SOLDERING MATERIAL

Abstract

This is a solder material with excellent elongation at break in high-temperature applications. The product includes a solder material containing 5.0 wt% or more and 10.0 wt% or less Sb, 2.0 wt% or more and 6.0 wt% or less Ag, 0.1 wt% or more and 0.5 wt% or less Ni, 3.0 wt% or more and 8.0 wt% or less Cu, the remainder being Sn and unavoidable impurities; a solder bonding section comprising a bond layer in which the solder material is melted; and a semiconductor device comprising the bonding section.

Description

solder material

[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.

[0002] Power semiconductor modules are widely used in fields requiring efficient power conversion. For example, their application range 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 a laminated substrate with a bonding material, which has 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.

[0006] JP 2005-324257 A International Publication WO 2020 / 122253

[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.

[0008] As a result of extensive research, the present 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 the reliability of the module, 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, and 3.0 mass % or more and 8.0 mass % or less of Cu, with the remainder being Sn and inevitable 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-described solder materials, the solder material preferably contains Sb in an amount of 6.0 mass % or more and 8.5 mass % or less.

[0012] In any of the above-described solder materials, the solder material 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.

[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."

[0018] 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. Fig. 2A is a diagram showing the shape of a test piece used in the examples. Fig. 2B is a cross-sectional view taken along line X-X in Fig. 2A. Fig. 3 is a graph showing the relationship between the Cu content in a solder material and normalized rupture elongation. Fig. 4 is a graph showing the relationship between the Cu content in a solder material and 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, which 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 inevitable 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 treated 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 a 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. By including Sb in this range, the Sn phase is solid-solution strengthened to form a Sn-Sb phase, improving elongation characteristics (creep characteristics) without fracture. This is preferable because cracks and peeling are less likely to occur at the solder joint. If the Sb content is less than 5.0% by mass, the solid-solution strengthening of the Sn layer by Sb is insufficient, which induces deformation of the Sn, resulting in deterioration of creep characteristics. If the Sb content exceeds 10.0% by mass, SbSn, a compound of Sn and Sb, becomes coarse, causing stress concentration, which can lead to fracture and reduced creep characteristics. More preferably, the Sb content is 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. The above-mentioned composition range makes the Sn—Sb phase approach a peritectic composition, improves wettability, maximizes the solid solution strengthening effect of Sb addition, and is also preferable in terms of void reduction and creep characteristics. The Sb content is more preferably 6.0 mass % or more and 7.0 mass % or less, because a peritectic composition is achieved within this range.

[0023] The Ag content is set to 2.0 mass % or more and 6.0 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 that the initial void rate is low and it is easy to ensure the joining quality. If the Ag content is less than 2.0 mass %, Ag ３ The small amount of dispersed Sn phase has the disadvantage of easily inducing localized deformation during deformation in a high-temperature environment of 175°C, while a content exceeding 6.0% by mass can result in disadvantages such as decreased 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 standpoint. 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 approaches a eutectic composition more closely, which is even more preferable from the standpoint 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. The above range is an almost eutectic composition, which is even more preferable in terms of bondability, creep characteristics and manufacturing, as described above.

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

[0025] The Cu content is 3.0% by mass or more and 8.0% by mass or less. By including Cu in this range, the solder material can be provided with slip properties. Therefore, after stress is applied, slip deformation occurs, improving creep properties and ultimately resulting in 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 material cannot be provided with sufficient slip properties. If the Cu content exceeds 8.0% by mass, it is believed that excessive CuSn-based hard structures are formed, resulting in a decrease in creep properties. Furthermore, 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 increased by 2.5 times or more (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 increased by 3.0 times or more (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 solder material has excellent processability and high mechanical strength as a joining layer.

[0027] When a Ni-containing layer (such as plating) is applied to the joint portion, 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 formation 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 joint portion 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 joint portion may be any material containing 50% by mass or more 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 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 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 following 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 x 10 －５ (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 larger the creep rupture elongation value, the more excellent 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 as a plate-shaped preform material, or as a powdered material that is mixed with flux to form a cream solder. When processed into a powdered material 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. For example, the average particle size can be 25 to 50 μm when measured using a general laser diffraction / scattering particle size distribution measuring device. 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 laminate 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 laminate 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] The formation of a solder joint is achieved 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 it can be joined in an active atmosphere such as hydrogen or formic acid. When using sheet solder, it is preferable to use a gas with reducing properties, such as hydrogen or formic acid. When joining in an active gas atmosphere with reducing properties, it is preferable to set the 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 to the melting point (Tm) of the solder material + (50°C ± 10°C). In actual joining, a temperature distribution of several degrees Celsius or more is achieved, so maintaining a certain temperature and time above a certain 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, electric 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, making it less susceptible to cracking and resulting in 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. The space defined by the case 16 and the heat sink 13 is filled with a sealing material 20 in contact with 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 portion 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 can be composed of an insulating substrate 122, a first conductive plate 121 formed on one of its main surfaces, and second conductive plates 123a and 123b formed on the other main surface. The insulating substrate 122 can be made of a material with excellent electrical insulation and thermal conductivity. Examples of the material for the insulating substrate 122 include Al. ２ O ３, AlN, SiN, etc. In particular, for high-voltage applications, materials that combine electrical insulation and thermal conductivity are preferable, and AlN and SiN can be used, but are not limited to these. Metal materials such as Cu and Al, which are easy to process, can be used for the first conductive plate 121 and the second conductive plates 123a and 123b. Furthermore, the conductive plates may be Cu or Al that have been subjected to surface treatment such as Ni plating for rust prevention purposes, and have a Ni-containing layer containing Ni or a Ni alloy formed on the surface. Specific Ni-containing layers formed by Ni plating include, but are not limited to, layers containing NiP and NiB. Methods for disposing the first conductive plate 121 and the second conductive plates 123a and 123b on the insulating substrate 122 include direct copper bonding and active metal brazing. In the illustrated embodiment, two second conductive plates 123a and 123b are discontinuously provided on the insulating substrate 122, with one second conductive plate 123a functioning as an electrode to be bonded to the semiconductor element 11 and the other second conductive plate 123b functioning as an electrode to be connected to the lead frame 18. However, the number and connection mode of the first and second conductive plates are not limited to the illustrated embodiment.

[0043] The lead frame 18 is a conductive connecting member connecting the semiconductor element 11 to the second conductive plate 123b, etc. Specifically, the lead frame 18 can be bonded to the front surface electrode of the semiconductor element 11, which is located on the opposite side from the electrode (back surface electrode) that contacts the laminate 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 a copper alloy. 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 the like. In this case, the film thickness of the Ni or Ni alloy layer or the Cr or Cr alloy layer may be approximately 20 μm or less. The lead frame may be connected to an output terminal (not shown). Wires may also be used as conductive connecting members for connecting the semiconductor element 11 and the second conductive plate 123b, etc. When bonding by wire bonding, solder material may not be used.

[0044] The heat sink 13 is made of a metal such as copper or aluminum, which has excellent thermal conductivity. To prevent corrosion, the 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 a case 16 is attached to the laminated substrate 12 and the heat sink 13 is not attached to the 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. Since 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 molten solder according to the first embodiment and the optional bonding layer 17 in the illustrated semiconductor device is merely exemplary. The semiconductor device according to the third embodiment of the present invention may be any device that includes a bonding layer made of molten solder according to the first embodiment between any of the components that make up the device. Therefore, the bonding layer between the laminated substrate 12 and the heat sink 13 may be the bonding layer made of molten solder 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 resin, phenol resin, maleimide resin, cyanate resin, oxazine resin, etc. Among them, epoxy resin having at least two epoxy groups per molecule is particularly preferred because of its high dimensional stability, water resistance, chemical resistance, and electrical insulation. Specifically, it is preferable to use an aliphatic epoxy resin, an alicyclic epoxy resin, or a mixture thereof. Note that, as the encapsulating resin, a thermosetting resin is preferred because it requires heat resistance and high insulation, and epoxy resin is particularly preferred because of its 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 refer to 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 metal nitride, such as, but 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. Furthermore, 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 together. In particular, inorganic fillers with an average particle size of approximately 0.2 to 20 μm are preferably used. The amount of inorganic filler added to the sealing material 20 is preferably 100 to 600 parts by weight, and more preferably 200 to 400 parts by weight, based on 100 parts by weight of the total weight 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 the extrusion moldability worse.

[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 per 100 parts by mass of the thermosetting resin (epoxy resin) base, and more preferably 80 to 150 parts by mass. If the amount of curing agent is less than 50 parts by mass, the glass transition temperature may decrease due to insufficient crosslinking, while if it is more than 170 parts by mass, 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, taken as 100%. 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 a derivative thereof, a tertiary amine, a boric acid ester, a Lewis acid, an organometallic compound, and an organic acid metal salt. The amount of the curing accelerator added is preferably 0.01 parts by mass or more and 50 parts by mass or less, and more preferably 0.1 parts by mass or more and 20 parts by mass or less, 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 electrodes of the semiconductor element by implant pins, and the printed circuit board may be insulated and sealed.

[0058] The semiconductor device is manufactured by, for example, bonding the semiconductor element 11 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 bonding is performed using aluminum wire 14. Depending on the configuration of the semiconductor device, implant pins and a printed circuit board may also be optionally bonded. Next, the 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. Then, full curing can be performed by further heating at 175 to 185°C for 1 to 2 hours. 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 the laminated substrate and the semiconductor element with the solder material according to the first embodiment, attaching conductive connecting members such as lead frames and aluminum wires, 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 bonded by the solder material according to the first embodiment, which makes it difficult for cracks to occur, resulting in a highly reliable device.

[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] [Examples] (1) Evaluation of Mechanical Properties of Solder Material Samples were manufactured by adding 0 to 8 mass% of Cu to an alloy containing 6 mass% of Sb, 4 mass% of Ag, 0.25 mass% of Ni, and the remainder consisting of Sn and unavoidable impurities, and the mechanical properties of the alloy were evaluated, including the fracture elongation, Young's modulus, and creep rupture elongation at 175°C.

[0062] Based on JIS Z2241:2011, tensile testing method for metallic materials, 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 elongation at break (%) was calculated from the strain at break (when the stress became 0). The shape of the test specimen is shown in FIG. 2A, and a cross-sectional view taken along line X-X in FIG. 2A is shown in FIG. 2B. In FIG. 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, 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 × 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, creep curves were obtained, and 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 under a constant load of 7 N (8.92 MPa) at a temperature of 175°C, using the same equipment as in the tensile test.

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

[0065] The relationship between the Cu content in the solder material and the normalized creep rupture elongation is shown in Figure 4. The normalized creep rupture elongation is expressed as a relative value when the creep rupture elongation when the Cu content is 0 is set to 1. As can be seen from Figure 4, when the Cu content is 3 mass% or more, the normalized creep rupture elongation increased by 40% or more compared to when the Cu content was 0. Furthermore, when the Cu content was in the range of 5 to 7 mass%, the creep properties were maximized, and the normalized creep rupture elongation was more than double that when the Cu content was 0. When the Cu content exceeded 7 mass%, the creep properties decreased, but when the Cu content was 8 mass% or less, the normalized creep rupture elongation remained at a level that was 40% or more higher compared to when the Cu content was 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 of Samples 1 to 18 having the compositions shown in Tables 1 and 2 below were prepared. A power semiconductor module was manufactured having a solder joint layer 10 in which the solder material was melted 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 as follows: ２ The holding temperature and holding time in a reducing environment were 260°C to 290°C and 100 to 160 seconds. The solder between the laminated substrate and the semiconductor element was applied using a metal mask with an opening of 3.2 mm square and a thickness of 200 μm. The solder paste used in this example was a flux whose main component was a common rosin. The solder paste used was a 1:1 mixture of this rosin-based flux and solder powder with a particle size of 25 to 45 μm.

[0068] [T ｊ Evaluation of P / C tolerance] The reliability of the power semiconductor module is evaluated by ｊ Power cycle resistance (T ｊ The power cycle test was performed at 75 to 150°C (ΔT ｖｊ One cycle consisted of one second of energization (current set to 150°C) and 9 to 15 seconds of rest (current set to 75°C). The number of cycles until an abnormality in the current or voltage was detected was defined as the P / C tolerance. The evaluation criteria for P / C tolerance 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, conforming to JIS Z 3198-4). This measurement method involves immersing a test piece in a liquid and then lifting it out, and determining the contact angle from the force applied to the periphery of the test piece where the tip of the test piece contacts 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 sec. 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. Note that a surface tension of less than 500 mN / m, 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 higher 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 flaw detection (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 of the power semiconductor module, ｊ The P / C resistance and the evaluation results, as well as the wettability evaluation results of the solder material, are shown.

[0071]

[0072] From the results of Samples 1 to 6, when the Cu content was increased from 3.2% by mass 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% by mass 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 the power semiconductor module using the solder material of sample 8, which had a reduced amount of Sb, as 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 the insufficient Sb content weakening the effect of solid solution strengthening and causing damage in the Sn portion, and that the low Sb content caused stress relaxation due to Sn deformation to occur preferentially. A higher Sb content is considered preferable. The solder material of sample 9, which had an increased amount of Sb, did not have good wettability, and voids were observed in some areas. In the power semiconductor module using the solder material of sample 12 as the bonding layer, minute cracks of about 30 to 50 μm were observed, centered around the Sn phase of the solder bonding layer. When the Ag content was 2% by mass or more, the Ag densely dispersed in the solder structure ３ Sn is considered to be preferable because it suppresses local deformation (deformation of the Sn phase). Furthermore, it was confirmed from Sample 10 that when Ag is present in a large amount, extremely small voids with a diameter of about 1.0 mm are generated. Although these voids do not pose a practical problem, the presence of the voids is considered to have resulted in a slightly lower P / C resistance than Sample 11.

[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 withstand capacity of 50k cycles or more can be obtained. With a composition within the specified range, it is thought that the slippage exhibited by the addition of Cu during P / C testing improved the stress relaxation effect of the solder material, thereby improving the withstand capacity.

[0075]

[0076] In Table 2, "-" indicates that the wettability was poor and that the P / C test was difficult to carry out due to voids in the initial structure and scattered solder balls (solder solidification). From Table 2, in the 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 (grain boundaries of β-Sn) in the solder bonding layer after the power cycle test. Samples 13 and 14 were Cu ６ Sn ５ Since the structure is not formed, the creep property improvement due to the improvement in slip property is not realized. ６ Sn ５ Although Cu was generated, its distribution was sparse, and it is believed that the effect of adding Cu was not fully realized. As a result, cracks occurred at the grain boundaries of β-Sn in Samples 13 to 16, leading to the end of their life. In Samples 17 and 18, the Ag content deviated from the eutectic composition (3.5 mass% or more and 5.0 mass% or less), which is thought to have reduced wettability, resulting in the formation of voids and solder balls. In the solder joint of Sample 18, Ag ３ Excessive amounts of AgSn compounds such as Sn were produced, 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 large amount 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.

[0078] REFERENCE SIGNS LIST 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. 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.

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 the 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 having 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 the surface of the laminated substrate opposite to the surface to which the semiconductor element is bonded, the semiconductor device comprising the solder joint of claim 9 between the laminated substrate and the cooler.

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