Wearout reliability and diffusion kinetics of Au and Pd-coated Cu (PdCu) ball bonds are useful technical information for Cu wire deployment in nanoscale semiconductor device packaging. This paper discusses the HAST (with bias) and UHAST (unbiased HAST) wearout reliability performance of Au and PdCu wires used in fine pitch BGA packages. In-depth failure analysis has been carried out to identify the failure mechanism under various wearout conditions. Intermetallic compound (IMC) diffusion constants and apparent activation energies (Eaa) of both wire types were investigated after high temperature storage life test (HTSL). Au bonds were identified to have faster IMC formation, compared to slower IMC growth of PdCu. PdCu wire was found to exhibit equivalent or better wearout reliability margin compared to conventional Au wire bonds. Failure mechanisms of Au, Cu ball bonds post-HAST and UHAST tests are been proposed, and both Au and PdCu IMC diffusion kinetics and their characteristics are discussed in this paper.