Research On Electroplating Techonology And Mechanism Of Sn-Ag-Cu Alloy Coatings

In surface mount technology (SMT), electronic apparatus need to electroplating lead-free soldering coatings to adapt the requirement of lead-free mounting because of the toxic of Pb. In this paper, electroplating processes and electrodeposit mechanism of lead-free Sn-Ag-Cu ternary alloy coatings from weak acid baths were studied.The effects of baths compositions and electroplating conditions on baths and coatings properties were investigated, and optimized baths compositions and electroplating conditions were determined as follows: 0.18~0.22mol/L Sn(CH3SO3)2 , 3.5~5.5mmol/L AgI , 1.0~2.0mmol/L Cu(CH3SO3)2 ,0.54~0.66mol/L K4P2O7 ,1.05~1.65mol/L KI ,0.15~0.3mol/L TEA,1~3g/L hydroquinone,4~9.6mmol/L heliotropin,20~30℃temperature,5.0~6.5 pH value,4-6A/dm2 current density. The coatings electrodeposited from above baths were composed ofβ-Sn, Ag3Sn and Cu6Sn5, which determined by X-ray diffraction (XRD). X-ray fluorescence (XRF) analysis showed that the coatings were 2mass%-6mass% Ag, 0.5mass%-2mass% Cu and rest Sn contained. Scanning electron microscopy (SEM) showed that the surface morphology of the coatings was dense and smooth. Banding tests and nicking tests results showed that the coatings integrated with Fe/Ni plates well. The corrode resistance and solderable properties of Sn-Ag-Cu alloy coatings were similar with 63Sn-37Pb alloy coatings.Electroplating bath disperse ability was 11.5% (T=5), which determined by Far-Near Cathode Method. Inner Hole Method result was that the bath overlay ability was d/φ=2. These results were not very good, so, the electroplating process needs to improve later.Liner sweep voltammetry (LSV) testing results showed that the addition of complexing agents (K4P2O7, KI and TEA) and bright agent (heliotropin) made well co-electrodeposition of Sn, Ag and Cu.