Silver pick-up during tail formation and its effect on free air ball in thermosonic copper ball bonding

The tail breaking process is studied by investigating tail bond imprint micrographs. The imprints were made on Ag metallization (diepad) with Cu wire (Cu-Ag) and with Au wire (Au-Ag). In the Au-Ag case, Au wire residue was found on the diepad indicating fracture occurred in the Au wire. In the Cu-Ag case, no Cu residue was found on the imprint, but material fracture of the substrate metallization. A more detailed investigation of the Cu wire tail end clearly shows the Ag pick-up. The FAB formation from tails with and without pick-up is studied with various electrical flame off (EFO) parameters and electrode to wire distances (EWDs). Hardness of FAB cross-sections is a microhardness tester. The hardness change of the FAB is important for stress developed on the bond pad during ball bonding process. The EWD parameter is found to be the main parameter to obtain spherical and oxide free FABs. As the EWD increases above 0.3 mm, oxide lines containing 1.09+/-0.47 wt%Ag are found on the FAB surface. The oxide lines further contain roughly 3 wt% of O2 and 96 wt% of Cu. Etching the sample showed that the oxide lines locations agree with those of the boundaries of the FAB grains, leading to the conclusion that the Ag pick-up material is concentrated on the grain boundaries inside the FAB. The results show that the FAB diameter is not significantly changed by the Ag pick-up. The FAB hardness is reduced by about 3%.