| Sn-Pb alloy deposit was widely used as a solder coating in traditional electronic packaging technology. However, Pb has been banned to use in electronic products due to its toxicity. Therefore, electroplating of lead-free solder coating is one of investigation hotspots in electronic electroplating field. At present, pure tin deposit has a bad solder reliability, tin-based binary alloy deposits have big brittleness or bad wettability, and noble metal deposits have high price. A general analysis shows that, Sn-Ag-Cu ternary alloy that used as a lead-free solder coating has a nice application foreground. So, it has an important theoretic and practical meaning to study on electrodeposition of Sn-Ag-Cu ternary alloy.Since acidic baths and alkaline baths have a lot of shortcomings, a weakly acidic bath containing methylsulfonic acid salts and iodide was developed to electrodeposit Sn-Ag-Cu alloy coating. Electrodeposition of Sn-Ag-Cu alloy on Fe or Cu substrate is employed on a direct current power supply. Scanning electron microscopy was used to observe morphologies of coatings and X-ray fluorescence was used to analyze compositions of coatings. On base of systemic studies, influencing rules of compositions of electrolyte and process conditions were determined, and optimized compositions of electrolyte and process conditions are determined for electrodeposit bright, smooth and compact Sn-Ag-Cu alloy with mass contents of Sn, Ag and Cu are 92%~97%, 2%~6% and 0.5%~2%, respectively.A bright Sn-Ag-Cu alloy coating can be electrodeposited from the electrolyte modified by adding both HT additive that was self-made and triethanolamine (TEA). Effect of HT and TEA on cathodic polarization of baths and morphologies and compositions of Sn-Ag-Cu coatings was studied. The results show that, HT increases cathodic polarization and makes crystal of the coating finer, and TEA decreases cathodic polarization and makes crystal of the coating denser. So, HT is considered as main brightener and TEA is considered as brightening promoter for electrodeposition of Sn-Ag-Cu alloy coating. The compute results of molecular kinetics show that, aromatic aldehyde molecules in HT are strongly adsorbed on surface of cathodic electrode, however TEA molecules have weak adsorption. Based on above analysis, a brightener corporate action model is put forward, which explains the form reason of bright deposits well.Electrodeposition behaviors of Sn-Ag-Cu alloy in electrolyte were studied by testing of cyclic voltammetry, linear sweet voltammetry, chronoamperometry, and electrochemical impedance spectrum. Co-deposition type of Sn-Ag-Cu alloy is determined as canonical. With cathodic potential towards to negative direction, electrodeposition process is parted to three periods, which were controlled by diffusion, both diffusion and electrochemical step, and electrochemical step, respectively. Competed discharge mechanism of metal complexing ions is put forward. Multiform metal complexing ions have nearly equal graining potential under a certain cathodic potential, and discharge on cathodic surface vies with each other and forms Sn-Ag-Cu alloy coating.Solder properties of Sn-Ag-Cu alloy coating were evaluated by differential scanning calorimetry, Tafel plots, extending, spreading, and high temperature and humidity tests. The results show that, melt points of Sn-Ag-Cu alloy coatings are 221~223℃, Sn-Ag-Cu alloy coatings have good compatibility with Sn-3.0Ag-0.5Cu solder and its corrosion resistance is as good as Sn-Pb alloy. Sn-Ag-Cu alloy coating has good wettability while its morphology is smooth and fine. Tin whiskers easily grow on surface of Sn-Ag-Cu coating when Cu is used as substrate, while growth of tin whisker is inhibited effectively when Ni electrodeposited from sulfamate electrolyte is used as preplating.Investigations in this paper show that, eutectic Sn-Ag-Cu ternary alloy deposit electrodeposited from weakly acidic bath is a new lead-free solder coating. The coating has excellent solder properties and can be used on the surfaces of printed circuit board and patch-like electronic apparatus in order to carry out lead-free electronic packing. The brightener corporate action model affords theory guidance on choosing brighteners used in bath for tin-based deposits, and the mechanism of competed discharge of complexing ions enriches theory of co-deposition of alloys.
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