JP7791641B2 - Sintering ink, metal particles, their manufacturing method, and circuit board manufacturing method - Google Patents

Description

The present invention relates to a technology for forming wiring patterns by sintering ink in which conductive particles are dispersed in a solvent.

A known technique involves applying a paste-like composition (ink) in which conductive particles are dispersed in a solvent to a substrate, then heating it to evaporate the solvent and sinter the conductive particles, forming wiring and other structures on the substrate. By using fine particles ranging in size from a few nanometers to several tens of micrometers, the conductive particles can be sintered at relatively low temperatures, making it possible to use a resin substrate.

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 makes it possible to obtain 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.

Patent Document 2 also 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 specified ratio and dispersed in a reducing organic solvent in order to reduce the volumetric shrinkage rate associated with heating, thereby preventing cracks 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 volumetric shrinkage rate and preventing 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.

JP 2014-55332 A JP 2015-11899 A

To enable the supply of large currents, it is desirable to form thick-film (e.g., 10 μm or thicker) wiring patterns by sintering conductive particles. However, experiments by the inventors have shown that 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 heating and sintering the resulting material, cracks are likely to occur in the wiring, and the thermal shock resistance of the wiring is reduced.

Furthermore, compositions in which nano-sized conductive particles are mixed with micro-sized metal microparticles, such as the technology in Patent Document 2, require a high sintering temperature to sinter the micro-sized conductive particles, and are therefore subject to the constraint that a highly heat-resistant substrate material must be used.

The object of the present invention is to provide a sintering ink that can prevent cracks from occurring in wiring after sintering.

To achieve the above objective, the present invention provides 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 measured while the metal particles are heated using differential thermal analysis, the differential thermal analysis chart showing the relationship between the temperature and the temperature difference has a peak below 200°C, has no inflection point in the temperature range from 100°C to the peak temperature, and is linear in the temperature range from 100°C to at least 20°C lower than the peak temperature.

When wiring is produced by applying and sintering the sintering ink of the present invention, the occurrence of cracks is suppressed, preventing breakage and improving reliability.

(a) TG/DTA chart of Example 9, (b) TG/DTA chart of Comparative Example 10. TG/DTA charts of Examples 9 and 10 and Comparative Examples 9 and 10. 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. (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.

One embodiment of the present invention will be described below. First, the baking ink of this embodiment will be described.

<<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 described in detail below.

[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.

When these metal particles, the surface of which is 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 below 200°C, as shown in Figure 1(a), for example. Furthermore, 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. Furthermore, if there are multiple peaks below 200°C, the lowest-temperature peak has the above characteristics.

The reference material for the above DTA method is a material whose temperature changes in accordance with the temperature of the heating furnace, and is a reference material commonly used in DTA.

The term "inflection point" used here refers not only to a general inflection point, which is defined as a point where a curve changes from concave to convex, or from convex to concave, but also to a point where the slope changes. Therefore, in this embodiment, when a differential thermal analysis chart has no inflection point, this means that the slope of the chart is constant (linear) or changes only gradually, and that the slope does not suddenly increase or have two steps.

The slope of the linear portion of the differential thermal analysis chart should desirably be 1 μV/°C or less, particularly 0.5 μV/°C or less, and even more desirably 0.3 μV/°C or less. The sample weight used for measurement is an average of 10 mg, and measurements are taken using samples adjusted to 5-15 mg.

The slope near the apex of peak P1 may be gentler than the slope at lower temperatures. In other words, the slope of the change in temperature difference (differential thermal) in the temperature range from a temperature 20°C lower than the peak temperature to the peak temperature 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 peak temperature.

The metal particles are preferably silver particles, copper particles, or gold particles, with silver particles being particularly preferred. Furthermore, the organic dispersant that coats the metal particles is preferably polyvinylpyrrolidone.

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

The particle size was calculated from images observed using a field emission scanning electron microscope (FE-SEM).

[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.

When sintering ink with dispersed metal particles like this is applied to a wiring-shaped film and heated to 200°C for sintering, 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 undergo a gentle exothermic reaction with a constant gradient, which allows sintering to proceed slowly and reduces the shrinkage (shrunkness) of the metal particles.

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

First, we will explain the method for manufacturing metal particles.

[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.).

Next, a metal ion solution containing metal ions is added dropwise to the organic dispersant solution, causing metal particles coated with the organic dispersant to precipitate.

Then, the metal particles are extracted from the organic dispersant solution. This allows for the production of metal particles coated with organic dispersant.

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.

The boiling point of the solvent is preferably below 260°C. The boiling point of the solvent here refers to the temperature at which the temperature difference between the solvent and a reference substance peaks, as measured, for example, using a differential thermal analyzer.

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. Polyvinylpyrrolidone is particularly preferred.

The inventors discovered that by setting the temperature of the organic dispersant solution to less than 170°C when the metal ions are dropped, it is possible to produce metal particles that, when subjected to differential thermal analysis, have a peak P1 at or below 200°C, and exhibit a linear change in the temperature difference with respect to the rise in temperature from a reference material in the temperature range from 100°C to the peak temperature of peak P1.

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

The ink solvent used 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 inkjet methods, etc., is low concentration, and that the sintering ink used in screen printing, etc., is high concentration. In particular, when forming thick wiring, it is preferable to use a sintering ink adjusted to a high concentration for screen printing. Furthermore, if necessary, polymers or organic substances that improve the dispersibility of the metal particles may be added to the ink solvent.

This allows the sintering ink to be produced.

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 believed 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.

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

The sintering ink is applied to the substrate in the shape of a wiring to form a film in the shape of a wiring. Application methods that can be used include, for example, inkjet, dispensing, flexography, gravure, gravure offset, 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 can be evaporated by heating at a temperature lower than the temperature of the subsequent sintering process, or the solvent can be evaporated to the extent that the ink is concentrated.

Next, the wiring-shaped film is heated to a predetermined temperature (e.g., 200°C) above the peak apex 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 apex temperature of 200°C or less is linear with increasing temperature, so sintering is presumed to proceed slowly.

The above process allows for the formation of wiring on the substrate with reduced cracking, enabling the manufacture of circuit boards. In particular, cracking can be suppressed even when the wiring is 10 μm or thicker.

Materials for the substrate used in the above circuit boards 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, etc., as well as metals coated with an insulating layer on the surface.

The following describes an example of the present invention.

<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.

(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) was dissolved in 200 g of diethylene glycol as a solvent by stirring, and the PVP solution was heated to a reaction temperature of 150°C or 160°C.

8g of silver nitrate and 40g of diethylene glycol were stirred to dissolve the silver nitrate in the diethylene glycol, preparing a solution containing silver ions.

The silver ion-containing solution was added dropwise to the PVP solution heated to the reaction temperature, and the solution was then stirred for the specified time while being maintained at the reaction temperature.

After the reaction, the temperature is lowered, the silver particles are collected, diluted with alcohol, etc., and then filtered or centrifuged to obtain silver particles coated with PVP. Centrifugation is performed to remove unnecessary substances, and methods such as filtration can also be used.

The amount of PVP and reaction temperature for each of Examples 1-10 are as shown in Table 1.

(Comparative Examples 1-10)
In Comparative Example 1-10, the reaction temperature was set to 170° C. or 180° C. Except for this, silver particles were produced in the same manner as in Example 1-10.

The amount of PVP and reaction temperature for each of Comparative Examples 1-10 are as shown in Table 1.

<Measurement of particle size>
The silver particles of Examples 9 and 10 and Comparative Examples 9 and 10 were photographed at approximately 10,000 to 100,000 magnifications using a field emission scanning electron microscope (FE-SEM), and the resulting images were analyzed to measure the particle size of the silver particles. The particle sizes were 15 to 70 nm for Example 9, 15 to 80 nm for Example 10, 15 to 80 nm for Comparative Example 9, and 15 to 85 nm for Comparative Example 10.

<Thermal analysis using a TG/DTA device>
The sintering compositions (conductive inks) of Examples 9 and 10 and Comparative Examples 9 and 10 were placed in the heating furnace of a TG/DTA apparatus 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 apparatus, and the atmosphere was air. An empty aluminum cup was used as the reference (i.e., air was used as the reference material). Note that while air was used as the reference material here, it is of course possible to use reference materials commonly used in differential thermal analysis, such as aluminum oxide (α-alumina) or potassium chloride.

The TG charts and DTA charts for Example 9 and Comparative Example 10 are shown in Figures 1(a) and (b).

A characteristic of the silver particles of Example 1-10 was that they had 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 Example 1-10, only exhibited a gentle DTA peak below 200°C. However, silver particles produced at a reaction temperature of 170°C or 180°C, as in Comparative Example 1-10, exhibited an exothermic reaction with a steep rise (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 amount (10-30g) and reaction temperature (150-180°C) (Examples 1-10 and Comparative Examples 1-10).

The gently rising exothermic reaction below 200°C is due to the sintering of the metal particles. As in Comparative Example 1-10, the slope is two-stage, and the steeper slope 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.

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

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. Furthermore, the DTA chart for the silver particles of Comparative Example 1-10 had an inflection point (the point where the slope changes) at a temperature 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.

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 this is thought to be due to the influence of the PVP that adheres to the silver particles, with the PVP changing quality or adhering differently depending on the conditions.

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

Silver particles were added to the solvent polyethylene glycol (average molecular weight 200) and stirred to an Ag concentration of 85 wt%-88 wt%. This produced the sintering inks of Examples 1-10 and Comparative Examples 1-10.

<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 the silver particles were sintered by heating at a temperature of 200°C for 1 hour to form wiring. The thickness was set to be 10 µm or more after sintering.

<Crack measurement>
The sintered product (wiring) was photographed using a stereo microscope. This confirmed whether cracks had occurred in the sintered product. In addition, the area of the cracks was measured on the image to calculate the area of the cracks relative to the entire sintered product. The image of the sintered product (wiring) and the area ratio of the cracks are shown in Figure 3.

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

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. In other words, 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.

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, and 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, is sintered at 200°C, the sintering ink of Comparative Example 1-10 is sintered more rapidly, resulting in greater stress during sintering and making cracks more likely to appear in the wiring. On the other hand, the sintering ink of Example 1-10, which has a gentle DTA peak, is sintered more slowly, allowing for a sintered body to be created with lower stress during sintering, which is thought to make the wiring less susceptible to cracks.

<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.

It is believed that the difference in porosity is due to the wiring produced with the sintered ink of Example 9 being sintered slowly and therefore suffering from little shrinkage, while the wiring produced with the sintered ink of Comparative Example 10 being sintered rapidly and therefore suffering from greater shrinkage.

By producing a sintered product using the silver particles of the above-described embodiments and examples, cracks are less likely to occur, making it suitable for use in forming wiring used in printed electronics, wiring for touch panels and transparent screens, and wiring for circuit boards in automotive equipment, lighting devices, communication devices, gaming machines, office automation equipment, industrial equipment, and general consumer electronics devices.

Furthermore, by using the silver particles of the embodiments and examples not only in wiring but also in joints, it becomes possible to create joints with a low crack occurrence rate. For example, by using them in the joints of LEDs, the forward voltage VF is lowered, the thermal resistance is lowered, and the amount of heat generated by the LED can also be reduced.

Claims (7)

a step of 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. a step of 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 or below 200°C, 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 at least 20°C lower than the peak temperature of said peak. a step of 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. 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. 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. 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. The method for manufacturing a circuit board according to claim 4 , wherein the thickness of the wiring is 10 [mu]m or more.