LEAD-FREE SOLDER FOIL FOR DIFFUSION SOLDERING AND METHOD FOR PRODUCING THE SAME

Abstract

The invention relates to a lead-free solder foil for diffusion soldering and to the method for its production, with which method metallic structural parts and/or metallized/metal-coated structural parts, i.e. metallic surface layers of adjacent structural parts, may be bonded to one another. The task of the invention is to provide an economic and environmentally friendly lead-free solder foil that is not hazardous to health for diffusion soldering, with which the structural parts to be soldered can be bonded to one another in such a way, in a process temperature range typical of the soft soldering, i.e. at approximately 240° C. and in soldering times of shorter than 5 minutes, without a subsequent heat treatment and without the exertion of a pressing force during the soldering, that a continuous layer of a high-melting bonding zone is obtained in the form of an intermetallic phase having a remelting temperature of higher than 400° C. The lead-free solder foil (1) according to the invention for diffusion soldering contains a solder composite material (4), which is produced by roll-plating and which is then constructed in such a way that, in a lead-free soft-solder environment of a soft-solder matrix (5), compact particles (6) of a high-melting metal component (7) are completely surrounded by lead-free soft solder (8), wherein the dispersedly distributed particles (6) of the high-melting metal component (7) have a thickness of 3 μm to 20 μm in the direction of the foil thickness, the spacings of the particles (6) relative to one another in the soft-solder matrix (5) are 1 μm to 10 μm, each of the particles of the high-melting metal component (7) is enveloped all around by a layer, 1 μm to 10 μm thick, of the lead-free soft solder (8), and the solder foil (1) has, adjacent to the metallic surface layers (3) of the structural parts (2) to be joined, an outer cladding layer (10), the layer thickness of which is 2 μm to 10 μm and which consists of soft solder (8).

Claims

1-6. (canceled)

7. A method for production of a lead-free solder foil (1), produced by means of a rolling method, for diffusion soldering, in order to bond metallic structural parts (2) and/or metallized/metal-coated structural parts (2), i.e. metallic surface layers (3) of adjacent structural parts (2) to one another; wherein, for production of the lead-free solder foil (1), the roll-plating method is repeated numerous times and is used dispersingly in such a way that a compact solder composite material (4) is obtained in which, in a lead-free soft-solder environment, i.e. a soft-solder matrix (5), particles (6) of a high-melting metal component (7), i.e. a hard-solder component, are dispersedly distributed in such a way that each of the particles (6) is completely surrounded by lead-free soft solder (8); and wherein, for production of a solder composite material (4), soft-solder components and metal components are first joined alternately, by means of the roll-plating method, as a layer composite, corresponding to the provided/intended percentage composition of the solder composite material (4), in such a way that the metal component always becomes bonded on both sides with the soft-solder component, wherein the layer thicknesses, to be used, of the components are in such a ratio to one another on the whole that, in the subsequent soldering process, the soft-solder content is incorporated completely in the intermetal.lic phase; and wherein, with the once plated layer composite, further roll-plating steps are subsequently repeated numerous times, in which the respective plated material is plated with itself, so that the number of layers in the material is increased but their thickness is simultaneously reduced; and wherein the number of roll-plating steps up to the finished solder composite material (4) is repeated numerous times in dependence on the chosen material combination of soft-solder and hard-solder components and on the desired total thickness for the shaped solder pads, such that, as a consequence of the numerously repeated roll-plating of the layer composite, intermingling of the individual components in the solid state takes place; and that, in the process, due to tearing of the layers of one of the two components, their fragments then become dispersedly distributed, i.e. dispersed in the other, i.e. the softer component, so that, due to the numerously repeated dispersing roll-plating, a structure with particle spacings smaller than or equal to 10 μm is obtained.

8. A lead-free solder foil (1) for diffusion soldering, which was produced by the method according to claim 7, in order to bond metallic structural parts (2) and/or metallized/metal-coated structural parts (2), i.e. metallic surface layers (3) of adjacent structural parts (2) to one another; wherein the lead-free solder foil (1) comprises compact solder composite material (4), and this compact, i.e. substance-to-substance bonded, solid solder composite material (4) is constructed in such a way that, in a lead-free soft-solder environment, i.e. a soft-solder matrix (5), particles (6) of a high-melting metal component (7), i.e. a hard-solder component, are dispersedly distributed by the numerously repeated dispersing roll-plating in such a way that each of the particles (6) is completely surrounded by lead-free soft solder (8), in order to bring about, in a customary soft-soldering process, with a soldering profile typical of the lead-free soft soldering, a complete transformation of the soft solder (8) of the soft-solder matrix (5) into intermetallic phases (9), which have a melting temperature of higher than 400° C.; and wherein the particles (6) of the high-melting metal component (7) dispersedly distributed in the soft-solder matrix (5) have a thickness of 3 μm to 20 μm in the direction of the foil thickness, wherein the spacings of the particles (6) relative to one another in the soft-solder matrix (5) are 1 μm to 10 μm, and each of the particles (6) of the high-melting metal component (7) is enveloped all around by a layer of the lead-free soft solder (8) that is 1 μm to 10 μm thick; and wherein the soft-solder content, i.e. the soft-solder matrix (5), is not higher in relationship to the content of high-melting metal component (7) than is necessary in the intermetallic phases (9) to be constructed, wherein this ratio of the percentage content of the particles (6) of the high-melting component (7) disposed in the solder composite material (4) to the percentage content of the soft solder (8) of the lead-free soft-solder matrix (5) surrounding the particles (6) is determined in such a way according to the stoichiometric formula of the intermetallic phases (9) to be formed from the respective starting materials that all soft solder (8) of the lead-free soft-solder matrix (5) is always transformed into the intermetallic phases (9) to be respectively constructed; and wherein the total thickness of the lead-free solder foil (1) is 20 μm to 0.5 mm; and wherein the solder foil (1), i.e. the solder composite material (4) has, adjacent to the metallic surface layers (3) of the structural parts (2) to be joined, an outer cladding layer (10), the layer thickness of which is 2 μm to 10 μm and which comprises soft solder (8).

9. The lead-free solder foil (1) for diffusion soldering according to claim 8, wherein the solder foil (1) is constructed as a multi-layer solder foil (11); and wherein the individual layers of the multi-layer foil (11) comprise alternately the solder composite material (4) and layers, 2 μm to 100 μm thick, of a high-melting metal component (7), i.e. an intermediate layer (23); and wherein the multi-layer solder foil (11) has, adjacent to the metallic surface layers (3) of the structural parts (2) to be joined, an outer cladding layer (10), the layer thickness of which is 2 μm to 10 μm and which comprises soft solder (8); and wherein the total thickness of the multi-layer solder foil (11) is 40 μm to 1.0 mm.

10. The method for production of the lead-free solder foil (1) for diffusion soldering according to claim 8, wherein the solder foil (1) is constructed as a multi-layer solder foil (11), the individual layers of which are bonded to one another by means of roll-plating in such a way that these individual layers of the multi-layer solder foil (11) comprise alternately the solder composite material (4) and layers of a high-melting metal component (7), i.e. an intermediate layer (23), wherein the multi-layer foil (11) has, adjacent to the metallic surface layers (3) of the structural parts (2) to be joined, an outer cladding layer (10) that comprises soft solder (8).

11. A use of the lead-free solder foil (1) for diffusion soldering according to claim 8, wherein this lead-free solder foil (1) is used as a shaped solder pad (12) in a lead-free soft-soldering process and, in the process, with use of a soldering profile typical of the lead-free soft soldering, the adjacent structural parts (2) are bonded to one another in such a way that the bonding zone (16) has a remelting temperature of higher than 400° C. after the soldering process.

12. The use of the lead-free solder foil (1) for diffusion soldering according to claim 8, wherein the lead-free solder foil (1) is partly disposed at the junctions (15) of a metallic conductor ribbon (13), which functions as an electrical conductor in the product (14) to be joined, in such a way that the conductor ribbon (13) coated partly at its junctions (15) bonds, in a lead-free soft-soldering process, the structural parts (2) to be bonded with the conductor ribbon (13), to one another at the junctions (15) in such a way that, after the lead-free soft-soldering process, the bonding zone (16) has a remelting temperature of higher than 400° C.

Description

[0078] In the following, the approach according to the invention will be explained in more detail on the basis of an exemplary embodiment in conjunction with 5 figures.

[0079] FIG. 1 shows the schematic structure of a semiconductor power switch.

[0080] The chip/semiconductor module 21 is soldered onto a conductor track, i.e. a metallic surface layer 3, which is carried by an electrically insulating layer of ceramic (DCB), the ceramic substrate 20. Its upper side is bonded to another conductor track/metallic surface layer 3 likewise situated on the substrate, which is normally realized in a bonding process using thin aluminum or copper wires/conductor ribbons 13. The ceramic substrate 20 is soldered onto a base plate 19, which is mounted on a heat sink/a cooling block 17. All surfaces/surface layers 3 to be bonded must be metallic, and the bonding zones 16 themselves must ensure heat flow to the heat sink as effectively as possible.

[0081] In the following, the use of the solder foil 1 according to the invention will be explained in more detail in conjunction with the joining process, a diffusion process for construction of the semiconductor power switch illustrated in FIG. 1.

[0082] In this connection, the lead-free solder foil 1 according to the invention in the design as a solder composite material 4 is used on the one hand for achievement of a current terminal of the semiconductor module 21 having a conductor ribbon 13 and on the other hand is also used as a shaped solder pad 12 for soldering of the semiconductor module 21 onto the DCB, the ceramic substrate 20.

[0083] FIG. 2 shows, in a sectional diagram, the arrangement of solder foil 1 in the embodiment as a solder composite material 4 between the metallic surface layers 3, to be bonded, of the joining partners with like or different metallic surfaces/surface layers 3. In the solder composite material 4, particles 6 of copper are distributed dispersedly in a lead-free Sn soft solder matrix 5, wherein the spacing between the particles 6 is smaller than or equal to 10 μm and the uppermost and lowermost layer, the cladding layers 10, are respectively formed by the soft solder 8.

[0084] FIG. 3 schematically represents the arrangement according to FIG. 2 after the soldering process. The Sn soft solder 8 is completely transformed into intermetallic compounds/intermetallic phases 9 having a melting point higher than 400° C., wherein residues (residual metal 22) of the high-melting metal particles 6 of Cu are dispersedly distributed. Thereby it is ensured that the entire bonding zone 16 melts only at temperatures above 400° C. and in addition ensures not only the high electrical conductivity but also a very good thermal conductivity.

[0085] In the following, the lead-free solder foil according to the invention is used in the design as a multi-layer solder foil 11 for system soldering, i.e. in this case for achievement of a solder bond between the DCB, the ceramic substrate 20 and the base plate 19.

[0086] FIG. 4 shows, in a schematic sectional drawing, the arrangement of the solder foil 1 in one possible embodiment as a multi-layer solder foil 11, in the form of shaped solder pads 12, in their location relative to the joint partners, i.e. between the like or different metallic surface layers 3 to be joined of the structural parts to be joined.

[0087] In this multi-layer solder foil 11, two layers of a high-melting metal component 7, such as Cu, for example, the intermediate layers 23, are disposed between three layers of the solder composite material 4. In the solder composite material 4, Cu particles 6 are distributed dispersedly in a lead-free Sn soft solder matrix 5, wherein the spacing between the particles 6 is smaller than or equal to 10 μm, wherein the uppermost and lowermost layer of the multi-layer solder foil 11, the cladding layers 10, are again respectively formed by the soft solder 8.

[0088] FIG. 5 now schematically shows the arrangement according to FIG. 4 after the soldering process. In the material layers comprising solder composite material 4, the Sn soft solder 8 is completely transformed into intermetallic compounds/intermetallic phases 9 having a melting point higher than 400° C., wherein, however, residues of the high-melting metal particles 6 of Cu are also dispersedly distributed.

[0089] Between them, bonded by the intermetallic phases 9, the residual metal 22, such as Cu, for example, of the intermediate layers 23 of the high-melting component 7, is present, whereby the entire bonding zone 16 melts only at temperatures above 400° C., and a very good thermal conductivity and also an adapted resulting thermal expansion of the same are ensured.

[0090] In the following, the soldering process for production of the bonding zones 16, illustrated in FIGS. 3 and 5, comprising the lead-free solder foil 1 according to the invention, will now be explained in more detail.

[0091] For chip soldering, the semiconductor modules 21, such as, for example, Si chips, SiC chips or IGBT modules, are soldered together with a DCB, a ceramic substrate 20. The said semiconductor modules 21 are normally coated with Ni or Ni(Ag), and the DCB, the ceramic substrate 20, is coated with a surface layer 3 of Cu and often additionally also with Ni. Heretofore, usually high-lead-content soldering alloys have usually been used for chip soldering, since their melting temperature ranges from 290° C. to 305° C. and the solder bond created in this way is not intended to remelt, in view of the stage-wise soldering that is standard in series production, wherein the second soldering process for system soldering takes place at temperatures of higher than 240° C. During series production, the chip soldering is usually performed in a first stage, and the system soldering takes place with a lead-free solder in a second stage. Since the high-lead-content solder has a higher melting temperature than the lead-free solder, this stage-wise soldering in the described sequence ensures that the chip-solder bond does not melt during the system soldering.

[0092] According to the present invention, a shaped solder pad 12 of solder composite material 4 having an Sn soft-solder matrix 5 and copper particles 6 distributed dispersedly therein is used for chip soldering, wherein the solder composite material 4 has, adjacent to the metallic surface layers 3 of the structural parts 2 to be joined, an outer cladding layer 10 of soft solder 8, which on one side bears on the metallic surface 3 of the chip/semiconductor module 21 and on the other side of the solder composite material 4 bears on the metallic surface/surface layers 3 of the DCB/of the ceramic substrate 20, i.e. comes in contact with these.

[0093] Compared with the chip-soldering process performed in conjunction with the high-lead-content soldering, a much lower process temperature is possible during use of the approach according to the invention, so that the heating up to 240° C., which is usual in a lead-free soft-soldering process, is sufficient here.

[0094] The Sn soft solder 8 melts at approximately 220° C., the molten phase reacts with the metallic surfaces/surface layer 3 of the adjacent structural parts 2 and within 2 minutes dissolves so much dispersed copper that the molten phase is completely transformed into the solid intermetallic phases 9, i.e. into CuSn3 and Cu6Sn5.

[0095] In this way the pore-free bonding zone 16 is obtained, the melting temperature of which lies above 400° C.

[0096] For system soldering, the DCB, the ceramic substrate 20, which is now already carrying the chip/the semiconductor module 21, is soldered together with the base plate 19. For this purpose, the base plate 19 is normally coated with a surface layer 3 of Cu, Ni, Ni(P) or Ni(Ag), and the DCB/the ceramic substrate 20, is coated with a surface layer 3 of Cu, N, Ni(P) or Ni(Ag).

[0097] According to the invention, a shaped solder pad 12 of multi-layer solder foil 11 is processed in a lead-free soft-soldering process for system soldering. The use of the multi-layer solder foil 11 offers the possibility, via the layer structure of multi-layer solder foil 11, of increasing the mechanical flexibility of the bonding zone 16 obtained after the soldering process. In the present exemplary embodiment, the shaped solder pad 12 consists of layers of a solder composite material 4 containing an Sn soft-solder matrix 5 and particles 6 of a copper metal component 7 distributed dispersedly in this Sn soft-solder matrix 5, wherein these layers alternate with layers of a high-melting metal component 7, such as copper, for example, while the outer layers of the solder composite material 4, the cladding layers 10, consist only of the Sn soft solder 8.

[0098] These outer layers of the solder composite material 4 come into contact with the metallic surfaces/ surface layers 3 of the substrate 20 and of the base plate 19, i.e. of the structural parts 2.

[0099] The Sn soft-solder 8 melts in turn at approximately 220° C. The now molten cladding layer 10 forms, together with the metallizations of the substrate 20 and of the base plate 19, the intermetallic phases 9 of CuSn3 and Cu6Sn5.

[0100] Simultaneously, the soft solder 8, which now is likewise molten, dissolves so much dispersed copper (the particles 6 of the metal component 7) in the multi-layer solder foil 11 within 2 minutes that this is completely transformed into the solid intermetallic phases of CuSn3 and Cu6Sn5. These same phases are additionally formed at the interface with the intermediate layers 23 of the high-melting metal component 7. In this way, a pore-free bonding zone 16, the melting temperature of which lies above 400° C., and which, due to remaining metallic residual layers 22, has an adapted, resulting thermal expansion coefficient, is formed after the soldering process in the region of the originally disposed multi-layer solder foil 11.

[0101] In the prior art, the chip upper side is usually joined (bonded) by fine aluminum or copper wires to the conductor track on the substrate in an ultrasonic welding process. By means of the solder foil according to the invention, this joining method may likewise be replaced by a diffusion soldering process, which takes place by analogy with the aforementioned soldering processes.

[0102] According to the invention, a conductor ribbon 13, comprising an electrical conductor such as aluminum or copper, is used for contacting the chip, and on its two connecting faces to be joined the solder composite material 4 was applied beforehand in such a way that its outer layer, consisting of Sn soft solder 8, contacts the metallic surface layer 3 of the chip/semiconductor module 21 on one side and the metallic surface layer 3 of the DCB/substrate. During heating to the corresponding temperature of a lead-free soft-soldering process, the soft solder 8 of the solder composite material 4 melts.

[0103] In the interior of the solder composite material 4, the now molten soft solder 8 dissolves so much dispersed copper (the particles 6 of the metal component 7) within 2 minutes that it is completely transformed into the solid intermetallic phases of CuSn3 and Cu6Sn5. At the interface to the metallizations (metallic surface layers 3) of the chip upper side and of the substrate, the intermetallic phases CuSn3 and Cu6Sn5 are likewise formed. Thus here also a bonding zone 16 is formed that is equivalent to that in chip and system soldering.

LIST OF REFERENCE SYMBOLS

[0104] 1 Solder foil

[0105] 2 Structural parts

[0106] 3 Surface layer

[0107] 4 Solder composite material

[0108] 5 Soft-solder matrix

[0109] 6 Particles

[0110] 7 Metal component

[0111] 8 Soft solder

[0112] 9 Intermetallic phases

[0113] 10 Cladding layer

[0114] 11 Multi-layer solder foil

[0115] 12 Shaped solder pad

[0116] 13 Conductor ribbon

[0117] 14 Product

[0118] 15 Junctions

[0119] 16 Bonding zone

[0120] 17 Cooling block

[0121] 18 Thermal interface materials

[0122] 19 Base plate

[0123] 20 Ceramic substrate (DCB)

[0124] 21 Semiconductor module (chip)

[0125] 22 Residual metal (high-melting)

[0126] 23 Intermediate layer (high-melting)