Routing passes, displayed in the group box by the same name, are processed in the order listed. The simpler and faster passes are run first, followed by the more complex strategies.
Note: A directional bias tells
TraxMaker which way to route connections on each layer.
Routing Pass What It Does
Pre-Router Before routing begins, the pre-router checks to see if any connections have been routed manually by the user and marks these connections as already routed.
SMD to Pwr/Gnd Automates the routing of Surface Mount component power and/or ground pins to internal Power and Ground Plane layers. When this option is selected, the autorouter places “stringers” or short track/pad segments, which are con- nected to the appropriate midplane layer using a thermal relief. See About SMD
Routing later in this chapter for informa-
tion about settings to improve comple- tion rates.
SMD Fan Out Special pass to preserve routing room around SMD components by routing each pad in a “fan out” configuration. Memory A fast heuristic pattern router that
places short horizontal and vertical tracks, typically used to connect parallel rows of memory ICs. Short 45 degree track segments are placed to “wave” around blocking pads, clearances permitting. No vias are placed. Line Probe This option enables a number of line
probe and pattern routing passes. These include the basic L, C or Z shapes, with zero to three vias. The Line Probe router is very fast and can provide excellent results, provided good parts placement and correct settings are used. However, it is best suited for lower density boards and probably won’t provide the same degree of completion as the more advanced Maze router.
SMD to Pwr/Gnd
Memory
Maze This is a gridded wave expansion router with rip-up-and-retry capabilities. The maze router can take some time to complete on a complex board, but it will always provide the highest degree of completion of any of the routing passes, provided the correct settings are used. Maze routing is primarily used for increasing completion. It will rip-up and reroute other connections which block its path. Maze routing places a high priority on layer biasing. When the Maze router rips-up an obstacle, it doesn’t necessarily rip up the entire connection. In most cases it will only rip-up and reroute a single track. Sometimes this reroute will result in some fairly obvious backtracking, so it is always best to run at least one smoothing pass after maze routing. The Maze router is an iterative router, meaning that after it has completed one pass of routing attempts, it can try again to see if any of the rip-ups from the previous pass have cleared a way for any new routes which were previously unsuccessful. The Maze Passes setting tells the router how may times it should go through the list of unrouted connec- tions before giving up.
About SMD Routing
The SMD to Pwr/Gnd option on the Setup Router dialog box automates the routing of Surface Mount component power and/or ground pins to internal Power and Ground plane layers. When this option is selected, the autorouter places “stringers”—short track/pad segments which are connected to the appropriate midplane layers, using a thermal relief.
Maze
To use the SMD to Pwr/Gnd option,
1 Choose Netlist > Pwr/Gnd Plane Nets.
2 If a netlist has been previously loaded, select Power plane and Ground plane nets from those listed. If a netlist hasn’t been previously loaded and you want to select the Power/Ground plane nets from a list of all nets, you will have to load a netlist prior to performing this step.
OR
Type the names of the Power and Ground plane net if you know which ones they are.
3 Choose OK.
4 Choose Netlist > Load Netlist, then open the netlist into your design. The nets specified in step 2 will be
designated as already having been routed, so they will not display in the list of loaded nets or as part of the ratsnest.
5 Choose Options > Setup and click on the Router tab, then select the SMD to Pwr/Gnd option.
6 Proceed with routing as usual. Note that short stringer tracks (a short track and a 40 mil pad) will be autorouted to the SMD power and/or ground pads and connected to the appropriate plane using a thermal relief as in Figure 7.4.
If you want a direct (rather than thermal relief) connection, you will have to reedit the pads after routing.
Stringers are short track segments connected to a midplane layer with a thermal relief.
Figure 7.4. One of the pads on SMD component A4 is connected to a separate through-hole thermal relief pad by way of a stringer.
SMD designs provide additional challenges for autorouting, because pads can normally only be routed on a single layer. The following settings can be used to improve completion rates:
Option Setting
Router Grid 5 mil Track Width 8 mil
Via Size 40 mil
Track to Track 8 mil separation Track to Via 8 mil separation Via to Pad/Via 8 mil separation Via/Pad to Pad 8 mil separation
Smoothing
Use the options in the Smoothing Passes group box to clean up routed connections. Smoothing passes are performed immediately following the routing passes. They can also be done at any time, independent of the routing passes to perform a “clean up only” action.
It is recommended that you run the routing passes sepa- rately from the smoothing passes. This lets you conve- niently revert back to your “routed” file if you don’t like the result of the smoothing pass.
Smoothing Option What It Does
Smooth User Routed Applies the smoothing passes to all connections, regardless of how or when they were placed. If not
checked, only the tracks routed by the netlist-based routing tools will be smoothed.
Basic Includes Via Minimization, Diagonal Remover, Loop Remover, Double- back Remover, Dual Stub Remover, Two-Via Remover and Arc Replacer (if enabled.)
Advanced The advanced smoother uses the Maze router to find better routing paths. Multiple passes with the advanced smoother will generally result in better track placement. Arc Replacer The Arc Replacer converts 90
degree “corners” to 90 degree arcs, clearances permitting. You can specify an arc radius in the Arc
Radius text box. The arc line width
depends on the Track Width router variable setting. Only tracks of the same (matching) width will be modified during a single pass. You can reset the Track Width variable and run the arc replacer on other routed or pre-routed tracks.
Tip: Run routing passes
separately from smoothing passes.
Variables
The Variable settings define the grid, track width, via size and via hole size that are used by the autorouter.
Clearances
The Clearance settings define the minimum separations maintained between placed and routed primitives such as tracks to tracks, tracks to vias, vias to vias, vias to pads and pads to pads. To change a separation value, type it in the appropriate text box.