Allegro - Tip of the Week: Adding auto-voids under SMD pads

Padstack Editor supports adding route keepout geometry as part of its definition. These keepout objects can be controlled on each layer of the pad structure or on adjacent layers that can extend beyond the begin/end layers. When considering an adjacent layer keepout strategy, you must first define the geometry in the pad definition or instance and then apply the following properties to pins or vias at the board level:

• Adjacent_Layer_Void_Above

• Adjacent_Layer_Void_Below

The maximum property value is 8 (layers) and is applied to consecutive layers. The librarian controls the adjacent layer geometry in the pad definition while the layout designer adds the layer depth control via listed properties. All pad figures, except Donut Pad, may be used to define the keepout figure.

Follow these steps to define ADJACENT LAYER keepouts:

1. Define the Keep Out geometry and size in Padstack Editor.

2. In the layout, go to Edit > Properties, enable ‘Pins’ in the Find Filter, and add the “Adjacent_Layer_Void_Above” or “Adjacent_Layer_Void_Below” properties to the pins with the required layer value.

3. Keepouts are created on consecutive layers as per the number defined for the properties.

Voids under SMD pads can be used to control impedance when the trace enters the pad and voids associated with buried/blind vias. These keepouts are associated with the symbols and therefore move with the symbols.

Team PCBTech

Cadence Design Systems