Using the Fanout Command Effectively

Allegro PCB Router Application NoteUsing the fanout command effectivelySource: Cadence Engineering March 1998Revised by: Geoffrey Indrajo March 1998Edited by: David Busdeicker August 2001This application note explains when and how you can best use the basic fanout commandin ALLEGRO PCB ROUTER.OverviewThe fanout command generates escape wires and vias for SMD pins. Because SMDcomponents are so common in modern PCB designs, fanout is one of the most frequentlyused commands in ALLEGRO PCB ROUTER. This application note discusses when andhow you can best use the basic fanout command in ALLEGRO PCB ROUTER.Using FanoutFanout is one of the first commands you should use if your PCB design incorporatesSMD components. If you do not generate escape wires and vias for the pins of SMDcomponents, those pins will probably not be connected during autorouting.After running fanout, you should investigate any remaining unroutes to assure that theautorouter will achieve 100% completion overall during the main routing phase.The following tips about using fanout should improve your success in autorouting.• Do not use fanout if the design does not have SMD components.• Do not use fanout if the design is restricted to only one or two signal layers. The vias placed during fanout willblock much of the routing area that is needed by the autorouter in designs that only have one or two signallayers. This situation will likely prevent the autorouter from achieving 100% completion. You should only usefanout on designs that have four or more signal layers.• If the design contains buses (groups of pins that belong to the same net and that are located on the same X orY coordinate), and you are using the bus_diagonal command, do not use the fanout command first. The fanoutcommand must follow the bus_diagonal command in all ALLEGRO PCB ROUTER versions prior to version 8.0.• Generally, you will see very little improvement by attempting to run more than five fanout passes. If certain SMDcomponents are not fanned out after five passes, problems with placement, pin spacing, or other clearancerules are inhibiting the fanout.Investigating fanout unroutes

One way to resolve fanout unroutes is to highlight and inspect all of the fanout violations.To do this, use the command highlight no_fanout or choose Highlight from the Viewmenu, and then select SMD Pads Without Fanouts.After considering this overall view of the fanouts, you now have the option to globallychange a parameter on your design, or to closely investigate each problem individually,correcting them one at a time.To look at each fanout unroute1. From the View menu, choose Visit.This displays the Visit dialog box.2. Click the SMD tab.3. Select SMD Pads Without Fanouts.4. Click First to visit the first instance, and then click Next to visit the subsequent instances.The count shown in the Current text box indicates which instance you are currently visiting, out of a total count.Each failure you visit will display the pad in the center of the screen. These should be highlighted if you invokedthe highlight no_fanout command earlier.In the example Visit dialog box shown here, there are 91 pads without fanouts to visit.However, the highlight no_fanout command would report that there are 119 pads withoutfanouts. This difference is due to the fact that Visit does not count power and ground netsthat are not fanned out. This might be corrected in a future release.

Another discrepancy you may encounter is an occasional small difference between thenumber of fanout failures listed in the status file report (report status window) ascompared to the number of failures noted in the highlight command. This differenceoccurs primarily if you use multiple fanout passes. The difference between the tworeports should be small. The number of fanouts reported by the highlight command isalways the correct number, and you should rely on that count.Fixing fanout problemsThe following guidelines will help you resolve some of the more common problems youmay encounter with fanout.• Check the via grid size. A via grid that is too large will significantly reduce the number of places where vias arepermitted. Consider setting a via grid of 1 and using via-to-via clearance rules.• Use smaller vias. This solution will yield more board area for placing vias around dense SMD componentpatterns.• Check the component placement and try to adhere to the following guidelines for placement:ooPlace similarly sized SMD components back-to-back on opposite sides of the board, where possible.If similarly sized SMD components are not available, place as many SMD discrete components aspossible on the opposite side of the larger SMD footprints.Both of these placement tips should yield more area for via placement.• Using the interactive editor, manually edit the existing fanouts to make space for the escape vias that are stillneeded to complete the fanout of the unrouted SMD pins.ConclusionHigh fanout completion is essential to achieving good autorouting results. If you cannotfanout certain SMD components in the design, you will probably not achieve 100%completion during autorouting. ALLEGRO PCB ROUTER provides tools to find,investigate and correct areas where you might have problems with fanout. By applyingthese effectively, you will create better PCB designs and achieve higher success rates inautorouting them.