We have studied a thermo-compression bonding method for high density interconnections. Fluxes are commonly used in conventional solder bonding. However, flux applications have several issues such as the void generation in solder and the flux residue remaining between bumps. These could degrade their reliabilities seriously when the bump pitch becomes small since these features do not scale to bump-pitch dimensions. In this paper, we present the experimental results on the investigation of flux-less bonding with a hydrogen radical plasma treatment in fine-pitch bump joining, in order to eliminate these issues. Experimental results of 10 μm-pitch solder bonding showed good metallic continuities at joining interfaces without any organic residues, showing advantages of flux-less bonding on fine-pitch solder bonding. © 2014 JIEP.