The Study Of Sequential Electroplating Au/Sn Bumps For FC-LEDs

Along with the increase of luminescence efficiency of LEDs, it was estimated that the dimensions of market of whole global HB-LED(High Brightness LED, HB-LED) market will amount to 9 billion US dollars by 2011, and white light HB-LED solid-state lighting will became true in the near future. Higher requirements were submitted to the optics, thermology, electricity and mechanical structure of the packaging of LEDs. In order to decrease the thermal resistance and increase the luminescence efficiency of the packaging structures, totally new design proposal must be applied.Since the flip chip(FC) technology could improve both of the luminescence efficiency and heat dissipation potential of LED chips, FC packaging for white light HB-LED was one of the hotspots of research. However, bumping was one critical step in FC technology. This paper will focus on the bumping process. Au-Sn eutectic solder was chosen for the bumps, and sequential electroplating was adopted to manufacture the bumps. To achieve green manufacturing, traditional cyanide gold electrolytes and tin solutions containing fluorine and lead in them will must be replaced by green solutions in the near future.A stable non-cyanide electroplating solution for gold and a non-toxic solution for tin were worked out separately, besides, the electroplating process and parameters were decided for both of the solutions. On this basis, Au/Sn double-layer structure was successfully prepared. The results indicated that:1. A stable non-cyanide electroplating solution for Au was developed, it can be stable when the pH=6～9 and the electroplating temperature T=13℃～90℃. It consisted chiefly of NaAuCl₄·2H₂O, Na₂SO₃, ethylenediamine tetracetic acid (EDTA). An electrolyte for tin was developed, too, its chief ingredients were SnSO₄, K₄P₂O₇·3H₂O and C₆H₈O₆. This electrolyte for tin had the best combination property when its pH value was 8.0. Pure gold and pure tin bumps were successfully prepared.2. The grain(minor particle) size and the roughness of the film surface increased with the increasing temperature, so did the growth rate of the deposited films.3. If the other parameters were fixed, such as electrolyte, temperature, current density, the films electroplated by the periodic reversal current were much better than the ones electroplated by the pulsed current. The former had higher compactness, better planeness, and faster growth rate.4. As far as electroplating gold was concerned, when pH=8.0, T=80℃, J= 0.5～2.5A/dm², the higher the J was, the faster the Au films growth rate was, the surface of the Au bumps became coarsing and the density of the bumps kept reducing, dentrites were formed when J=2.0A/dm² . Regard to electroplating tin, when J= 0.2～3.0A/dm² , the size of the tin grains kept growing when J increased, but no dentrite was observed.5. When T=80℃, pH=8.0, J=1.0 A/dm², a good liner relationship existed between the gold film thichness and the depositing time. Similarly , when T=45℃, pH=8.0, J=1.0 A/dm², a good liner relationship existed between the gold film thichness and the depositing time.