Study the oxidation resistance on palladium-coated Free Air Ball

Abstract

The use of Palladium-coated Copper (Pd-Cu) wire helps prevent Copper (Cu) oxidation before and during the bonding process. Different EFO current, firing time and ball size ratio (BSR), affect the roundness of the Free Air Balls (FABs) formed and the distribution of Pd on the surface of the FABs. In this paper, Pd-Cu wires of diameter 0.8 mil were used and 3 settings of Electronic Flame Off (EFO) current and firing time were used. Optical and scanning electron microscope (SEM) inspection were used to observe the shape of the FABs formed. Energy-dispersive X-ray spectroscopy (SEM-EDX) was carried out to observe the Pd distribution as well as the oxidation condition on the surface of the FABs. Other than the optical and SEM inspections, Focused Ion Beam (FIB) and Transmission electron microcopy (TEM) were carried out after a Pressure Cooker Test (PCT) to further investigate the corrosion phenomenon of Cu on FAB surfaces. It was found that for FAB surfaces which contain Pd, even if there is not complete coverage of Pd, the Cu oxidation is a slow process and that FABs with higher Pd content have better corrosion resistance performance. Cu Grain boundaries show the weakest location for corrosion resistance in the FAB.