Investigation of Palladium Distribution on the Free Air Ball of Pd-coated Cu wire

Abstract

The application of copper wire coated with Palladium is a solution to prevent copper oxidation during the bonding process. This is true in the Second Bond but may not be the situation when it comes to First Bond. This is because during the Free Air Ball (FAB) formation, the Pd coated Cu wire is re-melted and the Pd distribution on the FAB is depending on various factors. For FAB formation, the simplest is that with gold wire. This is because gold wire FAB can be formed easily without any protective gas. Even with a layer of insulating coating as in X-Wire ™, gold wire does not need any protective environment during its re-melting and formation of spherical FAB [1]. For copper wire, its FAB formation need protective gas environment as copper will oxidize at high temperature and thereby its FAB. This has increased the complexity considerably as it needs to take into consideration the type of protective gas, its flow and cooling effect. For metallic coating copper wire such as Pd coated Cu wire, its FAB formation also need protective gas. This is because during the re-melting, the core copper will oxidize at high temperature. As Pd has been reported to oxidize at 600–800°C [2] and during re-melting, Cu will have a temperature above 1083°C, hence Pd will also be oxidized. It has also been reported that Pd distribution on the FAB is very much dependent on the effect of EFO (Electronic Flame Off) current [3–4]. To ensure best protection of first bond against HAST and PCT tests, it is best to have the Pd distributed over the entire surface of the FAB, thereby forming protective armour against corrosion attack by halogen ion present in mold compound chemistry. In this paper, various analysis techniques, Optical Scope, SEM, EDM and FIB were used to analyze Pd distributions in Pd-coated Cu FAB with three different types of Pd-coated Cu wires. It has been shown that Pd distribution change with different types of Pd-coated Cu wires under the same EFO parameters.