Thiazolyl Blue as a High-Efficiency Leveler for Conformal Copper Electrodeposition in High-Aspect-Ratio PCB Through-Holes

Driven by the rapid development of 5G/6G communications, high-performance computing, and artificial intelligence, the metallization of high-aspect-ratio through-holes (THs) in printed circuit boards (PCBs) imposes stringent requirements on the thickness uniformity and interconnect reliability of electroplated copper layers. This study systematically investigates the application of a novel leveler, thiazolyl blue (MTT), for PCB THs’ electroplating, successfully realizing high-quality conformal deposition in high-aspect-ratio (10:1) THs. Electrochemical analyses demonstrate that MTT significantly enhances the cathodic polarization and effectively inhibits the reduction of Cu²⁺. Meanwhile, microscopic morphological characterizations confirm that MTT exhibits excellent leveling capability, reducing the surface roughness of the copper deposit to as low as 0.147 μm. Furthermore, density functional theory (DFT) calculations reveal that the high inhibitory efficiency of MTT originates from the dominant interfacial adsorption of MTT at the cathode, with the bulk cation−π interaction with Cu²⁺ contributing as a minor secondary effect at the microscopic level. Crystallographic and interfacial wettability analyses further indicate that MTT effectively refines the grain size and promotes the preferential growth of the Cu(200) plane while improving the wettability of the plating bath. In practical high-aspect-ratio through-hole plating tests, the introduction of MTT substantially increases the throwing power (TP) from 25.2% to 96.8%. Based on these results, a competitive interfacial polarization remodeling mechanism governed by multiphysics coupling is proposed to elucidate the intrinsic role of MTT in promoting conformal through-hole deposition. This work provides a solid theoretical foundation and practical engineering guidance for the fabrication of highly reliable, high-frequency 3D interconnected PCB structures.