Understanding Molecular Properties of Bismarck Brown R and Its Effects on the Electroreduction of Copper Ions at a Polarized Interface: Toward Void-Free Filling of Microvia

Printed circuit boards (PCBs) serve as fundamental building blocks of modern electronics, and the quality of copper electroplating in microvias is crucial to the reliability and performance of high-density interconnect products. Achieving void-free bottom-up filling of microvias remains a key technical challenge due to the nonuniform current distribution and mass transport constraints inherent in electroplating. In this work, Bismarck Brown R (BBR), an aromatic azo-containing organic molecule, is introduced as a leveler for acidic sulfate copper electroplating. The molecular structure, adsorption mechanism, electrochemical behavior, and synergistic effects of BBR are systematically elucidated using DFT calculations, MD simulations, electrochemical measurements, and in situ FTIR spectroscopy. DFT and MD results reveal that BBR adsorbs strongly onto the copper surface in a planar configuration via combined physical and chemical interactions with an adsorption energy of −693.67 kcal/mol. Electrochemical analysis demonstrates that BBR alone accelerates copper deposition at the bottom and inhibits it at the mouth of the microvias. More importantly, BBR synergistically enhances the inhibiting effect of EO-PO and the accelerating effect of SPS, with such synergy being more pronounced at the microvia mouth. In situ FTIR confirms that both EO-PO and SPS promote BBR adsorption on the copper surface. The ternary additive system (EO-PO + SPS + BBR) achieves a positive relative charge difference (Δ Q = 6.95%), indicative of a superior bottom-up filling performance. Electroplating experiments validate that void-free, dense copper filling of microvias is achieved exclusively in the presence of BBR. Furthermore, BBR modulates the preferred crystallographic orientation of the copper coating from (200) to (111). This work provides a comprehensive molecular-level understanding of BBR as a high-performance additive and offers an effective strategy for void-free microvia filling in advanced PCB manufacturing.