Metal thin films are used as major components in microelectromechanical systems (MEMS). However, problems with long-term reliability that set limits on the lifetime of MEMS applications have been. observed. The more efficiently the thin films resist stress relaxation, the longer the lifetime of the MEMS device. In particular, Al thin films have been used as capacitance switches, but are shown to have a low resistance to stress relaxation, thus hindering overall performance and leading to shorter lifetime. Using bulge testing, this study investigates the viscoelastic behavior of pure Al thin film in comparison with thin Al alloy films with 12% and 16% Mg. The results show that the addition of Mg to Al films significantly decreases the relaxation behavior and increases the strengthening mechanism of such thin films. The greater the Mg content in an Al film, the greater the resistance of the film. The normalized modulus decreases less with a greater Mg content. Al-Mg thin films have better relaxation resistance than pure Al thin films and thus serve as a better material for MEMS capacitance switches. (C) 2016 Elsevier B.V. All rights reserved.