JPS6361991B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a silver conductive paste characterized by containing an acid. Specifically, the present invention relates to a silver conductive paste with an improved balance between curing properties and conductivity of the resulting cured film. In the present invention, conductivity refers to initial conductivity and stability over time. Silver conductive paste, which uses silver as the main conductive material, has excellent conductivity and processability, so with the recent development of electronics, it is widely used as electrodes for resistors, capacitors, etc., and as wiring conductors for printed wiring boards. It is being used. Against this background, the demand for rationalization of processing steps is increasing, and the curing characteristics of silver conductive paste are becoming an important factor. However, the resins used as binders for silver conductive paste are usually thermosetting resins such as phenol resins, epoxy resins, melamine resins, and urethane resins, but it is difficult to cure these resins. There are methods of curing with heat alone without using a curing catalyst, and methods of curing with heat using a curing catalyst such as acid or alkali. A method using a curing catalyst is effective. However, when a curing catalyst such as an acid or alkali is used, the curing time can certainly be shortened and the curing temperature can be lowered, but on the other hand, the interaction between the silver particles and the curing catalyst can be reduced. Therefore, a defective conductive film is formed on the surface of the silver particles, resulting in an increase in electrical resistance. On the other hand, when a curing catalyst is not used, under normal conditions of use, curing is inevitably slow, resulting in high electrical resistance and poor stability against heat and humidity. For these reasons, at present no silver conductive paste has been obtained that satisfies the balance between curing properties and conductivity. In view of these circumstances, the present inventors conducted extensive research with the aim of obtaining a silver conductive paste with excellent curing properties and good conductivity in the resulting cured film. The present invention was completed based on the discovery that the balance between curing properties and electrical conductivity can be improved by combining it with an acid. That is, the present invention is an acid having an ionization constant pka of 2 or less and/or 3 or more, which is one or more acids selected from stearic acid, glycolic acid, oxalic acid, maleic acid, and pyrophosphoric acid. The present invention provides a silver conductive paste comprising a mixed resin of a butylated melamine resin and an epoxy resin, a solvent, and silver powder. The silver powder in the present invention is spherical powder, flake powder,
Any dendritic powder may be used, or a mixture of these silver powders may be used. The binder resin used in the present invention is made of a mixed resin of a butylated melamine resin and an epoxy resin, and the butylated melamine resin may be either n-butylated melamine resin or iso-butylated melamine resin. -Butylated melamine resins are more common. On the other hand, although there are no particular regulations regarding epoxy resins, they are usually bisphenol A type epoxy resins with an epoxy equivalent (equivalent amount per epoxy resin) of 400 to 400.
Use 700. Further, the mixing ratio of the butylated melamine resin and the epoxy resin is preferably a weight ratio of butylated melamine resin/epoxy resin = 9/1 to 3/7, and the amount added is 10 to 30 parts by weight per 100 parts by weight of silver. is preferred. The solvents used in the present invention include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol diethyl ether, diethylene glycol monobutyl ether acetate, benzyl alcohol, α -Tarpineol is a typical example. The acids with an ionization constant pka of 2 or less used in the present invention are oxalic acid, maleic acid, and pyrophosphoric acid, and the acids with a pka of 3
The above acids are stearic acid and glycolic acid. In the present invention, these acids can be used alone or in a mixture of two or more. Other acids cannot be used in the present invention because the curing speed is slow, or even if the curing speed is fast, the conductivity decreases. The amount of the acid added is 0.1 to 5 parts by weight per 100 parts by weight of the paint.
Parts by weight are preferred. In the present invention, the acid used may be added to the paste immediately before use or may be added to the paste in advance, but in the latter case, it is better to store it at a low temperature of 10°C or less. preferable. Further, when adding, it may be diluted as appropriate with a solvent used in the paste of the present invention. The paste of the present invention is usually prepared using a kneader such as a crusher or a roll. When preparing the paste of the present invention, reducing agents such as catechol and hydroquinone, complexing agents such as ethylenediaminetetraacetate, acetylacetone, and methylamine, noble metals belonging to Group B and Groups of the Periodic Table other than silver such as gold and palladium, and These compounds, viscosity modifiers such as polyvinyl alcohol and polyvinylpyrrolidone, surfactants such as potassium oleate, sodium dodecylbenzenesulfonate, and polyoxyethylene lauryl ether, carbon black, lubricants, flame retardants, thixotropic agents, etc. Can be added in small amounts. When curing the paste of the present invention, various substrates such as paper-phenolic resin laminates, glass epoxy resin laminates, organic substrates such as polyester films and polyimide films, and inorganic substrates such as glass and alumina can be used. It may be applied or printed on a surface such as the like and cured, or it may be inserted into a mold such as a through hole and cured. The curing conditions of the present invention vary depending on the type of acid, the blending ratio of the paste, etc., but are usually 110 to 150°C,
Conditions of 30-60 minutes are used. Also, before starting the main curing, heat treatment at a low temperature of 80 to 100℃ for 5 to 50 minutes.
May be precured for 20 minutes. A hot air electric constant temperature oven is usually used for curing. The silver conductive paste of the present invention has excellent curing properties and the obtained cured film has good conductivity, so it is extremely effective in streamlining the processing steps of printed wiring boards. The present invention will be explained in detail below using examples. The performance of the present invention was evaluated by the following method. Formation of print on a 225 mesh Tetron screen.
The emulsion was applied to a thickness of 10 micrometers. Directly onto this screen by photolithography,
A plate film having a rectangular pattern with a width of 2 mm and a length of 7 mm was formed. This pattern portion becomes an opening because it is not exposed to light. Next, a paper-phenol laminate measuring 100 mm x 100 mm was prepared with copper foil etched as terminals on both ends. The screen was placed on this laminate, an appropriate amount of silver conductive paste was placed in front of the opening of the strip pattern, and a pattern was printed using a squeegee (made of urethane rubber with a rubber hardness of No. 70). Thereafter, a printed body was formed under predetermined curing conditions. Using a conductivity milliohmmeter (MILLIOHMMETER 4328A model manufactured by Yokogawa Heuretsu Card Co., Ltd.),
Electrical resistance was measured by applying each terminal of a milliohmmeter to the copper foil portions at both ends of the printed body, and the thickness, length, and width of the printed body were measured to calculate the specific resistance value. In addition, the thickness can be measured using a surface roughness meter (manufactured by Kosaka Laboratory).
Type SE). Curing time While curing the printed material at a specified temperature, the electrical resistance value of the cured material was measured every 10 minutes, and the time required for the rate of change in electrical resistance value to become 1% or less was defined as the curing time. . Humidity Resistance As a measure of the stability of electrical conductivity over time, we used a constant temperature and humidity tester (ETAC's tabletop temperature and humidity tester JLH-400).
-40 type) under the conditions of 40℃ and RH95%, and the rate of change of the electrical resistance value ρ after 1000 hours with respect to the initial value ρ ₀ ρ - ρ ₀ /ρ ₀ ×100 was determined. . Example 1 14 grams of flaky silver powder with an average particle size of 1.2μ, 6 grams of spherical silver powder with an average particle size of 0.7μ, 1.2 grams of butylated melamine resin (Yuban-21R manufactured by Mitsui Toatsu Chemical Co., Ltd.), epoxy resin (oil 0.8 g of Epicoat-1001 (manufactured by Kasiel Co., Ltd.), 6.6 g of diethylene glycol monobutyl ether acetate, and 0.29 g of stearic acid (PKa = 5.77) as a curing catalyst were added to a 200 ml mortar and kneaded well with a pestle to obtain a silver paste. . Using the thus obtained paste, screen printing was performed using the method described above, and the curing temperature was 150°C.
It was cured. During curing, the electrical resistance value was measured while sampling every 10 minutes, and curing was continued until the rate of change in resistance value became 1% or less. The obtained cured product was subjected to the moisture resistance test described above to evaluate its moisture resistance. The results are shown in Table 1. Example 2 The same procedure as in Example 1 was followed except that 0.29 g of oxalic acid (PKa=1.27) was used as the curing catalyst. The results are shown in Table 1. Example 3 Stearic acid was used as a curing catalyst in Example 1.
The results are shown in Table 1 when a mixture of 0.145 grams and 0.145 grams of oxalic acid was used. Comparative Example 1 in Table 1 corresponds to the case of Example 1 in which no curing catalyst was used. As can be seen from the description in Table 1, the silver conductive pastes of the present invention all have a faster curing speed and at the same time significantly improved conductivity and moisture resistance.

[Table] *Additional amount is "weight%" (relative to silver paste)
represents.
Example 4 This corresponds to the case where 0.29 g of glycolic acid (PKa=4.76) was used as the curing catalyst in Example 1. Example 5 This corresponds to the case where 0.29 g of maleic acid (PKa=1.92) was used as the curing catalyst in Example 1. Example 6 This corresponds to the case where pyrophosphoric acid (PKa=0.85) was used as the curing catalyst in Example 1. Comparative Example 2 This corresponds to the case where phosphoric acid (PKa=2.12) was used as the curing catalyst in Example 1. Comparative Example 3 This corresponds to the case where malonic acid (PKa=2.69) was used as the curing catalyst in Example 1. The evaluation results of Examples 4 to 6, Comparative Examples 2 and 3 are shown in Table 2. When phosphoric acid or malonic acid having a PKa of 2 to 3 is used, the initial conductivity is significantly reduced.

[Table] *Additional amount represents weight% (relative to silver paste)

Claims (1)

[Claims] 1 An acid with an ionization constant pka of 2 or less and/or 3 or more selected from stearic acid, glycolic acid, oxalic acid, maleic acid, and pyrophosphoric acid.
A mixed resin of a butylated melamine resin and an epoxy resin, characterized by containing one or more acids;
Silver conductive paste consisting of solvent and silver powder.