Methods for forming a stacking interposer are provided that create a more compact and/or reliable interposer cavity. According to one method, a segmentation process that partially cuts a multi-cell, multi-layer PCB panel to a controlled depth along the internal walls/edges of a cavity region with each of the interposer cell sites defined within the PCB panel is used. The material within the cavity region is then removed (by routing) to a controlled depth to form the internal cavity for each interposer cell site. Pillars may then be removed from the PCB panel. As a result of the initial partial cuts of the internal walls of the cavity region, the corners of the cavities may have a square configuration for fitting over the top of a BGA/memory device (which has very square corners).