Lead-free solder foil

Abstract

The invention relates to a lead-free solder foil for joining metallic and / or metal-coated components. The task of the invention is to develop a lead-free solder foil which enables a defined joining zone geometry and achieves a high-temperature-resistant soldered joint with a minimum of pores and / or shrinkage cavities, which guarantees high reliability even in a multistage soldering process and improves the thermal conductivity of the joining zone. The lead-free solder foil (1) according to the invention is constructed in such a way that two or more composite wires (3) are individually, mutually parallel and parallel to the band edges between the two solder bands, coated in a solder matrix (2) by means of a roll-coating process, wherein the composite wires (3) have a core (4) which consists of a more refractory metal / metal alloy than the solder matrix (2) and around which a sheath (5) consisting of another metal / metal alloy is arranged, and 5 to 15 μm of solder material is arranged above and below at least one of the cores (4) after the roll-coating process.

Description

Technical Field

[0001] The present invention relates to a lead-free solder foil, which enables the interconnection of the surface layers of metal components and / or metallized / metal-coated components, i.e., adjacent components. Background Technology

[0002] In order to connect components in the electronics industry, solder materials, mostly so-called soft solders, are used in the prior art not only in the form of solder paste but also in the form of pre-partitioned, pure metal solder assemblies.

[0003] Today, the reliability of solder joints in electronic devices, especially in power electronic devices, requires solder materials with excellent mechanical, electrical, and thermal properties, as well as the bonding area produced by the solder material. The durability of the solder material should now be extended to increasingly higher temperature ranges.

[0004] For reasons of environmental protection and health and safety, the international trend is to use environmentally friendly and safe lead-free solder materials.

[0005] In the transition to lead-free solder, a large number of solder variants (mostly based on tin) were developed, which, while possessing good mechanical, electrical, and thermal properties compared to lead-containing alloys, melt in the range of approximately 214°C to approximately 250°C.

[0006] For higher operating / use temperatures, there is currently no lead-free solder that can balance the temperature resistance required for power electronics with the necessary reliability and economy.

[0007] Therefore, for example in high-temperature applications, especially at operating temperatures exceeding 250°C, there is a need to develop novel (preferably lead-free) solder foils that can be used at low cost, meeting the temperature control requirements of power electronic devices. This is to prevent damage to the structural assemblies to be joined during the soldering process, and also to achieve high-temperature soldering connections from an economic perspective, particularly under the following conditions: typical soldering characteristics for solder, typical process temperature range for lead-free soldering (preferably 250 to 300°C), but at a temperature at least 20-30 K higher than the liquidus temperature of the solder used, and below 5 K as commonly used in the prior art. The brazed joint also ensures high thermal reliability of the joint area between adjacent components during multi-stage brazing, without subsequent heat treatment and without applying additional clamping force to the joint during brazing, and with minimal formation of porosity and / or shrinkage cavities in the joint area, without depending on the atmosphere to be set in the brazing furnace during brazing.

[0008] In order to establish such a connection between components, the solder must always be in a molten state.

[0009] After the solder cools and solidifies, a strong material-locked connection is established between the components. The disadvantage of prior art solder foil is that once the solder is in a molten state, it has no noteworthy internal resistance (surface tension of liquid solder) relative to the shape changes caused by external forces.

[0010] These externally acting forces can be gravity generated by the weight of the components to be connected, but they can also be thrust and pressure generated by the connection technology, such as capillary forces, or forces caused by the effects of surface tension, such as the forces caused by the interaction of atoms during brazing when the joint / connection area is joined.

[0011] These forces can cause the components to be joined to tilt relative to each other during the brazing process.

[0012] Therefore, after brazing, when viewed in cross-section, the solder thickness is much thinner at one edge of the structure than at the opposite edge. These different cross-sectional thicknesses of the solder material on the surface to be brazed negatively affect the service life of the brazed joint, while also increasing the thermal resistance in the "thick" areas, thus creating undesirable so-called "hot spots".

[0013] In extreme cases, if the surface-mount (SMD) component stands upright in the liquefied solder area during the "tombstone effect," it may result in a complete loss of functionality of the surface-mount component.

[0014] In the field of power electronic device construction and connection technology, lead-free solder joints generally have a significantly reduced lifespan compared to their lead-rich counterparts, characterized by a smaller number of temperature load change cycles until the corresponding brazed structural assembly fails.

[0015] Therefore, even though the toxicity of lead to humans and nature is well known, solders with high lead content are still used today in sensitive fields such as aerospace or military technology.

[0016] Other solutions, such as sintering with copper or silver powder in paste form or using very brittle, high-gold-content solder, are very expensive and therefore rarely considered as alternatives in these fields.

[0017] In addition to these thermomechanical advantages, lead-containing solders, especially those with high lead content, have the advantage that they only melt at higher temperatures than other conventional lead-free solders. Therefore, it is possible to reliably design so-called multi-stage brazing processes using high-lead-content solders, in which different layers of a module are constructed sequentially by brazing.

[0018] Therefore, for example, the first brazing part can be performed using a high-melting-point solder with a high lead content, while subsequent brazing parts can be designed using a low-melting-point tin-based solder.

[0019] Due to the temperature difference in the melting range of 80 K (Kelvin) and above, there is in principle no danger in such a design that a component that has been brazed will come loose in a subsequent brazing step for the next component, because the temperature interval relative to the solidus temperature of the first solder is large enough and thus can be well controlled.

[0020] If limited to lead-free solders of general existing technology, then this temperature interval between the remelting temperature of an already soldered joint and the melting temperature of the solder joint to be re-soldered is reduced to slightly more than 30 K for different lead-free tin-based solders.

[0021] However, in industrial practice, when brazing in vacuum reflow soldering equipment, overheating usually occurs about 20-30 K above the liquidus temperature of the solder in order to produce a high-quality bond.

[0022] Therefore, in the prior art, indium and bismuth-containing solders also exist as a low-melting-point alternative, which can be used for reliable lead-free designs for multi-stage brazing.

[0023] However, the very low melting temperature of the indium and bismuth-containing solder, coupled with its poor creep resistance, greatly limits its use in power electronic devices.

[0024] In addition, indium is a very expensive metal, so its use as a solder for large-area, large-volume systems is generally uneconomical.

[0025] Because the special regulations for the further use of lead-containing solder in economic zones that are not subject to special regulations on lead will soon be terminated (Auslaufen), there is a lack of alternative solder materials that can make up for the gap with lead-containing solder in terms of toughness and higher desoldering temperature.

[0026] Among existing build-and-join techniques, the following approaches are known, in which attempts are made to prevent tilting of the components to be brazed during the brazing process and to produce a constant solder layer thickness by inserting shaped bodies, meshes, or rings. A drawback of these methods is that they cannot eliminate air bubbles that can lead to brazed joints containing shrinkage cavities.

[0027] Therefore, DE 2228703 A1 or the corresponding US 3900153 A describes a solder material in which metallic, round or cylindrical particles are incorporated into the surrounding soft solder by melting or by rolling, with the aim of ensuring a constant spacing between the components to be brazed in an electrical structural element so as to prevent the components to be brazed from tilting during the brazing process.

[0028] In this solution, the metal particles added to the solder material are not allowed to melt during the brazing process, so as to ensure a minimum spacing between components after the brazing process.

[0029] Since the particles are geometrically arbitrarily and absolutely irreproducibly integrated into the solder material, this solution allows for setting at most a minimum layer thickness associated with the height of the particles after the components are joined together.

[0030] Not only when the particles are incorporated into the solder surrounding the particles by melting, but also when the particles are rolled into solder foil or solder strip, serious disadvantages arise for areas subsequently brazed with such materials. These disadvantages are characterized in particular by an increase in the number of cavities in the joint.

[0031] JP 2004174522 A pre-describes another lead-free composite solder and its manufacturing method.

[0032] The use of lead-free composite solder according to JP 2004174522 A requires the application of pressure during the soldering process, i.e., the application of weight to the joint. As a result, this bonding process is associated with high manufacturing costs, and the mandatory extrusion pressure (because semiconductor structural elements are becoming increasingly thinner) may damage or even break the semiconductor structural elements, thereby causing the loss of electrical properties of the structural elements to be soldered.

[0033] In the composite solders described in JP 2004174522 A, a metal mesh is always used, in which copper wires with a wire diameter of 25 μm to 100 μm are interwoven with each other in a mesh-like manner. These copper wires must either be costly cleaned in a vacuum or under inert argon at a temperature of about 400°C to 600°C for a period of 30 minutes before being bonded to the solder foil, or they must be coated with a layer of tin with a thickness of about 1 μm before being bonded to the solder foil.

[0034] Then, these cleaned or Sn-coated meshes are stacked one or more times in a sandwich manner between two solder foils made of tin, each having a foil thickness of 20 μm to 250 μm. Subsequently, either discontinuously by composite extrusion in a very uneconomical manner or more efficiently by continuous roll cladding, in the case of a "percentage height reduction" of all the sandwich-stacked components of the composite solder, that is, pressing the solder foil and mesh (the difference between the total initial height and the final height of all the sandwich-stacked components of the composite solder as a percentage of the total initial height of all the sandwich-stacked components of the composite solder) to a degree of 20% to 30% to form a composite solder.

[0035] According to the bonding method used in conjunction with JP 2004174522 A, due to the complex geometry of the metal mesh, especially in the area of ​​the bonding point, under given process conditions (with a height reduction of 20%-30%), it is not possible to guarantee complete material locking around the wire interlayer through the surrounding solder matrix (refer to JP2004174522 A). Figure 5 and Figure 6 Therefore, gaps remain in the hard-to-reach areas of the mesh's connection points. Furthermore, the solder foil makes only slight contact within the mesh openings at best under specified process conditions. However, under the process conditions proposed in JP 2004174522 A, it is impossible to establish a strong, material-locked bond between the Sn foils used.

[0036] Therefore, by means of the solution described in advance in JP 2004174522 A, the metal mesh arranged in the Sn solder is integrated into the Sn solder with only a small adhesion strength and also has a lot of voids / cavities.

[0037] For example, it is impossible to seal the gaps between the wire joints of the metal mesh with soft solder by galvanizing the metal mesh or by pressing or rolling the cladding.

[0038] In composite solders according to the solution of JP 2004174522 A, air bubbles embedded in the wire joints are preferably concentrated and fixed there, and can only be removed in a very small area during soft soldering, even under vacuum.

[0039] Therefore, when using this solder foil according to JP 2004174522 A, a high proportion of porosity remains in the bonding layer (even after costly brazing processes under vacuum).

[0040] These bubbles, due to their insulating effect, inevitably impair thermal conductivity, causing the thermal conductivity of the joints bonded according to JP2004174522 A to lag behind the overall thermal conductivity of the solder.

[0041] Other drawbacks are caused by the fact that the connection points in the metal mesh to be integrated, with their double wire thickness, are always directly adjacent to non-overlapping, single wire segments, and both are also adjacent to the mesh cavities.

[0042] When solder foil is pressed into these grid structures of metal mesh, the solder foil material used must directly overcome forming resistance to material flow that is therefore of completely different magnitude, side by side. This results in a localized, direct, and therefore unhelpful stress state (shear stress, tensile stress) between the mating components, which directly resists material locking.

[0043] Furthermore, for the 20% to 30% percentage reduction in height proposed in JP 2004174522 A, contaminants on the strip surface accumulate between the strips that are extruded into composite solder and subsequently result in low-quality brazing during brazing.

[0044] Furthermore, all the shortcomings already described in the teachings presented in JP 2004174522 A result in the following: the mating parts entering the working connection, namely the metal mesh and the Sn foil, tend to "separate" from each other under the mechanical stress of the composite structure.

[0045] Therefore, in the subsequent processing steps of the composite foil, minimal mechanical stress, such as the shearing force during foil punching, leads to delamination of the components, that is, on the one hand, it causes the mesh to delaminate from the foil, but on the other hand, it also causes the foil to separate from each other.

[0046] This inevitably leads to a very high and costly scrap rate.

[0047] More precisely, the disadvantage of too low adhesion strength can be mitigated at the cost of the same desired increase in remelting temperature by, for example, limiting the combination to only thick soft solder foil with thin wire braids.

[0048] Therefore, in accordance with the teachings of JP 2004174522 A, it is also proposed to reduce the disadvantages of left cavities or areas not flowed by solder at the cost of increasing the remelting temperature. The method is to specify that a thick solder foil is combined with a thin wire braid so that more wire area is flowed by solder.

[0049] However, the solder foil constructed in this way can no longer guarantee the effect of building the intermetallic phase with an increased remelting temperature as described in JP 2004174522 A, because the soft solder coating on the metal mesh is too high for the case where the intermetallic phase is connected to the mating parts during brazing.

[0050] Therefore, in this solution according to JP 2004174522 A, although it is possible to improve the surface area of ​​the wire braid in the solder matrix where the solder flows around it, this is inevitably accompanied by a significant reduction in the high remelting temperature desired for a reliable multi-stage brazing process, because the metal mesh is not connected to the mating parts through the intermetallic phase.

[0051] However, since the solution described in JP 2004174522 A can only ensure a higher remelting temperature and a longer mechanical life when an intermetallic phase is constructed between the metal mesh and the bonding mating parts, a relatively small layer thickness of soft solder between the copper wire and the bonding mating parts is mandatory, because the intermetallic phase is constructed only with the mesh material of the metal mesh, Cu, under these framework conditions.

[0052] However, due to the very large surface area of ​​the Cu braid used in JP 2004174522 A, the formation of the intermetallic phase within such a large volume in the brazing area causes embrittlement of the brazing area. This is because too little malleable solder remains after the brazing process, which is capable of compensating for the thermomechanical stresses that occur during the brazing process, caused by the different coefficients of thermal expansion of the structural components / materials involved in the joint. This leads to the subsequent fracture of the weakest mating component, in this case, the electronic component.

[0053] For solder joints with a high proportion of intermetallic phases, the problem is that only the substrate can bond to electrical components that do not differ significantly from each other in terms of their coefficients of thermal expansion. Otherwise, shear stress in the semiconductor could cause it to break during soldering, as there is no malleable soft solder available in the solder joint to absorb stress peaks / excessive stress. In solder joints that are advantageous for thermomechanical stress, preferably consisting entirely of soft solder that absorbs thermal stress through deformation during soldering, the problem is obtaining a high-temperature resistant solder joint that ensures high thermal reliability of the connection area between adjacent components even during multi-stage soldering processes.

[0054] In this regard, JP 2004174522 A states that the optimal thickness of the composite solder is 150 μm, and it can be used at a thickness of up to 250 μm, because at a thickness of 250 μm or greater, the resistance and thermal resistance of the connection become too large, such that, for example, the heat can no longer be adequately dissipated from the electronic components.

[0055] It was also found in JP 2004174522 A that stress relief could no longer be guaranteed from a thickness of 80 μm or less in the composite solder, which then led to cracking.

[0056] This is because, according to JP 2004174522 A, when using one or more wire braids, the solder layer is weakened over a large area by the wire and / or the intermetallic phase formed during brazing, so that only a thin area consisting of "residual solder" remains after the brazing process in a distributed manner.

[0057] Because it is very difficult to ensure the defined geometry of the wire braid using the material composite manufactured according to the teachings of JP 2004174522 A, it is impossible to ensure the precise construction of the brazing part, especially in the case of a geometrically complex forming space, since the minimum spacing can only be defined by the periodically repeating wire connection points.

[0058] Another material for diffusion brazing is described in advance in WO 2011 / 112722 A1. This material consists of a metallic carrier strip and a thin, lead-free, double-sided soft solder capping layer, which is converted into an intermetallic phase as completely as possible during the time-consuming brazing process. As explained in patent document WO 2011 / 112722 A1, it should always be considered that materials with at least similar coefficients of thermal expansion should always be joined together.

[0059] This presents significant limitations, beyond the time-consuming brazing process, when bonding traditional silicon-based semiconductor structural elements together. These elements can only be brazed onto a substrate with a ceramic interlayer that compensates for the imbalance caused by the different coefficients of thermal expansion of the various materials. These ceramic interlayers are known as AMB (Active Metal Brazed), DCB (Direct Copper Bonded), or DBC (Direct Bonded Copper), thus limiting the use of only these expensive ceramic substrates. Furthermore, as also described in WO 2011 / 112722 A1, the surfaces to be bonded must be individually prepared in terms of roughness, waviness, and flatness to ensure adequate wetting.

[0060] Furthermore, DE 10 2017 004 626 A1 or the corresponding WO 2018 / 210361 A1, deposited by the applicant, discloses a proven lead-free solder foil for diffusion brazing and a method for manufacturing the same, which enables the metal surface layers of metal components and / or metallized / metal-coated components, i.e., adjacent components to be brazed, to be joined together in a typical process temperature range for soft brazing and in a brazing time of less than 5 minutes without subsequent heat treatment and without the application of extrusion pressure during brazing, thereby producing a continuous layer of high-melting-point bonding zones in the form of an intermetallic phase having a remelting temperature above 400°C.

[0061] The lead-free solder foil disclosed in this teaching for diffusion soldering comprises a solder composite material manufactured by roll-coating such that only compact particles of the high-melting-point metal component are completely embedded in the lead-free solder within a lead-free solder matrix, such that these dispersed particles of the high-melting-point metal component in the solder matrix have a thickness of 3 μm to 20 μm along the foil thickness direction, wherein the spacing between the particles in the solder matrix is ​​1 μm to 10 μm, and each particle of the high-melting-point metal component is covered on each side by a 1 μm to 10 μm thick layer of lead-free solder, and the solder foil further has an outer sheath layer adjacent to the surface layer of the metal of the component to be bonded, the outer sheath layer having a layer thickness of 2 μm to 10 μm and the outer sheath layer being composed of solder.

[0062] While this lead-free solder foil can ensure a reliable and cost-effective lead-free design for multi-stage soldering, this solution cannot provide geometric control over the solder layer, especially when dealing with more complex geometries for the solder joint, in order to, for example, prevent tilting of the component to be soldered during the soldering process.

[0063] As is known from the teachings disclosed in DE 10 2013 110 812 B3, and as a variant of the teachings disclosed in DE 2228703 A1 or the corresponding US 3900153 A, it is possible to add higher melting point particles to the surface of a solder strip and then press the particles into the surface of the solder strip by rolling or linear extrusion in order to produce a brazing material containing higher melting point particles, which should then ensure a uniform solder layer thickness after brazing.

[0064] This rolling or pressing manufacturing method also leads to serious drawbacks in this solution, as in the solutions described earlier, because only a weak bonding force is generated between the particles and the solder material by rolling or pressing.

[0065] The persistent danger in loose composite structures established by rolling or pressing is that the higher melting point particles are lost either during transport or in other operations before the brazing process due to falling or dropping.

[0066] Furthermore, the surface of the particles is only wetted by the solder when the particles sink into the liquid solder, which leads to bubbles depending on the amount and type of material. These bubbles inevitably result in brazing connections with shrinkage cavities, which cause a non-negligible weakening of the connection due to wetting defects.

[0067] Other solutions, such as those described in DE 3442537 A1, use metal braids or structured foils made of metals with higher melting points (the melting temperature of these metal braids or structured foils is higher than the melting temperature of the solder involved) to ensure the solder area has the most uniform thickness possible.

[0068] Here, these metal braids or structured foils are placed either under or on top of the solder before brazing.

[0069] It is also known that such a mesh is placed between structures and filled with solder paste.

[0070] The main disadvantage of applying mesh and structured foil separately is that these meshes and foils must be wetted by solder during the brazing process, or that these meshes and foils are in contact with at least one of the components to be joined at the beginning and thus prevented from being wetted by solder.

[0071] The brazing features must be designed in such a way that the thermal mass of the mesh and foil is taken into account, which means a considerable expense for the fabrication of the new brazing features. In addition, it also requires a longer brazing time compared to conventional soft brazing features (common in the prior art).

[0072] Each time the brazing area is redesigned, the brazing features must be readjusted.

[0073] Here, the wetting time of the solder material must be taken into account, and the sinking time of the mesh or the formed foil in the molten solder must also be taken into account.

[0074] Furthermore, there is always a design issue regarding the thickness of the mesh and foil compared to the thickness of the solder layer.

[0075] Due to the complexity of the mesh and foil, a very low-viscosity solder must be selected.

[0076] Furthermore, the density difference between the mesh material and the solder material must be taken into account to ensure the mesh sinks / deposits into the liquid solder material. A common consequence of this is wetting defects, which in turn reduce thermal conductivity and significantly impair / reduce the mechanical stability of the solder layer, thereby significantly impairing / reduceing the reliability of the resulting brazed joint.

[0077] Because the mesh or structured foil is not integrated into the preform (solder form), but is loosely inserted during the construction of the stack, this requires a separate setup step, which also significantly increases the overall manufacturing time of the brazing process.

[0078] A solder material is known from WO 2018 / 209237 A1, wherein a soft solder strip is fed to a wire having a diameter of 5 μm to 200 μm by melting or rolling, with the aim of improving the thermal conductivity of the brazed joint manufactured by such a material composite structure.

[0079] Using a material manufacturing method not described in detail in WO 2018 / 209237 A1, but detected by keywords (which must start with the rolling of wire in a strip or foil), the surface area of ​​the wire is openly located on the surface of the composite structure.

[0080] Rolling cannot achieve a truly robust, material-locked integration of the wire in the solder strip.

[0081] Therefore, there is always a risk in subsequent processing steps, such as punching or bending, that the wire may "come off" from the solder before brazing.

[0082] Furthermore, this means that the wire must first sink / immerse in liquid solder during brazing, which results in voids and wetting defects.

[0083] Therefore, this solution also suffers from the common drawback of all the solutions described so far: small gaps, voids, etc., filled with atmosphere always remain in the solder material manufactured according to WO 2018 / 209237 A1, which lead to shrinkage cavities and porosity with the drawbacks already described during brazing.

[0084] However, the materials / solder forming parts specified in WO 2018 / 209237 A1 can also be produced by adding wire to molten solder.

[0085] Here, (for common wire / solder combinations (copper wire, tin-based solder)) a large portion of the wire has already been converted into a solution in the liquid solder before brazing.

[0086] Furthermore, this manufacturing method, which involves adding wire to molten solder, is very production-intensive because only discontinuous and lower-quality modification methods, such as ingot casting, can be considered, while continuous methods, such as continuous casting, cannot be considered.

[0087] In addition to improving thermal conductivity, the material described in WO 2018 / 209237 A1 also has the positive side effect of controlling the thickness of the solder layer during the brazing process.

[0088] In this regard, it can be determined that, according to current technology, the solder joints of power semiconductor modules between the base plate or cooler and the substrate are provided with a solder layer thickness ranging from 250 μm to approximately 400 μm at the cost of increased thermal resistance for the sake of long-term thermomechanical stability.

[0089] The materials described in WO 2018 / 209237 A1 are completely unsuitable for solder layer thicknesses of 250 μm to about 400 μm, which are common in the prior art, because even for a wire diameter of 200 μm, the components may still be tilted relative to each other by at least 50 μm (for a solder layer thickness of 250 μm) to a maximum of 200 μm (for a solder layer thickness of 400 μm).

[0090] In the latter case, that is, 50% of the thickness of the specified solder layer.

[0091] However, by incorporating wire, the purpose (task) that the present invention seeks to achieve through the addition of wire in this "tilting at the brazing point" renders the improvement of the thermal conductivity of the brazing point ineffective, since the tilted brazing point is the main cause of hot spots in terms of heat.

[0092] In principle, it can be determined that a solder joint that is favorable to thermomechanical stress should, on the one hand, include a metal-to-metal connection with a higher temperature stability within a certain range, along with a metal with a higher melting point (composed of Cu or similar metals) enclosed within it; on the other hand, it should also be malleable, creep-resistant, and insensitive to changes in thermal load. Summary of the Invention

[0093] Therefore, the objective of this invention is to develop a lead-free solder foil with a thickness of 50 μm to 600 μm, which can set a defined and reproducible connection geometry after the soldering process, regardless of whether the configuration is simple or more complex, wherein the soldering process can also be performed within the typical process temperature range for lead-free soldering, preferably at a temperature of 250 to 300°C, but at least 20-30 K above the liquidus temperature of the solder used, and below the temperature typically used in prior art for lead-free soldering. The soldering time is minimized, and no subsequent heat treatment is required, nor is additional clamping force applied to the mating parts during soldering, nor is the atmosphere in the soldering furnace to be set during soldering. High-temperature solder joints should be achieved with minimal porosity and / or shrinkage in the joint area. The lead-free solder foil should also ensure high thermal reliability of the joint area between adjacent components during multi-stage soldering processes where there is a risk of remelting of previously soldered areas. Furthermore, the lead-free solder foil ensures a geometrically precise joint area with high dimensional accuracy. This joint area also significantly improves the overall thermal conductivity of the joint area and simultaneously prevents / prevents crack propagation in the joint area during material fatigue. It also absorbs thermal stress introduced by soldering but also generated during the use of the component due to its ductility. Thus, in terms of overall performance, the service life of the joint area is increased many times over compared to joint areas manufactured with soldering materials according to existing construction and joining techniques, thanks to the solutions described herein.

[0094] According to the present invention, this task is achieved by a lead-free solder foil 1 with a thickness of 50 μm to 600 μm, utilizing a metal component and / or a metallized / metal-coated component, i.e., the surface layer of the metal of adjacent components, in a novel manner itself achieving a high-temperature resistant solder connection during the soldering process under the following conditions: within / of the typical process temperature range for lead-free soldering, i.e., at a temperature preferably 250 to 300°C, but at a temperature at least 20-30 K above the liquidus temperature of the solder used, and at a temperature typically below 5 K for lead-free soldering in the prior art. The high-temperature solder joint ensures high thermal reliability of the joint area between adjacent components even in multi-stage brazing processes where there is a risk of remelting of previously brazed areas, while minimizing the formation of porosity and / or shrinkage cavities in the joint area, without depending on the common brazing time of min and the atmosphere to be set during brazing in the brazing furnace. This solves the problems of the prior art mentioned above due to the characteristics of the novel solder joint that needs to be ensured by using solder foil, and because the possible layout according to the invention can realize complex geometric structures, such structures may also be found in stacks with integrated mechanical functions in the future.

[0095] Importantly, the solder foil 1 according to the invention does not require subsequent heat treatment or additional clamping force to be applied to the mating parts during brazing, when necessary.

[0096] Importantly for this invention, the solder foil 1 is compactly constructed such that two or more composite wires 3 are individually, parallel to each other and parallel to the strip edges, integrated in a soft solder substrate 2 made of either lead-free tin-based solder, pure tin, pure indium, or an indium-based alloy, such as InSn48, in such a way that the composite wires 3 are longitudinally oriented in the rolling direction by means of a rolling cladding method, with a "percentage reduction in the height of the initial strip" (the difference between the total initial height of the strip 6 (obtained without considering the height of the composite wires 3) and the final height H of the composite foil 1 (with the embedded composite wires 3) is obtained as a percentage of the total initial height of the strip 6, and is in the range of greater than 30% to a maximum of 95%, preferably 50% to 85%) and thus arranged in a material-locked manner in the soft solder substrate.

[0097] Importantly, the minimum permissible spacing between the cores 2 of the composite wire 3 after the rolling process is approximately 500 μm.

[0098] The result is that a sufficient amount of malleable solder remains between the wires coated with the intermetallic phase, so as to compensate for the thermomechanical stress caused by the different coefficients of thermal expansion of the mating parts during brazing and under use through elastic-plastic deformation.

[0099] At the same time, at this spacing, there is no risk that the reaction products formed during brazing in a reactive atmosphere due to capillary effect will be suppressed in the volume of the solder.

[0100] Furthermore, it is characterized that, after the rolling process, the minimum permissible spacing of the core 2 of the composite wire 3 relative to the outer edge of the solder foil is approximately 500 μm.

[0101] As a result, the tensile stress that occurs in the edge region during the roll cladding process does not negatively affect the material bonding integration of the composite wire 3, and adhesion or bonding defects that may occur in the case of brazing may lead to increased porosity or shrinkage formation.

[0102] Here, the maximum spacing between the cores 2 of the composite wire 3 and between them and the outer edge of the solder foil after the rolling process depends on the width of the soft solder strip used.

[0103] The result is that the quantity of composite wire 3 and its precise positioning within the width range of the band 6 can be achieved very flexibly and individually, within the aforementioned limitations and boundaries, depending on the application, and thus a special solution can be provided for all common tasks of building and connecting technologies.

[0104] In this solder foil according to the invention, composite wires 3, individually and parallel to the edge and oriented longitudinally to the rolling direction, are integrated into the soft solder matrix 2 in a fully material-locked manner, that is, with maximum adhesion strength, ensuring that the forces that occur in subsequent processing steps, such as when punching the foil, will absolutely not cause delamination of the components.

[0105] Meanwhile, the complete material-locking integration of the composite wire 3 into the solder substrate 2 according to the present invention results in the following: after the roll cladding according to the present invention, which has a “percentage reduction in the height of the starting band” in the range of more than 30% to 95%, no voids remain inside the composite structure, so that it is no longer necessary to remove gas from the solder foil 1 according to the present invention during the subsequent brazing process. Thus, the solder in the solder substrate 2 of the solder foil 1 according to the present invention always has the same technical characteristics as the integral starting-solder material.

[0106] Importantly, the composite wires 3, each individually arranged in the solder foil 1, have a core 4 made of a metal or metal alloy that has a higher melting point and is stronger than the soft solder matrix 2, such as copper or copper-based alloy, silver or silver-based alloy, nickel or nickel-based alloy, gold or gold-based alloy, and a sheath 5 made of another metal or another metal alloy, such as pure tin or tin-based alloy or indium or indium-based alloy, is arranged around the core.

[0107] Characteristically, the sheath 5 of the composite wire 3 has a layer thickness of 2% to 20% relative to the total diameter of the composite wire 3.

[0108] Another important feature of the present invention is that, after the rolling cladding process, a layer made of solder material is arranged above and below at least one core of the core 4 of the composite wire 3 encased in the solder matrix 2. This layer consists of the region of the solder matrix 2 and the layer made of the sheath 5 of the composite wire 3. The sheath layer is at least 5 μm thick at its thinnest point, but at most 15 μm, preferably from 5 μm to about 10 μm.

[0109] The solder foil 1 according to the present invention is produced by conveying an upper strip 6 made of soft solder, a lower strip 6 made of soft solder, and a plurality of composite wires 3 located between the strips 6 in a defined manner to a roll gap for performing a roll cladding.

[0110] The force that causes the shape change of the cross section results in the following: the various component materials of the solder foil 1 according to the invention are interlocked with each other to form a material composite structure.

[0111] The composite material structure produced from this rolling cladding process, in the form of solder foil 1 according to the invention, has a rectangular cross-section appearance transverse to the strip length of solder foil 1, and the composite wire (3) is embedded in the soft solder matrix 2 of the rectangular cross-section.

[0112] By utilizing the material and wire geometry design of the composite wire 3, which can have a circular or elliptical cross-section in its initial state, combined with the specific selection of the geometry of the strip in its initial state, and in conjunction with the adjustment of the geometry of the roll gap, a large number of complex geometric arrangements (hereinafter referred to as layouts) can be generated for the interior of the solder foil. However, by rolling cladding according to the invention, a flat product is generally always produced first, which approximately corresponds to a polygonal geometry.

[0113] The spacing between the composite wires 3 and the roll gap geometry can always be adjusted in a defined and reproducible manner.

[0114] Because the initial product uses "quasi-continuous semi-finished products" and composite wires in the form of solder foil or solder strip, the manufacturing method of solder foil 1 according to the present invention is very efficient and economical.

[0115] The solder foil 1 according to the invention has a composite structure in which all materials are fully material-locked and no gaps, voids or similar structures filled with atmosphere are left.

[0116] Thus, during the transport of the solder foil according to the invention or during other operations using the solder foil according to the invention, the risk of the composite wire falling off, slipping, or otherwise misaligning is completely eliminated.

[0117] According to the present invention, only circular or elliptical composite wire cross-sections are used as semi-finished products, thereby ensuring that the geometry of these semi-finished products is always well “flowed” around the solder during roll cladding.

[0118] The composite wire 3 is completely surrounded during the roll cladding process, thereby making the shrinkage and porosity characteristics when brazing with the solder foil 1 according to the invention as small as when brazing with an integral solder foil or a solder mold made therefrom.

[0119] At the same time, the stress state that occurs between the composite wire and the soft solder of strip 6 during roll forming will not result in exceeding the forming capacity of strip 6.

[0120] The forming ability is the ability of a material to plastically change its shape to a sufficiently large extent under the influence of an external force (which generates stress in the material) such that the "material bond" is not lost or cannot be formed at all.

[0121] Because of this method of integrating the composite wire into the solder according to the present invention through the rolling cladding process according to the present invention, the composite wire is completely surrounded by the solder in any case.

[0122] According to the invention, it is thus ensured that if the solder becomes molten, all component surfaces are immediately and completely wetted, and thus wetting defects can be completely eliminated.

[0123] However, with the solution according to the present invention, the integrated composite wire 3 can also be intentionally set to be non-uniform in terms of its cross-sectional geometry so as to be able to mimic the very complex geometry of the brazing gap.

[0124] Currently, for example, in the case of power semiconductor modules with a structure that incorporates recessed or protruding brazing seams between the substrate and the base plate or coolant, this very complex geometry of the brazing seam is required.

[0125] However, much more complex geometries can be achieved in construction and connection techniques using solder foil according to the present invention.

[0126] This will become necessary in the future if the module, in addition to its electrical functions, also has to perform mechanical functions and no longer has to be manufactured in the common “flat” configuration, but rather in a complex 3D structure.

[0127] Due to the freedom to choose the geometry of the cross-sectional shape of the solder matrix, as well as the freedom to choose the shape and quantity of the composite wires 3 integrated in the solder matrix, there are almost no restrictions on the layout.

[0128] Then, solder forming parts 8, so-called preforms, for corresponding usage geometries can be produced cost-effectively from this solder foil 1 according to the invention through a blanking or combined blanking-forming process.

[0129] By means of the solder foil 1 according to the invention, in a conventional lead-free soldering process using soldering characteristics common to soft soldering in the prior art, within the typical process temperature range for soft soldering, i.e., preferably 250°C to 300°C, but at least 20-30K above the liquidus temperature of the solder used and for a soldering time of less than 5 minutes, and also at a soldering temperature up to 280°C and for a soldering time of 5 minutes, and then during the soldering process, due to the metallization effect, the solder column is transformed, above and below, in the sheath area of ​​the composite wire into a higher melting point intermetallic phase surrounding the core 4 of the corresponding composite wire 3, such that until the components to be joined thus form a complete bridge composed of the higher melting point intermetallic phase, so that after soldering, only a fixed bridge with a higher melting point is arranged around the core 4 of the composite wire 3 in a precisely defined cross-sectional area in the connection area 13. According to the present invention, after the brazing process, a soft solder material of the original soft solder matrix is ​​also arranged in the connection area 13 between these fixed bridges and next to these fixed bridges.

[0130] According to the invention, the fixed bridges with higher melting points formed during the brazing process, that is, the material-locked connections between the components to be joined, have significantly higher strength in the areas of the solder matrix adjacent to these bridges compared to the solder material of the solder matrix, but also have a much higher remelting temperature.

[0131] Therefore, the bridge composed of intermetallic phases according to the present invention solves two fundamental problems in multi-stage brazing using conventional soft solders in the aforementioned construction and connection techniques.

[0132] First, the components to be connected are prevented from slipping during the multi-stage brazing process by the “higher temperature stable” bridge constructed according to the invention between them, because the bridge formed according to the invention will not remelt during the soft soldering process at the stated temperature.

[0133] However, according to the present invention, these bridges also result in the following: due to their higher mechanical stability, crack propagation at these bridges is prevented / inhibited in the event of material fatigue of the solder.

[0134] Because a malleable solder matrix is ​​always arranged between these very robust bridges that serve as crack block-elements, the solder matrix, according to the invention, can also simultaneously reduce excessive stress in the joint area through plastic deformation.

[0135] Meanwhile, these very robust, rail-shaped bridges enable the existence of "venting open" areas during brazing, either beside or along the "rails" or between adjacent "rails" and the surfaces to be joined of adjacent components. These "venting open" areas can, on the one hand, allow for the venting of reaction products generated during brazing (i.e., gases (water vapor or similar) formed by the reducing medium used during brazing and the surface oxides of the joining components) from these surface areas of the joining components (i.e., between the surfaces to be joined of the components, such as the ceramic substrate 9, the base plate 10, and the solder foil 1). On the other hand, these "venting open" areas also enable the following idea: that voids, which are usually formed due to the geometry of the surfaces to be joined that deviate from the ideal surface (due to roughness, waviness, grooves, scratches, etc.), existing between the surfaces of the joining components, can then be eliminated during brazing.

[0136] Overall, a completely new type of brazing connection is generated by means of the solution according to the present invention, which, in addition to the advantage of precise geometric control of the brazing site, significantly improves the process control of the brazing process in multi-stage brazing and also prevents material fatigue. The service life of the connection area 13 produced by the solder foil 1 according to the present invention is significantly better (higher) than that of the connection area 13 produced by conventional brazing materials using construction and connection techniques. Attached Figure Description

[0137] Advantageous embodiments, details and other features of the invention will become apparent from other parts of this disclosure and from the following description of embodiments according to the invention.

[0138] The invention will now be explained in detail with reference to six illustrations of the solutions according to the invention. Wherein:

[0139] Figure 1 An apparatus according to the invention is shown for manufacturing solder foil 1 according to the invention;

[0140] exist Figure 2 The cross-sectional view shows a solder forming component made of solder foil 1 according to the invention, which is arranged between substrate 9 and base plate 10 before the brazing process;

[0141] Figure 3 It shows in Figure 2 The device shown in the image is in the state immediately following the soldering process;

[0142] Figure 4 It shows in Figure 3 The device shown is in operation after a long period of use;

[0143] exist Figure 5 The figure shows a solder forming part 8 made of solder foil 1 according to the invention, which has two composite wires 3 integrated in a soft solder substrate 2.

[0144] Figure 6 A perspective view shows a solder forming part 8 made of solder foil 1 according to the invention, the solder forming part having three composite wires 3 integrated in a soft solder substrate 2. Detailed Implementation

[0145] Figure 1 An apparatus according to the invention is shown for manufacturing lead-free solder foil 1 according to the invention.

[0146] As in Figure 1 As shown, the initial materials are initially two foils or strips 6 made of soft solder, which exist with a degreased and contaminated surface quality.

[0147] These belts 6 can be brushed before rolling the cladding for improved further processing, in order to remove the outer, resilient passivation layer and, in addition, to have a surface roughness that is beneficial for the subsequent bonding to be implemented in the roll gap.

[0148] These foils or strips 6 are arranged on the inlet side of the rolls 7 of the mill stand so that the lower strip 6, which is made of solder material, passes through the roll gap first.

[0149] Subsequently, the composite wire 3, which is suitably wound on a spool, is applied in the middle to the corresponding spool.

[0150] The composite wire 3 can exist in a circular or elliptical cross-sectional shape.

[0151] Based on the number of composite wires 3 to be integrated into the solder foil 1, these composite wires are arranged between the strips on the inlet side of the rolling mill stand.

[0152] In a simple layout, the number of composite wires to be integrated is two.

[0153] However, more composite wires 3, such as five or more composite wires 3, can also be used in the solder foil 1 according to the present invention. Here, the geometry of the composite wires 3 can be selected differently, such as their thickness.

[0154] A simple example specifies the use of two composite wires with an initial circular shape and the same diameter.

[0155] In a more complex instance, it is also possible to use three or more composite wires with different shapes and, in particular, different diameters.

[0156] As the initial material for the band 6 used to generate the solder matrix 2 after the roll cladding process, all commonly used lead-free tin-based alloys can be considered, especially: Sn; SnAg3.5; SnCu3; SnCu0.7; SnSb5; SnSb8; SnAg0.3Cu0.7; SnAg1Cu0.7; SnAg3.8Cu0.7; SnAg3.0Cu0.5; SnAg0.4Cu0.5.

[0157] However, it is also possible to use indium-based metals and alloys, such as pure indium or InSn48.

[0158] For the core 4, metals and alloys consisting of pure copper and copper-based alloys, pure silver and silver-based alloys, and nickel and nickel-based alloys are particularly suitable, especially in combination with tin-based alloys used for the solder foil to be used. However, gold and gold-based alloys can also be used in combination with indium-based solder, although this is an example of a costly combination of materials.

[0159] Characterized not only for the connection process during roll cladding but also for the diffusion brazing process, the sheath 5 of the composite wire 3 is made of a different metal than the core 4.

[0160] The edge layer (mainly tin oxide) of the sheath 5, which is very brittle compared to the metal of the core 4, supports the connection structure with the strip 6 during the roll-coating process because these brittle layers of the sheath 5 crack at small deformations and thus provide space for connection-friendly materials inside the sheath layer.

[0161] It is not important here whether the sheath 5 of the composite wire 3 is produced by electroplating or by impregnation of the core 4 in molten metal.

[0162] The thickness of the sheath layer can be approximately 2% to 20% of the total diameter of the composite wire 3.

[0163] As a "diffusion brazing region" surrounding the core 4 of the composite wire 3 used accordingly, a phase with a higher melting point may appear as an intermetallic phase or combination thereof constructed according to the prior art, depending on the choice of materials used.

[0164] In order to achieve a remelting temperature of ≥400°C for the intermetallic phase, we always strive for material combinations, such as the combination of tin-based solder as the solder matrix 2 with the core 4 of a composite wire 3 made of copper, silver or nickel wire, or the combination of indium-based solder as the solder matrix 2 with the core 4 of a composite wire 3 made of nickel or gold wire.

[0165] The corresponding brazing characteristics, namely the temperature-time-state during the brazing process, always correspond to the common soft soldering characteristics for the corresponding material combinations in the prior art, which have a typical process temperature range for lead-free soft soldering, that is, for example, at a temperature up to about 280°C and a brazing time of less than 5 minutes.

[0166] According to the present invention, the solder foil 1 is in Figure 1 The embodiment shown depicts a very thin composite wire 3 in the middle, with two thicker, here equally thick, composite wires 3 arranged adjacent to each other on either side. This embodiment is used to mimic the recessed brazing seam and to ensure the highest possible geometric accuracy after brazing.

[0167] However, depending on the requirements for the geometry of the subsequent brazing gap, any number of composite wires with different diameters can also be used.

[0168] according to Figure 1 In the next process step, the composite wire 3 is passed through the guide tool and then through the roll gap and aligned with the lower strip 6 that has already been passed through.

[0169] During the subsequent roll-coating process, the guide tool ensures that the composite wires 3 are coated only at the desired locations and thus also defines the spacing between the composite wires 3 and the spacing between the composite wires 3 relative to the longitudinal edge of the solder foil 1.

[0170] After aligning the composite wire 3 with the lower strip 6 in this manner, the composite wire is mechanically fixed to the exit side of the roll 7.

[0171] Next, the upper strip 6, which enters the gap between the rolls 7 on the composite wire, passes through the rolls and aligns with the lower strip 6 side by side.

[0172] Advantageously, a foil or strip of the same width made of soft solder can be used.

[0173] For continuous processes, it is equally suitable to use materials in the form of winding, so-called rolls.

[0174] After the strip 6 and the composite wire 3 pass through the roll gap in the desired arrangement, the roll gap is closed.

[0175] The thickness of the resulting composite material strip is adjusted by setting the minimum spacing between the rolls.

[0176] If the rolling process begins now, due to the plasticization of the metal in the roll gap, the softer material of the solder flows around the rounded area of ​​the harder composite wire 3, shapes it, and connects with each other, so that the material-locked composite structure, i.e., the solder foil 1 according to the invention, is joined. The "percentage reduction in height of the starting strip" necessary for this during the rolling cladding (obtained as a percentage of the quotient of the difference between the total starting height from the strip 6 (taken without considering the height of the composite wire 3) and (i.e., subtracted) the final height H of the composite foil 1 (with the embedded composite wire 3) about the total starting height of the strip 6) is in the range of greater than 30% to 95% of the total height of the starting strip, depending on the choice of materials used for the solder foil / solder strip and the correspondingly used individual composite wires oriented longitudinally to the rolling direction and parallel to each other and parallel to the strip edge.

[0177] If, for example, round wire is used as the initial material for composite wire 3, the round wire will approach an elliptical geometry when it is deformed in the roll gap.

[0178] As already mentioned, the already elliptical composite wire 3 can also be used as the initial material shape.

[0179] When producing the cladding, it is important to have the composite wire surface rounded.

[0180] Within the scope of the solution according to the present invention, composite wire cross-sectional geometry with obvious edge shapes is not used.

[0181] Only when the sheath surface of the composite wire 3 is rounded can the softer material of the solder matrix 2 flow around the composite wire 3 in a material-locking manner during the forming process, without encountering excessive resistance in the material flow.

[0182] Here, the solder material of the solder matrix 2 undergoes a very large degree of local shaping at the location where the composite wire 3 is surrounded.

[0183] The circular shape of the surface of the composite wire 3 improves the enclosure of the composite wire without losing the material bond of the solder (fracture / crack), and the circular shape of the surface achieves the following result in that: it is also possible to integrate very thick composite wires 3, such as composite wires 3 with a diameter 2.5 times the thickness of the (initial) band 6 made of solder material, without taking into account internal or external damage to the composite structure.

[0184] Only in this way can the necessary, extremely thin solder layer of 5 μm to a maximum of 15 μm be achieved, which is located above and below the region of at least one of the integrated wire cores 4 after the cladding process and is necessary for diffusion brazing.

[0185] Within the scope of the process flow according to the present invention, there is no problem in integrating composite wires 3 of very different thicknesses, or composite wires 3 that are only one-third of the original diameter of the thickest composite wire 3, or even smaller in exceptional cases, into the solder matrix.

[0186] Different alloys can also be used for the core 4 of the integrated composite wire 3, such as copper alloys of different hardness or pure copper with different degrees of cold curing.

[0187] The reason for using alloys with different hardness is that different composite wires 3 deform to different degrees compared with solder during the roll cladding process.

[0188] The composite wire 3 with a softer wire core alloy becomes flatter and always more elliptical during roll cladding, and occupies less height in the overall composite structure compared to harder metals and alloys.

[0189] Therefore, by selecting alloys with different hardness, different geometric layouts of the solder foil 1 according to the present invention can also be produced.

[0190] Depending on the metals used for the strip 6, the solder matrix 2, and the composite wire 3, it may be necessary to heat the initial material before or during entry into the roll gap in order to obtain improved formability.

[0191] In the subsequent rolling step, the solder foil 1 can also be rolled into an even thinner solder foil 1.

[0192] The solution according to the invention also specifies that a solder forming part is manufactured from the solder foil 1 thus produced according to the invention, so as to perform very precise pre-division of the solder foil 1 for the desired brazing area. This can be done, for example, by methods such as punching, laser cutting, micro-etching or conventional shearing.

[0193] In addition, it is also possible to use combination methods to manufacture solder-formed parts with complex geometries, which include segmentation and simultaneous forming (deep drawing, stamping).

[0194] Because it is known that brazing areas with sharp inclinations are more prone to mechanical failure and the formation of undesirable hot spots than layers of uniform thickness, many applications require only two composite wires 3 of equal height, which are present as thickly as possible in the solder foil 1 and are positioned at precise intervals between each other, and, for example, in solder moldings, are present as close as possible to the edge in an integrated manner.

[0195] Then, during the brazing process, the tilting of the component was prevented and the brazing layer was made as uniform as possible.

[0196] Figure 2 A cross-sectional view prior to the brazing process is shown, in which a solder forming element 8, consisting of a lead-free solder foil 1 according to the invention, is arranged between a ceramic substrate 9 and a base plate 10, the ceramic substrate being used in the embodiments described herein and also known as DCB, DBC, or AMB.

[0197] According to the present invention, the solder forming part 8, which is made of solder foil 1 according to the present invention and punched from solder foil 1, is constructed such that two composite wires 3 are encased in the soft solder matrix 2, which in this embodiment is made of lead-free tin-based solder, i.e., compactly integrated in the soft solder matrix. The composite wires according to the present invention have a core 4, which is made of a metal or metal alloy that has a higher melting point and is harder than the soft solder matrix 2, here made of copper. A sheath 5 made of another metal or another metal alloy, here made of tin-based alloy, is arranged around the core.

[0198] According to the invention, after the “stack” is formed, the arrangement is heated in a soldering process with typical soldering characteristics for soldering, within a typical process temperature range for soldering (i.e., at a temperature up to about 280°C and a soldering time of less than 5 minutes), and the solder matrix is ​​transformed into a molten state.

[0199] This can be carried out, for example, in a vacuum furnace under the action of reducing gases, as is common in existing technologies.

[0200] Here, the liquid solder diffuses on the surface it contacts.

[0201] According to the present invention, as in Figure 3 As shown, due to the diffusion process between the metal atoms from the composite wire region and the hydraulic solder surrounding the composite wire, a diffusion zone is formed between the solder and the material to be wetted, extending all the way to the surface of the adjacent component (that is, extending all the way to the substrate 9 and the base plate 10). This is a region with new chemical composition, namely the so-called intermetallic phase 11.

[0202] After soldering, in cross-section, the joint area brazed with monolithic solder has a joint area composed of the initial components, which has the same chemical composition as before soldering.

[0203] If the solder foil 1 according to the present invention is used for brazing, a region with chemically changed composition is formed around the core 4 due to the diffusion process between the composite wire 3 and the liquid solder melt during the soft soldering process with soft soldering characteristics according to the present invention.

[0204] In this embodiment, solder SnCu3 is used as the solder matrix.

[0205] The composite wire 3 is made of non-alloy pure copper in the core 4 and has a sheath 5 made of tin-based alloy.

[0206] During soldering, according to the present invention, an intermetallic phase composed of Cu3Sn and Cu6Sn5 is formed around these cores 4.

[0207] Because the gap between the elliptical core integrated on the edge side of the composite wire and the surface of the area to be joined is only 5 μm to a maximum of 15 μm at the thinnest part in this embodiment (12.5 μm in this embodiment), these areas grow very rapidly with the intermetallic phase.

[0208] Here, a structure is formed around the core 4 and between the core 4 and the substrate 9, and also between the core 4 and the base plate 10. Figure 3 The bridge shown is composed of intermetallic phase 11.

[0209] These intermetallic phases 11 are characterized by higher strength and hardness, as well as a higher melting point relative to the solder matrix 2 used (the melting point of the intermetallic phase 11 depends on the material used accordingly and is mostly ≥ 415°C).

[0210] After brazing with the solder foil 1 according to the invention, compared with brazing with integral soft solder material, the connection area between the substrate 9 and the base plate 10 is not only composed of malleable soft solder, but also on the one hand composed of bridges (the bridges are composed of very strong, temperature-stable intermetallic phase 11) and on the other hand composed of malleable soft solder of the soft solder matrix 2 adjacent to these bridges.

[0211] These properties, which are unified in a single material, are referred to as "toughness" by material customers.

[0212] After the structural assembly is completed, it is then used as usual, whereby the power loss generated in the semiconductor structural element is discharged as heat to the substrate 10 via the substrate 9 and the corresponding solder joint area of ​​the prior art.

[0213] The different materials of each component expand to different degrees according to their respective coefficients of thermal expansion.

[0214] If the operation stops for a shorter or longer period of time as is usually the case, then the power loss in the form of heat is no longer emitted by the semiconductor structural elements, and the material subsequently cools and contracts.

[0215] Given this continuous change in thermal load, stress is generated in the material due to varying degrees of expansion. These stresses are particularly concentrated in the edge regions of the connection area.

[0216] These edge regions of the connection area are subjected to high loads during the service life due to a large number of heat load changes.

[0217] The degradation of solder is also discussed in this regard.

[0218] In traditional monolithic solder joints, only the edge areas of the brazed area / joint zone suffer cracking in the initial stage.

[0219] In the initial stage, the functional capabilities of the entire component are not yet limited.

[0220] In conventional solder joints, these cracks in the solder of the joint area extend beneath the region where the power semiconductor element is located during further operation.

[0221] The heat was then only able to produce very poor results.

[0222] As a result, the semiconductor structural element becomes too hot and then the entire structural assembly fails.

[0223] Now, by means of a novel brazing connection formed during the soldering process using the solution according to the present invention, a significantly longer service life can be guaranteed for the entire structural assembly, such as, for example, the entire power semiconductor module, compared to the prior art.

[0224] Figure 4 It shows in Figure 3 The figure shows the solder connection (along with the power semiconductor module) according to the invention after a long period of continuous operation.

[0225] What becomes clear from this illustration is how the solution according to the invention can prevent the further propagation of crack 12.

[0226] The stretchable solder matrix 2 incorporates a stable bridge composed of intermetallic phase 11 at the center of the connection area 13 to ensure optimal relief of thermal stress.

[0227] At the edge of the connection area 13, cracks 12 may still form due to fatigue of the solder material after multiple thermal load cycles.

[0228] In this regard, such as in Figure 4 As shown, the bridge now described, which is composed of a very strong intermetallic phase 9 formed around the copper wire 3 according to the invention, prevents these cracks 12 from spreading further.

[0229] Therefore, it is necessary and necessarily advantageous to integrate the composite wire 3 as close as possible or as close as necessary to the edge / boundary of the solder form 8, so as to prevent cracks (progress) from occurring below the placement area of ​​the semiconductor structural element.

[0230] Because the composite wire 3, which is clad in solder foil 1 and has an integrated core 4 made of copper, silver or gold, further significantly improves the thermal conductivity of the connection area 13 relative to the surrounding soft solder, thereby significantly increasing the lifespan of the power semiconductor module again.

[0231] Because of the significantly improved heat dissipation achieved according to the present invention, the operating temperature of the entire structural assembly is further reduced, and the occurrence of thermomechanical stress is also significantly reduced again.

[0232] exist Figure 5 and Figure 6The perspective view shows two different solder forming parts 8 made of different solder foils 1 according to the present invention.

[0233] Figure 5 A perspective view shows a solder forming part 8 made of solder foil 1 according to the invention, which has two composite wires 3 integrated / clad in a soft solder matrix 2 to ensure a uniform solder layer thickness after the brazing process.

[0234] To, for example, solder semiconductor structural elements with back-side metallization onto a Cu-ceramic substrate, using Figure 5 The solder forming part 8 shown is made of solder foil according to the invention and has dimensions of, for example, B = 15 mm × L = 15 mm.

[0235] In this embodiment, it should be ensured that there is a connection area 13 with a height H = 100 μm.

[0236] Since the semiconductor material has no noteworthy bending and the substrate, through the stretching of this region, also does not provide any noteworthy deviation from the flat surface profile, it is sufficient to stabilize the brazing area having two composite wires 3 of equal height placed on the edge side.

[0237] The solder forming part 8 required for this purpose is made from the solder foil 1 manufactured according to the present invention.

[0238] Two soft solder strips 6, each 0.340 mm thick and 70.0 mm wide, made of SnCu3, are combined with six separately laid composite wires 3 as initial materials. The composite wires have a copper core 4 and a tin sheath 5.

[0239] The strip 6 and the composite wire 3 are fed into the roll gap by the corresponding guiding tools as explained, and are aligned with each other such that the solder strip 6 enters the roll gap overlapping each other and the composite wire 3 is precisely spaced between them.

[0240] Here, each of the extended composite wires 3 is oriented relative to each other at a spacing of 10.0 mm.

[0241] The spacing relative to the edge is also set at 10.0 mm.

[0242] The composite wire 3 is made of tensile copper with a diameter of 500 μm in the core 4 and has a sheath 5 made of tin with a thickness of 25 μm.

[0243] The composite structure was clad to a thickness of 220 μm and then thinned to a final thickness of 100 μm using two subsequent rolling passes.

[0244] Now, it is possible to continuously punch out three solder forming parts with dimensions of 15.0 mm × 15.0 mm per punch stroke from the solder foil 1 according to the invention within a width range.

[0245] The blanking tool and the support for the punch are made such that the distance between the punch and the edge is 7.5 mm and the distance between the punches is 5 mm.

[0246] Thus, a preform / solder form 8 is punched out from the solder foil 1, wherein the two integrated composite wires have a spacing of 2.5 mm relative to their outer edges.

[0247] The geometry of one of these solder forming parts is in Figure 5 It is shown in three dimensions.

[0248] Here, the height H of the solder forming part 8 is 0.100 mm; the width B of the solder forming part 8 is 15.0 mm; and the length L of the solder forming part 8 is 15.0 mm.

[0249] The composite wire 3, which is deformed into an elliptical geometry, has the following extensions: a larger diameter of approximately 430 μm and a smaller diameter with an extension of approximately 84 μm. This smaller diameter is then decisively responsible for controlling the uniform solder layer thickness.

[0250] The solder column / solder thickness above the composite wire 3 covered in the solder foil 1 (height H = 100 μm) according to the present invention is approximately 8 μm at its thinnest point.

[0251] According to the present invention, these approximately 8 μm thick solder layers, when brazed using typical brazing characteristics for soft soldering, i.e., within the typical process temperature range for soft soldering, completely transform into higher melting point intermetallic phases, specifically Cu6Sn5 and Cu3Sn, at temperatures up to approximately 280°C and in a brazing time of less than 5 minutes.

[0252] According to the present invention, a complete bridge composed of a high-melting-point intermetallic phase is established along two composite wires 3 integrated during the brazing process, not only toward the substrate surface but also toward the back metallization structure of the semiconductor. The bridge mainly connects the core of the wire to prevent the brazed semiconductor structural element from tilting relative to the substrate and at the same time ensures high-precision fixation of the semiconductor structural element relative to the substrate. This is not only beneficial for transportation but also for the subsequent brazing process in the next furnace chamber.

[0253] In this subsequent brazing process, the substrate 9, which is connected to the semiconductor structural element, is then brazed together with the coolant.

[0254] The solution according to the present invention achieves the following result: the semiconductor material that has been previously brazed to the substrate 9 will neither "slip" when transported to the next furnace chamber nor during the next brazing process.

[0255] The invention described herein is used to illustrate a solution in which only the region surrounding the cladding composite wire 3 is transformed into a robust intermetallic phase 11 with a higher melting point.

[0256] The remaining portion of the connection area 13 is composed of soft solder of the original composition.

[0257] Therefore, there exists a sufficient region that, through its malleable material properties, compensates for the thermomechanical stress that occurs during bonding and under operating conditions, which is impossible to achieve with the connection region 13, which is mainly composed of brittle intermetallic phase 11, and may lead to the failure of the semiconductor structural element during the brazing process.

[0258] Figure 6 Solder forming part 8 with three composite wires 3 integrated in a solder matrix 2 is now shown.

[0259] With the aid of this solder forming part 8, the recessed brazing portion should be stabilized during the "brazing of substrate 9 to base plate 10", which is generated due to the use of the pre-bent base plate 10 and the allocated biconvex substrate 9.

[0260] Here, the two thick, edge-integrated composite wires 3 should be responsible for the smallest possible tilt of the substrate 9 and thus for the most uniform solder layer thickness of approximately 180 μm in the connection area 13.

[0261] The third composite wire 3, which has a relatively small thickness and is located in the center, should stabilize the recessed brazing area.

[0262] In order to manufacture the solder foil 1 according to the invention as the initial material for manufacturing the solder forming part 8, two brushed strips 6 made of SnAg3.5 with a thickness of 0.470 mm and a width of 70 mm and three composite wires 3 are used in the tensile state. The composite wires have copper as the material for the core 4 and have a tin-plated layer of the core 4 as the sheath 5.

[0263] The two composite wires 3 arranged on the edge side are round, have an outer diameter of 0.8 mm before the roll-coating, and have a layer thickness of about 25 μm for the sheath 5 made of tin.

[0264] The third composite wire, namely the intermediate composite wire 3, has a diameter of 0.55 mm and also has a sheath 5 made of tin with a thickness of about 25 μm.

[0265] The brushed strip 6 at the bottom passes through the unwinding device, through the open roller gap, the guide tool, and is then conveyed to the winding unit.

[0266] The intermediate composite wire 3 is passed through a guide tool by a spool and is oriented precisely in the center relative to the brushed strip 6 below.

[0267] The centrally located composite wire 3 is fixed on the outlet side.

[0268] Subsequently, the composite wire 3 with an outer diameter of 0.800 mm on the edge side is unwound from the spool and passed through the guide tool and positioned on the right side of the centrally extended composite wire.

[0269] In this embodiment, the spacing relative to the composite wires extending in the center should be 20 mm.

[0270] This creates a 15 mm gap relative to the right edge of the SnAg3.5 strip 6 located below the composite wire.

[0271] The third composite wire 3 should extend to the left of the center of the belt 6 and also have a diameter of 0.800 mm. The third composite wire is wound through the guide tool by a spool and arranged at the same 20 mm interval relative to the composite wire that extends in the center.

[0272] The composite wire 3 extending to the left and right sides of the center is also fixed to the exit side of the roll.

[0273] The upper SnAg3.5 belt 6 is arranged laterally to overlap with the lower SnAg3.5 belt 6, so that the belt passes through the roll gap as the last belt via a guide tool.

[0274] Then, the gap between the rolls 7 is adjusted in such a way that a solder foil 1 according to the invention with a thickness of 0.350 mm is produced.

[0275] Here, the cross-section of the circular composite wire between the SnAg3.5 foils is shaped and surrounded by soft solder.

[0276] The thickness of the solder foil 1 according to the present invention is further reduced to 180 μm by using two successive rolling passes.

[0277] The solder foil 1 manufactured in this way according to the invention has a rectangular cross-section having a height H = 0.18 mm and a width of approximately 70 mm, and has a soft solder matrix composed of SnAg3.5.

[0278] The composite wire 3, which is upset / deformed into an elliptical shape, is embedded in the solder matrix in a symmetrical manner with its centroid centered relative to the rectangular cross-section.

[0279] The intermediate wire has a smaller diameter of approximately 120 μm and a larger diameter of approximately 395 μm.

[0280] The two composite wires 3 arranged on the edge side are also shaped into an elliptical specification, with a smaller diameter of about 155 μm and a larger diameter of about 620 μm.

[0281] Here, in the solder foil 1 according to the present invention, the centroid of the elliptical surface is always in the middle, that is, at half of the total thickness / total height.

[0282] The guiding tool also ensures that the distance between the centroids of the elliptical surfaces of the composite wire remains equidistant throughout the entire rolling cladding process.

[0283] Subsequently, a blanked part having the following dimensions, namely a so-called "preform" (see [reference]), is punched out from this solder foil 1 according to the invention. Figure 5 ): Height H = 180 μm; Width B = 46.5; Length 38.0 mm.

[0284] The stamped part can be manufactured from solder foil 1 according to the invention, as explained above, the solder foil having a minimum width B of 43.0 mm (meaningful) up to a maximum width B of 70.0 mm. The length L of the solder forming part 8 can theoretically be from 5.0 mm up to more than 100 mm in a meaningful manner.

[0285] During the soldering process, the solder foil 1 according to the present invention operates at the peak temperature of the soldering characteristics at 250°C-260°C.

[0286] Here, the SnAg3.5 solder becomes molten.

[0287] The same applies to the tin sheath of copper wires.

[0288] By transforming a portion of the solder in the region of the composite wire 3 into intermetallic phases Cu6Sn5 and Cu3Sn according to the present invention, the proportion of the liquid solder above and below the composite wire on the edge side is reduced, so that the substrate can be tilted by an absolute value ≤10 μm.

[0289] Because the intermediate composite wire is designed to be slightly thinner in its structure, it allows for a shorter gap between the lower edge of the substrate 9 and the upper edge of the base plate, thus enabling the recessed shape for the brazing seam. Simultaneously, it prevents the lower side of the substrate from sinking too far towards the upper side of the base plate in the middle.

[0290] Therefore, the substrate 9 is supported by three composite wires 3.

[0291] If the overall structure is subjected to strong compressive force during brazing, the middle composite wire 3 also prevents the composite wire 3 on the edge side from being squeezed out during brazing.

[0292] The material according to the invention also combines the following advantages: on the one hand, it prevents crack growth at the location surrounding the wire during material fatigue and thereby improves the service life of the brazed part; on the other hand, it allows the material to bond due to the large area of ​​the malleable soft solder, which causes large thermomechanical stress due to the different thermal expansion characteristics of the material.

[0293] Therefore, by means of the teachings of the present invention, a novel solder foil 1 is provided, which can achieve a defined and reproducible connection zone geometry, whether a simpler or more complex shape is achieved after the brazing process, and is also suitable for multi-stage brazing processes to utilize typical brazing characteristics for soft soldering, that is, within the typical process temperature range for soft soldering, that is, at a temperature preferably 250°C to 300°C and below 5°C. Within a short brazing time, without subsequent heat treatment and without the application of extrusion pressure during brazing, and within the scope of a so-called multi-stage brazing process that avoids the formation of porosity and / or shrinkage cavities in the joint area while also preventing the risk of remelting of previously brazed areas, the brazed components are prevented from slipping by: a complete bridge, precisely defined in size and spacing, fixedly connected with the metal / metallized surface layer of the component to be brazed during the brazing process with high geometric precision, is constructed of a high-melting-point intermetallic phase, the bridge having a remelting temperature above 400°C, and the bridge ensuring high dimensional precision and geometric accuracy. The connection zone, in addition to the mechanical strength of these temperature-stable bridges, simultaneously prevents / prevents crack propagation in the connection zone during solder material fatigue, located adjacent to the bridges. The soft solder surrounding the bridges, i.e., the soft solder matrix, simultaneously absorbs the thermomechanical stress introduced by brazing but also generated during the use of the component, thereby resisting material fatigue. Thus, in terms of overall effect, the solution presented here significantly improves the service life of the connection zone compared to existing connection zones manufactured using conventional brazing materials according to existing construction and connection techniques, and at the same time, significantly improves the thermal conductivity of the entire connection zone due to the embedded Cu wire (or Ag or Ni, etc.).

[0294] List of reference numerals in the attached diagram:

[0295] 1 Solder foil

[0296] 2. Solder matrix

[0297] 3. Composite wire

[0298] 4 cores

[0299] 5. Sheath

[0300] 6. Strip (foil)

[0301] 7 rolls

[0302] 8 Solder forming parts

[0303] 9 Ceramic substrate

[0304] 10. Base plate

[0305] 11 Intermetallic phases

[0306] 12 Cracks

[0307] 13 Connection Area

[0308] H height

[0309] B width

[0310] L length

[0311] A. Edge Spacing

[0312] c. Spacing.

Claims

1. A lead-free solder foil (1) having a thickness of 50 μm to 600 μm, for connecting the surface layers of metal components and / or metallized / metal-coated components, i.e., adjacent components, to each other, wherein, The lead-free solder foil (1) is constructed such that two or more wires are arranged individually, parallel to each other and parallel to the edge in a soft solder matrix (2) composed of lead-free tin-based solder, pure tin, and pure indium, characterized in that these individual wires arranged in the lead-free solder foil (1) are constructed as composite wires (3), the composite wires having a core (4) made of a metal or metal alloy with a higher melting point and greater strength than the soft solder matrix (2), a sheath (5) made of another metal or other metal alloy arranged around the core, and the sheath (5) of the composite wire (3) having a diameter of 2% to 20% relative to the total diameter of the composite wire (3). The composite wire (3) is oriented longitudinally in the rolling direction and is wrapped between two solder foils or two solder strips by means of a rolling cladding method with a "percentage reduction in the height of the initial strip" and thus arranged in the solder matrix (2) with material locking. After the rolling cladding process, a layer of solder material is arranged above and below at least one core of the core (4) of the composite wire (3) wrapped in the solder matrix (2). The layer consists of the area of ​​the solder matrix (2) and a layer of sheath (5) of the composite wire (3). The layer of sheath of the composite wire is then at least 5 μm at the thinnest part, but at most 15 μm.

2. The lead-free solder foil (1) according to claim 1, characterized in that, The solder matrix (2) is composed of an indium-based alloy.

3. The lead-free solder foil (1) according to claim 1, characterized in that, The sheath (5) of the composite wire (3) is produced by electroplating or by immersing the core (4) in molten metal.

4. The lead-free solder foil (1) according to claim 2, characterized in that, The indium-based alloy is InSn48.

5. The lead-free solder foil (1) according to claim 1, characterized in that, The percentage reduction in the initial strip height is calculated as a percentage of the difference between the total initial height of the strip (6) and the final height H of the composite foil with respect to the total initial height of the strip (6), wherein the total initial height of the strip (6) is calculated without considering the height of the composite wire (3), wherein the final height H of the composite foil includes the embedded composite wire (3).

6. The lead-free solder foil (1) according to claim 5, characterized in that, The initial belt's height reduction, calculated as a percentage, ranged from greater than 30% to a maximum of 95%.

7. The lead-free solder foil (1) according to claim 1, characterized in that, These individual wires arranged in the lead-free solder foil (1) constitute the composite wire (3), which has a core (4) made of copper or copper-based alloy, silver or silver-based alloy, nickel or nickel-based alloy, gold or gold-based alloy, and a sheath (5) made of pure tin or tin-based alloy or indium or indium-based alloy arranged around the core.

8. The lead-free solder foil (1) according to any one of claims 1 to 7, characterized in that, Composite wires (3) with circular or elliptical cross sections are used as composite wires (3).

9. The lead-free solder foil (1) according to any one of claims 1 to 7, characterized in that, The composite wires (3) used in the lead-free solder foil (1) all have the same cross-sectional dimensions in the initial state.

10. The lead-free solder foil (1) according to any one of claims 1 to 7, characterized in that, The composite wire (3) used in the lead-free solder foil (1) has different cross-sectional dimensions in the initial state.

11. The lead-free solder foil (1) according to any one of claims 1 to 7, characterized in that, After the rolling process, the minimum permissible spacing between the cores (4) of the composite wire (3) is 500 μm, and after the rolling process, the minimum permissible spacing between the cores (4) of the composite wire (3) relative to the outer edge of the solder foil is 500 μm.

12. The lead-free solder foil (1) according to any one of claims 1 to 7, characterized in that, The core (4) of all composite wires (3) used in lead-free solder foil (1) is made of a uniform core material, and the sheath (5) of all composite wires (3) used in lead-free solder foil (1) is made of a uniform sheath material.

13. The lead-free solder foil (1) according to any one of claims 1 to 7, characterized in that, The core (4) of all composite wires (3) used in lead-free solder foil (1) is made of different core materials, and the sheath (5) of all composite wires (3) used in lead-free solder foil (1) is also made of different sheath materials.

14. The lead-free solder foil (1) according to any one of claims 1 to 7, characterized in that, The percentage reduction in height of the initial strip is in the range of 50% to 85%.

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