Skin Depth Considerations: How Copper Roughness Degrades Differential Signal Propagation Above 100Gbps

This paper analyses the high speed effect of copper foil roughness based on High Density Interconnect Printed Circuit Boards (HDI PCBs) with Co-Packaged Optics (CPO) substrate using Semi-Additive Process (SAP). The lower surface roughness of the copper foil is determined by the type of copper foil, while the upper surface is related to electrodeposition and roughening. The effects of signal and reference surface roughness on insertion loss and impedance of microstrip GCPW and strip GCPW lines are investigated using the Groiss model and Sa parameter modelling simulations. The results show that the roughness of the lower surface of the copper foil significantly affects the insertion loss, especially in the microstrip GCPW line, where the 3 µm VLP-2 copper foil increases the S21 by about 7% and decreases the TDR to 97 Ω at 50 GHz; whereas the impedance of the strip GCPW line rises but the insertion loss does not change much. The upper surface roughness has a more significant effect on the insertion loss, with S21 rising sharply before the roughness of 0. 48 µm and then stabilising. The TDR of microstrip GCPW line fluctuates more (100 Ω to 96 Ω), while that of strip is smaller (100 Ω to 101 Ω), indicating that the former impedance is more sensitive. The increase in the roughness of the reference surface causes the insertion loss to rise, and the TDR varies from 95 Ω to 101 Ω from 0. 18 μm to 0. 52 μm for the microstrip GCPW line, with a smaller effect for the strip (100. 2 Ω to 100. 9 Ω). This work is instructive for signal integrity design and optimization in high-speed communications, data centers, and high-performance computing.