Sn-plated copper alloy strip having improved fatigue characteristics

1. An Sn-plated copper alloy strip comprising an Sn or Sn alloy plating film formed on the surface of a copper alloy strip by electroplating and reflowing treatment, wherein the average hydrogen concentration in the Sn-plated copper alloy strip is 0.1 to 2 mass ppm, and the fatigue limit is about 300 to about 500 MPa when measured by:

taking a rectangular piece with 10 mm width such that the longitudinal direction of the piece is parallel with a rolling direction of the strip;

applying alternating bending stresses repeatedly on the piece in a direction perpendicular to a thickness of the piece, and determining a number of vibrations repeated until the piece is snapped, the vibration waveform being 50 Hz sine wave, the stress σ applied to the piece being calculated by the equation: L=(3·t·E·f/(2·σ) 1/2 , where:

L is the length of the vibrating section, t is the thickness of the piece, f is the amplitude of the vibration, and E is Young's modulus;

calculating N for different values of σ to determine an SN curve;

determining from the SN curve a value of σ at which N=10 million; and

taking the value of σ at that point as the fatigue limit.

2. The Sn-plated copper alloy strip of claim 1 , wherein the plating film comprises an Sn phase and an Sn—Cu alloy phase in this order from the plating film surface toward inside, and the thickness of the Sn—Cu alloy phase being about 0.1 to about 2.0 μm.

3. The Sn-plated copper alloy strip of claim 1 , wherein the plating film comprises an Sn phase, an Sn—Cu alloy phase and a Cu alloy phase in this order from the plating film surface toward inside, the thickness of the Sn—Cu alloy phase being about 0.1 to about 2.0 μm, and the thickness of the Cu alloy phase being up to about 2.0 μm.

4. The Sn-plated copper alloy strip of claim 1 , wherein the plating film comprises an Sn phase, an Sn—Ni alloy phase, and a Ni alloy phase in this order from the plating film surface toward inside, the thickness of the Sn—Ni alloy phase being about 0.1 to about 2.0 μm, and the thickness of the Ni alloy phase being up to about 2.0 μm.

5. The Sn-plated copper alloy strip of claim 1 , wherein the plating film comprises an Sn phase, a Cu—Sn alloy phase, and a Ni alloy phase in this order from the plating film surface toward inside, the thickness of the Cu—Sn alloy phase being about 0.1 to about 2.0 μm, and the thickness of the Ni alloy phase being about 0.1 to about 2.0 μm.

6. The Sn-plated copper alloy strip of any one of claims 1 to 5 , wherein the copper alloy strip comprises about 1.0 to about 4.5 mass % of Ni, about 0.2 to about 1.2 mass % of Si, optionally one or more elements selected from the group consisting of Sn, P, Zn, Ti, Mg, Al, Zr, Cr, Co, Mo, Fe, Ag and Mn in about 2.0 mass % or less in total, and the balance being copper and unavoidable impurities.

7. The Sn-plated copper alloy strip of any one of claims 1 to 5 , wherein the copper alloy strip comprises about 22 to about 40 mass % of Zn, optionally one or more elements selected from the group consisting of Ni, Si, Sn, P, Ti, Mg, Al, Zr, Cr, Co, Mo, Fe, Ag and Mn in about 2.0 mass % or less in total, and the balance being copper and unavoidable impurities.

8. The Sn-plated copper alloy strip of any one of claims 1 to 5 , wherein the copper alloy strip comprises about 2 to about 22 mass % of Zn, optionally one or more elements selected from the group consisting of Ni, Si, Sn, P, Ti, Mg, Al, Zr, Cr, Co, Mo, Fe, Ag and Mn in about 2.0 mass % or less in total, and the balance being copper and unavoidable impurities.