Effect of EFO settings on microstructure and hardness in the FAB of copper bonding wire

Continuously increasing price of gold becomes the major driving force for substituting the gold wire with copper wire in the field of wire bonding. However, since the hardness of copper is higher than that of gold, the potential risk for copper wire bonding is that Cu FAB would cause serious splash on aluminum pad and, even worse, damage of Si die. Since free air ball (FAB) formation is controlled by an electric flame-off (EFO) process, EFO settings would have an influence on the FAB microstructure, thus fabricating FAB with different mechanical properties. However, few researches have focused on the correlation between EFO settings and mechanical properties of FAB, let alone the microstructure evolution in the FAB. In the present paper, we try to find the EFO dependence of hardness in copper FAB. Since the microstructure determines the material´s properties, the effect of EFO settings on microstructure evolution is studied to try to give an explanation on the FAB hardness variation. Columnar grains in the FAB would make the material properties of FAB anisotropic. Since the FAB hardness along the wire drawing direction has a direct influence on bonding pads, FAB cross-sections are acquired perpendicular to the wire drawing direction. Electron backscatter diffraction (EBSD) technology is employed to investigate the microstructure evolution. In the orientation imaging microscopic (OIM) maps, subgrain boundaries are only found at the peripheries of FAB, mainly due to the quench effect by shield gas. Low EFO current results in larger grain size in both FAB and ball neck. Besides, low EFO current produces FAB with larger Taylor factor in the wire drawing direction. Vickers hardness tests are conducted on these cross-sections. Surprisingly, EFO settings have less influence on the FAB hardness in the wire drawing direction. This phenomenon may be attributed to the trade-off between the grain size strengthening and texture hardening.