Fabrication and characteristics of electroplated Sn-0.7Cu micro-bumps for flip-chip packaging

The current study investigates the electroplating characteristics of Sn-Cu eutectic micro-bumps electroplated on a Si chip for flip chip application. Under bump metallization (UBM) layers consisting of Cr, Cu, Ni and Au sequentially from bottom to top with the aim of achieving Sn-Cu bumps 10 × 10 × 6 μm in size, with 20 μm pitch. In order to determine optimal plating parameters, the polarization curve, current density and plating time were analyzed. Experimental results showed the equilibrium potential from the Sn-Cu polarization curve is -0.465 V, which is attained when Sn-Cu electro-deposition occurred. The thickness of the electroplated bumps increased with rising current density and plating time up to 20 mA/cm² and 30 min respectively. The near eutectic composition of the Sn-0.72wt%Cu bump was obtained by plating at 10 mA/cm²for 20 min, and the bump size at these conditions was 10 × 10 × 6 μm. The shear strength of the eutectic SnCu bump was 9.0 gf when the shearing tip height was 50% of the bump height.