Abstract
An electrically conducting material including a substrate composed of copper or a copper alloy, and a coating composed of at least one layer. The coating has an outermost layer consisting to an extent of at least 90 Vol % of an intermetallic phase which is or includes Cu.sub.6Sn.sub.5. The surface of the outermost layer that faces away from the substrate has insular, silver-rich precipitations with an area fraction of 7 to 20%.
Claims
1. An electrically conducting material comprising a substrate composed of copper or a copper alloy, and a coating composed of at least one layer, where the coating has an outermost layer consisting to an extent of at least 90 vol % of an intermetallic phase which is or comprises Cu.sub.6Sn.sub.5, wherein the surface of the outermost layer that faces away from the substrate has insular, silver-rich precipitations with an area fraction of 7 to 20%.
2. The electrically conducting material according to claim 1, wherein more than 50% of the area fraction of the insular, silver-rich precipitations is formed by precipitations which have a size of at least 0.4 μm.
3. The electrically conducting material according to claim 1, wherein between the outermost layer and the substrate there is an interlayer which comprises a copper-rich intermetallic phase.
4. The electrically conducting material according to claim 3, wherein the coating consists of the interlayer and of the outermost layer with the insular, silver-rich precipitations.
5. The electrically conducting material according to claim 1, wherein the fraction of silver in the coating is 2 to 10 wt % of the total amount of tin and silver in the coating.
6. The electrically conducting material according to claim 3, wherein the copper-rich intermetallic phase is or comprises CU.sub.3Sn.
Description
[0022] The invention is elucidated in more detail using schematic drawings and surface pictures.
[0023] FIG. 1 shows a cross section of a material with a tin-silver layer according to the prior art
[0024] FIG. 2 shows an SEM picture of the surface of a tin-silver layer according to the prior art
[0025] FIG. 3 shows a cross section of a material with a coating according to the invention
[0026] FIG. 4 shows an SEM picture of the surface of a coating according to the invention.
[0027] Parts corresponding to one another are provided with the same reference symbols in all of the figures.
[0028] FIG. 1 shows schematically a cross section through a material 1 having a layer 5, known from the prior art, composed of a tin-silver alloy with approximately 4 wt % of silver. The layer 5 was applied to the substrate 10 by hot-dip galvanization. The layer 5 consists of a matrix of free tin 51 and finely distributed Ag.sub.3Sn precipitations 52 embedded therein. In FIG. 1, these finely distributed Ag.sub.3Sn precipitations 52 are represented symbolically by the white dotting of the layer 5, whereas the tin 51 is represented by the black background. During the hot-dip galvanization itself, tin is transformed, at the interface between substrate 10 and the tin-silver alloy layer 5, into a relatively thin, tin-rich, intermetallic phase 41 having the composition Cu.sub.6Sn.sub.5.
[0029] FIG. 2 shows a picture generated using a scanning electron microscope (SEM picture) of the surface of a tin-silver layer 5 according to the prior art. FIG. 2 therefore represents, so to speak, the outer surface of the material 1 of FIG. 1. The Ag.sub.3Sn precipitations 52 are apparent as small light dots or regions, which are present in fine distribution in the darkly represented matrix of free tin 51. The area fraction of the Ag.sub.3Sn precipitations 52 is approximately 4%. The size of the majority of the Ag.sub.3Sn precipitations 52 is below 0.3 μm.
[0030] FIG. 3 shows schematically a cross section through a material 1 having a coating 2 according to the invention. The coating 2 has an outer layer 21 and an interlayer 22 composed of copper-rich intermetallic phase 42 Cu.sub.3Sn, which is located between the substrate 10 and the outer layer 21. The outer layer 21 contains intermetallic phase 41 with the composition Cu.sub.6Sn.sub.5. Located on the surface of the outermost layer 21 facing away from the substrate 10 are insular, silver-rich precipitations 3. These precipitations 3 are embedded at least partly in the outermost zone of the intermetallic phase 41. They partially protrude beyond the outer surface of the intermetallic phase 41.
[0031] FIG. 4 shows a picture generated using a scanning electron microscope (SEM picture) of the surface of the outermost layer 21 of a coating 2 according to the invention. FIG. 4 therefore represents, so to speak, the outer surface of the material 1 of FIG. 3. The silver-rich precipitations 3 are apparent as light regions which are arranged like islands on the darkly represented intermetallic phase 41. The area fraction of the silver-rich precipitations 3 is approximately 11%. The size of the majority of the silver-rich precipitations 3 is between 0.4 and 3 μm.
[0032] The coating represented in FIG. 4 was produced by subjecting the material 1 represented in FIG. 2, having a 3 μm layer of a tin-silver alloy with 4 wt % of silver, to a heat treatment at 200° C. for a duration of 30 hours. In laboratory experiments which imitate the use of the material as a plug-in connector, a coating produced in this way exhibits a contact resistance which is approximately at the same low level as that of a tin-silver layer which has not been subjected to any heat treatment. On the other hand, the insertion forces and pulling forces ascertained for the coating according to the invention were up to 25% lower than those ascertained on a non-heat-treated sample.
List of Reference Symbols
[0033] 1 material
[0034] 10 substrate
[0035] 2 coating
[0036] 21 outermost layer
[0037] 22 interlayer
[0038] 3 silver-rich precipitations
[0039] 41 intermetallic phase
[0040] 42 copper-rich intermetallic phase
[0041] 5 layer of tin-silver alloy
[0042] 51 tin
[0043] 52 Ag.sub.3Sn precipitations
