In this paper, a deep bandgap behavior analysis of the vertical cascaded electromagnetic-bandgap (EBG) structure is made. It is shown that the vertical cascaded EBG structure can be decomposed into two EBG structures cascaded horizontally, one with the bigger patches and the other with the smaller patches. The design guidelines of the vertical cascaded EBG structure are drawn. Furthermore, the vertical cascade concept is extended to 3-D cascade for wideband simultaneous switching noise (SSN) suppression. The number of rows of patches for noise coupling reduction is investigated. Building SSN isolation walls along a printed circuit board for wideband electromagnetic-interference reduction and along sensitive devices for SSN isolation using a 3-D cascaded EBG structure is proposed. Simulations and measurements are performed to verify the SSN suppression. High performance is observed.