Solder alloys, solder paste, printed circuit boards and electronic control devices

Abstract

This invention relates to solder alloys, solder pastes, printed circuit boards, and electronic control devices. A solder alloy is provided that can suppress the occurrence and propagation of cracks within solder joints caused by thermal fatigue, and can suppress the fracture of solder joints caused by instantaneous and concentrated strong external forces. One solder alloy comprises 2% by mass and 4.2% by mass of Ag, more than 0% by mass and 1% by mass of Cu, 3% by mass and 5.5% by mass of Sb, 4% by mass and 5.9% by mass of In, 0.01% by mass and 0.005% by mass of Co, wherein the total content of Ni and Co is 0.05% by mass and less, and the remainder consists of Sn and unavoidable impurities.

Description

Technical Field

[0001] The present invention relates to a solder alloy, a solder paste, a printed circuit board and an electronic control device. Prior Art

[0002] Solder alloys are widely known as materials used to join materials (e.g., printed circuit boards and electronic components). In soldering using solder alloys, the materials are electrically connected via the solder joint. Furthermore, lead-free solder alloys have become increasingly common in recent years to reduce environmental impact.

[0003] With the advancements in electronic equipment performance and the expansion in the types and uses of electronic control devices, the internal environment of these devices is becoming more severe than ever. In particular, the internal temperature of electronic devices fluctuates significantly between when in use and when not in use. This repetitive temperature fluctuation, known as thermal cycling, exerts repeated stress on electronic control devices and the printed circuit boards (PCBs) incorporated into these devices. Furthermore, due to differences in the linear expansion coefficients of the components comprising the PCB, thermal stress concentrated in solder joints causes repeated plastic deformation, leading to the initiation and expansion of cracks in these joints caused by thermal fatigue.

[0004] As a solder alloy that suppresses the occurrence and expansion of cracks in solder joints caused by thermal fatigue, for example, a solder alloy containing elements such as Sb and In added to a Sn-Ag-Cu solder alloy has been proposed (see Patent Document 1). Specifically, Patent Document 1 discloses a solder alloy comprising 2.5 to 4.5% by weight of Ag, 0.6 to 2.0% by weight of Cu, 2.5 to 9.0% by weight of Sb, and the remainder being Sn. This solder alloy improves the thermal fatigue resistance of the solder joint under extremely severe thermal cycle or thermal shock test conditions.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application No. 2018-518368 Summary of the Invention

[0008] Problems to be solved by the invention

[0009] In recent years, with the advancement of electronic equipment's performance, the density of electronic devices, electronic control units, and printed circuit boards has been increasing, leading to a trend toward higher-density packaging and smaller sizes. Consequently, electronic components mounted on printed circuit boards have also been miniaturized and simplified (e.g., eliminating lead wires). Furthermore, solder joining methods, which utilize "footless" solder joints that eliminate the need for solder fillets, have begun to be adopted.

[0010] However, in these methods, in an environment where the above-mentioned hot and cold cycles occur, thermal stress is more likely to concentrate on the solder joint, and cracks caused by thermal fatigue become more likely to expand.

[0011] Furthermore, depending on the type of electronic device, a sudden and concentrated strong external force may be applied to the electronic device (printed circuit board, etc.) due to dropping during use, etc. This external force may also cause the solder joints to break.

[0012] Therefore, an object of the present invention is to provide a solder alloy that can suppress the occurrence and expansion of cracks in a solder joint due to thermal fatigue and can suppress the breakage of the solder joint due to a sudden and concentrated strong external force.

[0013] Solutions for solving problems

[0014] The present invention has the following aspects.

[0015] (1) A solder alloy comprising 2 mass % or more and 4.2 mass % or less of Ag, more than 0 mass % and 1 mass % or less of Cu, 3 mass % or more and 5.5 mass % or less of Sb, 4 mass % or more and 5.9 mass % or less of In, 0.01 mass % or more of Ni, and 0.005 mass % or more of Co, the total content of Ni and Co being 0.05 mass % or less, and the remainder consisting of Sn and unavoidable impurities.

[0016] (2) The solder alloy according to (1), wherein the Ag content is 2.5 mass % or more and 4 mass % or less.

[0017] (3) The solder alloy according to (1) or (2), wherein the Cu content is 0.4 mass % or more and 0.8 mass % or less.

[0018] (4) The solder alloy according to any one of (1) to (3), wherein the Sb content is 3 mass % or more and 5 mass % or less.

[0019] (5) The solder alloy according to any one of (1) to (4), wherein the Sb content is 3.2 mass % or more and 4.5 mass % or less.

[0020] (6) The solder alloy according to any one of (1) to (5), wherein the content of In is 4.5 mass % or more and 5.9 mass % or less.

[0021] (7) The solder alloy according to any one of (1) to (6), wherein the content of In is 5 mass % or more and 5.9 mass % or less.

[0022] (8) A solder paste comprising: a powder of the solder alloy described in any one of (1) to (7); and a flux comprising a base resin, a thixotropic agent, an activator, and a solvent.

[0023] (9) A printed circuit board having a solder joint formed using the solder alloy described in any one of (1) to (7).

[0024] (10) An electronic control device comprising the printed circuit board described in (9).

[0025] Effects of the Invention

[0026] The solder alloy of the present invention can suppress the occurrence and expansion of cracks in a solder joint due to thermal fatigue, and can suppress the breakage of the solder joint due to a sudden and concentrated strong external force. Simple diagram description

[0027] FIG1 is a schematic cross-sectional view of a printed circuit board produced in (1) the crack resistance test (without solder fillets).

[0028] FIG2 is a schematic cross-sectional view of a printed circuit board produced in the (2) crack resistance test (with solder fillets). Implementation Method

[0029] The solder alloy, solder paste, printed circuit board, and electronic control device of the present invention are described below. It should be noted that the present invention is not limited to the following embodiments.

[0030] 1. Solder alloy

[0031] The solder alloy of this embodiment contains Ag in an amount of not less than 2 mass % and not more than 4.2 mass %, Cu in an amount of not less than 0 mass % and not more than 1 mass %, Sb in an amount of not less than 3 mass % and not more than 5.5 mass %, In in an amount of not less than 4 mass % and not more than 5.9 mass %, Ni in an amount of not less than 0.01 mass %, and Co in an amount of not less than 0.005 mass %, wherein the total content of Ni and Co is not more than 0.05 mass %, and the remainder is composed of Sn and unavoidable impurities.

[0032] The occurrence and expansion of cracks in solder joints due to thermal fatigue can be attributed to a variety of factors.

[0033] Specifically, the repeated stresses generated within the solder joint due to thermal cycling cause plastic deformation, which in turn leads to cracks on the solder joint surface caused by grain boundary sliding. Furthermore, the repeated concentration of stress on these cracks facilitates their propagation. As a result, the solder joint fractures under stresses lower than the (maximum) tensile strength of the solder joint. Furthermore, among the intermetallic compounds precipitated within the solder joint, some are coarsened by thermal cycling. Furthermore, cracks propagate along this coarsened intermetallic compound layer.

[0034] To suppress plastic deformation in solder joints, it is considered desirable to add alloying elements (Sb, In) to the solder alloy that dissolve in the Sn phase and strengthen the solder joint (solid solution strengthening). On the other hand, solder alloys that are difficult to plastically deform may be prone to brittle fracture.

[0035] Furthermore, Sb and In can form intermetallic compounds with Sn through the solder joint, which can strengthen the solder joint through precipitation, thereby increasing the tensile strength of the solder joint. However, as mentioned above, the stress concentrated in the solder joint due to thermal cycling reduces the (maximum) tensile strength of the solder joint. Therefore, even if the tensile strength of the solder joint is high before thermal cycling, cracks in the solder joint, where the tensile strength has been reduced due to thermal cycling, are likely to propagate, ultimately leading to fracture.

[0036] Furthermore, as mentioned above, sudden and concentrated strong external forces can be applied to electronic devices during use, such as when they are dropped, sometimes resulting in fracture of solder joints. To withstand the stress caused by these external forces, solder joints must have high energy absorption. Specifically, the solder joints must have high yield strength and tensile strength, and high ductility, the amount of strain required to fracture after a stress exceeding the tensile strength is applied. As mentioned above, solder alloys with high yield strength are more susceptible to brittle fracture, so it is also necessary to suppress brittleness in the solder alloy.

[0037] Therefore, there is a concern that the solid solution strengthening and precipitation strengthening of the solder joints achieved solely by the addition of these alloying elements may not be sufficient to fully withstand these external forces. In particular, the trade-off between Sb and In solid solution strengthening and increased brittleness makes the solder joints susceptible to brittle fracture caused by these external forces.

[0038] On the other hand, the solder alloy of this embodiment, through the above-described method, can improve the yield strength of the solder joint through solid solution strengthening of Sb and In, while simultaneously improving the tensile strength of the solder joint through precipitation strengthening of Sb, In, Ni, and Co. Furthermore, by adjusting the type and content of each alloying element, embrittlement can be suppressed and the ductility of the solder joint can be improved.

[0039] Therefore, the solder alloy of this embodiment can suppress the occurrence and expansion of cracks in the solder joint due to thermal fatigue.

[0040] Furthermore, external forces applied to solder joints, such as from dropping electronic devices, generate multiple stresses (tensile, compressive, shearing, bending, and torsion) almost simultaneously, subjecting the solder joints to significant loads. However, the solder alloy of this embodiment suppresses embrittlement of the solder joints, achieving a well-balanced improvement in yield strength, tensile strength, and ductility. This allows it to withstand these external forces, demonstrating excellent impact resistance. Therefore, the solder alloy of this embodiment is also suitable for electronic devices designed to be dropped during use, electronic devices subject to sudden impact, electronic control units, and printed circuit boards.

[0041] In addition, the solder alloy of this embodiment can also be used for miniaturization and simplification, especially for the installation of leadless electronic components such as BGA (Ball Grid Array) and QFN (Quad Flat Non-leaded package), and the formation of solder joints without solder foot shapes.

[0042] Specifically, on printed circuit boards mounted with electronic components such as BGAs and QFNs, stress tends to concentrate on the solder joints. Furthermore, this stress concentration not only promotes plastic deformation and the resulting cracking of the solder joints, but also promotes brittle failure and the development of brittle cracks (brittle cracks) in the solder joints. Furthermore, the stress generated in the cracks increases, making them more likely to expand.

[0043] On the other hand, the solder alloy of this embodiment can suppress plastic deformation in the solder joint through the above-mentioned method, while also preventing brittle fracture and brittle cracking. Furthermore, it can form a solder joint with high tensile strength that is not easily degraded. Therefore, even if cracks occur in the solder joint, their expansion can be slowed. Furthermore, due to its excellent ductility, even if thermal stress exceeding the tensile strength of the solder joint is generated in the solder joint due to stress concentration, the expansion of the crack until the solder joint breaks can be suppressed. Therefore, the solder alloy of this embodiment is also suitable for mounting electronic components such as BGAs and QFNs.

[0044] Furthermore, solder joints using fillet-less solder joints reduce the volume of the solder joint itself, and the contact area between the solder joint and the joined material (electronic components, substrates, etc.) is also reduced. Consequently, it is believed that the yield strength and tensile strength, particularly the tensile strength, of solder joints without fillets are significantly reduced. Consequently, cracks that develop in the solder joints of printed circuit boards with fillet-less solder joints are more likely to propagate when placed in a hot / cold cycle environment.

[0045] On the other hand, the solder alloy of this embodiment, through the aforementioned mechanism, provides excellent tensile strength in the solder joint and suppresses the reduction in tensile strength caused by thermal cycling. Furthermore, the solder alloy of this embodiment has excellent yield strength, thus slowing the growth of cracks even if they develop in the solder joint. Furthermore, its excellent ductility suppresses the growth of cracks that eventually lead to fracture of the solder joint. Therefore, the solder alloy of this embodiment is also suitable for forming solder joints without fillets.

[0046] (1) Ag

[0047] The solder alloy of this embodiment contains Ag at a concentration of 2% to 4.2% by mass. This allows for the well-balanced precipitation of fine Ag3Sn intermetallic compounds at the Sn grain boundaries of the solder joint, thereby improving the yield strength and tensile strength of the solder joint without impairing ductility. This also enhances the thermal fatigue resistance and impact resistance of the solder joint. Furthermore, this embodiment prevents the Ag3Sn intermetallic compounds from coarsening, thereby suppressing a decrease in the tensile strength and ductility of the solder joint.

[0048] The Ag content is more preferably 2.5 mass % or more and 4 mass % or less, and even more preferably 2.8 mass % or more and 3.8 mass % or less.

[0049] By keeping the Ag content within this range, the yield strength and tensile strength of the solder joint can be further increased without impairing ductility, thereby further improving the thermal fatigue resistance and impact resistance of the solder joint. Furthermore, this can further suppress embrittlement of the solder joint and further reduce the reduction in tensile strength and ductility caused by thermal cycling.

[0050] (2)Cu

[0051] The solder alloy of this embodiment contains more than 0% by mass and less than 1% by mass of Cu. This approach allows for excellent joint reliability in the solder joint. Furthermore, the well-balanced precipitation of the Cu6Sn5 intermetallic compound at the Sn grain boundaries in the solder joint improves the yield strength and tensile strength of the solder joint without impairing ductility. This also enhances the thermal fatigue resistance and impact resistance of the solder joint. Furthermore, according to the above approach, the concentrated precipitation of the Cu6Sn5 intermetallic compound at the joint interface suppresses coarsening, thereby preventing embrittlement of the solder joint and suppressing the reduction in tensile strength and ductility caused by thermal cycling.

[0052] A more preferable content of Cu is 0.4 mass % or more and 0.8 mass % or less.

[0053] By setting the Cu content within this range, the yield strength and tensile strength of the solder joint can be further improved without impairing ductility, thereby further improving the thermal fatigue resistance and impact resistance of the solder joint.

[0054] (3)Sb

[0055] The solder alloy of this embodiment contains Sb at a concentration of 3% to 5.5% by mass. This allows for solid solution strengthening of Sb and precipitation strengthening of fine SnSb and ε-Ag3(Sn, Sb) intermetallic compounds at Sn grain boundaries without impairing the ductility of the solder joint. This suppresses grain boundary sliding deformation in the solder joint and improves the tensile strength of the solder joint.

[0056] In addition, according to the above embodiment, the SnSb and ε-Ag 3 (Sn, Sb) intermetallic compounds are less likely to coarsen, thereby preventing embrittlement of the solder joint and suppressing the reduction in tensile strength and ductility caused by thermal cycling.

[0057] The Sb content is preferably 3 mass % or more and 5 mass % or less, more preferably 3.2 mass % or more and 4.5 mass % or less, and still more preferably 3.5 mass % or more and 4.1 mass % or less.

[0058] By setting the Sb content within this range, the yield strength and tensile strength of the solder joint can be further improved without impairing ductility, thereby further improving the thermal fatigue resistance and impact resistance of the solder joint.

[0059] (4)In

[0060] The solder alloy of this embodiment contains 4% to 5.9% In by mass. This allows for solid solution strengthening of In and precipitation strengthening of fine AgSnIn and InSb intermetallic compounds at the Sn grain boundaries without impairing the ductility of the solder joint. This suppresses grain boundary sliding deformation in the solder joint and improves the tensile strength of the solder joint. Furthermore, the solder alloy of this embodiment suppresses embrittlement of the solder joint, thereby improving the thermal fatigue resistance and impact resistance of the solder joint.

[0061] Furthermore, according to the above aspect, the generation of γ-InSn 4 in the solder joint portion and the resulting deformation of the solder joint portion can be suppressed, thereby suppressing a decrease in the tensile strength and ductility of the solder joint portion.

[0062] The In content is preferably 4.5% by mass or more and 5.9% by mass or less, more preferably 5% by mass or more and 5.9% by mass or less, and still more preferably 5.5% by mass or more and 5.9% by mass or less.

[0063] By setting the In content within this range, the yield strength and tensile strength of the solder joint can be further improved without impairing ductility, and the thermal fatigue resistance and impact resistance of the solder joint can be further improved.

[0064] (5)Ni, Co

[0065] The solder alloy of this embodiment contains at least 0.01% Ni by mass and at least 0.005% Co by mass. Furthermore, the total content of Ni and Co is 0.05% by mass or less. This method allows for the precipitation of fine (Cu, Ni, Co) 6Sn 5 intermetallic compounds within the solder joint during soldering and further dispersion within the solder joint. Consequently, the yield strength and tensile strength of the solder joint are increased without impairing ductility, thereby further enhancing the thermal fatigue resistance and impact resistance of the solder joint. Furthermore, according to the above method, the (Cu, Ni, Co) 6Sn 5 intermetallic compounds are less likely to coarsen, thereby preventing embrittlement of the solder joint and suppressing the reduction in tensile strength and ductility caused by thermal cycling.

[0066] Furthermore, solder alloys containing Ni and Co may form needle-like structures composed of Ni, Co, Sn, and Cu within the solder ingot. Furthermore, when a solder ingot containing these needle-like structures is powdered using an atomizer, for example, the needle-like structures may precipitate within the powder. This, when used in solder paste, may reduce the printability of the solder paste. However, the solder alloy of the present application, through the aforementioned method, suppresses the precipitation of needle-like structures and improves the thermal fatigue resistance and impact resistance of the solder joint.

[0067] A more preferable Ni content is 0.01 mass % or more and 0.04 mass % or less, and a further preferable Ni content is 0.02 mass % or more and 0.04 mass % or less.

[0068] Furthermore, a more preferable content of Co is 0.005 mass % or more and 0.02 mass % or less, and a further more preferable content is 0.005 mass % or more and 0.01 mass % or less.

[0069] By setting the respective contents of Ni and Co within the above ranges and setting their total content within the above ranges, the yield strength and tensile strength of the solder joint can be further improved without impairing ductility, thereby further improving the thermal fatigue resistance and impact resistance of the solder joint.

[0070] It should be noted that the inventors conducted statistical analysis on the relationship between the content of each alloying element of Ag, Cu, Sb and In and the yield strength, tensile strength and ductility of the solder alloy, as well as the reduction in tensile strength and ductility caused by thermal cycling, and obtained a regression equation.

[0071] As a result, it was found that if the value calculated from the regression equation shown in the following formula (1) is 12 or more and 36 or less, the thermal fatigue resistance and impact resistance of the solder joint can be further improved. 12≤58.046+(-4.238×Ag content)+(-11.371×Cu content)+(-2.133×Sb content)+(-1.024×In content)≤36…(1)

[0072] In the above formula (1), the units of the contents of Ag, Cu, Sb and In are all mass %.

[0073] In addition, when the value calculated by the regression equation shown in the above formula (1) is 18 or more and 30 or less, the thermal fatigue resistance and impact resistance of the solder joint can be further improved.

[0074] In addition, the remainder of the solder alloy of this embodiment is composed of Sn and inevitable impurities.

[0075] 2. Bonding materials

[0076] The bonding material of this embodiment comprises the aforementioned solder alloy and can be used in the following forms, such as solder paste, solder balls, solder wire, solder preforms, and resin-core solder. The bonding material form can be appropriately selected based on the size, type, and application of the materials to be bonded, as well as the solder bonding method.

[0077] Furthermore, the bonding material of this embodiment can suppress the occurrence and expansion of cracks in the solder joint due to thermal fatigue by containing the above-mentioned solder alloy, and can also suppress the breakage of the solder joint due to instantaneous and concentrated strong external force.

[0078] 3. Solder paste

[0079] The solder paste of this embodiment includes powder formed of the above-mentioned solder alloy (hereinafter referred to as "alloy powder"), and is produced by, for example, kneading the above-mentioned alloy powder with flux to form a paste.

[0080] <Flux>

[0081] The aforementioned flux contains, for example, a base resin, a thixotropic agent, an activator, and a solvent.

[0082] Examples of the base resin include rosin-based resins; acrylic resins obtained by polymerizing at least one monomer such as acrylic acid, methacrylic acid, various esters of acrylic acid, various esters of methacrylic acid, crotonic acid, itaconic acid, maleic acid, maleic anhydride, esters of maleic acid, esters of maleic anhydride, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl chloride, and vinyl acetate; epoxy resins; and phenolic resins. These can be used alone or in combination.

[0083] Examples of the thixotropic agent include hardened castor oil, hydrogenated castor oil, diamide-based thixotropic agents (such as saturated fatty acid diamides, unsaturated fatty acid diamides, and aromatic diamides), oxygen-containing fatty acids, and dimethyldibenzylidene sorbitol. These agents may be used alone or in combination.

[0084] Examples of the activator include organic acids (monocarboxylic acids, dicarboxylic acids, and other organic acids), halogen-containing compounds, and amine-based activators. These can be used alone or in combination.

[0085] Examples of the solvent include alcohol-based, butyl cellosolve-based, glycol ether-based, and ester-based solvents, which may be used alone or in combination.

[0086] Furthermore, the flux may be blended with an antioxidant. Examples of such antioxidants include hindered phenol-based antioxidants, phenol-based antioxidants, bisphenol-based antioxidants, and polymer-based antioxidants.

[0087] In addition, additives such as matting agents and defoaming agents may be further added to the aforementioned flux.

[0088] The alloy powder and flux blend ratio (mass %) when preparing the solder paste of this embodiment can be set to 65:35 to 95:5 in terms of alloy powder:flux ratio. Alternatively, the blend ratio can be set to 85:15 to 93:7 or 87:13 to 92:8, for example.

[0089] It should be noted that the particle size of the alloy powder can be set to 1 μm or more and 40 μm or less. Alternatively, the particle size can be set to 5 μm or more and 35 μm or less, or 10 μm or more and 30 μm or less.

[0090] Furthermore, the solder paste of this embodiment, by containing the aforementioned alloy powder, can suppress the occurrence and expansion of cracks in the solder joint due to thermal fatigue, and can also suppress the breakage of the solder joint due to instantaneous and concentrated strong external force.

[0091] 4. Joint

[0092] The joint in this embodiment is formed using the aforementioned solder alloy and joins the materials to be joined. It should be noted that, in this specification, a joint formed using a joining material containing the aforementioned solder alloy is also included in the term "joint formed using a solder alloy." The joint in this embodiment can be formed using any method, such as reflow soldering or flow soldering, as long as it can be formed using the aforementioned solder alloy. Furthermore, the joining material used can be appropriately selected based on the size, type, and application of the materials to be joined, as well as the method of formation.

[0093] 5. Joint structure

[0094] The joint structure of this embodiment includes a first material to be joined, a joint portion, and a second material to be joined. The joint portion is formed using the solder alloy, which is the joint portion, and joins the first material to be joined and the second material to be joined.

[0095] Examples of combinations of the first and second materials to be joined include two or more selected from substrates (having a surface formed of any of ceramic, metal, alloy, or resin, and not having an electronic circuit formed thereon), printed circuit boards (substrates having an electronic circuit formed thereon, but not having electronic components mounted thereon), printed circuit boards (printed circuit boards having electronic components mounted thereon), electronic components, silicon wafers, semiconductor packages, and semiconductor chips. Specific examples of combinations include printed circuit boards and electronic components, printed circuit boards and semiconductor chips, semiconductor packages and printed circuit boards, and printed circuit boards and printed circuit boards.

[0096] In addition, the bonding structure of this embodiment is produced, for example, by the following method.

[0097] When using a printed circuit board as the first material to be joined and an electronic component as the second material to be joined, first, the joining material (in the case of solder paste, this is applied) is placed at a predetermined location on the first material to be joined, such as on an electronic circuit, and the second material to be joined is placed thereon. Then, these are reflowed at a predetermined heating temperature, such as a peak temperature of 245°C, to form a joint between the first and second materials to be joined. This produces the joined structure (printed circuit board) of this embodiment.

[0098] It should be noted that when a preform is used as the aforementioned joining material, flux is applied to the surface of the preform, it is placed at a predetermined position on the aforementioned first joining material, the aforementioned second joining material is placed thereon, and heating is performed.

[0099] In addition, when an electronic component with solder balls such as a BGA is used as the second bonding material, solder paste is applied to the surface of the BGA at a predetermined position of the first bonding material, and the second bonding material is placed on the predetermined position of the first bonding material and heated.

[0100] Furthermore, the joint structure of this embodiment includes a joint formed using the aforementioned solder alloy. This suppresses the occurrence and expansion of cracks within the solder joint due to thermal fatigue, and also suppresses breakage of the solder joint due to sudden, concentrated, strong external forces. Thus, this embodiment provides a highly reliable joint structure.

[0101] 6. Electronic control device

[0102] The electronic control device of this embodiment includes the joint structure of this embodiment. For example, a printed circuit board (PCB) composed of electronic components and a printed circuit board is placed within a housing, controlling the operation of components that make up the electronic device. Furthermore, the electronic control device of this embodiment can suppress the occurrence and expansion of cracks in the solder joints caused by thermal fatigue, and can also suppress the breakage of the solder joints caused by instantaneous, concentrated, strong external forces. Thus, according to this embodiment, a highly reliable electronic control device can be provided.

[0103] Example

[0104] Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples. However, the present invention is not limited to these Examples.

[0105] Preparation of flux

[0106] The following components were mixed to obtain the fluxes of Examples and Comparative Examples. Hydrogenated acid-modified rosin (product name: KE-604, manufactured by Arakawa Chemical Industries, Ltd.) 49% by mass Glutaric acid 0.3 mass% Suberic acid 2% by mass Malonic acid 0.5% by mass Dodecanedioic acid 2% by mass Dibromobutylene glycol 2% by mass Fatty acid amide (product name: Slipacks ZHH, manufactured by Nippon Chemical Co., Ltd.) 6% by mass Diethylene glycol monohexyl ether 35.2% by mass Hindered phenol antioxidant (product name: Irganox 245, manufactured by BASF Japan Co., Ltd.) 3% by mass

[0107] Preparation of solder paste

[0108] Powders of the solder alloys listed in Tables 1 and 2 were prepared by atomization. Subsequently, the solder alloy powders (powder particle size 20 μm to 38 μm) were mixed with the aforementioned flux at the following blending ratios (mass %) to prepare solder pastes according to Examples and Comparative Examples.

[0109] Solder alloy powder: flux = 89:11

[0110] Table 1 quality% Sn Ag Cu Sb In Ni Co Example 1 The remainder 3.5 0.7 3.0 5.5 0.04 0.005 Example 2 The remainder 3.5 0.7 3.2 5.5 0.04 0.005 Example 3 The remainder 3.5 0.7 3.5 5.5 0.04 0.005 Example 4 The remainder 3.5 0.7 4.1 5.5 0.04 0.005 Example 5 The remainder 3.5 0.7 4.5 5.5 0.04 0.005 Example 6 The remainder 3.5 0.7 5.0 5.5 0.04 0.005 Example 7 The remainder 3.5 0.7 3.5 4.0 0.04 0.005 Example 8 The remainder 3.5 0.7 3.5 5.0 0.04 0.005 Example 9 The remainder 3.5 0.7 3.5 5.9 0.04 0.005 Example 10 The remainder 2.5 0.7 3.5 5.5 0.04 0.005 Example 11 The remainder 2.8 0.7 3.5 5.5 0.04 0.005 Example 12 The remainder 3.8 0.7 3.5 5.5 0.04 0.005 Example 13 The remainder 4.0 0.7 3.5 5.5 0.04 0.005 Example 14 The remainder 3.5 0.1 3.5 5.5 0.04 0.005 Example 15 The remainder 3.5 0.4 3.5 5.5 0.04 0.005 Example 16 The remainder 3.5 0.8 3.5 5.5 0.04 0.005 Example 17 The remainder 3.5 1.0 3.5 5.5 0.04 0.005 Example 18 The remainder 3.5 0.7 3.5 5.5 0.01 0.005 Example 19 The remainder 3.5 0.7 3.5 5.5 0.04 0.01 Example 20 The remainder 3.5 0.7 3.5 5.5 0.03 0.02 Example 21 The remainder 3.5 0.7 3.5 5.5 0.02 0.01 Example 22 The remainder 3.5 0.7 3.5 5.5 0.02 0.02 Example 23 The remainder 2.5 0.1 3.0 4.0 0.01 0.005 Example 24 The remainder 4.0 0.7 5.0 5.9 0.04 0.01 Example 25 The remainder 2.0 0.1 3.0 4.0 0.01 0.005 Example 26 The remainder 4.2 1.0 5.5 5.9 0.01 0.005

[0111] Table 2 quality% Sn Ag Cu Sb In Ni Co Comparative Example 1 The remainder 3.5 0.7 2.0 5.5 0.04 0.005 Comparative Example 2 The remainder 3.5 0.7 6.0 5.5 0.04 0.005 Comparative Example 3 The remainder 3.5 0.7 3.5 3.0 0.04 0.005 Comparative Example 4 The remainder 3.5 0.7 3.5 7.0 0.04 0.005 Comparative Example 5 The remainder 1.0 0.7 3.5 5.5 0.04 0.005 Comparative Example 6 The remainder 4.5 0.7 3.5 5.5 0.04 0.005 Comparative Example 7 The remainder 3.5 1.5 3.5 5.5 0.04 0.005 Comparative Example 8 The remainder 3.5 0.7 3.5 5.5 - 0.01 Comparative Example 9 The remainder 3.5 0.7 3.5 5.5 0.06 0.01 Comparative Example 10 The remainder 3.5 0.7 3.5 5.5 0.01 - Comparative Example 11 The remainder 3.5 0.7 3.5 5.5 0.01 0.06 Comparative Example 12 The remainder 1.0 0.7 2.0 3.0 0.04 0.05 Comparative Example 13 The remainder 4.5 1.5 6.0 7.5 0.04 0.05

[0112] (1) Crack resistance test (without weld leg)

[0113] Prepare the following tools. Chip capacitor: 2.0mm×1.2mm Glass epoxy substrate: Surface treated with Cu-OSP, with an insulating layer and electrode pattern (1.3mm x 0.45mm) capable of mounting the aforementioned chip capacitors. Thickness: 1.6mm. Metal mask: corresponds to the electrode pattern above. Thickness is 150 μm.

[0114] Then, each solder paste was tested through the following steps.

[0115] First, solder paste was printed on a glass epoxy substrate using a metal mask. Then, ten chip capacitors were placed at predetermined locations on the printed solder paste. The thickness of the printed solder paste was adjusted using the metal mask. Next, the glass epoxy substrate with the chip capacitors was reflowed in a reflow oven (product name: TNP-538EM, manufactured by Tamura Manufacturing Co., Ltd.) to create a printed circuit board (PCB) comprising chip capacitors, a glass epoxy substrate, and solder joints connecting them. Two solder joints were formed for each chip capacitor. A schematic cross-sectional view of the resulting printed circuit board is shown in Figure 1.

[0116] The reflow soldering conditions described above include preheating at 170°C to 190°C for 110 seconds, a peak temperature of 245°C, a time above 220°C for 45 seconds, and a cooling rate from the peak temperature to 200°C of 1°C to 8°C / second. The oxygen concentration was set to 1,500±500 ppm.

[0117] Next, using a thermal shock tester (product name: ES-76LMS, manufactured by HITACHI APPLIANCES, INC.), the printed circuit board was exposed to an environment in which one cycle consisted of -40°C (30 minutes) to 125°C (30 minutes). This thermal shock cycle was repeated 1,000 times to obtain a test substrate.

[0118] Then, the target portion of the test substrate was cut out and sealed with epoxy resin (product name: Epomount (base agent and curing agent), manufactured by Refine Tech Co., Ltd.).

[0119] Next, a wet grinder (TegraPol-25, manufactured by Marumoto Struas Corporation) was used to prepare a central cross-section of each chip capacitor mounted on the test substrate. A scanning electron microscope (TM-1000, manufactured by Hitachi High Technologies Co., Ltd.) was used to observe the presence and condition of cracks in all solder joints on the test substrate.

[0120] Then, based on the observation results, the total length of the cracks and the total length of the imaginary cracks were calculated for each solder joint, and the crack rate (%) was calculated based on the following formula.

[0121] Crack rate (%) = (total length of cracks) / (total length of imaginary cracks) × 100

[0122] It should be noted that the total crack length is the sum of the lengths of all cracks occurring in the solder joint. Furthermore, the total imaginary crack length is the length of the path of the cracks occurring in the solder joint, hypothetically rupturing the solder joint. This also applies to the following tests.

[0123] Then, the maximum value of the solder joint portion with the largest cracking rate (%) was evaluated according to the following criteria. The results are shown in Tables 3 and 4. ○: The maximum value of the crack rate exceeds 30% and is 40% or less △: The maximum crack rate is more than 40% and less than 50% ×: The maximum crack rate exceeds 50%

[0124] (2) Crack resistance test (with weld foot)

[0125] A test substrate was prepared under the same conditions as in the (1) Crack Occurrence Confirmation Test (without solder fillets), except that the following equipment was used. The crack rate (%) was determined under the same conditions. A schematic cross-sectional view of the printed circuit board prepared in this test is shown in Figure 2.

[0126] Glass epoxy substrate: Surface treated with Cu-OSP, with an insulating layer and electrode pattern (1.3mm x 0.8mm) capable of mounting the aforementioned chip capacitors. Thickness: 1.6mm.

[0127] Then, the maximum value of the solder joint portion with the largest cracking rate (%) was evaluated according to the following criteria. The results are shown in Tables 3 and 4. ◎: The maximum crack rate is less than 20% ○: The maximum value of the crack rate exceeds 20% and is 30% or less △: The maximum crack rate is more than 30% and less than 40% ×: The maximum crack rate exceeds 40%

[0128] (3) Crack resistance confirmation (QFN)

[0129] Prepare the following tools.

[0130] QFN: 0.5mm pitch (9mm in height x 9mm in width x 0.9mm in thickness, number of terminals: 64 pins)

[0131] Glass epoxy substrate: A pattern capable of mounting the aforementioned QFN is formed on a glass epoxy substrate surface treated with Cu-OSP. The thickness is 1.6 mm.

[0132] Metal mask: Corresponding to the above pattern. Thickness: 150 μm.

[0133] Then, test each solder paste according to the following steps.

[0134] First, solder paste was printed on a glass epoxy substrate using a metal mask. Five QFNs were then placed at predetermined locations on the printed solder paste. The thickness of the printed solder paste was adjusted using the metal mask. The glass epoxy substrate with the QFNs was then reflowed in a reflow oven (product name: TNV30-508EM2-X, manufactured by Tamura Manufacturing Co., Ltd.) to create a printed circuit board consisting of the QFNs, the glass epoxy substrate, and the solder joints connecting them. Two solder joints were formed for each QFN.

[0135] The reflow soldering conditions described above include preheating at 170°C to 190°C for 110 seconds, a peak temperature of 245°C, a time above 220°C for 45 seconds, and a cooling rate from the peak temperature to 200°C of 1°C to 8°C / second. The oxygen concentration was set to 1,500±500 ppm.

[0136] Next, using a thermal shock tester (product name: ES-76LMS, manufactured by HITACHI APPLIANCES, INC.), the printed circuit board was exposed to an environment in which one cycle consisted of -40°C (30 minutes) to 125°C (30 minutes). This thermal shock cycle was repeated 1,000 times to obtain a test substrate.

[0137] The target portion of the test substrate was cut out and sealed with epoxy resin (product name: Epomount (base agent and curing agent), manufactured by Refine Tech Co., Ltd.).

[0138] Then, a wet grinder (product name: TegraPol-25, manufactured by Marumoto Struas Corporation) was used to prepare the state of the central cross-section of each QFN mounted on the test substrate. A scanning electron microscope (product name: TM-1000, manufactured by Hitachi High Technologies Co., Ltd.) was used to observe the presence and state of cracks in all solder joints on the test substrate.

[0139] Then, based on the observation results, the total length of the cracks and the total length of the imaginary cracks were calculated for each solder joint, and the crack rate (%) was calculated based on the following formula.

[0140] Crack rate (%) = (total length of cracks) / (total length of imaginary cracks) × 100

[0141] Then, the maximum value of the solder joint portion with the largest cracking rate (%) was evaluated according to the following criteria. The results are shown in Tables 3 and 4. ◎: The maximum crack rate is less than 50% ○: The maximum value of the crack rate exceeds 50% and is 60% or less △: The maximum crack rate is more than 60% and less than 70% ×: The maximum crack rate exceeds 70%

[0142] (4) Confirmation test for needle-like substance production

[0143] Solder ingots formed of each solder alloy were produced.

[0144] Then, for each solder ingot, solder alloy powder was produced under the following conditions.

[0145] First, 50 g of the solder ingot, 890 g of castor oil, and 10 g of hydrogenated acid-modified rosin (product name: KE-604, manufactured by Arakawa Chemical Industries, Ltd.) were placed in a 2 L stainless steel beaker and continuously heated using a hooded heater.

[0146] When the temperature of the contents in the stainless steel beaker reached 160°C, stirring was started using a homogenizer (manufactured by SMT Co., Ltd.) at a rotation speed of 2,000 rpm. Heating with a hooded heater was continued during stirring.

[0147] When the temperature of the contents of the stainless steel beaker reached 270°C, heating was stopped, the homogenizer speed was increased to 10,000 rpm, and the contents of the stainless steel beaker were stirred for 5 minutes. After stirring, the contents of the stainless steel beaker were cooled to room temperature.

[0148] The solder alloy powder that had settled in the castor oil was removed from the stainless steel beaker and washed with ethyl acetate to remove any deposits. The solder alloy powder was then observed at 200x magnification using a digital microscope. The results were evaluated according to the following criteria. The results are shown in Tables 3 and 4. ○: No needle-like substances are generated in the solder alloy powder ×: Needle-shaped substances are generated in the solder alloy powder

[0149] (5) Charpy impact test

[0150] The test was conducted under the following conditions according to the conditions specified in JIS Z2242.

[0151] Test piece: U-notch test piece

[0152] Test temperature: 25℃

[0153] The impact energy required to break each test piece was divided by the cross-sectional area of ​​the test piece, and the impact value was evaluated according to the following criteria. The results are shown in Tables 3 and 4. ◎: Impact value is 15.0J / cm2 or more ○: Impact value is 12.5 J / cm2 or more and less than 15.0 J / cm2 △: Impact value is 10.0 J / cm2 or more and less than 12.5 J / cm2 ×: Impact value less than 10.0 J / cm 2

[0154] (6) Comprehensive evaluation

[0155] The test results of (1) to (5) above were scored according to the following criteria. The overall scores are shown in Tables 3 and 4. It should be noted that solder alloys with an overall score of 10 or less were considered to have failed to achieve the desired properties. ◎:5 points 0:3 points △: 1 point ×: -10 points

[0156] Table 3 (1) Crack resistance test (without weld leg) (2) Crack resistance test (with solder pins) (3) Crack resistance confirmation (QFN) (4) Confirmation test for needle-like substance production (5) Charpy impact test (6) Comprehensive evaluation Example 1 △ ○ ○ ○ ◎ 15 Example 2 ○ ○ ○ ○ ◎ 17 Example 3 ◎ ◎ ◎ ○ ◎ twenty three Example 4 ◎ ◎ ◎ ○ ◎ twenty three Example 5 ◎ ◎ ◎ ○ ○ twenty one Example 6 ◎ ◎ ◎ ○ △ 19 Example 7 △ ○ △ ○ ◎ 13 Example 8 ○ ◎ ○ ○ ◎ 19 Example 9 ◎ ◎ ◎ ○ ◎ twenty three Example 10 △ ○ ○ ○ ◎ 15 Example 11 ○ ○ ◎ ○ ◎ 19 Example 12 ○ ◎ ◎ ○ ◎ twenty one Example 13 △ ○ ○ ○ ○ 13 Example 14 ○ ○ ○ ○ ◎ 17 Example 15 ◎ ◎ ◎ ○ ◎ twenty three Example 16 ◎ ◎ ◎ ○ ◎ twenty three Example 17 ○ ○ ○ ○ ◎ 17 Example 18 ○ ○ △ ○ ◎ 15 Example 19 ◎ ◎ ◎ ○ ◎ twenty three Example 20 ◎ ◎ ◎ ○ ◎ twenty three Example 21 ◎ ◎ ◎ ○ ◎ twenty three Example 22 ◎ ◎ ◎ ○ ◎ twenty three Example 23 △ ○ △ ○ ◎ 13 Example 24 ○ ◎ ◎ ○ △ 17 Example 25 △ △ △ ○ ◎ 11 Example 26 ○ ○ △ ○ △ 11

[0157] Table 4 (1) Crack resistance test (without weld leg) (2) Crack resistance test (with weld foot) (3) Crack resistance confirmation (QFN) (4) Confirmation test for needle-like substance production (5) Charpy impact test (6) Comprehensive evaluation Comparative Example 1 × △ △ ○ ◎ 0 Comparative Example 2 ◎ ◎ ◎ ○ × 8 Comparative Example 3 × △ × ○ ◎ -11 Comparative Example 4 △ △ △ ○ × -4 Comparative Example 5 × × × ○ ◎ -twenty two Comparative Example 6 × △ △ ○ ○ -2 Comparative Example 7 × △ × ○ ◎ -11 Comparative Example 8 ○ ○ × ○ ◎ 4 Comparative Example 9 ◎ ◎ ◎ × ◎ 10 Comparative Example 10 ○ ○ × ○ ◎ 4 Comparative Example 11 ◎ ◎ ◎ × ◎ 10 Comparative Example 12 × × × × ◎ -35 Comparative Example 13 × × × × × -50

[0158] As shown above, the solder alloy of this embodiment can suppress the occurrence and expansion of cracks in the solder joint caused by thermal fatigue, regardless of whether the solder joint is formed into a solder joint with a fillet shape or a solder joint without a fillet shape.

[0159] Furthermore, it was found that the solder alloy of this example can suppress the occurrence and expansion of cracks in the solder joint due to thermal fatigue even when mounting a QFN without leads.

[0160] Furthermore, it can be seen that the solder alloy of this embodiment can suppress the fracture of the solder joint even when a strong external force is applied instantaneously and concentratedly to the solder joint, that is, it has good durability against impact.

[0161] It can be seen that the solder joint formed using the solder alloy of this embodiment not only has good yield strength and tensile strength, but also has good ductility.

[0162] Furthermore, since needle-like substances are not generated in the solder alloys of the examples, good printability can be ensured even when used in solder paste.

[0163] Therefore, printed circuit boards manufactured using this solder alloy and electronic control devices incorporating the same can ensure higher reliability regardless of their application.

[0164] 100,200:Printed circuit board 10A, 10B: Glass epoxy substrate 11A, 11B: Electrodes 12A, 12B: Insulation layer 20: Chip capacitors 30A, 30B: Solder joint

Claims

1. A solder alloy comprising 2% by mass and 4.2% by mass of Ag, more than 0% by mass and 1% by mass of Cu, 3% by mass and 5.5% by mass of Sb, 4.5% by mass and 5.9% by mass of In, 0.01% by mass of Ni, and 0.005% by mass of Co, wherein the total content of Ni and Co is 0.05% by mass or less, and the remainder consists of Sn and unavoidable impurities.

2. The solder alloy as described in claim 1, wherein, The content of Ag is 2.5% by mass or more and 4% by mass or less.

3. The solder alloy as described in claim 1, wherein, The Cu content is above 0.4% by mass and below 0.8% by mass.

4. The solder alloy as described in claim 2, wherein, The Cu content is above 0.4% by mass and below 0.8% by mass.

5. The solder alloy as claimed in any one of claims 1 to 4, wherein, The content of Sb is 3% by mass or more and 5% by mass or less.

6. The solder alloy as claimed in any one of claims 1 to 4, wherein, The content of Sb is 3.2% by mass or more and 4.5% by mass or less.

7. The solder alloy as claimed in any one of claims 1 to 4, wherein, The In content is 5% by mass or more and 5.9% by mass or less.

8. The solder alloy as described in claim 5, wherein, The In content is 5% by mass or more and 5.9% by mass or less.

9. The solder alloy as described in claim 6, wherein, The In content is 5% by mass or more and 5.9% by mass or less.

10. A solder paste comprising: a powder of a solder alloy as described in any one of claims 1 to 9; and a flux comprising a base resin, a thixotropic agent, an activator, and a solvent.

11. A printed circuit board having a solder joint formed using any one of claims 1 to 9.

12. An electronic control device comprising the printed circuit board described in claim 11.

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