Conductive paste and multilayer substrate using same

Abstract

The present invention provides: a conductive paste which has excellent thermal conductivity, electrical conductivity, fillability of a hole that is formed in a substrate, and long-term reliability; and a multilayer substrate which uses this conductive paste.　This conductive paste contains, per 100 parts by mass of a liquid epoxy resin (A): 2,300 to 5,000 parts by mass of a metal filler (B) which contains two or more metal powders including at least one metal powder (B1) that comprises a high-melting-point metal which contains silver and / or copper, while having a melting point of 800°C or more, and at least one metal powder (B2) that comprises a low-melting-point metal which is composed of an alloy of two or more metals selected from the group consisting of tin, lead, bismuth and indium, while having a melting point of 180°C or less; 10 to 40 parts by mass of a curing agent (C) which contains a hydroxyl group-containing aromatic compound; and 60 to 230 parts by mass of a flux (D).

Description

[Technical Field]

[0001] The present invention relates to a conductive paste that is excellent in thermal conductivity, electrical conductivity, ability to fill holes formed in a substrate, and long-term reliability, and to a multilayer substrate using the same. [Background technology]

[0002] Patent Document 1 describes a conductive paste used for filling holes in substrates, etc., which is a conductive paste made by compounding a thermosetting resin with a conductive filler, flux, and a curing agent, and which, when heated under certain conditions, hardens the resin and melts the metal powder to form a metallized paste.

[0003] However, the conductive paste described in Patent Document 1 has a problem in that the thermal conductivity of the conductive paste is insufficient due to the low metal powder content. On the other hand, if the metal powder content is increased to improve the thermal conductivity, the viscosity of the conductive paste increases, resulting in a problem in that the conductive paste is less able to fill holes in the substrate.

[0004] Patent Document 2 describes a conductive paste having thermal conductivity, but there is room for improvement in long-term reliability when used to fill holes in a multilayer substrate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-108629 [Patent Document 2] Japanese Patent Application Publication No. 2019-165155 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above, and aims to provide a conductive paste that has excellent thermal conductivity, electrical conductivity, ability to fill holes formed in a substrate, and long-term reliability, and a multilayer substrate using the same. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention includes the following embodiments. [1] A conductive paste comprising, relative to 100 parts by mass of (A) a liquid epoxy resin, (B) 2,300 to 5,000 parts by mass of a metal filler containing two or more types of metal powder, including at least one type of metal powder (B1) having a high melting point metal containing silver and / or copper and having a melting point of 800°C or higher, and at least one type of metal powder (B2) having a low melting point metal having a melting point of 180°C or lower, the metal powder being an alloy of two or more types selected from the group consisting of tin, lead, bismuth, and indium, (C) 10 to 40 parts by mass of a curing agent containing a hydroxyl group-containing aromatic compound, and (D) 60 to 230 parts by mass of a flux. [2] The conductive paste according to [1], wherein the content ratio ((B1) / (B2)) of the metal powder (B1) having a high melting point metal and the metal powder (B2) having a low melting point metal in the metal filler (B) is 0.3 to 1.0 in mass ratio. [3] The conductive paste according to [1] or [2], wherein the hydroxyl group-containing aromatic compound is at least one selected from the group consisting of phenol-based curing agents and naphthol-based curing agents. [4] A multilayer substrate formed by alternately stacking a plurality of conductive layers and insulating layers, wherein a hole is formed through at least one of the insulating layers, and the hole is filled with the conductive paste according to any one of [1] to [3] and cured, and the conductive layers located on both ends of the hole are electrically connected to each other via the cured conductive paste. [Effects of the Invention]

[0008] The conductive paste of the present invention provides excellent thermal conductivity, electrical conductivity, filling ability into holes formed in a substrate, and long-term reliability. DETAILED DESCRIPTION OF THE INVENTION

[0009] As described above, the conductive paste according to the present invention contains, relative to 100 parts by mass of (A) liquid epoxy resin, (B) 2,300 to 5,000 parts by mass of a metal filler containing two or more types of metal powder, including at least one type of metal powder (B1) having a high melting point metal containing silver and / or copper and a melting point of 800°C or higher, and at least one type of metal powder (B2) having a low melting point metal having a melting point of 180°C or lower, the metal powder being an alloy of two or more types selected from the group consisting of tin, lead, bismuth, and indium, (C) 10 to 40 parts by mass of a curing agent containing a hydroxyl group-containing aromatic compound, and (D) 60 to 230 parts by mass of a flux.

[0010] The use of this conductive paste is not particularly limited, but it is suitably used as a composition for filling holes formed in a multilayer substrate in which multiple conductive layers and insulating layers are alternately laminated. The hole formed in the multilayer substrate may be a hole that penetrates at least one insulating layer, or may be a hole that penetrates multiple conductive layers and insulating layers. When the conductive paste of the present invention is used in a multilayer substrate, the conductive paste of the present invention is filled into the hole, and the conductive layers located at both ends of the hole are mutually conductive via the cured product obtained by curing the conductive paste.

[0011] The liquid epoxy resin is not particularly limited as long as it contains an epoxy group in the molecule and is liquid at room temperature (25°C), but specific examples include bisphenol A type epoxy resin, bisphenol F type epoxy resin, glycidyl amine type epoxy resin, and glycidyl ether type epoxy resin.

[0012] The epoxy equivalent of the liquid epoxy resin is not particularly limited, but is preferably 100 to 500 g / eq, and more preferably 200 to 400 g / eq. When the epoxy equivalent is within the above range, a conductive paste with excellent filling ability into holes formed in a substrate is easily obtained.

[0013] The metal filler (B) contains a metal powder (B1) having a high-melting point metal with a melting point of 800°C or higher and a metal powder (B2) having a low-melting point metal with a melting point of 180°C or lower, and when heated, the metal powder (B2) melts and becomes metallized.

[0014] The content of the metal filler (B) is not particularly limited as long as it is 2300 to 5000 parts by mass relative to 100 parts by mass of the liquid epoxy resin, but is preferably 2500 to 4500 parts by mass, and more preferably 3100 to 4000 parts by mass. When the content of the metal filler (B) is within the above range, excellent thermal conductivity, electrical conductivity, and long-term reliability are likely to be obtained.

[0015] The content of the metal powder (B1) is not particularly limited, but is preferably 300 to 3,000 parts by mass, and more preferably 500 to 2,500 parts by mass, relative to 100 parts by mass of the liquid epoxy resin. When the content of the metal powder (B1) is within the above range, excellent thermal conductivity, electrical conductivity, and long-term reliability are likely to be obtained.

[0016] The content of the metal powder (B2) is not particularly limited, but is preferably 1,000 to 4,000 parts by mass, and more preferably 1,000 to 2,500 parts by mass, relative to 100 parts by mass of the liquid epoxy resin. When the content of the metal powder (B2) is within the above range, excellent thermal conductivity, electrical conductivity, and long-term reliability are likely to be obtained.

[0017] The ratio of the metal powder (B1) to the metal powder (B2) in the metal filler (B) ((B1) / (B2)) is not particularly limited, but is preferably 0.3 to 1.0 by mass, and more preferably 0.4 to 0.8. When the ratio of the metal powder (B1) to the metal powder (B2) is within the above range, excellent thermal conductivity, electrical conductivity, and long-term reliability are likely to be obtained.

[0018] The form of the metal in the metal powder (B1) or the metal powder (B2) is not limited, but examples thereof include a mixture of a certain type of metal powder with a metal powder made of another type of metal, a powder of a certain type of metal coated with another type of metal, or a mixture of these.

[0019] The high-melting point metal may be a single metal or an alloy of two or more metals. Preferred examples of the high-melting point metal include one or more of silver (melting point: 961°C), copper (melting point: 1083°C), and silver-coated copper powder.

[0020] The low-melting-point metal may be an alloy of two or more metals. Preferred examples of low-melting-point metals include alloys of two or more of tin (melting point: 231°C), lead (melting point: 327°C), bismuth (melting point: 271°C), and indium (melting point: 156°C), which have a melting point of 180°C or less. The low-melting-point metal preferably contains tin, and particularly preferably an alloy of tin (Sn) and bismuth (Bi), with the alloy ratio being Sn:Bi = 80:20 to 42:58.

[0021] The shape of the metal powder is not particularly limited, and examples include flake (scale-like), dendritic, spherical, fibrous, and amorphous (polyhedral) shapes. However, spherical shapes are preferred from the viewpoint of obtaining a composition with lower resistance and improved thermal conductivity.

[0022] The average particle size of the metal filler (B) is not particularly limited, but is preferably 0.5 to 20 μm, and more preferably 1 to 10 μm. When the average particle size of the metal filler (B) is within the above range, excellent thermal conductivity and long-term reliability are likely to be obtained.

[0023] The average particle size of the metal powder (B1) is not particularly limited, but is preferably 0.5 to 10 μm, more preferably 1 to 5 μm. When the average particle size of the metal powder (B1) is within the above range, there is increased contact between the metal powder particles, making it easier to obtain excellent thermal conductivity.

[0024] The average particle size of the metal powder (B2) is not particularly limited, but is preferably 1 to 20 μm, more preferably 5 to 10 μm. When the average particle size of the metal powder (B2) is within the above range, excellent filling properties and long-term reliability are likely to be obtained.

[0025] In this specification, the average particle size means the particle size (primary particle size) at an integrated value of 50% in the particle size distribution obtained by the laser diffraction scattering method.

[0026] The tap density of the metal filler (B) is not particularly limited, but is preferably 3.0 to 7.0 g / cm 3 When the tap density is within the above range, excellent thermal conductivity and electrical conductivity are likely to be obtained.

[0027] The tap density of the metal powder (B1) is not particularly limited, but is preferably 5.0 to 7.0 g / cm 3 When the tap density is within the above range, excellent thermal conductivity and electrical conductivity are likely to be obtained.

[0028] The tap density of the metal powder (B2) is not particularly limited, but is preferably 3.0 to 5.0 g / cm 3 When the tap density is within the above range, excellent thermal conductivity and electrical conductivity are likely to be obtained.

[0029] The curing agent (C) is not particularly limited as long as it contains a hydroxyl group-containing aromatic compound, but it is preferable that the hydroxyl group-containing aromatic compound is a phenolic compound or a naphthol compound. That is, such curing agents include phenolic curing agents and naphthol curing agents, but curing agents that do not fall into these categories may also be contained within a range that does not contradict the object of the present invention.

[0030] The phenol-based hardener is a phenol novolak or a derivative thereof that can be used as a hardener, and the naphthol-based hardener is a naphthol or a derivative thereof that can be used as a hardener.

[0031] The content of the curing agent (C) is not particularly limited as long as it is 10 to 40 parts by mass relative to 100 parts by mass of the liquid epoxy resin, but is preferably 15 to 30 parts by mass. When it is 10 parts by mass or more, sufficient curing of the conductive paste is likely to be achieved, and when it is 40 parts by mass or less, the pot life is unlikely to be shortened, and excellent electrical conductivity, thermal conductivity, and long-term reliability are likely to be achieved.

[0032] The flux (D) promotes the metallization of the metal powder, and examples thereof include zinc chloride, lactic acid, citric acid, oleic acid, stearic acid, glutamic acid, benzoic acid, oxalic acid, glutamic acid hydrochloride, aniline hydrochloride, cetylpyridine bromide, urea, hydroxyethyl laurylamine, polyethylene glycol laurylamine, oleyl propylene diamine, triethanolamine, glycerin, hydrazine, rosin, 8-ethyloctadecanedioic acid, etc. Among these, 8-ethyloctadecanedioic acid is preferred from the viewpoint of obtaining excellent filling properties and long-term reliability.

[0033] The content of the flux (D) is not particularly limited as long as it is 60 to 230 parts by mass relative to 100 parts by mass of the liquid epoxy resin, but is preferably 80 to 200 parts by mass, and more preferably 100 to 180 parts by mass. When it is 60 parts by mass or more, metallization of the metal powder is easily achieved, and excellent long-term reliability and filling of holes formed in the substrate are easily achieved, while when it is 230 parts by mass or less, excellent electrical and thermal conductivity are easily achieved.

[0034] The conductive paste according to the present invention may contain known additives such as antifoaming agents, thickeners, adhesives, fillers, flame retardants, colorants, etc., within the scope of the invention.

[0035] The conductive paste according to the present invention preferably has a low viscosity so that it can be filled into holes formed in a multilayer substrate by a dispensing method, an atmospheric printing method, a vacuum printing method, or the like.

[0036] Here, the dispensing method refers to a method of applying a conductive paste by extruding it from the tip of a syringe-shaped nozzle. The atmospheric printing method refers to a method of stencil printing in which a synthetic fiber screen is stretched over a plate, an optical film is formed on the screen to block all pores except for the necessary image lines, a plate is made, and ink is rubbed through the pores in the film under atmospheric pressure to print on the printing surface of a substrate placed below the plate. The vacuum printing method refers to a method of stencil printing in which a synthetic fiber screen is stretched over a plate, an optical film is formed on the screen to block all pores except for the necessary image lines, a plate is made, and ink is rubbed through the pores in the film under vacuum to print on the printing surface of a substrate placed below the plate.

[0037] The viscosity of the conductive paste according to the present invention is preferably adjusted appropriately depending on the application and the equipment used for application, and is not particularly limited, but as a general guideline, it is preferably 1200 to 2500 dPa·s at a conductive paste temperature of 25° C. The viscosity can be measured in accordance with JIS K7117-1 using a single cylindrical rotational viscometer (a so-called B-type or BH-type viscometer) with a rotor No. 7 at 10 rpm.

[0038] From the viewpoint of preventing the occurrence of voids, the conductive paste according to the present invention preferably does not contain a solvent. [Example]

[0039] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following. In the following, "parts" and "%" are based on mass unless otherwise specified.

[0040] [Examples 1 to 7, Comparative Examples 1 to 8] Conductive pastes were prepared by mixing the components according to the blending ratios (parts by mass) shown in Tables 1 and 2. Details of the components used are as follows.

[0041] Liquid epoxy resin: Mitsubishi Chemical Corporation "jER871", epoxy equivalent = 390g / eq High-melting-point metal powder 1: Silver-coated copper powder (average particle size 3 μm), manufactured by DOWA Electronics Co., Ltd. High-melting-point metal powder 2: Silver powder (average particle size 3 μm), manufactured by DOWA Electronics Co., Ltd. High-melting-point metal powder 3: Copper powder (average particle size 3 μm) manufactured by Fukuda Metal Foil and Powder Co., Ltd. Low melting point metal powder: Sn-Bi alloy metal powder (Sn:Bi=42:58, melting point 138°C, average particle size 6μm) Phenol-based hardener: Arakawa Chemical Industries, Ltd., "Tamanol 758" Flux: Okamura Oil Mills, 8-ethyloctadecanedioic acid

[0042] The obtained conductive paste was evaluated for resistivity, thermal conductivity, filling property, and long-term reliability 1 to 4. The evaluation results are shown in Tables 1 and 2. The evaluation methods are as follows.

[0043] <Resistivity(×10 -5 Ω·cm) The conductive paste was printed on a glass epoxy substrate using a metal plate in lines (length 60 mm, width 1 mm, thickness approximately 100 μm), and then heated at 180°C for 60 minutes to fully harden it, producing an evaluation substrate with a conductive pattern. Next, the resistance between both ends of the conductive pattern was measured using a tester, and the cross-sectional area (S, cm 2 The resistivity was calculated from the square root of the square root of the conductive pattern (x, cm) and the length (L, cm) using the following formula (1). Five lines were printed on each of three glass epoxy boards to form a total of 15 conductive patterns, and the average resistivity of these was calculated. -5 When the electrical conductivity was Ω·cm or less, the electrical conductivity was evaluated as excellent. Specific resistance=(S / L)×R...(1)

[0044] <Thermal conductivity> The thermal conductivity of the conductive paste was evaluated using a Thermowave Analyzer TA-33 (manufactured by BETHEL). Specifically, a Teflon (registered trademark) sheet (100 mm × 100 mm × 3 mm) was prepared, and the center of the sheet was masked with polyimide tape to form an opening 50 mm wide and 50 mm long. The conductive paste according to each Example and Comparative Example was then line-printed. The conductive paste was then cured by heating at 180°C for 60 minutes, and the polyimide tape was peeled off to form a coating film (50 mm wide, 50 mm long, approximately 100 μm thick). The resulting coating film was peeled off from the Teflon (registered trademark) sheet to prepare a cured sample. The thermal diffusivity α (m 2 / S) and the density ρ (kg / m 3 The thermal conductivity K (W / m K) was calculated from the specific heat Cp (J / Kg K) using the following formula (2). Thermal conductivity of 30 W / m K or higher was evaluated as excellent. Thermal conductivity K = thermal diffusivity α × density ρ × specific heat Cp (2)

[0045] <Samples used in long-term reliability evaluation> A 169-hole interconnection pattern with a diameter of 100 μm was formed on a prepreg (Panasonic Corporation, "R-1551") approximately 100 μm thick using a CO2 laser, and the holes were filled with conductive paste using a printing method. Samples were then produced by pressing using a vacuum press under the following pressure and temperature conditions. The conductive pastes of Examples 1 to 7 and Comparative Examples 2, 4 to 8 were all able to fill the holes formed in the prepreg without overflowing, and were therefore evaluated as having excellent filling properties, as indicated by an "O" in Tables 1 and 2. Comparative Examples 1 and 3 were evaluated as not having sufficient filling properties due to the occurrence of unfilled areas and / or cracks during curing, and were therefore indicated by an "X" in Table 2.

[0046] Pressure: 0kg / cm 2 Surface pressure of 10.2 kg / cm for 17 minutes 2 The pressure was increased to 30.6 kg / cm and held there for 10 minutes. 2The pressure was increased to 0 kg / cm and held there for 46 minutes, then reduced to 0 kg / cm over 23 minutes. 2 The pressure was reduced to .

[0047] Temperature: The temperature was raised from 30°C to 130°C over 17 minutes and held at that temperature for 10 minutes.The temperature was then raised to 180°C over 24 minutes, held at that temperature for 46 minutes, and then cooled to 30°C over 23 minutes.

[0048] <Long-term reliability 1 (resistance change rate before and after heat cycle (HC) test)> As a heat cycle test, each sample obtained above was subjected to 1000 cycles of heat cycles of 30 minutes at -65°C and 30 minutes at 125°C. For measurement of resistance value, the resistance value between both ends of the connection pattern was measured before and after the heat cycle test, and the resistance value was divided by the number of holes to determine the resistance value per hole, and the average value was calculated.

[0049] The rate of change in resistance before and after the heat cycle test was calculated using the following formula, where a is the resistance measured before the test and b is the resistance measured after the test. If the rate of change in resistance was within ±10%, the reliability was evaluated as excellent. Resistance change rate (%) = (ba) × 100 / a

[0050] <Long-term reliability 2 (resistance change rate before and after reflow test)> As a reflow test, each of the samples obtained above was subjected to a reflow process five times under the conditions of 10 seconds at 260° C. The rate of change in resistance value before and after the reflow test was measured in the same manner as in the heat cycle test.

[0051] <Long-term reliability 3 (rate of change in resistance value before and after heat resistance test)> As a heat resistance test, each sample obtained above was left standing for 1000 hours at an environmental temperature of 100° C. The rate of change in resistance value before and after the heat resistance test was measured in the same manner as in the heat cycle test.

[0052] <Long-term reliability 4 (rate of change in resistance value before and after humidity resistance test)> For the moisture resistance test, each sample obtained above was left standing at an environmental temperature of 85°C and humidity of 85% for 1000 hours. The rate of change in resistance value before and after the moisture resistance test was measured in the same manner as in the heat cycle test.

[0053] [Table 1]

[0054] [Table 2]

[0055] The results shown in Table 1 show that all of Examples 1 to 7 are excellent in electrical conductivity, thermal conductivity, filling properties, and long-term reliability.

[0056] From the results shown in Table 2, Comparative Example 1, which is an example in which the content of the metal filler (B) exceeds the upper limit, was inferior in long-term reliability 1 to 4.

[0057] Comparative Example 2, which is an example in which the content of the metal filler (B) is less than the lower limit, was inferior in long-term reliability 1 and 2.

[0058] Comparative Example 3, which is an example in which the content of the metal filler (B) exceeds the upper limit, was inferior in electrical conductivity, thermal conductivity, packing property, and long-term reliability 1 and 2.

[0059] Comparative Example 4, which is an example in which the content of the metal filler (B) was less than the lower limit, was poor in electrical conductivity, thermal conductivity, and long-term reliability 2.

[0060] Comparative Example 5, which is an example in which the content of the curing agent (C) was less than the lower limit, was poor in thermal conductivity and long-term reliability 1 to 4. Note that the resistance value could not be measured for long-term reliability 1, but cross-sectional observation of the inside of the hole confirmed that this was due to the occurrence of cracks.

[0061] Comparative Example 6, which is an example in which the content of the curing agent (C) exceeded the upper limit, was poor in electrical conductivity and thermal conductivity.

[0062] Comparative Example 7, which is an example in which the content of flux (D) is less than the lower limit, was inferior in long-term reliability 1 to 4. Note that, although the resistance value could not be measured for long-term reliability 1, it was confirmed that this was due to the occurrence of cracks when the cross section inside the hole was observed.

[0063] Comparative Example 8, which is an example in which the content of the flux (D) exceeded the upper limit, was poor in electrical conductivity and thermal conductivity.

Claims

1. (A) per 100 parts by mass of liquid epoxy resin, (B) 2,500 to 5,000 parts by mass of a metal filler containing two or more types of metal powder, including at least one type of metal powder (B1) having a high melting point metal containing silver and / or copper and having a melting point of 800°C or higher, and at least one type of metal powder (B2) having a low melting point metal having a melting point of 180°C or lower, which is made of an alloy of two or more types selected from the group consisting of tin, lead, bismuth, and indium; (C) 10 to 40 parts by mass of a curing agent containing a hydroxyl group-containing aromatic compound; (D) A conductive paste containing 100 to 200 parts by mass of a flux.

2. The metal filler (B) has a mass ratio of the metal powder (B1) to the metal powder (B2) ((B1) / (B2)) of 0.3 to 1.

0. The conductive paste according to claim 1.

3. 3. The conductive paste according to claim 1, wherein the hydroxyl group-containing aromatic compound is at least one selected from the group consisting of phenol-based curing agents and naphthol-based curing agents.

4. A multilayer substrate formed by alternately stacking a plurality of conductive layers and insulating layers, A hole is formed through at least one of the insulating layers; The hole is filled with the conductive paste according to claim 1 or 2 and hardened; a multilayer substrate, wherein the conductive layers located on both ends of the hole are electrically connected to each other via the hardened conductive paste;

Images