Investigation of palladium coverage on free air balls of palladium-coated copper wires

Recently, copper wire bonding has been given increasing attention due to its low cost and excellent material performance. Compared with pure copper wire, Palladium-coated copper (Pd-Cu) wire gets more extensive application partly due to the Pd-coated layer protection for Cu FAB from oxidation in the bonding process. Therefore the research of Pd coverage on free air balls (FAB) is an important aspect for Pd-Cu wires. FAB of Pd-Cu wires may have exposed Cu region on top side of FAB where it is to be bonded, this Pd uncovered part could be possibly extended and then exposed to the outside of bonding area. After mold encapsulation, the exposed Cu region on FAB may have higher risk to be subject to corrosion compared with the Pd covered region and lead to failure of first bonding. This paper presents a study for Pd coverage issues on FAB of 0.8mil Pd-Cu wires. A chemical etching method with FeCl₃ solution for FAB was adopted to analyze the Pd coverage on FAB. The etching condition had been optimized through careful examinations on the surface morphology of etched FAB. With various FAB forming process parameters, e.g., electronic flame-off (EFO) current intensity and heating, Pd coverage has been studied. The Cu exposure, before and after FAB formation, was also compared and analyzed. This analysis results will provide an important reference for the optimizing FAB process parameters in Cu wire bonding process.