JPH0262022A - Laminated ceramic capacitor - Google Patents

Abstract

PURPOSE:To make a surface layer dense, to harden the layer without changing the composition material and to prevent this layer from being broken or chipped off by an external force by a method wherein the particle diameter of the laminated surface layer of a dielectric ceramic is made smaller than the particle diameter inside the layer. CONSTITUTION:Internal electrodes 3 are buried to be layer-like between a plurality of laminated dielectric ceramics 2; these are sandwiched between dielectric ceramics 2 which do not have the electrodes 3; a laminated dielectric 1 is formed. A particle diameter of a surface layer of the ceramics 2 is made smaller than a particle diameter at the inside. It is preferable that the surface of the dielectric 1 having the electrodes 3 is coated with a paste containing a rare-earth oxide and is baked. Then, only the surface layer can be made dense and hardened without changing a composition material. Thereby, it is possible to prevent the layer from being broken or chipped off by an external force.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a multilayer ceramic capacitor in which internal electrodes are embedded between dielectric ceramics.

Conventional multilayer ceramic capacitors generally have internal electrodes embedded in layers between dielectric ceramics to form a multilayer dielectric, and external electrodes are disposed at both ends of the multilayer dielectric.

In addition, these multilayer ceramic capacitors have recently been made into chip-type and extremely miniaturized to increase mounting efficiency.Furthermore, with recent advances in surface mounting technology, they have been automatically mounted on wiring boards. Directly attached.

Problems that Ming was trying to solve However, when this type of multilayer ceramic capacitor was mounted on a wiring board, there were problems such as chipping, accelerated deterioration, and changes in electrical characteristics. In other words, when an automatic mounting device grips a capacitor and solders it to a board, if the gripping force is too strong, damage such as chips or cracks may occur on the surface of the capacitor, which has poor chipping resistance, that is, strength against external forces such as shock. Ru. Furthermore, when the wiring board is subjected to heat during soldering or thermal stress during mounting, it deforms and warps, albeit slightly. At this time, bending stress acts on the multilayer ceramic capacitor directly attached to the substrate, and in conventional multilayer ceramic capacitors having poor mechanical strength, cracks and chips occur on the surface, causing so-called chipping. Furthermore, this chipping causes a change in dielectric constant, resulting in a decrease in capacitance.

Therefore, attempts have been made to compose the laminated dielectric with a material having excellent chipping resistance to increase the mechanical strength of the entire laminated ceramic capacitor.

However, when the composition of the laminated dielectric is changed, electrical properties such as dielectric constant also change, so there is a limit to its modification.

Furthermore, the material that improves chipping resistance must be compatible with the composition of the laminated dielectric, and cannot be commonly applied to a wide variety of laminated ceramic capacitors, so its range of application is extremely narrow.

In view of this point, a method of barrel polishing multilayer ceramic capacitors has recently been adopted. This is intended to improve mechanical strength such as chipping resistance and bending strength by chamfering the ridgeline portions of multilayer ceramic capacitors where cracks and cracks are likely to occur. In addition, during manufacturing, while the laminated dielectric ceramic after firing is used as the material to be polished, powder such as ZrO is put into a container as an abrasive material, and the container is vibrated or rotated to polish the surface of the dielectric ceramic. are doing.

However, although the multilayer ceramic capacitor obtained in this manner may have slightly improved bending strength, it is difficult to obtain desirable effects such as deterioration of electrical characteristics. That is, in barrel polishing, the surface finish is greatly affected by the rotation speed and rotation time of the container.

For example, if the rotation time is short, the amount of polishing of the laminated dielectric ceramic material to be polished will be reduced, and the ridgeline portions will not be chamfered sufficiently, resulting in a decrease in bending strength. On the other hand, if the rotation time becomes longer, polishing will be performed more than necessary, and not only the ridgeline portion but also the surface of the laminated dielectric ceramic will be deeply scraped off, causing problems such as a decrease in bending strength and a change in capacitance. was. In this way, with the barrel polishing method, it is difficult to perform optimal polishing without affecting the electrical characteristics, and furthermore, it requires fine adjustment depending on the type, shape, quantity, etc. of the capacitor, so Production efficiency is also extremely low.

The present invention has been made in view of the above-mentioned problems, and has been improved to improve bending strength and chipping resistance without deteriorating electrical characteristics, and to be applicable to a wide variety of laminated dielectrics. Our goal is to provide high-quality multilayer ceramic capacitors.

Means for Solving the Problems In order to solve the above problems, the multilayer ceramic capacitor according to the present invention is characterized in that the particle size of the surface layer of the multilayer dielectric ceramic is smaller than the particle size inside.

Preferably, a paste containing a rare earth oxide is applied to the surface of the laminated dielectric having internal electrodes and then fired.

Due to the above configuration, the particle size of the surface layer of the laminated dielectric ceramic is smaller than that of the inside, so only the surface layer is densified and hardened without changing the composition material. . Therefore, the chipping resistance and bending strength are increased without deteriorating the electrical characteristics as a capacitor, and the occurrence of cracks, chips, etc. due to external forces is prevented, and a long life is maintained.

Example Embodiments of the present invention will be described below based on the drawings.

As shown in FIG. 1, the multilayer ceramic capacitor according to the present invention has internal electrodes 3 embedded in a layered manner between a plurality of laminated dielectric ceramics 2, and a dielectric ceramic having no internal electrodes. 2a.

A laminated dielectric 1 is formed by sandwiching the two layers at 2a. moreover,
It has a configuration in which external electrodes 4, 4 are provided at both ends. This multilayer ceramic capacitor is manufactured in the following process order, and by making it a rectangular parallelepiped and small chip type, it can be directly attached to a wiring board via solder and can be read directly.

Next, the manufacturing process will be explained according to FIGS. 2 to 4.

First, create a green sheet. Here, BaZr0m was added to powdered BaTi0m: 84.35mol1%.
: MnC0 is added at a ratio of 0.07 wt% to 100 wt% of 15.65 mo1% and mixed. Then, PV is added to these raw powders as a binder.
Using A (polyvinyl alcohol), a surfactant, a dispersant, water, etc. are added and kneaded to obtain a slurry.

Subsequently, this slurry is formed into a sheet by a doctor blade method to obtain a green sheet with a thickness of 35 μm.

Thereafter, each green sheet is dried, and internal electrodes 3 are formed on its negative side. The internal electrodes are made by printing a mixture of metal powder such as palladium, silver, or the like with a suitable binder on each of the green sheets, and then drying it. A plurality of green sheets are stacked according to the required capacitance, and the green sheets are pressed together while being sandwiched between green sheets having no internal electrodes. Next, as shown in Figure 2, the laminated green sheets are cut to a predetermined size.
A laminated dielectric 1 is obtained.

Next, as shown in FIG. 3, an oxide is applied to the surface of the laminated dielectric 1. As this oxide, Dy! Oa, CeOz
, Sm, O, , Gd, O, and the like. And this oxide 30%
Ethyl cellulose and butyl cellosolve acetate are added to make a paste, and this paste is applied to the upper and lower surfaces of the laminated dielectric 1. Subsequently, this laminated dielectric 1 is dried and then fired at 1300°C for 2 hours. Thereafter, a conductive paste is applied and baked on both end faces of the laminated dielectric 1 to complete the formation of external electrodes 4.4, thereby constructing the laminated ceramic capacitor shown in FIG. 1.

The multilayer ceramic capacitor thus obtained is as follows.

External dimensions Width: 1.6mm Length:
3.2 mm Thickness: 1.2 mm Ceramic unit thickness: 20 μm - Opposed pole area per layer: 1.3 mm2 Average grain size of dielectric inner layer: 4.5 μm Average grain size of dielectric surface layer: 2. 5 μm The above average particle size was measured by visual observation using a scanning electron microscope, and it became clear that the particle size of the surface layer was smaller than that of the inner layer. This is thought to be because the oxide suppresses the growth of crystals in the ceramic surface layer.

Next, in order to find out the chipping resistance, bending strength, and electrical characteristics of this multilayer ceramic capacitor, a conventional product was used as a comparative example and tested under the same conditions. In this test, monolithic ceramic capacitors having the same weight and shape were used as samples, and the same number of capacitors were used.

First, in the chipping resistance test, in order to detect defects such as cracks and chips that occur when impact force is applied to multilayer ceramic capacitors, 100 multilayer ceramic capacitors and 100 microcapacitors were each placed in a 250 m1 vinyl chloride capacitor. After the samples were placed in a pot and rotated for about 16 hours, the weight was measured to determine the weight loss (%) of each sample. The formula for determining this weight loss is as follows.

A test (two-point support central concentrated load) was conducted.

Furthermore, when measuring the electrical characteristics, the capacitance (C) and dielectric loss (tan δ) of the multilayer ceramic capacitor are
To find out, first, a voltage of 1 kHz, I Vrms was applied to each sample using an automatic bridge measuring device. Next, to measure the insulation resistance (R), a voltage of 50V was applied for 60 seconds using an insulation resistance meter, and the CR product (Ω・
F) was measured.

The measurement results from the above tests are shown in the table.

[Leave below]Next, when testing the flexural strength, the yield point is defined as the point at which a crack occurs when a load is applied to a sintered dielectric material without embedded internal electrodes, and the load at that time is determined as the yield point. was measured. In addition, the outer dimensions of the test piece used here are 3 mm thick and 4 m wide.
Five rectangular sintered dielectrics each having a length of 40 mm were prepared, and after the surface was finished by barrel polishing, three-point bending was performed using a bending tester. As can be understood from the above table, the present invention example It has become clear that the multilayer ceramic capacitor of 2008 has only improved mechanical strength, with almost no effect on electrical characteristics.

That is, as an electrical characteristic, the electrostatic capacitance 1t (C) is 0.14 μF for both the comparative example and the present invention example, and the dielectric loss (tan
s) is only 0.2% higher in the example of the present invention. In addition, the insulation resistance (R) is 5000 for both the comparative example and the present invention example.
ΩF, and there was no change at all.

On the other hand, the weight loss, which is used to determine chipping resistance as mechanical strength, was 2.3% for the comparative example, while it was 1.1% for the inventive example, which was suppressed to about half.

In addition, the bending strength was 1560 kg/cm2 for the comparative example, while it was 2170 kg/cm2 for the inventive example, which was 1.4 times the strength and was found to be excellent in bending stress. .

As described above, the multilayer ceramic capacitor of this example is
When the laminated dielectric 1 is coated with a paste mainly composed of rare earth oxides and fired, the particle size in the surface layer of the laminated dielectric 1 becomes larger than the particle size inside the dielectric, resulting in densification. . As a result, the surface layer was hardened, and the chip resistance, 2-pin resistance, and bending strength could be improved without affecting the electrical properties. Therefore, this multilayer ceramic cocidenser has excellent mechanical strength, and when it is automatically mounted on a wiring board, the surface layer is not chipped or damaged as in the conventional case. Further, even if the substrate on which the multilayer ceramic capacitor is mounted is deformed and warped due to thermal stress and bending stress is applied to the multilayer ceramic capacitor, no cracks will occur in the ridgeline portion, and the occurrence of cracks or the like is prevented.

It should be noted that the multilayer ceramic capacitor according to the present invention is not limited to the above embodiments, and can be modified in various ways within the scope of the gist, and in particular, the ceramic composition material, manufacturing process, etc. can be widely applied. Furthermore, the type of oxide applied to reduce the particle size of the surface layer may be, in addition to rare earths, other substances that are effective in reducing the particle size of the surface layer.

Effects of the Invention As detailed above, according to the present invention, the particle size of the surface layer of the multilayer dielectric ceramic is smaller than the internal particle size and is fully densified. is hardened. As a result, the mechanical strength such as chipping resistance and bending strength of the multilayer ceramic capacitor can be improved without affecting the electrical characteristics. Therefore, when an external force is applied to the capacitor itself, cracks, chips, etc. can be prevented from occurring, the characteristics of the capacitor will not deteriorate, and deterioration will be prevented, thereby maintaining a long life. Moreover, it can be applied to various multilayer capacitors, and the range of application is greatly expanded.

[Brief explanation of the drawing]

The drawings show embodiments of the present invention; FIG. 1 is a sectional view of a multilayer ceramic capacitor, and FIGS. 2 to 4 are sectional views showing manufacturing steps. DESCRIPTION OF SYMBOLS 1... Laminated dielectric material, 2, 2a... Dielectric ceramic, 3... Internal electrode, 4... External electrode. Patent applicant Murata Manufacturing Co., Ltd.

Claims (1)

[Claims] 1. A multilayer ceramic capacitor in which internal electrodes are embedded between a plurality of dielectric ceramic sheets, characterized in that the particle size of the surface layer of the laminated dielectric ceramic is smaller than the particle size of the internal particles.