a phenomenon in that the insulation resistance is lessened on application of a voltage under a high temperature atmosphere even if water or water vapor is not present. The phenomenon is described below with reference to the results of experiments conducted by the inventors.
The phenomenon can be summarized as follows. For example, when wiring parts containing silver are arranged on an insulating substrate with a narrow distance of about 100 &mgr;m therebetween, a voltage of about 16 V (the maximum standard for electric devices installed in an automobile) is applied between the wiring parts, and it is put under a high temperature atmosphere of about 150° C., the insulating resistance between the wiring parts is greatly lowered with the lapse of time, and at last, insulation failure therebetween occurs.
The phenomenon occurs under an atmosphere in that no water content is present, such as at a temperature of 100° C. or higher, or in vacuum. Furthermore, when the wiring parts are in a state of substantially short-circuit, dendrites of silver is not observed between the wiring parts, which must be observed on generation of the migration under the presence of water content, and polarity is also not observed in the phenomenon (i.e., the phenomenon occurs irrespective of the polarity of a positive electrode part and a negative electrode part). Accordingly, it can be determined that this is a phenomenon newly found, which is different from the migration occurring under the presence of water content. Such a phenomenon is called a high temperature leakage phenomenon hereinafter.
Although the mechanism of the phenomenon is not completely clarified, the summary is as follows according to the investigation by the inventors.
Conventionally, wiring parts on a general thick film circuit board are obtained by printing a conductive paste composition on an insulating substrate such as a ceramic substrate formed by alumina, followed by baking. The conductive paste composition comprises conductive metallic powder containing silver and an inorganic binder such as glass frit, which are dispersed in an organic vehicle. The conductive paste composition is generally baked at a temperature of from 800° C. to 900° C. During the baking, parts of the silver and the inorganic binder are evaporated from the wiring parts formed on the insulating substrate by printing, and they are attached as a reaction product containing silver on the insulating substrate. Since the reaction product is generally present in an extremely thin form, it is generally not observed. The reaction product has an insulating property, which is the same as the underlying insulating substrate.
However, when a voltage is applied between the wiring parts between which the reaction product is present and it is put under a high temperature atmosphere without influence of water content (which is called a high temperature atmosphere hereinafter), silver contained in the reaction product comes to have conductive nature, and the insulating property between the wiring parts is lessened with the lapse of time.
The mechanism of the high temperature leakage phenomenon summarized above was clarified by the following some verification experiments (verification experiment A and verification experiment B).
The verification experiment A is described below. A circuit board was prepared, on which wiring parts having plural wiring distances different from each other were formed by using a general conductive paste composition containing silver by printing and baking on a ceramic substrate formed by alumina. A voltage was applied between the wiring parts, and it was put under a high temperature atmosphere. In the verification experiment A, the voltage applied between the wiring parts had a value (for example 60 V), which was higher than the voltage generally applied, for reaction enhancement.
It was found as shown in
FIG. 1
that the insulating resistance value between the wiring parts was lowered with the lapse of time under the high temperature atmosphere.
FIG. 1
is a graph showing the relationship between the time lapsing under a high temperature atmosphere and the insulating resistance value (Log10X, unit:&OHgr;). In the plot marks in the figure, white circles and black circles are for the case of the temperature 125° C., white triangles and black triangles are for the case of the temperature 150° C., white circles and white triangles are for the case of a wiring distance 100 &mgr;m, and black circles and black triangles are for the case of a wiring distance 50 &mgr;m.
At these cases, appearance change such as the known dendrites of silver generated by water content was not observed at the wiring parts between which insulating resistance value lowers. Therefore, it was found that the phenomenon had nature influenced by conditions, such as the wiring distance, the applied voltage and the ambient temperature.
Furthermore, the time when the insulating resistance value lowers to 100 M&OHgr; was investigated as an insulation failure time in the relationship with the electric field intensity, which could be obtained from the voltage applied between the wiring parts. The results are shown in FIG.
2
.
FIG. 2
is a graph showing the relationship between the electric filed intensity and the insulation failure time. In the plot marks in the figure, circles are for the case of a temperature 150° C., and triangles are for the case of a temperature 125° C.
It was found from the results of the verification experiment A shown in
FIGS. 1 and 2
that the deterioration of insulating property between the wiring parts depends on the wiring distance and the electric field intensity determined by the applied voltage, and is farther accelerated, the higher the ambient temperature is.
The detail observation was carried out to investigate the reason of the deterioration of insulating property.
No appearance change was observed before and after the insulation deterioration by the observation using a scanning electron microscope (SEM) with respect to the wiring parts, between which the insulation deterioration was observ
Miyairi Masayuki
Nagasaka Takashi
Naito Kazumasa
Oka Kengo
Ootani Yuji
Denso Corporation
Gupta Yogendra
Hamlin Derrick G.
Pillsbury Madison & Sutro LLP
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