A study on the fatigue behaviour of electro-plated NiCo thin film for probe tip applications

In this study, fatigue tests were performed for NiCo alloy which is commonly used in probe tips of test machines of semiconductors. The smooth specimens were fabricated by MEMS process including electro-plating process on a single-side polished 6 inches (100) silicon wafer. The NiCo thin film consists of about 66.2 % Ni and 33.8 % Co. The specimen is 2000 μm long, 9 μm thick and 100 μm wide. The tests were performed using a novel test procedure, a structure of the specimen is easy to manipulate, align and grip a thin-film and a test machine proposed by the authors. The closed-loop feedback control of the magnetic-electric actuator allowed load-controlled and high cycle fatigue tests with 5 Hz frequency, in ambient environment and with a stress ratio of R = 0.1. The fatigue strength coefficient and exponent were 1324 MPa and -0.0558, respectively. In a fatigue test of thin film, it is difficult to perform the fatigue test at R = -1. However, the result of the fatigue test at R = -1 should be required when fatigue life prediction is performed under variable loading. Therefore, in this study, the modified Goodman method, Gerber method and Soderberg method were considered to investigate which method is most reasonable when mean stress is considered.