A first semiconductor package of a POP structure has a first body and a plurality of first solder balls. A second semiconductor package of the POP structure has a second body and a plurality of second solder balls. A stand-off mechanism is utilized to maintain a minimum gap between the first body and the second body while a reflow soldering process is performed. By performing the reflow soldering process, the first solder balls and the second solder balls are heated and engaging with one another so as to solder the first solder balls and the second solder balls to form a plurality of interposer solder balls. Each interposer solder ball has a height substantially equal to the minimum gap and a cross sectional width less than a pitch between two adjacent interposer solder balls. Thereby, the POP structure would be a fine pitch package.