router

A router in PCB design is a software tool that automatically runs copper traces between component pads to complete electrical connections. After a placement tool positions each IC, resistor, capacitor, and other component on the board, the router takes over: it calculates optimal paths for every signal that must travel from one pad to another, threading wires (traces) through the available space on one or more layers of the board while respecting clearance rules, layer stackup constraints, and electrical requirements.

Routers work by solving what is essentially a massive constraint satisfaction problem. The algorithm must avoid shorts (traces touching where they should not), maintain minimum clearances from other traces and from board edges (typically 5 to 10 mils for conventional PCBs), and ensure that high-speed signals stay on preferred layers or follow length-matching rules. Different routers use different strategies: some employ grid-based pathfinding similar to maze-solving algorithms, while others use ripup-and-reroute logic that tears up partially laid traces and replaces them if a conflict arises or if optimization becomes necessary.

Routing challenges and variants

A design may be unroutable if component placement is too dense or if the layer count is too low. Complex high-speed boards (20+ layers) often require differential pair routing, where two related traces must maintain precise spacing and equal length to preserve signal integrity. BGAs with hundreds of pins demand escape routing, in which traces fan out from the pad array in patterns that allow subsequent layers to handle longer connections. Some boards use blind vias (connecting only outer layers to inner ones) or buried vias (connecting only inner layers) to free up routing space.

Autorouters trade speed for quality. A fully automatic router may complete a simple board in minutes but leave a poor routing with excess trace length, numerous layer changes, or inefficient use of space. Design teams often use interactive routing instead, where the autorouter handles routine connections and engineers manually clean up critical paths, power distribution, and high-speed channels. In very dense designs, humans may route the whole board by hand, treating the autorouter as a starting point or verification tool.

The term "router" borrows from network terminology, where a router directs packets between destinations. In PCB design, it serves the analogous role of directing signals from source to destination. The quality of a routed design directly affects manufacturing yield, electromagnetic compliance, signal integrity, and thermal performance; a poor routing may work in simulation but fail in production or in the field.