COPPER STRIP FOR MAKING ELECTRICAL CONTACTS AND PROCESS FOR PRODUCING A COPPER STRIP AND CONNECTOR

Abstract

The present invention concerns a copper strip for the making of electrical contacts, with a base material of copper or a copper alloy and a coating of tin, wherein the coating of tin has a proportion of insoluble, precipitation-forming elements of 0.003 to 1.0, preferably up to 0.5, preferably up to 0.05, particularly preferably up to 0.01 percent by weight. The invention further relates to a process for producing a copper strip for the manufacture of electrical contacts and a connector.

Claims

1-14. (canceled)

15. A copper strip for making electrical contacts, comprising: a base material of copper or a copper alloy; and a coating of tin, wherein the coating of tin has a proportion of insoluble, precipitation-forming elements of 0.003 to 1.0 weight percent.

16. The copper strip according to claim 15, wherein the proportion of insoluble, precipitation-forming elements is up to 0.5 weight percent.

17. The copper strip according to claim 15, wherein the proportion of insoluble, precipitation-forming elements is up to 0.05 weight percent.

18. The copper strip according to claim 15, wherein the proportion of insoluble, precipitation-forming elements is up to 0.01 weight percent.

19. The copper strip according to claim 15, wherein the insoluble, precipitation-forming elements are formed by one or more of the following elements: silver, germanium, nickel, and cobalt in a proportion of 0.003 to 0.5 weight percent each.

20. The copper strip according to claim 15, wherein the insoluble, precipitation-forming element silver is present in the coating of tin in a proportion of 0.08 to 0.5 weight percent in the form of an Ag.sub.3Sn phase.

21. The copper strip according to claim 20, wherein the Ag.sub.3Sn phase is present in the coating of tin in particle sizes with a mean area value of 0.01 to 0.03 m.sup.2.

22. The copper strip according to claim 20, wherein the Ag.sub.3Sn phase is present in the coating of tin in particle sizes with a mean area value of 0.0140 to 0.0180 m.sup.2.

23. The copper strip according to claim 20, wherein the Ag.sub.3Sn phase is present in the coating of tin in particle sizes with an average circumference of 0.2 to 0.8 m.

24. The copper strip according to claim 20, wherein the Ag.sub.3Sn phase is present in the coating of tin in particle sizes with an average circumference of 0.4 to 0.6 m.

25. The copper strip according to claim 20, wherein in the coating of tin and/or the base material a portion of the copper is present in a Cu.sub.6Sn.sub.5 phase, and the Ag.sub.3Sn phase is present in the area of or envelops the Cu.sub.6Sn.sub.5 phase.

26. The copper strip according to claim 25, wherein the Cu.sub.6Sn.sub.5 phase is arranged adjacent to an Ag.sub.3Sn phase in or adjacent to a boundary layer between the base material and the coating of tin.

27. The copper strip according to claim 15, wherein the insoluble, precipitation-forming elements are formed by one or more of the following elements: antimony and bismuth in a proportion of 0.02 to 1.0 weight percent each.

28. The copper strip according to claim 15, wherein the coating of tin has a higher hardness than the base material.

29. A process for producing a copper strip for the manufacture of electrical contacts, comprising: providing a base material of copper or a copper alloy; and applying a coating of tin to the base material in a dipping process and/or an electroplating process, wherein the coating of tin has a proportion of insoluble, precipitation-forming elements of 0.003 to 1.0 percent by weight.

30. The process according to claim 29, wherein the insoluble, precipitation-forming elements are deposited on the base material in a first step in the electroplating process, and the coating of tin is applied in a second step by the dipping process.

31. The process according to claim 29, wherein the coating of tin is applied by the electroplating process and is subjected to a reflow treatment after application.

32. The process according to claim 29, wherein the copper strip is exposed to air.

33. A connector made from a copper strip according to claim 15.

34. A connector made from a copper strip produced by a process according to claim 29.

Description

[0026] The invention is explained in more detail below on the basis of preferred forms of execution with reference to the attached figure. Show

[0027] FIG. 1 a section through a copper strip with a tin coating according to the invention; and

[0028] FIG. 2 a cutout of a microstructure of the coating formed by the alloy of the invention in an enlarged view; and

[0029] FIG. 3 different microstructures near the boundary layer and at a distance from the boundary layer after different cooling processes.

[0030] FIG. 1 shows a cutout of a copper strip 1 according to the invention for the manufacture of electrical contacts with a tin (Sn) coating 3, 4 on both sides in sectional view. The copper strip 1 has a base material 2 made of a known copper alloy, which does not have to meet any special requirements for the realization of the solution according to the invention and in particular does not have to be modified.

[0031] The base material 2 is coated on both sides in a dipping process with a coating 3, 4 consisting of a thin layer of tin (Sn). This tin contains the proportion of silver suggested by the invention. Silver is an element which is insoluble in tin even in the smallest amounts and thus deliberately forms precipitates, in this case the intermetallic intermediate phase Ag3Sn. The intermetallic intermediate phase is formed during the application of tin in the dipping process, when the tin solidifies on the base material. Furthermore, the particle size and shape can also be influenced subsequently, e.g. by thermal treatment. The quantity ratio of silver to tin is such that the weight proportion of silver in relation to tin is less than 1.0, preferably less than 0.5 weight percent. For cost reasons, the weight percentage of the expensive silver can be further reduced to below 0.1 weight percent or even below 0.05 weight percent, preferably to 0.01 weight percent. Even these very small amounts of silver in the tin coating are sufficient to form a sufficient intermediate phase Ag3Sn, which inhibits the diffusion of the copper and thus the formation of the Cu3Sn phase, thus increasing the adhesive strength due to the avoided Kirkendall pores.

[0032] Antimony (Sb), germanium (Ge), nickel (Ni), bismuth (Si) and cobalt (Co) can also be considered as alternative insoluble, precipitation-forming elements. The term insoluble and precipitation-forming is used to describe the property of producing precipitates in the coating, which can be formed either by intermetallic intermediate phases of the element together with the tin or by precipitates of the elements themselves. These precipitates serve to inhibit diffusion and the resulting formation of the Cu3Sn phase, which is responsible for the formation of the Kirkendall pores, so that the added elements reduce the formation of Kirkendall pores and increase the adhesive strength.

[0033] The precipitation-forming, insoluble element silver has been applied to the base material 2 by a dipping process, preferably by a hot-dip tinning process. However, it is also conceivable to apply the silver and tin in one or two steps, each in an electroplating process and/or in a combination of a dipping process and an electroplating process.

[0034] In this case, the coating 3, 4 can additionally be re-melted for a short time in a reflow process, whereby unfavorable structural properties can be eliminated.

[0035] FIG. 2 shows an enlarged microstructure of the tin-based coating, which can be achieved by adding silver in a proportion of 0.08 to 0.5 weight percent in coating 3, 4. Due to the addition of silver an Ag3Sn phase 6 is formed in the coating 3, 4 and especially in the boundary layer 5 to the base material 2, which can be seen in FIG. 3, which forms itself preferably around the Cu6Sn5 phase 5. The Ag3Sn phase 6 thus forms a kind of barrier or inhibition layer between the Cu6Sn5 phase 5 and the tin 7, which prevents or at least inhibits the formation of the Cu3Sn phase that causes the Kirkendall pores.

[0036] The Ag3Sn phase 6 forms by the addition in the proposed proportion range grains with a grain size with a mean area value of 0.01 to 0.03 m.sup.2, preferably of 0.0140 to 0.0180 m.sup.2 and an average circumference of 0.2 to 0.8 m, preferably of 0.4 to 0.6 m, which is optimal for the purpose of inhibiting the Ag3Sn phase while at the same time keeping the amount of silver as low as possible. The Ag3Sn phase 6 inhibits the diffusion of the copper necessary for the formation of the Cu3Sn phase in the area adjacent to the Cu6Sn5 phase 5, so that the formation of the Cu3Sn phase is inhibited. It is not inevitably necessary for the purpose to be achieved that the Cu6Sn5 phase 5 is completely envelooped by the Ag3Sn phase 6. It is also not a disadvantage if single grains of Cu6Sn5 phase 5 are not enveloped by Ag3Sn phase 6.

[0037] FIG. 3 shows different microstructures of Cu6Sn5 phase 5 and Ag3Sn phase 6, which are formed in the upper two illustrations by cooling with water, in the middle illustrations by cooling in air and in the lower figures by cooling in a furnace. In the left images, the microstructure can be seen at a distance from a Cu6Sn5 phase 5 not shown, while in the right images the microstructure near the Cu6Sn5 phase 5 can be seen.

[0038] In the upper illustrations it can be seen that the Ag3Sn phase 6 is formed in a particle structure between a residual portion 7 of the tin during rapid cooling or ageing by water, whereby the microstructure near Cu6Sn5 phase 5 is even finer. In the middle illustration, the microstructure can be seen after aging of copper strip 1 in air, in which the remaining portion 7 of tin has grown into coarser grains due to the longer cooling phase, and the Ag3Sn phase 6 is arranged in a fine needle structure between the remaining portion 7 tin. As can be seen in the middle, right illustration, the needle structure of Ag3Sn phase 6 is more pronounced near Cu6Sn5 phase 5. In the lower illustration it can be seen that the Ag3Sn phase 6 also grows into a more pronounced needle structure during aging in the furnace, i.e. with even longer cooling times, and that it also grows into a more pronounced needle structure.

[0039] The microstructures show that a desired needle-like morphology of Ag3Sn phase 6 and a sufficient encapsulation of Cu6Sn5 phase 5 by Ag3Sn phase 6 can be achieved by aging in air. Furthermore, since ageing in air is the most cost effective method of ageing, ageing in air is the preferred method for this purpose.

[0040] The boundary layer here is formed by a layer of Cu6Sn5 phase 5 and has a thickness of 10 to 100 m. Depending on the amount of silver, this boundary layer can be thicker or thinner, and it can contain additional grains of Ag3Sn phase 6.

[0041] Another positive effect of adding the insoluble, precipitation-forming elements is that the hardness of the coating 3,4 is increased after thermal treatment or ageing and is ideally higher than the hardness of the base material 2. If silver is added as an insoluble, precipitation-forming element, the Ag3Sn phase 6 is formed during ageing according to the principle described above and thus leads to an increase in hardness. Preferably, the ageing is carried out in a temperature range of 80 to 150 C. over a period of 500 to 1500 hours to achieve the desired effect and the formation of the microstructure.