Lead-free soldering foil

Abstract

A lead-free soldering foil, for connecting metal and/or metal-coated components. allows the setting of a defined connecting-zone geometry and, with pores and/or voids being formed only to a minimal extent, achieves a high-temperature-resistant soldered connection that ensures great reliability even in staged soldering processes and increases the thermal conductivity of the connecting zone. The lead-free soldering foil is constructed so that, in a soft-solder matrix, two or more composite wires are each individually sandwiched by roll cladding between two soft-solder strips, parallel to one another and parallel to the edges of the strips. These composite wires include a core, which contains a higher-melting, stronger metal/metal alloy in comparison with the soft-solder matrix and around which a shell of another metal/metal alloy is arranged, and, after the roll-cladding operation, there is still 5 pm to 15 pm of soft-solder material arranged above and below at least one of the cores.

Claims

1. A lead-free soldering foil (1) having a thickness of 50 μm to 600 μm, in order to connect metallic devices (2) and/or metallized/metal-coated devices (2) with one another, wherein the soldering foil (1) has tape edges and is structured such that, in a soft solder matrix (2), two or more wires are respectively disposed individually, parallel to one another and parallel to the tape edges, wherein these wires respectively disposed individually in the soldering foil (1) are formed as composite wires (3), which have a core (4), which comprises a metal that is higher-melting and at the same time stronger compared with the soft-solder matrix (2), or a metal alloy, of copper or a copper-base alloy, silver or silver-base alloys, nickel or nickel base alloys, gold or gold-base alloys, around which a jacket (5) of a different metal or of a different metal alloy, of pure tin or a tin-base alloy, or of indium or an indium-base alloy is disposed; and the jacket (5) of the composite wires (3) has a layer thickness of 2% to 20% relative to the total diameter of the composite wire (3); and the composite wires (3), aligned along the rolling direction between two soft-solder foils or two soft-solder tapes, are clad in place by means of roll cladding in a “percentage height reduction of the starting tapes”, determined from the difference between the total starting height of the tapes (6) (determined without including the height of the composite wires (3)) and the final height H of the composite foil (1) (with embedded composite wires (3)) relative to the total starting height of the tapes (6), in percent, in the range from greater than 30% to at most 95% and thereby are disposed in substance-to-substance relationship in the soft-solder matrix; and after the roll-cladding process, over and under at least one of the cores (4) of the composite wires (3) clad in place in the soft-solder matrix (2), a layer of soft-solder materials is still disposed, which is composed of the region of the soft-solder matrix (2) and the layer of the jacket (5) of the composite wires (3), and which in total then measures at least 5 μm but at most 15 μm at the thinnest location.

2. The lead-free soldering foil (1) according to claim 1, wherein the soft-solder matrix (2) comprises either lead-free tin-base solders, pure tin, pure indium or alloys on the basis of indium.

3. The lead-free soldering foil (1) according to claim 1, wherein the jacket (5) of the composite wires (3) is produced galvanically or by dipping the core (4) in metal melts.

4. The lead-free soldering foil (1) according to claim 1, wherein composite wires (3) having round or oval cross section are used as the composite wires (3).

5. The lead-free soldering foil (1) according to claim 1, wherein the composite wires (3) used in a soldering foil (1) all have the same cross-sectional dimensions in the initial condition.

6. The lead-free soldering foil (1) according to claim 1, wherein the composite wires (3) used in a soldering foil (1) have different cross-sectional dimensions in the initial condition.

7. The lead-free soldering foil (1) according to claim 1, wherein the minimum permissible spacing between the cores (2) of the composite wires (3) is approximately 500 μm after the rolling process, and the minimum permissible spacing of the cores (2) of the composite wires (3) relative to the outer edge of the soldering foil is approximately 500 μm after the rolling process.

8. The lead-free soldering foil (1) according to claim 1, wherein the cores (4) of all composite wires (3) used in a soldering foil (1) comprise a uniform core material and the jackets (5) of all composite wires (3) used in a soldering foil (1) comprise a uniform jacket material.

9. The lead-free soldering foil (1) according to claim 1, wherein the cores (4) of all composite wires (3) used in a soldering foil (1) comprise various core materials and also the jackets (5) of all composite wires (3) used in a soldering foil (1) comprise different jacket materials.

10. The lead-free soldering foil (1) according to claim 1, wherein the “percentage height reduction of the starting tapes” lies in the range of 50% to 85%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the invention will now be explained in more detail in conjunction with six diagrams of the solution according to the invention.

(2) FIG. 1 shows the arrangement according to the invention for manufacture of the soldering foil 1 according to the invention.

(3) In FIG. 2, a solder preform consisting of a soldering foil 1 according to the invention and disposed between a substrate 9 and a base plate 10 prior to the soldering process is illustrated in a sectional diagram.

(4) FIG. 3 shows the arrangement illustrated in FIG. 2 immediately after the soft-soldering process.

(5) FIG. 4 shows the arrangement illustrated in FIG. 3 after a longer service time in continuous operation.

(6) In FIG. 5, a solder preform 8 manufactured from the soldering foil 1 according to the invention and having two composite wires 3 integrated in a soft-solder matrix 2 is shown in a 3D representation.

(7) FIG. 6 shows a solder preform 8 manufactured from the soldering foil 1 according to the invention and having three composite wires 3 integrated in a soft-solder matrix 2 in a 3D representation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) FIG. 1 shows the arrangement according to the invention for manufacture of the lead-free soldering foil 1 according to the invention.

(9) As illustrated in FIG. 1, starting materials to begin with are two foils or tapes 6 of soft solder, which are provided in degreased and in dirt-free surface quality.

(10) For improved further processing, these tapes 6 may be brushed prior to roll cladding, in order to remove external elastic passivation layers and additionally to have the surface roughness necessary for the connecting to be performed subsequently in the roll gap.

(11) These foils or tapes 6 are so disposed on the inlet side of the rolls 7 of a roll stand that the lower tape 6 of soldering material is threaded first through the roll gap.

(12) Then the composite wires 3, which expediently are provided wound on spools, are applied centrally on the corresponding unwinder shafts.

(13) The composite wire 3 may be provided in round or else oval cross sections.

(14) Corresponding to the number of composite wires 3 to be integrated in the soldering foil 1, these are disposed between the tapes on the inlet side of the roll stand.

(15) In a simple layout, the number of composite wires to be integrated is equal to two.

(16) However, even more composite wires 3, for example five and more composite wires 3, may be used in a soldering foil 1 according to the invention. For this purpose, the geometric shape of the composite wires 3 and also their thickness may be selected differently.

(17) A simple example provides that two composite wires, round starting shape and equal diameters will be used.

(18) In a more complex example, three or more composite wires of different shape and above all different diameter may also be used.

(19) As the starting material for the tapes 6, which produce the soft-solder matrix 2 after the roll-cladding process, all common, lead-free tin-base alloys may 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.

(20) However, metals and alloys on the basis of indium may also be used, for example pure indium or InSn48.

(21) For the core 4, especially metals and alloys of pure copper and copper-base alloys, pure silver and silver-base alloys as well as nickel and nickel-base alloys may be considered, above all in combination with tin-base alloys for the soft-solder foils to be used. However, gold and gold-base alloys may also be used in combination with indium-base solders, although this represents a cost-intensive example for a material combination.

(22) For the connecting process during roll-cladding and also for the diffusion-soldering process, it is characterizing that the jacket 5 of the composite wires 3 consists of a different material than the core 4.

(23) The peripheral layers of the jacket 5 (mainly zinc oxides), which are very brittle in comparison with the metal of the core 4, support the establishment of connection to the tapes 6 during the roll-cladding process, since these brittle layers of the jacket 5 already rupture at slight deformation and thereby make space for the connection-friendly material in the interior of the jacket layers.

(24) In this case, it is immaterial whether the jacket 5 of the composite wire 3 was produced galvanically or else by dipping the core 4 in metal melts.

(25) The thickness of the jacket layer may be from approximately 2% to 20% of the thickness of the total diameter of the composite wire 3.

(26) As the “diffusion-soldered regions” around the core 4 of the respective composite wire 3 being used, the intermetallic phases formed according to the prior art or their combinations will represent the higher-melting phases, depending on choice of the materials being used.

(27) In order to achieve remelting temperatures of the intermetallic phases of ≥400° C., material combinations such as tin-base solder as the soft-solder matrix 2 with cores 4 of the composite wires 3 of copper, silver or nickel wire, or indium-base solder as the soft-solder matrix 2 with cores 4 of the composite wires 3 of nickel or gold wire are desirable.

(28) The respective soldering profile, i.e. the temperature-time regime during the soldering process, then always corresponds to the soft-soldering profile standard in the prior art for the respective material combination, with the process-temperature range typical for the lead-free soft soldering, i.e. at up to approximately 280° C. and in soldering times of shorter than 5 minutes, for example.

(29) The embodiment of a soldering foil 1 according to the invention illustrated in FIG. 1 shows at the center a very thin composite wire 3, next to which two thicker, in this case equally thick composite wires 3 are disposed, one on each side. This embodiment is used in order to conform to a concave soldering gap and to ensure a geometric exactness that is as high as possible after the soldering.

(30) Depending on requirement applicable to the geometry of the later soldering gap, however, any suitable number of composite wires 3 having different diameters may also be used.

(31) According to FIG. 1, the composite wires 3 are threaded in a next process step through a guide die and then through the roll gap, and in the process are aligned with the lower tape 6, which has already been threaded through.

(32) In the subsequent roll cladding, the guide die ensures that the composite wires 3 are clad in place only at the desired locations, and thus it also defines the spacing of the composite wires 3 relative to one another as well as the spacing of the composite wires 3 relative to the long edges of the soldering foil 1.

(33) After the composite wires 3 have been aligned in this way with the lower tape 6, they are mechanically fixed on the outlet side of the rolls 7.

(34) Next, the upper tape 6, which enters the roll gap between the rolls 7 above the composite wires, is threaded through the rolls and then aligned edge-to-edge with the lower tape 6.

(35) It is advantageous to use foils or tapes 6 of soft solder having equal width.

(36) For a continuous process, it is likewise expedient to use the material being used in wound form, in so-called coils.

(37) After the tapes 6 and the composite wires 3 have been threaded in desired arrangement through the roll gap, the roll gap is closed.

(38) The thickness of the resulting material composite tape is adjusted by the adjustment of the minimum spacing of the rolls relative to one another.

(39) If the rolling process is now begun, the softer material of the solder flows, due to plastification of the metals in the roll gap, around the rounded regions of the stronger composite wires 3, molds these in place and reconnects with itself, so that a substance-to-substance material composite, the soldering foil 1 according to the invention, is joined. The “percentage height reductions of the starting tapes” necessary for this purpose during roll cladding (determined from the ratio, from the difference between the total starting height of the tapes 6 (determined without including the height of the composite wires 3) and (i.e. minus) the final height H of the composite foil 1 (with embedded composite wires 3), relative to the total starting height of the tapes 6, in percent), lie in the range from greater than 30% to 95% (of the total height of the starting tapes), depending on choice of the materials used for the soldering foils/soldering tapes and the respectively used composite wires, which are aligned individually and parallel to the tape edges, along the rolling direction.

(40) If, for example, a round wire is used as the starting material of the composite wire 3, it will approximate an oval geometry in the process of forming in the roll gap.

(41) As already mentioned, composite wires 3 that are already oval may also be used as the starting-material shape.

(42) The rounded shape of the composite-wire surface is important for the production of the cladding.

(43) Composite-wire cross-sectional geometries with pronounced edge shapes have no use within the scope of the solution according to the invention.

(44) It is only with rounded shapes of the jacket surfaces of the composite wires 3 that the softer material of the soft-solder matrix 2 is able to flow in substance-to-substance relationship around the composite wire 3 during the forming process, without encountering too large resistances in the flow of substance while doing so.

(45) In the process, the soldering material of the soft-solder matrix 2 is exposed to very large local degrees of deformation at the locations at which the composite wire 3 is surrounded by flow.

(46) The round shape of the surface of the composite wires 3 improves an enclosing of the composite wires without losing the material cohesion of the solder (break/crack) and in this way makes it possible for the first time for even very thick composite wires 3, such as, for example, composite wires 3 having a diameter equal to 2.5 times the thickness of the (starting) tapes 6 of soldering material, to be integrated in substance-to-substance relationship without having to expect an internal or external damage to the composite.

(47) It is only in this way that it is possible to obtain the necessary, very thin layers of soft solder of 5 μm to at most 15 μm, which are situated over and under the regions at least of one of the integrated wire cores 4 after the cladding process and are necessary for a diffusion soldering.

(48) Within the scope of the process workflow according to the invention, it does not represent any problem to integrate, in the solder matrix, even composite wires 3 of very different thickness, or composite wires 3 that merely amount to only one third of the original diameter of the thickest composite wire 3, or in exceptions are even smaller.

(49) Different alloys may also be used for the cores 4 of the integrated composite wires 3.

(50) Examples are copper alloys of different strength or pure copper having different degrees of strain hardening.

(51) The background for the use of alloys of different strength is that the different composite wires 3 are also deformed to different extents during roll cladding in comparison with the solder.

(52) The composite wires 3 having the softer wire-core alloys become flatter and increasingly oval during roll cladding and in comparison take up less height in the overall composite in comparison with stronger metals and alloys.

(53) By choosing alloys of different strength, it is therefore also possible to produce various geometric layouts of the soldering foil 1 according to the invention.

(54) Depending on the metals used for the tapes 6, the soft solder matrix 2 and the composite wires 3, it may be necessary to heat the starting materials prior to or during entry into the roll gap, in order to achieve better formability.

(55) The soldering foil 1 may also be rolled out to an even thinner soldering foil 1 in subsequent rolling steps.

(56) The solution according to the invention likewise provides for manufacturing solder preforms from the soldering foils 1 according to the invention produced in this way, in order to undertake a very exact proportioning of the soldering foil 1 for the desired soldered joints. This may take place, for example, by methods such as stamping, laser cutting, microetching or conventional shearing methods.

(57) Moreover, combined methods, which include cutting and simultaneous forming (deep-drawing, embossing) may also be employed for the manufacture of complex geometric solder preforms.

(58) Since it is known that severely tilted soldered joints tend to fail mechanically and to lead to formation of undesired hot spots as homogeneously thick layers, many application situations require only two equally high composite wires 3, which are as thick as possible in the soldering foil 1 and thus are placed at an exact spacing relative to one another, and that the composite wires are integrated as close to the edge as possible, for example in a solder preform.

(59) During the soldering process, a tilting of the components is then suppressed in this way and a solder layer as homogeneous as possible is ensured.

(60) FIG. 2 shows a sectional drawing prior to the soldering process, in which a solder preform 8 consisting of the lead-free soldering foil 1 according to the invention is disposed between a ceramic substrate 9, which is used in the exemplary embodiment presented here and is also known as DCB, DBC or AMB, and a base plate 10.

(61) This solder preform 8 consisting of the of the soldering foil 1 according to the invention and stamped out of the soldering foil 1 is constructed according to the invention in such a way that two composite wires 3 are clad in place in the soft-solder matrix 2, which in this exemplary embodiment consists of a lead-free tin-base solder, i.e. are integrated compactly in the soft-solder matrix, and according to the invention have a core 4, which consists of a metal or a metal alloy, in this case of copper, which is higher-melting and at the same time stronger compared with the soft-solder matrix 2, and around which a jacket 5 of a different metal or a different metal alloy, in this case of a tin-base alloy, is disposed.

(62) According to the invention, this arrangement, after the “stack” has been produced, is heated in a soft-soldering process with a soldering profile typical for soft-soldering processes in a process-temperature range typical for soft soldering (i.e. at up to approximately 280° C. and in soldering times of shorter than 5 minutes), and in the process the soft-solder matrix is transformed to the molten state.

(63) This may take place, as is standard in the prior art, for example in vacuum ovens, usually under the effect of reducing gases.

(64) In the process, the molten solder spreads on the surfaces in contact with it.

(65) According to the invention, as illustrated in FIG. 3, diffusion zones, i.e. regions with new chemical composition, so-called intermetallic phases 11, extending up to the neighboring device surfaces (i.e. up to the substrate 9 and the base plate 10) and disposed between the solder and the materials to be wetted, are then formed around the composite wires due to diffusion processes between metal atoms from the composite-wire region and the molten solder.

(66) Connection zones soldered with monolithic soft solders have, after the soft soldering, and considered over the cross section, a connection zone that consists of the starting ingredients and possesses the same chemical composition as before the soldering.

(67) In contrast, if soldering is carried out with the soldering foil 1 according to the invention, regions having chemically changed composition are formed around the core 4 due to the diffusion processes between the composite wire 3 and the molten soft-solder melt that take place according to the invention during the soft-soldering process with soft-soldering profiles.

(68) In the present exemplary embodiment, the SnCu3 solder is used as the solder matrix.

(69) The composite wires 3 consist in the core 4 of unalloyed, pure copper with a jacket 5 of a tin-base alloy.

(70) During soft soldering, intermetallic phases of Cu3Sn and Cu6Sn5 are formed according to the invention around these cores 4.

(71) Since the spacings between the peripherally integrated oval cores of the composite wires and the surfaces of the regions to be joined in the present exemplary embodiment are only 5 μm up to at most 15 μm (12.5 μm in the exemplary embodiment), these regions grow very rapidly with intermetallic phases.

(72) In the process, the bridges of intermetallic phase 11 illustrated in FIG. 3 are formed around the core 4 and between the core 4 and the substrate 9, as well as between the core 4 and the base plate 10.

(73) These intermetallic phases 11 are characterized by a higher strength and hardness and also a higher melting point compared with the soft-solder matrix 2 being used (melting point of the intermetallic phases 11 is dependent on the respective materials being used and is generally ≥415° C.).

(74) After the soldering with the soldering foil 1 according to the invention, and in comparison with soldering using monolithic soft-soldering material, the connection zone between the substrate 9 and the base plate 10 consists not only of ductile soft solder but instead on the one hand of bridges consisting of very strong, temperature-stable intermetallic phases 11 and on the other hand of ductile soft solder of the soft-solder matrix 2 adjoining these bridges.

(75) When these properties are combined in one material, they are known as “tough” in materials science.

(76) After completion of the assembly, this is then used as is generally standard. In the process, the power loss produced in the semiconductor devices in the form of heat is removed via the substrate 9 and the respective soft-solder connection zone of the prior art into the base plate 10.

(77) In the process, the different materials of the individual devices expand to different extents according to their respective coefficients of thermal expansion.

(78) If the operation is stopped intermittently, as is generally standard, for a short or even long time, then power loss in the form of heat is no longer emitted by the semiconductor device, and the materials then cool down and in the process contract.

(79) As a consequence of this constant thermal load cycling, stresses are generated in the materials due to the different expansions. These are concentrated in particular at the peripheral regions of the connection zone.

(80) These peripheral regions of the connection zone are exposed to high stress and strain due to the large number of thermal load cycles over the service life.

(81) This phenomenon is also known as degradation of the solder.

(82) In conventional monolithic soft-solder connections, only the edge regions of the soldered joint/connection zone are affected by cracks in the initial stage.

(83) In the initial stage, the functional capability of the overall structural part is not yet restricted.

(84) In the further continuous operation, these cracks in the soft solder of the connection zone then grow in conventional soft-solder connections until they are under the regions above which the power semiconductor devices sit.

(85) From then on the heat can be removed only very poorly.

(86) Thereby the semiconductor device becomes too hot, then leading to a failure of the entire assembly.

(87) By means of the novel soldered connection formed with the solution according to the invention in the soft-soldering process, a substantially longer life of the entire assembly, such as the entire semiconductor module, for example, can now be assured compared with the prior art.

(88) FIG. 4 shows the soldered connection according to the invention illustrated in FIG. 3 (in conjunction with a power semiconductor module) after longer continuous operation.

(89) From this diagram, it is obvious how the further growth of the cracks 12 is prevented by means of the solution according to the invention.

(90) In conjunction with the stable bridges of intermetallic phases 11, the ductile soft-solder matrix 2 ensures an optimum dissipation of the thermal stresses in the middle of the connection zone 13.

(91) Nevertheless, formation of cracks 12 as a consequence of fatigue phenomena of the soft-solder material can occur at the peripheries of the connection zone 13 after a large number of thermal load cycles.

(92) In this situation, as illustrated in FIG. 4, the bridges of the very strong, intermetallic phases 9 that were formed according to the invention around the copper wires 3 ensure that these cracks 12 are prevented from a further propagation.

(93) Therefore it is imperative and necessarily advantageous to integrate the composite wires 3 as close as possible or as close as needed to the edges/peripheries of the solder preforms 8, in order thereby to prevent a crack (propagation) underneath the regions of the seat of the semiconductor devices.

(94) As a consequence of the composite wires 3 having integrated cores 4 of copper, silver or gold, clad in place in the soldering foil 1, a further, significant improvement of the thermal conductivity of the connection zone 13 is additionally ensured simultaneously compared with the surrounding soft solder, whereby the life of the power semiconductor module is again significantly enhanced.

(95) As a consequence of the significantly improved heat dissipation according to the invention, it is possible that the operating temperatures of the entire assembly will be further lowered and also that thereby the development of thermomechanical stresses will again be significantly reduced.

(96) In FIGS. 5 and 6, two different solder preforms 8 manufactured from different soldering foils 1 according to the invention are illustrated in 3D views.

(97) FIG. 5 shows, in a 3D representation, a solder preform 8 manufactured from the soldering foil 1 according to the invention having two composite wires 3 integrated/clad in place in a soft-solder matrix 2 for assurance of a homogeneous solder-layer thickness after the soldering process.

(98) In order, for example, to solder a semiconductor device having metallization on the back side on a Cu-ceramic substrate, the solder preforms 8 illustrated in FIG. 5 and manufactured from the soldering foil according to the invention are used with the dimensions of, for example, B=15 mm×L=15 mm.

(99) With the present exemplary embodiment, a connection zone 13 having a height H=100 μm will be ensured.

(100) Since the semiconductor material has no noteworthy sag and the substrate likewise exhibits no noteworthy deviation from a planar surface configuration over the extent of this region, it is sufficient to stabilize the soldered joint with two peripherally located, equally high composite wires 3.

(101) The solder preforms 8 needed for this purpose are fabricated from a solder foil 1 according to the invention, which is manufactured as follows.

(102) Two soft-solder tapes 6 of SnCu3 having the thickness 0.340 mm and width 70.0 mm in conjunction with six separately routed composite wires 3 having a core 4 of copper and a jacket 5 of tin are used as the starting material.

(103) The tapes 6 and composite wires 3 are fed via appropriate guide dies as already explained to the roll gap, and in the process are so aligned relative to one another that the soft-solder tapes 6 again enter the roll gap while coinciding with one another and the composite wires 3 are disposed with exact spacing between them.

(104) In the process, the individually routed composite wires 3 are aligned at a spacing of 10.0 mm relative to one another.

(105) The spacing relative to the tape edges is likewise set at 10.0 mm.

(106) The composite wires 3 consist in the core 4 of hard-drawn copper having a diameter of 500 μm and they have a jacket 5 of 25 μm thick tin.

(107) The composite is clad to a thickness of 220 μm and is reduced to the final thickness of 100 μm with two subsequent roll passes.

(108) From this soldering foil 1 according to the invention, it is now possible to stamp out three solder preforms with the dimensions 15.0 mm×15.0 mm per stamping stroke, continuously over the width.

(109) For this purpose, the stamping die and the seat of the stamp are designed such that a spacing of the stamp relative to the tape edges is 7.5 mm and the stamps have a spacing of 5 mm among one another.

(110) In this way, preforms/molded solder parts 8, in which the two integrated composite wires have a spacing of 2.5 mm relative to their outer edges, are stamped out from the soldering foil 1.

(111) The geometric structure of one of these solder preforms is illustrated in 3D in FIG. 5.

(112) Therein the height of the preform 8 is H=0.100 mm, the width of the preform 8 is B=15.0 mm, the length of the preform 8 is L=15.0 mm.

(113) The composite wires 3 deformed to an oval geometry have the following extent, wherein the larger diameter is approximately 430 μm and the shorter diameter, which subsequently is definitive for the control of a homogeneous solder-layer thickness, has an extent of approximately 84 μm.

(114) The soft-solder column/soft-solder thickness above the composite wires 3 clad in place in the soldering foil 1 according to the invention (with height H=100 μm) is approximately 8 μm at the thinnest location.

(115) According to the invention, these approximately 8 μm thick solder layers are transformed completely into the higher-melting intermetallic phases, in this case Cu6Sn5 and Cu3Sn, during soldering with the soft-soldering profiles typical for the soft soldering, i.e. with the in a process-temperature range typical for the soft soldering, at up to approximately 280° C. and in soldering times of shorter than 5 minutes.

(116) According to the invention, complete bridges of higher-melting intermetallic phases are created in the process along the two composite wires 3 integrated during the soldering process, both toward the substrate surface and toward the back-side metallization of the semiconductor, which bridges, among other possibilities in conjunction with the cores of the wires, ensure that a tilting of the soldered-on semiconductor device in comparison with the substrate is avoided and simultaneously that a highly exact fixation of the semiconductor device relative to the substrate is guaranteed, both for the transport and for a subsequent soldering process in the next oven compartment.

(117) In this subsequent soldering process, the substrate 9 connected with the semiconductor device, for example, is then further soldered with a cooling element.

(118) In the process, the solution according to the invention ensures that neither during transport to a next oven compartment nor during the next soldering process is the semiconductor material already soldered previously with the substrate 9 able to “slip”.

(119) With the invention presented here, a solution is presented in which only the regions around the composite wires 3 that have been clad in place will be transformed to higher-melting, strong intermetallic phases 11.

(120) The rest of the connection zone 13 consists of soft solder of the original composition.

(121) Thus sufficient regions exist that compensate by ductile material behavior for the thermomechanical stresses developed during joining and under service conditions, which is not possible in conjunction with a connection zone 13 that consists predominantly of brittle intermetallic phases 11 and may already lead to failure of the semiconductor devices during the soldering process.

(122) FIG. 6 now shows a solder preform 8 having three composite wires 3 integrated in a soft-solder matrix 2.

(123) By means of this solder preform 8, a concave soldered joint will be stabilized during a “substrate 9—to—base plate 10 soldering”, which results due to the use of a pre-bent base plate 10 and an associated biconvex substrate 9.

(124) In the process, the two thick composite wires 3 integrated close to the edge will ensure the smallest possible tilting of the substrate 9 and thus the most homogeneous possible solder-layer thickness in a connection zone 13 of approximately 180 μm.

(125) A third, centrally situated composite wire 3 having smaller thickness will stabilize the concave soldered joint.

(126) For the manufacture of the soldering foil 1 according to the invention as starting material for the manufacture of the solder preforms 8, two brushed tapes 6 of SnAg3.5 having a thickness of 0.470 mm and a width of 70 mm are used, as are three composite wires 3 with copper as the material for the core 4 and a galvanic tin coating of the core 4 as the jacket 5, in the hard-drawn condition.

(127) The two peripherally disposed composite wires 3 are round, prior to the roll cladding possess an outside diameter of 0.8 mm and have a layer thickness of the jacket 5 of tin of approximately 25 μm.

(128) The third, the central composite wire 3 possesses a diameter of 0.55 mm and likewise has a jacket 5 of tin of approximately 25 μm.

(129) The lower brushed tape 6 is threaded from an unwinder through the opened roll gap, past a guide die, and fed to a winding unit.

(130) The central composite wire 3 is threaded from a spool through a guide die and aligned exactly at the center of the lower brushed tape 6.

(131) The central composite wire 3 becomes fixed on the outlet side.

(132) Thereupon a peripheral composite wire 3 having the outside diameter of 0.800 mm is unwound from a spool and threaded through a guide die then disposed to the right of the centrally routed composite wire.

(133) The spacing relative to the centrally routed composite wire will be 20 mm in this exemplary embodiment.

(134) Thereby a spacing of 15 mm results relative to the right periphery of the SnAg3.5 tape 6 situated under the composite wire.

(135) The third composite wire 3, which will be routed to the left of the center of the tape 6 and which likewise has a diameter of 0.800 mm, is unwound from a spool, threaded through a guide die and disposed at a spacing of likewise 20 mm relative to the centrally routed composite wire.

(136) The composite wires 3 routed on the left and right of the center likewise become fixed on the outlet side of the rolls.

(137) Finally, the upper SnAg3.5 tape 6 is threaded through the roll gap in a manner coinciding with the lower SnAg3.5 tape 6 as seen in side arrangement, and in the process likewise passes a guide die.

(138) The roll gap between the rolls 7 is then adjusted such that a soldering foil 1 according to the invention having a thickness of 0.350 mm is obtained.

(139) In the process, the round composite wire cross sections are molded in place between the SnAg3.5 foils and enclosed by soft solder.

(140) By means of two successive roll passes, the thickness of the soldering foil 1 according to the invention is then further reduced to 180 μm.

(141) The soldering foil 1 according to the invention, manufactured in this way, has a rectangular cross section with a height H=0.18 mm and a width of approximately 70 mm, with a soft-solder matrix of SnAg3.5.

(142) The composite wires 3 squeezed/deformed to oval shape are embedded in the soft-solder matrix, centrally/symmetrically relative to the centroid of the rectangular cross section.

(143) The smaller diameter of the middle wire is approximately 120 μm and its largest diameter is approximately 395 μm.

(144) The two peripherally disposed composite wires 3 are likewise deformed to oval formats, with approximately 155 μm for the smaller diameter of the oval and approximately 620 μm for the larger diameter of the oval.

(145) The centroids of these ovals are always in the middle, meaning at half height of the total thickness/total height, in the soldering foil 1 according to the invention.

(146) Due to the guide dies, it is simultaneously ensured that the spacings of the centroids of the composite wire ovals are maintained at equal distances over the entire roll-cladding process.

(147) Subsequently, stamped parts, so-called “preforms”, having the following dimensions (see FIG. 5): height H=180 m; width B=46.5; length 38.0 mm, are stamped out of this soldering foil 1 according to the invention.

(148) The stamped parts can be manufactured from this soldering foil 1 according to the invention, manufactured as explained above, from a (practical) minimum width B of 43.0 mm up to the maximum width B of the tape of 70.0 mm.

(149) Theoretically, the length L of the solder preforms 8 could usefully be 5.0 mm up to greater than 100 mm.

(150) In the soft-soldering process, a peak temperature in the soldering profile of 250° C.-260° C. is used for working with this soldering foil 1 according to the invention.

(151) Under these conditions the SnAg3.5 solder becomes molten.

(152) The same is true for the tin jacket of the copper wires.

(153) Due to the transformation according to the invention of a part of the soft solder in the region of the composite wires 3 into the intermetallic phases Cu6Sn5 and Cu3Sn, the proportion of the molten soft solder that is situated over and under the peripheral composite wires decreases, so that the value by which the substrate is absolutely able to tilt is ≤10 μm.

(154) The central composite wire, since it is dimensioned to be somewhat thinner in its structure, permits a shortening of the spacing between the lower edge of the substrate 9 and the upper edge of the base plate, so that a concave shape for the soldering gap is made possible. Simultaneously, it prevents excessive sinking, i.e. solution of the substrate underside toward the base-plate upper side at the center.

(155) In this way the substrate 9 is braced by three composite wires 3.

(156) The central composite wire 3 also prevents the peripheral composite wires 3 from being able to be squeezed out during soldering, if the overall structure were to be exposed to a high pressing pressure in the soldering process.

(157) With the material according to the invention, the following advantages are also combined with one another: on the one hand, that the crack growth in case of material fatigue is blocked at the locations around the wires and thus the life of the soldered joints is prolonged; on the other hand, that, due to the presence of large regions of ductile soft solder, a joining of materials that cause large thermomechanical stresses on the basis of different thermal expansion behavior is permitted.

(158) By means of the teaching according to the invention, therefore, a novel soldering foil 1 is provided that permits the adjustment of a defined and reproducible connection-zone geometry, regardless of simple or else complex configuration after the soldering process, and that is also suitable for stepwise soldering processes, in order that, with a soldering profile typical for the soft soldering, i.e. in a process-temperature range typical for the soft soldering, i.e. at preferably 250 to 300° C., and in soldering times of shorter than 5 minutes, and also without a subsequent heat treatment and without the exertion of a pressing pressure during the soldering, with simultaneous prevention of the formation of pores and or blowholes in the connection zone, the slipping of soldered components even during so-called stepwise soldering processes, in which the danger of a remelting of previously soldered regions exists, is prevented by the fact that complete bridges of higher-melting, intermetallic phases, which are strongly bound with high geometric exactness to the metallic/metallized surface layers of the devices to be soldered and are exactly defined in their dimensions and their spacings are formed during the soldering process, which bridges have a remelting temperature of higher than 400° C., and which ensure a connection zone that is highly accurate and geometrically exact in the dimensions and that, in addition, due to the mechanical strength of these temperature-stable bridges, simultaneously prevent/stop a crack propagation in the connection zone in case of material fatigue of the solder directly at the bridges, wherein the soft solder surrounding the bridges, the soft-solder matrix, simultaneously absorbs the thermomechanical stresses introduced by the soldering but also developed during the device service and thereby counteracts a material fatigue, whereby, in operative general context, the solution presented here significantly extends the life of the connection zone compared with a connection zone manufactured with conventional soldering materials according to the prior art in mounting and connection technology, and in the process significantly improves the thermal conductivity of the entire connection zone as a consequence of the embedded wires of Cu (or Ag, or Ni, etc.).

SUMMARY OF THE REFERENCE SYMBOLS

(159) 1 Soldering foil 2 Soft-solder matrix 3 Composite wire 4 Core 5 Jacket 6 Tape (foil) 7 Roll 8 Solder preform 9 Ceramic substrate 10 Base plate 11 Intermetallic phase 12 Crack 13 Connection zone H Height B Width L Length a Spacing relative to periphery c Spacing