Effect of EFO parameters on Cu FAB hardness and work hardening in thermosonic wire bonding

Effects of the electrical flame off (EFO) parameters on the hardness and work hardening properties of Cu free air balls (FABs) are reported and compared to those of Au. Cu FABs were characterised using an online deformability method that measures the insitu deformability of the Cu FAB and by traditional off-line Vickers microhardness testing of cross-sectioned samples. The online deformability measurements are about twice as precise as the microhardness measurements. Cu FABs produced using EFO current (IEFO) and EFO time (tEFO) of 250 mA and 0.118 ms, respectively, have 14.29 HV (21%) higher hardness than those produced with IEFO = 45 mA and tEFO = 1.03 ms. While the Cu FABs with IEFO = 250 mA are the hardest, they are also the most deformable and end up the softest after bonding due to less work hardening than observed with lower IEFO FABs.