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Local hardening behavior of free air balls and heat affected zones of thermosonic wire bond interconnections

The local deformation behavior of the free air ball (FAB) and the heat affected zone (HAZ) of thermosonic wire bond interconnections is of great interest in reliability considerations for current highly integrated microelectronic devices. The mechanical properties of the HAZ have significant influence on the loop stability, which is very critical in fine pitch and long loop applications. On the other hand, knowledge of the hardening behavior of the FAB is essential to avoid chip damage risks, particularly if bonding is applied to state-of-the-art ICs containing mechanically sensitive low K dielectric materials. The significance of this reliability risk is even more increased if gold wires were replaced by copper wires. In this study, we characterized the hardening behavior of FABs from three typical gold bonding wires using a modified micro compression test. The stress/strain behavior was calculated via inverse finite element simulations from the experimental force/displacement curves. The mechanical properties of the FABs were compared to these of the related HAZ and the unaffected wire, and were correlated to the wire microstructure that was investigated by electron backscatter diffraction (EBSD). Since the EBSD results for the FAB grain structure indicate a possible anisotropic behavior, a capillary compression test setup was developed allowing to mechanically characterize the FAB in a loading situation comparable to the bond process itself and the results were compared to the experiments with a loading in perpendicular direction. Both these approaches allow an extended characterization of bonding wires considering also the FAB properties, and support therefore material selection for application and wire material development.