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This study investigates interfacial reaction and joint strength of Sn-Ag and Sn-Ag-Cu solder balls bonded to electroless Ni-P plating with various thicknesses of Au coating (50, 250 and 500 nm). For the Sn-Ag solder joints, a P-rich layer formed after reflow-soldering at the joint interface, regardless of thickness of the Au coating. However, for the Sn-Ag-Cu solder joints, a P-rich layer formed at the joint interface only in those samples with Au coating of 250 and 500 nm. Fractures were found to occur at the interface, and the joint strength degraded at Au thickness of >250 nm for both the Sn-Ag and Sn-Ag-Cu solder joints. An Au thickness of 50 nm resulted in the best joint strength.

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