PCB auto-routing workflow -- Freerouting (open-source) integration with KiCad, interactive router tips, design rule prep, and post-route cleanup checklist. Use after component placement to route a board automatically or semi-automatically.
| Method | Best for | Avoid when |
|---|---|---|
| Freerouting (autorouter) | General digital I/O, non-critical signal nets, high net count boards | Any impedance-controlled or differential signal |
| KiCad interactive router | Guided routing with DRC enforcement, semi-manual cleanup | Hundreds of nets (too slow) |
| Manual routing | All critical signals — see table below | Bulk single-ended digital nets |
Decision rule: Route critical signals manually first, then hand the remaining nets to Freerouting. A board where 20% of nets are critical and 80% are general digital can use Freerouting productively for the 80%.
| Signal type | Reason | Correct method |
|---|---|---|
| Differential pairs (USB, LVDS, Ethernet, HDMI, PCIe) | Requires precise length matching and controlled impedance; autorouter cannot guarantee pair skew within spec | KiCad differential pair router with interactive length tuning |
| Crystal oscillator traces | Must be short, have minimal parasitic capacitance, and often need a guard ring; length and topology matter | Manual — route crystal as close to IC as possible, keep symmetric |
| RF traces (antenna feed, LNA input/output, matching networks) | Controlled impedance (typically 50 Ω); trace width calculated from stackup; any detour changes characteristic impedance | Manual — calculate width from stackup, route directly |
| Power rails and planes | Current capacity determines trace width (IPC-2221); wide traces or filled zones needed | Manual or copper pour — set by IPC-2221 current tables |
| High-current switching node (buck/boost) | Minimizing loop area is critical for EMC; autorouter maximizes completion, not loop area | Manual — identify the critical switching loop and route it first |
| Decoupling capacitor bypass paths | Must be the shortest physical path between IC power pin and decoupling cap | Manual — place cap before routing, connect before autorouting runs |
| DDR/LPDDR address and data buses | Fly-by topology, length matching within byte lanes, Vref routing | Manual with KiCad length tuning |
| Clock distribution networks | Star topology or matched-length tree; autorouter uses shortest path, not matched path | Manual with KiCad length tuning |
Work through every item before starting the router. Skipping steps causes incomplete routes or DRC violations that are expensive to fix after routing.
Net class assignment:
Design rule import:
.kicad_dru constraint file before routing..kicad_dru.Component placement:
E on component → Lock).Board outline:
Copper pours disabled:
B fills, unfill is under Edit menu or use Ctrl+B).Ratsnest verification:
Teardrops:
Download: