Investigation of palladium coverage on bonded balls of palladium-coated copper wires

The application of Palladium coated-Copper (Pd-Cu) wires is a solution to prevent copper oxidation during the copper wire bonding process. This is true for the Second Bond but may not be the situation when it comes to the First Bond, because during the Free Air Ball (FAB) formation, the Pd-Cu wire tip is melted and the Pd distribution on the FAB surface is dependent on several processing factors. The type of the wires used and the electrical current applied during the EFO (Electronic Flame Off) process to form the FAB are believed to be the two major factors affecting the resulting Pd distribution on the FAB. Without an optimized EFO process to form ideal, spherical FABs with nice palladium surface coverage, the copper may be exposed after the First bond. Consequently, after molding, it is subject to corrosion due to the presence of halogen ions inside the mold compound chemistry. Typically, after a pressure cooker test or HAST test, these first bonds may fail. In this paper, we will first present the study of palladium surface coverage during FAB formation and also bonded ball using various EFO parameters with different Pd-Cu wire sizes. During first bond bonding, pure impact of the FAB will determine 75%-80% of the final shape of the bonded ball bond; the subsequent ultrasonic USG vibration accounts only for about 20% of the final shape of the ball bond, depending on the bonding parameters. To reveal the various degrees of palladium coverage on the first bond, we prepared samples and subjected them to weak chloride acids to etch away the exposed copper regions and compare the results using SEM photomicrographs. We found that, even if when the palladium surface coverage on a FAB is incomplete, it is still possible to "seal" off the copper during a good first bond. Therefore, we will present the guidelines on how to achieve improved EFO parameters to form good palladium coverage during FAB formation, as well as how to differentiate and select a palladium coverage in FAB that could still form good first bond with concealed copper not exposed to corrosion attack from the molding compounds, hence forming a more reliable bonding. Such allowable exposed copper regions are presented for three different wire diameters: 0.8, 0.9, and 1.0 mil wires.