PCB Design Blogs

When designing a PCB layout, all the constraints and design rules must be followed to avoid design issues and ensure that the board works as intended. Constraints are rules that you apply to different objects in your design, nets, and XNets to ensure that there are no manufacturing problems in the board. For example, to maintain a 10mils distance between all the nets in a design, you can create a spacing constraint, specify its value as 10mils, and then apply the constraint to all the nets in the design.

Allegro PCB design layout tools are equipped with an advanced constraint manager system, Allegro® Constraint Manager that provides several checks and constraints to govern the electrical, spacing, and physical behavior of a design. In this blog post, we will discuss how to capture and manage spacing constraints sets in Allegro Constraint Manager.

A spacing constraint is a rule that defines spacing between two physical objects of a PCB to avoid interference with the signal of adjacent objects. For example, line-to-line spacing constraint is an example that applies between two lines or nets on a board. Before we move any further, let’s first understand what a constraint set is. By definition, a constraint set or a CSet is a named, reusable collection of constraint values. This essentially translates to a group of constraints or rules that are bundled together for quick and easy assignment on design objects. 

Creating Spacing Constraint Sets

You can open Allegro Constraint Manager directly in the Spacing domain where you define SCSets. You can also open Allegro Constraint Manager from the Cmgr icon on the toolbar and then navigate to the Spacing domain.

To create an SCSet, follow these steps:

1. In the layout editor, choose Setup – Constraints – Spacing or click the Cmgr icon.
2. Ensure that you are in the All Layers sheet under the Spacing Constraint Set.
3. Right-click the design name cell under the Name column and choose Create – Spacing CSet from the pop-up menu.
4. Specify a name for the SCSet, say 10MILSPACE, and click OK.

As you can see in the following image, a new row with the name 10MILSPACE is created which is the SCSet you just created. You can change the values of all the constraints in this row, for example to 10, if you want all the nets to be 10mils apart while routing in the design. Easy, isn’t it?

All new designs created in Allegro® PCB Editor have a DEFAULT CSet available. But, don’t worry, you can always change the values of these default CSets to suit your requirements.

Applying Spacing Constraint Sets

Applying SCSets to objects is a matter of a couple of clicks. To assign the 10MILSPACE SCSet to an individual net, click the cell under the Referenced Spacing CSet column for a net and select 10MILSPACE. Now, all the other nets while routing will be 10mils away from this net on the design.

Instead of assigning SCSet to individual nets, you can assign an SCSet to multiple nets in one go. Let’s extend this example here. To assign this SCSet to the nets with names starting with the letter A, follow these steps:

1. Create a class by selecting the nets in the Name column that start with A.
2. Right-click and choose Class from the pop-up menu.
3. Type the name as Anets and click OK.

           A new Net Class ANETS is created.

4. Now, click the cell under the Referenced Spacing CSet for ANETS and select 10MILSPACE.
5. Expand ANETS.

As you can see in the following image that the SCSet 10MILSPACE is assigned to all the nets that start with the letter A.

Similarly, you can apply SCSets to other objects in your design.

To delete an SCSet, in the All Layers sheet under the Spacing Constraint Set folder, right-click the SCSet in the Name column and choose Delete from the pop-up menu. Also, note that you cannot delete the DEFAULT CSets.

To learn in detail about this flow, watch the Creating and Applying Spacing Constraint Sets within the Constraint Manager training byte on the Cadence Support portal. Click the training byte link now or visit Cadence Support and search for this training byte under Video Library.

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