Effects of organic acids on through-hole filling by copper electroplating

Through-hole (TH) filling of a printed circuit board (PCB) by copper electroplating was performed using formic acid, acetic acid and propionic acid instead of H2SO4. Tetranitroblue tetrazolium chloride (TNBT) was used as an inhibitor to perform a fast copper protruding deposition at the hole center during plating. The use of an organic acid instead of H2SO4 overcomes two issues caused by H2SO4: poor copper filling performance at high H2SO4 concentrations and a chap-like copper structure in the filled TH at low H2SO4 concentrations. The filling performance using an organic acid was evaluated through the experimental examination of cross-sections of THs. The inhibition mechanism of TNBT on copper deposition was characterized by galvanostatic measurements with real-time injection of various acids, cyclic voltammetry (CV), linear sweep voltammetry (LSV) and UV-vis spectrophotometry. The results show that TNBT is electrochemically reducible only in the presence of protons. Its inhibiting strength and mechanism depend on the pH value, the chloride ion concentration and the cathodic potential. An electrochemical reaction mechanism was proposed herein to explain the filling plating behaviors and results.