Using thermal reliefs

Thermal relief pads are used as contacts to plane layers or copper zones, for applications such as the connection to power and ground on a multilayer board. There are two things you must do before defining thermal reliefs:

· designate the target layer for the thermal reliefs as a plane layer in the Layers spreadsheet

· assign a net to the layer.

Defining thermal reliefs

You can specify relative dimensions for small and large thermal reliefs by editing the default values in the Thermal Relief Settings dialog box. The dimension options include the sizes for annular over drill, isolation width, and spoke width. Small thermal reliefs are used throughout the board by default. You can assign large thermal reliefs to a particular padstack using the Edit Padstack dialog box (accessed by double-clicking in a cell in the Padstacks spreadsheet).

To specify dimensions for the thermal reliefs

1 From the Options menu, choose Thermal Relief Settings. The Thermal Relief Settings dialog box appears.

2 Edit the settings for the following options in both the Small Thermal Relief group box and the Large Thermal Relief group box, then choose the OK button.

Annular over drill After drilling, the width remaining between the drilled hole and the inside

of the isolation ring.

Isolation Width The width of the isolation ring that surrounds the pad.

Spoke Width The width of the copper tie that connects the pad to the plane.

To assign large thermal reliefs

1 Choose the spreadsheet toolbar button, then choose Padstacks. The Padstacks spreadsheet appears.

2 Double-click on the name of the padstack to which you want to assign a large thermal relief. The Edit Padstack dialog box appears.

3 Select the Large Thermal Relief option, then choose the OK button. Layout assigns a large thermal relief to the padstack. It will have the relative dimensions that you specified in the Thermal Relief Settings dialog box.

Rules that apply to creating thermal reliefs

Layout follows the rules below to determine which pads are assigned thermal reliefs on the plane layers and in what order.

· If the entire net is unrouted, all through-hole pads attached to nets are assigned a thermal relief.

· Routed sections of nets are considered subnets. Each subnet must have at least one thermal relief. Subnets employ the following search order for assigning a thermal relief.

· Vias are always assigned thermal reliefs. For example, if you route between a capacitor on the bottom of the board and an IC on the top of the board, the via will have a thermal relief.

· If the subnet does not find a via, any pad marked as a forced thermal relief becomes the thermal relief for that subnet.

· If the subnet does not find a via or a pad marked as a forced thermal relief, the first pad marked as a preferred thermal relief becomes the thermal relief for that subnet.

· If the subnet does not find a via or a pad marked as a forced or preferred thermal relief, global or standard pads receive thermal reliefs.

· If the subnet does not find a via, a forced or preferred thermal relief, or a global or standard

pad, the pad for the thermal relief is picked at random.

· If no pad fits the correct criteria, a design rule check for dispersion creates an error at each pad that fails to connect to the plane.

Forced thermal reliefs and preferred thermal reliefs

If you designate a footprint pad as a forced thermal relief, then as long as the pad is attached to the appropriate net, the pad is assigned a thermal relief on the plane layers that are attached to that net.If you designate a footprint pad as a preferred thermal relief, then as long as the pad is attached to the appropriate net, the pad will be the first in each subnet (routed portion of the net) to be assigned a thermal relief on the plane layers that are attached to that net. If there is already a via on the subnet, the via will receive a thermal relief, because vias are always assigned thermal reliefs.