solder material

Abstract

The present invention is a solder material having outstanding elongation at break in high-temperature environments.　Provided are: a solder material containing 5.0-10.0 mass% Sb, 2.0-6.0 mass% Ag, 0.1-0.5 mass% Ni, and 3.0-8.0 mass% Cu, the remainder being Sn and unavoidable impurities; a solder joint comprising a joint layer obtained by melting the solder material; and a semiconductor device comprising said joint.

Description

[Technical Field]

[0001] The present invention relates to a solder material, and more particularly to a solder material that has improved elongation at break in a high-temperature environment and is suitable for joining semiconductor elements. [Background technology]

[0002] Power semiconductor modules are widely used in fields that require efficient power conversion. For example, their application is expanding to power electronics fields such as industrial equipment, electric vehicles, and home appliances. These power semiconductor modules contain built-in switching elements and diodes, and the elements use Si (silicon) semiconductors or SiC (silicon carbide) semiconductors. These semiconductor elements are bonded to laminated substrates with bonding materials, which have traditionally been solder.

[0003] In recent years, due to environmental concerns, Pb-free solders that do not contain lead have been adopted as an alternative to Sn-Pb solders. Of the various Pb-free solder compositions currently known, Sn-Ag Pb-free solders are widely used as solder materials for power semiconductor modules such as IGBT modules, as they offer a relatively good balance in terms of solder wettability, mechanical properties, and heat transfer resistance, and have a proven track record in products.

[0004] A solder composition is known that contains 0.01% by weight or more and 0.5% by weight or less of Ni, more than 2% by weight but not more than 5% by weight of Cu, and the remainder being Sn, and that may further contain at least one element selected from the group consisting of Ag, In, Zn, Sb, Ge, and P (see, for example, Patent Document 1). The solder composition disclosed in Patent Document 1 is said to suppress conductor breakage due to corrosion of a conductor whose main component is Cu, and to have properties similar to those of conventional Sn-Pb solder compositions.

[0005] A solder alloy is known that has an alloy composition, in mass %, of Sb: 9.0 to 33.0%, Ag: more than 4.0% but less than 11.0%, Cu: more than 2.0% but less than 6.0%, and the balance being Sn (see, for example, Patent Document 2). The solder alloy disclosed in Patent Document 2 is said to suppress chip cracking during cooling, improve the heat dissipation characteristics of the solder joint, and exhibit high joint strength at high temperatures. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-324257 [Patent Document 2] International Publication No. WO 2020 / 122253 Summary of the Invention [Problem to be solved by the invention]

[0007] Recent trends in power semiconductor modules, driven by higher currents and operating in high-temperature environments, have led to the occurrence of cracks in the solder joints between semiconductor elements and laminated substrates. These cracks are thought to be caused by thermal stress. When cracks occur in solder joints, electrical resistance increases, leading to further temperature rises. Previously, efforts have been made to improve the strength and Young's modulus of solder materials at room temperature, but these improvements are still not sufficient for use in high-temperature environments. [Means for solving the problem]

[0008] As a result of extensive research, the inventors have discovered that the reliability of solder joints can be improved by improving creep properties and elongation at break at high temperatures of around 175° C. Specifically, they came up with the idea that by adding a predetermined amount of Cu to a solder material containing Sn, Sb, Ag, and Ni, the creep properties and elongation at break are improved at high temperatures while maintaining the same Young's modulus, thereby improving module reliability, and have completed the present invention.

[0009] That is, according to one embodiment, the present invention relates to a solder material containing 5.0 mass % or more and 10.0 mass % or less of Sb, 2.0 mass % or more and 6.0 mass % or less of Ag, 0.1 mass % or more and 0.5 mass % or less of Ni, 3.0 mass % or more and 8.0 mass % or less of Cu, with the remainder being Sn and unavoidable impurities.

[0010] The solder preferably contains Cu in an amount of 4.5 mass % or more and 8.0 mass % or less.

[0011] In any of the above solder materials, the Sb content is preferably 6.0 mass % or more and 8.5 mass % or less.

[0012] Any of the above-described solder materials preferably contains Ag in an amount of 3.0 mass % or more and 6.0 mass % or less.

[0013] It is preferable that any of the above-mentioned solder materials is a solder material used for joining semiconductor elements.

[0014] According to another embodiment, the present invention relates to a solder joint comprising a solder joint layer in which any of the above-mentioned solder materials is melted, and a joint body having a metal layer on a surface in contact with the solder joint layer.

[0015] According to another embodiment, the present invention relates to a semiconductor device including a semiconductor element bonded onto a laminated substrate and a conductive connecting member bonded to the semiconductor element, the semiconductor device having the aforementioned solder joint between the laminated substrate and the semiconductor element or between the semiconductor element and the conductive connecting member.

[0016] According to yet another embodiment, the present invention relates to a semiconductor device including a semiconductor element bonded onto a laminated substrate and a cooler bonded to a surface of the laminated substrate opposite to the surface to which the semiconductor element is bonded, the semiconductor device having the aforementioned solder joint between the laminated substrate and the cooler. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a solder material that has improved fracture elongation and creep rupture elongation at high temperatures, an excellent stress relaxation effect, and high wettability. This also makes it suitable for use in joining semiconductor elements in semiconductor devices that are used in high-temperature environments and generate large currents of up to several thousand amperes, thereby suppressing cracks at the joint and contributing to improving the reliability of the semiconductor device. Hereinafter, in this specification, fracture elongation and creep rupture elongation may be referred to as "elongation properties." [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a conceptual cross-sectional view showing the cross-sectional structure of a semiconductor device according to one embodiment of the present invention. [Figure 2A] FIG. 2A is a diagram showing the shape of a test piece used in the examples. [Figure 2B] FIG. 2B is a cross-sectional view taken along line XX in FIG. 2A. [Figure 3] FIG. 3 is a graph showing the relationship between the Cu content in the solder material and the normalized elongation at break. [Figure 4] FIG. 4 is a graph showing the relationship between the Cu content in the solder material and the normalized creep rupture elongation.

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the embodiments described below.

[0020] [First embodiment: solder material] According to a first embodiment, the present invention provides a solder material that is an alloy containing 5.0 mass % or more and 10.0 mass % or less of Sb, 2.0 mass % or more and 6.0 mass % or less of Ag, 0.1 mass % or more and 0.5 mass % or less of Ni, and 3.0 mass % or more and 8.0 mass % or less of Cu, with the remainder being Sn and unavoidable impurities.

[0021] In the solder material according to the first embodiment, the inevitable impurities mainly refer to Cu, Ni, Zn, Fe, Al, As, Cd, Au, In, P, Pb, etc. The upper limit of the content of each element considered as an inevitable impurity varies depending on the main components of the solder material and is determined by international standards. For example, the upper limit of the amount of inevitable impurities in the solder material according to this embodiment may be the value determined by the international standards for Sn-Ag solder and Sn-Sb solder. Furthermore, the solder material according to the first embodiment is a Pb-free solder alloy that does not contain lead.

[0022] The Sb content is 5.0% by mass or more and 10.0% by mass or less. This range of Sb strengthens the Sn phase, forming an Sn-Sb phase, improving elongation (creep properties) without fracture. This is preferable because cracks and peeling are less likely to occur at solder joints. If the Sb content is less than 5.0% by mass, the Sn layer is not sufficiently strengthened by Sb, which induces deformation of the Sn, resulting in poor creep properties. If the Sb content exceeds 10.0% by mass, SbSn, a compound of Sn and Sb, coarsens, causing stress concentration, which can lead to fracture and reduced creep properties. The Sb content is more preferably 5.5% by mass or more and 8.5% by mass or less, or 5.5% by mass or more and 7.0% by mass or less, or 6.0% by mass or more and 8.5% by mass or less. Within the above composition range, the Sn-Sb phase approaches a peritectic composition, improving wettability and maximizing the solid solution strengthening effect of Sb addition, which is also preferable in terms of reducing voids and creep characteristics. The Sb content is more preferably 6.0 mass% or more and 7.0 mass% or less, because within this range, a peritectic composition is achieved.

[0023] The Ag content is 2.0% by mass or more and 6.0% by mass or less. By including Ag in this range, the pinning effect against dislocation motion is maintained even at a high temperature of 175°C, and wettability is also excellent. Therefore, there is an advantage in that the initial void fraction is low, making it easier to ensure bonding quality. If the Ag content is less than 2.0% by mass, there is a disadvantage in that the amount of Ag3Sn phase dispersed is small, which makes it easier to induce localized deformation during deformation in a high-temperature environment of 175°C. If the Ag content exceeds 6.0% by mass, there may be disadvantages such as reduced wettability and an elevated melting point. The Ag content is more preferably 3.0% by mass or more and 6.0% by mass or less. By including Ag in this range, the Sn-Ag phase approaches a eutectic composition, improving wettability. Furthermore, the fine Sn-Ag phase is uniformly dispersed around the Sn phase, which is preferable in terms of bondability and creep properties. Furthermore, the uniform properties are also preferable from a manufacturing perspective. The Ag content is more preferably 3.5% by mass or more and 5.0% by mass or less. By including Ag in this range, the Sn-Ag phase becomes closer to a eutectic composition, which is even more preferable from the standpoints of creep properties and manufacturing, as described above. The Ag content is even more preferably 3.7% by mass or more and 5.0% by mass or less. Within this range, the Sn-Ag phase becomes almost a eutectic composition, which is even more preferable from the standpoints of bondability, creep properties, and manufacturing, as described above.

[0024] The Ni content is 0.1% by mass or more and 0.5% by mass or less. By including Ni in this range, particularly when used for Cu joining applications, the product at the solder / Cu substrate interface changes from Cu6Sn to (Cu,Ni)6Sn, suppressing deterioration of joint toughness (elongation characteristics) after high-temperature aging. Furthermore, while Ni is expected to improve strength by refining crystal grains and suppress variation in mechanical properties by polycrystallization, if the Ni content is less than 0.1% by mass, the above effects may not be fully achieved. However, if it exceeds 0.5% by mass, there is a concern that the melting point may rise. It is more preferable that the Ni content be 0.15% by mass or more and 0.4% by mass or less. This range improves toughness and reduces cracking. Furthermore, it is preferable because it reduces the amount of initial voids generated during large-area joining.

[0025] The Cu content is 3.0% by mass or more and 8.0% by mass or less. This range of Cu content provides the solder with slip properties. This results in slip deformation after stress is applied, improving creep properties and ultimately providing a stress relaxation effect. By including 3.0% by mass or more of Cu, the fracture mode changes to slip originating from grain boundaries, improving creep properties by at least 40%. If the Cu content is less than 3.0% by mass, the solder cannot be provided with sufficient slip properties. If the Cu content exceeds 8.0% by mass, excessive hard CuSn structures are thought to be formed, resulting in reduced creep properties. From the viewpoint of wettability, the Cu content is set to 8.0% by mass or less.

[0026] The Cu content is preferably 4.5% by mass or more and 8.0% by mass or less. By including Cu in this range, the creep properties at high temperatures, particularly at 175°C, can be improved by more than two times compared to when Cu is not included (see the extrapolated line in Figure 4). In particular, by including a Cu content of 5.2% by mass or more and 7.8% by mass or less, the creep properties can be improved by more than 2.5 times (Figure 4). Furthermore, by including a Cu content of 5.9% by mass or more and 7.0% by mass or less, the creep properties can be improved by more than 3.0 times (Figure 4). Furthermore, by including a Cu content of 4.8% by mass or more and 5.3% by mass or less, deformations such as burrs and localized irregularities are reduced when processed into a predetermined shape, making plastic processing easier. In other words, the material has excellent processability as a solder material and high mechanical strength as a joining layer.

[0027] When a Ni-containing layer (such as plating) is applied to the portion to be joined, Cu, a component of the solder material, may be consumed in a reaction with Ni dissolved in the solder. The Ni-containing layer is a layer containing 50% or more by mass of Ni, and may be a base material or a layer containing Ni or a Ni alloy formed by a film-forming method such as plating. Specifically, it may be, for example, a NiP plating layer containing more than 0% but not more than 10% by mass of P. Furthermore, when the portion to be joined is a Cu layer, Sn, a component of the solder material, may be consumed in a reaction with Cu dissolved in the solder. Therefore, when used to join a Ni-containing layer or a Cu layer, the Cu content range in the solder material is preferably, for example, 5.1% by mass or more and 8.0% by mass or less, and preferably 5.2% by mass or more and 7.5% by mass or less. The Cu layer of the portion to be joined may be any material containing 50% or more by mass of Cu, including Cu or a Cu alloy.

[0028] Note that adding Bi to the solder material according to the first embodiment having the above composition may be undesirable because a low melting point phase is generated, reducing mechanical strength, and Bi is easily oxidized. In particular, adding Bi in an amount of 2% by mass or more is undesirable. Furthermore, adding Zn may be undesirable because it is easily oxidized, deteriorating wettability. In particular, adding Zn in an amount of 2% by mass or more is undesirable.

[0029] In this specification, the creep characteristics of a solder material are defined as the value of strain × stress required until the solder material breaks, and can also be referred to as strain energy. The strain energy required until the solder material breaks is called creep rupture elongation. In this specification, creep rupture elongation refers to the value of elongation until breakage when a constant load (constant stress) is applied to a test piece at a specific temperature. In the rest of this specification, creep rupture elongation refers to the creep rupture elongation at 175°C unless otherwise specified. In addition, a small test piece with a gauge diameter of 1 mm and a gauge length of 2.4 mm is used as the test piece, and the constant load is 7 N (8.92 MPa). This load is determined so that the deformation rate of the solder joint layer is applied to the test piece in a heat cycle test of a semiconductor device to which the solder material according to this embodiment is applied. Specifically, in order to set the constant load to 7 N, a 1.0 × 10 -5 ~5.0×10 -5 A strain rate in the range of approximately (1 / s) can be applied. The conditions for the heat cycle test to apply the above deformation rate are a high temperature of 150°C, a low temperature of -55°C, a holding time of 60 minutes, and a temperature change of 7°C / min. Stress relaxation properties and fatigue properties can be evaluated by evaluating creep rupture elongation, and it is considered that the higher the creep rupture elongation value, the better the stress relaxation properties and fatigue properties. In the above measurement method, since the test is performed under a constant load, creep properties (strain energy) and creep rupture elongation are proportional to each other and can be considered to be the same.

[0030] The solder material according to the present invention can be prepared by melting raw materials selected from Sn, Sb, Ag, Ni, and Cu, or master alloys containing the raw materials, in an electric furnace according to a conventional method. Each raw material preferably has a purity of 99.99% by mass or higher.

[0031] Furthermore, the solder material having the above composition can be processed into a plate-shaped preform material, or into a powder form and mixed with flux to form a cream solder. When processed into a powder form and mixed with flux to form a cream solder, the particle size (particle diameter) of the solder powder preferably has a particle size distribution in the range of 10 to 100 μm, and more preferably in the range of 20 to 50 μm. The average particle size, for example, can be 25 to 50 μm when measured using a general laser diffraction / scattering particle size distribution analyzer. Any flux can be used as the flux, but rosin-based flux is particularly preferred.

[0032] The solder material according to this embodiment has excellent elongation at break and creep characteristics at high temperatures of about 175°C to 200°C, and can be suitably used for joining components in electronic devices, particularly for joining the back electrode of a semiconductor element and the electrode of a laminated substrate.

[0033] [Second embodiment: solder joint] According to a second embodiment, the present invention provides a solder joint, which includes a solder joint layer in which the solder material of the first embodiment is melted, and a jointed body having a metal layer on a surface in contact with the solder joint layer.

[0034] The solder joint layer constituting the solder joint according to this embodiment may be a layer formed by melting the solder material of the first embodiment. The thickness and shape of the solder material used to form the solder joint layer can be appropriately determined by those skilled in the art according to the purpose and application, and are not particularly limited. As an example, when joining the front surface of a semiconductor element to a wiring member such as a lead frame, a thickness of 50 μm or more is preferred, but is not limited to this range. When joining the back electrode of a semiconductor element to a conductive plate of a laminated substrate, the thickness of the solder joint layer can be approximately 200 to 300 μm, and preferably approximately 200 to 250 μm, but is not limited to this range. Furthermore, when joining a laminated substrate to a heat sink, a thickness of approximately 400 to 500 μm is preferred.

[0035] The bonded object constituting the solder joint according to this embodiment is a member having a metal layer on the surface in contact with the solder joint layer. The type of metal constituting the metal layer is not particularly limited, but the bonded object may generally be a member having an electrode, and the electrode may be made of Cu, Ni, Al, Ti, Au, Ag, or an alloy thereof. The bonded object may also be a member having a plating film containing these metals on the surface in contact with the solder joint layer.

[0036] A solder joint is formed by placing a solder material of a predetermined shape and thickness in contact with the metal layer of the object to be joined and heating it at a predetermined temperature. The joining atmosphere can be a nitrogen atmosphere, or bonding can be performed in an active atmosphere such as hydrogen or formic acid. When using sheet solder, it is preferable to use a reducing gas such as hydrogen or formic acid. When joining in an active gas atmosphere with a reducing effect, the temperature is preferably set at a temperature at which the gas effectively reduces oxides, for example, 250 to 300°C. The joining temperature for sheet solder and cream solder may be set at the melting point (Tm) of the solder material + (50°C ± 10°C). In actual joining, a temperature distribution of several degrees or more is required, so maintaining a certain temperature and time above this level ensures stable joint quality. The molten solder is then solidified by cooling at a predetermined temperature drop rate, forming a solder joint layer. The temperature rise rate for this heat treatment can be approximately 1°C / sec, and the temperature drop rate is preferably 5°C / sec or higher, and more preferably 8°C / sec or higher and 15°C / sec or lower. By setting the temperature increase rate and temperature decrease rate in the heat treatment within this range, the crystal grains become finer and each phase precipitates uniformly, thereby reducing variations in quality.

[0037] The solder joint according to the second embodiment may constitute a part of an electronic device, and examples of the electronic device include, but are not limited to, electrical and power equipment such as inverters, mega solar power plants, fuel cells, elevators, cooling systems, and automotive semiconductor devices. Typically, the electronic device is a semiconductor device. The joint in the semiconductor device may be a die-bond joint, a joint between a conductive plate and a heat sink, a joint between terminals, a joint between a terminal and another member, or any other joint, particularly a joint through which a large current flows, but is not limited to these. The semiconductor device will be described in detail in the third embodiment.

[0038] The solder joint according to the second embodiment has excellent stress relaxation properties at high temperatures, which makes it less susceptible to cracking and makes it a highly reliable solder joint.

[0039] [Third embodiment: semiconductor device] According to a third embodiment of the present invention, a semiconductor device includes a semiconductor element bonded onto a laminated substrate, and includes a solder joint layer formed by melting the solder material of the first embodiment. In other words, the semiconductor device according to the third embodiment includes the solder joint according to the second embodiment, in which the object to be bonded is a semiconductor element and / or any member constituting the semiconductor device.

[0040] FIG. 1 is a conceptual cross-sectional view of a power semiconductor module, which is an example of a semiconductor device according to this embodiment. The illustrated power semiconductor module has a laminated structure in which a back electrode of a semiconductor element 11 is bonded to a laminated substrate 12 by a bonding layer 10, and the laminated substrate 12 is bonded to a heat sink 13 by a bonding layer 17. A case 16 incorporating external terminals 15 is attached to the heat sink 13. Front electrodes of the semiconductor element 11 and electrodes of the laminated substrate 12 are connected by a lead frame 18, which is a conductive connecting member. The semiconductor element 11 and external terminals 15 are also connected by aluminum wires 14. A sealing material 20 is filled in the space defined by the case 16 and the heat sink 13, contacting the semiconductor element 11, the laminated substrate 12, the lead frame 18, and the aluminum wires 14, which are conductive connecting members. FIG. 1 illustrates an embodiment in which bonding layers 10 formed by melting solder material according to the first embodiment are provided between the back electrodes of the semiconductor element 11 and the laminated substrate 12, and between the front electrodes of the semiconductor element 11 and the lead frame 18. However, the solder material according to the first embodiment can be used to join any part of a semiconductor device, and the present invention is not limited to the illustrated embodiment.

[0041] The semiconductor element 11 is a power chip such as an IGBT (Insulated Gate Bipolar Transistor) or a diode chip, and may be a Si device or a wide-gap semiconductor device such as a SiC device, a GaN device, a diamond device, or a ZnO device. A combination of these devices may also be used. For example, a hybrid module using a Si-IGBT and a SiC-SBD may be used. The number of semiconductor elements mounted may be one or more.

[0042] The laminated substrate 12 may be composed of an insulating substrate 122, a first conductive plate 121 formed on one of its principal surfaces, and second conductive plates 123a and 123b formed on the other principal surface. The insulating substrate 122 may be made of a material with excellent electrical insulation and thermal conductivity. Examples of materials for the insulating substrate 122 include Al2O3, AlN, and SiN. For high-voltage applications, a material that combines electrical insulation and thermal conductivity is preferable, such as AlN or SiN, but is not limited to these. The first conductive plate 121 and the second conductive plates 123a and 123b may be made of metal materials with excellent workability, such as Cu or Al. Furthermore, the conductive plates may be Cu or Al that have undergone surface treatment, such as Ni plating, to provide a Ni-containing layer containing Ni or a Ni alloy on their surface for rust prevention or other purposes. Specific examples of Ni-containing layers formed by Ni plating include, but are not limited to, NiP-containing layers and NiB-containing layers. Methods for disposing the first conductive plate 121 and the second conductive plates 123a, 123b on the insulating substrate 122 include a direct copper bonding method and an active metal brazing method. In the illustrated embodiment, two second conductive plates 123a, 123b are discontinuously provided on the insulating substrate 122, with one second conductive plate 123a functioning as an electrode bonded to the semiconductor element 11 and the other second conductive plate 123b functioning as an electrode connected to the lead frame 18. However, the number and connection mode of the first conductive plates and second conductive plates are not limited to those in the illustrated embodiment.

[0043] The lead frame 18 is a conductive connecting member that connects the semiconductor element 11 to the second conductive plate 123b and other components. Specifically, the lead frame 18 can be bonded to the front surface electrodes of the semiconductor element 11, which are located on the opposite side of the electrodes (rear electrodes) that contact the laminated substrate 12, using, for example, a bonding layer 10 formed by melting a solder material according to the first embodiment. The lead frame 18 can also be bonded to wiring portions such as the second conductive plate 123b using a bonding layer 17 made of a solder material. The lead frame 18 may be made of a metal such as copper or an alloy containing copper. A Ni-containing layer, such as a Ni or Ni alloy layer (such as a NiP alloy or a NiB alloy) or a Cr or Cr alloy layer, may be formed on the surface of the lead frame 18 by plating or other methods. In this case, the thickness of the Ni or Ni alloy layer or the Cr or Cr alloy layer can be approximately 20 μm or less. The lead frame may be connected to an output terminal (not shown). Wires can also be used as conductive connecting members for connecting semiconductor element 11 to second conductive plate 123b, etc. When bonding by wire bonding, solder material may not be used.

[0044] Heat sink 13 is made of a metal with excellent thermal conductivity, such as copper or aluminum. To prevent corrosion, heat sink 13 may be coated with Ni or a Ni alloy (such as a NiP alloy or a NiB alloy) as a Ni-containing layer by plating or the like. The heat sink may also be a cooler having a water-cooling or air-cooling function. A semiconductor device may also be used in which case 16 is attached to laminated substrate 12 and heat sink 13 is not attached to case 16.

[0045] The bonding layer 10 that bonds the front electrode of the semiconductor element 11 to the lead frame 18 and the bonding layer 10 that bonds the back electrode of the semiconductor element 11 to the second conductive plate 123a can be formed using the solder material according to the first embodiment. Because cracking has traditionally been a problem at these joints, using the solder material according to the first embodiment can improve the reliability of these bonding layers. The bonding layer 17 between other components, such as the second conductive plate 123b and the heat sink 13, can be formed using the solder material according to the first embodiment, but can also be formed using any Pb-free solder. Examples of suitable materials include, but are not limited to, Sn-Ag-Cu, Sn-Sb, Sn-Sb-Ag, Sn-Cu, Sn-Sb-Ag-Cu, Sn-Cu-Ni, and Sn-Ag. Alternatively, the bonding layer can be formed using a connecting material containing fine metal particles, such as a sintered body of nanosilver particles. The arrangement of the bonding layer 10 made of melted solder material according to the first embodiment and the optional bonding layer 17 in the illustrated semiconductor device is merely an example. The semiconductor device according to the third embodiment of the present invention may be provided with a bonding layer made of melted solder material according to the first embodiment between any of the components constituting the device. Therefore, the bonding layer between the laminated substrate 12 and the heat sink 13 may be a bonding layer made of melted solder material according to the first embodiment.

[0046] The case 16 can be made of a thermoplastic resin such as polyphenylene sulfide (PPS) or polybutylene terephthalate (PBT).

[0047] The sealing material 20 may be a cured thermosetting resin obtained by curing a thermosetting resin composition that contains a thermosetting resin base and an inorganic filler, and may optionally contain a curing agent, a curing accelerator, and additives.

[0048] The thermosetting resin base is not particularly limited, and examples thereof include epoxy resins, phenolic resins, maleimide resins, cyanate resins, and oxazine resins. Among these, epoxy resins having at least two epoxy groups per molecule are particularly preferred due to their high dimensional stability, water resistance, chemical resistance, and electrical insulation. Specifically, it is preferable to use aliphatic epoxy resins, alicyclic epoxy resins, or mixtures thereof. As the encapsulating resin, thermosetting resins are preferred because of their heat resistance and high insulation properties, and epoxy resins are particularly preferred due to their high elasticity.

[0049] Aliphatic epoxy resins are epoxy compounds in which the carbon atom directly bonded to the epoxy group is a carbon atom constituting an aliphatic hydrocarbon. Therefore, even if the main skeleton contains an aromatic ring, compounds that satisfy the above conditions are classified as aliphatic epoxy resins. Examples of aliphatic epoxy resins include, but are not limited to, bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol AD ​​epoxy resins, biphenyl epoxy resins, naphthalene epoxy resins, cresol novolac epoxy resins, and trifunctional or higher polyfunctional epoxy resins. These can be used alone or in combination. Furthermore, naphthalene epoxy resins and trifunctional or higher polyfunctional epoxy resins have high glass transition temperatures and are therefore also referred to as high-heat-resistant epoxy resins. The inclusion of these high-heat-resistant epoxy resins can improve heat resistance.

[0050] Alicyclic epoxy resins are epoxy compounds in which the two carbon atoms constituting the epoxy group constitute an alicyclic compound. Examples of alicyclic epoxy resins include, but are not limited to, monofunctional epoxy resins, bifunctional epoxy resins, and trifunctional or higher polyfunctional epoxy resins. Alicyclic epoxy resins can be used alone or in combination with two or more different alicyclic epoxy resins. Mixing an alicyclic epoxy resin with an acid anhydride curing agent and curing it increases the glass transition temperature, so mixing an alicyclic epoxy resin with an aliphatic epoxy resin can improve heat resistance.

[0051] The thermosetting resin base used in the encapsulant 20 may be a mixture of the above-mentioned aliphatic epoxy resin and alicyclic epoxy resin. When mixed, the mixing ratio may be any, and the mass ratio of the aliphatic epoxy resin to the alicyclic epoxy resin may be about 2:8 to 8:2, or may be about 3:7 to 7:3, and is not limited to a specific mass ratio. In a preferred embodiment, the thermosetting resin base used in the encapsulant 20 is a mixture of a bisphenol A type epoxy resin and an alicyclic epoxy resin in a mass ratio of 1:1 to 1:4.

[0052] The inorganic filler may be a metal oxide or a metal nitride, and examples thereof include, but are not limited to, fused silica, silica (silicon oxide), alumina, aluminum hydroxide, titania, zirconia, aluminum nitride, talc, clay, mica, and glass fiber. These inorganic fillers can increase the thermal conductivity and reduce the thermal expansion coefficient of the sealing material 20. These inorganic fillers may be used alone or in combination of two or more. The inorganic filler may be a microfiller or a nanofiller, and two or more inorganic fillers with different particle sizes and / or types may be mixed and used. In particular, it is preferable to use an inorganic filler with an average particle size of approximately 0.2 to 20 μm. The amount of inorganic filler added to the sealing material 20 is preferably 100 to 600 parts by mass, and more preferably 200 to 400 parts by mass, based on 100 parts by mass of the total mass of the thermosetting resin base and the optional curing agent. If the blending amount of the inorganic filler is less than 100 parts by mass, the thermal expansion coefficient of the sealing material 20 may become high, which may make peeling or cracking more likely to occur. If the blending amount is more than 600 parts by mass, the viscosity of the composition may increase, which may make it difficult to extrude.

[0053] The thermosetting resin composition constituting the encapsulant 20 may optionally contain a curing agent. The curing agent is not particularly limited as long as it reacts with the thermosetting resin base, preferably the epoxy resin base, and can be cured. However, an acid anhydride curing agent is preferably used. Examples of the acid anhydride curing agent include aromatic acid anhydrides, specifically phthalic anhydride, pyromellitic anhydride, and trimellitic anhydride. Alternatively, examples of the acid anhydride curing agent include cyclic aliphatic acid anhydrides, specifically tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, and methylnadic anhydride; and aliphatic acid anhydrides, specifically succinic anhydride, polyadipic anhydride, polysebacic anhydride, and polyazelaic anhydride. The amount of curing agent is preferably 50 to 170 parts by mass, more preferably 80 to 150 parts by mass, per 100 parts by mass of the thermosetting resin (epoxy resin) base. If the amount of curing agent is less than 50 parts by mass, the glass transition temperature may decrease due to insufficient crosslinking. If the amount of curing agent is more than 170 parts by mass, the moisture resistance, high heat distortion temperature, and heat resistance stability may decrease. When using a bisphenol A epoxy resin alone or a mixture of a bisphenol A epoxy resin and a high-heat-resistant epoxy resin as exemplified above as the thermosetting resin base, it may be preferable not to use a curing agent, as this improves heat resistance. The blending ratio of the high-heat-resistant epoxy resin may be, for example, 10 to 50% by mass, more preferably 10 to 25% by mass, based on the total mass of the thermosetting resin base. This range is preferable because it improves heat resistance and does not increase viscosity.

[0054] A curing accelerator may be added as an optional component to the thermosetting resin composition constituting the encapsulant 20. Examples of the curing accelerator include imidazole or its derivatives, tertiary amines, borate esters, Lewis acids, organometallic compounds, and organic acid metal salts. The amount of the curing accelerator added is preferably 0.01 to 50 parts by mass, more preferably 0.1 to 20 parts by mass, per 100 parts by mass of the thermosetting resin base.

[0055] The thermosetting resin composition constituting the encapsulant 20 may also contain optional additives to the extent that the additives do not impair its properties. Examples of additives include, but are not limited to, flame retardants, pigments for coloring the resin, and plasticizers and silicone elastomers for improving crack resistance. These optional components and their amounts can be appropriately determined by those skilled in the art depending on the specifications required for the semiconductor device and / or the encapsulant.

[0056] The sealing material 20 may be a single layer made of one type of resin having the composition exemplified above, or may be a layer including two or more layers. An additional layer of thermoplastic resin or silicone rubber may be provided on one or more thermosetting resin layers.

[0057] For the purpose of explaining the present invention, the cross-sectional structure of a specific module has been shown in FIG. 1, but the configuration of the semiconductor device is not limited to the illustrated form. For example, the conductive connecting member may be only one of a lead frame, a wire, and an implant pin, or may include two or more of these. The semiconductor device may also be a caseless semiconductor device that does not include a case. Furthermore, the semiconductor device may further include a printed circuit board connected to the front surface electrodes of the semiconductor element by implant pins, and the printed circuit board may be insulated and sealed.

[0058] In manufacturing a semiconductor device, for example, the semiconductor element 11 is bonded to the heat sink 13 and the laminated substrate 12 using the solder material according to the first embodiment. Next, the case 16 is attached and fixed to the heat sink 13 using resin or the like. After that, the lead frame 18 is bonded and wire-bonded using aluminum wire 14. Depending on the configuration of the semiconductor device, implant pins and a printed circuit board may also be bonded optionally. Next, a thermosetting resin composition constituting the encapsulant 20 is injected into the case 16 and heat-cured. The heat-curing process can be, for example, a two-stage curing process. When an epoxy resin is used as the thermosetting resin base, the resin is heated at 90 to 120°C for 1 to 2 hours to achieve a semi-cured state. Thereafter, the resin is further heated at 175 to 185°C for 1 to 2 hours to perform full curing. However, the temperature and time are not limited to specific values, and two-stage curing may not be necessary. The caseless semiconductor device can be manufactured by joining a laminated substrate and a semiconductor element with the solder material according to the first embodiment, attaching conductive connecting members such as a lead frame and aluminum wire, and then placing these in a mold and filling it with a sealing material.

[0059] In the semiconductor device according to the third embodiment, the semiconductor element, the laminated substrate, or the cooler is joined by the solder material according to the first embodiment, which makes it difficult for cracks to occur, resulting in a highly reliable device. [Example]

[0060] The present invention will be described in more detail below with reference to examples of the present invention, but the present invention is not limited to the scope of the following examples.

[0061] [Example] (1) Evaluation of the mechanical properties of solder materials Samples were produced by adding 0 to 8 mass% of Cu to an alloy containing 6 mass% Sb, 4 mass% Ag, 0.25 mass% Ni, and the remainder being Sn and unavoidable impurities, and the mechanical properties of these samples were evaluated, including the fracture elongation, Young's modulus, and creep rupture elongation at 175°C.

[0062] Based on JIS Z2241:2011, Metallic Materials Tensile Testing Methods, a stress-strain curve from the tensile test was obtained, and the Young's modulus was calculated from the slope of the curve, and the fracture elongation (%) was calculated from the strain at fracture (when the stress became 0). Figure 2A shows the shape of the test specimen, and Figure 2B shows a cross-sectional view taken along line XX in Figure 2A. In Figure 2A, a dumbbell-shaped specimen was used, with a length L1 of 12.0 mm, both end lengths L2 of 3.9 mm, both end diameters φ1 of 2 mm, and a gauge length L3 of 2.4 mm and a diameter φ2 of 1 mm. The tensile test was performed at a strain rate of 2.0 x 10 -3 (1 / s) at a temperature of 175°C using a micro-tensile testing device (manufactured by Saginomiya Seisakusho, model: LMH-207).

[0063] Based on JIS Z2271:2010, creep and creep rupture test methods for metallic materials, a creep curve was obtained, and the creep rupture elongation was calculated from the elongation at rupture. The same test specimens as those used in the tensile test were used. The creep rupture test was performed at a constant load of 7N (8.92MPa) and a temperature of 175°C, using the same equipment as the tensile test.

[0064] The relationship between the Cu content in solder material and normalized fracture elongation is shown in Figure 3. The normalized fracture elongation is expressed as a relative value when the fracture elongation when the Cu content is 0 is set to 1. Figure 3 shows that the fracture elongation value improves when the Cu content is in the range of 3% by mass to 8% by mass.

[0065] Figure 4 shows the relationship between the Cu content of solder materials and normalized creep rupture elongation. The normalized creep rupture elongation is expressed relative to the creep rupture elongation when the Cu content is 0, which is set to 1. As can be seen from Figure 4, when the Cu content is 3% or more by mass, the normalized creep rupture elongation increases by more than 40% compared to when the Cu content is 0. Furthermore, when the Cu content is in the range of 5 to 7% by mass, the creep properties are maximized, and the normalized creep rupture elongation is more than double that when the Cu content is 0. When the Cu content exceeds 7% by mass, the creep properties decrease, but when the Cu content is 8% or less by mass, the normalized creep rupture elongation remains at a level that is more than 40% higher than when the Cu content is 0.

[0066] Even when the Cu content in the solder material was changed from 0 to 8 mass %, the value of the normalized Young's modulus hardly changed (graph not shown).

[0067] (2) Reliability evaluation of semiconductor devices Solder materials for Samples 1 to 18 having the compositions shown in Tables 1 and 2 below were prepared. A power semiconductor module was manufactured, including a solder joint layer 10 formed by melting the solder material at the joint of a laminated substrate including a Si semiconductor element as the semiconductor element 11 shown in FIG. 1 and a first conductive plate 121 and second conductive plates 123a and 123b made of Cu as the laminated substrate 12. The joining conditions were a N2 reducing environment, a holding temperature of 260°C to 290°C, and a holding time of 100 to 160 seconds. The solder between the laminated substrate and the semiconductor element was applied using a metal mask with a 3.2 mm square opening and a thickness of 200 μm. The paste solder used in this example was a flux primarily composed of ordinary rosin. The paste solder was prepared by kneading this rosin-based flux with solder powder having a particle size of 25 to 45 μm at a mass ratio of 1:1.

[0068] [T j Evaluation of P / C tolerance] The reliability of power semiconductor modules is j Power cycle resistance (T j The power cycle test was performed at 75 to 150°C (ΔTvj One cycle consisted of one second of energized operation (current set to achieve 150°C) and 9 to 15 seconds of rest (current set to achieve 75°C) at a temperature of 150°C. The number of cycles until an abnormality in the current or voltage was detected was defined as P / C resistance. The evaluation criteria for P / C resistance were A for 50k cycles or more, B for 40k cycles or more but not more than 50k cycles, C for 25k cycles or more but less than 40k cycles, and D for less than 25k cycles.

[0069] [Evaluation of wettability] Surface tension was measured using a Wilhelmy method wetting tester (Rhesca; SAT-5100, compliant with JIS Z 3198-4). This method involves immersing a test piece in a liquid and then lifting it out, and then determining the contact angle from the force applied to the periphery of the test piece where the tip of the test piece makes contact with the liquid surface. The test conditions were an immersion temperature of 270°C, a 5 x 30 x 0.3 mm phosphorus-deoxidized copper (C1220) test piece, and an appropriate amount of rosin-based flux applied to the test piece. The immersion speed was 5 mm / sec, the immersion depth was 2 mm, and the immersion time was 10 seconds. The evaluation criteria were as follows: A: surface tension less than 500 mN / m, B: 500 to 525 mN / m, and D: 525 mN / m or greater. A surface tension of less than 500 mN / m, which is rated A, is the surface tension of Sn3.0Ag0.5Cu (a solder material containing 3% Ag by mass, 0.5% Cu by mass, and the remainder consisting of Sn and unavoidable impurities), an industrially common solder material. A surface tension of less than 500 mN / m is preferable because no defects such as voids are observed at the solder joint or its interface. A surface tension of 500 to 525 mN / m is not a practical problem, but may result in the formation of minute voids of approximately 1.0 mm, resulting in a rating of B. On the other hand, a surface tension of 525 mN / m or more (D) is more likely to result in voids at the joint and its interface during module assembly (after soldering), resulting in large void diameters and a large occurrence rate. A surface tension of 525 mN / m or more is undesirable because the maximum void diameter exceeds 1.5 mm when observed using ultrasonic testing (SAT), leading to reduced module reliability.

[0070] Tables 1 and 2 below show the composition of the solder material used as the evaluation sample, the T j The P / C tolerance and the evaluation results, as well as the wettability evaluation results of the solder material, are shown below.

[0071] [Table 1]

[0072] The results for samples 1 to 6 show that when the Cu content was increased from 3.2% to 8% by mass, the lifespan of the semiconductor module was extended by more than 1.5 times. This is presumably due to an improvement in creep characteristics of more than 1.3 times. Furthermore, when the Cu content was increased from 5.2% to 8% by mass, the lifespan of the semiconductor module was extended by more than 1.7 times. This is presumably due to an improvement in creep characteristics of more than 2.0 times.

[0073] On the other hand, in a power semiconductor module using solder material (sample 8) with reduced Sb content in the bonding layer, cracks were observed in the Sn phase of the solder bonding layer after the power cycle test. This is thought to be the result of damage occurring in the Sn portion due to the weakening of solid-solution strengthening due to the insufficient Sb content. It is presumed that the low Sb content allowed stress relaxation due to Sn deformation to occur preferentially. A higher Sb content is considered preferable. The solder material (sample 9) with increased Sb content did not exhibit good wettability, and voids were observed in some areas. In a power semiconductor module using solder material (sample 12) in the bonding layer, microcracks of approximately 30 to 50 μm were observed, centered around the Sn phase of the solder bonding layer. A Ag content of 2% or more by mass is considered preferable because the densely dispersed Ag3Sn in the solder structure suppresses local deformation (deformation of the Sn phase). Furthermore, sample 10 confirmed that a high Ag content caused the formation of very small voids approximately 1.0 mm in diameter. These voids do not pose any practical problems, but it is believed that the P / C resistance is slightly lower than that of sample 11 due to the presence of the voids.

[0074] From Table 1, it was confirmed that by constructing a solder joint layer using a solder material containing Sb, Ag, Ni, and Cu within the specified ranges, a highly reliable power semiconductor module with a P / C resistance of 50k cycles or more can be obtained. With a composition within the specified range, it is thought that the slippage that occurs when Cu is added during P / C testing improves the stress relaxation effect of the solder material, thereby improving the resistance.

[0075] [Table 2]

[0076] In Table 2, "-" indicates that poor wettability, voids in the initial structure, and scattered solder balls (solder solidification) made it difficult to conduct the P / C test. Table 2 shows that in power semiconductor modules using the solder materials of Samples 13 to 16 in the bonding layer, cracks were observed at the grain boundaries of the Sn phase (β-Sn grain boundaries) in the solder bonding layer after the power cycle test. Samples 13 and 14 did not exhibit improved creep properties due to improved slip properties because Cu6Sn5 was not formed in the structure. Samples 15 and 16 did not exhibit the full effect of Cu addition because Cu6Sn5 was formed but its distribution was sparse. As a result, Samples 13 to 16 developed cracks at the β-Sn grain boundaries, leading to the end of their life. In Samples 17 and 18, the Ag content deviated from the eutectic composition (3.5% by mass or more and 5.0% by mass or less), which is thought to have reduced wettability and caused voids and solder balls to form. In the solder joint of Sample 18, excessive AgSn compounds such as Ag3Sn were formed, resulting in a non-uniform structure.

[0077] The solder alloy of the present invention can be applied to general soldered joints. It is particularly suitable for use in components that generate a lot of heat, such as LED elements and power semiconductor devices such as power diodes. The solder alloy of the present invention has excellent fracture elongation and creep properties (creep fracture elongation) at high temperatures, improving the reliability of soldered joints. In particular, improved durability was confirmed in P / C tests using IGBT modules. The increase in electrical resistance due to cracking, which is a problem during high-temperature operation, can be resolved by improving fracture elongation and creep properties. [Explanation of symbols]

[0078] 10 bonding layer 11 semiconductor element 12 laminated substrate 121 first conductive plate, 122 insulating substrate, 123a, 123b second conductive plate 13 heat sink, 14 aluminum wire, 15 external terminal, 16 case 17 bonding layer, 18 lead frame, 20 sealing layer

Claims

1. 5.0% by mass or more and less than 10.0% by mass of Sb; 2.0% by mass or more and 6.0% by mass or less of Ag; 0.1% by mass or more and 0.5% by mass or less of Ni; 3.0 mass% or more and 8.0 mass% or less of Cu; Contains does not contain 2.0% by mass or more of Bi and 2.0% by mass or more of Zn; The balance is a solder material consisting of Sn and unavoidable impurities.

2. The solder material according to claim 1 , containing Cu in an amount of 4.5 mass % or more and 8.0 mass % or less.

3. The solder material according to claim 1 , containing Sb in an amount of 6.0 mass % or more and 8.5 mass % or less.

4. The solder material according to claim 2 , containing Sb in an amount of 6.0 mass % or more and 8.5 mass % or less.

5. The solder material according to claim 1 , containing Ag in an amount of 3.0 mass % or more and 6.0 mass % or less.

6. The solder material according to claim 2 , containing Ag in an amount of 3.0 mass % or more and 6.0 mass % or less.

7. The solder material according to claim 3 , containing Ag in an amount of 3.0 mass % or more and 6.0 mass % or less.

8. The solder material according to claim 1, which is used for joining semiconductor elements.

9. A soldered joint comprising: a solder joint layer in which the solder material according to claim 1 is melted; and a jointed body having a metal layer on a surface in contact with the solder joint layer.

10. A semiconductor device including a semiconductor element bonded onto a laminated substrate and a conductive connecting member bonded to the semiconductor element, the semiconductor device comprising the solder joint described in claim 9 between the laminated substrate and the semiconductor element or between the semiconductor element and the conductive connecting member.

11. A semiconductor device including a semiconductor element bonded onto a laminated substrate and a cooler bonded to a surface of the laminated substrate opposite to the surface to which the semiconductor element is bonded, the semiconductor device comprising the solder joint described in claim 9 between the laminated substrate and the cooler.

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