| Au-30at.% Sn eutectic solder has high strength, good wettability, high thermal conductivity, high creep resistance, good thermal fatigue resistance and other advantages. That make it very suitable for high-reliability packages, especially in the high-power LED flip chip package. Compared to the non-plated preparation method of Au-30 at.%Sn eutectic solder bump, the plating method is low cost, simple process, high degree of consistency and so on. But the study of non-cyanide plating bath is difficult, especially in the bath stability. Therefore a more stable bath is prepared in this paper and the effect of bath composition and pating parameters on the coatings was studied systematically. At lats the Au-30at.% Sn eutectic solder was obtained after bath composition and pating parameters optimization.The main conclusions of this paper are as follows:(1) The plating bath consitited of Au(DMH)4", sodium sulfite, ammonium citrate, stannous pyrophosphate, potassium pyrophosphate and catechol, gold ions and stannum(II) ions has been fully complexed. Thus the bath is more stable, it remains colorless at a relatively high temperature during plating and placed for nearly 20 days at room temperature still no precipitate.(2) In the bath components:Catechol can improve the quality of coatings and has a great effect on tin content of the coating, the suitable concentration is between 0.02 M and 0.04 M. Sn2+concentration has great effect on the coating surface morphology and composition and it plays a vital role on Sn content, the suitable Sn2+ concentration should be 0.02 M (the Sn2+ to Au ions concentration ratio should be 2). sodium sulfite concentration had no effect on morphology and composition of the coatings, then sodium sulfite concentration can greater than or equal to 0.12 M (the sodium sulfite to Au ions concentration ratio is greater than or equal to 12). Ammonium citrate concentration has a larger effect on the coating morphology but no effect on the coating composition, the suitable concentration should between 0.20 M and 0.24 M (the Ammonium citrate to Au ions concentration ratio is between 20 and 24). Pyrophosphate concentration has great effect on coating morphology, less effect on coating composition, the suitable concentration should between 0.24 M and 0.32 M (the Pyrophosphate to Sn ions concentration ratio should between 6 and 8). The pH value of the bath has little effect on coating morphology and composition, considering the bath stability and application environment, the appropriate value should between 7 and 8.(3) In the plating parameters:peak current density has great effect on coating morphology and the effect on coating composition is not large, the suitable value should between 10 mA/cm2 and 20 mA/cm2. On-time has great effect on the coating morphology and composition, the suitable value should be less than 3 ms. Off-time also has great effect on the coating morphology and composition, the suitable value should be 3 ms or above. Stirring speed has greater effect on the quality of the coating and the larger speed can increase the internal stress to result in the abscission, it had no effect on the coating composition, the suitable value should between 250 rpm and 300 rpm. Plating temperature have a great effect on the coating morphology and composition, the suitable temperature is about 45℃.(4) After orthogonal experiment and robust parameter experiment, the composition and plating parameters are optimized. When the peak current density is 15 mA/cm2, on-time is 1.5 ms, off-time is 5 ms, catechol concentration is 0.028 M, the Au-30 at.% Sn eutectic solder can be obtained. The coating is dense, silvery white and bright. The plating rate can reaach 12μm/h. |
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