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22 DESIGN007 MAGAZINE I AUGUST 2021 As frequency and rise times increase, SerDes design is becoming more critical. Losses caused by the skin effect, fiber weave effect, surface roughness, vias, and connectors are oen too excessive for reliable communications. PCB dielectric materials such as AGC (Nelco) N4000-13SI, Isola I-Speed and Panasonic Meg- tron 6 are reasonably priced, low loss, temper- ature stable, moderately flat weave laminates applicable to the 2, 4 and 10 Gbps channels. However, for such applications as PCIe-Gen 4 (16Gbps) and higher, one may need to con- sider alternatives for stackup construction to meet the higher performance requirements. So far, we have been able to deal with fiber weave skew, using a variety of physical lay- out and laminate construction techniques, while still using the more common existing PCB materials. Isola GigaSync material, for instance, was developed in 2013 to specifically reduce fiber skew, but unfortunately its dis- sipation factor (Df ) and dielectric constant (Dk) are too high for today's serial channels. ese homogenous materials try to match the Dk of the polymer to that of the glass, with some boundary modification also being per- formed where the polymer and fibers contact. is effectively eliminates glass weave skew. Unfortunately, glass is still fairly high Dk when compared to the newer polymers that come in the 2.5-2.8 range. A better alternative is to either use hybrid stackups with mixed woven and non-woven materials to lower the cost or use spread-glass material. Isola I-Speed MS, for instance, is mechanically spread glass, expanded in both the warp and fill direction, which is an improvement especially if two-ply construction is employed. But as frequency continues to soar, cutting edge interconnects running signals at 28 Gbps Fly-over Technology— When It All Gets Too Fast Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Twinax Cable Flyover TM Technology. (Source: Samtec Inc.)

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