Solder paste

Abstract

The present invention relates to a flux containing rosin, an active agent, a solvent, a thixotropic agent containing polyethylene glycol, the content of the polyethylene glycol being 10 to 20 mass% relative to the total mass of the flux, and the content of the thixotropic agent other than the polyethylene glycol being 5 mass% or less relative to the total mass of the flux.

Description

Technical Field

[0001] This invention relates to a flux and solder paste that can suppress scorching after heating and the separation of powder and flux. Background Technology

[0002] Solder paste consists of flux and solder powder. Electrode pads are provided on a substrate on which electronic components are mounted. Solder paste is printed onto the electrode pads, and a reflow soldering process is used to form a solder joint. Solder paste printing is performed as follows: a metal mask with openings is placed on the substrate, and a squeegee is pressed against the metal mask while moving it, applying solder paste to the electrode pads of the substrate in a single pass through the openings of the metal mask.

[0003] In recent years, with the miniaturization of electronic components and the resulting reduction in the size of electrode pads, the time it takes to use up purchased solder paste has increased. During prolonged storage, depending on the storage conditions, the viscosity of the solder paste may increase, making it difficult to apply the paste through the openings of the metal mask, and resulting in a loss of the original printing performance. Furthermore, the smaller electrode pads and narrower printing area necessitate a reduction in the particle size of the solder powder used in the solder paste. This increases the surface area of ​​the solder powder, leading to surface oxidation and subsequent deterioration of the solder paste over time.

[0004] Therefore, for example, Patent Documents 1 to 3 disclose a flux containing less than 7% by weight of polyethylene glycol (hereinafter, appropriately referred to as "PEG") to suppress the increase in solder paste viscosity caused by the formation of complexes between the constituent elements of the solder powder and the rosin in the flux, thereby improving printability and storage stability.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 5-228690

[0008] Patent Document 2: Japanese Patent Application Publication No. 10-109188

[0009] Patent Document 3: Japanese Patent Application Publication No. 10-128573 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] In the inventions described in Patent Documents 1-3, a specified amount of PEG is included to suppress the increase in viscosity of the solder paste. However, Patent Document 1 states that when the PEG content exceeds 7% by mass, the flowability decreases. Patent Document 2 states that if the PEG content exceeds 3% by mass, not only does the risk of migration increase due to the hydrophilicity of the flux itself, but also the overprotection of the solder metal particles hinders activity and increases the formation of solder balls. Patent Document 3 states that if the PEG content exceeds 2% by mass, the formation of solder balls increases. Thus, in the inventions described in Patent Documents 1-3, the PEG content is determined with regard to the drawbacks caused by suppressing viscosity increase and improving printability and storage stability, specifically to reduce flowability and the formation of solder balls.

[0012] However, as described in Patent Documents 1-3, although it is believed that increasing the thixotropic agent content can suppress sag and solder ball formation, extending the melting time during reflow soldering to reduce voids will increase the peak temperature. As a result, the thixotropic agent will burn and become difficult to clean. Furthermore, these patent documents completely lack any research on reducing the void formation rate that must be suppressed in brazed joints.

[0013] Even if scorching occurs, it can be cleaned using a high-cleaning-power cleaning solution, such as a Freon-based cleaning solution. However, using such a cleaning solution results in a high environmental impact, so its use should be avoided. Furthermore, in the inventions described in Patent Documents 1-3, when the amount of thixotropic agent is limited to prevent scorching, there is a concern that the solder powder may separate from the flux due to reduced thixotropy and viscosity.

[0014] Thus, existing solder pastes cannot simultaneously address issues such as the separation of solder powder and flux, the environmental impact during cleaning, and the generation of voids; therefore, further research is needed.

[0015] The objective of this invention is to provide flux and solder paste that can be easily cleaned with an environmentally friendly cleaning solution, do not separate from the powder and flux, and suppress the formation of voids.

[0016] Solution for solving the problem

[0017] In order to ensure sufficient melting time and easily remove residues using a cleaning fluid with low environmental impact even with reduced void formation, the inventors focused on the causative substances of the residues. As mentioned above, the causative substances of the residues are produced by the burning of the thixotropic agent. Therefore, the inventors conceived of intentionally reducing the thixotropic agent content, unlike conventional fluxes. However, since the thixotropic agent still burns even with reduced content, residues remain unchanged if a cleaning fluid with low environmental impact and low cleaning power is used. Furthermore, it is also necessary to avoid the separation of solder powder from the flux due to a decrease in solder paste viscosity.

[0018] Therefore, in-depth research was conducted to simultaneously reduce void formation rate, improve cleanability, and suppress separation of solder powder and flux. In addition to keeping the thixotropic agent content low, the PEG content was deliberately increased, a level previously avoided to suppress sagging and solder ball formation. As a result, the PEG's peak heating temperature rises due to the longer heating time, and because it remains in a liquid state without decomposition, thixotropic agent burning is suppressed, and residues can be easily removed using environmentally friendly cleaning solutions such as aqueous or semi-aqueous cleaning solutions. Simultaneously, the heating time is ensured to reduce void formation rate. Furthermore, the higher PEG content compared to previous methods also suppresses separation of solder powder and flux, thus completing this invention.

[0019] The present invention, derived from these insights, is described below.

[0020] (1) A flux, characterized in that it contains rosin, an activator, a solvent, and a thixotropic agent containing polyethylene glycol, wherein the content of polyethylene glycol is 10 to 20% by mass relative to the total mass of the aforementioned flux, and the content of the thixotropic agent other than polyethylene glycol is 5% by mass or less relative to the total mass of the flux.

[0021] (2) According to the flux described in (1) above, wherein polyethylene glycol is solid at room temperature.

[0022] (3) A solder paste comprising the flux and solder powder described in (1) or (2) above. Detailed Implementation

[0023] The present invention will now be described in more detail. In this specification, "%" means "by weight" unless otherwise specified. The components of the flux constituting the present invention will be described in detail.

[0024] 1. Rosin

[0025] The flux of the present invention contains rosin. By containing rosin, metal oxides can be removed. Examples of rosin used in the present invention include, for example, raw material rosin such as resin rosin, wood rosin, and tall oil rosin, and derivatives obtained from such raw material rosin. Examples of such derivatives include, for example, purified rosin, hydrogenated rosin, disproportionated rosin, polymerized rosin, and α,β-unsaturated carboxylic acid modified compounds (acrylated rosin, maleated rosin, fumarated rosin, etc.), as well as purified, hydride, and disproportionated compounds of such polymerized rosin, and purified, hydride, and disproportionated compounds of such α,β-unsaturated carboxylic acid modified compounds, etc., and more than two of these may be contained simultaneously. From the perspective of high heat resistance and improved flux cleaning properties, polymerized rosin is preferred as the rosin used in the present invention.

[0026] The rosin content relative to the total mass of the flux is preferably 15% to 50%. More preferably, the rosin content relative to the total mass of the flux is more than 15%, further preferably more than 20%, and particularly preferably more than 25%. Furthermore, the rosin content relative to the total mass of the flux is more preferably less than 50%, further preferably less than 48%, and particularly preferably less than 46%.

[0027] Rosin is a highly effective material for preventing the re-oxidation of powder during reflow soldering (in heating), and it is commonly used as a flux material for soldering, optimizing basic mounting characteristics. However, from a cleaning point of view, there is a concern that poor cleaning may occur if a poorly soluble cleaning solution is used to remove rosin residue as flux residue. In this invention, the reduced cleaning performance caused by the addition of rosin can be eliminated by adding polyethylene glycol, and rosin can be contained within the aforementioned range.

[0028] 2. Surfactant

[0029] The flux of the present invention contains an activator to remove metal oxides. Examples of activators used in the present invention include organic acids, organic halogen compounds, and amine hydrohalides, with organic acids being preferred.

[0030] Examples of organic acids include malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, phthalic acid, pimelic acid, octanoic acid, azelaic acid, sebacic acid, dimer acid, propionic acid, 2,2-dihydroxymethylpropionic acid, tartaric acid, malic acid, glycolic acid, diethanolic acid, thioglycolic acid, dithioglycolic acid, stearic acid, 12-hydroxystearic acid, palmitic acid, and oleic acid. Malonic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, and sebacic acid are preferred, and adipic acid or 2,2-dihydroxymethylpropionic acid are more preferred. Two or more of these acids may also be present simultaneously.

[0031] Examples of organohalogen compounds include trans-2,3-dibromo-1,4-butenediol, triallyl isocyanurate hexabromide, 1-bromo-2-butanol, 1-bromo-2-propanol, 3-bromo-1-propanol, 3-bromo-1,2-propanediol, 1,4-dibromo-2-butanol, 1,3-dibromo-2-propanol, 2,3-dibromo-1-propanol, 2,3-dibromo-1,4-butanediol, and 2,3-dibromo-2-butenediol.

[0032] Amino halides are compounds obtained by reacting amines with hydrogen halides. Examples of amines that are amino halides include ethylamine, ethylenediamine, triethylamine, methylimidazole, 2-ethyl-4-methylimidazole, diphenylguanidine, and xylylguanidine. Examples of hydrogen halides include hydrides of chlorine, bromine, iodine, and fluorine (hydrogen chloride, hydrogen bromide, hydrogen iodide, and hydrogen fluoride).

[0033] The content of the activator relative to the total mass of the flux is preferably 1 to 10%. More preferably, the content of the activator relative to the total mass of the flux is 1.5% or more, further preferably 2.0% or more, and particularly preferably 2.5% or more. Furthermore, the content of the activator relative to the total mass of the flux is more preferably 8% or less, further preferably 6% or less, and particularly preferably 4% or less.

[0034] 3. Solvent

[0035] The flux of the present invention contains a solvent in order to melt and uniformly disperse rosin and the like. Examples of solvents used in the present invention include alcohol-based solvents, ester-based solvents, glycol ether-based solvents, terpineol-based solvents, etc., with glycol ether-based solvents being preferred.

[0036] Examples of ester-based solvents include diisobutyl succinate, dibutyl succinate, dimethyl adipate, diethyl adipate, dibutyl adipate, diisopropyl adipate, diisobutyl adipate, diisodecyl adipate, dibutyl maleate, dimethyl sebacate, diethyl sebacate, dibutyl sebacate, dioctyl sebacate, and diisopropyl sebacate.

[0037] Examples of alcohol-based solvents include isopropanol, 1,2-butanediol, isobornylcyclohexanol, 2,4-diethyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 2,5-dimethyl-3-hexyn-2,5-diol, 2,3-dimethyl-2,3-butanediol, 1,1,1-tris(hydroxymethyl)ethane, 2-ethyl-2-hydroxymethyl-1,3-propanediol, and 2,2'-oxobis(methylene) Bis(2-ethyl-1,3-propanediol), 2,2-bis(hydroxymethyl)-1,3-propanediol, 1,2,6-trihydroxyhexane, bis[2,2,2-tri(hydroxymethyl)ethyl] ether, 1-ethynyl-1-cyclohexanol, 1,4-cyclohexanediol, 1,4-cyclohexanediol, erythritol, threitol, guaiacol glycerol ether, 3,6-dimethyl-4-octyne-3,6-diol, 2,4,7,9-tetramethyl-5-decyn-4,7-diol, etc.

[0038] Examples of glycol ether solvents include diethylene glycol mono-2-ethylhexyl ether, ethylene glycol monophenyl ether, 2-methylpentane-2,4-diol, diethylene glycol monohexyl ether, diethylene glycol dibutyl ether, triethylene glycol monobutyl ether, 1,3-butanediol, phenylethylene glycol, and hexanediol, with diethylene glycol monohexyl ether or diethylene glycol mono-2-ethylhexyl ether being preferred. They may also contain two or more of these solvents simultaneously.

[0039] The solvent content relative to the total mass of the flux is preferably 20-75%. More preferably, it is 25% or more, even more preferably 30% or more, and particularly preferably 35% or more. Furthermore, the solvent content relative to the total mass of the flux is more preferably 72% or less, even more preferably 70% or less, and particularly preferably 50% or less.

[0040] 4. Thixotropic agents containing polyethylene glycol

[0041] The flux of the present invention contains a thixotropic agent comprising PEG.

[0042] Regarding the molecular weight of PEG contained in thixotropic agents, it is desirable that the molecular weight of PEG is lower than that of aqueous cleaning solutions from a cleaning perspective. However, in the case of low molecular weight, the properties are mostly liquid, raising concerns about its potential deterioration as a viscosity adjuster for solder paste and its heat-induced sagging properties. Considering these concerns, the optimal molecular weight is preferably 2700–3400. However, there are also properties where PEGs of different molecular weights can be used together, or where they can be covered by other flux materials; therefore, it is also suitable to use combinations with molecular weights between 250 and 9000.

[0043] PEG is preferably liquid in a temperature range above 240°C.

[0044] Furthermore, from the viewpoint of suppressing the generation of voids and improving cleanability, the boiling point of PEG is preferably 200°C or higher, more preferably 230°C or higher, and even more preferably 250°C or higher.

[0045] If PEG evaporates during reflow soldering, it can lead to voids. Furthermore, since it often remains as flux residue, it becomes advantageous for cleaning. Therefore, a high boiling point is preferred, especially one with a boiling point of 300°C or higher. With this type of PEG, the high boiling point allows it to remain in a liquid state at the melting temperature of soldering alloys known as high-melting-point solders.

[0046] If the PEG is kept in a liquid state during heating, the liquid PEG flows, thus suppressing the adhesion of scorch caused by the thixotropic agent. Residue can also be easily cleaned with a cleaning solution that has low environmental impact and low cleaning power. Furthermore, PEG is thixotropic; therefore, it can be used as a substitute for the thixotropic agent that causes scorch, reducing the thixotropic agent content and suppressing the amount of scorch adhesion. Residue can then be easily cleaned with the aforementioned cleaning solution. Moreover, even if the peak temperature rises due to prolonged heating time, residue can still be easily cleaned, resulting in a reduction in the void generation rate. Additionally, when using PEG, which is solid at room temperature, the flowability of the solder paste at room temperature can be adjusted in the same way as with thixotropic agents, thus suppressing the separation of solder powder and flux.

[0047] To achieve this effect, the solder paste of the present invention contains 10 to 20% PEG by mass relative to the flux.

[0048] When the PEG content is below 10%, the aforementioned effects cannot be achieved. On the other hand, when the PEG content exceeds 20%, the amount of rosin that can be contained to achieve a suitable viscosity is limited, thereby reducing reflow solderability and failing to suppress void formation. Furthermore, PEG is hydrophilic; therefore, if a semi-aqueous cleaning solution suitable for cleaning rosin-based fluxes is used, the cleaning performance will be inconsistent. The PEG content is preferably 10–16%, more preferably 12–16%.

[0049] The PEG used in this invention imparts sufficient thixotropy to the solder paste and increases its viscosity, suppressing the separation of flux and solder powder that may occur during storage. Therefore, it is preferably solid at room temperature.

[0050] In addition to PEG, the solder paste of the present invention may also contain a thixotropic agent. Examples include wax-based thixotropic agents and amide-based thixotropic agents, with wax-based thixotropic agents being preferred.

[0051] Examples of wax-based thixotropic agents include hydrogenated castor oil.

[0052] Examples of amide-based thixotropic agents include lauramide, palmitamide, stearamide, behenamide, hydroxystearamide, oleamide, mustardamide, methylene bisstearamide, ethylene bislaurate, ethylene bisstearamide, ethylene bishydroxystearamide, and hydroxymethylstearamide. They may also contain two or more of these agents simultaneously.

[0053] The content of thixotropic agents other than PEG must be less than 5% of the total mass of the flux, preferably less than 3%, or it may be absent.

[0054] 5. Other ingredients

[0055] The flux of the present invention may contain amines, antioxidants, or defoamers.

[0056] As amines, aliphatic amines, amino alcohols, imidazoles, etc. can be used.

[0057] Examples of aliphatic amines include dimethylamine, ethylamine, 1-aminopropane, isopropylamine, trimethylamine, n-butylamine, diethylamine, sec-butylamine, tert-butylamine, N,N-dimethylethylamine, isobutylamine, and cyclohexylamine.

[0058] Examples of amino alcohols include 2-aminoethanol, 2-(ethylamino)ethanol, diisopropanolamine, triethanolamine, N-butyldiethanolamine, triisopropanolamine, N,N-bis(2-hydroxyethyl)-N-cyclohexylamine, N,N,N',N'-tetra(2-hydroxypropyl)ethylenediamine, and N,N,N',N”,N”-penta(2-hydroxypropyl)diethylenetriamine.

[0059] Examples of imidazoles include 2-methylimidazolium, 2-undecylimidazolium, 2-pentadedecylimidazolium, 1,2-dimethylimidazolium, 2-ethyl-4-methylimidazolium, 2-phenylimidazolium, 2-phenyl-4-methylimidazolium, 1-benzyl-2-methylimidazolium, 1-benzyl-2-phenylimidazolium, 1-cyanoethyl-2-methylimidazolium, 1-cyanoethyl-2-undecylimidazolium, and 1-cyanoethyl-2-ethyl-4-methylimidazolium. Imidazole, 1-cyanoethyl-2-phenylimidazolium, 1-cyanoethyl-2-undecylimidazolium trimellitate, 1-cyanoethyl-2-phenylimidazolium trimellitate, 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-triazine, 2,4-diamino-6-[2'-undecylimidazolyl-(1')]-ethyl-triazine, 2,4-diamino-6-[2'-ethyl-4'-methylimidazolium] [2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-triazine, 2-phenylimidazolyl-(1')]-ethyl-triazine isocyanuric acid adduct, 2-phenylimidazolyl isocyanuric acid adduct, 2-phenyl-4,5-dihydroxymethylimidazolyl, 2-phenyl-4-methyl-5-hydroxymethylimidazolyl, 2,3-dihydro-1H-pyrrolo[1,2-a]benzimidazole, 2-methylimidazoline, 2-phenylimidazoline 2,4-Diamino-6-vinyl-triazine, 2,4-diamino-6-vinyl-triazine isocyanuric acid adduct, 2,4-diamino-6-methacryloyloxyethyl-triazine, epoxy-imidazolium adduct, 2-methylbenzimidazole, 2-octylbenzimidazole, 2-pentylbenzimidazole, 2-(1-ethylpentyl)benzimidazole, 2-nonylbenzimidazole, 2-(4-thiazolyl)benzimidazole, benzimidazole, etc.

[0060] As antioxidants, hindered phenolic antioxidants can be cited as an example.

[0061] Examples of defoamers include acrylic polymers, vinyl ether polymers, butadiene polymers, and organosilicon.

[0062] 6. Solder paste

[0063] The solder paste of the present invention contains the above-mentioned flux and solder powder.

[0064] The solder powder is not particularly limited, and solder powders with various solder alloy compositions, such as Sn-3Ag-0.5Cu, Pb-10Sn, and Sn-Sb-based solder alloys, can be used. Because the solder paste of the present invention uses the aforementioned flux, it solves the problems encountered in conventional solder pastes made by mixing flux and high-melting-point solder powder, even when using high-melting-point solder powder. In the solder paste of the present invention, even if the melting point of the solder powder is 240°C or higher, or 280°C or higher, the presence of a specified amount of PEG suppresses poor cleaning caused by scorching, separation of the solder powder and flux, and consequently, the formation of voids.

[0065] The particle size of the solder powder is preferably 10–50 μm. The content of the solder powder relative to the total mass of the solder paste is preferably 85–92%.

[0066] The solder paste of the present invention can be manufactured using methods generally available in the art. First, the solder powder can be manufactured using known methods such as: dropwise addition of heated molten solder to obtain particles, centrifugal spraying, or pulverizing the main soldering material. In the dropwise addition and spraying methods, dropwise addition and spraying are preferably carried out in an inactive atmosphere or solvent to form particles. Then, the components are heated and mixed to prepare a flux, and the aforementioned solder powder is introduced into the flux and stirred and mixed to manufacture the solder paste.

[0067] Example

[0068] The present invention will be described based on the following embodiments, but the present invention is not limited to the following embodiments. In the embodiments, "%" means "mass %" unless otherwise specified. In addition, the numerical values ​​of the alloy composition of the solder powder are expressed as a content in mass %

[0069] Prepare solder powders with an alloy composition of Sn-3Ag-0.5Cu (Ag: 3% by mass, Cu: 0.5% by mass, balance: Sn and impurities, melting point: 220℃) and Pb-10Sn (Sn: 10% by mass, balance: Pb and impurities, melting point: 302℃) and a particle size of 20–38 μm. Mix the fluxes prepared as shown in Tables 1 and 3 with the solder powders to prepare solder paste. The values ​​in Tables 1 and 3 are in "% by mass".

[0070] The alloy composition of the solder powder used in Examples 1-7 and Comparative Examples 1-4 shown in Table 1 is Sn-3Ag-0.5Cu, and the alloy composition of the solder powder used in Examples 8-14 and Comparative Examples 5-8 shown in Table 3 is Pb-10Sn.

[0071] The ratio of flux to solder powder is as follows: when using Sn-3Ag-0.5Cu solder powder, the ratio of flux to solder powder is 12:88; when using Pb-10Sn solder powder, the ratio of flux to solder powder is 9:91.

[0072] For each solder paste, evaluate 1. cleanability, 2. separation of flux and solder powder (separation), and 3. presence or absence of voids. Details are as follows.

[0073] 1. Cleanability

[0074] 1) Printing

[0075] The solder paste prepared as described above is printed on a 50mm×50mm Cu plate using a metal mask with an opening size of 5mm×5mm and a thickness of 0.4mm.

[0076] 2) Reflow soldering conditions

[0077] The case of Sn-3Ag-0.5Cu

[0078] Set the preheating temperature to 150-180℃ and hold for 200 seconds. Set the heating rate from the preheating temperature to the peak temperature to 1.34℃ / second, the peak temperature to 250℃, the heating time above 220℃ to 250 seconds, and the oxygen concentration to 2000ppm. Then, heat the product and cool it to room temperature.

[0079] • Pb-10Sn

[0080] The heating rate from room temperature to peak temperature was set to 4℃ / second, the peak temperature to 380℃, the heating time above 302℃ to 50 seconds, and the oxygen concentration to below 100ppm. The mixture was then cooled to room temperature.

[0081] 3) Cleaning conditions

[0082] • Status of water-based cleaning solutions

[0083] Use the cleaning agent brand "VIGON A200" (manufactured by Zestron) and dilute it with ion-exchanged water at a ratio of 30:70 (by mass) to obtain a cleaning solution. Then, perform the cleaning as follows.

[0084] (1) Perform ultrasonic cleaning in an ultrasonic bath filled with cleaning solution (50°C, 10 minutes).

[0085] (2) Rinse in an ultrasonic bath filled with ion-exchange water (room temperature, 2 times for 3 minutes each time).

[0086] (3) Dry in a constant temperature bath at 50℃ for 10 minutes.

[0087] • Situation of semi-aqueous cleaning solutions

[0088] Use the cleaning solution with the brand name "Cleanthrough 750J" (manufactured by Kao Corporation) and follow the steps below to clean.

[0089] (1) Perform ultrasonic cleaning in an ultrasonic bath filled with cleaning solution (40°C, 10 minutes).

[0090] (2) Rinse in an ultrasonic bath filled with ion-exchange water (room temperature, 3 minutes).

[0091] (3) Dry in a constant temperature bath at 50℃ for 10 minutes.

[0092] 4) Judgment

[0093] The dried test pieces were observed using a scanning electron microscope (SEM). Cases where no cleaning residue was observed were marked with “○”, and cases where cleaning residue was observed were marked with “×”.

[0094] 2. Separation

[0095] After 3 days at room temperature, visually inspect the surface condition of the solder paste. Mark "〇" for cases where no separation is observed and "×" for cases where separation is visible (where flux floats due to separation).

[0096] 3. The presence or absence of gaps

[0097] 1) Printing

[0098] The solder paste prepared as described above is printed on a 50mm×50mm Cu plate using a metal mask with an opening size of 5mm×5mm and a thickness of 0.4mm.

[0099] 2) Reflow soldering conditions

[0100] The case of Sn-3Ag-0.5Cu

[0101] Set the preheating temperature to 150-180℃ and hold for 200 seconds. Set the heating rate from the preheating temperature to the peak temperature to 1.34℃ / second, the peak temperature to 250℃, the heating time above 220℃ to 250 seconds, and the oxygen concentration to 2000ppm. Then, heat the product and cool it to room temperature.

[0102] • Pb-10Sn

[0103] The heating rate from room temperature to peak temperature was set to 4℃ / second, the peak temperature to 380℃, the heating time above 302℃ to 50 seconds, and the oxygen concentration to below 100ppm. The mixture was then cooled to room temperature.

[0104] 3) Observation of gaps

[0105] The transmission image of the solder joint after reflow soldering was observed using a Microfocus X-ray System XVR-160 manufactured by UNi-HiTE SYSTEM, and the void generation rate was determined.

[0106] The void generation rate was determined by transmitting a transmission image of the brazed joint onto the Cu plate surface perpendicular to the 3mm solder joint. Then, based on the contrast of the transmission image's hue, the void area ratio was automatically calculated by identifying the metal-filled areas and void areas (unfilled metal areas), and this ratio was used as the void generation rate.

[0107] 4) Judgment Criteria

[0108] A void generation rate exceeding 3% is marked as “×”, a void generation rate exceeding 1% but less than 3% is marked as “〇”, and a void generation rate less than 1% is marked as “◎”.

[0109] The evaluation results are shown in Tables 2 and 4.

[0110] [Table 1]

[0111]

[0112] [Table 2]

[0113]

[0114] [Table 3]

[0115]

[0116] [Table 4]

[0117]

[0118] As shown in Tables 2 and 4, it can be seen that the solder pastes of the embodiments all meet the characteristics of the present invention, thus exhibiting excellent cleanability and suppressing the generation of separation and voids.

[0119] In contrast, the solder pastes of Comparative Examples 1, 2, 5, and 6 contained no PEG or had a low PEG content. Therefore, residue was visible in aqueous cleaning solutions with a higher hydrophilicity than semi-aqueous cleaning solutions. The solder pastes of Comparative Examples 3 and 7 had a high PEG content but limited rosin content, resulting in reduced reflow solderability and high porosity. Furthermore, since PEG is hydrophilic, the solder pastes of Comparative Examples 3 and 7 exhibited poor cleaning performance with semi-aqueous cleaning solutions.

[0120] The solder pastes of Comparative Examples 4 and 8 contain a high content of thixotropic agents other than PEG, which results in charring and reduced cleanability in any cleaning solution.

[0121] Industrial availability

[0122] According to the present invention, flux and solder paste that can be easily cleaned with an environmentally friendly cleaning solution, do not cause separation of powder and flux, and suppress the formation of voids can be provided.

[0123] Although the present invention has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

[0124] This application is based on Japanese patent application filed on October 4, 2019 (Japanese Patent Application No. 2019-183500), the contents of which are incorporated herein by reference.

Claims

1. A solder paste, characterized in that, It contains flux and solder powder. The flux contains rosin, activator, solvent, and thixotropic agent including polyethylene glycol. The content of polyethylene glycol relative to the total mass of the flux is 12-20% by mass; the content of thixotropic agents other than polyethylene glycol relative to the total mass of the flux is less than 5% by mass; and the content of rosin relative to the total mass of the flux is more than 15% by mass but less than 50% by mass. The solvent is at least one selected from the group consisting of alcohol solvents, ester solvents, glycol ether solvents, and terpineols. The alcohol solvent is selected from isopropanol, 1,2-butanediol, isoborneol, 2,4-diethyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 2,5-dimethyl-3-hexyn-2,5-diol, 2,3-dimethyl-2,3-butanediol, 1,1,1-tris(hydroxymethyl)ethane, 2-ethyl-2-hydroxymethyl-1,3-propanediol, 2,2'-oxobis(methylene)bis( The group consisting of 2-ethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)-1,3-propanediol, 1,2,6-trihydroxyhexane, bis[2,2,2-tri(hydroxymethyl)ethyl] ether, 1-ethynyl-1-cyclohexanol, 1,4-cyclohexanediol, 1,4-cyclohexanediol, erythritol, threitol, guaiacol glycerol ether, 3,6-dimethyl-4-octyne-3,6-diol, and 2,4,7,9-tetramethyl-5-decyn-4,7-diol. The ester solvent is selected from the group consisting of diisobutyl succinate, dibutyl succinate, dimethyl adipate, diethyl adipate, dibutyl adipate, diisopropyl adipate, diisobutyl adipate, diisodecyl adipate, dibutyl maleate, dimethyl sebacate, diethyl sebacate, dibutyl sebacate, dioctyl sebacate, and diisopropyl sebacate. The glycol ether solvent is selected from the group consisting of diethylene glycol mono-2-ethylhexyl ether, ethylene glycol monophenyl ether, 2-methylpentane-2,4-diol, diethylene glycol monohexyl ether, diethylene glycol dibutyl ether, triethylene glycol monobutyl ether, 1,3-butanediol, phenylethylene glycol, and hexanediol.

2. The solder paste according to claim 1, wherein, The polyethylene glycol is a solid at room temperature.

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