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An Electrochemical Investigation of Deposit Initiation used to Develop a Low Corrosion, Non-Cyanide Immersion Gold for UBM Applications
Keywords: Immersion Gold, Under Bump Metallization, Electrochemical Technique
Electroless nickel/immersion gold (ENIG) is commonly used as the under bump metallurgy (UBM) in low-cost flip-chip 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. Cyanide based immersion gold processes have been used for the metallization of printed circuit boards in surface mount applications for many years. Due to photoresist incompatibility, increasing restrictions on waste treatment, and its inherent toxicity, there is significant pressure to eliminate cyanide from the electronics and semiconductor industries. As a result, non-cyanide based environmentally friendly processes have been developed. 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.
Yu Luo, Research Chemist
Rohm and Haas Electronic Materials
Freeport, NY

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