Characterization of oxidation of electroplated Sn for advanced flip-chip bonding

Sn based Pb-free solder material is often used for flip-chip bonding. However, Sn is prone to be oxidized in ambient due to its low standard Gibbs free energy. In this paper, we investigate the oxidation of electroplated Sn at room temperature and the temperature ramp-up during flip-chip bonding by XPS. It is found that the intial oxide of our electroplated Sn is about 1.43 nm thick, and the oxide thickness increases with time at room temperature. Acid wet clean can reduce the oxide thickness. After cleaning, the oxide thickness is reduced to less than 1.4 nm within 10 minutes, but after that the growth of oxide follows a similar trend as the as-deposited Sn. Moreover, oxide grows fast during the temperature ramp-up. It is found that about 1.0 nm oxide is grown when the temperature reaches 250 degrees C in less than 30 seconds. Therefore, it is important to find a solution to control the total amount of oxide for fluxless soldering.