The presently claimed invention is to provide a package for compact RF signal system, and a method to form the package thereof in order to miniaturize the size of package, improve signal integrity, and reduce manufacturing cost. The package comprises a hybrid substrate with a sandwiched structure, in which the hybrid substrate comprises an upper layer and a lower layer with different dielectric properties being separated by an interposer for improving electrical isolation and mechanical stiffness. Metal layers are formed on the sidewalls of the opening to surround an active component, such that the metal sidewalls together with two ground plates in the upper and lower layers constitute a self-shielding enclosure inside the package to protect the active component.