Poster Session

Microstructural Characterization of Pb-free, Electrodeposited Sn-based Solder Films

Chunfen Han, PhD student in Materials Engineering

Supervisor - Dr. Douglas Ivey and Dr. Qi Liu

A simple and stable, cyanide-free, slightly acidic, chloride-based solution has been developed for the electrodeposition of Sn-0.7wt% Cu eutectic solder films. The electrolyte constituents are SnCl2 2H2O (0.22 mol/L), CuCl2 2H2O (0.003 mol/L) and tri-ammonium citrate (0.30 mol/L) as the only additive. Dense and uniform Sn-Cu eutectic films (Sn-0.7wt% Cu) can be deposited on copper and nickel substrates. The plating rate for this system is about 20 m/hr. Copper is evenly distributed (~0.7 wt% Cu) through most of the thickness, with higher Cu levels (~5.5wt%Cu) at the solder/Ni interface. After solder reflow at 260°C, two phases, Sn and a (Cu,Ni)6Sn5 intermetallic compound (IMC), are present. The (Cu,Ni)6Sn5 phase has two different morphologies, one is quite coarse while the other is fine. A Cu/Sn-Cu/Ni sandwich structure formed during reflow and both Cu and Ni are dissolved in the solder and diffuse significant distances from the substrate. At the Sn-Cu/Ni interface, the IMC layer is much thinner than at the Cu/Sn-Cu interface, and the IMC has a composition corresponding to (Cu0.9Ni0.1)6Sn5. The tin matrix phase has ~1.5 wt% Cu and ~1.1 wt% Ni.