Protect Differential Pair Reference Planes During Shape Degassing

Copper shape degassing is commonly used to improve manufacturability, but when voids are generated beneath high-speed differential pairs on adjacent layers, they can unintentionally disrupt return current paths and reduce reference plane continuity. For SI-sensitive interfaces, maintaining solid copper under differential pairs is often desirable to preserve predictable impedance behavior.

To avoid this, Allegro X Advanced Package designer/APD provides the Degas_No_Void property, allowing designers to exclude critical differential pairs from degassing operations and maintain a more uniform copper reference environment.

How to?

1. Identify and select the differential pair nets.

2. Apply the Degas_No_Void property to the selected differential pairs.

3. Run the Shape Degassing command and configure the desired degassing pattern and click Generate.

4. Review the degassing results and verify that voids are excluded beneath the protected differential pair regions while standard degassing is maintained elsewhere.

Have you encountered SI concerns caused by degassing beneath critical routing, or do you use other techniques to preserve reference-plane integrity around differential pairs? Share your experience below.