An automatic custom block composition tool for composing custom blocks of an integrated circuit (IC) design that may include non-standard library cells. The tool includes program instructions that are executable to create and use a placement control file that includes instructions for use by the custom block composition tool to place the one or more non-standard library cells into the custom block layout. In addition, the program instructions may instantiate a leafcell for each non-standard and each standard library cell included in a netlist. The program instructions may access the placement control file and place each leafcell in a row of the custom block layout according to the placement control file. The program instructions may also pre-route power, clock and critical network signals, and generate a router control file used during remaining routing of the custom block by a conventional router tool.