| The flip-chip technology can improve the luminescence efficiency and thermal dissipation of high-power LEDs. Since the electric and thermal conductivities of flip-chip are carried out by solder bump, the capability of the bump has a direct impact on the chip reliability. Au-30at.%Sn eutectic solder is the idea material for flip-chip bump due to its excellent mechanical properties and high reliability. Electroplating of Au-30at.%Sn eutectic alloy bump has the advantages of low cost, simple process, precise position control and small size bump preparation compared with evaporation, sputtering and electroless. However, cyanide Au-Sn electroplating solution being currently used is highly toxic and does seriously harm to environment and personal safety, hence it is urgent to develop a non-cyanide Au-Sn electroplating solution.In the present work, the effects of solution ingredients and process parameters of non-cyanide sulfite-pyrophosphate Au-Sn solution system on the morphology and composition of Au-Sn coating were systematically investigated, and then the response surface methodology was used to precisely control the composition of Au-Sn coating. The influence law of solution ingredients on Au-Sn co-electrodeposition behavior was analyzed by electrochemical means. Finally, the properties of the Au-30at.%Sn eutectic coating were evaluated. The Au-30at.%Sn eutectic alloy bumps prepared by co-electrodeposition were compared with those prepared by sequential electroplating. The interfacial reactions between co-electrodeposited Au-30at.%Sn eutectic alloy bumps and copper and nickel substrates were also investigated. The main conclusions are as follows:1. The weakly alkaline sulfite-pyrophosphate Au-Sn solution system was chosen to co-electrodeposit Au-Sn alloy coating. The morphology and composition of Au-Sn coating were regarded as the evaluation indexes to optimize the solution ingredients and process parameters. The results were as follows:the molar ratio of gold to sodium sulfite and tin to potassium pyrophosphate were1:12-1:24and1:6～1:9, respectively; the concentration of EDTA, catechol, ascorbic acid and nickel chloride were0.01-0.08mol/L,0.01-0.07mol/L,0.09mol/L and0.005mol/L, respectively; the range of the pH value of the solution was restricted between7.5～8.5; the temperature range was35～45℃; the stirring speed was set at200rpm; the pulse frequency was greater than10Hz; the forward on time was not shorter than2ms, the forward off time was not shorter than4ms, the reversal on time was not longer than2ms, and the ratio of forward current to reverse current was larger than4:1when the duty ratio of periodic reversal pulse was100Hz.2. Response surface methodology was adopted to adjust the Au-Sn co-electrodeposition solution ingredients and process parameters to precisely control the composition of Au-Sn coating. It was shown that the impact of main factors and interactions on the composition of Au-Sn coating fell in order of C>A>E>AB>D>AD>B in the fractional factorial design experiment. Central composite design experiment, containing pi I value, EDTA concentration and catechol concentration, was carried out to built the model correlating the response and the factors. Then the optimum solutions of solution ingredient and process parameter for the predicted values of Au-Sn coating can be determined based on the model. According to the model, the optimum solution for the Au-30at.%Sn eutectic coating was also determined. It was indicated that the Au-30at.%Sn eutectic alloy can be obtained under the optimum condition by repeated experiments. The eutectic alloy composition was accurate, and the experiment had repeatability.3. The influence law of solution ingredients on Au-Sn co-electrodeposition behavior was analyzed by electrochemical means. After adding complexing agent into the solution, the deposition potential of Au was close to that of Sn, which was benefit for the co-electrodeposition of Au-Sn alloy. After adding EDTA into the solution, the reduction peak potential of Au-Sn moved towards positive direction; after adding catechol into the solution, the reduction peak potential of Au-Sn was shifted negatively; the reduction peak potential of Au-Sn was further shifted to negative direction with the addition of both EDTA and catechol. The additives made the Au-Sn eathodie reduction peaks move towards to positive or negative position, which changed the co-electrodeposition behavior of Au-Sn. The Au content in the coating increased when the eathodie reduction peak moved towards to positive direction, while the Sn content in the coating increased when the eathodie reduction peak moved towards to negative direction. The co-electrodcposition of Au-Sn under the eathodie peak potential was controlled by diffusion step. With cathodic potential towards to negative direction, the co-electrodeposition process was controlled by both diffusion and electrochemical step.4. The properties of the co-electrodeposiled Au-30at.%Sn eutectic coatings were evaluated, such as roughness, bonding force, melting point, corrosion resistance and so on. The results showed that the roughness of the co-electrodeposited Au-30at.%Sn eutectic alloy coatings was only tens of nanometer, and the bonding force between the Au-30at.%Sn alloy eutectic coating and Si/TiW/Au substrate was good. The melt point was280.66℃. Moreover, the co-electrodeposited Au-30at.%Sn eutectic alloy coatings had good corrosion resistance. It is obvious that the quality of the Au-30at.%Sn eutectic bump prepared by co-electrodeposition was improved compared with that prepared by sequential electroplating. After the co-electrodeposited Au-30at.%Sn coatings reacted with Ni and Cu substrates at310oC, it was shown that (Ni,Au)3Sn2intermetallic compound formed at the interface of Au-30at.%Sn/Ni, while two-layer intermetallic compound, i.e.,(Au,Cu)5Sn close to the matrix of solder and AuCu close to the Cu substrate, formed at the interface of Au-30at.%Sn/Cu. The good bonding between the Au-30at.%Sn eutectic alloy coatings and Ni and Cu substrates indicated that the co-electrodeposited Au-30at.%Sn eutectic coating had good solderability. |
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