Challenges of using flex cables in high speed serial links

Blade servers are constantly moving towards higher data rates and smaller form factor resulting in complex routing choices to accommodate various chassis configurations. Flexible printed circuit (FPC) cables or flex cables serve as a good choice for interconnect medium in such densely configured server systems. In this paper, various challenges involved in designing a high speed serial link using flex cables are discussed. The impact of flex cable stack-up choice on signal integrity (SI) performance (impedance and loss) is studied. Sensitivity analysis on flex cable impedance and loss are performed by taking various flex cable design parameters into account using 3-D full wave modeling results. Best design practices needed for FPC cables are discussed in detail. Finally, SATA 3.0 interface is used as an example to demonstrate the design trade-offs of flex cable.