QSFP28 connector footprint optimization of crosstalk and ILD_rms_dB for 25+ Gbps signaling

This paper discusses optimization of the QSFP28 connector printed circuit board (PCB) footprint area. Crosstalk with baseline footprint was checked, and optimized to meet the CEI-28G-VSR mated HCB-MCB specification. Then the PCB via to trace transition area was modified to improve channel performance. Return loss, and ILD_rms_dB (root-mean-squared of ILD across the frequency range in dB) were used as metrics to find optimal channel. Lastly, channel simulation with AMI models was performed with varied ILD_rms_dB channels. The simulation showed that ILD_rms_dB is an effective metric to optimize the channel by choosing the best Tx/Rx equalization settings. This study also shows a significant improvement in crosstalk by increasing the diameter of ground vias in the circuit board. This simple change yields a reduction in crosstalk and should be considered while optimizing connector footprints.