A manufacturing method of bottom substrate of package. A bottom substrate of a package on package electrically connected to a top substrate by means of a solder ball, including a core board, a solder ball pad formed on a surface of the core board in correspondence with a location of the solder ball, an insulation layer laminated on the core board, a through hole formed by removing a part of the insulation layer such that the solder ball pad is exposed, and a metallic layer filled in the through hole and connected electrically with the solder ball, allows the number of ICs mounted on a bottom substrate to be increased without increasing the size of a solder ball, and allows the size and pitch of the solder balls to be made smaller by controlling the thickness of the insulation layer laminated on the bottom substrate, whereby more signal transmission is possible between a top substrate and a bottom substrate.