Development of Simple Electrolytes for the Electrodeposition and Electrophoretic Deposition of Pb-free, Sn-based Alloy Solder Films

Chunfen Han*, Qi Liu and Douglas G. Ivey Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6

*Email: chunfen@ualberta.ca, Tel: 780-492-8902

Keywords: Electrodeposition, Electrophoretic Deposition, Polarization Curves, Tin-Copper Eutectic Solder

Abstract

A large number of Pb-free solders have been developed due to restrictions on the use of Pb in conventional Pb-Sn solders. The most promising examples of these solders are Sn-based; e.g., the Sn-0.7wt%Cu alloy and eutectic and near-eutectic SAC (Sn-Ag-Cu) alloys. Alloy solder films can be deposited from electrolytes containing several additives which are used to improve solution stability and deposit morphology. However, these additives make the electrodeposition process difficult to control. Two approaches have been studied in our laboratory to simplify the process and to make the electrolytes environmentally friendly. In the first approach, Sn-based, slightly acidic electrolytes have been developed, which contain only one additive (tri-ammonium citrate) other than the metal salts. The optimum citrate concentration was 0.30 mol/L for a solution containing 0.22 mol/L of SnCl2•2H2O. The addition of 0.003 mol/L of CuCl2•2H2O to this solution can give near eutectic Sn-Cu deposits at current densities of 10-12.5 mA/cm2 with dense and uniform Sn-Cu deposits and a plating rate of 20-27.5 µm/hr at room temperature. The solution is stable for more than 36 days. In the second approach, nano-size Cu powders are suspended in the simple Sn solution with the aid of a dispersant. Near eutectic Sn-Cu films can be obtained with the addition of 0.004 mol/L (0.25 g/L) of Cu particles.

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