Using the Edit Pad dialog box (shown in Figure 4.9), you can change the characteristics of a pad, such as designation, layer placement, size, hole size, and shape.
Figure 4.7. Select from several different shapes of pads. From left, Circular, Rectangular, Octagonal, Rounded Rectangle, Cross Hair Target, and Moire Target.
You can also assign pads to special Power and Ground layers. Pads can be connected to these solid copper layers directly or with a thermal relief plotted as four arc segments (see Figure 4.8).
Figure 4.8. From left, a Relief to Power Plane, Direct to Power Plane, Relief to Ground Plane, Direct to Ground Plane, and Thermal Relief pad.
The colors used for the Power and Ground Plane pads cross hair lines are the same colors defined for the Power and Ground layers. When plotted, Relief pads appear as four arc segments. For more information, see About Thermal Relief
Pads later in this section.
To edit a pad,
1 Select the Arrow Tool from the Tool Bar.
3 Change the pad’s characteristics as desired (see the following table) then choose OK.
4 Choose the scope of the pad changes (just this pad, or other pads as well). See the following table of options.
Figure 4.9. Use the Edit Pad dialog box to change the attributes of one or more pads.
Option Lets You
Designation Assign the pad a designation.
When importing PCB netlists from schematic programs like CircuitMaker, the pad designa- tions of a component footprint must match the pin designations of the corresponding part in the netlist.
Shape Specify a shape for the pad. Choose Circular, Rectangular, Octagonal, Rounded Rectangle, Cross Hair Target, or Moire Target (see Figure 4.7). Layer Type Select whether the pad will be
surface mounted or through-hole. X-Size Stretch or contract the pad
Y-Size Stretch or contract the pad relative to the Y coordinate. Hole size Specify a hole size used to create
the Excellon N/C drill guide. Holes can be defined up to 2 mils larger than the current pad diameter. This provides a convenient way to define a mechanical hole, with enough tolerance to ensure that it will be free of any copper when manu- factured. If you try to enter a hole size that is more than 2 mils larger than the current pad diameter, TraxMaker will truncate the value so that it is exactly 2 mils larger. The maximum hole size is 1000 mils (1 inch).
X Position Change the x coordinate of the pad by typing a value.
Y Position Change the y coordinate of the pad by typing a value.
No Connection Keep the pad as a “free” pad, unconnected or untagged to another layer.
Relief to Power Plane Convert the pad to one that is connected to the Power Plane and also provides a thermal relief. Available when Multi-Layer is selected. Refer to Figure 4.8. Direct to Power Plane Connect the pad directly to the
Power Plane.
Relief to Ground Plane Convert the pad to one that is connected to the Ground Plane and also provides a thermal relief. Available when Multi-Layer is selected.
Direct to Ground Plane Connect the pad directly to the Power Plane.
Tagged to Power Plane Tag or “mark” the pad to the Power Plane for proper routing. See About Thermal Relief Pads below for more information. Tagged to Ground Plane Tag or “mark” the pad to the
Ground Plane for proper routing. See About Thermal Relief Pads below for more information. Only this pad Restrict the changes to the
currently selected pad.
Matching pads Restrict the changes to matching in this component pads on the same component as
the original selection. Changes are made only if the size and shape attributes match the selected pad.
All matching pads Apply the attribute changes to same hole size pads with matching hole size, as
well as same size and shape. All matching pads Apply the attribute changes to all
component or free pads of the same size and shape.
About Thermal Relief Pads
Thermal relief pads are used to facilitate quick and easy soldering and unsoldering of pins that connect to Power or Ground planes. Because thermal relief pads have sections that cut away from the surrounding copper (see Figure 4.8), heat can be concentrated to the small area around the pad without dissipating across the entire Power or Ground plane. A through-hole component pad, because of its multilayer configuration, naturally connects to the Power and Ground planes. Designating such through-hole pads as thermal relief pads requires no special techniques besides selecting the appropriate option in the Edit Pad dialog box.
SMD (surface mounted) component pads, on the other hand, do require a special technique. Because an SMD pad
cannot penetrate through the Board to the Power or Ground planes, the pad must be connected to a separate through- hole pad by a “stringer” track. There are two ways to create the extra stringer track and pad:
•
Place the stringer and through-hole pad manually using the tools on the Tool Bar.•
Select the nets that are to be connected using the Pwr/Gnd Plane Nets option on the Netlist menu (see
Chapter 6: Nets and Netlists for more information), turn
on the SMD to Pwr/Gnd routing option, then route the Board.
Tagging an SMD Pad to Ground or Power Plane
Suppose you want to designate one of the pads on an SMD component as a thermal relief pad connected to the Power or Ground Plane.
To tag a pad to the Power or Ground Plane,
1 Double click on the pad, select Tagged to Power Plane or Tagged to Ground Plane, then choose OK.
If you want to tag a multilayer pad, you first must change it to be either a Top or Bottom layer pad, then you can select one of the tagging options.
2 Choose Options > Setup, then click on the Router tab.
3 Select SMD to Pwr/Gnd, then choose OK.
4 Choose File > Save to save your work.
5 Choose Route > Board to route connections.
Notice that the SMD pad you designated as tagged is now connected to a relief pad (either Power or Ground) connected by a stringer as in Figure 4.10.
Figure 4.10. One of the pads on SMD component A4 is connected to a separate through-hole thermal relief pad via a stringer.