Gold paste suitable for dip coating and dip coating method using said gold paste

Abstract

To provide a gold paste having good coating properties in dip coating and capable of suppressing angularity and uneven film thickness of a coated material during pulling up and surface roughness of the metal paste in a dip tank.SOLUTION: This invention relates to a gold paste, comprising: a gold powder having purity of 99.9 mass% or more and an average particle size of 0.1 μm to 0.5 μm or less; and an organic solvent. In the gold paste of the present invention, the organic solvent is applied which has: a distance Ra of Hansen solubility parameter to gold powder of 7.0 MPa1 / 2 or more; and intrinsic viscosity value, measured by a rotational viscometer at a shear rate of 4 / s at 25°C, of 1.5 mPa_s or more and 6.5 mPa_s or less. The gold paste of the present invention is useful for dip-coating a gold film on an appropriate coated material, and has good coating characteristics with no angularity of the coating film.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a gold paste suitable for applications such as electrode and wiring formation, bonding, and sealing in the electronics field, including semiconductor devices and semiconductor elements. In particular, the present invention relates to a gold paste suitable for application by dip coating, which can improve the shape stability of the coating film after application. The present invention also relates to a method for applying the gold paste by a dip coating method using this gold paste. [Background technology]

[0002] Brazing filler metals and solders were once widely used to form, join, and seal electrodes (bumps) and wiring for various applications, such as electrical and electronic components, semiconductor devices, semiconductor elements, power devices, and MEMS. However, in recent years, the use of metal pastes (metal slurries) has expanded. The present applicant has also previously proposed that gold pastes (gold slurries) made by mixing high-purity (99.9% by mass or higher) gold (Au) and other metal powders on the submicron order (1 μm or less) with an organic solvent are useful for these applications (see, for example, Patent Documents 1 and 2).

[0003] Forming or joining electrodes and wiring using a metal paste requires that the paste be applied to a substrate or other substrate and then dried to volatilize components other than the metal powder. The gold paste developed by the applicant is essentially composed only of gold powder and an organic solvent, making it relatively easy to remove the organic solvent by volatilization. Furthermore, the gold powder in this gold paste is made of highly pure, fine gold particles, which can be sintered at low temperatures (below 300°C) to form a dense sintered body. This low-temperature sintering property is advantageous in the manufacture of semiconductor elements and devices, where low-temperature processes are recommended.

[0004] To date, methods for applying gold paste to substrates, elements, and other components to be coated have widely been used, such as spin coating, screen printing, inkjet printing, and dropping, which supply an appropriate amount of gold paste to the surface of the component to be coated. The gold paste developed by the present applicant, as described above, focuses on and optimizes the thixotropy index (TI) value as a criterion for optimizing the formability of gold paste applied by these coating methods. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5613253 specification [Patent Document 2] Japanese Patent Application Publication No. 9-20903 [Patent Document 3] Patent No. 6254015 specification Summary of the Invention [Problem to be solved by the invention]

[0006] In addition to the methods mentioned above, dip coating is also known as a method for applying metal paste. Dip coating is a method in which a member to be coated is immersed (dipped) into a tank (dip tank) containing metal paste and then pulled out to apply the metal paste. Dip coating is a method that can apply metal paste uniformly and efficiently to a member to be coated, and is an application method that has excellent area efficiency and minimizes the inclusion of foreign matter. Dip coating of metal paste is used, for example, for partial application to wire-shaped or chip-shaped members to be coated in semiconductor elements such as sensor elements.

[0007] One requirement for applying metal paste by dip coating is the uniformity of the thickness of the coating film applied to the coated component. During dip coating, the metal paste is spread out like a drop when the coated component is pulled up, which tends to result in a thicker film near the center of the lower part of the coated component. This can result in the formation of sharp corners at the bottom of the coated component. These sharp corners are not only undesirable from an appearance standpoint, but also reduce the dimensional accuracy of the component, affecting the yield of electrical and electronic components. Therefore, metal pastes suitable for dip coating must have dimensional stability in the coating film thickness when pulled up.

[0008] Furthermore, when the coated member is removed, the surface of the metal paste in the dipping tank may become rough, such as with corners or dents. If the surface of the metal paste becomes rough, it is necessary to flatten the surface by squeegeeing after each dipping. In this case, the metal paste may adhere to the squeegee, resulting in a decrease in yield.

[0009] In dip coating, the above-mentioned sharp corners on the bottom of the coated component and the surface roughness of the metal paste in the dipping tank are affected by the lifting speed after dipping the component to be coated. If the lifting speed is too fast, these phenomena are likely to occur. Therefore, while sharp corners can be suppressed by reducing the lifting speed, this reduces the manufacturing efficiency of devices, etc. Therefore, there is a demand for improvements in the applicability of the metal paste.

[0010] An example of an improved metal paste that takes into consideration the problem of angularity of the coating film during dip coating is the metal paste (conductive paste) described in Patent Document 3. This metal paste contains flaky silver powder, silver nanoparticles, and a thermosetting resin, and has a TI value of 1.5 to 4.5. This metal paste uses flaky silver powder of 2 μm to 20 μm in size and further contains a thermosetting resin used as an adhesive, thereby increasing the thixotropy of the metal paste and suppressing poor appearance such as angularity during dip coating.

[0011] According to the present applicant, the above-mentioned gold pastes (Patent Documents 1 and 2) developed by the present applicant also have been found to have areas that need improvement when dip coating, such as the shape stability of the coating film and the surface roughness of the metal paste in the dip bath. In comparison with the metal paste of Patent Document 2, although the metal type is different, it is possible to produce a metal paste suitable for dip coating by adopting a similar composition. However, the use of micron-order flake powder or the addition of a thermosetting resin, as in Patent Document 2, is not desirable considering the characteristics of the gold paste developed by the present applicant.

[0012] In other words, making the particle size of the gold powder that makes up the gold paste on the order of microns affects the low-temperature sintering properties of the gold paste developed by the applicant. The low-temperature sintering properties of metal pastes are extremely important in the manufacturing process of semiconductor devices, etc., and cannot be exchanged for the application properties of dip coating.

[0013] Furthermore, adding high-boiling-point resins, such as thermosetting resins, to metal pastes is also not a desirable strategy. If a metal paste contains high-boiling-point resins, the resin may not completely decompose even when heated for drying and sintering after application, and may remain in the electrodes and joints after heating. Residual organic compounds derived from such resins may cause contamination inside semiconductor devices or in the manufacturing atmosphere, potentially affecting semiconductor performance.

[0014] The present invention has been made against the background described above, and provides a gold paste that is based on the gold paste developed by the applicant described above, but which has good application properties in dip coating without impairing the original properties and advantages of the paste, and which can suppress sharp corners and uneven film thickness on the coated member when pulled up, as well as surface roughness of the metal paste in the dipping bath. [Means for solving the problem]

[0015] The present invention, which solves the above-mentioned problems, provides a gold paste comprising gold powder and an organic solvent, wherein the gold powder is made of gold with a purity of 99.9 mass% or more and has an average particle size of 0.1 μm or more and 0.5 μm or less, and the organic solvent has a Hansen solubility parameter distance Ra of 7.0 MPa for the gold powder. 1 / 2 The gold paste is characterized by the above and an intrinsic viscosity of 1.5 mPa·s or more and 6.5 mPa·s or less, measured at a temperature of 25°C and a shear rate of 4 / s using a rotational viscometer.

[0016] To solve the above-mentioned problems while maintaining the low-temperature sinterability and other properties of the gold paste developed by the applicant (Patent Documents 1 and 2), the gold powder must be similar. The addition of resins such as thermosetting resins must also be avoided. Considering these assumptions, optimizing the organic solvent and adjusting the properties of the gold paste accordingly is an appropriate solution. After extensive research based on this approach, the inventors decided to use an organic solvent whose Hansen solubility parameter distance Ra for gold powder falls within a specified range, as described above.

[0017] The Hansen Solubility Parameter (hereinafter sometimes abbreviated as HSP) is one method for defining the solubility parameter of a solvent. HSP is a geometric position (vector) expressed in three-dimensional space by dividing the so-called Hildebrand solubility parameter (SP value) into three components: a dispersion term (δd), a polar term (δp), and a hydrogen bonding term (δh). HSP has traditionally been used as an index for estimating the affinity between a solvent and a solute, and is a parameter used to estimate the degree to which a given solvent can dissolve a given solute. In the gold paste of the present invention, gold particles are dispersed in the solvent without dissolving, so the traditional HSP concept has rarely been applied to this material. However, in recent years, HSP has been recognized as useful for estimating the affinity of solid dispersions with solvents. Furthermore, techniques for measuring the HSP (δd, δp, δh) of solid dispersions have also become publicly known. Taking these points into consideration, the inventors have found that HSP analysis of organic solvents and gold powder is useful for evaluating the coating properties of gold paste, and have decided to apply this method to the present invention.

[0018] Furthermore, the inventors have determined that the intrinsic viscosity of the organic solvent to be applied should be set within a suitable range, since it influences the hardness of the coating film, in addition to the HSP distance Ra relative to the gold powder. These features of the present invention allow the shape of the gold paste applied to the substrate by dipping to be controlled, suppressing hardness and other issues, while optimizing the condition of the gold paste in the dipping bath. The gold paste and dip coating method according to the present invention are described in detail below.

[0019] (A) Gold paste according to the present invention The gold paste according to the present invention is composed of gold powder and an organic solvent. Each of these components will be explained below.

[0020] (1)Gold powder The gold powder in the gold paste of the present invention is composed of gold with a purity (gold concentration) of 99.9% by mass or higher and an average particle size of 0.1 μm to 0.5 μm. As with conventional techniques, the gold paste of the present invention forms a sintered body after application when used as an electrode or a bonding material. When using the sintered metal powder as a bonding material, it must be further compressed and densified. The purity and average particle size of the gold powder in the metal paste are limited to clarify the optimal conditions for the formation and use of these sintered metal powders. The reason for limiting the purity of the gold powder to 99.9% by mass or higher is that gold with a low purity has a high hardness and is less susceptible to plastic deformation when the sintered body is compressed and densified. The reason for limiting the average particle size of the gold powder to 0.1 μm to 0.5 μm is that gold powder with a particle size exceeding 0.5 μm requires a high sintering temperature, which prevents the low-temperature sintering characteristic of the gold paste of the present invention from being exhibited. The reason for setting the lower limit at 0.1 μm is that particles with a particle size less than this size tend to aggregate when made into a paste.

[0021] The purity and average particle size of the gold powder in the gold paste can be measured by collecting the gold paste and volatilizing the organic solvent. The purity of the gold powder can be measured by inductively coupled plasma optical emission spectroscopy (ICP), energy dispersive X-ray spectroscopy (EDX), X-ray fluorescence analysis (XRF), and other methods. The average particle size of the gold powder can be determined by observing and photographing the gold powder using a microscope (optical microscope, electron microscope (SEM, TEM), etc.), measuring the particle sizes of multiple gold powder particles in the photograph or image, and averaging these values. For particle size measurement, it is preferable to measure the major and minor axes of each particle and calculate the particle size using a biaxial method. Computer software such as image analysis software may also be used as appropriate.

[0022] As mentioned above, the gold powder is made of high-purity gold, but the inclusion of unavoidable impurities is permitted. Examples of unavoidable impurity elements include Na, Al, Fe, Cu, Se, Sn, Ta, Pt, Bi, Pd, S, Ag, Br, and Si. The total amount of unavoidable impurity elements is preferably 500 ppm or less, and more preferably 300 ppm or less. These unavoidable impurities may exist in a state of being adsorbed or attached to the gold powder surface, or may exist in a state of reacting with or alloying with the gold powder.

[0023] Furthermore, as disclosed in Patent Document 2, the gold powder in the gold paste preferably has a reduced chloride ion content. This is because chloride ions may not completely gasify even after drying and sintering the gold paste, and may become an acidic liquid in the presence of moisture, corroding electrodes and joints. The chloride ion content in the gold powder is preferably 100 ppm or less. Patent Document 2 also cites the aggregation of gold powder when paste is formed as a drawback of reducing chloride ions in the gold powder. Furthermore, it cites treating the gold powder with a cyanide solution as a method for improving aggregation. This treatment is also effective for the gold powder in the gold paste of the present invention. Therefore, the gold powder may contain a certain amount of cyanide ions. In this case, the cyanide ion content is preferably 10 ppm or more and 1000 ppm or less.

[0024] However, in the present invention, there is no problem even if the content of chloride ions or cyanide ions in the gold powder is outside the above range. This is because the application characteristics of the gold paste by the dip coating method of the present invention (such as the angularity of the coating film and the roughness of the gold paste surface) are not affected by chloride ions or cyanide ions. Gold powder components, including but not limited to chloride ions, can affect the surface condition of the gold powder and therefore may affect the HSP of the gold powder. Even in this case, the object of the present invention can be achieved by setting the HSP distance Ra between the gold powder and the organic solvent within the above range. Measurement of the HSP of gold powder will be described later.

[0025] There are no particular limitations on the method for producing gold powder. Preferably, it is produced by a wet reduction method, as in conventional techniques. The production of gold powder by wet reduction is based on a process of precipitating gold by adding a reducing agent to a gold compound solution. The production of gold powder by wet reduction involves performing gold reduction and precipitation once or multiple times to produce gold powder of a predetermined particle size. A preferred method is to adjust the particle size by performing one or more steps of adding a gold compound solution and a reducing agent to a solution in which ultrafine gold particles are dispersed as core particles, thereby precipitating gold on the surface of the core particles. The gold compound solution used in the wet reduction method is preferably a chloroauric acid solution, which is inexpensive and stably supplied. Furthermore, it is preferable to wash the gold powder produced by wet reduction with an appropriate solvent.

[0026] (2) Organic solvents In order to improve its coating characteristics (such as the angularity of the coating film and the roughness of the gold paste surface), the gold paste of the present invention has the following characteristics: (a) the distance Ra of the HSP to the gold powder is 7.0 MPa 1 / 2 The method is characterized by using an organic solvent that satisfies the following two requirements: (a) the intrinsic viscosity, measured using a rotational viscometer at a temperature of 25°C and a shear rate of 4 / s, is 1.5 mPa·s or more and 6.5 mPa·s or less.

[0027] (a) HSP distance Ra of organic solvent to gold powder The distance Ra of the Hansen solubility parameters for gold powder is the distance between the coordinates of the HSP of the gold powder and the HSP of the organic solvent. When the HSPs are close to each other, that is, when the value of the distance Ra is small, the interaction distance becomes small and the affinity between the gold powder and the organic solvent increases, which affects the dispersibility and wettability. The distance Ra of the HSP is calculated by the distance between the HSP of the gold powder (δd G ,δp G ,δh G ) and the HSP of organic solvents (δd S ,δp S ,δh S ) can be calculated using the following formula 1.

[0028]

number

[0029] Numerical values ​​and calculation methods for the dispersion term δd, polar term δp, and hydrogen bond term δh used in HSP calculations are described, for example, in "INDUSTRIAL SOLVENT SHANDBOOK" (pp. 35-68, Marcel Dekker, Inc., 1996), "HANSEN SOLUBILITY PARAMETERS: A USER'S HANDBOOK" (pp. 1-41, CRC Press, 1999), and "DIRECTORY OF SOLVENTS" (pp. 22-29, Blackie Academic & Professional, 1996). Each component of HSP can also be calculated using group contributions, such as the VKH method or S&P method. Generally, a simple method of investigation is to use values ​​calculated by software and compiled into a database. Examples of such software include the calculation software "Hansen Solubility Parameters: A User's Handbook" (pp. 1-41, CRC Press, 1999). Solubility Parameters The values ​​in the database included in "HSPiP in Practice (HSPiP) Version 4.1.03" (by Steven Abbott, Charles M. Hansen, and Hiroshi Yamamoto) can be used.

[0030] On the other hand, solid particles such as gold powder often do not have reference values ​​for HSP in the databases mentioned above. Known methods for measuring the HSP of solid particles such as gold particles include visual observation based on the Hansen method, interfacial sedimentation velocity (ISV), dynamic light scattering, and inverse gas chromatography (IGC). Measurement methods based on the Hansen method use multiple solvents with known HSP values ​​and determine the HSP by observing the behavior of the solid particles in these solvents. For example, in the visual observation method, the target solid particles are dispersed in multiple solvents with known HSP values, and the dispersibility and affinity of the particles are visually scored. The HSP value of the solid particles is estimated based on the HSP values ​​of solvents with a certain score or higher. Dynamic light scattering involves dispersing particles in multiple solvents and measuring their particle size. The degree of dispersion and aggregation of particles in each solvent varies depending on the affinity between the solvent and the particles, resulting in different measured particle sizes. The affinity between the solvent and the particles is evaluated based on this particle size. In these Hansen method measurements, the HSP (δd, δp, δh) of a solvent evaluated to have a high affinity with the target solid particle is plotted in coordinate space, and the smallest sphere (Hansen sphere) that can encompass the HSP of a solvent whose score or affinity meets a reference value (threshold) is determined. The central coordinates of this smallest sphere (Hansen sphere) are then used as the HSP (δd, δp, δh) of the particle. The IGC method involves filling a column with solid particles and flowing probe molecules with known HSP values ​​through the column to evaluate their adsorption to the solid particles. Specifically, probe molecules such as diglucol, toluene, and chloroform are used, and the retention volume for each probe molecule is measured to calculate the HSP value of the solid particle.

[0031] The organic solvent used in the present invention has a HSP distance Ra of 7.0 MPa. 1 / 2 The above organic solvents. Ra is 7.0 MPa. 1 / 2 Organic solvents with a viscosity of less than 1000 ppm have too high an affinity for gold particles, causing the gold paste to behave like an elastic body, making it difficult to control the shape of the applied gold paste and also causing the gold paste surface to become rough in the dipping bath.

[0032] The upper limit of the HSP distance Ra of the organic solvent used in the present invention is not particularly limited, but is 20 MPa. 1 / 2 It is preferable that the HSP distance Ra is 20 MPa or less. Organic solvents with an excessively large HSP distance Ra have poor affinity with gold powder, and there is a risk of separation during storage or use. Furthermore, from the investigations conducted by the present inventors, it is preferable that the HSP distance Ra is 20 MPa or less. 1 / 2 Organic solvents exceeding this value include those containing halogens (fluorine, etc.) as constituent elements or those with too low a boiling point, and many of these are undesirable for the purposes of the present invention. Within the scope of investigations by the present inventors, the upper limit of the HSP distance Ra is 18.3 MPa. 1 / 2 The following is more preferable, and within this range, separation of the gold paste is unlikely to occur and favorable coating properties can be exhibited.

[0033] (b) Intrinsic viscosity of organic solvent The organic solvents used in this invention are also required to have an intrinsic viscosity within a specified range, as measured using a rotational viscometer at 25°C and a shear rate of 4 / s. In particular, using an organic solvent with an excessively high intrinsic viscosity can affect coating properties, such as roughness, during gold paste application. On the other hand, organic solvents with too low an intrinsic viscosity may cause separation between the gold powder and the organic solvent. The inventors' studies have confirmed that organic solvents with an intrinsic viscosity of 1.5 mPa·s or more and 6.5 mPa·s or less can achieve favorable coating properties without causing separation of the gold paste. Based on this, the intrinsic viscosity range was set.

[0034] The preferred range of the intrinsic viscosity of the organic solvent in the present invention is based on the actual measured value of the organic solvent. The intrinsic viscosity of the organic solvent is measured using a rotational viscometer at a temperature of 25°C and a shear rate of 4 / s. In this case, it is preferable to measure multiple times (preferably three or more times) and use the average value as the intrinsic viscosity of the organic solvent.

[0035] (c) Other compositions and specific examples of organic solvents Furthermore, the organic solvent used in the present invention preferably has a boiling point of 140°C or higher and 360°C or lower. Organic solvents with a boiling point below 140°C evaporate quickly and are not suitable for handling during dip coating at room temperature. Furthermore, considering the drying process of the gold paste after dip coating, it is preferable that the boiling point of the organic solvent be 140°C or higher. On the other hand, organic solvents with a boiling point above 360°C may remain in the formed electrode / junction even after the applied gold paste is dried and fired.

[0036] Furthermore, considering the preferred range of the organic solvent from the viewpoint of its chemical structure, the organic solvent used in the present invention is preferably an organic solvent that does not contain a hydroxyl group at the end of its structural formula. Organic solvents that contain a hydroxyl group in their structure (primary alcohols) may undergo structural changes due to catalytic reactions with gold powder. In consideration of stable application of the paste, organic solvents without a hydroxyl group are preferred. Furthermore, considering the preferred range of the boiling point described above, organic solvents with a carbon number of 5 to 20 are preferred.

[0037] Based on the above description of the organic solvent to be used in the present invention, specific examples of preferred organic solvents include bis(2-butoxyethyl) ether (DGDE), dodecylbenzene, 2-ethylhexyl acetate, pentadecane, and the like.

[0038] The organic solvent used in the present invention is preferably a single solvent consisting of one type of organic solvent, but may also be a mixed solvent consisting of a mixture of multiple organic solvents. A mixed solvent may also be used as long as it satisfies the above HSP conditions, and it is preferable that the intrinsic viscosity value and other factors also satisfy these preferred conditions. Regarding the HSP of a mixed solvent, the HSP of the organic solvents to be mixed can be calculated from their volume ratio. For example, the HSP (δd m ,δp m ,δh m ) is the HSP of solvent A (δd A ,δp A ,δh A ) and HSP of solvent B (δd B ,δp B ,δhB ) and the mixing ratio of solvent A and solvent B (volume ratio: a, b (a+b=1)) using the following formula 2.

[0039]

number

[0040] Furthermore, the organic solvent used in the gold paste according to the present invention can be identified (analyzed) by any one of known analytical methods, such as column chromatography (GC), column chromatography-mass spectrometry (GC-MS), Fourier transform infrared spectroscopy (FT-IR), differential thermogravimetry-mass spectrometry (TG-DTA), and nuclear magnetic resonance analysis (NMR), or a combination thereof, to determine the type and structure of the organic solvent. Then, for the identified organic solvent, it is possible to calculate the HSP and measure the intrinsic viscosity using the methods described above.

[0041] (3) Method for manufacturing gold paste according to the present invention and other configurations The gold paste according to the present invention described above is prepared by dispersing the gold powder described above in an organic solvent. The gold paste according to the present invention can be produced by mixing the gold powder described above with an organic solvent. The gold powder and organic solvent can be mixed at room temperature. Furthermore, when adding the following additives, they can be added simultaneously with the gold powder and organic solvent or after the gold powder and organic solvent are mixed.

[0042] The gold powder content (content) in the gold paste is preferably 85% by mass or more and 97% by mass or less, based on the total mass of the gold paste. If the content is below 85% by mass, the gold powder tends to settle and separate from the solvent. Furthermore, a gold paste with a low gold content also reduces the amount of gold powder applied to the substrate, requiring multiple dipping and drying processes, reducing productivity. On the other hand, if the content exceeds 97% by mass, the gold powder may aggregate, making it difficult to apply a uniform amount of gold powder to the substrate during dipping. Furthermore, if the gold powder content exceeds 97% by mass, the surface of the gold paste tends to become rough, even when an organic solvent with appropriate HSP and intrinsic viscosity is used. The gold powder content of the gold paste is more preferably 95% by mass or more and 96% by mass.

[0043] Furthermore, the gold paste according to the present invention basically comprises two components, gold powder and an organic solvent, but may contain additives as appropriate. The additives may include one or more selected from acrylic resins, cellulose resins, and alkyd resins. The addition of these resins prevents aggregation and separation of the gold powder in the paste, resulting in the formation of a more uniform coating film. Examples of acrylic resins include methyl methacrylate polymers, examples of cellulose resins include ethyl cellulose, and examples of alkyd resins include phthalic anhydride resins. Among these, ethyl cellulose is particularly preferred.

[0044] There are no particular limitations on the viscosity of the gold paste according to the present invention. The viscosity of the gold paste is adjusted not only by the intrinsic viscosity of the organic solvent but also by the content of gold powder in the gold paste. The viscosity of the gold paste does not directly affect the coating properties and morphological stability of the coating film, which are the objectives of the present invention. Even if the viscosity of the gold paste is low, if the intrinsic viscosity of the organic solvent exceeds the above range, sharp corners may occur. Furthermore, if the intrinsic viscosity of the organic solvent is appropriate, good coating properties can be obtained even with a gold paste having a relatively high viscosity. When gold powder is dispersed at an appropriate content while using the above organic solvent, the viscosity of the gold paste tends to be 3000 Pa·s or less when measured using a rotational viscometer at 25°C and a shear rate of 0.4 / s, and this range is preferred.

[0045] (B) Dip coating method according to the present invention Next, a method for applying gold paste by dip coating using the gold paste of the present invention will be described. The application of the gold paste of the present invention can basically follow conventional dip coating methods. That is, at least a portion of a member to be coated is immersed in gold paste stored in a dip tank, and then the member is pulled out, and the gold paste is applied to the member to be coated. Therefore, the method is the same as the conventional method, except that the gold paste supplied and stored in the dip tank is the gold paste of the present invention. Various dip coating devices are commercially available for applying pastes such as gold paste by dip coating, and these can be used without restriction.

[0046] In the present invention, there are no limitations on the material, shape, or dimensions of the coated member to which the gold paste is applied. The material of the coated member can be metal, semiconductor, or ceramic materials used in electrical and electronic equipment, semiconductor devices, and elements, as well as organic materials such as resins and plastics. Furthermore, the shape and dimensions of wire- or chip-shaped members and plate-shaped members such as substrates are not important. Furthermore, there is no limit to the number of coated members that can be applied in a single application operation (dipping and lifting). One of the advantages of dip coating is its efficiency in forming a uniform coating film on multiple coated members.

[0047] When applying gold paste using the dip coating method, the frequency of sharp edges in the applied coating film and surface roughness of the gold paste in the dipping bath tends to depend on the lifting speed after dipping. An excessively high lifting speed makes sharp edges more likely to occur, while a slower lifting speed increases work time and reduces productivity due to a decrease in the amount of gold paste adhered to the coated member. In the present invention, the lifting speed is preferably 1 mm / s or more and 50 mm / s or less.

[0048] When the coated object is pulled up, an appropriate amount of gold paste adheres to the object, completing the dip coating. A drying process can then be carried out as needed. The drying process is carried out to volatilize and remove the organic solvent from the applied gold paste.

[0049] The gold paste of the present invention can be used not only as a material for bumps, electrodes, and wiring, but also as a bonding material for electronic and semiconductor elements and components. When using the gold paste for these purposes, it can be in the coated state described above or in a dried state after drying treatment, or the metal powder can be sintered to form a dense metal powder sintered compact. The metal powder sintered compact obtained from the gold paste has a density similar to that of thin film or bulk gold materials. Furthermore, the gold paste of the present invention is made from high-purity gold powder and does not contain impurities such as resins that can cause contamination, making the sintered compact useful as a conductive material for electrodes and other applications. Furthermore, the gold powder sintered compact also functions as a bonding material, becoming further densified by heating and pressure, and in the process bonding closely to the materials being joined.

[0050] When applying gold paste to a substrate to be coated and then forming a metal powder sintered body, the heating temperature in the sintering process is preferably between 200°C and 400°C. This is because temperatures below 200°C weaken the point contact and bonding between the metal particles, while temperatures above 400°C cause excessive bonding between the metal particles, resulting in necking between the gold powder particles and a strong bond that is too hard. Furthermore, heating above 400°C may result in a deterioration in the performance of semiconductor devices and other devices due to thermal effects. The heating time in the sintering process is preferably between 10 minutes and 120 minutes. The sintering process may be performed in air, a vacuum, or an inert gas atmosphere. This sintering process is also performed without pressure. [Effects of the Invention]

[0051] As described above, the gold paste of the present invention is suitable for application by dip coating, suppressing shape defects such as sharp edges in the coating film after the coated member is removed and reducing roughness of the gold paste in the dipping bath. Furthermore, this gold paste has good stability without causing separation of the gold powder in the dipping bath. These effects are achieved by selecting an organic solvent whose HSP distance Ra and intrinsic viscosity are within the appropriate ranges based on the gold powder. Furthermore, the gold paste of the present invention has good sinterability at low temperatures and can form metal powder sintered compacts useful for various applications such as electrodes, bonding, and sealing. [Brief explanation of the drawings]

[0052] [Figure 1] FIG. 4 is a diagram for explaining a method for evaluating the sharpness of a gold paste in the present embodiment. [Figure 2] FIG. 2 is a diagram for explaining a method for evaluating the surface roughness of a gold paste in this embodiment. [Figure 3] 6 is a photograph of samples (Example 1, Comparative Example 4, Comparative Example 2) after coating, showing an example of the results of an evaluation test of coating properties in dip coating of the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0053] A preferred embodiment of the present invention will be described below. In this embodiment, gold paste was produced by mixing gold powder produced by a wet reduction method with various organic solvents. The gold paste was then applied by dip coating, and the shape of the coating film (presence or absence of sharp edges) was evaluated. Furthermore, its usefulness as a bonding material was confirmed.

[0054] [Preparing gold powder] Gold powder was produced according to the manufacturing method of the prior art (Patent Document 2) by the applicant of the present application. A chloroauric acid solution was prepared as a solution of gold compounds, to which hydroxylammonium chloride was added as a reducing agent and stirred. The chloroauric acid solution was then added, and the solution was stirred at 80°C for 1 hour to obtain a purple-colored, transparent gold colloid solution. The chloroauric acid solution and hydroxylammonium chloride were then added to this gold colloid solution, and the solution was stirred at 80°C for 0.5 hours to adjust the particle size. After the reaction, the gold powder was filtered and washed with isopropyl alcohol and a cyanide solution to obtain gold powder. This gold powder had a purity of 99.99% by mass and an average particle size of 0.3 μm.

[0055] The HSP of the gold powder prepared above was measured by visual observation based on the Hansen method using the following procedure. The freeze-dried gold powder is dispensed into containers, and several dozen solvents (acetone, ethanol, etc.) with known HSP values ​​are added one by one to each container. After treating in an ultrasonic water bath for 60 seconds, mix the gold powder and solvent by hand shaking. After leaving the container undisturbed for 10 minutes and 16 hours, the degree of dispersion of the gold powder in the solvent was visually observed and scored (on a 5-point scale from 1 (good dispersibility) to 5 (poor dispersibility)). A score of 3 or higher is considered to be a good dispersion of the solvent, and a Hansen sphere is created based on the HSP values ​​of those solvents to determine the HSP value of the gold powder.

[0056] By the above measurement method, the HSP (δd G , δp G , δh G ) is δd G =16.5MPa 1 / 2 , δp G =12.7MPa 1 / 2 , δh G =13.1MPa 1 / 2 was measured.

[0057] [Gold paste manufacturing] In this embodiment, the gold paste was manufactured using the seven organic solvents shown in Table 1 below. These organic solvents were selected from the database of the spreadsheet software "Hansen Solubility Parameters in Practice (HSPiP) Version 4.1.03" and used as the HSP (δd S , δp S , δh S ) and the HSP (δd G , δp G , δh G The distance Ra is calculated based on the viscosity of each organic solvent measured using a rotational viscometer (TA The intrinsic viscosity was measured using a DHR-2 (manufactured by Instruments, geometry: stainless steel φ20 mm parallel plates, gap 0.5 mm) at a measurement temperature of 25°C and a shear rate of 0.4 / s (held for 60 seconds).

[0058] [Table 1]

[0059] The gold paste was produced by mixing the gold powder described above with the organic solvent listed in Table 1. The gold powder content in the gold paste was set to between 90% and 97% by mass. For organic solvents S-1 and S-2, several types of gold pastes with different gold powder contents were produced. The viscosity of the produced gold pastes was measured using a rotational viscometer under the same conditions as above, except that the shear rate was 0.4 / s and the gap was 0.05 mm.

[0060] The gold paste was subjected to a dip-coating test to evaluate its application characteristics. A nano-dip coater (ND-0407-S5, manufactured by SDI) was used as the dip-coating device, and a 0.2-0.3 mm diameter Dumet wire with a gold dip coating film pre-formed near its tip was used as the substrate. In the evaluation test, gold slurry (φ5 mm x height 2.5 mm: 0.05 mL) was stored in the dip tank of the dip-coating device. The substrate was immersed 2.5 mm into the gold paste and immediately pulled up at a set pull-up speed. The pull-up speed was set between 0.1 and 50 mm / s. After application, the gold paste was dried by heating in an electric furnace at 160°C for 20 minutes, and then sintered by firing at 400°C for 20 minutes.

[0061] The gold paste application area and bottom end of the Dumet wire, which was the coated component, were then observed at 100x magnification using a digital microscope (Keyence VHX-950F). The gold paste surface in the dipping bath immediately after being pulled out during the application process was also observed using the digital microscope. Based on these observation results, the presence or absence of sharp edges, the amount of adhesion, and the roughness of the paste surface were evaluated. The evaluation methods and criteria for each evaluation item were as follows:

[0062] ·Angularity evaluation Using the observation image obtained as shown in Figure 1, the tip of the wire with the paste attached is designated as point C. From point C, a vertical base line is drawn parallel to the wire and passing through a point moved 100 μm in the pulling direction, with the wire as the reference point. The intersections of this vertical base line and the outer periphery where the paste is attached are designated as points A and B, and the angle θ is measured in relation to the triangle connecting these points. θ of 90° or more was judged as excellent (◎), θ of less than 90° and 80° or more was judged as good (◯), and θ of less than 80° was judged as poor (×). - Evaluation of adhesion amount Using the obtained observation images, the area ratio of the wire to which the paste was attached was visually calculated. 90% or more was judged as excellent (◎), less than 90% and 80% or more was judged as good (◯), and less than 80% was judged as poor (×). -Evaluation of paste surface roughness The depth composition function of a digital microscope was used to obtain data on the shape of the irregularities on the gold paste surface in the dipping bath (Figure 2(a)). A profile line was then drawn so that it passed through the center of the irregularities (the dipped portion) (Figure 2(b)). For this profile line, a portion other than the dipped portion was used as the reference point, and the length from the reference point to the lowest point of the concave portion was measured, and the length from the reference point to the highest point of the convex portion was measured (concave portions are measured as negative values, and convex portions are measured as positive values). The maximum absolute value of the measured length was recorded, and a maximum length of less than 100 μm was judged as excellent (◎), 100 μm to 500 μm was judged as good (◯), and 500 μm or more was judged as poor (×).

[0063] The evaluation results of the application characteristics of the dip coating performed in this embodiment are shown in Table 2. Examples of the results of this evaluation test are shown in Figure 3. Figure 3 shows the results of Example 1, which showed good results with no sharp edges, Comparative Example 4, which showed sharp edges, and Comparative Example 2, which was poor in terms of the amount of adhesion.

[0064] [Table 2]

[0065] The results of the evaluation tests for this embodiment demonstrate that optimizing both the HSP distance Ra and the intrinsic viscosity of the applied organic solvent relative to the gold powder ensures the application characteristics and morphological stability of the gold paste. The gold pastes of Examples 1 to 9, which met these requirements, were evaluated as "good" (◯) or better in terms of the degree of hardening during dipping, adhesion amount, and paste surface roughness in the dipping tank. In contrast to each example, Comparative Examples 1 and 2 had an organic solvent HSP distance Ra of less than 7.0, resulting in unfavorable results in terms of adhesion amount and paste surface roughness. Furthermore, Comparative Examples 3 and 4 had organic solvents with intrinsic viscosities that were too high, making hardening more likely to occur.

[0066] Furthermore, when comparing Examples 1 to 3 and Examples 4 to 6, which use the same organic solvent, the viscosity of the gold paste tends to be significantly affected by the gold powder content. Furthermore, there is no proportional relationship between the intrinsic viscosity of the organic solvent and the viscosity of the gold paste. Based on the results of this embodiment, it is believed that the viscosity of the gold paste as a whole has little effect on the application properties. Comparing each example with Comparative Example 2, it appears that a lower viscosity of the gold paste tends to be preferable to a certain extent, but comparing Comparative Examples 2 to 4, this is not necessarily the case. As mentioned above, when it comes to application properties during dipping into the gold paste, optimizing the HSP and intrinsic viscosity of the organic solvent should be prioritized.

[0067] Next, a bonding strength test was carried out on the gold pastes of Examples 1 to 3, 4, 5, and 7. In this bonding test, a Si chip (area 9 mm ) on which a 300 nm gold film had been formed in advance was used. 2 Apply gold paste to the center of the surface (application area: approx. 1mm) 2 ), and on top of that, a Si chip (area 1 mm ) with the same gold film deposited as above. 2 ) was placed on the Si chip to create a sample (a laminated structure of Si chip / gold paste / Si chip). The gold paste was placed on the Si chip immediately after application, and was not dried or sintered at this stage. The sample was then heated to sinter and bond the gold paste to form a bonded body. This heating process was carried out at 230°C using heat transferred from a heating tool, and the heating time was 30 minutes.

[0068] Then, a shear test was carried out on the fabricated joint. In the shear test, a blade was moved at a constant speed from the side of the joint to the upper tip (area 1 mm 2 The fracture stress (N) was measured when fracture (peeling of the chip) occurred. The relationship between this stress value and the bonded area (1mm 2 ) and the bond strength (MPa) per unit area was calculated. This test was conducted multiple times with five samples, and the average values ​​of the breaking stress and bond strength were calculated. The results are shown in Table 3. In this shear test, the pass standard was set at 15 MPa or more, assuming the bond strength of the bond in electronic components.

[0069] [Table 3]

[0070] As can be seen from Table 3, the joints formed with the gold pastes of each example have a suitable joint strength for use in joining electronic components. Therefore, it can be said that the gold paste of the present invention can form a coating film with a suitable shape during dip coating and has suitable functions as a joining material after application. [Industrial Applicability]

[0071] The gold paste of the present invention has an appropriate HSP distance Ra relative to the gold powder, and by selecting an organic solvent with a suitable intrinsic viscosity, it is suitable for application by dip coating. The present invention enables paste application while reducing defects in the shape of the coating film (such as sharp edges) and roughness of the gold paste in the dipping bath when applying the gold paste by dip coating. The gold paste of the present invention is also compatible with low-temperature processes and is effective for applications such as bonding, electrode and wiring formation, and sealing in a variety of applications, including electronic components, semiconductor devices, power devices, and MEMS.

Claims

1. In a gold paste comprising gold powder and an organic solvent, The gold powder is made of gold with a purity of 99.9% by mass or more and has an average particle size of 0.1 μm or more and 0.5 μm or less, The organic solvent has a Hansen solubility parameter Ra of 7.0 MPa for the gold powder. １／２ That's all, and A gold paste having an intrinsic viscosity of 1.5 mPa·s or more and 6.5 mPa·s or less, as measured at a shear rate of 4 / s at a temperature of 25°C using a rotational viscometer.

2. 2. The gold paste according to claim 1, wherein the organic solvent has a boiling point of 140°C or higher and 360°C or lower.

3. 3. The gold paste according to claim 1, wherein the content of the gold powder is 85% by mass or more and 97% by mass or less based on the total mass of the gold paste.

4. A gold paste dip coating method comprising immersing at least a portion of a member to be coated in gold paste stored in a dipping tank and then lifting the member out of the tank, and applying the gold paste to the member to be coated, 4. A method for dip-coating a gold paste, wherein the gold paste is the gold paste according to claim 1.

5. 5. The method for dip-coating a gold paste according to claim 4, wherein the member to be coated is pulled up at a speed of 1 mm / s or more.

Images