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The optimal electroplating parameters for a pulse-current co-electroplating system of Au-Sn deposits in a non-cyanide electrolyte were investigated using experimental strategies, including fractional factorial design (FFD) and central composite design (CCD) coupled with the response surface methodology. pH value, ethylene diamine tetraacetic acid (EDTA) concentration, catechol concentration and metallic ions molar ratio (i.e., [Au]/[Sn]) were identified as the key factors affecting the composition of Au-Sn deposits in the FFD study. A reliable model between the response variable and the key factors of pH value, EDTA concentration and catechol concentration was established for the composition control of Au-Sn alloys in the CCD study. The standard deviation of the response variable (tin content) was set at the minimum level to determine the optimal co-electroplating parameters for the predicted composition value of Au-Sn deposits. Pair T test was conducted to validate both predicted and observed composition values under the optimal electroplating parameters, and the composition of Au-Sn deposits can be precisely controlled based on the established model. Scanning electron microscope observation and X-ray diffractometer analysis revealed that the morphology and crystalline of the Au-Sn deposits were compositiondependent.

Au−30 at-Sn eutectic alloy was fabricated by sequentially pulse electroplating Au and Sn films on Si chips. Three kinds of Au/Sn/Au triple layer films were prepared in the present work: Au/Sn/Au (6/6/1 μm) films, Au/Sn/Au (6/6/6 μm) films and Au/Sn/Au (8/6/1 μm) films. The microstructure and phase transformation in Au/Sn/Au films during aging and reflow soldering were investigated. For Au/Sn/Au (6/6/1 μm) films during aging at 100 and 150°C, the layered AuSn/AuSn2/AuSn4 structure formed in the reaction region. Furthermore, the Sn film was completely consumed, and AuSn4 finally transformed into AuSn and AuSn2 after aging at 150°C for 15 h. For Au/Sn/Au (6/6/6 μm) films during aging at 150°C, the electroplating sequence had an important effect on the formation of Au−Sn phases. An Au5Sn layer was present at the Au II/Sn interface but not at the Au I/Sn interface. For Au/Sn/Au (8/6/1 μm) films, the micropores that formed preferentially along the Au5Sn/AuSn interface remarkably decreased with increasing reflow temperature from 280 to 310°C. After reflowing for 10 s, the microstructure was not an Au−Sn eutectic; however, after reflowing for 60 s, coarsened primary Au5Sn phase and typical Au−30 at-Sn eutectic microstructure of fine eutectic phases (AuSn+Au5Sn) formed.

The Au-Sn alloy was successfully pulse electrodeposited in a new non-cyanide and non-toxic stable electroplating bath which basically was composed of Au compound and tin sulfate (SnS04). The composition and morphology of electrodeposited Au-Sn alloy were investigated in terms of current density, temperature and pH value in the developed bath. It was possible to control tin content in the electrodeposits by means of current density. Au-Sn alloy with Sn concentration of about 20 to 45 at.% was deposited at peak current density ranging from 3 mA/cm 2 to 45 mAzcm". Although the microstructure and surface morphology of the electrodeposits were influenced by the peak current density, dense and smooth Au-Sn films with fine grain had been electroplated at different current density. With increasing pH value, the tin content in the alloy was obviously increased from about 10 to 45 at.%. When the pH value was greater than 8.5, the deposits became porous and incompact mainly owing to the hydrogen evolution reaction during plating. With increasing temperature, the tin content was slightly changed, however, the grain size became bigger and the films had a tendency towards coarser because of the rapid consumption of metallic ions. Therefore, it can be concluded that the optimum condition of pH value for electroplating of Au-Sn alloys was ranging from 7.5 to 8 and that of temperature was 35 to 45°C.