autorouter

An autorouter is a software tool that automatically generates the copper trace patterns connecting electrical components on a printed circuit board (PCB). Instead of manually drawing each wire path between pins, a designer loads a netlist (the electrical connectivity requirements) and the autorouter calculates and places conductive traces on multiple copper layers, working within constraints like minimum trace width, spacing rules, and the physical locations of drilled vias.

The core task is path-finding under tight constraints. The autorouter must connect thousands of nets while respecting design rules: typical minimum trace widths are 0.1 to 0.25 mm for consumer boards, 0.05 mm for high-density designs. It avoids creating electrical shorts, minimizes trace length to reduce signal degradation and electromagnetic interference, and distributes traces across available layers (modern boards have 4, 6, or more copper layers). Most autorouters use variants of maze-routing or other graph-search algorithms, often with passes that gradually refine solutions.

When autorouting fails

Autorouters regularly get stuck in congested areas, leaving unrouted nets despite available board space. A designer then manually routes critical or problematic traces by hand (interactive routing), then re-runs the autorouter on remaining nets. Densely packed boards with hundreds of pins, tight spacing between components, or asymmetrical layouts frustrate automated tools. Power and ground planes complicate matters: they must carry high current and low impedance, so manual design and layer-stack planning typically precedes autorouting. High-speed signals (clock lines, differential pairs) are almost always routed by hand.

The autorouter sits between layout (component placement) and physical verification in the PCB design flow. A well-placed layout makes autorouting succeed; poor placement forces manual workarounds. Professional design suites like Cadence Allegro, Mentor Graphics xpedition, and Altium Designer include integrated autorouters. Open-source tools like FreeRouting provide basic autorouting for hobbyist designs. Turnaround time matters: for a 500-net board, a capable autorouter can complete routing in seconds to minutes, whereas manual routing of the same board takes hours.

The name reflects its automation function: it routes (connects traces) on its own, without interactive user guidance for each connection. The term has remained consistent since the 1980s as PCB design moved from drafting table to computer. Autorouter quality is measured by completion rate (percentage of nets successfully routed) and design metrics: total trace length, number of vias, number of layer changes, and maximum via density in congested regions.