Resin flux solder paste and mounting structures

Abstract

To provide a resin flux solder paste which suppresses occurrence of a side ball while forming a connection part which is stronger and excellent in connection stability, and a mounting structure on which an electronic component is mounted using the same.SOLUTION: A resin flux solder paste contains solder powder and a flux, wherein the flux contains at least an epoxy resin, a curing agent, a curing accelerator and an activator, the curing agent is a phenol novolac resin, the activator is at least one selected from the group consisting of an organic base, an organic acid and a salt thereof, the solder powder contains a plurality of solder particles, the solder particles are covered with the activator, a percentage content of the solder powder is 86-97 wt.% with respect to 100 wt.% of the total weight of the resin flux solder paste.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention mainly relates to a solder paste containing an epoxy resin in a flux component and a mounting structure, which are used when soldering semiconductor components or electronic components to a circuit board.

Background Art

[0002] In recent years, miniaturization and high functionality of mobile devices such as mobile phones or PDAs (Personal Digital Assistants) have been progressing. As a mounting technology capable of coping with this, mounting structures such as BGA (Ball Grid Array) or CSP (Chip Scale Package) are widely used. Mobile devices are likely to be subjected to mechanical loads such as dropping impacts. In QFP (Quad Flat Package), the leads absorb the impact. However, in the case of BGA or CSP that do not have leads for mitigating the impact, it has become important to ensure shock resistance reliability. Therefore, high solder connection reliability in device mounting has become essential, and structural methods and solder materials that can achieve this are desired.

[0003] Therefore, as a further countermeasure, a semiconductor mounting structure using a solder paste containing a thermosetting resin in the flux and a manufacturing method thereof have been proposed (see, for example, Patent Document 1). [[ID=十七]]

[0004] A solder paste containing a thermosetting resin in the flux (hereinafter referred to as a resin flux solder paste) can form a reinforcing structure in which the resin contained in the flux separates from the solder and the resin covers the periphery of the solder connection part in the process of heating and melting and connecting the solder. As a result of this reinforcement, it becomes possible to increase the strength of the connection part of the solder.

[0005] Figure 3 is a cross-sectional view of the connection joint joined using the solder paste described above. As shown in Figure 3, the electrode 2 provided on the circuit board 1 and the electrode 4 provided on the circuit board 3 are joined with a solder conductive part 9A, and the surrounding area is reinforced with a reinforcing part 6b which is a hardened solid resin.

[0006] In this assembly process using resin flux solder paste, the resin flux solder paste is printed onto the wiring electrodes and other predetermined positions on the circuit board using a metal mask, and then heated in a reflow oven. At this time, the flux chemically removes the oxide film on the metal surface to be soldered and the oxide film on the solder powder surface through a reduction reaction, i.e., the flux action is at work, enabling the molten connection of the solder. Subsequently, the curing of thermosetting resins such as epoxy resin proceeds, and the joining of the wiring electrodes on the circuit board and electronic components, as well as reinforcement with resin, is performed in a single heating reflow process.

[0007] On the other hand, while lead-based eutectic solder was conventionally used as the solder material for regular (non-thermosetting resin) solder paste, lead-free solder is now being used due to environmental concerns. Lead-free solders include Sn-Bi solder, Sn-Ag-Cu solder (hereinafter also simply referred to as SAC solder), and Sn-Cu solder. In assembly using SAC solder, solders containing In, which have different metal compositions, have also been put into practical use as a measure to achieve high connection reliability. As representative examples of SAC solder, SAC305 (Sn-3.0Ag-0.5Cu) solder (hereinafter also simply referred to as SAC305 solder) and SAC105 (Sn-1.0Ag-0.5Cu) solder (silver ratio 1%) (hereinafter also simply referred to as SAC105 solder), which has a lower silver ratio, have been investigated and are gradually being put into practical use.

[0008] As mentioned above, using resin flux solder paste allows for improved connection reliability through a reinforced structure formed by resin, without causing process delays or cost issues. However, the resin flux solder pastes that have been put into practical use are those using low-melting-point solders such as Sn-Bi solder, as shown in Patent Document 1. For example, solder pastes containing thermosetting resins that use high-melting-point solders such as SAC solder have hardly been put into practical use yet. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 5204241 [Overview of the project] [Problems that the invention aims to solve]

[0010] The reason the above-mentioned products have not been put into practical use is their low reliability. One of the reasons for this is the occurrence of "chip side balls" (hereinafter also simply referred to as "side balls") when using resin flux solder paste. Specifically, when mounting chip-type electronic components (hereinafter also simply referred to as "chip components") onto a circuit board, resin flux solder paste is printed onto the circuit board, the chip components are mounted, and then reflow heating is performed. At this time, small solder balls of several hundred microns in size (i.e., side balls) may be generated that protrude from the side of the chip component. When "side balls" are generated, external stress (more specifically, external vibrations, etc.) can cause the side balls to detach from the chip component and adhere to electrical circuits near the chip component, potentially causing problems such as short circuits. Therefore, suppressing the generation of "side balls" is a very important item in quality control.

[0011] Furthermore, suppressing the generation of side balls sometimes resulted in problems such as reduced connection strength or inability to properly form solder balls.

[0012] Therefore, the objective of the present invention is to provide a resin flux solder paste with excellent reliability. In other words, the objective of the present invention is to provide a resin flux solder paste that suppresses the generation of side balls while forming a stronger connection with excellent connection stability. Another object of the present invention is to provide a mounting structure on which electronic components are mounted using such a resin flux solder paste. [Means for solving the problem]

[0013] According to the first gist of the present invention, A resin flux solder paste containing solder powder and flux, The flux comprises at least an epoxy resin, a curing agent, a curing accelerator, and an activator. The curing agent is a phenol novolac resin, The activator is at least one selected from the group consisting of organic bases, organic acids, and salts thereof. The aforementioned solder powder contains a plurality of solder particles, and the solder particles are covered with the aforementioned activator. The content of the solder powder is 86 to 97% by weight relative to 100% by weight of the total weight of the resin flux solder paste, in a resin flux solder paste.

[0014] According to the second gist of the present invention, A mounting structure in which electronic components are mounted on a circuit board using the above-mentioned resin flux solder paste, A mounting structure comprising a conductive portion formed by metal bonding of the electronic component and the circuit board, and a reinforcing portion formed by covering at least a part of the area surrounding the conductive portion with a hardened flux. [Effects of the Invention]

[0015] The present invention provides a resin flux solder paste that suppresses the generation of side balls while forming a connection part that is stronger and has excellent connection stability, and a mounting structure in which electronic components are mounted using such a resin flux solder paste.

Brief Description of the Drawings

[0016] [Figure 1] It is a cross-sectional view of a solder joint portion of a CSP joined using the resin flux solder paste of the present invention. [Figure 2A] It is a cross-sectional explanatory view schematically showing the bonding process of the ball portion of a CSP using the resin flux solder paste of the present invention. [Figure 2B] It is a cross-sectional explanatory view schematically showing the bonding process of the ball portion of a CSP using the resin flux solder paste of the present invention. [Figure 2C] It is a cross-sectional explanatory view schematically showing the bonding process of the ball portion of a CSP using the resin flux solder paste of the present invention. [Figure 2D] It is a cross-sectional explanatory view schematically showing the bonding process of the ball portion of a CSP using the resin flux solder paste of the present invention. [Figure 3] It is a cross-sectional view of a solder joint portion of a CSP joined using a conventional solder paste.

Embodiments for Carrying Out the Invention

[0017] Hereinafter, embodiments of the present invention will be specifically described. The present invention is not limited to such embodiments. The present invention can be appropriately modified and implemented within the scope of the object of the present invention. Hereinafter, the resin flux solder paste and the implementation structure will be described in detail.

[0018] The numerical ranges referred to in this specification are intended to include the lower limit value and the upper limit value itself. That is, taking a numerical range such as 86 to 97% by weight as an example, that numerical range is interpreted as including the lower limit value "86% by weight" and the upper limit value "97% by weight".

[0019] Hereinafter, embodiments of the present invention will be specifically described with reference to the figures.

[0020] <Resin flux solder paste> The resin flux solder paste according to an embodiment of the present invention is A resin flux solder paste containing solder powder and flux, The flux comprises at least an epoxy resin, a curing agent, a curing accelerator, and an activator. The curing agent is a phenol novolac resin, The activator is at least one selected from the group consisting of organic bases, organic acids, and salts thereof. The aforementioned solder powder contains a plurality of solder particles, and the solder particles are covered with the aforementioned activator. The content of the solder powder is 86 to 97% by weight relative to 100% by weight of the total weight of the resin flux solder paste.

[0021] The resin flux solder paste (hereinafter also referred to as solder paste) according to this embodiment can form a stronger and more stable connection while suppressing the generation of side balls. In other words, the solder paste according to this embodiment can achieve both "excellent suppression of side ball generation" and "excellent bonding stability and adhesion of the formed connection". The reason for this is presumed to be as follows. The solder paste according to this embodiment contains at least an epoxy resin flux, a phenol novolac resin as a curing agent, and a curing accelerator, thereby exhibiting excellent epoxy resin adhesion and achieving a high resin reinforcement effect on the solder joint. Furthermore, the solder paste according to this embodiment has a solder powder content of 86 to 97% by weight of 100% by weight of the total weight of the solder paste, and the solder particles (surface) of the solder powder are covered with an activator. As a result, the cohesive force of the molten solder is increased, preventing the scattering of fine particles from the molten solder and suppressing the generation of side balls. Therefore, the solder paste according to this embodiment can form a connection that is stronger and has superior connection stability, while suppressing the generation of side balls.

[0022] Conventionally, it has been difficult to achieve both effective suppression of side ball formation and the formation of solder joints with excellent connection stability and adhesion. As a result of our investigation, we found that the reason why it is difficult to achieve both is that there is a trade-off relationship between suppression of side ball formation and excellent connection stability and adhesion of the joint. Specifically, to effectively suppress the generation of side balls, it is effective to increase the content of solder powder in the photocurable composition. However, increasing the content of solder powder necessitates increasing the content of the activator to sufficiently melt the solder powder. As a result, the content of epoxy resin in the flux decreases, leading to a decrease in the connection stability and adhesion of the formed joint. On the other hand, to form a joint with excellent connection stability and adhesion, it is effective to increase the epoxy resin content in the photocurable composition. However, increasing the epoxy resin content relatively decreases the content of solder powder and activator, making it difficult to sufficiently suppress the generation of side balls.

[0023] The inventors believed that achieving both of the above-mentioned properties, which are in a trade-off relationship, could not be adequately addressed by simply extending the conventional formulation design philosophy, and therefore focused on the action of activators in the flux. Specifically, they focused on the fact that activators in contact with the oxide film on the surface of solder particles contribute significantly to the reduction and removal of the oxide film on the surface of the solder particles. From the viewpoint of improving the reduction and removal action of the activator, they conceived of the characteristic that "solder particles are covered with activator" in the solder paste. The inventors also found that by improving the reduction and removal action of the oxide film by the activator, the activator content can be reduced, and thereby the content of solder powder can be relatively increased, suppressing the generation of side balls.

[0024] From the above, the solder paste according to this embodiment can form a connection part that is stronger and has excellent connection stability, while suppressing the generation of side balls.

[0025] [Solder powder] The solder powder contained in the solder paste according to this embodiment is not particularly limited, but it is preferable to use solder powder with a melting point of 180°C or higher, particularly 200°C or higher. The composition of the solder powder is not particularly limited, but may be in the form of elemental Sn or a solder alloy. Examples of solder alloys include Sn-based Sn-Ag-Cu (SAC solder), Sn-Cu solder, or alloys of Sn-Ag solder. Examples of SAC solder include SAC305 (Sn-3.0Ag-0.5Cu) solder with a melting point of 220°C, or SAC105 (Sn-1.0Ag-0.5Cu) solder with a melting point of 225°C. Examples of Sn-Ag solder include Sn-3.5Ag solder with a melting point of 221°C. Examples of Sn-Cu solders include Sn-0.7Cu solder, which has a melting point of 227°C. Of these solder alloys, SAC305 solder or SAC105 solder is preferred, and SAC305 is more preferred. This is because SAC305 solder is currently used in general consumer electronics, achieving high connection reliability and low cost, and is also commonly used for solder balls in CSP and BGA packages.

[0026] In this specification, the composition of solder powder is expressed by hyphenating the elemental symbols of the elements contained in the solder powder. In this specification, when describing the metallic composition of solder powder, a numerical value or range of numerical values ​​may be shown immediately before the metallic element, which indicates the mass % (= mass %) of each element in the metallic composition, as is commonly used in the art. Solder powder may contain trace metals that are inevitably present, insofar as it is substantially composed of the listed elements, such as Ni, Ge, Zn, Sb, Cu, etc.

[0027] In this specification, the melting point of solder powder (or solder) refers to the temperature at which melting ends when the state change during the heating process of the sample is observed, and can be measured using differential scanning calorimetry (DSC), differential thermogravimetric analysis (TG-DTA), etc.

[0028] The solder powder contains a plurality of solder particles. The solder particles are coated with an activator. Preferably, the entire surface of the solder particles is coated with the activator. In one embodiment, the solder particles are coated with a coating film of the activator.

[0029] The presence of an activator coating on solder particles, and the presence of an activator coating film, can be confirmed using a scanning electron microscope (SEM). More specifically, this can be confirmed by acquiring and observing SEM images. Furthermore, if the activator is composed of nitrogen atoms (more specifically, triethanolamine (TEA), etc.), this can be confirmed by observing the signal originating from the nitrogen atoms constituting the activator on the surface of the solder particles using energy-dispersive X-ray spectroscopy (SEM-EDX).

[0030] The solder powder content in this embodiment is 86 to 97% by mass or less, relative to 100% by weight of the total mass of the solder paste. When the solder powder content is 86% by weight or more, the generation of side balls can be efficiently suppressed. Although not bound by any particular theory, the reason is presumed to be as follows. The inventors first focused on the mechanism of side ball generation and carefully observed the soldering process. During reflow heating, the oxide film of the solder particles is removed by the reducing action of the activator and melts. In this melting process, the moisture contained in the solder particles turns into water vapor, the volume of the moisture rapidly expands, and the water vapor escapes from the molten solder. At that time, solder fine particles separate from the molten solder and scatter, generating side balls. Thus, the inventors discovered that side balls are generated when solder fine particles scatter from the molten solder due to the phase transition (liquid phase → gas phase) of the moisture contained in the solder particles.

[0031] Next, based on the technical knowledge regarding the mechanism of side ball generation described above, the inventors focused on increasing the cohesive force of the solder in order to prevent the scattering of solder fine particles. After diligent investigation, they found that increasing the contact area between solder particles during the melting process enhances the cohesive force of the molten solder, and thus arrived at the characteristic that "the solder particle content is 86% by weight or more relative to 100% by weight of the total weight of the resin flux solder paste."

[0032] Furthermore, considering the balance between the activator for reducing and removing the oxide film on the surface of the solder particles during the solder melting process, and the epoxy resin, curing agent, and curing accelerator for forming a stronger and more stable connection, the inventors came up with the characteristic that "the solder powder content is 97% by weight or less relative to 100% by weight of the total weight of the resin flux solder paste."

[0033] The solder powder content is higher than the usual content. As a result, the solder paste according to this embodiment can achieve both "formation of a stronger connection with superior connection stability" and "suppression of side ball generation," which are in a trade-off relationship. The reason why the solder powder content can be higher than usual is that, as mentioned above, by adopting a form in which the activator functions efficiently (a form in which the solder particles (surface) are covered with the activator), the activator content can be reduced compared to the usual.

[0034] [Flux] In this embodiment, the flux comprises at least an epoxy resin, a curing agent, a curing accelerator, and an activator. In addition to these components, the flux may further contain other components. Each essential component of the flux is described in more detail below.

[0035] (Epoxy resin) In this embodiment, the flux contains epoxy resin. When the flux contains epoxy resin, other components such as solder powder are easily dispersed in the epoxy resin. The epoxy resin is preferably liquid at room temperature, but it may be liquefied by mixing a solid epoxy resin at room temperature with a liquid epoxy resin, or by adding a solvent to a solid epoxy resin at room temperature.

[0036] As for epoxy resins that are liquid at room temperature, any resin having two or more epoxy groups in one molecule is acceptable, and its molecular weight and molecular structure are not particularly limited; various types can be used. Specifically, various liquid epoxy resins such as glycidyl ether type, glycidylamine type, glycidyl ester type, or olefin oxidation type (alicyclic type) can be used. Examples of commercially available products include bisphenol A type epoxy resin (manufactured by Mitsubishi Chemical Corporation: product number jER828) and YD8125 (manufactured by Nippon Steel Chemical & Material Co., Ltd., epoxy equivalent 170).

[0037] As mentioned above, it is preferable to add a solvent to the liquid epoxy resin in order to disperse the solder powder (more specifically, SAC solder). In other words, in one preferred embodiment, from the viewpoint of further improving the dispersibility of the components of the solder paste, it is preferable that the flux further contains a solvent and the epoxy resin is liquid at room temperature.

[0038] (Hardening agent) In this embodiment, the flux contains a curing agent. The curing agent is a phenol novolac resin. The phenol novolac resin, as a curing agent, reacts with the epoxy resin when the solder paste is heated to, for example, 220°C, thereby curing the epoxy resin. The phenol novolac resin preferably has at least two or more phenolic hydroxyl groups and allyl groups within one molecule (or one molecular chain). One type of phenol novolac resin may be used, or two or more types may be used. When there are two or more types of phenol novolac resins, preferably one of them is a phenol novolac resin having both phenolic hydroxyl groups and allyl groups in the repeating unit (in the phenol unit) (see, for example, [Chemical Formula 1]), and the other one is a biphenyl aralkyl resin.

[0039] The former, phenol novolac resin, is presumed to have a lower viscosity due to the steric hindrance effect of the allyl group, which prevents the alignment of molecular chains by hydrogen bonding between phenolic hydroxyl groups. In particular, low molecular weight dimers of this phenol resin (for example, the compound n=0 in [Chemical Formula 1]) can take on a liquid form. Specifically, commercially available products include MEH-8000H (viscosity 1500~3500 mPa·s, hydroxyl equivalent 139~143) and MEH-8005 (viscosity 4500~7500 mPa·s, hydroxyl equivalent 133~138) manufactured by Meiwa Kasei Co., Ltd. In this disclosure, hydroxyl equivalent refers to the value measured by the neutralization titration method in accordance with Japanese Industrial Standard (JIS)-K0070.

[0040] [ka]

[0041] Phenol novolac resins, which have both phenolic hydroxyl groups and allyl groups in their repeating units, exhibit a low glass transition temperature (TG) of 60-70°C in their cured products with epoxy resins. Therefore, to achieve a high TG of 80°C or higher, it is preferable to use a phenol novolac resin with a rigid skeleton, similar to the case of epoxy resins. By using such a resin, the crosslinking density of the cured product can be increased, thereby improving the rigidity of the cured product.

[0042] Therefore, the inventors diligently investigated phenol novolac resins with a rigid skeleton and discovered that biphenyl aralkylphenol resin (the latter phenol novolac resin mentioned above) is the most superior. Furthermore, as with epoxy resins, it is desirable that the resin does not harden until the SAC solder melts. From this viewpoint, in one preferred embodiment, the curing agent contains 30 to 95% by weight of biphenyl aralkylphenol resin with a hydroxyl group equivalent of 150 to 350, based on 100% by weight of the total amount of the curing agent, and 5 to 70% by weight of phenol novolac resin having allyl groups with a hydroxyl group equivalent of 100 to 200, based on 100% by weight of the total amount of the curing agent. Specifically, this solid phenol novolac resin is biphenyl aralkyl resin: product number MEH-7851 (manufactured by Meiwa Kasei, hydroxyl group equivalent 210).

[0043] (Curing accelerator) A curing accelerator accelerates the curing reaction between the epoxy resin and the curing agent (phenol novolac resin). The curing accelerator is preferably a boron-containing compound. The curing reaction between epoxy resin and phenol novolac resin is relatively unreactive. Therefore, if the solder paste does not contain a curing accelerator, the curing reaction will hardly occur even at high temperatures. It is desirable for the curing reaction to proceed relatively quickly at high temperatures. For this reason, the addition of a curing accelerator is necessary.

[0044] The inventors diligently investigated types of curing accelerators and arrived at the conclusion that boron-containing compounds are effective. Specifically, commonly used curing accelerators include imidazole, DBU-based salts, and tertiary amine compounds. However, these actively accelerate curing even at low temperatures, causing the resin to react before the SAC solder melts, resulting in thickening and solidification of the solder paste, and preventing proper soldering.

[0045] The inventors further investigated and derived a boron-containing compound as a curing accelerator that hardly proceeds in the low temperature range and proceeds in the high temperature range (preferably, the curing reaction proceeds rapidly in a relatively short time). In this embodiment, the curing accelerator includes a boron compound. Examples of boron-containing compounds as curing accelerators include arylborane complexes and arylborate salts (more specifically, phosphorus-containing arylborane complexes and arylborate salts). Phosphorus-containing arylborane complexes and arylborate salts correspond to either organophosphine compounds and their salts, or quaternary phosphonium compounds and their salts. In other words, the boron-containing compound may include one or more of the organophosphine compounds and their salts, or quaternary phosphonium compounds and their salts. Examples of organophosphine compounds and their salts, and quaternary phosphonium compounds and their salts, include triphenylphosphine-triphenylborane, tetraphenylphosphonium-tetraphenylborate, and tetraphenylphosphonium-P-tolylborites. Of these multiple boron-containing compounds, tetraphenylphosphonium tetraphenylborate (TPP-K) is particularly preferred as a curing accelerator. Since tetraphenylphosphonium tetraphenylborate (TPP-K) has a high melting point of 305°C, the solder paste according to this embodiment has excellent applicability to high-melting-point solders as a curing accelerator.

[0046] [ka]

[0047] The content of the curing accelerator is preferably 0.5 to 3.0 phr relative to the total flux amount. If the content of the curing accelerator is less than 0.5 phr relative to the total flux amount, the curing reaction will be very weak, requiring exposure to high temperatures for a long time. This long heating process may cause problems such as thermal degradation of surrounding parts, which is undesirable. Also, if the content of the curing accelerator is more than 3.0 phr relative to the total flux amount, the curing reaction will be very vigorous, leading to problems such as uneven curing and increased curing stress, which is undesirable. A curing accelerator content of 0.5 to 3.0 phr relative to the total flux amount is preferable because it allows for a reaction that matches the reflow time of several minutes, eliminates the need for prolonged exposure to high temperatures, and enables a uniform and stable curing reaction. Here, the unit of content "phr" is the total flux weight excluding the solvent. In quantity This shows the weight percentage of the additive relative to the total weight.

[0048] (Activating agent) The activator removes oxide films that may be present on the surface of the components to be joined (more specifically, electrodes, wiring, and / or solder powder of electronic components) in the temperature range in which the solder paste is heated. Generally, activators include organic acids, organic bases, halogens, or salts thereof (more specifically, amine salts, etc.) that have reducing power to remove oxide films that may be present on the surface of the components to be joined in the temperature range in which the solder paste is heated. In electrical and electronic applications, halogen compounds are undesirable because they can cause insulation failures if they are included in the epoxy resin cured product. The activator is at least one selected from the group consisting of organic acids, organic bases (more specifically, amine compounds), and salts thereof (more specifically, amine salts, etc.).

[0049] Organic acids possess particularly excellent fluxing properties (where fluxing refers to the reducing action that removes oxide films formed on the metal surface to which the solder paste is applied, and the action that lowers the surface tension of the molten solder, thereby promoting the wettability of the solder to the joined metal surface). Furthermore, their reactivity with epoxy resins is not as strong as that of amine salts at room temperature, but they can react with epoxy resins when heated. After reducing the solder and removing the oxide film, organic acids are incorporated into the cured epoxy resin, and therefore cause almost no adverse effects such as corrosion. Examples of organic acids include dicarboxylic acids (more specifically, sebacic acid, adipic acid, glutaric acid, succinic acid, malonic acid, and pimelic acid), tricarboxylic acids, or their derivatives (more specifically, levulinic acid as a derivative of dicarboxylic acid, and citric acid as a derivative of tricarboxylic acid). Among these, dicarboxylic acids (dibasic acids) that do not reduce the crosslinking density are preferred when considering the reaction with epoxy.

[0050] Furthermore, amine compounds (more specifically, tertiary amines such as triethylamine and triethanolamine) and amine salts (more specifically, triethanolamine-adipate, etc.) also exhibit excellent effects in removing the oxide film (solder coating) from solder particles. Among these, triethanolamine and triethanolamine-adipate (powder, melting point 120°C, see [Chemical Formula 3]) are preferred from the viewpoint of relatively low reactivity with epoxy resin and excellent storage at room temperature. In addition, triethanolamine is preferred because it is liquid at room temperature and therefore easy to handle.

[0051] [ka]

[0052] Based on the above, the activator preferably includes at least one selected from the group consisting of levulinic acid, triethanolamine, triethanolamine adipate, and sebacic acid.

[0053] The activator preferably exhibits a liquid state at temperatures below 120°C in air. This is because it facilitates the preparation of solder particles coated with the activator, as will be described later. Examples of activators that exhibit a liquid state at temperatures below 120°C in air include levulinic acid, triethanolamine, and triethanolamine adipate. More preferably, the activator exhibits a liquid state at or near room temperature (e.g., 25-35°C). This is because it facilitates the preparation of solder particles whose entire surface is covered with the activator, thereby improving the wettability of the activator with the surface of the solder particles. Furthermore, it is even more preferable that the activator coating the solder particles has low viscosity at room temperature. To adjust and reduce viscosity, an activator solution with added solvent may be used to coat the solder particles. Examples of activators that exhibit a liquid state at or near room temperature include levulinic acid and triethanolamine. Activators that exhibit a liquid state at or near room temperature are also preferable because they can form a relatively strong bond with the oxide film on the surface of the solder particles, thus maintaining the state of being coated with the activator. For example, these include hydrogen bonds between the components (tin oxide) that make up the oxide film on the surface of solder particles and the activator, as well as bonds between the components of the oxide film and the polar groups of the activator (more specifically, amino groups, etc.). The presence of these bonds can be confirmed, for example, by measuring the signals originating from these bonds using infrared spectroscopy.

[0054] (Solder particles coated with an activator) In the solder paste according to this embodiment, the solder particles of the solder powder are covered with an activator. In other words, the activator is concentrated at a high concentration (i.e., unevenly distributed) on the surface of the solder particles. Based on the technical idea that the activator in contact with the oxide film surface contributes significantly to oxide film removal, this embodiment is adopted to improve the activator's ability to remove oxide film from the solder particle surface. Therefore, even if the activator content is lower than the usual content, the oxide film on the solder particle surface can be efficiently reduced and removed.

[0055] The solder particles are coated with an activator. Preferably, the entire surface of the solder particles is coated with the activator. Solder particles whose entire surface is coated with an activator have the appearance of moist powder particles.

[0056] Since the solder particles can maintain a state of being covered with the activator, it is preferable that they form a relatively strong bond with the activator covering the surface of the solder particles. Maintaining a state of being covered with the activator means, for example, in a solder paste manufacturing method, even if epoxy resin comes into contact with the surface of the activator coating film during a step such as mixing the solder particles covered with the activator into flux after coating them with the activator, the activator constituting the coating film is less likely to diffuse and dissolve. As will be discussed later, this has been demonstrated by examples that show excellent solder meltability even with a lower activator content compared to that of ordinary solder paste.

[0057] It is preferable that the solder particles are covered with a coating of an activator. When the solder particles are covered with a coating of an activator, the activator content can be brought closer to the amount necessary to efficiently reduce and remove the oxide film on the surface of the solder particles. Therefore, the activator content can be reduced while effectively removing the oxide film of the solder particles during melting, and at the same time, the solder particle content can be further increased. Thus, in this case, a stronger connection with superior connection stability can be formed, while further suppressing the generation of side balls.

[0058] When the solder surface is pre-treated with a liquid activator, the thickness of the coating film is approximately 0.1 to 20 μm. When preparing a coating film, the film thickness can be adjusted, for example, by the size of the solder particles or (if a solvent is used) by the concentration of the activator. When preparing a coating film with a liquid activator, the film thickness is several 0.1 to several μm when using a liquid activator at room temperature. By dispersing and adding a solid (powdered) activator to a liquid activator at room temperature, a coating film with a thickness of several tens of μm can be easily prepared on the solder surface.

[0059] (Method for manufacturing solder particles coated with an activator) This shows an example of a method for manufacturing solder particles whose surface is covered with an activator (surface-coated solder). Prepare an activator solution by mixing a predetermined amount of liquid activator with a solvent. While stirring the solder powder in a container, drop the activator solution into the container. Stir until the entire mixture has a moist, granular appearance to create a surface-coated solder.

[0060] In order to coat the surface of fine solder particles with an activator, good wetting (adherence) between the activator and the surface of the solder particles is necessary. From the viewpoint of improving the wetting of the activator with the surface of the solder particles, it is preferable that the activator be liquid at around room temperature (for example, 25-35°C).

[0061] From the viewpoint of more easily covering the surface of solder particles, the activator preferably exhibits a liquid state at a temperature of 120°C or lower in air, and more preferably at room temperature in air. In other words, the melting point of the activator is preferably 120°C or lower, and more preferably room temperature or lower. The following describes the workability of surface coating of solder particles according to the melting point of the activator.

[0062] Even powdered activators at room temperature, such as triethanolamine adipate, which has a low melting point of 120°C, can be dissolved with a solvent at a temperature below 120°C to form a homogeneous solution. Solder particles can then be immersed in this solution, or the solder surface can be coated by spraying or dropping the solution onto the solder particles. When using this heated triethanolamine adipate solution, it is preferable to pre-mix the solder particles in powder form while spraying or dropping the triethanolamine adipate solution onto them, as this eliminates the need for a grinding step.

[0063] Even powder activators with melting points higher than 120°C, such as succinic acid (melting point 186°C), adipic acid (melting point 152°C), coric acid (melting point 142°C), and sebacic acid (melting point 133°C), can be made liquid by increasing the temperature. However, above 130°C, the carboxyl groups of the organic acids react with the oxide film on the surface of the solder particles upon heating, leading to increased viscosity. From the viewpoint of suppressing the increase in viscosity of the solder paste and suppressing the deterioration of printing properties, it is preferable that the melting point of the activator be 120°C or lower. However, powder activators with melting points higher than 120°C can be made into a solution in which the powder is dispersed in a liquid activator, and then dropped onto the solder powder to cover the oxide film of the solder particles. In this case, a composite layer is formed on top of the liquid activator covering the oxide film of the solder particles, further covered by the powder activator. As a result, solder particles covered with a composite layer of liquid and powder activators are produced.

[0064] (Other ingredients) In this embodiment, the flux may further contain other components in addition to the epoxy resin, curing agent, curing accelerator, and activator. Examples of other components include commonly used modifiers (more specifically, rosin), solvents, and thixotropes. An example of a solvent is diethylene glycol monohexyl ether. By adding a solvent to the flux, the viscosity and fluidity of the solder paste can be adjusted (more specifically, by reducing its viscosity and imparting fluidity, etc.), thereby improving the dispersibility of the components of the solder paste (e.g., solder powder). Examples of thixotropes include hydrogenated castor oil, stearic acid amide, sorbitol compounds, and hydrogenated castor oil. By adding a thixotrope to the flux, the shape of the solder paste (e.g., printed shape) can be maintained. An example of a thixotrope is Gelol MD manufactured by Shin Nippon Rika Co., Ltd.

[0065] (Method of manufacturing solder paste) Next, we will show an example of a method for preparing the solder paste according to the embodiment described above, and a specific method for creating (or manufacturing) a mounting structure by mounting electronic components on a circuit board using the solder paste.

[0066] The epoxy resin and curing agent (phenol novolac resin) mentioned above are heated and melted to prepare a homogeneous resin mixture. After cooling, the curing accelerator is weighed and mixed into the resin mixture to prepare a flux. The surface coating solder prepared by the method described above is added to the flux and mixed and kneaded. Solder paste is thus prepared. Thus, in the method for manufacturing solder paste, the solder powder is pre-treated by coating it with an activator, and then mixed with components such as epoxy resin to prepare the paste. The reason for this is not bound by any particular theory, but can be inferred as follows: The surface of the solder powder has affinity not only for the activator but also for other components in the solder paste (more specifically, the epoxy resin). For example, when solder powder is added to a mixture of activator and epoxy resin, the activator and epoxy resin compete to form weak chemical bonds with the tin oxide on the surface of the solder powder, covering the surface of the solder particles. Therefore, by pre-treating the surface of the solder powder with an activator, the surface of the solder powder can be preferentially and reliably covered with the activator in the solder paste according to this embodiment.

[0067] Furthermore, a thixotropic agent may be added as needed to improve printability. Additionally, an activator may be added to the flux mixed with the surface coating solder.

[0068] <Implementation Structure> An embodiment of the present invention is a mounting structure in which electronic components are mounted on a circuit board using the above-described solder paste, comprising a conductive portion in which the electronic components and the circuit board are metal-joined, and a reinforcing portion formed by covering the area around the conductive portion with a hardened flux.

[0069] The mounting structure according to this embodiment will be described with reference to Figure 1. Figure 1 is a cross-sectional view of a solder joint portion of a CSP joined using solder paste. As shown in Figure 1, the mounting structure (e.g., semiconductor device) comprises a circuit board 1 and a circuit board 3, electrodes 2 and 4 provided on the circuit boards 1 and 3, a solder conductive part (joint portion) 9 that metal-bonds electrodes 2 and 4, and a reinforcing portion 6b that covers at least a part of the surface of the solder conductive part 9. The circuit board 1 on which electrode 2 is provided is an electronic component (e.g., a chip component and a semiconductor component). Examples of chip components are chip resistors and chip capacitors. The circuit board 3 on which electrode 4 is provided is, for example, a printed circuit board. Examples of semiconductor components are semiconductor packages such as CSPs or BGAs formed with solder balls as terminals, QFPs formed with leads as terminals, and semiconductor elements (bare chips) formed with terminals without being housed in a package. The circuit boards 1 and 3 have conductive wiring. Electrodes 2 are terminals of electronic components. Electrodes 2 and 4 are electrically connected via the solder conductive part 9. The reinforcing portion 6b is composed of a cured flux product and reinforces the solder conductive portion 9 by covering at least a portion of its surface (surroundings), thereby improving its mechanical strength. The cured flux product is a cured product of epoxy resin in the solder paste that has been heat-cured.

[0070] (Method of manufacturing the implementation structure) Using the solder paste according to an embodiment of the present invention, an assembly structure according to this embodiment can be manufactured by mounting electronic components (more specifically, semiconductor components, etc.) on a circuit board having conductive wiring, for example. In this assembly, the solder paste is used to join the terminals of the electronic components to the electrodes of the circuit board, forming a solder conductive part (joint part) 9. An example of a manufacturing method for the assembly structure will be described with reference to Figures 2A to 2D. Figures 2A to 2D are schematic cross-sectional explanatory diagrams showing the joining process of the ball portion of a CSP using the solder paste according to an embodiment of the present invention. The manufacturing method for the assembly structure includes, for example, a coating step, a mounting step, and a joining step.

[0071] The coating process involves applying solder paste 7 to the electrodes 4 of the circuit board 3, as shown in Figure 2A. The solder paste 7 can be applied, for example, by placing a metal mask with through-holes in the same positions as the electrodes 4 onto the circuit board 3, supplying solder paste 7 to the surface of the metal mask, and filling the through-holes with a squeegee. After that, by removing the metal mask from the circuit board 3, a circuit board 3 can be obtained with solder paste 7 applied to each electrode 4.

[0072] As shown in Figure 2B, the mounting process involves mounting the electronic components (circuit board 1) onto the circuit board 3 using a chip mounter or the like, so that the electrodes 4 to which solder paste 7 has been applied and the electrodes 2 of the circuit board 1 face each other. Specifically, the electronic components are placed on top of the circuit board so that the electrodes 4 of the circuit board 3, to which the solder paste 7 has been applied, and the terminals of the electronic components (e.g., chip components or semiconductor components) on top of the circuit board 3 face each other while the applied solder paste 7 is still in an uncured state. Solder bumps 5 are formed on the electrodes 2 of the circuit board 1 used in the mounting process.

[0073] In the bonding process (reflow process), as shown in Figure 2C, the circuit board 1 on which the electronic components are mounted is heated using a heater (drying oven) 8. For example, a printed wiring board on which chip components are arranged is heated in a reflow oven to a predetermined heating temperature. As a result, as shown in Figure 2D, a solder conductive part 9 is formed where the solder powder has melted and become one with the solder ball, and a reinforcing part 6b is formed on the surface of the conductive part 9, which is made of a hardened flux (hardened epoxy resin). The electronic components and the circuit board 1 are electrically bonded via the solder conductive part 9.

[0074] In the joining process, it is necessary to sufficiently melt and agglomerate the solder powder, and then allow the curing reaction of the flux resin component to proceed sufficiently and appropriately. Specifically, if the curing reaction of the epoxy resin, which is the flux component in the solder paste, proceeds before the solder powder has sufficiently melted during the joining process, the flux will become thicker. This will inhibit the agglomeration and melting of the solder particles, preventing proper metallic conductivity. To avoid this situation and obtain proper conductivity and good mechanical strength, it is necessary to slow down the curing reaction of the flux resin (suppress the progress of the curing reaction) until the temperature of the heater 8 (e.g., reflow oven) rises to the melting point of the solder powder being used. Furthermore, once the temperature of the heater 8 reaches the melting point of the solder powder, the solder powder has sufficiently melted and fused with the solder balls of the electronic component (e.g., semiconductor component), and fused with the electrode metal of the circuit component, the flux resin needs to complete its curing reaction in a short time (e.g., a few minutes). [Examples]

[0075] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited in any way by the following examples. Furthermore, unless otherwise specified, "parts" and "%" in the examples refer to mass.

[0076] <Making solder paste> First, the following components for the solder paste (solder powder, flux raw materials, etc.) were prepared.

[0077] [Solder powder] • SAC305: Sn-3.0Ag-0.5Cu, average particle size 10-25 μm, melting point 219°C (manufactured by Mitsui Mining & Smelting Co., Ltd.) • SAC105: Sn-1.0Ag-0.5Cu, average particle size 10-25 μm, melting point 225°C (manufactured by Mitsui Mining & Smelting Co., Ltd.)

[0078] [Flux] (Epoxy resin: Base epoxy resin) • Bisphenol A type epoxy resin, YD8125 (manufactured by Nippon Steel Chemical & Material Co., Ltd., epoxy equivalent weight 170) (Hardening agent: Phenol novolac resin containing allyl groups (liquid phenolic resin)) • Allylated phenol novolac resin (allyl-modified phenol novolac resin): MEH8000H (manufactured by Meiwa Chemicals Co., Ltd., hydroxyl group equivalent 140) (Hardening agent: Biphenyl aralkylphenol resin (high-rigidity phenolic resin)) • Biphenyl aralkyl type phenol novolac resin, MEHC7851S (manufactured by Meiwa Chemicals Co., Ltd., hydroxyl group equivalent 210) (Curing accelerator) • Tetraphenylphosphonium tetraphenylborate (TPP-K) (Activator: Organic base) • Triethanolamine (TEA: manufactured by Yoneyama Pharmaceutical Co., Ltd.) (Activating agent: Organic acid) • Sebacic acid (manufactured by Fujifilm Wako Pure Chemical Corporation) • Levulinic acid (manufactured by Tokyo Chemical Industry Co., Ltd.) • Adipic acid (manufactured by Tokyo Chemical Industry Co., Ltd.) (Activator: Salt of an organic acid and an organic base) Triethanolamine adipate (TEA adipate) (manufactured by Showa Chemical Co., Ltd.)

[0079] [others] (solvent) • Diethylene glycol monohexyl ether (manufactured by Nippon Emulsifier Co., Ltd.) (Tixotropic agent) • Gel-ol MD (manufactured by Shin-Nippon Rika Co., Ltd.)

[0080] (Preparation of solder powder) Next, the activators (more specifically, activator I and activator II) and solvents shown in Tables 1-3 below were weighed and mixed in the amounts shown in Tables 1-3. This dispersed or dissolved the activators in the solvent to prepare an activator mixture. The resulting activator mixture was either a liquid-powder mixture or a liquid-liquid mixture.

[0081] Specifically, in Examples 1-2 and 7-8, TEA as an activator was mixed with a solvent to prepare a mixture. The resulting mixture was heated, and powdered sebacic acid was added and mixed to prepare an activator mixture. In Examples 4 and 6, TEA adipate was mixed with a solvent and dissolved at 120°C to prepare a mixture (mixed solution). TEA was added to the resulting mixed solution and mixed to prepare an activator mixture. In Example 5, powdered sebacic acid and levulinic acid were added to a heated solvent and mixed to prepare an activator mixture.

[0082] Next, the solder powder was placed in a container. While stirring the solder powder slowly in the container, the previously prepared activator mixture was added dropwise. The mixture was stirred until the entire mixture became moist. As a result, solder powder was prepared. The surface of the solder powder was covered with the activator.

[0083] (Preparation of flux) Next, the epoxy resin, curing agent, curing accelerator, activator, and thixotropic agent were weighed out to the compositions shown in Tables 1-3. These were then heated and mixed to prepare the flux. Note that the amount of activator in the flux is the amount excluding the mass of activator used to coat the solder powder.

[0084] (Preparation of solder paste) The obtained flux was mixed with solder powder having a coating film and kneaded for a predetermined time using a planetary mixer to prepare resin flux solder pastes for Examples 1-8 and Comparative Examples 1-2, respectively. The composition of the obtained resin flux solder pastes is summarized in Tables 1-3. The amount of each compound is expressed in parts by mass. The unit phr represents the total flux weight excluding the solvent. In quantity This shows the weight percentage of the additive relative to the total weight.

[0085] [Table 1]

[0086] [Table 2]

[0087] [Table 3]

[0088] <Measurement method, observation method> For Examples 1-8 and Comparative Examples 1-2, the following items were observed and measured. The results are shown in Tables 1-3 as characteristics of the solder paste in each example.

[0089] [Method for observing the coating film] Images were captured using a scanning electron microscope (SEM), and the images were observed to confirm that the solder particles were covered with an activator. The observation results are shown in Tables 1-3.

[0090] <Evaluation Method> Examples 1-8 and Comparative Examples 1-2 were evaluated based on the following items. The evaluation results are shown in Tables 1-3 as characteristics of the solder paste in each example.

[0091] [Connection stability (solder meltability) evaluation: Visual observation of solder balls] The appearance of the solder balls was observed. Based on the obtained appearance results, the connectivity (connection stability) between electrodes via the solder balls was evaluated according to the following criteria. The evaluation results are summarized in Tables 1 and 2. Connection stability was evaluated in accordance with Japanese Industrial Standard (JIS) Z3197. The evaluation results indicate increasing connectivity stability in the order of ×, △, ○, and ◎. The evaluation results are summarized in Tables 1 to 3. (Judgment criteria) ◎(Level 1): The solder melts and forms a large sphere, but there are no solder balls around it. ○ (Level 2): ​​The solder has melted and formed a large sphere, with three or fewer solder balls with a diameter of 75 μm or less surrounding it. △ (Level 3): The solder has melted into a large sphere, surrounded by four or more solder balls with a diameter of 75 μm or less, and they are not arranged in a semi-continuous ring. × (Level 4): The solder has melted and formed a large sphere, with numerous smaller spheres arranged in a semi-continuous ring around it.

[0092] [Evaluation of the suppression of side balls: Measurement of the number of side balls generated] The solder pastes from Examples 1-8 and Comparative Examples 1-2 were printed onto glass epoxy substrates using a metal mask to form printed areas. 1005 chip components (resistors) were mounted on these printed areas and reflow soldered. Evaluation samples were prepared in this manner. The number of side balls generated on the sides of the chip components was counted visually. The number of measurement samples, n, was 10. (Judgment criteria) ×: The number of side balls generated is one or more. ○: The number of side balls generated is 0 (no side balls were generated).

[0093] [Evaluation of adhesion strength: Measurement of shear adhesion strength] The solder pastes from Examples 1-8 and Comparative Examples 1-2 were printed onto glass epoxy substrates using a metal mask to form printed areas. 1005 chip resistors were mounted on the printed areas as components and reflow soldered. Evaluation samples were prepared in this manner. The shear strength (Kgf / chip) of the connection points of the mounted components was measured using a bond tester. The number of measurement samples n was 5, and the average value of the 5 measurements was calculated. The resulting average value was defined as the shear adhesion force. The adhesion force was evaluated based on the following evaluation criteria. The judgment results indicate increasing adhesion force in the order of ×, △, and ○. (Judgment criteria) ×: The adhesion strength is less than 1.0. △: The adhesion strength is between 1.0 kgf and less than 2.0 kgf. ○: The adhesion strength is 2.0 kgf or more.

[0094] [Overall assessment] Based on the three evaluations above, an overall assessment of the solder paste was made according to the following evaluation criteria. (Judgment criteria) ○: Of the three evaluation results, all are either ◎ or ○. △: Of the three evaluation results, none are ×, and at least one is △. ×: At least one of the three evaluation results is ×.

[0095] <Configuration and evaluation results> [Example 1] (composition) As shown in Table 1, the solder paste of Example 1 contained 520 parts by mass of SAC305 solder as solder powder and 33.23 parts by mass of flux (excluding solvent). The solder particles in the solder paste had a coating film coated with an activator. The ratio of solder powder to the total amount of solder paste (excluding solvent) of 553.23 parts by mass was 94.0% by weight. The flux contained 15.0 parts by mass (45.14 phr) of YD8125 as an epoxy resin, a phenol novolac resin as a curing agent, 0.500 parts by mass (1.50 phr) of a boron-containing compound (tetraphenylphosphonium tetraphenylborate) as a curing accelerator, 2.58 parts by weight (7.76 phr, 0.5% of solder) of triethanolamine (TEA) as a liquid activator, and 2.58 parts by weight (7.76 phr, 0.5% of solder) of sebaciate as a powder activator. The aforementioned phenol novolac resin consisted of 5.95 parts by weight (17.91 phr) of the liquid phenol MEH8000H and 5.95 parts by weight (17.91 phr) of the solid phenol MEHC7851S. The flux of Example 1 further contained 0.67 parts by weight (2.02 phr) of gelol MD as a thixotropic agent and 17.12 parts by weight (3.0% of the total solder paste) of diethylene glycol monohexyl ether as a solvent. Example 1 was an example that fell within the scope of the invention as described in claim 1.

[0096] (Evaluation results) In the evaluation results for Example 1, the appearance evaluation of the solder balls was level 2, resulting in a positive (○) rating. Furthermore, the number of side balls was 0, also resulting in a positive (○) rating. Additionally, the shear adhesion force was 2.4 kgf, also resulting in a positive (○) rating. Based on these results, the overall evaluation for Example 1 was positive (○).

[0097] [Examples 2-8] (composition) As shown in Tables 1 and 2, solder pastes were prepared in the same manner as in Example 1, except that at least one of the following was changed: the type and ratio of solder, the type and amount of activator, the type and amount of curing accelerator, the amount of solvent, and the amount of thixotropic agent. In Examples 4 and 6, a liquid activator mixture was prepared by dissolving the solid activator triethanolamine adipate in a solvent at 120°C. Examples 2-8, as shown in Tables 1-2, were also examples that fell within the scope of the invention according to claim 1. For example, the solder particles in Examples 2-8 had a coating film coated with an activator.

[0098] (Evaluation results) The overall evaluation for Examples 2-8 was either ○ or △. For example, in the evaluation results for Example 2, the appearance evaluation of the solder balls was level 2, resulting in a ○. Also, the number of side balls was 0, resulting in a ○. Furthermore, the shear adhesion force was 1.9 kgf, resulting in a △. As a result of these factors, the overall evaluation for Example 2 was △.

[0099] [Comparative Examples 1-2] Comparative Examples 1 and 2 were prepared in the same manner as in Example 1, except that at least one of the following was changed: the type and ratio of solder, the type and amount of activator, the type and amount of curing accelerator, the amount of solvent, and the amount of thixotropic agent, as shown in Table 3, and the surface of the solder particles was not coated with an activator. In other words, in Comparative Examples 1 and 2, solder paste was prepared by mixing solder powder that had not been coated with an activator using a conventional method with flux to achieve the composition shown in Table 3.

[0100] (Comparative Example 1) In Comparative Example 1, the solder powder content was 82.0% by weight, which was outside the range of 86-97% by weight. Furthermore, the solder particles did not have a coating film that covered the entire surface of the solder particles. In other words, Comparative Example 1 was an example that did not fall within the scope of the invention as described in Claim 1. The evaluation results for Comparative Example 1 showed that the appearance evaluation of the solder balls was level 3, resulting in a judgment of △. Furthermore, the number of side balls was 9, resulting in a judgment of ×. Additionally, the shear adhesion force was 1.9 kgf, also resulting in a judgment of △. As a result, the overall judgment for Comparative Example 1 was ×.

[0101] (Comparative Example 2) In Comparative Example 2, the solder particles did not have a coating film that covered the entire surface of the solder particles. In other words, Comparative Example 2 was an example that was not included in the scope of the invention as described in Claim 1. The evaluation results for Comparative Example 2 were as follows: the solder balls were at level 4, resulting in a "fail" (×) rating. Additionally, 6 side balls were generated, also resulting in a "fail" rating. Furthermore, the shear adhesion force was 0.9 kgf, which also resulted in a "fail" rating. As a result, the overall evaluation for Comparative Example 2 was "fail" (×).

[0102] Based on the results in Tables 1-3, as shown in Examples 1-8, a solder paste comprising solder powder and flux, wherein the flux comprises at least epoxy resin, a curing agent, a curing accelerator, and an activator, the curing agent being a phenol novolac resin, the activator being at least one selected from the group consisting of organic bases, organic acids, and their salts, the solder powder comprising a plurality of solder particles, the solder particles having a coating film coated with the activator, and the solder powder content being 86-97% by weight relative to 100% by weight of the total weight of the resin flux solder paste, can suppress the generation of side balls while forming a stronger connection with superior connection stability. [Industrial applicability]

[0103] The solder paste and mounting structure of the present invention can be used in a wide range of applications in the field of electrical / electronic circuit formation technology. For example, it can be used for connecting various electronic components and for bonding them to a substrate. Furthermore, it can be used in products that incorporate these elements, components, or substrates, such as DVDs, mobile phones, general-purpose home appliances, and automotive electronic products. [Explanation of symbols]

[0104] 1 CSP board 2 CSP electrodes 3 Circuit board 4. Electrodes of the circuit board 5 CSP solder bumps 7. Resin flux solder paste 6b Reinforcement section 8 Heating machine (drying oven) 9. Solder conductive part

Claims

1. A resin flux solder paste containing solder powder and flux, The flux comprises at least an epoxy resin, a curing agent, a curing accelerator, and an activator. The curing agent is a phenol novolac resin, The activator is at least one selected from the group consisting of organic bases, organic acids, and salts thereof. The aforementioned solder powder contains a plurality of solder particles, and the solder particles are covered with the aforementioned activator. The content of the solder powder is 86 to 97% by weight relative to 100% by weight of the total weight of the resin flux solder paste. The content of the activator is 0.5 to 3.0% by weight relative to 100% by weight of the total weight of the solder powder. The aforementioned curing accelerator is a boron-containing compound, The resin flux solder paste contains 0.5 to 3.0 phr of the curing accelerator based on 100% by weight of the total flux excluding the solvent.

2. The resin flux solder paste according to claim 1, wherein the entire surface of the solder particles is covered with the activator.

3. The resin flux solder paste according to claim 1 or 2, wherein the solder particles are covered with a coating film of the activator.

4. The resin flux solder paste according to claim 3, wherein the thickness of the coating film is 0.1 to 20 μm.

5. The resin flux solder paste according to any one of claims 1 to 4, wherein the activator exhibits a liquid state at a temperature of 120°C or lower in air.

6. The resin flux solder paste according to any one of claims 1 to 5, wherein the activator comprises at least one selected from the group consisting of levulinic acid, triethanolamine, triethanolamine adipate, and sebacic acid.

7. The resin flux solder paste according to any one of claims 1 to 6, wherein the boron-containing compound is tetraphenylphosphonium tetraphenylborate.

8. The resin flux solder paste according to any one of claims 1 to 7, wherein the phenol novolac resin has a phenolic hydroxyl group and an allyl group in its molecule.

9. The aforementioned solder powder has a melting point of 200°C or higher. The resin flux solder paste according to any one of claims 1 to 8, wherein the solder powder comprises Sn alone, or an alloy selected from the group consisting of Sn-Ag alloys, Sn-Cu alloys, and Sn-Ag-Cu alloys, further comprising an additional metal, such as a Sn-Ag alloy system, a Sn-Cu alloy system, a Sn-Ag-Cu alloy system, or an additional metal.

10. A mounting structure comprising a circuit board on which electronic components are mounted using the resin flux solder paste described in any one of claims 1 to 9, A mounting structure comprising a conductive portion formed by metal bonding of the electronic component and the circuit board, and a reinforcing portion formed by covering at least a part of the area surrounding the conductive portion with a hardened flux.

Images