On-Line Hardness Characterization of Novel 2-mil Copper Bonding Wires

Abstract

In order to eliminate the chip cratering for copper wire applications in IC packaging, it is worthwhile to develop new Cu wire chemistries to obtain a soft copper wire with a soft free-air ball (FAB). The conventional hardness characterization of a new bonding wire is a labour intensive, time-consuming work. Therefore an on-line hardness characterization method is presented that enables the hardness comparison of a larger number of different wires within a shorter time. The influences of capillary change, bonding substrate metallization and temperature on this method is quantified. It is found these influences need to be held constant during a hardness comparison study. With this method, the wire and FAB hardness comparison of nine novel 2-mil copper bonding wires, Cu 1 to Cu 9, and one 2-mil Au wire are performed. The wire hardness (wireside) and FAB hardness are characterized. It is found that the Cu 4 and Cu 5 have the softest wireside hardness and FAB hardness.

Free-air ball formation and deformability with Pd coated Cu wire

“…15 mN touchdown) where the FAB experiences negligible deformation, the data points are the FAB height (H FAB ) and wire height (H W ), respectively. The measured H FAB is lower than the FAB diameter due to the capillary geometry [4][5][6]. A "force for target deformation" value is determined by interpolation.…”

“…For each wire, EFO parameters are set to obtain a target FAB diameter of ≈40 µm. A previously reported method is used for investigating and comparing deformability [4][5][6][7] using an Esec 3100 wirebonder with custom software options. For the controlled deformation, the nominal forces range from 100 to 900 mN.…”