Effect of [Au]/[Na2SO3] molar ratio on co-electroplating Au-Sn alloys in sulfite-based solution

LEDs (light emitting diodes) that assembled using flip-chip technology are today used as long-life, energy efficient, environmentally friendly light sources. However, the flip chip solder joints have to meet high requirements. Therefore, their performance and quality are crucial for the integrity of the assembly, which in turn is vital to the overall function of the LED. Au-30at.%Sn eutectic alloy is the generally used solder in electro-optical assemblies due to its excellent thermal and mechanical properties. Au-Sn solder bumps can be obtained by sequential electroplating of Au and Sn layers or by co-electroplating Au-Sn alloys from a single solution. In the present work, Au-Sn alloys have been co-electroplated from a non-cyanide, sulfite-based stable solution which contains Na₃Au (SO₃)₂ (gold sodium sulfite) as the source of gold and SnSO₄ (stannous sulfate) as the source of Sn. Na₂SO₃ (sodium sulfite) is added as the complexing agent for gold and an additional commercial complexing agent for Sn. The effect of the [Au]/[Na₂SO₃] molar ratio in the plating solution on the composition of the deposits, surface morphology and plating rate has been investigated. It was shown that the [Au]/[Na₂SO₃] molar ratio of 1/24 proved to be the best one with respect to plating rate and surface morphology in the present experiment. When the Sn²⁺ concentration is 0.03 mol/L, the optimum concentration for co-electroplating Au-Sn alloys is Au(I) concentration of 0.02 mol/L and Na₂SO₃ concentration of 0.48 mol/L, corresponding to the [Au]/[Na₂SO₃] molar ratio of 1/24.