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

4.7.2. General Guidelines for GTS Transceiver Ethernet Interface

  • Intra pair skew: ±1 mil

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 and so forth for microstrip routing. Altera recommends to use a 3D simulation for optimization the BGA breakout. 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 31. 90 Ω Top Layer Breakout StructureThis figure shows the detailed structures on SoM for the top layer.
Figure 32. 90 Ω Layer 3 Breakout StructureThis figure shows the detailed structures on SoM for the Layer 3 Fan Out.
Figure 33. 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 34. 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 following figure, the left diagram shows simulation results for 18 mil pad with the same cut-out size as 16 mil pad. The impedance is lower due to large capacitance caused by larger pad size, and the return loss is approximately 3 to 4 dB higher, however still meeting -15 dB requirement. A larger anti-pad size, such as 28 mil improves return loss performance as shown in the right diagram.

Figure 35. Simulation Results of Layer 3 Breakout (18 mil Pad Size)This figure shows the simulation results using the same cut-out size (left) and larger anti-pad size (28 mil diameter) (right).

The following figure shows Layer 5 breakout structure, where there is no Layer 5 breakout on SoM. This is for simulation study reference.

Figure 36. 90 Ω Layer 5 Breakout Structure
Figure 37. Simulation Results of Return Loss for Layer 5 Breakout