Effect of [Au]/[Na<inf>2</inf>SO<inf>3</inf>] molar ratio on co-electroplating Au-Sn alloys in sulfite-based solution

Abstract

LEDs (Light Emitting Diodes) that assembled using flipchip 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 coelectroplating 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 Na 3 Au (SO 3 ) 2 (gold sodium sulfite) as the source of gold and SnSO 4 (stannous sulfate) as the source of Sn. Na 2 SO 3 (sodium sulfite) is added as the complexing agent for gold and an additional commercial complexing agent for Sn. The effect of the [Au]/[Na 2 SO 3 ] 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]/[Na 2 SO 3 ] 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 Sn 2+ 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 Na 2 SO 3 concentration of 0.48 mol/L, corresponding to the [Au]/[Na 2 SO 3 ] molar ratio of 1/24. IntroductionLEDs, fabricated with III-V compound semiconductors, are capable of transforming electric energy into light energy with high efficiency and a life-span up to thousands of hours. In order to decrease the thermal resistance and improve heat transport away from the LED, flip chip technology has been developed to fabricate high-performance of LEDs. Thus bumping becomes a critical step in FC technology. The performance and quality of the solder bumps are crucial for the integrity interconnection, which in turn is vital to the overall function of the LEDs.As a joining material, solder provides electrical, thermal and mechanical continuity in electronics assemblies. Pb-Sn solders are well-known as the bumping material. However, with the increase of the environmental concerns, the use of lead free solders for electronic products has become a major driving force for technology developments [1,2]. The Au30at.%Sn alloy appears to be an attractive alternative, with a melting temperature of 280°C. Moreover, the Au-30at.%Sn alloy owns superior mechanical and thermal properties to conventional Pb-Sn solders, and have no negative effect on the environment.

Effects of plating parameters on the Au-Sn co-electrodepositon for flip chip-LED bumps

“…Thus, the choice of bump materials is rather important for the understanding of the reliability of the flip chip packaging. Pb-Sn solders are well-known as the bumping material, however, lead and lead compounds have been cited by the Environmental Protection Agency (EPA) as one of the chemicals posing the greatest threat to human life and the environment [1].Of the gold eutectic solders, the Au-30 at.%Sn eutectic alloy, with a melting temperature of 280 DC, appears to be an attractive alternative in the optoelectronic packaging due to the superior mechanical and thermal properties [2][3][4].…”