Auto-Router
Understanding Auto-Router
After auto-place positions components on the board, the auto-router connects them with copper traces. The router reads the netlist (every pin-to-pin connection), applies design rules (minimum trace width, clearance, via size), and attempts to route 100% of the connections without violating any constraints. A board is considered "fully routed" when every net is connected and every rule is satisfied.
Maze Routing (Lee's Algorithm)
The oldest and most reliable approach. The router overlays a fine grid on the board and uses Breadth-First Search to find the shortest path from source pad to target pad, one cell at a time. It guarantees a connection if one exists, but it is catastrophically slow for dense boards. A 10-layer board with 5,000 nets can require billions of grid cell evaluations.
Memory: O(W x H) per layer (grid cells)
Time: O(W x H x N) worst case
Where:
W, H = Board width and height in grid units
N = Number of nets to route
Example: A 100mm x 100mm board at 25 μm grid resolution = 4,000 x 4,000 = 16 million cells per layer. With 6 layers, that is 96 million cells the algorithm must evaluate for every single net.
Shape-Based Routing
Modern tools abandoned pure grid-based routing in favor of shape-based engines that work with geometric primitives (line segments, arcs, polygons) instead of individual grid cells. This allows 45-degree diagonal traces, smooth curved bends, and variable-width tapering, all of which reduce trace length and improve signal integrity.
Why Auto-Routing Fails for RF Traces
| Failure Mode | What the Router Does | What RF Requires |
|---|---|---|
| Ground return path | Routes a 50-ohm trace over a ground plane split without warning. | Continuous, unbroken ground plane directly beneath every RF trace, verified manually. |
| Bend geometry | Creates 90-degree corners to minimize routing area. | Mitered 45-degree bends or curved arcs to prevent impedance discontinuities above 6 GHz. |
| Via transitions | Drops a single via to change layers when congested. | Coaxial via transitions with ground stitching vias surrounding the signal via to maintain impedance. |
| Trace coupling | Runs two signal traces in parallel for long distances to save space. | Minimum 3x trace-width separation between RF signals to prevent crosstalk below -40 dB. |
| Length matching | Takes the shortest possible path. | Differential pairs matched to within 5 mils; I/Q paths matched to within 1 degree of phase. |
The professional workflow is to manually route all RF signal paths (antenna to LNA, mixer IF output, PLL loop filter), lock them, then allow the auto-router to handle I2C, SPI, UART, GPIO, and power management connections.
Frequently Asked Questions
Can an auto-router create a proper 50-ohm trace?
Modern auto-routers can maintain a specified trace width for impedance-controlled nets if you define the constraint rules correctly. However, they cannot manage the ground plane return path beneath the trace. If the router pushes the trace across a ground plane split or over a void, the impedance changes even though the trace width is correct. Manual routing with continuous ground reference verification is required for any trace above 1 GHz.
What is the difference between maze routing and shape-based routing?
Maze routing (Lee's algorithm) treats the PCB as a grid and explores every possible path cell-by-cell, guaranteeing a solution if one exists but consuming massive memory. Shape-based routing works with geometric shapes instead of grids, allowing diagonal 45-degree traces and curved paths that produce shorter, lower-loss routes. Most modern tools like Cadence Allegro use shape-based engines.
Should I ever auto-route an entire RF board?
No. Even with perfect constraint rules, an auto-router cannot manage the electromagnetic interactions between adjacent traces, the thermal via placement under power devices, or the ground stitching via fences needed between RF stages. The industry standard is to manually route all RF signal paths, power planes, and clock nets first, then auto-route only the remaining low-speed digital and I2C/SPI control signals.