Characterization of cu free air ball constitutive behavior using microscale compression test

In recent years, there is increasing interest in copper wirebond technology as an alternative to gold wirebond in microelectronic devices due to its superior electrical performance and low cost. At present, validated constitutive models for the strain rate and temperature dependent behavior of Cu free air ball (FAB) appear to be largely missing in the literature. The lack of reliable constitutive models for the Cu FAB has hampered the modeling of the wirebonding process and the ability to assess risk of fracture in ultra low-k dielectric stacks. The challenge to FAB characterization is primarily due to the difficulty in performing mechanical tests on spherical FAB of micrometers in size. To address this challenge, we perform compression tests on FAB using custom-built microscale tester in the current study. Specifically, the tester has three closed-loop controlled linear stages with submicron resolutions, a manual tilt stage, a six-axis load cell with sub-Newton load resolution for eliminating misalignment, a milliNewton resolution load cell, a capacitance sensor to estimate sample deformation and to control the vertical stage in closed loop, a high working depth camera for viewing the sample deformation, and controllers for the stages implemented in the LabVIEW environment. We compress the FAB between tungsten carbide punches and develop a constitutive model for the copper of FAB through an inverse modeling procedure. In the inverse procedure, the assumed constitutive model parameter values are iterated until the load-displacement response matches the experimentally observed response.