Via Structures – A ‘must have’ utility to speed up the design process

In all modern high-speed digital board designs, every discontinuity on the board must be considered carefully, especially the vias, which are abundantly used in digital design. As frequency increases and signal rise time reduces, via causes impedance discontinuities, resulting in signal reflections, hence, deterioration of signal integrity (SI) and system performance.

Via Structures in Allegro PCB Editor are user-defined sets of objects for replication and reuse across one or more designs. Unlike modules, via structures do not store components or logic, making them less powerful but easily reusable. Exporting and maintaining structures in a library format makes it easy to reuse. Once created, you can import and export these structures for reuse in other designs between different databases, such as Allegro PCB Editor and Allegro Package Designer Plus (APD+).

A single unified Create Structure form combines the Standard, HighSpeed, and L Comp forms into one easy-to-use form while adding descriptions and walk-throughs on how each can be created. Graphics provide visual examples of each structure, making it easier to identify possible use cases for each. You can create three types of via-structure symbols in Allegro PCB Editor.

1. HighSpeed Via Structure: High-speed via structures combine vias, connect lines (clines) or traces, static shapes without voids, and route keepouts. A high-speed via structure can comprise differential pairs with return path vias and route keepouts for custom voiding.

2. L Comp: The inductive compensation loop (L Comp) structures are created by combining connect lines and route keepouts. L Comp is a ¾-turn circular loop structure added to the end of traces before they are connected to the socket pins of the DIMM. These structures are used to balance out the effect of socket capacitance to gain higher performance and also require plane voiding in the surrounding ground or power supply reference planes.

3. Standard Via Structure: You can combine patterns of vias and clines to create a standard via structure symbol. Chosen vias and clines must all be connected and belong to the same net. They may be connected to only one pin, whose location becomes the symbol origin. Duplicate symbol names are not created. A via structure must start and end on the subclasses specified in the create fanout command’s Start and End subclass fields when creating fanouts.

Team PCBTech

Cadence Design Systems