WO2005057731A1

WO2005057731A1 - Tin coating for contact purposes

Info

Publication number: WO2005057731A1
Application number: PCT/FI2004/000744
Authority: WO
Inventors: Tag Hammam; Richard Mac Key
Original assignee: Outokumpu Copper Products Oy
Priority date: 2003-12-09
Filing date: 2004-12-08
Publication date: 2005-06-23
Also published as: FI20031791A0; FI20031791A; FI118874B

Abstract

The invention relates to a tin coating to be used in an electric connector. According to the invention at least one oxidizing agent is added to the tin in order to achieve an essentially tin oxide free metal-to-metal contact.

Description

TIN COATING FOR CONTACT PURPOSES

The invention relates to a tin coating to be used in an electric connector in which coating at least one oxidizing agent is added to the tin to achieve an essentially tin oxide free metal-to-metal contact.

The ultimate connection between two conductors is a welded joint. A cold welded spot will restrict detrimental microsliding and wear during vibration. Moreover, a cold welded contact spot will prevent the ingress of air or moisture, which could cause corrosion. Provided that the cold welded contact spot can be maintained the contact resistance will be low and stable during a long term operation.

With a soft coating, e.g. tin, tin alloys or silver coatings, it is possible to achieve a cold welded contact spot during insertion. In order to achieve a cold welded contact spot it is of decisive importance to achieve an oxide-free metal-to-metal contact at the interface during insertion. A high contact force and a thick and soft coating will in general promote a cold welded contact spot, since any free space at the interface is restricted, and subsequently a gas-tight joint contact spot is achieved during insertion.

The trend with increasing number of contact terminals in a connector requires a reduced insertion force for each contact terminal. A reduced insertion force is in general achieved by using a low contact force in combination with a soft thin coating.

The FI patent application 20012453 relates to a contact terminal to be used for electrical purposes, which contact terminal contains a metallic substrate with good conductivity and coated with a metallic element. The coating in the contact terminal is doped with at least one additive, such as phosphorus in order to improve the electrical stability of the coating. With a higher hardness of the thin coating in combination with a low contact force will result in infinitesimal small spaces at the contact interface containing oxygen. This result in a thin surface oxides at parts of the contact interface, with a subsequently mechanical weak cold-welded contact spot. This cold-welded contact spot will easily be ruptured when subjected to cyclic mechanical stress, such as vibration.

The object of the present invention is to eliminate some drawbacks of the prior art and to achieve an improved tin coating to achieve an essentially tin oxide free metal-to-metal contact in electric contact purposes by adding an oxidizing agent to the tin coating. The essential features of the invention are enlisted in the appended claims.

According to the invention at least one oxidation agent is added to the tin coating to achieve an essentially tin oxide free metal-to-metal contact in an electric connector during insertion. In order to achieve the essentially oxide free metal-to-metal contact the preferential oxidation agent in the tin coating will react with the possible residual oxygen left at the contact interface before the tin starts to form a tin oxide with this possible residual oxygen at the contact interface.

The additive to the tin metal coating suitable as an oxidation agent in accordance with the invention is at least one element of the group containing manganese, titanium, lithium or boron. The amount of the oxidation agent in the tin metal coating is between 0.02 % and 1 % by weight. In the embodiment of the invention when two or more elements from that group are added to the tin metal coating, the amount of the oxidation agent is the total amount for all these elements.

The addition of the oxidation agent to the tin metal coating in order to achieve an essentially tin oxide free metal-to-metal contact between the base metal and the tin metal coating is effective so that the formation of oxide of the solute additive is thermodynamically favoured over the formation of the tin oxide. The oxide of the additive will then be formed before the possible oxygen in the contact interface will react with the tin metal. The additive suitable as the oxidation agent in accordance with the invention has also a property that the additive forms discrete oxide particles in the initial stage of the oxidation. These discrete oxide particles further prevent the formation of a continuous oxide film on the contact interface. When the formation of the continuous oxide film on the contact interface is prevented in accordance with the invention, the formation of a mechanical strong cold welded contact spot is promoted during insertion. Thus a significant improvement in the performance of a contact terminal is achieved when comparing with a conventional tin coating.

The invention is described in more details referring in the attached drawing in which Fig. 1 shows the test results for the contact voltage drop in fretting tests as a function of time. The following calculations in Table 1 show how stable the present oxides are at T=25 °C. As lower as the value of the standard heat of formation of the compound (ΔH_T) value is for the oxide of the additive compared to the tin oxide as more thermodynamically favoured is the oxidation of the additive. The software used is Thermo-Calc and the thermodynamic database is the SGTE1 (Scientific Group Thermodata Europe) substance database.

Table 1.

The present invention to use a preferential oxidation agent in the tin coating to restrict the formation of tin oxide at the contact interface was tested in the fretting tests in a test bench, which consists of an electronic controlled shaker and a measurement system. Before the fretting tests all contacts were subjected to one long sliding stroke to wipe off the initial surface layer. During the fretting tests, the contacts were subjected to a vibration displacement stroke of 20 μm at a frequency of 100 Hz, and also a current loads of 2 A DC. The contact geometry was crossed cylinders with a diameter of 10mm.

In the test the samples constituted of solid rods in order to avoid uncontrolled influence of the substrate material during fretting. Tin with 0.04 % by weight titanium was tested.

According to Fig.1 the contact voltage drop was tested in fretting tests as a function of time. A normal load of 30 N was applied. As shown the increase in contact resistance is significant restricted when a preferential oxidation agent is used, due to the formation of tin oxide at the contact interface is restricted.

Claims

1. Tin coating to be used in an electric connector, characterized in that at least one oxidizing agent is added to the tin in order to achieve an essentially tin oxide free metal-to-metal contact.

2. Tin coating according to the claim 1 , characterized in that the amount of the oxidizing agent is between 0,02 % and 1 % by weight.

3. Tin coating according to the claim 1 or 2, characterized in that the oxidizing agent forms discrete oxide particles in the initial stage of the oxidation.

4. Tin coating according to any of the preceding claims, characterized in that the oxidizing agent is manganese.

5. Tin coating according to any of the preceding claims 1 to 3, characterized in that the oxidizing agent is titanium.

6. Tin coating according to any of the preceding claims 1 to 3, characterized in that the oxidizing agent lithium.

7. Tin coating according to any of the preceding claims 1 to 3, characterized in that the oxidizing agent is boron.

8. Tin coating according to the claim 1 , characterized in that the oxidizing agent is at least two elements from the group of manganese, titanium, lithium or boron.