Insulating ceramic composition and ceramic inductor using the same

Abstract: An insulating ceramic composition which comprises 62-75 wt% of silicon oxide (in terms of SiO 2 ), 4-22 wt% of barium oxide (in terms of BaCO 3 ), 0.5-2.5 wt% of aluminum oxide (in terms Al 2 O 3 ), 0-0.8 wt% of chromium oxide (in terms of Cr 2 O 3 ), 0.2-0.8 wt% of calcium oxide (in terms of CaCO 3 ), 8-18 wt% of boron oxide (in terms of B 2 O 3 ), and 0-2.5 wt% of potassium oxide (in terms of K 2 O). A ceramic inductor made of said insulating ceramic composition.

Description: The present invention relates to an insulating ceramic composition, preferrably to a ceramic substrate for miniaturized electric cercuits, and particularly to a ceramic inductor using this insulating ceramic composition. The miniaturization of electronic machines and equipment is generally realized by the use of ceramic substrates on which are mounted electronic parts to constitute electric circuits. Efforts to increase the mounting density further are being directed to the development of multilayer ceramic substrates which contain internal circuits and electronic circuit components, such as capacitor and inductor. The multilayer ceramic substrates are made of aluminium oxide ("alumina"). Having a sintering temperature as high as 1500-1600°C, alumina needs a large amount of energy for firing. In addition, alumina necessitates the internal conductors to be made of high-melting material, such as tungsten and molybdenum, which endures the firing of alumina. The result is a high resistance of internal circuits which limits the current capacity. There is proposed a material for substrates which sinters at low temperatures in JP-A-4-16551. The ceramic composition composed mainly of SiO 2 . The ceramic composition can be fired at comparatively low temperatures ranging from 940°C to 1000°C, which permits internal conductors to be made of copper. Unfortunately, copper needs a reducing or non-oxidizing firing atmosphere. This disadvantage can be eliminated by replacing copper with silver resistant to oxidation in the air at high temperatures. Silver, however, makes it necessary to fire the ceramic composition at temperatures lower than 900°C. It is an object of the present invention to provide an insulating ceramic composition which permits firing at temperatures lower than 900°C with a broad range of optimum firing temperatures and yields a ceramic product having a high insulation resistance and a low dielectric constant This object is resolved by an insulating ceramic composition which comprises 62-75 wt% of silicon oxide (in terms of SiO 2 ), 4-22 wt% of barium oxide (in terms of BaCO 3 ), 0.5-2.5 wt% of aluminum oxide (in terms Al 2 O 3 ), 0-0.8 wt% of chromium oxide (in terms of Cr 2 O 3 ), 0.2-0.8 wt% of calcium oxide (in terms of CaCO 3 ), 8-18 wt% of boron oxide (in terms of B 2 O 3 ), and 0-2.5 wt% of potassium oxide (in terms of K 2 O). Another object of the invention is to provide a ceramic inductor made of said ceramic composition having a ceramic sintered body, an internal conductor formed as an inductor in said ceramic sintered body, and external electrodes which are formed on the end surfaces of said ceramic sintered body and electrically connected to said internal conductor, wherein said ceramic sintered body comprises 62-75 wt% of silicon oxide (in terms of SiO 2 ), 4-22 wt% of barium oxide (in terms of BaCO 3 ), 0.5-2.5 wt% of aluminum oxide (in terms Al 2 O 3 ), 0-0.8 wt% of chromium oxide (in terms of Cr 2 O 3 ), 0.2-0.8 wt% of calcium oxide (in terms of CaCO 3 ), 8-18 wt% of boron oxide (in terms of B 2 O 3 ), and 0-2.5 wt% of potassium oxide (in terms of K 2 O). Fig. 1 is a perspective view showing one embodiment of the ceramic inductor pertaining to the present invention. The invention will be explained with reference to the following examples. Example 1 There were mixed together silicon oxide, barium carbonate, aluminum oxide, chromium oxide, calcium carbonate, boron oxide, and potassium oxide (as starting materials) in the ratio shown in Table 1. The resulting mixture was crushed by wet process using a ball mill, followed by dehydration, drying, and calcination at 800-900°C. The calcined powder was crushed again by wet process using a ball mill to give a prepared powder. The prepared powder was mixed with water or organic solvent (as a binder) to give a slurry, which was subsequently formed into a green sheet (1 mm thick) by the doctor blade process. The green sheet was cut into square pieces (10 mm by 10 mm). Both surfaces of each piece were coated by printing with silver paste composed of silver powder and organic vehicle in the ratio of 80:20 by weight. The coated pieces were fired in the air at 850-950°C for 90 minutes. The samples obtained as mentioned above were tested for characteristic properties. Dielectric constant and insulation resistance were measured with an impedance analyzer and IR meter, and flexural strength was measured with a strength tester. Dielectric constant (ε) was calculated from electrostatic capacity (C) which was measured with an automatic bridge tester at 1 kHz, 1 V ms , and 25°C. Insulation resistance (R) was measured with an insulation resistance tester by application of 16V dc for 2 minutes at 25°C. Flexural strength was measured by the 3-point bending method and calculated according to JIS. The results are shown in Table 1. The optimum firing temperature shown in Table 1 is the firing temperature at which the maximum shrinkage is reached. (Shrinkage is due to sintering and evaporation of binder which occur as the temperature increases.) The range of optimum firing temperatures shown in Table 1 is the difference between the limits above and below the above-mentioned optimum firing temperature, with the lower limit being the temperature at which shrinkage is greater by 0.5% than that at the optimum firing temperature, and the upper limit being the temperature at which the sample begins to become poor in solderability. To test solderability, the sample was dipped in a lead-solder bath at 230"10°C for 2 seconds after preheating at 150°C for 20 seconds, with the silver electrode coated with a chloride-based flux. Solderability was rated, by visual observation, as good if more than 90% of the surface of the silver electrode was covered with solder; otherwise, it was rated as poor. Incidentally, samples not conforming to the present invention are asterisked in Table 1. According to the present invention, the insulating ceramic composition is restricted in the ratio of its components for the reasons given below. With SiO 2 less than 62 wt%, the sample has a dielectric constant greater than 6, which adversely affects the characteristic properties of the electronic circuit for high frequencies, as in Sample No. 1. With SiO 2 more than 75 wt%, the sample needs firing at temperatures higher than 900°C, as in Sample No. 22. With BaCO 3 less than 4 wt%, the sample has a low flexural strength of 820 kg/cm 2 and needs firing at temperatures higher than 900°C, as in Sample No. 20. With CaCO 3 more than 22 wt%, the sample has a dielectric constant greater than 6, as in Sample No. 2. With Al 2 O 3 less than 0.5 wt%, as in Sample No. 7, or with Al 2 O 3 more than 2.5 wt%, as in Sample No. 15, the sample needs firing at temperatures higher than 900°C. With Cr 2 O 3 more than 0.8 wt%, the sample is low in insulation resistance, as in Sample No. 6. With CaCO 3 less than 2 wt%, the sample has a narrow range (less than 15°C) of optimum firing temperatures. With CaCO 3 more than 0.8 wt%, the sample needs firing at temperatures higher than 900°C as in Sample No. 7. With B 2 O 3 less than 8 wt%, the sample needs firing at temperatures higher than 900°C, as in Sample No. 22. With B 2 O 3 more than 18 wt%, the sample is low in insulation resistance, as in Sample No. 20. With K 2 O more than 2.5 wt%, the sample is low in insulation resistance, as in Sample No. 8. In the meantime, silver for electrodes in this example may be replaced by silver alloy or silver-palladium alloy. Example 2 This example demonstrates a ceramic inductor made of the ceramic composition in Example 1. The slurry prepared in Example 1 was made into a green sheet by the doctor blade process. On one surface of the green sheet was formed by printing a conductor pattern from an electrically conductive paste of silver-palladium alloy such that when printed green sheets are placed one over another the conductor pattern constitutes a coiled conductor within their laminate through via holes in them. The laminate of green sheets was fired in the air at 860°C for 1 hour to give a bisque ware. On the end surface of the bisque ware was formed an external electrode composed of a first layer of amalgam plated silver, a second layer of electroplated nickel, and a third layer of plated tin. The external electrode is electrically connected to the internal coiled conductor. Fig. 1 is a perspective view of one embodiment of the ceramic inductor obtained as mentioned above. In Fig. 1, there are shown the ceramic inductor 1, the insulating ceramic 2, the internal coiled conductor 3, and the external electrode 4. The insulating ceramic (bisque ware) is made of the SiO 2 -BaO-B 2 O 3 insulating ceramic composition of the present invention. Since the ceramic composition is capable of firing at low temperatures, the internal conductor can be made of a silver-rich alloy having a high conductivity. Therefore, the resulting chip-type ceramic inductor is suitable for the high-frequency range. The material to form the internal conductor pattern on the ceramic sheet surface may be silver or silver alloy such as silver-palladium alloy. The method for pattern forming may be any of printing, coating, vapor deposition, and sputtering. The material to form the external electrode may be not only silver (mentioned above) but also silver-palladium alloy or such metal (or alloy thereof) as nickel and copper. Printing, vapor deposition, or sputtering may be employed to form the external electrode. The external electrode formed on the end surface of the laminate may be fired simultaneously with the firing of the laminate. The ceramic inductor 1 is not specifically restricted in shape. In addition, the coil is not specifically restricted in the number of turns. A desired number of turns may be established by properly selecting the number of insulating ceramic layers 2 carrying the internal conductor pattern. Although this example demonstrates a ceramic inductor having one inductor, the same idea may be applied to composite components of ceramic inductors such as transformer and LC filter. (Effect of the invention) The insulating ceramic composition of the present invention is capable of firing in the air at temperatures lower than 900°C. The insulating ceramic composition of the present invention has such a broad range of optimum firing temperature that it yields good products even when the temperature fluctuates in the batch furnace or continuous firing furnace. Therefore, it is suitable for commercial mass production. The insulating ceramic composition of the present invention yields a ceramic product having a high insulation resistance, a low dielectric constant, and a high flexural strength. The ceramic inductor of the present invention has the internal conductor of silver or silver alloy because it is made of the insulating ceramic composition of the present invention. The ceramic inductor of the present invention has a high self-resonance frequency and a high Q (quality factor).

Claims: Insulating ceramic composition preferrably ceramic substrate for miniaturized electric circuits, characterized in that it comprises 62-75 wt% of silicon oxide (in terms of SiO 2 ), 4-22 wt% of barium oxide (in terms of BaCO 3 ), 0.5-2.5 wt% of aluminum oxide (in terms Al 2 O 3 ), 0-0.8 wt% of chromium oxide (in terms of Cr 2 O 3 ), 0.2-0.8 wt% of calcium oxide (in terms of CaCO 3 ), 8-18 wt% of boron oxide (in terms of B 2 O 3 ), and 0-2.5 wt% of potassium oxide (in terms of K 2 O). A ceramic inductor of the type having a ceramic sintered body, an internal conductor formed as an inductor in said ceramic sintered body, and external electrodes which are formed on the end surfaces of said ceramic sintered body and electrically connected to said internal conductor, characterized in that the ceramic sintered body comprises 62-75 wt% of silicon oxide (in terms of SiO 2 ), 4-22 wt% of barium oxide (in terms of BaCO 3 ), 0.5-2.5 wt% of aluminum oxide (in terms Al 2 O 3 ), 0-0.8 wt% of chromium oxide (in terms of Cr 2 O 3 ), 0.2-0.8 wt% of calcium oxide (in terms of CaCO 3 ), 8-18 wt% of boron oxide (in terms of B 2 O 3 ), and 0-2.5 wt% of potassium oxide (in terms of K 2 O).

Inventors: IMADA KATSUHISA, KOBAYASHI TAKASHI, MAEDA EIICHI, NISHII MOTOI, NISHINAGA YOSHIHIRO, IMADA, KATSUHISA, KOBAYASHI, TAKASHI, MAEDA, EIICHI, NISHII, MOTOI, NISHINAGA, YOSHIHIRO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1998
Main classification: C04B35
IPC codes: C04B35, H01F17, H01B3