Copper ink and conductive film

Abstract

This copper ink contains copper powder and a binder resin. The binder resin contains an acrylic resin. According to thermogravimetric analysis of the crushed product obtained by heating the copper ink for 30 minutes at 200°C under a nitrogen atmosphere and then cooling and finally crushing the ink, the percentage weight loss within a range from 200°C to 300°C is 0.5% to 2.0%.

Description

Copper Ink and Conductive Film

[0001] This specification describes a copper ink and a conductive film.

[0002] A copper ink containing copper powder is, for example, printed in a predetermined pattern on a substrate, and then used in applications such as forming a conductive film of a sintered body of copper powder by sintering the copper powder in some cases.

[0003] Here, photofiring, which causes instantaneous sintering and dense shrinkage of copper powder by light irradiation, may be employed for sintering copper powder. A technology related to this is described in, for example, Patent Document 1.

[0004] Patent Document 1 proposes a "copper fine particle dispersion characterized by comprising: first copper fine particles having a center particle diameter of 1 nm or more and 100 nm or less; second copper fine particles having a center particle diameter of 0.3 µm or more and less than 2 µm; a resin having a concentration of 0.05 mass% or more and less than 8 mass% relative to the first copper fine particles and the second copper fine particles; a solvent; and a dispersant that disperses the first copper fine particles and the second copper fine particles in the solvent", for the purpose of "providing a copper fine particle dispersion that can achieve both good printability and low resistance, a method for forming a conductive film, and a circuit board".

[0005] Japanese Unexamined Patent Application Publication No. 2017-168248

[0006] In the applications as described above, copper ink may be required to be capable of forming a predetermined pattern with high accuracy in a thick conductive film. Particularly in photofiring, when an attempt is made to form a thick conductive film, cracks occur in the conductive film, and it is often difficult to form a thick conductive film.

[0007] This specification provides a copper ink that can form a conductive film having a relatively large thickness and is excellent in printability, and a conductive film.

[0008] The copper ink disclosed in this specification comprises copper powder and a binder resin, wherein the binder resin contains an acrylic resin, and the crushed material obtained by heating the copper ink to 200°C for 30 minutes under a nitrogen atmosphere, then cooling and crushing, shows a weight loss rate of 0.5% or more and 2.0% or less in the range of 200°C to 300°C as determined by thermogravimetric analysis.

[0009] The conductive film disclosed in this specification includes a sintered body of the copper powder in the copper ink described above.

[0010] The copper ink described above can form a relatively thick conductive film and exhibits excellent printability.

[0011] This graph shows the change in weight loss rate with increasing temperature for each copper ink in Comparative Example 3 and Example 3, as measured by thermogravimetric analysis. This figure shows the relationship between the amount of resin relative to copper in the copper inks of Examples 1 to 3 and the weight loss rate in the range of 200°C to 300°C as measured by thermogravimetric analysis. This figure shows the shape and dimensions of the S-shaped circuits printed in the examples. These are photographs of the S-shaped printing results for Examples 1 and 2 and Comparative Example 1. This figure shows the shape and dimensions of the straight circuits printed in the examples. These are photographs of the straight circuit printing results for Examples 1 and 2. These are SEM images of the fired samples for Examples 2 and 3 and Comparative Examples 1 and 4. This is an SEM image of the fired sample for Comparative Example 2.

[0012] The embodiments of the copper ink and conductive film described above will be explained in detail below. One embodiment of the copper ink contains copper powder and a binder resin, and may also contain a solvent, additives, and a photocatalytic curing modifier. The binder resin in the copper ink contains an acrylic resin. Furthermore, when the crushed material obtained by heating this copper ink to 200°C for 30 minutes under a nitrogen atmosphere, then cooling and crushing it is subjected to thermogravimetric analysis (TG), the weight loss rate in the range of 200°C to 300°C is 0.5% or more and 2.0% or less.

[0013] Acrylic resin is suitable for firing because it produces few carbonized components (residues) during thermal decomposition. However, even if the binder resin is a copper ink containing acrylic resin, if the weight loss rate of the copper ink in the range of 200°C to 300°C in thermogravimetric analysis exceeds 2.0%, the sinterability when forming a thick film will decrease, and if it is less than 0.5%, the printability when forming a thick film will decrease.

[0014] If the weight loss rate in the range of 200°C to 300°C is 0.5% or more and 2.0% or less, the copper ink is considered to contain a resin with a relatively small molecular weight and intricately intertwined molecular chains, and therefore has a relatively low resin content. A resin with intricately intertwined molecular chains can make the copper ink more viscous even in small amounts. This makes fine printing possible even with a small resin content, ensuring the required printability. Furthermore, copper ink containing a resin with a relatively small molecular weight is less likely to generate voids during firing, and even if voids do occur, their size is likely to be small. Small voids, even if adjacent ones connect, are unlikely to form large cracks that would cause disconnection. For this reason, if the binder resin of the copper ink contains a small amount of acrylic resin with a relatively small molecular weight and intricately intertwined molecular chains, even if the copper ink is used in light firing, where the copper powder sintersects and shrinks densely in a relatively short time, a thick conductive film can be formed while suppressing the occurrence of cracks.

[0015] (Ingredients) The copper powder contained in the copper ink contains copper (Cu) and does not necessarily have to be made of pure copper, but it is preferable that 90% or more by mass of the metal components contained in the copper ink is Cu.

[0016] To confirm that 90% or more by mass of the metallic components in copper ink is Cu, the copper ink can be heated to 700°C or higher for 5 hours or more in an inert atmosphere (e.g., nitrogen, vacuum, or argon) or a reducing atmosphere (e.g., hydrogen, or a gaseous atmosphere composed of a mixture of hydrogen, nitrogen, or argon). The resulting sintered body can then be analyzed by X-ray diffraction (XRD). By identifying the peaks obtained by X-ray diffraction and calculating the abundance ratio from the area ratio, the Cu content in the metallic components of the copper ink can be confirmed. Sintering in a reducing atmosphere is more preferable because it reduces metal oxides and removes unwanted peaks.

[0017] The average particle size D50 of the copper powder is, for example, 50 nm to 1000 nm, and preferably 100 nm to 300 nm. If the average particle size D50 of the copper powder is too small, the proportion of the organic protective layer on the surface of the copper particles increases, leading to increased volume shrinkage after sintering. Increased volume shrinkage can lead to problems such as being unable to sinter in the desired shape or the occurrence of cracks after sintering. If the average particle size D50 of the copper powder is too large, sintering by light firing and fine printing may become difficult, and the resistance of the conductive film may increase.

[0018] The average particle size D50 of copper powder is determined by measuring the particle size of copper particles in the copper powder using a laser diffraction / scattering particle size distribution analyzer. The resulting particle size histogram (particle size distribution graph) shows that the cumulative frequency of copper particles based on volume reaches 50%, and this measurement is performed in accordance with JIS Z8825 (2013).

[0019] The binder resin contained in the copper ink shall include acrylic resin. As mentioned earlier, since acrylic resin produces fewer carbonized components during thermal decomposition, copper ink containing it will have excellent sinterability.

[0020] The above-mentioned acrylic resin preferably has at least one of a carboxyl group, an amino group, a hydroxyl group, and an alkyl group. Acrylic resins having such substituents are thought to have high viscosity at low shear due to intermolecular interactions caused by the substituents. For this reason, when an acrylic resin having the above substituents is included, the thixotropic index (TI) tends to be 7.0 or higher, as will be described later.

[0021] Specific examples of acrylic resins include, for instance, NOF Corporation's Marproof® MH-059 and Marproof® MH-08541. Copper ink may contain at least one such acrylic resin as a binder resin.

[0022] The binder resin may consist solely of acrylic resin, but it may also contain at least one of the following in addition to acrylic resin: cellulose resin, alkyd resin, polyvinyl alcohol resin, polyvinyl acetal, ketone resin, urea resin, melamine resin, polyester, polyamide, or polyurethane.

[0023] The presence of acrylic resin in copper ink can be confirmed as follows: Dilute the copper ink with a solvent that readily dissolves acrylic resin (such as toluene, acetone, or methyl ethyl ketone). Stir the mixture with a stirring bar using a magnetic stirrer. Then, place the mixture in a centrifuge container and centrifuge at 3000 rpm for about 15 minutes, collecting the supernatant. By measuring this supernatant using pyrolysis GC-MS, if acrylic resin is present, fragments derived from methacrylic acid (C4H6O2) or acrylic acid (C3H4O2) can be detected.

[0024] Solvents that may be contained in copper ink include alcohol solvents (e.g., one or more selected from the group consisting of terpineol, dihydroterpineol, isopropyl alcohol, diethylene glycol monobutyl ether, terpinel oxyethanol, and dihydroterpinel oxyethanol), glycol ether solvents (e.g., diethylene glycol monobutyl ether), acetate solvents (e.g., one or more selected from the group consisting of diethylene glycol monobutyl ether acetate, dihydroterpineol acetate, dihydrocarbitol acetate, carbitol acetate, linaleel acetate, and terpinyl acetate), ketone solvents (e.g., methyl ethyl ketone), hydrocarbon solvents (e.g., one or more selected from the group consisting of toluene and cyclohexane), cellosolves (e.g., one or more selected from the group consisting of ethyl cellosolve and butyl cellosolve), diethyl phthalate, or propineoate-based solvents (e.g., one or more selected from the group consisting of dihydroterpinylpropineoate, dihydrocarbylpropineoate, and isovonylpropineoate).

[0025] Dispersants that may be included in copper ink include, for example, saturated fatty acids (formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, stearic acid, nonadesylic acid, arachidic acid) and unsaturated fatty acids (alpha-linolenic acid, stearidonic acid, eicosatetraenoic acid, eicosapentaenoic acid, docosapentaenoic acid, docosahexaenoic acid, linoleic acid, gamma-linolenic acid, dihomo-gamma-linolenic acid, arachidonic acid, docosatetraenoic acid, docosapentaenoic acid, palmitoleic acid, vaccenic acid, pauric acid, oleic acid, elaidic acid, erucic acid, nervonic acid, sapienic acid).

[0026] To manufacture copper ink, at least copper powder and a binder resin may be mixed, and optionally a solvent, a photocatalytic curing modifier, and / or a dispersant. Copper powder may be obtained by purchasing, etc., but it can also be produced by generating copper particles using a liquid-phase method such as chemical reduction or disproportionation.

[0027] In the chemical reduction method, for example, an aqueous solution of copper salt, such as an aqueous solution of copper sulfate, is used as a raw material solution. After adjusting the pH by adding sodium hydroxide or other alkalis, a reducing agent such as hydrazine is added to generate cuprous oxide powder in the slurry based on a reduction reaction such as 4CuSO4 + N2H4 + 8NaOH → 2Cu2O + 4Na2SO4 + 6H2O + N2. Next, this slurry is heated and the pH is adjusted while adding hydrazine to reduce the cuprous oxide to copper through a reaction such as 2Cu2O + N2H4 → 4Cu + 2H2O + N2, generating copper particles in the liquid.

[0028] The reducing agent, such as hydrazine, added to the copper sulfate aqueous solution is used to reduce divalent copper to monovalent copper (cuprous oxide). When the reducing agent is added all at once, the resulting cuprous oxide particles tend to be very fine. After relatively fine cuprous oxide particles have formed, the reducing agent can be added in stages. After the formation of cuprous oxide particles, the first addition of the reducing agent is primarily used to create nuclei of metallic copper, while the second addition can be used to grow those nuclei.

[0029] In the disproportionation method, for example, an aqueous solution containing a dispersant such as gum arabic, gelatin, or collagen peptide is mixed with a slurry containing cuprous oxide powder, and sulfuric acid is added to the mixture. This causes a disproportionation reaction of Cu₂O + H₂SO₄ → Cu↓ + CuSO₄ + H₂O, which generates copper particles in the liquid.

[0030] (Thermogravimetric Analysis) When copper ink is heated to 200°C for 30 minutes under a nitrogen atmosphere, then cooled and crushed, the resulting crushed material is subjected to thermogravimetric analysis, and the weight loss rate in the range of 200°C to 300°C is 0.5% or more and 2.0% or less.

[0031] If the weight loss rate in the range of 200°C to 300°C exceeds 2.0%, even if the binder resin has a small molecular weight, it is thought that the binder resin content is high, and due to the increase in voids during firing, it becomes difficult to sinter thick films by light firing. On the other hand, if the weight loss rate is below 0.5%, even if the binder resin has a complex intertwining of molecular chains, it is thought that the binder resin content is low, and due to the low viscosity of the copper ink, it is thought that printability of thick films cannot be guaranteed. From this viewpoint, it is preferable that the weight loss rate in the range of 200°C to 300°C is 1.0% or more, more preferably 1.2% or more, and also 1.8% or less, and more preferably 1.5% or less.

[0032] Furthermore, in the thermogravimetric analysis described above, the weight loss rate within the range of 200°C to 400°C is preferably 5.0% or less, more preferably 4.5% or less, and preferably 1.0% or more, and more preferably 2.5% or more.

[0033] In thermogravimetric analysis, the weight loss within the 200°C to 400°C range is considered to be almost entirely due to the volatilization of the binder resin in the copper ink. Therefore, the weight loss rate within the 200°C to 400°C range may correspond to the binder resin content relative to the copper powder content. When the weight loss rate within the 200°C to 400°C range is 5.0% or less, it is considered that the binder resin is contained in an appropriate amount relative to the copper powder. If there is too much binder resin, the resistivity may increase due to residual charcoal after firing. Also, if there is too much binder resin, the volume of voids that may occur during firing may increase, raising concerns about cracks that could lead to wire breakage. On the other hand, if there is too little binder resin, the printability may decrease.

[0034] Furthermore, the temperature at which the weight loss rate increases from 0% to 0.5% for the first time in the above-mentioned thermogravimetric analysis is preferably 300°C or lower. This can reduce the amount of charcoal remaining after firing.

[0035] The weight loss rate and temperature measurements for the thermogravimetric analysis described above were performed as follows: The mirror surface of a 6-inch silicon wafer was wiped with ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 99.5%) and Bencot®. A metal mask with an opening of 1 cm x 8 cm and a thickness of 50 μm (manufactured by Taku Giken Co., Ltd., product name: Additive Mask) was used to apply copper ink with a metal squeegee (manufactured by Taku Giken Co., Ltd., product name: Kyokukyo Squeegee) that was 0.2 mm thick, 100 mm wide, and 50 mm long, to prepare a printed sample. The copper ink was stirred at 2000 rpm for 30 seconds in a rotation-orbit mixer (product name: Awatori Rentaro®, model number: ARE-310, manufactured by Thinky Co., Ltd.) immediately before application.

[0036] Next, the printed sample is fired in a heating device (manufactured by Unitem Japan Co., Ltd., product name: RSS-210-S). The printed sample is placed on the stage, and N2 is flowed at a flow rate of 5 L / min to replace the inside of the chamber with N2 gas for 500 seconds. While continuing to flow N2 at a flow rate of 5 L / min, the temperature is raised from 22°C (room temperature) to 200°C at 1 K / sec, and after reaching 200°C, it is heated for 30 minutes. After that, while continuing to flow N2 at a flow rate of 5 L / min, the stage is cooled until it reaches 35°C or below, and the sample (a copper sintered product formed on a silicon wafer) is removed. The copper sintered product is scraped off the silicon wafer with a spatula, collected with weighing paper, and crushed in an agate mortar for 5 minutes to obtain crushed material.

[0037] Subsequently, the crushed material is weighed into an aluminum measuring pan and heated in a nitrogen atmosphere in a TG-DTA device (Rigaku Corporation, TG-DTA8122) at a rate of 10°C / min from 25°C, until the measurement is completed at 400°C. The weight at 300°C and 400°C are subtracted from the weighed weight to obtain the weight change between 200°C and 300°C and between 200°C and 400°C, respectively. Since the material is heated to 200°C beforehand, the weighed weight is treated as the weight at 200°C. For example, if the weighed weight is 16.0 mg and the weight at 300°C is 15.9 mg, the weight loss rate between 200°C and 300°C is (16.0 - 15.9) / 16.0 × 100 = 0.6%. Furthermore, since heating at 200°C in nitrogen for 30 minutes is thought to have completely evaporated the solvent, the powder measured for TG-DTA measurement is thought to consist mainly of acrylic resin and copper powder.

[0038] (Thixotropic Index) The thixotropic index (also called "TI") of copper ink is preferably 7.0 or higher. A higher TI is considered to indicate better printability. For finer printing, copper ink with a high TI is desirable.

[0039] TI is measured using a rheometer (Anton Paar, MCR102). A 25 mm diameter parallel plate is used as the measurement jig, with a scraping position of 0.52 mm, a measurement position of 0.5 mm, and a measurement temperature of 25°C. In addition, to standardize the initial conditions, a pre-shear rate of 1 sec is used. -1 The solution is applied for 30 seconds, followed by a 150-second rest period, and then a multi-step measurement is performed in which the shear rate is changed in steps. The measured results can be graphed as a viscosity curve with shear rate on the x-axis and viscosity on the y-axis. Low shear rate (6.3 seconds) -1 The viscosity of ) is high shear rate (62.8 sec -1 The value obtained by dividing the viscosity at ) by the viscosity is defined as TI.

[0040] Many acrylic resins tend to decrease in viscosity as the shear rate increases. In addition, the viscosity of an acrylic resin at low shear may change due to the influence of entanglement of molecular chains and the influence of intermolecular interactions exerted by substituents (carboxy group, amino group, hydroxy group, alkyl group, etc.) of the acrylic resin. On the other hand, at high shear, the influence of molecular chain entanglement and intermolecular interactions weaken. For this reason, the TI obtained from low shear rate and high shear rate can vary to a certain extent greatly depending on the type of acrylic resin contained in the copper ink.

[0041] (Application) The above-described copper ink is suitably used for forming a conductive film by photo-firing, but is not limited thereto, and may also be subjected to heat-firing.

[0042] In photo-firing, for example, after drying a copper ink printed in a pattern, the copper powder is sintered by irradiating light with a xenon lamp or a laser toward the dried copper ink. Thereby, a conductive film including a sintered body of copper powder is formed. The conductive film has conductivity and can be effectively used as a wiring. Photo-firing can be performed over a large area depending on the size of the firing device. The copper ink photo-fired on a substrate can be used as a substitute for copper foil, so it can also be used for the purpose of wiring formation by a subtractive method after laminating a dry film.

[0043] In addition, when the copper ink is applied onto a substrate or the like, it may be used for screen printing or screen offset printing. Screen printing and screen offset printing are suitable in that printing can be performed even when the line width is about 200 µm and the printing thickness is 10 µm or more. However, the printing method applied to the copper ink is not limited to these. The copper ink may be printed using, for example, gravure offset printing, transfer printing, dispenser printing, stencil printing, bar coating, an applicator, letterpress printing, or intaglio printing (gravure printing).

[0044] (Potential contribution to SDGs) According to the embodiments described above, a copper ink that can form a relatively thick conductive film and has excellent printability can be provided, so that the occurrence of wire breakage can be suppressed, and there is a possibility that product yield can be improved. An improvement in product yield leads to a stable supply of products and a reduction in the loss of metal raw materials, which are limited resources. Therefore, the copper powder of one embodiment may contribute to Goal 9 "Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation" and Goal 12 "Ensure sustainable consumption and production patterns" of the Sustainable Development Goals (SDGs) led by the United Nations.

[0045] Next, the above-mentioned copper ink was prototyped, and its effect was confirmed, so this will be described. However, the description herein is for illustrative purposes only, and is not intended to be limited to this.

[0046] (Manufacturing conditions) Copper powder having an average particle diameter D50 of 150 nm, an acrylic resin dissolved in terpineol, and solvents (terpineol, γ-butyrolactone) were weighed into a plastic cup, and stirred at 2000 rpm for 2 minutes using a rotation-revolution mixer (trade name: Awatori Rentaro, model number: ARE-310, manufactured by Thinky Corporation). Subsequently, kneading was performed for 10 passes at a roll gap of 5 µm and a roll speed of 130 rpm using a three-roll mill (model 80E, manufactured by Nagase Screen Printing Research Laboratory Co., Ltd.), and the product recovered in a plastic cup was used as the copper ink.

[0047] In each of the examples and comparative examples, as shown in Table 1, the type of acrylic resin (A or B) having different molecular weights and the amount of resin relative to copper were varied. Note that in all of the examples and comparative examples, the copper concentration (concentration of copper powder) in the copper ink was adjusted to be 75% by mass.

[0048] (Thermogravimetric Analysis, TI) The copper ink obtained as described above was subjected to TI measurement and thermogravimetric analysis according to the previously described method to determine the 0.5% weight loss temperature, weight loss rate at 200°C to 300°C, weight loss rate at 200°C to 400°C, minimum weight loss rate, and TI. The results are shown in Table 1. The minimum weight loss rate represents the ratio of the smallest weight during the heating process up to 400°C to the weight of the crushed material at the start of measurement in thermogravimetric analysis. If the binder resin content is the same, a binder resin with a smaller minimum weight loss rate means that the binder resin is more easily thermally decomposed. In Table 1, "-" indicates that the measurement was not taken. For reference, Figure 1 shows the change in weight loss rate with increasing temperature in the thermogravimetric analysis for each copper ink of Comparative Example 3 and Example 3. The weight loss rate at 200°C to 300°C for Comparative Example 1 is an estimated value. Figure 2 shows the relationship between the amount of resin to copper and the weight loss rate at 200°C to 300°C for Examples 1 to 3. As shown in Figure 2, a strong positive proportional relationship is observed between the amount of resin relative to copper and the weight loss rate at 200°C to 300°C. From the relationship shown in Figure 2 and the amount of resin relative to copper in Comparative Example 1 (8 wt%), the weight loss rate of Comparative Example 1 at 200°C to 300°C can be estimated.

[0049] (Confirmation of Printability) The printability of copper ink printed in an S-shape was confirmed. Specifically, the following was done: Copper ink was printed on a substrate using a screen printing machine (Mino Group Co., Ltd., Cube 1515), a screen plate (Takeda Tokyo Process Service Co., Ltd., stainless steel mesh, mesh count 250 lines / inch, wire diameter 30 μm, mesh thickness 62 μm), and a squeegee blade (Mino Group Co., Ltd., hardness 80-85°, flat type 9 x 42 x 1500 mm, MP series). The squeegee speed and doctor blade speed were set to 30 mm / sec, and the clearance was 1.5 mm. SPME (Squeegee Pressure Mechanical Equalizer; squeegee parallelism and negative pressure setting device) was used to press the squeegee. After printing, the wiring was dried on a hot plate set to 80°C for about 60 minutes. This was used as the print sample.

[0050] The printed S-shape was created by five adjacent wires extending in parallel in an S-shape, forming an overall S-shape. As shown in Figure 3, the line width of each wire is 200 μm, and the spacing between adjacent wires is 200 μm. (When defined by the inscribed position) Two circles with radii of curvature R (mm) = 5.7, 6.1, 6.5, 6.9, and 7.3 were drawn, and for the five wires, a structure (A) was created by erasing the 3 o'clock to 6 o'clock portion of the clock, and a structure (B) was created by erasing the 9 o'clock to 12 o'clock portion of the clock. The 6 o'clock portion of structure (A) and the 12 o'clock portion of structure (B) were connected at the same radius of curvature to form an S-shape. Note that in Figure 3, dimensions are expressed in mm (millimeters). Photographs of the S-shape printing results for Examples 1 and 2 and Comparative Example 1 are shown in Figure 4.

[0051] Furthermore, the printability of copper ink when printed in a linear pattern was confirmed. Specifically, the following was done: Copper ink was printed onto a substrate using a screen printing machine (Mino Group Co., Ltd., Cube 1515), a screen plate (Takeda Tokyo Process Service Co., Ltd., stainless steel mesh, 250 mesh lines / inch, wire diameter 30 μm, mesh thickness 62 μm), and a squeegee blade (Mino Group Co., Ltd., hardness 80-85°, flat type 9 × 42 × 1500 mm, MP series). The squeegee speed and doctor blade speed were set to 30 mm / sec, and the clearance was 1.5 mm. SPME (Squeegee Pressure Mechanical Equalizer; squeegee parallelism and negative pressure setting device) was used to press the squeegee. After printing, the wiring was dried on a hot plate set to 80°C for about 60 minutes. This was used as the print sample.

[0052] As shown in Figure 5, the printed linear shape is an I-shape with a wiring width of 100 μm, a length of 30 mm, and 1 mm square terminal sections at both ends, with 10 wires arranged in a row at intervals of 2.5 mm.

[0053] These results are shown in Table 1 as "Printability S-shape" and "Printability Straight line," respectively. In Table 1, "×" for "Printability S-shape" and "Printability Straight line" means that the wiring was unable to maintain its shape and was crushed, and adjacent wiring came into contact with each other, resulting in poor printability. "△" means that although some adjacent wiring came into contact with each other, the remaining wiring was printed cleanly, resulting in reasonably good printability. "〇" means that no contact was observed between adjacent wiring in the entire area, resulting in excellent printability. Note that S-shapes are extremely difficult to print, so if the printability results for straight lines are good, it is considered to have excellent printability.

[0054] As can be seen from Table 1, Examples 1 to 3 showed good printability for straight lines and S-shapes. This is thought to be because the weight loss rate within the 200°C to 300°C range of thermogravimetric analysis was within the specified range. Furthermore, the results of the S-shape printability in Examples 1 to 3 show that printability improves as the resin content increases.

[0055] (Confirmation of Sinterability) For Examples 1 to 3 and Comparative Examples 1 and 4, the S-shaped printed coating film was subjected to light sintering in an air environment at room temperature, voltage: 2500V, pulse width: 2ms, and irradiation distance: 150mm using a light sintering apparatus (Xenon, S2210) to prepare a sintered sample. For Comparative Example 2, the S-shaped printed coating film was subjected to light sintering in an air environment at room temperature, voltage: 2700V, pulse width: 3ms, and irradiation distance: 120mm using a light sintering apparatus (Ushio Inc., SUS980) to prepare a sintered sample.

[0056] The sinterability of the fired samples was confirmed by imaging with a scanning electron microscope (SEM) under the following conditions, and the film thickness was measured with a laser microscope (Lasertec Corporation, OPTELICS HYBRID). For SEM imaging, the fired samples were placed in a resin casting container and poured with a two-part epoxy resin, taking care to avoid air bubbles. The shape was then adjusted with a polisher to a size suitable for ion milling, and the surface was polished with waterproof abrasive paper of increasing grit (#100, #400, #800, #1200, #1500) to expose the joint cross-section, and finally the surface was smoothed with diamond paste (5 μm). The polished surface was then removed by cross-section polishing with an argon ion beam using an ion milling device (Hitachi High-Tech Corporation, IM-4000). Next, platinum was deposited on the surface using a sputtering device and observed with an FE-SEM (Hitachi High-Tech Corporation, SU70). Figure 7 shows SEM images of the calcined samples from Examples 2 and 3 and Comparative Examples 1 and 4. The numbers in parentheses in Figure 7 indicate the SEM magnification. Figure 8 shows the SEM image of the calcined sample from Comparative Example 2.

[0057] Table 1 shows the results of observing the fired samples as "S-shaped light firing sintering properties". In Table 1, "○" in "S-shaped light firing sintering properties" means that no cracks were observed and the sintering properties were excellent; "△" means that although some cracks were observed, many parts were sintered cleanly in the film thickness direction and the sintering properties were good; and "×" means that many cracks were observed and the sintering properties were poor.

[0058] Table 1 shows that all three examples (1-3) exhibited good sinterability. Examples 1 and 2, in particular, with low resin content, showed excellent sinterability. In Comparative Example 1, it is presumed that the high resin content resulted in poor sinterability. In Comparative Example 4, in addition to the high resin content, the use of a high molecular weight acrylic resin led to the formation of large voids after the acrylic resin evaporated during light firing. These voids connected with each other, resulting in cracks and thus low sinterability. Indeed, comparing Comparative Example 1 and Comparative Example 4, which had the same conditions except for the type of acrylic resin, Comparative Example 4, which used a high molecular weight acrylic resin (resin B), showed larger cracks than Comparative Example 1, which used a low molecular weight acrylic resin (resin A), suggesting a higher likelihood of wire breakage. Numerous cracks were observed in the fired sample of Comparative Example 2, confirming that its sinterability was poor. Regarding light firing conditions, a longer pulse width allows energy to penetrate more easily into the interior, and a higher voltage and shorter irradiation distance promote sintering more effectively. In that sense, the light firing conditions for Comparative Example 2 tend to result in better sinterability of the fired sample than the light firing conditions for the other examples. However, even with such favorable light firing conditions, the sinterability was still "X," so it is highly likely that if the fired sample of Comparative Example 2 were prepared using the light firing conditions for the other examples, the sinterability would also be "X." Comparative Example 3 does not have sinterability results, but it uses the same resin B as Comparative Example 4, and it is thought that the sinterability would be "X" because it is prone to the formation of large voids and cracks.

[0059] When comparing Example 2 with Comparative Example 2 and Example 3 with Comparative Example 3, both having the same resin content, the minimum weight loss rate was smaller for Examples 2 and 3 than for Comparative Examples 2 and 3. From this, it can be concluded that resin A produces less resin residue after sintering than resin B, and thus enables the realization of lower-resistance wiring.

[0060]

[0061] Based on the above, it was suggested that the aforementioned copper ink can form a relatively thick conductive film and has the potential to exhibit excellent printability.

Claims

1. A copper ink comprising copper powder and a binder resin, wherein the binder resin comprises an acrylic resin, and the copper ink is heated to 200°C for 30 minutes under a nitrogen atmosphere, then cooled and crushed, and the weight loss rate of the crushed material in the range of 200°C to 300°C is 0.5% or more and 2.0% or less, as determined by thermogravimetric analysis.

2. The copper ink according to claim 1, wherein the temperature at which the weight loss rate increases from 0% to 0.5% for the first time in the thermogravimetric analysis is 300°C or less.

3. The copper ink according to claim 1 or 2, wherein the weight loss rate in the range of 200°C to 300°C is 1.0% or more as determined by the thermogravimetric analysis.

4. The copper ink according to claim 1 or 2, wherein the weight loss rate in the range of 200°C to 400°C is 5.0% or less as determined by the thermogravimetric analysis.

5. The copper ink according to claim 1 or 2, wherein the weight loss rate in the range of 200°C to 400°C is 1.0% or more as determined by the thermogravimetric analysis.

6. The copper ink according to claim 5, wherein the weight loss rate in the range of 200°C to 400°C is 2.5% or more as determined by the thermogravimetric analysis.

7. Shear rate 6.3 sec -1 Shear rate of 62.8 sec for viscosity -1 The copper ink according to claim 1 or 2, wherein the thixotropic index, defined by the viscosity ratio in the 300ml, is 7.0 or higher.

8. The copper ink according to claim 1 or 2, wherein the acrylic resin has at least one of a carboxyl group, an amino group, a hydroxyl group, and an alkyl group.

9. The copper ink according to claim 1 or 2, wherein the average particle size D50 of the copper powder is 50 nm to 1000 nm.

10. The copper ink according to claim 1 or 2, used for forming a conductive film by light firing.

11. The copper ink according to claim 1 or 2, used for screen printing or screen offset printing.

12. A conductive film comprising a sintered body of copper powder in the copper ink according to claim 1 or 2.

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