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Direct Palladium on Bonding Wires
Keywords: Palladium Coating, Bonding Wire, Direct Coating
Copper has been used to replace Gold bonding wires in several applications. If coated by a nanolayer of Palladium, Copper bonding wire has even more applications than in its bare form. However, the Pd nanolayer comes with a cost premium, and as wire bonding is regularly reviewed for the goals of more efficiency and cost effectiveness (like all packaging processes), new solutions are required for the future challenges. The development of new coated bonding wires with improve performance and lower cost are one possible way towards those goals. Microbonds has introduced a low cost direct Palladium Coating Cu (DPCC) Process that can be applied to Copper, Silver, and other bonding wires. In this process, the Pd coating is applied directly to any bare wire at any diameter avoiding the use of electroplating and co-drawing which can be costly and inconsistent. Direct Pd coated Cu (DPCC) wire has been reported to form good Free-Air-Balls and strong second bonds. In case of a 15 micron diameter Copper wire, DPCC can achieve nanolayers with a wide range of thickness up to at least 100 microns. Forming gas as wll as Nitrogen gas can be used for shielding. Based on ball pull results, the bond reliability performance is similar to that obtained with non-direct Pd coated Cu wires that are commercially available. Cross sections of the ball bonds reveal the Palladium distribution. For the usual wire, more Palladium can be seen at the ball neck region which is remote from the bond interface. For the DPCC wire, no particular concentration of Palladium was observed, indicating a more homogeneous distribution. Based on these results, DPCC wire is a good candidate to reduce cost in existing Pd-Cu wire bonding processes. Recently, bare Ag wire has received some interest as a low cost replacement for Au. Free-Air-Balls can be formed under nitrogen shielding with varying levels of consistency. When coated with Pd, consistent Free-Air-Balls can be produced in air. This can be explained by the constriction of the electrical flame-off discharge to the small break-off spot at the tail end with pure Ag, surrounded by Pd coating. The lower thermionic work function of Ag compared to Pd makes it easier for electrons to exit through the Ag surface than the Pd coated surface. Pd-Ag wire bondability is at least as good as that of Au wire.
Michael Mayer,
University of Waterloo
Waterloo, Ontario

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