Mechanical properties and microstructure of heavy aluminum bonding wires for power applications

Heavy aluminum wire bonding is of great interest as a first level packaging technology for automotive and power electronic devices. The related mechanical and thermo-mechanical cyclic loading conditions require an adequate consideration of reliability aspects and lifetime estimations based on in-depth knowledge of material properties, involving an understanding of the correlation between mechanical deformation behavior and microstructure. In this paper, we present results of mechanical and microstructure investigations using advanced data evaluation approaches. For characterization of static mechanical properties, tensile tests were performed to determine the stress/strain behavior of selected wire materials. The results derived showed a pronounced hardening effect which could be adequately described by classical hardening laws. The grain structure of the wires was analyzed using the electron backscattered diffraction method (EBSD). It is shown, that data statistics and data evaluations have considerable effects on the results represented in terms of grain size. Furthermore, the correlation between deformation properties and microstructure could be described by the Hall-Petch relation predicting a linear dependence of the initial yield stress on the square root of grain size. Since in many cases a knowledge of local changes in the mechanical properties due to either process-induced effects or due to application conditions is required, we also compared results of microindentation and micro compression experiments on small wire cylinders with those of the tensile tests. A very good agreement of the flow curves determined from the micro compression test with the tensile test data was found while the correlation with the indentation results was rather limited. Thus, the newly developed micro compression test is a very powerful tool for characterization of local material properties of bonding wires used in real components. It may also serve as a basis for more sophistica- ted cyclic hardening investigations as it is necessary for establishing advanced lifetime predictions in future.