Sintering ink, metal particles, their manufacturing method, and circuit board manufacturing method

Abstract

To provide sintering ink which can suppress occurrence of cracking in wiring after sintering.SOLUTION: Sintering ink is obtained by dispersing metal particles in a solvent. In the metal particles, when a temperature difference between a reference substance and the metal particles is determined while heating them by a differential thermal analysis method, a differential thermal analysis chart indicating a relation between the temperature and the temperature difference has a peak at 200°C or lower, has no inflection point in a temperature range between 100°C and a maximum temperature of the peak, and is linear in at least a range between 100°C and a temperature lower than the maximum temperature of the peak by 20°C.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a technique for forming a wiring pattern by sintering an ink in which conductive particles are dispersed in a solvent. [Background technology]

[0002] A known technique involves applying a paste-like composition (ink) in which conductive particles are dispersed in a solvent to a substrate, and then heating the composition to evaporate the solvent and sinter the conductive particles, thereby forming wiring or the like on the substrate. By using fine particles with a size of several nanometers to several tens of micrometers as the conductive particles, the conductive particles can be sintered at a relatively low temperature, making it possible to use a resin substrate.

[0003] Patent Document 1 discloses that the surfaces of conductive particles are covered with an organic substance to prevent aggregation during production, and that the organic substance is then replaced with a polymer dispersant having acidic and basic functional groups. Patent Document 1 states that this allows for the production of a paste-like composition that can be sintered at low temperatures and has sufficient adhesion to the substrate. The degree of substitution is such that no exothermic peak due to thermal decomposition of the organic substance is detected in the differential thermal analysis of the conductive microparticles.

[0004] Furthermore, Patent Document 2 discloses a paste in which conductive particles with an average primary particle size of 1 to 150 nm and conductive particles with an average primary particle size of 1 to 10 μm are mixed in a predetermined ratio and dispersed in a reducing organic solvent in order to reduce the volume shrinkage rate due to heating, thereby suppressing cracking in the sintered compact and improving corrosion resistance. The micro-sized conductive particles restrict the free movement of the nano-sized conductive fine particles, thereby reducing the volume shrinkage rate and suppressing the occurrence of coarse voids and cracks. Furthermore, by using nano-sized conductive particles coated with an organic dispersant, an organic compound layer remains on the surface of the sintered compact, improving corrosion resistance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-55332 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-11899 Summary of the Invention [Problem to be solved by the invention]

[0006] In order to enable the supply of large currents, it is desirable to form a thick-film (for example, 10 μm or thicker) wiring pattern by sintering conductive particles. However, according to experiments by the inventors, when a thick-film wiring is formed by applying a thick layer of a composition (paste) in which conductive particles are dispersed in a solvent to a substrate and then sintering it by heating, cracks are likely to occur in the wiring, and the thermal shock resistance of the wiring is reduced.

[0007] Furthermore, compositions in which nano-sized conductive particles are mixed with micro-sized metal microparticles, such as the technology of Patent Document 2, have the constraint that a high sintering temperature is required to sinter the micro-sized conductive particles, and that a substrate material with high heat resistance must be used.

[0008] An object of the present invention is to provide a sintering ink that can suppress the occurrence of cracks in wiring after sintering. [Means for solving the problem]

[0009] In order to achieve the above object, according to the present invention, there is provided a sintering ink in which metal particles are dispersed in a solvent, and when the temperature difference between the metal particles and a reference material is determined while the metal particles are heated by differential thermal analysis, the differential thermal analysis chart showing the relationship between the temperature and the temperature difference has a peak at or below 200°C, does not have an inflection point in the temperature range from 100°C to the apex temperature of the peak, and is linear in the temperature range from 100°C to a temperature at least 20°C lower than the apex temperature of the peak. [Effects of the Invention]

[0010] When wiring is produced by applying and sintering the sintering ink of the present invention, the occurrence of cracks is suppressed, wire breakage and the like are prevented, and reliability can be improved. [Brief explanation of the drawings]

[0011] [Figure 1] (a) TG / DTA chart of Example 9, (b) TG / DTA chart of Comparative Example 10. [Figure 2] TG / DTA charts of Examples 9 and 10 and Comparative Examples 9 and 10. [Figure 3] FIG. 1 is an explanatory diagram showing images of sintered bodies produced using silver particles of Examples 1-10 and Comparative Examples 1-10 and the crack area ratios. [Figure 4] (a) Photograph of wiring produced using the sintering ink of Example 9, (b) Photograph of wiring produced using the sintering ink of Comparative Example 10. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the present invention will be described below. First, an ink for baking according to the embodiment will be described.

[0013] <<Sintering ink>> The sintering ink of this embodiment is a dispersion of metal particles in a solvent. This sintering ink is applied to a substrate in a desired shape to form a film, and by heating it to a predetermined temperature, the metal particles contained in the sintering ink are sintered to form wiring. This is explained in detail below.

[0014] [Metal particles] In this embodiment, the surfaces of the metal particles dispersed in the sintering ink are coated with an organic dispersant to prevent aggregation during production.

[0015] When the metal particles whose surfaces are coated with an organic metal dispersion material are heated by differential thermal analysis (DTA) to determine the temperature difference with a reference material, the differential thermal analysis chart has a peak P1 at 200°C or less, as shown in an example of Figure 1(a). The differential thermal analysis chart has no inflection points in the temperature range from 100°C to the apex temperature of peak P1, and is linear in the temperature range from 100°C to at least 20°C lower than the apex temperature of the peak. When there are multiple peaks at 200°C or less, the lowest-temperature peak has the above characteristics.

[0016] The reference substance for the DTA method is a substance whose temperature changes in accordance with the temperature of the heating furnace, and a reference substance that is generally used in DTA is used.

[0017] The inflection point referred to here includes not only a general inflection point defined as a point where a curve changes from concave to convex or from convex to concave, but also a point where the slope changes. Therefore, in this embodiment, a differential thermal analysis chart having no inflection point means that the slope of the chart is constant (linear) or changes only gently, and does not suddenly increase or have two steps.

[0018] The gradient of the linear portion of the differential thermal analysis chart is preferably 1 μV / °C or less, more preferably 0.5 μV / °C or less, and even more preferably 0.3 μV / °C or less. The sample weight used for the measurement is 10 mg on average, and measurements are performed using samples adjusted to 5 to 15 mg.

[0019] The slope near the apex of peak P1 may be gentler than the slope at lower temperatures. That is, the slope of the change in temperature difference (differential thermal) in the temperature range from a temperature 20°C lower than the apex temperature of the peak to the apex temperature of the peak may be gentler than the slope of the change in temperature difference in the temperature range from 100°C to a temperature 20°C lower than the apex temperature of the peak.

[0020] The metal particles are preferably silver particles, copper particles, or gold particles, and are particularly preferably silver particles. The organic dispersant that coats the metal particles is preferably polyvinylpyrrolidone.

[0021] The particle size of the metal particles is preferably 250 nm or less, and more preferably 200 nm or less.

[0022] The particle size was calculated from an image observed using a field emission scanning electron microscope (FE-SEM).

[0023] [Ink solvent] Examples of solvents that can be used to disperse metal particles include glycols such as ethylene glycol, diethylene glycol, triethylene glycol, and polyethylene glycol, diols such as propanediol, butanediol, and benzenediol, and ethers such as diethyl ether and diphenyl ether. Ethylene glycol, diethylene glycol, triethylene glycol, and polyethylene glycol are particularly preferred.

[0024] When a sintering ink with dispersed metal particles is applied to a film in the shape of a wiring and sintered by heating to 200°C, it can prevent cracks from occurring in the sintered wiring. This is thought to be because, as shown in Figure 1(a), the metal particles cause a gentle exothermic reaction with a constant gradient, which allows sintering to proceed slowly and reduces the shrinkage of the metal particles (sintering shrinkage).

[0025] <<Method of manufacturing sintering ink>> Next, a method for producing the sintering ink of this embodiment will be described.

[0026] First, a method for producing metal particles will be described.

[0027] [Metal particle manufacturing method] First, an organic dispersant solution prepared by dissolving an organic dispersant in a solvent is heated to a predetermined reaction temperature (less than 170°C).

[0028] Next, a metal ion solution containing metal ions is dropped into the organic dispersant solution to precipitate metal particles coated with the organic dispersant.

[0029] The metal particles are then removed from the organic dispersant solution, thereby producing metal particles coated with the organic dispersant.

[0030] Examples of solvents that can be used include glycols such as ethylene glycol, diethylene glycol, triethylene glycol, and polyethylene glycol, diols such as propanediol, butanediol, and benzenediol, organic solvents such as amines, alcohols, ethers, aromatic compounds, ketones, and nitriles, and water. Ethylene glycol, diethylene glycol, and propanediol are particularly preferred.

[0031] The boiling point of the solvent is desirably less than 260° C. The boiling point of the solvent here is the temperature at which the temperature difference between the solvent and a reference material reaches a peak, as measured by, for example, a differential thermal analyzer.

[0032] Examples of organic dispersants that can be used include polymers such as polyvinylpyrrolidone and polyvinyl alcohol (PVA), amines such as oleylamine and butylamine, and compounds containing functional groups such as hydroxyl groups, carboxyl groups, alkoxy groups, carbonyl groups, ester groups, and mercapto groups. In particular, polyvinylpyrrolidone is preferably used.

[0033] The inventors discovered that by setting the organic dispersant solution to a temperature below 170°C when dripping metal ions, it is possible to produce metal particles that, when subjected to differential thermal analysis, have a peak P1 below 200°C, and the change in temperature difference from a reference material in the temperature range from 100°C to the peak temperature of peak P1 is linear with increasing temperature.

[0034] [Dispersion of metal particles in ink solvent] The metal particles produced by the above-described production method are dispersed in an ink solvent.

[0035] The ink solvent is the one described above. The dispersion concentration of the metal particles in the ink solvent can be, for example, about 10 wt% to 95 wt%. In this case, it is preferable that the sintering ink used in the inkjet method or the like is low concentration, and the sintering ink used in screen printing or the like is high concentration. In particular, when forming thick wiring, it is preferable to perform screen printing using a sintering ink adjusted to a high concentration. Furthermore, if necessary, polymers or organic substances that improve the dispersibility of the metal particles may be added to the ink solvent.

[0036] In this way, the sintering ink can be produced.

[0037] It is also possible to extract metal particles from the sintering ink. Specifically, a solvent that dissolves the ink solvent is mixed with the sintering ink to dissolve the ink solvent, and then the metal particles can be extracted by filtering or centrifuging. Because the extracted metal particles are coated with an organic dispersant, when a DTA chart is obtained by differential thermal analysis, it is thought that there is a peak P1 below 200°C as mentioned above, and the slope of the rising edge has the linear characteristic as mentioned above.

[0038] <<Circuit board manufacturing method>> A method for manufacturing a circuit board using the sintering ink of this embodiment will be described.

[0039] The sintering ink is applied to the substrate in the shape of a wiring to form a film in the shape of a wiring. Examples of application methods that can be used include inkjet printing, dispensing, flexography, gravure printing, gravure offset printing, and screen printing. The film is applied to a predetermined thickness so that the desired thickness is achieved after sintering. After application, the ink solvent may be evaporated by heating at a temperature lower than the temperature of the subsequent sintering step, or the solvent may be evaporated to a concentration level.

[0040] Next, the wiring-shaped film is heated to a predetermined temperature (e.g., 200°C) above the peak temperature. This sinters the metal particles contained in the sintering ink. At this time, the temperature difference (differential) between the metal particles and the DTA reference material in the temperature range from 100°C to the peak temperature of 200°C or less is linear with increasing temperature, so sintering is expected to proceed slowly.

[0041] By the above process, wiring that is suppressed from cracking is formed on the substrate, and a circuit board can be manufactured. In particular, even if the wiring thickness is 10 μm or more, the occurrence of cracks can be suppressed.

[0042] Materials for the substrate used in the above circuit board include polyimide, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic, epoxy, silicone, liquid crystal polymer, glass epoxy, paper phenol, ceramic, glass-containing silicone glass, glass, and metals with an insulating layer coated on the surface. [Example]

[0043] Examples of the present invention will be described below.

[0044] <Synthesis of silver particles> First, silver particles were synthesized as metal particles for the examples and comparative examples. The surfaces of these silver particles were coated with polyvinylpyrrolidone, an organic dispersant, to prevent aggregation during the manufacturing process.

[0045] (Examples 1-10) The silver particles were synthesized by the polyol method. First, 10 to 30 g of polyvinylpyrrolidone (molecular weight 10,000, referred to as PVP) and 200 g of diethylene glycol as a solvent were stirred to dissolve the PVP in the solvent, and the PVP solution was heated to a reaction temperature of 150°C or 160°C.

[0046] 8 g of silver nitrate and 40 g of diethylene glycol were stirred to dissolve the silver nitrate in the diethylene glycol, preparing a silver ion-containing solution.

[0047] The silver ion-containing solution was dropped into the PVP solution heated to the reaction temperature, and then the solution was continuously stirred while maintaining the reaction temperature for a predetermined time.

[0048] After the reaction, the temperature was lowered, the silver particles were recovered, diluted with alcohol or the like, and then filtered or centrifuged to obtain silver particles coated with PVP. Centrifugation is performed to remove impurities, and methods such as filtration can also be used.

[0049] The amounts of PVP and reaction temperatures in Examples 1-10 are as shown in Table 1.

Table 1

[0050] (Comparative Examples 1-10) In Comparative Examples 1-10, the reaction temperature was set to 170 °C or 180 °C. Otherwise, silver particles were produced in the same manner as in Examples 1-10.

[0051] The amounts of PVP and reaction temperatures in Comparative Examples 1-10 are as shown in Table 1.

[0052] <Measurement of particle size> The silver particles of Examples 9, 10, Comparative Examples 9, and 10 were photographed at about 10,000 to 100,000 times with a field emission scanning electron microscope (FE-SEM), and the particle size of the silver particles was measured by analyzing the obtained images. Example 9 was 15 nm to 70 nm, Example 10 was 15 nm to 80 nm, Comparative Example 9 was 15 nm to 80 nm, and Comparative Example 10 was 15 nm to 85 nm.

[0053] <Thermal analysis by TG / DTA apparatus> The sintering compositions (conductive inks) of Examples 9-10 and Comparative Examples 9-10 were placed in the heating furnace of a TG / DTA device together with a reference material and heated at a temperature increase rate of 5°C / min. The weight loss rate in the range of 30°C to 600°C and the TG / DTA chart in the range of 30°C to 600°C were measured. A simultaneous differential thermal and thermogravimetric analyzer (Shimadzu Corporation, Model DTG-60A) was used as the TG / DTA device, and the atmosphere was air. An empty aluminum cup was used as the reference (i.e., air was used as the reference material). While air was used as the reference material here, it is of course possible to use a reference material commonly used in differential thermal analysis, such as aluminum oxide (α-alumina) or potassium chloride.

[0054] The TG chart and DTA chart of Example 9 and Comparative Example 10 are shown in FIGS.

[0055] A characteristic of the silver particles of Examples 1-10 was the presence of a DTA peak P1 (exothermic reaction) with a gentle rise below 200°C. Silver particles produced at a reaction temperature of 150°C or 160°C, as in Examples 1-10, exhibited only a gentle DTA peak below 200°C. However, silver particles produced at a reaction temperature of 170°C or 180°C, as in Comparative Examples 1-10, exhibited an exothermic reaction with a steep rise (having a two-step slope) below 200°C, and the weight ratio also decreased during this exothermic reaction. This was a characteristic common to all 20 combinations of PVP amounts (10-30 g) and reaction temperatures (150-180°C) (Examples 1-10 and Comparative Examples 1-10).

[0056] The gently rising exothermic reaction below 200°C is due to the sintering of the metal particles. The steeper part of the two-stage slope, as in Comparative Example 1-10, is thought to be due to the significant influence of the PVP attached around the metal particles on the exothermic reaction of sintering the metal particles. For example, it is speculated that the two-stage slope may be due to the PVP attached around the metal particles being altered. Furthermore, although the reason is unknown, it is thought that the combustion temperature of the PVP on the particle surface varies depending on the reaction temperature during silver particle synthesis.

[0057] Furthermore, the slope of the DTA chart for the silver particles of Example 1-10 was calculated in the range from 100°C to a temperature 20°C lower than the peak temperature, and was found to be 0.18 μV / °C or more and 0.21 μV / °C or less, as shown in Table 1.

[0058] In contrast, when the slope of the DTA chart for the silver particles of Comparative Example 1-10 was calculated in the range from 100°C to a temperature 20°C lower than the peak apex temperature, it was 0.23 or greater and 0.25 or less, as shown in Table 1, which was a steeper slope than the slope of Example 1-10. The DTA chart for the silver particles of Comparative Example 1-10 also had an inflection point (the point where the slope changes) at temperatures 13°C to 23°C lower than the peak apex temperature. Furthermore, when the slope of the DTA chart was calculated in the range from a temperature 20°C lower than the peak apex temperature to the peak apex temperature, it was 0.88 or greater and 1.98 or less, as shown in Table 1, which was a steeper slope than the slope of Example 1-10.

[0059] It is not clear why the slope of the DTA peak below 200°C becomes gentler or steeper (having a two-step slope) depending on the reaction temperature, but it is thought that this is due to the influence of the PVP that adheres to the silver particles, and that the PVP changes or adheres differently depending on the conditions.

[0060] <Production of sintering ink> Next, sintering inks were produced using the silver particles of Examples 1-10 and Comparative Examples 1-10.

[0061] Silver particles were added to a solvent, polyethylene glycol (average molecular weight 200), and stirred to an Ag concentration of 85 wt% to 88 wt%, thereby producing sintering inks of Examples 1-10 and Comparative Examples 1-10.

[0062] <Manufacturing of wiring boards> The sintering composition inks containing silver particles of Examples 1-10 and Comparative Examples 1-10 were screen-printed onto a substrate and heated at 200°C for 1 hour to sinter the silver particles, forming wiring. The thickness was set to be 10 µm or more after sintering.

[0063] <Crack measurement> The sintered product (wiring) was photographed using a stereo microscope. This allowed us to confirm whether cracks had occurred in the sintered product. Furthermore, by measuring the area of ​​the cracks on the image, we calculated the area that the cracks accounted for in the entire sintered product. Figure 3 shows an image of the sintered product (wiring) and the area ratio of the cracks.

[0064] As shown in Figure 3, no cracks occurred in the wiring produced using the sintering ink of Example 1 to 10. In Comparative Example 1 to 10, cracks occurred at an area ratio of 13% or more.

[0065] The reason for this large difference in crack occurrence between Example 1-10 and Comparative Example 1-10 is thought to be due to the difference in the slope of the DTA peak below 200° C. in the DTA chart. That is, the silver particles of Comparative Example 1-10, which have a steep slope of the DTA peak below 200° C., generate heat rapidly when heated, causing rapid sintering and making them more susceptible to crack occurrence.

[0066] To explain further, when the sintering ink of Example 1-10, which contains silver particles with a gentle DTA peak below 200°C, is sintered at 200°C, compared with the sintering ink of Comparative Example 1-10, which contains silver particles with a DTA peak with a steep rise (two-step slope) below 200°C, the sintering ink of Comparative Example 1-10 is sintered more rapidly, resulting in greater stress during sintering and making the wiring more susceptible to cracks. On the other hand, the sintering ink of Example 1-10, which has a gentle DTA peak, is sintered more slowly, allowing for the creation of a sintered body with reduced stress during sintering, which is thought to make the wiring less susceptible to cracks.

[0067] <Porosity measurement> In addition, SEM (scanning electron microscope) images were taken of areas of the wiring in Example 9 and Comparative Example 10 where no cracks had occurred. The images are shown in Figure 4. From these images, it can be seen that voids exist between the silver particles. When the proportion of the void area to the total area (void ratio) was calculated, it was 14.5% for Example 9 and 4.9% for Comparative Example 10.

[0068] It is presumed that the difference in porosity is due to the fact that the wiring produced with the sintered ink of Example 9 was sintered slowly and therefore had little shrinkage, while the wiring produced with the sintered ink of Comparative Example 10 was sintered rapidly and therefore had a large shrinkage.

[0069] By producing a sintered product using the silver particles of the above-described embodiments and examples, cracks are less likely to occur, and the product is therefore suitable for applications such as forming wiring used in printed electronics, wiring for touch panels and transparent screens, and wiring for circuit boards in in-vehicle equipment, lighting devices, communication devices, gaming machines, office automation equipment, industrial equipment, and general consumer electronics devices.

[0070] Furthermore, the silver particles of the embodiments and examples can be used not only for wiring but also for junctions, making it possible to produce junctions with a low cracking rate. For example, by using them in the junctions of LEDs, the forward voltage VF can be lowered, the thermal resistance can be lowered, and the amount of heat generated by the LED can be reduced.

Claims

1. A method for producing a polymerizable organic dispersant, comprising: heating an organic dispersant solution obtained by dissolving an organic dispersant in a solvent to a predetermined reaction temperature; a step of dropping a metal ion solution containing metal ions into the organic dispersant solution to precipitate metal particles coated with the organic dispersant; and removing the metal particles coated with the organic dispersant from the organic dispersant solution. the organic dispersant is polyvinylpyrrolidone, A sintering ink in which metal particles produced by the method for producing metal particles, in which the reaction temperature is less than 170°C, are dispersed in an ink solvent.

2. A method of producing a polymerizable organic dispersant, comprising: heating an organic dispersant solution obtained by dissolving an organic dispersant in a solvent to a predetermined reaction temperature; a step of dropping a metal ion solution containing metal ions into the organic dispersant solution to precipitate metal particles coated with the organic dispersant; and removing the metal particles coated with the organic dispersant from the organic dispersant solution. The organic dispersant is polyvinylpyrrolidone, and the reaction temperature is less than 170°C. A sintering ink in which metal particles are dispersed in an ink solvent, and when the temperature difference with a reference material is determined while heating by differential thermal analysis, the differential thermal analysis chart showing the relationship between temperature and said temperature difference has a peak at 200°C or less, has no inflection point in the temperature range from 100°C to the apex temperature of said peak, and is linear in the temperature range from 100°C to at least 20°C lower than the apex temperature of said peak.

3. A method of producing a dispersion medium comprising: heating an organic dispersant solution obtained by dissolving an organic dispersant in a solvent to a predetermined reaction temperature; a step of dropping a metal ion solution containing metal ions into the organic dispersant solution to precipitate metal particles coated with the organic dispersant; and removing the metal particles coated with the organic dispersant from the organic dispersant solution. The organic dispersant is polyvinylpyrrolidone, and the reaction temperature is less than 170°C. A sintering ink in which metal particles are dispersed in an ink solvent, and when the temperature difference with a reference material is determined while heating using differential thermal analysis, a differential thermal analysis chart showing the relationship between temperature and said temperature difference has a peak indicating sintering of the metal particles, has no inflection point in the temperature range from 100°C to the peak temperature of said peak, and is linear in the temperature range from 100°C to a temperature at least 20°C lower than the peak temperature of said peak.

4. A step of applying the sintering ink according to claim 1 onto a substrate to form a film; and heating the film to a predetermined temperature to sinter the metal particles contained in the sintering ink to form wiring.

5. A step of applying the sintering ink according to claim 2 onto a substrate to form a film; and heating the film to a temperature equal to or higher than the peak temperature to sinter the metal particles contained in the sintering ink and form wiring.

6. A step of applying the sintering ink according to claim 3 onto a substrate to form a film; and heating the film to a temperature equal to or higher than the peak temperature to sinter the metal particles contained in the sintering ink and form wiring.

7. 7. The method for manufacturing a circuit board according to claim 4, wherein the thickness of the wiring is 10 [mu]m or more.

Images