Study On Co-electrodepositing Au-Sn Bumps In Non-cyanide Solution

As a new type of green lighting source, LED (Light Emitting Diodes) would surely lead the 21st century trends since their unmatched advantages at power consumption, luminance and lifetime compared with traditional lighting sources. The flip-chip technology improved the thermal dissipation and luminescence efficiency of High-power LEDs. However, higher requirements were submitted to the packaging technology.Au-30at.%Sn eutectic solder was chosen for the bumps, and a stable non-cyanide electroplating solution for Au-Sn alloys must be developed to replace the traditional solutions containing cyanide. Besides, the solution component and the electroplating process parameters were decided for co-electrodepositing Au-30at.%Sn eutectic bumps. The results indicated that:1. A stable non-cyanide electroplating solution with pH of 6 to 9 for Au-Sn alloys was developed. The solution contained Na3Au(SO3)2 (gold sodium sulfite) as the source of gold and SnSO4 (stannous sulfate) as the source of tin. Na2SO3 and EDTA were added as the complexing agent for gold, and an additional complexing agent for tin.2. The plating temperature had an obvious effect on the surface planarization of the Au-Sn films. When the temperature was in the range from 25℃to 65℃, the lower the temperature the rougher the films; however, higher temperature would degrade the stability of the solution. The suitable plating temperature for the newly developed solution was 45-55℃.3. Peak current density affected the tin content of Au-Sn films electroplated from the newly developed solution:Au-Sn films containing 50at.%Sn could be electroplated at low peak current density, while Au-Sn films containing 16at.%Sn electroplated at high peak current density.4. When [Au (I)]/[Na2SO3] molar ratio varied from 1:8 to 1:20 respectively, the plating rate of the Au-Sn films electroplated under T=45℃, J=27.5 mA/cm2 had no significant change. However, when the ratio varied to 1:24, the plating rate had a sharply increase. High Au (I) concentration in the solution could not result in a higher gold content in the Au-Sn films and higher plating rate, but might degrade the stability of the solution and increase the gold waste. The decreasing of Na2SO3 concentration would degrade the solution stability, while the increasing Na2SO3 concentration would result in a higher viscosity. The suitable concentration for Au (I) and Na2SO3 in solution was 0.02 mol/L and 0.48 mol/L, i.e., the corresponding molar ratio of [Au (I)]/[Na2SO3] was 1:24.