Characterization of Through-Hole Filling by Copper Electroplating Using a Tetrazolium Salt Inhibitor

Abstract

Through-hole (TH) filling of an advanced printed circuit board (PCB) by copper electroplating is performed using a direct current (DC) plating method and a simple copper plating formula composed of a single organic additive, namely, nitrotetrazolium blue chloride monohydrate (NTBC). Copper is preferentially deposited at the hole center to form a butterfly-shaped copper cross-section in the filled TH during plating. Three key factors are identified for enhancing the filling performance via a center protrusion deposition in the TH and are characterized by practical filling plating results and cyclic voltammetry (CV) analyzes. The filling performance of the copper plating formula for the TH is sensitive to forced convection, chloride ion concentrations and H2SO4 concentrations. An appropriate forced convection combined with weak accelerators [3-(Benzothiazolyl-2-mercapto)-propylsulfonic acid, sodium salt (ZPS) or 3-S-Isothiuronium propyl sulfonate (UPS)] pre-adsorbed on the copper seed layer of the TH can effectively shorten the fill plating time from 9 h to 5 h. The center-protrusion filling mechanism and the role of NTBC in the TH filling are discussed in this work based on the plating results and electrochemical analyzes.