Investigation on FAB Morphology Evolution and Pd Redistribution Behavior in Palladium-Coated Copper Wires During Electronic Flame-Off (EFO) Process

Compared with bare copper wire, palladium-coated copper (PCC) wire is widely used in microelectronic packaging due to its improved oxidation resistance and enhanced reliability. However, the formation mechanism of free air balls (FABs) and the redistribution behavior of Pd during the electronic flame-off (EFO) process, particularly under different Pd coating thicknesses and processing conditions, have not yet been fully understood. In this work, four types of 1 mil PCC wires with Pd coating thicknesses of 60, 80, 100, and 120 nm were systematically investigated to study the influence of EFO parameters on FAB morphology and Pd redistribution behavior. SEM, FIB, and EDS analyses were employed to provide experimental insights into the coupled relationship between transient thermal input, internal pore distribution and elemental segregation evolution, and Pd redistribution behavior. The results show that the preferred FAB morphology is obtained at 54 mA and 580 μs, with a diameter-to-wire ratio of approximately 2. With increasing Pd coating thickness, the exposed copper area on the FAB surface decreases from 13% to 6%, while the Pd-deficient region gradually shifts toward the bottom of the FAB. This study provides experimental insights into the Pd redistribution behavior during FAB formation under different EFO conditions, which may contribute to the optimization of Pd-coated Cu bonding wires.