Mechanical characterization of gold and copper free air balls in thermosonic wire bond interconnections

In current highly integrated microelectronic devices, the thermosonic wire bond interconnections play a fundamental part in terms of reliability considerations. To avoid chip damage risks, especially if bonding is applied to state-of-the-art ICs containing mechanically sensitive low-K dielectric materials, knowledge of the local deformation behavior of the free air ball (FAB) is essential. The significance of damage risks increases even more if gold wires are to be replaced by copper wires. In this study, the hardening behavior of FABs from two typical gold bonding wires and one copper wire was characterized using a modified micro compression test. In this test, the FAB has been loaded in a direction perpendicular to the wire axis. We calculated the stress/strain behavior via parameter identification using finite element simulations from the experimental force/displacement curves. The mechanical properties of the FABs were compared to those of the unaffected wire. To mechanically characterize the FAB in a loading situation rather comparable to a real bond process, a capillary compression test setup was additionally developed. Both approaches allow an extended deformation characterization of bonding wires considering also the FAB properties. Therefore, they support material selection for wire material development and wire bonding application.