Copper nanoink, substrate for printed wiring board, and method for producing substrate for printed wiring board

Abstract

A copper nanoink which comprises copper nanoparticles and a solvent, and wherein: the copper nanoparticles have a first average particle diameter of 1-200 nm inclusive, the first average particle diameter being a volume-based average diameter obtained by analyzing a scanning electron microscope image of the copper nanoparticles; the copper nanoparticles contain at least one of a copper oxide and a copper hydroxide, and in the copper nanoink, a first ratio, which is the ratio of the total mass ratio M2 of the copper oxide and the copper hydroxide to the mass ratio M1 of copper, as measured by a reference intensity ratio method, is 5.0% or less.

Description

Technical Field

[0001] This disclosure relates to copper nano-inks, substrates for printed wiring boards, and methods for manufacturing substrates for printed wiring boards.

[0002] This application claims priority based on Japanese Application No. 2023-039679, filed on March 14, 2023, and incorporates all the contents of that Japanese application. Background Art

[0003] In recent years, copper nanoparticle inks, in which copper nanoparticles are dispersed in solvents such as water, have been used in the formation of metal layers on substrates for printed wiring. The metal layer comprises a sintered body containing copper nanoparticles. The metal layer is formed by firing a coating film formed on the surface of a base film by coating with copper nanoparticle ink (Patent Document 1).

[0004] Existing technical documents

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2021-17641 Summary of the Invention

[0007] The copper nanoparticle ink disclosed herein is a copper nanoparticle ink comprising copper nanoparticles and a solvent, wherein the first average particle size of the copper nanoparticles is 1 nm or more and 200 nm or less, the first average particle size being the average diameter based on volume obtained by analyzing a scanning electron microscope image of the copper nanoparticles, the copper nanoparticles comprising at least one of copper oxide and copper hydroxide, and wherein in the copper nanoparticle ink, the ratio of the total mass ratio M2 of copper oxide and copper hydroxide determined by the reference intensity ratio method to the mass ratio M1 of copper, i.e., the first ratio, is 5.0% or less. Attached Figure Description

[0008] Figure 1 This is a schematic cross-sectional view showing a printed wiring board substrate according to one embodiment of the present disclosure. Detailed Implementation

[0009] [The technical problem this disclosure aims to solve]

[0010] In the manufacturing process of copper nanoparticle inks, the copper nanoparticles contained in the inks are easily oxidized, readily forming copper oxides (CuO, Cu2O). Furthermore, due to the reaction between the alkaline components used in the manufacturing process and the copper ions dissolved from the copper nanoparticles, copper hydroxides (Cu(OH)2) are easily formed. Therefore, the concentrations of copper oxides and copper hydroxides in the copper nanoparticle inks tend to increase. If the concentrations of copper oxides and copper hydroxides in the copper nanoparticle inks increase, there is a tendency for the conductivity of the sintered copper nanoparticle layer obtained by sintering the coated film of the copper nanoparticle ink to decrease. Therefore, there is a need for a copper nanoparticle ink that can produce a sintered copper nanoparticle layer with high conductivity.

[0011] The purpose of this disclosure is to provide a copper nanoparticle ink capable of obtaining a sintered body layer of copper nanoparticles with high conductivity, a printed wiring board substrate having a sintered body layer of copper nanoparticles with high conductivity, and a method for manufacturing a printed wiring board substrate having a sintered body layer of copper nanoparticles with high conductivity.

[0012] [The Effects of This Disclosure]

[0013] According to this disclosure, a method for manufacturing a copper nanoparticle ink capable of obtaining a sintered body layer of copper nanoparticles with high conductivity, a printed wiring board substrate having a sintered body layer of copper nanoparticles with high conductivity, and a printed wiring board substrate having a sintered body layer of copper nanoparticles with high conductivity can be provided.

[0014] [Description of embodiments of this disclosure]

[0015] First, the implementation methods of this disclosure are listed and explained.

[0016] (1) The copper nano-ink disclosed herein is a copper nano-ink containing copper nanoparticles and a solvent, wherein the first average particle size of the copper nanoparticles is 1 nm or more and 200 nm or less, the first average particle size is the average diameter of the volume reference obtained by analyzing the scanning electron microscope image of the copper nanoparticles, the copper nanoparticles contain at least one of copper oxide and copper hydroxide, and in the copper nano-ink, the ratio of the total mass ratio M2 of copper oxide and copper hydroxide determined by the reference intensity ratio method to the mass ratio M1 of copper, i.e., the first ratio, is 5.0% or less.

[0017] According to this disclosure, it is possible to provide copper nanoparticle ink that can obtain a sintered body layer of copper nanoparticles with high conductivity, and a substrate for printed wiring boards having a sintered body layer of copper nanoparticles with high conductivity. "The first ratio is 5.0% or less" can also be described as "M2 / M1 is 0.05 or less".

[0018] (2) In (1) above, the ratio of the content of chlorine as a mass standard C2 determined by ion chromatography to the content of copper as a mass standard C1 determined by ICP emission spectroscopy, i.e., the second ratio, can be less than 0.3%.

[0019] Therefore, copper nanoparticles are not easily aggregated. "The second ratio is less than 0.3%" can also be recorded as "C2 / C1 is less than 0.003".

[0020] (3) In (1) or (2) above, the second average particle size of the copper nanoparticles can be 5 nm or more and 400 nm or less. The second average particle size is the average diameter based on the number of particles measured using an ultrasonic attenuation particle size analyzer.

[0021] This improves the dispersibility and stability of copper nanoparticles in copper nano-inks.

[0022] (4) In any of (1) to (3) above, the first proportion may be 0.1% or more.

[0023] Therefore, a thin copper oxide layer and a thin copper hydroxide layer tend to exist around the copper nanoparticles, which have an increased affinity for the solvents and dispersants of the copper nanoparticles.

[0024] (5) The substrate for printed wiring board disclosed herein comprises: a base film including a first main surface; and a sintered body layer of copper nanoparticles formed on the first main surface, wherein the Cu2O content of the sintered body layer is 3.0% by mass or less, and the gloss of the sintered body layer is 300 or more.

[0025] The printed wiring board substrate disclosed herein can have a sintered body layer of copper nanoparticles with high conductivity.

[0026] (6) In (5) above, the resistivity of the sintered body layer can also be 100 μΩ·cm or less. This further improves the conductivity of the sintered body layer. When the sintered body layer is electroplated, if the resistivity of the sintered body layer is 100 μΩ·cm or less, good electrical conductivity is obtained.

[0027] (7) The method for manufacturing a printed wiring board substrate disclosed herein is a method for manufacturing a printed wiring board substrate as described in (5) or (6) above, comprising: a step of coating the copper nano-ink of any one of (1) to (4) above onto the base film; a step of drying the coated film of the copper nano-ink; and a step of firing the dried film of the copper nano-ink. In the firing step, the sintered body layer is formed on the base film.

[0028] According to this manufacturing method, it is possible to manufacture a substrate for printed wiring boards having a sintered body layer of copper nanoparticles with high conductivity.

[0029] [Details of the embodiments disclosed herein]

[0030] The following describes specific examples of the copper nano-ink and the substrate for printed wiring boards disclosed herein.

[0031] In this disclosure, when more than one value is recorded as the lower limit and the upper limit of the numerical range, a combination of any value recorded in the lower limit and any value recorded in the upper limit is also disclosed. For example, when a1, b1, and c1 are recorded as the lower limit and a2, b2, and c2 are recorded as the upper limit, the following combinations are disclosed: a1 and below a2, a1 and below b2, a1 and below c2, b1 and below a2, b1 and below b2, b1 and below c2, c1 and below a2, c1 and below b2, and c1 and below c2.

[0032] [Implementation Method 1: Copper Nanoparticle Ink]

[0033] One embodiment of this disclosure (hereinafter also referred to as "Embodiment 1") relates to a copper nanoparticle ink comprising copper nanoparticles and a solvent. The first average particle size of the copper nanoparticles is 1 nm or more and 200 nm or less. The first average particle size is the average diameter based on volume obtained by analyzing a scanning electron microscope image of the copper nanoparticles. The copper nanoparticles comprise at least one of copper oxide and copper hydroxide. In the copper nanoparticle ink, the first ratio, i.e., the ratio of the total mass ratio M2 of copper oxide and copper hydroxide as determined by the reference intensity ratio method to the mass ratio M1 of copper, is 5.0% or less.

[0034] In copper nanoparticle inks, the first proportion is less than 5.0%, indicating a low content of copper oxides and copper hydroxides. Therefore, using this copper nanoparticle ink, a sintered layer of copper nanoparticles with high conductivity can be obtained.

[0035] <Copper Nanoparticles>

[0036] First average particle size

[0037] The first average particle size of copper nanoparticles is greater than 1 nm and less than 200 nm. The lower limit of the first average particle size is 1 nm, but it can be 10 nm or 20 nm. If the first average particle size is greater than 1 nm, the dispersibility and stability of the copper nanoparticles in the copper nano-ink can be improved. The upper limit of the first average particle size is 200 nm, but it can also be 150 nm. If the first average particle size is less than 200 nm, the porosity in the sintered layer obtained by firing the copper nano-ink is less likely to increase, and the conductivity of the sintered layer is improved. The first average particle size of copper nanoparticles can be greater than 10 nm and less than 150 nm, or greater than 20 nm and less than 150 nm.

[0038] The first average particle size of copper nanoparticles is the average diameter of a volume reference obtained by resolving scanning electron microscope (SEM) images of copper nanoparticles. The first average particle size is equivalent to the average diameter of a volume reference based on the particle size of each copper nanoparticle. The method for determining the first average particle size of copper nanoparticles is as follows: Prepare a polyimide film, coat one side of the polyimide film with copper nanoparticle ink, and allow the coating film to dry to form a dried film. Observe the surface of the dried film using a scanning electron microscope (SEM) (Hitachi High-Tech "SU-8020" (trademark)) to obtain an SEM image. The magnification is set to 100,000x. In the SEM image, set two rectangular measurement fields of 900nm × 1300nm. The two measurement fields are set to be non-overlapping. Using image processing software (Image-Pro "Image-Pro" (trademark) manufactured by Pixnet), measure the equivalent circle diameter of all copper nanoparticles in the two measurement fields, and calculate the average diameter of the volume reference. "Equivalent circle diameter of copper nanoparticles in the field of view" refers to the diameter of a circle in the field of view with an area equal to that of the copper nanoparticles. The average diameter of the volume reference is also denoted as the mean volume diameter (MV), defined by Equation 1 below.

[0039] MV=Σ(Vi·di) / Σ(Vi) Equation 1

[0040] In Equation 1, Vi represents the volume of each particle, and di represents the particle size.

[0041] In the copper nano-ink produced simultaneously, even if multiple measurement fields are set arbitrarily, it can be confirmed that there are no significant differences between these measurement values.

[0042] The First Ratio

[0043] The copper nanoparticles contain at least one of copper oxide and copper hydroxide. In the copper nano-ink, the first ratio, determined by the reference strength ratio method, is 5.0% or less, where the total mass ratio M2 of copper oxide and copper hydroxide is relative to the mass ratio M1 of copper. The total mass ratio M2 of copper oxide and copper hydroxide is the sum of the mass ratios of copper oxide and copper hydroxide. The copper oxide is CuO and Cu2O. The copper hydroxide is Cu(OH)2. It is presumed that the copper oxide (CuO and Cu2O) contained in the copper nanoparticles is formed by the oxidation of copper in the copper nanoparticles during the manufacturing process of the copper nano-ink. It is presumed that the copper hydroxide (Cu(OH)2) contained in the copper nanoparticles is formed by the reaction of copper in the copper nanoparticles with copper ions partially dissolved from the copper nanoparticles during the manufacturing process of the copper nano-ink.

[0044] The upper limit of the first proportion is 5.0%, but it can be 4.5%, 4.0%, 3.5%, 3.0%, 2.4%, or 2.0%. If the first proportion is below 5.0%, the content of copper oxide and copper hydroxide in the copper nanoparticle ink is low, thus a sintered body layer of copper nanoparticles with high conductivity can be obtained. There is no particular limitation on the lower limit of the first proportion. The first proportion can be above 0%, above 0%, or above 0.1%. If the first proportion is above 0.1%, a thin copper oxide layer and copper hydroxide layer are likely to exist around the copper nanoparticles, which tends to increase the affinity with the solvent and dispersant of the copper nanoparticles. The first percentage can be above 0% and below 5.0%, or above 0% and below 5.0%, or above 0.1% and below 5.0%, or above 0.1% and below 4.0%, or above 0.1% and below 3.0%, or above 0.1% and below 2.4%, or above 0.1% and below 2.0%.

[0045] The presence of copper nanoparticles comprising at least one of copper oxide and copper hydroxide can be confirmed by the following steps: A polyimide film is prepared, and copper nanoparticle ink is coated onto one side of the polyimide film. The coated film is dried in a vacuum to form a dried film. For the copper nanoparticles contained in the dried film, mapping analysis is performed under vacuum using an energy dispersive X-ray specttroscope (EDX) attached to a scanning transmission electron microscope (STEM: Scanning Transmission Electron Microscope, JEOL Ltd. "ARM200F" (trademark)) (STEM-EDX). If at least one of copper oxide and copper hydroxide is confirmed at the edge of the copper nanoparticles in the mapping image, it is determined that the copper nanoparticles comprise at least one of copper oxide and copper hydroxide.

[0046] The first ratio is obtained by XRD (X-ray diffraction) of the copper nano-ink, based on the XRD spectrum obtained using the Reference Intensity Ratio (RIR) method. The conditions for X-ray diffraction measurement are as follows.

[0047] Measurement apparatus: Rigaku Corporation "SmartLab"

[0048] Using X-rays: Cu-Ka

[0049] Excitation conditions: 45kV, 200mA

[0050] Incident optical system: CBO-f

[0051] Sample stage: XY worktable

[0052] Slit size: 0.8mm

[0053] Mask: 0.5mm

[0054] Light-receiving optical system: HyPix-3000 (two-dimensional)

[0055] Scanning method: 2θ-θ scan

[0056] Measurement range: 2θ is 30° to 80°

[0057] Step size: 0.03

[0058] Scanning speed: 2° / min

[0059] In the XRD spectrum of copper nano-ink, the integrated intensities of the strongest lines for Cu, Cu₂O, CuO, and Cu(OH)₂ were determined. Using the RIR values ​​registered in the database, the mass ratios of each component were calculated based on these integrated intensities. In the XRD spectra, the strongest line for Cu is located at a diffraction angle of 2θ = 43.30°, for Cu₂O at 2θ = 36.44°, for CuO at 2θ = 35.54° or 38.73°, and for Cu(OH)₂ at 2θ = 23.83°. The RIR values ​​for each component were determined with reference to the powder X-ray diffraction database published by the International Center for Diffraction Data (ICDD).

[0060] The mass ratio of Cu is equivalent to the mass ratio of copper M1, and the total mass ratio of Cu2O, CuO, and Cu(OH)2 is equivalent to the total mass ratio of copper oxides and copper hydroxides M2. The first ratio is calculated based on the mass ratio of copper M1 and the total mass ratio of copper oxides and copper hydroxides M2.

[0061] Second average particle size

[0062] The second average particle size of copper nanoparticles can be above 5 nm and below 600 nm. The lower limit of the second average particle size of copper nanoparticles can be 5 nm, 25 nm, or 80 nm. If the second average particle size of copper nanoparticles is above 5 nm, the dispersibility and stability of copper nanoparticles in the copper nano-ink can be improved. The upper limit of the second average particle size of copper nanoparticles can be 600 nm, 400 nm, or 250 nm. If the second average particle size of copper nanoparticles is below 600 nm, especially below 400 nm, the porosity in the sintered layer obtained by firing the copper nano-ink is less likely to increase, and the conductivity of the sintered layer is improved. The second average particle size of copper nanoparticles can be above 25 nm and below 400 nm, or above 80 nm and below 250 nm.

[0063] The second average particle size of the copper nanoparticles is the average diameter based on the number of particles, as measured using an ultrasonic attenuation particle size analyzer. The second average particle size is equivalent to the average diameter based on the equivalent sphere diameter of the copper nanoparticle aggregates in the copper nanoparticle ink. The "equivalent sphere diameter of the copper nanoparticle aggregates" refers to the diameter of a sphere with the same volume as the copper nanoparticle aggregates. The ultrasonic attenuation particle size analyzer used is the Dispersion Technology "DT-100" (trademark). The measurement range is 5 nm to 1000 μm. The average diameter based on the number of particles is also denoted as the mean number diameter (MN), defined by Equation 2 below.

[0064] MN=Σ(ni·di) / Σni Equation 2

[0065] In Equation 2, ni represents the number of particles and di represents the particle size.

[0066] <Second proportion>

[0067] Copper nanoparticle inks may also contain chlorine. It is presumed that the chlorine in copper nanoparticle inks originates from titanium trichloride or copper(II) chloride (CuCl2) used in the production of copper nanoparticles. In copper nanoparticle inks, the ratio of the chlorine content (C2, mass %) determined by ion chromatography to the copper content (C1, mass %) determined by ICP emission spectroscopy—that is, the second ratio—can be less than 0.45%, less than 0.3%, less than 0.2%, less than 0.15%, less than 0.06%, or less than 0.02%. A second ratio of less than 0.45%, where less than 0.3%, can improve the dispersibility and stability of copper nanoparticles in the copper nanoparticle ink. There is no particular limitation on the lower limit of the second ratio. For example, the second ratio can be greater than 0%, more than 0%, or greater than 0.02%. The second ratio can be greater than 0% and less than 0.45%, or greater than 0.02% and less than 0.30%.

[0068] The second ratio was determined according to the following steps. The copper nano-ink was diluted with water at a dilution ratio of 50,000 times to obtain a diluted solution. The mass percentage (in mass%) of copper in the diluted solution was determined by ICP emission spectroscopy. The determination based on ICP emission spectroscopy was performed using the "iCAP6300" (trademark) instrument manufactured by Thermo Fisher Scientific. The mass percentage (in mass%) of chlorine in the diluted solution was determined by ion chromatography. The determination based on ion chromatography was performed using the "ICS-2100" (trademark) ion chromatography system manufactured by Thermo Fisher Scientific. The second ratio was equal to {mass percentage (in mass%) of chlorine in the diluted solution / mass percentage (in mass%) of copper in the diluted solution}.

[0069] <Solvent>

[0070] There are no particular limitations on the solvent for copper nanoparticle inks. Water can also be used as a solvent for copper nanoparticle inks.

[0071] When using water as a solvent, the water content in the copper nanoparticle ink can be between 20 and 1900 parts by mass relative to 100 parts by mass of copper nanoparticles. If the water content is 20 parts by mass or more, the copper nanoparticle content in the copper nanoparticle ink will not be excessive, allowing for uniform coating of the copper nanoparticle ink on the base film. If the water content is 1900 parts by mass or less, the copper nanoparticle content in the copper nanoparticle ink is sufficient, enabling the formation of a well-bonded layer with the necessary thickness and density on the surface of the base film of the printed wiring board substrate.

[0072] <Dispersant>

[0073] Copper nanoparticle inks can also contain dispersants. Examples of dispersants include polyethyleneimine, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene glycol. By including dispersants in copper nanoparticle inks, the particles become less prone to aggregation and sedimentation. One or more of these dispersants can be used. Hereinafter, polyethyleneimine, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene glycol will be collectively referred to as "dispersants."

[0074] In copper nano-inks, the mass ratio of the dispersant to the copper can be 0.01% or more and 2.5% or less, or 0.1% or more and 1.5% or less. When using two or more dispersants, the mass of the dispersant is the total mass of the dispersants.

[0075] <Additives>

[0076] To the extent that it does not impair the effectiveness of this disclosure, copper nano-inks may also contain additives. Additives may be, for example, ascorbic acid or amine polymers.

[0077] <Manufacturing Method of Copper Nano-Ink>

[0078] The manufacturing method of copper nanoparticle ink is described. The manufacturing method of copper nanoparticle ink includes a step of precipitating copper nanoparticles, a step of cleaning copper nanoparticles, and a step of dispersing copper nanoparticles in a solvent.

[0079] Process for precipitating copper nanoparticles

[0080] In the process of precipitating copper nanoparticles, the copper nanoparticles are precipitated using a liquid-phase reduction method. In this process, copper ions are reduced in an aqueous solution containing a reducing agent, and copper nanoparticles are precipitated. An example of a liquid-phase reduction method is the titanium redox method.

[0081] The process for precipitating copper nanoparticles includes a process for preparing a reducing agent aqueous solution and a process for reducing copper ions.

[0082] In the process of preparing the reducing agent aqueous solution, an aqueous solution containing reducing copper ions is prepared. In the process of reducing copper ions, an aqueous solution containing copper ions is added to the reducing agent aqueous solution, or a water-soluble metal compound that generates copper ions through ionization is added to the reducing agent aqueous solution.

[0083] In liquid-phase (aqueous solution) reaction systems, various reducing agents can be used to reduce copper ions and precipitate them. Examples of reducing agents include sodium borohydride, sodium hypophosphite, hydrazine, trivalent titanium ions (e.g., titanium trichloride) and divalent cobalt ions (transition metal ions), ascorbic acid, reducing sugars such as glucose and fructose, and polyols such as ethylene glycol and glycerol. Trivalent titanium ions can also be used as reducing agents. The liquid-phase reduction method using trivalent titanium ions as the reducing agent is the titanium redox method. In the titanium redox method, copper ions are reduced by the redox reaction when trivalent titanium ions are oxidized to tetravalent ions, causing copper nanoparticles to precipitate. According to the titanium redox method, copper nanoparticles with fine and uniform particle sizes are easily formed.

[0084] In the reducing agent aqueous solution, for example, complexing agents, dispersants such as polyvinyl alcohol, or pH adjusters may be further added.

[0085] Various well-known complexing agents can be used as complexing agents. Examples of such agents include sodium citrate, sodium tartrate, sodium acetate, gluconic acid, sodium thiosulfate, ammonia, and ethylenediaminetetraacetic acid. One or more of these agents may also be used. Sodium citrate can also be used as a complexing agent.

[0086] Dispersants that are incorporated into the reducing agent aqueous solution include, for example, polyethyleneimine, polyvinylpyrrolidone, and polyvinyl alcohol.

[0087] The pH adjuster used in the reducing agent aqueous solution is a common acid or base such as hydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide, sodium carbonate, or ammonia. The pH of the reducing agent aqueous solution is, for example, above 5 and below 13. If the pH of the reducing agent aqueous solution is too low, the metal precipitation rate slows down, and the particle size distribution tends to widen. If the pH of the reducing agent aqueous solution is too high, the metal precipitation rate becomes too high, and the precipitated copper nanoparticles agglomerate, potentially forming clusters or chains of coarse particles.

[0088] In the process of reducing copper ions, copper ions are added to an aqueous reducing agent solution. In this solution, the copper ions are reduced by the reducing agent, and copper nanoparticles precipitate out.

[0089] If a water-soluble copper compound is dissolved in water, copper ions are produced through the ionization of the water-soluble copper compound. Examples of water-soluble copper compounds include copper(II) nitrate trihydrate (Cu(NO3)2·3H2O), copper(II) sulfate pentahydrate (CuSO4·5H2O), copper(II) chloride (CuCl2), and copper(II) sulfate pentahydrate (CuSO4·5H2O).

[0090] If a water-soluble copper compound is directly added to a reducing agent aqueous solution, the reaction proceeds locally immediately after the compound is added. This results in non-uniform particle size distribution of the copper nanoparticles. Therefore, it is also possible to dissolve the water-soluble copper compound in water and add the aqueous solution containing copper ions to the reducing agent aqueous solution.

[0091] The lower limit of the temperature of the reducing agent aqueous solution in the copper ion reduction process can be 0℃ or 15℃. A good reduction reaction efficiency can be obtained if the temperature is above 0℃. The upper limit of the temperature of the reducing agent aqueous solution in the precipitation process can be 100℃, 60℃, or 50℃. If the temperature is below 100℃, the growth rate of copper nanoparticles becomes moderate, and the particle size is easily adjusted.

[0092] Process for cleaning copper nanoparticles

[0093] In the process of cleaning copper nanoparticles, the copper nanoparticles precipitated in the reducing agent aqueous solution are cleaned.

[0094] The reaction solution containing the precipitated copper nanoparticles was left to stand for at least 12 hours to allow the copper nanoparticles to precipitate. The supernatant was then removed by decantation. Approximately 95% of the supernatant was removed.

[0095] After removing the supernatant, the solution containing the copper nanoparticle precipitate is washed with an aqueous solution containing a dispersant. Specifically, an aqueous solution containing a dispersant is added to the solution containing the copper nanoparticle precipitate, and the mixture is stirred for 60 minutes to obtain a mixture. Hereinafter, the aqueous solution containing the dispersant is also referred to as "dispersant aqueous solution". The same dispersant used in the process of precipitating the copper nanoparticles can be used. Examples of dispersants include polyethyleneimine, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene glycol. Next, the stirred mixture is subjected to solid-liquid separation by centrifugation to recover the dehydrated solid material containing the copper nanoparticles and the dispersant. Hereinafter, the process of washing the solution containing the copper nanoparticle precipitate with an aqueous solution containing a dispersant is also referred to as "dispersant washing".

[0096] Pure water is added to the solid material and dispersed using a stirrer. The mixture is then centrifuged to recover the dehydrated solid material. This process will be referred to as "pure water washing" below. A second pure water wash is then performed to recover the solid material containing copper nanoparticles.

[0097] The process of cleaning copper nanoparticles can remove ionic components that are not adsorbed onto the copper nanoparticles.

[0098] The recovered solid material containing copper nanoparticles can also be pulverized into powder through processes such as drying and crushing. To prevent agglomeration, the solid material can be dispersed in a high concentration in an aqueous solution instead of being pulverized.

[0099] Process for dispersing copper nanoparticles

[0100] In the process of dispersing copper nanoparticles, a solid material containing copper nanoparticles is dispersed in a solvent to achieve an appropriate concentration of copper nanoparticles. This yields copper nanoparticle ink.

[0101] Water can also be used as a solvent for copper nanoparticle inks. Alternatively, water-compatible organic solvents can also be used in conjunction with water. Examples of organic solvents include alcohols such as ethanol, IPA, ethylene glycol, and glycerol, as well as glycol ethers such as diethylene glycol monobutyl ether.

[0102] The process of adding additives

[0103] The manufacturing method of copper nano-ink can also include the step of adding additives such as ascorbic acid or amine polymers to the dispersion of copper nanoparticles.

[0104] [Implementation Method 2: Substrate for Printed Wiring Board]

[0105] refer to Figure 1 A printed wiring board substrate according to one embodiment of this disclosure (hereinafter also referred to as "Embodiment 2") will be described. The printed wiring board substrate 1 includes a base film 2 including a first main surface 4 and a sintered body layer 3 of copper nanoparticles formed on the first main surface 4. The Cu2O content of the sintered body layer is 3.0% by mass or less. The gloss of the sintered body layer 3 is 300 or more.

[0106] The substrate 1 for printed wiring boards can have high conductivity.

[0107] <Base membrane>

[0108] The base film 4 of the substrate 1 for the printed wiring board is insulating. There are no particular limitations on the base film 4. Conventionally known base films can also be used as the base film 4. For example, the base film 4 is a polyimide film, a PET (polyethylene terephthalate) film, a PEEK (polyether ether ketone) film, or a fluoropolymer film.

[0109] <Sintered Body Layer>

[0110] The Cu2O content of sintered body layer 3 is 3.0% by mass or less. The upper limit of the Cu2O content of sintered body layer 3 can be 2.0% by mass or 1.5% by mass. The lower limit of the Cu2O content of sintered body layer 3 can also be 0% by mass. The Cu2O content of sintered body layer 3 can be more than 0% by mass and less than 3.0% by mass, more than 0% by mass and less than 2.0% by mass, or more than 0% by mass and less than 1.5% by mass.

[0111] The Cu2O concentration in sintered layer 3 was determined by X-ray diffraction using "Empyrean" (trademark) manufactured by Malvern Panalytical.

[0112] The lower limit of the gloss of the sintered body layer 3 is 300, but it can be 310 or 320. If the gloss of the sintered body layer 3 is above 300, the conductivity of the sintered body layer 3 increases. There is no particular limit to the upper limit of the gloss of the sintered body layer 3; for example, it can be 500. The gloss of the sintered body layer 3 can be above 300 and below 500, above 310 and below 500, or above 320 and below 500.

[0113] The gloss of sintered layer 3 was measured using a "Gloss Checker IG-410" (trademark) manufactured by Horiba Manufacturing Co., Ltd.

[0114] The upper limit of the resistivity of the sintered body layer 3 can be 100 μΩ·cm, 90 μΩ·cm, 80 μΩ·cm, or 60 μΩ·cm. There is no particular limitation on the lower limit of the resistivity of the sintered body layer 3; for example, it can be 1.6 μΩ·cm. The resistivity of the sintered body layer 3 can be above 1.6 μΩ·cm and below 100 μΩ·cm, above 1.6 μΩ·cm and below 90 μΩ·cm, above 1.6 μΩ·cm and below 80 μΩ·cm, or above 1.6 μΩ·cm and below 60 μΩ·cm.

[0115] The resistivity of sintered layer 3 was measured using a Loresta-GX MCP-T700 instrument manufactured by Nitto Seiko Analytech.

[0116] The average thickness of the sintered body layer 3 can be 0.01 μm or more and 5 μm or less, or 0.02 μm or more and 3 μm or less. The method for determining the average thickness of the sintered body layer 3 is as follows: Using a fluorescence X-ray apparatus (measuring apparatus: Hitachi High-Tech "FT160S" (trademark)), the thickness is measured at five locations on the sintered body layer 3. The average thickness of the five locations is the average thickness of the sintered body layer 3.

[0117] The substrate 1 for printed wiring boards can be used to manufacture printed wiring boards by subtractive or semi-additive methods. Printed wiring boards manufactured using the substrate 1 have conductive patterns including a layer that has been patterned onto the sintered body layer 3.

[0118] <Manufacturing Method of Substrate for Printed Wiring Boards>

[0119] The manufacturing method of the substrate 1 for printed wiring boards includes, for example, a process of coating the copper nano-ink of Embodiment 1 onto a base film 4 (coating process); a process of drying the coated film of copper nano-ink (drying process); and a process of firing the dried film of copper nano-ink (firing process). In the firing process, a sintered body layer 3 is formed on the base film 4.

[0120] Coating Process

[0121] In the coating process, copper nano-ink is coated onto the base film 2 to form a coating film.

[0122] As a method for coating copper nano-inks onto a base film, conventionally known coating methods such as spin coating, spray coating, rod coating, die coating, slot coating, roller coating, and dip coating can be used. For example, copper nano-inks can also be coated only on a portion of the surface of the base film 4 using screen printing, a dispenser, or inkjet printing.

[0123] Drying Process

[0124] In the drying process, the coating film of copper nano-ink is dried to form a dried film.

[0125] In the drying process, the coated film can also be dried by heating or blowing air. The coated film can also be dried by blowing hot air onto it. The temperature of the hot air should be such that it does not cause the solvent of the copper nano-ink to boil. For example, the temperature of the hot air is above 15°C and below 80°C. The wind speed of the hot air should also be such that it does not cause fluctuations in the coated film. For example, the wind speed of the hot air on the surface of the coated film is above 5 m / s and below 10 m / s.

[0126] Firing Process

[0127] In the firing process, the dried film is fired after the drying process. As a result, the solvent and dispersant of the copper nano-ink in the dried film evaporate or thermally decompose, and the copper nanoparticles are sintered. Thus, a sintered body layer 3 is formed on one side (first main surface 4) of the base film 4. The firing process can also be carried out in nitrogen atmosphere.

[0128] In the case of copper nano-inks containing polyethyleneimine, polyethyleneimine has a reducing effect during the firing process, thus the copper oxides in the copper nano-ink are reduced to copper through a reduction reaction. Furthermore, the carbon constituting polyethyleneimine is oxidized and released as carbon dioxide.

[0129] When copper nano-inks contain polyvinyl alcohol, polyvinyl alcohol is resistant to oxidation, so the copper in the dried film is not easily oxidized.

[0130] The lower limit of the firing temperature in the firing process can be either 300℃ or 325℃. If the firing temperature is above 300℃, copper oxide can be fully reduced, and copper nanoparticles can be completely sintered. The upper limit of the firing temperature can be either 390℃ or 375℃. If the firing temperature is below 390℃, copper is less likely to oxidize.

[0131] The minimum firing time in the firing process can be either 10 minutes or 20 minutes. If the firing time is more than 10 minutes, the copper oxide can be fully reduced, and the copper nanoparticles can be completely sintered. The maximum firing time can be either 6 hours or 4 hours. If the firing time is less than 6 hours, the copper is less likely to oxidize.

[0132] In the manufacturing method of the substrate 1 for printed wiring board, metal can also be laminated on the sintered body layer of copper nano-ink formed after the firing process by electroless plating or electroplating. This metal layer can also be thicker than the sintered body layer 4.

[0133] Example

[0134] This embodiment will be more fully described through examples. However, this embodiment is not limited to these examples.

[0135] [Experiment 1]

[0136] In Experiment 1, the performance of the copper nano-ink of Embodiment 1 and the printed wiring board substrate 1 and the copper nano-ink and printed wiring board substrate manufactured by conventional manufacturing methods were evaluated.

[0137] <Manufacturing of Copper Nano Ink>

[0138] <Sample 1 to Sample 7>

[0139] Process for precipitating copper nanoparticles

[0140] Titanium trichloride (as a reducing agent), sodium hydroxide (as a pH adjuster), sodium citrate (as a complexing agent), and a dispersant were dissolved in pure water to obtain a reducing agent aqueous solution. The types of dispersants used in each sample are shown in the "Dispersant Type" column of Table 1. The reducing agent aqueous solution was prepared in a 60L reaction vessel. Then, while stirring the reducing agent aqueous solution, copper nitrate trihydrate, maintained at the same temperature as the reducing agent aqueous solution, was added to the reducing agent aqueous solution to precipitate copper nanoparticles.

[0141] Process for cleaning copper nanoparticles

[0142] The solution containing copper nanoparticles after the reaction was left to stand for more than 12 hours to allow the copper nanoparticles to precipitate. Subsequently, approximately 95% of the supernatant was removed by decantation. An aqueous solution containing a dispersant in pure water (dispersant aqueous solution) was added to the solution containing the copper nanoparticle precipitate, and the mixture was stirred for 60 minutes to obtain a mixture. The types of dispersants used in each sample are shown in the "Dispersant Type" column of Table 1. The dispersant content of the dispersant aqueous solution was 30% by mass. The amount of dispersant aqueous solution added to the solution containing the copper nanoparticle precipitate was 300 g.

[0143] Next, the stirred mixture was subjected to solid-liquid separation by centrifugation to recover the dehydrated solid material containing copper nanoparticles and dispersant (dispersant washing). The centrifugation speed was 15,000 rpm.

[0144] 5L of pure water was added to the solid material and dispersed using a stirrer. The mixture was then centrifuged to recover the dehydrated solid material (after rinsing with pure water). The solid material containing copper nanoparticles was then recovered through a second pure water rinse.

[0145] Process for dispersing copper nanoparticles

[0146] Next, pure water was added to the solid material containing copper nanoparticles and dispersed to obtain copper nano-ink.

[0147] <Sample 1-1 to Sample 1-5>

[0148] Copper nanoparticles were precipitated using the same method as for samples 1 to 7.

[0149] Process for separating copper nanoparticles

[0150] Next, the reducing agent aqueous solution containing the precipitated copper nanoparticles is centrifuged and separated into a copper nanoparticle concentrate and a liquid phase by a centrifuge.

[0151] Process for dispersing copper nanoparticles

[0152] In samples 1-2 to 1-4, after filtering the copper nanoparticle concentrate containing copper nanoparticles, pure water was added to obtain copper nanoparticle ink.

[0153] In samples 1-1 and 1-5, polyethyleneimine (PEI) and citric acid were added to the dispersion of copper nanoparticles. The content of each component was then adjusted with pure water to obtain copper nanoparticle inks with the composition listed in the "Composition of Copper Nanoparticle Ink" column of Table 1.

[0154] In samples 1-6, polyethyleneimine, polyvinyl alcohol, and citric acid were added to the dispersion of copper nanoparticles. The content of each component was adjusted with pure water to obtain copper nanoparticle inks with the composition described in the "Composition of Copper Nanoparticle Ink" column of Table 1. Samples 1-6 are equivalent to sample 18 of the embodiment of Patent Document 1.

[0155] [Table 1]

[0156] Table 1

[0157]

[0158] <Determination of Copper Nano-Ink>

[0159] In the freshly manufactured copper nanoparticle ink, the first average particle size, the second average particle size, the first proportion, and the second proportion of copper nanoparticles were measured. The methods for measuring each item were as described in Embodiment 1. The results are shown in Table 2. In each sample, the component with the largest mass ratio of copper oxide and copper hydroxide is represented as the "maximum mass ratio component" in Table 2. In all samples, the copper nanoparticles comprise at least one of copper oxide and copper hydroxide.

[0160] [Table 2]

[0161] Table 2

[0162]

[0163] <Fabrication of substrates for printed wiring boards>

[0164] Coating and drying processes

[0165] A polyimide film with an average thickness of 25 μm was used as the base film. Copper nano-ink for each sample was coated onto one side of the polyimide film using a rod coating method. The coated film was then dried by air drying at room temperature to form a dried film.

[0166] Firing Process

[0167] Next, the dried film was fired in a nitrogen-filled hot blast furnace at a firing temperature of 350°C for 30 minutes to form a sintered body layer with an average thickness of 0.2 μm. This yielded the substrate for the printed wiring board of each sample.

[0168] <Determination of substrates for printed wiring boards>

[0169] In the freshly manufactured printed wiring board substrate, the gloss of the sintered body layer, the Cu2O content of the sintered body layer, and the resistivity of the sintered body layer were measured. The measurement methods for each item were as described in Embodiment 1. The results are shown in Table 2. When the resistivity of the sintered body layer is 100 μΩ·cm or less, it is determined that the resistivity of the sintered body layer is low, and the printed wiring board substrate having this sintered body layer is determined to have high conductivity.

[0170] <Inspection>

[0171] The copper nano-inks and printed wiring board substrates of Samples 1 to 7 are examples. The copper nano-inks and printed wiring board substrates of Samples 1-1 to 1-6 are comparative examples. The sintered body layer of the printed wiring board substrates of Samples 1 to 7 (examples) has a lower resistivity than the sintered body layer of the printed wiring board substrates of Samples 1-1 to 1-6 (comparative examples). Therefore, the printed wiring board substrates of Samples 1 to 7 (examples) have a higher conductivity than the printed wiring board substrates of Samples 1-1 to 1-6 (comparative examples).

[0172] The first average particle size of the copper nanoparticles in Sample 1 and Sample 1-2 is 70 nm. The second average particle size of the copper nanoparticles in Sample 1 is 80 nm, and the second average particle size of the copper nanoparticles in Sample 1-2 is 100 nm. Therefore, compared with Sample 1-2, the copper nanoparticles in Sample 1 are less prone to aggregation, and the dispersion of the copper nanoparticles is improved. In addition, compared with Sample 1-2, the first and second proportions of Sample 1 are smaller.

[0173] The first average particle size of the copper nanoparticles in Sample 2 and Samples 1-3 is 20 nm. The second average particle size of the copper nanoparticles in Sample 2 is 25 nm, and the second average particle size of the copper nanoparticles in Samples 1-3 is 40 nm. Therefore, compared with Samples 1-3, the copper nanoparticles in Sample 2 are less prone to aggregation, and the dispersion of the copper nanoparticles is improved. In addition, compared with Samples 1-3, the first and second proportions of Sample 2 are smaller.

[0174] The first average particle size of the copper nanoparticles in Sample 3 and Samples 1-4 is 150 nm. The second average particle size of the copper nanoparticles in Sample 3 is 175 nm, and the second average particle size of the copper nanoparticles in Samples 1-4 is 500 nm. Therefore, compared with Samples 1-4, the copper nanoparticles in Sample 3 are less prone to aggregation, and the dispersion of the copper nanoparticles is improved. In addition, compared with Samples 1-4, the first and second proportions of Sample 3 are smaller.

[0175] Samples 1 and 1-2 were compared, samples 2 and 1-3 were compared, and samples 3 and 1-4 were compared. From these comparisons, it can be inferred that the process of cleaning copper nanoparticles helps to prevent copper nanoparticles from agglomerating, improves the dispersion of copper nanoparticles, reduces the first proportion, and reduces the second proportion.

[0176] [Experiment 2]

[0177] In Experiment 2, after sealing and storing the copper nano-ink of Sample 1 for 3 months, the first average particle size, second average particle size, first proportion, and second proportion of the copper nano-ink were measured. Using this copper nano-ink, a substrate for printed wiring boards was fabricated using the same method as Sample 1, and the gloss and resistivity of the sintered layer were measured. The results are shown in Table 3. In Table 3, the copper nano-ink of "Sample 2-1" is the copper nano-ink of Sample 1 after being sealed and stored for 3 months. The sintered layer of "Sample 2-1" is the sintered layer formed using the copper nano-ink of Sample 1 after being sealed and stored for 3 months.

[0178] [Table 3]

[0179] Table 3

[0180]

[0181] <Inspection>

[0182] In the copper nano-ink of Sample 1, which was sealed and stored for 3 months (the copper nano-ink of Sample 2-1), agglomeration and oxidation occurred slightly. However, the resistivity of the sintered body layer of the printed wiring board substrate made using the copper nano-ink stored for 3 months was low, and the printed wiring board substrate had high conductivity.

[0183] The embodiments and examples of this disclosure have been described, but from the outset it is contemplated that appropriate combinations and various modifications can be made to the above embodiments and examples.

[0184] All aspects of the embodiments and examples disclosed herein are illustrative and should not be considered limiting. The scope of the invention is defined not by the above embodiments and examples, but by the claims, and is intended to include all modifications equivalent in meaning and scope to the claims.

[0185] Explanation of reference numerals in the attached figures

[0186] 1. Substrate for printed wiring boards

[0187] 2. Base film

[0188] 3 Sintered body layer

[0189] 4. First main face.

Claims

1. A copper nano-ink, comprising copper nanoparticles and a solvent, The first average particle size of the copper nanoparticles is greater than 1 nm and less than 200 nm. The first average particle size is the average diameter based on volume, obtained by analyzing the scanning electron microscope image of the copper nanoparticles. The copper nanoparticles comprise at least one of copper oxide and copper hydroxide. In the copper nano-ink, the first ratio, namely the ratio of the total mass ratio M2 of copper oxide and copper hydroxide as determined by the reference strength ratio method to the mass ratio M1 of copper, is 5.0% or less.

2. The copper nano-ink according to claim 1, wherein, In the copper nano-ink, the ratio of the chlorine content C2 (as determined by ion chromatography) to the copper content C1 (as determined by ICP emission spectroscopy), i.e., the second ratio, is less than 0.3%.

3. The copper nano-ink according to claim 1 or 2, wherein, The second average particle size of the copper nanoparticles is greater than 5 nm and less than 400 nm. The second average particle size is the average diameter of the number of particles measured using an ultrasonic attenuation particle size analyzer.

4. The copper nano-ink according to any one of claims 1 to 3, wherein, The first proportion is 0.1% or higher.

5. A substrate for a printed wiring board, comprising: Base film, including a first main surface; and A sintered layer of copper nanoparticles is formed on the first main surface. The Cu2O content of the sintered body layer is less than 3.0% by mass. The gloss level of the sintered body layer is above 300.

6. The substrate for a printed wiring board according to claim 5, wherein, The resistivity of the sintered body layer is below 100 μΩ·cm.

7. A method for manufacturing a substrate for a printed wiring board, comprising the method for manufacturing a substrate for a printed wiring board as described in claim 5 or 6, and including: The process of coating the base film with the copper nano-ink as described in any one of claims 1 to 4; The process of drying the coating film of the copper nano-ink; and The process of firing the dried film of the copper nano-ink. In the firing process, the sintered body layer is formed on the base film.

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