An electrochemical investigation of deposit initiation used to develop a low corrosion, non cyanide immersion gold for UBM applications

Electroless nickel/immersion gold (ENIG) is commonly used as the under bump metallurgy (UBM) in low-cost flipchip applications. A number of variables, including substrate pre-cleaning, catalyzation, electroless nickel initiation, phosphor content in the electroless nickel film, and immersion gold chemistry all affect the quality of the final finish. As for the gold chemistry, an electrolyte that provides good adhesion and low corrosion of the base metal is preferred. By definition, an immersion process relies on the corrosion of the underlying substrate to provide the electrons for deposition of the plated metal layer. This paper introduces an electrochemical analytical technique that studies the relative corrosivity of immersion gold electrolytes on the underlying electroless nickel. A new, low corrosion, immersion gold process will be described that has been developed based on the use of this electrochemical measurement technique. This process is capable of consistently plating 0.05 to 0.15 micron gold films over a wide operating window. Electrochemical characterization of this new process will be presented to illustrate the effect of the operating parameters such as Au concentration, pH, and temperature on process repeatability. Process performance with respect to wire bonding and solderability will also be shown.