Signal integrity analysis of a 26 layers board with emphasis on the effect of non-functional pads

Aim of this paper is the signal integrity analysis of multilayer boards with emphasis on the effect of non-functional pads. In particular it is demonstrated how removing the nonfunctional pads to reduce the coupling mechanisms of the pads with the planes above and/or below can not be considered a general rule of thumb due to possible larger impedance variation (TDR). Accurate and reliable numerical simulations are in this case really important in order to correctly determine the impact of those non-functional pads on the signal integrity. A 26 layers board is analyzed by simulating with a three dimensional EM field solver a portion of the original Allegro file.