Evaluation of the low-cycle fatigue strength of Sn3.0Ag0.5Cu solder at 313 and 353 K using a small specimen

Abstract

Abstract This study performs fatigue tests by using a small specimen of Sn3.0Ag0.5Cu solder at 313 and 353 K. The specimen has a 3 mm-diameter gage section and a gage length and total length of 6 and 55 mm The specimen is heated in an electric furnace to control the temperature within ±1 K from the target temperature. The strain ranges employed are 0.5 %, 0.7 %, 1.0 %, and 1.5 %. A triangular waveform with 0.5 % × s−1 strain rate is adopted. The number of cycles to failure (failure life) Nf is defined as the number of cycles that causes a 25 % decrease of stress amplitude from that at the middle of the lifespan or at specimen breakage. The relationship between the strain range and failure life at 313 and 353 K coincides well, and no variation of the failure life with the test temperature is identified. Stress amplitudes decrease with increasing cycles and decrease faster for higher strain ranges because of higher crack propagation rates. The degree of relaxation in the stress amplitude is larger at 353 K than at 313 K. Crack observation of the fatigued specimens is performed to discuss the relationship between crack propagation and the failure life of solders. The relationship among slips, microcracks, and main cracks is discussed, and the mechanisms for fatigue failure are proposed. Almost no change in the crack morphology is observed at 313 and 353 K, and this result is related to the lack of changes in the failure mechanism because of the change in the test temperature.