Fatigue response of polyimide thin film under cyclic loading

Fatigue characteristics of polyimide thin film strips prepared with realistic wafer-level redistribution processes were investigated experimentally. Uniaxial tensile tests were first conducted on the thin film specimens to characterize the stress-strain relationship, and to measure ultimate strength and elongation. Both strain- and stress-controlled fatigue cycling experiments were then performed. Under strain-controlled cyclic fatigue loading an obvious stress relaxation behavior was observed. The stress relaxation characteristic depends only on the applied strain range, but not on the level of the average strain. Under stress-controlled cyclic fatigue loading the polyimide thin film exhibited both viscoelastic and plastic responses, and the peak plastic strain followed a power-law increasing trend as the fatigue cycle increased. A fatigue strain evolution model was developed by statistically fitting the stress-controlled fatigue responses with a physics-based mathematical model. The fatigue response model can be applied for developing design rules for extending the polyimide based redistribution interconnect technology to the regimes of larger chip size and higher input/output densities.