JP7666578B1 - Bonding paste and bonded body - Google Patents

Abstract

[Problem] To provide a bonding paste that suppresses voids in a coating film after sintering even under pressure-free conditions, exhibits high thermal conductivity, has excellent bonding strength, and can suppress a decrease in bonding strength due to thermal cycles, and a bonded body using the bonding paste.
[Solution] The above problem is solved by a joining paste containing metal particles (A) and a dispersion medium (B), in which the metal particles (A) contain 50 mass % or more of metal particles (a) having an average particle diameter of 100 nm or more and 500 nm or less, based on the total mass of the metal particles (A), and when the joining paste is heated at a heating rate of 3°C/min, and the weight loss at 200°C, M200, is 88 or more and 98 or less, assuming that the weight loss at 650°C, M650, is 100.
[Selection diagram] None

Description

The present invention relates to a bonding paste that has excellent thermal conductivity and bonding strength under non-pressure conditions and can suppress the decrease in bonding strength caused by thermal cycles, and a bonded body using the bonding paste.

Conventionally, solder has been used as a bonding material for bonding metal members together, between metal members and semiconductor elements, between metal members and light-emitting diode (LED) elements, etc. In recent years, in the field of next-generation power electronics technology, there has been a demand for devices such as SiC that can operate at high temperatures. As a bonding material for manufacturing such devices, there is a demand for alternative materials to solder from the viewpoint of high-temperature operating reliability. For example, as shown in Patent Documents 1 and 2, bonding materials such as bonding pastes using sinterable metal particles have been proposed.

Patent Document 1 discloses a bonding material containing metal nanoparticles and a solvent, in which the values of heat loss at 100°C, 150°C, and 200°C are specified, with the heat loss L700 when the material is heated from 40°C to 700°C at a heating rate of 3°C/min in a nitrogen atmosphere being taken as 100%.
Patent Document 2 discloses a bonding composition that contains inorganic particles and a specific organic substance attached to at least a portion of the surface of the inorganic particles, and whose weight loss rates when heated from room temperature to 200°C and when heated from 200°C to 300°C are specified by thermal analysis.

JP 2020-164895 A International Publication No. 2013-061527

There are two types of bonding processes using such bonding materials: a method performed without pressure (hereinafter referred to as pressureless bonding) and a method performed under pressure (hereinafter referred to as pressure bonding). Pressure bonding is effective in reducing voids, improves thermal conductivity, and promotes sintering of metal particles, making it possible to form strong bonds, and has the advantage of excellent bonding strength and thermal cycle characteristics. However, pressure bonding requires dedicated equipment, and the objects to be bonded must be able to withstand a pressurized environment, so the objects to be bonded and their applications are limited. Therefore, there is a demand for reducing voids and achieving high thermal conductivity in pressureless bonding, as well as realizing strong bonds and improving bonding strength and thermal cycle characteristics.
On the other hand, in recent years, the area of semiconductor elements has been increased, but it is known that the larger the area of the element, the more difficult it is for gases and the like inside to escape, and voids tend to occur. In addition, the difference between the linear expansion coefficient of Si elements and SiC elements and the linear expansion coefficient of copper used as silver particles and base material is very large, and during a thermal cycle test, distortion occurs between the copper base material / bonding layer containing silver particles / Si or SiC element layers, and cracks tend to occur, which tends to reduce the thermal cycle characteristics. Furthermore, although the linear expansion coefficient of SiC elements is about the same as that of Si elements, the hardness is higher than that of Si elements, so that the distortion during thermal cycles is larger than that of Si elements, and cracks tend to occur in the bonding layer. That is, the level of requirements for improving the bonding strength and thermal cycle characteristics of SiC semiconductor elements has been increasing in recent years.

However, the inventions described in Patent Documents 1 and 2 do not meet the constituent elements of the present invention, and cannot resolve the issues of bonding strength and thermal cycle characteristics when large-area SiC elements are bonded without pressure.

Therefore, the problem that this disclosure aims to solve is to provide a joining paste that suppresses voids in the coating film after sintering even under pressure-free conditions, exhibits high thermal conductivity, has excellent joining strength, and can suppress the decrease in joining strength due to thermal cycles, and a joined body using the joining paste.

As a result of extensive investigations, the present inventors have found that the above-mentioned problems can be solved.
[1]: A bonding paste comprising metal particles (A) and a dispersion medium (B), the metal particles (A) containing metal particles (a) having an average particle diameter of 100 nm or more and 500 nm or less, based on the total mass of the metal particles (A), and when the bonding paste is heated at a heating rate of 3°C/min, a weight loss M650 at 650°C is taken as 100, and a weight loss M200 at 200°C is 88 or more and 98 or less.
[2]: When the weight loss M650 is 100 and the weight loss at 150°C is M150, the ratio of the weight loss M150 to the weight loss M200 (M150/M200) is 0.975 or more. The joining paste according to [1].
[3]: The dispersion medium (B) has a content of a dispersion medium (b1) having a boiling point of 250 ° C. or more and 300 ° C. or less, based on the total mass of the dispersion medium (B). [1] or [2]. The joining paste described.
[4]: The bonding paste according to [3], wherein the dispersion medium (b1) contains at least one selected from the group consisting of terpenes and glycol ethers.
[5]: The dispersion medium (B) has a content of a dispersion medium (b2) having a boiling point exceeding 300 ° C. of 10 mass% or less based on the total mass of the dispersion medium (B). [1] to [4]. The joining paste according to any one of the above.
[6]: The dispersion medium (B) has a content of a dispersion medium having a hydroxyl group of 90 mass% or more based on the total mass of the dispersion medium (B). The joining paste according to any one of [1] to [5].
[7]: A bonded body in which a first bonded portion and a second bonded portion are bonded by the bonding paste according to any one of [1] to [6].
[8]: The bonded body according to [7], wherein the first bonded portion is an untreated substrate.
[9]: The bonded body according to [7] or [8], wherein the second bonded portion is SiC.

The present invention provides a bonding paste that suppresses voids in the coating film after sintering even under pressure-free conditions, exhibits high thermal conductivity, has excellent bonding strength, and can suppress the decrease in bonding strength due to thermal cycling, and a bonded body using the bonding paste.

The bonding paste of the present disclosure is a bonding paste containing metal particles (A) and a dispersion medium (B), wherein the metal particles (A) have an average particle diameter of 100 nm or more and 500 nm or less in a ratio of metal particles (a) of 50 mass% or more, and wherein, when the bonding paste is heated at a heating rate of 3°C/min, the weight loss M650 at 650°C is taken as 100, and the weight loss M200 at 200°C is 88 to 98.
The bonding paste of the present disclosure contains 50 mass% or more of metal particles (a) having an average particle size of 100 nm or more and 500 nm or less in metal particles (A), and the ratio of the weight loss M200 at 200°C to the weight loss M650 at 650°C is within a predetermined range. Therefore, even under strict bonding conditions of no pressure and when the members to be bonded are untreated substrates such as plating-less substrates or SiC semiconductor elements, for which stable bonding is difficult, the bonding paste can exhibit excellent bonding strength and bonding strength after thermal cycles.
In particular, the present invention provides a bonding paste having high thermal conductivity, high bonding strength at the bonded portion, and suppressed decrease in bonding strength due to thermal cycles, which can be bonded without pressure, and a bonded body using the bonding paste. In addition, the present invention provides a bonding paste having high thermal conductivity, excellent bonding strength, and thermal cycle characteristics, even when a large-area SiC element is bonded without pressure.

<Metal Particles (A)>
The metal particles (A) are responsible for the electrical conductivity and thermal conductivity of the bonded body and for bonding the bonded bodies during the sintering process, and include silver, copper, alloys containing silver and/or silver, silver oxide, copper oxide, and coated particles in which a core body made of a metal (except silver and copper) is coated with silver and/or silver. By using the metal particles (A), a bonded body having excellent strength can be obtained. In addition, the metal particles (A) can be used at a wide range of firing temperatures and in various firing environments such as atmospheric pressure, nitrogen atmosphere, vacuum, or reducing atmosphere.

Metal particles (A) contain a predetermined amount or more of metal particles (a) (hereinafter referred to as metal particles (a)) having an average particle size of 100 nm or more and 500 nm or less, and thereby exhibit the function of melting or bonding (hereinafter also referred to as sintering) the particles to each other in the temperature range of 200°C to 350°C at which the joining paste is heated and sintered, and can be changed into bulk metal. As a result, the objects to be joined are joined. Hereinafter, the portion formed by sintering the metal particles (A) present between the objects to be joined is referred to as the joining layer.
From the viewpoints of electrical conductivity and thermal conductivity, the metal particles (A) and the metal particles (a) are preferably particles using silver or copper, and from the viewpoint of oxidation resistance, they are more preferably silver particles.

In the present disclosure, it is important that the metal particles (A) contain 50% by mass or more of metal particles (a) having a specific average particle size. In this specification, the "average particle size" means the 50% cumulative particle size distribution particle size (d50) on a volume basis determined by the measurement method described in the Examples. The d50 of the metal particles (a) is 100 nm or more and 500 nm or less, preferably 150 nm or more, and more preferably 200 nm or more. In addition, the d50 of the metal particles (a) is preferably 400 nm or less, and more preferably 300 nm or less.

The metal particles (A) and the metal particles (a) may have their surfaces coated with an organic component. For example, the silver particles (A) and the silver particles (a) may be particles in which the surfaces of the silver particles are coated with an organic component. The organic component is also called a protective agent. When the bonding paste is coated with an organic component, the storage stability is improved. Examples of the organic component include fatty acids, aliphatic amines, and aliphatic alcohols. Saturated or unsaturated fatty acids are preferable, saturated or unsaturated fatty acids having 3 to 18 carbon atoms are more preferable, and saturated or unsaturated fatty acids having 6 to 18 carbon atoms are even more preferable. The organic component may contain one or more types.

The content of the metal particles (a) having a specific average particle size in the metal particles (A) is preferably 70 mass% or more, more preferably 85 mass% or more. The metal particles (A) and the metal particles (a) may each be independently used alone or in combination.
The metal particles (A) may contain metal particles other than the metal particles (a) as long as the effects of the present invention are not impaired. The metal particles (A) may contain metal particles having an average particle diameter of more than 500 nm, may be combined with metal particles having an average particle diameter of more than 1000 nm, or may be of different metal species.

<Dispersion medium (B)>
The bonding paste of the present disclosure contains a dispersion medium (B). The dispersion medium (B) has a function of dispersing the metal particles (A) and a role of imparting fluidity to the coating film in the silver sintering step.
Examples of the dispersion medium (B) include terpineol, dihydroterpineol, dihydroterpinyl acetate, Tersolve TOE100, Tersolve MTPH (manufactured by Nippon Terpene Co., Ltd.), Texanol (2,2,4-trimethylpentane-1,3-diol monoisobutyrate), carbitol, carbitol acetate, butyl carbitol, isophorone, γ-butyrolactone, dipropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol methyl-n-propyl ether, 3-methoxy-3-methylbutyl acetate, ethylene glycol, propylene glycol diacetate, and dipropylene glycol methyl ether acetate. Examples of the dispersing medium include isoparaffin-based dispersing mediums contained in hydrocarbon-based dispersing mediums, and these may be used alone or in combination of two or more.

The dispersion medium (B) preferably contains 50% by mass or more of the dispersion medium (b1) (hereinafter referred to as the dispersion medium (b1)) having a boiling point of 250°C or more and 300°C or less, based on the total mass of the dispersion medium (B). By containing 50% by mass or more of the dispersion medium (b1), the dispersion medium is more likely to remain in the silver sintering process, and it becomes easier to set the weight loss M200 value at 200°C to 88 or more and 98 or less. The content of the dispersion medium (b1) is preferably 60% by mass or more, more preferably 70% by mass or more, based on the total mass of the dispersion medium (B). The boiling point of the dispersion medium (b1) is preferably 260°C or more.

In addition, the dispersion medium (B) preferably contains 10% by mass or less of a dispersion medium (b2) (hereinafter referred to as dispersion medium (b2)) having a boiling point exceeding 300° C., based on the total mass of the dispersion medium (B). By containing 10% by mass or less of the dispersion medium (b2), the dispersion medium does not remain excessively in the silver sintering step, and it becomes easy to set the weight loss M200 value at 200° C. to 88 or more and 98 or less.
If the weight loss M200 value at 200°C is 88 to 98, the dispersion medium remains at the sintering stage, increasing the fluidity of the coating film and suppressing the generation of voids in the coating film after sintering, allowing high thermal conductivity to be exhibited. In addition, the wettability of the bonded parts is improved and the contact area is increased, allowing for strong bonding. This allows for excellent bonding strength and thermal cycle resistance.

Examples of dispersion media (b1) with a boiling point of 250°C or higher and 300°C or lower include terpenes such as Tersolve TOE-100 (2-(1-methyl-1-(4-methyl-3-cyclohexenyl)ethoxy)ethanol) (manufactured by Nippon Terpene Chemical Co., Ltd.); glycol ethers such as diethylene glycol monohexyl ether, triethylene glycol monoethyl ether, and triethylene glycol monobutyl ether; Texanol (2,2,4-trimethylpentane-1,3-diol monoisobutyrate) and 1,6-diacetoxyhexane. Among these, at least one selected from the group consisting of terpenes and glycol ethers is preferably used.

The dispersion medium (B) preferably contains a dispersion medium containing a hydroxyl group. When the dispersion medium (B) has a hydroxyl group, it is adsorbed to the metal particles (A) and the dispersibility is improved. Furthermore, the evaporation of the dispersion medium is slowed down, so that the film shrinkage that occurs during sintering is slowed down. This increases the fluidity of the coating film, suppresses the generation of voids in the coating film after sintering, and exhibits high thermal conductivity. In addition, the wettability to the bonded parts is improved and the contact area is increased, so that a strong bond can be formed. This allows excellent bonding strength and thermal cycle resistance to be exhibited.
From the above viewpoints, the content of the hydroxyl group-containing dispersion medium is preferably 90 mass % or more, and more preferably 95 mass % or more, based on the total mass of the dispersion medium (B).

Examples of dispersion media containing hydroxyl groups include terpineol-based media such as terpineol, dihydroterpineol, dihydroterpinyl acetate, Tersolve TOE-100 (2-(1-methyl-1-(4-methyl-3-cyclohexenyl)ethoxy)ethanol), and Tersolve MTPH (isobornylcyclohexanol); and glycol ether-based media such as diethylene glycol monohexyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, and polyethylene glycol monobutyl ether.

<Weight loss>
As described above, it is important that the bonding paste of the present disclosure has a weight loss M200 of 88.0 or more and 98.0 or less when the weight loss M650 at 650 ° C. is taken as 100 when the temperature is increased at a heating rate of 3 ° C. / min. When the weight loss M200 is within the above range, the dispersion medium remains at the sintering stage, so that the fluidity of the coating film is increased, the generation of voids in the coating film after sintering is suppressed, and high thermal conductivity can be exhibited. In addition, the wettability to the bonded parts is improved and the contact area is increased, so that a strong bond can be formed. This allows excellent bonding strength and thermal cycle resistance to be exhibited. When M650 is taken as 100, the value of M200 is preferably 90.0 or more and 96.0 or less.
In this specification, when the weight loss at 650° C., M650, is taken as 100, the weight losses at 150° C., 250° C. and 300° C. are taken as M150, M250 and M300, respectively.
The weight loss can be measured using a thermogravimetric differential thermal analysis method (hereinafter, also referred to as a TG-DTA analysis method). For example, the weight loss can be measured using a thermogravimetric differential thermal analyzer (TG/DTA8122 (manufactured by RIGAKU Corporation)) under conditions of heating 10 mg of a sample in a nitrogen atmosphere at a heating rate of 3° C./min and a starting temperature of 30° C.

In terms of the thermal conductivity, bonding strength, and thermal cycle characteristics described above, the bonding paste of the present disclosure may have an M150 of 85.0 or more and 96.0 or less, an M250 of 93.0 or more and 98.5 or less, and an M300 of 96.0 or more and 99.0 or less.

In addition, the bonding paste of the present disclosure preferably has a ratio of M150 to M200 (M150/M200) of 0.975 or more. By having the value of M150/M200 of 0.975 or more, the rapid volatilization of the dispersion medium and other organic substances is suppressed during the sintering process between 150°C and 200°C, and the shrinkage of the coating film that occurs during sintering is gradual. This increases the fluidity of the coating film, suppresses the generation of voids in the coating film after sintering, and exhibits high thermal conductivity. In addition, the wettability of the bonded parts is improved and the contact area is increased, allowing for the formation of a strong bond. This allows for excellent bonding strength and thermal cycle resistance to be exhibited.

From the viewpoint of the thermal conductivity, bonding strength, and thermal cycle characteristics described above, the bonding paste of the present disclosure may have a ratio of M200 to M250 (M200/M250) of 0.95 or more, and a ratio of M150 to M250 (M150/M250) of 0.93 or more.

<Compound (C)>
The bonding paste of the present disclosure may contain a compound (C) (hereinafter referred to as compound (C)) having 20 to 80 carbon atoms and having two or more functional groups (c) selected from the group consisting of hydroxyl groups, carboxy groups, and amino groups. By including such a compound (C), the composition can exist as a liquid composition even after a part or all of the dispersion medium (B) has evaporated, so that a good bonding interface integrated with the bonded parts can be formed. In addition, each functional group (c) has high binding ability with metal particles and exhibits excellent dispersibility.
The number of carbon atoms in compound (C) includes the number of carbon atoms in functional group (c). Therefore, when functional group (c) in compound (C) is a carboxy group, the number of carbon atoms including the carbon atoms in the carboxy group is considered to be the number of carbon atoms in compound (C).

In compound (C), the skeleton (partial structure) excluding functional group (c) is an organic residue, but is preferably a hydrocarbon group or a group in which multiple hydrocarbon groups are bonded with a linking group containing a heteroatom. Examples of such linking groups containing a heteroatom include an -O- group (ether group), a -C(=O)- group (carbonyl group), a -C(=O)-O- group (ester group or oxycarbonyl group), and a -C(=O)-NH- group (amide group or iminocarbonyl group). Compound (C) preferably does not have any functional groups other than functional group (c).

When the number of functional groups (c) in compound (C) is n, compound (C) preferably has an n-valent hydrocarbon group. In compound (C), the skeleton excluding functional group (c) is more preferably composed only of n-valent hydrocarbon groups, since this allows a strong bond to be obtained.

The number of functional groups (c) in compound (C) is preferably 2 or 3. Compound (C) may have a linear structure or a branched and/or cyclic structure, but preferably has a branched and/or cyclic structure. When compound (C) has a branched and/or cyclic structure, it is preferable that compound (C) has a branched and/or cyclic structure, since it tends to be a liquid with low crystallinity and good fluidity.

Examples of the compound (C) having a linear structure include eicosane diacid, heneicosane diacid, docosane diacid, tetracosane diacid, triacontanedioic acid, dotriacontanedioic acid, tetracontanedioic acid, pentacontanedioic acid, hexacontanedioic acid, and batyl alcohol. Examples of the compound (C) having a branched and/or cyclic structure include dimer acid, trimer acid, tetramer acid, dimer diol, trimer triol, tetramer tetraol, dimer diamine, trimer triamine, tetramer tetramine, and phytantriol.
The compound (C) is more preferably a compound selected from the group consisting of dimer acid, trimer acid, dimer diol, trimer triol, dimer diamine, and trimer triamine.

In the bonding paste of the present disclosure, the content of compound (C) is preferably 0.05 to 2.0 mass%, more preferably 0.1 to 1.0 mass%, based on the mass of metal particles (A), from the viewpoints of initial bonding strength and thermal cycle properties.

<Compound (D)>
The joining paste of the present disclosure may contain a compound (D) (hereinafter referred to as compound (D)) having at least one type of nitrogen atom selected from the group consisting of secondary nitrogen atoms and tertiary nitrogen atoms and having four or more hydroxyl groups.
From the viewpoint of bonding strength and thermal cycle properties, the compound (D) preferably has a tertiary nitrogen atom. The number of tertiary nitrogen atoms is preferably 1 to 3, more preferably 2 to 3. When the number of tertiary nitrogen atoms is 2 or more, the bondability with the metal particles (A) is enhanced, and the dispersibility is significantly improved. In addition, the tertiary nitrogen atom has a function of reducing the metal particles (A) and the bonded portion, and when the number of tertiary nitrogen atoms is 2 or more, the reduction function is enhanced.
In addition, from the viewpoint of dispersibility and reduction function, the number of hydroxyl groups in the compound (D) is preferably 4 to 6. In particular, hydroxyl groups have excellent bonding properties with the metal particles (A), and therefore tend to improve dispersibility, and a number of hydroxyl groups of 4 or more enhances bonding properties with the metal particles (A), and the dispersibility is significantly improved. In addition, hydroxyl groups have a function of reducing the metal particles (A) and the bonded portion, and a number of hydroxyl groups of 4 or more greatly improves the reduction function.

The content of the compound (D) is preferably 0.02% by mass or more based on the mass of the metal particles (A). If it is 0.02% by mass or more, the dispersibility of the metal particles (A) is improved, and sintering proceeds with the compound (D) remaining in the coating film, and the coating film has fluidity, so that the adhesion to the bonded part and the defects of the bonding layer are reduced. From the viewpoint of dispersibility, adhesion to the bonded part, and reduction of defects in the bonding layer, it is more preferably 0.05% by mass or more, and even more preferably 0.10% by mass or more.
The content of the compound (D) is preferably 2.00% by mass or less based on the mass of the metal particles (A). By being 2.00% by mass or less, the amount remaining in the coating film after sintering can be reduced, and the bonding strength and thermal cycle properties are excellent. From the viewpoint of reducing the amount remaining, it is more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less.

<Production of bonding paste>
The bonding paste of the present disclosure may contain at least metal particles (A) and a dispersion medium (B), and the manufacturing method thereof is not particularly limited and may be prepared using a known method. Examples of devices for preparing the bonding paste from the metal particles (A) and the dispersion medium (B) include a disperser, a three-roller, a bead mill, an ultrasonic disperser, and a self-revolving mixer.

The mass ratio of the metal particles (A) based on the mass of the bonding paste is preferably 80% by mass to 95% by mass, and more preferably 85% by mass to 94% by mass. By including the metal particles (A) in the above range, the bonding paste exhibits good printability, while suppressing the residue of the dispersion medium (B) in the bonded body and suppressing the occurrence of voids originating from the dispersion medium (B), thereby exhibiting good bonding strength.

The joining paste of the present disclosure may contain additives, such as sintering accelerators, binder resins, resin-type dispersants, and reducing agents.

<Jointed body and method for manufacturing the joined body>
The bonding paste of the present disclosure can bond a first bonded part and a second bonded part to obtain a bonded body. The bonded body can be manufactured, for example, by the following manufacturing method (I) using pressureless bonding or manufacturing method (II) using pressure bonding. The bonding paste of the present disclosure has a controlled weight loss during sintering, and can suppress voids in the coating film after sintering even under pressureless conditions, thereby exhibiting high thermal conductivity. In addition, the bonding paste has excellent bonding strength and can suppress a decrease in bonding strength due to thermal cycles.

[Production method (I)]
The manufacturing method (I) is a pressure-free bonding method, and preferably includes, for example, the following steps (1) to (3).
(1) A step of applying a bonding paste to a first object to be bonded.
(2) A step of placing a second part to be joined on the first part to be joined that has been applied with the joining paste.
(3) A step of sintering the placed laminate in a pressureless environment.

(1) Coating process The method of applying the bonding paste to the bonded body is not particularly limited as long as it can be applied uniformly to the member, and examples thereof include various printing methods such as screen printing, flexographic printing, offset printing, gravure printing, metal mask printing, gravure offset printing, and the like, and a discharge method using a dispenser, etc. The bonding paste of the present disclosure has excellent fluidity even when it contains metal particles at a high concentration, and is therefore preferably used in combination with metal mask printing.

(2) Placing Step Next, the second part to be joined is placed on the first part to be joined to which the joining paste of the present disclosure has been applied. When the joining paste of the present disclosure is used, the second part to be joined can be placed without applying pressure. In the non-pressure joining, the placing step is preferably also performed without applying pressure, but it is also possible to place the second part while applying pressure. When applying pressure, the pressure is appropriately set depending on the viscosity of the joining paste and the drying state of the paste, and is preferably 0.001 to 40 MPa, more preferably 0.003 to 30 MPa.

(3) Sintering process The sintering conditions for pressureless bonding of a laminate in which a second bonded part is placed on a first bonded part are appropriately changed, and examples of such conditions include atmospheric pressure, a nitrogen atmosphere, a vacuum, or a reducing atmosphere at 200 to 350°C. Examples of the sintering device include a hot air oven, a sintering furnace, an electric furnace, an infrared oven, a reflow oven, a microwave oven, a hot plate, and an optical sintering device. These devices can be used alone or in combination as appropriate. The pressureless sintering conditions are preferably such that the temperature is raised to a set temperature by measuring the temperature rise at 2°C to 30°C/min, and then the temperature is maintained at or above the set temperature for about 10 minutes to 2 hours. The set temperature is preferably 200 to 350°C, more preferably 230°C to 300°C, and even more preferably 250 to 280°C.

(2a) Pre-drying step In the manufacturing method (I) using pressureless bonding, it is preferable to provide a pre-drying step (2a) between steps (2) and (3) for the purpose of removing organic components in the bonding coating film. Since the bonding paste of the present disclosure contains the compound (B), the coating film can flow and form a bonding interface even after the dispersion medium (B) has been dried and removed in advance, so a pre-drying step can be provided. By providing a pre-drying step, the residual dispersion medium (B), which is one of the causes of defects (voids) in the bonding layer, can be suppressed, and the bonding layer has excellent density, which is preferable.
The preliminary drying can be carried out, for example, using an apparatus similar to the calcination apparatus at a temperature in the range of 60 to 220° C. for 1 to 300 minutes, preferably at a temperature in the range of 70 to 100° C. for 30 to 120 minutes.

That is, the production method (I) is particularly preferably one comprising the following steps:
(1) A step of applying a bonding paste to a first object to be bonded.
(2) A step of placing a second part to be joined on the first part to be joined that has been applied with the joining paste.
(2a) A step of pre-drying the placed laminate by heating.
(3) A step of sintering the placed laminate in a pressureless environment.

[Manufacturing method (II)]
The manufacturing method (II) is a pressure bonding method, and preferably includes, for example, the following steps (10) to (30).
(10) A step of applying a bonding paste to a first object to be bonded.
(10a) A step of pre-drying the coated laminate by heating.
(20) A step of placing a second part to be joined on a first part to be joined that has been coated with a joining paste and pre-dried.
(30) A step of sintering the placed laminate in a pressurized environment.

(10) Coating process The method of applying the bonding paste to the bonded body is not particularly limited as long as it can be uniformly applied to the member, and examples thereof include various printing methods such as screen printing, flexographic printing, offset printing, gravure printing, metal mask printing, gravure offset printing, and the like, and a discharge method using a dispenser, etc. The bonding paste of the present disclosure has excellent fluidity even when it contains metal particles at a high concentration, and is therefore preferably used in combination with metal mask printing.

(10a) Pre-drying step In the manufacturing method (II) using pressure bonding, a pre-drying step (10a) can be provided between steps (10) and (20) for the purpose of removing organic components in the bonding coating film. By providing the pre-drying step, the pre-drying can be carried out, for example, using an apparatus similar to a baking apparatus at a temperature in the range of 60 to 220° C. for 1 to 300 minutes.

(20) Placing step Next, the second part to be joined is placed on the first part to be joined that has been coated with the bonding paste of the present disclosure and pre-dried. In the case of pressurized bonding, the placing step may also be performed under pressure. The pressure is appropriately set depending on the viscosity of the bonding paste and the drying state of the paste, but is preferably 0.1 to 40 MPa, more preferably 0.3 to 30 MPa.

(30) Sintering step The sintering conditions for pressure-bonding a laminate in which a second bonded part is placed on a first bonded part may be appropriately changed, and examples of the sintering conditions include atmospheric pressure, a nitrogen atmosphere, a vacuum, or a reducing atmosphere at 200 to 350° C.
The pressure is appropriately set depending on the viscosity of the bonding paste and the drying state of the paste, but is preferably 0.1 to 40 MPa, and more preferably 1 to 30 MPa.

In either manufacturing method, there is no restriction on the thickness of the bonding layer formed when the parts to be bonded are bonded with the bonding paste, and the thickness of the bonding layer is preferably 3 μm to 500 μm, more preferably 10 μm to 200 μm, and even more preferably 20 μm to 100 μm.

[Part to be joined]
The type of the joined part is not particularly limited, and examples thereof include metal materials, semiconductor materials, plastic materials, ceramic materials, and electronic elements. Examples of metals include copper, gold, and aluminum. Examples of semiconductor materials include silicon, germanium, gallium arsenide, gallium phosphide, cadmium sulfide, silicon nitride, graphite, yttrium oxide, magnesium oxide, silicon carbide, and gallium nitride. Examples of plastic materials include polyimide, polyethylene, polypropylene, polyethylene terephthalate, polycarbonate, and polyethylene naphthalate. Examples of ceramic materials include glass and silicon. Examples of electronic elements include semiconductor elements, LED elements, and power device elements.

The first and second joined parts may be of different types or may not be the same type. The surfaces of the joined parts may be subjected to a corona treatment, a plating treatment, or the like in order to increase the joining strength of the joined parts.
The bonding paste of the present disclosure can reduce voids and achieve strong bonding, and can achieve high thermal conductivity, excellent bonding strength, and excellent thermal cycle characteristics even for substrates that have not been treated with corona treatment, plating treatment, etc. (also called untreated substrates or plating-less substrates.) For example, the bonding paste of the present disclosure is suitable for a case where the first bonded portion is an untreated substrate (e.g., an untreated copper substrate).
In addition, the bonding paste of the present disclosure can reduce voids and achieve strong bonding, and can achieve high thermal conductivity, excellent bonding strength, and excellent thermal cycle characteristics even in a configuration such as "copper base material/bonding layer containing silver particles/SiC element" that is prone to distortion during thermal cycle testing. For example, the bonding paste of the present disclosure is suitable when the second bonded part is SiC.

The present disclosure will be described in detail below using examples and comparative examples, but the technical scope of the present disclosure is not limited thereto. In the examples, "parts" and "%" represent "parts by mass" and "% by mass", respectively, unless otherwise specified. Numerical values in tables represent "parts" unless otherwise specified.

[Weight loss]
The weight loss of the bonding paste of the present disclosure was measured using a thermogravimetric differential thermal analyzer (TG/DTA8122 (manufactured by RIGAKU Corporation)) by heating 10 mg of a sample in a nitrogen atmosphere from a starting temperature of 30°C to 650°C at a heating rate of 3°C/min. The weight loss at each temperature was determined. When the weight loss at 650°C (M650) was taken as 100, the weight loss at 150°C was taken as M150, the weight loss at 200°C was taken as M200, the weight loss at 250°C was taken as M250, and the weight loss at 300°C was taken as M300.

<Production of Metal Particles>
(Production Example 1) Metal particles A1
Under a nitrogen atmosphere, 200 parts of toluene and 22.3 parts of silver hexanoate were mixed while stirring at 25 ° C. to obtain a 0.5 M solution, and then 1.6 parts of diethylaminoethanol and 0.28 parts of oleic acid were added as a dispersant and dissolved. Then, 73.1 parts of a 20% aqueous solution of succinic acid dihydrazide (hereinafter, SUDH) was dropped as a reducing agent, and the liquid color changed from light yellow to dark brown. In order to further promote the reaction, the temperature was raised to 40 ° C., and the reaction was allowed to proceed. After standing and separating, the aqueous phase was removed to remove excess reducing agent and impurities, and distilled water was added to the toluene layer several times, washing and separation were repeated, and then the process of adding toluene, centrifuging, and removing the supernatant was repeated twice. The precipitate was dried to obtain metal particles A1 in which silver particles were coated with hexanoic acid and oleic acid. The particle size of metal particles A1 was determined by the method described below, and d50 was 210 nm.

(Production Example 2) Metal particles A2
Metal particles A2 were obtained in the same manner as in Production Example 1, except that the amount of diethylaminoethanol was 1.8 parts and the amount of oleic acid was 0.31 parts. The d50 was 185 nm.

(Production example 3) Metal particles A3
Metal particles A3 were obtained in the same manner as in Production Example 1, except that the amount of diethylaminoethanol was 1.2 parts and the amount of oleic acid was 0.18 parts. The d50 was 290 nm.

(Production Example 4) Metal particles A4
Metal particles A4 were obtained in the same manner as in Production Example 1, except that the amount of diethylaminoethanol was 1.0 part and the amount of oleic acid was 0.14 part. The d50 was 390 nm.

(Production Example 5) Metal particles A5
Metal particles A5 were obtained in the same manner as in Production Example 1, except that the amount of diethylaminoethanol was 2.4 parts and the amount of oleic acid was 0.42 parts. The d50 was 120 nm.

(Production Example 6) Metal particles A6
Metal particles A6 were obtained in the same manner as in Production Example 1, except that the amount of diethylaminoethanol was 0.7 parts and the amount of oleic acid was 0.12 parts. The d50 was 450 nm.

(Production Example 7) Metal particles A7
Metal particles A7 were obtained in the same manner as in Production Example 1, except that the silver hexanoate was replaced with 17.0 parts of copper pentanoate, the amount of diethylaminoethanol was 1.8 parts, and the amount of oleic acid was 0.45 parts. The d50 was 150 nm.

(Production Example 8) Metal particles A10
Metal particles A10 were obtained in the same manner as in Production Example 1, except that the amount of diethylaminoethanol was 2.1 parts and the amount of oleic acid was 0.71 parts. The d50 was 85 nm.

(Production Example 9) Metal particles A11
Metal particles A11 were obtained in the same manner as in Production Example 1, except that the amount of diethylaminoethanol was 1.4 parts and the amount of oleic acid was 0.09 parts. The d50 was 600 nm.

(Production Example 10) Metal particles A12
Metal particles A12 were obtained in the same manner as in Production Example 1, except that the amount of diethylaminoethanol was 0.6 parts and the amount of oleic acid was 0.07 parts. The d50 was 1100 nm.

[Method for measuring the average particle size of metal particles]
Isopropyl alcohol was added to each metal particle and dispersed in an ultrasonic disperser to obtain a 0.5% by mass dispersion. The particle diameter of the metal particles in the obtained dispersion was measured using Nanotrac UPA-EX150 (manufactured by Nikkiso Co., Ltd.) to determine the average particle diameter (d50). Of the metal particles produced by the above method, metal particles A1-A7 correspond to metal particles (a) of the present disclosure, and metal particles A10-A12 correspond to metal particles other than metal particles (a).

<Dispersion medium (B)>
The following material was used as the dispersion medium (B). The manufacturer's name, supplementary information, and boiling point are given in parentheses.
Dispersion medium B1: Triethylene glycol monobutyl ether (glycol ether type, containing hydroxyl group, boiling point 278°C)
Dispersion medium B2: Tersolve TOE-100 (manufactured by Nippon Terpene Chemical Co., Ltd., terpene-based, containing hydroxyl groups, boiling point 268° C.)
Dispersion medium B3: 1,6-diacetoxyhexane (Tokyo Chemical Industry Co., Ltd., no hydroxyl group, boiling point 260° C.)
Dispersion medium B4: Dihydroterpineol (manufactured by Nippon Terpene Chemical Co., Ltd., terpene type, containing hydroxyl group, boiling point 210° C.)
Dispersion medium B5: Diethylene glycol monomethyl ether (glycol ether type, containing hydroxyl group, boiling point 193° C.)
Dispersion medium B6: Tersolve MTPH (manufactured by Nippon Terpene Chemical Co., Ltd., terpene-based, containing hydroxyl groups, boiling point 308° C.)

<Compound (C)>
Compound C1: Pripol 2033 (manufactured by Croda Japan Co., Ltd., a dimer diol having 36 carbon atoms. It is composed of two hydroxyl groups and a divalent hydrocarbon group having a branched and cyclic structure. It is liquid.)
Compound C2: Priamine 1071 (manufactured by Croda Japan Co., Ltd., a mixture of a dimer triamine having 36 carbon atoms (composed of two amino groups and a divalent hydrocarbon group having a branched and cyclic structure, liquid) and a trimer triamine having 54 carbon atoms (composed of three amino groups and a trivalent hydrocarbon group having a branched and cyclic structure, liquid))
Compound C3: Pripol 1009 (manufactured by Croda Japan Co., Ltd., hydrogenated dimer acid having 36 carbon atoms. Consists of two carboxy groups and a divalent hydrocarbon group having a branched and cyclic structure. Liquid at 25° C.)
Compound C4: Pripol 1040 (manufactured by Croda Japan Co., Ltd., a trimer acid having 54 carbon atoms. It consists of three carboxy groups and a trivalent hydrocarbon group having a branched and cyclic structure. It is liquid at 25° C.)

<Compound (D)>
Compound D1: Alkanolamine (number of hydroxyl groups: 4, number of tertiary nitrogen atoms: 2)

<Production of bonding paste>
[Example 1]
Metal particles A1 (88 parts) and triethylene glycol monobutyl ether (12 parts) were mixed using a planetary stirrer to prepare a bonding paste.

[Examples 2 to 29, Comparative Examples 1 to 6]
A joining paste was obtained in the same manner as in Example 1, except that the types of materials and the amounts (parts) of the materials were changed according to the compositions shown in Tables 1 to 3. In the tables, blank spaces indicate that no materials were added.

<Evaluation of bonding paste>
Using the obtained bonding paste, a bonded body was produced by the manufacturing method described in Tables 1 to 3. Note that, in Examples 1 to 27 and Comparative Examples 1 to 6, the following manufacturing method 1 was used, in Example 28, the following manufacturing method 2 was used, and in Example 29, the following manufacturing method 3 was used. Details of the manufacturing methods and printing conditions for the bonding paste are as follows.
[Printing conditions (metal mask printing)]
Metal mask: opening 7.5 mm square, plate thickness 100 μm (manufactured by Ceria Corporation)
Metal squeegee: 40 mm x 250 mm, thickness 1 mm (manufactured by Seria Corporation)

[Production method 1]
The bonding paste was printed once under the above printing conditions on the first bonded part (copper base material (plating-free): 20 mm × 20 mm × 3 mm), and then the plated surface of the second bonded part (gold-plated SiC element: 8 mm × 8 mm × 0.3 mm) was placed with the bonding paste surface facing the bonding paste surface, and bonded without pressure under the sintering condition 1 described below to obtain bonded bodies.
[Sintering condition 1] The laminate was placed in a sintering furnace in a nitrogen atmosphere, and the temperature was increased from 25° C. to 80° C. at a rate of 5° C./min, and pre-dried at 80° C. for 90 minutes. Thereafter, the temperature was increased to 300° C. at a rate of 8° C./min, and after reaching 300° C., the laminate was held at 300° C. for 2 hours.

[Production method 2]
The bonding paste was printed once under the above printing conditions on the first bonded part (copper base material (plating-free): 20 mm × 20 mm × 3 mm), and then the second bonded part (gold-plated SiC element: 8 mm × 8 mm × 0.3 mm) was placed with the plated surface facing the bonding paste surface, and bonded without pressure under the sintering conditions described below to obtain a bonded body.
[Sintering condition 2] The laminate was placed in a sintering furnace in a nitrogen atmosphere, and the temperature was raised from 25° C. to 300° C. at a rate of 8° C./min. After reaching 300° C., the laminate was held at 300° C. for 2 hours.

[Production method 3]
The bonding paste was printed once on the first bonded part (copper base material (plating-free): 20 mm x 20 mm x 3 mm) under the above printing conditions, and then placed in a hot air oven and pre-dried at 180°C for 10 minutes. Next, the plated surface of the second bonded part (gold-plated SiC element: 8 mm x 8 mm x 0.3 mm) was placed facing the pre-dried bonding paste surface, and pressure-bonded under sintering condition 3 below to obtain a bonded body.
[Sintering condition 3] In a nitrogen atmosphere, a pressure of 30 MPa was applied from above the second bonded portion, and the temperature was raised from room temperature to 300°C at a rate of 20°C/min. After reaching 300°C, the temperature was maintained for 5 minutes.

The obtained bonded body was used to evaluate the bonding strength and thermal cycle characteristics. In addition, the thermal conductivity was evaluated under the conditions described below. The results are shown in Tables 1 to 3.
[Bonding strength]
The bonded body was fixed at the site of the first bonded part, and pushed toward the second bonded part at a height of 100 μm from the interface between the first bonded part and the bonding layer at a speed of 500 μm/s, and the bond strength (die shear strength) at which the bond was broken was determined and evaluated based on the following evaluation criteria. The larger the die shear strength value, the better, and 10 MPa or more is within the practical range. The measurement conditions are shown below.
[Measurement conditions]
Measurement device: Universal bond tester (manufactured by Daisy Japan, 4000 series)
Measurement height: 100 μm
Measurement speed: 500 μm/s

(Evaluation Criteria)
S: Die shear strength is 35 MPa or more. A: Die shear strength is 25 MPa or more and less than 35 MPa. B: Die shear strength is 15 MPa or more and less than 25 MPa. C: Die shear strength is 10 MPa or more and less than 15 MPa. D: Die shear strength is less than 10 MPa.

[Cold-heat cycle characteristics]
The bonded body was subjected to the following cycle test, and the bond strength (die shear strength) of the bonded body after the cycle test was measured and evaluated in the same manner as in the above-mentioned [Bonding strength]. A practical range is 10 MPa or more.
[Cycle test]
The bonded body was stored at −40° C. for 30 minutes, followed by storing at 150° C. for 30 minutes, which constituted one cycle. Storage was carried out for 500 cycles.

[Thermal conductivity]
The thermal conductivity was calculated from the thermal diffusivity, specific heat, and density according to the following formula.
Thermal conductivity (W/m·K) = density (g/cm ³ ) × specific heat (J/kg·K) × thermal diffusivity (mm ² /s)
The thermal diffusivity was determined as follows. The bonding paste was printed once on the first bonded portion (copper base material (plating-less): 20 mm x 20 mm x 3 mm) using a metal mask (opening 20 mm square, plate thickness 200 μm) metal squeegee: 40 mm x 250 mm, thickness 1 mm (manufactured by Ceria Corporation), and then the second bonded portion (copper base material (plating-less): 20 mm x 20 mm x 3 mm) was placed on it, and the laminate placed in a sintering furnace in a nitrogen atmosphere was placed on it, and the temperature was raised from 25 ° C. to 300 ° C. at a rate of 8 ° C./min., and after reaching 300 ° C., it was held at 300 ° C. for 2 hours to obtain a bonded body. The obtained bonded body was carbon-coated with carbon spray. Next, the thermal diffusivity was measured using a xenon flash analyzer LFA447 Nano Flash (manufactured by NETZSCH). The higher the thermal conductivity value, the better, and a value of 80 W/(m·K) or more is within the practical range.

(Evaluation Criteria)
S: 250W/(m.K) or more A: 200W/(m.K) or more but less than 250W/(m.K) B: 150W/(m.K) or more but less than 200W/(m.K) C: 80W/(m.K) or more but less than 150W/(m.K) D: Less than 80W/(m.K)

According to the results in Tables 1 to 3, when the bonding paste of the present disclosure was used, the thermal conductivity and bonding strength were very high, even in a configuration in which stable bonding is extremely difficult, such as a pressureless, plating-free substrate and a large-area SiC element, and a decrease in bonding strength was suppressed even after a thermal cycle test was performed.
In particular, the bonding paste having a value of M150/M200 of 0.975 or more, the bonding paste having a dispersion medium (b1) having a boiling point of 250 to 300 ° C. inclusive of 50 mass% or more, the bonding paste having at least one dispersion medium (b1) selected from the group consisting of terpenes and glycol ethers, the bonding paste having a content of dispersion medium (b2) having a boiling point exceeding 300 ° C. of 10 mass% or less, and the bonding paste having a content of dispersion medium having a hydroxyl group of 90 mass% or more based on the total mass of the dispersion medium (B) are excellent in bonding strength and suppressed deterioration of bonding strength due to thermal cycles even under non-pressurized conditions (Examples 1 and 7, Examples 7 and 17, Examples 2 and 6, Examples 1 and 13, Examples 2 and 6).
On the other hand, when the bonding paste of the comparative example was used, the thermal conductivity and bonding strength were significantly low, and the bonding strength after the thermal cycle test also decreased significantly.

Claims (10)

A bonding paste comprising metal particles (A) and a dispersion medium (B),
The metal particles (A) contain metal particles (a) having an average particle diameter of 100 nm or more and 500 nm or less in an amount of 50 mass% or more based on the total mass of the metal particles (A),
The metal particles (A) contain silver and/or copper,
When the weight loss M650 of the bonding paste at 650°C is taken as 100, the weight loss M200 at 200°C is 88 or more and 98 or less (wherein the weight loss M650 and the weight loss M200 are determined by a thermogravimetric differential thermal analysis method by heating a 10 mg sample in a nitrogen atmosphere at a starting temperature of 30°C and a heating rate of 3°C/min). 2. The joining paste according to claim 1, wherein when the weight loss at 150°C is defined as M150 when the weight loss M650 is defined as 100, the ratio of the weight loss M150 to the weight loss M200 (M150/M200) is 0.975 or more (wherein the weight loss M150 is determined by a thermogravimetric differential thermal analysis method by heating a 10 mg sample in a nitrogen atmosphere at a starting temperature of 30°C and at a heating rate of 3°C/min). The dispersion medium (B) may be terpineol, dihydroterpineol, dihydroterpinyl acetate, 2-(1-methyl-1-(4-methyl-3-cyclohexenyl)ethoxy)ethanol , isobornylcyclohexanol , texanol, carbitol, carbitol acetate, butylcarbitol, isophorone, γ-butyrolactone, dipropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol methyl-n-propyl ether, 3-methoxy-3-methylbutyl acetate, ethylene glycol, propylene glycol diacetate, dipropylene glycol methyl ether acetate, 1,3-butylene glycol, 1,3 butanediol, 2. The bonding paste according to claim 1, comprising at least one selected from the group consisting of 1,4 butanediol, 2-ethyl-1,3 hexanediol, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, polyethylene glycol monobutyl ether, diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, and an isoparaffin-based dispersion medium. The joining paste according to claim 1, wherein the content of the dispersion medium (b1) having a boiling point of 250°C or more and 300°C or less in the dispersion medium (B) is 50 mass% or more based on the total mass of the dispersion medium (B). The joining paste according to claim 4, wherein the dispersion medium (b1) contains at least one selected from the group consisting of terpenes and glycol ethers. The joining paste according to claim 1, wherein the content of the dispersion medium (B) having a boiling point exceeding 300°C (b2) is 10 mass% or less based on the total mass of the dispersion medium (B). The joining paste according to claim 1, wherein the dispersion medium (B) has a content of a dispersion medium having a hydroxyl group of 90 mass% or more based on the total mass of the dispersion medium (B). A bonded body in which a first bonded part and a second bonded part are bonded with the bonding paste according to claim 1. The bonded body according to claim 8, wherein the first bonded portion is an untreated substrate. The bonded body according to claim 8 or 9, wherein the second bonded portion is SiC.