Method for producing electronic component-mounted substrate, and solder flux

Abstract

A method for producing an electronic component-mounted substrate according to the present disclosure comprises: a first step for preparing a circuit substrate that has a plurality of lands, on at least part of which is formed a solder precoat coated with an organic film; a second step for placing an electronic component of not more than a JIS-specified 0402 size on the solder precoat coated with the organic film; and a third step for heating the circuit substrate on which the electronic component was placed in the second step and soldering the electronic component to the plurality of lands. The electronic component has: an electronic component body which has a pair of end faces, an upper surface, a lower surface, and a pair of side surfaces; and a pair of terminals which are respectively disposed on both end-face sides of the electronic component body so as to contact at least the lower surface. The pair of terminals are soldered to a first land by melting and solidifying the solder precoat in the third step. The organic film contains three or more thermoplastic resins that have differing softening points. The softening points of the three or more thermoplastic resins are each not more than the melting point of solder contained in the solder precoat.

Description

Method for manufacturing an electronic component mounting substrate and solder flux

[0001] The present invention relates to a method for manufacturing an electronic component mounting substrate and a solder flux. More specifically, the present invention relates to a method for manufacturing an electronic component mounting substrate and a solder flux used in the 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 mount minute electronic components on a circuit board at a high density with a narrow pitch. As a technique for mounting minute electronic components at a high density, a method using solder precoat has been studied (Patent Document 1).

[0003] International Publication No. 2022 / 195937

[0004] However, there remains room for further improvement in order to improve the production yield of electronic component mounting substrates, and in particular, it has been necessary to suppress the formation of solder bridges between adjacent electronic components.

[0005] Therefore, the present disclosure provides a method for manufacturing an electronic component mounting substrate capable of suppressing the formation of solder bridges between adjacent electronic components, and a solder flux.

[0006] One 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 on which a solder precoat, at least a portion of which is covered with an organic film, is formed; a second step of mounting electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat covered with the organic film; and a third step of heating the circuit board on which the electronic components are mounted in the second step to solder the electronic components to each of the plurality of lands, wherein the electronic components have an electronic component body having a pair of end faces, an upper surface, a lower surface, and a pair of side surfaces, and a pair of terminals arranged on both end faces of the electronic component body in contact with at least the lower surface, the pair of terminals are soldered to the lands by melting and solidifying the solder precoat in the third step, and the organic film contains three or more thermoplastic resins with different softening points, the softening point of each of the three or more thermoplastic resins being below the melting point of the solder precoat.

[0007] Another aspect of the present invention relates to a solder flux used in a method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands on which a solder precoat covered with a flux residue film is formed; a second step of mounting electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat; and a third step of heating the circuit board on which the electronic components are mounted in the second step to solder the electronic components to the plurality of lands, wherein the solder flux is disposed on the surface of the flux residue film in the second step to temporarily fix the electronic components to each of the solder precoats, and comprises a first thermoplastic resin and a second thermoplastic resin, wherein the softening point T1 of the first thermoplastic resin and the softening point T2 of the second thermoplastic resin are different.

[0008] According to this disclosure, a method for manufacturing an electronic component mounting substrate and a solder flux can be provided that can suppress the formation of solder bridges between adjacent electronic components.

[0009] 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 showing an example of a circuit board used in 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 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] (Method for manufacturing an electronic component mounting substrate) The method for manufacturing an electronic component mounting substrate according to the embodiment of the present disclosure comprises: a first step of preparing a circuit board having a plurality of lands on which a solder precoat, at least a portion of which is covered with an organic film, is formed; a second step of mounting electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat covered with the organic film; and a third step of heating the circuit board on which the electronic components were mounted in the second step to solder the electronic components to each of the plurality of lands.

[0014] In the method for manufacturing an electronic component mounting substrate according to the embodiment of the present disclosure, the electronic component comprises an electronic component body having a pair of end faces, an upper surface, a lower surface, and a pair of side surfaces, and a pair of terminals arranged on both end faces of the electronic component body in contact with at least the lower surface. In the method for manufacturing an electronic component mounting substrate according to the embodiment of the present disclosure, the pair of terminals are soldered to the lands by melting and solidifying a solder precoat in the third step.

[0015] In the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, the organic film contains three or more thermoplastic resins with different softening points, and the softening point of each of the three or more thermoplastic resins is below the melting point of the solder precoat.

[0016] In the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, it is important that (i) in the first step, a circuit board having a plurality of lands on which a solder precoat is formed, at least a portion of which is covered with an organic film, is prepared; (ii) the organic film contains three or more types of thermoplastic resins with different softening points; and (iii) the softening point of each of the three or more types of thermoplastic resins is below the melting point of the solder precoat. The reasons for this are explained below.

[0017] When using a circuit board with multiple lands, where a solder precoat is applied to cover each of the lands, and at least a portion of the solder precoat is covered with an organic film, and when using electronic components mounted on the lands, each of the pair of terminals is provided on both end faces of the electronic component body, in contact with at least the bottom surface, in the electronic component, each of the pair of terminals is mounted on the solder precoat via the organic film that covers at least a portion of the solder precoat. The electronic component mounted in this way is soldered to the pair of lands via the pair of terminals by melting and solidifying the solder precoat. In this way, an electronic component mounting board with electronic components mounted on a circuit board is manufactured.

[0018] The organic films described above are formed by solder flux or by a combination of flux residue film and solder flux. When the organic film is formed by a combination of flux residue film and solder flux, the flux residue film is arranged to cover at least a portion of the solder precoat, and the solder flux is arranged to cover at least a portion of the flux residue film. Therefore, in electronic components, each of the pair of terminals is mounted on the solder precoat via the solder flux contained in the organic film. The flux residue film is, for example, a residue film formed when a solder paste containing solder particles and solder flux is supplied to a land when forming a solder precoat on the land, and then the solder flux contained in this solder paste is heated. Such a flux residue film is a solid component in which much of the liquid component contained in the solder flux has evaporated due to heating.

[0019] Solder flux contains a thermoplastic resin as its main component. When an organic film is formed with solder flux containing one type of thermoplastic resin as its main component, heating the circuit board during the manufacturing process of an electronic component mounting board as described above causes the thermoplastic resin to reach its softening point before the solder precoat melts, resulting in a sudden increase in the fluidity of the entire organic film.

[0020] In this case, when tiny electronic components smaller than the 0402 size specified by JIS standards are mounted on the pads, the organic film, whose fluidity has suddenly increased, can cause the tiny electronic components to move. For example, adjacent tiny electronic components may end up in close proximity to each other. In high-density mounting, where the spacing between adjacent tiny electronic components is kept as narrow as possible, if the tiny electronic components move and end up in close proximity to each other, a portion of the molten solder precoat can get between the opposing end faces of adjacent tiny electronic components, making it easier for solder bridges to form between them.

[0021] In high-density mounting, the distance between the lands corresponding to each of the tiny electronic components soldered adjacent to each other on a land, or in other words, the distance between opposing lands in a pair of lands on which tiny electronic components are mounted adjacent to each other, is designed to be extremely short (for example, 0.02 mm to 0.10 mm). In this case, the occurrence of solder bridges becomes apparent.

[0022] However, the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure has the configurations (i) to (iii) above. Here, when the organic film is formed with a solder flux containing three or more thermoplastic resins with different softening points as the main resin, the fluidity of the organic film can be gradually improved each time the softening point of each of the three or more thermoplastic resins is reached. That is, the fluidity of the organic film can be improved in stages. As a result, compared to when the fluidity of the entire organic film is improved all at once, it is possible to suppress large fluctuations in the position of minute electronic components, and thus suppress the positional relationship between adjacent minute electronic components. In addition, when the organic film is formed with a solder flux containing two thermoplastic resins with different softening points as the main resin, the fluidity of the organic film can be improved in two stages according to the softening points of the two thermoplastic resins. However, since minute electronic components are extremely lightweight, when the fluidity of the organic film is improved in two stages, it is not always possible to suppress large fluctuations in the position of minute electronic components, and adjacent minute electronic components may end up in close proximity.

[0023] Furthermore, when an organic film is formed by combining a flux residue film and solder flux, the flux residue film is a solid component as explained above, so its fluidity is not easily improved by heating. Therefore, even if the solder flux contains one type of thermoplastic resin and the flux residue film contains the above-mentioned thermoplastic resin and two or more thermoplastic resins with different softening points, it is possible to suppress the improvement in the overall fluidity of the organic film. Consequently, even in this case, it is possible to suppress large fluctuations in the position of minute electronic components compared to when the fluidity of the entire organic film is improved all at once, thus preventing adjacent minute electronic components from being in close proximity to each other.

[0024] As described above, in the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, the fluidity of the entire organic film is dramatically improved, and as a part of the solder precoat is interposed between the opposing end faces of adjacent minute electronic components, the occurrence of solder bridges between them can be suppressed.

[0025] Hereinafter, a method for manufacturing an electronic component mounting substrate according to one embodiment of this disclosure will be described step by step with reference to the drawings. Before describing each step, an example of a circuit board used in the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure will be described. Furthermore, in the following, electronic components of size 0402 or smaller as defined by the JIS standard will be described as first electronic components, and electronic components with a larger volume than the first electronic components will be described as second electronic components.

[0026] [Circuit Board] As shown in Figure 2, one example of a circuit board 1 has a substrate portion 1a and a plurality of lands 2 arranged on the main surface of the substrate portion 1a. As shown in Figure 2, the plurality of lands 2 have at least a plurality of first lands 2A and a plurality of second lands 2B. In Figure 2, an example is shown in which the circuit board 1 has four first lands 2A and two second lands 2B, with a pair of first lands 2A arranged adjacent to each other, but the number of first lands 2A and second lands 2B is not limited to this. Also, in the circuit board 1, a pair of second lands 2B may be arranged adjacent to each other.

[0027] 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 a circuit board 1 is a printed circuit board.

[0028] The first land 2A is a land for mounting a first electronic component of size 0402 or smaller as defined by the JIS standard. As shown in Figure 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 faces of the electronic component body 5a. In other words, the first electronic component 5 is a chip component. Examples of chip components include chip resistors, chip capacitors, and chip inductors. Details of the configuration of the first electronic component 5 will be described later. 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.

[0029] As shown in Figure 2, a solder precoat 4 is formed on at least a portion of the surface of the first land 2A, and the solder precoat 4 is covered with a flux residue film 7. Furthermore, as shown in Figure 3, at least a portion of the flux residue film 7 on the first land 2A is covered with solder flux 3. In other words, on the first land 2A, an organic film 9 is formed by combining the flux residue film 7 and the solder flux 3. Note that, as shown in Figure 2, the flux residue film 7 may be connected across multiple first lands 2A. Since the first land 2A is a land on which a minute-sized first electronic component is mounted, the distance between adjacent first lands 2A is extremely small. For this reason, the flux residue film 7 may be formed to be connected across multiple first lands 2A.

[0030] In adjacent pairs of first lands 2A, the distance between the opposing first lands 2A (distance L shown in Figure 2) may be 0.02 mm or more and 0.10 mm or less. A distance L of 0.02 mm or more suppresses the formation of solder bridges between the respective end faces of adjacent first electronic components 5, and a distance L of 0.10 mm or less allows for high-density mounting of the first electronic components 5 on the circuit board 1. The distance L may also be 0.02 mm or more and 0.07 mm or less. A distance L of 0.07 mm or less allows for even higher density mounting of the first electronic components 5 on the circuit board 1.

[0031] Furthermore, as shown in Figure 4, when the first electronic component 5 is mounted on the first land 2A, distance L is also the distance between the first lands 2A corresponding to each of the first electronic component 5 soldered adjacent to each other on the first land 2A. In addition, distance L is also the distance between the opposing first lands 2A in a pair of first lands 2A on which the first electronic component 5 is mounted adjacent to each other.

[0032] The second land 2B is a land on which the second electronic component 8 is mounted. The second electronic component 8 is an electronic component with a larger volume than the first electronic component 5. As shown in Figure 4, the second electronic component 8 may have an electronic component body 8a and a pair of second terminals 8b arranged on both end faces of the electronic component body 8a. In other words, the second electronic component 8 may be a chip component. Details of the configuration of the second electronic component 8, which is a chip component, will be described later. When the second electronic component 8 is a chip component, the second electronic component 8, like the first electronic component 5, has each of its pair of second terminals 8b mounted on each of the pair of second lands 2B.

[0033] As shown in Figure 2, a solder precoat 4 is formed on at least a portion of the surface of the second land 2B, and the solder precoat 4 is covered with a flux residue film 7. Also, as shown in Figure 3, at least a portion of the flux residue film 7 on the second land 2B is covered with solder flux 3. In other words, on the second land 2B, an organic film 9 is formed by combining the flux residue film 7 and the solder flux 3.

[0034] In the following, an example of manufacturing an electronic component mounting board according to one embodiment of this disclosure will be described using a circuit board 1 shown in Figure 2. Furthermore, in the following, an example will be described in which a second electronic component 8, which is a chip component, is used as the electronic component mounted on the second land 2B.

[0035] <First Step> In the first step of the method for manufacturing an electronic component mounting substrate according to one embodiment of the present disclosure, a circuit board 1 having a plurality of first lands 2A on which a solder precoat 4, at least a portion of which is covered with an organic film 9, is formed, as shown in Figure 3. The first step may be carried out by purchasing a commercially available circuit board having a plurality of first lands 2A on which a solder precoat 4 covered with an organic film 9 is formed (first method). Alternatively, the first step may be carried out by forming an organic film 9 on each of the plurality of first lands 2A on the circuit board 1 having a plurality of first lands 2A (second method). The second method will be described below. Furthermore, an example in which the organic film 9 is formed by combining a flux residue film 7 and solder flux 3 will be described below.

[0036] In the second method, as shown in Figure 3, in addition to covering at least a portion of each of the multiple first lands 2A with the organic film 9, at least a portion of each of the multiple second lands 2B is also covered with the organic film 9. Specifically, in the second method, first, solder paste is applied to at least a portion of each of the multiple first lands 2A, and then solder paste is applied to at least a portion of each of the multiple second lands 2B. After that, the circuit board 1 is heated to form a solder precoat 4 and a flux residue film 7 that covers at least a portion of the solder precoat 4. Then, solder flux 3 is applied to each of the multiple first lands 2A and the multiple second lands 2B so as to cover at least a portion of the flux residue film 7.

[0037] Preferably, the organic film 9 is composed of a flux residue film 7 and solder flux 3 covering at least a portion of the flux residue film 7, as described above. Since the flux residue film 7 is a solid component as explained above, its fluidity does not easily improve with heating. Therefore, by including the flux residue film 7 in the organic film 9, it is possible to suppress the improvement of the overall fluidity of the organic film.

[0038] The solder precoat 4 is formed in an amount sufficient to ensure adequate bonding strength with the electronic components. The solder precoat 4 is formed by coating the surface of the land with solder for soldering electronic components. For example, it can be formed by supplying solder paste containing solder particles to the land, melting the solder particles contained in the solder paste, wetting and spreading it on the land, and then solidifying the solder. Therefore, the solder precoat 4 does not include the solder paste that was supplied to the land but before the solder particles were melted. The solder precoat 4 may be formed with solder obtained by an electrolytic plating method, or with solder generated by heating a special material applied to the land.

[0039] The method for applying the solder paste is not particularly limited, and various known application methods can be used. For example, stencil printing, dispensing, and spraying methods can be used. Stencil printing is preferable because it allows for the formation of a solder paste layer of uniform thickness in one step. Stencil printing may also be a screen printing method. Screen printing may be performed using a mask with through holes corresponding to the land positions. Screen printing may also be a contact printing method using a metal mask, for example. Contact printing is suitable for precisely controlling the amount of solder paste applied. Screen printing may also be a printing method using a mesh screen (off-contact printing method). Off-contact printing can print solder paste at a higher speed than contact printing. Therefore, by adopting off-contact printing as the screen printing method, productivity can be increased compared to adopting contact printing.

[0040] The typical thickness of a mask used in screen printing is about 60 μm. On the other hand, when screen printing is performed on a circuit board 1 having a first land 2A on which a tiny electronic component such as a first electronic component is mounted, the thickness of the mask is preferably 40 μm or less, and more preferably 30 μm or less.

[0041] The solder precoat 4 can be formed by heating a circuit board 1 having a plurality of first lands 2A and a plurality of second lands 2B to which solder paste has been applied, melting the solder particles contained in the solder paste in each of the plurality of first lands 2A and a plurality of second lands 2B, and then cooling the circuit board 1. At this time, the solder precoat 4 is covered with a flux residue film 7 formed by the solder flux contained in the solder precoat 4. As a result, the circuit board 1 has a plurality of first lands 2A and a plurality of second lands 2B to which the solder precoat 4 covered with the flux residue film 7 has been formed, as shown in Figure 2. The flux residue film 7 can be removed by washing if necessary.

[0042] Solder paste contains solder particles and solder flux. Solder particles have a melting point of 200°C or higher. The melting point of solder particles may be 210°C or higher, or 215°C or higher. The melting point of solder particles may be 240°C or lower, 230°C or lower, or 225°C or lower. It is preferable to use lead-free solder particles. Solder paste contains solder particles and solder flux in a volume ratio (volume ratio at 25°C) of, for example, solder particles:solder flux = 8:2 to 6:4. The median diameter (D50) in the volume-based particle size distribution of solder particles is desirable to be as small as possible.

[0043] Solder flux comprises at least a main resin and an activator. In the method for manufacturing an electronic component mounting substrate according to the embodiments of this disclosure, the solder flux contained in the solder paste includes a thermoplastic resin as the main resin. In addition to the main resin and activator, the solder flux may also contain a thixotropic agent and a solvent. Solder flux can be prepared by mixing these components.

[0044] As the main resin, for example, it is preferable to use a rosin-based resin that itself has reducibility, but other resins may also be used. The main resin may be used alone or in combination of two or more kinds.

[0045] Examples of the rosin-based resin include natural rosins such as gum rosin and wood rosin, and derivatives thereof (polymerized rosin, hydrogenated rosin, disproportionated rosin, acid-modified rosin, rosin ester, etc.).

[0046] Examples of other resins other than the rosin-based resin include terpene resin, terpene phenol resin, styrene resin, xylene resin, acrylic resin, polyester resin, polyolefin resin, polyamide, polyamine, phenol resin, phenoxy resin, and epoxy resin, etc.

[0047] Incidentally, each of the various main resins described above is classified as a thermoplastic resin. Among the various main resins described above, from the viewpoint of having reducibility itself and exhibiting excellent adhesiveness, it is preferable to use a rosin-based resin.

[0048] Examples of the thixotropic agent (thixotropy-imparting agent) include wax-based thixotropic agents, amide-based thixotropic agents, and sorbitol-based thixotropic agents, etc. The thixotropic agent may be used alone or in combination of two or more kinds.

[0049] As the activator, a compound that reduces the oxide film covering the surface of the solder used for soldering electronic components is used. Examples of the solder include solder precoat 4. Also, when solder plating is provided on the surface of the terminals of electronic components such as the first electronic component, the solder contained in such solder plating is also included. Incidentally, the main resin such as the rosin-based resin may have a certain degree of activation effect. Therefore, in this specification, the activator means a compound other than the main resin such as the rosin-based resin and having an activation effect.

[0050] 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. Organic acids are preferred as the activator.

[0051] 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.

[0052] Examples of solvents include water, alcohol-based solvents, glycol-based solvents, ketone-based solvents, hydrocarbon-based solvents, ester-based solvents, glycol ether-based solvents, and terpionelles. A single solvent may be used, or two or more may be used in combination.

[0053] Solder flux may contain other components besides those listed above. Other components include surfactants, silane coupling agents, antioxidants, and colorants.

[0054] In the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, the organic film 9 covering at least a portion of the solder precoat 4 may be formed by solder flux 3 supplied so as to cover at least a portion of the solder precoat 4. In addition, a flux residue film 7 is usually formed on the solder precoat 4 by solder flux so as to cover at least a portion of it. Therefore, if a flux residue film 7 is formed, the flux residue film 7 is removed, and then solder flux 3 is applied so as to cover at least a portion of the solder precoat 4 to form the organic film 9.

[0055] In the method for manufacturing an electronic component mounting substrate according to the embodiments of this disclosure, the organic film 9 includes three or more thermoplastic resins with different softening points. The thermoplastic resin is a thermoplastic resin included as a main component resin in solder paste or solder flux. The thermoplastic resin included as a main component resin in solder paste is included in the flux residue film 7 of the organic film 9, and the thermoplastic resin included as a main component resin in solder flux is included in the solder flux film 3 of the organic film 9.

[0056] In the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, the softening points of each of the three or more thermoplastic resins are below the melting point of the solder precoat 4. In this case, when soldering the first electronic component 5 to the first land 2A, the solder precoat 4 can be sufficiently wetted and spread over the surface of the first terminal 5b of the first electronic component 5. This allows the first electronic component 5 to be sufficiently bonded to the first land 2A.

[0057] When the organic film 9 is composed of a flux residue film 7 and solder flux 3 covering at least a portion of the flux residue film 7 as described above, it is preferable that the flux residue film 7 contains at least one of three or more thermoplastic resins with different softening points, and that the solder flux 3 contains at least one of three or more thermoplastic resins with different softening points. For example, when the organic film 9 contains three thermoplastic resins with different softening points, the flux residue film 7 may contain one thermoplastic resin and the solder flux 3 may contain two thermoplastic resins, or the flux residue film 7 may contain two thermoplastic resins and the solder flux 3 may contain one thermoplastic resin. Furthermore, if the organic film 9 contains four types of thermoplastic resins with different softening points, the flux residue film 7 may contain one type of thermoplastic resin and the solder flux 3 may contain three types of thermoplastic resins, or the flux residue film 7 may contain two types of thermoplastic resins and the solder flux 3 may contain two types of thermoplastic resins, or the flux residue film 7 may contain three types of thermoplastic resins and the solder flux 3 may contain one type of thermoplastic resin.

[0058] When the organic film 9 is composed of a flux residue film 7 and solder flux 3 covering at least a portion of the flux residue film 7, as described above, it is particularly preferable that the flux residue film 7 contains one of three or more thermoplastic resins with different softening points, and the solder flux 3 contains two or more of the three or more thermoplastic resins with different softening points. In this case, in addition to further improving the fluidity of the organic film 9 in stages, the first electronic component 5 can be bonded to the first land 2A even more securely.

[0059] In the first step, as shown in Figure 3, a flux placement substep is performed in which solder flux 3 is placed so as to cover at least a portion of the flux residue film 7 formed on a plurality of first lands 2A and a plurality of second lands 2B. This makes it possible to form an organic film 9 in which the flux residue film 7 and solder flux 3 are combined. The flux placement substep may also be performed on a solder precoat 4 from which the flux residue film 7 has been removed. This makes it possible to form an organic film 9 with solder flux 3.

[0060] The flux placement substep can be carried out by applying solder flux so as to cover at least a portion of the flux residue film 7 formed on the plurality of first lands 2A and the plurality of second lands 2B. The method of applying the solder flux is not particularly limited, and various known application methods can be used. For example, stencil printing, dispensing, and spraying can be used. Among these, stencil printing is preferred. Stencil printing may be a screen printing method. Screen printing can be carried out in the same manner as described in the section on the method of applying the solder precoat 4.

[0061] <Second Step> In the second step of the method for manufacturing an electronic component mounting substrate according to one embodiment of the present disclosure, as shown in Figure 4, in addition to mounting a first electronic component 5 of size 0402 or smaller as defined by JIS standards on each of the plurality of first lands 2A via a solder precoat 4, a second electronic component 8 is mounted on each of the plurality of second lands 2B. The mounting of these electronic components can be carried out by a known component mounting device.

[0062] Now, let's explain the first electronic component 5 and the second electronic component 8.

[0063] The size of the first electronic component 5 is defined by JIS standards such as 0402, 03015, 0201, and 01005. Note that an electronic component of size 0402 means an electronic component with dimensions of 0.4 mm in length and 0.2 mm in width; an electronic component of size 03015 means an electronic component with dimensions of 0.3 mm in length and 0.15 mm in width; an electronic component of size 0201 means an electronic component with dimensions of 0.25 mm in length and 0.125 mm in width; and an electronic component of size 01005 means an electronic component with dimensions of 0.1 mm in length and 0.05 mm in width. The first electronic component 5 is smaller in size and extremely lightweight compared to the second electronic component 8, which will be described later.

[0064] The first electronic component 5 typically has a rectangular shape and comprises an electronic component body 5a having a pair of end faces, an upper surface, a lower surface, and a pair of side surfaces, and a pair of first terminals 5b disposed on at least one of the end faces and the lower surface of the electronic component body 5a and having solder plating 6 on their surfaces (see Figures 4 and 5). The end faces are the side surfaces in the longitudinal direction of the electronic component. The lower surface is the surface mounted on the circuit board 1, and the upper surface is the surface opposite to the lower surface. Furthermore, the side surfaces are surfaces perpendicular to all of the end faces, upper surface, and lower surface.

[0065] The second electronic component 8 is an electronic component with a larger volume than the first electronic component 5. The second electronic component 8 may be a chip component, as shown in Figure 4. When the second electronic component 8 is a chip component, the dimension of the longer side (length) exceeds 0.4 mm, and the dimension of the shorter side (width) exceeds 0.2 mm. The chip-type second electronic component 8 also usually has a rectangular shape.

[0066] If the second electronic component 8 is a chip component, its size is, for example, 0603 and 1005 as defined by JIS standards. A 0603 size electronic component means an electronic component with dimensions of 0.6 mm in length and 0.3 mm in width, and a 1005 size electronic component means an electronic component with dimensions of 1.0 mm in length and 0.5 mm in width.

[0067] The second electronic component 8, which is a chip component, has a rectangular shape as described above and comprises an electronic component body 8a having a pair of end faces, an upper surface, a lower surface, and a pair of side surfaces, and a pair of second terminals 8b arranged on both end faces of the electronic component body 8a in contact with at least the lower surface (see Figure 4).

[0068] The second electronic component 8 may be an electronic component other than a chip component, which has a larger volume than the first electronic component 5. In this case, the second electronic component 8 may be, for example, a flat package, a connector, a ball grid array, or a cylindrical capacitor.

[0069] <Third Step> In the third step of the method for manufacturing an electronic component mounting substrate according to one embodiment of the present disclosure, the circuit board 1 on which the first electronic component 5 and the second electronic component 8 are mounted in the second step is heated, and in addition to soldering the first electronic component 5 to each of the plurality of first lands 2A, the second electronic component 8 is soldered to each of the plurality of second lands 2B. In the third step, 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 8b are soldered to the second lands 2B in the third step by melting and solidifying the solder precoat 4.

[0070] The circuit board 1 on which the first electronic component 5 and the second electronic component 8 are mounted can be heated, for example, using a heating device such as a reflow oven.

[0071] In the third step, the solder precoat 4 applied to 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 precoat 4 is then spread over the contact surface with the first terminal 5b and the contact surface with the second terminal 8b, respectively, and the temperature inside the reflow oven is lowered. As a result, the spread solder precoat 4 solidifies, soldering the first terminal 5b to the first land 2A and soldering the second terminal 8b to the second land 2B. In Figure 5, the molten and solidified solder precoat 4 is shown as the solder portion 4a.

[0072] As explained above, a solder precoat 4 covered with an organic film 9 is formed on the first land 2A, and the first electronic component 5 is mounted on the first land 2A via the organic film 9. The organic film 9 contains three or more thermoplastic resins with different softening points. Therefore, when the solder precoat 4 is melted, the fluidity of the organic film 9 can be improved in stages, which suppresses the positional relationship between adjacent first electronic components 5. As a result, the solder precoat 4 intervenes between adjacent first electronic components 5, preventing them from connecting to each other. In other words, it is possible to suppress the formation of solder bridges between adjacent first electronic components 5.

[0073] (Solder Flux) The solder flux according to the embodiment of this disclosure is used in a method for manufacturing an electronic component mounting substrate, which comprises: a first step of preparing a circuit board having a plurality of lands on which a solder precoat covered with a flux residue film is formed; a second step of mounting electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat; and a third step of heating the circuit board on which the electronic components were mounted in the second step to solder the electronic components to the plurality of lands.

[0074] The solder flux according to the embodiment of this disclosure is placed on the surface of the flux residue film in the second step of the manufacturing method described above to temporarily fix electronic components to each of the solder precoats. The solder flux according to the embodiment of this disclosure comprises a first thermoplastic resin and a second thermoplastic resin. In the solder flux according to the embodiment of this disclosure, the softening point T1 of the first thermoplastic resin and the softening point T2 of the second thermoplastic resin are different. Note that the softening points T1 and T2 may satisfy the relationship T1 < T2.

[0075] A solder flux containing a first thermoplastic resin and a second thermoplastic resin is used, for example, to cover a solder precoat covered with a flux residue film. In this case, the flux residue film must contain a third thermoplastic resin having a different softening point than the first and second thermoplastic resins. In this case, the relationship T2 < T3 may be satisfied between the softening point T2 and the softening point T3 of the third thermoplastic resin.

[0076] The softening points T1, T2, and T3 may differ by 10°C or more, and may be lower than the melting point of the solder precoat. A difference of 10°C or more between the softening points T1, T2, and T3 results in a more gradual improvement in the fluidity of the organic film. Furthermore, because the softening points T1, T2, and T3 are lower than the melting point of the solder precoat, the activator contained in the softened organic film can sufficiently remove the oxide film formed on the surface of the solder precoat before the solder precoat melts. This allows for suitable soldering of electronic components (e.g., the first electronic component 5) to the land (e.g., the first land 2A).

[0077] A solder flux containing a first thermoplastic resin and a second thermoplastic resin may be used to cover a solder precoat from which the flux residue film has been removed. In this case, the solder precoat needs to be further covered with a solder flux containing a thermoplastic resin (e.g., a third thermoplastic resin) with a different softening point than the first and second thermoplastic resins.

[0078] (Note) The following technologies are disclosed in accordance with the above description. (Technology 1) A method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands on which a solder precoat, at least a portion of which is covered with an organic film, is formed; a second step of mounting an electronic component of size 0402 or smaller as defined by JIS standards onto the solder precoat covered with the organic film; and a third step of heating the circuit board on which the electronic component is mounted in the second step to solder the electronic component to each of the plurality of lands, wherein the electronic component has an electronic component body having a pair of end faces, an upper surface, a lower surface, and a pair of side surfaces, and a pair of terminals arranged on both end faces of the electronic component body in contact with at least the lower surface, the pair of terminals are soldered to the lands by melting and solidifying the solder precoat in the third step, the organic film contains three or more thermoplastic resins with different softening points, and the softening point of each of the three or more thermoplastic resins is below the melting point of the solder precoat. (Technical 2) The method for manufacturing an electronic component mounting substrate according to Technical 1, wherein the organic film is formed by solder flux supplied to the solder precoat so as to cover at least a portion of the solder precoat. (Technical 3) The method for manufacturing an electronic component mounting substrate according to Technical 1, wherein the organic film is formed by a flux residue film generated when forming the solder precoat and solder flux supplied so as to cover at least a portion of the flux residue film. (Technical 4) The method for manufacturing an electronic component mounting substrate according to Technical 3, wherein the flux residue film contains at least one of three or more thermoplastic resins with different softening points, and the solder flux contains at least one of three or more thermoplastic resins with different softening points. (Technical 5) The method for manufacturing an electronic component mounting substrate according to Technical 4, wherein the solder flux contains two or more of the three or more thermoplastic resins with different softening points.(Technical 6) A solder flux used in a method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands on which a solder precoat covered with a flux residue film is formed; a second step of mounting electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat; and a third step of heating the circuit board on which the electronic components are mounted in the second step to solder the electronic components to the plurality of lands, wherein the solder flux is placed on the surface of the flux residue film in the second step to temporarily fix the electronic components to each of the solder precoats, and comprises a first thermoplastic resin and a second thermoplastic resin, wherein the softening point T1 of the first thermoplastic resin and the softening point T2 of the second thermoplastic resin are different. (Technical 7) The solder flux according to Technical 6, wherein the flux residue film comprises a third thermoplastic resin, and the softening point T1, the softening point T2, and the softening point T3 of the third thermoplastic resin are different. (Technical 8) The solder flux according to Technical 7, wherein the softening points T1, T2, and T3 differ by 10°C or more and are lower than the melting point of the solder precoat.

[0079] 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.

[0080] (Example 1) In a circuit board having a plurality of first lands, solder paste (solder cream) was applied to each of the plurality of first lands by screen printing, and then the circuit board was heated to melt the solder particles contained in the solder paste. The solder paste contained solder flux containing one type of thermoplastic resin (softening point 102.0°C) in addition to the solder particles. Next, the circuit board was cooled to form a solder precoat covered with a flux residue film on each of the plurality of first lands.

[0081] Next, the solder flux according to Example 1 was applied to multiple first lands by screen printing so as to cover the flux residue film. This formed an organic film on multiple first lands having a flux residue film and solder flux covering the flux residue film. The solder flux according to Example 1 contained three types of rosin-based resins (rosin 1 to 3 (first rosin to third rosin)) with different softening points as thermoplastic resins, an activator, a thixotropic agent, and a solvent. Rosin 1 was Shiragiku rosin (manufactured by Arakawa Chemical Industries, Ltd.) with a softening point of 82.0°C, rosin 2 was Aradaimm 95 (manufactured by Arakawa Chemical Industries, Ltd.) with a softening point of 96.6°C, and rosin 3 was Marquid No. 1 with a softening point of 142.5°C. The solvent was 31 (manufactured by Arakawa Chemical Industries, Ltd.), the activator was adipic acid (manufactured by Tokyo Chemical Industries Wako Pure Chemical Industries, Ltd.), the thixotropic agent was castor wax (manufactured by NOF Corporation), and the solvent was hexyl carbitol (manufactured by Tokyo Chemical Industries Wako Pure Chemical Industries, Ltd.). In other words, the organic film according to Example 1 contained a total of four thermoplastic resins: one thermoplastic resin contained in the flux residue film and three thermoplastic resins contained in the solder flux. The mixing ratio of each component is shown in Table 1A below. In Table 1A below, "○" indicates that the organic film has a flux residue film, and "×" indicates that the organic film does not have a flux residue film.

[0082] (Example 2) In Example 2, an organic film was formed on multiple first lands in the same manner as in Example 1, except that the solder flux used contained rosin 1 and 2 in the mixing ratios shown in Table 1A below. The organic film had a flux residue film and solder flux covering the flux residue film. That is, the organic film in Example 2 contained a total of three thermoplastic resins: one thermoplastic resin contained in the flux residue film and two thermoplastic resins contained in the solder flux.

[0083] (Example 3) In Example 3, an organic film was formed on multiple first lands in the same manner as in Example 1, except that the solder flux used contained rosin 1 and 3 in the mixing ratios shown in Table 1A below. The organic film had a flux residue film and solder flux covering the flux residue film. That is, the organic film in Example 3 contained a total of three thermoplastic resins: one thermoplastic resin contained in the flux residue film and two thermoplastic resins contained in the solder flux.

[0084] (Example 4) In Example 4, an organic film was formed on multiple first lands in the same manner as in Example 1, except that the solder flux used in Example 4 contained rosin 2 and 3 in the mixing ratios shown in Table 1A below. The organic film had a flux residue film and solder flux covering the flux residue film. That is, the organic film in Example 4 contained a total of three thermoplastic resins: one thermoplastic resin contained in the flux residue film and two thermoplastic resins contained in the solder flux.

[0085] (Example 5) Except for removing the flux residue film and covering the solder precoat with solder flux, an organic film containing solder flux was formed on multiple first lands in the same manner as in Example 1. That is, the organic film according to Example 5 contained three types of thermoplastic resins contained in solder flux. The mixing ratios of each component of the solder flux according to Example 5 are shown in Table 1A below.

[0086]

[0087] (Comparative Example 1) Except for removing the flux residue film and covering the solder precoat with solder flux, an organic film containing solder flux was formed on multiple first lands in the same manner as in Example 2. That is, the organic film according to Comparative Example 1 contained two types of thermoplastic resins contained in solder flux. The mixing ratio of each component of the solder flux according to Comparative Example 1 is shown in Table 1B below. In Table 1B below, "○" indicates that the organic film has a flux residue film, and "×" indicates that the organic film does not have a flux residue film.

[0088] (Comparative Example 2) Except for removing the flux residue film and covering the solder precoat with solder flux, an organic film containing solder flux was formed on multiple first lands in the same manner as in Example 3. That is, the organic film according to Comparative Example 2 contained two types of thermoplastic resins contained in solder flux. The mixing ratios of each component of the solder flux according to Comparative Example 2 are shown in Table 1B below.

[0089] (Comparative Example 3) Except for removing the flux residue film and covering the solder precoat with solder flux, an organic film containing solder flux was formed on multiple first lands in the same manner as in Example 4. That is, the organic film according to Comparative Example 3 contained two types of thermoplastic resins contained in solder flux. The mixing ratios of each component of the solder flux according to Comparative Example 3 are shown in Table 1B below.

[0090] (Comparative Example 4) Except for removing the flux residue film and covering the solder precoat with solder flux containing rosin 1, an organic film having solder flux was formed on multiple first lands in the same manner as in Example 1. That is, the organic film according to Comparative Example 4 contained one type of thermoplastic resin contained in the solder flux. The mixing ratio of each component of the solder flux according to Comparative Example 4 is shown in Table 1B below.

[0091] (Comparative Example 5) Except for covering the solder precoat with solder flux containing rosin 2, an organic film having solder flux was formed on multiple first lands in the same manner as in Comparative Example 4. That is, the organic film according to Comparative Example 5 contained one type of thermoplastic resin contained in the solder flux. The mixing ratio of each component of the solder flux according to Comparative Example 5 is shown in Table 1B below.

[0092] (Comparative Example 6) Except for covering the solder precoat with solder flux containing rosin 3, an organic film having solder flux was formed on multiple first lands in the same manner as in Comparative Example 4. That is, the organic film according to Comparative Example 6 contained one type of thermoplastic resin contained in the solder flux. The mixing ratio of each component of the solder flux according to Comparative Example 6 is shown in Table 1B below.

[0093]

[0094] [Evaluation] For each example of the manufacturing of an electronic component mounting board, the circuit board was prepared by mounting a first electronic component on multiple first lands (second step). The circuit board with the first electronic component mounted was then heated, and the first electronic component was soldered to each of the first lands to obtain the electronic component mounting board for each example. The first electronic component 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 first terminals arranged only on the pair of end faces and the bottom surface of the electronic component body. The first electronic component also had the 0402 size as defined by the JIS standard. In other words, the first electronic component was a chip component having the 0402 size. The first electronic component was soldered to the first lands via a pair of terminals. The shortest distance (distance L in Figure 2) between the first lands corresponding to each first electronic component soldered adjacent to each other on the first lands was 0.07 mm (70 μm). Soldering was performed using a reflow oven. In addition, 200 first electronic components were soldered to the circuit board in each example.

[0095] - For each example of suppressing solder bridge formation, the electronic component mounting substrate was evaluated to determine whether the formation of solder bridges between adjacent first electronic components was suppressed. The presence or absence of suppression of solder bridge formation was evaluated according to the following criteria: ◎: The distance between adjacent first electronic components is greater than 60 μm but less than 70 μm. ○: The distance between adjacent first electronic components is greater than 50 μm but 60 μm or less. ×: The distance between adjacent first electronic components is 50 μm or less.

[0096] The results of evaluating whether or not solder bridge formation was suppressed are shown in Table 2 below. In Table 2 below, the presence or absence of solder bridge formation suppression will be abbreviated as "solder bridge suppression".

[0097]

[0098] Table 2 shows that the evaluation of solder bridge suppression for the electronic component mounting substrates in each embodiment was good, with a score of ○ or higher. Furthermore, the evaluation of solder bridge suppression for the electronic component mounting substrate in Example 1 was particularly good, with a score of ◎. In contrast, the evaluation of solder bridge suppression for the electronic component mounting substrates in each comparative example was poor, with a score of ×. From these results, it can be seen that the organic film covering the solder precoat contains three or more thermoplastic resins with different softening points, thereby suppressing the formation of solder bridges between adjacent first electronic components.

[0099] 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.

[0100] The method for manufacturing an electronic component mounting substrate and the solder flux described herein can be used in applications where it is required to suppress the formation of solder bridges between adjacent electronic components.

[0101] 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: Flux residue film, 8: Second electronic component, 8a: Electronic component body, 8b: Second terminal, 9: Organic film

Claims

1. A method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands on which a solder precoat, at least partially covered with an organic film, is formed; a second step of mounting electronic components of size 0402 or smaller as defined by JIS standards, onto the solder precoat covered with the organic film; and a third step of heating the circuit board on which the electronic components are mounted in the second step to solder the electronic components to each of the plurality of lands, wherein the electronic component has an electronic component body having a pair of end faces, an upper surface, a lower surface, and a pair of side surfaces, and a pair of terminals arranged on both end faces of the electronic component body in contact with at least the lower surface, the pair of terminals are soldered to the lands by melting and solidifying the solder precoat in the third step, the organic film contains three or more thermoplastic resins with different softening points, and the softening point of each of the three or more thermoplastic resins is below the melting point of the solder precoat.

2. The method for manufacturing an electronic component mounting substrate according to claim 1, wherein the organic film is formed by solder flux supplied to the solder precoat so as to cover at least a portion of the solder precoat.

3. The method for manufacturing an electronic component mounting substrate according to claim 1, wherein the organic film is formed by a flux residue film generated when forming the solder precoat and solder flux supplied so as to cover at least a portion of the flux residue film.

4. The method for manufacturing an electronic component mounting substrate according to claim 3, wherein the flux residue film comprises at least one of three or more thermoplastic resins with different softening points, and the solder flux comprises at least one of three or more thermoplastic resins with different softening points.

5. The method for manufacturing an electronic component mounting substrate according to claim 4, wherein the solder flux comprises two or more of the three or more thermoplastic resins with different softening points.

6. A solder flux used in a method for manufacturing an electronic component mounting substrate, comprising: a first step of preparing a circuit board having a plurality of lands on which a solder precoat covered with a flux residue film is formed; a second step of mounting electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat; and a third step of heating the circuit board on which the electronic components are mounted in the second step to solder the electronic components to the plurality of lands, wherein the solder flux is placed on the surface of the flux residue film in the second step to temporarily fix the electronic components to each of the solder precoats, and comprises a first thermoplastic resin and a second thermoplastic resin, wherein the softening point T1 of the first thermoplastic resin and the softening point T2 of the second thermoplastic resin are different.

7. The solder flux according to claim 6, wherein the flux residue film contains a third thermoplastic resin, and the softening point T1, the softening point T2, and the softening point T3 of the third thermoplastic resin are different.

8. The solder flux according to claim 7, wherein the softening points T1, T2, and T3 differ by 10°C or more and are lower than the melting point of the solder precoat.

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