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In this work, a novel, simple, and green citrate-based copper electronic electroplating bath with the characteristics of weak corrosion, low copper ion concentration, and simple composition is developed in microvia void-free filling for printed circuit board application. Succinimide (SM), regarded as a leveler, is the core component of the bath. The action mechanisms of SM on microvia void-free copper filling are carefully expounded. UV–vis spectroscopy and theoretical calculations demonstrate that only the [Cu2(Cit)2]2– coordination ion exists in the bath containing SM at pH 9.0. Linear sweep voltammetry and in situ Fourier transform infrared spectroscopy combined with X-ray photoelectron spectroscopy experiments illustrate that SM can inhibit the electroreduction of the [Cu2(Cit)2]2– coordination ion and promote the electroreduction of intermediate Cu(I) to Cu(0), resulting in the improvement of coating quality. Galvanostatic measurements reveal that the reduction of the [Cu2(Cit)2]2– coordination ion is electrochemically controlled, and SM is diffusion-controlled. The leveler properties of SM (diffusion control and inhibition of [Cu2(Cit)2]2– coordination ion electroreduction) promote copper in bottom-up growing and void-free filling of microvias.

In this work, an electronic copper electroplating bath with low copper ion concentration and preeminent throwing power in through hole (TH) thickening of the printed circuit board (PCB) was successfully developed. Based upon the weak alkaline bath containing the composite complexants of citrate (Cit) and ethylenediamine (En), their synergistic effects on the preeminent bath throwing power were carefully investigated and theoretically proved. The UV‐vis spectroscopy results illustrated that En exhibited a stronger coordination ability with Cu (II) ion than citrate, and the stoichiometric coordination ratio between En and Cu (II) ion was 2 : 1. The linear sweep voltammetry (LSV) results showed that the electro‐reduction peak potential of Cit−Cu coordination ion was −0.55 V (vs. Hg/HgO), whereas that of En−Cu ion was at a more negative potential of −0.82 V. The in situ Fourier transform infrared (FTIR) spectroscopy revealed that the adsorptions of the Cit−Cu and En−Cu coordination ions were potential dependent on the copper electrode. The Cit−Cu coordination ion was easily adsorbed at the interior sites of through holes with lower over‐potential to promote the growth of copper coating. Moreover, the En−Cu coordination ion adsorbed preferably on the surface and the edge of through hole for its higher electro‐reduction overpotential, to hinder the germinating of copper nuclear. The cross‐section observation of optical microscope proved that the throwing power of novel copper electroplating bath behaved surpassingly, up to 105.8 %.

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