solder paste

Abstract

To provide a solder paste that can prevent the occurrence of voids in solder joints or solder bumps while ensuring solder wettability.SOLUTION: A solder paste contains a rosin-based resin (A), an activator (B), and a solvent (C). The solvent (C) contains a flux containing a fatty acid ester (C-1) having 12 or more carbon atoms, with its viscosity of 200 mPa s or more and 400 mPa s or less at normal temperature, and alloy powder (D) composed of a solder alloy.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a solder paste. [Background technology]

[0002] BACKGROUND ART Solder alloys have been widely used as bonding materials for bonding electronic components to electronic circuits formed on substrates such as printed wiring boards and silicon wafers. The joining method using a solder alloy includes, for example, a method using a solder paste made by mixing a solder alloy powder with flux, and a method using a solder bump made of a solder alloy. Note that the solder paste is also sometimes used to form the solder bump.

[0003] In the case of a soldering method using solder paste and a method of forming solder bumps using solder paste, the solder paste is generally printed on a substrate in a predetermined pattern, heated at a predetermined temperature (preheating and reflow), and then the flux residue formed on the substrate is washed away (in the case of soldering using solder paste, washing may be omitted).

[0004] However, in this case, during heating, the flux contained in the solder paste and gas generated by the volatilization of the flux components may become trapped within the molten solder alloy and not be expelled. The flux and gas remaining within the resulting solder joints and solder bumps are called voids, and they may reduce the reliability of the solder joints and solder bumps. This may also lead to a reduction in the reliability of semiconductors and electronic devices.

[0005] Examples of fluxes that can suppress the generation of voids in solder joints and solder bumps include those with a reduced amount of organic solvent, such as the soldering flux disclosed in Patent Document 1. This solder flux uses a specific organic solvent and a thixotropic agent to improve the viscosity stability and printability of the solder paste and reduce the amount of organic solvent used, which has the advantage that the gas generated by the evaporation of the organic solvent during heating and the organic solvent itself are less likely to be absorbed into the molten solder alloy. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-209690

[0007] On the other hand, reducing the amount of solvent or activator contained in the flux may reduce the activity of the flux and solder wettability, which may result in insufficient solder bonding, reduced reliability of the solder joint, and failure to form solder bumps (occurrence of missing bumps). Summary of the Invention [Problem to be solved by the invention]

[0008] The problem to be solved by the present invention is to provide a solder paste that can suppress the generation of voids in solder joints and solder bumps while ensuring solder wettability. [Means for solving the problem]

[0009] The solder paste according to the present invention contains a rosin-based resin (A), an activator (B), and a solvent (C), and the solvent (C) contains a flux containing a fatty acid ester (C-1) having 12 or more carbon atoms and having a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature, and an alloy powder (D) made of a solder alloy.

[0010] The fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature preferably contains in its structure an ester bond between a polyhydric alcohol (C-1a) having divalent or more hydroxyl groups and a fatty acid (C-1b) having 12 or more and 22 or less carbon atoms.

[0011] The polyhydric alcohol (C-1a) having di or more hydroxyl groups is preferably at least one of trimethylolpropane, pentaerythritol, and polyglycerol, and the fatty acid (C-1b) having 12 or more and 22 or less carbon atoms is preferably isostearic acid.

[0012] The fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature is preferably a triester or tetraester of the polyhydric alcohol (C-1a) having dihydroxyl groups or more and the fatty acid (C-1b) having 12 or more and 22 carbon atoms.

[0013] The fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity at room temperature of 200 mPa·s or more and 400 mPa·s or less is preferably at least one selected from trimethylolpropane triisostearate, pentaerythritol tetraisostearate, and diglyceryl triisostearate.

[0014] The solder bumps of the present invention are formed using the solder paste. [Effects of the Invention]

[0015] The solder paste of the present invention can suppress the occurrence of voids in solder joints and solder bumps while ensuring solder wettability. [Brief explanation of the drawings]

[0016] [Figure 1] 1A to 1C are schematic cross-sectional views showing the changes in a printing process, a heating process, and a cleaning process when forming solder bumps on a substrate. [Figure 2]10A to 10C are schematic cross-sectional views showing the changes in the printing process, heating process, and cleaning process when a missing bump occurs when forming solder bumps on a substrate. DETAILED DESCRIPTION OF THE INVENTION

[0017] An embodiment of the solder paste of the present invention will be described in detail below, although it should be understood that the present invention is not limited to this embodiment.

[0018] 1. Flux The flux of this embodiment contains a rosin resin (A), an activator (B), and a solvent (C).

[0019] Rosin resin (A) Examples of the rosin-based resin (A) include rosins such as tall oil rosin, gum rosin, and wood rosin, as well as modified rosin resins such as hydrogenated rosin, polymerized rosin, heterogenized rosin, acrylic acid-modified rosin, maleic acid-modified rosin, and formylated rosin, and derivatives thereof, which may be used alone or in combination. As the rosin-based resin (A), among hydrogenated rosins, fully hydrogenated rosin, polymerized rosin, or rosin esters obtained by esterifying rosins or rosin-based modified resins are preferably used.

[0020] The blending amount of the rosin-based resin (A) is preferably 15% by mass or more and 50% by mass or less, more preferably 18% by mass or more and 45% by mass or less, and particularly preferably 22% by mass or more and 40% by mass or less, based on the total amount of the flux.

[0021] Activator (B) Examples of the activator (B) include organic acids, halogen-containing compounds, and amine-based activators, which can be used alone or in combination.

[0022] The organic acids include monocarboxylic acids, dicarboxylic acids and other organic acids.

[0023] Examples of monocarboxylic acids include propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, capric acid, lauric acid, myristic acid, pentadecylic acid, palmitic acid, margaric acid, stearic acid, tuberculostearic acid, arachidic acid, behenic acid, lignoceric acid, and glycolic acid.

[0024] Examples of dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, eicosanedioic acid, fumaric acid, maleic acid, tartaric acid, diglycolic acid, and 1,4-cyclohexanedicarboxylic acid.

[0025] Other organic acids include dimer acid, levulinic acid, lactic acid, acrylic acid, benzoic acid, salicylic acid, anisic acid, citric acid, picolinic acid, and anthranilic acid.

[0026] Examples of compounds containing halogen include non-dissociative halogen compounds (non-dissociative activators) and dissociative halogen compounds (dissociative activators). Examples of non-dissociative activators include non-salt organic compounds in which halogen atoms are covalently bonded.The organic compounds may be, for example, compounds in which a single element, such as chlorine, bromine, iodine, or fluorine, is covalently bonded, such as chlorides, bromides, iodides, or fluorides, or compounds in which two or more different halogen atoms are covalently bonded.Furthermore, in order to improve the solubility of the organic compounds in aqueous solvents, it is preferable that the organic compounds have a polar group such as a hydroxyl group, such as halogenated alcohols.

[0027] Examples of the amine-based activators include organic amines, amine salts (inorganic acid salts or organic acid salts) such as hydrogen halide salts of organic amines, organic acid salts, and organic amine salts.

[0028] As the activator (B), tris(2,3-dibromopropyl)isocyanurate, 1,3-diphenylguanidine hydrobromide, glutaric acid, sebacic acid, dodecanedioic acid, eicosanedioic acid, and the like are preferably used.

[0029] The amount of the activator (B) is preferably 1% by mass or more and 30% by mass or less, more preferably 1% by mass or more and 25% by mass or less, and particularly preferably 1.5% by mass or more and 22% by mass or less, based on the total amount of the flux.

[0030] Solvent (C) Examples of the solvent (C) include alcohols, ethanols, acetones, toluenes, xylenes, ethyl acetates, ethyl cellosolves, butyl cellosolves, glycol ethers, esters, etc. These can be used alone or in combination.

[0031] The flux according to this embodiment preferably contains, as the solvent (C), a fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity at room temperature of 200 mPa·s or more and 400 mPa·s or less.

[0032] In this specification, "room temperature" for a fatty acid ester (C-1) having 12 or more carbon atoms and having a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature refers to a temperature between approximately 15°C and 30°C, and the viscosity is measured using a Brookfield rotational viscometer (5 rpm) at a measurement temperature of 25°C.

[0033] The flux according to this embodiment contains the fatty acid ester (C-1) having 12 or more carbon atoms and having a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature, thereby ensuring solder wettability while suppressing the generation of voids in solder joints and solder bumps.

[0034] That is, solvents usually have the property of easily volatilizing when heated during soldering. Therefore, depending on the amount of solvent blended, there may not be a sufficient amount of solvent remaining during heating, which may reduce the amount of flux coated on the molten solder alloy surface and result in insufficient removal of oxidized components from the solder alloy surface. In such cases, the wettability of the solder alloy may decrease, which may result in a decrease in solderability with the electrodes on the substrate.

[0035] On the other hand, the fatty acid ester (C-1) with 12 or more carbon atoms and a viscosity of 200 mPa·s to 400 mPa·s at room temperature is resistant to volatilization, even when heated. Therefore, when a flux containing this is used in a solder paste, a significant reduction in the amount of flux coated on the molten solder alloy surface during solder joining (heating) can be suppressed without increasing the amount of activator or solvent. This allows the viscosity (fluidity) of the solder paste (flux) to be maintained constant, resulting in good solder wettability.

[0036] Furthermore, the fatty acid ester (C-1) having a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature and a carbon number of 12 or more has excellent compatibility with other components contained in the flux. This, combined with the ability of the flux to maintain a certain level of fluidity when heated, means that even if the flux is temporarily entrapped in the molten solder alloy, it is easily expelled, thereby suppressing the occurrence of voids in the solder joints and solder bumps.

[0037] Furthermore, the flux of this embodiment contains the fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature, which makes it possible to suppress the occurrence of missing bumps during the formation of solder bumps, as will be described later.

[0038] The fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature preferably contains in its structure an ester bond between a polyhydric alcohol (C-1a) having divalent or more hydroxyl groups and a fatty acid (C-1b) having 12 or more and 22 or less carbon atoms.

[0039] The polyhydric alcohol (C-1a) having di- or higher hydroxyl groups more preferably has three or more alcoholic hydroxyl groups, and is particularly preferably at least one of trimethylolpropane, pentaerythritol, and polyglycerol. In the polyhydric alcohol (C-1a) having divalent or more hydroxyl groups, it is preferred that the hydroxyl groups are ester-bonded and no hydroxyl groups remain.

[0040] The fatty acid (C-1b) having 12 or more and 22 or less carbon atoms preferably has a branched alkyl group or a branched alkenyl group in its structure, and is particularly preferably isostearic acid.

[0041] The fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature is preferably a triester or tetraester of the polyhydric alcohol (C-1a) having dihydroxyl groups or more and the fatty acid (C-1b) having 12 or more and 22 carbon atoms.

[0042] As the fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity at room temperature of 200 mPa·s or more and 400 mPa·s or less, trimethylolpropane triisostearate, pentaerythritol tetraisostearate, diglyceryl triisostearate, etc. are preferably used. The fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity at room temperature of 200 mPa·s or more and 400 mPa·s or less can be used alone or in combination of two or more.

[0043] The amount of the fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity at room temperature of 200 mPa·s to 400 mPa·s is preferably 1% by mass to 20% by mass of the total amount of the flux, more preferably 2% by mass to 10% by mass, and particularly preferably 4% by mass to 8% by mass.

[0044] The total amount of the solvent (C) is preferably 20% by mass or more and 70% by mass or less, more preferably 25% by mass or more and 65% by mass or less, and particularly preferably 30% by mass or more and 60% by mass or less, based on the total amount of the flux.

[0045] As a solvent other than the fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s to 400 mPa·s at room temperature, a solvent having a viscosity of 50 mPa·s or less at room temperature can be preferably used in combination from the viewpoint of maintaining viscosity. As such a solvent, alcohols having an aryl group or a branched alkyl group are preferred. In particular, alcohols having an aryl group include phenyl glycol.

[0046] As alcohols having a branched alkyl group, it is particularly preferable to use them in combination with higher alcohols having 12 or more carbon atoms, and it is particularly desirable to use them in combination with isostearyl alcohol. Such higher alcohols with branched alkyl groups are highly compatible with the fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature. This allows the fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature to be uniformly dispersed in the flux. As a result, the hydrophobic bonds formed between the long alkyl chains encapsulate the surface of the solder alloy powder, preventing oxidation of the solder surface during reflow and reducing the occurrence of missing bumps. The solvents other than the fatty acid ester (C-1) having 12 or more carbon atoms and having a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature can be used alone or in combination.

[0047] Thixotropic Agent The flux according to this embodiment can be blended with a thixotropic agent. Examples of the thixotropic agent include hydrogenated castor oil, bisamide-based thixotropic agents (saturated fatty acid bisamides, unsaturated fatty acid bisamides, aromatic bisamides, etc.), and dimethyldibenzylidene sorbitol. These can be used alone or in combination.

[0048] Among these, hydrogenated castor oil is particularly preferably used as the thixotropic agent. Hydrogenated castor oil has a lower softening point than other thixotropic agents, and therefore can improve the fluidity of the flux during soldering (heating).

[0049] The blending amount of the thixotropic agent is preferably 3% by mass or more and 15% by mass or less, and more preferably 5% by mass or more and 10% by mass or less, based on the total amount of the flux.

[0050] The flux of this embodiment may contain an antioxidant in order to suppress oxidation of the alloy powder (D) made of a solder alloy. Examples of the antioxidant include hindered phenol-based antioxidants, phenol-based antioxidants, bisphenol-based antioxidants, polymer-type antioxidants, etc. Among these, hindered phenol-based antioxidants are particularly preferred. The antioxidant is not limited to these, and the amount of the antioxidant to be added is not particularly limited. The amount of the antioxidant to be added is generally about 0.5% by mass to 5% by mass of the total amount of the flux.

[0051] The flux of this embodiment may further contain additives such as a matting agent, an antifoaming agent, etc. The amount of the additives to be added is preferably 10% by mass or less, more preferably 5% by mass or less, based on the total amount of the flux. The flux of this embodiment may contain resins other than the rosin-based resin within the range that does not impair the effect of the flux.

[0052] 2. Solder paste The solder paste of this embodiment can be prepared by mixing the above flux with alloy powder (D) made of a solder alloy by a known method.

[0053] Alloy powder (D) consisting of a solder alloy Examples of the solder alloy used in the alloy powder (D) made of the solder alloy include a combination of multiple elements such as Sn, Ag, Cu, Bi, Zn, In, Ga, Sb, Au, Pd, Ge, Ni, Cr, Al, P, In, and Pb.

[0054] The amount of alloy powder (D) made of the solder alloy is preferably 80 to 94 mass % of the total amount of the solder paste, more preferably 84 to 92 mass %, and particularly preferably 88 to 90 mass %.

[0055] As described above, the solder paste of this embodiment contains a fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature in the flux. Therefore, even during solder joining (heating), the amount of flux coated on the surface of the molten solder alloy is not significantly reduced, and a constant viscosity (fluidity) can be maintained, thereby demonstrating good solder wettability. Furthermore, with the solder paste of this embodiment, even if flux is temporarily absorbed into the molten solder alloy when heated, this flux is easily expelled, thereby suppressing the occurrence of voids within the solder joints and solder bumps.

[0056] 3. Solder bumps Furthermore, the solder paste of this embodiment can suppress the occurrence of missing bumps when forming solder bumps.

[0057] That is, solder bumps are typically formed through a process including the following (see also FIG. 1): 1) Printing process (Figure 1(a)) Solder paste 30 (solder alloy 32 (powder form) and flux 34) is printed in a predetermined pattern on electrodes 20 formed on substrate 10 (in openings formed by insulating layer 40).

[0058] 2) Heating process (Figure 1(b)) The substrate 10 on which the solder paste 30 is printed is heated to a predetermined temperature (preheating and reflow). Normally, the solder alloy 32 melts and coagulates, and the flux 34 becomes more easily flowable onto the insulating layer 20 due to a decrease in viscosity caused by heating.

[0059] 3) Cleaning process (Figure 1(c)) After heating the substrate 10, flux residue (not shown) is removed by cleaning the substrate 10 with a cleaning liquid. Solder bumps 36 are also formed on the substrate 10 (electrodes 20).

[0060] As described above, in the heating step 2), the viscosity of the flux 34 decreases with heating, making it easier for the flux 34 to flow onto the insulating layer 20. Therefore, if the viscosity of the flux is increased before heating to prevent this, or if the decrease in the viscosity of the flux during heating is prevented, once the flux is taken into the molten solder alloy 32, it tends to remain in the solder alloy 32 (or, conversely, it becomes difficult to expel), which makes it easier for voids to occur in the formed solder bumps 36.

[0061] On the other hand, if the viscosity of the flux is reduced before heating to prevent voids or if the viscosity of the flux is made easier to reduce during heating, as shown in Figure 2, in the 2) heating step, the flux 34 flows out onto the insulating layer 20 and also tends to flow underneath the molten solder alloy 32 (Figure 2(b)). If the amount of solder alloy 32 is large, it may sink toward the electrode 20 due to its own weight, but if the solder bumps themselves must be fine, for example, when mounting a semiconductor chip with a fine electrode pitch, the solder alloy 32 tends to float up, resulting in the state shown in Figure 2(b). In this case, the molten solder alloy 32 agglomerates without spreading (joining) onto the electrode 20, and therefore, 2) after the heating step, the solder alloy solidifies in the state shown in Figure 2(b) and flux residue is formed. Therefore, in the cleaning step 3), the solidified solder alloy is removed from the substrate 10 together with the flux residue (the occurrence of so-called missing bumps, FIG. 2(c)). This phenomenon is likely to occur when plating, particularly Sn plating (Sn plating 50), is applied to the substrate 10 (electrode 20), as shown in Figure 2. This is because Sn plating is prone to oxidation, making it difficult for the molten solder alloy to spread. Therefore, even if the molten solder alloy 32 comes into contact with the Sn plating 50 during the heating step, there is a risk that the solder alloy 32 will float up due to the flow of the flux 34 before it has time to spread sufficiently.

[0062] However, as described above, the solder paste of this embodiment contains a flux containing a fatty acid ester (C-1) having 12 or more carbon atoms and having a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature. Therefore, even during the heating process, the amount of flux coated on the surface of the molten solder alloy is not significantly reduced, and a constant viscosity (fluidity) can be maintained, thereby enabling good solder wettability to be exhibited. Furthermore, even if the flux is temporarily absorbed into the molten solder alloy, it is easily expelled, thereby suppressing the occurrence of voids in the solder joints and solder bumps. In this way, the solder paste of this embodiment can suppress the occurrence of voids in the solder bumps and the occurrence of missing bumps.

[0063] 4. Electronic circuit boards The electronic circuit board of this embodiment has solder bumps formed as described above.

[0064] The electronic circuit mounting board of this embodiment also has solder joints formed using the solder paste or solder bumps.

[0065] Such solder joints are formed, for example, by the following method. 1) When using solder paste The solder paste is printed at a predetermined position on an electronic circuit board, and then electronic components (electronic components of different sizes and types may be mixed; the same applies below) are mounted at predetermined positions on the electronic circuit board, and then reflowed to form the solder paste.

[0066] 2) When using solder bumps The solder bumps are formed at predetermined positions on an electronic circuit board, and then flux is applied to the solder bumps. Electronic components are then mounted at predetermined positions on the electronic circuit board, and the components are then reflowed to form the solder bumps.

[0067] The solder joint formed in this manner is formed using the above-mentioned solder paste or a solder bump formed using the solder paste, so that the occurrence of voids in the solder joint can be suppressed and its reliability can be maintained. An electronic circuit mounting board having such solder joints can exhibit high reliability and can be suitably used in semiconductors, electronic devices, and the like. [Example]

[0068] EXAMPLES The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0069] Measurement of viscosity of solvent (C) The results of measurements of each solvent (C) listed in Table 1 using a Brookfield rotational viscometer (5 rpm) at a measurement temperature of 25°C are also listed in Table 1. The viscosity units listed in Table 1 are mPa·s.

[0070] Flux preparation Each flux according to the examples and comparative examples was prepared by kneading the components according to the composition and blending ratio shown in Table 1. In Table 1, the units of the numerical values representing the composition are parts by mass unless otherwise specified.

[0071] Preparation of solder paste Next, 11.5 mass % of the flux according to the examples and comparative examples and 88.5 mass % of Sn-0.7Cu solder alloy powder (powder particle size 1 μm to 12 μm) were kneaded in a planetary mixer to obtain solder paste according to the examples and comparative examples.

[0072] [Table 1]

[0073] *1 Fully hydrogenated rosin manufactured by Eastman Chemical Company *2 Polymerized rosin manufactured by Arakawa Chemical Industries, Ltd. *3 Rosin ester manufactured by Harima Chemicals Co., Ltd. *4 Tris(2,3-dibromopropyl)isocyanurate manufactured by Nippon Kasei Co., Ltd. *5 Trimethylolpropane triisostearate, manufactured by Kokyu Alcohol Kogyo Co., Ltd. *6 Pentaerythrityl tetraisostearate, manufactured by Kokyu Alcohol Kogyo Co., Ltd. *7 Diglyceryl triisostearate, manufactured by Kokyu Alcohol Kogyo Co., Ltd. *8 Tributyl citrate, manufactured by Tokyo Chemical Industry Co., Ltd. *9 Octyldodecyl 12-stearoyl stearate, manufactured by Kokyu Alcohol Kogyo Co., Ltd. *10 Polyglyceryl-2 diisostearate, manufactured by Kokyu Alcohol Kogyo Co., Ltd. *11 Diisostearyl malate, manufactured by Kokyu Alcohol Kogyo Co., Ltd. *12 Bis(16-methylheptadecyl) 2-hydroxybutanedioate, manufactured by Nikko Chemicals Co., Ltd. *13 Isostearyl alcohol, manufactured by Kokyu Alcohol Kogyo Co., Ltd. *14 12-Hydroxystearic acid triglyceride, manufactured by KF Trading Co., Ltd.

[0074] (1) Maximum void area ratio confirmation test The following tools were prepared: Printed wiring board (surface treatment: Sn plating, solder resist opening diameter: 100 μm) Metal mask (thickness: 30 μm, opening diameter: 130 μm) Metal squeegee

[0075] Six printed wiring boards were prepared for each type of solder paste, and each solder paste was screen-printed into the solder resist openings using the metal mask and metal squeegee. Each printed wiring board was then reflowed in a nitrogen atmosphere under the following conditions to form solder joints on each printed wiring board. Reflow oven: Product name: TNP-25-538EM, manufactured by Tamura Corporation Preheat: 130℃ to 180℃ for 40 to 50 seconds Peak temperature: 238℃ to 240℃ Time above 227°C: 26 to 30 seconds

[0076] Next, each printed wiring board was washed under the following conditions to prepare test substrates on which solder bumps were formed. Cleaning solution: Semi-aqueous cleaning solution (product name: CleanThrough 750HS, manufactured by Kao Corporation) Cleaning method: ultrasonic cleaning Washing conditions: Wash with cleaning solution (60°C for 2 minutes), then wash with ion-exchanged water (60°C for 2 minutes)

[0077] For each test substrate, 700 random bumps were observed using an X-ray inspection device (product name: XD7600 Diamond, manufactured by Nordson Dage), and the maximum total void area ratio (total void area ratio = (total void area) / (solder bump area) x 100) was measured and evaluated according to the following criteria (the number of evaluation test substrates was 6 per example (Example and Comparative Example)). The results are shown in Table 2. ○: Maximum total void area ratio is over 1% and 7% or less △: Maximum total void area ratio is over 7% and 10% or less ×: Maximum total void area ratio exceeds 10%

[0078] (2) Missing bump occurrence confirmation test Each test substrate prepared in the above (1) maximum void area ratio confirmation test was observed using an X-ray inspection device (product name: XD7600 Diamond, manufactured by Nordson Dage), the number of missing bumps was counted, and the results were evaluated according to the following criteria (the number of evaluation test substrates was six per example (Example and Comparative Example)). The results are shown in Table 2. ○: No missing bumps ×: One or more missing bumps

[0079] [Table 2]

[0080] As shown above, the solder paste according to each example contains a fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity at room temperature of 200 mPa·s or more and 400 mPa·s or less, thereby ensuring solder wettability while suppressing the occurrence of voids in the solder joints and solder bumps. [Explanation of symbols]

[0081] 10 Substrate 20 electrodes 30 Solder paste 32 Solder alloys 34 Flux 40 insulating layer 50 Sn plating

Claims

1. a flux containing a rosin-based resin (A), an activator (B), and a solvent (C), wherein the solvent (C) contains a fatty acid ester (C-1) having 12 or more carbon atoms and a viscosity of 200 mPa s or more and 400 mPa s or less at room temperature; and alloy powder (D) made of a solder alloy, The fatty acid ester (C-1) having 12 or more carbon atoms and having a viscosity of 200 mPa·s or more and 400 mPa·s or less at room temperature is a triester or tetraester of a polyhydric alcohol (C-1a) having divalent or more hydroxyl groups, which is at least one of trimethylolpropane, pentaerythritol, and polyglycerol, and a fatty acid (C-1b) having 12 or more carbon atoms and 22 or less, which is isostearic acid; the blending amount of the rosin-based resin (A) is 15% by mass or more and 50% by mass or less based on the total amount of the flux, The blending amount of the activator (B) is 1 mass % or more and 30 mass % or less based on the total amount of the flux, the amount of the fatty acid ester (C-1) having 12 or more carbon atoms and having a viscosity of 200 mPa s or more and 400 mPa s or less at room temperature is 1 mass % or more and 20 mass % or less based on the total amount of the flux, The solder paste contains the alloy powder (D) made of the solder alloy in an amount of 80 mass % to 94 mass % based on the total amount of the solder paste.

2. 2. The solder paste according to claim 1, wherein the fatty acid ester (C-1) having 12 or more carbon atoms and having a viscosity at room temperature of 200 mPa s or more and 400 mPa s or less is at least one selected from the group consisting of trimethylolpropane triisostearate, pentaerythritol tetraisostearate, and diglyceryl triisostearate.

3. A solder bump formed using the solder paste according to claim 1 or 2.

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