Solder flux, solder paste, and method for manufacturing electronic component mounting substrate

Abstract

A solder flux according to an embodiment of the present disclosure includes a rosin-based resin, a volatile solvent that dissolves the rosin-based resin, and a hardly volatile liquid having an evaporation rate of 0.001 or less when the evaporation rate of n-butyl acetate is 1, the content of the hardly volatile liquid being 5-40 parts by mass with respect to 100 parts by mass of the solder flux.

Description

Soldering flux, solder paste, and method for manufacturing an electronic component mounting substrate

[0001] The present invention relates to a soldering flux, a solder paste, and a method for manufacturing an electronic component mounting substrate.

[0002] In recent years, with the miniaturization of electronic devices, miniaturization of electronic component mounting substrates has also been demanded. In order to meet such a demand for miniaturization of electronic component mounting substrates, it is necessary to densely mount minute electronic components on a circuit board at a narrow pitch. As a technique for densely mounting minute electronic components, a method using solder precoat has been studied (Patent Document 1).

[0003] In the method using solder precoat, usually, a soldering flux is disposed on the surface of the solder precoat, and an electronic component is temporarily fixed by this soldering flux.

[0004] International Publication No. 2022 / 195937

[0005] Among circuit boards on which high-density mounting is performed, there are some in which the number of electronic components mounted on one circuit board is extremely large. In this case, the time required from when a soldering flux is applied to the circuit board until the mounting of all the electronic components is completed becomes long. When the time required for mounting the electronic components becomes long like this, the soldering flux on which the electronic components have not yet been mounted may dry during the mounting of the electronic components. And since the adhesive force (tack force) decreases in the dried soldering flux, mounting defects may occur in some of the electronic components mounted on one circuit board. For example, an electronic component mounted on a soldering flux with a reduced tack force is likely to be displaced even by a slight vibration because the temporary fixing effect by the soldering flux is small. Therefore, there has been a problem that mounting defects due to displacement occur when soldering is performed using a reflow apparatus. Such mounting defects become prominent in minute components (electronic components having a size of 0402 or less defined by JIS standards) with a small contact area with the soldering flux.

[0006] Therefore, the present disclosure provides a soldering flux, a solder paste, and a method for manufacturing an electronic component mounting substrate that can suppress mounting defects of electronic components in high-density mounting.

[0007] One aspect of the present invention relates to a solder flux comprising a rosin-based resin, a volatile solvent for dissolving the rosin-based resin, and a non-volatile liquid whose evaporation rate is 0.001 or less when the evaporation rate of n-butyl acetate is set to 1, wherein the content of the non-volatile liquid is 5 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the solder flux.

[0008] Another aspect of the present invention relates to a solder paste comprising solder particles and the solder flux described above.

[0009] A further aspect of the present invention relates to a method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands for mounting electronic components or a plurality of solder precoats formed on the plurality of lands; a second step of applying the solder flux to the plurality of lands or the plurality of solder precoats; a third step of mounting electronic components, including electronic components of size 0402 or smaller as defined by JIS standards, on the plurality of lands or the plurality of solder precoats to which the solder flux has been applied, after the second step; and a fourth step of heating and soldering the circuit board on which the electronic components are mounted.

[0010] A further aspect of the present invention relates to a method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands for mounting electronic components or a plurality of solder precoats formed on the plurality of lands; a second step of applying the solder paste to the plurality of lands or the plurality of solder precoats; a third step of mounting electronic components, including electronic components of size 0402 or smaller as defined by JIS standards, on the plurality of lands or the plurality of solder precoats to which the solder paste has been applied, after the second step; and a fourth step of heating the circuit board on which the electronic components are mounted and soldering them.

[0011] According to this disclosure, it is possible to provide a solder flux, solder paste, and a method for manufacturing an electronic component mounting substrate that can suppress mounting defects of electronic components in high-density mounting.

[0012] This is a flowchart of a method for manufacturing an electronic component mounting substrate according to an embodiment of this disclosure. This is a side cross-sectional view illustrating an example of the first step of the method for manufacturing an electronic component mounting substrate according to an embodiment of this disclosure. This is a side cross-sectional view illustrating an example of the second step of the method for manufacturing an electronic component mounting substrate according to an embodiment of this disclosure. This is a side cross-sectional view illustrating an example of the third step of the method for manufacturing an electronic component mounting substrate according to an embodiment of this disclosure. This is a side cross-sectional view illustrating the state in which electronic components have been soldered to a circuit board.

[0013] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. Notwithstanding, known components may be applied to components of parts that are characteristic of this disclosure. In this specification, when "the range of numerical values ​​A to numerical values ​​B" is used, that range includes numerical values ​​A and B.

[0014] In the following explanation, when examples are given for the lower and upper limits of numerical values ​​related to specific physical properties or conditions, any combination of either of the given lower limits and any of the given upper limits is permitted, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, unless otherwise specified, one type may be selected and used alone, or two or more types may be used in combination.

[0015] This disclosure includes any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims. In other words, any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims is possible, as long as it does not result in a technical inconsistency.

[0016] (Solder Flux) The solder flux according to the embodiment of this disclosure comprises a rosin-based resin, a volatile solvent for dissolving the rosin-based resin, and a non-volatile liquid whose evaporation rate is 0.001 or less when the evaporation rate of n-butyl acetate is set to 1. In the solder flux according to the embodiment of this disclosure, the content of the non-volatile liquid is 5 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the solder flux.

[0017] In the solder flux according to the embodiments of this disclosure, it is important that (i) the solder flux contains a non-volatile liquid whose evaporation rate is 0.001 or less when the evaporation rate of n-butyl acetate is set to 1, and (ii) the content of the non-volatile liquid is 5 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the solder flux. The reasons for this will be explained below.

[0018] Solder flux contains a main resin and a volatile solvent that dissolves the main resin. The main resin provides the tack force necessary for temporarily fixing electronic components. Solder flux is also required to reduce and remove oxide films formed on the surface of solder precoats. Therefore, rosin-based resins, which themselves have reducing properties, are often used as the main resin in solder flux.

[0019] When high-density mounting of electronic components onto a circuit board using solder flux, the number of electronic components mounted on a single circuit board can become abnormally large (for example, more than 10,000). In this case, the time required from applying solder flux to the circuit board until all electronic components are mounted becomes longer. When the time required for mounting electronic components is extended in this way, the solder flux that has not yet had any electronic components mounted may dry out during the mounting process. As a result, the adhesive force (tack force) of the solder flux decreases, which can lead to mounting defects in some of the electronic components mounted on a single circuit board. For example, electronic components mounted on solder flux with reduced tack force are less likely to be temporarily fixed by the solder flux, making them susceptible to displacement even with slight vibrations. Therefore, when soldering using a reflow soldering machine, there is a problem of mounting defects caused by displacement. Such mounting defects are particularly noticeable in tiny components with a small contact area with the solder flux (electronic components of size 0402 or smaller as defined by JIS standards).

[0020] To suppress the drying of solder flux, it is conceivable to include non-volatile components (e.g., non-volatile liquids) in the solder flux. However, simply including non-volatile components may still not be sufficient to adequately suppress the drying of the solder flux. Furthermore, there is a concern that the non-volatile components may hinder the soldering of electronic components by solder precoating, leading to mounting defects. In other words, there is a concern that mounting defects may occur due to poor soldering.

[0021] However, the solder flux according to the embodiment of this disclosure has the configurations of (i) and (ii) above. That is, the type and content of the non-volatile liquid are appropriately balanced in the solder flux. Therefore, even when the time required for mounting electronic components is long in high-density mounting, it is possible to sufficiently suppress the drying of the solder flux on which electronic components have not yet been mounted. As a result, a decrease in the adhesive force (tack force) due to drying in the solder flux can be suppressed, and thus misalignment of electronic components on the solder flux can be suppressed. Therefore, mounting defects of electronic components caused by misalignment can be suppressed. In addition, mounting defects caused by the solder precoating or the like being hindered by the presence of an excessive amount of non-volatile liquid in the solder flux can be suppressed.

[0022] The specific configuration of the solder flux according to the embodiment of this disclosure will be described below.

[0023] Solder flux is used to mount electronic components on circuit boards. As mentioned above, solder flux has an appropriate balance of the type and content of non-volatile liquids. Therefore, solder flux is suitable for high-density mounting of electronic components. Specifically, solder flux is preferably used in circuit boards having multiple lands or solder precoats formed on multiple lands, where it is pre-applied to multiple lands or multiple solder precoats, and used to mount electronic components, including electronic components of size 0402 or smaller as defined by JIS standards, onto the circuit board.

[0024] As explained above, solder flux contains a rosin-based resin, a volatile solvent that dissolves the rosin-based resin (hereinafter also simply referred to as the volatile solvent), and a non-volatile liquid with an evaporation rate of 0.001 or less when the evaporation rate of n-butyl acetate is set to 1 (hereinafter also simply referred to as the non-volatile liquid). Solder flux exhibits adhesive force (tack force) due to the inclusion of the rosin-based resin. This adhesive force (tack force) allows electronic components to be temporarily fixed on a circuit board. In solder flux, the rosin-based resin is classified as the main resin.

[0025] Rosin-based resins themselves possess reducing properties. Therefore, when solder flux contains rosin-based resin, it can reduce oxide films formed on the surface of solder precoats and other areas.

[0026] Examples of rosin-based resins include natural rosins such as gum rosin and wood rosin, and their derivatives (polymerized rosin, hydrogenated rosin, disproportionated rosin, acid-modified rosin, and rosin esters).

[0027] A volatile solvent is defined as a solvent whose evaporation rate is greater than 0.001 when the evaporation rate of n-butyl acetate is set to 1. Examples of volatile solvents include water, alcohol-based solvents, glycol-based solvents, ketone-based solvents, hydrocarbon-based solvents, ester-based solvents, glycol ether-based solvents, and terpionelles. Among these volatile solvents, alcohol-based solvents are preferred, and among alcohol-based solvents, diethylene glycol monobutyl ether (with an evaporation rate of 0.004) is preferred.

[0028] Examples of non-volatile liquids include epoxy resins, acrylic resins, silicone resins, ionic liquids, amphiphilic liquids, oils, and glycol ethers. It is preferable to use at least one selected from the group consisting of silicone resins, ionic liquids, amphiphilic liquids, and glycol ethers as the non-volatile liquid. Examples of silicone resins include alkyl silicone resins, examples of ionic liquids include imidazolium-based ionic liquids, examples of amphiphilic liquids include sorbitan monostearate, and examples of glycol ethers include heptaethylene glycol monomethyl ether. It is preferable that the non-volatile liquid has an OH group in its molecule. When the non-volatile liquid has an OH group in its molecule, a hydrogen bond is formed between the non-volatile liquid and the alcohol-based solvent when the volatile solvent is an alcohol-based solvent. This hydrogen bond suppresses the volatilization of the alcohol-based solvent. This further sufficiently suppresses the drying of the solder flux, even when the number of electronic components mounted on a single circuit board is unusually large (e.g., 10,000 or more).

[0029] In solder flux, the content of the non-volatile liquid is 5 parts by mass or more and 40 parts by mass or less per 100 parts by mass of solder flux. By having a non-volatile liquid content of 5 parts by mass or more in the solder flux, the decrease in the adhesive strength (tack force) of the solder flux due to drying can be suppressed, thereby suppressing misalignment of electronic components caused by a decrease in adhesive strength (tack force). Furthermore, by having a non-volatile liquid content of 40 parts by mass or less, mounting defects caused by poor soldering can be suppressed.

[0030] In solder flux, the mass ratio of the volatile solvent to the non-volatile liquid is preferably 0.4 to 8. By including the non-volatile liquid and volatile solvent in the above mass ratio, the decrease in the adhesive strength (tack force) of the solder flux due to drying can be suppressed, thereby suppressing displacement of electronic components caused by the decrease in adhesive strength (tack force).

[0031] Solder flux may contain resins other than rosin-based resins as the main resin component. Examples of resins other than rosin-based resins include terpene resins, terpene phenol resins, styrene resins, xylene resins, polyester resins, polyolefin resins, polyamides, polyamines, phenol resins, and phenoxy resins. Solder flux may contain one type of resin other than rosin-based resins alone, or two or more types.

[0032] Solder flux may contain thixotropes and activators in addition to rosin-based resins, resins other than rosin-based resins, volatile solvents, and non-volatile liquids.

[0033] Examples of thixotropic agents include wax-based thixotropic agents, amide-based thixotropic agents, and sorbitol-based thixotropic agents. A single thixotropic agent may be used, or two or more may be used in combination. Among these thixotropic agents, wax-based thixotropic agents are preferred, and among wax-based thixotropic agents, castor wax is preferred.

[0034] The activator used is a compound that reduces the oxide film covering the surface of solder (e.g., solder precoat) used for soldering electronic components. As explained above, rosin-based resins themselves have a reducing effect, and resins other than rosin-based resins may also have a certain degree of activating effect. Therefore, in this specification, the term "activator" refers to a compound other than the main resin, such as a rosin-based resin, that has an activating effect.

[0035] The activator reduces the oxide film and assists in the formation of a good junction. Examples of activators with reducing properties include organic acids, amines, and halides. The activator may be used alone or in combination of two or more. It is preferable to use an organic acid as the activator.

[0036] Examples of organic acids used as activators include succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dimer acid, propionic acid, 2,2-bishydroxymethylpropionic acid, tartaric acid, malic acid, glycolic acid, diglycolic acid, thioglycolic acid, dithioglycolic acid, stearic acid, 12-hydroxystearic acid, palmitic acid, and oleic acid. Among these, adipic acid is preferred.

[0037] Solder flux may contain components other than those listed above. Other components include silane coupling agents, antioxidants, and colorants.

[0038] (Solder Paste) The solder paste according to the embodiment of this disclosure comprises solder particles and solder flux according to the embodiment of this disclosure. In other words, the solder paste may be constructed by incorporating solder particles into the solder flux according to the embodiment of this disclosure. Alternatively, the solder paste according to the embodiment of this disclosure may be pre-applied to a plurality of lands or a plurality of solder precoats formed on a circuit board having a plurality of lands for mounting electronic components, instead of the solder flux according to the embodiment of this disclosure.

[0039] (Method for Manufacturing an Electronic Component Mounting Substrate) The method for manufacturing an electronic component mounting substrate according to the embodiments of this disclosure can be carried out using the solder flux according to the embodiments of this disclosure or the solder paste according to the embodiments of this disclosure. Hereinafter, the method for manufacturing an electronic component mounting substrate using the solder flux according to the embodiments of this disclosure will be described as the first embodiment, and the method for manufacturing an electronic component mounting substrate using the solder paste according to the embodiments of this disclosure will be described as the second embodiment.

[0040] [First Embodiment] The method for manufacturing an electronic component mounting substrate according to the first embodiment, as shown in Figure 1, comprises: a first step of preparing a circuit board having a plurality of lands or a plurality of solder precoats formed on a plurality of lands for mounting electronic components; a second step of applying solder flux according to the present disclosure to the plurality of lands or a plurality of solder precoats; a third step of mounting electronic components, including electronic components of size 0402 or smaller as defined by JIS standards, on the plurality of lands or a plurality of solder precoats to which the solder flux has been applied after the second step; and a fourth step of heating the circuit board on which the electronic components are mounted and soldering them.

[0041] In the following, an example of a method for manufacturing an electronic component mounting substrate according to the first embodiment will be described with reference to Figures 2 to 5. Furthermore, in the following, an example of manufacturing an electronic component mounting substrate using a circuit board having multiple lands on which solder precoats have been formed will be described.

[0042] A solder precoat is formed by coating the surface of a land with solder for soldering electronic components. For example, it can be formed by supplying a solder paste containing solder particles to the land, melting the solder particles contained in the solder paste, wetting and spreading it over the land, and then solidifying the solder. Therefore, the solder precoat does not include the solder paste that was supplied to the land but before the solder particles were melted. The solder precoat may be formed with solder obtained by a plating method, or with solder generated by heating a special material applied to the land.

[0043] <First Step> In the first step, a circuit board having multiple lands to which electronic components will be soldered is prepared. As shown in Figure 2, one example of a circuit board 1 has a substrate portion 1a and multiple lands 2 arranged on the main surface of the substrate portion 1a. As shown in Figure 2, the multiple lands 2 have at least multiple first lands 2A and multiple second lands 2B. For convenience, Figure 2 shows an example in which the circuit board 1 has four first lands 2A and two second lands 2B.

[0044] The substrate portion 1a may be composed of various known substrates such as a laminated substrate, a resin substrate, a ceramic substrate, and a silicon substrate. An example of the circuit board 1 is a printed circuit board.

[0045] The first land 2A is a land for mounting an electronic component with a size of 0402 or less defined by the JIS standard. Hereinafter, such an electronic component is also referred to as a first electronic component. The 0402-size electronic component means an electronic component with a length of 0.4 mm × a width of 0.2 mm. As shown in FIG. 4, the first electronic component 5 is mounted on a pair of first lands 2A. The first electronic component 5 has an electronic component body 5a and a pair of first terminals 5b arranged on both end face sides of the electronic component body 5a. That is, the first electronic component 5 is a chip component. Examples of the chip component include a chip resistor, a chip capacitor, and a chip inductor. In the first electronic component 5, each of the pair of first terminals 5b is mounted on each of the pair of first lands 2A.

[0046] As shown in FIG. 2, the first land 2A is a land having a solder precoat 4 formed on its surface. As described above, the solder precoat 4 can be formed by a so-called solder paste printing method in which a solder paste is supplied to the first land 2A and then the solder particles contained in the solder paste are melted. The solder precoat 4 is used for soldering an electronic component. FIG. 2 shows an example in which the entire surface of the first land 2A is covered with the solder precoat 4, but the solder precoat 4 only needs to cover at least a part of the surface of the first land 2A. The formation mode of the solder precoat 4 is the same for the second land 2B described later.

[0047] As shown in FIG. 2, the second land 2B is a land having a solder precoat 4 formed on its surface. The solder precoat 4 can be formed by a so-called solder paste printing method. An electronic component is mounted on the second land 2B. The electronic component has at least one terminal. The electronic component mounted on the second land 2B may be a second electronic component having a larger volume than the first electronic component 5. The second electronic component may be a second electronic component 7 having an electronic component body 7a and a pair of second terminals 7b arranged on both end face sides of the electronic component body 7a, as shown in FIG. 4. That is, the second electronic component 7 may be a chip component. When the second electronic component 7 is a chip component, each of the pair of second terminals 7b of the second electronic component 7 is mounted on each of the pair of second lands 2B, similarly to the first electronic component 5.

[0048] The second electronic component 7 may be an electronic component other than a chip component having a larger volume than the first electronic component 5. In this case, the second electronic component 7 may be, for example, a flat package, a connector, a ball grid array, a cylindrical capacitor, or the like.

[0049] Hereinafter, an example in which the second electronic component 7, which is a chip component, is used as the electronic component mounted on the second land 2B will be described. Such chip components include, for example, electronic components of the 0603 size defined by the JIS standard.

[0050] In an example of the first step of the method for manufacturing an electronic component mounting substrate according to the first embodiment, as shown in FIG. 2, a circuit board 1 having a plurality of first lands 2A and a plurality of second lands 2B on which a solder precoat 4 is formed is prepared. The first step may be carried out by purchasing a commercially available circuit board having a plurality of first lands 2A and a plurality of second lands 2B on which a solder precoat 4 is formed (the first method).

[0051] <Second Step> In an example of the second step of the method for manufacturing an electronic component mounting substrate according to the first embodiment, a solder flux 3 is applied so as to cover at least a part of each of the solder precoats 4 of the plurality of first lands 2A and the plurality of second lands 2B (see FIG. 3). As the solder flux 3, the solder flux according to the embodiment of the present disclosure is used.

[0052] The method for applying the solder flux 3 is not particularly limited, and various known application methods can be used. For example, stencil printing, dispensing, and spraying methods can be used. Among these, stencil printing is preferred. Stencil printing may also be a screen printing method. Screen printing may be carried out using a mask having through holes corresponding to the land positions. Screen printing may be, for example, a contact printing method using a metal mask. Contact printing is suitable for precisely controlling the amount of solder flux applied. Screen printing may also be a mesh screen printing method (off-contact printing method). Off-contact printing can print solder flux at a higher speed than contact printing. Therefore, by adopting the off-contact method as the screen printing method, productivity can be increased compared to adopting the contact printing method.

[0053] <Third Step> In an example of the third step of the method for manufacturing electronic components according to the embodiment of this disclosure, as shown in Figure 4, in addition to mounting first electronic components 5 of size 0402 or smaller as defined by JIS standards onto the solder flux 3 on a plurality of first lands 2A, second electronic components 7 onto the solder flux 3 on a plurality of second lands 2B. In the third step, the first electronic components 5 are mounted onto the solder flux 3 of each of the plurality of first lands 2A via the first terminal 5b, and the second electronic components 7 are mounted onto the solder flux 3 of each of the plurality of second lands 2B via the second terminal 7b. The mounting of these electronic components can be carried out by a known component mounting device.

[0054] The mounting of the first electronic component 5 onto multiple first lands 2A and the mounting of the second electronic component 7 onto multiple second lands 2B may be performed using one component mounting device or using multiple component mounting devices. In both cases, the first electronic component 5 and the second electronic component 7 are mounted one by one onto multiple first lands 2A and multiple second lands 2B in sequence. Therefore, if the total number of first electronic components 5 and second electronic components 7 is 10,000 or more, it will take an extremely long time to mount all of these electronic components.

[0055] However, since the solder flux 3 of this disclosure is configured as described above, drying of the solder flux can be suppressed even if the circuit board is exposed to the operating space of the component mounting device for a long period of time. Therefore, a rapid decrease in the adhesive force (tack force) of the solder flux can be suppressed.

[0056] <Fourth Step> In the fourth step, the circuit board 1 on which the first electronic component 5 and the second electronic component 7 were mounted in the third step is heated, and in addition to soldering the first electronic component 5 to each of the first lands 2A, the second electronic component 7 is soldered to each of the second lands 2B (see Figure 5). The pair of first terminals 5b are soldered to the first lands 2A by melting and solidifying the solder precoat 4, and the pair of second terminals 7b are soldered to the second lands 2B by melting and solidifying the solder precoat 4.

[0057] The circuit board 1 on which the first electronic component 5 and the second electronic component 7 are mounted can be heated, for example, using a heating device such as a reflow oven.

[0058] In the fourth step, the solder precoat 4 on the first land 2A and the second land 2B is melted by heating in a heating device such as a reflow oven. The molten solder that has melted the solder precoat 4 is then spread over the contact surface with the first terminal 5b and the contact surface with the second terminal 7b, respectively, and the circuit board 1 is then cooled. As a result, the spread molten solder solidifies, soldering the first terminal 5b to the first land 2A and soldering the second terminal 7b to the second land 2B. The solidified molten solder is shown as the solder portion 4a in Figure 5.

[0059] [Second Embodiment] The method for manufacturing an electronic component mounting substrate according to the second embodiment, as shown in Figure 1, comprises: a first step of preparing a circuit board having a plurality of lands or a plurality of solder precoats formed on a plurality of lands for mounting electronic components; a second step of applying solder paste according to the present disclosure to the plurality of lands or a plurality of solder precoats; a third step of mounting electronic components, including electronic components of size 0402 or smaller as defined by JIS standards, on the plurality of lands or a plurality of solder precoats to which the solder paste has been applied after the second step; and a fourth step of heating the circuit board on which the electronic components are mounted and soldering them.

[0060] In the second step, a solder paste according to the embodiment of the present disclosure is applied to a plurality of lands or a plurality of solder precoats instead of the solder flux according to the embodiment of the present disclosure. The solder paste according to the embodiment of the present disclosure can be applied to the lands 2 (first land 2A and second land 2B) or solder precoat 4 of the circuit board 1 using the same means as in the case of the solder flux according to the embodiment of the present disclosure. In the third step, the electronic components (first electronic component 5 and second electronic component 7) are mounted so that their terminals are in contact with the solder paste. In the fourth step, the electronic components are soldered to the lands 2 of the circuit board 1 by melting and solidifying the solder particles contained in the solder paste.

[0061] The solder paste according to the embodiment of this disclosure is constructed by incorporating solder particles into the solder flux according to the embodiment of this disclosure. Therefore, even if the circuit board 1 is exposed to the operating space of the component mounting device for a long period of time, drying of the solder paste can be suppressed. Consequently, the rapid decrease in the adhesive force (tack force) of the solder paste is suppressed, and the same effect as when using the solder flux according to the embodiment of this disclosure is achieved.

[0062] As an example of a method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, a circuit board 1 in which all lands 2 are covered with solder precoat 4 has been described. However, a circuit board 1 in which not all lands 2 are covered with solder precoat 4, or a circuit board 1 in which lands 2 covered with solder precoat 4 and lands 2 not covered with solder precoat 4 are mixed may also be used. In this case, in the second step, solder flux 3 is directly applied to the lands 2 that are not covered with solder precoat 4. Furthermore, the electronic components (first electronic component 5 and second electronic component 7) mounted on the lands 2 that are not covered with solder precoat 4 are those that have solder plating (tin plating, or metal plating that functions as solder, etc.) or solder bumps on the surface of their terminals. In this case, the electronic components are soldered to the lands 2 by the molten solder plating or solder bumps in the fourth step.

[0063] In one example of a method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, an example of performing the first step (first method) is described in which a commercially available circuit board having a plurality of first lands 2A and a plurality of second lands 2B with a solder precoat 4 formed on it is purchased. However, the example of performing the first step is not limited to this. The first step may be performed by applying solder paste to each of the plurality of first lands 2A and the plurality of second lands 2B of the circuit board 1 having a plurality of first lands 2A and a plurality of second lands 2B (second method). In this case, the circuit board 1 may be heated after the solder paste has been applied to each of the plurality of first lands 2A and the plurality of second lands 2B.

[0064] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0065] First, a circuit board with 4,000 lands covered with solder precoat was prepared. Next, solder flux was applied to the 4,000 lands using a screen printing method to cover the solder precoat. Then, the mounting state of electronic components and the soldering state were evaluated using the circuit board with the solder flux applied (Evaluation 1 and 2, respectively). Evaluations 1 and 2 were performed using solder fluxes with different compositions (Examples 1 to 8 and Comparative Examples 1 to 13; see Table 1).

[0066] In the solder flux for each example, the mass ratio of rosin resin, adipic acid, caster wax, and diethylene glycol monobutyl ether was rosin resin: adipic acid: caster wax: diethylene glycol monobutyl ether = 60:5:5:30. As the rosin resin, the product name "Aradaimm R-95" (polymerized rosin) manufactured by Arakawa Chemical Industries, Ltd. was used; as the diethylene glycol monobutyl ether, the product name "Butylcarbitol" manufactured by Tokyo Chemical Industries, Ltd. was used; and as the caster wax, the product name "Caster Wax A Flake" manufactured by NOF Corporation was used.

[0067] In Table 1, the first solvent is a volatile solvent that dissolves rosin-based resins, the second solvent is a non-volatile liquid with an evaporation rate of 0.001 or less when the evaporation rate of n-butyl acetate is set to 1, and the third solvent is a liquid with an evaporation rate of 0.002 or more when the evaporation rate of n-butyl acetate is set to 1, and which does not belong to either the first solvent (volatile solvent) or the second solvent (non-volatile liquid).

[0068]

[0069] [Evaluation] ・Evaluation 1 (Evaluation of mounting state) The circuit boards for each example were stored in a desiccator at room temperature (23±2℃) for 24 hours. Using a component mounting device, electronic components were mounted on 4000 pads on the circuit boards for each example after storage. The electronic components had an electronic component body having a pair of end faces, a top surface, a bottom surface, and a pair of side surfaces, and a pair of terminals arranged on both end faces of the electronic component body. The electronic components also had the 0402 size as defined by the JIS standard. In other words, the electronic components were chip components of the 0402 size. The electronic components were mounted on a pair of pads via a pair of terminals. Therefore, in each example, 2000 electronic components were mounted on the circuit board.

[0070] For each example of a circuit board with mounted electronic components, the number of misaligned electronic components on the pads was counted by microscopic observation. Then, the ratio R1 (ND / NT × 100, in %) of the number of misaligned electronic components (ND) to the total number of electronic components (NT) was calculated. The mounting condition of the electronic components was then evaluated for each example of a circuit board according to the following evaluation criteria. The evaluation results are shown in Table 2 below. Evaluation Criteria ◎: R1 is less than 0.1% 〇: R1 is 0.1% or more and less than 1% ×: R1 is 1% or more

[0071] • Evaluation 2 (Evaluation of soldering condition) First, for each example of circuit board that was not stored in a desiccator, 2,000 electronic components were mounted on 4,000 pads using a component mounting device, as described above. Next, each example of circuit board was heated in a reflow oven to solder the electronic components to pairs of pads.

[0072] For each example of a circuit board with soldered electronic components, the number of electronic components with soldering defects (electronic components with poor connections) was counted. The ratio R2 (NS / NT × 100, in %) of the number of electronic components with poor connections to the total number of electronic components (NT) was then calculated. The soldering condition of the electronic components was then evaluated for each example of a circuit board according to the following evaluation criteria. The evaluation results are shown in Table 2 below. Evaluation Criteria ◎: R2 is less than 0.1% 〇: R2 is 0.1% or more and less than 1% ×: R2 is 1% or more

[0073]

[0074] Table 2 shows that in each example (Examples 1 to 8), the circuit boards have both Evaluation 1 and Evaluation 2 at or above ○. In contrast, in each comparative example (Comparative Examples 1 to 13), the circuit boards have either Evaluation 1 or Evaluation 2 at or below ×.

[0075] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0076] The solder flux, solder paste, and method for manufacturing electronic component mounting substrates relating to this disclosure can be used in applications where it is required to suppress mounting defects of electronic components in high-density mounting.

[0077] 1: Circuit board, 1a: Board portion, 2: Land, 2A: First land, 2B: Second land, 3: Solder flux, 4: Solder precoat, 4a: Solder portion, 5: First electronic component, 5a: Electronic component body, 5b: First terminal, 7: Second electronic component, 7a: Electronic component body, 7b: Second terminal

Claims

1. A solder flux comprising a rosin-based resin, a volatile solvent for dissolving the rosin-based resin, and a non-volatile liquid whose evaporation rate is 0.001 or less when the evaporation rate of n-butyl acetate is set to 1, wherein the content of the non-volatile liquid is 5 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the solder flux.

2. A solder flux according to claim 1, which is pre-applied to the plurality of lands or the plurality of solder pre-coats formed on the plurality of lands, and is used to mount electronic components, including electronic components of size 0402 or smaller as defined by JIS standards, onto the circuit board.

3. The solder flux according to claim 1 or 2, wherein the mass ratio of the volatile solvent to the non-volatile liquid is 0.4 or more and 8 or less.

4. The solder flux according to claim 1 or 2, wherein the non-volatile liquid is at least one selected from the group consisting of epoxy resins, acrylic resins, silicone resins, ionic liquids, amphiphilic liquids, oils, and glycol ethers.

5. The solder flux according to claim 4, wherein the non-volatile liquid is at least one selected from the group consisting of silicone resins, ionic liquids, amphiphilic liquids, and glycol ethers.

6. The solder flux according to claim 4, wherein the non-volatile liquid has an OH group in its molecule.

7. A solder paste comprising solder particles and the solder flux described in claim 1 or 2.

8. A method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands for mounting electronic components or a plurality of solder precoats formed on the plurality of lands; a second step of applying the solder flux described in claim 1 to the plurality of lands or the plurality of solder precoats; a third step of mounting electronic components, including electronic components of size 0402 or smaller as defined by JIS standards, on the plurality of lands or the plurality of solder precoats to which the solder flux has been applied, after the second step; and a fourth step of heating the circuit board on which the electronic components are mounted and soldering them.

9. A method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands for mounting electronic components or a plurality of solder precoats formed on the plurality of lands; a second step of applying the solder paste described in claim 7 to the plurality of lands or the plurality of solder precoats; a third step of mounting electronic components, including electronic components of size 0402 or smaller as defined by JIS standards, to the plurality of lands or the plurality of solder precoats to which the solder paste has been applied, after the second step; and a fourth step of heating and soldering the circuit board on which the electronic components have been mounted.

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