Characterization of Rate-Dependent Constitutive Behavior of Copper Free-Air Ball and Its Use for Wire Bonding Process Simulation

Copper (Cu) wire bond technology is an increasingly popular alternative to gold (Au) wire bond in microelectronic devices due to its superior electrical performance and low cost. Cu wires are also approximately 25% more thermally conductive than gold wires, resulting in better heat dissipation. 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 wire bonding process and the ability to assess risk of fracture in ultralow-k dielectric stacks. The challenge to FAB characterization is primarily due to the difficulty in performing mechanical tests on spherical FAB of micrometer size. To address this challenge, we describe a custom-built microscale tester and carry out compression tests on the FAB in this paper. The Anand viscoplastic constitutive model is developed for Cu FAB through an inverse modeling procedure. In the inverse procedure, constitutive model parameter values are iterated through an automated optimization procedure, until the load-displacement response matches the experimentally observed response. Using the estimated Anand model parameters, a finite-element analysis of the impact and ulatrasonic vibration stages of wire bonding process is carried out, and the resulting dielectric fracture risk is analyzed.