Pb-free Sn-Ag alloy solder bumps for flip-chip bonding were formed by electroplating with the aid of a photolithography. Pyrophosphate-iodide baths were used for the plating bath. Smooth, fine-grained Sn-Ag alloy films were obtained using a bath containing both polyethylene glycol (mean molecular weight 600, PEG600) and formaldehyde (HCHO) additives. Sn-Ag alloy films with near eutectic compositions (Sn-3.8 atom % Ag) were obtained under both galvanostatic and potentiostatic conditions. All of the Sn-Ag alloy films deposited consisted of β-Sn phase and phase. Straight-walled bumps and mushroom bumps were formed under both galvanostatic and potentiostatic conditions from the pyrophosphate-iodide bath containing both PEG600 and HCHO. Mushroom bumps with smooth surfaces and homogeneous composition were obtained under potentiostatic conditions.
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Arai et al. (2003) studied this question.
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