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Comparison of Bare Copper and Palladium Coated Copper Wire in Wire Bonding
Keywords: Pd coated copper, 1st bond reliability, 2nd bond strength
Recent emergence of palladium (Pd) coated copper wire as a 2nd alternative to gold wire for low cost bonding wire solution has created great interest on the performance of the coated wire in terms of bondability and reliability over bare copper. Comparison of the two wires in the process condition to form free air ball (FAB) and bondability and reliability was conducted. The fact that palladium coating provides better corrosion protection of the copper core of the wire means longer floor life and shelf life of the wire for end users. Because of the noble metal protection, it enables the use of lower cost inert gas, like nitrogen, over the higher cost forming gas used for free air ball (FAB) formation during bonding process. It has also been assumed that, due to the Pd layer on the wire, there is always a layer of Pd or Pd rich phase protecting the surface of FAB and hence, the bonded ball, from corrosion attack by halogen ion presented in mold compound chemistry during highly accelerated stress test (HAST) or pressure cooker test (PCT) whereby high humidity environment can aggravate oxidation of copper and render 1st bonder failure after long exposure. Devices bonded with both wires in un-molded condition were subjected to high temperature storage test up to minimum of 1000 hrs and compare their bond strength. 1st bond reliability of both wires were assessed in molded device and subjected to highly accelerated stress test (HAST) in green mold compound. The intermetallic formation of Pd coated copper was also compared with bare copper to review if Pd has an impact on the growth rate. The obvious benefit of Pd coating on second bond strength is unquestionable as this is demonstrated by the higher stitch pull strength over bare copper.
Johnny Yeung, R&D Director
Heraeus Materials Singapore Pte Ltd
Singapore 569871,
Singapore


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