ELECTRICALLY CONDUCTIVE RESIN PASTES AND MULTILAYER CERAMIC CAPACITORS HAVING A TERMINAL ELECTRODE COMPRISED OF THE SAME

Abstract: Disclosed is an electrically conductive resin paste for use in the formation of a terminal electrode for a multilayer ceramic chip capacitor with no need of a high temperature firing and a multilayer ceramic chip capacitor formed thereon. The resin paste comprises an admixture of an electrically conductive filler consisting of a noble metal powder, a resin binder, and a curing agent dispersed in an organic medium, the resin binder comprising a mixture of at least two resins of an epoxy resin and a thermosetting or thermoplastic resin and the weight ratio of the noble metal powder to the thermosetting resin being about 100:5 to 100:45.

Description: TITLE ELECTRICALLY CONDUCTIVE RESIN PASTES AND MULTILAYER CERAMIC CAPACITORS HAVING A TERMINAL ELECTRODE COMPRISED OF THE SAME FIELD OF THE INVENTION This invention relates to an electrically conductive resin composition and in particular to such composition useful as a terminal for multilayer ceramic chi capacitors . BACKGROUND OF THE INVENTION Electrically conductive compositions for the formation of a terminal electrode for multilayer ceramic chip capacitor (MLC) are prepared in the form of pastes having suitable consistency and rheology, which are formed by admixing metal powders of gold, silver, palladium or alloys thereof, glass frits, inert organic vehicles and resins and dispersing them by a mechanical mixing. Such electrically conductive resin pastes are coated onto the opposing surfaces for taking out an inner electrode in MLC element, dried and fired at a high temperature of about 700 to 860°C to form a terminal electrode. This firing causes glass frits in the paste to disperse into a dielectric layer constituting the MLC element, thus effecting fused binding of the particles. At the same time, fused metal powders in the paste permit mechanical, electrical connection and fixing of the termina electrode comprised of electrically conductive compositions and the MLC element . In general, soldering is employed in attaching such chip type of circuit board to the patterned surface. fused solder is applied to a round area in a circuit patter on which a terminal for tip parts is situated, to effect a prolonged fixing of a copper-clad laminate for circuit boar thereto. When a terminal electrode is formed by firing electrically conductive pastes at a high temperature onto the MLC element, a stress occurs in a joint between a terminal electrode and a capacitor element, in particular a capacitor element area corresponding to the periphery of th terminal electrode due to a sinter shrinkage of metal powders in the paste, diffusion of a glass component in the paste into a dielectric constituting a capacitor element an diffusion of metal powders in the paste into an inner electrode of a capacitor element. Further, a crack occurs due to a rapid temperature change when soldering a chip capacitor to a circuit substrate and "deflection" occurs in the circuit due to the external bending force exerted on the substrate. In that case, a terminal of a chip type has less degree of freedom and the strength of an inner electrode and a capacitor element formed of a dielectric is lower than that of the substrate comprised of e.g., a glass fabric based epoxy resin. Therefore, "deflection" or forced bending in the circuit substrate produces a crack in a chip capacitor body after package, thus giving no sufficient terminal strength. In recent years, it has been extensively performed to connect and fix a chip part to both surfaces of the substrate made of alumina, glass fabric or epoxy resin for the purpose of increasing a package density. In such, a both surface package, the occurrence of crack in the capacitor element becomes an important issue. Thus, a technical approach has been demanded to form a terminal electrode without a high temperature firing. In view of the problems encountered in the application of such prior electrically conductive pastes to the MLC element followed by a high temperature firing, the present inventors have made an investigation in an effort to develop an electrically conductive paste which permits a connection and fixing to the MLC element by dry curing with no need of a high temperature firing and is capable of using as a terminal electrode. As a result, it has been found that lowering the elasticity of the electrically conductive composition constituting the terminal electrode can absorb the stress caused by "deflection" or "bending" in the circuit substrate due to an external bending force or the difference in expansion coefficient by heat, thus increasing the degrees of freedom to connect a chip part to a substrate and further removing the diffusion of the particles from the terminal electrode composition into the capacitor element which causftsTa crack in a joint at the periphery of the terminal electrode for the prior capacitor element, thereby inhibiting the occurrence of a crack in the body of a capacitor element due to a rapid heat at the time of soldering. DETAILED DESCRIPTION OF THE INVENTION The present invention provides an electrically conductive resin composition for the formation of a terminal electrode for a multilayer ceramic chip capacitor with no need of a high temperature firing and a multilayer ceramic chip capacitor formed of the same. The electrically conductive resin composition of the invention is paste, comprising an admixture of an electrically conductive filler consisting of a noble metal powder, a resin under, and a curing agent dispersed in an organic medium, the resin binder comprising a mixture of at least two resins of an epoxy resin and a thermosetting or thermoplastic resin and the weight ratio of the noble metal powder to the thermosetting resin being about 100:5 to 100:45. The present invention also provides a multilayer ceramic chip capacitor formed from the electrically conductive resin paste serving as a terminal electrode for the multilayer ceramic chip capacitor. The multilayer ceramic chip capacitor is formed by laminating alternately a plurality of dielectric sheets having an electrically conductive inner electrode adhered thereon in such manner that the areas for taking out the inner electrode are arranged oppositely, laminating the outmost layer with a dielectric sheet for a protective layer to integrate them, applying the electrically conductive resin paste for the multilayer ceramic chip capacitor to said areas in the integrated laminate and.fixing it to form a terminal electrode. The electrically conductive fillers used in the invention can be any of noble metal powders. In general, all metals referred to as noble metals can be used, but gold, silver, platinum, palladium, rhodium, the mixtures thereof and the alloys thereof are particularly used. The resin binders constituting the conductive resin pastes of the invention comprise a mixture of at least two resins of an epoxy resin and a thermosetting or thermoplastic resin. An epoxy resin which is one component of the resin binder refers to that consisting of a compound containing two or more epoxy groups in the molecule which cures by the action of a curing agent or a catalyst. Examples of the epoxy resins can include bisphenol A type epoxy resins, i.e., glycidyl etherified compounds of bisphenol A or its analogous compounds, diglycidyl ester type resins, novolak epoxy resins, glycidyl amine type resins, cycloaliphatic epoxy resins or the like. Those epoxy resins are capable of curing with the curing agents described later on. A thermosetting resin which is another component of the resin binder can include phenol resins, melamine resins, alkyd resins, unsaturated polyester resins, diarylphthalate resins or the like. Other components of the resin binder may be a thermoplastic resin which can include phenoxy resins, acryl resins or the like. The ratio of resin solid component in the dispersions to conductive filler can vary considerably. As a matter of course, larger amount of resin solid component provides improved mechanical strength, improved adhesion strength of the coated film and improved resistance to wear, but gives an adverse effect on capacitance and dissipation factor which are important among characteristics of capacitor, capacitance being a constant of proportionality between an electrical charge and an applied voltage and dissipation factor being the tangent of the angle (δ) at which current lags from 90° vector relative to voltage and expressed as 100 x tan δ. Therefore, less than 45 parts, preferably less than 35 parts of the resin solid component per 100 parts by weight of the conductive filler are required when one coat is applied onto the surface taking out the inner electrode of the capacitor element to form a terminal electrode portion. Less than 5 parts by weight will sometimes produce unfavorable problems such as deterioration of adhesion or the like. The elastic modulus of the terminal electrode for a multilayer ceramic chip capacitor fabricated from the compositions of the invention is measured in the range of 2.5-3.0 x 10^ MPa. On the other hand, it was confirmed by measurement that the elastic modulus of the prior terminal electrode for the same capacitor is more than the above range, which is fabricated by coating a paste composition having silver powder and glass frit dispersed in an inert organic solvent, drying and baking at high temperature. The organic media used for the conductive resin paste of the invention are added in order to reduce the viscosity rising during the milling of the binder and the conductive filler and provide improved workability. It is important to select those having the solubility according to kinds of the binder. The media free from the solubility produces an agglomeration of resin which results in no formation of chain combination of conductive filler, making the electrical conductivity unstable and losing physical and chemical stability of the coated film. The organic media which can be used for the conductive resin paste include'aliphatic alcohols, e.g., ethanol, i-propanol, n-propanol, butanol; esters of these alcohols, e.g., their acetate and propionate; caritol solvents, e.g., methyl carbitol, ethyl carbitol, butyl carbitol, butyl carbitol acetate; cellosolve solvents, e.g., cellosolve, butyl cellosolve, isoamyl cellosolve, hexyl cellosolve, butyl cellosolve acetate; ketone solvents, e.g., acetone, methyl ethyl ketone, 2-pentanone, 3-pentanone, cyclohexanone and hydrocarbon solvents, e.g., benzene, toluene, xylene, ethylbenzene, terpin, cyclohexane, methylcyclohexane, methylpentane. As the curing agents in the conductive resin, compositions of the invention can be used including: polyamide curing agents, aliphatic polyamine curing agents, cycloaliphatic polyamine curing agents, aromatic polyamine curing agents, dicyandiamide or the like. Preferably, the curing agents which initiate a curing reaction at a temperature below 40°C are used. Preferable low-temperature curing agents in the present invention are aliphatic polyamine curing agents and dicyandiamide. However, high-temperature curing agents can also be used, example of which include those which do not react at ordinary temperature, e.g., aromatic polyamine curing agents, HY 932, HT 972, HY 974, HT 976, NX 11014 (available from Ciba Geigy) ; acid anhydride curing agents, HY 920 (available from Ciba Geigy) and amide curing agents or the like. In the present invention, the electrically conductive resin pastes are coated in one layer onto the opposing surfaces for taking out an inner electrode in MLC element to form a terminal electrode area. In addition, th terminal electrode area having a multilayer structure of coated films is thought to form. In the formation of the terminal electrode having the multilayer structure, the ratio of the conductive filler component to the resin solid component can be varied. In the layer, directly bonding to the surfaces for taking out thέ inner electrode in MLC element (electrode bonding layer) , there is ensured good electrical connection of the chip capacitor element to the inner electrode is ensured and there is inhibited a deterioration of the characteristics such as capacitance an dissipation factor which are important electrical characteristics for capacitor. In the outmost layer (soldering layer) of the multilayer structure which is soldered to a circuit substrate, improved adhesion strength and lowered elastic modulus are achieved. In the coating of the electrode bonding layer, the proportion of the resin solid component in the paste is reduced so as to provide the range of 100:5 to 100:35 by the weight ratio to noble metal powder as described previously. On the other hand, the proportion of the resin solid component is increased in the paste for the coating formation of the soldering layer. The range of 100:20 to 100:40 by the weight ratio to noble metal powder is preferable. In the preparation of the present invention, the composition is formed in the form of paste by mixing and milling a conductive filler component consisting of a noble metal powder, a resin binder, a curing agent, and other additives with an inert organic solvent. The conductive resin paste, thus produced, can be applied to the area for taking out the inner electrode in MLC, for example by coating, screen-printing or dipping. When the conductive resin paste is deposited by dipping, air (bubble) in the paste is trapped, thus forming an air gap in the tailored terminal electrode. Therefore, defoaming is preferably performed by applying vacuum during or after the step of depositing the conductive resin paste. Reducing the air gaps by defoaming can prevent the penetration of a plating solution which may be occurred in the subsequent plating treatment. A cured surface of the terminal electrode tailored by depositing the conductive resin paste may contain a large number of voids, since it is not a sintered body. Depending upon a plating condition to the surface, the plating solution may penetrate into those voids. This surface can be polished for the removal of the voids. This polishing extends the metal on the surface, which results in good plate adhesion. As a polishing method may be employed a barrel polish with a small rubber ball. The invention is further illustrated by the following examples in which all parts and proportions are by weight, unless otherwise stated. EXAMPLES 1-10 Silver powders, an epoxy resin (available from Ciba Geigy Japan Co., Ltd. under the trade name of YAC 5020) , a phenol resin (available from Dainippon Ink Chemical Industry Co., Ltd. under the trade name of TB 2090) , a phenoxy resin (available from Union Carbide Co., Ltd. under the trade name of PKHH) , a curing agent (available from Ajinomoto Co., Ltd. under the trade nmae of MY-24) and a solvent were mixed in the weight proportions indicated in Table 1 and sufficiently milled with a three roll milling machine. The resultant conductive resin paste was adjusted to a viscosity suitable for the application of dipping. The silver powder used herein has a particle diameter of 0.2-10 μm. The viscosity of the paste deposited in the terminal by the application of dipping is preferably in the range of 6 to 9 at 16-30 PA.S. 0.5/10 rpm (viscosity ratio) at 10 rpm when Brookfield rotational viscometer (spindle No. 14) was used. Table 1 Example No. Composition _lfl 1_L 12 -12-. Silver Powder 100 100 100 100 100 100 100 100 100 100 100 100 100 Epoxy resin 6.75 9 11.25 3.7 4.9 6.1 7.4 8.6 8.8 SO Dicyandiamide 0.75 1 1.25 0.2 0.3 0.3 0.9 0.4 0.2 Phenol resin 2.2 3.0 3.7 4.4 5.2 9 Phenoxy resin 7.5 10 12.5 10 8.9 11.9 14.8 17.8 20.7 27 Terpineol 30 Butylcarbitol 30 25 20 25 25 25 25 25 25 25 40 40 Glass frit 3.8 Ethyl cellulose 3.0 Each of the conductive resin pastes thus prepared was coated onto a capacitor chip using a Palomer machine manufactured by Palomer Co., Ltd. After coating those pastes onto a capacitor chip, the pastes of Examples 1-4 were cured at 250°C for 60 minutes. The pastes of Examples 5-10 were cured at 200°C for 60 minutes or 230°C for 60 minutes or 250°C for 60 minutes. The pastes of Comparative Examples 11-12 as described later were cured at 200°C for 60 minutes and the paste of Example 13 was fired at 850°C. Capacitance and Dissipation Factor, tan δ (%) of the resultant capacitors (X7R (BaTiθ3 type) ) were measured in the following manner. A. Capacitance Capacitance is defined as a proportionality factor between a charge and an applied voltage (C = Q/V) . For a parallel plate capacitor, capacitance can be calculated from the formula C = 4d in which K is dielectric constant, A is a play area in cm^ and d is thickness of dielectric layer in cm. The units of capacitance are farads (1 farad = 9 x 10 11 static unit) . Capacitance is measured at a frequency of 120 or 1 KHz and IV (A.C.) using a general radioautomatic RLC bridge model 1683. Capacitance was measured between the cathode coating generally soldered to an anode lead. In some case, the lead was soldered to cathode and used for the measurement . B. Dissipation Factor Dissipation Factor (DF) is the tangent of the angle (δ) of the current lugged from a 90° vector relative to voltage, which is expressed herein as % DF (100 x tan δ) . DF was measured using the same general radioautomatic bridge as mentioned above for the capacitance. Further, the post strength and elasticity modulus of the cured film were measured. The post strength refers to a force requied for bringing down a post after the conductive resin paste was coated on an aluminum substrate and a small disc with a grasp (post) was placed on a coated surface which was in turn cured. The results are shown in Table 2. Table 2 Post Elasticity Strength Modulus Electrical Properties Electrical Properties Electrical Properties Cured at of Cured Example Cured at 200°C, 60 min. Cured at 230°C, 60 min. Cured at 250°C, 60 min.250°C Film No. Capacitance (nF) tanδ(%) Capacitance (nF) tanδ (%) Capacitance (nF) tanδ(%) 60 min. (g) (MPA. ) 1 109 1.46 148 -~ to 2 108 1.48 217 2.6 10 4 MPA 3 104 6.49 525 2.6 10 MPA 4 Unmeasurable 994 2.5 10 4 MPA 5 108 1.44 108 1.43 108 1.42 6 107 1.44 108 1.43 108 1.43 7 108 1.47 108 1.43 109 1.42 8 107 1.43 9 Unmeasurable 108 1.51 108 1.43 10 108 1.48 11 13 4.3-4.6 10 4 MP The conductive pastes were prepared by mixing silver powders, phenoxy resin and solvent in the proportions indicated in Table 1 and milling the mixture by the similar procedure as in Examples 1-10. The paste used in Example 13 is a firing type of paste. The composition of the pastes is shown in Table 1. The curing condition and the electrical properties of the product are shown in Table 2.

Claims: £L M£ What is claimed: 1. An electrically conductive resin paste for use in the formation of a terminal electrode for a multilayer ceramic chip capacitor which comprises an admixture of an electrically conductive filler consisting of a noble metal powder, a resin binder, and a curing agent dispersed in an organic medium, the resin binder comprising a mixture of at least two resins of an epoxy resin and a thermosetting or thermoplastic resin and the weight ratio of the noble metal powder to the thermosetting resin being from about 100:5 to 100:45. 2. A multilayer ceramic chip capacitor which is formed by laminating alternately a plurality of dielectric sheets having an electrically conductive inner electrode adhered thereon in such a manner that the areas for taking out the inner electrode are arranged oppositely, laminating the outmost layer with a dielectric sheet for a protective layer to integrate them, applying the electrically conductive resin paste of claim 1 to said areas in the integrated laminate and fixing it to form a terminal electrode.

Inventors: INABA AKIRA, OBA TAKAYUKI, INABA, AKIRA, OBA, TAKAYUKI
Applicants: DU PONT, INABA AKIRA, OBA TAKAYUKI, E.I. DU PONT DE NEMOURS AND COMPANY, INABA, AKIRA, OBA, TAKAYUKI
Application year: 1993
Main classification: H01G1
IPC codes: H01B1, H01G4