Manufacturing method for electronic component mounting boards

Abstract

The present invention provides a method for manufacturing an electronic component mounting substrate that can suppress the formation of solder bridges between adjacent electronic components. [Solution] The method for manufacturing an electronic component mounting substrate according to the present disclosure comprises: a first step of preparing a circuit board 1 having a plurality of first lands 2A on which a solder precoat 4 covered with a flux residue film 7 is formed; a second step of mounting first electronic components 5 of size 0402 or smaller onto the solder precoat 4 on the plurality of first lands 2A; and a third step of heating the circuit board on which the first electronic components 5 are mounted and soldering the first electronic components 5 to each of the first lands 2A. The first electronic component 5 has an electronic component body 5a and a pair of first terminals 5b. The pair of first terminals 5b are soldered to the first lands 2A in the third step by melting and solidifying the solder precoat 4. In the second step, the first terminals 5b are brought into direct contact with the flux residue film 7 covering the solder precoat 4 of each of the plurality of first lands 2A to mount the first electronic component 5.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an electronic component mounting substrate.

Background Art

[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).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, there is still 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.

Means for Solving the Problems

[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 first lands on which a solder precoat covered with a flux residue film is formed; a second step of mounting first electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat on the plurality of first lands; and a third step of heating the circuit board on which the first electronic components are mounted in the second step to solder the first electronic components to each of the first lands, wherein the first 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 first terminals arranged on both end faces of the electronic component body in contact with at least the lower surface, the pair of first terminals being soldered to the first lands by melting and solidifying the solder precoat in the third step, and the first terminals being brought into direct contact with the flux residue film covering the solder precoat of each of the plurality of first lands in the second step to mount the first electronic component. [Effects of the Invention]

[0007] According to this disclosure, it is possible to provide a method for manufacturing an electronic component mounting substrate that can suppress the formation of solder bridges between adjacent electronic components. [Brief explanation of the drawing]

[0008] [Figure 1] This is a flowchart of a method for manufacturing an electronic component mounting substrate according to an embodiment of the present disclosure. [Figure 2] This is a side cross-sectional view showing an example of a circuit board used in a method for manufacturing an electronic component mounting substrate according to an embodiment of the present disclosure. [Figure 3] This is a side cross-sectional view showing the flux placement process in which solder flux is placed on the second land. [Figure 4] This is a side cross-sectional view illustrating an example of a second step in a method for manufacturing an electronic component mounting substrate according to an embodiment of the present disclosure. [Figure 5] This is a side cross-sectional view illustrating an example of a third step in a method for manufacturing an electronic component mounting substrate according to an embodiment of the present disclosure. [Modes for carrying out the invention]

[0009] 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.

[0010] In the following explanation, when a lower and upper limit is given for 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 material may be selected and used alone, or two or more materials may be used in combination.

[0011] This disclosure includes any combination of two or more claims, which may be arbitrarily selected from the claims set forth in the attached claims. In other words, any combination of two or more claims, which may be arbitrarily selected from the claims set forth in the attached claims, is permitted, provided that no technical inconsistency arises.

[0012] (Manufacturing method for electronic component mounting boards) A method for manufacturing an electronic component mounting substrate according to the embodiment of the present disclosure, as shown in Figure 1, comprises: a first step of preparing a circuit board having a plurality of first lands on which a solder precoat covered with a flux residue film is formed; a second step of mounting first electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat on the plurality of first lands; and a third step of heating the circuit board on which the first electronic components were mounted in the second step to solder the first electronic components to each of the first lands.

[0013] In the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, the first 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 first 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 this disclosure, the pair of first terminals are soldered to the first land by melting and solidifying a solder precoat in the third step.

[0014] In the method for manufacturing an electronic component mounting substrate according to the embodiment of this disclosure, in the second step, the first terminal is brought into direct contact with the flux residue film covering the solder precoat of each of the plurality of first lands to mount the first electronic component.

[0015] 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 first lands on which a solder precoat covered with a flux residue film is formed, and (ii) in the second step, the pair of first terminals of the first electronic component are brought into direct contact with the flux residue film covering the solder precoat of each of the plurality of first lands, thereby mounting the first electronic component. In short, it is important that nothing other than the flux residue film is interposed between the solder precoat and the first terminals on the first land. The reasons for this will be explained below.

[0016] When using a circuit board with multiple lands, where a solder precoat is applied to cover each of the lands, and when using electronic components mounted on the lands, each component has a pair of terminals that are in contact with at least the bottom surface on both end faces of the component body, the electronic components are mounted on the solder precoat via solder flux applied to the surface of the solder precoat. The mounted electronic components are then 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 is manufactured with electronic components mounted on the circuit board.

[0017] When heating the circuit board during the manufacturing process of the electronic component mounting substrate as described above, before the solder precoat melts, the viscosity of the solder flux decreases and spreads over the surfaces of the solder precoat and the terminals, removing the oxide films formed on these surfaces. Thereafter, the solder precoat melts and spreads over the surfaces of the terminals from which the oxide films have been removed.

[0018] Here, when the electronic component mounted on the land is a first electronic component with a size of 0402 or less defined by the JIS standard, the electronic component may move due to the fluidized solder flux. When the electronic component moves in high-density mounting where the distance between adjacent electronic components is made as narrow as possible, solder bridges are likely to occur where the spread solder precoat connects between the opposing end faces of adjacent electronic components.

[0019] In high-density mounting, the distance between lands corresponding to each of the electronic components soldered adjacent to the land, in other words, the distance between opposing lands in a pair of lands on which the first electronic components are mounted adjacent to each other, is designed to be extremely short (for example, 0.02 mm or more and 0.10 mm or less). In this case, the occurrence of solder bridges becomes apparent. As a countermeasure to prevent this, although a method of reducing the amount of solder flux applied to the solder precoat can be considered, it is difficult to reduce the application amount due to the relationship between productivity and cost. In addition, some of the solder precoats are covered with a flux residue film generated during the manufacturing process. Although the flux residue film is difficult to fluidize as described later, a part of it may mix with the fluidized solder flux by heating, increasing the volume of the solder flux. In this case, it becomes even more difficult to take countermeasures against solder bridges.

[0020] The manufacturing method of an electronic component mounting substrate according to an embodiment of the present disclosure has the configurations of (i) and (ii) above. That is, in the manufacturing method of an electronic component mounting substrate according to an embodiment of the present disclosure, at the first land, nothing other than the flux residue film is interposed between the solder precoat and the first terminal. Here, the flux residue film is, for example, a residue film formed by heating the solder flux contained in the solder paste after supplying the solder paste containing solder particles and solder flux to the first land when forming the solder precoat on the first land. Such a flux residue film is a solid content in which most of the liquid components contained in the solder flux have been volatilized by heating. The solder particles form the solder precoat on the first land.

[0021] Therefore, even when the circuit board is heated to melt the solder precoat covering at least a part of the first land, the flux residue film, which is a solid content, has lower fluidity when softened by heating compared to the solder flux. Therefore, by bringing the first electronic component into direct contact with the flux residue film, it is possible to suppress the movement of the first electronic component on the first land. That is, it is possible to suppress the distance between adjacent first electronic components from becoming close. Also, the movement (flow) of the melted solder precoat (molten solder) can be restricted. Thereby, it is possible to suppress the formation of solder bridges between adjacent first electronic components. In particular, even when the first electronic components are mounted on the first circuit board at high density, the occurrence of solder bridges can be reduced between adjacent first electronic components.

[0022] The flux residue film contains a main resin derived from solder flux, and this main resin has adhesive properties. Therefore, the first electronic component can be temporarily fixed in place by the adhesive properties derived from the main resin. The main resin derived from solder flux softens and flows when heated. In addition, some of the activator derived from solder flux remains in the flux residue film. Therefore, when the main resin derived from solder flux softens when heated and comes into contact with the surface of the solder precoat and the surface of the terminal (the contact surface with the solder precoat), this activator can reduce (remove) the oxide film formed on the contacted area. Thus, the first electronic component can be soldered to the first land.

[0023] 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.

[0024] [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. Note that Figure 2 shows an example 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. In addition, the circuit board 1 may have a pair of second lands 2B arranged adjacent to each other.

[0025] 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.

[0026] 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.

[0027] 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. That is, the first land 2A is a land on which the first electronic component 5 is mounted, and it is a land on which a solder precoat 4 covered with a flux residue film 7 is formed. 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. Therefore, the flux residue film 7 may be formed to be connected across multiple first lands 2A.

[0028] In adjacent pairs of first lands 2A, the distance between the opposing first lands 2A (distance L shown in Figure 2) may be between 0.02 mm and 0.10 mm. 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 between 0.02 mm and 0.07 mm. 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.

[0029] 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.

[0030] As shown in Figures 2 and 3, the second land 2B has a solder precoat 4 formed on at least a portion of its surface, and solder flux 3 is arranged to cover at least a portion of each solder precoat 4. In the example shown in Figures 2 and 3, the solder precoat 4 in the second land 2B is covered with a flux residue film 7. Therefore, in the example shown in Figures 2 and 3, the solder flux 3 in the second land 2B covers the solder precoat 4 via the flux residue film 7.

[0031] 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 that is larger in volume than the first electronic component 5. The second electronic component may be a second electronic component 8 having an electronic component body 8a and a pair of second terminals 8b arranged on both end faces of the electronic component body 8a, as shown in Figure 4. 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. If the second electronic component 8 is a chip component, then, similar to the first electronic component 5, each of the pair of second terminals 8b is mounted on each of the pair of second lands 2B.

[0032] 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.

[0033] 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.

[0034] <1st process> In the first step of a 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 A on which a solder precoat 4 covered with a flux residue film 7 is formed is prepared, as shown in Figure 2. 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 a flux residue film 7 is formed (first method). Alternatively, the first step may be carried out by applying solder paste to each of the plurality of first lands 2A on the circuit board 1 having a plurality of first lands 2A, and then heating the circuit board 1 (second method). The second method will be described below.

[0035] In the second method, as shown in Figure 2, in addition to applying solder paste to multiple first lands 2A, solder paste is also applied to multiple second lands 2B, and then the circuit board 1 is heated.

[0036] 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 does not include the solder paste that was supplied to the land but before the solder particles were melted. The solder precoat may be a film-like solder obtained by an electrolytic plating method, or it may be formed from solder generated by heating a special material applied to the land.

[0037] 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.

[0038] Solder flux comprises at least a main resin and an activator. In addition to the main resin and activator, solder flux may also contain a thixotropic agent and a solvent. Solder flux can be prepared by mixing these components.

[0039] As the main resin, it is preferable to use a rosin-based resin that has reducing properties itself, but other resins may also be used. The main resin may be used alone or in combination of two or more types.

[0040] 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).

[0041] Other resins besides rosin-based resins include terpene resins, terpene phenol resins, styrene resins, xylene resins, acrylic resins, polyester resins, polyolefin resins, polyamides, polyamines, phenolic resins, phenoxy resins, and epoxy resins.

[0042] Examples of thixotropic agents include wax-based thixotropic agents, amide-based thixotropic agents, and sorbitol-based thixotropic agents. Thixotropic agents may be used individually or in combination of two or more types.

[0043] The activator used is a compound that reduces the oxide film covering the surface of solder used for soldering electronic components. An example of solder is solder precoat 4. Ma Furthermore, if a solder plating is applied to the surface of the terminals of electronic components such as the first electronic component, the solder contained in such solder plating is also included. Note that the main resin, such as rosin-based resin, may also have a certain degree of activating effect. Therefore, in this specification, "activator" refers to a compound other than the main resin, such as rosin-based resin, that has an activating effect.

[0044] 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.

[0045] 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.

[0046] 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 solvents may be used in combination.

[0047] Solder flux may contain other components besides those listed above. Other components include surfactants, silane coupling agents, antioxidants, and colorants.

[0048] 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 carried out 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.

[0049] 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.

[0050] 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 solder flux. 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. For example, in a commercially available circuit board 1 prepared by the first method, although the solder precoat 4 is provided on the plurality of second lands 2B, the flux residue film 7 may not be provided.

[0051] The flux residue film 7 can be removed by washing. Although the flux residue film 7 is a solid component formed when much of the liquid components contained in the solder flux have evaporated due to heating, the main resin (e.g., rosin-based resin) contained in the flux residue film 7 has adhesive properties. Therefore, this adhesive property allows for particularly suitable temporary fixing of the small and lightweight first electronic component to the circuit board 1, as shown in Figure 4.

[0052] The first step is preferably carried out by heating the solder paste in an inert atmosphere. Specifically, in the first step, solder paste is supplied to a plurality of first lands 2A and a plurality of second lands 2B on the circuit board 1, and the solder paste is heated in an inert atmosphere to obtain a plurality of first lands 2A and a plurality of second lands 2B on which a solder precoat 4 covered with a flux residue film 7 is formed. The solder paste contains solder particles and solder flux, as described above. The solder flux preferably contains a rosin-based resin and an activator.

[0053] In the first step, when the solder paste is heated in an inert atmosphere, the reaction (reduction reaction) caused by the activator contained in the solder flux is less likely to proceed. Therefore, some of the activator remains in the flux residue film 7 without reacting when the solder paste is heated in the first step. This allows for a higher activator content in the flux residue film 7, so that when the first electronic component is mounted on the flux residue film 7 via the first terminal 5b (see Figure 4), the activator contained in the flux residue film 7 can further sufficiently reduce the oxide film formed on the surface of the solder precoat 4. In addition, the oxide film formed on the surface of the first terminal 5b that is in contact with the flux residue film 7 can also be further sufficiently reduced. This allows for a more thorough bonding of the first terminal 5b to the first land 2A via the solder precoat 4.

[0054] An inert atmosphere refers to an atmosphere with a low oxygen concentration (a low-oxygen atmosphere), such as under vacuum conditions (for example, under conditions where the pressure is 0.1 Pa or less) or under an inert gas atmosphere (for example, nitrogen, argon, etc.).

[0055] Preferably, the first step includes a first substep in which the circuit board is heated to a main heating temperature above the melting point of the solder particles without performing a preheating step in which the solder paste is heated to a temperature below the melting point of the solder particles, thereby melting the solder contained in the solder paste at the main heating temperature, and a second substep in which the temperature is lowered from the main heating temperature to a temperature below the melting point to form a solder precoat on each of the multiple first lands. By raising the temperature to the main heating temperature all at once in the first substep and then lowering the temperature to a temperature below the melting point of the solder particles all at once in the second substep, the amount of activator consumed when the solder particles melt can be reduced. This makes it possible to increase the activator content in the flux residue film 7. In addition, by performing the first and second substeps, the heating time can be shortened. Furthermore, when the heating of the first step is performed while the circuit board is being transported by a conveyor belt, the length of the conveyor belt can be shortened as the heating time is shortened. This makes it possible to miniaturize the heating device used for heating the first step. Furthermore, by performing the first and second substeps in the first process, it is possible to suppress the formation of solder bridges between adjacent first electronic components and the formation of solder balls on the circuit board after going through the subsequent third process.

[0056] The solder flux preferably further 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. 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. The mass ratio of the non-volatile liquid to the solder flux is preferably 8% or more. The above-mentioned non-volatile liquid remains in the flux residue film 7 without volatilizing due to heating in the first step, and can increase the adhesive strength of the flux residue film 7.

[0057] <Flux placement process> In a method for manufacturing an electronic component mounting substrate according to one embodiment of the present disclosure, a flux placement step is performed between the first step and the second step in which solder flux 3 is placed so as to cover at least a portion of the solder precoat 4 of a plurality of second lands 2B (see Figure 3). When using the circuit board 1 according to the example shown in Figure 2, at least a portion of the solder precoat 4 is covered with solder flux 3 via the flux residue film 7, as shown in Figure 3. If the solder precoat 4 is placed on the second lands 2B of the circuit board 1 but the flux residue film 7 is not placed, solder flux 3 is placed so as to cover at least a portion of the solder precoat 4.

[0058] The flux placement process can be carried out by applying solder flux 3 so as to cover at least a portion of each of the solder precoats 4 of the multiple second lands 2B. The method of 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 screen printing. Screen printing can be carried out in the same manner as described in the section on the method of applying the solder precoat.

[0059] <Second process> In the second step of a 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 first electronic components 5 of size 0402 or smaller as defined by JIS standards onto the solder precoat 4 on a plurality of first lands 2A, second electronic components 8 are mounted onto the solder precoat 4 on a plurality of second lands 2B via solder flux 3. In the second step, the first electronic components 5 are mounted by directly contacting the flux residue film 7 covering the solder precoat 4 of each of the plurality of first lands 2A with the first terminal 5b. Also in the second step, the second electronic components 8 are mounted onto the solder flux 3 of each of the plurality of second lands 2B via the second terminal 8b. The mounting of these electronic components can be performed by known component mounting equipment.

[0060] Here, we will explain the first electronic component 5 and the second electronic component 8.

[0061] The size of the first electronic component 5 is defined by JIS standards such as 0402, 03015, 0201, and 01005. Specifically, an electronic component of size 0402 is 0.4 mm in length and 0.2 mm in width; an electronic component of size 03015 is 0.3 mm in length and 0.15 mm in width; an electronic component of size 0201 is 0.25 mm in length and 0.125 mm in width; and an electronic component of size 01005 is 0.1 mm in length and 0.05 mm in width. The first electronic component 5 is smaller and extremely lightweight than the second electronic component, which will be described later.

[0062] As described above, the first electronic component 5 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 formed on both end faces of the electronic component body 5a (see Figure 4). 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 the surfaces perpendicular to all of the end faces, upper surface, and lower surface.

[0063] The second electronic component 8 is an electronic component with a larger volume than the first electronic component 5.

[0064] If the second electronic component 8 is a chip component, its size will be, 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.

[0065] If the second electronic component 8 is a chip component, the second electronic component 8 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 formed on both end faces of the electronic component body 8a (see Figure 4).

[0066] <3rd process> In the third step of the method for manufacturing an electronic component mounting substrate according to one embodiment of the present disclosure, as shown in Figure 5, 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 first lands A, the second electronic component 8 is soldered to each of the 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 in the third step, the pair of second terminals 8b are soldered to the second lands 2B by melting and solidifying the solder precoat 4.

[0067] 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. In this case, in the first land 2A, the flux residue film 7 covering the solder precoat 4 has lower fluidity when softened by heating compared to the solder flux 3, but some of the activators contained in the solder flux used to form the solder precoat 4 remain unreacted in the flux residue film 7.

[0068] Therefore, as described above, when the circuit board 1 on which the first electronic component 5 and the second electronic component 8 are mounted is heated, the activator remaining in the flux residue film 7 can reduce the oxide film formed on the surface of the solder precoat 4. In addition, the oxide film formed on the surface of the first terminal 5b that is in contact with the flux residue film 7 can also be reduced. This ensures sufficient bonding of the first terminal 5b to the first land 2A via solder (solder contained in the solder precoat).

[0069] In the third 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 formed from the 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 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 8b to the second land 2B. The solidified molten solder is shown as the solder portion 4a in Figure 5.

[0070] As explained above, a solder precoat 4 covered with a flux residue film 7 is formed on the first land 2A, and the first electronic component 5 is mounted on the first land 2A via the flux residue film 7. Since the flux residue film 7 is a solid, it can suppress the movement of the tiny and lightweight first electronic component on the first land 2A in the third step. In other words, it can suppress the proximity of adjacent first electronic components. It can also restrict the movement (flow) of molten solder. This prevents the formation of solder bridges between adjacent first electronic components 5.

[0071] Furthermore, in the method for manufacturing an electronic component mounting substrate according to one embodiment of the present disclosure, as shown in Figure 2, a flux residue film 7 is formed on the second land 2B so as to cover the solder precoat 4, and solder flux 3 is arranged so as to cover at least a portion of the flux residue film 7. However, when a second electronic component 8, which has a larger volume than the first electronic component 5, is placed on the second land 2B, the second electronic component 8 has a large self-weight, so it is possible to suppress the movement of the second electronic component 8 on the second land 2B due to the fluidization of the solder flux 3. This also suppresses the formation of solder bridges between adjacent second electronic components 8.

[0072] Furthermore, when the second electronic component 8 is placed on the second land 2B, the distance between two opposing pairs of second lands 2B is usually greater than 0.10 mm. Therefore, even if a portion of the flux residue film 7 on the second land 2B liquefies due to heating and mixes with the solder flux 3, increasing the volume of the solder flux 3, and causing a deformation in which a portion of the solder portion 4a bulges out from the end face of the second electronic component 8, as shown in Figure 5, it is possible to suppress the formation of a solder bridge by connecting the solder portions 4a between the end faces of adjacent second electronic component 8.

[0073] Furthermore, when the first electronic component 5 is mounted on the second land 2B, although the solder flux 3 causes the first electronic component 5 to move on the second land 2B, if the distance between two adjacent pairs of second land 2Bs is normally greater than 0.10 mm, it is possible to suppress the formation of a solder bridge by connecting the solder portions 4a between the end faces of adjacent first electronic components 5.

[0074] The present invention may be modified in any way that does not depart from the spirit of the invention. For example, although the second electronic component 8 has been used as an example of an electronic component mounted on the second land 2B, this does not prohibit the mounting of the first electronic component 5 on the second land 2B. That is, the electronic component mounted on the second land 2B may be the first electronic component 5. For example, in the circuit board 1, there may be a mixture of first electronic components 5 mounted directly on the flux residue film 7 covering the first land 2A and first electronic components 5 mounted on the solder flux 3 applied to the second land 2B. In this case, it is desirable that the first electronic components 5 mounted on the second land 2B be separated by a distance such that solder bridges do not occur between adjacent first electronic components 5.

[0075] (Note) The following technologies are disclosed as described above. (Technology 1) A first step is to prepare a circuit board having multiple first lands on which a solder precoat covered with a flux residue film is formed, A second step involves mounting a first electronic component of size 0402 or smaller as defined by JIS standards onto the solder precoat in the aforementioned plurality of first lands, The process includes a third step of heating the circuit board on which the first electronic component is mounted in the second step, and soldering the first electronic component to each of the first lands, The first 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 first terminals disposed on both end faces of the electronic component body, at least in contact with the lower surface. The pair of first terminals are soldered to the first land in the third step by melting and solidifying the solder precoat. In the second step, the first terminal is brought into direct contact with the flux residue film covering the solder precoat of each of the plurality of first lands, thereby mounting the first electronic component. A method for manufacturing electronic component mounting boards. (Technology 2) In the first step, solder paste is supplied to a portion of the plurality of lands on the circuit board, and the solder paste is heated in an inert atmosphere to obtain a plurality of first lands on which the solder paste covered with the flux residue film is formed. The aforementioned solder paste comprises solder particles and solder flux. The aforementioned solder flux comprises a rosin-based resin and an activator. A portion of the activator remains in the flux residue film without reacting when the solder paste is heated in the first step. A method for manufacturing an electronic component mounting substrate as described in Technical 1. (Technology 3) The first step described above is, A first substep involves heating the circuit board to a main heating temperature above the melting point of the solder particles without performing preheating, which involves heating the solder paste at a temperature below the melting point of the solder particles, thereby melting the solder particles contained in the solder paste at the main heating temperature. The system includes a second substep of lowering the temperature from the heating temperature to a temperature below the melting point to form the solder precoat on each of the plurality of first lands. A method for manufacturing an electronic component mounting substrate as described in Technology 2. (Technology 4) The solder flux further contains a non-volatile liquid whose evaporation rate is 0.001 or less, with the evaporation rate of n-butyl acetate being set to 1. The mass ratio of the non-volatile liquid to the solder flux is 8% or more. A method for manufacturing an electronic component mounting substrate as described in Technology 2 or 3. (Technology 5) The distance between the first lands corresponding to each of the first electronic components soldered adjacent to each other on the first land is 0.02 mm or more and 0.10 mm or less. A method for manufacturing an electronic component mounting substrate as described in any one of Technology 1 to 4. (Technology 6) The circuit board further has a plurality of second lands on which a solder precoat is formed, Between the first step and the second step, a flux placement step is performed in which solder flux is applied so as to cover at least a portion of the solder precoat of each of the plurality of second lands. In the second step, electronic components are further mounted onto the solder precoat of the plurality of second lands via the solder flux. In the third step, the electronic components are further soldered to each of the plurality of second lands. The electronic component has at least one terminal, which is soldered to the second land by melting and solidifying the solder precoat in the third step. A method for manufacturing an electronic component mounting substrate as described in any one of Technology 1 to 5. (Technology 7) The electronic components mounted on the solder precoat of the plurality of second lands in the second step are second electronic components that have a larger volume than the first electronic components soldered to the first lands. A method for manufacturing an electronic component mounting substrate as described in Technical 6.

[0076] 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. [Industrial applicability]

[0077] The method for manufacturing an electronic component mounting substrate according to this disclosure can be used in applications where it is required to suppress the formation of solder bridges between adjacent electronic components. [Explanation of Symbols]

[0078] 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

Claims

1. A first step is to prepare a circuit board having a plurality of first lands on which a solder precoat covered with a flux residue film is formed, A second step involves mounting first electronic components of size 0402 or smaller as defined by JIS standards onto the solder precoat in the aforementioned plurality of first lands, The process includes a third step of heating the circuit board on which the first electronic component is mounted in the second step, and soldering the first electronic component to each of the first lands, The first 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 first terminals disposed on both end faces of the electronic component body, at least in contact with the lower surface. The pair of first terminals are soldered to the first land in the third step by melting and solidifying the solder precoat. In the second step, the first terminal is brought into direct contact with the flux residue film covering the solder precoat of each of the plurality of first lands, thereby mounting the first electronic component. A method for manufacturing electronic component mounting boards.

2. In the first step, solder paste is supplied to a portion of the plurality of lands on the circuit board, and the solder paste is heated in an inert atmosphere to obtain a plurality of first lands on which the solder precoat covered with the flux residue film is formed. The aforementioned solder paste comprises solder particles and solder flux. The aforementioned solder flux comprises a rosin-based resin and an activator. A portion of the activator remains in the flux residue film without reacting when the solder paste is heated in the first step. A method for manufacturing an electronic component mounting substrate according to claim 1.

3. The first step is, A first substep involves heating the circuit board to a main heating temperature above the melting point of the solder particles without performing preheating, which involves heating the solder paste at a temperature below the melting point of the solder particles, thereby melting the solder particles contained in the solder paste at the main heating temperature. The system includes a second substep of lowering the temperature from the heating temperature to a temperature below the melting point to form the solder precoat on each of the plurality of first lands. The method for manufacturing an electronic component mounting substrate according to claim 2.

4. The solder flux further contains a non-volatile liquid whose evaporation rate is 0.001 or less, with the evaporation rate of n-butyl acetate being set to 1. The mass ratio of the non-volatile liquid to the solder flux is 8% or more. A method for manufacturing an electronic component mounting substrate according to claim 2 or 3.

5. The distance between the first lands corresponding to each of the first electronic components soldered adjacent to each other on the first land is 0.02 mm or more and 0.10 mm or less. A method for manufacturing an electronic component mounting substrate according to any one of claims 1 to 3.

6. The circuit board further has a plurality of second lands on which a solder precoat is formed, Between the first step and the second step, a flux placement step is performed in which solder flux is applied so as to cover at least a portion of the solder precoat of each of the plurality of second lands. In the second step, electronic components are further mounted on the solder precoat of the plurality of second lands via the solder flux. In the third step, the electronic components are further soldered to each of the plurality of second lands. The electronic component has at least one terminal, which is soldered to the second land by melting and solidifying the solder precoat in the third step. A method for manufacturing an electronic component mounting substrate according to any one of claims 1 to 3.

7. In the second step, the electronic components mounted on the solder precoat of the plurality of second lands are second electronic components that have a larger volume than the first electronic components soldered to the first lands. The method for manufacturing an electronic component mounting substrate according to claim 6.

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