Formation of Au Microbump Arrays for Flip-Chip Bonding Using Electroless Au Deposition from a Non-Cyanide Plating Bath

The formation of Au microbump arrays for flip-chip bonding was investigated using an electroless Au plating bath. Generally, electroless Au deposition gives a low deposition rate, preventing the fabrication of Au bumps with heights of more than 5 μm. To overcome this problem, we developed a new electroless Au deposition process that employs a non-cyanide bath. This process delivers a high deposition rate, especially for fine bump patterns (bumps of less than 10 μm in diameter). Au bumps with a diameter and height of 10 μm each could be fabricated with a high deposition rate using the new electroless Au plating bath. Moreover, Au microbump arrays were successfully fabricated on Cu wiring. The practical feasibility of this technique was demonstrated by forming arrays with bump heights and bump diameters of 10 μm each and an array pitch of 30 μm. All film-formation processes were performed under cyanide-free conditions; thus, this process has a high industrial potential for fabricating ultra-fine Au microbump arrays.