AN 766: Intel® Stratix® 10 Devices, High Speed Signal Interface Layout Design Guideline

ID 683132
Date 3/12/2019
Document Table of Contents

Comparison of Dog-bone with GND Cutout Under the BGA and Via-in-Pad Configurations

Figure 10. Comparison of TDR from the FPGA Pad of Via-in-Pad and Dog-bone ConfigurationsThe pink line indicates the dog-bone configuration and the green line indicates the via-in-pad configuration.

In terms of TDR, both configurations have similar performance.

Figure 11. Full channel scattering parameter performances for Dog-Bone Configuration with GND/Void cutout underneath of BGA (Option 2) vs Via-in-pad (Option 1) ConfigurationsFigure shows the identical channel but uses the via-in-pad or dog-bone configuration in the FPGA area.
  • Option 1 (Green): Via-in-pad.
  • Option 2 (Pink): Dog-bone with GND/Void cutout under BGA pad configuration.

In this example, the via-in-pad shows slightly better performance. The other advantage of via-in-pad is that it provides more space for fan-out routing.

Figure 12. TDR Performance and TL Impedance ImpactThe example uses a dog-bone fan-out configuration and one layer GND cutout under the BGA pad by various TL routing impedances.

This example shows that slightly lowering the TL routing impedance ensures a smoother transition from the BGA pad, which results in less reflection on the channel. Intel recommends a 95Ω TL routing impedance for high speed serial interface (HSSI) channels.