Through-hole plating of Cu by modulated current deposition

In the manufacturing of printed circuit boards, demands on the uniformity of copper deposits are increasing. Of special importance are modern surface-mounted boards where a uniform copper layer is also required in holes as small as 0.15 mm in diameter. For these applications, plating with a modulated current is a promising technique for providing the necessary uniformity. In this study, copper was deposited with a pulsed rectangular cathodic current and with a periodically reversed current. The current frequency was varied to study the influence on the through-hole plating capability of a commercial “high throw” copper bath. It was found that by pulse plating at high frequencies (above 1 kHz), the uniformity was improved compared with that of d.c. deposits. At lower current frequencies, the uniformity was worse than that by d.c. plating. The appearance of pulse plated deposits was as bright as that of deposits obtained by d.c. plating. Deposits prepared by periodic reverse plating exhibit a similar uniformity to the pulse plated deposits; the appearance was, however, completely dull. The effect of brighteners was lost as these were desorbed during the reversed part of the plating cycle. Furthermore, chlorine evolution occurred at the pure copper cathodes during the reversed pulses.