Multilayer high speed flex printed circuit boards (FLEX-PCBs) are disclosed including a dielectrics systems with the back-side trenches, adhesives, signal lines and ground planes, wherein the signal line and ground plane lane are located on the dielectrics. Using of the open trenches in the substrate help to reduce the microwave loss and dielectric constant and thus increasing the signal carrying speed of the interconnects. Thus, according to the present invention, it is possible to provide a simply constructed multiplayer high speed FLEX-PCB using the conventional material and conventional FLEX-PCB manufacturing which facilitates the design of circuits with controlled bandwidth based on the trench opening in the dielectrics, and affords excellent connection reliability. As the effective dielectric constant is reduced, the signal width is required to make wider or the dielectric thickness is required to make thinner keeping fixed characteristics impedance. The fundamental techniques disclosed here can also be used for high-speed packaging.