The Korean Society of Surface Science and Engineering

This study explores the microstructural evolution, crystallographic development, electrochemical stability, discoloration resistance, and electrical performance of non-cyanide silver (Ag) coatings engineered for high-reliability electric vehicle (EV) relay contacts. Ag coatings with thicknesses ranging from 0.25 to 1.50 μm were deposited using a thiosulfate-based electrolyte containing thiourea, polyoxyethylene, and amine-type additives. Surface characterization revealed progressive grain refinement—from 125 to 75 nm—and a reduction in surface roughness from 43 to 28 nm, resulting in a highly compact and uniform microstructure at a thickness of 1.00 μm. X-ray diffraction analysis further confirmed enhanced crystallinity and a strengthened Ag(111) preferred orientation at this optimum thickness, accompanied by reduced peak broadening. Electrochemical measurements demonstrated improved corrosion resistance with increasing thickness, as evidenced by a positive shift in corrosion potential and a significant decrease in corrosion current density. Impedance analysis showed the highest charge-transfer resistance (4.1 kΩ·cm²) at 1.00 μm, indicating the most stable electrochemical interface. Accelerated aging tests (85 ℃, 85% RH) revealed excellent discoloration resistance, with ΔE values below 1.5 for 1.00 μm and thicker coatings. Electrical assessments confirmed low surface resistivity (~0.43 mΩ/sq) and superior thickness uniformity (±4–7%), both critical for maintaining stable contact resistance under repeated relay switching. These findings demonstrate that the optimized 1.00 μm Ag coating achieves a balanced enhancement in microstructural compactness, crystallographic stability, corrosion resistance, and environmental durability. The results provide a practical design guideline for eco-friendly, non-cyanide Ag plating processes for EV relay contacts and other high-reliability electronic components.

키워드 Silver plating; Discoloration suppression; EV relay; Electrochemical analysis; Process optimizatio.