JP7628530B2 - Conductive paste and conductive pattern using same - Google Patents

Description

The present invention relates to a conductive paste used to form conductive patterns, such as electrodes of electronic devices, and a conductive pattern using the same.

Conductive pastes are used for a variety of purposes, such as mounting and fixing semiconductor elements (semiconductor chips) such as ICs and LSIs onto metal pieces called lead frames, forming circuits on substrates by printing or other methods, and forming electrodes for electronic components such as capacitors.

In addition, with the recent increase in the degree of integration of semiconductor chips and the increasing density of circuits on circuit boards, conductive pastes are required to enable circuit patterns to be printed with high precision with little variation in line width, and the printed circuit patterns must also have high electrical and thermal conductivity, high resistance to migration, and excellent workability due to their moderate viscosity and fluidity.

For example, Japanese Patent Application Laid-Open No. 2012-18783 (Patent Document 1) discloses a conductive paste that can form conductive film wiring with low volume resistivity and has improved adhesion to substrates and printability by adding fine silver particles with an average primary particle diameter of 10 nm or more and 200 nm or less to silver particles with an average particle diameter of 0.5 μm or more to suppress a decrease in the fluidity of the conductive paste.

Furthermore, JP 2019-102273 A (Patent Document 2) discloses a conductive paste that uses three types of fillers, including nanoparticles, to provide an appropriate viscosity for maintaining a fine line shape while maintaining low electrical resistance, thereby suppressing a decrease in viscosity.

However, in the conductive pastes described in Patent Documents 1 and 2, it is difficult to disperse nano-sized fillers, and they tend to have high fluidity. If the fluidity is too high, the conductive paste will bleed after printing, causing variations in line width and resulting in circuit shorts. In addition, there is also the problem that the conductive patterns formed using these conductive pastes do not necessarily have high migration resistance.

In addition, conductive powders such as silver particles become more difficult to disperse when the loading amount is large. Therefore, compositions highly loaded with conventional flake-shaped (or flat) silver powder or spherical silver powder are prone to problems such as poor appearance and reduced adhesive strength and workability.

JP 2012-18783 A JP 2019-102273 A

Therefore, the present invention aims to provide a conductive paste that has little variation in line width, enables printing of conductive patterns with high precision, and has high electrical conductivity and high migration resistance even in the printed conductive pattern, and has excellent workability due to its moderate viscosity and fluidity, and a conductive pattern (circuit pattern) using the same.

In view of the above problems, the inventors conducted extensive research into the types, shapes, and combinations of conductive powders that are effective in suppressing line width variations and migration caused by bleeding during printing, and that achieve high electrical conductivity and excellent workability. As a result, they discovered that the above problems can be solved by using silver-coated copper flakes as a base for a conductive paste to which silver-coated silica powder is added, thereby completing the present invention.

That is, according to the present invention, there is provided a conductive paste containing silver-coated copper flakes and silver-coated silica powder, and a conductive pattern using the same. The conductive paste of the present invention may further contain a binder resin, a solvent, and a hardener.

The conductive paste of the present invention is a composition formed into a paste by adding silver-coated copper flakes, silver-coated silica powder, and a binder resin to the above. As long as it contains silver-coated copper flakes, silver-coated silica powder, and a binder resin, it may contain other components such as a solvent and an antifoaming agent as necessary, provided that the effects of the present invention are not impaired.

The silver-coated copper flakes used in the present invention are not particularly limited, and any known flake-shaped copper powder coated with silver can be used. The volume average particle diameter (D ₅₀ ) of the silver-coated copper flakes is preferably 1.0 μm or more and 50 μm or less, more preferably 2.0 μm or more and 20 μm or less. In particular, if the volume average particle diameter (D ₅₀ ) of the silver-coated copper flakes is 2.0 μm or more and 20.0 μm or less, it becomes very easy to deal with thin lines when drawing a circuit. Note that, as the copper powder to be coated with silver, spherical or nearly spherical copper powder or flake-shaped copper powder is known, but in the present invention, it is preferable to use silver-coated copper flakes from the viewpoint of suppressing the decrease in electrical contacts after circuit formation and suppressing the increase in electrical resistance.

The silver-coated copper flakes may be completely coated with silver, or may have some copper exposed. Being completely coated with silver is preferable because it reduces the resistivity. The amount of silver-coated copper flakes is preferably 10% by volume or more and 40% by volume or less, and more preferably 30% by volume or more and 40% by volume or less, based on the total non-volatile content of the conductive paste. If the amount of silver-coated copper flakes is 10% by volume or more and 40% by volume or less, it is possible to improve workability by having appropriate viscosity and fluidity while keeping the resistivity low.

The silver-coated silica powder used in the present invention is not particularly limited, and any known silica powder can be used as long as it is a silver-coated silica powder.The volume average particle diameter ( _D50 ) of the silver-coated silica powder is preferably 0.050 μm or more and 50.0 μm or less, and more preferably 0.1 μm or more and 5.0 μm or less.In particular, if the volume average particle diameter ( _D50 ) of the silver-coated silica powder is 0.1 μm or more and 5.0 μm or less, a high filling rate can be achieved, thereby suppressing the specific resistance value to a low level.

The silver-coated silica powder may be completely coated with silver, or some of the silica may be exposed. Complete coating with silver is preferable because it reduces the resistivity. The amount of silver-coated silica powder is preferably in the range of 99:1 to 15:85 by volume ratio of silver-coated copper flakes to silver-coated silica powder, and more preferably in the range of 99:1 to 20:80.

If the volume ratio of silver-coated copper flakes to silver-coated silica powder is in the range of 99:1 to 15:85, the fluidity of the resulting conductive paste is particularly favorable, reducing line variation when printed, and improving the electrical conductivity and migration resistance of the formed circuit pattern. The shape of the silver-coated silica powder is not particularly limited as long as it is particulate. Particulate silica powder is particularly favorable because of its excellent fluidity.

In addition, in this specification, the numerical (ratio) ranges indicated using "from" and "to" indicate ranges that include the numerical (ratio) values written before and after "from" and "to" as the minimum value (ratio) and maximum value (ratio), respectively.

The binder resin used in the present invention is not particularly limited and any known resin can be used. Thermosetting resins include epoxy resins, phenolic resins, urea resins, unsaturated polyester resins, alkyd resins, polyurethanes, and thermosetting polyimides. Thermoplastic resins such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyester, and polyamide, which have functional groups remaining at their terminals, may also be used in combination with a curing agent.

In order to achieve workability and printability suitable for various printing methods such as screen printing, it is preferable to mix the resin binder in a ratio of 30 volume % or more and 60 volume % or less based on the total non-volatile content of the conductive paste.

There are no particular limitations on the solvent used in the conductive paste of the present invention. It can be selected appropriately depending on the solubility of the resin used and the type of printing method, etc. Examples of the solvent of the present invention include one or a mixture of two or more of ester-based solvents, ketone-based solvents, glycol ether-based solvents, aliphatic solvents, alicyclic solvents, aromatic solvents, alcohol-based solvents, water, etc.

Examples of ester-based solvents include ethyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, amyl acetate, ethyl lactate, and dimethyl carbonate. Examples of ketone-based solvents include acetone, methyl ethyl ketone, methyl isobutyl ketone benzene, diisobutyl ketone, diacetone alcohol, isophorone, and cyclohexanenone. Examples of glycol ether-based solvents include ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, and acetate esters of these monoethers, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, and acetate esters of these monoethers.

On the other hand, examples of aliphatic solvents include n-heptane, n-hexane, isohexane, isoheptane, etc. Examples of alicyclic solvents include methylcyclohexane, ethylcyclohexane, cyclohexane, etc. Examples of aromatic solvents include toluene, xylene, tetralin, etc. Examples of alcohol solvents (excluding the above-mentioned glycol ether solvents) include ethanol, propanol, butanol, etc.

Furthermore, in a circuit pattern in which multiple linear lines are printed on a PET resin sheet with intervals of approximately 100 μm using the conductive paste of the present invention described above, adjacent lines do not short circuit (contact) with each other due to small variation in line width, and a circuit pattern can be obtained in which each line has excellent electrical conductivity.

The conductive paste of the present invention has an appropriate viscosity and flowability, and therefore has excellent workability, and has the excellent effect of enabling circuit patterns to be printed with high precision with little variation in line width. The printed conductive pattern also has the excellent effect of having high electrical conductivity and high migration resistance.

1 is a micrograph of a circuit pattern (conductive pattern) printed using the conductive paste of Example 4. 1 is a micrograph of a circuit pattern (conductive pattern) printed using the conductive paste of Comparative Example 1. 1 is a micrograph of a circuit pattern (conductive pattern) printed using the conductive paste of Comparative Example 3.

A conductive paste according to one embodiment of the present invention and a conductive pattern using the same will be described in detail below with reference to the drawings. Note that the present invention is not limited to the examples shown below, and various modifications are possible within the scope of the technical concept of the present invention.

1. Preparation of Conductive Paste The conductive paste according to one embodiment of the present invention and the conductive paste of the comparative example were prepared using the following raw materials and conditions (see "Table 1").
[Example 1]
The conductive paste of Example 1 was prepared by mixing 65.1 g of Toyaltec Filler®/TFM-C05F (manufactured by Toyo Aluminum Co., Ltd.) having a volume average particle diameter ( _D50 ) of 6 μm as silver-coated copper flakes, 0.29 g of Toyaltec Filler®/TFM-S02P (manufactured by Toyo Aluminum Co., Ltd.) having a volume average particle diameter (D50) of ₂ μm as silver-coated silica powder, 12.0 g of Elitel®/UE-3210 (manufactured by Unitika Co., Ltd.) as binder resin, 1.7 g of blocked isocyanate (product name: 7992, manufactured by Baxenden Co., Ltd.) as hardener, and 24.9 g of a mixed solvent of ethyl carbitol acetate and isophorone in a weight ratio of 16:9 as solvent, and kneading the mixture using a disper and a three-roll mill.

[Example 2]
The conductive paste of Example 2 was produced under the same conditions as Example 1, except that 0.57 g of Toyal Tech Filler (registered trademark)/TFM-S02P (manufactured by Toyo Aluminum Co., Ltd.) having a volume average particle size (D ₅₀ ) of 2 μm was blended as the silver-coated silica powder.

[Example 3]
The conductive paste of Example 3 was produced under the same conditions as Example 1, except that 1.2 g of Toyal Tech Filler®/TFM-S02P (manufactured by Toyo Aluminum Co., Ltd.) having a volume average particle size (D ₅₀ ) of 2 μm was blended as the silver-coated silica powder.

[Example 4]
The conductive paste of Example 4 was produced under the same conditions as Example 1, except that 2.3 g of Toyal Tech Filler®/TFM-S02P (manufactured by Toyo Aluminum Co., Ltd.) having a volume average particle size (D ₅₀ ) of 2 μm was blended as the silver-coated silica powder.

[Example 5]
The conductive paste of Example 5 was produced under the same conditions as Example 1, except that 25.9 g of Toyal Tech Filler®/TFM-C05F (manufactured by Toyo Aluminum Co., Ltd.) with a volume average particle diameter ( _D50 ) of 6 μm was used as the silver-coated copper flakes, and 32.3 g of Toyal Tech Filler®/TFM-S02P (manufactured by Toyo Aluminum Co., Ltd.) with a volume average particle diameter ( _D50 ) of 2 μm was used as the silver-coated silica powder.

[Comparative Example 1]
A conductive paste of Comparative Example 1 was produced under the same conditions as in Example 1, except that no silver-coated silica powder was added.

[Comparative Example 2]
The conductive paste of Comparative Example 2 was produced under the same conditions as Example ₁ , except that no silver-coated copper flakes were added, and 47.9 g of Toyal Tech Filler (registered trademark)/TFM-S02P (manufactured by Toyo Aluminum Co., Ltd.) having a volume average particle diameter (D50) of 2 μm was added as silver-coated silica powder.

[Comparative Example 3]
The conductive paste of Comparative Example 3 was produced under the same conditions as Example ₁ , except that no silver-coated copper flakes were added, and 7.9 g of Toyal Tech Filler (registered trademark)/TFM-S02P (manufactured by Toyo Aluminum Co., Ltd.) having a volume average particle diameter (D50) of 2 μm was added as silver-coated silica powder.

[Comparative Example 4]
The conductive paste of Comparative Example ₄ was produced under the same conditions as in Example 1, except that 2.3 g of spherical silver powder (product name: HXR-Ag, manufactured by Nippon Atomize Processing Co., Ltd.) having a volume average particle diameter (D50) of 5.7 μm was added instead of the silver-coated silica powder.

[Comparative Example 5]
The conductive paste of Comparative Example 5 was produced under the same conditions as in Example 1, except that 65.1 g of silver flakes (product name: TCG-1, manufactured by Tokuriki Chemical Laboratory Co., Ltd.) having a volume average particle diameter ( _D50 ) of 4.8 μm was used instead of the silver-coated copper flakes, and 2.3 g of Toyal Tech Filler (registered trademark)/TFM-S02P (manufactured by Toyo Aluminum Co., Ltd.) having a volume average particle diameter ( _D50 ) of 2 μm was used as the silver-coated silica powder.

[Comparative Example 6]
A conductive paste of Comparative Example 6 was produced under the same conditions as in Example 1, except that 65.1 g of silver flakes (product name: TCG-1, manufactured by Tokuriki Chemical Laboratory Co., Ltd.) having a volume average particle diameter ( _D50 ) of 4.8 μm was added as a substitute for the silver-coated copper flakes, and no silver-coated silica powder was added.

Table 1 shows the amount (g) of each component added to the conductive pastes of Examples 1 to 5 and Comparative Examples 1 to 6, and the mixing ratio (Vol %) of each component relative to the total non-volatile content of the conductive paste.

2. Preparation of circuit pattern (conductive pattern) Using the conductive pastes of Examples 1 to 5 and Comparative Examples 1 to 6, a screen plate made of a circuit pattern with a material of stainless steel, a screen mesh number of 325 meshes, an emulsion thickness of 10 μm, a line width of 100 μm, and a spacing between each line of 100 μm was used to print on a PET resin sheet by a screen printer (product name: DP-320 type screen printer, manufactured by Newlong Precision Industry Co., Ltd.). Subsequently, the sheet on which the circuit pattern was printed was dried at 150 ° C for 30 minutes to prepare a circuit pattern for evaluation.

3. Evaluation of Conductive Paste and Circuit Pattern (1) Viscosity To investigate the relationship between the workability and spreading of the conductive paste, the viscosity of the conductive pastes of Examples 1 to 5 and Comparative Examples 1 to 6 was measured at a temperature of 25° C. and a rotation speed of 0.5 rpm using a Brookfield Type Viscometer (Model No. DV2THBCJ0). The results are shown in Table 2.

(2) Variation in line width The above-mentioned evaluation circuit patterns prepared using the conductive pastes of Examples 1 to 5 and Comparative Examples 1 to 6 were observed at a magnification of 50 times using an inspection microscope (product name: ECLIPSE L200, manufactured by Nikon Corporation), and images were taken. The obtained images were then binarized using image analysis software (product name: Winroof 2018, manufactured by Mitani Shoji Co., Ltd.). The line widths of 1,000 points were measured from the binarized image, and the value of 3σ, which is an index of the variation in line width, was obtained. 3σ means an interval three times the standard deviation (σ), and if the distribution is normal, about 99.7% of the samples fall within the range of the average value ±3σ.

The smaller the 3σ value of the line width variation in the circuit pattern, the better, but if the 3σ value exceeds 50 μm, adjacent lines may short-circuit when current is applied, etc., so it is rated as "x" (bad), and if it is 50 μm or less, it is rated as "o" (good). Also, if a short circuit (contact) was observed between some of the adjacent lines from the beginning of the circuit pattern formation, it was rated as "x" (bad) regardless of the 3σ value. The above results are shown in Table 2.

Of the above-mentioned evaluation circuit patterns, a micrograph of the circuit pattern produced using the conductive paste of Example 4 is shown in Figure 1, a micrograph of the circuit pattern produced using the conductive paste of Comparative Example 1 is shown in Figure 2, and a micrograph of the circuit pattern produced using the conductive paste of Comparative Example 3 is shown in Figure 3.

(3) Migration Resistance The migration resistance of the circuit pattern was evaluated by measuring the time until a short circuit occurred while each of the above-mentioned evaluation circuit patterns was kept under the conditions of 85° C., 85% humidity, and an applied voltage of 50 V. The presence or absence of a short circuit in the circuit pattern was confirmed using a migration tester (product name: MODEL MIG-87B, manufactured by IMV Corporation).

The longer the time until the circuit pattern shorts out, the better the migration resistance. In this embodiment, a time until a short circuit occurs of 800 hours or more was evaluated as "good," and a time of less than 800 hours was evaluated as "poor." The results are shown in Table 2.

(4) Specific resistance value The specific resistance value (Ω cm) of the circuit pattern was measured by using an evaluation screen plate made of polyester resin, screen mesh number 280 mesh, emulsion thickness 9 microns, square shape of 4.8 cm x 4.8 cm, printing the conductive paste on a PET film, and drying at 150 ° C for 30 minutes to form a coating film. The thickness of the coating film was confirmed by measuring with a Digimatic Standard Outside Micrometer (product name: IP65 COOLANT PROOF Micrometer, manufactured by Mitutoyo Corporation). It was confirmed by measuring with a four-probe surface resistance meter (product name: Loresta GP, manufactured by Mitsubishi Analytec Co., Ltd.). For each evaluation circuit pattern, five arbitrary points were measured, and the average value was taken as the specific resistance value. Specifically, the dimensions of the printed matter, the average thickness of the printed matter, and the coordinates of the measurement points were input into the four-probe surface resistance meter, and the value obtained by automatically calculating was taken as the specific resistance value of the conductor layer (circuit pattern / conductive pattern).

The smaller the resistivity value, the better the conductivity. The dimensions of the printed matter refer to the dimensions consisting of the maximum length and maximum width of the pattern of a given shape that the printed matter has. The smaller the resistivity value, the better it is, and a resistivity of 2.0×10 ⁻⁴ Ω·cm or less was evaluated as "◯" (good), and conversely, a resistivity of more than 2.0×10 ⁻⁴ Ω·cm was evaluated as "×" (bad). The results are shown in Table 2.

The overall evaluation of each evaluation circuit pattern was given a rating of "Good" (O) only if it received a rating of "O" in all of the above-mentioned evaluations of "Line width variation," "Migration resistance," and "Specific resistance value." If there was even one rating of "X" in any of the individual evaluations, the overall evaluation was also given a rating of "X" (Bad).

Table 2 shows the evaluation results of the conductive pastes of Examples 1 to 5 and Comparative Examples 1 to 6 and the circuit patterns (conductive patterns) produced using them.

4. Consideration Comparing the conductive pastes of Examples 1 to 5 and Comparative Examples 1 to 6 from Table 2, the conductive paste of the present invention is based on silver-coated copper flakes, to which silver-coated silica powder is added, and thus good results can be obtained in the evaluations of "line width variation,""migrationresistance," and "specific resistance value" of the printed circuit pattern (conductive pattern). As a result, the conductive pattern can be printed with high precision with little variation in line width. In addition, the printed conductive pattern has high electrical conductivity and thermal conductivity, high migration resistance, and excellent workability due to a viscosity of 30 Pa·s or more and 70 Pa·s or less.

In particular, a comparison of the conductive pastes of Examples 1 to 5 with the conductive pastes of Comparative Examples 1 to 3 showed that when the volume ratio of silver-coated copper flakes to silver-coated silica powder is preferably in the range of 99:1 to 15:85, and more preferably in the range of 99:1 to 20:80, the printed circuit pattern has small line width variations and excellent migration resistance and conductivity can be obtained, and further, when the volume ratio of silver-coated copper flakes to silver-coated silica powder is in the range of 99:1 to 90:10, as in the conductive pastes of Examples 1 to 4, even better migration resistance can be obtained.

In addition, the conductive pastes of Examples 1 to 5 contain silver-coated copper flakes in an amount between 10% and 40% by volume relative to the total non-volatile content of the conductive paste, thereby achieving small line width variations in the printed circuit pattern, excellent migration resistance and conductivity, and improving workability by maintaining appropriate viscosity and fluidity while keeping the resistivity low. Furthermore, it was found that even better migration resistance can be obtained by using the conductive pastes of Examples 1 to 4 in which the amount of silver-coated copper flakes is between 30% and 40% by volume relative to the total non-volatile content of the conductive paste.

In addition, in the conductive pastes of Examples 1 to 5, it was found that it was effective to incorporate a resin binder in a ratio of 30 volume % or more and 60 volume % or less relative to the total non-volatile content of the conductive paste in order to achieve workability and printability suitable for various printing methods such as screen printing.

In the conductive pastes of Examples 1 to 5, it was found that when the volume average particle diameter (D ₅₀ ) of the silver-coated copper flakes is preferably 1.0 μm or more and 50 μm or less, more preferably 2.0 μm or more and 20 μm or less, the printed circuit pattern has little variation in line width, and it is extremely easy to deal with thin lines when drawing the circuit pattern.

In addition, in the conductive pastes of Examples 1 to 5, it was found that when the volume average particle diameter (D ₅₀ ) of the silver-coated silica powder is preferably 0.050 μm or more and 50.0 μm or less, more preferably 0.1 μm or more and 5.0 μm or less, a high filling rate is achieved, thereby keeping the resistivity low and reducing the variation in line width in the printed circuit pattern.

Furthermore, Figures 1 to 3 show that a circuit pattern (conductive pattern) formed by printing multiple lines spaced about 100 μm apart using the conductive paste of the present invention has small variations in line width so that adjacent lines do not short circuit (contact) with each other, and also results in a circuit pattern with excellent electrical conductivity for each line.

L: Printed line G: Gap (PET resin sheet)
S: Short circuit part (contact part)

Claims (4)

The present invention includes a silver-coated copper flake and a silver-coated silica powder .
The average particle size of the silver-coated copper flakes is 2.0 μm or more and 20.0 μm or less, and
The conductive paste is characterized in that the average particle size of the silver-coated silica powder is 0.1 μm or more and 5.0 μm or less . The conductive paste according to claim 1, characterized in that the mixing ratio of the silver-coated copper flakes to the silver-coated silica powder is within the range of 99:1 to 15:85 by volume. 3. The conductive paste according to claim 1, further comprising a resin binder in an amount of 30% by volume or more and 60% by volume or less based on the total non-volatile content of the conductive paste. A conductive pattern formed by using the conductive paste according to claim 1 .

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