Conductive paste

Abstract

A conductive paste comprising metal particles, an epoxy resin, a curing agent and a flux, the metal particles comprising first metal particles containing a high-melting metal having a melting point of 800°C or higher, and second metal particles containing a low-melting metal having a melting point of 180°C or lower, the low-melting metal being an Sn-Bi alloy, and the mass ratio Sn / Bi of Sn to Bi in the Sn-Bi alloy being 1 to 5, inclusive.

Description

Conductive paste Cross-reference to related applications

[0001] This application claims the priority of Japanese Patent Application No. 2024-167477, and the description thereof is incorporated herein by reference.

[0002] The present invention relates to a conductive paste.

[0003] Conventionally, conductive pastes have been used to electrically connect various components. For example, conductive pastes are used to form vias that connect the layers of a multilayer wiring board. As a conductive paste, one containing metal particles and a thermosetting resin composition containing an epoxy resin is known. Such a conductive paste is configured such that when heated to the temperature at which the epoxy resin cures, the cured product of the epoxy resin reinforces the conductive path formed by the metal particles.

[0004] In addition, in order to reduce the thermal load on the substrate, etc., it has been proposed to use a conductive paste containing low melting point metal particles. For example, in Patent Document 1, a conductive paste containing low melting point metal particles and high melting point metal particles has been proposed. Such a conductive paste is heated to a relatively low temperature that is higher than the melting point of the low melting point metal particles and lower than the melting point of the high melting point metal particles after being filled into a via hole. At this time, the molten low melting point metal forms an alloy with the high melting point metal so as to coat the high melting point metal particles. Then, the high melting point metal particles are joined via the alloy, and conductivity is imparted to the via.

[0005] The conductive paste specifically evaluated in Patent Document 1 contains a silver-coated alloy as high melting point metal particles and low melting point metal particles made of an Sn—Bi-based alloy, and the Sn / Bi mass ratio is 42 / 58 (=0.72). This Sn / Bi mass ratio corresponds to the vicinity of the eutectic point in the phase diagram of the Sn—Bi-based alloy. And such an Sn—Bi-based alloy has a low liquidus temperature (the melting point indicated by the liquidus line in the phase diagram) and is likely to melt during the thermal curing of the epoxy resin, so it is considered that both reduction of the thermal load on the substrate and good conductivity can be achieved.

[0006] International Publication No. 2016 / 136204

[0007] Incidentally, when manufacturing printed circuit boards, reflow ovens are used to solder electronic components such as capacitors. Furthermore, lead-free solder, which has become increasingly popular in recent years, has a higher melting point than lead solder, so the heating temperature of the reflow oven is also relatively high when using lead-free solder. For this reason, conductive pastes are required not only to exhibit good conductivity stably over the long term, but also to have good heat resistance. It should be noted that the requirement for good heat resistance in addition to good conductivity and long-term reliability is common not only when used in printed circuit boards, but also for conductive pastes used in other applications. However, the situation has not yet reached one where such requirements are fully satisfied.

[0008] In view of the above circumstances, the object of the present invention is to provide a conductive paste that is excellent in conductivity, heat resistance, and long-term reliability.

[0009] The conductive paste according to the present invention is as follows: [1] A conductive paste comprising metal particles, epoxy resin, a curing agent, and flux, wherein the metal particles comprise first metal particles containing a high-melting-point metal with a melting point of 800°C or higher, and second metal particles containing a low-melting-point metal with a melting point of 180°C or lower, wherein the low-melting-point metal is a Sn-Bi alloy, and the mass ratio of Sn to Bi of the Sn-Bi alloy, Sn / Bi, is 1 or more and 5 or less.

[0010] [2] The conductive paste according to [1], wherein D50 in the particle size distribution of the second metal particles is 1 μm or more and 15 μm or less.

[0011] [3] The conductive paste according to [1] or [2] above, wherein the mass ratio of the flux activator to the second metal particles is 0.05 or more.

[0012] [4] The conductive paste according to any one of [1] to [3] above, wherein the curing agent is a phenolic curing agent.

[0013] [5] The conductive paste according to [4] above, wherein the phenolic curing agent is a novolac-type phenolic resin.

[0014] According to the present invention, a conductive paste with excellent conductivity, heat resistance, and long-term reliability can be provided.

[0015] This is a schematic cross-sectional view of a multilayer wiring board according to one embodiment.

[0016] The conductive paste according to an embodiment of the present invention will be described with an example of forming conductive paths in the stacking direction of a multilayer wiring board.

[0017] The conductive paste according to this embodiment comprises metal particles and a thermosetting resin composition. The metal particles include first metal particles (hereinafter referred to as high-melting-point metal particles) containing a high-melting-point metal with a melting point of 800°C or higher, and second metal particles (hereinafter referred to as low-melting-point metal particles) containing a low-melting-point metal with a melting point of 180°C or lower. The thermosetting resin composition comprises an epoxy resin, a curing agent, and a flux.

[0018] The conductive paste according to this embodiment can be used to form vias that connect layers of a multilayer wiring board. As shown in Figure 1, the multilayer wiring board 1 has a laminated structure of two or more layers including a first layer and a second layer, and each layer has a circuit 10 made of metal foil or conductive ink, and an insulating film 20 that supports the circuit 10. In the formation of the via 30, for example, via holes H are formed between the circuits 10 formed in the first layer and the second layer, respectively. The conductive paste according to this embodiment is filled into the via holes H by a printing method or the like, and then heated to a predetermined temperature, so that the low-melting-point metal particles melt and diffuse between the high-melting-point metal particles, forming an alloy of the low-melting-point metal and the high-melting-point metal so as to cover the high-melting-point metal particles, and joining the high-melting-point metal particles to each other via the alloy and the low-melting-point metal, thereby forming vias 30 that become conductive paths that electrically connect the circuits of the first layer and the circuits of the second layer.

[0019] The conductive paste of this embodiment is used under heating conditions that do not inhibit the formation of the alloy, that is, heating conditions that allow the low-melting-point metal to melt before the curing of the thermosetting resin composition is completed. For example, when differential scanning calorimetry is performed on the conductive paste under a heating condition of 10°C / min, it may show an endothermic peak temperature associated with the melting of the low-melting-point metal particles. The endothermic peak temperature is preferably 140 to 150°C. On the other hand, when differential scanning calorimetry is performed on the thermosetting resin composition under a heating condition of 10°C / min, it may show an exothermic peak temperature of 160 to 180°C.

[0020] The high-melting-point metal particles have a surface layer made of a metal that can form an alloy with the molten low-melting-point metal. Examples of the metal forming the surface layer include copper, silver, nickel, and gold. Examples of the high-melting-point metal particles include copper particles, nickel particles, silver particles, silver-coated copper particles, gold-coated copper particles, silver-coated nickel particles, and gold-coated nickel particles. The high-melting-point metal particles may contain unavoidable impurities. The metal particles may consist of only one type or two or more types of the high-melting-point metal particles.

[0021] The median diameter (D50) in the particle size distribution of the high-melting-point metal particles is preferably 0.5 μm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less. In this specification, the particle size distribution refers to the volume-based particle size distribution measured by the laser diffraction scattering particle size distribution measurement method.

[0022] The low-melting-point metal particles of this embodiment include a Sn-Bi alloy. The low-melting-point metal particles may contain unavoidable impurities. Furthermore, the low-melting-point metal particles may also contain an In-containing alloy, such as a Sn-In alloy, to the extent that it does not hinder the objectives of the present invention. The Sn-Bi alloy content in the low-melting-point metal particles is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more.

[0023] Furthermore, the mass ratio of Sn to Bi in the Sn-Bi alloy, Sn / Bi, is between 1 and 5. Preferably, the mass ratio is between 1.5 and 4. A mass ratio of Sn / Bi of 1 or higher suppresses the decline in conductivity, reflow resistance, and long-term reliability caused by Bi. Additionally, a mass ratio of Sn / Bi of 5 or lower suppresses the decrease in meltableness of low-melting-point metal particles caused by the high Sn content, thereby forming a good alloy and improving conductivity and other properties.

[0024] Sn-Bi alloys with a Sn / Bi mass ratio within the above range have a solidus melting point of 139°C in the phase diagram, which corresponds to the temperature of the eutectic point. On the other hand, the same Sn-Bi alloy has a liquidus melting point higher than 139°C in the phase diagram, generally between 160 and 220°C, which is higher than the eutectic point. Compared to Sn-Bi alloys with a mass ratio of about 0.7 (liquid phase temperature around 139°C), such Sn-Bi alloys have lower meltability at the heating temperature during via formation and are theoretically considered to be at a disadvantage. In contrast, according to the examples described later (experimentally), the aforementioned Sn-Bi alloy (mass ratio Sn / Bi between 1 and 5) can form a liquid phase that exhibits sufficient wettability to high-melting-point metal particles, thereby improving the conductivity of the via.

[0025] In the particle size distribution of the low-melting-point metal particles, D50 is preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less. This increases the specific surface area, which suppresses the decrease in via conductivity due to the effect of the oxide film. It also makes it easier to fill via holes with small pore diameters with conductive paste.

[0026] The D50 in the particle size distribution of the low-melting-point metal particles is preferably 0.3 to 4, and more preferably 0.5 to 3, relative to the D50 in the particle size distribution of the high-melting-point metal particles. This ensures that the low-melting-point and high-melting-point metal particles are blended in a well-balanced manner, forming a dense packed structure that can exhibit good conductivity.

[0027] The conductive paste can be suitably used for filling via holes with a diameter of 0.5 mm or less. The diameter of the via hole may be 0.4 mm or less, or 0.3 mm or less. The ratio of the length (L) of the via hole to the diameter (D) (L / D) may be 1.0 or more, 1.5 or more, 2.0 or more, or 3.0 or more. The conductive paste of this embodiment does not easily degrade in conductivity during heat cycle tests, etc., and therefore can exhibit long-term reliability even with elongated vias. The via hole may simply be a through-hole formed in an insulating film, or it may be a through-hole formed in an insulating film with plating applied to the inner surface. That is, the conductive paste of this embodiment may be used in the former type of via hole to facilitate the manufacture of multilayer wiring boards, but it may also be used in the latter type of via hole to form a conductive path together with the plating of the via hole.

[0028] Furthermore, the mass ratio of the high-melting-point metal particles to the low-melting-point metal particles is preferably 0.3 or higher, and more preferably 1 or higher. Here, the mass ratio is, for example, 3 or less, and preferably 2 or less.

[0029] The content of the high-melting-point metal particles is preferably 400 parts by mass or more and 3,300 parts by mass or less per 100 parts by mass of the epoxy resin. On the other hand, the content of the low-melting-point metal particles is preferably 600 parts by mass or more and 3,500 parts by mass or less per 100 parts by mass of the epoxy resin.

[0030] The shapes of the high-melting-point metal particles and the low-melting-point metal particles are not particularly limited, but may be spherical, dendritic, or flaky. Dendritic or flaky low-melting-point metal particles have the advantage of increasing the number of contact points with each other, making it easier to ensure conductivity. On the other hand, spherical low-melting-point metal particles have the advantage of reducing the influence of oxide films that can reduce conductivity because their specific surface area is relatively small.

[0031] The epoxy resin is preferably a liquid epoxy resin at room temperature (25°C). The epoxy equivalent of the epoxy resin is preferably 500 g / eq or less. For example, the epoxy equivalent of the epoxy resin is 90 g / eq or more.

[0032] Examples of the epoxy resin include bisphenol-type epoxy resins such as bisphenol A-type epoxy resin and bisphenol F-type epoxy resin; glycidyl ether-type epoxy resins such as 4-hydroxybutyl acrylate glycidyl ether; glycidylamine-type epoxy resins such as N,N-diglycidyl-4-glycidyloxyaniline and 4,4'-methylenebis(N,N-diglycidylaniline); and aliphatic epoxy resins. The thermosetting resin composition may also contain epoxy resins as reactive diluents that are liquid at room temperature (25°C), such as butyl glycidyl ether, 2-ethylhexyl glycidyl ether, and 1,6-hexanediol diglycidyl ether.

[0033] The epoxy resin content is preferably 0.01 or more and 0.1 or less relative to the mass of the metal particles.

[0034] The curing agent is preferably one that forms a cured product with the epoxy resin under heating conditions of 150°C to 200°C, more preferably 150°C to 160°C. Examples of such curing agents include phenolic curing agents and acid anhydride curing agents.

[0035] Examples of the phenolic curing agents include bisphenol A, bisphenol F, dihydroxynaphthalene, bisphenol sulfide, phenol novolac resin, and phenol aralkyl resin.

[0036] Examples of the acid anhydride-based curing agents include methyltetrahydrophthalic anhydride, methylbutenyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, and dodecenyl succinic anhydride.

[0037] The content of the curing agent is preferably 1 to 30 parts by mass, more preferably 1 to 20 parts by mass, relative to 100 parts by mass of the epoxy resin.

[0038] Examples of the activator of the flux include carboxylic acids such as dicarboxylic acids such as sebacic acid, adipic acid, glutaric acid, succinic acid, malonic acid, 8-ethyloctadecanedioic acid, and pimelic acid, tricarboxylic acids such as citric acid, and aromatic carboxylic acids such as benzoic acid; alkanolamines such as monoethanolamine, diethanolamine, and triethanolamine, and salts of the carboxylic acid and the alkanolamine.

[0039] The activator is preferably contained in the thermosetting resin composition such that the mass ratio to the low melting point metal particles is 0.05 or more. Thereby, the conductivity of the via can be improved. The mass ratio here is, for example, 0.1 or less.

[0040] The thermosetting resin composition may contain other additives. Examples of the additives include plasticizers, flame retardants, antioxidants, defoaming agents, leveling agents, rheology control agents, fillers, and the like. The content of the additives is, for example, 0.5 to 20 parts by mass with respect to 100 parts by mass of the epoxy resin.

[0041] The thermosetting resin composition preferably does not contain a curing accelerator. Thereby, the pot life of the conductive paste can be extended. In addition, the continuous printing performance of the conductive paste can be improved.

[0042] The conductive paste preferably does not contain a solvent. That is, the conductive paste is preferably a solventless system. Thereby, the generation of voids due to the volatilization of the solvent during thermosetting can be suppressed.

[0043] The viscosity of the conductive paste is preferably 30 to 70 Pa·s. The viscosity can be measured using a cone plate type viscometer under the conditions of 25°C and a shear rate of 10 (1 / sec).

[0044] Incidentally, although the embodiments have been shown as examples, the conductive paste according to the present invention is not limited to the configurations of the above embodiments. Further, the conductive paste according to the present invention is not limited by the above-described effects. The conductive paste according to the present invention can be variously modified without departing from the gist of the present invention.

[0045] Hereinafter, the present invention will be further described by way of examples, but the present invention is not limited thereto.

[0046] [Raw materials used] (A) Epoxy resin: Bisphenol A type epoxy resin (manufactured by ADEKA Corporation, EP-4400, liquid, epoxy equivalent 200 g / eq) (B1) High melting point metal particle 1: Silver-coated copper powder, spherical, D50: 5 μm High melting point metal particle 2: Silver powder, spherical, D50: 5 μm (B2) Low melting point metal particle 1: Sn-Bi alloy (Sn / Bi = 60 / 40 = 1.5), D50: 3 μm Low melting point metal particle 2: Sn-Bi alloy (Sn / Bi = 60:40 = 1.5), D50: 15 μm Low melting point metal particle 3: Sn-Bi alloy (Sn / Bi = 80:20 = 4), D50: 1.5 μm Low melting point metal particle 4: Sn-Bi alloy (Sn / Bi = 80:20 = 4), D50: 3 μm Low melting point metal particle 5: Sn-Bi alloy (Sn / Bi = 80:20 = 4), D50: 14 μm Low melting point metal particle 6: Sn-Bi alloy (Sn / Bi = 80:20), D50: 20 μm Low melting point metal particle 7: Sn-Bi alloy (Sn / Bi = 42:58 = 0.72), D50: 8 μm Low melting point metal particle 8: Sn-Bi alloy (Sn / Bi = 90:10 = 9), D50: 8 μm (C) Hardener: Phenol novolac resin (manufactured by Arakawa Chemical Industries, Ltd., Tamanol 758) (D) Flux (activator): 8-Ethyloctadecanedioic acid (manufactured by Okamura Oil Co., Ltd.)

[0047] [Production Example] Each component was blended at the blending ratios shown in Tables 1 to 2 to prepare a conductive paste.

[0048] [Evaluation 1: Conductivity] (Production of test substrate) A prepreg (manufactured by Panasonic Corporation, "R-1551") with a thickness of about 100 μm was used. ２A 169-hole pattern with a diameter of φ100 μm was fabricated using a laser. After filling the holes with conductive paste using a printing method, a test substrate was fabricated by pressing it using a vacuum press under the following pressure and temperature conditions: Pressure: 0 kg / cm² ２ From 17 minutes, a surface pressure of 10.2 kg / cm² was applied. ２ The pressure was increased to [a certain level] and maintained at that pressure for 10 minutes. Then, over 24 minutes, the surface pressure was increased to 30.6 kg / cm². ２ The pressure was increased to [a certain level] and maintained at that pressure for 46 minutes. Then, over the next 23 minutes, it was increased to 0 kg / cm². ２ The pressure was reduced to [amount missing]. Temperature: The temperature was raised from 30°C to 130°C over 17 minutes and maintained at that temperature for 10 minutes. Then, the temperature was raised to 180°C over 24 minutes and maintained at that temperature for 46 minutes. After that, it was cooled to 30°C over 23 minutes. (Initial via resistance value) The resistance value between both ends of the connecting pattern on the test substrate was measured, and the resistance value per hole was obtained by dividing that resistance value by the number of holes. (Evaluation criteria) If the resistance value is 8 mΩ / hole or less, it can be evaluated as having excellent conductivity.

[0049] [Evaluation 2: Heat Resistance (Reflow Resistance)] The test substrate was subjected to a reflow process at 260°C for 10 seconds for 5 cycles. The resistance value of the test substrate after the reflow test was determined in the same manner as in Evaluation 1. (Evaluation Criteria) Let the resistance value before the reflow test be a and the resistance value after the test be b. The rate of change in resistance was calculated using the following formula. If the rate of change in resistance is within ±10%, it can be evaluated as having excellent heat resistance. Rate of change in resistance (%) = (b - a) / a × 100

[0050] [Evaluation 3: Long-term reliability] The test substrate was subjected to 1000 heat cycles, consisting of 30 minutes at -65°C and 30 minutes at 125°C. The resistance value of the test substrate after the heat cycle test was determined in the same manner as in Evaluation 1. (Evaluation criteria) The average resistance value before and after the heat cycle test was determined in the same manner as in Evaluation 2. If the rate of change in resistance value is within ±10%, it can be evaluated as having excellent long-term reliability.

[0051] [Evaluation 4: Filling Properties] The cured conductive paste of the test substrate was observed using an X-ray apparatus to check for the presence or absence of voids. (Evaluation Criteria) "○" was used when almost no voids were observed, and "△" was used when some voids were observed.

[0052]

[0053]

Claims

1. A conductive paste comprising metal particles, epoxy resin, a curing agent, and flux, wherein the metal particles comprise first metal particles containing a high-melting-point metal with a melting point of 800°C or higher, and second metal particles containing a low-melting-point metal with a melting point of 180°C or lower, the low-melting-point metal being a Sn-Bi alloy, and the mass ratio of Sn to Bi of the Sn-Bi alloy, Sn / Bi, being 1 or more and 5 or less.

2. The conductive paste according to claim 1, wherein D50 in the particle size distribution of the second metal particles is 1 μm or more and 15 μm or less.

3. The conductive paste according to claim 2, wherein the mass ratio of the flux activator to the second metal particles is 0.05 or more.

4. The conductive paste according to any one of claims 1 to 3, wherein the curing agent is a phenolic curing agent.

5. The conductive paste according to claim 4, wherein the phenolic curing agent is a novolac-type phenolic resin.

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