Pitfalls and solutions of replacing gold wire with palladium coated copper wire in IC wire bonding

Abstract

There is a growing interest in copper wire bonding for IC interconnection due to the cost savings and better material properties. Recently, palladium-coated copper (Pd-Cu) wire has emerged as the preferred choice over bare Cu wire in fine wire diameter applications, especially with BGA packages. The application of Pd-Cu wire 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-Cu wire is re-melted and the Pd distribution on the FAB depends on various factors. It has been reported that Pd distribution on the FAB is very much dependent on the effect of EFO (Electronic Flame Off) current and also the wire type. 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 a protective armour against corrosion attack by halogen ion present in mold compound chemistry. Different types of Pd-Cu wires will need different FAB formation optimization. If the EFO parameter is not optimized properly, it will result in dimple FAB or/and inconsistent FAB that is not perfectly round-shaped ball, with a deep valley at the centre that shape like “blueberry”. In this paper, two different types of Pd-Cu wires and its FAB formation at different EFO conditions were investigated. Various analysis techniques, Optical Scope, SEM, EDX and FIB were used to analyze the Pd distributions in the Pd-Cu FAB. A general guideline of how to obtain consistent good round FAB for Pd-Cu wire is presented. A method of analyzing the good FAB with Pd layer protection has been demonstrated. This methodology will help to ensure a good and proper Pd-Cu wire FAB to start with before carrying out First Bond wire bond optimization.