Lead-free solder foil for diffusion soldering

Abstract

Lead-free solder foil (1) for diffusion soldering to join metallic components (2) and / or metallized / metal-coated components (2), ie metallic surface layers (3) of adjacent components (2), to one another, characterized in that - that the lead-free solder foil (1) is compactly constructed by roll bonding as a solid, consistently uniformly structured solder composite material (4) in such a way that in this solder composite material (4) in a lead-free soft solder environment, a soft solder matrix (5), particles (6) of a high-melting metal component (7), a hard solder component, are dispersed, finely distributed, arranged in such a way that each of the particles (6) is completely surrounded by the lead-free soft solder (8) in order to bring about, in a conventional soft soldering process, with the soldering times of less than 5 minutes customary for lead-free soft soldering processes, a complete conversion of the soft solder (8) of the soft solder matrix (5) into intermetallic phases (9) which have a melting temperature of higher than 400°C, and - that the particles (6) of the high-melting metal component (7) dispersed and finely distributed in the soft solder matrix (5) have a thickness of 3 µm to 20 µm in the direction of the film thickness, wherein the distances between the particles (6) in the soft solder matrix (5) are 1 µm to 10 µm, and each of the particles (6) of the high-melting metal component (7) is coated on all sides by a 1 µm to 10 µm thick layer of the lead-free soft solder (8), and - that the soft solder portion, the soft solder matrix (5), is not higher in relation to the portion of high-melting metal component (7) than is required in the intermetallic phases (9) to be built up, wherein this ratio of the percentage of the particles (6) of the high-melting metal components (7) arranged in the solder composite material (4) to the percentage of the soft solder (8) of the lead-free soft solder matrix (5) surrounding the particles (6) is determined according to the stoichiometric formula of the intermetallic phases (9) to be formed from the respective starting materials in such a way that all the soft solder (8) of the lead-free soft solder matrix (5) is always converted into the intermetallic phases (9) to be built up, and - that the total thickness of the lead-free solder foil (1) is 20 µm to 0.5 mm, and - that the soldering foil (1), the solder composite material (4), has an outer coating layer (10) adjacent to the metallic surface layers (3) of the components (2) to be joined, the layer thickness of which is from 2 µm to 10 µm and which consists of soft solder (8).

Description

[0001] The invention relates to a lead-free solder foil for diffusion soldering, with which metallic components and / or metallized / metal-coated components, ie metallic surface layers of adjacent components, can be joined together.

[0002] The reliability of solder joints in electronics, and especially in power electronics, today requires very good mechanical, electrical and thermal properties of the solder materials as well as of the connection zones created with them, with their resistance currently being extended to ever higher temperature ranges.

[0003] For reasons of environmental protection and health safety, the international trend is towards the use of environmentally friendly and harmless lead-free solder materials.

[0004] In the course of the transition to lead-free solders, numerous solder variations, mostly based on tin, were developed which, although they also have good mechanical, electrical and thermal properties compared to lead-containing alloys, melt in the range of approximately 214°C to 250°C, so that the durability of their good properties is limited to applications up to approximately 150°C.

[0005] For higher working / application temperatures, there are currently no lead-free solders that combine the temperature resistance properties required in power electronics with the necessary reliability and cost-effectiveness.

[0006] Therefore, for high-temperature applications, especially at operating temperatures above 250°C, there is a need to develop new, cost-effective lead-free solder foils that meet the temperature control requirements of power electronics. This is necessary to avoid damaging the components to be joined during the soldering process and, also from a cost-effective perspective, to achieve a high-temperature-resistant solder joint that ensures high thermal reliability of the connection zones between adjacent components. Currently, the cost-intensive eutectic Au80Sn20 solder with a melting point of 280°C is sometimes used in electronics and related industries.

[0007] However, a broad application of this Au80Sn20 solder, such as for soldering Si semiconductor circuits in power electronic switches, is not possible due to the high cost of the solder material.

[0008] In this context, US Pat. No. 7,659,614 B2 also describes the joining of gold and / or silver-coated substrates and electronic components using tin-containing gold or indium solders. When using these materials, gold and / or silver from the metallization layers with tin and / or indium form bonding zones with higher melting temperatures than the originally used solders. The joining process takes place at a minimum of 250°C and lasts 10 to 30 minutes. However, a slight contact pressure is always required, which also makes the soldering process more complex.

[0009] Therefore, the use of this teaching disclosed in patent US 7659614 B2 remains severely limited for large-scale application due to the technological complexity and the high material costs for the coatings and the solder.

[0010] Since no technically and economically viable lead-free alternative to gold-containing alloys is available in the industry to date, despite the international need for environmentally friendly and non-hazardous lead-free solder materials, exemptions have been issued whereby lead is still permitted in high-melting solders (i.e. lead-based solder alloys with a mass fraction of at least 85% lead) to this day, and is therefore still widely used in practice despite health and environmental concerns.

[0011] However, as a result of the increasing use of wide-band gap semiconductors, such as SiC or GaN, whose operating temperatures can rise well above 200°C, there is an increasing demand for solder joints that meet the technical requirements in the field of high-temperature applications, ie operating temperatures in the range of 150°C to 400°C.

[0012] To solve this problem, sintering technology was developed, among other things, which uses silver-containing pastes to join electronic components. Unlike soldering, however, this joining process requires a pressing force. This additional technological component, "pressing force," is also a key reason why sintering technology has not yet been widely adopted.

[0013] Another alternative is the use of reactive solders. These are reactive multilayer systems made up of layers of at least two different materials a few nanometers thick. After activation, diffusion between the layers begins, which quickly develops into an exothermic reaction. This provides the heat necessary to melt a solder. For this purpose, very thin (significantly less than 1 µm) layers of two matching metals must be deposited alternately on top of one another so often that films with a total thickness of 40 µm to 150 µm are built up, the outer layers of which consist of a solder. Solder preforms can be separated from these layered films. Alternatively, these metals can also be deposited alternately on a component to be soldered, whereby the outer layer must again be a solder.The joining process is started by igniting the reactive layers; the speed and amount of heat can only be controlled via the layer structure, and must therefore be determined individually for each conceivable soldering task during the production of the molded parts or coating of the components to be soldered, which represents a major obstacle to a broad and universal application of this technology.

[0014] A variation of the widely used soft soldering technique is diffusion soldering. This is achieved using the conventional technique but with the addition of various technological steps, such as the application of external pressing force or subsequent heat treatment or longer soldering profiles. The result of this process is that a substance is created during the soldering process which differs from the original composition of the soft solder and firmly bonds the components to be joined. The substance's melting point is higher than that of the solder material used. To form this new substance, the high-melting intermetallic phase, in addition to the low-melting metal commonly used in the solder material, such as tin, another high-melting metal, such as copper, is required. From these phases, the intermetallic phases are built up by diffusion into one another; their melting points are higher than that of the low-melting metal.

[0015] DE 10 2007 010242 A1 discloses a method for joining two metal layers using a diffusion soldering process. This solution disclosed in DE 10 2007 010242 A1 requires that each metal layer is initially structured in a special way, and at least one of them must additionally be provided with a solder layer. Only this very specific design of the layers, adapted to the respective components to be joined, ensures the formation of a compact, correctly positioned connection zone from such an intermetallic phase, without the need for additional pressing force during the soldering process. Therefore, this solution disclosed in DE 10 2007 010242 A1 is also limited to very specific applications, such as the soldering of chips to wafers.

[0016] From US 8348139 B2, multi-layer solder foils for diffusion soldering are also known, which are constructed from a metallic core consisting of pure metals or their alloys with a melting point higher than 280°C, and which is connected on both sides with similar or different layers consisting of tin- or indium-based solders, wherein the thickness of the solder layers used is at least 5 µm.

[0017] For these multilayer solder foils according to US 8348139 B2, the diffusion soldering process is carried out at 300°C to 380°C in 5 to 8 minutes. However, to ensure a continuous layer of intermetallic phases, the joined components must then be heat treated. With this solution, an unspecified layer thickness of the metallic core material remains after the heat treatment.

[0018] Furthermore, multilayer solder foils for diffusion soldering are also known from US 2006 186550 A1. These foils consist of a metallic core, which can be made of Ag, Au, Cu, or Ni, onto which layers consisting of tin, indium, or bismuth-based solders are applied on both sides. During the diffusion soldering process, the two soft solder layers melt and react with the full-surface core material. According to the solution according to US 2006 186550 A1, the applied layers are 1 µm to a maximum of 20 µm thick, so that within a reasonable soldering process duration (approximately 10 minutes at 240°C), the conversion of the liquid phase into intermetallic phases is achieved to such an extent that the adhesion of the soldered components is ensured in a subsequent process step at 260°C.

[0019] The diffusion soldering process itself, as well as the reliability of the resulting bonding layer, has also been investigated in publications, including those by N. Oeschler and C. Ehrhardt (N. Oeschler et al.: Diffusion Soldering Technology for Highly Reliable Chip-Substrate Connections, Weichlöten 2013, DVS Reports Volume 290, pp. 55-61 and C. Ehrhardt et al.: Test Methods for Bonding Technology in Power Electronic Modules, Weichlöten 2013, DVS Reports Volume 290, pp. 43-51). The results described in these publications are only applicable to copper / tin-coated semiconductor-substrate connections and could only be achieved using a compression force.

[0020] US 9 620 434 B1 also describes the joining of power electronic components using diffusion soldering, which is suitable for working temperatures of over 250°C. For this purpose, two layer systems are used, each consisting of a high-melting and a low-melting metal layer, which are placed on the components to be joined. If necessary, the entire system is built up with the addition of high-melting and low-melting metal particles between the metal layers and then heated. The disadvantage of this solution is that the complete conversion of the liquid phase of the solder material into intermetallic compounds / intermetallic phases is only possible by significantly extending conventional soldering times, i.e. when using the solution according to US 9 620 434 B1, a process duration of over 30 minutes is mandatory.

[0021] According to US 2017 / 0 080 682 A1, for joining substrates, here in particular power electronics components which are exposed to thermal cycles with working temperatures of over 250°C, a composite bonding layer is used which has an inner bonding region and an outer bonding region which is positioned around the inner bonding region, wherein the material of the inner bonding region has a greater modulus of elasticity than the material of the outer bonding region, and this material has a metal matrix, wherein a part of the metal matrix is ​​positioned in the outer bonding region and a part of the metal matrix is ​​positioned in the inner bonding region, wherein the modulus of elasticity of the metal matrix is ​​greater than the modulus of elasticity of the soft material elements, but smaller than the modulus of elasticity of the hard material elements.The focus of this solution presented in US 2017 / 0 080 682 A1 is on balancing stresses between materials with different thermal expansion coefficients.

[0022] A special case of this middle substrate, referred to as a composite bonding layer, also includes a diffusion solder joint. However, since the description of the aforementioned invention does not provide any information on the joining process and joining time, as well as on the structure of the resulting joint, this solution is also assumed to have a soldering process time that is typical in the prior art, i.e., a soldering process time of over 30 minutes, which is required according to the prior art to achieve complete conversion of the liquid solder material into intermetallic phases.

[0023] Another publication by A. Syed-Khaja (A. Syed-Khaja et al.: Process optimization in transient liquid phase soldering (TLPS) for an efficient and economical production of high temperature power electronics, CIPS 2016, pp. 187-193) describes the use of solder preforms made of conventional solder alloys for diffusion soldering of substrates to a semiconductor component. This publication demonstrates that the use of thin solder preforms (25 µm) of a conventional SnCu solder, containing no more than 3% copper, leads to complete formation of the high-melting intermetallic bond zone without the application of pressing force. However, this requires somewhat longer soldering times and at least one copper-metallized component with an adapted roughness.However, when using solder preforms consisting of copper plated on both sides with pure tin (Sn20µm / Cu35µm / Sn20µm), only a partial conversion to a high-melting phase was achieved, meaning a soft solder component with a correspondingly low melting temperature remained in the joint zone. Only when using copper-plated components did this conversion occur completely. All results were achieved only after a soldering time of 22 minutes at a temperature of 260°C with components of an adjusted roughness.

[0024] In the publication "Testing Methods for the Connection Technology of Power Electronic Modules", Soft Soldering 2013, DVS Reports Volume 290, pp. 43-51, C. Ehrhardt et al. describe that conventional lead-free solder pastes, such as SnAgCu, must also be homogeneously mixed with high-melting powders, such as copper, to implement the diffusion soldering process. The molten tin-based solder of the lead-free solder paste dissolves the copper powder, allowing the intermetallic phases Cu6Sn5 and Cu3Sn to form. Using these lead-free solder pastes mixed with high-melting powders, the liquid phase was completely converted into intermetallic phases during the diffusion soldering process under the application of a pressing force. The melting points of the two phases formed in this way are 415°C and 676°C, respectively.However, their pore-free formation is dependent not only on the pressing force during the soldering process but also on a very homogeneous mixing of the two required components, solder paste and powder.

[0025] Patent EP 1337376 B1 describes a solder paste used as a solder. This solder paste contains, in addition to the solder material, metal-coated insulating cores with a high melting point. According to the solution according to EP 1337376 B1, the solder metal reacts completely with the metallization of the cores during the soldering process and, based on the diffusion soldering process, forms intermetallic phases that then enclose the high-melting cores. The resulting solder seam has an overall heterogeneous structure, which negatively impacts the thermal conductivity of the connection zone achieved with this solution. WO 96 / 19 314 A1 describes a powder mixture in which the solder metal consists of high-melting and low-melting metal components, with granular or platelet-shaped filler components added as additives.In general, metal powders or metal granules are very expensive to produce and also have a wide range of dimensions, so that intermediate classification processes are necessary. Furthermore, homogeneous mixing of metal powder is not without problems and is therefore very complex. The powder mixture itself should then, according to WO 96 / 19 314 A1, preferably be used as a suspension with a liquid organic solvent or as a paste. A filler component used in this context has the task of limiting the thickness of the intermetallic phases forming during diffusion soldering to a few µm. Therefore, depending on the wettability, it must be provided with appropriate coatings that promote or inhibit bonding and be mixed very homogeneously with the metal components. In special embodiments / special designs, according to WO 96 / 19 314 A1, the above-mentionedSolder metal consisting of powders can also be pressed into foils, from which preformed solder parts are punched out and placed between the objects to be joined. The production of such foils with a homogeneous distribution of the powder used, ie, powder metallurgy, is very complex and cost-intensive, and during compression, ie, in the powder metallurgical process, the theoretical density is either not achievable or only very difficult, ie, at high cost.

[0026] The disadvantage of all embodiments of this solution according to WO 96 / 19 314 A1 is that, in addition to the two metal components described above, a filler component is required to achieve the desired intermetallic phases. Furthermore, according to the description of this solution, this solution also requires a soldering process duration of over 30 minutes to achieve complete conversion of the liquid solder into intermetallic phases, or alternatively, a subsequent annealing process.

[0027] To improve surface wetting, the addition of a flux during the soldering process is also considered advantageous. However, with regard to occupational health and safety, this flux has the disadvantage that, according to the description in WO 96 / 19 314 A1, it produces organic acid, which must be removed in an additional step following the soldering process.

[0028] In summary, it can be stated that the cost-effective lead-free soft solders currently used in power electronics and other applications can only cover an operating temperature range of up to approximately 150°C. For the operating temperature range of soldered components above 150°C, there is currently no technically and economically viable lead-free solder alternative to gold-containing solder alloys that combines the temperature resistance required in power electronics with the necessary reliability and reasonable cost-effectiveness, i.e., without additional process parameters, such as additional pressing force or additional subsequent heat treatment.

[0029] In this context, there is therefore a need to provide new lead-free solders, if possible in the form of solder foil, so that they can then also be used cost-effectively in the form of solder preforms.

[0030] The object of the invention is therefore to develop an economically viable, environmentally friendly and harmless, lead-free solder foil for diffusion soldering, which, with a soldering profile typical for soft soldering, i.e., while avoiding long soldering times, as well as without subsequent heat treatment and without exerting a pressing force during soldering, while simultaneously avoiding the formation of pores, is intended to connect the metallic / metallized surface layers of the components to be soldered in such a way that a high-melting connection zone with a remelting temperature of higher than 400°C is created, wherein by means of the lead-free solder foil to be developed, even electrically conductive ribbons can be coated in the connection area, so that in the connection area of ​​the ribbons, the remelting temperature of the high-melting connection zone formed after the soldering process is higher than 400°C,and, in addition, for special applications, in a special design, the lead-free solder foil should also be provided with an adapted, resulting, thermal expansion coefficient in order to absorb thermal stresses introduced by soldering as well as those arising during component use, and at the same time to increase the mechanical flexibility of the connection zone created after the soldering process.

[0031] According to the invention, this object is achieved by a lead-free solder foil 1 for diffusion soldering, by means of which metallic components 2 and / or metallized / metal-coated components 2, i.e. metallic surface layers 3 of adjacent components 2, can be joined to one another, and which is characterized in that the lead-free solder foil 1 is constructed compactly as a solder composite material 4 in such a way that, in a lead-free soft solder environment, a soft solder matrix 5, particles 6 of a high-melting metal component 7, a hard solder component, are dispersedly arranged in such a way that each of the particles 6 is completely surrounded by the lead-free soft solder 8 in order to bring about, in a conventional soft soldering process, a complete conversion of the soft solder 8 of the soft solder matrix 5 into intermetallic phases 9 which have a melting temperature of higher than 400°C.

[0032] The compact, lead-free solder foil 1 according to the invention, produced as a solid composite, contains all the material required for the construction of the high-melting intermetallic phase, wherein the inventive distribution of the material required for the construction of the high-melting intermetallic phase in conjunction with the compact design according to the invention, as solder foil 1, causes a very rapid and pore-free formation of a high-melting intermetallic connection zone 16 with remelting temperatures of over 400°C to be achieved in a lead-free soft soldering process at temperatures of approximately 240°C.What is essential to the invention in this context is that the particles 6 of the high-melting metal component 7 dispersed in the soft solder matrix 5 have a thickness of 3 µm to 20 µm in the direction of the film thickness, wherein the distances between the particles 6 in the soft solder matrix 5 are 1 µm to 10 µm, and each of the particles of the high-melting metal component 7 is coated on all sides by a 1 µm to 10 µm thick layer of the lead-free soft solder 8.

[0033] By means of the lead-free compact solder foil 1 according to the invention with particles 6 of hard solder (hard solder particles) arranged in a soft solder matrix 5, soft solder environment, in conjunction with their dispersed distribution and at the same time compact embedding in the soft solder matrix 5, diffusion soldering is effected in a process area typical for lead-free soft soldering, without long soldering times, as well as without subsequent heat treatment, and without the exertion of a pressing force, which at the same time avoids the formation of pores, and connects the metallic / metallized surface layers 3 of the components 2 to be soldered to one another in such a way that a continuous pore-free layer of a high-melting connection zone 16 in the form of an intermetallic phase 9 is formed between the components 2 to be joined, the remelting temperature of which is above 400°C.What is characteristic here is that the proportion of soft solder, the soft solder matrix 5, is not higher in relation to the proportion of high-melting metal component 7 than is required in the intermetallic phases 9 to be built up. This ratio of the percentage proportion of the particles 6 of the high-melting metal components 7 arranged in the solder composite material 4 to the percentage proportion of soft solder 8, the lead-free soft solder matrix 5 surrounding the particles 6, is determined according to the stoichiometric formula of the intermetallic phases 9 to be formed from the respective starting materials in such a way that all soft solder 8 of the lead-free soft solder matrix 5 is always converted into the intermetallic phases 9 to be built up.

[0034] The ratio of the soft solder content to the proportion of particles 6 of high-melting metal component 7 in the soft solder matrix 5 thus depends on the stoichiometric formula of the intermetallic phase 9 to be formed. For example, when using the Sn / Cu combination with 50% Sn, this would be CuSn3 and Cu6Sn5.

[0035] The decisive factor for a remelting temperature of higher than 400°C is that the entire soft solder matrix 5 is always converted, otherwise there would still be areas in the connection zone 16 that would have a lower melting temperature, and this is not desired for the task.

[0036] With a different combination, for example when using the Sn / Ni combination with 43% Sn, Ni3Sn4 are formed as intermetallic phases.

[0037] In this context, however, it should also be noted that after the soldering process, particles 6 of the high-melting metal component may still remain in the connection zone 16, and the remelting temperature still remains higher than 400°C.

[0038] Through this higher proportion of particles 6, i.e. by means of the high-melting residual metal which is embedded in the intermetallic phase 9 as islands, e.g. made of copper, after the soldering process, it is possible to influence the mechanical, electrical and thermally conductive properties of the connection zone 16 achieved after the soldering process.

[0039] According to the teaching of the invention, it is therefore only crucial that the entire soft solder component consumed in the soldering process must be converted into intermetallic phases 9 in order to ensure a reflow temperature of higher than 400°C after the soldering process. For example, when using an In / Ag combination, a very high silver content would be necessary to achieve a reflow temperature of higher than 400°C. However, since the object of the invention is to develop an economically viable, lead-free solder foil for diffusion soldering, this combination will not be considered in more detail. It is also essential that the total thickness of the lead-free solder foil 1 is 20 µm to 0.5 mm, in accordance with the technological framework / desired properties of the connection zone 16.It is further characteristic that the solder foil 1, the solder composite material 4, has an outer coating layer 10 adjacent to the metallic surface layers 3 of the components 2 to be joined, the layer thickness of which is from 2 µm to 10 µm, and which consists of soft solder 8.

[0040] This coating layer 10 consisting of soft solder 8 serves during the soldering process to completely wet the surfaces / surface layers 3 of the adjacent components 2 to be joined and to form intermetallic phases 9 with these metallizations (e.g. Cu, Ni, Ni(P), Ni(Ag)) of the surfaces of the components 2 to be joined.

[0041] This lead-free solder foil 1 for diffusion soldering enables, with a soldering profile typical for lead-free soft soldering, for example when using solder foils 1 with a thickness of 30 µm to 250 µm at a soldering temperature of approximately 240°C and with soldering times of less than 5 minutes, without any subsequent heat treatment, as well as without exerting a pressing force during soldering, while at the same time avoiding the formation of pores, that the metallic / metallized surface layers 3 of the components 2 to be soldered are connected to one another in such a way that a continuous layer of a high-melting connection zone 16 in the form of an intermetallic phase 9 is created, which has a remelting temperature of higher than 400°C.

[0042] It is also essential to the invention that the lead-free solder foil 1 for diffusion soldering is constructed as a multi-layer solder foil 11 in accordance with special technical or technological requirements, as well as for economic reasons, wherein the individual layers of the multi-layer solder foil 11 alternately consist of the above-described solder composite material 4 and of 2 µm to 100 µm thick layers of a high-melting metal component 7, an intermediate layer 23, wherein the multi-layer solder foil 11 in turn has an outer cladding layer 10 adjacent to the metallic surface layers 3 of the components 2 to be joined, the layer thickness of which is from 2 µm to 10 µm, and which consists of soft solder 8, and the total thickness of the multi-layer solder foil 11 is from 40 µm to 1.0 mm.

[0043] Using this special design, the multilayer solder foil 11, the lead-free solder foil 1 can also be provided with an adapted, resulting, thermal expansion coefficient in order to absorb thermal stresses introduced by soldering as well as those arising during component use, while simultaneously increasing the mechanical flexibility of the connection zone created after the soldering process. It is also important that the lead-free solder foil 1 can be used for diffusion soldering both as a solder composite material 4 and as a multilayer solder foil 11 in the form of a solder preform 12. It can act as a diffusion solder between metallic surfaces / surface layers 3 in a lead-free soft soldering process and connect the adjacent components 2 to one another in such a way that the reflow temperature is higher than 400°C.

[0044] The solder preforms 12 are formed from the solder foil 1 into the desired preform geometry by cutting, punching, or punching-bending processes. They are thus universally applicable in many common soft soldering processes, which become diffusion soldering processes solely through the use of the distributed particles 6 of the solder composite material 4 (composite material). In this way, the reflow temperature of the connection zones is significantly increased compared to components conventionally soldered with soft solder. When using tin soft solder components and copper as a refractory metal component, the components 2 soldered with solder preforms 12 made of the solder composite material 4 can be used for operating temperatures up to 400°C, combining the temperature resistance of the properties required in power electronics with the necessary reliability and cost-effectiveness.It is also characteristic that a metallic conductor strip 13, which serves as an electrical conductor in the product 14 to be joined, is partially coated at the joints 15 with the lead-free solder foil 1, both in the embodiment as a solder composite material 4 and in the embodiment as a multi-layer solder foil 11, so that after the soft soldering process, the partially coated conductor strip 13 connects the adjacent components 2 to one another in such a way that after the soft soldering process, a connection zone 16 is created between the coated conductor strip 13 and the components 2 to be connected thereto, which connection zone has a remelting temperature of higher than 400°C. In this case, the lead-free solder foil 1 according to the invention, produced for diffusion soldering, is applied on one side by partial plating to a material with good electrical conductivity, such as copper or aluminum. Conductor strips 13 can then be manufactured from this partially plated material, which, for example,can be used instead of the usual bonding wire to build power modules.

[0045] The lead-free solder foil 1 for diffusion soldering according to the invention can be produced, for example, by roll bonding as described below.

[0046] Depending on the intended percentage composition, soft solder and metal components are alternately joined to form a layered composite by roll bonding, with the metal component being plated on both sides with the soft solder component.

[0047] The plating is started in such a way that the layer thicknesses of the components to be used are in such a relationship to one another that in the subsequent soldering process the soft solder component is completely incorporated into the intermetallic phase as intended.

[0048] This is followed by further roll-cladding steps, in which the clad material is clad with itself, increasing the number of layers in the material while simultaneously reducing their thickness. The number of clad steps required to produce the finished brazing composite material 4 depends on the selected material combination of soft and hard solder components and the desired overall thickness of the brazing preforms. During multiple clad coatings of the composite, the individual components are mixed in the solid state in such a way that, when the layers of one of the two components are torn open, their fragments are dispersed in the other, softer component.The resulting structure, with particle spacings of less than or equal to 10 µm according to the invention, ensures the desired short diffusion paths, which lead to the complete conversion of the soft solder component into the intermetallic phase in a short time during the subsequent lead-free soft soldering process, creating a compact, pore-free, high-melting point bond zone. The short diffusion paths according to the invention enable the applicability of conventional soft soldering profiles.

[0049] These intermetallic phases (9) are formed from a low-melting soft solder component and a high-melting metal component / braze component, which are used in mass proportions corresponding to their stoichiometric formula. The components are selected so that the melting point of their intermetallic phase lies between the melting points of the two components used. The melting temperature of the soft solder component is in the range of up to 240°C when tin is used as the base, while the melting temperature of the intermetallic phases (9) is above 400°C when copper is used as the high-melting metal component.

[0050] The solder composite material 4 produced by multiple forming processes can, if required, be applied to a high-melting metallic base material in further plating steps, whereby layers of solder composite material 4 and metallic intermediate layers 23 with specific desired mechanical properties alternate and thereby a multi-layer solder foil 11 is built up, but a soft solder component as the outer cladding layer 10 always forms the two outer layers.

[0051] Such a multi-layer soldering foil 11 can then, for example, with an adapted, resulting, thermal expansion coefficient, absorb thermal stresses introduced by soldering as well as those arising during component use.

[0052] The thickness of the solder foil 1 in the embodiment as a solder composite material 4 can always be adjusted to the exact thickness of the solder foil 1 or the solder preforms 12 to be produced therefrom by the initial thicknesses of the two components, the number of plating steps and the final rolling steps.

[0053] The thickness of the solder foil 1 in the embodiment as a multi-layer solder foil 11 can also be adjusted to the desired precise dimension of the solder foil 1 or the solder preforms 12 to be produced therefrom by the initial thicknesses of the metallic intermediate layer and the layers with solder composite material 4, the number of plating steps, and the final rolling steps. According to the invention, the high-melting metal component / brazing component is dispersed in the soft solder component with particle spacings of less than or equal to 10 µm.

[0054] As already explained, the outer layers of the lead-free solder foil according to the invention are always formed by the soft solder component.

[0055] The solution according to the invention will be explained in more detail below using an exemplary embodiment in conjunction with 5 figures.

[0056] The Fig. 1 shows the schematic structure of a semiconductor power switch.

[0057] The chip / semiconductor component 21 is soldered onto a conductor track, i.e., a metallic surface layer 3, which is supported by an electrically insulating ceramic layer (DCB), the ceramic substrate 20. Its upper surface is connected to another conductor track / metallic surface layer 3, also located on the substrate, which is usually achieved in a bonding process with thin aluminum or copper wires / conductor strips 13. The ceramic substrate 20 is soldered onto a base plate 19, which is mounted on a heat sink / heat sink 17. All surfaces / surface layers 3 to be connected must be metallic, and the connection zones 16 themselves must ensure the most effective heat flow possible to the heat sink.

[0058] The following describes the use of the soldering foil 1 according to the invention in connection with the joining process, a diffusion soldering process for the construction of the Fig. 1 will be explained in more detail.

[0059] The lead-free solder foil 1 according to the invention is used in the form of a solder composite material 4 on the one hand to achieve a current connection of the semiconductor component 21 with a conductor strip 13, and on the other hand also to solder the semiconductor component 21 to the DCB, the ceramic substrate 20, as a solder preform 12.

[0060] The Fig. 2 shows a sectional view of the arrangement of the solder foil 1 in the embodiment as a solder composite material 4 between the metallic surface layers 3 of the joining partners to be joined with identical or different metallic surfaces / surface layers 3. In the solder composite material 4, particles 6 of copper are dispersed in a lead-free Sn soft solder matrix 5, wherein the distance between the particles 6 is less than or equal to 10 µm and the uppermost and lowermost layer, the cladding layers 10, are each formed by the soft solder 8.

[0061] The Fig. 3 represents the arrangement according to the Fig. 2 schematically shows the soldering process. The Sn soft solder 8 is completely converted into intermetallic compounds / intermetallic phases 9 with a melting point higher than 400°C, dispersed within which are residues (residual metal 22) of the high-melting metal particles 6 made of Cu. This ensures that the entire connection zone 16 only melts at temperatures above 400°C and, in addition to the high electrical conductivity, also ensures very good thermal conductivity.

[0062] Subsequently, the lead-free solder foil according to the invention is used in the form of a multi-layer solder foil 11 for system soldering, ie here to achieve a solder connection between the DCB, the ceramic substrate 20 and the base plate 19.

[0063] The Fig. 4 shows in a schematic sectional view the arrangement of the solder foil 1 in a possible embodiment as a multi-layer solder foil 11, in the form of preformed solder parts 12, in its position with respect to the joining partners, ie between the identical or different metallic surface layers 3 of the components to be joined.

[0064] In this multilayer solder foil 11, two layers of a high-melting metal component 7, such as Cu, the intermediate layers 23, are arranged between three layers of the solder composite material 4. In the solder composite material 4, Cu particles 6 are dispersed in a lead-free Sn soft solder matrix 5, with the distance between the particles 6 being less than or equal to 10 µm. The top and bottom layers of the multilayer solder foil 11, the cladding layers 10, are each formed by the soft solder 8.

[0065] The Fig. 5 now shows schematically the arrangement according to Fig. 4 after the soldering process. In the material layers of solder composite 4, the Sn soft solder 8 is completely converted into intermetallic compounds / intermetallic phases 9 with a melting point higher than 400°C, but dispersed therein are also residues of the high-melting metal particles 6 Cu.

[0066] In between, connected by the intermetallic phases 9, there is the residual metal 22 of the intermediate layers 23 made of the high-melting metal component 7 such as Cu, whereby the entire connection zone 16 only melts at temperatures above 400°C and ensures very good thermal conductivity, as well as an adapted resulting thermal expansion thereof.

[0067] The following describes the soldering process for producing the Fig. 3 and Fig. 5 shown connecting zones 16 made of the lead-free solder foil 1 according to the invention will be explained in more detail.

[0068] For chip soldering, the semiconductor components 21, such as Si chips, SiC chips or IGBT modules, are soldered to a DCB, a ceramic substrate 20. The semiconductor components 21 are usually coated with Ni or Ni(Ag), and the DCB, the ceramic substrate 20, is coated with a surface layer 3 made of Cu and often additionally with Ni. To date, solder alloys with a high lead content have usually been used for chip soldering because their melting temperature is between 290°C and 305°C and the solder connection created in this way should not melt again at temperatures above 240°C due to the step soldering customary in series production in the second soldering process for system soldering. In series production, chip soldering is usually carried out in a first stage, and system soldering is carried out using a lead-free solder in a second stage.Since the high-lead solder has a higher melting temperature than the lead-free solder, this step-by-step soldering in the described sequence prevents the chip solder connection from melting during system soldering.

[0069] According to the present invention, a solder preform 12 made of solder composite material 4 with an Sn soft solder matrix 5 and copper particles 6 dispersed therein is used for chip soldering, wherein the solder composite material 4 has an outer cladding layer 10 made of soft solder 8 adjacent to the metallic surface layers 3 of the components 2 to be joined, which outer cladding layer 10 rests on the metallic surface 3 of the chip / semiconductor component 21 on the one hand and on the metallic surface / surface layers 3 of the DCB / ceramic substrate 20 on the other hand of the solder composite material 4, ie comes into contact with them.

[0070] Compared to the chip soldering processes carried out in conjunction with the high-lead solders, a significantly lower process temperature is possible when using the solution according to the invention, so that the heating up to 240°C usual in a lead-free soft soldering process is sufficient.

[0071] The Sn soft solder 8 melts at approximately 220°C, the liquid phase reacts with the metallic surfaces / surface layer 3 of the adjacent components 2 and dissolves so much dispersed copper within 2 minutes that the liquid phase is completely converted into the solid intermetallic phases 9, i.e. into CuSn3 and Cu6Sn5.

[0072] This creates the pore-free connection zone 16, whose melting temperature is above 400°C.

[0073] For system soldering, the DCB, the ceramic substrate 20, which already supports the chip / semiconductor component 21, is soldered to the base plate 19. The base plate 19 is usually coated with a surface layer 3 made of Cu, Ni, Ni(P), or Ni(Ag), while the DCB / ceramic substrate 20 is coated with a surface layer 3 made of Cu, Ni, Ni(P), or Ni(Ag). According to the invention, a solder preform 12 made of multilayer solder foil 11 is processed in a lead-free soft soldering process for system soldering. The use of the multilayer solder foil 11 offers the possibility of increasing the mechanical flexibility of the connection zone 16 created after the soldering process via the layer structure of the multilayer solder foil 11.In the present exemplary embodiment, the preformed solder part 12 consists of layers of a solder composite material 4 with a Sn soft solder matrix 5 and particles 6 of a copper metal component 7 dispersed in this Sn soft solder matrix 5, wherein these layers alternate with layers of a high-melting metal component 7, such as copper, the outer layers of the solder composite material 4, the cladding layers 10, consist only of the Sn soft solder 8. These outer layers of the solder composite material 4 come into contact with the metallic surfaces / surface layers 3 of the substrate 20 and the base plate 19, i.e. the components 2.

[0074] The Sn soft solder 8 melts again at approximately 220°C. The now liquid coating layer 10 forms the intermetallic phases 9 CuSn3 and Cu6Sn5 with the metallizations of the substrate 20 and the base plate 19. At the same time, the now also liquid soft solder 8 dissolves so much dispersed copper (the particles 6 of the metal component 7) within the multilayer solder foil 11 within 2 minutes that it is completely converted into the solid intermetallic phases CuSn3 and Cu6Sn5. The same phases are also formed at the interface with the intermediate layers 23 of the high-melting metal component 7. Thus, after the soldering process, a pore-free connection zone 16 is created in the area of ​​the originally arranged multilayer solder foil 11. The melting temperature of this connection zone is above 400°C and, due to the remaining metallic residual layers 22, has an adapted resulting thermal expansion coefficient.

[0075] In the prior art, the chip top side is typically bonded to the conductor track on the substrate using fine aluminum or copper wires in an ultrasonic welding process. Using the solder foil according to the invention, this joining method can also be replaced by a diffusion soldering process, which is analogous to the aforementioned soldering processes.

[0076] According to the invention, a conductor strip 13 made of an electrical conductor such as aluminum or copper is used to contact the chip. The solder composite material 4 has previously been applied to both of the connecting surfaces to be joined, such that its outer layer, consisting of Sn soft solder 8, contacts the metallic surface layer 3 of the chip / semiconductor component 21 on the one hand, and the metallic surface layer 3 of the DCB / substrate on the other. When heated to the appropriate temperature of a lead-free soft soldering process, the soft solder 8 of the solder composite material 4 melts.

[0077] Within the solder composite 4, the now liquid soft solder 8 dissolves so much dispersed copper (the particles 6 of the metal component 7) within 2 minutes that it completely converts into the solid intermetallic phases CuSn3 and Cu6Sn5. The intermetallic phases CuSn3 and Cu6Sn5 also form at the interface between the metallizations (metallic surface layers 3) of the chip top and the substrate. This creates a connection zone 16 equivalent to that found in chip and system soldering. Reference symbol compilation 1 solder foil 2 components 3 Surface layer 4 Solder composite material 5 Soft solder matrix 6 particles 7 Metal component 8 soft solder 9 Intermetallic phases 10 sheath layer 11 Multilayer soldering foil 12 Solder preform 13 Ladder tape 14 Product 15 joints 16 Connection zone 17 heat sinks 18 thermal interface materials 19 Base plate 20 ceramic substrate (DCB) 21 Semiconductor component (chip) 22 Residual metal (high-melting) 23 Intermediate layer (high-melting)

Claims

[1] Lead-free solder foil (1) for diffusion soldering to join metallic components (2) and / or metallized / metal-coated components (2), ie metallic surface layers (3) of adjacent components (2), characterized by , - that the lead-free solder foil (1) is compactly constructed by roll bonding as a solid, consistently uniformly structured solder composite material (4) in such a way that in this solder composite material (4) in a lead-free soft solder environment, a soft solder matrix (5), particles (6) of a high-melting metal component (7), a hard solder component, are dispersed, finely distributed, arranged in such a way that each of the particles (6) is completely surrounded by the lead-free soft solder (8) in order to bring about, in a conventional soft soldering process, with the soldering times of less than 5 minutes customary for lead-free soft soldering processes, a complete conversion of the soft solder (8) of the soft solder matrix (5) into intermetallic phases (9) which have a melting temperature of higher than 400°C, and - that the particles (6) of the high-melting metal component (7) dispersed and finely distributed in the soft solder matrix (5) have a thickness of 3 µm to 20 µm in the direction of the film thickness, wherein the distances between the particles (6) in the soft solder matrix (5) are 1 µm to 10 µm, and each of the particles (6) of the high-melting metal component (7) is coated on all sides by a 1 µm to 10 µm thick layer of the lead-free soft solder (8), and - that the soft solder portion, the soft solder matrix (5), is not higher in relation to the portion of high-melting metal component (7) than is required in the intermetallic phases (9) to be built up, wherein this ratio of the percentage of the particles (6) of the high-melting metal components (7) arranged in the solder composite material (4) to the percentage of the soft solder (8) of the lead-free soft solder matrix (5) surrounding the particles (6) is determined according to the stoichiometric formula of the intermetallic phases (9) to be formed from the respective starting materials in such a way that all the soft solder (8) of the lead-free soft solder matrix (5) is always converted into the intermetallic phases (9) to be built up, and - that the total thickness of the lead-free solder foil (1) is 20 µm to 0.5 mm, and - that the soldering foil (1), the solder composite material (4), has an outer coating layer (10) adjacent to the metallic surface layers (3) of the components (2) to be joined, the layer thickness of which is from 2 µm to 10 µm and which consists of soft solder (8). [2] Method for producing a lead-free solder foil (1) for diffusion soldering in order to join metallic components (2) and / or metallized / metal-coated components (2), ie metallic surface layers (3) of adjacent components (2), together, characterized by , - that the roll-cladding process is used to produce a solder composite material (4) contained in the solder foil (1), wherein, to produce the solder composite material (4), soft solder and metal components are first alternately joined to form a layered composite in accordance with the intended percentage composition, such that the metal component always comes into contact with the soft solder component on both sides, wherein the layer thicknesses of the components to be used are in relation to one another in such a way that in the subsequent soldering process the soft solder component is completely incorporated into the intermetallic phase, all of this is ensured, plating is started, and - that once the layered composite has been plated, further roll-cladding steps are subsequently carried out, in which the plated material is plated with itself, so that the number of layers in the material increases, but their thickness is simultaneously reduced, and - that the number of roll-cladding steps is repeated many times depending on the selected material combination of soft and hard solder components and the desired total thickness for the solder preforms until the finished solder composite material 4 is obtained, and - that when the layer composite is plated multiple times, the individual components are mixed in the solid state, and - that by tearing open the layers of one of the two components, their fragments are dispersed in the other, softer component, so that roll bonding produces a structure with particle spacing of less than or equal to 10 µm. [3] Lead-free solder foil (1) for diffusion soldering according to claim 1, characterized by , - that the soldering foil (1) is constructed as a multi-layer soldering foil (11), and - that the individual layers of the multi-layer soldering foil (11) consist alternately of the solder composite material (4) described in claim 1 and of 2 µm to 100 µm thick layers of a high-melting metal component (7), an intermediate layer (23), and - that the multi-layer soldering foil (11) has an outer coating layer (10) adjacent to the metallic surface layers (3) of the components (2) to be joined, the layer thickness of which is from 2 µm to 10 µm and which consists of soft solder (8), and - that the total thickness of the multilayer solder foil (11) is 40 µm to 1.0 mm. [4] Method for producing a lead-free solder foil (1) for diffusion soldering according to claim 3, characterized bythat the soldering foil (1) is constructed as a multi-layer soldering foil (11), the individual layers of which are joined to one another by means of roll bonding, wherein these individual layers of the multi-layer soldering foil (11) alternately consist of the solder composite material (4) described in claims 1 and 2 and of layers of a high-melting metal component (7), an intermediate layer (23), wherein the multi-layer soldering foil (11) has an outer jacket layer (10) adjacent to the metallic surface layers (3) of the components (2) to be joined, which outer jacket layer consists of soft solder (8). [5] Lead-free solder foil (1) for diffusion soldering according to claim 1 or 3, characterized bythat this lead-free solder foil (1) is used as a preformed solder part (12) in a lead-free soft soldering process and, with the soldering times of less than 5 minutes typical for lead-free soft soldering processes, connects the adjacent components (2) to one another in such a way that the connection zone (16) has a reflow temperature of higher than 400°C after the soldering process. [6] Lead-free solder foil (1) for diffusion soldering according to claim 1 or 3, characterized bythat the lead-free soldering foil (1) is used for coating the joining points (15) of a metallic conductor strip (13) which serves as an electrical conductor in the product (14) to be joined, whereby the joining points (15) of the metallic conductor strip (13) are partially coated with the lead-free soldering foil (1) according to one of claims 1 or 3, in such a way that the conductor strip (13) partially coated at its joining points (15) connects the components (2) to be connected to the conductor strip (13) to one another at the joining points (15) after a lead-free soft soldering process with soldering times of less than 5 minutes which are customary for lead-free soft soldering processes, in such a way that the connecting zone (16) has a remelting temperature of more than 400°C.

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