The effects of the plating current density (j) on the electroplated Cu microstructure, impurity distribution, and the solderability of the electroplated Cu were investigated. Analyses of electron backscatter diffraction and transmission electron microscopy showed that the Cu grain size (D) decreased from microscale to submicron-scale, and the predominant crystallographic orientation translated from 111||ND+101||ND into 101||ND (ND: Cu deposition direction) with increasing j. Time-of-flight secondary ion mass spectrometry analysis showed that the impurity content in the Cu platings (especially the Cl content) increased as a function of j. These investigations showed that j posed a significant influence on the characteristics of the electroplated Cu. Two intermetallic compound (IMC) species (η-Cu6Sn5 and Cu3Sn) accompanied with some material defects (e.g., nanovoids and microcracks) formed at the solder/Cu pillar interface after reflow. The IMC (Cu6Sn5) growth morphology and orientation, and the defect distribution were also strongly j-dependent. These j-dependent interfacial microstructures were closely related to different D and impurity contents of the Cu platings. The results of this study advanced our understanding of the effects of j on electrochemical metal deposition, and they were helpful in the development of the high-speed Cu electrodeposition technology. •High-speed Cu electrodeposition and its solderability•Strong effect of plating current density (j) on Cu pillar bumps•Strong dependences of Cu grain size and Cu orientation on j•Nanocrystallized Cu microstructure was obtained with a high j.•High j caused nanovoid formation at the interface, degrading joint reliability.