<p>The outbreak period during microvias filling by copper deposition was identified and characterized via staged electroplating analysis of copper layer deposition dynamics. Leveraging this critical phase, an innovative method was developed to improve filling performance by controlling accelerator addition during the outbreak period. Experimental results demonstrated that adding 0.4 mg L^− 1 bis-(3-sulfopropyl)-disulfide (SPS) at the start of the outbreak period effectively redistributed additives between the interiors and exteriors of microvias, thereby mitigating the decline in intra-microvia copper deposition rate. This approach significantly improved filling performance from 81.0 to 91.1%. Chronopotentiometric characterization revealed that SPS addition had a more pronounced effect on copper deposition inside microvias than in external regions. The experimental findings validated the proposed secondary additive redistribution model, which elucidated the fundamental mechanism behind the improved filling performance. </p> Graphical abstract <p></p>