A Simple Self-Exciting Vibration High Cycle Fatigue Tester for Accelerated Stress Testing of Thin Films

We developed a simple and inexpensive feedback regulated high cycle fatigue (HCF) testing device intended to simplify the reliability assessment of macroscopic thin metal film specimens taken right out of the fabrication line of electronic devices and proved its effectiveness in inflicting fatigue damage in thin copper films under HCF regime within a reasonable amount of time. A cm-sized steel cantilever carrying a macroscopic thin film specimen is brought to its resonance vibration by cyclic excitation using an electromagnetic (EM) coil. The reflection of a laser beam, off the surface of the cantilever tip and some optical elements and a photodiode is used to calculate vibration amplitude using optical principles and a feedback control loop regulates that vibration amplitude during fatigue experiment. As a testing sample, an organic board with electroplated copper thin film is fatigue loaded using the developed device and intermittently studied under optical microscope (OM). The OM images show the fatigue damage evolution manifested as dark patches on the film surface and also development of fatigue macro cracks and their growth. The scanning electron microscope (SEM) micrographs of the sample at crack points confirm that only the copper film suffered fatigue cracking and the organic board underneath remained intact.