Copper paste composition

Abstract: A copper paste composition for low temperature firing on a ceramic substrate is comprised of inorganic components and organic vechicle components. The inorganic components mainly consist of copper powder, cuprous oxide powder, cupric oxide powder and glass powder. The mixing ratio of the inorganic components of this composition is 100 parts by weight of copper powder, about 10 to 15 parts by weight of cuprous oxide powder, about 12 to 17 parts by weight of cupric oxide powder, and about 7 to 12 parts by weight of glass powder. The mixing ratio of the inorganic components with the organic vehicle components is about 95 to 70 parts by weight of the inorganic components and about 5 to 30 parts by weight of the organic vehicle components. Also the the average particle size of the copper powder is in a range of about 0.5 to 3 µm and the oxygen content of the copper powder is about 0.05 to 0.15 weight percent.

Description: The present invention relates to copper paste compositions, in particular, copper paste compositions that are suited to low temperature firing at 550°C to 750°C. Copper pastes can be fired onto ceramic or other refractory substrates to form a "thick film" conductor. Copper pastes for low temperature firing are disclosed in Japanese Patent Provisional Publication No. SHO-62-110202, equivalent U.S. Patent No. 4,521,329 and equivalent EP-A-0 131 778, all relating to a conductor element. The conductor element described is made by kneading a composition including an organic medium containing an organic resin and firing the composition onto a ceramic substrate to pyrolyze the organic resin, leaving a conductive copper film. The composition is composed of metallic copper in the form of metallic copper coated with a copper oxide coating, copper oxide particles, and an inorgainc binder, as follows: (a) 0.2 weight percent or more of oxygen in the form of a copper oxide coating, (b) copper oxide particles in an amount of 15 weight percent or less of the metallic copper, (c) an inorganic binder including copper oxide particles (d), the weight ratio of the copper oxide particles (d) to the copper oxide coating being 0 to 0.5 and the ratio of the total sum of copper oxide, as particles and coating, to the weight of oxide coated copper metal being 0 to 15 parts by weight of copper oxide to 100 parts by weight of oxide coated copper metal. The publications describe the oxygen in the metallic copper oxide coating and the oxygen in the copper oxide particles as sources for supplying oxygen for pyrolysis or combustion of the resin in the organic medium of the paste composition. It is generally well known that copper oxide is used as an additive in copper pastes. For high temperature firing applications (850°C to about 1000°C) there is no need to distinguish cuprous oxide and cupric oxide since both equally form chemical bonds with ceramic substrates. Therefore, the mixing ratio of the cuprous oxide and cupric oxide does not matter as practiced in the prior art. However, for low temperature firing, as described below, it has been found that the effect of cuprous oxide on the solderability, initial joint strength, and strength after aging differs from that of cupric oxide. It should be noted that oxygen in copper powder is present in the form of cuprous oxide rather than cupric oxide, and that this cuprous oxide does not directly contribute to combustion of the organic medium as an oxidizing agent, and lowers the degree of sintering when present in excess. Hence prior art copper pastes using copper powder with an oxygen content of 0.2 % or more by weight do not produce sufficient sintering after low temperature firing. As a result, the product has low resistance to soldering and, in particular, its strength after aging is significantly deteriorated. The present invention has been made in view of such problems of the prior art. It is intended to provide copper paste compositions having, after low temperature firing, a high degree of sintering, good solderability, high initial joint strength, and high joint strength after aging. According to the present invention, there is provided a copper paste composition comprising about 95 to about 70 percent by weight of inorganic components and about 5 to about 30 percent by weight of organic vehicle components, the inorganic components comprising a powdered refractory material, and copper powder including cuprous oxide powder and cupric oxide powder in amounts such that about 10 to about 15 parts by weight of cuprous oxide powder, about 12 to about 17 parts by weight of cupric oxide powder, and about 7 to about 12 parts by weight of refractory powder, are included for each 100 parts by weight of copper powder. In a preferred embodiment of the average particle size of the copper powder is about 0.5 to about 3 µm and that the oxygen content of the copper powder is about 0.05 to about 0.15 weight percent. Also according to the invention there is provided a method of making a thick film conductor on a substrate, comprising applying to the substrate a paste comprising copper powder, a binder, a solvent and a powdered refractory material, the copper powder including about 10 to about 15 parts by weight of cuprous oxide powder and about 12 to about 17 parts by weight of cupric oxide powder for each 100 parts by weight of copper powder and firing at a temperature sufficient to remove the solvent and the binder from the copper powder and the powdered refractory material and to adhere the copper powder, bound in the refractory material, to the substrate. Copper powder having a sufficient oxygen content, as described below, may be obtained, for example, by reducing cuprous oxide with hydrazine and then reducing with hydrogen. Due to reasons discussed below the average particle size of the cuprous oxide powder is desirably within a range from about 0.5 to µm about 5 µm, and in particular, the range from about 0.5 µm to about 2 µm is specially desired. There is no special need to limit the average particle size of the cupric oxide powder. The range, however, is desirably from about 0.5 µm to about 5 µm. The softening point of the glass powder is preferably in a range from about 300°C to about 600°C due to reasons discussed below. The softening poing is more preferably in a range from about 300°C to about 550°C. There is no particular reason to limit the average particle size of the glass powder. The particle size range, however, is desirably between about 1 µm and about 10 µm. The organic vehicle may be any commonly used organic vehicle. The organic vehicle typically is comprised of a solvent and a resin. For reasons discussed below, desirably there will be about 10 parts oor less by weight of the resin for every 100 parts by weight of the organic vehicle. Suitable resins include ethyl cellulose, acrylic resins, methacrylic resins, terpene resins and mixtures thereof. Suitable solvents include terpineol, C₁₀ to C₁₈ alcohols and mixtures thereof. Additionally, the organic vehicle optionally can include a suitable plasticizer such as dibutyl phthalate. Additives to the above-mentioned components can include about 5 parts or less by weight of inorganic oxides, particularly metal oxides such as vanadium oxide, lead oxide, antimony oxide and mixtures thereof per 100 parts by weight of the copper powder. Furthermore, surfactants such as a dispersant for maintaining stability of the paste and a thickener optionally may be added. The paste is typically prepared by mixing the required amounts of the components in any mixing device designed for pastes, such as in a universal mixer, until they form a paste, and then continuing homoginization of the composition, such as in a three roller mill. One method for making a thick-film circuit from the prepared paste is to print the paste in an appropriate pattern on a refractory substrate (made of alumina in many cases) by means of a screen printing machine, dry the printed substrate, and then fire the printed substrate in a belt furnace filled with N₂ or other inert gas for a cycle of about 40 minutes to about 60 minutes. The peak temperature is from about 550°C to about 750°C, and preferably from about 580°C to about 650°C, and the time for keeping the printed substrate at the peak temperature is from about 5 minutes to about 10 minutes. In low temperature firing of a copper paste, proper attention should be given to raising the degree of sintering, and to allowing easier evaporation of the organic vehicle. To achieve these objectives, the oxygen content in the copper powder should be kept within the appropriate range. If the oxygen content in the copper powder exceeds about 0.15 weight percent, sintering will become deficient, and in turn, solder wettability will be degraded, and the strength after aging will be deteriorated significantly. On the other hand, it is difficult and not practical to produce copper powder having an oxygen content less than about 0.05 weight percent. The average particle size of the copper powder having the above-mentioned oxygen content should be within a range from about 0.5 µm to about 3 µm. If the size exceeds about 3 µm, sintering will be deficient, and if the size is less than about 0.5 µm, it will be difficult to make the powder into a paste, and evaporation of the organic vehicle will be deficient. Cuprous oxide has a strong affinity with copper. Hence, cuprous oxide functions as a sintering agent, and after sintering, remains in the sintered particle boundary and prevents penetration of solder, and partly reacts with the substrate to form chemical bonds and improve the joint strength. To secure these effects, it is necessary to add about 10 parts or more by weight of cuprous oxide powder to each 100 parts by weight of copper powder. However, if more than about 15 parts by weight of cuprous oxide powder are added, the solder wettability will be deteriorated, and the strength after aging will be degraded. If the average particle size of the cuprous oxide powder is too coarse, the above-mentioned functions and effects will not be achieved. It, therefore, is desirable to set the average particle size of the cuprous oxide powder at or less than about 5 µm, and perferably, at or less than about 2 µm. Cupric oxide provides oxygen to the organic vehicle in the firing process, and therefore will be reduced. In this way, cupric oxide assists the combustion of the organic vehicle. To this end, the addition of about 12 parts or more by weight of cupric oxide powder to each 100 parts by weight of copper powder achieves efficient pyrolysis of the organic vehicle during the firing process. If about 17 parts or more by weight of cupric oxide powder is added, sintering of the copper particle will be hindered. The glass powder or other powdered refractory material serves to wet the surface of the substrate and to facilitate the sintering of the copper powder through liquid-phase sintering. Therefore, it is desirable to add about 7 to about 12 parts by weight of glass powder, or other refractory powder, to each 100 parts by weight of copper powder. To attain an appropriate wettability, it is desirable to select glass powder having a softening point of about 600°C or less, preferably having a softening point of about 550°C or less. The amount of the organic vehicle components relative to all the inorganic components depends on the particular vehicle used. To assure complete evaporation of the organic vehicle, the amount of the organic vehicle components should be from about 5 to about 30 parts by weight organic vehicle components to about 95 to about 70 parts by weight of other inorganic components. To achieve the full advantage of the present invention in improving the wettability, and in assuring complete evaporation of the organic vehicle, the amount of the resin is desirably about 10 parts or less by weight to 100 parts by weight of the organic vehicle. The present invention will be explained with reference to preferred embodiments according to the present invention. It should be noted that the present invention is in no way limited by these embodiments. Example To form conductor pads, the pastes having the compositions as shown in Table 1 were printed in an appropriate pattern on 96 % alumina ceramic substrates by means of a screen printing machine, and then dried at 120°C for 10 minutes. The printed substrates then were fired in a nitrogen environment under the following conditions: peak firing temperature -- 600°C; peak temperature time --10 minutes; heating-cooling cycle time -- 60 minutes; and 0₂ concentration in the firing zone -- 5 ppm. The conductor pads obtained under these conditions were measured for solder wettability, initial joint strength, and joint strength after aging. The results of the measurements are shown in Table 3. Table 2 describes the characteristics of the materials listed in Table 1. A. Solder Wettability A sample to which a soldering flux (Tamura Kaken XA-100) had been applied was immersed in a solder bath at 230°C ± 5°C for 5 + 0.5 seconds, and then was taken out of the bath. The average solder wet area of ten conductor pads (each pad measuring 2 mm x 2 mm) was measured. A sample having a wet area ratio of 90% or more was judged to pass the test, and a sample having a ratio of less than 90% was judged to fail the test. B. Initial Joint Strength A sample to which the above-mentioned soldering flux had been applied was immersed in a solder bath at 230°C ± 5°C for 5 ± 0.5°C seconds. Then 0.65 mm diameter tinned copper wires were attached to eight 2 mm x 2 mm conductor pads by means of a soldering iron, as shown in the drawing. In the drawing, 1 represents a substrate, 2 a conductor pad, 3 copper wire, and 4 solder. The soldered copper wire 3 then was pulled by a tension tester in a direction perpendicular to the substrate 1 (in the direction of the arrow) at a velocity of 10 mm/sec to measure the strength needed to separate the conductor pad 2 from the substrate 1. A sample was judged to pass the test when the average of the eight pad measurements was 3 kg or more and the minimum pad measurement was 2.5 kg or more. C. Joint Strength After Aging In the same manner as the initial joint strength measurements, copper wires were attached to the conductor pads formed on the substrates and then the said substrates were subjected to a temperature of 150°C for 250 hours and then the strength of each joint was measured in the same manner as described for initial joint strength. A sample was judged to pass the test when the average of the eight pad measurements was 2.5 kg or more and the minimum pad measurement was 2.0 kg or more. The figure is an explanatory cross-sectional side view illustrating the method for measuring joint strength. Table 3 clearly indicates the following points: 1) Embodiments 1 through 10 included copper powder having a mean partticle size and an oxygen content within the ranges according to the present invention, and appropriate amounts of cuprous oxide powder, cupric oxide powder and glass powder were mixed, and the ratio of the inorganic components and the organic vehicle components was appropriate. Accordingly, the evaporation of the organic vehicle was complete, and sintering of the copper powder was sufficient. As a result, the solder wettability was satisfactory, and the initial joint strength and the strength after aging were high, and all of the values of such characteristics were at or above the pass level. 2) In the case of Control 1 (C1), the mixing ratio of cuprous oxide is less than the lower limit according to the present invention. Accordingly, the initial joint strength and the strength after aging were low, and failed to attain the respective pass levels. 3) In the case of Control 2 (C2), the mixing ratio of cupric oxide was less than the lower limit according to the present invention, and the evaporation of the organic vehicle was deficient, and the sample failed to pass the solder wettability test. As a result, both the initial joint strength and the strength after aging were low and failed to attain the respective pass levels. 4) In the case of Control 3 (C3), no cupric oxide powder was mixed. Hence the organic vehicle did not evaporate, and the solder wettability was extremely low and failed to pass the test. As a result, the initial joint strength and the strength after aging were considerably low and failed to pass the tests. 5) In the case of Control 4 (C4) , no cuprous oxide was mixed. As a result, both the initial joint strength and the strength after aging were extremely low and failed to pass the tests. 6) In the case of Control 5 (C5), the mixing ratio of cuprous oxide was greater than the upper limit according to the present invention. As a result, the solder wettability failed to pass the test, and the strength after aging was also low and failed to pass the test. The copper paste compositions according to the present invention are applicable, in addition to thick film circuits, as electrode materials for ceramic parts, such as various capacitors. It has therefore been shown that very useful copper paste compositions in practical applications, having satisfactory solder wettability and high initial joint strength and joint strength after aging, can be provided by using copper powder having an oxygen content and a mean particle size within the respective ranges according to the present invention and by controlling the mixing ratios of cuprous oxide powder to cupric oxide powder, glass powder to said copper powder, and the ratio of the organic vehicle components to the inorganic components within the respective ranges according to the present invention.

Claims: A copper paste composition comprising 95 to 70 percent by weight of inorganic components and 5 to 30 percent by weight of organic vehicle components, the inorganic components comprising a powdered refractory material, and copper powder including cuprous oxide powder and cupric oxide powder in amounts such that 10 to 15 parts by weight of cuprous oxide powder, 12 to 17 parts by weight of cupric oxide power, and 7 to 12 parts by weight of refractory powder, are included for each 100 parts by weight of copper powder. The copper paste composition of Claim 1 wherein the copper powder has a mean particle size of 0.5 to 3 µm and an oxygen content between 0.05 and 0.15 percent by weight. The copper paste composition of Claim 2 wherein the cuprous oxide powder has a particle size range from 0.5 to 5 µm. The copper paste composition of Claim 3 wherein the cuprous oxide powder has a particle size range from 0.5 to 2 µm. The copper paste composition of any of claims 2 to 4 further comprising 5 parts or less by weight of an inorganic oxide per 100 parts by weight of the copper powder. The copper paste composition of Claim 5 wherein the inorganic oxide is vanadium oxide, lead oxide, antimony oxide or mixtures thereof. The copper paste composition of any preceding claim wherein the organic vehicle components include a solvent and a resin. The copper paste composition of Claim 7 wherein the solvent is terpineol, C₁₀ to C₁₈ alcohols, or mixtures thereof. The copper paste composition of Claim 7 or Claim 8 wherein the organic vehicle components further include a plasticizer. The copper paste composition of Claim 9 wherein the plasticizer is dibutyl phthalate. The copper paste composition of any of claims 7 to 10 wherein the resin is cellulosic resin, acrylic resin, methacrylic resin, terpene resin or mixtures thereof. The copper paste composition of any of claims 7 to 11 wherein the organic vehicle components include 10 percent or less by weight of resin. The copper paste composition of any preceding claim wherein the powdered refractory material is glass. The copper paste composition of Claim 13 wherein the glass powder has a softening point of between 300°C and 600°C A method of making a thick film conductor on a substrate, comprising applying to the substrate a paste comprising copper powder, a binder, a solvent and a powdered refractory material, the copper powder including 10 to 15 parts by weight of cuprous oxide powder and 12 to 17 parts by weight of cupric oxide powder for each 100 parts by weight of copper powder and firing at a temperature sufficient to remove the solvent and the binder from the copper powder and the powdered refractory material and to adhere the copper powder, bound in the refractory material, to the substrate. A method as claimed in Claim 15 in which the temperature of firing is 550°C to 750°C.

Inventors: ECHIGO MASASHI, OKADA SUSUMU, SAEKI SHUJI, SAKURABA MASAMI, SUEHIRO MASATOSHI, ECHIGO, MASASHI, OKADA, SUSUMU, SAEKI, SHUJI, SAKURABA, MASAMI, SUEHIRO, MASATOSHI
Applicants: DAI ICHI KOGYO SEIYAKU CO LTD, DOWA MINING CO, DAI-ICHI KOGYO SEIYAKU CO., LTD., DOWA MINING CO., LTD.
Application year: 1990
Main classification: H01B1
IPC codes: H01B1, C22C32, C04B35, B22F1, C04B41, H05K1