In recent years, the butterfly technique (BFT1) was developed to achieve through-hole (TH) filling of a printed circuit board (PCB) by copper electroplating.1It not only can operate in direct current plating but also perform a void-free TH filling. However, a thick copper layer is plated on the surface of a PCB to cause a dimple issue on the TH opening. The dimple on the TH opening has to be leveled off, otherwise the following build-up process cannot be performed because there are a lot of dimples on the inner copper layer. In this work, we adjusted the plating formula to overcome the dimple issue of BFT for TH filling. The plating result shows that a thin copper layer plated on the PCB surface is obtained and the dimple is significantly improved. The enhancement in the TH copper filling performance is beneficial to the build-up process of advanced PCB. Keywords: Through-hole filling, Printed circuit board, Dimple, Copper Electroplating     Reference W. P. Dow, H. H. Chen, M. Y. Yen, W. H. Chen, K. H. Hsu, P. Y. Chuang, H. Ishizuka, N. Sakagawa, R. Kimizuka, “Through-Hole Filling by Copper Electroplating”, Journal of The Electrochemical Society, 2008, 155, D750-D757. Figure 1