PCB Design Guidelines (HSSI, EMIF, MIPI, True Differential, PDN) User Guide: Agilex™ 5 FPGAs and SoCs

ID 821801
Date 8/29/2025
Public
Document Table of Contents

4.6.2. General Guidelines for GTS Transceiver PCIe* Gen 4.0 Interface

  • Intra pair skew: ±1mil

BGA Breakout Optimization

Through hole vias with backdrill are used. As outlined in the General Design Considerations section, note the change of impedance control tolerance, total insertion loss, crosstalk and so forth for microstrip routing especially when implementing long microstrip length. Altera recommends 3D field solver simulation for BGA breakout optimization. Try to make return loss lower than -15 dB at Nyquist frequency (lower than -20 dB is better) and control the impedance changing of cut-out area as small as possible (better within ±5 Ω).

Simulation results for 18 mil pad size have been also presented. Both pad sizes can get good performance.

Figure 24. 85 Ω Top Layer Breakout StructureThis figure shows the detailed structures on SoM for the top layer.
Figure 25. 85 Ω Layer 3 Breakout StructureThis figure shows the detailed structures on SoM for the Layer 3 Fan Out.
Figure 26. Simulation Results of Return LossThis figure shows the simulation results of return loss for top layer breakout (left) and Layer 3 breakout (right) using 8 mil drill hole size and 16 mil pad size.
Figure 27. Simulation Results of TDRThis figure shows the simulation results of TDR for top layer breakout (left) and Layer 3 breakout (right) using 8 mil drill hole size and 16 mil pad size.

The impedance is lower due to the large capacitance caused by larger pad size of 18 mil. While the return loss is 3 to 4 dB higher, it meets the -15 dB requirement. A larger anti-pad size such as 30 mil improves return loss performance as shown in the Simulation Results of L3 Breakout Using 18 mil Pad Size figure.

Figure 28. Simulation Results of Layer 3 Breakout Using 18 mil Pad SizeThis figure shows the simulation results using the same cut-out size (left) and larger anti-pad with 30 mil diameter (right).