Maximum Via Count
Purpose: Set the maximum number of vias for a trace or net to maintain impedance control and minimize parasitic capaci-
tance. Each via introduces discontinuities and can degrade signal integrity, especially at high frequencies.
Steps:
1. Within the Constraint Manager, navigate to the Electrical > Electrical Constraint Set > Routing > Vias worksheet.
2. Then either by creating a constraint set or using an existing one (like the ECS1 below), set the Max Via Count to 2.
3. Now you can apply this constraint set to specific differential pairs or nets by going to the Electrical > Net > Routing > Vias
worksheet.
4. In our case, the DP_DATA1 has the ECS1 Constraint set and the via limits applied. Notice how the actual number of vias
on the DP_DATA1 differential pair are within the acceptable limit of vias established in the Electrical Constraint Set
(ECS1).
Impact: It is easy to forget so many important factors in PCB design that are all crucial for high-speed designs. For instance,
for USB 3.2, it's strongly recommended to use no vias or layer transitions. However, if transitions (vias) are needed, to use no
more than two per differential pair (i.e. one via per trace).
Constraining the number of vias ensures that we do not inadvertently introduce errors into our board because of forgetting
the via limits. Let the constraint manager do the remembering for you as a designer.
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OrCAD X High-Speed Digital Design Guide
