Many important things (i mean high speed up to 32Gbps): - laminate, for example for highest speed above 10-20Gbps FR4 should be change to MEGTRON4 or even MEGTRON6 - no star topologies, stubs should be as short as possible, best solution is to use uVIA to connect signal from chip on top to first signal layer - series capacitor should be small - 402 or even 201, under capacitor pads you need to remove gnd plane to reduce capacity - routing connection in groups: tx tx tx tx rx rx rx rx is better than tx rx tx rx tx rx - many stiching via (gnd) near signal via