When routing differential pairs through a copper plane or pour, voiding is dependent on the defined spacing constraint; however, in some situations, impedance may need to be improved by configuring the via voiding as a pair or group instead of individually. With the high-speed option in Allegro PCB Designer, users can easily define the desired return path configuration and control the voiding to improve the differential pair impedance.
This quick how-to will provide step-by-step instructions on how to adjust the voiding for differential pair return path vias with the High-Speed Option in Allegro PCB Designer.
How-To Video
Configuring Via Voiding
![](https://www.ema-eda.com/wp-content/uploads/2023/03/viavoiding_step1-799x1024.png)
Step 1: Open the desired design in Allegro PCB Designer with the High-Speed Option enabled.
Step 2: Select Shape > Global Dynamic Params… from the menu.
![](https://www.ema-eda.com/wp-content/uploads/2023/03/viavoiding_step3-1024x791.png)
Step 3: Select the Void Controls tab and choose DiffPair combined void for vias added with Return Path option.
Step 4: Click Apply and OK.
![](https://www.ema-eda.com/wp-content/uploads/2023/03/viavoiding_step4note.png)
Note: The voided shape for the trace with return path vias added has been adjusted.
Adding Return Path Vias During Routing
Step 5: Select Route > Connect from the menu or the Add Connect button on the toolbar.
Step 6: Click to select one of the nets in the differential pair.
![](https://www.ema-eda.com/wp-content/uploads/2023/03/viavoiding_step7-1.png)
Step 7: Right-click and select Return Path Vias > In Line.
Note: Other Return Path Via Configurations are available including:
- 1 via: This pattern adds 1 return path via next to one of the differential pair vias.
- In Line: This pattern adds 2 return path vias aligned in a straight line with the differential pair vias.
- Equidistant: This pattern adds 1 return path via placed equidistant between the differential pair vias.
- Offset: This pattern adds 2 return path vias offset with respect to the differential pair vias.
- Diamond: This pattern adds 2 return path vias placed equidistant on each side of the differential pair vias.
- Rectangular: This pattern adds 4 return path vias in a rectangular/square pattern centered on the differential pair vias.
![](https://www.ema-eda.com/wp-content/uploads/2023/03/viavoiding_step8-1-1024x411.png)
Step 8: If you receive a message that Return Path Vias is a prototype feature, click OK.
Note: If a return path via has not been configured, you will be prompted to set one up. Learn how to configure return path vias with this how-to.
Step 9: Double-click the PCB canvas to activate the via and return path configuration.
Note: The vias defined for the differential pair have been added to the cursor along with the selected return path vias.
Step 10. Click to place the vias on the PCB.
![](https://www.ema-eda.com/wp-content/uploads/2023/03/viavoiding_step11-1-1024x704.png)
Step 11: Right-click and select Done. The clearance around the vias is grouped together rather than individually voided.
Wrap Up & Next Steps
Improve the impedance on high-speed differential pairs by controlling the return path via voiding when routing through copper with the high-speed functionality of Allegro PCB Designer. Check out our e-book to learn more about solving common issues for high-speed designs.